基于三元相变控制的二氧化硅气凝胶制备

Preparation of Silica Aerogels Based on Ternary Phase Transformation Control

二氧化硅气凝胶具有极高的透光率,在辐射探测领域中可用作Cherenkov辐射探测器的关键组成部分。然而,二氧化硅气凝胶在10 mm厚度要求下,在550 nm波长处实现透光率大于80%仍然是一个颇具挑战性的问题。首先采用乙醇滴定实验构建了三元溶液互溶性相图,通过在不同相区域内选取合理参数,实现了从透明溶胶到透明气凝胶的制备。进一步研究了高透明二氧化硅气凝胶密度、微观结构及其透光率三者间的影响关系,结果表明二氧化硅气凝胶的粒径和孔径会影响气凝胶透光率。综上所述,采用溶胶–凝胶法,优化选取互溶区的相点参数成功制备了厚度为10 mm,在550 nm处透光率为80.3%的高透明二氧化硅气凝胶,满足了Cherenkov辐射探测器对气凝胶辐射体光学参数的基本要求。

Introduction Silica aerogel is a novel nanostructured material,and the exceptional transparency makes it invaluable for applications in photothermal conversion,energy storage,building insulation,solar collector,and advanced optical devices.Silica aerogels could serve as the key component of devices in field of radiation detection due to their exceptionally high transmittance.Although numerous studies have addressed the transparency of silica aerogels,there is a notable gap in research for regarding the relationship between the transmittance and aerogel thickness.Consequently,it still remains a challenging problem for effectively maintaining high aerogel transmittance(approaching 80%at 550 nm)at a specific thickness(approaching 10 mm).In this work,the titration experiments were first concluded to identify optimal synthesis parameters,which was beneficial to investigate the phase demarcation points in the ternary phase diagram.Subsequently,according to the optimal parameters,highly transparent silica aerogels were prepared by using the sol-gel method.Additionally,a comprehensive examination of silica aerogels,including density,microstructure and transmittance,was conducted in detail.Methods The synthesis of silica aerogels was conducted through the sol-gel method.The following materials were employed:Tetraethoxysilane(TEOS;98%)as the silica precursor,ethanol(EtOH)as the solvent,water(H_(2)O)as the hydrolysis reactant,and ammonium hydroxide solution(NH_(4)OH,25%)as the catalyst,respectively.The alcogels were dried by ethanol supercritical drying to obtain transparent silica aerogels.The morphology and microstructure of silica aerogels were observed by a Zeiss EVO-50 XVP scanning electron microscope(SEM).The UV-Vis spectrophotometer was used to test the aerogel transmittance in the wavelength range of 500–800 nm.The specific surface area of aerogels was calculated by Bruno-Emmett-Teller(BET)method.Under the relative pressure p/p_(0)=0.99,the total pore volume was calculated by nitrogen adsorption-desorption isotherm.The related pore size distribution was obtained by Barret-Joyner-Halenda(BJH)method.Results and discussion The solvent titration experiment data was normalized and subsequently used to construct the ternary phase diagram.The ternary phase diagram clearly demonstrated the phase transition boundary,single-phase region and multi-phase region,respectively for three solvents at room temperature.Thus,the ternary phase diagram could be employed to predict whether any mixture of three solvents results in a transparent solution,regardless of its proportions.The silica aerogels were prepared by using the parameters of ternary phase diagram.Specifically,it was seen that the higher initial transmittance the sols had,the similarly higher transmittance the aerogels also had.Moreover,the related transmittance of the sol-gel-aerogel system in the single-phase region demonstrated the highest value(~80.3%),which was better than that(~74%)on the phase transition boundary.The silica aerogel with the higher transmittance could be prepared in the single-phase region,and it provided a new idea for further study around the single-phase region of ternary phase diagram.In order to manipulate the phase boundary point for accessing the single-phase transparent interval,the ethanol volume fraction in the mixture increased along the extension of the selected phase boundary point.It illustrated that the aerogel transmittance first rose with the increase of ethanol volume fraction.However,it decreased with the excessive ethanol.With an increase of ethanol volume fraction,the average particle size of silica aerogels decreased first and then increased.Moreover,the relationship between average particle size and transmittance were further established.The average particle size had a linear relationship with the transmittance of silica aerogels,meaning that the smaller the average particle size was,the larger the aerogel transmittance was.Furthermore,the skeleton particle boundary size of Rayleigh scattering and Mie scattering could be calculated and the corresponding particle boundary size at a wavelength of 550 nm was calculated to be 57 nm.The aerogel transmittance at a wavelength of 550 nm was further compared with the cumulative frequency of skeleton particles whose size were the smaller than the boundary one(~57 nm).It could be concluded that the aerogel transmittance had a certain linear correlation with the cumulative frequency.Obviously,the largest cumulative frequency(~94.68%)corresponded to the highest transmittance(the transmittance of sample C3 was 79.6%),while the lowest cumulative frequency(~83.05%)corresponded to the lowest transmittance(the transmittance of sample C5 was 69.5%).Conclusions According to the ternary phase diagram,the highly transparent silica aerogels were precisely prepared by using the optimal synthesis parameters.The higher initial transmittance of the sols induced a higher transmittance of the aerogels.Furthermore,the smaller the average particle size,the larger the transmittance of silica aerogels could be,which was attributed to the larger cumulative frequency of particle size smaller than 57 nm due to the suppression of Mie scattering opaque effect.Besides,the higher pore size meant the lower aerogel transmittance,which was caused by the stronger light scattering effect.In a word,the highly transparent silica aerogel with transmittance(~80.3%at 550 nm)at a specific thickness(~10 mm)was successfully fabricated by the sol-gel method in the single-phase region,which would be satisfied with practical application requirements in Cherenkov radiation detector.