Investigation On The Microstructure Refinement And Optimization In Dielectric Properties Of Bi-based Pyrochlore Ceramics

Bismuth-based pyrochlores is a kind of new dielectric ceramics with excellentperformance, on the basis of the composition of (Bi1.5Zn0.5)(Zr1.5Nb0.5)O7and(Bi1.5Zn0.5)(Ti1.5Nb0.5)O7with high dielectric constant, different ions were chosen to occupy Aand B site in pyrochlore structure partially and completely. The influence of the structure andthe properties by the occupation of different ions was researched combined with the accuratemicro structure information abtained by Rietveld refinement. Moreover, in order to developthe dielectric pyrochlores with temperature stable characteristics, the complex phasecompositon of pyrochlores was prepared by selecting suitable single phase fundamentalsystem which has positive and negative temperature coefficient. The main results are asfollows:(1) The compositions with formula (Bi1.5Zn0.5-xMgx)(Zr1.5Nb0.5)O7(x=0.1,0.2,0.3,0.5)(BZMZN) and (Bi1.5M0.5)(Zr1.5Nb0.5)O7(M=Zn、Mg、Ca、Sr) were studied. There is traces ofthe second phase Bi12MgO19when Zn sites were substituted by the partial and complete Mg.With increasing amount of Mg substitution, the dielectric constant decrease slightly because ofthe decrease of the total polarizability. The relaxed degree of series BZMZN pyrochloresincreases first and decreases when Zn was completely substituted by Mg, it is related to thetype of ions in A sites and inhomogeneous distribution in local regions. The total polarizabilityand mutual relationship of BO6octahedra were used to explain that series samples of(Bi1.5M0.5)(Zr1.5Nb0.5)O7have the basically same dielectric constant. The dielectric relaxationwas found in all BZMZN and (Bi1.5M0.5)(Zr1.5Nb0.5)O7pyrochlores at about-150℃. Therelaxation behavior was fitted by Arrenhnius model, VF model and new glass model and the relaxation mechanism was further deduced with the bond-valence theory, the relaxation isrelated to inhomogeneous distribution of A site atoms and the local intrinsic hopping of atomsin the A and O′positions among several local potential minima.(2) Quaternary pyrochlores doped by the pentavalent cations with the formula(Bi1.5Zn0.5)(Ti1.5M0.5)O7(M=Nb, Ta and Sb) have been synthesized and studied. The XRDpatterns show that all of the three samples give single phase. SEM photos show all the sampleshave well sintering condition and regular grain shape, compared with Nb and Ta, Sbsubstituted sample is easy to crystal growth, so the grain size of BZTS is maximum. Therefined data by GSAS program and the bond valence theory were used to analyze differentdielectric properties with occupation of different ions in B site. The small dielectric constant ofBZTS is due to the total polarizability, especially from the contribution of BO6octahedradistortion degree, and weak correlation when Sb placed into the center of the octahedra wouldalso result in small αε. The relaxation of BZTM series pyrochlores is related to inhomogeneousdistribution of A site atoms and the local intrinsic hopping of atoms in the A and O′positionsamong several local potential minima based on the fitting results. The conduction study showsthat defect-related weakly bonded electrons hopping among the local region are reckoned to bethe main charge carriers in the low temperature range. When it comes to the moderatetemperature range, oxygen vacancies migration was thermally activated as the carriersmechanism, and the different activation energies is related to the association and thedisassociation of massive complex defects. It is reckoned that there are the most structuredefects in the BZTS by the refined data which should provide more oxygen vacancies and freeoxygen ions to participate the conduction, leading to the σ value being the largest of the threesamples.(3) Through choosing the single phase pyrochlores with positive and negative temperaturecoefficient. Alternative compositons0.3BZN-0.7BZZN,0.5BZT-0.5BZZN,0.6BZS-0.4BZZNand0.2BZTN-0.8BZZN were prepared according to the Lichtenecker law. The peak split ofthe strongest peak from XRD data shows that the complex phase was formed, but there aretrace of the second phase in0.3BZN-0.7BZZN. The results of dielectric temperature testindicate that temperature stability of sample is improved in the range of-55~125℃forindustrial applications, especially0.3BZN-0.7BZZN system with high dielectric constant(~126), low dielectric loss (~0.004) and temperature coefficient close to zero which is up tothe international standard of NP0so called ultra stable temperature stable type capacitormaterials. It could be applied widely to oscillator, resonator and the coupling capacitors in thehigh frequency circuit.