Synthesis of low thermal expansion ceramics based on CaZr4(PO4)6–Li2O system

Ultra-low thermal expansion ceramics based upon CaZr4(PO4)6–Li2O system were synthesized through solid state sintering. Lithium oxide was introduced to the CaZr4(PO4)6 lattice to promote liquid phase sintering to form CaZr4(PO4)6 with strongly negative thermal expansion. A small addition of Li2O has improved the densification rate, but also caused an excessive grain growth, which resulted in extensive microcracking. The microcracking lowered the bulk thermal expansion to −6×10−6°C−1. Excessive amount of Li2O content also led to formation of the secondary phase, Li2Zr(PO4)2, and also caused the swelling of the CaZr4(PO4)6 phase to lower the sintered density. An addition of over 5% of lithium oxide, the glass phase surrounding the CaZr4(PO4)6 grain, together with the secondary phase increased the thermal expansion. At 2.5% addition, there was an indication that the viscous glassy phase has partially relieved the stresses associated with the thermal expansion anisotropy during cooling from the processing temperature. It was demonstrated that the microstructure of the CaZr4(PO4)6 could be altered by adding varying amount of lithium oxide to tailor the thermophysical properties of the material.