Preparation and characterization of Gd³⁺ substituted nano-structured cobalt ferrites for humidity sensor

Magnetic nano-materials CoGd_xFe_2-xO₄, (x = 0.0, 0.1, 0.3 and 0.5) were synthesized by chemical co-precipitation method. Spinel ferrite crystalline phase formation of the developed materials was confirmed by the peak positions in X-ray diffraction pattern. The developed particles were annealed at 573 K to improve their crystalline properties. The crystallite size was calculated by Debye Scherrer's formula. The average crystallite size was found to be 7 nm in the Gd³⁺ substituted sample (with x = 0.5) compared to 27 nm in the case of the un-substituted cobalt ferrite sample. The developed nano-particle pallets have been studied for conductivity based humidity sensors at room temperature in the humidity range of 10% RH to 80% RH. The results of our investigations reveal a strong dependence of the material electrical conductivity on the humidity and the Gd³⁺ concentration. The humidity sensitivity coefficient has been calculated in terms of relative change in resistance of material with humidity and it was found that among all developed nano-structured ferrite materials CoGd_0.5Fe_1.5O₄ ferrite exhibits the highest sensitivity towards humidity with an exponential increase in electrical conductivity of more than thousand times as the humidity increases from 10% RH to 80% RH.