Journal of Science: Advanced Materials and Devices xxx (2017) 1e6 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd 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 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 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 (http://creativecommons.org/licenses/by/4.0/) Keywords: Nb-doped TiO2 Low IR-transmittance Glass window Self-cooling Transparent conducting thin film 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 not possess, * Corresponding author E-mail address: luumanhquynh@hus.edu.vn (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 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/) 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 Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.002 L.M Quynh et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e6 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% 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 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 Results and discussion TNO (with Nb at at%) films were sputtered-deposited on nonheating corning glass (CG) substrates The sputtering process was carried out under a total pressure of 7.5  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  10À5 Torr) within 30 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 Nb5ỵ ion compared to that of the Ti4ỵ ion and resonating with Vegard's law [14,17] Besides, no Nb2O5 impurity was detected, as we could reveal that the Nb5ỵ 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 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 Nb5ỵ ions, our TNO thin films have shown a high carrier concentration of  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 Schematic installation of heat transfer measurement 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 Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.002 L.M Quynh et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e6 Fig SEM image (a) and X-ray diffraction pattern (b) of TNO thin film deposited on corning glass Fig 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: 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 Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.002 L.M Quynh et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e6 k1 SP ỵ k2 S0 SịP ỵ T T0 ịẵs1 S ỵ s2 SB Sị ẳ 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 ĐSị=C ẳ k1 SP ỵ k2 S0 SịP=C, sSị=C ẳ s1 S ỵ s2 SB Sị=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 simplied as the following: ĐSị sSị dT ỵ T T0 ị ẳ C C dt (2) The solution of the first-order differential equation is an exponentially time-dependent function, which would be assumed as: T ẳ T1 ỵ T2 eÀkt , where T1 and T2 are constants Applying this to the equation (2) above, we arrive at the following equation for T: TS; tị T0 ẳ ĐSị §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 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 ĐSị=sSị and sSị=C values were estimated From these i.e ĐSị=C and sSị=C, heat transfer rates were then calculated and results are shown in Fig These heat transfer rates are linearly dependent on the window area, which agrees well with our model mentioned in equation (1) Rewriting the denitions: ĐSị=C ẳ k2 PS0 =C ỵ k1 k2 ịPS0 =CS=S0 ị and sSị=C ẳ 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 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 presents the ĐSị=C, sSị=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 nd, ĐS ẳ S0 ị=C ẳ PS0 =Cịk1 , 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 ẳ S0 ị=C ẳ S0 =Cs1 ỵ 5s2 ị 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 ĐTNO S ẳ S0 ị=Cị=ĐCG S ẳ S0 ị=C ¼ ðkTNO =k 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 sS ẳ S0 ị=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 as the boundary condition, we have ĐS ẳ 0ị=C ẳ PS0 =Cịk2 , sS ẳ 0ị=C ẳ 5S0 =Cịs2 At S ¼ 0, the heat transfer rates are independent of the window material Fig 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 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 Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.002 L.M Quynh et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e6 Fig Dependence of ĐSị=C and sSị=C heat transfer rates on S/S0 Table ĐSị=C, sSị=C heat transfer rates with full size windows (S ẳ S0) ĐSẳS0 ị C ¼ PSC0 k1 ( C minÀ1) sðS¼S0 Þ C ¼ SC0 s1 ỵ 5s2 ị (min1) 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 sS ẳ 0ị=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 ĐS ẳ 0ị=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 sS ẳ 0ị=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 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 ĐS ẳ 0ị=C and the sS ẳ 0ị=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 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 ĐSị=C, sSị=C heat transfer rates without window at S ẳ ĐSẳ0ị C ẳ PSC0 k2 ( C min1) sSẳ0ị C ẳ 5SC0 s2 (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) 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 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 Science: Advanced Materials and Devices (2017), http://dx.doi.org/10.1016/j.jsamd.2017.07.002 L.M Quynh et al / Journal of Science: Advanced Materials and Devices xxx (2017) 1e6 grateful to Dr Thu Le from the Riken Metamaterials Laboratory for the IR spectra measurement References [1] C.G Granqvist, Transparent conductors as solar energy materials: a panoramic review, Sol Energy Mater Sol Cells 91 (2007) 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Quynh, et al., Thermal resistant efficiency of Nb- doped TiO2 thin film based glass window, Journal of Science: Advanced Materials and Devices (2017) , http://dx.doi.org/10.1016/j.jsamd .2017. 07.002... Quynh, et al., Thermal resistant efficiency of Nb- doped TiO2 thin film based glass window, Journal of Science: Advanced Materials and Devices (2017) , http://dx.doi.org/10.1016/j.jsamd .2017. 07.002... glass window, Journal of Science: Advanced Materials and Devices (2017) , http://dx.doi.org/10.1016/j.jsamd .2017. 07.002 L.M Quynh et al / Journal of Science: Advanced Materials and Devices xxx (2017)