Liquid-phase sintering surface energy

The driving force for densification is the reduction in surface energy as the free surfaces of particles disappear and how this is accomplished defines the terms firing , solid state sintering and liquid phase sintering. 

If the densification of liquid-phase sintering is achieved due to the viscous flow of a liquid that is able to fill up the pore spaces between the solid grains, it is called vitrification [112-114]. The driving force for vitrification is the reduction of solid-vapor interfacial energy, because the flow of the liquid covers the surfaces of the solid. Traditional clay-based ceramics are usually densified through vitrification. However, it is very unlikely to be observed in the processing of transparent ceramics, because the content of liquid phase must be controlled to a limited level. 

In the case of S/L interfaces, two different types can be postulated. The first interface is between a solid and its own liquid phase, such as that between ice and water. This type of interface is relatively easy to handle, and provides an insight into the fundamental features of S/L interfaces. The second interface is between a solid and a liquid of different composition. In many practical situations, the S/L interfaces belonging to the latter type are particularly important Note that the liquid phase sintering is carried out clearly below the melting point of the major solid phase. Here, the initial discussions relate to some of the essential features of S/L interfaces, derivable from the former type of S/L interface. The discussion can then easily be extended to the effect of a liquid phase of a different chemistry on the stmctures and energies of crystal surfaces. 

Figure 10.2 Commonly observed micro structures of ceramics produced by liquid-phase sintering, (a) Rounded grains in a moderate amount of liquid (> 5 vol%) (b) grains with flat contact surfaces in a low volume fraction of liquid (<2-5 vol%) (c) prismatic grains dictated by anisotropic interfacial energy with a moderate to high liquid volume (>10 vol%) (d) elongated grains with flat or curved sides resulting from anisotropic interfacial energy with a low liquid content (<2-5 vol%).

In what follows surface is defined as the plane between condensed matter and a vapour phase or vacuum, such as solid/vapour and liquid/vapour interfaces. In a broader sense, the term interface is used for the dividing plane between any two different phases. The existence of an interface means, by itself, the presence of an excess interface energy over the bulk energy. Since the driving force for sintering is the reduction of the total interfacial energy of the system concerned, it will be useful to understand the thermodynamic characteristics of interfacial energy. 

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