| Monitoring of environmental humidity is a basic part in many fields such as meteorology,medical equipment,industry and agriculture.Therefore,humidity sensors play an indispensable role in our daily production and life.High performance humidity sensor should have excellent sensitivity,fast response/recovery time,small hysteresis loop and good repeatability.Among the sensing parameters that can be used to characterize humidity,the capacitance,resistance,and impedance are the most widely used.Therefore,humiditysensitive materials are required to have good electrical properties.Rutile TiO2 and SnO2,as important n-type semiconductors,play an important role in the field of dielectric and humidity sensing.However,it is difficult for single-phase materials to achieve satisfactory humidity sensitivity,complex defects and heterostructures need to be formed by doping and compounding,so as to increase the active sites of water absorption and improve the humidity sensitivity of materials.In this paper,rutile TiO2 and SnO2 are taken as research objects,and doping and second phase compositing are adopted to control the structure of the materials to improve the humidity sensing performance.The thesis is summarized as follows:(1)The(In+Nb)codoping was used to control the dielectric and humidity sensitivity of rutile TiO2.(In0.5+Nb0.5)xTii-xO2(INTO,x=0%,0.5%,1%,2%,5%,10%)ceramics were prepared by solid-state reaction method.it was found that there were two relaxation processes in the dielectric temperature spectrum,which were the low-temperature polaron relaxation caused by the electronic transition between Ti3+and Ti4+ions and the Maxwell-Wagner relaxation caused by the surface layer effect at near room temperature.The humidity sensing results show that the capacitance of all samples increases with the increase of relative humidity(RH%)and doping level,and the humidity sensing mechanism is attributed to oxygen vacancy and NbTi· defect.(2)The(K0.25+Ta0.75)xSn1-xO2(KTSO,x=0%,1%,2%,5%)powder prepared by solidstate method,and the humidity sensitivity was controlled by changing the deposition thickness of the sensing layer.The results show that the humidity response increases with the increase of doping level,which is attributed to the increase of oxygen vacancy concentration and alkali metal-like ion defects,promoting the adsorption of water molecules.On this basis,the humidity response of x=5%samples under different deposition thicknesses was studied.We found that there is an optimal thickness of about 62.41 ?m,and the sensor based on this thickness shows the best sensing performance.Specific surface area and micro-strain stress are considered to be the influencing factors related to thickness,and the humidity response increases with the increase of both.(3)Composites of TiO2/NaNbO3 and TiO2/(K,Na)NbO3 were synthesized by a hydrothermal method,and impedance humidity sensors were prepared based on the two composites,the humidity sensitivity characteristics of pure phase TiO2 and composite materials were compared.The results show that,compared with the humidity response of pure phase TiO2 of two orders of magnitude,the composite material exhibits ultra-high humidity response of more than five orders of magnitude,and the great improvement of this sensing performance is mainly due to the fact that the heterojunction barrier formed by composite is easily affected by environmental humidity. |
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