Effect of Yttrium and Lanthanum on the Final‐Stage Sintering Behavior of Ultrahigh‐Purity Alumina

Final‐stage sintering has been investigated in ultrahigh‐purity Al 2 O 3 and Al 2 O 3 that has been doped individually with 1000 ppm of yttrium and 1000 ppm of lanthanum. In the undoped and doped materials, the dominant densification mechanism is consistent with grain‐boundary diffusion. Doping with yttrium and lanthanum decreases the densification rate by a factor of ˜11 and 21, respectively. It is postulated that these large rare‐earth cations, which segregate strongly to the grain boundaries in Al 2 O 3 , block the diffusion of ions along grain boundaries, leading to reduced grain‐boundary diffusivity and decreased densification rate. In addition, doping with yttrium and lanthanum decreases grain growth during sintering. In the undoped Al 2 O 3 , surface‐diffusion‐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 trajectory 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 Al 2 O 3 grain boundaries.