The Dielectric And Ferroelectric Properties Of Ca And Sr Doping BaTiO₃ Films And Ceramics

Ferroelectric materials have been applied to many electronic and optical devices, utilizing their excellent dielectric, piezoelectric and optical properties. Specially, BaTiO3 and isovalent-substituted BaTiO3 are the promising candidates and have been actively studied. Among them, Ca doped BaTiO3 (Ba1-xCaxTiO3, BCT) crystals are considered to be one of the foremost potential candidates for the lead-free electro-optic modulators and memory devices. Modern micro-devices require high-performance materials in the form of films with thickness in the range of a few hundred and even tens of nanometers. However, the reports on the BCT films are lack in open literature. In this thesis, we report on a comparative study of the microstructure and dielectric properties between BCT ceramics and films in the whole Ca concentration range of x=0 to 1. In addition, we had a research on the Sr and Ca co-doping effect on the dielectric and ferroelectric properties of the BaTiO3 ceramics, due to the fact that there is no detailed work on the (Ba,Ca,Sr)TiO3 ceramics, to the best of our knowledge. A comparative study of the microstructure and dielectric and ferroelectric properties between (BaxSr1-x)0.77Ca0.23TiO3 and Ba1-xSrxTiO3 ceramics was performed with the Sr concentration of x=1, 0.9, 0.8 and 0.7, respectively.A comparative study of the microstructure and dielectric properties between Ba1-xCaxTiO3 (BCT) ceramics and films were performed in the whole Ca concentration range of x=0 to 1. The ceramics were prepared by conventional solid-state reaction technique and the films by the method of pulsed laser deposition. X-ray diffraction study of the BCT ceramics exhibited a pure tetragonal phase for x = 0 to 0.25, a tetragonal-orthorhombic diphase for x = 0.25 to 0.85 and a pure orthorhombic phase for x = 0.90 to 1.00. And the dielectric phase transition temperature from tetragonal to cubic was marginally affected by the Ca doping into BaTiO3. However, BCT films deposited on Pt/Si/SiO2/Si substrates showed a different microstructure and dielectric properties. Tetragonal-orthorhombic diphase was not found in the BCT films for x = 0.25 to 0.85, and a large decrease of the Curie point and diffuse phase transition were observed in the BCT films. Based on the compositional analysis, such phenomena were ascribed to the occupancy of some Ca2+ to the Ti4+ sites in the BCT films.The dielectric and ferroelectric properties of (BaxSr1-x)0.77Ca0.23TiO3 ceramics with x=1 to 0.7 were studied and compared with those of the BaxSr1-xTiO3 and Ba0.77Ca0.23TiO3 ceramics. It has been found that Sr doping into the Ba0.77Ca0.23TiO3 ceramics causes a drastic decrease of the Curie temperature, just like Sr doping into pure BaTiO3 ceramics, demonstrating a cell volume effect. However, the (BaxSr1-x)0.77Ca0.23TiO3 ceramics with x= 0.9 and 0.8 have larger spontaneous polarization than those of the corresponding BaxSr1-xTiO3 and Ba0.77Ca0.23TiO3 ceramics, along with a sufficient insulating property. The enhancement of the polarization was explained by the increase of the lattice parameter c/a ratio due to the lattice distortion and strain developed in the ceramics. .