Solid state sintering mechanisms

Liquid-phase sintering is significantly more complex tlian solid-state sintering in tliat tliere are more phases, interfaces, and material transport mechanisms to consider. In general, densification will occur as long as it is... 

In sintering, the green compact is placed on a wide-mesh belt and slowly moves through a controlled atmosphere furnace (Fig. 3). The parts are heated to below the melting point of the base metal, held at the sintering temperature, and cooled. Basically a solid-state process, sintering transforms mechanical bonds, ie, contact points, between the powder particles in the compact into metallurgical bonds which provide the primary functional properties of the part. 

There is a qualitative distinction between these two types of mass transfer. In the case of vapour phase transport, matter is subtracted from the exposed faces of the particles via dre gas phase at a rate determined by the vapour pressure of the solid, and deposited in the necks. In solid state sintering atoms are removed from the surface and the interior of the particles via the various diffusion vacancy-exchange mechanisms, and the centre-to-cenU e distance of two particles undergoing sintering decreases with time. 

Deactivation is a common and important phenomenon in FTS. Deactivation effects of water are recorded on all commonly used supports. The suggested mechanisms include oxidation, sintering and solid state reactions rendering cobalt inactive. 

The kinetic transport mechanisms that permit sintering are solid-state processes, and therefore sintering is an important forming process that does not require melting. Materials with high melting temperatures, such as ceramics, can be molded into a complex shape from a powder and subsequently sintered into a solid body.13... 

The value of E0 is probably a little high if coking were the only mechanism of deactivation, thus it probably reflects sintering or a solid state transformation (with water vapor ) as being an additional important factor. 

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. 

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