Correlation between surface area reduction and ultrasonic velocity in sintered zinc oxide powders

Specific surface area and longitudinal wave ultrasonic velocity were measured in zinc oxide (ZnO) powder compacts partially sintered to different pore fractions (P). A linear correlation was observed between the reduction in surface area and the increase in ultrasonic velocity at all P up to the onset of pore closure. During the initial stage of sintering, the ultrasonic velocity increased by as much as 300%, with negligible reduction in P, whereas during the intermediate stage, the surface area reduction was accompanied by increasing ultrasonic velocity and decreasing P. During the final stage of sintering, the pores closed and became isolated. The linear relation between the reduction in surface area and the increase in longitudinal wave velocity encompassed the entire range of porosity from completely unsintered to the onset of pore closure, thus spanning the transition from initial- to intermediate-stage sintering. Ultrasonically inferred surface areas were used to evaluate the kinetics of the initial stage of sintering, and the results compared favorably with previously published values.