| Nitrogen dioxide(NO2)is a typical harmful pollutant.It results in photochemical smog and acid rain.It is also the main source of PM 2.5.Therefore,it is very important for both environmental protection and human health to develop a fast,stable and reliable NO2 gas sensor.Wide band-gap n-type semiconductors,such as tungsten oxide(WO3),have exhibited to be promising candidates for NO2 gas detection.In this thesis,the single particle and coatings were deposited by atmospheric plasma spray using powder,solution precursor,or a combination of both.Tungsten oxide powder through a powder port and ammonium tungstate aqueous solution through a liquid port were injected into plasma stream respectively or together to deposit WO3 single particle and coatings.The influence of spraying parameters,such as substrate temperature,spraying distance and hydrogen flow rate,on the microstructure of W03 single particle and coatings were studied.The NO2 sensing performence of WO3 coatings was evaluated.Firstly,substrate temperature and spraying distance affect the morphology of single particle deposited by APS and SPPS.Experimental results show that the APS particles were splashed seriously on substrate when without preheating.With the increase of the substrate temperature,particles turned to round splat gradually,and the transformation temperature is about 200 C.For SPPS,the morphology is more complex.A large number of bubble-like particles appeared when the substrate was 200? and hollow spherical particles were formed when the substrate was 250?.Subsequently,the effect of spraying distance and hydrogen flow rate on the single particle were studied.When the spraying distance was 100 mm and the hydrogen flow rate was 4 L/min,the flattening degree was the best.Then,three different methods were used to fabricate WO3 coatings on substrates equipped with Au electrodes.The results of SEM test showed that all these WO3 coatings were porous and layer-structured.The porosity of SPPS coatings was the largest and that of APS coatings was the smallest,while APS+SPPS coatings was in the middle.Our results found that holes in APS coatings were large,and the coatings were stacked by micrometer particles.The holes in SPPS coatings were small and the coatings had large specific surface area.APS+SPPS coatings had a unique morphology that a lot of nano sized WO3 particles adsorbed on micro WO3 particles.Finally,the sensing properties of the sensors based on the plasma sprayed coatings exposed to lppm nitrogen dioxide gas were characterized in a homemade instrument.The sensor response of the SPPS coatings was the highest and that of the APS coatings was the lowest,which due to the fact that the SPPS coatings were porous structure and had the highest specific area.The morphology of the coatings had an important influence on the gas sensing properties.The coatings deposited by SPPS at the distance of 100 mm and the coatings deposited by APS+SPPS at hydrogen flow of 2 L/min had the optimal sensing responses.The influences of gas humidity and working temperature on the sensor responses were further studied.When the working temperature increased from 150 to 250 C,the sensitivity of all samples decreases,but the response/recovery time was shortened.When the gas humidity increased from 0 to 100%,the electric resistance decreased whereas the change of sensitivity is more complicated. |
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