Research On The Solid-electrolyte SO₂ And CO₂ Gas Sensors

In this thesis, NASICON solid-electrolyte material SO₂and CO₂ gas sensors are introduced. High temperature solid technique at 1100℃and sol-gel technique at 900℃have been adopted to synthesize NASICON material. The material has been analysed by XRD, IR, RAMAN, XPS and TEM to show the correlating characters about solid structure and elemental composition. Observing x-ray diffracting plots, the material takes phase changing from three directions structure to monoclinic structure when sinter temperature is 1100℃by high temperature solid technique. Observing infrared ray absorption spectrums, the simple spectrums which assumed by the material aren't alike the complex spectrums which assumed by other mul-polymer phosphate. The product presents pure phase. From these analyses, the relative good NASICON material was gotten in the experiment. NASICON is synthesized by sol-gel technique. The raw materials are ZrOCl₂·8H₂O,NaNO₃,Si(OC₂H₅)₄,(NH₄)₂HPO₄,C₂H₅OH and NH₃·H₂O, as well as HNO₃ is used as the resolving solution. The material has been analyzed by XRD, IR, RAMAN, XPS and resistance analysis. From the XRD of the samples sintered at different temperature, it is concluded that the sample sintered at 900℃has already been monoclinic structure and the crystal is tending to perfection. The infrared ray absorption peaks of the samples are simple, which are different from that of the mul-polymer phosphate, it is known that the phase structure is pure. Observing the XPS charts, we know that each element has a single electrovalence. The calculated molecular formula is similar to the theoretic Na₃Zr₂Si₂PO₁₂. Na⁺ moves slowly and the resistance of the material is very high when the temperature is under 250℃, once the temperature is higher than 300℃, Na+ moves quickly and the resistance decreases obviously, which is shown in the resistance analysis. To the SO2 gas sensor , we have adopted the tube-type method. NASICON is coated reelingly on a hollow Al2O3 ceramics tube and sintered at the high temperature, then a base-layer is formed. The sensitive and contact electrode are prepared on each side of the exterior, respectively. Mixtures Na2SO3-BaSO4 and Na2SO3-BaSO4 are formed as the auxiliary phase by melting and quenching method. In order to heat the element, a small heater coil penetrates through the tube. The output electromotive force (EMF) of the sensor is chosen as the performance parameter. Through imitate air comparing with air , find the most suitable sensitive electrode matching, it is 30% to account for one Mole of proportions by BaSO4. The essential capabilities of tube-type element are tested. These capabilities include the relation of EMF and the heating current (temperature), the sensitivity of the element to SO2, the response and recovery time, the influence of humidity, the property of resistance to other gases and the stability in 30 days. It is concluded that the absolute values of EMF increase with the augment of the heating current and the values are linear to the square of the current. In the SO2 volumetric ratio concentration range from 10 to 100×10-6, the linear of EMF to the logarithm of concentration is excellent, as fits well with the Nernst equation. With the increase of the current, the sensitivity of element increases. At200mA current, the sensitivity of the Na2SO3-BaSO4 sensor is 81.1mV/decade and the Na2SO4-BaSO4 sensor is 60.5mV/decade. Considering the impact of the vapor on sensors, we have mixed the sensitive electrode and modified NASICON material, and have reduced the impact of the vapor on device effectively .This current is used as the operate current. The 90% response and recovery time from 10 to 100 ×10-6, the Na2SO3-BaSO4 sensor are 28.2s and 57.7s and the Na2SO4-BaSO4 sensor are 39.8s and 67.4s pectively. The property of resistance to CH4,CO,CO2,C4H10 is measured. In the co-existing gases range from 500 to 5000×10-6, above four gases affect the element differently, especially CH4. In 30 days, the EMF values of the same element are tested. It is shown that EMFs fluctuate.When the SO2 volumetric ratio concentration is 100×10-6,the EMF of the Na2SO3-BaSO4 sensor has nearly been changed 40mV. It indicates that the stability of this sensoris not very good .This may be because Na2SO3 is unstable in the air, will oxidize into Na2SO4 gradually , thus make the component part of the auxiliary electrode changed. And the EMF of the Na2SO4-BaSO4 sensor has little change , is smaller than 10mV , and demonstrates the better stability. Adopt ZrO2 , SiO2 and Na3PO4 to formate NASICON material directly, and make plate-type CO2 sensor with it. And find that the sensitivity of the sensors sintered at 1200℃is the largest and is 49. 7mV/decade. We test the impact of the vapor on sensor and find its relative value of EMF changing is smaller than 10%, and demonstrates the better resistance to humidity. From the imitate air to 100×10-6 ,the 90% response time is about...