Tertiary NiCuZn ferrites for improved humidity sensors: A systematic study

Abstract

Numerous works on ferrites containing three or more divalent cations offer incomplete data thus failing to offer clear correlations between composition and properties. This study describes the magnetic and electrical behaviour of spinel ferrites containing Ni, Cu and Zn. Samples with the general formula Ni0.nCu0.cZn0.zFe2O4, with all metals varying between 0 and 0.5, were obtained by the co-precipitation method. The structure of the samples was investigated by X-ray diffraction and infrared spectroscopy, confirming the formation of pure spinel ferrites. The lattice parameter calculation and EDX analysis confirm the composition of the samples.The magnetic behaviour of the nanoparticles is, in general, superparamagnetic with few samples that present small coercivity, corresponding to larger crystallite size. The measurements indicate the superparamagnetic size limit to be of about 14 nm, which can be observed for samples with low zinc content. The electrical properties of the nanoparticles vary with the composition with dominant indirect effect through microstructural changes. All samples have a similar behaviour in relation to electrical current frequency, with variations of about one order of magnitude resistivity. The relative permittivity indicates several samples that change behaviour, such as nickel rich samples, that significantly decrease relative permittivity at higher frequencies compared to zinc rich samples with smaller variations. All samples present variations in the presence of water vapour but increased sensitivity towards humidity was observed for nickel rich samples characterized by larger crystallite size and density. Keywords: Ferrite, Co-precipitation, Superparamagnetic, Humidity sensors