Abstract
We have investigated the LPG and humidity sensing properties of chromium (Cr3+) and aluminium (Al3+) co-doped nano-scale (9 nm-17 nm) spinel magnesium ferrite of the composition Mg(Cr0.5-Al0.5)(x)Fe2-xO4 (for x = 0.00, 0.10, 0.15, 0.20, 0.25, 0.30) synthesized utilizing inexpensive materials and very easy one step sol gel combustion technique using urea as fuel. The so prepared nano-sized powders were characterized by powder XRD for confirmation of phase-purity and crystallite size determination. The FT-IR spectroscopy was done to confirm the various bonds present in the system and also to detect traces of impurity materials which are the by-products of the reaction. Morphological studies were carried out by atomic force microscopy (AFM) and FE-SEM, which revealed the reduced particle size and the enhanced porosity on doping. Response of the sensor is based on varying electrical resistance of the material. The MgFe2O4 is well known for its humidity sensing. The incorporation of Cr3+ and Al3+ ions in place of Fe3+ in the MgFe2O4 system significantly alters its micro-structural features and pores thereby enhancing its sensitivity towards Liquefied Petroleum Gas (LPG) while retaining its humidity sensing. The sensitivity of the sensor for both the LPG as well as humidity increases with dopant concentration and the maximum sensitivity shifts towards higher temperature. The peak humidity sensitivity was observed around 120-130 degrees C at 85% relative humidity (RH) while the peak sensitivity for LPG was observed around 60-70 degrees C at 400 ppm. The humidity sensing mechanism has been explained in considerable detail and the conduction mechanism explained on the basis of Grothuss chain mechanism has also been elaborated.