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SiO2 one-dimensional heterostructure photonic crystal with infrared spectrally selective low emissivity
Optical Materials 96 (2019) 109333
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Preparation and characterization of Ge/TiO2//Si/SiO2 one-dimensional heterostructure photonic crystal with infrared spectrally selective low emissivity
T
Weigang Zhang∗, Dandan Lv College of Materials and Chemical Engineering, Chuzhou University, Hui Feng Road 1, Chuzhou, 239000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Low infrared emissivity Spectrally selective Photonic crystal High vacuum electron beam coating technology
Ge/TiO2//Si/SiO2 one-dimensional heterostructure photonic crystal (1DHPC) was successfully designed and prepared by alternating deposition Ge, TiO2, Si and SiO2 on the quartz substrate via the high vacuum electron beam coating technology. The composition structure and spectral emissivity of the as-prepared 1DHPC were characterized via the scanning electron microscopy (SEM) and fourier transform infrared spectrometer (FTIR), respectively. The microstructure measure result shows that the as-prepared 1DHPC has obvious multilayer structural characteristics, and the thicknesses of Ge layer, TiO2 layer, Si layer and SiO2 layer are basically the same as the design values. The test results of spectral emissivity show that the as-prepared 1DHPC has low infrared emissivity in the 3–5 μm and 8–14 μm bands, the average emissivity can be as low as 0.060 and 0.239, respectively. But in the 5–8 μm band, the as-prepared 1DHPC has higher infrared emissivity (0.562). The results indicate that the as-prepared Ge/TiO2//Si/SiO2 1DHPC has obviously infrared spectrally selective low emissivity characteristic, basically meets the requirements of our design. The results of this paper show that low infrared emissivity materials can still be prepared by using non-infrared transparent materials through a reasonable onedimensional photonic structural design.
1. Introduction Low infrared emissivity materials are currently recognized as the most effective way to achieve infrared stealth of all types of aircraft. In the past decade, several kinds of low infrared emissivity materials such as core-shell composites [1,2], multilayer structures [3], nano-composite films [4,5] and composite coatings [6,7] have been developed. However, all of the above materials do not have infrared spectrally selective low emissivity characteristic, they reduce the infrared emissivity of the two atmospheric window bands of 3–5 μm and 8–14 μm while reducing the infrared emissivity of other bands [8,9]. Which easily leads to the fact that the heat-generating equipment such as fighters and missiles in the 10000-m high-altitude operating environment cannot effectively dissipate heat by heat radiation. Therefore, the accumulation of heat in the component makes the overall infrared radiation intensity of the aircraft still high, which is not conducive to the infrared stealth of the aircraft, and the accumulation of heat will greatly affect the service life of the component. Infrared spectrally selective low emissivity materials can achieve low emissivity in the two atmospheric window bands of 3–5 μm and 8–14 μm to achieve the infrared stealth of
∗
the aircraft and the non-atmospheric window bands such as 5–8 μm have high emissivity to achieve effective heat dissipation of the aircraft by composition structure's design and regulation [10]. Which can effectively solve the contradiction between achieving low infrared emissivity and heat dissipation of the aircraft, and finally greatly improving the infrared stealth performance of various aircraft. One-dimensional photonic crystal (1DPC), also known as multilayer film structure or multilayer reflector structure, which has a selective reflection characteristic for incident light, is expected to be applied to the infrared stealth field of weaponry [11,12]. In particular, the onedimensional heterostructure photonic crystal can realize the broadband reflection of multiple bands in the infrared interval by appropriate structural design [13], thus achieving low emissivity in the two atmospheric window bands of 3–5 μm and 8–14 μm, and high emissivity in non-atmospheric window bands such as 5–8 μm. In the end, it is expected to resolve the contradiction between the low infrared emissivity and effective heat dissipation of aircraft. Since the one-dimensional photonic crystal applied to the infrared stealth field has a reflection peak wavelength in the middle and far infrared bands, it is required to have a relatively thick film thickness, which is difficult to prepare.
Corresponding author. E-mail address: [email protected] (W. Zhang).
https://doi.org/10.1016/j.optmat.2019.109333 Received 14 July 2019; Received in revised form 17 August 2019; Accepted 20 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 96 (2019) 109333
W. Zhang and D. Lv
Therefore, the literature on the very small number of infrared stealth photonic crystal that have been reported is mainly focused on theoretical design [14,15]. In addition, the traditional view is that low emissivity 1DPCs must require the dielectric materials have good infrared transparency, which further limits the application of 1DPCs in the field of infrared stealth [16]. Therefore, research on the photonic crystals with low infrared emissivity has important theoretical and practical significance. In this paper, Ge/TiO2//Si/SiO2 1DHPC with infrared spectrally selective low emissivity was successfully prepared via high vacuum electron beam coating technology. The effect of the number of periods on the infrared emissivity of 1DHPC was systematically analyzed. The composition structure and spectral emissivity of as-prepared 1DHPC were systematically investigated. 2. Experimental 2.1. Materials Fig. 1. Calculated reflection spectra of Ge/TiO2&Si/SiO2 1DHPCs with different numbers of periods.
Ge particles (purity 99.999 wt%, refractive index 4.0), TiO2 particles (purity 99.99 wt%, refractive index 2.1), Si particles (purity 99.99 wt%, refractive index 3.4) and SiO2 particles (purity 99.99 wt%, refractive index 1.45) were purchased from Nanjing Chemical Reagent Limited Company, China. All reagents were analytical grade and were used as received without further treatment.
Table 1 Calculated infrared emissivity of Ge/TiO2&Si/SiO2 1DHPCs with different numbers of periods. Number of periods
2.2. Preparation of the 1DHPC 4 5 6
Round quartz substrate (surface roughness 10 nm, diameter 5 cm, thickness 1 mm), properly cleaned by ultrasonic bath, was used as the substrate to prepare the 1DHPC. The 1DHPC with low infrared emissivity in the 3–5 μm and 8–14 μm atmospheric window bands was designed via the characteristic matrix method, then the thickness and number of periods of each structural layer (Ge, TiO2, Si and SiO2) were obtained. Ge and TiO2 were alternating deposited on the quartz substrate by high vacuum electron beam coating machine (OTFC-900) to fabricate the 1DPC with low emissivity in the 8–14 μm band, then Si and SiO2 were alternating deposited on the 1DPC with low emissivity in the 8–14 μm band by high vacuum electron beam coating machine (OTFC-900) to fabricate the 1DPC with low emissivity in the 3–5 μm band. Pure Ge particles, TiO2 particles, Si particles and SiO2 particles were pressed into billets as targets. The target-to-substrate distance was 60 cm. The high energy electron beam bombarded the target to evaporate it, then gradually deposited on the surface of the quartz substrate to form a film. During the whole deposition process, the deposition rates of Ge layer, TiO2 layer, Si layer and SiO2 layer were 0.4 nm/s, 0.6 nm/s, 0.4 nm/s and 0.6 nm/s, respectively, the substrate temperature was maintained at 250 °C, the chamber pressure 0.9 × 10−3 Pa, the accelerating voltage and current were 6 kV and 24 mA, respectively.
Average emissivity 3–5 μm
5–8 μm
8–14 μm
0.031 0.025 0.021
0.539 0.546 0.552
0.116 0.077 0.073
Fig. 2. Structural model of Ge/TiO2&Si/SiO2 1DHPC with infrared spectrally selective low emissivity.
2.3. Characterization The Photographs of the quartz substrate and 1DHPC were recorded by Nikon digital camera (COOLPIXL22). The morphology, microstructure and energy spectrum of the 1DHPC were characterized by field emission scanning electron microscopy (S-4800). The sample was sputtered with a thin layer of Au prior to imaging. The normal spectral emissivity at the wavelength of 3–18 μm of the 1DHPC was measured by fourier transform infrared spectrometer (JASCO FTIR-6100) at 100 °C. The Landcal R1500T blackbody furnace was used as the nearblackbody source. The normal spectral emissivity is the ratio of the radiance of a sample to that of a blackbody at the same temperature level and for the same spectral and normal directional conditions. The infrared emissivity in this paper refers to the average emissivity of a certain infrared band.
Fig. 3. Photographs of quartz substrate (a) and Ge/TiO2&Si/SiO2 1DHPC according to the structural model (b).
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Optical Materials 96 (2019) 109333
W. Zhang and D. Lv
Fig. 4. (a) Cross-sectional SEM image of Ge/TiO2&Si/SiO2 1DHPC according to the structural model. (b) (c) partial magnified images of (a). (d)–(g) EDS patterns of point A, B, C and D in (b) and (c).
3. Results and discussion
of 1DHPC can be calculated too. In order to make the 1DHPC has the strongest reflecting peak intensity to achieve the lowest emissivity under the same conditions, the optical thicknesses of the two dielectric layers are designed equal to a quarter of center wavelengths (4 μm and 11 μm) of the reflection peaks at 3–5 μm and 8–14 μm [17], then the thicknesses of Ge layer (0.688 μm), TiO2 layer (1.31 μm), Si layer (0.294) and SiO2 layer (0.690) are calculated. The number of periods of 1DHPC has an important influence on the reflectivity, so the reflection spectra of Ge/ TiO2//Si/SiO2 1DHPCs with 4–6 periods were calculated (Fig. 1). It can be seen that the reflection peaks intensity at 3–5 μm and 8–14 μm grow slightly with increasing the number of periods from 4 to 6, at the same time, the photonic bandwidth is gradually widened. Which leads the average reflectivity at 3–5 μm and 8–14 μm increased slightly from 0.969 to 0.979 and 0.884 to 0.927, respectively. Leading to the average emissivity at the wavelength of 3–5 μm and 8–14 μm decreased slightly from 0.031 to 0.021 and 0.116 to 0.073, respectively (Table 1). However, in the 5–8 μm band, the 1DHPCs have higher emissivity under 4–6
3.1. Theoretical analysis and design of the 1DHPC The characteristic matrix method for one-dimensional photonic crystal was used to calculate the reflection spectra of 1DHPC with the calculation software of Translight 3.01b [17]. According to the Kirchhoff's law and Principle of Conservation of Energy, for 1DHPC, the higher the reflectivity, the lower the emissivity [18]. The relationship between the infrared emissivity (ε) and reflectivity (r) in a particular band of non-transparent material such as 1DHPC can be expressed as: ε = 1-r
(1)
As shown in Equation (1), the reflectivity has a decisive effect on the infrared emissivity of 1DPCs. According to these features, after the average reflectivity of a certain band obtained from the simulated reflection spectra of 1DHPC, the infrared emissivity of a particular band
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Optical Materials 96 (2019) 109333
W. Zhang and D. Lv
Fig. 5. Spectral emissivity of Ge/TiO2&Si/SiO2 1DHPC according to the structural model, (a) Measured, (b) Calculated.
addition, the dielectric materials used to prepare the 1DHPC, except for Ge (transparent zone 1.7–23 μm), the infrared transparent regions of TiO2 (transparent zone 0.4–10 μm), Si (transparent zone 1–9 μm) and SiO2 (transparent zone 0.9–9 μm) do not cover the entire 8–14 μm band, but the 1DHPC still has a low infrared emissivity (0.239) in the 8–14 μm band. It can be seen that the traditional concept of low infrared emissivity materials must have good infrared transparency is debatable. With a reasonable one-dimensional photonic structural design to achieve a certain degree of reflection intensity, whether it is infrared transparent or non-transparent material, it can achieve low infrared emissivity. Which provides a new idea for the design of low infrared emissivity materials.
periods due to the weak reflection intensity (Table 1). When the 1DHPC has 5 periods, it already has a low emissivity at 3–5 μm and 8–14 μm, so we designed the number of periods was 5 for Ge/TiO2//Si/SiO2 1DHPC (Fig. 2).
3.2. Composition structure and infrared emissivity property of the 1DHPC The Ge/TiO2//Si/SiO2 1DHPC according to the structural model of Fig. 2 was successfully prepared by using the high vacuum electron beam coating technology (Fig. 3(b)). It can be seen that the 1DHPC has a purple-red structural color which is not possessed by materials such as Ge, TiO2, Si and SiO2. This is caused by a special reflection peak of the 1DHPC in the visible light band, indicating that the as-prepared 1DHPC has the basic characteristics of a one-dimensional photonic crystal [19]. Fig. 4(a) shows the cross-sectional SEM image of the Ge/TiO2//Si/SiO2 1DHPC according to the structural model. It can be seen that the 1DHPC has obvious multilayer film structural characteristics, which is periodic stacked by different dielectric materials. The EDS patterns (Fig. 4(d)–(g)) of points A, B, C and D in Fig. 4(b), (c) illustrate that the corresponding dielectric layers are Ge layer, TiO2 layer, Si layer and SiO2 layer, respectively. Which indicating that the as-prepared 1DHPC is consisted by 8–14 μm low emissivity Ge/TiO2 photonic crystal and 3–5 μm low emissivity Si/SiO2 photonic crystal. The as-prepared 1DHPC has uniform thicknesses of different dielectric layers, the average thicknesses of Ge layer, TiO2 layer, Si layer and SiO2 layer are 0.647 μm, 1.231 μm, 0.324 μm and 0.652 μm, respectively. The measurement results are basically the same as the thicknesses of different dielectric layers in the structural model. Fig. 5 shows the normal spectral emissivity at the wavelength of 3–18 μm of Ge/TiO2//Si/SiO2 1DHPC according to the structural model. It can be seen that the shape of the emissivity spectrum substantially matches the shape of the simulated reflection spectra. The low emissivity characteristics of 3–5 μm and 8–14 μm in the emissivity spectrum are consistent with the strong reflection characteristics in the simulated reflection spectra, and the high emissivity characteristics of 5–8 μm are consistent with the weak reflection characteristics in the simulated reflection spectra. In addition, it can be seen that the emissivity has obvious selectivity in different bands. The as-prepared 1DHPC has ultra-low infrared emissivity in the 3–5 μm band, the average emissivity can be as low as 0.060. In the 8–14 μm band, the asprepared 1DHPC has low infrared emissivity, the average emissivity can be as low as 0.239. But in the 5–8 μm band, the as-prepared 1DHPC has higher infrared emissivity, the average emissivity can be as high as 0.562. The above results indicate that the as-prepared Ge/TiO2//Si/ SiO2 1DHPC has obviously infrared spectrally selective low emissivity characteristic, basically meets the requirements of our design. In
4. Conclusions In summary, Ge/TiO2//Si/SiO2 1DHPC with infrared spectrally selective low emissivity was successfully prepared by using the high vacuum electron beam coating technology. The as-prepared 1DHPC has obviously infrared spectrally selective low emissivity characteristic, which has low infrared emissivity at the wavelength of 3–5 μm and 8–14 μm, but high infrared emissivity at the wavelength of 5–8 μm. The low infrared emissivity in the 3–5 μm and 8–14 μm bands can achieve infrared stealth, and high infrared emissivity in the 5–8 μm band can achieve effective cooling for the aircraft. In addition, the results of this paper indicate that with a reasonable one-dimensional photonic structural design, whether it is infrared transparent or non-transparent material, it can achieve low infrared emissivity. Which is conducive to enriching the design concept of low infrared emissivity materials. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (61705029), Natural Science Foundation of Anhui Province (1808085MF187) and Demonstration Experiment and Training Center of Anhui Province (2017sxzx33). We appreciate Andrew L. Reynolds to share the software of translight. References [1] Y.J. Wang, Y.M. Zhou, T. Zhang, M. He, X.H. Bu, Chem. Eng. J. 266 (2015) 199. [2] A. Srinivasa Rao, S. Sakthivel, J. Alloy. Comp. 644 (2015) 906.
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H. Zhang, H.B. Li, W.Q. Wu, B. Yang, Adv. Funct. Mater. 20 (2010) 3784. [13] L.D. Bonifacio, B.V. Lotsch, D.P. Puzzo, F. Scotognella, G.A. Ozin, Adv. Mater. 21 (2009) 1641. [14] H.F. Zhang, S.B. Liu, X.K. Kong, L. Zhou, C.Z. Li, B.R. Bian, Optik 124 (2013) 751. [15] W.S. Li, Q. Zhang, Y.H. Fu, H.M. Huang, Infrared Laser Eng. 44 (2015) 3299 (in Chinese). [16] B.P. Jelle, S.E. Kalnaes, T. Gao, Energy Build. 96 (2015) 329. [17] Y.F. Gao, J.M. Shi, D.P. Zhao, Infrared Technol 32 (2010) 235 (in Chinese). [18] S. Enoch, J.J. Simon, L. Escoubas, Z. Elalmy, F. Lemarquis, P. Torchio, G. Albrand, Appl. Phys. Lett. 86 (2005) 261101. [19] C. Lertvachirapaiboon, T. Parnklang, P. Pienpinijtham, K. Wongravee, C. Thammacharoen, S. Ekgasit, J. Struct. Biol. 191 (2015) 184.
[3] A.A. Solovyev, S.V. Rabotkin, N.F. Kovsharov, Mater. Sci. Semicond. Process. 38 (2015) 373. [4] F.Y. Zhang, Y.M. Zhou, Y.Q. Sun, J. Chen, Mater. Res. Bull. 45 (2010) 859. [5] H.A. Babrekar, S.M. Jejurikar, J.P. Jog, K.P. Adhi, S.V. Bhoraskar, Appl. Surf. Sci. 257 (2011) 1824. [6] K.Z. Wang, C.X. Wang, Y.J. Yin, K.L. Chen, J. Alloy. Comp. 690 (2017) 741. [7] L. Phan, W.G. Walkup IV, D.D. Ordinario, E. Karshalev, J.M. Jocson, A.M. Burke, A.A. Gorodetsky, Adv. Mater. 25 (2013) 5621. [8] Z.B. Huang, W.C. Zhou, X.F. Tang, Appl. Surf. Sci. 256 (2010) 2025. [9] Z.B. Huang, W.C. Zhou, X.F. Tang, Appl. Surf. Sci. 256 (2010) 6893. [10] W.G. Zhang, G.Y. Xu, J.C. Zhang, H.H. Wang, H.L. Hou, Opt. Mater. 37 (2014) 343. [11] B. Li, J. Zhou, L.T. Li, Q. Li, S. Han, Z.B. Hao, Chin. Sci. Bull. 50 (2005) 1529. [12] Z.H. Wang, J.H. Zhang, J. Xie, C. Li, Y.F. Li, S. liang, Z.C. Tian, T.Q. Wang,
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Preparation and characterization of Ge/TiO2//Si/SiO2 one-dimensional heterostructure photonic crystal with infrared spectrally selective low emissivity
T
Weigang Zhang∗, Dandan Lv College of Materials and Chemical Engineering, Chuzhou University, Hui Feng Road 1, Chuzhou, 239000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Low infrared emissivity Spectrally selective Photonic crystal High vacuum electron beam coating technology
Ge/TiO2//Si/SiO2 one-dimensional heterostructure photonic crystal (1DHPC) was successfully designed and prepared by alternating deposition Ge, TiO2, Si and SiO2 on the quartz substrate via the high vacuum electron beam coating technology. The composition structure and spectral emissivity of the as-prepared 1DHPC were characterized via the scanning electron microscopy (SEM) and fourier transform infrared spectrometer (FTIR), respectively. The microstructure measure result shows that the as-prepared 1DHPC has obvious multilayer structural characteristics, and the thicknesses of Ge layer, TiO2 layer, Si layer and SiO2 layer are basically the same as the design values. The test results of spectral emissivity show that the as-prepared 1DHPC has low infrared emissivity in the 3–5 μm and 8–14 μm bands, the average emissivity can be as low as 0.060 and 0.239, respectively. But in the 5–8 μm band, the as-prepared 1DHPC has higher infrared emissivity (0.562). The results indicate that the as-prepared Ge/TiO2//Si/SiO2 1DHPC has obviously infrared spectrally selective low emissivity characteristic, basically meets the requirements of our design. The results of this paper show that low infrared emissivity materials can still be prepared by using non-infrared transparent materials through a reasonable onedimensional photonic structural design.
1. Introduction Low infrared emissivity materials are currently recognized as the most effective way to achieve infrared stealth of all types of aircraft. In the past decade, several kinds of low infrared emissivity materials such as core-shell composites [1,2], multilayer structures [3], nano-composite films [4,5] and composite coatings [6,7] have been developed. However, all of the above materials do not have infrared spectrally selective low emissivity characteristic, they reduce the infrared emissivity of the two atmospheric window bands of 3–5 μm and 8–14 μm while reducing the infrared emissivity of other bands [8,9]. Which easily leads to the fact that the heat-generating equipment such as fighters and missiles in the 10000-m high-altitude operating environment cannot effectively dissipate heat by heat radiation. Therefore, the accumulation of heat in the component makes the overall infrared radiation intensity of the aircraft still high, which is not conducive to the infrared stealth of the aircraft, and the accumulation of heat will greatly affect the service life of the component. Infrared spectrally selective low emissivity materials can achieve low emissivity in the two atmospheric window bands of 3–5 μm and 8–14 μm to achieve the infrared stealth of
∗
the aircraft and the non-atmospheric window bands such as 5–8 μm have high emissivity to achieve effective heat dissipation of the aircraft by composition structure's design and regulation [10]. Which can effectively solve the contradiction between achieving low infrared emissivity and heat dissipation of the aircraft, and finally greatly improving the infrared stealth performance of various aircraft. One-dimensional photonic crystal (1DPC), also known as multilayer film structure or multilayer reflector structure, which has a selective reflection characteristic for incident light, is expected to be applied to the infrared stealth field of weaponry [11,12]. In particular, the onedimensional heterostructure photonic crystal can realize the broadband reflection of multiple bands in the infrared interval by appropriate structural design [13], thus achieving low emissivity in the two atmospheric window bands of 3–5 μm and 8–14 μm, and high emissivity in non-atmospheric window bands such as 5–8 μm. In the end, it is expected to resolve the contradiction between the low infrared emissivity and effective heat dissipation of aircraft. Since the one-dimensional photonic crystal applied to the infrared stealth field has a reflection peak wavelength in the middle and far infrared bands, it is required to have a relatively thick film thickness, which is difficult to prepare.
Corresponding author. E-mail address: [email protected] (W. Zhang).
https://doi.org/10.1016/j.optmat.2019.109333 Received 14 July 2019; Received in revised form 17 August 2019; Accepted 20 August 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 96 (2019) 109333
W. Zhang and D. Lv
Therefore, the literature on the very small number of infrared stealth photonic crystal that have been reported is mainly focused on theoretical design [14,15]. In addition, the traditional view is that low emissivity 1DPCs must require the dielectric materials have good infrared transparency, which further limits the application of 1DPCs in the field of infrared stealth [16]. Therefore, research on the photonic crystals with low infrared emissivity has important theoretical and practical significance. In this paper, Ge/TiO2//Si/SiO2 1DHPC with infrared spectrally selective low emissivity was successfully prepared via high vacuum electron beam coating technology. The effect of the number of periods on the infrared emissivity of 1DHPC was systematically analyzed. The composition structure and spectral emissivity of as-prepared 1DHPC were systematically investigated. 2. Experimental 2.1. Materials Fig. 1. Calculated reflection spectra of Ge/TiO2&Si/SiO2 1DHPCs with different numbers of periods.
Ge particles (purity 99.999 wt%, refractive index 4.0), TiO2 particles (purity 99.99 wt%, refractive index 2.1), Si particles (purity 99.99 wt%, refractive index 3.4) and SiO2 particles (purity 99.99 wt%, refractive index 1.45) were purchased from Nanjing Chemical Reagent Limited Company, China. All reagents were analytical grade and were used as received without further treatment.
Table 1 Calculated infrared emissivity of Ge/TiO2&Si/SiO2 1DHPCs with different numbers of periods. Number of periods
2.2. Preparation of the 1DHPC 4 5 6
Round quartz substrate (surface roughness 10 nm, diameter 5 cm, thickness 1 mm), properly cleaned by ultrasonic bath, was used as the substrate to prepare the 1DHPC. The 1DHPC with low infrared emissivity in the 3–5 μm and 8–14 μm atmospheric window bands was designed via the characteristic matrix method, then the thickness and number of periods of each structural layer (Ge, TiO2, Si and SiO2) were obtained. Ge and TiO2 were alternating deposited on the quartz substrate by high vacuum electron beam coating machine (OTFC-900) to fabricate the 1DPC with low emissivity in the 8–14 μm band, then Si and SiO2 were alternating deposited on the 1DPC with low emissivity in the 8–14 μm band by high vacuum electron beam coating machine (OTFC-900) to fabricate the 1DPC with low emissivity in the 3–5 μm band. Pure Ge particles, TiO2 particles, Si particles and SiO2 particles were pressed into billets as targets. The target-to-substrate distance was 60 cm. The high energy electron beam bombarded the target to evaporate it, then gradually deposited on the surface of the quartz substrate to form a film. During the whole deposition process, the deposition rates of Ge layer, TiO2 layer, Si layer and SiO2 layer were 0.4 nm/s, 0.6 nm/s, 0.4 nm/s and 0.6 nm/s, respectively, the substrate temperature was maintained at 250 °C, the chamber pressure 0.9 × 10−3 Pa, the accelerating voltage and current were 6 kV and 24 mA, respectively.
Average emissivity 3–5 μm
5–8 μm
8–14 μm
0.031 0.025 0.021
0.539 0.546 0.552
0.116 0.077 0.073
Fig. 2. Structural model of Ge/TiO2&Si/SiO2 1DHPC with infrared spectrally selective low emissivity.
2.3. Characterization The Photographs of the quartz substrate and 1DHPC were recorded by Nikon digital camera (COOLPIXL22). The morphology, microstructure and energy spectrum of the 1DHPC were characterized by field emission scanning electron microscopy (S-4800). The sample was sputtered with a thin layer of Au prior to imaging. The normal spectral emissivity at the wavelength of 3–18 μm of the 1DHPC was measured by fourier transform infrared spectrometer (JASCO FTIR-6100) at 100 °C. The Landcal R1500T blackbody furnace was used as the nearblackbody source. The normal spectral emissivity is the ratio of the radiance of a sample to that of a blackbody at the same temperature level and for the same spectral and normal directional conditions. The infrared emissivity in this paper refers to the average emissivity of a certain infrared band.
Fig. 3. Photographs of quartz substrate (a) and Ge/TiO2&Si/SiO2 1DHPC according to the structural model (b).
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Optical Materials 96 (2019) 109333
W. Zhang and D. Lv
Fig. 4. (a) Cross-sectional SEM image of Ge/TiO2&Si/SiO2 1DHPC according to the structural model. (b) (c) partial magnified images of (a). (d)–(g) EDS patterns of point A, B, C and D in (b) and (c).
3. Results and discussion
of 1DHPC can be calculated too. In order to make the 1DHPC has the strongest reflecting peak intensity to achieve the lowest emissivity under the same conditions, the optical thicknesses of the two dielectric layers are designed equal to a quarter of center wavelengths (4 μm and 11 μm) of the reflection peaks at 3–5 μm and 8–14 μm [17], then the thicknesses of Ge layer (0.688 μm), TiO2 layer (1.31 μm), Si layer (0.294) and SiO2 layer (0.690) are calculated. The number of periods of 1DHPC has an important influence on the reflectivity, so the reflection spectra of Ge/ TiO2//Si/SiO2 1DHPCs with 4–6 periods were calculated (Fig. 1). It can be seen that the reflection peaks intensity at 3–5 μm and 8–14 μm grow slightly with increasing the number of periods from 4 to 6, at the same time, the photonic bandwidth is gradually widened. Which leads the average reflectivity at 3–5 μm and 8–14 μm increased slightly from 0.969 to 0.979 and 0.884 to 0.927, respectively. Leading to the average emissivity at the wavelength of 3–5 μm and 8–14 μm decreased slightly from 0.031 to 0.021 and 0.116 to 0.073, respectively (Table 1). However, in the 5–8 μm band, the 1DHPCs have higher emissivity under 4–6
3.1. Theoretical analysis and design of the 1DHPC The characteristic matrix method for one-dimensional photonic crystal was used to calculate the reflection spectra of 1DHPC with the calculation software of Translight 3.01b [17]. According to the Kirchhoff's law and Principle of Conservation of Energy, for 1DHPC, the higher the reflectivity, the lower the emissivity [18]. The relationship between the infrared emissivity (ε) and reflectivity (r) in a particular band of non-transparent material such as 1DHPC can be expressed as: ε = 1-r
(1)
As shown in Equation (1), the reflectivity has a decisive effect on the infrared emissivity of 1DPCs. According to these features, after the average reflectivity of a certain band obtained from the simulated reflection spectra of 1DHPC, the infrared emissivity of a particular band
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Optical Materials 96 (2019) 109333
W. Zhang and D. Lv
Fig. 5. Spectral emissivity of Ge/TiO2&Si/SiO2 1DHPC according to the structural model, (a) Measured, (b) Calculated.
addition, the dielectric materials used to prepare the 1DHPC, except for Ge (transparent zone 1.7–23 μm), the infrared transparent regions of TiO2 (transparent zone 0.4–10 μm), Si (transparent zone 1–9 μm) and SiO2 (transparent zone 0.9–9 μm) do not cover the entire 8–14 μm band, but the 1DHPC still has a low infrared emissivity (0.239) in the 8–14 μm band. It can be seen that the traditional concept of low infrared emissivity materials must have good infrared transparency is debatable. With a reasonable one-dimensional photonic structural design to achieve a certain degree of reflection intensity, whether it is infrared transparent or non-transparent material, it can achieve low infrared emissivity. Which provides a new idea for the design of low infrared emissivity materials.
periods due to the weak reflection intensity (Table 1). When the 1DHPC has 5 periods, it already has a low emissivity at 3–5 μm and 8–14 μm, so we designed the number of periods was 5 for Ge/TiO2//Si/SiO2 1DHPC (Fig. 2).
3.2. Composition structure and infrared emissivity property of the 1DHPC The Ge/TiO2//Si/SiO2 1DHPC according to the structural model of Fig. 2 was successfully prepared by using the high vacuum electron beam coating technology (Fig. 3(b)). It can be seen that the 1DHPC has a purple-red structural color which is not possessed by materials such as Ge, TiO2, Si and SiO2. This is caused by a special reflection peak of the 1DHPC in the visible light band, indicating that the as-prepared 1DHPC has the basic characteristics of a one-dimensional photonic crystal [19]. Fig. 4(a) shows the cross-sectional SEM image of the Ge/TiO2//Si/SiO2 1DHPC according to the structural model. It can be seen that the 1DHPC has obvious multilayer film structural characteristics, which is periodic stacked by different dielectric materials. The EDS patterns (Fig. 4(d)–(g)) of points A, B, C and D in Fig. 4(b), (c) illustrate that the corresponding dielectric layers are Ge layer, TiO2 layer, Si layer and SiO2 layer, respectively. Which indicating that the as-prepared 1DHPC is consisted by 8–14 μm low emissivity Ge/TiO2 photonic crystal and 3–5 μm low emissivity Si/SiO2 photonic crystal. The as-prepared 1DHPC has uniform thicknesses of different dielectric layers, the average thicknesses of Ge layer, TiO2 layer, Si layer and SiO2 layer are 0.647 μm, 1.231 μm, 0.324 μm and 0.652 μm, respectively. The measurement results are basically the same as the thicknesses of different dielectric layers in the structural model. Fig. 5 shows the normal spectral emissivity at the wavelength of 3–18 μm of Ge/TiO2//Si/SiO2 1DHPC according to the structural model. It can be seen that the shape of the emissivity spectrum substantially matches the shape of the simulated reflection spectra. The low emissivity characteristics of 3–5 μm and 8–14 μm in the emissivity spectrum are consistent with the strong reflection characteristics in the simulated reflection spectra, and the high emissivity characteristics of 5–8 μm are consistent with the weak reflection characteristics in the simulated reflection spectra. In addition, it can be seen that the emissivity has obvious selectivity in different bands. The as-prepared 1DHPC has ultra-low infrared emissivity in the 3–5 μm band, the average emissivity can be as low as 0.060. In the 8–14 μm band, the asprepared 1DHPC has low infrared emissivity, the average emissivity can be as low as 0.239. But in the 5–8 μm band, the as-prepared 1DHPC has higher infrared emissivity, the average emissivity can be as high as 0.562. The above results indicate that the as-prepared Ge/TiO2//Si/ SiO2 1DHPC has obviously infrared spectrally selective low emissivity characteristic, basically meets the requirements of our design. In
4. Conclusions In summary, Ge/TiO2//Si/SiO2 1DHPC with infrared spectrally selective low emissivity was successfully prepared by using the high vacuum electron beam coating technology. The as-prepared 1DHPC has obviously infrared spectrally selective low emissivity characteristic, which has low infrared emissivity at the wavelength of 3–5 μm and 8–14 μm, but high infrared emissivity at the wavelength of 5–8 μm. The low infrared emissivity in the 3–5 μm and 8–14 μm bands can achieve infrared stealth, and high infrared emissivity in the 5–8 μm band can achieve effective cooling for the aircraft. In addition, the results of this paper indicate that with a reasonable one-dimensional photonic structural design, whether it is infrared transparent or non-transparent material, it can achieve low infrared emissivity. Which is conducive to enriching the design concept of low infrared emissivity materials. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (61705029), Natural Science Foundation of Anhui Province (1808085MF187) and Demonstration Experiment and Training Center of Anhui Province (2017sxzx33). We appreciate Andrew L. Reynolds to share the software of translight. References [1] Y.J. Wang, Y.M. Zhou, T. Zhang, M. He, X.H. Bu, Chem. Eng. J. 266 (2015) 199. [2] A. Srinivasa Rao, S. Sakthivel, J. Alloy. Comp. 644 (2015) 906.
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