Structure, property, and processing relationships of CMZP - [(Ca_0.6,Mg_0.4)Zr₄(PO₄)₆]

Processing of (Ca_0.6, Mg_0.4)Zr₄(PO₄)₆ [CMZP] powder synthesized by sol-gel techniques was investigated. Amorphous powder was cold pressed/sintered and hot pressed. Cold pressing CMZP, followed by controlled drying and sintering, resulted in maximum relative densities of 96% when using 2wt% of a sintering aid (ZnO). When increasing ZnO content to ≥ 5wt%, a second phase ((ZrO)₂P₂O₇) was formed.

Hot pressing conditions were optimized to yield the highest room temperature flexure strength. Mechanical properties of the hot pressed CMZP at room temperature were: (1) a four-point flexure strength of 124.77 MPa, (2) an elastic modulus of 118.45 GPa, and (3) a Weibull modulus of 10.7.

At 850°C, the flexure strength and elastic modulus were 118.35 MPa and 68.63 GPa while at 1050°C they were 89.57 MPa and 78.27 GPa. Samples tested at 1250°C creeped during loading.

The fracture toughness K_c of hot pressed CMZP was determined to be 1.145 MPa·√m by indentation. Breaking multiple-indented bars in a four-point bend resulted in a fracture toughness value of 1.269 MPa·√m.

The coefficient of thermal expansion (CTE) on heating from 25°C to 1000°C was 2.64 x 10⁻⁶/°C and did not change on cycling ten times to 1000°C. No thermal expansion hysteresis was observed.

Elevated temperature X-ray diffraction (XRD) revealed that the hot pressed CMZP remained single phase when heated to 1200°C and cooled to room temperature.