Study On Modification Of SnO₂ And Its Composite Oxide SnO₂-MO_x Based Gas Sensors

Because of high sensitivity, short response time, small size and light weight, metal oxide based gas-sensors are extensively applied in many fields, such as chemistry-engineering, coal mine exploiting, environment conservation, agriculture development and health for detecting poisonous and flammable gas to avoid possible accident.However, SnO₂ is widely-used sensing material and easily affected by the change of operating temperature and humidity, so the long-term sensitivity and stability become worse and can not be applied in detecting a certain concentration of target gas.In the first part of this work that was supported by NSFC, some methods to enhance the selectivity and stability of the SnO₂ sensors were investigated. In the second part, SnO₂-M (M=Ti, Fe) was prepared and its gas sensing properties were also examined. Moreover, metal oxide based HCHO gas sensor was also studied.Sol-gel method was employed to prepare SnO₂ nanopowder as its grain size could be varied by some reaction factors including the solution concentration, the dripping rate speed of NH3 · H2O, the sorts of solvent, the sintering temperature and time. It was found that the grain size would be bigger when the solution concentration was higher or lower, the NH₃ · H₂O dripping rate faster or slower, different sorts of NH3 · H₂O, higher sintering-temperature than 600°CStability is the most important parameter in practical application and it is the most difficult part in exploiting chemical gas sensor. There are many factors that should be considered for getting a high stability, such as the temperature stability of the sensitive material itself and the environment temperature and humidity influence on the sensor performance. We have gotseveral methods that are effective to improve the performance of the SnO2-based CO sensor. (1) For the first time, different materials with positive or negative temperature coefficient were used to compensate the temperature stability of the gas sensors. When BaTiO3 was doped into SnO2, XRD and IR pattern indicated that the grain size was retrained, and the cross-linking between particles was broken, and the ratio surface of material and the concentration of oxygen-adsorption was broaden. As a result, the temperature stability of SnO2-based gas sensor was greatly improved. (2) Different conductive type materials were used with the compensate-theory to improve the temperature stability of gas sensors. For the first time, P-type Lai_xSxrFeO3 was added into N-type SnO2 powder, therefore, it is possible to directly compensate the element in the same gas sensor. By this way, the conduction change of the sensor can be obviously reduced when the operating temperature varied. (3) For the first time, the gas sensor modified by NH4H2PO4 was found to be sensitive to both CO and NH3 and its temperature stability can also be improved. Furthermore, the SnO2 sensors modified by high concentration NH4H2PO4 showed PTC effect. (4) For the first time, IR analysis of thionrea surface modification was investigated. It was found that the thionrea surface modification could reduce the humidity influence on the sensor stability, and SO2'4 could improve the long-term stability.In order to improve the selectivity, we add 4A molecule-sifter at the surface of SnO2 material and, for the first time, it is found that the employing of 4A molecule-sifter is advantage to reduce the influence from C2H5OH and increase the sensitivity to CO gas.SnO?-based CO gas sensors usually have a high sensitivity at low operating temperature because the desorption of CO gas is normally difficult to happen at this temperature. Because of this, the sensor should be operated at a relatively high temperature and the CO desorption could be conducted by complex equipment. In this work, CO gas sensor was prepared with the doping of RuO2-CeO2, which brought about a good response and recovery...