Fe2O3-activated Cr2O3 thick films as temperature dependent gas sensors

Thick films of pure Cr2O3 were prepared by screen-printing technique. Pure chromium oxide (Cr2O3) thick films were almost insensitive to reducing gases. The surfaces of these films were modified by dipping them into a 0.01 M aqueous solution of ferric chloride (FeCl3) for different intervals of time, followed by firing at 550 °C for 30 min. The firing resulted in the oxidation of the FeCl3 additive into Fe2O3. The characterizations and the gas sensing properties of pure and surface activated Cr2O3 thick films were investigated. The grains of Fe2O3 dispersed around the grains of Cr2O3 varied the barrier height between the grains. Upon exposure to reducing gases at a particular temperature, the barrier height decreased greatly, leading to a drastic increase in conductance. The sensor gave optimum responses to different gases at different operating temperatures. The surface misfits, calcination temperature and operating temperature can affect the microstructure and gas sensing performance of the sensor. The quick response and fast recovery are the main features of this sensor. The effects of microstructure and activator concentration on the gas response, selectivity, response time and recovery time of the sensor in the presence of reducing gases were studied and discussed.