The Preparation And Raman Spectra Of Negative And Near-zero Thermal Expansion Materials Of Ca_1-xSr_xZr₄P₆O₂₄

In general, most materials expand on heating and contract on cooling. They exhibit positive coefficients of thermal expansion (CTE). However, an increasing number of materials with negative thermal expansion (NTE) have been discovered in recent years. Interest in negative thermal expansion materials has increased greatly in recent years, especially when improved understanding of the underlying atomistic and molecular mechanisms, as well as a raft of new potential applications are concerned.The NaZr2P3O12 type structure (NZP) has emerged as a new family, and has attracted significant attention due to their stability to the low thermal expansion coefficients, extreme environmental conditions, good thermal shock resistance, fast ionic conductivity and catalytic activity. Of the NZP members, it has been observed that CaZr4P6024 and SrZr4P6024 exhibit an opposite anisotropy in their lattice thermal expansions. So it is possible to obtain near-zero expansion materials by means of forming the composition of miscible binary solid solution.In this thesis, low thermal expansion ceramics Ca1-xSrxZr4P6O24 (x=0,0.5,1)are synthesized by rapid solid state reactions. And also we study the structure and phase transitions of Ca1-xSrxZr4P6O24 compounds by Raman spectroscopy. The phonon modes for symmetric and asymmetric stretching, symmetric and asymmetric bending, translational and librational motions are assigned. It is shown that the Raman band positions are obviously affected by Ca2+ or Sr2+ cations due to their different cation size and electronic negativity. For most of the Raman modes, the shift amounts with temperature in Ca0.5Sr0.5Zr4P6O24 are between those in CaZr4P6O24 and SrZr4P6O24, suggesting the opposite anisotropic thermal expansion properties resulted from Ca2+ and Sr2+ cancelling each other in Cao.5Sro.5Zr4P6024. Temperature dependence of the Raman spectroscopic reveals that there is no phase transition of all the samples from 113K to 823K. The differential scanning calorimetry (DSC) show that all of the samples have no phase transition from 298K to 1423K and are not hygroscopic at room temperature.