Thermal resistant efficiency of Nb doped TiO2 thin film based glass window 2017 Journal of Science Advanced Materials and Devices

Journal of Science: Advanced Materials and Devices xxx (2017) 1e6

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Original Article

Thermal resistant efficiency of Nb-doped TiO2 thin film based glass
window
Luu Manh Quynh a, *, Nguyen Thi Tien a, Nguyen Ba Loc a, Vu Quang Tho b,
Nguyen Thi Lan a, Pham Van Thanh a, Nguyen Minh Hieu c, Ngoc Lam Huong Hoang c,
Nguyen Hoang Luong c
a
b
c

Faculty of Physics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam
Tan Trao University, Tuyen Quang, Viet Nam
Nano and Energy Center, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 22 March 2017
Received in revised form
20 June 2017
Accepted 9 July 2017

Available online xxx

The proportional relationship between the infrared (IR) transmittance of a transparent material and its
IR-induced heat transfer can be explained via a simple model. The agreement between theory and the
experimental work was examined by measuring the temperature rising inside a heat-insulated box with
glass windows under IR irradiation, where the material of the glass windows was modified from corning
glass (CG) to 9 at% Nb-doped TiO2 (TNO) fabricated by sputtering deposition. The fabricated TNO thin film
was mostly transparent in visible region and had low transparency in IR region, which produced the selfcooling effect inside the insulated box. In comparison to the window glass made by CG, the temperature
increase inside the box would be 24% less if the window was made by CG coated by TNO (TNO on CG).
This suggests a potential application for the manufacture of products which are effective in energy-cut
cooling. The energy-cut was found to decline proportionally to the decrease of the glass window area.
© 2017 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
Nb-doped TiO2
Low IR-transmittance
Glass window
Self-cooling
Transparent conducting thin film

1. Introduction
Increasing world energy consumption causes the rise of atmospheric CO2 level, which is one of the main causes of global
warming. The field of renewable energy and energy savings is a
challenging subject. The transparent conductors (TCs) e based on
both oxides as well as non-oxides e play an important role in
transmitting, converting as well as saving energy [1]. TCs are of
attention because of several reasons. Firstly, they are transparent in
the visible light range and absorb ultraviolet (UV) light due to excitations across an energy gap. In addition, they reflect IR radiations
of wavelengths longer than the plasma one [2].
IR-reflective properties of TCs have been reported earlier in

several oxide materials, such as Sn-doped In2O3 (ITO) [3e7],
Al-doped ZnO (AZO) [8e10], and F-doped SnO2 (FTO) [11e13]. Nbdoped TiO2 (TNO) is a newcomer TC [14e17] and TiO2 has attributes
that other conventional host materials of TCs do not possess,

* Corresponding author.
E-mail address: (L.M. Quynh).
Peer review under responsibility of Vietnam National University, Hanoi.

namely a high refractive index [18], the large static permittivity
[19], the high chemical stability especially in a reducing atmosphere [20], and the photocatalytic ability [21]. TNO thin films have
some other benefits, including its low materials cost, easy fabrication, and self-cleaning ability. As a result, they have a valuable
potential for application as an energy-saving coating layer for the
“cool” window glass, which aims to minimize the temperature rise
in the black interior of household appliances caused by IR-light
absorption from solar irradiations [22,23].
A well-known model of solar-reflective “cool” coatings was
introduced by Levinson et al. with a full complication of the relationship between the backscattering, absorption coefficient of the
coating material and the solar irradiation spectrum [24]. This model
was well applicable to different colored “cool” coating pigments
[25e27]. Later, Mohelnikova brought out a simplified in-lab technique targeting the evaluation of the heat-induction inside a glasswindow box [28]. It was widely used for different transparent IRreflective materials [29e31]. However, given the heat conductance and the outside surface temperature profile of the glass
window, the model was still complicated. Furthermore, the concrete relationship between the optical property of the material and
the heat-induction inside the box had not been discussed yet.

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In this paper, the thermal resistance effect of TNO thin films
fabricated on glass window was investigated. The air temperature
inside a thermally insulated box was measured. The front side of
the box had a small window with a glass sample. Furthermore, the
effect of the area of the glass window on the temperature rise inside
the box was evaluated. The theoretical relationship between the IR
transmission and the temperature increase was also discussed. By
using the TNO coated glass, the energy for cooling was calculated to
decrease by 24%.
2. Experiment and methods

The IR lamp was placed in front of the window within the same
distance in all measurements, so that the IR power irradiated to the
window and to the box could be considered as constant. The room
temperature and the temperature in the box, T, were measured by a
digital thermometer (Conotec, Korea) at different timings, respectively at 1, 2, 3, 5, 7, 10, 15, 20 and 30 min, after the lamp had been
turned on. The measurement was carried out with the area of the
window varied between 4, 9, 16 and 25 cm2. At each window's size,
the process was repeated within 3 different days to check the
reproducibility. During the experiments, “active cooling” of the
window surface was generated by using an air ventilator.

2.1. TNO thin film fabrication

3. Results and discussion

TNO (with Nb at 9 at%) films were sputtered-deposited on nonheating corning glass (CG) substrates. The sputtering process was
carried out under a total pressure of 7.5 Â 10À3 Torr in pure Ar

atmosphere. The RF sputtering power applied to the target was
kept constant at 90 W during the process. The as-deposited
amorphous films were annealed at 350  C in vacuum atmosphere
(~1 Â 10À5 Torr) within 30 min.
The thickness of TNO films was determined by the cross-section
scanning electron microscope (SEM-NOVA NANOSEM) measurement. Light absorption was observed by both the Shimadzu UV2450 spectrophotometer in the UVevisible region from 200 nm
to 900 nm, and the Shimadzu UV-3600 spectrophotometer in the
near infrared (NIR) region from 800 nm to 2600 nm. The crystal
structure of the thin films was examined using a BRUKER 5005 Xray diffraction (XRD) analyzer.
2.2. Installation of heat resistant measurement
The temperature increase in a closed box was generated by the
irradiation from an IR lamp (Medilamp 250 W, TNE Co., Vietnam,
shown in Fig. 1). The box of cubic structure was covered by heatresistant Styrofoam, whose S0 side area was 49 cm2. A window
was installed at the side to ensure that the IR rays can pass fully
through. The area of the window, S, could be modified. Inside the
box, black foam was fixed in order to create maximum IR
absorption.

With similar fabrication conditions, the thickness and XRD
pattern of the TNO thin films were the same as the other TNO thin
film products (Fig. 2) in our previous report [32]. The thickness of
the film was about 230 nm. In comparison to the characteristics of
standard anatase TiO2 (JCPDS No. 021-1272), all the detected XRD
peak positions of our sample are slightly shifted to smaller 2q angles. This shift corresponds to the larger length of the a- and c-axes
of the unit cell, originating from the larger radius of the Nb5þ ion
compared to that of the Ti4þ ion and resonating with Vegard's law
[14,17]. Besides, no Nb2O5 impurity was detected, as we could
reveal that the Nb5þ ions were fully incorporated to the TiO2 lattice.
The transmittance spectra of a CG sample and a TNO thin film
coated CG (TNO on CG) sample are shown in Fig. 3. In the whole

wavelength range from 400 nm to 2600 nm containing UVevis and
IR regions, the corning glass transmits more than 90% of light.
Regardless of the presence of doped Nb5þ ions, our TNO thin films
have shown a high carrier concentration of 8 Â 1021 cmÀ3 as
determined by the Hall measurement (data not shown). This generates plasmonic reflectivity in the IR region [14e17]. As a result,
the transmittance spectrum of our TNO thin film in the IR region is
as low as 70%. Besides, a wave-like spectrum is detected in the
UVevis region from 400 nm to 1000 nm, which might originate
from the light interference on the thin film. Moreover, considering
earlier works [8,22,33], the same strong interference effect corresponding to the high reflective index of TiO2-based thin film was
detected in the visible region from 400 nm to 1000 nm.

Fig. 1. Schematic installation of heat transfer measurement.

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Fig. 2. SEM image (a) and X-ray diffraction pattern (b) of TNO thin film deposited on corning glass.

Fig. 3. UVevis-IR transmittance spectra of the corning glass (CG) and TNO thin film on corning glass (TNO on CG) at visible and near infrared regions.

The mean near-IR transmittance was calculated by the formula
P
Pl2
nm
TIR ¼ 800

2600 nm Absi  li = l1 li , where Ti is the transmittance at
the li wavelength. By means of this formula, the mean near-IR
transmittance of CG and TNO on CG have been found as
CG ¼ 92:1% and T TNO ¼ 72:7%, respectively. If all the IR irradiaTIR
IR
tions in this range are completely absorbed by the black foam

inside the box (see Fig. 1) and are fully converted to produce heat,
the temperature increase inside the box depending on the window materials will vary proportionally to the mean near-IR
transmittance values.
The differential equation of the temperature inside the box
could be written as the following:

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k1 SP þ k2 ðS0 À SÞP þ ðT À T0 Þ½s1 S þ s2 ðSB À Sފ ¼ C

dT
dt

(1)

where P is the heat flux of the IR source; S and S0 are respectively
the area of the window and that of the irradiated side of the box. SB
is the total area of the box e in our case, the box is cubic, hence

SB ¼ 6S0. The heat transfer efficiencies, which mean the heat percentage that causes the temperature increase of the box internal,
are k1 relating to the window material and k2 relating to the box
material. As the temperature of the box internal rises, the heat
release process starts. This release is proportional to the difference
between the box internal temperature, T, and the room temperature, T0. The heat release efficiencies are labeled as s1 and s2
relating to the window and the box, respectively.
Two
new
quantities
are
defined,
namely
§ðSÞ=C ¼ k1 SP þ k2 ðS0 À SÞP=C, sðSÞ=C ¼ s1 S þ s2 ðSB À SÞ=C, which
are later also identified as the heat transfer rates regarding the
heat induction and the heat release processes. The differential
equation could be simplified as the following:

§ðSÞ sðSÞ
dT
þ
ðT À T0 Þ ¼
C
C
dt

(2)

The solution of the first-order differential equation is an exponentially time-dependent function, which would be assumed as:
T ¼ T1 þ T2 eÀkt , where T1 and T2 are constants. Applying this to the
equation (2) above, we arrive at the following equation for T:

TðS; tÞ À T0 ¼

§ðSÞ §ðSÞ ÀsðSÞt
À
e C
sðSÞ
sðSÞ

(3)

The temperature increase inside the box varies exponentially
with the irradiation time and the area of the window. Fig. 4(a) and
(b), respectively, show the time dependence of the temperature
increase inside the box with the different areas of the CG and TNO
on CG based glass windows. All the experiments were repeated 3
times on different days and the high reproducibility was revealed
with the mean standard error of the temperature increase smaller
than 2%.
As is clearly seen in Fig. 4(a) and (b), the temperature increase
varies exponentially with the time, which is well described with
equation (3). By fitting the experimental data into the exponential
function (3) using the Gnuplot program, the §ðSÞ=sðSÞ and sðSÞ=C
values were estimated. From these i.e. §ðSÞ=C and sðSÞ=C, heat

transfer rates were then calculated and results are shown in Fig. 5.
These heat transfer rates are linearly dependent on the window area,
which agrees well with our model mentioned in equation (1).
Rewriting the definitions: §ðSÞ=C ¼ k2 PS0 =C þ ðk1 À k2 ÞPS0 =CðS=S0 Þ
and sðSÞ=C ¼ s2 SB =C þ ðs1 À s2 ÞS0 =CðS=S0 Þ and applying a linear

fitting, the S-depending values were calculated, from those the
§ðSÞ=C, sðSÞ=Cvalues are considered at the boundary conditions,
namely S ¼ 0 for the case the box being constructed without the glass
window and S ¼ S0 for the case the glass window takes the full size of
one box-side.
Table 1 presents the §ðSÞ=C, sðSÞ=C heat transfer rates calculated with the full-size windows made from CG and TNO on CG,
respectively. At the S ¼ S0 boundary condition, we find,
§ðS ¼ S0 Þ=C ¼ ðPS0 =CÞk1 , which is proportional to only the heat
transfer efficiency of the window material, and equals to
(6.97 ± 0.53) C minÀ1 for CG window and (5.39 ± 0.53) C minÀ1
for the TNO on CG window. With this condition, the sðS ¼ S0 Þ=C ¼
S0 =Cðs1 þ 5s2 Þ rates are (0.22 ± 0.04) minÀ1 and (0.18 ± 0.04)
minÀ1. By replacing these heat transfer rates into equation (3), the
saturation temperature increase inside the box could be estimated
and was found to equal to (33.6 ± 3.1)  C and (27.5 ± 2.6)  C for CG
and TNO on CG window, respectively. In other words, in comparison to the box built with CG-window, the cooling energy required
to match the box internal temperature with the box outside temperature makes a 24% cut of that, if the window is made by TNO
on CG.
The ratio between the heat transfer rates of the two windows e
TNO
on
CG
and
CG
e
was
estimated
to
be
CG Þz77:3%, which is

ð§TNO ðS ¼ S0 Þ=CÞ=§CG ðS ¼ S0 Þ=C ¼ ðkTNO
=k
1
1
very close to the ratio between the mean IR-transmittance of the
TNO
CG
two materials, TIR =TIR ¼ 78:9%. The small difference between
these two values might correspond to the very low heat conduction
of corning glass, the surface temperature of the window being
passively cooled by air ventilator, and the correlation between the
IR-lamp irradiated spectrum and the IR-transmittance spectrum of
the material [24]. Besides, the heat release rate sðS ¼ S0 Þ=C contains two main parts: the heat conduction of the material and the
Boltzmann radiation heat release [28], which is relatively small
compared to the low heat release from the box to the external
environment.
By taking S ¼ 0 as the boundary condition, we have
§ðS ¼ 0Þ=C ¼ ðPS0 =CÞk2 , sðS ¼ 0Þ=C ¼ ð5S0 =CÞs2 . At S ¼ 0, the heat
transfer rates are independent of the window material

Fig. 4. Time dependence of temperature increase inside the box when the window was (a) corning glass (CG) and (b) TNO thin film on corning glass (TNO on CG) with different
areas.

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Fig. 5. Dependence of §ðSÞ=C and sðSÞ=C heat transfer rates on S/S0.

Table 1
§ðSÞ=C, sðSÞ=C heat transfer rates with full size windows (S ¼ S0).
§ðS¼S0 Þ
C

¼ PSC0 k1 ( C minÀ1)

sðS¼S0 Þ
C

¼ SC0 ðs1 þ 5s2 Þ (minÀ1)

CG

TNO on CG

CG

TNO on CG

6.96(6) ± 0.52(6)

5.38(5) ± 0.53(2)

0.21(7) ± 0.03(5)

0.18(1) ± 0.03(5)

corresponding to that no windows are built on the box. In particular, the §ðS ¼ 0Þ=C rate is proportional to the heat transfer efficiency of the box, while the sðS ¼ 0Þ=C rate depends only on the
heat release efficiency of the box. These rates should be the same
corresponding to the case only one box materials being used that is
Styrofoam. The experiment has revealed that §ðS ¼ 0Þ=C equals to
(0.77 ± 0.13)  C minÀ1 for CG window and (0.69 ± 0.12)  C minÀ1 for
TNO on CG window (Table 2). These values are almost identical in
their standard error range, which agrees with our suggested model.
The same result is achieved with the sðS ¼ 0Þ=C rate. Further, the
k1 =k2 z11 denotes that the heat transfer by the box materials is
very small in comparison to the heat transfer by the window
material.
Several models had been earlier introduced to explain the
temperature increase inside a closed box under both solar irradiation [24e26] and artificial IR light sources [27e29]. Few theories
have been applied on transparent roof and/or windows [24,28,29],
among those we have found that the one introduced by Mohelnikova [28,29] is similar to ours. In the Mohelnikova model, the heat
transfer between the internal space of the box and the outside was
complex incorporating the heat conduction/convection of the wall
materials, the StefaneBoltzmann heat emissivity and the irradiative flux from the IR-source. Hence, the heat transfer parameters
could not be easily estimated. In our approach, the heat transfer
efficiency is simplified with only two parameters for each material:
heat transfer efficiency rate and heat release efficiency rate. These
rates included heat conduction of material and the radiation heat
release. The heat conduction is proportional to the temperature
increase inside the box. The radiation heat release could be written
in StefaneBoltzmann equation: q ¼ εsðT 4 À T04 Þ, where ε and s are

the material emissivity and StefaneBoltzmann constant, respectively. As the temperature increase inside the box was small in
comparison with the actual absolute temperature of the box,
qz4εsT03 ðT À T0 Þ. Hence, the heat transfer efficiency rate and heat
release efficiency rate could be considered to be constants. Besides,

by changing the window area and measuring the time-dependence
of the box internal temperature increase, the heat transfer efficiency rates of the window materials and of the box can be determined (or evaluated).
In setting up the experiment, we have minimized the heat
conduction and Boltzmann radiation between the internal space
and external space of the box. As a result, the heat transfer rate
related to the IR-irradiation from the IR-lamp can be considered as
proportional to the mean IR-transmittance of the glass window
material. In further investigations, the heat transfer rates of
different box materials could be estimated via the §ðS ¼ 0Þ=C and
the sðS ¼ 0Þ=C rates, whereas the box-walls with different materials were used. This approach was applied in further investigations
on the “cooling effect” of other transparent materials as well.
4. Conclusion
We have brought out a simplified model to investigate the IRirradiation-generated temperature increase inside a glass window
heat insulated box. The experimental work was carried out on two
materials of glass window: CG and TNO-coated CG. The black foam
inside the box absorbs the IR-rays passing through the glass window, hence causes the temperature inside the box to rise. As it was
shown in the study, the temperature increase depends on the
irradiation period and the heat transfer rate of the glass window. In
summary, the study reveals that the IR-transmittance of the window material is proportional to the heat transfer rate from the IRlamp to the internal space of the box. Besides, CG coated with
TNO thin films are good for use as glass window materials in smart
constructions targeted to “self-cooling” applications. The model
applied so far is suitable only for insulated boxes with active
cooling surfaces.
Acknowledgments

Table 2
§ðSÞ=C, sðSÞ=C heat transfer rates without window at S ¼ 0.
§ðS¼0Þ
C

¼ PSC0 k2 ( C minÀ1)

sðS¼0Þ
C

¼ 5SC0 s2 (minÀ1)

CG

TNO on CG

CG

TNO on CG

0.77(1) ± 0.12(6)

0.68(7) ± 0.12(1)

0.099 ± 0.03(5)

0.08(8) ± 0.03(5)

This study is supported by Vietnam National University, Hanoi
(VNU) under the granted research project number QG.14.23. The
authors thank the Center for Materials Science- Faculty of Physics,
Nano and Energy Center, Hanoi University of Science for providing
us with relevant equipment and research facilities. We are also

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grateful to Dr. Thu Le from the Riken Metamaterials Laboratory for
the IR spectra measurement.
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Please cite this article in press as: L.M. Quynh, et al., Thermal resistant efficiency of Nb-doped TiO2 thin film based glass window, Journal of
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