Liquid-Phase Sintering of Nanocrystalline Titania Doped with Boron Oxide: Bulk Versus Thin Film

Boron oxide with a melting point of 450°C was used as an additive to study liquid‐phase sintering of nanocrystalline titania, both in bulk as well as in thin films. The liquid phase has two salient effects. (I) It leads to enhanced transformation of anatase to rutile in the bulk material, accompanied by strong grain growth, and thus to a reduction in the densification rate with the anatase. (II) In the rutile, the liquid phase leads to a strongly increased densification rate in the bulk, but a reduced densification rate in the thin film. The latter is suggested to be caused by excellent wetting and low grain‐boundary energies, leading to extended grain‐boundary grooving and very rough surfaces of the thin film. The transition of boron oxide, once enhancing and once reducing the densification rate, is suggested to be related to the ratio of grain size to film thickness.