Study On Defect Chemistry And Dielectric Properties Of Alkaline Earth Metal/BaTiO₃-based Ceramics

The samples were prepared by a mixed oxide method route at different sintering temperature. The crystal structure, dielectric properties, site occupations and defect chemistry of a series of alkaline earth metal/Ba Ti O₃-based ceramics were investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, electron paramagnetic resonance, and dielectric measurements.The study of ceramic’s defect chemistry become more and more convenient by means of the electron paramagnetic resonance（EPR） technique. Any ceramic is not absolutely insulative, with a certain concentration of electrons. When the odd electrons in ceramics are trapped in a vacancy of lattice site, a new signal shows a g-factor different from ge = 2.0023 for the free electron. So, it is a powerful technique for identifying vacancy defects.（1） The(Ba_1-xCa_x)Ti O₃（x = 0.03） ceramics（BCa₃T） ceramics were prepared at 1300-1500°C. In the rhombohedral phase below-100 °C, an EPR signal at g = 1.955 appeared in the insulating BCa₃ T with an electrical resistivity of 108 ?.cm and was assigned as ionized oxygen vacancy defects for the tetragonal BCa₃ T ceramics. BCa₃ T prepared at 1300 °C showed a temperature-stable X6 S dielectric specification（ε′ = 1750）. Three types of vacancy defects of Ba, Ti, and O vacancies could coexist in BCa₃ T due to the fact that the partial Ti-sites were occupied by Ca²⁺.（2） The study of(Ba_1-xMg_x)Ti O₃（x = 0.015）ceramics prepared at different sintering temperature found that it is easy for Mg²⁺ ions to occupy Ba sates and stably exist. With the increase of sintering temperature, the ceramics changed to tetragonal phase from the pseudo-cubic crystal, and finally changed into the structure of the mixed tetragona-hexagonal phase. The signal of Ti vacancies were detected by the electron paramagnetic resonance technique.（3） The(Ba_1–xEr_x)(Ti_1–x/2Mg_x/2)O₃ ceramics（BETM） were prepared at 1400°C. For the normal BETM ceramics, its dielectric-peak temperature（Tm） decreased linely with the doping amount x, a Y5V-type ceramic was obtained at x = 0.05. However, a better temperature stability of ε′ = 1660 – 1990 for T ≤ 60 °C was observed for the x = 0.02 sample and the minimum of ε′ occurred at room temperature（RT）. With decreasing T from RT, ε′ increased slowly. A tetragonal perovskite structure was formed up to the solution limit of x = 0.03 and a very small amount of a secondary phase of Er₂Ti₂O₇ appeared when x ≥ 0.04. In terms of the probe role of Ba-site Er³⁺ ions via a greatly enhanced fluorescent effect in Raman spectrum, Er³⁺ and Mg²⁺ were determined to occupy predominantly Ba sites and Ti sites, respectively. However, there was little evidence for Mg²⁺ ions to occupy Ba sates in this research.（4） The dielectric properties of(Ba_1-xLa_x)(Ti_1-x/2Mgx/2)O₃（x = 0.03） ceramics prepared at different sintering temperature（1250°C, 1350°C, and 1400°C） were investigated. The sintering temperatures had a large effect for the dielectric properties and the vacancies of these ceramics. The sample prepared at 1250 °C showed a temperature-stable X8 R dielectric specification.（5） The solid solubility of(Ba_1-xLax)(Ti_1-xEr_x)O₃ and(Ba_1-xLax)(Ti_1-y-x/4E_ry)O₃ ceramics at 1400°C was investigated. The Er₂Ti₂O₇ phase was one of the impurity phases in these ceramics. It was inevitable that a small amount of Er³⁺ ions occupied Ba-sites, then caused a fluorescent effect in Raman spectrum.