Optical Properties And Radition Stability Of Al₂O₃ Hollow Microspheres

Al₂O₃ is widely used in many fields such as microelectronics,optics,structural ma-terials,and catalytic chemistry etc due to its high melting point,high hardness,high resis-tivity and relatively wide band gap.The nanomaterial is obtained by using nanotechnology have excellent properties that are not possessed by the bulk phase materials.Therefore,the preparation of nano structured Al₂O₃ can further expand its application range.As a main additive of paints,the Al₂O₃ pigment has excellent reflection property in the ultraviolet and to near-infrared spectral range.At the same time,and silicone resins have good heat resistance and weather resistance.Thus the paints consisted by silicone resin binder and Al₂O₃ pigment have been widely used.With the development of science and technology,the performance requirements for paints have become increasingly higher,while the existing paint coatings have failed to meet the needs of product replacement and update.Therefore,developing new lightweight functional paints,of which has good opti-cal stability in extreme environments,has very important practical significance and broad application prospects.In space technology,thermal control coating is an important part of all passive ther-mal control systems in spacecraft.It is widely used on the inner and outer surfaces of spacecraft,which plays an important role to ensure the overall thermal balance of the spacecraft,regulate temperature gradients,and maintain the normal ambient temperature of devices.Under the extreme space environment factors such as the charged particles radiation from Earth's radiation band and solar ultraviolet,the optical and thermal proper-ties of thermal control coating will be continuously degraded,resulting an effect on the temperature balance level of spacecraft.Therefore,developing a new type of thermal con-trol coating which has stable performance to resist the space radiation environment has important practical significance and urgent requirements for ensuring the on-orbit service temperature regulation of long-life spacecraft.The thermal control coating materials are mostly prepared by using the metal oxide powders with excellent optical properties as pigments,and the resins?or modified resins?with high radiation resistance and good adhesion property.Under the influence of space low-energy charged particles,solar ultraviolet radiation,and other environmental factors,high concentrated radiation defects?e.g.color centers,carbonization,and precipitated metal ions,etc.?will be generated on the surface of the irradiated thermal control coating,leading to significant degradation of thermal and physical properties?e.g.solar absorption ratio and hemisphere emissivity?for thermal control coating under the interaction of space radiation environment.Both the optical and the colorimetric theories?for non-transparent materials?have shown that,the chromaticity and the optical property non-transparent ma-terials are primarily determined by the nature of the surface layer with thickness ranged in a few microns.Owing to that the traditional coating type thermal control coating materials use micro-and nanoscale solid metal oxide powder particles as the pigment component,and the space low-energy charged particles and solar ultraviolet radiation induce high con-centrated defects in the a-few-microns depth below the surface,the effective service per-formance of conventional coating type thermal control coating materials are greatly re-duced.Since radiative defects are the product of interaction between radiation environ-mental factors and materials,it may be one of effective ways to reduce the concentration of radiation-induced defects in the surface layer of thermal control coating under the in-fluence of the radiation environment by reducing the density of thermal control coating materials through using the hollow microspherical pigment and increasing the porosity to make interaction of incident particles occur below the coating.Based on this basic strategy,in this paper,the Al₂O₃ pigments which have wide application in the field of thermal con-trol and paints industry is chosen as the research object,and an Al₂O₃ hollow microsphere with micro/nano structures is synthesized using a hydrothermal method.The optical per-formance and the optical stability under low-energy proton and electron irradiation for micro/nano structured hollow microspheres are compared and investigated.The micro-scopic mechanism for Al₂O₃ micro/nano structured hollow microspheres resisting low-energy proton and electron radiation is analyzed and explained.For this study,four Al₂O₃ materials are were selected,i.e.Al₂O₃ micron powder,na-nopowder,commercial Al₂O₃ hollow microspheres?about 50?m in diameter,wall thick-ness of about 3 to 5?m?with?-phase structure,and Al₂O₃ hollow microspheres?about 5?m in diameter and about 50 nm in thickness?.The low-energy proton and electron radia-tion tests are carried out on a space complex radiation environment simulation equipment located in Key Laboratory of Space Environmental Material Behavior and Evaluation Technology of Harbin Institute of Technology.The optical properties of above four Al₂O₃pigments and coatings before and after low-energy proton and electron radiation are meas-ured and analyzed using Lambda-950 UV-Vis-NIR spectrophotometer,BET,XPS,and computer simulation software and hardware methods.The different intrinsic mechanisms of optical stability under charged particle radiation are explored and analyzed.In this paper,firstly,the "SRIM" and "Casino" software's were used to simulate and analyze the energy deposition behavior of 100 keV protons and 100 keV electrons,and the distribution rules of radiation induced defects in the Al₂O₃ solid micropowders and micro/nano structured hollow microspheres.The results of simulation calculations and analyses show that the energy deposition of 100 keV proton irradiation is mainly concen-trated in the surface layer of Al₂O₃ solid micropowder particles within 1.5?m depth,whereas for Al₂O₃ hollow microspheres with a diameter of 5?m and a wall thickness of50 nm,100 keV protons can directly penetrate 27 layers of densely packed Al₂O₃ hollow microspheres?corresponding to a penetration depth of about 150?m?,which means that the concentration of radiative defects in the Al₂O₃ hollow microsphere structure powder and/or coating will be greatly reduced under the same proton irradiation conditions.The simulation results for electron irradiation exhibit the same rules.The simulation results preliminarily showed that the use of hollow microsphere structure powder pigments is most likely an effective way to improve the space radiation resistance of coating thermal control coatings.In view of the above theoretic analysis and simulation results,in this paper,an Al₂O₃hollow microsphere with uniform size distribution?about 5?m in diameter and about 50 nm in wall thickness?has been successfully prepared with polystyrene microspheres as templates by employing a hydrothermal synthesis method.Then,the resulted Al₂O₃ hol-low microspheres are washed with deionized water for several times followed by dehy-dration and drying.The final product of Al₂O₃ hollow microspheres having an?-phase structure are obtained by heat treatment at 1300°C.Diffuse reflectance spectrum meas-urement results show that the hydrothermally synthesized Al₂O₃ hollow microspheres have excellent solar spectral diffuse reflectance in the range of 200²500 nm.The solar absorption ratio of the prepared Al₂O₃ hollow microsphere pigments reaches a very low value of 0.05.The coating,which is prepared with Al₂O₃ hollow microspheres as a pig-ment and silicone resin as binder,still has a solar absorption ratio as low as 0.11,implying that its optical performance fully meets the service requirements for a thermal control coating.For the prepared Al₂O₃ hollow microspheres,the Al₂O₃ nanopowders,and the com-mercial Al₂O₃ hollow microspheres,after the 100 keV proton irradiation,their optical property degradation mainly occurs in the near ultraviolet spectral range,while the optical degradation of the solid Al₂O₃ micron powders occurs mainly in the near ultraviolet and visible spectral range.In contrast,the optical degradation degree is much lower and the absorption peaks are more sharp for the irradiated Al₂O₃ hollow microspheres,showing very good optical stability to resist the low-energy proton.After 100 ke V electron radiation,the optical property degradation of the Al₂O₃ hollow microspheres mainly occurs in the near ultraviolet spectral range,while the optical degradation of the solid Al₂O₃ micropow-der,the nanopowders,and the commercial hollow microspheres occurs mainly in the near ultraviolet and visible spectral range.In comparison,the Al₂O₃ hollow microspheres have the lowest optical degradation and sharper absorption peaks,indicating the impressive optical stability to resist the low-energy electron radiation.The coating,which is prepared with Al₂O₃ hollow microspheres as pigments and silicone resin as binder,still maintains the optimal optical stability under the charged particles radiation conditions used in the present paper.The XPS measurement is used to conduct detailed analysis on the influence of low-energy proton and/or electron radiation on Al₂O₃ micropowder,nanopowders,self-made micro,nanostructured,hollow microspheres and commercial hollow microspheres.The XPS results show that,after 100 keV proton and electron radiation subjected to above four Al₂O₃ pigments,all the stoichiometric ratios of Al and O elements exhibit a significant shift comparing to that of the unirradiated pigments.After the 100 keV proton irradiation,the stoichiometric ratios of Al and O elements for the above four Al₂O₃ pigments reach values up to 1:8,indicating a distinct excess of O elements.The reason for that can be explained by that the proton irradiation induces formation of a large number radiation defects in the Al₂O₃ pigment,such as oxygen vacancies,interstitial aluminum,and color center structures etc.Due to the extreme strong adsorption characteristics of Al₂O₃ powder,when the proton irradiated Al₂O₃ pigment samples are exposed to the atmosphere,oxygen from the atmosphere will be quickly adsorbed by the proton radiation activated Al₂O₃pigments,and accommodates on the surface of Al₂O₃ pigments by chemical and/or phys-ical adsorption,resulting in distinct shift of chemical binding energy showing in the XPS spectrum of the irradiated Al₂O₃ pigments.The analysis is in agreement with the apparent shift in the stoichiometric ratios of the Al and O elements which are detected in the XPS spectra of the above four Al₂O₃ pigments after 100 keV proton irradiation.The XPS anal-ysis results from electron irradiated samples show the similar pattern and phenomenon as that observed in the proton irradiated samples.After the electron irradiation,the Al and O stoichiometric ratios of the above four Al₂O₃ pigments reach the same values up to 1:8,indicating a distinct radiation induced O enrichment in the surface layer.The above analysis results show that,low-energy proton and/or electron radiation has the essentially same damage mechanism for the solid Al₂O₃ micron powders,the nano powders,the prepared micro/nano structured hollow microspheres,and commercial hol-low micro spheres.The optical measurements also show that prepared micro/nano struc-tured Al₂O₃ hollow microspheres have better optical stability after low-energy proton and/or electron irradiation.The fundamental reason for that is due to the micro and nano structured Al₂O₃ hollow microspheres effectively change the range?also known as the energy deposition path?of charged particles.It not only leads to the radiation defects dis-tributing in a larger range,but also facilitates the relaxation of the radiation defects,which thereby effectively reduces the radiation defects concentration of the optical influence re-gion in surface layer of sample.This conclusion gives a theoretical support and provides a new strategy for design of new radiation resistant materials.