Dielectric Response Behavior And Photothermal Performance Regulation Mechanism Of Inorganic Thermal Control Coatings

Radiation thermal control materials refer to a class of functional materials that can selectively absorb the incident electromagnetic in the range of visible to mid-infrared wavelength and draw heat from surface via emitting thermal infrared ray.Inorganic dielectrics are attractive candidates in the field of radiative thermal control due to their high temperature resistance,friction resistance,aging resistance,and phonon-polarized vibration fundamental frequency of thermal infrared response.However,there was no clear mathematical and physical relationship between the phonon-electron dielectric response behavior,extinction absorption characteristics,and spectral emissivity in dielectrics.In addition to promoting infrared absorption/radiation,the polar response of the dielectric is also prone to strong phonon polarization resonance,resulting in impedance mismatch with the air interface and reduced emissivity.Besides,the research on radiation thermal control often only focuses on the photothermal conversion process of materials,while ignoring the internal conduction of heat,which is extremely unfavorable for high-temperature thermal protection of high-speed aircraft.In this thesis,from the perspective of macro-and micro-dielectric response behavior and mechanism of inorganic thermal control elements,the design criteria for inorganic thermal control coating materials are proposed through first-principles calculations and numerical analysis methods of photoelectric field transmission.Through the coating structure design and atomic intrinsic doping,the problem of strong reflection and emissivity drop caused by strong phonon polarization resonance is solved,and the theory and the coupling regulation of thermal radiation and thermal conductivity are explored.Thereby,the application of dielectric materials in the fields of radiation thermal control and aircraft high temperature thermal protection is developed.Based on the theory of radiation energy transfer,the spectral radiation characteristics of the thermal control coating material were optimized to achieve the maximum radiation heat dissipation efficiency.It is revealed that the broad-spectrum high emissivity in the range of 3-20μm has higher radiative heat dissipation efficiency than the narrow-band high emissivity in the atmospheric window（8-13μm）.Based on the macro and micro vibration response model,the complex refractive index criterion for the design of inorganic thermal control coating materials is proposed:when the extinction coefficient（k）and refractive index（n）of the material element are in the range of 0.01<k<1,0.5<n<2,it can meet the needs of high emissivity thermal control coating materials.Through the combination of first-principles calculations and experiments,the physical mechanism of the influence of the phonon and electronic structure of the inorganic dielectric thermal control material element（such as Ti O₂,Si O₂,Al₂O₃,h-BN and Y₃Nb O₇,etc.）on its dielectric response and emissivity is revealed,and the accuracy of design criteria of the inorganic thermal control coating material is verified.Based on the dielectric response characteristics of the above-mentioned thermal control materials and the design criteria of inorganic thermal control coating materials,a series of thermal control coatings are designed and fabricated according to different application requirements.From the perspective of coating micro-nano structure design,adjust the selective photothermal properties of the coating,improve the emissivity of the thermal control coating,and explore the coupling method and mechanism of high emissivity and low thermal conductivity.To solve the heat dissipation problem of power-intensive energy components of titanium and titanium-based alloys,a high emissivity Ti O₂ coating is designed and prepared on the surface of titanium alloy.A micro-patterned surface was designed to address the emissivity drop of Ti O₂ at 12-20μm,due to the polar vibration of Ti-O and the resultant excessive extinction coefficient（k>1）.The results show that when the diameter of the micropore is～6μm,the coating can localize the incident electric field in the micropores to fully excite the extinction absorption effect of surface phonon polaritons,thereby improving the emissivity to 0.85 across the whole thermal infrared wavelength（3-20μm）.In view of the heat dissipation and light transmission requirements of lighting structures such as solar cells,a Si O₂-TPX particle system coating was designed and prepared.According to the strong phonon polarization response of Si O₂ at 9.45μm and the resultant excessive extinction coefficient（k≈2.7）,we optimized both the sunlight transmission and infrared thermal irradiation by modeling the size-dependent scattering and absorption of light by Si O₂ spheres embedded in a TPX polymer matrix through a simple solution method.Experimental results show that the use of nanospheres（20nm）enabled the nanocomposite coating with both high sunlight transmittance（>90%）and infrared emissivity（～0.85）.Thus,the coating can guarantee the power conversion efficiency of the cell and reduce the cell temperature up to～5°C.Besides,according to the thermal control application requirements of aerospace satellites and deep space probes,a h-BN/Al₂O₃ double-layer coating was designed and fabricated.The selective spectral radiation and thermal conductivity characteristics of the coating are optimized by adjusting the thickness ratio of h-BN and Al₂O₃layers.When the thicknesses of the outer h-BN and inner Al₂O₃ layers are both 20μm,Al₂O₃ can compensate for the decrease in emissivity caused by h-BN’s large extinction coefficient（k≈3）at～7μm,and improve the thermal infrared emissivity of the coating greater than 0.86,the solar reflectance is 89%;Meanwhile,the coating exhibits anisotropic thermal conductivity with high thermal resistance in the longitudinal direction and high thermal conductivity in the lateral direction.Based on the atomic scale of phonon polaritons and thermal phonon conduction in inorganic dielectric materials,the thermal radiation and thermal conductivity properties of coating materials are regulated by intrinsic doping of atoms.From the perspective of chemical bond matching,a well-designed-Si-O-Al-O-P-O-multicomponent network structure allows the positive charge of the phosphorus oxygen tetrahedron to balance the negatively charged aluminum oxide tetrahedron,thereby reducing the polarization of the system and settling the vibration intensity in a suitable range（0.2<k<1）.Meanwhile,the multimode vibrations of Si-O,Al-O,and P-O bonds would prompt a broadband infrared absorption/emission with a high average hemispherical emissivity>0.95 and reflects nearly 90%of solar irradiance.Furthermore,through lattice atom doping,the high temperature resistant（up to1600℃）and low thermal conductivity Y₃Nb O₇ coating material with great application potential for thermal protection of high-speed aircraft is modified and designed.Ca²⁺and Cr³⁺ions are doped into the lattice of Y₃Nb O₇ to introduce impurity energy levels,lattice distortion and multi-mode vibration,improve the extinction coefficient（k）to satisfy the design criteria（0.01<k<1）,and increase the visible and thermal infrared band emissivity to 0.9.This breaking through the limitation that the polar resonance wavelengths of Y-O and Nb-O in undoped Y₃Nb O₇is greater than 14μm and does not work for high-temperature radiation（when the temperature is great than 1000K,the effective thermal radiation wavelength was concentrating on the 0.5-14μm）.Besides,the thermal conductivity of the material is as low as 1.7W/（m·K）,and has good high temperature performance stability.