NEW GREEN DIELECTRICS RELATED TO CALCIUM TITANATE: SYNTHESIS AND CHARACTERIZATIONS

摘要

Calcium titanate oxide CaTiO_3 (CTO) belongs to the novel engineering ceramics used in microwave technologies and is one of the post-PZT materials. CTO and its solid solutions exhibit a lot of electronic and structural properties that make them multifunctional. Therefore, CTO-based materials attracted more and more researchers. The main purpose of the present work is the preparation of new lead-free dielectrics by solid state reaction between two perovskites: CaTiO_3 and KMgF_3. Calcium titanate and potassium magnesium fluoride powders were previously synthesized respectively at 850 C and 700 °C. Several chemical compositions 1-x CaTiO_3 + x KMgF_3 were then prepared and dry-milled. The mixtures were pressed into pellets and sintered in free air at 900 °C for 2 h. The obtained ceramics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analyses (TGA, DTGA) and dielectric measurements (DE). The XRD study showed the formation of a fluorinated perovskite solid solution with formula Ca_(1-x)K_xTi_(1-x)Li_xO_(3-2x)F_(3x)(CTOF) in the initial composition range 0 ≤ x ≤ 0.20. The diffraction peaks of CTOF samples were indexed in an orthorhombic lattice and no significant change was observed in the unit cell parameters with increasing KMgF_3 amount. On the other hand, the addition of KMgF_3 to CTO allowed the ceramic's densification at low temperature. The shrinkage increased from 1.9 % to 9.5 % and the grain's size of the various ceramics was in the range 0.5 - 5 μm. Several thermal phenomena were detected by DSC and DE. No weight loss and no peak were depicted on the TGA and the DTGA curves over all the temperature range investigated; therefore, the phenomena observed by DSC and DE could be ascribed to phase transitions.