Thick film odor sensor with γ-Fe_2O_3 semiconductor oxide

摘要

γ-Fe_2O_3 is a n-type semiconductor oxide and it has been extensively studied as a gas sensing material. It is usually obtained by oxidizing Fe_3O_4 at 250 °C, temperature confirmed by the DTA curve. γ-Fe_2O_3 has a spinel type crystal structure with a lattice parameter of 8,3A and can be expressed in the following form: Fe~(3+).[□_(1/3).(Fe~(3+))_(5/3)]. O_4 where □: cation vacancy The γ-Fe_2O_3 and Fe_3O_4 can be converted from one to the other by a reversible oxidation-reduction process in the gases atmosphere The microstructure of the γ-Fe_2O_3 observed by scanning electron microscopy consist of columnar grains of 0,2-0,5 μm, a porosity of 57% and specific surface area of 16 m~2/g. The sensor is obtained by thick film technology. An alumina substrate was used. On one side of the substrate was screen-printed an interdigitated array electrode using a conductive Pd-Ag ink. A thick film heater by resistive ink was pasted on the other side of the substrate. The sensitive ink was prepared from γ-Fe_2O_3 with a little amounts by TiO_2 and PdCl_2. The sensitive ink was screen-printed on the side with electrode of the substrate in a thickness of 50 μm and heat treated at 380 °C for 2 hours. The sensor was tested in a vacuum oven DUO-VAC. The electrical resistance in pure air was 1,8 MΩ and decrease in gases atmosphere. For 1000 ppm acetone, ethanol, cigarette's smoke, mercaptan, toluene the sensibilities were 93%, 89%, 81%, 75% and 39%, for 300 °C working temperature. The response time of the sensor was 10 seconds.