Phase transitions of CaTiO 3 ceramics sintered with the aid of NaF and MgF 2

Abstract

New phases with initial composition (1 − x)CaTiO 3 − xNaF − xMgF 2 (0 ≤ x ≤ 0.20) have been prepared at low temperature (950 °C) from mixtures of CaTiO 3 and fluorides NaF and MgF 2. The oxyfluorides obtained have been characterized by X-ray diffraction at room temperature and indexed by isotypy with orthorhombic CaTiO 3. The microstructures of these phases are observed by scanning electron microscopy. Dielectric measurements have been carried out during cooling cycle from 500 °C to room temperature at two frequencies (100 Hz, 1 kHz). Differential scanning calorimetry (DSC), thermogravimetry (TG) and differential thermogravimetry (DTG) analyses have been performed, respectively, from room temperature up to 550 °C (DSC) and 920 °C (TG–DTG). The dielectric measurements revealed two anomalies which have been confirmed by DSC analyses. These phenomena are ascribed to second order phase transitions. The variation of the real permittivity with temperature is in agreement with the class I capacitor specifications. However, the dielectric losses have to be improved.