Research On Gas Sensing And Photoluminescent Properties Of NASICON Materials

With the rapid development of industry, the poison gases (CO, CO2, CH4, NOx, SOx…) produced by chemical production, combustion, smelting, mine and automobile offgas are increasing in the atmosphere. The environment is badly polluted. The existence and health of human being are seriously threatened. The influence of green house caused by CO2 is prominent, so exploring CO2 gas sensors with good performance become the focal subject. The sensitivity of metal oxide gas sensors for CO2 is very low because of the bad chemically activity of CO2. The solid electrolyte CO2 gas sensor is promising for its high sensitivity.This article combines the high and new technology development planning subject of Jilin Science and Technology Office"Research on new type CO2 sensor and testing instrument"and the subject of Natural Science Foundation of China"Research on characteristic, design and fabrication of the integrated multifunctional poisonous gas sensors". Against the high power consumption and dissatisfactory stability of solid electrolyte NASICON CO2 gas sensors at present, the preparation and characterization of NASICON materials and the fabrication and modifying of CO2 sensors were studied deeply, the obvious progress was obtained. At the same time, NASICON based NO2 gas sensors and the photoluminescence characteristics of NASICON materials were studied elementarily in order to exploit new investigation and application of NASICON materials.1. NASICON nanophase materials were synthesized by high temperature solid reaction process and sol-gel method. The resulting materials were characterized by means of XRD, IR, Raman and XPS. The following conclusions were gained:(1) NASICON materials synthesized by high temperature solid reaction process and sintered at 1100℃had monoclinic structure. The component of materials was pure. The mean crystallite size was 40nm.(2) Whether nitric acid dissolved or oxalic dissolved was used, the appropriate sintering temperature of NASICON synthesized by sol-gel method is 900℃, that was 200℃lower than the high temperature solid reaction process. The resulting materials showed the presence of monoclinic NASICON and traces of ZrO2. No other phase could be detected in these samples. The mean crystallite size was 10-20nm.(3) The stoichiometric of NASICON synthesized by two methods was perfect, that satisfied the request of solid electrolyte gas sensors.2. The tube-type and miniature CO2 gas sensors were designed and fabricated. The deep research was adopted to improve the sensing performance of sensors. The conclusions were as follows:(1) The tube-type CO2 gas sensors were fabricated. NASICON synthesized by sol-gel method was adopted as bulk material. Li2CO3-BaCO3 binary carbonates in molar ratio 1:1.5 were utilized as the sensing electrode. Within a wide range of CO2 volume ratio concentration from 500×10-6 to 5000×10-6, the sensitivity of the sensor was 61.5mV/decade. The sensors represented better selectivity. The response and recovery time (90%) corresponding to the switching change between 500×10-6 and 1000×10-6 volume ratio concentration CO2 were≤20s and≤2min, respectively.(2) For the first time, we found that the stability and humid resistance of the sensors were improved obviously and the steady time of the sensors could be reduced greatly from 30min to 5min, after the sensing electrode was doped with 10wt. % glass frit.(3) The miniature CO2 gas sensors with NASICON synthesized by sol-gel method as bulk material were fabricated. Li2CO3-BaCO3 binary carbonates in molar ratio 1:1.5 doped with 10wt. % glass frit was utilized as the sensing electrode. When the heating current was 120mA, the sensitivity of the sensor was 67.3mV/decade, which was closed to the sensitivity (61.5mV/decade) of tube-type sensor at the heating current of 180mA. The power consumption was reduced from 1.08W to 0.72W. The sensor performed perfect selectivity, better humid resistance and stability. The response and recovery time (90%) corresponding to the switching change between 500×10-6 and 5000×10-6 volume ratio concentration CO2 were≤20s and≤58s, respectively.3. The fabrication, properties and sensing mechanism of NASICON based NO2 gas sensors were investigated preliminarily. The following conclusions were obtained:(1) The sensitivity of NO2 gas sensors with AgNO3-Ba(NO3)2 binary nitrates in molar ratio 3:2 as sensing electrode was 83mV/decade, when the working current was 120mA. The sensors had better selectivity. The response and recovery time (90%) corresponding to the switching change between 5×10-6 and 50×10-6 volume ratio concentration NO2 were≤30s and≤60s, respectively. However, the humid resistance and stability of the sensors were dissatisfactory because of the instability, decomposability and easy oxidization of nitrates. The irreversible reaction happened after the sensing electrode contacted NO2. The sensing electrode materials were consumed, so the sensing electrode materials with better stability should be found in order to improve the stability of sensors.(2) WO3 was utilized as the sensing electrode of NO2 gas sensors. When the working current was 210mA, the sensitivity of the sensor was 41.4mV/decade. The response and recovery time (90%) corresponding to the switching change between 5×10-6 and 50×10-6 volume ratio concentration NO2 were≤40s and≤90s, respectively. The humid resistance and stability were improved obviously comparing with NO2 gas sensors using binary nitrates as sensing electrode. Moreover, the melting point of WO3 was higher and the sensors could bear high temperature, so WO3 was appropriate sensing electrode of NO2 gas sensors.4. For the first time, the photoluminescence (PL) properties of NASICON and doped-NASICON materials were investigated. NASICON was synthesized by sol-gel method and doped with Er3+, Tm3+, Cs+ and Dy3+ by 1wt. %, 3wt. % and 5wt. %, respectively. The ultraviolet light (325nm) excited luminescent emissions of the resulted powders were investigated preliminarily. The conclusions were as follows:(1) The main peak of the pure NASICON was found at the wavelength of 474nm (blue light). The reason of the PL of NASICON materials was that Zr4+ possessed optical activeness and results in luminescence as an activator.(2) The luminescent intensity was weakened obviously after doping with Er3+ and Tm3+, but which was increased after doping with 3wt. % Cs+ and Dy3+. Er3+ and Tm3+ worked as the quenching center, but Cs+ and Dy3+ possessed activation center. The PL peaks of NASICON: Dy3+ showed red shift about 12nm comparing with NASICON. But the wavelength of NASICON: Cs+ did not change.(3) The influence of sintering temperature on PL properties of NASICON: Dy3+ (1wt. %) was studied. The luminescent intensity of sample sintered at 900℃was the strongest.