Sintering viscous

Conventional Sintering. Ceramic sintering is usually accompHshed by heating a powder compact to ca two-thirds of its melting temperature at ambient pressure and hoi ding for a given time. Densification can occur by soHd-state, Hquid-phase, or viscous sintering mechanisms. 

P. Philip, L. Bonakdar, P. Poulin, J. Bibette, andF. Leal-Calderon Viscous Sintering Phenomena in Liquid-Liquid Dispersions. Phys. Rev. Lett. 84, 2018 (2000). 

G.W. Scherer Viscous Sintering in Inorganic Gels. Colloid Surface Sci. 14, 265 (1987). 

F. Placin, M. Feder, and F. Leal-Calderon Viscous Sintering Phenomena in Liquid-Liquid Dispersions Application to the Preparation of Sdicone Macroporous Aerogels. J. Phys. Chem. B 107, 9179 (2003). 

Since both aerogels and xerogels have high surface areas and small pore diameters they are used as ultrafiltration media, antireflective coatings, and catalysts supports. Final densi-fication is carried out by viscous sintering. 

Modern Manufacturing Techniques. Manufacturing techniques for making bulk vitreous silica are for the most part improved variations of the historical processes. The main exception is the sol—gel process (see Sol-gel technology). All processes involve the fusion or viscous sintering of silica particles. The particles can be in the form of a loose powder or a porous preform. The powders can be made from natural quartz or from the decomposition of chemical precursors, such as silicon tetrachloride, and tetraethylorthosilicate (1 EOS). In some approaches, such as flame hydrolysis, the powder is produced and fused in a single step. The improvements made to these techniques deal mainly with the procedures used to prepare the powders, that is, to control purity and particle size, and the specific conditions under which the powders are consolidated. 

The model of viscous sintering was developed by Frenkel (24), who derived the following expression for the rate of coalescence of spherical adjacent particles ... 

Viscous sintering is a process of densification driven by interfacial energy (81). Material moves by viscous flow in such a way as to eliminate porosity and thereby reduce the solid-vapor interfacial area. The rate of viscous sintering is proportional to the surface area and inversely propor-... 

Figure 13. Sintering parameter (1/K) and viscosity according to Scherer model (85) of viscous sintering for two-step acid-catalyzed xerogel. Samples were heated to indicated temperatures at 2 or 20 °C/min and held isothermally. Corresponding bulk densities are plotted on the abscissa. (Reproduced with permission from reference 79. Copyright 1984.)...

The low temperatures and correspondingly high viscosities involved in viscous sintering of gels (normally near the glass transition temperature Tg,... 

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