Liquid-phase sintering pore filling

In liquid phase sintering, pore filling was found to be the essential process of densification. What is the dependence of densification on scale ... 

Lee, S.-M. and Kang, S.-J. L., Theoretical analysis of liquid phase sintering pore filling theory, Acta Mater., 46, 3191-202, 1998. 

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. 

So far, two models and theories have been developed for explaining densifi-cation during liquid phase sintering. One is the classical three-stage model and theory and the other the pore filling model and theory. ... 

The pore filling model of liquid phase sintering was developed via a series of experimental observations. " Kwon and Yoon " first observed... 

When the gas pressure in an isolated pore is different to that outside the compact, pore filling is either retarded with an excess internal pressure or accelerated with an excess external pressure. The former is due to entrapped insoluble gases and, in this case, pore filling is not complete and gas bubbles remain within liquid-filled pores. (Gas bubbles are, in fact, often observed in liquid phase sintered compacts.) Even in compacts with entrapped gases, pore filling can be critical because the possibility of formation of sharp internal notches at the pore surface during solidification of the liquid must be considerably reduced by liquid filling. 

Explain the densification and shrinkage processes in the pore filling model and theory of liquid phase sintering. What are the fundamental differences between this model and Kingery s contact flattening model ... 

Cho, S.-J., Kang, S.-J. L. and Yoon, D. N., Effect of entrapped inert gas on pore filling during liquid phase sintering, Metall. Trans. A, 17A, 2175-82, 1986. 

Figure 10.19 The change in microstructure during liquid-phase sintering of a mixture of fine (10 fxm) W powder, 2 wt% of 30 gon Ni spheres and 2 wt% 125 fxm Ni spheres, showing sequential filling of the pores. (From Ref. 41.)...

Sometimes, a part of the ceramic material melts and provides a liquid phase during sintering. This liquid phase provides a medium for faster flow of material to fill the pores because the liquid can (1) act as a flux and dissolve the solid ceramic materials of the porous ceramic green body, (2) provide a pathway for fast diffiision compared to solid state diffusion, and (3) precipitate the final ceramic material in the pore thereby filling it. As a result sintering is a complex phenomena. 

Densification results from particle rearrangement under the influence of capillary forces and the filling of pores by the liquid phase. This process is very rapid, and if during the early stages of sintering, the liquid flows and completely fills the finer pores between the particles, 100 percent densification can result almost instantaneously. 

---