Principles of Solid State Sintering

Our approach to the firing stage will be to consider the physical principles of solid state sintering and then to consider how the sintering process is influenced by the key process variables, including the application of an external pressure and the presence of a liquid phase. 

For sintering to proceed, two conditions must be met. First, there must be a means or mechanism for materials transport that will allow material to flow to fill pores and create bonds between particles. Second, there must exist a source of energy to drive or activate that materials transport. There are several different forms of sintering, each with its own combination of conditions, as shown in Table 7.6. We examine solid-state sintering in depth as one of the most general methods, but similar principles can be seen to apply to all sintering methods. 

In contrast to solid-state sintering, the analysis of viscous sintering appears relatively simple in principle. Matter transport occurs by a viscous flow mecha-... 

Sintering may take place in the solid state or by the effect of a viscous liquid. Although sintering of multicomponent systems may involve concomitant chemical changes, it is in principle a physical process which is not directly related to chemical reactions. 

In the field of high temperature thermal insulation advantage is taken of the special powder properties and the amorphous character of fumed silicas. In principle, mixtures of fumed silicas, an opacifier to reflect the heat radiation and a small quantity of mineral fibers for reinforcing are used for temperatures up to 1000°C. It is necessary to density the mixture to approximately 200 g/1 to obtain the minimum of the superposition of gas and the solid state thermal conduction. Important is also the absence of mineralizing ions, like sodium or potassium, to prevent sintering effects. Commercial thermal insulation systems based on this principle have thermal conductivities of approximately 25 mW/mK at a mean temperature of 200°C and of approximately 30 mW/mK at a mean temperature of 4(X)°C. Therefore, they are... 

A two-step solid-state transformation by sintering powders has been described by Liipke and co-workers (Ltipke, 1998) and its principles are also apphcable to other PHA polymers than PHB. The anthors estimated that the rrltirrrate mechartical properties depend substantially on the initial molecular weight of polymer as well as on the processing temperature. A slight decrease of crystallirrity was also reported. 

Diffusion rates may, in principle, also be determined from any property or reaction which depends on atomic mobility. By way of illustration, ionic conductivity of the anion is directly proportional to the anion diffusion coefficient (see Electrical conductivity). From high temperature solid state reactions, sintering, oxidation of metals etc. diffusion coefficients may be evaluated provided the detailed mechanism of the processes are known. Examples of this will be given in Chapter 7. 

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