A 27Al, 29Si, 25Mg and 17O NMR investigation of alumina and silica Zener pinned, sol-gel prepared nanocrystalline ZrO2 and MgO

Abstract

Alumina and silica Zener pinning particles in sol-gel prepared nanocrystalline ZrO₂ and MgO have been characterised using ²⁷Al and ²⁹Si MAS NMR after annealing at various temperatures up to 1200 °C. The structures of the pinning phases were found to differ not just between the two metal oxide systems but also depending on the exact method of manufacture. Three distinct transitional alumina phases have been observed in different alumina-pinned samples annealed at 1200 °C, one in particular identified by a peak at a shift of 95 ppm in the ²⁷Al NMR spectrum. Both the alumina and silica pinning phases reacted with the MgO nanocrystals, forming spinel in the case of alumina, and enstatite and forsterite in the case of silica. Despite reacting readily with the MgO, the silica pinning particles were effective at restricting grain growth, with 11 nm MgO nanocrystals remaining after annealing at 1000 °C.