Phase Physical Sintering

H.E. Exner. Principles of single phase sintering. Reviews on Powder Metallurgy and Physical Ceramics, 1 1-237, 1979. 

Figure 3.5 shows the effect of calcination on the physical properties of HyCOM TiOz s. The crystallite size of anatase and the BET surface area of as-prepared sample were 11 nm and 140 m2g-1, respectively. Upon elevating the calcination temperature, the crystallite size was increased and the surface area was decreased, reflecting crystal growth and sintering of the anatase crystallites upon calcination. It should be noted that even after calcination at 973 K the sample remained in the anatase phase and had a large surface area of 34 m2g-1. The factor of adsorptivity, [Ag+]ads, was also reduced by the calcination (Fig. 3.6) and almost proportional to the BET surface area (Fig. 3.7). This shows that the density (ca-...

Sintering is an important mode of deactivation in supported metals. The high surface area support (carrier or substrate) in these catalysts serves several functions (l) to increase the dispersion and utilization of the catalytic metal phase, (2) to physically separate metal crystallites and to bind them to its surface, thereby enhancing their thermal stability towards agglomeration, and (3) in some cases to modify the catalytic properties of the metal and/or provide separate catalytic functions. The second function is key to the prevention or inhibition of thermal degradation of the catalytically active metal phase. 

An alternative to stochastic reconstruction of multiphase media is the reconstruction based on the direct simulation of processes by which the medium is physically formed, e.g., phase separation or agglomeration and sintering of particles to form a porous matrix. An advantage of this approach is that apart from generating a medium for the purpose of further computational experiments, the reconstruction procedure also yields information about the sequence of transformation steps and the processing conditions required in order to form the medium physically. It is thereby ensured that only physically realizable structures are generated, which is not necessarily the case when a stochastic reconstruction method such as simulated annealing is employed. 

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. 

In most cases such binary phases are not of importance for the production of alloys, because they exhibit completely different physical properties in comparison to the materials of the solid solution regions (higher hardness, brittleness, etc.) which are in use as alloys. A typical example are a-phase inclusions in powder metallurgically produced W-lORe alloy, which lead to a pronounced embrittlement They are die consequence of improper mixing of the two components prior to the sintering process. Re-enriched areas cannot be completely equalized by difiusion during sintering. 

---