Sintering of NaCl powder: Mechanisms and first stage kinetics
Isothermal NaCl sintering experiments are conducted between 500 and 790 degrees C on powders 75, 100, 150 and 400 mu m in average diameter. Along with literature data, the results are compared with theoretical predictions for the initial stage of sintering, to draw a general picture of the process over a wide spectrum of temperatures and particle sizes. Where densification is observed, the mechanism is grain boundary diffusion of material from boundary sources. As observed in previous work, there is a transition in dominance between this mechanism and the alternative, non-densifying, mechanism of evaporation-condensation. More detailed analysis shows that, after initial and effectively instantaneous plastic yielding driven by the surface energy, boundary diffusion will always dominate the initial stage of sintering for the conditions of this investigation, leading to some (possibly very small) degree of densification. For particles above about 150 mu m at temperatures within 300 degrees C of the melting point of NaCl this mechanism will rapidly be displaced by evaporation-condensation, halting densification. An approximate equation is derived for the rate of sintering by evaporation-condensation in the presence of a finite-pressure inert atmosphere; the rate of this mechanism is found to be significantly reduced when sintering is carried out at pressures approaching atmospheric.