Thermal and Thermomechanical Structuring Processes

Sintering is a very useful method to produce open porous glass or glass ceramic bodies with a controlled porosity in the micrometre range. Sintering processes are used for the preparation of carrier material for biotechnological applications and applications in pharmaceutical industry [166, 369, 466]. Furthermore, porous glasses and glass ceramics are used as filters in chemical processing [425] and as sound absorbers [177]. Sintering processes are also used to produce glass coatings with various properties [98].

The sintering process can be assisted by sintering additives, which are usually coated onto the particles to be sintered [177]. The activated surface of the glass particles melts during the sintering process. This liquid phase coexists with the solid beads at the sintering temperature and causes a point contact between them. The capillary action exerted by the liquid phase rearranges the particles during the early stages of sintering. The interaction between the liquid and the solid results in rapid densification because of the higher diffusivity and mass transport in the melt. An open porous glass body results because the temperature–time regime is chosen such that the particle core remains solid and, therefore, guarantees the stability of the body during the sintering. This process can be controlled much easier than the pure thermal sintering. The main disadvantage of liquid-assisted sintering is the presence of the additional phase which reduces the chemical and thermal stability of the porous glass at the contact points and at the surface of glass particles.