255 ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2020, Vol. 65, No. 2, pp. 255–262. © Pleiades Publishing, Ltd., 2020. Russian Text © The Author(s), 2020, published in Zhurnal Neorganicheskoi Khimii, 2020, Vol. 65, No. 2, pp. 252–260. Is Supercritical So Critical? The Choice of Temperature to Synthesize SiO 2 Aerogels S. A. Lermontov a , A. E. Baranchikov b, c, *, N. A. Sipyagina a , A. N. Malkova a , G. P. Kopitsa d, e , Kh. E. Yorov c , O. S. Ivanova b , A. Len f , and V. K. Ivanov b a Institute of Physiologically Active Substances, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia b Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia c Moscow State University, Moscow, 119991 Russia d Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, 199034 Russia e Konstantinov Institute of Nuclear Physics, Russian Academy of Sciences, Gatchina, 188300 Russia f Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, Konkoly-Thege Miklós út 29-33, Budapest, 1121 Hungary *e-mail: a.baranchikov@yandex.ru Received September 27, 2019; revised October 5, 2019; accepted October 5, 2019 Abstract―The structure of SiO 2 -based materials obtained by hydrolysis of tetraethoxysilane and subsequent drying of SiO 2 lyogels at temperatures from 85 to 265°C has been analyzed in detail. It has been found that drying conditions have no marked effect on the specific surface area of SiO 2 , which constitutes ~1000 m 2 /g in all cases. Average pore size and specific pore volume monotonically increase with drying temperature, the contribution of large mesopores (>20 nm) into total material porosity rises at high drying temperature. Dry- ing temperature has a considerable effect on the degree and character of individual SiO 2 particles aggregation. Keywords: sol–gel synthesis, specific surface area, pore size distribution, small-angle neutron scattering, mesostructure DOI: 10.1134/S0036023620020084 INTRODUCTION Aerogels based on SiO 2 are unique materials that have high porosity (up to 99% and more), specific sur- face area (∼1000 m 2 /g), low heat and sound conduc- tivity, which provides their wide practical application as sorbents, catalysts, and structural materials for heat and sound insulation [1–6]. Since Kistler prepared in 1931 the first aerogel [7], a number of approaches was developed to synthesize aerogels and aerogel-like materials of different chemi- cal composition: metal oxides, organic polymers, car- bon, etc. [4, 8–14]. Aerogels (especially metal-oxide aerogels) are commonly obtained using sol–gel tech- nique followed by supercritical drying, which provides solvent removal from matrix with minimal material shrinkage [9]. It is generally assumed that the use of supercritical drying allows one to avoid the emergence of liquid–solid interfaces and minimize capillary forces resulting in considerable mechanical stress in the gels that causes their destruction. The methods of supercritical drying of lyogels are usually divided into low-temperature and high-tem- perature supercritical drying. The most frequently used supercritical fluid for low-temperature supercrit- ical drying is carbon dioxide (critical temperature is 31°C, critical pressure is 7.3 MPa), while for high- temperature supercritical drying lower aliphatic alco- hols such as methanol, ethanol, isopropanol (the val- ues of critical temperature are 240, 243, and 235°C, the values of critical pressure are 7.9, 6.3, and 4.7 MPa, respectively) are used. We proposed other organic compounds as supercritical fluids for the synthesis of metal oxide aerogels, including ethers and esters [15– 18] and perfluorinated alcohols [19]. The analysis of literature shows that in almost all cases the choice of temperature for supercritical drying of aerogels is dic- tated by the solvent used and the drying is performed at temperatures deliberately higher than its critical tempera- ture. Available data on the structure of aerogels prepared under subcritical conditions are extremely scarce (for example, the works by Kirkbir et al. [20, 21]). At present, a method of lyogel drying under atmo- spheric pressure (so-called ambient pressure drying), which requires no special equipment (autoclaves and pressure control systems necessary for drying at ele- vated temperature and pressure) becomes widespread [14, 22–24]. At the same time, this approach is INORGANIC MATERIALS AND NANOMATERIALS