The Preparation And Gas Sensing Properties Of MFeO₃ Based Semiconductor Materials

With the improvement of social productivity and the life level of most people, gas sensors are used more and more widely. Semiconductor-typed gas sensor is one kind of most widely used gas sensors, which becomes a research hotspot at present. In this study, MFeO₃(M=Sm, La) thin films was prepared by inorganic salt sol-gel method; The indirectly heated and sintered type elements were prepared with traditional process. The synthetical processes and gas sensitive properties to acetone have been discussed. Relation of the composition, synthetical process, structure and sensitive property to acetone gas of LaFeO₃ thin films was investigated in this paper. The gas sensing properties of these two kinds of structures were compared.With citric acid used as complex agent , the precursor solution was prepared by dissolving Sm(NO₃)₃?6H2O(La(NO₃)₃?6H2O), Fe(NO₃)₃?9H2O and other inorganic salts into water, of which the concentration was 0.3mol/L. MFeO₃ thin films with perovskite structure nano-crystalline grains were fabricated after the dip-coating, drying, pre-heating and sintering. FT-IR, TG-DSC, XRD and SEM were used to analyze the precursor and the thin films. Resistance vs. temperature properties of MFeO₃ thin films were analyzed, which showed the characteristics of semiconductor. According to the curves the conducting activation energy was figure out, which was coincided with the theory.The MFeO₃ thin films exposed to low concentration acetone gas showed good sensing properties. The sensitivity of SmFeO₃ to 30ppm acetone gas was 20, the response time was 15s, and the recovery time was 16s at the testing temperature of 450°C. The sensitivity of LaFeO₃ thin film was higher. The sensitivity was 22.5, the response time and the recovery time were 30s and 20s respectively. Its conductive and sensitive mechanism was proposed, with the types and concentration of defects and conductive carrier discussed. The effects of testing temperature, gas concentration, doping and film thickness to the properties of MFeO₃ thin films were deeply studied. The effects of testing voltage, glass frit, noble metal Pt and doping to the properties of the indirectly heated and sintered elements were systematic studied. It was found that the SmFeO₃ element has optimal gas sensing properties to 10ppm acetone, which doped 1mol% Pt at testing voltage 4.5V.Comparing to the properties of the thin film and sintered elements, it was seen that the sensitivity to acetone of thin film was higher because of high surface activity and specific surface area. The selectivity, the stability and the gas sensing mechanisms of MFeO₃ was discussed. It is pointed that the developing direction of gas sensor is thin film kinds elements. Materials of low resistance at low temperature are selected, of which the selectivity is improved by little doping methods.