Thermoelastic properties of polycrystalline strontium titanium silicate Sr₂TiSi₂O₈ synthesized via the reactive sintering method

Abstract

The aim of the work presented in this paper is to investigate the thermoelastic properties of polycrystalline Sr-fresnoite Sr₂TiSi₂O₈ (STS). On that purpose, Sr₂TiSi₂O₈ samples were synthesized by solid state reactive sintering from SrCO₃, TiO₂ and SiO₂ powders. The thermal expansion behaviour of the so-obtained material was characterized at the microscale by using High Temperature X-Ray Diffraction (HT – XRD) to follow the changes in the unit cell parameters of STS. The results were correlated to the macroscopic expansion behaviour characterized by dilatometry, and to the evolution of the Young’s modulus with temperature measured by the impulse excitation technique (IET). It is evidenced that STS presents a commensurate-to-incommensurate phase transformation characterized by thermal hysteresis and leading to a minimum peak in CTE at 200 °C during the heating and at 150 °C during the cooling. From 300 °C to 900 °C, the thermal expansion exhibits an almost linear behaviour with an average CTE of $12\times {10}^{-6}{K}^{-1}$. Between 900°C and 1000 °C, a maximum peak in CTE, also characterized by thermal hysteresis, is observed and seems to be attributed to a reverse transformation to a commensurate phase. The value of Young’s modulus E is 105 ± 0.5 GPa at room temperature. The commensurate-to-incommensurate phase transformation also affects the Young’s modulus by inducing a weak minimum peak at the same temperatures as those observed for the CTE. At higher temperature, the Young’s modulus remains stable up to 500 °C and slowly decreases over this temperature. It remains equal to 96 ± 0.5 GPa at 950 °C.