Synthesis, sintering and thermal expansion of Ca_1-x Sr _x Zr₄P₆O₂₄ — an ultra-low thermal expansion ceramic system

Abstract

Ca_1-x Sr _x Zr₄P₆O₂₄ (O ⩽ × ⩽ 1.0) system which belongs to a new large family of low thermal expansion materials known as NZP or CTP, was synthesized by the solid state and the sol-gel methods. The conventional sol-gel method was modified by introducing a seeding step which resulted in significant improvement in the sintering characteristics and the microstructure of the sintered material. Sintering data were compared with those obtained by the powder mixing technique. Thermal expansion of the sintered samples was measured by classical dilatometry and by high-temperature X-ray diffractometry. It was found that CaZr₄P₆O₂₄ (x= 0) and SrZr₄P₆O₂₄ (x= 1) phases had opposite anisotropies in their respective axial thermal expansions. This behaviour led to the development of a crystalline solution composition of nearly zero expansion characteristic. Microstructures of the sintered specimens were examined by scanning electron microscopy.