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TiO2-doped NiFe2O4 samples were prepared via ball-milling and two-step sintering processes. Besides NiFe2O4 phase, two new phases, NiTiO3 and Fe2TiO5, formed in TiO2-doped samples. The temperature of sintering onset for 1.0 wt% TiO2-doped samples is 230°C lower than that of undoped samples. Early-stage synthesis process of TiO2-doped NiFe2O4 ceramics is controlled by grain boundary diffusion mechanism. Increasing TiO2 content from 0 to 1.0 wt%, the apparent activation energy decreased from 813.919 KJ/mol to 639.361 KJ/mol. The values of relative density and bending strength reached their maximum value with 1.0 wt% TiO2. Saturation magnetization, residual magnetization ratio and coercivity decrease with increasing TiO2 content. [ABSTRACT FROM AUTHOR]

Abstract: Samples with small amounts of V2O5 (0, 0.5, 1.0, 2.0, 2.5wt%, respectively) were prepared via ball-milling and two-step sintering from commercial powders (i.e., Fe2O3, NiO and V2O5). Micro-structural features, phase transformation, sintering behavior and magnetic properties of V2O5-doped NiFe2O4 composite ceramics have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermal dilatometer and vibrating sample magnetometer (VSM), respectively. Morphology and the detecting result of thermal dilatometer show that V2O5 can promote sintering process, the temperature for 2.5wt% V2O5-doped samples to reach the maximum shrinkage rate is 59°C lower than that of un-doped samples. With an increase of V2O5 from 0wt% to 2.5wt%, the apparent activation energy decreased from 813.919kJ/mol to 298.806kJ/mol. The early-stage synthesis process of un-doped ceramic samples and 2.5wt% V2O5-doped samples are mainly controlled by grain boundary diffusion and volume diffusion, respectively. Increasing V2O5 content, residual magnetization ratio (Mr/Ms) and coercivity (Hc) elevate, meanwhile, saturation magnetization decreases. [Copyright &y& Elsevier]

Abstract: NiFe2O4 composite ceramics reinforced with carbon fibers (Cf), nickel-coated carbon fibers (NiCf) and ZrO2 fibers (ZrO2f) were fabricated by the conventional high-temperature solid state reaction at 1300°C for 6hs. The effects of the three kinds of fibers on microstructure and mechanical properties of NiFe2O4 composite ceramics were investigated. The fracture surfaces of NiFe2O4 composite ceramics were observed by scanning electron microscopy (SEM). Elemental analysis of micro-structural phases was performed using energy dispersive spectroscopy (EDS), attached with SEM. Flexural strength by three-point bending techniques and fracture toughness were measured. It was found that leaving holes after reactions between the carbon fiber and matrix led to decreased mechanical properties. The ZrO2f-doped composite had an improved fracture toughness of 3.05±0.1MPam1/2 compared to NiFe2O4 composite ceramics having 1.91±0.2MPam1/2 due to fiber debonding, fiber pull-out and fiber bridging as well as microcracking toughening. The results presented here show that ZrO2f can serve as a potential toughening agent for NiFe2O4 composite ceramics. [Copyright &y& Elsevier]

Abstract: Two-step sintering (TSS) has been employed in the current study to suppress the accelerated grain growth of NiFe2O4 nanopowder compacts in the final sintering stage. Experiments are conducted to determine the appropriate temperatures for each step. The temperature range from 1200°C to 1300°C is effective for the first-step sintering (T 1) due to its highest densification rate. The second-step sintering temperature (T 2) should be within the kinetic window, where grain boundary diffusion is maintained but grain boundary migration is suppressed. The grain sizes of high density (≥98% theoretical density) NiFe2O4 compacts produced by TSS are smaller than 700nm, while that of those formed by CS are over 2.5μm. The evidence indicates that the saturation magnetization of nearly full NiFe2O4 ceramics is independent of grain size and likewise high, with the corresponding values of approximately 54emu/g. The Vickers hardness and fracture toughness both increase with the decrease of grain size and porosity. [Copyright &y& Elsevier]