Liquid-phase sintering of SmCo5

Abstract

The initial sintering kinetics of stoichiometric SmCo5 powder containing a samarium-rich sintering addition (60 wt% Sm plus 40 wt% Co) have been investigated as a function of amount of liquid phase, time, temperature, and particle size. The shrinkage as a function of time exhibits the classical three stages of liquid-phase sintering, i.e., rearrangement, solution precipitation, and solid phase. The rate-controlling step during the solution-precipitation stage corresponds to a phase-boundary (solid/liquid) reaction leading to dissolution. Evidence for this conclusion is partially based on the logarithm-shrinkage—logarithm-time slopes being equal to 1/2 instead of 1/3. A 1/3 slope is predicted by the liquid-diffusion-controlled sintering model while the phase-boundary sintering model predicts a slope of 1/2. An activation energy of 52.8 kcal/mole was obtained for the temperature dependence, and this also suggests a phase-boundary reaction rather than rate control by diffusion in the liquid phase. The rate of sintering follows an r−1 particle-size dependence instead of an r−4/3 dependence, again suggesting that the solution-precipitation stage of sintering of SmCo5 is controlled by a phase-boundary reaction leading to dissolution.