EFFECT OF YTTRIUM AND LANTHANUM ON THE FINAL-STAGE SINTERING BEHAVIOROF ULTRAHIGH-PURITY ALUMINA

Final-stage sintering has been investigated in ultrahigh-purity Al2O3 and Al2O3 that has been doped individually with 1000 ppm of yttrium an d 1000 ppm of lanthanum, In the undoped and doped materials, the domin ant densification mechanism is consistent with grain-boundary diffusio n, Doping with yttrium and lanthanum decreases the densification rate by a factor of similar to 11 and 21, respectively. It is postulated th at these large rare-earth cations, which segregate strongly to the gra in boundaries in Al2O3, block the diffusion of ions along grain bounda ries, leading to reduced grain-boundary diffusivity and decreased dens ification rate. In addition, doping with yttrium and lanthanum decreas es grain growth during sintering. In the undoped Al2O3, surface-diffus ion-controlled pore drag governs grain growth; in the doped materials, no grain-growth mechanism could be unambiguously identified, Overall, yttrium and lanthanum decreases the coarsening rate, relative to the densification rate, and, hence, shifted the grain-size-density traject ory to higher density for a given grain size. It is believed that the effect of the additives is linked strongly to their segregation to the Al2O3 grain boundaries.