Preparation And Study Of The Low-thermal-expansion Material Ca_1-xSr_xZr₄P₆O₂₄

The general nature of materials is their thermal expansion on heating and contraction on cooling, but in nature there are very few materials with opposite properties. Expansion on cooling and contraction on heating refer to that at a certain temperature range the coefficient of thermal expansion is less than zero, also known as negative thermal expansion. With negative thermal expansion materials the thermal stress can be effectively reduced and the thermal properties of materials can be enhanced. Therefore, the negative thermal expansion materials have become a hot topic in materials science community in recent years.Ca1-xSrxZr4P6O24 has a similar crystal structure as NaZr2P3O12 of NZP family phosphate ceramics. The family of materials has properties of anisotropic thermal expansion which is negative on certain axis. Because of its flexibility in cation substitution, making preparation of zero thermal expansion materials or controlled expansion materials possible. However, preparation of these materials using traditional techniques (solid state reaction or sol-gel method) is quite cumbersome and expensive, and some of the procedures may be environmental unfriendly and not conducive to mass production. Besides, Ca1-xSrxZr4P6O24 ceramics have usually very low density which cannot be used for many applications. To solve these problems, we concentrate on the rapid synthesis of Ca1-xSrxZr4P6O24 and on the enhancement of its density in this thesis. The main results obtained are as follows.1. A series of Ca1-xSrxZr4P6O24 (x=1.0,0.5,0.0) materials are successfully prepared by high temperature one-step fast solid-phase sintering method using CaO, SrCO3, ZrO2 and NH4H2PO4 powders as raw materials. The optimum preparation conditions are obtained. Compared with other synthetic methods, this method is simple energy and time saving and suitable for industrial mass production.2. Room temperature XRD analyses of Ca1-xSrxZr4P6O24 (x=1.0,0.5,0.0) series of ceramic materials show that the best sintering conditions for Ca1-xSrxZr4P6O24 (x=1.0,0.5,0.0) are:sintering at 1400℃for 16 hours (x=0.0), at 1500℃for 8 hours and by rapid cooling (x=0.5), and sintering 1400℃for 16 hours (x=1.0). 3. Temperature-dependent Raman spectroscopy and DSC analysis show that Ca1-xSrxZr4P6O24 (x=1.0,0.5,0.0) series of ceramic materials have no phase change in the temperature range from-160 to 1150℃and maintain appropriate stability. 4. Ca1-xSrxZr4P6O24 (x=0.5) materials has a low density of about 76.69% of theoretical density without sintering additive. In order to increase its density, we add 1.0 wt.% MgO to the raw materials and sinter it at 1500℃for 8 hours. The density is increased to 3.098g/cm3,96.22% of the theoretical density. The mechanisms of the density increase are explained with the help of microstructure analysis by SEM.