VO<sub>2</sub>-based superposed Fabry-Perot multilayer film with a highly enhanced infrared emittance and emittance tunability for spacecraft thermal control

Xie Bowei; Zhang Wenjie; Zhao Junming; Liu Linhua; Liu Linhua

doi:10.1364/OE.464266

1. Introduction

Spacecraft thermal management systems often cope with the violent alternation of high and low temperature working conditions in space, caused by the orbital eclipses, seasonal changes in solar intensity and sun angle variation, and dynamic internal thermal loads [1,2]. Thermal radiation is the only way for spacecraft to dissipate heat into outer space. However, the emittance of traditional thermal control coatings for spacecraft cannot be adjusted. A great amount of thermal compensation power and weight resources of the thermal control subsystem is needed for purpose of thermal control, which strongly restricts the life and reliability of the spacecraft [1–3]. By using an intelligent thermal control film which can switch its infrared emittance in orbit, the thermal compensation power and weight resources of thermal control subsystem can be reduced by more than 90 percent and 75 percent, respectively [3]. What’s more, the primary benefit of the intelligent thermal control film for crewed applications is to eliminate the need for dual loop thermal control architecture by preventing freezing of the radiator, and for robotic applications the benefit is to reduce the need for survival heaters [4]. Therefore, the development of intelligent thermal control films is critical for the rapidly increasing demand of spacecraft missions.

An intelligent thermal control film is mainly based on the far-field thermal radiation. Such films might be of three types: the thermochromic film [5,6], the electrochromic film [7,8] and the shutter regulation technology [9]. Most recently, a new principle of smart skin based on electric tuning of near-field radiative heat flux is proposed for spacecraft thermal control [10]. Among these technologies, the thermochromic film based on thermally driven phase change materials is the only scheme that can automatically switch its emittance without additional energy consumption, which shows great potential for the application of spacecraft thermal control. Typical thermally driven phase change materials include chalcogenides (such as Ge₂Sb₂Te₅) with amorphous-crystalline transition [11,12], and perovskite oxides (A_1-XB_XMo₃) [13,14] and vanadium dioxide (VO₂) [15,16] with metal-insulator transition. The phase changing temperature of Ge₂Sb₂Te₅ is too high (∼931 K) [11,12], and the temperature range of perovskite oxides during the phase transition is too large (∼200 K) [13], both of which face huge obstacles in the practical application of spacecraft thermal control. The requirement for transition temperature of the thermochromic film for human spacecraft is typically dictated by the needs of the avionics downstream of the radiator outlet. Avionics and similar electronic components can only work around room temperature (268∼318 K) [1,2]. Therefore, VO₂ is a more favorable phase change material for spacecraft applications due to its dramatic change of optical properties at phase change point (∼340 K) near room temperature [16]. Meanwhile, the transition temperature of VO₂ can be decreased by doping VO₂ with tungsten [16,17].

In addition to the phase transition temperature, the radiative characteristic of the thermochromic film also plays a vital role in the application of spacecraft thermal control. Firstly, the infrared emittance ɛ of the thermochromic film at high temperature is expected to be as large as possible in order to efficiently dissipate the internal thermal loads into the space, which is the basis of the spacecraft thermal control [2]. Generally, the infrared emittance of the traditional thermal control coatings is larger than 0.80, such as an optical solar reflector (ɛ>0.80), a white paint (ɛ>0.87) and a black paint (ɛ>0.88) [2]. Meanwhile, the solar absorptance a_solar of the thermal control coatings needs to be small enough (a_solar< 0.20) especially when the radiating surface is in the sunward side of the spacecraft, since the solar radiance is the main source of the external heat flux. When the radiating surface is in the nightside, low solar absorptance of the coatings is not a necessary requirement. However, there are very few missions where the radiating surface can be completely kept out of sun. For thermochromic film with large solar absorption, some optimal scheme is needed to reduce the solar absorptance, such as adding layers with high solar reflectivity and infrared transmittance. Moreover, the emittance tunability Δɛ of the thermochromic film needs to be large enough to deal with the violent alternation of thermal environment. However, both the infrared emittance at high temperature and emittance tunability of the thermochromic film made solely of phase change materials are far from satisfying the needs of spacecraft. In order to enlarge the emittance tunability of the thermochromic film, thermochromic materials are usually combined with other functional materials to form composite micro/nano structures, including multilayer films [18–23], micro/nano particle composite films [24,25] and photonic crystal films [26–28].

The VO₂-based Fabry-Perot (FP) multilayer films attract tremendous attention due to its simple structure, suitable phase change temperature and enhanced emittance tunability by FP resonance. Hendaoui et al. [18] synthesized a VO₂ based smart film which is made of 850 nm thick SiO₂ layer sandwiched between a thin VO₂ front layer with a thickness of 30 nm and a highly reflecting back Au layer. The infrared emittance of the device was found to be 0.22 and 0.71 at 25 °C and 100 °C, respectively. Taylor et al. [19] numerically elucidated the physical mechanisms of the absorption enhancements of the VO₂-based FP multilayer films by examining the total phase shift in the multilayer structure and the phonon modes of VO₂. Kim et al. [20] synthesized a multilayer structure (Si/VO₂/BaF₂/Au) for spacecraft thermal control. The emittance tunability of the multilayer structure can reach about 0.49 with an infrared emittance of 0.64 at high temperature. Shrewsbury et al. [21] proposed a device based on a multilayer stack of VO₂, ZnSe and gold. The emittance tunability of the multilayer stack is 0.69 with a high temperature emittance equal to 0.77. However, the FP resonance is limited in a narrow band, which restricts the further increase of the infrared emittance and emittance tunability of the VO₂-based FP multilayer film. Table 1 illustrates the performances of existing typical thermochromic thermal control films. As observed in Table 1, the infrared emittance of most existing thermochromic films under metallic state is less than 0.77, which is far from that of traditional thermal control coatings. Moreover, there is still much room for improvement in the aspect of large-range emittance tuning.

Table 1. The performances of existing typical thermochromic thermal control films.

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Herein, a VO₂-based superposed FP multilayer film is proposed for spacecraft thermal control, which supports three FP resonances at different peak wavelengths. The superposed FP film is constructed by encapsulating three thin VO₂ layers in four lossless BaF₂ spacers on an Al substrate. The effect of each layer thickness on the radiation characteristics of the superposed FP film is studied, and the thickness of each layer is optimized in terms of the infrared emittance and emittance tunability. For the typical growth techniques of the VO₂ layers considered here, the emittance tunability of the superposed FP film can exceed 0.70 with total normal emittance larger than 0.91 at high temperature. The largest total normal emittance of the superposed film can reach 0.95 with emittance tunability of 0.78. In addition, the infrared emittance and emittance tunability of the superposed FP film remain large in a very wide incident angle range.

2. Design of multilayer film

Figure 1(a) shows the sketch of the FP multilayer structure. As shown in Fig. 1(a), a dielectric spacer is placed between a thin top mirror and a reflecting substrate to form a FP resonance which dramatically enhances the absorption of the film but is limited in a narrow band. Figure 2 illustrates the refractive index n and absorption index κ of the VO₂ film as a function of wavelength λ under dielectric state and metallic state [15,31,32]. As can be seen in Fig. 2, VO₂ switches between dielectric state (low loss, semitransparent) and metallic state (lossy, more reflective) in the infrared spectral range. For the VO₂ layer under dielectric state, the multilayer structure is highly reflective due to the reflecting substrate, since the insulating VO₂ layer and the spacer are highly transparent in the mid infrared spectral range. However, when VO₂ is under metallic state, the VO₂ layer serves as a top mirror to form a FP resonant material, generating an enhanced absorption in the mid-infrared spectral range.

Fig. 1. (a) The sketch of the FP multilayer structure; (b) The sketch of the VO₂-based superposed FP multilayer film. Three VO₂ layers with different thicknesses are encapsulated in four BaF₂ layers with the same thickness on an Al substrate to construct three FP resonances at different peak wavelengths.

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Fig. 2. (a) The refractive index n and (b) absorption index κ of the VO₂ film as a function of wavelength λ under dielectric state and metallic state. The VO₂ film samples used in the complex refractive index measurements were synthesized by using magnetron sputtering [15], atomic layer deposition [31] and sol-gel [32], respectively.

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In order to enlarge the infrared emittance (in the whole mid-infrared spectral range) and emittance tunability, the VO₂-based superposed FP multilayer film is designed to form multiple FP resonances. Figure 1(b) shows the sketch of the VO₂-based superposed FP multilayer film. As can be seen in Fig. 1(b), the superposed FP film is composed of three VO₂ layers, four BaF₂ layers and an Al substrate. The Al substrate is chosen as the reflecting substrate of the FP resonant material, and the complex refractive index of Al published in Ref. [34] is used in our calculation. For the dielectric spacer, a broadband transparent material is preferred for the enhancement of the emittance tunability. BaF₂ is chosen as the dielectric spacer which has almost no absorption in the spectral range from 2.5 to 20 µm. Figure 3 shows the refractive index n and absorption index κ of BaF₂ film as a function of wavelength λ [33]. In addition, the outermost BaF₂ layer is designed to serve as anti-reflective layer to further increase the absorption of the superposed FP film.

Fig. 3. The refractive index n and absorption index κ of the BaF₂ [33] film as a function of wavelength λ.

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In order to construct a multiple FP resonances VO₂-BaF₂-Al stack, three VO₂ layers are encapsulated in four BaF₂ layers, as shown in Fig. 1(b). In general, the amplitude and the width of the FP resonance can be tuned by the top mirror thickness, and the resonance wavelength is dependent on the spacer thickness [19,20,35]. The required spacer thickness l to achieve a FP resonance at the target wavelength λ can be predicted by [35]:

(1)$$l = \frac{{m\lambda }}{{4n}}$$

where m is an odd integer, and n is the refractive index of the spacer. In this work, the thicknesses of each VO₂ layers are different, but the thicknesses of the four BaF₂ layers are designed to be uniform, thus the peak wavelengths of the three FP resonances can be almost uniformly spaced. For spacecraft thermal control, the spectral emittance is expected to be close to unity in the mid-infrared spectral range. In order to obtain a large infrared emittance, the three FP resonances should be uniformly spaced in the spectral range where the spectral exitance of a blackbody is strong. The spectral exitance of a blackbody at a temperature of 300 K peaks at the wavelength of about 9 µm and exhibits a relatively large exitance in the spectral range 5∼25 µm. Therefore, according to formula (1), the thickness of the BaF₂ layer should vary around 1.5 µm with m = 1, corresponding to the resonance wavelengths of about 9, 15 and 20 µm. It should be noted that the target wavelengths of the FP resonances are just estimated values so that the thickness of the BaF₂ layer can be limited in a proper range to avoid brute-force calculations. In this work, VO₂ layers with thicknesses in the range of 5∼50 nm (interval is 5 nm), and BaF₂ layers with thicknesses in the range of 0.5∼2.0 µm (interval is 0.1 µm) are modeled to optimize the infrared emittance and emittance tunability of the superposed FP film.

Spectral directional emittance can be obtained by calculating the spectral directional hemispherical reflectance of a film. According to Kirchhoff’s law, the spectral directional emittance ɛ_λ_,θ and spectral directional absorptance a_λ_,θ of a body are equal at any specified temperature and wavelength at thermal equilibrium conditions. Therefore, the spectral directional emittance of an opaque material can be expressed by:

(2)$${\varepsilon _{\lambda ,\theta }} = 1 - {R_{\lambda ,\theta }}$$

where R_λ_,θ is the spectral directional hemispherical reflectance with incident angle θ (including the specular and the diffuse components of the reflectance). The spectral directional hemispherical reflectance of the multilayer film is calculated by the Rigorous Coupled Wave Analysis (RCWA) method [36,37]. The total directional emittance ɛ_θ can be obtained by integrating the spectral directional emittance in the infrared spectral range:

(3)$${\varepsilon _\theta } = \frac{{\int_{2.5\mathrm{\mu}\textrm{m}}^{30\mathrm{\mu}\textrm{m}} {{\varepsilon _{\lambda ,\theta }}{I_\textrm{B}}({\lambda ,T} )d\lambda } }}{{\int_{2.5\mathrm{\mu}\textrm{m}}^{30\mathrm{\mu}\textrm{m}} {{I_\textrm{B}}({\lambda ,T} )d\lambda } }}$$

where I_B(λ, T) is the spectral exitance of a blackbody at temperature T given by Planck’s function. The spectral range from 2.5 to 30 µm is considered in this work, corresponding to main emission band of a blackbody at a temperature of 300 K. The emittance tunability Δɛ_θ is defined as the difference of total directional emittance between metallic and dielectric state of the VO₂ layers:

(4)$$\Delta {\varepsilon _\theta } = {\varepsilon _{\textrm{m},\theta }} - {\varepsilon _{\textrm{d,}\theta }}$$

where ɛ_m,θ is the total directional emittance of the thermochromic film under metallic state, and ɛ_d,θ is the total directional emittance of the thermochromic film under dielectric state.

3. Results and analysis

In this section, the complex refractive index of a VO₂ film synthesized by magnetron sputtering [15] (see Fig. 2) was used to comprehensively show the radiative properties of the superposed FP film and interpret the physical mechanism. Figure 4(a) shows the total normal emittance ɛ_normal and emittance tunability Δɛ_normal of the VO₂-based superposed FP multilayer film as a function of the thickness l₁ of the first VO₂ layer with l₂= 10 nm, l₃= 30 nm and l₀= 1.5 µm. As shown in Fig. 4(a), the total normal emittance of the superposed FP film for VO₂ layers under metallic state decreases monotonously with increasing thickness of the first VO₂ layer, while for VO₂ layers under dielectric state it increases monotonously, leading to a decreased emittance tunability. Thus, the smaller the thickness l₁ of the first VO₂ layer is, the larger the total normal emittance and emittance tunability is. Figure 4(b) presents the total normal emittance ɛ_normal and emittance tunability Δɛ_normal of the VO₂-based superposed FP multilayer film as a function of the thickness l₂ of the second VO₂ layer with l₁= 10 nm, l₃= 30 nm and l₀= 1.5 µm. As shown in Fig. 4(b), with increasing thickness of the second VO₂ layer, the total normal emittance of the superposed FP film for VO₂ layers under metallic state decreases monotonously, while for VO₂ layers under dielectric state it increases monotonously, leading to a decreased emittance tunability. Therefore, similar to the first VO₂ layer, a smaller thickness l₂ of the second VO₂ layer is preferred to obtain a larger total normal emittance and emittance tunability of the superposed FP film.

Fig. 4. (a) The total normal emittance ɛ_normal and emittance tunability Δɛ_normal of the VO₂-based superposed FP multilayer film as a function of the thickness l₁ of the first VO₂ layer with l₂= 10 nm, l₃= 30 nm and l₀= 1.5 µm; (b) The total normal emittance ɛ_normal and emittance tunability Δɛ_normal of the VO₂-based superposed FP multilayer film as a function of the thickness l₂ of the second VO₂ layer with l₁= 10 nm, l₃= 30 nm and l₀= 1.5 µm; (c) The total normal emittance ɛ_normal and emittance tunability Δɛ_normal of the VO₂-based superposed FP multilayer film as a function of the thickness l₃ of the third VO₂ layer with l₁= l₂= 10 nm and l₀= 1.5 µm; (d) The total normal emittance ɛ_normal and emittance tunability Δɛ_normal of the VO₂-based superposed FP multilayer film as a function of the thickness l₀ of the BaF₂ layer with l₁= l₂= 10 nm and l₃= 30 nm. The total normal emittance is obtained by integrating the spectral normal emittance in the infrared spectral range from 2.5 to 30 µm.

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Figure 4(c) shows the total normal emittance ɛ_normal and emittance tunability Δɛ_normal of the VO₂-based superposed FP multilayer film as a function of the thickness l₃ of the third VO₂ layer with l₁= l₂= 10 nm and l₀= 1.5 µm. As observed in Fig. 4(c), with increasing thickness of the third VO₂ layer, the total normal emittance of the superposed FP film for VO₂ layers under metallic state firstly increases when l₃<30 nm, and then decreases slightly when l₃>30 nm. While for VO₂ layers under dielectric state it increases monotonously, leading to a decreased emittance tunability. Therefore, it seems that the superposed FP film with l₃ equal to 30 nm tends to obtain a large total normal emittance and emittance tunability. Overall, the effect of the thickness of the outer VO₂ layer on the total normal emittance and emittance tunability of the superposed FP film is significant. On the contrary, the total normal emittance and emittance tunability of the superposed FP film are almost independent on the thickness of the inner VO₂ layer. It can be explained by the fact that the outer VO₂ layer interacts with the primary incident wave, i.e., the strongest intensity of incident wave for the FP resonance, leading to a more dramatical interference between the film and the electromagnetic wave.

Figure 4(d) shows the total normal emittance ɛ_normal and emittance tunability Δɛ_normal of the VO₂-based superposed FP multilayer film as a function of the thickness l₀ of the BaF₂ layer with l₁= l₂= 10 nm and l₃= 30 nm. As shown in Fig. 4(d), with increasing thickness of the BaF₂ layer, the total normal emittance of the superposed FP film for VO₂ layers under both states increases monotonously. For VO₂ layers under metallic state, the thickness of the BaF₂ layer primarily influences the resonant wavelength of the FP resonance. Therefore, the total normal emittance tends to increase until perfect thickness of the BaF₂ layer is achieved. For VO₂ layers under dielectric state, on the other hand, the increased thickness of the BaF₂ layer leads to an increased emittance in the spectral range from 20 to 30 µm, thus the total normal emittance of the superposed FP film increases. Moreover, the emittance tunability decreases slightly with increasing thickness of the BaF₂ layer. In summary, for VO₂-based superposed FP multilayer film with l₀= 1.0 µm, l₁= l₂= 10 nm, and l₃= 15 nm, the emittance tunability can exceed 0.70 with total normal emittance larger than 0.91, simultaneously. For VO₂-based superposed FP multilayer film with l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, a maximum total normal emittance (ɛ_normal= 0.95) is obtained with emittance tunability of 0.67.

In order to better understand the enhancement mechanism, Fig. 5 illustrates the spectral normal emittance ɛ_λ_,normal of the VO₂-based superposed FP multilayer film with a maximum total normal emittance at high temperature in the spectral range from 2.5 to 30 µm with normal plane wave incidence. The thickness of each layer of the superposed FP film in Fig. 5 is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively. As can be seen in Fig. 5, for VO₂ layers under metallic state, the spectral normal emittance of the superposed FP film is larger than 0.95 in the spectral range from 5 to 22 µm (i.e., the major emission band of a blackbody at a temperature of 300 K), resulting in a total normal emittance of 0.95. In the spectral range from 20 to 30 µm, the spectral normal emittance decreases monotonously with increasing wavelength due to the large complex refractive index of VO₂ layers. For VO₂ layers under dielectric state, the superposed FP film only exhibits significant spectral normal emittance when the wavelength is larger than 15 µm, while in the spectral range from 2.5 to 15 µm there is almost no absorption/emission, resulting in a total normal emittance of 0.28.

Fig. 5. The spectral normal emittance ɛ_λ_,normal of the VO₂-based superposed FP multilayer film for VO₂ layers under metallic and dielectric state in the spectral range from 2.5 to 30 µm, the thickness of each layer is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively. The shaded area is the spectral exitance I_B(λ, T) of a blackbody at a temperature of 300 K given by Planck’s function.

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The large emittance of the superposed FP film is attributed to the absorption of the three metallic VO₂ layers due to the superposed FP resonances at specific wavelengths. Figure 6 presents the spectral normal emittance ɛ_λ of each layer of the superposed FP film for VO₂ layers under (a, c) metallic and (b, d) dielectric state in the spectral range from 2.5 to 30 µm with normal plane wave incidence. The thickness of each layer of the superposed FP film in Fig. 5 is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively. As shown in Fig. 6(a), due to the FP resonance, the spectral normal emittance of the first, second and third VO₂ layer peaks at wavelengths of about 20, 15 and 9 µm, respectively, corresponding to the three FP resonances VO₂-BaF₂-Al stacks in Fig. 1(b) with dielectric spacer thicknesses of about 4.5, 3.0 and 1.5 µm. The intensity of each FP resonance is weakened due to the decreased intensity of the reflected wave by the other two thin VO₂ layers, resulting in lower absorption of each VO₂ layer. However, by combining the three FP resonances, a nearly perfect absorption/emission (i.e., a spectral normal emittance close to 1) of the total multilayer film is obtained in the spectral range from 5 to 20 µm. For VO₂ layers under dielectric state, the superposed FP film becomes more reflective due to the bottom reflecting Al substrate and highly transparent VO₂ and BaF₂ layers. The total normal emittance of the superposed FP film is primarily attributed to the absorption of the VO₂ layers in the spectral range from 10 to 20 µm and the BaF₂ layers in the spectral range from 20 to 30 µm as shown in Fig. 6(b) and 6(d).

Fig. 6. The spectral normal emittance ɛ_λ_,normal of each layer of the VO₂-based superposed FP multilayer film for VO₂ layers under (a, c) metallic and (b, d) dielectric state in the spectral range from 2.5 to 30 µm with normal plane wave incidence. The thickness of each layer is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively.

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Figure 7 shows the normalized electric field |E/E₀| (E₀ is the incident electric field) of the VO₂-based superposed FP multilayer film for VO₂ layers under (a-d) metallic state and (e-h) dielectric state at the wavelengths of 4.4, 9, 15 and 20 µm, respectively. The thickness of each layer of the superposed FP film in Fig. 7 is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively. As shown in Fig. 7(a) and 7(e), the incident and reflected waves dramatically interfere with each other at the wavelength of 4.4 µm for VO₂ layers under both states, form an obvious enhanced and restricted electric field compared to the incident electric field. The VO₂ layers are just located at the restricted regions with electric field of almost zero, leading to an extremely small emission of the total multiple film as shown in Fig. 5. For the superposed FP film with VO₂ layers under metallic state at the resonance wavelengths, as shown in Figs. 7(b-d), the electric field tends to decrease monotonously along Z axis direction (i.e., the incident direction). Almost no interference between the incident and reflected waves can be observed due to the perfect absorption caused by the superposed FP resonances in the spectral range from 5 to 22 µm. While for VO₂ layers under dielectric state, the whole VO₂ and BaF₂ layers become semi-transparent, strong interferences between the incident and reflected waves are observed in Figs. 7(f-h).

Fig. 7. The normalized electric field |E/E₀| of the VO₂-based superposed FP multilayer film for VO₂ layers under (a-d) metallic state and (e-h) dielectric state at the wavelengths of 4.4, 9, 15 and 20 µm, respectively. The thickness of each layer is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively. E₀ is the incident electric field, and the incident direction is normal to the layers. The white lines represent the boundaries of each layer. Only the BaF₂ layers can be clearly seen in the figures, due to the VO₂ layers too thin to be represented.

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Figure 8 shows the spectral and incident angle dependent emittance contours of the VO₂-based superposed FP multilayer film for VO₂ layers under metallic state with (a) Transverse Electric (TE) wave and (b) Transverse Magnetic (TM) wave, and dielectric state with (c) TE wave and (d) TM wave. The thickness of each layer of the superposed FP film in Fig. 8 is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively. As can be seen in Figs. 8(a-b), for the superposed FP film with VO₂ layers under metallic state, the spectral directional emittance varies little with increasing incident angle in the spectral range from 5 to 20 µm when the incident angle is less than 45° for both TE and TM waves, while in the spectral range from 20 to 30 µm it is more sensitive to the incident angle. For incident angles larger than 45°, the spectral directional emittance tends to decrease obviously with increasing incident angle. For the superposed FP film with VO₂ layers under dielectric state, as shown in Figs. 8(c-d), the spectral directional emittance seems to be independent on the incident angle when the incident angle is less than 60° and keeps at a relatively small value (<0.5) in the spectral range from 2.5 to 30 µm. Discontinuities in the spectral directional emittance of the superposed FP film are observed at a wavelength of about 22.5 µm in Fig. 8. It is due to the sudden increase of the spectral directional emittance of the BaF₂ layers as can be seen in Figs. 6(c-d). Spectral directional emittance with values close to unity can be observed for VO₂ layers under dielectric state with both TE and TM waves, but it is still confined in a narrow spectral range and incident angle range. The areas of high spectral directional emittance for VO₂ layers under dielectric state can be explained as below. The reflectance of the dielectric VO₂-BaF₂ interface increases with the incident angle. With such increased interfacial reflectance, it tends to form a FP resonance in the spectral range from 15 to 30 µm where the absorption index is large enough as shown in Fig. 2(b). Similar phenomenon and explanations can be seen in Refs. [19,38].

Fig. 8. The spectral and incident angle dependent emittance contours of the VO₂-based superposed FP multilayer film for VO₂ layers under metallic state with (a) TE wave and (b) TM wave, and dielectric state with (c) TE wave and (d) TM wave. The thickness of each layer of the multilayer film is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively.

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Figure 9 shows the total directional emittance ɛ_θ of the VO₂-based superposed FP multilayer film for VO₂ layers under metallic state and dielectric state with TE wave and TM wave as a function of the incident angle. The thickness of each layer of the superposed FP film in Fig. 9 is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively. As shown in Fig. 9, for VO₂ layers under metallic state, the total directional emittance of the superposed FP film with TE and TM waves remains larger than 0.9 for incident angles up to 45° and 60°, respectively, indicating that the total directional emittance with TE wave is more sensitive to incident angle than that of TM wave. For VO₂ layers under dielectric state, the total directional emittance of the superposed FP film seems to be independent of the incident angle for both TE and TM waves for incident angles up to 60°. Generally, for both TE and TM waves, a large total directional emittance at high temperature and high emittance tunability of the superposed FP film is obtained even with incident angles up to 60°. These results show that the total directional emittance and emittance tunability of the superposed FP film possesses excellent insensitivity against both the incident angle and wave polarization.

Fig. 9. The total directional emittance ɛ_θ of the VO₂-based superposed FP multilayer film for VO₂ layers under metallic and dielectric state with TE wave and TM wave as a function of the incident angle. The thickness of each layer is l₀= 1.5 µm, l₁= l₂= 10 nm, and l₃= 30 nm, respectively.

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Last but not least, the complex refractive index of VO₂ layers in the visible and infrared spectral range obtained by experimental measurements are different throughout the literature [15,20,31,32], as shown in Fig. 2. It can be explained by the fact that the growth technique influences the optical properties due to the quality of the thin crystalline films [31,32]. Despite the variation of the complex refractive index of VO₂, the physical mechanism of the superposed FP film is general. Therefore, the limited difference in the complex refractive index of the VO₂ layers will not affect the acquisition of excellent performance of the superposed FP film. Table 2 illustrates the performances and the optimal film structure parameters of the superposed FP film for VO₂ layers synthesized by typical growth techniques, including magnetron sputtering, atomic layer deposition and sol-gel. As shown in Table 2, for VO₂ layers synthesized by different growth techniques, the excellent performances of the superposed FP film can be obtained by adjusting the layer thickness. Generally, the emittance tunability of the superposed FP film can exceed 0.70 with total normal emittance larger than 0.91 at high temperature. For VO₂ layers synthesized by sol-gel method, the largest total normal emittance of the superposed FP film can reach 0.95 with emittance tunability of 0.78.

Table 2. The performances and the optimal film structure parameters of the superposed FP film for VO₂ layers synthesized by typical growth techniques.

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It should be noted that lowering the VO₂ phase transition temperature is also essential for making VO₂-based thermochromic film viable for spacecraft applications. Generally, spacecraft applications will be targeting a transition temperature between room temperature and ∼10 °C. The transition temperature of VO₂ can be decreased by doping VO₂ with tungsten [16,17]. It seems that doping W would not significantly affect the complex refractive index of VO₂ [16,17,39], and thus would not affect much the excellent radiation characteristic of the superposed FP film.

4. Conclusion

In this work, a VO₂-based superposed FP multilayer film with simple structure, nearly perfect infrared emission (i.e., an infrared emittance close to 1) and large emittance tunability was proposed for spacecraft thermal control. The superposed FP film is constructed by encapsulating three thin VO₂ layers in four lossless BaF₂ spacers on the Al substrate to form multiple FP resonances. The effect of the thickness of each layer on the infrared emittance and emittance tunability of the superposed FP film was studied to achieve a maximum infrared emittance and emittance tunability. Generally, for the typical growth techniques of the VO₂ layers considered here, the emittance tunability of the superposed FP film can exceed 0.70 with total normal emittance larger than 0.91 at high temperature, simultaneously. For VO₂ layers synthesized by sol-gel method, the largest total normal emittance of the superposed FP film can reach 0.95 with emittance tunability of 0.78. The spectral normal emittance of the superposed FP film is near unity for VO₂ layers under metallic state in the spectral range from 5 to 20 µm, due to the three superposed FP resonances. While for VO₂ layers under dielectric state, the superposed FP film becomes more reflective especially in the spectral range from 2.5 to 11.5 µm with a spectral normal emittance less than 0.2. In addition, the infrared emittance and emittance tunability performances remain large for an incident angle up to 60°. The structure proposed in this work outperforms the existing thermochromic films in literature, which could accelerate the application of thermochromic films in the field of spacecraft thermal control.

Funding

National Natural Science Foundation of China (52106103, 52076123); Natural Science Foundation of Shandong Province (ZR2020QE194).

Disclosures

The authors declare that there are no conflicts of interest related to this paper

Data availability

Data underlying the results presented in this paper are not publicly available but may be obtained from the authors upon reasonable request.

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Film types	Film structure and materials	ɛ^a	Δɛ^c	Spectral range	EXP/SIM^d
Traditional thermal control coatings	optical solar reflector	0.80	–	Entire infrared	EXP
	white paint	0.87	–	Entire infrared	EXP
	black paint	0.88	–	Entire infrared	EXP
Particle composite film	VO₂ particle composite film [5]	0.65^b	0.45	Entire infrared	EXP
	VO₂ nanopowder pellets [24]	0.36	−0.46	8∼14 µm	EXP
	CaF₂@VO₂ particles composite film [25]	0.83	0.36	4∼14 µm	EXP
Photonic crystal film	patterned VO₂ meta-surface [29]	0.75	0.48	2.5∼25 µm	EXP
Photonic crystal film	trapezoidal VO₂-Ge multilayer [30]	0.87	0.67	8∼13 µm	SIM
Multilayer film	VO₂/SiO₂/Au multilater [16,18]	0.71	0.49	2.5∼25 µm	EXP
	Si/VO₂/BaF₂/Au multilayer [20]	0.64	0.49	2∼25 µm	EXP
	VO₂/ZnSe/VO₂/ZnSe/Au multilayer [21]	0.77	0.69	2∼30 µm	SIM
	VO₂/MgF₂/W + Ge/MgF₂ multilayer [22]	0.636	0.582	8∼13 µm	SIM
	Mo/VO₂/nanocrystal multilayer [23]	0.867	0.568	8∼13 µm	SIM
This work	superposed FP film (sol-gel)	0.95	0.78	2.5∼30 µm	SIM
This work	superposed FP film (magnetron sputtering)	0.91	0.70	2.5∼30 µm	SIM

Film types	Film structure and materials	ɛ^a	Δɛ^c	Spectral range	EXP/SIM^d
Traditional thermal control coatings	optical solar reflector	0.80	–	Entire infrared	EXP
	white paint	0.87	–	Entire infrared	EXP
	black paint	0.88	–	Entire infrared	EXP
Particle composite film	VO₂ particle composite film [5]	0.65^b	0.45	Entire infrared	EXP
	VO₂ nanopowder pellets [24]	0.36	−0.46	8∼14 µm	EXP
	CaF₂@VO₂ particles composite film [25]	0.83	0.36	4∼14 µm	EXP
Photonic crystal film	patterned VO₂ meta-surface [29]	0.75	0.48	2.5∼25 µm	EXP
Photonic crystal film	trapezoidal VO₂-Ge multilayer [30]	0.87	0.67	8∼13 µm	SIM
Multilayer film	VO₂/SiO₂/Au multilater [16,18]	0.71	0.49	2.5∼25 µm	EXP
	Si/VO₂/BaF₂/Au multilayer [20]	0.64	0.49	2∼25 µm	EXP
	VO₂/ZnSe/VO₂/ZnSe/Au multilayer [21]	0.77	0.69	2∼30 µm	SIM
	VO₂/MgF₂/W + Ge/MgF₂ multilayer [22]	0.636	0.582	8∼13 µm	SIM
	Mo/VO₂/nanocrystal multilayer [23]	0.867	0.568	8∼13 µm	SIM
This work	superposed FP film (sol-gel)	0.95	0.78	2.5∼30 µm	SIM
This work	superposed FP film (magnetron sputtering)	0.91	0.70	2.5∼30 µm	SIM

VO₂-based superposed Fabry-Perot multilayer film with a highly enhanced infrared emittance and emittance tunability for spacecraft thermal control

Abstract

1. Introduction

2. Design of multilayer film

3. Results and analysis

4. Conclusion

Funding

Disclosures

Data availability

References

Data availability

Cited By

Figures (9)

Tables (2)

Equations (4)

Optics Express

Growth technique	Film structure parameters				Total normal emittance ɛ_m,normal (metallic)	Emittance tunability Δɛ
Growth technique	l₀ (µm)	l₁ (nm)	l₂ (nm)	l₃ (nm)	Total normal emittance ɛ_m,normal (metallic)	Emittance tunability Δɛ
Magnetron sputtering [15]	1.0	10	10	15	0.91	0.70
Atomic layer deposition [31]	1.5	20	30	50	0.94	0.73
Sol-gel [32]	1.5	10	10	30	0.95	0.78