Study On Preparation Of Nano ZnO-based Gas Sensors And Application To Combusive And Explosive Detection

Recently, there is a considerable attention in ZnO-based gas sensors due to their simple manufacture, low cost and convenient test, and new gas-sensing materials, new sensor technology and the artificial intelligence (electronic nose) are still the three main research directions in this field. With the increasing development of the social life, a high healthy environment and body security is highly desirable. Focused on the three directions, systemic works were carried out in the dissertation including nano ZnO-based gas sensors preparation from zinc nanoparticles as the zinc resources, gas sensor security improvement by using UV light exposure and application on the detection of volatile organic compounds (VOCs), ethanol and solid explosives.Zinc nanoparticles with mean size of 35nm were prepared by thermal evaporation method. The pure bulk material was melted and evaporated in a vacuum chamber by renovated hybrid induction and laser heating. The ZnO specimens with different morphologies were obtained from the two-step oxidation of the zinc nanoparticles, namely, the zinc nanoparticles were pre-oxidized by H₂O₂ and then thermally oxidized in air. SEM and XRD results showed that the bushiest ZnO nanoneedles can be obtained after using H₂O₂ pre-oxidization at the Zn/H₂O₂ ratio of 20mg/ml and themal oxidation at 650℃for 2h and ZnO nanoparticles at the Zn/H₂O₂ ratio of 2mg/ml, and the morphologies were evolved from nanoneedles to nanoparticles between the two Zn/H₂O₂ ratios. Whatever the Zn/H₂O₂ ratio is, there is only chestnut bur-like structure appeared after thermal oxidation at 850℃for 8h. DSC-TG results showed that while the content of the ZnO film shell on the zinc nanoparticle surface is about 50.4%, the melting and oxidation processes happened almost at the same time during the thermal oxidation process and the bushiest ZnO nanoneedles can be formed. The H₂O₂ pre-oxidation has less effect on the zinc microparticles and thermal oxidation controls the whole oxidation process. The zinc crystal growth models for nanoparticles and microparticles were also built according to the PBR theory of metal oxidation; meanwhile, the growth process was verified by VS mechanism in the experiment. The gas-sensing performance for VOCs was carried out and the busiest ZnO nanoneedle exhibited the highest sensitivities among the nano ZnO materials, whose sensitivity is 9.9, 11.8 and 13 for benzene, toluene and xylene at 320℃, respectively. However, the ZnO specimens from the oxidation of zinc microparticles performed insensitivity to the VOCs due to the film too thick.The performance of TiO2-doped and WO3-doped ZnO-based and pure ZnO gas sensors to ethanol analyte at room temperature by using UV light exposure at the wavelength of 365nm was investigated. The results revealed that all the sensors are sensitive to UV light and ethanol analyte under the UV light exposure simultaneously at room temperature, but insensitive to ethanol analyte without UV exposure at room temperature, that is, the gas-sensing performance at room temperature must be based on the UV light exposure. There are rapid response and recovery time within 8s for all the sensors. The effect of heating voltage under UV light exposure on sensors were studied, which indicated that the effect UV light become less with the increasing of heating voltage and only UV light exposure but no heater can substitute for about 3.0V heating voltage resulting in a safer working environment for the sensors.The pure ZnO and doped ZnO-based sensor specimens with dopants of Sb₂O₃, TiO₂, V₂O-5 and WO₃ were employed to detect the four explosives such as NH4NO3, mineral explosives (ME), picric acid (PA) and 2,6-dinitrotoluene (2,6-DNT). Among these four explosives, only the fingerprint gases of 2,6-DNT displayed reducing signal but the others oxidizing signals. According to the two rules of small change of base-line resistance and huge change of extreme voltage of the sensors tested by many times and taken these two parameters as feature values, six better sensors were picked out with the demand of system hardware. The detection capability of the sensor array was studied by using static sampling method and the results showed that with the increase of concentration of the analytes, the sensitivities of the sensors increased as an exponential function, and the detection concentration of samples were low to 3.34μg/L. At last, for the sake of approaching to practical application, these four explosives were also studied with full dynamic sampling method and the results demonstrated that all the analytes performed small signals at 1.7μg/L. With the help of discriminant functional analysis (DFA) method, the four explosives could be completely identified at the concentration of 83.4μg/L and 15.3μg/L when appropriate items were extracted as the feature parameters. The four explosives were also well identified at the concentration of 3.34μg/L with the help of radial basis function method and the error rate only 3.12%.Humidity is an important effect factor on sensor's use. Testing the base-line resistances of the six sensors in air within a range of humidity revealed that the humidity has less effect on the resistances of the six picked sensors within the range of 75-97RH%, therefore, the specific testing condition is very important to ensure the reliability of analyse result. The correlation of the content of explosive and sensitivity displayed that the sensitivity of different volume of fingerprint gases from the same mass of explosve is the exponential function of the content whereas the sensitivity of the same volume of fingerprint gases from the different masses of explosves is the quasi-linear function of the content.