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 (H2O) as the hydrolysis reactant, and ammonium hydroxide solution (NH4OH, 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/p0=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.