Thermal expansion property of sintered polycrystals and unit cell of celsian solid solution (Ba,Sr)Al₂Si₂O₈

Abstract

The purpose of this work is to clarify the transformation mechanism from hexacelsian to celsian and the thermal expansion properties of celsian solid solutions (Ba_1−xSr_xAl₂Si₂O₈, x = 0–1.0) using high-temperature powder X-ray diffraction and dilatometry of sintered polycrystals. Hexacelsian polycrystals were prepared from BaCO₃, SrCO₃ and finely purified kaolin powder (Al₂O₃·2SiO₂·2H₂O) by heat-treatment at 950–1400 °C. Sr-hexacelsian polycrystals transformed to celsian via nucleation and growth from inner to surface to develop cracks at surface at 1100–1200 °C. Single phase of celsian polycrystals in composition of Ba_1−xSr_xAl₂Si₂O₈ (x = 0–0.8) were prepared by heat-treatment at 1400 °C for 1 h and their dilatometric thermal expansion coefficient (TEC) were almost 4.0–4.2 × 10⁻⁶/°C from 25 to 900 °C. On the other hand, average TEC of unit cell of celsian measured by high temperature X-ray diffraction were lower than the values of dilatometry and increased from 3.0 × 10⁻⁶/°C to 3.8 × 10⁻⁶/°C with the Sr content.