Studies in Densification Dynamics

Abstract

Whereas conventional kinetics studies determine how rapid a densification rate can be under given conditions, in densification dynamics one is concerned with learning how fast it should be in each successive stage of densification, and thereafter, with controlling the density-time profile to yield an optimum densification cycle. For a given grade of starting material, densification rates attained in the intermediate ranges of densities (∼0.75–0.90 pth) are closely linked with gas entrapment and retention in the final stages: the faster the rate in this critical region, the greater the likelihood of gas retention and its attendent problems. By avoiding excessive densification rates, gas evolution along branching, connecting pores is favored, premature pore closure is minimized and grain growth is inhibited. It is postulated that for each stage of densification a critical rate exists which cannot be exceeded without jeopardy to continued densification in some subsequent stage. The existence of such a critical rate (∼1.7 × 10−3 min−1 at D≈0.94) has been demonstrated in spinel. Determinations of critical densification rates over ranges of fractional densities in alumina are now in progress; initial findings are cited as examples in discussing rate controlled sintering apparatus, research techniques, and their applications to development of rate controlled sintering regimes which have been optimized in terms of densification dynamics.