Consolidation to the final membrane structure by heating

The dried xerogel must be stabilised by heating to a sufficiently high temperature to obtain a microstructural, mechanical and chemical stable ceramic membrane layer. 

In this overall process two sub-processes operate which partly overlap. At relatively low temperature, usually up to about 300 00°C, the strongly hydrated amorphous gel particles are transformed to more crystalline, mainly dehydrated particles. This step is usually called calcination. In this step organic additives also have to be burned out. Usually considerable volume changes of the constituting crystallites occur. 

The particles formed, usually oxide, form a packing with contact points, the number of which per unit of volume is determined by their coordination number and particle radius (see e.g. Ref. [24]). On these contact points a physical reaction takes place which results in the formation of necks between the particles as shown in Fig. 8.12. This process is called the initial state of sintering. 

In a rough approximation this driving force between sintering of two particles is given by the gradient of the sintering pressure Pg, with Pg given by  

The pore size can be increased at the cost of a decreasing porosity by controlled heating of the membrane in the temperature range of 400-1000°C for the most common membrane materials. 

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