Materials Science and Engineering - Electronic and Mechanical Properties of Materials Part 9 pps

3.225
15
Designer Wavelengths
Variation of band-gap energy with composition x of In
1-x
Ga
x
As.
© H.L. Tuller, 2001
3.225
16
Band-Gap Colors
Mixed crystals of yellow cadmium sulfide CdS and black
cadmium selenide CdSe.
© H.L. Tuller, 2001
8
3.225
17
Light Sources
• Photoluminescence
• Cathodoluminescence
• Electroluminescence
© H.L. Tuller, 2001
3.225
18
♦ Energy
• onversion
• torage
• onservation
♦ Emissions
• Smoke stack

• Automotive
Challenges for New Millenium
Needed: dvances in sensors,
actuators and energy conversion
devices.
C
S
C
a
9
3.225
19
Harsh Environments
Ceramic Sensors
Automotive
Emissions
Factory
Emissions
Process Control
Aerospace
Performance
3.225
20
Electroceramics
Ceramics:
• Traditionally admired for their stability
•Mechanical
•Chemical
•Thermal
• Exhibit other key functional properties

• Electrical, Electrochemical, Electromechanical
•Optical
•Magnetic
10
3.225
21
Electroceramics Versatility
Atmosphere dependent conductivity (I.
Kosacki and H.L. Tuller, Sensors &
Actuators B
24-25
, 370 (1995).)
High Strain (Pb
0.98
La
0.02
(Zr
0.7
Hf
0.3
)
1-x
Ti
x
O
3
AFE-
FE System (C. Heremans and H.L. Tuller, J. Euro.
Ceram. Soc.,
19

, 1139 (1999).)
Semiconducting; Electrochemical; Piezoelectric;
Electro-optic; Magnetic, …
3.225
22
Feedback Control System
System
Actuator
Sensor
Chemical
Signal
Signal
Electrical
Power
Chemical
Species
Micro-Processor
Other Input
11
3.225
23
Sensors for Exhaust Gas Monitoring
Requirements
• clear dependence on pO
2
• short response times < 100 ms
• 700<T<1000°C
• long term stability …
3.225
24

Sensor Materials
Gas Sensor
Electrical Signal compatible
with Microprocessor System
Chemical
Environmental Stimulus
Requirements of Gas Sensor Materials
• High sensitivity
• High selectivity
•Reproducibility
• Fast response time
• Compatibility with Si microelectronics
•Long term stability
• Small size
•Low cost
12
3.225
25
3-Way Catalyst Conversion Efficiency
3.225
26
Potentiometric Gas Sensor
PO
2
(Ref)PO
2
(Exhaust)
E
E = (kT/4q) ln {PO
2

/ PO
2
(Ref)}
Nernst Potential
13
3.225
27
Auto Exhaust Sensor
• Requirements
• Sensitivity
•Reproducibility
• Robust
•Low cost
3.225
28
•Miniaturization (e.g. biological implants)
•Integration - logic, amplification, telemetry
•Portability - low power dissipation
•Rapid response
•Cost
Sensor Trends and Challenges
Neural recording/stimulation microprobe. Probes
15µm thick 2-4mm long. (Najafi and Hetke, IEEE
Trans. Biomed. Eng.
37
, 474 (1990).)
14
3.225
1
© H.L. Tuller-2001

Measurement of Gas Sensor Performance
Si wafer
ZnO film
H
2
H
2
H
2
H
2
Pt electrode
SiO
2
layer
Electrical
Measurement
• Gas sensing materials:
1. Sputtered ZnO film (150 nm
(Massachusetts Institute of Technology)
2. Sputtered SnO
2
film (60 nm)
(Fraunhofer Institute of Physical Measurement Techniques)
• Target gases:
H
2
, CO, NH
3
, NO

2
, CH
4
• Operating temperature:
320 - 460 ºC
3.225
2
© H.L. Tuller-2001
Mechanisms in Semiconducting Gas Sensor
• Bulk:
O
O
= 2e
’
+ V
O

+ 1/2 O
2
(g)
Induce shallow donors: density related to PO
2
n
2
[V
O

] PO
2
1/2

= K
R
(T)
⇒
n = (2 K
R
(T))
1/3
PO
2
-1/6
Bulk electronic conduction
modulate
Change in stoichiometry
1
3.225
3
© H.L. Tuller-2001
Resistive Oxygen Sensors Based on SrTiO
3
m
1
2
kT
E
pOe
A
±
−
∝σ

semiconducting oxide
Electrode
U
I
2
O
p
Exhaust
3.225
4
© H.L. Tuller-2001
Influence of Dopants on Electrical Conductivity of SrTiO
3
Sr
2+
Ti
4+
O
2-
3
Acceptors: Al, Ni, Fe
Donors: Nb, Ta, Sb, Y, La, Ce, Pr, Nd, Pm, Sm, Gd
λ
log(σ / (Ωcm)
-1
)
T = 800 °C
log(pO
2
/ bar)

0.995 1,005
-16 -4 0
-5
-4
-3
-2
-1
0
1
donor
acceptor
-20 -12 -8
donor doped
acceptor doped
2
3.225
5
© H.L. Tuller-2001
Temperature Independence: High Acceptor Concentration in SrTiO
3
10
-20
10
-15
10
-10
10
-5
10
0

0,1
1
750°C
800°C
900°C
850°C
950°C
electrical conductivity / (Ω cm)
-1
pO
2
/ bar
m = 0,2
Sr(Ti
0,65
Fe
0,35
)O
3
Response times
T / °C t
90
/ ms
900 6.5
800 26
750 83
700
[1] Menesklou et al, MRS fall
meeting, Vol. 604, p. 305-10 (1999).
185

3.225
6
© H.L. Tuller-2001
Oxygen Sensor in Thick Film Technology
3