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

By replacing these heat transfer rates into equation3, the saturation temperature increase inside the box could be estimated and was found to equal to 33.6± 3.1C and 27.5± 2.6C for CG an

Original Article

window

a Faculty of Physics, Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

b Tan Trao University, Tuyen Quang, Viet Nam

c 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

Article history:

Received 22 March 2017

Received in revised form

20 June 2017

Accepted 9 July 2017

Available online xxx

Keywords:

Nb-doped TiO 2

Low IR-transmittance

Glass window

Self-cooling

Transparent conducting thin film

a b s t r a c t

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 thinfilm was mostly transparent in visible region and had low transparency in IR region, which produced the self-cooling 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 (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Increasing world energy consumption causes the rise of

atmo-spheric CO2 level, which is one of the main causes of global

warming Thefield of renewable energy and energy savings is a

challenging subject The transparent conductors (TCs)e based on

both oxides as well as non-oxidese 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

ex-citations 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]

Nb-doped TiO2(TNO) is a newcomer TC[14e17]and TiO2has attributes

that other conventional host materials of TCs do not possess,

namely a high refractive index [18], the large static permittivity

[19], the high chemical stability especially in a reducing atmo-sphere[20], and the photocatalytic ability[21] TNO thinfilms have some other benefits, including its low materials cost, easy fabrica-tion, 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 rela-tionship 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 tech-nique targeting the evaluation of the heat-induction inside a glass-window box[28] It was widely used for different transparent

IR-reflective materials [29e31] However, given the heat conduc-tance and the outside surface temperature profile of the glass window, the model was still complicated Furthermore, the con-crete relationship between the optical property of the material and the heat-induction inside the box had not been discussed yet

* Corresponding author.

E-mail address: luumanhquynh@hus.edu.vn (L.M Quynh).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2017.07.002

2468-2179/© 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 ( http://creativecommons.org/licenses/by/4.0/ ).

carried out under a total pressure of 7.5
103Torr in pure Ar

atmosphere The RF sputtering power applied to the target was

kept constant at 90 W during the process The as-deposited

amorphousfilms were annealed at 350C in vacuum atmosphere

(~1
105Torr) within 30 min.

The thickness of TNOfilms was determined by the cross-section

scanning electron microscope (SEM-NOVA NANOSEM)

measure-ment Light absorption was observed by both the Shimadzu

UV-2450 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 thinfilms was examined using a BRUKER 5005

X-ray 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 inFig 1) The box of cubic structure was covered by

heat-resistant 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

absorption

films were the same as the other TNO thin film products (Fig 2) in our previous report[32] The thickness of thefilm 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 an-gles This shift corresponds to the larger length of the a- and c-axes

of the unit cell, originating from the larger radius of the Nb5þion compared to that of the Ti4þion and resonating with Vegard's law

[14,17] Besides, no Nb2O5 impurity was detected, as we could reveal that the Nb5þions were fully incorporated to the TiO2lattice The transmittance spectra of a CG sample and a TNO thinfilm coated CG (TNO on CG) sample are shown inFig 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 Nb5þions, our TNO thinfilms have shown a high carrier concentration of 8
1021 cm3 as determined by the Hall measurement (data not shown) This gen-erates plasmonic reflectivity in the IR region[14e17] As a result, the transmittance spectrum of our TNO thinfilm 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 thinfilm Moreover, considering earlier works[8,22,33], the same strong interference effect corre-sponding to the high reflective index of TiO2-based thinfilm was detected in the visible region from 400 nm to 1000 nm

Fig 1 Schematic installation of heat transfer measurement.

The mean near-IR transmittance was calculated by the formula

TIR¼P800 nm

2600 nmAbsi
li=Pl2

l1li, where Ti is the transmittance at theli wavelength By means of this formula, the mean near-IR

transmittance of CG and TNO on CG have been found as

TCG

IR ¼ 92:1% and TTNO

IR ¼ 72:7%, respectively If all the IR irradia-tions in this range are completely absorbed by the black foam

inside the box (seeFig 1) and are fully converted to produce heat, the temperature increase inside the box depending on the win-dow 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:

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.

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

C þsðSÞ

C ðT  T0Þ ¼dT

The solution of thefirst-order differential equation is an

expo-nentially time-dependent function, which would be assumed as:

T¼ T1þ T2ekt, where T1and T2are constants Applying this to the

equation(2)above, we arrive at the following equation for T:

TðS; tÞ  T0¼§ðSÞsðSÞ 

§ðSÞ

sðSÞe

s ðSÞt

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 inFig 4(a) and (b), the temperature increase

varies exponentially with the time, which is well described with

equation(3) Byfitting the experimental data into the exponential

function (3) using the Gnuplot program, the§ðSÞ=sðSÞ andsðSÞ=C

values were estimated From these i.e §ðSÞ=C and sðSÞ=C, heat

for the TNO on CG window With this condition, thesðS ¼ S0Þ=C ¼

S0=Cðs1þ 5s2Þ rates are (0.22 ± 0.04) min1 and (0.18 ± 0.04) min1 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 compari-son to the box built with CG-window, the cooling energy required

to match the box internal temperature with the box outside tem-perature 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 windowse

ð§TNOðS ¼ S0Þ=CÞ=§CGðS ¼ S0Þ=C ¼ ðkTNO

1 =kCG

very close to the ratio between the mean IR-transmittance of the two materials, TTNO

IR =TCG

IR 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 ratesðS ¼ S0Þ=C con-tains 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

§ðS ¼ 0Þ=C ¼ ðPS0=CÞk2,sðS ¼ 0Þ=C ¼ ð5S0=CÞs2 At S¼ 0, the heat

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.

corresponding to that no windows are built on the box In

partic-ular, the§ðS ¼ 0Þ=C rate is proportional to the heat transfer

effi-ciency of the box, while thesðS 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 min1for CG window and (0.69± 0.12)C min1for

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 thesðS ¼ 0Þ=C rate Further, the

k1=k2z11 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

irradi-ation[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

Mohelni-kova[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

irradia-tiveflux 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 εsðT4 T4Þ, where ε andsare

the material emissivity and StefaneBoltzmann constant, respec-tively As the temperature increase inside the box was small in comparison with the actual absolute temperature of the box,

qz4εsT3ðT  T0Þ 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 ef fi-ciency rates of the window materials and of the box can be deter-mined (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 thesðS ¼ 0Þ=C rates, whereas the box-walls with different mate-rials 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 IR-irradiation-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 win-dow, 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 win-dow material is proportional to the heat transfer rate from the IR-lamp to the internal space of the box Besides, CG coated with TNO thinfilms 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 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

Fig 5 Dependence of §ðSÞ=C andsðSÞ=C heat transfer rates on S/S 0

Table 1

§ðSÞ=C,sðSÞ=C heat transfer rates with full size windows (S ¼ S 0 ).

§ðS¼S 0 Þ

C ¼ PS 0

C k 1 (C min1) s ðS¼S 0 Þ

C ¼ S 0

C ðs1 þ 5s2 Þ (min 1 )

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)

Table 2

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

§ðS¼0Þ

C ¼ PS 0

C k 2 (  C min1) s ðS¼0Þ

C ¼ 5S 0

Cs2 (min1)

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)

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