CN110422345B - OSR thermal control coating based on photonic crystal - Google Patents

Abstract

The invention relates to the technical field of thermal control coatings, in particular to an OSR thermal control coating based on photonic crystals, which consists of an emitting layer, an ultraviolet light reflecting layer and a visible infrared light reflecting layer, wherein the ultraviolet light reflecting layer is used as a connecting layer, a medium C in the visible infrared light reflecting layer is used as an isolating layer, so that the strong adhesive force and environmental adaptability of the photonic crystals are ensured, the high reflection of the solar ultraviolet-visible-infrared full spectrum is realized, the low absorption of the solar wide spectrum is realized, the high emission physical property of the original OSR thermal control coating is kept, the state density of the photonic crystals to electromagnetic waves in a forbidden band frequency domain is low, the interaction of the electromagnetic waves with a loss medium and an absorption medium is reduced, the solar radiant heat received by a spacecraft is reduced, the surface temperature of the spacecraft is reduced, and the forbidden band property of 200nm-2000nm is finally realized, the forbidden bandwidth of the photonic crystal is widened. The forbidden bandwidth of the original technology in the infrared band is about 200nm at the widest.

Description

A Photonic Crystal-Based OSR Thermal Control Coating

technical field

The present invention relates to the technical field of thermal control coatings, and more particularly, to an OSR thermal control coating based on photonic crystals.

Background technique

Thermal control coating is one of the important safeguard systems for spacecraft. Solar radiation is the largest thermal radiation received by spacecraft, which covers ultraviolet, visible and infrared. When the spacecraft is operating in space, the temperature on the sunny side can reach up to 250°C, and the lowest temperature on the back side can reach -200°C. In this case, the temperature non-uniformity of the internal structural components and instruments of the spacecraft can reach ±50-100 °C. Most of the spacecraft equipment has strict temperature requirements. Generally, electronic equipment is kept at -15℃-﹢50℃; while Ni-Cd batteries can withstand -10℃-﹢40℃; for some special equipment, in addition to the In addition to the temperature range requirements, there are also requirements for the rate of temperature change, such as space telescopes and high-precision ground observation cameras. High temperatures and huge temperature change rates are unacceptable for spacecraft equipment. The spacecraft mainly use light alloy materials such as aluminum alloy and titanium alloy. The emissivity ε _H of metals is very small, so that the thermal equilibrium temperature of the spacecraft will be very high when operating under solar radiation. The thermal control coating is coated on the surface of the spacecraft, just like the skin of the spacecraft, to control the surface temperature of the spacecraft to ensure the normal operation of the spacecraft and its internal equipment. The thermal equilibrium temperature expression of the surface of the spacecraft is as follows:

where S is the solar constant, σ is the Stefan-Boltzmanncha constant, _AP is the effective area of the spacecraft perpendicular to the direction of solar radiation, A is the effective area of the spacecraft, α _S is the solar energy absorption rate on the spacecraft surface, ε _H is the spacecraft surface The hemispherical outward infrared emissivity. The emissivity of an object is defined as the ratio of the radiation ability E of the object to the radiation force E _b of the black body at the same temperature. For a particular spacecraft, its S, σ, _AP , A are all constants. It can be seen that the equilibrium temperature of the spacecraft surface can be thermally controlled by selecting different α _S /ε _H . The smaller the value of the absorption-emission ratio α _{S /ε H of the thermal control coating, the greater the cooling degree of the spacecraft; the larger the value of the absorption-emission ratio α S /ε H of the} thermal control coating, the higher the temperature of the spacecraft. big. Spacecraft mainly use light alloy materials, such as aluminum alloys, titanium alloys, etc. The emissivity ε _H of metals is very small, so that the thermal equilibrium temperature of the spacecraft will be very high when operating under solar radiation. The thermal control coating applied on the surface of the spacecraft reduces the absorption of solar energy on the surface of the spacecraft, increases the thermal radiation on its surface, reduces the equilibrium temperature of the spacecraft, prolongs the service life of the spacecraft, and ensures the maintenance of the spacecraft and its instruments and equipment. within the normal operating temperature range. Therefore, the thermal control coating with low absorption-emission ratio is the key technology to ensure the normal operation of spacecraft and its equipment.

At present, thermal control coatings can be divided into the following four types according to the composition of the coating: uncoated metal surfaces, such as polished surfaces, sandblasted surfaces; paint-type coatings, various organic and inorganic coatings; electrochemical coatings, such as Anodized coatings and electroplating coatings; secondary surface mirror type thermal control coatings such as optical sun reflectors (OSA), plastic film type secondary surface mirrors, and paint secondary surface mirrors; existing use of white paint and secondary surface mirrors Subsurface mirrors are the main technical route to obtain low absorption and high emission. Among them, the white paint is mainly composed of ZnO, ZrO ₂ and other white pigments and organic resins, such as Z-93, YB71, etc. The secondary surface mirror mainly includes F-46 and optical reflector (OSR). In view of the current demand for thermal control coatings for spacecraft, the current OSR metal reflective layer has technical problems such as strong solar spectrum absorption ability, poor reflection ability, and narrow reflection frequency band. This makes OSRs have difficult technical problems such as a large solar absorption ratio α _S (0.13), a low solar spectral reflectance ratio ρ _S , and a narrow reflection spectral band.

At present, the main material is a thermal control coating composed of quartz glass and metal film layers. The main problem of using this coating is that it is difficult to take into account the ultraviolet, visible and infrared multi-bands in the reflection frequency band, and it is also difficult to achieve low absorption in a wide frequency range of 200nm-2000nm. Moreover, due to the action of the electric field, the metal will generate electromagnetic oscillations, which will result in energy loss. Even with a thin skin depth, the electromagnetic energy loss is not negligible. On the other hand, the adhesion and environmental adaptability of the metal film layer are also poor.

SUMMARY OF THE INVENTION

In order to overcome the deficiencies in the prior art, the present invention provides an OSR thermal control coating based on photonic crystals, which solves the problems that the current OSR structure is difficult to achieve high reflection and low absorption of the full spectrum of ultraviolet-visible-infrared solar energy, and the like, Realize the wide-band total reflection of 200nm-2000nm.

In order to solve the above-mentioned technical problems, the technical scheme adopted in the present invention is:

A photonic crystal-based OSR thermal control coating is composed of an emission layer, an ultraviolet light reflection layer, and a visible infrared light reflection layer;

The emission layer is made of materials with an infrared emissivity > 0.8, which can transmit more than 90% of ultraviolet light, visible light and 200nm-2000nm infrared light, with high transmission rate, high temperature resistance, and extremely small thermal expansion coefficient;

The ultraviolet light reflective layer adopts a dielectric photonic crystal composed of alternating dielectric films A and B, and the alternating period is 3-7. The dielectric film A and dielectric film B are non-metallic materials, and the ultraviolet light reflective layer 200nm-400nm is the forbidden band, reflecting 200nm-400nm ultraviolet light, has strong reflection ability to ultraviolet light, and has good transmission ability to visible light and infrared light;

The visible-infrared light reflective layer is composed of medium C and medium D alternately, and the alternating period is 4.5-7.5. The medium C and medium D are metal photonic crystal films, and the visible-infrared light reflective layer is in visible light and 400nm-2000nm infrared light. The equivalent permittivity of light is less than 0, preventing electromagnetic waves in this frequency domain from entering the visible-infrared reflective photonic crystal.

Further, the emission layer adopts any one of quartz glass and cerium glass, and the thickness is 0.1 mm-0.2 mm.

Further, the dielectric film A adopts a non-metallic dielectric material whose photon absorption wavelength is outside the frequency band of the solar spectrum, that is, less than 200 nm and greater than 2000 nm, or a non-metallic dielectric material with low absorption and high pass through visible light and infrared light; the dielectric film B uses materials with low loss and high dielectric constant.

Further, the dielectric film A is any one of Al ₂ O ₃ , BaF ₂ , KBr, SiO ₂ , SiC, MgF ₂ and TiO ₂ ; the dielectric film B is any one of Si and Ge.

Further, the dielectric constant of the dielectric film B is higher than the dielectric constant of the dielectric film A by at least 1.5.

Further, the ultraviolet light reflective layer dielectric film A is made of Al ₂ O ₃ film, and the film thickness is d _A =80nm-120nm; the dielectric film B is Si film, and the film thickness is d _B =5nm-10nm; alternating period Set to 4.

Further, the medium C is a high dielectric constant dielectric, and the real part of the dielectric constant is greater than 1.5; the medium D is a metal material with high reflectivity, and the real part of the dielectric constant of visible light and infrared light is less than zero.

Further, the medium C is any one of Al ₂ O ₃ , SiO ₂ , TiO ₂ and ITO; the medium D is any one of Al and Ag. Wherein ITO is indium tin oxide.

Further, the medium C adopts an ITO film; the medium D adopts an Ag film, and the film thickness of the metal Ag is greater than 100 nm; the thickness ratio of the Ag film and the ITO film is greater than 1.5.

Compared with the prior art, the present invention has the following beneficial effects:

The invention provides an OSR thermal control coating based on photonic crystal, which is composed of an emission layer, an ultraviolet light reflection layer and a visible infrared light reflection layer, the ultraviolet light reflection layer is used as a connecting layer, and the medium C in the visible infrared light reflection layer is used as an isolation layer. layer, which ensures the strong adhesion and environmental adaptability of the photonic crystal, realizes the high reflection (reflectivity greater than 80%) of the solar ultraviolet-visible-infrared full spectrum (200nm-2000nm), and at the same time realizes its broad solar energy spectrum and low absorption, At the same time, the high emission physical properties of the original OSR thermal control coating are maintained, and the use of photonic crystals has a low density of states for electromagnetic waves in the bandgap frequency domain, reducing the interaction between electromagnetic waves and lossy media and absorbing media, so as to reduce aerospace The solar radiation heat received by the spacecraft reduces the temperature of the spacecraft surface. The metal thin film applied to OSR has two technical problems, one is the poor adhesion with quartz glass, and the method for preparing OSR mainly includes magnetron sputtering. The first layer of dielectric films such as ITO is mainly deposited at high temperature, and then other films of OSR are sequentially deposited at room temperature. After all the layers are deposited, high temperature annealing is carried out in a vacuum environment of 300℃-450℃, and the annealing time is 30min-45min. Using this method, the technical problem of poor adhesion of OSR can be well solved; on the other hand, the environmental adaptability is strong, and a protective layer needs to be deposited. On the one hand, the photonic crystal film composed of A and B acts as an ultraviolet reflection layer; The aspect acts as an isolation layer between D and air, avoiding the interaction between the environment and D, and for this reason, the UV reflective layer acts as a protective layer. This scheme realizes the integrated design of the reflection layer, the connection layer and the protective layer through the structural design. The performance parameters such as dielectric constant, electrical conductivity and light transmittance of photonic crystal film materials will have certain influence on the transmittance of photonic crystals in ultraviolet, infrared and visible light. The invention realizes the forbidden band characteristic of 200nm-2000nm by continuously optimizing the design of its material properties, and widens the forbidden band width of the photonic crystal. The forbidden band width of the original technology in the infrared band is about 200nm at the widest.

Description of drawings

1 is a schematic structural diagram of a photonic crystal-based OSR thermal control coating provided by the present invention;

2 is a working principle diagram of a photonic crystal-based OSR thermal control coating provided by the present invention;

Fig. 3 is the microstructure of the structure of the emission layer and the ultraviolet reflection layer and the ultraviolet reflection layer;

Fig. 4 is the microstructure of the emissive layer, the ultraviolet reflective layer, the visible-infrared light reflective layer structure and the visible-infrared light reflective layer;

5 is a schematic diagram of the reflectivity and transmittance of the ultraviolet reflective layer;

Fig. 6 is the reflectivity and absorptivity of the visible infrared light reflection layer;

Fig. 7 Reflectance and absorption of different metals in UV, visible and IR;

In the figure: 1 is the emission layer, 2 is the ultraviolet light reflection layer, and 3 is the visible infrared light reflection layer.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, but not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

As shown in Figure 1-4, a photonic crystal-based OSR thermal control coating consists of an emission layer 1, an ultraviolet light reflection layer 2, and a visible infrared light reflection layer 3;

The emission layer is made of materials with an infrared emissivity > 0.8, which can transmit more than 90% of ultraviolet light, visible light and 200nm-2000nm infrared light, with high transmission rate, high temperature resistance, and extremely small thermal expansion coefficient;

The ultraviolet light reflecting layer adopts a dielectric type photonic crystal which is alternately formed by a dielectric film A and a dielectric film B, and the alternating period is 3-7. The dielectric film A and the dielectric film B are non-metallic materials, and the ultraviolet light reflecting layer 200nm-400nm is the forbidden band, reflecting 200nm-400nm ultraviolet light, which has strong reflection ability to ultraviolet light, and has good transmission ability to visible light and infrared light; the dielectric film A adopts the absorption photon wavelength outside the solar spectrum frequency band. , that is, non-metallic dielectric materials less than 200nm and greater than 2000nm, or non-metallic dielectric materials with low absorption and high pass in visible light and infrared light; : Absorption wavelength=hc/Eg=1240nm/Eg, the component Eg>4.1eV or Eg<0.62eV of the ultraviolet photonic crystal. The dielectric film B is made of materials with low loss and high dielectric constant. The dielectric film A is any one of Al ₂ O ₃ , BaF ₂ , KBr, SiO ₂ , SiC, MgF ₂ and TiO ₂ ; the dielectric film B is any one of Si and Ge. The dielectric constant of the dielectric film B is higher than that of the dielectric film A by at least 1.5.

The visible-infrared light reflective layer is alternately composed of medium C and medium D, and the alternating period is 4.5-7.5. The medium C and medium D are metal photonic crystal films, and the visible-infrared light reflective layer is in visible light and 400nm-2000nm infrared. The equivalent permittivity of light is less than 0, preventing electromagnetic waves in this frequency domain from entering the visible-infrared reflective photonic crystal. The medium C is a high dielectric constant dielectric, and the real part of the dielectric constant is greater than 1.5; the medium D is a metal material with high reflectivity, and the real part of the dielectric constant in visible light and infrared light is less than zero. The medium C adopts any one of Al ₂ O ₃ , SiO ₂ , TiO ₂ and ITO; the medium D adopts any one of Al and Ag.

In this embodiment, the emission layer is made of any one of quartz glass and cerium glass, with a thickness of 0.1 mm-0.2 mm, and the size of the glass sheet can be 40 mm×40 mm, 40 mm×20 mm, or 20 mm×20 mm. The thickness of the emissive layer is designed according to the emissivity requirements of the thermal control coating.

In this embodiment, the ultraviolet light reflective layer dielectric film A is an Al ₂ O ₃ film, and the film thickness is d _A =80nm-120nm; the dielectric film B is a Si film, and the film thickness is d _B =5nm-10nm ;Alternate period set to 4. Wherein, the structure of the second photonic crystal film layer is not limited to the above structure, as long as it is ensured that the ultraviolet photonic crystal is a forbidden band at 200nm-400nm.

In this embodiment, the medium C adopts an ITO film; the medium D adopts an Ag film, and the film thickness of metal Ag is greater than 100 nm; the thickness ratio of the Ag film and the ITO film is greater than 1.5.

When the solar spectrum passes through the coating, the emission layer has a strong transmission capability for the entire solar spectrum. In this way, the ultraviolet-visible-infrared will be incident on the surface of the ultraviolet reflective photonic crystal through the emission layer. Due to the band gap characteristic of the ultraviolet reflective photonic crystal, the theoretically 100% ultraviolet (200nm-400nm) will be reflected back, and through the emission layer, reflected into space; while visible-infrared (400nm-2000nm) electromagnetic waves continue to be incident on the surface of ultraviolet and visible-infrared reflective photonic crystals. Since the designed visible-infrared photonic crystal has a negative equivalent dielectric constant at 400nm-2000nm, this frequency domain electromagnetic wave is reflected into space. Therefore, it will produce the effect of low absorption-emission ratio and high reflection of the full spectrum of solar energy. The path of electromagnetic waves in all frequency bands is a medium with low absorption (no band transition) and low loss (no free electron oscillation), and solar energy absorbs relatively low.

The implementation effect of this program. Using the time domain finite element difference method to calculate the ultraviolet and visible-infrared spectral solar spectral reflection and transmission spectra of the structure designed in this scheme are shown in Figure 5 and Figure 6: In Figure 5, (a) is the ultraviolet reflection photon The reflectivity of the crystal, (b) is the transmittance of the ultraviolet reflective photonic crystal; the reflectivity is >75% (200nm-400nm), >90% (250nm-400nm) and the transmittance is <3% (250nm-375nm); The sum of reflectance and transmittance in the ultraviolet region is about 1, and its absorption to ultraviolet is negligible. In Figure 6, (a) is the visible-infrared (400nm-2000nm) reflectance, (b) is the visible-infrared (400nm-2000nm) absorptivity, and the visible-infrared reflectance > 85% (400nm-2000nm), Absorptivity < 10%; transmittance almost zero.

Figure 7 shows the reflectance and absorptivity of different metals in the ultraviolet, visible and infrared. The solid line in the figure is the reflectance, and the dotted line is the absorptivity. When the wavelength of Ag with strong visible-infrared reflectivity is less than 400nm, the reflectivity suddenly increases. drop, and metal Ag has strong absorption in visible light, and the absorption rate is greater than 70%. The metal Al has the characteristics of broad spectral reflection in the ultraviolet-visible-infrared, but its reflection performance is significantly lower than that of the metal Ag, and its reflection ability is poor, and the metal Al also has obvious absorption in the infrared. However, metals such as Cu and Pt have poor reflectivity and strong absorption in the solar spectrum, and the absorption is >80% in a wide spectrum range. For this reason, the inherent physical properties of metal thin films make the current OSR have the shortcomings of poor solar spectrum reflectivity, narrow reflection frequency band and large absorption ratio. To this end, using a new method and new principle to design a new type of solar reflective layer to replace the metal reflective layer of the OSR is the key to solving this technical problem. In addition to metal, the current new type of solar reflective film also uses indium tin oxide layered reflective film. The reflection frequency band of this structure can be freely controlled through structural design, and it has high reflection performance, and the reflection rate can reach more than 90%. However, its shortcomings are mainly due to its narrow reflection frequency band, which makes it difficult to achieve the purpose of high reflection of the full spectrum of the solar spectrum. In addition, the frequency domain superposition of photonic crystals and disordered photonic crystals are used to expand the reflection frequency band. Compared with a single reflective material, the photonic crystal-based mirror has the advantages of designing its reflection frequency band, its reflection frequency band and superposition. This scheme designs a new type of OSR structure constructed by surface layer, ultraviolet reflection type and visible-infrared reflection type photonic crystal. The OSR thermal control coating solar full-spectrum high reflectance and low absorption is achieved.

Only the preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the above-mentioned embodiments, and within the scope of knowledge possessed by those of ordinary skill in the art, various aspects can also be made without departing from the purpose of the present invention. Various changes should be included within the protection scope of the present invention.

Claims (5)

1. an OSR thermal control coating based on photonic crystal, is characterized in that: be made up of emission layer, ultraviolet light reflection layer, visible infrared light reflection layer; The emission layer is made of materials with an infrared emissivity > 0.8, which can transmit more than 90% of ultraviolet light, visible light and 200nm-2000nm infrared light, has a high transmission rate, is resistant to high temperature, and has a very small thermal expansion coefficient; The emission layer adopts any one of quartz glass and cerium glass; The ultraviolet light reflective layer adopts a dielectric photonic crystal composed of alternating dielectric films A and B, and the alternating period is 3-7. The dielectric film A and dielectric film B are non-metallic materials, and the ultraviolet light reflective layer The forbidden band is at 200nm-400nm; The visible-infrared light reflective layer is composed of medium C and medium D alternately, and the alternating period is 4.5-7.5. The medium C and medium D are metal photonic crystal films, and the visible-infrared light reflective layer is in visible light and 400nm-2000nm infrared light. The equivalent dielectric constant of light is less than 0; The dielectric film A adopts a non-metallic dielectric material whose photon absorption wavelength is outside the frequency band of the solar spectrum, that is, less than 200 nm and greater than 2000 nm, or a non-metallic dielectric material with low absorption and high pass through visible light and infrared light; Materials with high loss and high dielectric constant; The dielectric film A adopts any one of Al ₂ O ₃ , BaF ₂ , KBr, SiO ₂ , SiC, MgF ₂ and TiO ₂ ; the dielectric film B adopts any one of Si and Ge; The medium C is a dielectric with a high dielectric constant, and the real part of the dielectric constant is greater than 1.5; the medium D is a metal material with high reflectivity, and the real part of the dielectric constant in visible light and infrared light is less than zero; Described medium D adopts any in Al and Ag; The medium C is any one of Al ₂ O ₃ , SiO ₂ , TiO ₂ and ITO. 2 . The photonic crystal-based OSR thermal control coating according to claim 1 , wherein the thickness of the emission layer is 0.1 mm-0.2 mm. 3 . 3 . The photonic crystal-based OSR thermal control coating according to claim 1 , wherein the dielectric constant of the dielectric film B is at least 1.5 higher than the dielectric constant of the dielectric film A. 4 . 4. a kind of OSR thermal control coating based on photonic crystal according to claim 1, is characterized in that: described ultraviolet light reflection layer dielectric film A adopts Al ₂ O ₃ film, film thickness d _A =80nm-120nm ; The dielectric film B is a Si film, and the film thickness is d _B =5nm-10nm; the alternating period is set to 4. 5. A photonic crystal-based OSR thermal control coating according to claim 1, characterized in that: said medium C adopts ITO film; said medium D adopts Ag film, and the film thickness of metal Ag is greater than 100 nm; The thickness ratio of the Ag film and the ITO film is greater than 1.5.

Images