Development And Characteristics Of Series Poisonous Gas Sensors

Along with the advancement of modern society and development of science, more and more poisonous and harmful gases release from the industrial production, automobile exhaust or interior decoration. These gases could form great threat to the people's physical fitness and the safety of life and their properties. The principal pollutant of the atmosphere and indoor pollution includes NO2, H2S, CO, SO2, C7H8, and so on. It is very important to develop the sensors for detecting and controlling these toxic and harmful gases. This paper develops series of poisonous and harmful gas sensors in allusion to these gases.The current research of this paper is carried out from two kinds of gas sensors: metal oxides semiconductor type and solid electrolyte type. Starting from the synthesis and characterization of the sensing materials, several types of gas sensors have been studied, for example, semiconductor NO2 sensors; solid electrolyte potentiometric SO2, C7H8, NO, H2S, CO sensors; solid electrolyte double function CO-C7H8 sensor; solid electrolyte amperometric NO2 sensor. The gas sensing properties of the fabricated sensors have been measured and studied. The effects of sensing materials selection, materials preparation method, materials and devices sintering temperature, dopants selection, doping ratio, operating temperature etc on the sensing characteristics have been investigated.1. Thick film NO2 gas sensors have been fabricated with NiO synthesized by ammonia precipitation, homogeneous precipitation and citric acid precipitation process respectively. The results exhibit that the best sensing properties to NO2 occur for the sensor with NiO synthesized by homogeneous precipitation process sintered at 600°C as the basis material and 3 wt. % WO3 as the adultant. At 180°C, the sensitivity to 100 ppm NO2 is 15.8. The response time of the sensor to 100 ppm NO2 is 15 s.2. The NASICON solid electrolyte is prepared by sol-gel and solid reaction processes, respectively. With the help of XRD and SEM, the synthesized materials are analyzed and characterized. The results show that monocline structure has been seen for the NASICON prepared by sol-gel process at 1000°C. The mean crystalline size of NASICON synthesized by sol-gel process is about 10-22 nm. The NASICON prepared by solid reaction process at 1125°C exhibits the monocline structure also. The mean crystalline size of NASICON synthesized by solid reaction process is about 7-8 nm. 3. Solid electrolyte potentiometric SO2, toluene, NO, H2S, CO sensors and double function CO-C7H8 sensor have been fabricated with NASICON prepared by sol-gel process and different metal oxides or composite oxides sensing electrodes.(1) At 375°C, the sensitivity of the sensor with ZnSnO3 as the sensing electrode to SO2 is 255 mV/decade. Well response-recovery characteristics have been observed for the sensor to SO2. The response and recovery time to 5, 10, 20, 50 ppm SO2 is 18, 10, 8, 5 s and 27, 99, 184, 243 s, respectively. Also, the sensor exhibits well selectivity and water resistance performances to SO2. ZnSnO3 sintered at different temperatures have the single quadrilateral structure.(2) At 430°C, the sensitivity of the sensor with Sm2O3 as the sensing electrode to toluene is -75 mV/decade. The response and recovery time to 5, 10, 20, 50 ppm toluene is 45, 42, 38, 35 s and 8, 32, 50, 60 s. Also, well selectivity and stability have been observed for the sensor. The variation for the sensitivity of the sensor to 50 ppm toluene is less than 5 mV between 30 days.(3) At 350°C, the variation of EMF value for the sensor with NiWO4 as the sensing electrode to 5-500 ppm NO is 126 mV. This type of sensor shows well response-recovery properties to NO. The response and recovery time to NO with different concentrations is all less than 30 s. Also the sensor exhibits well selectivity and stability to NO. The variation for the sensitivity of the sensor to 5 or 500 ppm NO is less than 5 mV between 50 days. NiWO4, ZnWO4, CoWO4, CuWO4, MgWO4 prepared by solid reaction process all belong to scheelite structure.(4) At 350°C, the sensitivity of the sensor with Nb2O5 as the sensing electrode to NO is 68 mV/decade. Fast response-recovery rates to NO have been observed for the sensor. The response and recovery time to 5, 10, 20, 50 ppm NO is 20, 18, 14, 10 s and 10, 22, 33, 43 s. Also this kind of sensor shows well selectivity to NO.(5) At 320°C, the variation of EMF value for the sensor with Ni-Ti oxides (mole ratio for Ni and Ti is 1: 3) as the sensing electrode is -132 mV. The response and recovery time to 5, 20, 50 ppm H2S is 10, 8, 4 s and 20, 28, 40 s. Also the sensor exhibits well selectivity and water resistance properties to H2S. Rutile structure has been observed for Ni-Ti composite oxide synthesized by sol-gel process.(6) At 400°C, the sensitivity of the sensor with Y2O3 as the sensing electrode to CO is -45 mV/decade. The response and recovery time of the sensor to 5, 10, 20, 50 ppm CO is 25, 22, 20, 18 s and 16, 22, 25, 39 s. This type of sensor shows well selectivity and stability to CO. The variation for the sensitivity of the sensor to CO is less than 3 mV between 30 days.(7) The optimal operating temperature of the single sensor for the device with Y2O3 and ZnO-TiO2 as the sensing electrodes is all 400°C. The change direction of EMF for the two sensors to CO and toluene is opposite. At 400°C, the sensitivities for sensor A and B to CO and toluene occurs and are -45, 64 mV /decade, respectively. The device exhibits well selectivity to CO and toluene, too. The device can be considered to be the assemble device of single CO and toluene sensor. The heater and Au reference electrode are public for the two sensors. This structure increases the heating efficiency of the heater.(8) The sensing mechanisms of the above solid electrolyte potentiometric gas sensors can be explained by the mixed potential theory.4. A solid electrolyte amperometric NO2 sensor has been fabricated with NASICON prepared by solid reaction process as the ionic conduction layer and Au, Au, Au+carbonates+NaNO2 as the sensing, reference, counter electrodes, respectively. The results show that the variation ofΔI for the sensor with 9 wt. %SrCO3 as the adultant is 3.7×103 nA at 200 oC. The sensitivity of the sensor to 10 ppb NO2 is 300 nA. The tested range for the sensor to NO2 is 10-100 ppb. The sensing mechanism for the sensor could be explained by the synthesis and decomposition reactions of NaNO2 at the sensing and counter electrodes.