Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) WW Part 10 ppt

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NONDESTRUCTIVE EVALUATION 713
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0.001
0.002
0.003
0.004
0.005
0.006
200 240 280 320 360 400 440 480 520 560 600 640 680
Time (microsec)
Rel. amp. (power units)Rel. amp. (power units)
0
0.01
0.02
0.03
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0.05
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100 140 180 220 260 300 340
Time (microsec)
Figure 53. Noncontact ultrasound transmission through a hu-
man heel using 250-kHz (top) and 500-kHz (bottom) frequency
transducers. The first peak corresponds to ultrasound transmis-
sion through air, skin, tissue, and heel bone. Other peaks are not
identified.
the material surface in ambient air. The ultrasound re-
ceived by this transducer was amplified by a 64-dB gain.
Figure 55 shows the time and frequency domain of the

ultrasound detected (heard) by the NC transducer. By
sweeping the frequency across a wide range, the frequency-
dependent response from the source (vibrating system) can
be investigated and related to its characteristics or condi-
tion. In this mode, we successfully interrogated frequencies
Non-contact
passive “Listener”
3.5 MHz 12.5 mm
diameter
Broadband
amplifier
3 mm Ambient air
25 mm
Steel
Single burst
16 volt sine wave
Ultrasound source
transducer
800 KHz to 8 MHz
Bandwidth at −6 dB
Figure 54. Experimental setup for passive operation of noncon-
tact transducer.
Figure 55. Time and frequency domains of ultrasound detected
by noncontact transducer, per Fig. 54 setup.
as high as 7 MHz in ambient air. This opens the door to
noncontact acoustic emission, acoustoultrasonics, and any
other situation where detection of high frequency ultra-
sound is desired. Applications of the passive use of NC
transducers are dynamics of vibration, materials cutting,
testing of railroad, highways, bridges, runways, etc.

Other Noncontact Ultrasound Applications
Besides the applications of NCU described here, this mode
can also be used for level detection; dimensional and
proximity analysis; high temperature material evaluation;
analysis of liquid-sensitive and hazardous material, and
analysis of gases and liquids. Finally, it suffices to say that
if ultrasound can be propagated through a medium or re-
flected from an interface, then much information about the
medium and the interface can be obtained.
CONCLUSIONS
In this paper, we outlined the significance of ultrasound for
nondestructive characterization of materials and for non-
invasive diagnostic applications in the medical field. We
have also shown the feasibility of noncontact ultrasonic
measurements in the time, frequency, and image domains,
analogous to other wave-based methods.
Underscoring the significance of the noncontact ultra-
sound mode, we presented a detailed discussion about the
difficulty of achieving this mode. We have also shown that
this work ultimately resulted in very high transduction
noncontact transducers, thus making the noncontact ul-
trasound mode a reality. Applications of these transducers
in industry and the medical field have been described by
using documentary evidence.
We also provided an introduction to a novel ultrasonic
noncontact analyzer and its applications for characterizing
industrial and biomedical materials and products.
We believe that the noncontact ultrasound mode is
among the most significant developments for characteriz-
ing and analyzing all states of matter. Though we have

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714 NONDESTRUCTIVE EVALUATION
provided selected examples of its applications, there is no
doubt that the users of this technology will further enhance
its use in materials quality, process control, and health care
in our increasingly complex world. This advancement in
the field of ultrasound and materials characterization has
opened much needed and unprecedented opportunities in
research and education.
ACKNOWLEDGEMENTS
The author gratefully acknowledges the assistance of
M. Langron, Ultran Laboratories, in producing the trans-
ducers used for this paper. The enthusiastic support
and valuable suggestions of E. Blomme, Katholieke
Hogeschool, Belgium and M. Landa, Academy of Sciences,
Czech Republic, are acknowledged in kind. The work pre-
sented in this article was supported by the continuing
efforts of SecondWave and Ultran Laboratories for the ad-
vancement of industry and medical science through inno-
vative developments in ultrasound.
BIBLIOGRAPHY
1. J. Curie and P. Curie, Bull. no. 4 Soc. Mineral. France 3:90
(1880), C.R. Acad. Sci. Paris 91:294 (1880).
2. Apparatus for Warning a Ship at Sea of its Nearness to Large
Objects Wholly or Partially under Water, Brit. Pat. Specifica-
tion 11,125, March 27, 1913, R.L. Richardson.
3. R.E. Green, in Materials Analysis by Ultrasonics, A. Vary, ed.,
Noyes Data, NJ, 1987, p. 6.
4. Z. Cho, J.P. Jones, and M. Singh,Foundations of Medical Imag-

ing. Wiley, NY, 1993, pp. 477–486.
5. R.M. White, J. Appl. Phys. 34: 3559–3567 (1963).
6. A.A. Bondarenko, Y.B. Drobat, and S.V. Kruglov, Soviet J. NDT
12: 655–658 (1976).
7. H.M. Ledbetter and J.C. Moulder, J. Acoust. Soc. Am. 65: 840–
842 (1979).
8. A.M. Aindow, R.J. Dewhurst, S.B. Palmer, and C.B. Scruby,
NDT Int. 17: 329–335 (1984).
9. G.A. Allers, in Intelligent Processing of Materials and Ad-
vanced Sensors, H.N.G. Wadley, P.A. Parish, B.B. Rath, and
S.M. Wolf, eds., Metallurgical Society, PA, 1986, pp. 17–27.
10. J.A. Brunk, Allied Signal, private communication, 1999.
11. J.A. Brunk, C.J. Valenza, and M.C. Bhardwaj, in Acousto-
Ultrasonics, Theory and Applications, J.C. Duke, Jr., ed.,
Plenum Press, NY, 1988, pp. 231–238.
12. M.C. Bhardwaj and A. Bhalla, J. Mater. Sci. Lett. 10 (1991).
13. N. Kulkarni, B. Moudgil, and M. Bhardwaj, Am. Ceram. Soc.,
Ceram. Bull 73(6): (1994).
14. J.D. Fox, B.T. Khuri-Yakub, and G.S. Kino, 1983 IEEE Ultra-
sonics Symp., 1983, pp. 581–592.
15. T. Yano, M. Tone, A. Fukumoto, IEEE Trans. UFFC 34(2): 222–
236 (1987).
16. M.I. Haller and B.T. Khuri-Yakub, IEEE Ultrasonics Symp.,
1992, pp. 937–939.
17. D. Reilly and G. Hayward, IEEE Ultrasonic Symp., 1991,
pp. 763–766.
18. Ultrasonic Transducer for High Transduction in Gases and
Method for Ultrasound NonContact Transmission into Solid
Materials, US and international patents pending and in pro-
cess, 1997–1999, M.C. Bhardwaj.

19. D.W. Schindel, D.A. Hutchins, L. Zou, and M. Sayer, IEEE
Trans. Ultrasonics Ferroelectic Frequency Control 42:42–51
(1995).
20. I. Ladabaum, B.T. Khuri-Yakub, and D. Spoliansky, Appl.
Phys. Lett. 68:7–9 (1996).
21. M. Castaings and B. Hosten, Ultrasonics 36: 361–365 (1998).
22. M. Landa, M.C. Bhardwaj, and I. Neeson, Institute of Ther-
momechanics, Academy of Sciences of the Czech Republic,
Prague, CZ, Report no. Z1266/99 (1999).
23. M.C. Bhardwaj, Mater. Res. Innovation 1: 188–196 (1997).
24. J.P. Jones, D. Lee, M. Bhardwaj, V. Vanderkam, and
B. Achauer, Acoust. Imaging 23: (1997).
25. M.C. Bhardwaj, Proc. Am. Ceram. Soc. 89: (1998).
26. T. Carneim, D.J. Green, and M.C. Bhardwaj, Ceram. Bull.
(1999).
27. B.R. Tittmann, M.C. Bhardwaj, V. Vandervalk, and I.
Neeson, Proc. 23rd Annu. Conf. Composites Adv. Ceram. Mater.
Struct. The American Ceramic Society, Westerville, OH, 1999.
28. M.C. Bhardwaj, I. Neeson, M.E. Langron, and V. Vandervalk,
24th Annu. Conf. Composites Adv. Ceram. Mater. Struct. The
American Ceramic Society, Westerville, OH (2000).
29. R.Y. Vun, Q. Wu, M. Bhardwaj, and G. Stead, Proc. 12th
Int. Symp. Nondestructive Test. Wood, University of Western
Hungary, Sopron, Hungary, 2000.
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PAINTS
SHIGENORI EGUSA
∗

Japan Atomic Energy
Research Institute
Takasaki-shi, Gunma, Japan
INTRODUCTION
Paints are used everywhere in an industrialized society
(1,2). The most important functions of paints are protec-
tion and decoration of a substrate. Paints can protect sub-
strates against corrosion, oxidative aging, weathering, and
mechanical damage and can also provide pleasant color
contrasts or a lustrous appearance, hide imperfections in
the substrate such as knots in wood, or enhance the beauty
of the substrate by using a wood grain. In other words,
paints can add to the useful life of materials and also to
their attractiveness (1).
Smart paints are an innovative type of paint that has
a sensor function as well as the protective and decora-
tive functions of conventional paints. Smart paints can de-
tect abnormal vibration of a structural material by mon-
itoring the natural frequencies and mode shapes of the
material. They can also detect damage generated in the
material by monitoring the acoustic emission (AE) wave
traveling from the damage location to the material sur-
face. Vibration and AE can be monitored in real time, thus
enabling health monitoring of the material even during
operation.
Smart paints are used in large-scale structures such as
vehicles operated at high speeds, civil infrastructures of
huge mass and volume, and special facilities that contain
large amounts of petroleum, nuclear fuel, and explosive
substances. An accident in these facilities can be cata-

strophic because an enormous amount of energy stored in
the form of kinetic, potential, or internal energy is released
suddenly by the accident. Smart paints can possibly pre-
vent such a disaster by warning of abnormal vibration and
damage generated in a structural material. Hence, one ref-
erence goes so far as to say “Brush with disaster—Smart
paint warns of impending doom” (3).
The frequency of health monitoring needed for struc-
tural materials increases steadily as age increases be-
cause the corrosion of steel and concrete progresses gradu-
ally during the service period of several decades. Smart
paints can be applied to a structural material at any
time before and after the construction of the structure,
thus making health monitoring quite, easy even for a
structure already in active service. Smart paints can
make a significant contribution to increasing the service
life of a structure, and consequently to saving natural
resources.
∗
Deceased
BASIC CONCEPTS OF SMART PAINTS
The frequency range covered by vibrational measurements
is the low-frequencyrangebelow ∼20 kHz (4),whereas that
covered in AE wave monitoring is the ultrasonic frequency
range above ∼20 kHz (5). Therefore, if the sensitivity of
a smart paint is high enough in both frequency ranges,
the paint can be used as a vibrational and AE sensor inte-
grated into a structural material. Such a sensor function
of a smart paint is analogous to the action of a sponge
that discharges and soaks up water in response to the

application and release of external pressure (6). In this
analogy, a smart paint is a sponge that repeats the cycle of
releasing and drawing an electrical charge at the natural
frequency of a structural material or at a frequency of the
AE wave traveling through the material.
A smart paint isapplied directly to the surface ofastruc-
tural material when the material is a conductor like metal
or carbon fiber composite. In this case, the conducting ma-
terial can be used as a bottom electrode for the smart paint.
When the structural material is an insulator like concrete
or ceramic, on the other hand, an electroconductive paint
is first applied to the material surface, thus forming a thin
conducting layer as a bottom electrode. Then, the smart
paint is applied to the surface of the bottom electrode.
Whether the structural material is conducting or insulat-
ing, an electroconductive paint is applied to the surface of
the smart paint film, thus forming a thin conducting layer
as a top electrode. Then, a high voltage is applied to the
smart paint film using the top and bottom electrodes, thus
making the film piezoelectrically active. This poling proce-
dure is usually performed in air at room temperature.
Smart paints are piezoelectric composites that consist of
piezoceramic and polymer phases (see Characterization of
Piezoelectric Ceramic Materials; Piezoelectricity in Poly-
mers). Thus, smart paints and piezoelectric composites
have essentially the same nature with respect to many fac-
tors such as the ceramic/polymer composition, the method
of preparation, the poling procedure, and the mechanical,
electrical, and piezoelectric properties. An essential differ-
ence exists in that a piezoelectric composite is used as a

discrete point sensor or actuator, but a smart paint is used
as a continuously distributed sensor that can cover a large
surface area of a structural material.
PIEZOELECTRIC COMPOSITES
Piezoceramics such as barium titanate (BaTiO
3
) and
lead zirconate titanate (PZT) are typical piezoelectric
materials that have excellent properties such as a high
electromechanical coupling coefficient and a moderate
dielectric constant (7,8). Piezoceramics, however, have
the problem that the high density inherent in ceramics
makes the specific acoustic impedance much higher than
that of water or human tissue, thus causing impedance
mismatch (7). Brittleness common to all ceramics is
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another drawback of piezoceramics. Piezoelectric polymers
such as poly(vinylidene fluoride) (PVDF), on the other
hand, do not have the problems of brittleness and
impedance mismatch, and furthermore have the excellent
property that they can be formed into thin, broad films.
However, the electromechanical coupling coefficients and
the dielectric constants of piezoelectric polymers are much
lower than those of piezoceramics (8).
A solution to these problems is the previously men-
tioned piezoelectric composites that consist of piezoceramic
and polymer phases. The polymer phase in the composites

increases the composite toughness and also decreases the
composite density and dielectric constant, thus solving
the problems of piezoceramics and piezoelectric polymers
simultaneously (9–11). The electrical and mechanical
properties of piezoelectric composites are determined
primarily by the fraction of the piezoceramic and polymer
phases and by the properties of these constituent materials
(12–14). Composite properties are affected also by the con-
nectivity pattern of the piezoceramic and polymer phases
(15–20).
COMPOSITION OF SMART PAINTS
The smart paints reported so far are piezoelectric compos-
ites made up of piezoceramic particles dispersed in a poly-
mer matrix. The polymer matrix need not be piezoelectri-
cally active, and hence popular polymers such as alkyd,
acrylic, and epoxy resins can be used as the matrix resin.
The preparation of smart paints and the application pro-
cedures are essentially the same as those of conventional
paints, except for poling for a dried film of smart paint. As
a result, most of the fundamental characteristics and func-
tions of conventional paints are imparted to smart paints,
thus enabling smart paints to have protective, decorative,
and sensor functions simultaneously.
Smart paints can form continuous paint films covering a
large surface area of a structural material. Because of the
electrically insulating nature of the paint film, however,
the electrical charge actually detected is only that gener-
ated in a region that has an electrode on the surface of
the paint film. Therefore, if a set of separate electrodes is
formed on the paint film surface, the electrical charge gen-

erated in each region can be detected and analyzed sepa-
rately. This feature of smart paints enables the application
of the paints as a vibrational modal sensor that can deter-
mine the natural frequencies and mode shapes of a struc-
tural material (21,22). Furthermore, this feature enables
another application of smart paints as an AE sensor that
can determine the damage location in a structural mate-
rial quite easily without using the conventional technique
based on the arrival time difference of an AE wave (5).
Paints in general can be applied to all kinds of materi-
als such as metals, composites, concrete, and ceramics; the
material surface can be flat, curved, or even irregularly
shaped. Furthermore, paints can be applied and reapplied
at any time, when necessary. Final dry films of paints are
generally light, flexible, and tough. These excellent prop-
erties of paint in general are imparted to smart paints as
well, thus giving the smart paints further useful features
as vibrational and AE sensors integrated into a structural
material.
FORMATION OF SMART PAINT FILMS
Paint Preparation, Application, and Curing
Paints in general are made up of three components: pig-
ment, binder, and volatile liquid (1,2). The volatile liquid
is a solvent or a nonsolvent that provides a practical vis-
cosity for packaging and application and does not normally
become part of the dried paint film. The binder is a film-
forming substance which is mostly a polymeric material
such as alkyd, acrylic, or epoxy resin. The binder is used
as a solution in a solvent or as a dispersion of fine particles
in a nonsolvent. Such a solution or dispersion is called a

vehicle. Paint pigments are finely divided, insoluble, solid
particles such as titanium dioxide (TiO
2
), zinc oxide (ZnO),
and calcium carbonate (CaCO
3
). The pigment particles are
dispersed stably in the paint vehicle before application and
the pigment particles are distributed uniformly through-
out the binder resin in the dried paint film. The decora-
tive functions of a paint are due, for the most part, to the
pigment.
The basic components of smart paints are essentially
the same as those of conventional paints, except that piezo-
ceramics such as PZT and BaTiO
3
are used as pigments in
smart paints. The piezoceramics used in the smart paints
so far are PZT (23–30) and lead titanate (PbTiO
3
) (23), and
the binders used are acrylic resin (23), polyurethane (23),
and epoxy resin (25–29). Smart paints made up of these
components are prepared by essentially the same proce-
dure as used for conventional paints. Smart paints are
applied by using familiar coating tools such as brushes,
rollers, or spray guns. Smart paints are also cured in the
usual way in air at ambient temperature or at elevated
temperatures.
Electrode Formation and Poling

A simple method for forming an electrode on the surface
of a paint film is to apply an electroconductive paint by us-
ing a coating tool such as a brush or roller. A more elaborate
method is to deposit a vapor of gold or aluminum onto the
paint film surface (30). A screen mask technique is also ef-
fective for this purpose, especially when the electrode pat-
tern is complicated. The main advantage of this technique
is that leads as well as electrodes can be printed on the
paint film surface, as shown in Fig. 1. This technique, how-
ever, has the disadvantage that it cannot be used for large
structures such as airplanes, trains, or bridges.
For such large structures, an ordinary coating method
by brush, roller, etc. may be the most practical for forming
an electrode on the paint film surface. As a lead for the
electrode, on the other hand, a thin electrical wire or tape
∼50 µm thick or so may be the most practical choice for a
large structure because such a thin wire or tape is compa-
rable in thickness to a paint film and hence, can be buried
in the paint film or under a topcoat. Note that when smart
paints are put into practical use, the electrodes and leads
are covered by a topcoat, thus making the appearance ex-
actly the same as that of conventional paints.
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756 PAINTS
Figure 1. Electrodes and leads printed on a PZT/epoxy paint film formed on one surface of an alu-
minum beam. The left end of the beam where the leads come together is wrapped in an electrically
insulating material. The aluminum beam is clamped at this section for vibrational measurements.
Piezoelectric composites are usually poled in an oil bath
at elevated temperatures because poling at a higher tem-

perature achieves saturation poling in a lower poling field.
For smart paints, on the other hand, poling is done in air
at room temperature because even room temperature pol-
ing can achieve high enough piezoelectric activity for the
paint application to serve as vibrational and AE sensors
integrated into a structural material (25–29).
EVALUATION OF SMART PAINT FILMS
The sensor function of smart paints relies heavily on the
piezoelectric activity of the poled paint film. Usually, the
activity is expressed in terms of a piezoelectric constant
which is the ratio of the charge developed per unit sur-
face area or the voltage developed per unit film thickness
to the stress or strain applied externally. The charge-to-
stress, voltage-to-stress, charge-to-strain, and voltage-to-
strain ratios are the piezoelectric constants d, g, e, and h,
respectively (7).
Piezoelectric materials are inherently anisotropic, and
hence two subscripts are attached to the piezoelectric
constant to describe the anisotropic properties. The first
subscript is used to indicate the direction of the charge or
voltage development, and this is always the film thickness
direction for a piezoelectric film such as PVDF or a smart
paint film. The second subscript is used to indicate the di-
rection of the stress or strain applied externally, and this
direction is any of the 1, 2, and 3 axes of the film which
correspond to the length, width, and thickness directions,
respectively (7).
Sensitivity as a Vibrational Sensor
When a structural material is deformed, strain is devel-
oped in all directions of the material, including the direc-

tion tangent to the material surface. This is also true when
the structural material is vibrating. For a smart paint used
as a vibrational sensor, therefore, one of the most impor-
tant sensitivities to be evaluated is the piezoelectric con-
stant e
31
because this constant is the ratio of the charge
per unit surface area to the strain in the direction tangent
to the paint film surface.
The e
31
constant is evaluated from vibrational measure-
ment on a cantilever beam like that shown in Fig. 1. A
typical example of the measurement is shown in Fig. 2
10
−6
10
−7
10
−8
10
−9
10
−10
10
−8
10
−7
10
−6

10
−5
10
−4
0 50 100 150 200 250
Strain amplitude, m/m
Output charge, C/m
2
Frequency, Hz
Figure 2. Frequency spectra of output signals from a PZT/epoxy
paint film formed on one surface of an aluminum beam and from
a strain gauge bonded to the opposite surface of the beam.
for a paint film which has the PZT/epoxy composition of
53/47 by volume and is formed on the surface of an alu-
minum beam 3.0 mm thick, 30 mm wide, and 460 mm long
(350 mm long as a cantilever beam) (27). This example is
for a 109-µm thick paint film cured at room temperature
and poled at 240 kV/cm for 5 min. The spectrum shape ob-
tained from the paint film is similar to that obtained from
a strain gauge which is bonded to the opposite surface of
the beam to monitor the strain developed in the direction
of the cantilever length. Then, the e
31
constant is evaluated
from the charge-to-strain ratio at a natural frequency of 18
or 112 Hz.
The e
31
constant thus evaluated depends on many fac-
tors such as the poling field, the film thickness, the cure

temperature, and the PZT/epoxy composition (26,27). A
typical example of the poling-field and film-thickness de-
pendence is shown in Fig. 3 for paint films cured at room
temperature that have the PZT/epoxy composition of 53/47
by volume (27). The e
31
constant increases steadily as the
poling field increases for all of the paint films shown here,
and saturation poling is not achieved, even at a high pol-
ing field of ∼150 kV/cm. The e
31
constant obtained at a
particular poling field, say, 100 kV/cm, increases as film
thickness increases from 33 to 152 µm, thus exhibiting a
clear film-thickness dependence. This point is further de-
scribed later.
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40
30
20
10
0
e
31
(mC/m
2
)/(m/m)
0 50 100 150 200

Poling field (kV/cm)
Figure 3. Plots of the piezoelectric constant e
31
vs. the poling
field for PZT/epoxy paint films cured at room temperature and
evaluated as a vibrational sensor.
Sensitivity as an Acoustic Emission Sensor
In many cases, eventual failure of a structural material oc-
curs after a certain amount of damage accumulates within
the material. The generation of such damage is almost al-
ways accompanied by the emission of an AE wave, and
hence the damage generated and accumulated can be de-
tected by monitoring the AE wave (5). The AE wave is emit-
ted in all directions, and consequently, an AE wave that
arrives at the material surface and enters the smart paint
film on the material surface always exists. Furthermore,
an AE wave that enters the paint film nearly perpendi-
cularly always exists. Such an AE wave develops strain in
the paint film in the direction normal to the film surface be-
cause the AE wave is a compression wave in which particle
motion is in the same direction as the propagation of the
wave. For a smart paint used as an AE sensor, therefore,
the sensitivity to be evaluated is the piezoelectric constant
h
33
because the h
33
constant refers to the ratio of the volt-
age per unit film thickness to the strain in the direction
normal to the paint film surface.

For a conventional AE sensor, the sensitivity s is usu-
ally given by s = V/v
0
, where V is the output voltage of the
sensor and v
0
is the velocity amplitude of AE waves (31).
The strain amplitude of AE waves ε
0
is given by ε
0
= v
0
/v,
where v is the phase velocity of AE waves. Combining these
equations with h
33
= (V/d)/ε
0
leads to s = h
33
d/v, where
d is the film thickness. This equation indicates that the
paint film sensitivity as an AE sensor s is independent
of the frequency of AE waves and that the sensitivity in-
creases linearly as film thickness increases. This equa-
tion also indicates that the h
33
constant is calculated from
h

33
= sv/d.
The paint film sensitivity as an AE sensor is evaluated
from measurement using an ultrasonic transducer to pro-
duce AE waves and a laser Doppler vibrometer to moni-
tor the velocity amplitude of the AE waves (28). A typical
example of the measurement is shown in Fig. 4 for a paint
film that has the PZT/epoxy composition of 53/47 by vol-
ume and is formed on the surface of square aluminum plate
10
−3
10
−4
10
−5
10
−6
10
−7
10
−8
10
−7
10
−6
10
−5
10
−4
10

−3
10
−2
0 0.2 0.4 0.6 0.8 1.0 1.2
Output voltage, V
Frequency, MHz
Velocity amplitude (m/s)
Figure 4. Frequency spectra of output signals from a PZT/epoxy
paint film formed on one surface of an aluminum plate and from
a laser Doppler vibrometer that monitors the velocity amplitude
of AE waves.
0.2 mm thick that has 50 mm sides. This example is for a
152-µm thick paint film cured at room temperature and
poled at 184 kV/cm for 5 min. The spectral shape obtained
from the paint film is similar to that obtained from the
laser vibrometer in the frequency range above ∼0.3 MHz.
Such a similarity of spectral shapes reflects a nearly flat
frequency response of the paint film to AE waves. Then,
the paint film sensitivity as an AE sensor is evaluated
from the average ratio of the output voltage of the paint
film to the velocity amplitude of AE waves in the frequency
range 0.3–1.0 MHz.
The paint film sensitivity thus evaluated, s can be con-
verted into the h
33
constant by using the relationship
h
33
= sv/d, where v is the phase velocity of AE waves in the
PZT/epoxy paint film. The h

33
constant calculated by using
an assumed value of v = 2850 m/s (6) is plotted in Fig. 5 as
a function of film thickness for paint films cured at room-
temperature that have the PZT/epoxy composition of 53/47
0 50 100 150
Film thickness, µm
200 250 300
120
100
80
60
40
20
0
h
33
(MV/m)/(m/m)
Figure 5. Plots of the piezoelectric constant h
33
at 50 (◦), 100 (
•
),
150 (), and 250 kV/cm ()vs.film thickness for PZT/epoxy paint
films cured at room temperature and evaluated as an acoustic
emission sensor.
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by volume (28). It is seen that the h

33
constant obtained
at a poling field of 50, 100, 150, or 250 kV/cm increases
steadily as film thickness increases, thus exhibiting a
clear film-thickness dependence. Such a film-thickness de-
pendence is also observed for the e
31
constant shown in
Fig. 3.
FACTORS DETERMINING POLING BEHAVIOR OF SMART
PAINT FILMS
The poling behavior of a PZT/epoxy paint film depends
on the film thickness, as shown in Figs. 3 and 5. Fur-
thermore, the poling behavior also depends on the cure
temperature and the PZT/epoxy composition (26–29). Such
complicated poling behavior is virtually determined by the
electric field that acts on the PZT particles dispersed in the
epoxy matrix. The most important factors that determine
the electric field and, consequently, the poling behavior of
the paint film are the electrical conductivities of the PZT
particles and the epoxy matrix, the connectivity pattern of
the PZT phase, and the space charge accumulated at the
PZT/epoxy interface.
Electrical Conductivities of Constituent Materials
It is now well established that in poling a composite speci-
men made of piezoceramic particles dispersed in a polymer
matrix, the electric field that acts on the ceramic parti-
cles is very low compared with that applied externally to
the composite specimen (14,32). This occurs because the
electrical conductivity of polymeric materials in general is

much lower than that of ceramic materials, and hence the
polymer matrix in the composite specimen bears almost all
of the externally applied electric field at the expense of the
electric field that acts on the ceramic particles. As a result,
the piezoelectric activity of the ceramic/polymer composite
specimen is very low, compared with a pure piezoceramic
specimen poled in thesameelectric field. This idea explains
why saturation poling is not achieved, even in a high poling
field of ∼150 kV/cm, as seen in Fig. 3. Saturation poling
for a pure PZT ceramic specimen, on the other hand is
achieved in a low poling field of ∼10 kV/cm (12).
A promising solution to this problem is to increase the
electrical conductivity of the polymer matrix up to that of
the ceramic particles, so that the electric field distribution
becomes uniform throughout the composite specimen. This
can be achieved by adding a small amount of a semicon-
ductor filler such as carbon, germanium, or silicon to the
composite specimen (32). This can also be achieved by pol-
ing at a high temperature where the electrical conductivity
of the polymer matrix becomes equal to that of the ceramic
particles (33).
Connectivity Pattern of Ceramic Phase
Figure 6 is a scanning electron microscopy (SEM) picture
that shows the internal microstructure of a paint film that
has the PZT/epoxy composition of 53/47 by volume (27). It
is seen that the size of PZT particles ranges from ∼0.5 to
∼1.5 µm, and that a substantial fraction of the PZT parti-
cles are in contact with each other, so that the PZT phase
10 µm
Figure 6. SEM picture of a paint film that has the PZT/epoxy

composition of 53/47 by volume. This example is a 49-µm thick
paint film cured at 150
◦
C.
is practically self-connected in three dimensions. The
self-connectivity of the PZT phase is one of the most im-
portant factors that determines the poling behavior of a
PZT/epoxy paint film. In fact, the paint film is hardly poled
when the PZT volume fraction is decreased to such a level
that the PZT particles are isolated from one another by the
continuous phase of the epoxy matrix (26).
Figures 3 and 5 show that the poling behavior of a
PZT/epoxy paint film depends on the film thickness even
when the PZT volume fraction remains constant at 53%.
A SEM picture like that shown in Fig. 6, however, detects
no observable difference in the PZT phase connectivity for
paint films that have different thicknesses. The difference
in the PZT phase connectivity is reflected much more ex-
plicitly in the current–voltage characteristic of the paint
film rather than in the SEM picture, as described here.
Space Charge at the Ceramic/Polymer Interface
The current–voltage characteristic of a PZT/epoxy paint
film shows that the conduction is ohmic in a low electric
field, whereas in a high electric field, the space-charge-
limited (SCL) conduction predominates over ohmic conduc-
tion (28). Furthermore, the current–voltage characteristic
shows that the critical electric field at which the ohmic-to-
SCL transition takes place decreases as the film thickness
decreases. The result is that conduction during the poling
process is mostly SCL for a thin film, whereas conduction

is mostly ohmic for a thick film.
The SCL conduction becomes predominant when a
space charge of electrons is injected into the PZT/epoxy
paint film during the poling process. The space charge has
a tendency to build up preferentially at the interface be-
tween the PZT and epoxy phases in the paint film (28).
The space charge decreases the electric field acting on the
PZT phase, and hence decreases the piezoelectric activity
of the paint film obtained in a given poling field. This ef-
fect of the space charge becomes significant, particularly
for a thin film, because SCL conduction becomes more
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predominant as the film thickness decreases. Therefore,
the film-thickness dependence of the piezoelectric constant
shown in Figs. 3 and 5 is ascribed to the space charge of
electrons injected into the paint film during the poling pro-
cess.
The fact that the current–voltage characteristic of a
PZT/epoxy paint film depends on the film thickness is
closely related to the drying rate of the wet paint film. In
fact, it is well known that the thickness of a wet paint film
has a significant influence on the rate of solvent evapora-
tion and, consequently, on film formation during curing (3).
Thus, it is quite possible that the degree of self-connectivity
of the PZT phase depends on the thickness of the dried
paint film. Therefore, the drying rate of the wet paint film
is another important factor that determines the poling be-
havior of a PZT/epoxy paint film.

TECHNIQUES FOR APPLYING SMART PAINT FILMS
Techniques for applying smart paint films as vibrational
and AE sensors are essentially the same as those for a
PZT ceramic or PVDF film bonded to the surface of a
structural material. Theories, models, methods, and sys-
tems constructed for use of the PZT and PVDF sensors
(21,22,34) can also be applied to smart paint films used
as vibrational and AE sensors integrated into a structural
material.
Vibrational Modal Sensor
One example of an application of smart paints is a vibra-
tional modal sensor integrated into a structural material.
As noted before, the sensitivity of the paint film used for
this purpose is the e
31
constant which is the ratio of the
charge per unit surface area to the strain in the direc-
tion tangent to the paint film surface. Figure 7 shows a
result of vibrational modal testing of a cantilever beam
like that shown in Fig. 1 by using a PZT/epoxy paint film
0 5 10 15 20 25 30 35
Longitudinal coordinate, cm
Modal strain, 10
−6
m/m
150
100
50
0
−50

−100
Figure 7. Modal strain shapes of a cantilever aluminum beam
for the first (
◦), second (
•
), and third modes () determined by a
PZT/epoxy paint film formed on the beam surface.
that has an e
31
constant of 9.0 × 10
−3
(C/m
2
)/(m/m) (26). A
set of vibrational measurements is carried out for all of the
electrodes formed on the paint film surface: an identical ex-
citatory force is applied at a fixed point on the cantilever
beam. Then, the output charge of the paint film at each
electrode is converted into the strain using the e
31
constant
and is plotted against the distance from the clamped end of
the beam to the center of each electrode. The modal strain
shapes thus obtained are shown in Fig. 7 for the first three
modes at 18, 112, and 315 Hz.
It is worth nothing that the modal strain shapes shown
in Fig. 7 can be converted into modal displacement shapes
by d
2
φ/dx

2
=−ε/η, where φ is the transverse displace-
ment of a uniform cantilever beam, x is the longitudinal
coordinate of the beam, ε is the longitudinal strain in the
beam surface, and η is the half-thickness of the beam (35).
Modal displacement shapes determined by this equation
are identical to those determined by a laser Doppler vi-
brometer that measures the transverse movement of the
beam surface (26). Thus, smart paints offer an interesting
and promising alternative to conventional sensors such as
accelerometers and laser vibrometers (1).
FUTURE DIRECTIONS
Smart Paints
The highest sensitivity of smart paint films achieved so
far is e
31
=∼40 × 10
−3
(C/m
2
)/(m/m) as a vibrational sen-
sor and h
33
=∼100 × 10
6
(V/m)/(m/m) as an AE sensor,
as shown in Figs. 3 and 5. For commercially available
PVDF films, the sensitivity is e
31
=∼66 × 10

−3
(C/m
2
)/
(m/m), e
32
=∼6.8 × 10
−3
(C/m
2
)/(m/m), and h
33
=∼50 ×
10
6
(V/m)/(m/m), determined in essentially the same way
described before for smart paint films. This indicates that
the sensitivity of smart paint films is comparable to that
of PVDF films. So far as sensitivity is concerned, there-
fore, smart paints have already reached a level suitable
for practical use.
For smart paints to be put into practical use, however,
the paints must meet performance requirements such as
exterior durability and sensitivity stability. Exterior dura-
bility is the paint films resistance to environmental factors
such as uv radiation, heat, moisture, oxygen, and ozone (2).
These environmental factors can cause mechanical degra-
dation of paint films, thus leading to the failure of the pro-
tective and decorative functions of smart paints. These en-
vironmental factors may also cause electrical degradation

of paint films, thus leading to the failure of the sensor func-
tion of smart paints. Considering that smart paints are
truly appreciated when used in severe and isolated envi-
ronments, the evaluation of exterior durability and sensi-
tivity stability is absolutely necessary for the paints to be
put into practical use.
Smarter Paints
According to a concept of intelligent materials in Japan,
the intelligence in materials is classified into three cat-
egories; intelligence from the human standpoint, intelli-
gence inherent in materials, and intelligence at the most
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primitive levels in materials (36). The intelligence from
the human standpoint is a relative concept based on the
value of a material and its utility in relation to all as-
pects of society such as economy, conservation of resources,
intensiveness of information, human friendliness, relia-
bility, harmony with the environment, and optimum life
span.
Water-borne piezoelectric paints are smarter paints
from the standpoint of harmony with environment (37).
A paint that can spontaneously become a piezoelectric film
after the usual drying process will also be a smarter paint
from the standpoint of human friendliness. In fact, poling
a paint film at a high voltage is dangerous work and should
be avoided if possible. A feasibility study of a poling-free
piezoelectric paint shows that a paint made of PVDF par-
ticles and epoxy resin does not need poling for the final

dry film to be piezoelectrically active (38). At the present
stage, however, the piezoelectric activity is not enough for
practical use of the paint film. Studies are currently un-
der way to increase the piezoelectric activity of the paint
film.
From the standpoint of intensiveness of information, a
smarter paint of the future will have a sensor function for
material conditions such as vibration and damage gener-
ation and also for atmospheric variables such as temper-
ature, pressure, moisture, and wind velocity. Such a paint
resembles human skin in that the skin has a sensor func-
tion for the external stimuli imposed on the human body
and also for the surrounding conditions such as tempera-
ture, humidity, wind, and rain. The ultimate goal of smart
paints, therefore, should be to mimic the human skin as
closely as possible.
ACKNOWLEDGMENTS
The work in smart paints by S. Egusa and N. Iwasawa was
supported by the Japan Atomic Energy Research Institute
through the Special Program for Fundamental Researches
(1991–1994) and through REIMEI Research Resources
(1998).
BIBLIOGRAPHY
1. J.H. Lowell, in Coatings, J.I. Kroschwitz, ed., Encyclopedia of
Polymer Science and Engineering, 2e., Wiley-Interscience, NY,
1985, Vol. 3, pp. 615–675.
2. Z.W. Wicks, Jr., in Coatings, J.I. Kroschwitz, ed., Encyclopedia
of Polymer Science and Engineering, 2e., Wiley-Interscience,
NY, 1989, Supplement Vol. pp. 53–122.
3. O. Graydon, New Scientist, p. 20, October 17, 1998.

4. D.J. Ewins, Modal Testing: Theory and Practice. Research
Studies Press, Taunton, 1984.
5. C.B. Scruby, J. Phys. E: Sci. Instrum. 20: 946–953 (1987).
6. KYNAR Piezo Film Technical Manual, Pennwalt Corporation,
Valley Forge, PA, 1987, p. 6.
7. A.J. Moulson and J.M. Herbert, Electroceramics. Chapman &
Hall, London, 1990, Chap. 6.
8. M.V. Gandhi and B.S. Thompson, Smart Materials and
Structures. Chapman & Hall, London, 1992, Chap. 5.
9. T. Kitayama and S. Sugawara, Proc. Gr. Inst. Electr. Comm.
Eng. Jpn., 1972, CPM 72-17 (in Japanese).
10. L.A. Pauer, IEEE Conf. Res., pp. 1–5 (1973).
11. W.B. Harrison, Proc. Workshop Sonar Transducer Mater.
Naval Research Laboratories, November 1975, p. 257.
12. T. Furukawa, K. Fujino, and E. Fukada, Jpn. J. Appl. Phys.
15(11): 2119–2129 (1976).
13. T. Furukawa, K. Ishida, and E. Fukada, J. Appl. Phys. 50(7):
4904–4912 (1979).
14. T. Furukawa, K. Suzuki, and M. Date, Ferroelectrics 68:33–44
(1986).
15. H. Banno and S. Saito, Jpn. J. Appl. Phys. 22 (Supplement
22-2): 67–69 (1983).
16. H. Banno, Ferroelectrics 50:3–12 (1983).
17. R.E. Newnham, D.P. Skinner, and L.E. Cross, Mater. Res. Bull.
13: 525–536 (1978).
18. R.E. Newnham, L.J. Bowen, K.A. Klicker, and L.E. Cross,
Mater. Eng. 2:93–106 (1980).
19. R.E. Newnham, Ferroelectrics 68:1–32 (1986).
20. R.E. Newnham and G.R. Ruschau, J. Am. Ceram. Soc. 74(3):
463–480 (1991).

21. C K. Lee and F.C. Moon, J. Appl. Mech. 57: 434–441 (1990).
22. S.A. Collins, D.W. Miller, and A.H. von Flotow, Sensors
for Structural Control—Applications Using Piezoelectric
Polymer Film. Space Engineering Research Center #12-
90, Massachusetts Institute of Technology, Cambridge, MA,
1990.
23. K.A. Hanner, A. Safari, R.E. Newnham, and J. Runt, Ferro-
electrics 100: 255–260 (1989).
24. C.A. Rogers and S.C. Stein, Proc. 1st Int. Conf. Intelligent
Mater. 1993, pp. 87–93.
25. S. Egusa and N. Iwasawa, Proc. 1st Int. Conf. Intelligent Mater.
1993, pp. 101–104.
26. S. Egusa and N. Iwasawa, J. Mater. Sci. 28: 1667–1672
(1993).
27. S. Egusa and N. Iwasawa, Ferroelectrics 145:45–60 (1993).
28. S.S. Egusa and N. Iwasawa, J. Appl. Phys. 78: 6060–6070
(1995).
29. S. Egusa and N. Iwasawa, J. Smart Mater. Struct. 7: 438–445
(1998).
30. J.M. Haleand J. Tuck, A NovelStrain TransducerUsing Piezo-
electric Paint. Proc. Mech. Eng. in press.
31. ASTM E1106-86, Standard Method for Primary Calibration of
Acoustic Emission Sensors. American Society for Testing and
Materials, Philadelphia, PA, 1986, pp. 489–498.
32. G. Sa-Gong, A. Safari, S.J. Jang, and R.E. Newnham,
Ferroelectrics Lett. 5: 131–142 (1986).
33. J.P. Dougherty and Y. Chen, Proc. 2nd Int. Conf. Intelligent
Mater. 1994, pp. 462–473.
34. C.A. Rogers, ME 4016, Virginia Polytechnic Institute and
State University, Blacksburg, VA (private communication,

1991).
35. S.H. Crandall, N.C. Dahl, and T.J. Lardner, An Introduc-
tion to the Mechanics of Solids. McGraw-Hill, NY, 1972,
p. 628.
36. T. Takagi, Proc. Int. Workshop Intelligent Mater., Tsukuba,
Japan, 1989, pp. 1–10.
37. J.M. Hale, University of Newcastle, Newcastle, England
(private communication, 1999).
38. S. Egusa, 1998 REIMEI Conf., Japan Atomic Energy
Research Institute, Tokai, Japan, July 14–15, 1999.
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PEST CONTROL APPLICATIONS 761
PEST CONTROL APPLICATIONS
SHERRY DRAISEY
Good Vibrations Engineering, Ltd
Nobleton, Ontario, Canada
INTRODUCTION
The smart aspects of the piezoceramic ultrasonic appli-
cation being used for pest control are just beginning to
evolve. Pest control, using ultrasonics, is based on devel-
oping a pressure environment which is extremely unpleas-
ant or deadly to the pests in question. The feedback as-
pect of smart structure applications involves three types of
sensing:
r
motion sensors (designed to power up the ultrasonic
device when large pest groups have been detected)
r
pressure sensors (these are used in fluid media to

sense if pressure levels have risen enough to gener-
ate structural instability)
r
sound sensors (for antinoise generation to stop the
sound from being externally transmitted) that coordi-
nate the antinoise generation
Airborne or land pests, such as some insects, spiders,
rodents, and small cats and dogs are driven away by the
unpleasant sound created by the noise generated by the
ceramic elements. For fluid-borne pests, the ceramic is
driven to create a pressure field that includes cavitation.
The release of energy from the collapse of cavitating bub-
bles provides the source deadly to small microorganisms.
Table 1 lists the types of pests that have been effec-
tively deterred by ultrasonic measures. The table lists the
frequency range that has been successful for these pests,
as well as the approximate coverage (or flow rate) across
which they are effective. The coverage is directly related to
the system size and power.
The Environmental Protection Agency (EPA) has sug-
gested that pest control devices have a deterrent effect of
>60% to be considered viable.
SOUND AS A PEST DETERRENT
The control of airborne and land pests is based on gen-
erating high-frequency noise. This is done to disturb and
confuse the species, making the environment generally un-
pleasant. The sound levels are in the range of 90+ dB at
1 meter from the source.
Table 1. Pests Effectively Controlled by Ultrasonic Devices
Coverage (varies with power

Pest Frequency Range consumption)
Dogs, cats, skunks 14–25 kHz 278.8 m
2
(4000 ft
2
)
Mice 26–50 kHz 46.4 sq m
2
(500 ft
2
)
Moths 40 kHz 5.7 m
3
(200 ft
3
)
Rodents, spiders, some insects 26–42 kHz 74.3 m
2
(800 ft
2
)
Microorganisms 23 kHz 273.6 liters/h (60 imp. gal/h)
The concept behind ultrasonic pest control is to alter
the behavior patterns of the pests to the extent that they
are forced to leave the area. Some devices have been de-
signed for operation within buildings, others for outdoors.
Versions of the devices target specific pest groups (mice),
and more sophisticated versions have settings that allow
selecting particular pest groups.
The power supplies for the designs varies from plug-in

wall units (110 or 220/240 V) to battery operated systems.
Motion sensors are used for detecting larger size pests.
This reduces power consumption and eliminates unneces-
sary noise pollution.
Test Results
The test data presented here were provided by the
Weitech company, a manufacturer of a variety of ultrasonic
deterring devices designed to produce ultrasonic sound
in air.
Mosquitoes. At least one company’s test results of the
high-frequency ultrasonic deterrent effect on mosquitoes
has suggested that it does not meet the EPA suggested
deterrent level.
Small Rodents. The available test results (1) for small
rodents depend on the particular rodent. Two types of ro-
dents are considered. For each test set, there were six ro-
dents in the sample—three males and three females. They
were housed in two adjoining chambers, one exposed to the
ultrasonic sound (∼90 dB), the other at much lower noise
levels ( 30 to 35 dB or lower).
Two parameters areusedto evaluate the influence of the
ultrasound—food consumption (measurement of the daily
food consumption in the treated and untreated chambers)
and activity (animal track evidence in the treated and un-
treated chambers). Before the introduction of ultrasonic
treatment, healthy mice that had good hearing (hearing
test—Preyer’sreflex, a reaction to loud noise) are housed
in the two chambers, and their activity and food consump-
tion levels are measured.
The effect of the ultrasonic deterrent on the Norway rat

(Rattus norvegicus) is more pronounced than on wild house
mice (Mus musculus) . The average weight of the Norway
rats in the test was 237 grams (8.4 oz). The average weight
of the wild house mice was 17 grams (0.6 oz). The results
are shown in Figs. 1 and 2 as an index (the ratio of the
treated measurements to the total measurements). Food
consumption influence is shown in black bars, and tracking
activity is shown in gray.
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Influence on wild norway rat population
Pre treatment Post treatment
Treatment
1
0.8
0.6
Index of treated results vs total
0.4
0.2
0
−2642
Days
0
Figure 1. The influence of ultrasonic noise on the Norway rat
population.
Figure 1 shows the effect of treatment on the Norway
rat. Figure 2 shows the effect of the treatment on wild
house mice. The influence on both populations is most sig-
nificant for food consumption. The tracking activity of the

wild house mice is not heavily influenced by the ultrasonic
effect.
The rodents’ hearing was checked before and after the
testing. Only rodents that had good hearing were selected
for the study. It has been postulated that the rodents might
eventually become accustomed to the noise, but this was
not the case. There were instances where rodents were not
influenced, but this was due to hearing loss.
The sound patterns (frequency and amplitude) of four
of the pace electronic pest repeller units were measured.
1
0
−20 2 4 6
Days
81012
0.2
0.4
0.6
Index of treated results vs total
0.8
Pre treatment
Treatment
Post treatment
Influence on wild housemice population
Figure 2. The influence of ultrasonic treatment on the wild house
mice population.
The primary source of total sound output was at 40 kHz
and above. The sound output dropped slightly at 31.5 kHz.
Sound output below 20 kHz was negligible.
CAVITATION AS A DESTRUCTOR

Piezoceramic elements are commonly used to induce cavi-
tation in fluids in biological applications for scaling in-
struments, but killing microorganisms is normally done by
high-temperature sterilization. The erosive effect of cavi-
tation is what is useful in removing a variety of type of
scales. Cavitation is caused when the localized pressure
drops below the fluid vapor pressure. This results in cavi-
tating bubbles.
The collapse of cavitating bubbles is accompanied by a
rapid release of energy. It is the collapse of the cavitat-
ing bubbles that is used to destroy microorganisms. It is
not clear whether the microorganism population is imme-
diately killed by the bubble collapse, or if the population is
just weakened enough to limit its viability.
The generation of cavitation is limited to areas fairly
close to the pressure/sound source. Cavitation can be ap-
plied to a large volume of fluid either by moving the source
through the fluid or by moving the fluid past the source.
The application described here moves the fluid past the
source by pumping the volume through tubing to ensure
fairly even exposure of the liquid to the pressure field. This
does not sterilize the fluid, but it does eliminate a signifi-
cant portion of the microorganism population.
The biological test results available indicate that cavita-
tion does significantly reduce the population in both water
and diesel fuel, but theeffectvaries for the types of microor-
ganisms tested. The population reduction is of the order of
50%.
It is expected that piezoceramically induced cavitation
could be used to reduce zebra mussel population in nuclear

reactor water intake tubes by interfering with the zebra
mussels during an early stage of their development, such
as the larval stage.
The specific engineering design that follows was based
on controlling microbial growth in military marine diesel
tanks. These populations are currently controlled by “good
housekeeping” of ships’ tanks and by using environmen-
tally harmful biocides. If an ultrasonic cavitation system
were to be installed on a ship, it would be necessary to in-
clude an antinoise system to cancel the ultrasonic sound
that creates the cavitation. This would be needed to mini-
mize the likelihood that the vessel would be detected by
unfriendly ships.
Engineering Application/Design
The cavitation of a fluid is induced when local pressure
drops below its vapor pressure. It involves the release of
relatively small amounts of energy (compared to boiling),
so that though there is a temperature change in the fluid;
it is small (of the order of 1–2
◦
C, depending on exposure
time and volume).
One of the well-known side effects of cavitation is its ero-
sive effects on materials. This presents a practical problem
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Driver
electronics
Cavitation bubbles

Inner tube
Working medium
Piezoceramic rings
Transmission medium
Figure 3. Schematic of cavitation concept.
in trying to use cavitation. The components used to cause
the cavitation need specialconsiderationto survive the ero-
sive environment.
A general requirement for pest control is that it is
needed for large volumes. Cavitation is a fairly local ef-
fect. To apply it to a large liquid volume, the fluid must
be brought into a fairly local range. One way of achiev-
ing this is a flow-through system. The liquid is pumped
through tubes that are exposed to the cavitating field. Such
an arrangement could involve expenditures of significant
amounts of power.
A flow-through configuration was studied analytically
to achieve maximum fluid cavitation at minimum power
consumption. The particular system modeled was based
on a two-fluid system to avoid the electrode erosion that
would be induced by cavitation. Figure 3 shows the con-
ceptual arrangement. The fluid immediately adjacent to
the electrodes is pressurized to eliminate cavitation. This
fluid is used to transmit energy through a thin-walled pipe
(stainless steel) into the fluid that contains the microor-
ganism. The analytical model of the system was a piezo-
dynamic field modeled by using finite elements. It is based
on a finite element formulation of the piezoceramic ele-
ments, the physical piping structure, a liquid transmis-
sion medium, and the sound pressure field experienced

by the microorganism-borne fluid (either water or diesel
fuel).
The model was then test verified before applying it to a
specific design.
Finite Element Formulation. The finite element method
is an analytic technique for solving general field problems.
It offers a number of advantages over competing meth-
ods. It can handle arbitrary geometries and both static
and dynamic problems. It uses matrix numerical methods
for which very efficient and general algorithms have been
developed.
The special purpose FE formulation developed to han-
dle both the fluid characteristics and the electrical input
(as well asthenormal structural characteristics) was based
on the principles of the FE method in (2). The code mod-
eled the structural behavior of the elements that represent
the piezoelectric components, as outlined in (2, p. 22). The
piezoelectric behavior was included using the approach of
(3, p. 86). The fluid areas of the model were analyzed using
the approach described in (2, p. 540).
The degrees of freedom of the model are the group of
r
nodal displacements of the solid components,
r
nodal pressures of the fluid components,
r
nodal electrical potentials of the piezoelectric compo-
nents, and
r
the junction voltages of an external electrical circuit

connected to the piezoelectric components (this latter
capability was not used, though it is included for pos-
sible future use).
Then, the defining equations of the finite element approach
used are
[A
2
]

d
2
w
dt
2

+ [A
1
]

dw
dt

+ [A
0
]{w}+[A
−1
]

{w}dt
+ [A

−2
]

{w}dt.dt ={b}, (1)
where
[A
2
] =




M 000
SG00
0000
0000




, [A
1
] =




c 000
0 f 00
00 00

00 00




,
[A
0
] =





K
1
ρ
S
T
E 0
0 H 00
E
T
0 −∇
2
0
00 0C






,
[A
−1
] =




0000
0000
0000
000R




, [A
−2
] =




0000
0000
0000
000I





,
{b}=







F
0
Q
Q
N







, {w}=








U
P

ν







.
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In these equations,
M =

[N
s
]
T
ρ
s
[N
s
]dV
s

S =

S
[N
f
]
T
ρ
f
[N
s
]dS
sf
G =

[N
f
]
1
a
2
[N
f
]dV
f
c =

[N
s
]

T
µ
s
[N
s
]dV
s
f =

[N
f
]
T
µ
f
[N
f
]dV
f
K =

[B]
T
[D][B]dV
p
E =

[B
e
]

T
[][B
e
]dV
p
H =

[∇N
f
]
T
[∇N
f
]dV
f
I = external circuit inductance
C = external circuit capacitance
R = external circuit resistance
U = solid element nodal displacements
P = fluid element nodal pressures
V = external circuit voltages
F = externally imposed force on solid element nodes
Q = externally imposed charges on piezoelectric
elements
Q
N
= externally imposed charges on external circuit
φ = piezoelectric element nodal potentials
a = speed of sound in fluid
where

[N
s
] = shape function matrix for solid elements
[
N
f
]
= shape function matrix for fluid elements
[
B
]
= shape function derivatives giving strain in solid
elements
[
B
e
]
= derivatives of potential shape function in piezo-
electric elements
ρ = mass density (subscript s for solid, f for fluid)
µ = damping (subscript s for solid, f for fluid).
The model assumed axisymmetry which was imple-
mented as described in (2, p. 119). The elements describe
the cross section of the complete unit from the centerline
out, that is, that section which is rotated about the axis
of symmetry to sweep out the 3-D geometry of the unit.
The elements used were eight-node, isoparametric quadri-
laterals, using quadratic shape functions for all fields (2-D
solid displacements, fluid pressures, and electrical fields).
Third-order Gaussian numerical integration was used for

all element integrals. The integrals across volume are
done by the usual finite element approach of integrating
across each element independently, followed by assembling
the resulting equations into matrix form, as described in
(2, p. 9).
Damping was included in the model by adding mate-
rial damping to the fluid regions, as described in the pre-
ceding equations. Based on experimental measurements,
enough damping was included to give a resonant amplifica-
tion (Q factor) of 5 to 8. Two extreme conditions were used.
In the first, damping was distributed across both the trans-
mission and working media. In the second, damping was
concentrated in the working medium. The first case corre-
sponds most closely to low excitation levels, whereas the
second should more closely match high excitations when
cavitation is occurring. Then, the energy dissipation will
be concentrated in the working medium because of the
cavitation.
The model is linear. This is expected to give good re-
sults up to the point at which cavitation begins. Beyond
that point, the response of the system is no longer linear
because the fluid behaves effectively less stiff on the nega-
tive side of the pressure wave than on the positive side due
to the formation of cavitating bubbles. In principle, this
effect could be modeled using the nonlinear approaches
described in (2, p. 450). This simplification was accepted
because the objective was to compare alternative designs,
rather than to analyze the behavior in absolute terms. It is
assumed that systems that give a greater linear response
will also give a greater nonlinear response. This may not

be true in unusual cases, and it may not represent the ef-
fect of changes in the spatial distribution of the acoustic
field in all cases (it would be expected that the “softening”
nonlinearity which will occur here would tend to make the
energy distribution more uniform in the system, compared
to the linear case).
Figure 4 shows typical results from the model. These
show the pressure distribution across the fluid cross sec-
tion for 100 volt peak–peak excitation of the piezo rings for
various excitation frequencies. It can be seen that the en-
ergy in the working medium in all cases is concentrated at
the center. At low frequencies, only a single pressure peak
occurs. At higher frequencies, when the wavelength of the
sound waves in the fluid becomes comparable to the di-
mensions of the device, two and then three pressure peaks
Figure 4. Finite element predictions of cavitating field.
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Table 2. Finite Element Model Parameters
Parameter Material Dimensions
Inner tubing Stainless steel tube 1.5 in outer diameter
(E = 30E6 psi) 0.012 in wall thickness
Piezoceramic rings PZT4 2 in diameter
(stack of four) 0.125 in wall thickness
0.5 in height
Transmission fluid SAE 10W30 motor oil Density,
speed of sound
Working fluid Water or diesel fuel Density,
speed of sound

occur axially along the centerline. These observations are
consistent with qualitative results. These results were ob-
tained by suspending an aluminum foil strip in the cavi-
tating field. Because it is known that cavitation erodes alu-
minum, the distribution and degree of perforation provide
an indication of the cavitating intensity.
The specific parameters of the model are listed in
Table 2.
Test Verification of Analytical Model. Modeling a com-
bined electrical/piezoelectric/structural/fluid system is
complex. A number of approximations and simplifications
were made. For this reason, some model correlation was
done in advance of prototype development (experimental
data taken from breadboard unit). The FE model was done
for a four-ring prototype. The experimental testing was
done on a three-ring arrangement.
There were two type of measurements made for the
correlation exercise, the current–voltage relationship and
sound pressure measurements. The predicted and mea-
sured current versus voltage relationship for the system is
shown in Figure 5. Measured values are shown at 22.7 kHz
10
0
10
0
10
1
10
2
10

−1
10
−2
P-P Piezo current (A)
P-P Piezo voltage (V)
Piezo current vs voltage
Measured at 22.7 kHz
Model at 26.5 kHz
Model at 22.7 kHz
Figure 5. Measured and predicted current vs voltage.
which gives the peak piezo current. Model values are
shown for both this frequency and for 26.5 kHz, which is
the frequency at which the model shows peak current. It
can be seen that the measured values at low voltages are
about 60% of the modeled values. This is mainly due to
the four rings in the model versus three in the breadboard.
The sound pressure field was measured using the Specialty
Engineering Associates needle hydrophone, Model SPRH-
2-0500.
Figure 6 shows the response of the hydrophone at two
different excitatory voltage levels, as captured on a digi-
tal storage oscilloscope. Note that the two cases were
at slightly different frequencies. These frequencies corre-
spond to the peak responses at each excitatory level. That
they are different indicates nonlinearity in the model. It
can be seen that the hydrophone response waveform is un-
symmetrical and has pressure spikes on the positive volt-
age (low pressure) side. This is an indication of cavitation.
It is more prominent at the higher excitatory voltage.
The model predicts that the peak pressure in the unit

should be 1 kPa per volt of excitation. The transducer out-
put should be 0.25 mV per volt of excitation. The results
in Fig. 6 show a 20-mV peak-to-peak response at 130-V
peak-to-peak excitation in (a) and 65 mV response at 240 V
excitation, or 0.16 mV/V and 0.27 mV/ V, respectively. This
agreement is reasonable given the uncertainty of the hy-
drophone (it was being used somewhat out of its design fre-
quency range). Themodelpredicts that the pressureshould
lead the voltage by 10 to 20
◦
, and it can be seen that this
is reasonable, though the experimental measurements do
not really allow testing this.
Figure 7 shows the pressure distribution measured
along the centerline of the device for low voltage excita-
tion (where the nonlinearity of the system does not con-
fuse the results), and Fig. 8 shows the pressure distribu-
tion measured across the centerline at the midheight of the
piezo rings. The hydrophone readings in these figures have
been converted to acoustic pressures. The model predic-
tions are alsoshown. It can beseenthat the model andmea-
sured values show the same trends and the differences are
1–3dB.
Design Studies
Outer Diameter of Transmission Medium. A design was
studied to optimize the outer diameter of the transmission
medium on the sound intensity in the working medium.
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0
−150
150
(a)
100
50
0
−50
50
0
−50
−100
20 40 60
Time (µ sec)
Response at 26.8 kHz
80 100 120
0 20406080100120
Piezo excitation (V)Hydrophone output (mV)
100
(b)
50
0
−50
Response at 26.2 kHz
0 20 40 60 80 100 120
−100
20
10
0
−10

−20
0 204060
Time (µ sec)
80 100 120
Piezo excitation (V)Hydrophone output (mV)
Figure 6. Hydrophone response at (a) 130 V p–p excitation;
(b) 240 V p–p excitation.
The integral of acoustic pressure across the volume of the
working medium was used as a performance indicator.
Two extremes of damping models were used—damping
concentrated in the working medium and damping dis-
tributed over both working and transmission media. Fig-
ure 9 shows the results for both cases (as the integral
of pressure vs. the outer diameter, (OD) of the transmis-
sion medium. It can be seen that when damping is concen-
trated in the working medium, the optimum occurs at an
OD of 113 mm because the spacing between the outside
of the piezo ring and the OD of the transmission medium
is about one-half an acoustic wavelength. Such a condition
would be expected to result in translating the high acoustic
impedance condition at the rigid outer wall to a low acous-
tic impedance at the ring [see (8), p. 18 for an example].
This low acoustic impedance of the transmission medium
Rings
Model at 25.0 kHz
13 V P−P Excitation
Measured at 23.7 kHz
Measured at 26.0 kHz
84
82

80
78
76
74
72
70
68
66
−50 500
Z (mm)
Axial pressure distribution on centerline
P−P Pressure (dB re 1 Pa)
Figure 7. Acoustic pressure distribution along centerline.
at the ring is mismatched to that of the ring so that the
coupling between the ring and transmission medium is
poor at the outside of the ring. Little energy is launched
outward from the ring, leaving more to be launched inward
to the working medium.
The figure also shows that when damping is distributed
across both transmission and working media, the optimum
occurs at a lower OD. This may be due to the fact that
when damping is included in the transmission medium,
the increase in transmission medium volume, which oc-
curs as its OD is increased, results in more energy losses
in the system, thus biasing the optimum to a smaller
diameter.
84
82
80
78

76
P−P Pressure (dB re 1 Pa)
74
72
70
68
66
−10
r/R
1
13 V P-P Excitation
Measured at 26.0 kHz (assumed symmetrical)
Measured at 23.7 kHz (assumed symmetrical)
Model at 26.0 kHz
Radial pressure distribution at ring mid-height
Figure 8. Acoustic pressure distribution across diameter at ring
midheight.
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0
30
25
20
15
10
5
OD (mm)
Effect of outer diameter
10

0
70 75 80 85 90 95 100 105 110 115 120
70 75 80 85 90 95 100 105 110 115 120
2
4
6
8
Integral (PdV) (Pa.m
^3
)Integral (PdV) (Pa.m
^3
)
Distributed damping
Prototype design
Working fluid only damping
Figure 9.
F
f

0
Power
Acousti c
νs φ.
Electronics Concept. Three electronics concepts were
considered, and two were experimentally evaluated:
r
a function generator to produce a sinusoidal (or other)
waveform and a power amplifier to generate a final
high-power output signal to be sent through a trans-
former to thepiezo elements in the mechanical module

r
a high-power oscillator
r
a switching power supply
The first approach was used in prototype testing and de-
velopment. It was not continued in the higher power, high
flow-rate evaluation unit because the readily available
Switched
voltage
source
3 - Pole
butterworth
low-pass
filter
Coil to
produce
tuned circuit
with piezo
Piezo
model
1.53 mH
L
1
L
T
8.49nF 8.49nF
C
1
C
2

R
T
R
P
C
P
100
21.2nF
1.91mH
Figure 10. Electronics concept.
power amplifiers are limited in power (so would have to
be ganged to drive the larger system) and the class A am-
plifier action used is relatively inefficient, making cooling
of the electronics an issue.
The high-power oscillator was not developed because
of concerns of achieving high power without instability
problems.
The switching power supply was used for designing
the evaluation unit. It is in line with current methods of
driving high-power motors using pulse-width modulation
(PWM). Digital circuitry is used to generate square wave-
forms. These may be duty-cycle modulated and are used
to switch power MOSFET transistors on and off rapidly
so that the average voltage presented to the equipment
as a result of the variable duty-cycle appears sinusoidal.
Such an approach is efficient because the transistors are
always completely on or completely off (except during short
switching transients), and they dissipate little power in ei-
ther of these states. In our case, the output frequencies
are too high for true PWM, but square waves can be gen-

erated at these frequencies and filtered to eliminate higher
harmonics.
Figure 10 shows an electronic filtering concept evalu-
ated by analysis. A high voltage supply that has positive
and negative polarity and a 33% duty cycle is switched on
and off. The fundamental frequency of the source is 25 kHz.
This is followed by a three-pole low-pass filter that has
a cutoff at 62.5 kHz. The output from this filter feeds a
tuned circuit that represents the piezo rings (21.2-nF ca-
pacitance and a 100-ohm resistor to simulate a system Q
of 3) in series with an inductance chosen to tune the cir-
cuit to the 25 kHz fundamental. This makes the driven
system of this tuned circuit appear resistive at the funda-
mental frequency and so matches the low-pass filter’s out-
put impedance expectation. Note that no transformer is
shown, though by adding a transformer between the filter
and the piezo, lower voltages would exist in the left-hand
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10
1
10
2
10
0
10
−2
10
1

10
2
10
1
10
2
Freq (kHz)
10
0
10
−1
Voltage across piezo (V)Power spectral density (arbitrary units)
10
−2
10
−3
Spectral content for 33% duty cycle +/− square wave PWM
Frequency response (voltage across piezo for 1 V PWM input)
PWM input voltage
Piezo voltage
Figure 11. Frequency response function of electronics concept.
side of the circuit which would probably ease component
choice.
Figure 11 showsthecalculated frequency response func-
tion. It also shows the spectral content of the voltage out of
the switched power supply and into the piezo. The output
from the switched power supply it is assumed, is both posi-
tive and negative in the 33% duty cycle and has switching
transients 25% aslongas the on-time, thatis, 1.67 µs. Sum-
ming all power above the fundamental to 250 kHz gives a

total harmonic distortion figure of 71% for the switched
power supply output that has this waveform, but only 4%
for the voltage across the piezo.
A breadboard of this system was built and tested. It was
felt that the advantages of the switching amplifier concept
outweighed its disadvantages for a production application.
A commercial supplier (Instruments Inc. of San Diego CA)
was found.
Implementation Issues. The thin walled stainless steel
tube that contains fluid-borne microorganisms was de-
signed to be as thin as possible to maximum the pressure
transmitted through to the fluid. The thickness is limi-
ted by the pressure in the transmission medium. The thin
walled tube is fairly close to buckling under the pressure
of the transmission medium.
In the prototype system, there was no pressure sensor to
ensure that the pressure of the transmission medium was
maintained between 30–100 psi. The small temperature
change (1–2
◦
C) that results from the excitation of the
system causes the pressure to vary. The temperature
change is kept to this low level by pumping the working
fluid continuously past the transmission medium. During
biological evaluation of the prototype system, the pressure
did drift above 100 psi. After completing of prototype
testing, the system was dismantled, and it was discovered
that the tubing had buckled.
The evaluation unit which was built as a follow-on to
the prototype includes both a temperature and pressure

sensor as part of the design. This ensures that the system
will shut down before the critical pressure is exceeded. In
an early version of the evaluative design (which contained
16 piezo rings, rather than the original four), the stainless
steel tubing did buckle because the unsupported length of
the tubing hadmorethan doubled. Modifications of thetub-
ing boundary conditions were madetoensure that buckling
did not occur but at the same time maintained as thin a
profile as possible to maximize the energy transfer to the
microorganism-borne fluid.
Another significant issue that arose during early test-
ing of the evaluative system relates to the importance of
tolerancing the rings themselves. After short runs of the
16-ring stack system, failures in the rings occurred. They
were failing mechanically—breaking into two pieces. The
initiation of the crack seemed to be associated with a burn
mark on the ring. It was postulated that the set of rings be-
ing used was not sufficiently well toleranced for roundness.
The system was rebuilt using rings of improved tolerance
(proved by Sensor Technologies of Collingwood, Ontario).
There have been no ring failures since the system was
rebuilt.
The original electronic driveforthe system was basedon
square wave input switching. When this was implemented,
switching noise was feeding back to the input, causing
noise spikes that were outside the acceptable range of the
microprocessor. To eliminate this problem, the signal gen-
erator was rebuilt to use sine wave excitation.
Figure 12 shows a drawing of the cavitation portion
of the system. The elements of the figure are as listed in

Table 3.
Effectiveness of Cavitation in Destroying Microorganisms
The effectiveness of using a cavitation field to destroy mi-
croorganisms was measured for two types of fluid hosts
(water and diesel fuel) (9) and three types of microorgan-
isms:
r
Serratia marcescens
r
Pseudomonas aeruginosa
r
Saccharomyces cerevisiae (yeast)
The fitted results are shown in Fig. 13, plotted as a function
of exposure time to the cavitation field. Regression analysis
was used to fit the data to the following equation:
log

Irradiated
Control

= (Slope × Time) + const. (2)
These test results were for microorganisms exposed to
cavitation while the working medium was moving (be-
ing pumped) through the cavitation field. Earlier test re-
sults were performed while the medium was static during
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11
12

13
14
15
16
17
18
I
10
9
8
7
6
5
4
3
2
1
Figure 12. Cavitation unit—16 ring.
exposure to the cavitation field. The cavitation effect was
more pronounced on the moving population than on the
static population. It was hypothesized that the motion en-
sured improved distribution of the microorganisms in the
cavitation field.
There were two different strains of Pseudonomas aeru-
ginosa used in the study. Tests in water were done using
ATCC 10145. A strain of Pseudonomas aeruginosa was
isolated from a sample of marine diesel fuel. This strain
would not survive at elevated temperatures (37
◦
C) where

the ATCC 10145 thrived.
Table 3. Parts of Cavitation Unit
Drawing Label Part
1 Lower sealing flange
2 Hydraulic O-ring
3 Lower flange
4 Hydraulic O-ring
5 Body
6 Body assembly rods
7 Flow-through tubing
8 Supporting ring
9 Hydraulic O-ring
10 Hydraulic O-ring
11 Upper flange
12 Upper supporting ring
13 Hydraulic O-ring
14 PZT ring, 2.0 in OD
15 Middle PZT supporting ring
16 PZT Assembly rods
17 Self-locking nuts
18 Lower PZT supporting ring
10
0.001
0 5 10 15 20
0.01
0.1
1
Treated/control
Exposure time(s)
Flow through testing

Saccharomyces
(yeast)
Pseuds in water
Serratia in water
Pseud in diesel
Serratia in diesel
Pseud 'isolate'
in diesel
Figure 13. Biological test results.
The results were based on a flow-through testing system
that involved recirculating the population to obtain the re-
quired exposure time. Figure 14 shows a schematic of the
experimental facility. The contaminated working fluid was
recirculated during testing. This eliminated the need for
disposal of large volumes of contaminated fluid. The re-
circulating effect underestimates the effectiveness of the
method because the population is being gradually reduced
for each pass through the cavitation field.
It had been postulated that the pumping action itself
might influence the microorganism population, but that
effect was studied and found insignificant on either the
Serratia marcescens or the Pseudomonas aeruginosa.
There did seem to be a small effect on the yeast results.
An attempt was made to predict the kill efficiency of a
single pass of the population through the cavitation field.
Kill efficiency e is the ratio of microorganisms per unit vol-
ume of fluid killed in one pass to microorganisms present
in an untreated unit volume of fluid.
6
UDM experimental facility

1
8
7
5
4
3
2
1 − Cavitator
2 − Tank for treated water
3 − Tank for contaminated water
4 − Control valves
5 − Pump
6 − Power supply
7 − Hydraulic cylinder
8 − Screw
Figure 14. Schematic of flow-through experimental facility.
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NOTATION
C
o
= initial concentration (microorganism’s/litre)
C
n
= concentration after n passes through cavitation
field
e = kill efficiency
n = number of times sample passed through
cavitation field

V = volume of cavitation field
X = holding tank volume
C
n
C
o
=

X − e × V
X

n
(3)
When this equation is applied to the yeast test data ob-
tained, the resulting kill efficiency is 0.49. When it is ap-
plied to the test results for Pseudomonas aeruginosa in
diesel fuel, the resulting kill efficiency is 0.45. These re-
sults were based on an exposure time of 3.15 seconds in
the cavitation field.
BIBLIOGRAPHY
1. A.D. Ashton. Laboratory Evaluation of Ultrasonic Devices:
Weitech Electronics,
2. O.C. Zienkiewicx, The Finite Element Method. McGraw-Hill,
NY, 1977.
3. K. Ragulskis, R. Bansevicius, R. Barauskas, and G.
Kulvietis, Vibromotors for Precision Microrobots. Hemisphere,
NY, 1988.
4. Modern Piezoelectric Ceramics, Morgan Matroc Vernitron
Division, Bedford, OH, 1988.
5. J.R. Frederick, Ultrasonic Engineering. Wiley, NY, 1965.

6. S.S. Save, A.B. Pandit, and J.B. Joshi, Chem. Eng. J. 55 B67–
B72 (1994).
7. A.J. Chapman, Heat Transfer. Macmillan, NY, 1967.
8. G.L. Gooberman, Ultrasonics: Theory and Application. Hart P,
NY, 1969.
9. S. Draisey. Ultrasonic Destruction of Microorganisms in Ship-
board Fuels: Biology Report. Canadian National Defence Re-
port, DREA CR 98/426.
PHOTOCHROMIC AND
PHOTO-THERMO-REFRACTIVE GLASSES
L.B. GLEBOV
University of Central Florida
Orlando, FL
INTRODUCTION
Inorganic glasses are the main transparent material,
which people have long used for observation (windows
in buildings, windshields in cars, eyeglasses, prisms and
lenses in optical instruments), light delivery (light bulbs,
projectors, lasers, optical fibers), and fine arts (crockery,
bijouterie, jewelry). The ability of glasses to change colo-
ration after exposure to sunshine was well known since
the last century. A new era in glass application was started
in 1949 by S.D. Stookey’s publication (12) in which record-
ing a permanent photographic image in silicate glass was
described. This two-step process of exposure to UV radia-
tion and thermal developmentthatresulted in a crystalline
phase precipitation in the exposed areas was similar to
the classical photographic process. As a result of inten-
sive research during a long period of time, a great number
of different photosensitive glasses were developed, which

have found very wide application in different branches of
industry and personal use. When exposed to optical radia-
tion, these glasses (and glassceramics)change their optical
properties (absorption, refraction, or scattering) instantly
or after thermal development, permanently or transiently.
Among the great variety of photosensitive glasses, we em-
phasize only the two most widely used types.
The largest commercial application was obtained for
so-called “photochromic glasses,” which exhibit reversible
coloration after exposure to UV or visible light and can
vary their absorption depending on the illumination level.
Glasses that contained small concentrations of microcrys-
tals of silver and copper halides, proposed by Armistead
and Stookey in 1965 became the most widely used for
reversible coloration (13). A peculiarity of these materi-
als is that they are produced by glassmaking technology
whereas the photochromic processes occur in microcrystals
distributed in the glass matrix. Several hundred original
papers were dedicated to different aspects of heteroge-
neous photochromic glasses in those years. The vast biblio-
graphy and detailed descriptions of these heterogeneous
photochromic glasses were collected in books (3,4), and
therefore we will not include a list of original publications
in this article.
Another type of photosensitive glass, which is beginning
its application in optics and photonics right now, is “photo-
thermorefractive (PTR)” glass. If this glass is exposed to
UV radiation followed by heat treatment, it varies in re-
fractive index. A phase hologram in the volume of this glass
was recorded in 1990 by Glebov and coauthors (5). The fea-

ture of this process is that homogeneous glass is exposed
to light and a microcrystalline phase is produced in the
volume of the glass matrix by a thermodevelopment pro-
cess. No books have been written on this subject. The main
results concerning phase hologram recording in glasses
can be found in a few original papers (5–7) and a survey
(8). Similar processes of photoionization followed by ther-
moinduced crystallization were studied for single- and full-
color photography in polychromatic glasses, as described in
(1, 9–12). Thus, these references can also be used for
learning the basic physical phenomena that result from
irradiation and development of PTR glasses. Some basic
data concerning intrinsic absorption, electronic excitation,
and nonlinear photoionization in multicomponent glasses
can be found in (13,14).
PHYSICAL PRINCIPLES OF PHOTOSENSITIVITY
IN GLASSES
Photosensitivity is the variation in glass properties from
exposure to optical radiation. Photoinduced processes can
be caused by the absorption of light and consequent
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10
3
10
2
10
1
10

0
10
−1
1234
Photon energy, eV
Wavelength, nm
4
56
1
2
Absorption, cm
−1
1000 500 400 300 200
3
Figure 1. Absorption spectra of 25Na
2
O–75SiO
2
glass. 1: intrin-
sic absorption; 2 and 3: extrinsic absorption of 0.1 wt.% of Fe
3+
and Fe
2+
, respectively; and 4: color center generation spectrum
(arbitrary units).
excitation of electrons from groundtoupperlevels by which
these electrons can be delivered to other places (we will
not consider heating and posterior melting or ablation).
Absorption spectra of solids may be conventionally divided
into three groups. Absorption due to electron transitions

in defect-free substances of stoichiometric composition is
called “intrinsic,”“basic,” or “fundamental” absorption. The
absorption in atoms or molecules that are present as small
additives is called “extrinsic,” or “dopant,” or “impurity” ab-
sorption. The absorption by defects in the host substance
created by chemical or physical effects is called “induced,”
or “additional,” or “defect” absorption.
The absorption spectra of widespread alkali silicate
glass, which is the basis of the majority of technical glasses,
are presented in Fig. 1. Intrinsic absorption (curve 1) is in
the range of 210 nm (6 eV) and exhibits an exponential
dependence of the absorption coefficient on photon energy
(or wave number). This absorption is caused by basic struc-
tural units of silicate glass (Si–O–Na), which are called L
centers. An example of extrinsic absorption in 25Na
2
O–
75SiO
2
glass is shown by curves 2 and 3 for ferric (Fe
3+
)
and ferrous (Fe
2+
) ions, which determine the actual ab-
sorption of commercial silicate glasses in the near IR, visi-
ble, and near UV spectral regions. Induced absorption pro-
duced by UV and γ radiation (Fig. 2) is caused by ionization
in the glass matrix and further trapping of electrons and
holes at different glass matrix defects. The presence of dif-

ferent dopants and impurities results additional induced
absorption bands. Extrinsic absorption can be caused by
additional ions distributed in the glass matrix and also
by bigger units, for example, microcrystals. The absorp-
tion spectra of borosilicate glass doped with copper and
chlorine, which has undergone heat treatment, are shown
in Fig. 3. Instead of absorption of copper ions in the glass in
the far UV region, a narrow absorption peak near 380 nm
(3.25 eV) is seen in these spectra, which corresponds to
excitons in CuCl crystals precipitated in the glass matrix
as the result of heat treatment. Induced absorption can
0.6
0.4
Optical density
0.2
12345
2
6
300 K
77 K
1
1000 400 300 200
H
E
Wavelength, nm
Photon energy, eV
Figure 2. Induced absorption spectra of 25Na
2
O–75SiO
2

glass.
1: exposure to UV at 77 K; 2: γ irradiation at 300 K. Arrows show
the positions of the absorption bands of electron (E) and hole (H)
color centers.
0
50
100
400 350 250300
Wavelength, nm
3.2 3.6 4.0 4.4 4.8
Photon energy, eV
Absorption, cm
−1
1
2
3
Figure 3. Absorption spectra of borosilicate glass doped with cop-
per and chlorine after 2 hours of treatment at T(
◦
C): (12) 550, (13)
600, (3) 650.
also be produced by relatively big particles. Photoinduced
precipitation of microcrystals of such metals as gold, silver,
and copper causes additional absorption, usually calledcol-
loidal coloration.
Glass exposure to radiation whose photon energy is
more than the intrinsic absorption edge (curve 1 in Fig. 1)
causes photoionization in the glass matrix followed by the
generation of both electron and hole color centers. The
dependence of the induced absorption on the photon en-

ergy (or wavelength) is called the color center generation
spectrum or the spectrum of photosensitivity (curve 4 in
Fig. 1). Photoionization in the glass matrix (generation of
both electron and hole centers) is impossible if the pho-
ton energy of the exciting radiation is less than a bandgap,
which is determined by the position of the intrinsic absorp-
tion (curve 1 in Fig. 1). In other words, the long wavelength
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edge of the color center generation spectrum (curve 4 in
Fig. 1) coincides with the intrinsic absorption edge (curve 1
in Fig. 1).
The photosensitivity spectrum can be shifted to the long
wavelength side if the glass is doped with some ions in a
lower valence state, and the dopant’s excited level is placed
above the threshold of the charge carrier’s mobility. In this
case, a mobile electron can be trapped either by defect at an
intrinsic electron center formation or by another dopant,
that is, to recharge the activators. The depth of the dopant
ground level in Na
2
O–3SiO
2
glass is 5.2 eV for Fe
2+
, 5.0 eV
for Tb
3+
, and 3.6 eV for Ce

3+
. Comparison of these values
with curve 3 in Fig. 1 shows that the ionization threshold of
Fe
2+
corresponds to the longwavelength edge of theabsorp-
tion band whose maximum is at 6.5 eV (191 nm). Excita-
tion using smaller photon energy causes tunnel ionization
whose efficiency is about one to two orders of magnitude
less than that of over-barrier ionization. The thresholds
of tunnel ionization of dopants in Na
2
O–3SiO
2
glass are
3.5 eV for Fe
2+
, 3.1 eV for Tb
3+
, and 3.1 eV for Ce
3+
. Refer-
ring Fig. 1, one can see that the tunnel ionization of Fe
2+
is obtained at an excitation of the long wavelength bands
whose peaks are at 5.1 and 4.4 eV (243 and 282 nm) up to
3.5 eV (350 nm). Unlike intrinsic ionization that inevitably
produces electron and hole centers, the only hole center
generated from the excitation of dopant absorption bands
is the same (but oxidized) dopant ion. All newly created

centers are electron centers (either intrinsic or extrinsic).
The other way to shift photosensitivity to the long wave-
length side is to use nonlinear ionization produced by pow-
erful optical irradiation. In silicate glass exposed to pulsed
radiation whose photon energy is more than half of the
bandgap (hν>3eV,λ<400 nm) and whose irradiance is
more than 1 MW/cm
2
, both electron and hole color centers
appear as a result of two-photon ionization in the glass
matrix. The final concentration of color centers is deter-
mined by equilibrium between two-photon generation and
single-photon bleaching of color centers.
INDUCED COLORATION OF REVERSIBLE
PHOTOCHROMIC GLASSES
Generally, the term photochromism may be treated as any
variation of color induced by optical radiation, but usu-
ally people use a narrower definition, which excludes irre-
versible color changes. So, photochromism is a reversible
variation in color (i.e., of the absorption spectrum or spec-
trum of attenuation) of a material under optical radiation
that relaxes when exposure stops. Naturally, when experi-
mental conditions are changed, for example, a temperature
change, the magnitude of the photochromic effect can vary
(even to complete disappearance). Therefore, we shall call
a photochromic material one that, under specified operat-
ing conditions, becomes colored by optical radiation and
restores its transparency after radiation ceases.
Relaxation of induced absorption after illumination
ceases is usually caused by thermal fading of color cen-

ters, which are not stable at a given temperature. This
is the most important feature of photochromic materials
because reversibility of the photochromic effect means the
absence of any stable induced centers generated by illu-
mination. A great number of electron and hole color cen-
ters in silicate glasses produced by UV radiation (Fig. 2)
leads to fatigue because of the progressive accumulation of
stable color centers. This is the reason that these glasses
are not used as photochromic materials, although pho-
tochromism was discovered in cerium-doped, reduced sili-
cate glasses. Glasses doped with microcrystals of silver and
copper halides (Fig. 3) show complete reversibility of colo-
ration at room temperature and therefore have the widest
commercial application.
The main feature of photochromic glasses, variable op-
tical density both observed during exposure and upon its
cessation, has to betaken into account to determine charac-
teristics such as integral and spectral sensitivity, darken-
ing degree and rate, thermal fading, and optical bleaching
rates. Let us define the main concepts required for pho-
tochromic material characterization. Light absorption (or,
more exactly, light attenuation or losses, that is the sum
of absorption and scattering) is characterized by the trans-
mittance, τ = I
tr
/I
0
(where I
tr
and I

0
are the intensities of
transmitted and incident light, respectively), or the opti-
cal density, D =−log
10
τ . The optical density of a sample
before irradiation (original absorption, clear glass) is D
0
(Fig. 4). The optical density of the sample at the moment
exposure ceases (induced absorption, dark glass) is D
exp
.
The optical density in t seconds of the thermal fading pro-
cess (induced absorption, partially relaxed glass) is D
t
. The
spectral dependences of τ
0
and D
0
are the transmission
or absorption spectra of clear glass. The spectral depen-
dences of τ
exp
and D
exp
are the transmission or absorption
spectra of dark glass. Glass has a gray color if the absorp-
tion (transmission) spectrum is flat in the visible region. A
brown color means that the absorption in the blue region

is greater than that in the red region.
The dependences of D
exp
and D
t
on the time of illumi-
nation or aging are the kinetics of coloration and relax-
ation, respectively (Fig. 4). D
exp
increases when the expo-
sure time increases and comes to the equilibrium level D
e
D
exp
D
t
D
0
t
0
t
exp
Time
Illumination Aging
Optical density
Figure 4. Kinetics of photochromic glass darkening under illu-
mination and fading in the aging process. D
0
, D
exp

, and D
t
are the
optical densities of clear, dark, and relaxed glass, respectively.
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when the rate of color center generation is equal to the
rate of thermal fading. The criterion of relaxation charac-
terizes the degree of thermal fading in a certain time after
illumination ceases:
K
rel
=
D
exp
− D
t
D
exp
− D
0
(1)
The value of that time interval should be selected on the
basis of the practical applications of a photochromic glass.
Thus, for photochromic lenses used as sunglasses, a time
interval of 180 s is recommended. From Eq. (12), it is ob-
vious that, if a glass has faded completely in that time,
K
rel

= 1. Contrariwise, if the induced absorption has not
reduced at all in that time, K
rel
= 0. Now, there are pho-
tochromic glasses whose K
rel
vary in the entire range from
zero to about one. K
rel
for a particular glass can be changed
by temperature variation.
An important parameter is the spectral sensitivity of
a photochromic material, the dependence of the saturated
photoinduced optical density (D
e
) on the photon energy of
the exciting radiation. This dependence is called the color
center generation spectrum. The absorption edge of pho-
tochromic glass determines the position of the color cen-
ter generation spectrum because photosensitive crystals
absorb exactly in that region (compare curves 1 and 2 in
Fig. 5). The short wavelength edge of the color center gener-
ation spectrum is connected with the decrease of the thick-
ness of the layer containing color centers, that is due to the
increase of the glass absorption coefficient. The long wave-
length edge is caused by a decrease in the absorption and
in the efficiency of photosensitive center ionization. These
photosensitive centers are usually copper centers in silver
halide crystals or excitons in a crystalline phase of copper
chloride. Owing to that, the position of the maximum in

the color center formation spectrum does not coincide with
that of any maximum in the photochromic glass absorp-
tion spectrum. Moreover, its position is determined by the
spectral shape of the photochromic glass absorption edge,
1000
1.0
0.8
0.6
Optical density
0.4
0.2
0.0
12345
500
400
300
Photon energy, eV
Wavelength, nm
4
3
2
1
Figure 5. Spectra of glass doped with AgCl(Br). Absorption of
original glass (12) and color centers (3), color center generation
(13) and bleaching (4) efficiency. Sample thickness 5 mm.
is a function of the sample thickness, and drifts to the short
wavelength side as the thickness decreases. The absorption
spectrum of an exposed glass doped with AgCl microcrys-
tals is presented in Fig. 5, curve 3. This absorption repre-
sents a wide band in the visible spectral range. The spec-

tral shape of this band is usually ascribed to precipitation
of colloidal silver particles on the surface of halide micro-
crystals. Curve 4 in Fig. 5 shows that excitation of the ab-
sorption band of color centers destroys these centers and
causes optical bleaching. Thus, optical bleaching by visi-
ble light is a process additional to thermal fading, which
accelerates the relaxation of darkened silver halide photo-
chromic glass.
The photosensitivity of photochromic glasses doped
with CuCl can be shifted from the UV region to the long
wavelength side. Virgin photochromic glass is photosensi-
tive only to UV irradiation and cannot be darkened by vis-
ible light. Excitation of glasses doped with CuCl that are
exposed to UV radiation does not produce optical bleach-
ing, as shown in Fig. 5 (curve 4) for silver halide glasses.
On the contrary, initial additional absorption (induced by
UV radiation) can be intensified by additional exposure to
visible and even IR radiation having photon energy much
below the ionization threshold of copper centers. Note that
the power density of long wavelength irradiation must be
high enough to produce this intensification. It is shown in
Fig. 6 that the spectra of additional absorption produced
in this glass after irradiation at various wavelengths are
the same. Consequently, this long wavelength sensitivity
results from generating new color centers by exciting the
same color centers. Therefore this process is called “coop-
erative breeding of color centers.”
The mechanismoftwo-photon cooperative breedingisas
follows. Initial exposure to UV radiation causes ionization
0.4

0.3
0.2
0.1
Optical density
4005006008001000
321
3
2
1
1.5 2.0
Photon energy, eV
Wavelength, nm
2.5
Figure 6. Spectra of induced absorption in copper halide pho-
tochromic glass (thickness 5 mm) after exposure to radiation at
different wavelengths: (12)440 nm(2.78 eV),(13) 633nm (1.96eV),
and (3) 1060 nm (1.17 eV).
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Figure 7. Energy diagram of the first stage
of photochromic glass coloration at (a) short
wavelength coloration, (b) two-photon coopera-
tive breeding, and (c) three-photon cooperative
breeding.
Cu
0
Cu
0
Cu

0
Cu
0
Cu
0
Cu
0
Cu
2−
Cu
+
Cu
2−
Cu
2−
Cu
+
Cu
+
Cu
0
Cu
0
3.25 eV
(a)
hv hv hv
1.96 eV 1.17 eV
(b)(c)
ee e
Valence band

Conduction band
of a photosensitive center (Cu
+
) and generates electrons
and hole centers (Cu
2+
). Then released electrons produce
color centers by reducing copper (Cu
+
) or silver (Ag
+
) ions.
The initial concentration of color centers (Fig. 7a) is deter-
mined by the number of UV-ionized photosensitive centers.
This concentration can be rather small and even invisible
to the naked eye. Linear absorption of two photons of visi-
ble light by two color centers causes a transition of these
centers to excited states (Fig. 7b). Further, these centers
simultaneously transfer the accumulated energy to the
photosensitive centers (Cu
+
) and return to their ground
states. An excited photosensitive center releases an elec-
tron and converts to its ionized state in the same man-
ner as after linear excitation, as illustrated in Fig. 7a. The
released electron is trapped by an acceptor, converts to a
reduced state (Cu
0
), and this is a first stage in generat-
ing a new color center. Thus, the number of color centers

increases after each cycle. This means that induced ab-
sorption increases in the process of exciting previously in-
duced color centers without altering the spectrum of the
induced absorption. The efficiency of this nonlinear pro-
cess is proportional to the squared intensity of the exciting
long wavelength radiation.
The coloration caused by exposure to pulsed IR radia-
tion can be explained similarly to the three-photon cooper-
ative breeding of color centers (Fig. 7c). The latter process
obeys the cubical dependence of efficiency on the intensity
of the exciting radiation. There are several important fea-
tures of cooperative breeding of color centers. The first is a
very high level of additional absorption because photosen-
sitivity in this case is not connected with the sharp absorp-
tion edge of glass (Fig. 5) and a thick slab can be homoge-
neously colored. The second is the opportunity of localizing
colored spots in arbitrary places of the bulk glass. The spots
are produced by focusing the exciting beam because photo-
sensitivity is proportional to the squared or cubical inten-
sity of the exciting radiation and therefore, is concentrated
near the focal plane. The third is an opportunity to store
a latent image produced by UV radiation that can be re-
vealed by photodevelopment.
HETEROGENEOUS PHOTOCHROMIC GLASSES
Photochromic glasses co-doped with silver and copper
halides are heterogeneous materials. They represent
two-phase systems that consist of a vitreous host and dis-
persed photosensitive microcrystals. This is important be-
cause microcrystals show a reversible photochromic effect
without fatigue. However, in a two-phase system, light at-

tenuation is caused by absorption of each phase and also by
scattering produced by the difference between the refrac-
tive indexes of the crystalline and vitreous components.
Therefore, the parameters of the crystalline phase should
be chosen to prevent strong scattering. The size of the par-
ticle of most photosensitive microcrystals, whose refractive
index is about 2, should be no more than 10–20 nm to keep
scattering below the level of acceptability for optical appli-
cations.
The main approach to producing dispersed microcrys-
tals in a vitreous host is crystalline phase growth as a
result of host glass heat treatment at temperatures from
500–700
◦
C, depending on host composition. These temper-
atures correspond to a viscosity range from 10
10
–10
13
poise.
To secure crystalline phase precipitation, special require-
ments are applied to the host glass. First, this glass should
be an oversaturated solution of the photosensitive phase
(silver and copper halides) that allows effective diffusion
of these components in the temperature range mentioned.
Second, the solubility of the photosensitive components
must drop quickly when cooling to allow the homogeneous
glass to melt at high temperature and the crystalline phase
to precipitate in the secondary heat treatment process. The
last is usually connected with phase separation (immisci-

bility) and altered coordination of different components in
the host glass.
The best glass, which satisfies the requirements men-
tioned before, is alkaline borosilicate glass. This glass ma-
trix is the basis for almost all commercial photochromic
glasses manufactured by a number of companies in differ-
ent countries. Halides (Cl, Br, I) of silver and copper are
photosensitive components, which are added to the batch.
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Cations such as Mg, Ca, Ba, Zn, Cd, Al, and Pb, or anions
such as P and S are used by different companies as addi-
tions to modify technical and end use properties. These
compositional changes lead to variations in photosensi-
tivity, the criterion of relaxation, and induced absorption
spectra. Photochromic glasses can be divided into two large
groups: silver halide glasses that have small concentra-
tions of copper, which usually exhibit faster relaxation and
lower sensitivity and copper halide glasses that have small
concentration of silver, which exhibit slower relaxation and
higher sensitivity. In silver halide glasses, small additions
of copper are a sensitizer.
The traditional schedule for photosensitive phase cre-
ation, “bottom-to-top,” consists of four stages: melting,
rough annealing and cooling to room temperature, addi-
tional heat treatment (roasting), and final annealing. Final
annealing is necessary for stress relaxation because crys-
talline phase precipitation occurs at temperatures above
the glass transition temperature. The other method of sen-

sitization is “top-to-bottom,” which is used for mass pro-
duction because of heat energy saving. In the latter, the
glass casting cools down to roasting temperature but not
to room temperature. It requires the other schedule (time
and temperature) because the most effective growth of nu-
cleation centers occurs at temperatures below the roasting
temperature.
OPTICAL WAVEGUIDES IN PHOTOCHROMIC GLASSES
The largest commercial application of photochromic
glasses is for sunglasses. Tens of millions of photochromic
lenses are produced worldwide each year for this purpose.
However, the alkaline borosilicate origin of photochromic
glasses allows some other applications in modern optics
and photonics. It is well known that these glasses are suit-
able for ion exchange and, consequently, planar and chan-
nel waveguides can be created on this glass. Besides that,
the mildly sloping dependence of photochromic glass vis-
cosity on temperature allows creating of optical fibers. The
optical properties of photochromic waveguides compared
with bulk photochromic glasses are unusual because of
structural transformations in the ion-exchanged layers or
in the drawn fibers and the peculiarities of light propaga-
tion in waveguides. An important feature of ion-exchanged
glass is incompleteness of structural relaxation. The ex-
change of ions that have different radii creates stresses in
glass. These stresses produce strong differences between
the refractive indexes of waveguide modes that are or-
thogonally polarized (birefringence). Compression of sil-
ver halide photochromic glass after substituting Na
+

by
K
+
at temperatures below the glass transition tempera-
ture reaches 1 GPa and produces birefringence up to 20%
of the total refractive index variation, as shown in Fig. 8.
Exposure of waveguides in photochromic glasses to UV
radiation produces reversible coloration. This means that
ion-exchange treatment does not destroy the photosensi-
tive crystalline phase and this technology is available for
photosensitive waveguide fabrication. However, parame-
ters of coloration and relaxation of photochromic wave-
guides are different compared to bulk glass. For silver
1.502
1.498
1.494
Refractive index
15
0510
Distance from surface, mm
TE
TM
Figure 8. Refractive index profiles of photochromic glass after
Na
glass
–K
melt
ion exchange. TE or TM polarizations mean electric
or magnetic field oriented along the surface, respectively.
halide glasses, the criterion of relaxation in waveguides is

more than that in bulk glass. This means that relaxation
in waveguides occurs faster. For copper halide glasses, re-
laxation in the waveguide was not detected, which means
that the coloration of these waveguides is stable. There
is a difference in photosensitivity between different wave-
guide modes. Modes Whith low numbers propagate near
the surface and have lower sensitivity than modes that
have a large number and propagate in deep layers. This dif-
ference is caused by copper (which is a sensitizer) depletion
in the surface layer as result of copper exchange for potas-
sium or other ions. This phenomenon can be used for mode
selection.
The other feature of photochromic waveguides is ani-
sotropy of photosensitivity and induced coloration. This
phenomenon is connected with ion-exchange stresses.
Dichroism (the difference between induced absorption for
orthogonal polarizations) is proportional to birefringence
in a waveguide. It is important to note that photosensi-
tive microcrystals are plastic or melted at the tempera-
tures of ion exchange. Therefore, dichroism is determined
by stresses and also by orientation of liquid drops of the
photosensitive phase caused by ion-exchange stresses.
The discrete structure of light propagation in photo-
sensitive planar waveguides gives one more opportunity
for multiplexing by mode selection. If a mode in such a
waveguide (Mode #1 in Fig. 9) is excited by actinic radi-
ation, the waveguide becomes colored. The spatial profile
of induced absorption is determined by the spatial profile
of the exciting modes intensity. As a result, a sort of dis-
tributed absorbing mask will be formed in the waveguide

whose absorption profile is similar to that of the intensity
distribution of actinic radiation in the waveguide. Conse-
quently, losses for mode #1 increase after excitation of this
mode by actinic radiation. The attenuation of other modes
is determined by overlapping of their fields by the dis-
tributed mask, that is, by the field of the mode that induced
this absorption. Because field profiles for the modes that
have different numbers essentially differ from each other