Formation Mechanism of In Situ Intergranular CaZrO3 Phases in Sintered Magnesia Refractories

Abstract

Sintered magnesia refractories combined by in situ intergranular CaZrO3 phases were synthesized using natural MgO-containing natural minerals with CaO and SiO2 impurities and nano-sized ZrO2 additive. A homogenous distribution of intergranular CaZrO3, independent from the intergranular CaO-MgO-SiO2 phases, was formed in situ within the sintered magnesia aggregates by introducing 0.75 wt pct nano-sized ZrO2 into the magnesite. The formation mechanism of the in situ intergranular CaZrO3 phases was determined. The nano-sized ZrO2 was introduced and uniformly distributed at grain boundaries of the magnesia due to the micron-nano-sized particles composite system and wetting grinding process. Then the CaO in impurities were prior to SiO2 to react with the ZrO2 for generating CaZrO3 at the grain boundaries by increasing sintering temperature. Nevertheless, the nano-sized ZrO2 particles were encapsulated in the MgO crystallites with similar particle size decomposed from brucite and prevented from reacting with CaO impurities in magnesite. The mixing homogeneity of magnesite particles and ZrO2 particles and the direct contact between ZrO2 and CaO impurities in magnesite has a crucial effect on the formation of intergranular CaZrO3 phases. Furthermore, the intergranular CaZrO3 phases could enhance the bonding of magnesia grains and have great potential for improving the service performance of magnesia.