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

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.