The monosized, spherical powder (MP) was prepared by homogeneous precipitation in aquecus solutions through reaction with the thermal decomposition products of urea; a complete description of the process is described elsewhere.' The precipitate was calcined to oxide at 750°C for 24 h and then passed through a 200-mesh nylon screen. The raw powder (RP) used to prepare the MP powder was also used as the comparison in the present study. The RP material was purchased and was used as received.' Figure 1 shows the morphology of the two powders. The MP powder is very uniform with a mean size of 0.4 pm, while the RP powder is much larger with a broad size distribution between 0.4 and 12.0 pm and a mean particle diameter of 4.2 pm.'