Preparation of monolithic silica aerogels, the aerogels thus obtained and their use for the preparation of silica glass articles and of heat-insulating materials

In a process for the preparation of a silica aerogel which is monolithic and free of fissures and cracks and has at least one of its dimensions greater than 10 cm, in which process water and a non-aqueous solvent are added to a hydrolyzable silicon compound, the amount of water being sufficient to hydrolyze and polycondense the said compound to form a gel, the silicon compound being present in an amount of at least 40% by volume, relative to the total of the silicon compound plus the non-aqueous solvent, the whole, placed in a container of the desired shape, is subjected, in an autoclave containing an additional amount of the non-aqueous solvent used to form the solution, to temperature and pressure conditions which are such that the critical point of the solvent of the solution remaining after gelling is exceeded, without the liquid/vapor equilibrium curve of the solvent being broken at any time, and the solvent and any residual water present are then removed in order to obtain a silica aerogel,

• the improvement wherein:
• (a) the water and non-aqueous solvent are added to the hydrolyzable silicon compound in such amounts that the resultant solution is homogeneous;
• (b) the autoclave treatment is carried out without the prior removal of water from the gel;
• (c) the autoclave is heated at a rate of at least 50.degree. C./hour to a temperature above the critical temperature, and once the desired maximum temperature has been reached, the solvent is removed under substantially isothermal conditions, over a period of at most 5 hours;
• and
• (d) after the autoclave has returned to atmospheric pressure, the residual solvent is purged from the autoclave, the heating of the autoclave is stopped and the aerogel is withdrawn from the autoclave after cooling.