Phase Diagram, and Structural and Electrical Properties of Pyrochlores in Bi2o3-Zno-Nb2o5 Ternary System

A comprehensive investigation of phase diagram, structural and electrical properties of pyrochlores in Bi2O3-ZnO-Nb2O5 ternary system was presented. A thorough and complete literature review was carried out in order to gather background information on bismuth zinc niobate (BZN) phases and related materials. Thus, a better understanding in the phase formation, research problems, electrical and thermal properties of the investigated materials and their potential application is achieved. BZN pyrochlores and related materials were prepared via conventional solid state reaction at sintering temperatures ranging from 700 oC to 1200 oC using high purity oxides. Analysis and characterization were performed using a combination of techniques including diffraction, microscopy, spectroscopy, thermal analysis and physical property measurements. X-ray diffraction (XRD) was used for phase identity and purity determination. Detailed analysis was carried out on single phase materials. The surface structure and morphology were characterized using scanning electron microscopy (SEM). Structural analysis was carried out using Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy and Rietveld refinement using neutron and X-ray diffraction data. Electrical properties were determined by a.c impedance spectroscopy in the frequency range of 5 Hz to 13 MHz and temperature range of ~28 oC to 850 oC. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were employed to study thermal properties. Other analysis such as inductively coupled plasma atomic emission spectrometry (ICP-AES) and density measurement were carried out on selected samples. A complete phase diagram including two solid solution areas for cubic and monoclinic phases in BZN ternary system has been constructed. The cubic pyrochlore solid solutions do not include the so-called ideal composition P, Bi3Zn2Nb3O14. It may be described in terms of two compositional variables: ZnO deficiency compared to P together with variable Bi: Nb ratio with general formula, Bi3+yZn2-xNb3-yO14-x-y: -0.11(1)  y  0.14(1) and –0.03(1)  x  0.31(1). Selected BZN materials have been characterized by a.c impedance spectroscopy. These materials exhibited excellent dielectric properties: permitivity, ε’ = ~ 80-100, dielectric loss (tan δ) = ~ 0.002 - 0.009 and temperature coefficient, Tcc = ~ 400 ppm/oC, at ~28 oC in the frequency region of 1x105 Hz. Chemical doping was carried out in order to elucidate relative ability of cubic pyrochlore Bi3Zn1.84Nb3O13.84 to accommodate various dopants in forming new solid solutions and in the search for better performance materials. However, chemically doped BZN materials did not show extensive solid solutions limit or significant improvement in electrical properties. In conclusion two structurally related phases i.e. cubic and monoclinic phases exist in the BZN ternary system. These materials display interesting electrical properties: the cubic phase, P has a large negative temperature coefficient of permittivity while the monoclinic phase, M has a positive value for the temperature dependency. Given the opposite signs of the temperature coefficients of these phases, it may be possible to make composites of P and M so as to achieve controllable or almost zero temperature coefficient of capacitance (TCC) values. In addition both of these phases have high dielectric constants. Hence, these materials have potential applications in high frequency multilayer devices including LC filters and low temperature, co-fired ceramic, LTCC system.

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