Properties of NiFe2O4 ceramics from powders obtained by auto-combustion synthesis with different fuels

Abstract

Nickel ferrite (NiFe2O4) powders derived by auto-combustion synthesis using three different fuels (citric acid, glycine and dl-alanine) have been characterized. The sintering behavior of ceramics using these powders has been compared. Oxygen balance (OB) setting for the chemical reaction is found to regulate the combustion reaction rate. A rapid reaction rate and a high flame temperature are achieved with dl alanine fuel yielding single phase NiFe2O4 powder in the as-burnt stage, whereas powders derived with citric acid and glycine fuels show poor crystallinity and necessitate post-annealing. The powder particles are largely agglomerated with a non-uniform distribution in shape and size, and the average particle size is estimated in the range ~ 54–71nm. Powders derived from dl-alanine fuel show better phase purity, smaller crystallite size, larger surface area and superior sintering behavior. Additional Raman modes discerned for dl-alanine derived powder support a 1:1 ordering of Ni2+ and Fe3+ at the octahedral sites relating to microscopic tetragonal P4122 symmetry expected theoretically for the formation of NiFe2O4 with inverse spinel structure. Microstructure of sintered ceramics depends on the precursor powders that are used and sintering at 1200°C is found to be optimum. Citric acid and glycine derived powders yield high saturation magnetization (Ms~47–49emu/g), but poor dielectric properties, whereas dl-alanine derived powders yield ceramics with high resistivity (~3.4×108Ωcm), low dielectric loss (tanδ~0.003 at 1MHz) and high magnetization (46emu/g). Dielectric dispersion and impedance analysis show good correlation with the changes in the ceramic microstructure.