Silica Glass from Aerogels

by Michel Prassas

Aerogel evolution during sintering

Chemistry Let's take for the simplicity of the purpose  the example of a single component glass SiO2 Quartz the major natural crystalline form of SiO2 is a periodic structure of  tetrahedra where  Si occupy the center and oxygens are on the apex . A regular network in a plane representation is given schematically below

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Figure 1: Ordered SiO₂ structure

Silica glass in the other hand is made from the same elemental tetrahedron except that there is no apparent regularity  in the construction of this network as shown below.

Figure 2: Random network of SiO₂

One needs to go up to more than 1800°C in order to melt the quartz crystal and transform one structure to another. In addition -due to the high viscosity of silica- successive melting is required before achieving optical quality. Glass chemists overcame the difficulty by building the amorphous silica network at room temperature using liquid silicon precursors known as Silicon alkoxides. The general formula of this molecular precursors is Si (OR)₄ . 
Alcoxy groups (OR where R= C_nH _2n+1) are readily hydrolyzed by water (especially those with n<2)  and progressively replaced by OH groups. Silanols (Si-OH) can further react between them or /and with non hydrolyzed  alcoxy groups to form a siloxane bond (Si-O-Si ), which is the beginning of the silica network formation. These reactions are schematically represented below.

POLYCONDENSATION

An amorphous 3D silica network is those progressively formed by chemical reactions at room temperature. The viscosity of the solution is continuously increasing up to the point were the entire solution is gelified. 
The resulted gel (alcogel) is  constituted by a coherent continuous  solid silica network impregnated by a liquid phase representing the  solvent and the reaction by-products.

Although simple in its illustration the system is rather complex. A  number of parameters can change the reactivity scheme and  ultimately  the properties of the final product. 
Among them

	pH,
	Water content,
	Concentration,
	Temperature,
	Drying conditions

Hopefully all these  parameters, give additional degree of freedom to tailor specific properties such as pore volume, pores size, specific surface area etc.