Electrical Properties of Sr(Fe0.5Nb0.5)O3 and BiFeO3 Doped with Holmium Oxide and Antimony Oxide

Abstract

In this research project, properties of some perovskite oxide based ceramics were investigated. The modified strontium iron niobate and bismuth ferrite ceramics, which were prepared by a solid-state, were selected for the studies. The project can be dividing into five parts. For the first part of the work, the effects Ho2O3 on the properties of SFN (Sr(Fe1-xHox)0.5Nb0.5O3) ceramics synthesized via a solid state reaction technique were investigated. The undoped ceramic showed an orthorhombic phase, but it transformed to a pseudo cubic phase for higher Ho concentrations. A low solubility limit of Ho in SFN caused a formation of second phase for the x=0.15 ceramic. Dielectric behavior of undoped ceramic exhibited high dielectric constant over a wide temperature range. However, the doping shifted this region to a higher temperature. The doping also shifted the peak of dielectric loss to a higher temperature. Activation energy of dielectric relaxation increased with increasing Ho concentration. In addition, complex impedance analysis was applied to determine the behaviors of grain boundary and grain after doping. The second part of the work was to study the processing parameters that affected on the properties of BiFeO3. The BiFeO3 was prepared by a mixed oxide, solid-state reaction method. Excess Bi2O3 (1-7 wt.%) was introduced prior to powder calcination to compensate for any Bi2O3 that may have been lost from the samples due to volatilization during heat treatments. Various heating rates (1-10C/min) were performed at the calcination state. X-ray diffraction analysis revealed that pure phase BFO was observed for the samples calcined at a low heating rate (1C/min) and contained lower amount of Bi2O3.The higher levels of excess Bi2O3 produced an increased in dielectric constant and dielectric loss. Further, ferroelectric behavior was improved for higher amount Bi2O3 contented samples. For the third part, properties of the pure and modified BiFeO3 (BFO) ceramics (BFO doped with Sb) were determined. Phase formation, microstructure, and dielectric properties were investigated. The samples showed a main phase of rhombohedral BFO. The additive inhibited grain growth, with average grain size decreasing from ~14 µm for pure BFO to 3µm for the modified BFO samples .The dielectric constant of the modified samples tended to improve with the additive. This improvement can be related a conduction mechanism in the studied samples For the fourth part, properties of Ga doped BaFe0.5Nb0.5O3 (Ba(Fe1-xGax)0.5Nb0.5O3) ceramics were investigated. All ceramics showed perovskite structure with cubic symmetry and the solubility of Ga in BFN ceramics had a limit at x = 0.2. Examination of the dielectric spectra indicated that all ceramic samples presented high dielectric constants which were frequency dependent. The x=0.2 ceramic showed a very high dielectric constant (r>240,000 at 1 kHz) while the x=0.4 sample exhibited high thermal stability of dielectric constant with low loss tangent from room temperature to 100oC with r>28,000 (at 1 kHz) when compared to other samples. By using a complex impedance analysis technique, bulk grain, grain boundary, and electrode response were found to affect the dielectric behaviour which could be related to the Maxwell–Wagner polarization mechanism For the final part of the present work strontium iron niobate Sr(Fe0.5Nb0.5)O3 doped with BFO ceramics were synthesized via a solid state reaction technique. Properties of the ceramics were determined. All ceramic showed perovskite structure with an orthorhombic phase. Examination of the dielectric spectra indicated that all ceramic samples presented high dielectric constants which were frequency dependent. The x=0.050 ceramic showed a very high dielectric constant (r>40,680 at 1 kHz). By using a complex impedance analysis technique, bulk grain, grain boundary, and electrode response were found to affect the dielectric behaviour which could be related to the Maxwell–Wagner polarization mechanism. Further, ferroelectric behavior was improved for higher amount BFO contented samples