Study On Preparation And Property Of Metal Oxide Gas Sensor Arrays

Metal oxide gas sensors as the "gas-electricity" transducer to detect gas and control industry production are one of the most widely researched and used gas sensors due to their advantageous features, such as low price, prompt response and simple structure. However, with the development of modern industry and information technology, the use of gas sensors is no longer only limited to annunciating and alarming. Gas sensors are called for not only qualitative discrimination but also quantitative analysis of gas in applications. A single sensor can not competent to do it. At present, metal oxide semiconductor sensors are single devices which have the defects and deficiency as big bulk, high power consumption, poor sensitivity, thermal stability, reliability, selectivity, anti-interference and coherency. Simple multi-devices combined can not meet the needs of industry production. In this dissertation, to increase the sensitivity, selectivity and stability of gas sensor arrays, the effect of doping, pretreatment of powders, pretreatment of thick films and sintering craft were discussed and complex impedance analysis was used to study the thick film micro-structure change according to the sensor stability change, on the basis of the technology to manufacture ZnO based semiconductor gas sensor arrays.Firstly, the application and development trend of gas sensors array were introduced in this paper. The structure and gas-sensing mechanisms of metal oxide semiconductor sensor arrays were expatiated. Improvements and current research evolvements of technology to manufacture metal oxide micro-structure gas sensors and their arrays were summarized. Sensor Performance was analyzed. At last, factors to effect stability, ways to improve stability and methods to research it were particularly discussed.Secondly, gas sensor arrays were manufactured by laser micromachining and screen-printing technique and features of each method were compared. Research was focused on effects of the preparation of gas-sensing materials paste and screen-printing techniques to the morphology of thick films and the resolution of samples. Both laser micromachining and the screen-printing techniques have good stability and reliability. The laser micromachining technique is a simple preparation method and a tiny fluctuation of temperature distribution was brought out on array substrate. The resistance heater on the array substrate prepared by screen-printing technique provides even temperature distribution, and the screen-printing technique is more suitable for volume-produce. The organic vehicle mixed in the paste has important effects to the stability, the printing property of the paste, and coherency of thick film. The main solvent composed of terpineol. butyl-carbitol-acetate and di-n-butyl phthalate with the ration of each 6:3:1 can meet the real needs with gradual volatility. 1wt.% 1,4-butyrolactone, 4wt.% span-85, 0.5wt.% hydrogenated castor oil added serve as rheological agent, surfactant, thixotropic agent. Well-proportioned and stable paste was then prepared by mixing solid powders and organic vehicle together with the weight ration 7:3. The most appropriate ration of solid powders and organic vehicle has small adjustments according to the difference of powder properties. The results show that highest resolution of the printing films can be achieved with printing speed 60mm/s, nets high 3.5mm, pressure of scraper 2.0kgf, and scraping angle 72°.Thirdly, sensitivity and stability of nano-ZnO sensors were researched in aspects as sintering temperature of thick films, pretreatment of powder, and hydrothermal treatment of thick films. Effects of sintering temperature to sensitivity have different features according to different kinds of gas. The higher sintering temperature is, the more stable sensitivity is. Relative humidity (RH) is another important factor to sensor stability. In testing and storage circumstance of 50%RH, sensors show best sensitivity and stability. Pretreatment of nano-ZnO at 700℃for 3h and hydrothermal treatment of thick films at 200℃have control function to the sensitivity of sensors and also increase their stability.Fourthly, single-doped and multi-doped ZnO sensor arrays were fabricated with different sintering craft. And the corresponding sensitivity and stability were studied. The results show that the sensors based on 5wt.% Ni-doped T-ZnO with magnetic-field induced in parallel direction to the thick film surface have the optimized sensitivity and selectivity to 100ppm formaldehyde together with nice stability. ZnO+5wt% Al₂O₃ sensors have comparatively higher sensitivity to methanol, formaldehyde, and acetone; the trinary doped ZnO+5wt.% Al₂O₃+10wt.% TiO₂+1wt.% V₂O₅ sensors are more sensitive to benzene, toluene and xylene and the stability of thick films is also increased; binary doped ZnO+5wt. % Al₂O₃+10wt. % TiO₂ sensors have comparatively lower sensitivity and stability. 10wt.% Cu₂O doped nano-ZnO have optimized sensitivity to all kinds of gas. 5wt.% Bi₂O₃ and Cu₂O each doped ones improve sensitivity and stability.Fifthly, microwave sintering technique was employed to study the sensing property and stability of single doped or multi-doped ZnO sensor arrays. The longer the ZnO and doped ZnO thick films are sintered, the lower the sensor conductivity and sensitivity are. The sensors sintered by microwave for 20min have the highest sensitivity and lowest temperature for optimized sensitivity. Those sintered for 60min have the best stability. The single Al₂O₃ and trinary Al₃O₂, TiO₂ and V₂O₅ doped nano-ZnO sensors have higher sensitivity and stability to 100ppm formaldehyde. Microwave sintering technique can effectively modify and control sensibility and stability of gas sensors and thus is worth further research.Sixthly, complex impedance analysis was employed to study the microstructure of thick films and its change according to the change of stability of sensors. For the ZnO and 5wt.% Bi₂O₃ doped ZnO sensors sintered in electric cooker, the Impedance curve have comparatively large fluctuation In the early continuous high temperature and heating cycle operation, and the phase-frequency characteristics curve tends to high frequencies. The grain boundary resistance and capacitance of the corresponding sensors decrease and increase gradually, respectively. The impedance curve and phase-frequency curve of the 5wt.% Cu₂O doped ZnO sensors is stable in stability testing process, which are contributed to p-n junction micro-structure in the thick films. The stability of the sensors is improved. The ZnO and ZnO+Bi₂O₃ sensors sintered by microwave have the same characteristics of impedance curve, which is a flat half circular arc composed of two quarters of circular arc with different diameters. The thick films are sintered well, so the sensors resistance is stable. For Cu₂O doped ZnO thick film, Cu₂O suppress of the ZnO particle agglomeration, which lower the sensor stability. The impedance curve of the ZnO and doped ZnO sensors by the hydrothermal treatment had remained unchanged in continuous high temperature operation, which was a quarter of circular arc. It indicated that the more uniform, more stable grain and grain boundary structure in the thick films are obtained. However, in the heating cycle operation, the impedance curves are fluctuant. It indicated that the thick films can not effectively absorb stress from themselves and the substrate. The grain boundary micro-structure in the thick films is spoiled and the stability of the sensors is decreased.