Synthesis and characterization of dense and fine nickel ferrite ceramics through two-step sintering

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 (T1) due to its highest densification rate. The second-step sintering temperature (T2) 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 700 nm, while that of those formed by CS are over 2.5 μm. 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 54 emu/g. The Vickers hardness and fracture toughness both increase with the decrease of grain size and porosity.