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

Abstract Nickel ferrite (NiFe 2 O 4 ) 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 d l alanine fuel yielding single phase NiFe 2 O 4 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 ~ 54–71 nm. 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 Ni 2+ and Fe 3+ at the octahedral sites relating to microscopic tetragonal P 4 1 22 symmetry expected theoretically for the formation of NiFe 2 O 4 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 ( M s ~47–49 emu/g), but poor dielectric properties, whereas dl -alanine derived powders yield ceramics with high resistivity (~3.4×10 8 Ω cm), low dielectric loss (tan δ ~0.003 at 1 MHz) and high magnetization (46 emu/g). Dielectric dispersion and impedance analysis show good correlation with the changes in the ceramic microstructure.