Humidity and Temperature Sensing of Mixed Nickel–Magnesium Spinel Ferrites

Abstract

Temperature- and humidity-sensing properties were evaluated of NixMg1-x spinel ferrites (0 ≤ x ≤ 1) synthesized by a sol-gel combustion method using citric acid as fuel and nitrate ions as oxidizing agents. After the exothermic reaction, amorphous powders were calcined at 700 °C followed by characterization with XRD, FTIR, XPS, EDS and Raman spectroscopy and FESEM microscopy. Synthesized powders were tested as humidity- and temperature-sensing materials in the form of thick films on interdigitated electrodes on alumina substrate in a climatic chamber. The physicochemical investigation of synthesized materials revealed a cubic spinel Fd3¯m phase, nanosized but agglomerated particles with a partially to completely inverse spinel structure with increasing Ni content. Ni0.1Mg0.9Fe2O4 showed the highest material constant (B30,90) value of 3747 K and temperature sensitivity (α) of −4.08%/K compared to pure magnesium ferrite (B30,90 value of 3426 K and α of −3.73%/K) and the highest average sensitivity towards humidity of 922 kΩ/%RH in the relative humidity (RH) range of 40–90% at the working temperature of 25 °C.