Sinter viscous forces

Porous Media Packed beds of granular solids are one type of the general class referred to as porous media, which include geological formations such as petroleum reservoirs and aquifers, manufactured materials such as sintered metals and porous catalysts, burning coal or char particles, and textile fabrics, to name a few. Pressure drop for incompressible flow across a porous medium has the same quahtative behavior as that given by Leva s correlation in the preceding. At low Reynolds numbers, viscous forces dominate and pressure drop is proportional to fluid viscosity and superficial velocity, and at high Reynolds numbers, pressure drop is proportional to fluid density and to the square of superficial velocity. 

Viscous forces in sinter bridges between amorphous particles. 

Viscous Forces in Sinter Bridges Between Annorphous Particles... 

Diffusion models have been developed by Coble (1970) for initial, intermediate and final stages of pressure sintering including both the applied pressure and the surface energy as driving forces. The Mackenzie-Shuttleworth model is considered suitable for Newtonian viscous materials. 

Initial Stage in Ash Sintering. The sulphate phase can initiate ash sintering by bringing the silicate particles to close contact as a result of the surface tension force. Subsequent sintering proceeds by viscous flow and the rate of sinter bond growth is proportional to the surface tension of silicate glassy phase and inversely proportional to the particle size and the viscosity. The latter... 

As discussed above, the presence of the liquid layer applies a compressive capillary force to the particles. The liquid layer evolves during the sintering process. Once the densification through the solution-precipitation mechanism starts, the thickness of the liquid layer gradually decreases with time. When the liquid capillary becomes too narrow for the liquid to flow, the solution-precipitation nearly stops. In this case, the rate of reduction in thickness of the liquid layer due to viscous flow will compete with the rate of densification. Therefore, because the flow of the liquid though the capillary is sufficiently slow, there is a thin layer of the liquid to be remained after densification, which has a thickness of 0.5-2 nm for most ceramics. 

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. 

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