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Thermal stability and optical properties of low emissivity multilayer coatings for energy-saving applications
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
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Thermal stability and optical properties of low emissivity multilayer coatings for energy-saving applications Du-Cheng Tsaia, Zue-Chin Changb, Bing-Hau Kuoa, Erh-Chiang Chena, Yen-Lin Huangc, Tsung-Ju Hsiehd, Fuh-Sheng Shieua,∗ a
Department of Materials Science and Engineering, National Chung Hsing University, Taichung, 40227, Taiwan, Republic of China Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung, 41170, Taiwan, Republic of China c Metal Industries Research and Development Center, 81160, Kaohsiung, Taiwan, Republic of China d GCL System Integration Technology Company Limited, Suzhou, People's Republic of China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Coating materials Thin films Vapor deposition Optical properties Transmission electron microscopy TEM
In this study, the deposition and characteristics of Si- and Ti-series multilayer coatings with respective glass/ Si3N4/NiCr/Ag/NiCr/Si3N4 and glass/TiO2/ZnSnO3/ZnO/Ag/NiCrOx/ZnSnO3/Si3N4 structures were investigated. Experimental analyses were performed, and the optical performance and thermal stability of the coatings were assessed. The as-deposited Si- and the Ti-series samples exhibited optimal shading coefficient of 0.49 and 0.57, respectively. Thermal annealing in air was performed to analyze the thermal stability and oxidation resistance of the samples. The Si- and Ti-series samples prevented the failure of low-emissivity properties at 700 and 600 °C, respectively. In addition to high transparency, the Si-series samples exhibited improved lowemissivity properties, reduced light pollution, and superior thermal stability compared to the Ti-series samples. The use of Si3N4 and NiCr layers effectively prevented the oxidation and cracking of the Ag layer upon heat treatment. This study presents the characteristics of Si- and Ti-series samples for energy-saving building applications.
1. Introduction Low-emissivity coatings have high reflectance at the near infrared (IR) spectrum and high transmittance at the visible region. They are widely used as windows in air conditioning in transportation and buildings. Therefore, they are important from ecological and sustainability viewpoints [1,2]. Metal-oxide/metal/metal-oxide multilayer coatings are simple designs that present good low-emissivity properties. In these structures, Ag is the best choice for metal layers owing to its high IR reflectance without significant absorption of visible light [3]. Typical TiO2, ZnO, MoO3, SnO2, and Nb2O5 have gained considerable attention as metal oxide layers because they suppress the reflection from the metal layers and are selectively transparent [4–8]. However, a simple sandwiched design cannot simultaneously achieve high performance and environmental stability. Si3N4 is an extremely transparent, hard, durable, and corrosion-resistant material [9]. A Si3N4 layer can prevent the oxidation of Ag and the scratching of its surface, without the need for an additional protective barrier layer. However, its high degree of stress tends to reduce adhesion. Therefore, deposition of a thin NiCr barrier is necessary to
∗
increase the adhesion between the Ag and Si3N4 layers [10]. Multilayer coatings with glass/Si3N4/NiCr/Ag/NiCr/Si3N4 structures, named Siseries samples, are expected to have good low-emissivity properties and thermal stability. TiO2 is known for its extremely high index of refraction, hence it has been alternately laminated with various materials of low refractive index to lower visible light reflectance [11]. ZnO layers can increase grain size and subsequently enhance the resistivity and absorption of Ag layers [12]. Additionally, ZnSnO3 layers retard the interdiffusion of different layers during deposition and under hightemperature treatments [13]. The functionality of NiCrOx is the same as that of the NiCr layer. A drawback of using a NiCr layer is its high visible light absorption. The displacement of NiCr by NiCrOx is of significant benefit to transparency [14]. Multilayer coatings with glass/ TiO2/ZnSnO3/ZnO/Ag/NiCrOx/ZnSnO3/Si3N4 structures, named Tiseries samples, exhibit superior visible light transmittance without considerably sacrificing other properties. In this study, we investigated sputter-deposited Si- and Ti-series samples and evaluated their thermal stability and optical properties. Ag-based low-emissivity multilayer coatings were developed by tuning the thickness and structures of multilayer composites through thermal treatment.
Corresponding author. E-mail address: [email protected] (F.-S. Shieu).
https://doi.org/10.1016/j.ceramint.2019.12.021 Received 16 September 2019; Received in revised form 29 November 2019; Accepted 2 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Du-Cheng Tsai, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.021
Ceramics International xxx (xxxx) xxx–xxx
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Fig. 1. Schematic diagram of the multilayer structure of the Si50, Si60, Ti70, and Ti80 samples.
2. Experimental Four sets of low-emissivity multilayer coatings (Fig. 1) were deposited through a vacuum magnetron sputtering technique. As substrates, clear float glass (CLR; Taiwan Glass Industry Corporation, Taiwan) and tinted float glass (FGR; Taiwan Glass Industry Corporation, Taiwan) were used. They were thoroughly cleaned with cerium oxide and distilled water with an ultrasonic cleaner and subsequently dried under dry nitrogen gas. Si3N4 films from Si cathodes were deposited in an Ar and N2 atmosphere. In contrast, TiO2, ZnO, ZnSnO3, and NiCrOx films from Ti, Zn, ZnSn, and NiCr cathodes, respectively, were deposited in an Ar and O2 atmosphere. Reactive sputtering was utilized in both cases. NiCr and Ag films were formed in a pure Ar atmosphere. The targets sizes were 3.0 m of length and 1.2 m of width. The deposition procedure was performed in the CLR at a working pressure of 3 mTorr and substrate temperature of 80 °C. Details of the process parameters of different materials are listed in Table 1. The thickness of each layer was measured using an ellipsometer. The asdeposited low-emissivity multilayer coatings were annealed at different temperatures (200–800 °C) for 2 h in air, and a furnace was used to examine their thermal stability. The crystal structures were analyzed using a glancing-incidence (1°) X-ray diffractometer (GIXRD, Bruker D8 Discover) with Cu Kα radiation at a scanning speed of 1°/min. The scanning step was 0.02°, and the scanning range was 20–80°. Microstructural examinations were conducted using an analytical transmission electron microscope (TEM, JEOL JEM-2100F). Optical transmittance and reflectance properties were examined using a UV–vis–NIR spectrophotometer (Shimadzu UV3600).
Fig. 2. (a) Transmittance and (b) reflectance spectra of the as-deposited CLR, FGR, Si50, Si60, Ti70, and Ti80 samples.
was irradiated onto the surface of the Ti-series samples, high visible light reflection caused light pollution in the form of reflected glare [15]. The simultaneous increase of visible light transmittance and heat insulation is challenging. Nevertheless, the Ti70 sample presented both features. However, despite the good low-emissivity performance of this sample, its high visible light reflectance caused a mirror-effect problem [16]. The surface of the brighter side acts as a mirror because the amount of light passing through the window from the darker side is lower than the amount being reflected by the lighter side. This effect can be observed from the outside during the day and from the inside during the night. In other words, it might be difficult to see the outside due to the mirror effect, while the inside can be easily seen on a cloudy or rainy day, or at night. That is, there is a low possibility to enjoy the view during the day and little privacy at night. Therefore, surface reflection at the visible region is usually undesirable. In this context, the Ti80 sample improved the view during the day and privacy at night compared to the Ti70 sample. Thermal annealing is an efficient method to verify the stability of films. Therefore, we investigated the effects of thermal annealing in air on the Ag-based multilayer films that were sputter-deposited on glass. Fig. 3 shows the measured transmittance and reflectance spectra of Si50, Si60, Ti70, and Ti80 samples annealed at various temperatures. Their optical properties are summarized in Table 2. When Si50 and Si60 samples were annealed at 600 °C and Ti60 and Ti70 samples were annealed at 500 °C, the visible light transmittance was improved, but IR reflectance decreased. The increased visible light transmittance was caused by the improvement in the crystalline quality of the deposited layers [17]. The IR decrease indicates that a slight chemical reaction occurred in the Ag layer, possibly due to an interdiffusion of the Ag layer and the neighbor layer, or due to oxidation of the Ag layer. When
3. Results Fig. 2 shows the transmittance and reflectance spectra of the asdeposited CLR, FGR, Si50, Si60, Ti70, and Ti80 samples. The spectra indicate that high visible light transmittance and IR reflectance can be attained in the Ag-based films. Thus, the samples fulfill the requirements for a transparent heat reflector. The optical properties of the samples are summarized in Table 2. The Ti-series samples revealed a higher and wider visible-light transmittance band and higher IR reflectance compared to the Si-series samples. However, the shading coefficient increased with visible light transmittance, which can result in a poor energy-saving effect of heat insulation. When direct sunlight Table 1 Deposition conditions of the low emissivity multilayer coatings. Process parameter Film Target Discharge current (A) Ar flow (SCCM) O2 flow (SCCM) N2 flow (SCCM)
Values Si3N4 Si 110 180 – 50
TiO2 Ti 110 205 25 –
ZnO Zn 110 205 25 –
ZnSnO3 ZnSn 110 205 25 –
NiCrOx NiCr 60 205 25 –
NiCr NiCr 60 230 – –
Ag Ag 60 230 – –
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Table 2 Average visible light transmittance, reflectance, and shading coefficient of the CLR, FGR, Si50, Si60, Ti70, and Ti80 samples as a function of annealing temperature. Sample
Annealing temperature (°C)
Visible region (380 nm–780 nm)
IR region (780 nm–2500 nm)
Transmittance (%)
Reflectance (%)
Transmittance (%)
Reflectance (%)
Shading coefficient
CLR
As-deposited
88.13
8.52
83.07
7.42
0.92
FGR
As-deposited
69.32
6.90
46.47
5.22
0.82
Si50
As-deposited 200 300 400 500 600 700 800
48.50 48.50 48.61 48.51 49.36 48.52 0.56 24.09
6.82 6.89 6.77 6.44 5.73 6.22 1.10 2.35
17.84 17.84 18.27 18.31 17.80 15.82 0.25 37.60
56.64 57.19 57.63 57.58 53.11 52.70 11.05 2.15
0.49 0.49 0.49 0.49 0.49 0.49 0.75 0.75
Si60
As-deposited 200 300 400 500 600 700 800
58.17 58.02 58.84 59.02 60.37 59.95 1.19 26.07
5.24 5.06 5.22 4.97 5.61 5.46 1.29 1.71
21.99 22.00 22.20 22.27 21.05 20.37 0.67 38.74
54.98 54.43 55.39 55.58 53.84 52.18 13.16 2.33
0.56 0.56 0.56 0.56 0.56 0.56 0.78 0.78
Ti70
As-deposited 200 300 400 500 600 700 800
65.81 65.21 68.03 68.61 68.84 51.37 15.46 14.19
15.85 16.34 16.78 15.20 17.80 4.77 1.91 3.24
18.84 17.55 19.48 17.65 17.66 79.95 31.38 24.96
73.79 75.13 73.46 69.97 69.37 4.46 1.98 2.83
0.57 0.57 0.57 0.57 0.57 0.80 0.81 0.81
Ti80
As-deposited 200 300 400 500 600 700 800
74.89 75.06 76.40 78.58 78.86 59.56 31.96 23.50
8.95 8.96 9.14 7.83 10.71 10.54 2.16 3.05
25.31 24.82 25.18 24.75 24.52 82.60 62.13 52.52
66.38 67.31 67.66 64.81 66.64 9.07 2.84 2.89
0.66 0.66 0.66 0.66 0.66 0.80 0.81 0.81
intensities increased due to the enhanced crystallinity and grain size of the Ag layer. However, when the annealing temperature increased to 700 °C, the Ag peaks decreased. After 800 °C, the crystalline SiO2 phase emerged, and the Ag phase nearly disappeared. In the Ti70 and Ti80 samples (Fig. 5c and 5d), the as-deposited film revealed an Ag phase accompanying an SnO2 minor phase. In this study, ZnSnO3 or other ZnO–SnO phases did not occur, which is consistent with a previous report by Mihaiu et al. [19], who confirmed that the crystal ZnO–SnO phases did not occur until 900 °C. As the annealing temperature increased to 500 °C, an obvious improvement in the crystallinity of Ag was presented. Nevertheless, a further increase of the annealing temperature led to a decrease or even disappearance of the Ag phase. Therefore, Si50 and Si60 samples present superior thermal stability than Ti70 and Ti80 samples. TEM was used to perform a detailed evaluation of microstructures and further verify the failure behavior of the Ag-based films. Fig. 6 shows the high resolution TEM images of the as-deposited Si60 and Ti70 samples. The energy-dispersive spectroscopy line scan revealed their chemical composition distribution (not shown). The Si60 and Ti70 samples showed clear 10- and 16-nm continuous Ag layers, respectively. The stack structure of the Si60 sample was maintained even after annealing at 700 °C (Fig. 7a). A 10 nm-thick Si oxide layer was formed on the film surface, indicating the high stability of the Si3N4 layer oxidation barrier. However, the high temperature promoted the aggregation of Ag, followed by the appearance of voids. Moreover, interdiffusion occurred in the Ag and NiCr layers, thus forming a mixed
the annealing temperatures of Si50 and Si60, and Ti60 and Ti70 samples further increased to 700 and 600 °C, respectively, the visible light transmittance and IR reflectance declined rapidly. Moreover, the shading coefficient increased considerably, which implied a severe structural change. The resulting samples are shown in Fig. 4. When the annealing temperature was up to 700 °C, the glass surface began to soften and acted as a stiff liquid; thus, visible surface cavities were formed. A further increase to 800 °C caused nucleation and growth of crystals. Consequently, the devitrification effect accompanying rounded edges of glass occurred [18]. These phenomena deformed the film structure and deteriorated the light transmittance of samples. The topography and color of Si50 and Si60 samples annealed at 800 °C and of Ti70 and Ti80 samples annealed at 700 °C were completely different from those of the samples annealed at lower temperatures. The Ti70 and Ti80 samples exhibited poorer thermal stability compared to the Si50 and Si60 samples. The effects of layer structure design and annealing temperature on the phase evolution of Ag-based films were evaluated. The XRD patterns of the films with various layer structure designs post-annealed at 200–800 °C were obtained to evaluate their phase structures, as shown in Fig. 5. In the Si50 and Si60 samples (Fig. 5a and 5b), four diffraction peaks corresponding to the (111), (200), (220), and (311) lattice planes of a face-centered cubic Ag crystal structure were observed. This finding indicates that the as-deposited NiCr and Si3N4 layers presented an amorphous structure. After annealing at 600 °C, the patterns remained unchanged. Therefore, the Ag peaks became sharper, and their 3
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Fig. 3. Transmittance and reflectance spectra of the (a) Si50, (b) Si60, (c) Ti70, and (d) Ti80 samples after annealing in air at different temperatures.
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Fig. 4. Photographs of the Si50, Si60, Ti70, and Ti80 samples after annealing in air at different temperatures.
Fig. 5. XRD patterns of the (a) Si50, (b) Si60, (c) Ti70, and (d) Ti80 samples after annealing in air at different temperatures. 5
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Fig. 6. Cross-sectional TEM micrographs of the as-deposited (a) Si60 and (b) Ti70 samples.
Fig. 7. Cross-sectional TEM micrographs of (a) 700 °C-annealed Si60 and (b) 600-annealed Ti70 samples.
Ag–NiCr layer. These phenomena, along with surface cavities caused by glass softening, resulted in the failure of the low-emissivity property of the Si50 and Si60 samples. After the Ti70 sample was annealed at 600 °C (Fig. 7b), oxidation occurred on the surface of the Si3N4 layer. The Si3N4 bulk layer was partially oxidized and became fragmental, indicating the failure of the Si3N4 barrier layer during annealing in air. The interface between oxide layers became obscured possibly due to interdiffusion at high temperatures. The original Ag layer nearly disappeared while Ag oxides and a thick crack layer were formed. The underlying oxide layer in the Ti-series samples generated cracks during the thermal treatment. These cracks resulted in a barrier failure of the upper Si3N4 layer. The NiCr layer in the Si-series samples provided good adhesion and thus prevented severe aggregation growth or crack formation of Ag during the thermal treatment. In contrast, the ZnO and NiCrO layers in the Ti-series samples provided insufficient adhesion for the Ag layer, which facilitated the formation of a thick crack layer under high temperatures. In conclusion, the Si3N4 and NiCr layers increased the thermal stability of the low-emissivity coatings.
respectively. The Ti-series samples simultaneously exhibited high visible light transmittance and high IR reflectance. However, their poorer low emissivity property compared to that of the Si-series was caused by the increase in visible light transmittance and heavier light pollution due to increased visible light reflectance. Therefore, the application of the Ti-series is limited. In contrast, the Si-series samples have better low-emissivity properties and reduced light pollution but lower visible light transmittance. In the Ti-series samples that were annealed at elevated temperatures (~600 °C), oxygen could out-diffuse into the metal layer and oxidize the thin Ag layer; and a thick crack layer was formed. Visible transmittance and IR reflectance degraded owing to the above effects. In contrast, at higher temperatures up to 700 °C, the oxidation barrier of the Si-series samples remained thermally stable. Nevertheless, Ag aggregation and glass softening caused a failure of the low-emissivity property. This study demonstrates the promising characteristics of Si- and Ti-series multilayer coatings. Moreover, the effects of the thermal treatment were used to evaluate their applications under various environments for the energy-saving building industry.
4. Conclusion Declaration of competing interest This study reports the preparation and investigation of Si-series and Ti-series multilayer coatings with glass/Si3N4/NiCr/Ag/NiCr/Si3N4 and glass/TiO2/ZnSnO3/ZnO/Ag/NiCrOx/ZnSnO3/Si3N4/structures,
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence 6
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our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript.
[7] L. Liu, S. Ma, H. Wu, B. Zhu, H. Yang, J. Tang, X. Zhao, Effect of the annealing process on electrical and optical properties of SnO2/Ag/SnO2 nanometric multilayer film, Mater. Lett. 149 (2015) 43–46. [8] A. Dhar, T.L. Alford, Optimization of Nb2O5/Ag/Nb2O5 multilayers as transparent composite electrode on flexible substrate with high figure of merit, J. Appl. Phys. 112 (2012) 103113. [9] L. Weng, X. Zhang, J. Han, W. Han, The effect of Si3N4 on microstructure, mechanical properties and oxidation resistance of HfB2-based composite, J. Compos. Mater. 43 (2009) 113–123. [10] R.J. Martín-Palma, J.M. Martínez-Duart, Ni–Cr passivation of very thin Ag films for low-emissivity multilayer coatings, J. Vac. Sci. Technol. A 17 (1999) 3449–3451. [11] S.B. Khan, S. Irfan, Z. Zhuanghao, S.L. Lee, Influence of refractive index on antireflectance efficiency of thin films, Materials 12 (2019) 1483. [12] G. Ding, C. Clavero, D. Schweigert, M. Le, Thickness and microstructure effects in the optical and electrical properties of silver thin films, AIP Adv. 5 (2015) 117234. [13] F. Landucci, Q. Jeangros, E. Rucavado, C. Spori, M. Morales‐Masis, C. Ballif, C. Hébert, A. Hessler‐Wyser, The microstructure of ZnSnO and its correlation to electrical and optical properties, European Microscopy Congress 2016: Proceedings, American Cancer Society, 2016, pp. 368–369. [14] C. Schaefer, G. Bräuer, J. Szczyrbowski, Low emissivity coatings on architectural glass, Surf. Coat. Technol. 93 (1997) 37–45. [15] S. Fu, Z. Lai, W. Zhong, X. Zhu, Preparation of TiN solar spectrally selective absorbing thin film by DC magnatron sputtering, 4th International Symposium on Advanced Optical Manufacturing and Testing Technologies: Advanced Optical Manufacturing Technologies, International Society for Optics and Photonics, 2009, p. 72821P. [16] W. Hoffman, High Shading Performance Coatings, (2004) US20040016202A1. [17] G.K. Dalapati, S. Masudy-Panah, S.T. Chua, M. Sharma, T.I. Wong, H.R. Tan, D. Chi, Color tunable low cost transparent heat reflector using copper and titanium oxide for energy saving application, Sci. Rep. 6 (2016) 20182. [18] N.L. Bowen, Devitrification of glass, J. Am. Ceram. Soc. 2 (1919) 261–281. [19] S. Mihaiu, A. Toader, I. Atkinson, O.C. Mocioiu, C. Hornoiu, V.S. Teodorescu, M. Zaharescu, Advanced ceramics in the SnO2–ZnO binary system, Ceram. Int. 41 (2015) 4936–4945.
Acknowledgments The authors gratefully acknowledge the financial support for this research by the Ministry of Science and Technology of Taiwan under Grant No. NSC 106-2221-E-005 -026 -MY3. The present work was also supported in part by the Center for Micro/Nano Science and Technology of the National Cheng Kung University. References [1] J. Zhou, Y. Gao, Z. Zhang, H. Luo, C. Cao, Z. Chen, L. Dai, X. Liu, VO2 thermochromic smart window for energy savings and generation, Sci. Rep. 3 (2013) 3029. [2] G. Leftheriotis, P. Yianoulis, Characterisation and stability of low-emittance multiple coatings for glazing applications, Sol. Energy Mater. Sol. Cells 58 (1999) 185–197. [3] M. Miyazaki, E. Ando, Durability improvement of Ag-based low-emissivity coatings, J. Non-Cryst. Solids 178 (1994) 245–249. [4] Z. Zhao, T.L. Alford, The optimal TiO2/Ag/TiO2 electrode for organic solar cell application with high device-specific Haacke figure of merit, Sol. Energy Mater. Sol. Cells 157 (2016) 599–603. [5] E.H. Nezhad, H. Haratizadeh, B.M. Kari, Influence of Ag mid-layer in the optical and thermal properties of ZnO/Ag/ZnO thin films on the glass used in Buildings as insulating glass unit (IGU), Ceram. Int. 45 (2019) 9950–9954. [6] J.-Y. Park, H.-K. Kim, Flexible metal/oxide/metal transparent electrodes made of thermally evaporated MoO3/Ag/MoO3 for thin film heaters, Phys. Status Solidi AAppl. Mat. 215 (2018) 1800674.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Thermal stability and optical properties of low emissivity multilayer coatings for energy-saving applications Du-Cheng Tsaia, Zue-Chin Changb, Bing-Hau Kuoa, Erh-Chiang Chena, Yen-Lin Huangc, Tsung-Ju Hsiehd, Fuh-Sheng Shieua,∗ a
Department of Materials Science and Engineering, National Chung Hsing University, Taichung, 40227, Taiwan, Republic of China Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung, 41170, Taiwan, Republic of China c Metal Industries Research and Development Center, 81160, Kaohsiung, Taiwan, Republic of China d GCL System Integration Technology Company Limited, Suzhou, People's Republic of China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Coating materials Thin films Vapor deposition Optical properties Transmission electron microscopy TEM
In this study, the deposition and characteristics of Si- and Ti-series multilayer coatings with respective glass/ Si3N4/NiCr/Ag/NiCr/Si3N4 and glass/TiO2/ZnSnO3/ZnO/Ag/NiCrOx/ZnSnO3/Si3N4 structures were investigated. Experimental analyses were performed, and the optical performance and thermal stability of the coatings were assessed. The as-deposited Si- and the Ti-series samples exhibited optimal shading coefficient of 0.49 and 0.57, respectively. Thermal annealing in air was performed to analyze the thermal stability and oxidation resistance of the samples. The Si- and Ti-series samples prevented the failure of low-emissivity properties at 700 and 600 °C, respectively. In addition to high transparency, the Si-series samples exhibited improved lowemissivity properties, reduced light pollution, and superior thermal stability compared to the Ti-series samples. The use of Si3N4 and NiCr layers effectively prevented the oxidation and cracking of the Ag layer upon heat treatment. This study presents the characteristics of Si- and Ti-series samples for energy-saving building applications.
1. Introduction Low-emissivity coatings have high reflectance at the near infrared (IR) spectrum and high transmittance at the visible region. They are widely used as windows in air conditioning in transportation and buildings. Therefore, they are important from ecological and sustainability viewpoints [1,2]. Metal-oxide/metal/metal-oxide multilayer coatings are simple designs that present good low-emissivity properties. In these structures, Ag is the best choice for metal layers owing to its high IR reflectance without significant absorption of visible light [3]. Typical TiO2, ZnO, MoO3, SnO2, and Nb2O5 have gained considerable attention as metal oxide layers because they suppress the reflection from the metal layers and are selectively transparent [4–8]. However, a simple sandwiched design cannot simultaneously achieve high performance and environmental stability. Si3N4 is an extremely transparent, hard, durable, and corrosion-resistant material [9]. A Si3N4 layer can prevent the oxidation of Ag and the scratching of its surface, without the need for an additional protective barrier layer. However, its high degree of stress tends to reduce adhesion. Therefore, deposition of a thin NiCr barrier is necessary to
∗
increase the adhesion between the Ag and Si3N4 layers [10]. Multilayer coatings with glass/Si3N4/NiCr/Ag/NiCr/Si3N4 structures, named Siseries samples, are expected to have good low-emissivity properties and thermal stability. TiO2 is known for its extremely high index of refraction, hence it has been alternately laminated with various materials of low refractive index to lower visible light reflectance [11]. ZnO layers can increase grain size and subsequently enhance the resistivity and absorption of Ag layers [12]. Additionally, ZnSnO3 layers retard the interdiffusion of different layers during deposition and under hightemperature treatments [13]. The functionality of NiCrOx is the same as that of the NiCr layer. A drawback of using a NiCr layer is its high visible light absorption. The displacement of NiCr by NiCrOx is of significant benefit to transparency [14]. Multilayer coatings with glass/ TiO2/ZnSnO3/ZnO/Ag/NiCrOx/ZnSnO3/Si3N4 structures, named Tiseries samples, exhibit superior visible light transmittance without considerably sacrificing other properties. In this study, we investigated sputter-deposited Si- and Ti-series samples and evaluated their thermal stability and optical properties. Ag-based low-emissivity multilayer coatings were developed by tuning the thickness and structures of multilayer composites through thermal treatment.
Corresponding author. E-mail address: [email protected] (F.-S. Shieu).
https://doi.org/10.1016/j.ceramint.2019.12.021 Received 16 September 2019; Received in revised form 29 November 2019; Accepted 2 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Du-Cheng Tsai, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.021
Ceramics International xxx (xxxx) xxx–xxx
D.-C. Tsai, et al.
Fig. 1. Schematic diagram of the multilayer structure of the Si50, Si60, Ti70, and Ti80 samples.
2. Experimental Four sets of low-emissivity multilayer coatings (Fig. 1) were deposited through a vacuum magnetron sputtering technique. As substrates, clear float glass (CLR; Taiwan Glass Industry Corporation, Taiwan) and tinted float glass (FGR; Taiwan Glass Industry Corporation, Taiwan) were used. They were thoroughly cleaned with cerium oxide and distilled water with an ultrasonic cleaner and subsequently dried under dry nitrogen gas. Si3N4 films from Si cathodes were deposited in an Ar and N2 atmosphere. In contrast, TiO2, ZnO, ZnSnO3, and NiCrOx films from Ti, Zn, ZnSn, and NiCr cathodes, respectively, were deposited in an Ar and O2 atmosphere. Reactive sputtering was utilized in both cases. NiCr and Ag films were formed in a pure Ar atmosphere. The targets sizes were 3.0 m of length and 1.2 m of width. The deposition procedure was performed in the CLR at a working pressure of 3 mTorr and substrate temperature of 80 °C. Details of the process parameters of different materials are listed in Table 1. The thickness of each layer was measured using an ellipsometer. The asdeposited low-emissivity multilayer coatings were annealed at different temperatures (200–800 °C) for 2 h in air, and a furnace was used to examine their thermal stability. The crystal structures were analyzed using a glancing-incidence (1°) X-ray diffractometer (GIXRD, Bruker D8 Discover) with Cu Kα radiation at a scanning speed of 1°/min. The scanning step was 0.02°, and the scanning range was 20–80°. Microstructural examinations were conducted using an analytical transmission electron microscope (TEM, JEOL JEM-2100F). Optical transmittance and reflectance properties were examined using a UV–vis–NIR spectrophotometer (Shimadzu UV3600).
Fig. 2. (a) Transmittance and (b) reflectance spectra of the as-deposited CLR, FGR, Si50, Si60, Ti70, and Ti80 samples.
was irradiated onto the surface of the Ti-series samples, high visible light reflection caused light pollution in the form of reflected glare [15]. The simultaneous increase of visible light transmittance and heat insulation is challenging. Nevertheless, the Ti70 sample presented both features. However, despite the good low-emissivity performance of this sample, its high visible light reflectance caused a mirror-effect problem [16]. The surface of the brighter side acts as a mirror because the amount of light passing through the window from the darker side is lower than the amount being reflected by the lighter side. This effect can be observed from the outside during the day and from the inside during the night. In other words, it might be difficult to see the outside due to the mirror effect, while the inside can be easily seen on a cloudy or rainy day, or at night. That is, there is a low possibility to enjoy the view during the day and little privacy at night. Therefore, surface reflection at the visible region is usually undesirable. In this context, the Ti80 sample improved the view during the day and privacy at night compared to the Ti70 sample. Thermal annealing is an efficient method to verify the stability of films. Therefore, we investigated the effects of thermal annealing in air on the Ag-based multilayer films that were sputter-deposited on glass. Fig. 3 shows the measured transmittance and reflectance spectra of Si50, Si60, Ti70, and Ti80 samples annealed at various temperatures. Their optical properties are summarized in Table 2. When Si50 and Si60 samples were annealed at 600 °C and Ti60 and Ti70 samples were annealed at 500 °C, the visible light transmittance was improved, but IR reflectance decreased. The increased visible light transmittance was caused by the improvement in the crystalline quality of the deposited layers [17]. The IR decrease indicates that a slight chemical reaction occurred in the Ag layer, possibly due to an interdiffusion of the Ag layer and the neighbor layer, or due to oxidation of the Ag layer. When
3. Results Fig. 2 shows the transmittance and reflectance spectra of the asdeposited CLR, FGR, Si50, Si60, Ti70, and Ti80 samples. The spectra indicate that high visible light transmittance and IR reflectance can be attained in the Ag-based films. Thus, the samples fulfill the requirements for a transparent heat reflector. The optical properties of the samples are summarized in Table 2. The Ti-series samples revealed a higher and wider visible-light transmittance band and higher IR reflectance compared to the Si-series samples. However, the shading coefficient increased with visible light transmittance, which can result in a poor energy-saving effect of heat insulation. When direct sunlight Table 1 Deposition conditions of the low emissivity multilayer coatings. Process parameter Film Target Discharge current (A) Ar flow (SCCM) O2 flow (SCCM) N2 flow (SCCM)
Values Si3N4 Si 110 180 – 50
TiO2 Ti 110 205 25 –
ZnO Zn 110 205 25 –
ZnSnO3 ZnSn 110 205 25 –
NiCrOx NiCr 60 205 25 –
NiCr NiCr 60 230 – –
Ag Ag 60 230 – –
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Table 2 Average visible light transmittance, reflectance, and shading coefficient of the CLR, FGR, Si50, Si60, Ti70, and Ti80 samples as a function of annealing temperature. Sample
Annealing temperature (°C)
Visible region (380 nm–780 nm)
IR region (780 nm–2500 nm)
Transmittance (%)
Reflectance (%)
Transmittance (%)
Reflectance (%)
Shading coefficient
CLR
As-deposited
88.13
8.52
83.07
7.42
0.92
FGR
As-deposited
69.32
6.90
46.47
5.22
0.82
Si50
As-deposited 200 300 400 500 600 700 800
48.50 48.50 48.61 48.51 49.36 48.52 0.56 24.09
6.82 6.89 6.77 6.44 5.73 6.22 1.10 2.35
17.84 17.84 18.27 18.31 17.80 15.82 0.25 37.60
56.64 57.19 57.63 57.58 53.11 52.70 11.05 2.15
0.49 0.49 0.49 0.49 0.49 0.49 0.75 0.75
Si60
As-deposited 200 300 400 500 600 700 800
58.17 58.02 58.84 59.02 60.37 59.95 1.19 26.07
5.24 5.06 5.22 4.97 5.61 5.46 1.29 1.71
21.99 22.00 22.20 22.27 21.05 20.37 0.67 38.74
54.98 54.43 55.39 55.58 53.84 52.18 13.16 2.33
0.56 0.56 0.56 0.56 0.56 0.56 0.78 0.78
Ti70
As-deposited 200 300 400 500 600 700 800
65.81 65.21 68.03 68.61 68.84 51.37 15.46 14.19
15.85 16.34 16.78 15.20 17.80 4.77 1.91 3.24
18.84 17.55 19.48 17.65 17.66 79.95 31.38 24.96
73.79 75.13 73.46 69.97 69.37 4.46 1.98 2.83
0.57 0.57 0.57 0.57 0.57 0.80 0.81 0.81
Ti80
As-deposited 200 300 400 500 600 700 800
74.89 75.06 76.40 78.58 78.86 59.56 31.96 23.50
8.95 8.96 9.14 7.83 10.71 10.54 2.16 3.05
25.31 24.82 25.18 24.75 24.52 82.60 62.13 52.52
66.38 67.31 67.66 64.81 66.64 9.07 2.84 2.89
0.66 0.66 0.66 0.66 0.66 0.80 0.81 0.81
intensities increased due to the enhanced crystallinity and grain size of the Ag layer. However, when the annealing temperature increased to 700 °C, the Ag peaks decreased. After 800 °C, the crystalline SiO2 phase emerged, and the Ag phase nearly disappeared. In the Ti70 and Ti80 samples (Fig. 5c and 5d), the as-deposited film revealed an Ag phase accompanying an SnO2 minor phase. In this study, ZnSnO3 or other ZnO–SnO phases did not occur, which is consistent with a previous report by Mihaiu et al. [19], who confirmed that the crystal ZnO–SnO phases did not occur until 900 °C. As the annealing temperature increased to 500 °C, an obvious improvement in the crystallinity of Ag was presented. Nevertheless, a further increase of the annealing temperature led to a decrease or even disappearance of the Ag phase. Therefore, Si50 and Si60 samples present superior thermal stability than Ti70 and Ti80 samples. TEM was used to perform a detailed evaluation of microstructures and further verify the failure behavior of the Ag-based films. Fig. 6 shows the high resolution TEM images of the as-deposited Si60 and Ti70 samples. The energy-dispersive spectroscopy line scan revealed their chemical composition distribution (not shown). The Si60 and Ti70 samples showed clear 10- and 16-nm continuous Ag layers, respectively. The stack structure of the Si60 sample was maintained even after annealing at 700 °C (Fig. 7a). A 10 nm-thick Si oxide layer was formed on the film surface, indicating the high stability of the Si3N4 layer oxidation barrier. However, the high temperature promoted the aggregation of Ag, followed by the appearance of voids. Moreover, interdiffusion occurred in the Ag and NiCr layers, thus forming a mixed
the annealing temperatures of Si50 and Si60, and Ti60 and Ti70 samples further increased to 700 and 600 °C, respectively, the visible light transmittance and IR reflectance declined rapidly. Moreover, the shading coefficient increased considerably, which implied a severe structural change. The resulting samples are shown in Fig. 4. When the annealing temperature was up to 700 °C, the glass surface began to soften and acted as a stiff liquid; thus, visible surface cavities were formed. A further increase to 800 °C caused nucleation and growth of crystals. Consequently, the devitrification effect accompanying rounded edges of glass occurred [18]. These phenomena deformed the film structure and deteriorated the light transmittance of samples. The topography and color of Si50 and Si60 samples annealed at 800 °C and of Ti70 and Ti80 samples annealed at 700 °C were completely different from those of the samples annealed at lower temperatures. The Ti70 and Ti80 samples exhibited poorer thermal stability compared to the Si50 and Si60 samples. The effects of layer structure design and annealing temperature on the phase evolution of Ag-based films were evaluated. The XRD patterns of the films with various layer structure designs post-annealed at 200–800 °C were obtained to evaluate their phase structures, as shown in Fig. 5. In the Si50 and Si60 samples (Fig. 5a and 5b), four diffraction peaks corresponding to the (111), (200), (220), and (311) lattice planes of a face-centered cubic Ag crystal structure were observed. This finding indicates that the as-deposited NiCr and Si3N4 layers presented an amorphous structure. After annealing at 600 °C, the patterns remained unchanged. Therefore, the Ag peaks became sharper, and their 3
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Fig. 3. Transmittance and reflectance spectra of the (a) Si50, (b) Si60, (c) Ti70, and (d) Ti80 samples after annealing in air at different temperatures.
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Fig. 4. Photographs of the Si50, Si60, Ti70, and Ti80 samples after annealing in air at different temperatures.
Fig. 5. XRD patterns of the (a) Si50, (b) Si60, (c) Ti70, and (d) Ti80 samples after annealing in air at different temperatures. 5
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Fig. 6. Cross-sectional TEM micrographs of the as-deposited (a) Si60 and (b) Ti70 samples.
Fig. 7. Cross-sectional TEM micrographs of (a) 700 °C-annealed Si60 and (b) 600-annealed Ti70 samples.
Ag–NiCr layer. These phenomena, along with surface cavities caused by glass softening, resulted in the failure of the low-emissivity property of the Si50 and Si60 samples. After the Ti70 sample was annealed at 600 °C (Fig. 7b), oxidation occurred on the surface of the Si3N4 layer. The Si3N4 bulk layer was partially oxidized and became fragmental, indicating the failure of the Si3N4 barrier layer during annealing in air. The interface between oxide layers became obscured possibly due to interdiffusion at high temperatures. The original Ag layer nearly disappeared while Ag oxides and a thick crack layer were formed. The underlying oxide layer in the Ti-series samples generated cracks during the thermal treatment. These cracks resulted in a barrier failure of the upper Si3N4 layer. The NiCr layer in the Si-series samples provided good adhesion and thus prevented severe aggregation growth or crack formation of Ag during the thermal treatment. In contrast, the ZnO and NiCrO layers in the Ti-series samples provided insufficient adhesion for the Ag layer, which facilitated the formation of a thick crack layer under high temperatures. In conclusion, the Si3N4 and NiCr layers increased the thermal stability of the low-emissivity coatings.
respectively. The Ti-series samples simultaneously exhibited high visible light transmittance and high IR reflectance. However, their poorer low emissivity property compared to that of the Si-series was caused by the increase in visible light transmittance and heavier light pollution due to increased visible light reflectance. Therefore, the application of the Ti-series is limited. In contrast, the Si-series samples have better low-emissivity properties and reduced light pollution but lower visible light transmittance. In the Ti-series samples that were annealed at elevated temperatures (~600 °C), oxygen could out-diffuse into the metal layer and oxidize the thin Ag layer; and a thick crack layer was formed. Visible transmittance and IR reflectance degraded owing to the above effects. In contrast, at higher temperatures up to 700 °C, the oxidation barrier of the Si-series samples remained thermally stable. Nevertheless, Ag aggregation and glass softening caused a failure of the low-emissivity property. This study demonstrates the promising characteristics of Si- and Ti-series multilayer coatings. Moreover, the effects of the thermal treatment were used to evaluate their applications under various environments for the energy-saving building industry.
4. Conclusion Declaration of competing interest This study reports the preparation and investigation of Si-series and Ti-series multilayer coatings with glass/Si3N4/NiCr/Ag/NiCr/Si3N4 and glass/TiO2/ZnSnO3/ZnO/Ag/NiCrOx/ZnSnO3/Si3N4/structures,
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence 6
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our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript.
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Acknowledgments The authors gratefully acknowledge the financial support for this research by the Ministry of Science and Technology of Taiwan under Grant No. NSC 106-2221-E-005 -026 -MY3. The present work was also supported in part by the Center for Micro/Nano Science and Technology of the National Cheng Kung University. References [1] J. Zhou, Y. Gao, Z. Zhang, H. Luo, C. Cao, Z. Chen, L. Dai, X. Liu, VO2 thermochromic smart window for energy savings and generation, Sci. Rep. 3 (2013) 3029. [2] G. Leftheriotis, P. Yianoulis, Characterisation and stability of low-emittance multiple coatings for glazing applications, Sol. Energy Mater. Sol. Cells 58 (1999) 185–197. [3] M. Miyazaki, E. Ando, Durability improvement of Ag-based low-emissivity coatings, J. Non-Cryst. Solids 178 (1994) 245–249. [4] Z. Zhao, T.L. Alford, The optimal TiO2/Ag/TiO2 electrode for organic solar cell application with high device-specific Haacke figure of merit, Sol. Energy Mater. Sol. Cells 157 (2016) 599–603. [5] E.H. Nezhad, H. Haratizadeh, B.M. Kari, Influence of Ag mid-layer in the optical and thermal properties of ZnO/Ag/ZnO thin films on the glass used in Buildings as insulating glass unit (IGU), Ceram. Int. 45 (2019) 9950–9954. [6] J.-Y. Park, H.-K. Kim, Flexible metal/oxide/metal transparent electrodes made of thermally evaporated MoO3/Ag/MoO3 for thin film heaters, Phys. Status Solidi AAppl. Mat. 215 (2018) 1800674.
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