Research On The Microstructure And Dielectric Relaxation Behavior Of Bi-based Pyrochlore Ceramics

Cubic Bi-based pyrochlores, a kind of dielectric material with low sintering temperature, high dielectric constant and low dissipation factor, has been paid increasing attentions worldwidely. Taken Materials Science as the viewpoint and the relationship of structures and properties the springboards, groups of Bi-based pyrochlore materials were prepared by conventional solid-state reaction methods. Advanced test methods were applied to verify the micro-defects information of series ceramics. The defects information and dielectric relaxation properties had been studied. The detailed microstructure information related to the dielectric relaxation of materials had been summarized. The influence of chemical composition on the microstructure and dielectric properties of pyrochlores materials had been investigated. We have got some conclusion as following.Firstly, Positron Annihilation technique (PAT) had been introduced to obtain the structural defects information of the quarternary (Bi_1.5Zn_0.5)(M_1.5Nb_0.5)O7 (M=Ti⁴⁺, Sn⁴⁺, Zr⁴⁺, and Ce⁴⁺) samples for the first time. The result shows that the defect of the material is depend on the ions radii of the substituted ions in B-site to great extent. The bigger the difference of weighted ionic radii in B-site of BZNM to (Bi_1.5Zn_0.5)(Zn_0.5Nb_1.5)O7 material, the higher the positron lifetime value is, while the total defect concentration is decreased due to the formation of complex defect [M_Zn¨·V_Zn″] and [2M_Nb'·Z_ni¨]. Combined with the XRD result of BZNM, it could be concluded that the ions radii in B-site of Bi-based pyrochlores, which causes little influence to the structure, is between 0.60 and 0.72 (A|°). Ti⁴⁺, Nb⁵⁺, Ta⁵⁺, Sn⁴⁺ and Zr⁴⁺ are some of the suited occupied ions.Secondly, the high temperature electrical property of BZNM had also been studied. At low temperature range, the electrons are the main charge carriers while the local defects play a subordinate role. At high temperature range, the acceleration effect from the aggravation of local defects thermal motion and the lowering of grain boundary resistance are more significant than the retardation effect from the increase of activation energy, so the electrical conductivities of the material rise. The relationship of electrical conductivities of four samples get reverse in low temperature range and high temperature range, which can be ascribed to the different local defects concentration of series materials.Thirdly, the dielectric relaxation properties of (Bi_1.5Zn_0.5-xSr_x)(Ti_1.5Nb_0.5)O7 (BZSTN, x=0^<sub>0.5) ceramics had been studied. Three samples (x=0.3^<sub>0.5) had showed obvious dielectric relaxation behavior. With the increase of Sr/Zn ratio from 3:2 to 5:0, the relaxivities of three materials decreased. It was indicated that the relaxation properties could be ascribed to the bending of O'-A-O' chain of the pyrochlore structure. The influence of Sr²⁺-O′-Zn²⁺ bending on Zn²⁺-O′-Zn²⁺ orderly structure would get stronger if the Sr/Zn ratio get smaller. So the relaxivities of three materials decreased with the increase of Sr/Zn ratio. By appropriate Sr²⁺ introduction, it's possible to obtain low dielectric constant temperature coefficient, even zero, and the problem of dielectric constant sensitivity to the environment temperature of Bi-based pyrochlore materials could be solved.Lastly, high properties of new Bi-based pyrochlore dielectric ceramics had been prepared. Compared with the three groups of ceramics we studied, the polarizability of B-site ions, instead of A-site ions, has great affects on the dielectric properties of Bi-based pyrochlores, which proves that the contribution from B-site ions to its dielectric properties is more significant.