Surface diffusion in the initial sintering of alumina.

In the previous paper1), a kinetic equation for the initial combined sintering of surface and volume diffusions was developed. The result is shown in Eq. (1), (ƒ¢L/L0)712/(t-ƒÂt)=K(ƒ¢L/L0)+H•c•c(1) and the second term of the right side of Eq. (1) gives the surface diffusion coefficient Ds as shown in Eq. (2). Ds=(4kTm7/7ƒÂƒÁƒ¶)Ha4•c•c(2) Eq. (2) shows that the surface diffusion coefficient strongly depends on particles size. In the previous work no effort, however, was made for obtaining powder with narrow size distribution. The intention of the present work is to supplement the previous work and to show experimentally the effect of particle size distribution on the value of Ds. High purity ƒÁ-Al2O3 (99.99%) for the synthesis of a single crystal purchased from Shinkosha was fired at 1200•Ž for 50h and completely converted to ƒ¿-Al2O3 of nearly spherical form. The resulting particle was repeatedly elutriated in water containing 4vol% of NH4OH. The sample obtained had the narrow distribution of 0.2•`0.4ƒÊ in diameter, while the previous one the wide distribution giving the average size of 0.2ƒÊ The powder was hand pressed into a compact (10mm in diameter and about 4mm thick) at 1t/cm2 without binder. The