Preparation and Characterisation of Bismuth Zinc Tantalate Pyrochlore Materials

Pyrochlore materials in Bi2O3-ZnO-Ta2O5 ternary system were synthesized by solid-state reaction at 1050oC for 48 hours. The X-ray diffraction (XRD) pattern of the material of composition Bi3Zn2Ta3O14 could be fully indexed on a cubic cell with a = 10.5437(9) Å. A study on the phase formation mechanism indicated that Bi2O3 played an important role only in the initial stage of the formation of the pyrochlore phase. The subsolidus phase diagram of Bi2O3-ZnO-Ta2O5 in the region of the cubic pyrochlore has been determined at 1050oC. This phase forms a solid solution area that includes the ideal composition P, Bi3Zn2Ta3O14. Density measurement was used to identify the possible mechanism of solid solution formation. The solid solution area in the phase diagram can be described by the combination of two mechanisms: Bi3Zn2-xTa3O14-x and Bi3+yZn2Ta3-yO14-y, to yield the formula Bi3+yZn2-xTa3-yO14-x-y, -0.20 ≤ y ≤ 0.16 and 0.00 ≤ x ≤ 0.40. XRD in combination with neutron diffraction data were used for Rietveld Refinement for the elucidation of the crystal structure of selected materials. The results show that there is a degree of insensitivity of the refinement to certain changes in the cation content and the oxygen stoichiometry is refined to a value similar to the expected values. Various possible sources of error and variation in permittivity measurements were investigated and the results showed that pellet density was the parameter that had the greatest effect on the determination of capacitance value. Optimisation of sintering conditions was carried out to obtain pellets with highest density and capacitance value. Cubic Bi3Zn2Ta3O14 (BZT) has ε’ of 58, dielectric loss (tan δ) of 0.0023 at 30oC and 1 MHz; temperature coefficient of capacitance (TCC) of -156 ppm/oC in the range of 30oC to 300oC at 1 MHz. Slight variations of permittivity and dielectric loss with compositions were observed in the solid solutions. Conductivities of the solid solutions are higher than that of BZT with activation energy, Ea, in the range of 1.55 – 1.67 eV. The structurally related monoclinic phase Bi2(Zn1/3Ta2/3)2O7 has ε’ of 62, tan δ of 0.0084 at 30oC and 1 MHz and TCC of +110 ppm/oC in the range of 30oC to 300oC at 1 MHz. The conductivity at 649oC is 6.68x10-6 ohm-1 cm-1 with Ea = 1.75 eV. Chemical doping using divalent, tetravalent, trivalent, tetra/hexavalent and pentavalent cations was carried out in the search for better performance materials; only di-, tetra- and pentavalent dopants could be successfully introduced into BZT. Results show that permittivity and dielectric loss of di- and tetravalent doped solid solutions did not vary greatly with dopant concentration. On the other hand, ’ and tan greatly increased or decreased with variations in composition for pentavalent doped materials. Conductivities of the doped materials are higher than that of Bi3Zn2Ta3O14 with Ea of 1.36 – 1.62 eV. Elemental analysis using inductively-coupled plasma optical emission spectrometry (ICP-OES) confirmed the stoichiometric compositions of single phase materials. Fourier transform infra red (FTIR) spectroscopy was used to identify functional groups of the materials and Raman spectroscopy was used as a complement to FTIR. Four absorption bands were observed in FTIR and Raman spectra which can be assigned to different bond stretching or bending modes. There is a correlation between the frequency of the absorption bands and dopant concentration in pentavalent doped materials. Thermal analysis showed no phase transition and weight loss in the temperature range of 35 to 1000oC. Scanning Electron Microscopy was performed to study the morphology of the materials. All the materials show grains of polyhedral shapes which are randomly distributed with visible open pores in the materials.

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