Sintering high purity,magnesia with additions of hafnium dioxide

Conclusions The introduction of hafnium dioxide into high-purity active magnesia accelerates its sintering and with 0.1–0.3 mol % HfO₂ a ceramic with an apparent density of 99–100% of the theoretical (3.56–3.63 g/cm³) can be obtained after firing at 1300–1400°C. The optimum quantity of HfO₂ additive is close to 0.25 mol %.The calcination temperature of the mixture of magnesium hydroxide and additive obtained by precipitation from a solution of MgCl₂ and Hf(SO₄)₂ with ammonia, and the fabrication pressure do not greatly affect the final density of the ceramics.Sintering of spectrally pure MgO containing 0.25 mol. % HfO₂ begins at 950°C, then the apparent density grows rapidly with rise in firing temperature, approaching 3.40 g/cm² at 1100°C. Selective recrystallization at these low temperatures is slow and sintering is not accompanied by a substantial grain growth. At 1300°C and higher firing temperatures densification of the ceramics approaches the limit in several minutes.The mechanism of the transfer of substance during the sintering of these specimens is volume self-diffusion from the grain boundaries to the surfaces of the bridges formed between them. The energy of activation of this process 3.9 eV (6.2 × 10^–16 J) in the 1000–1300°C range] and the coefficient of self-diffusion for MgO calculated in accordance with this (6 × 10^–14 cm²/sec at 1100°C) correspond to existing data on the diffusion of magnesium into MgO.With the incorporation of 0.25 mol. % HfO₂ in less pure magnesia obtained from chemically pure MgCl₂, sintering is little different from the sintering of spectrally pure MgO, but the limiting apparent density of the ceramics in this case is somewhat lower-of the order of 99% of the theoretical.