Preparation And Characterizations Of Thick Film CO₂ Gas Sensors Based On Nano Materials

Due to continuous deforestation of forests and abusing of fossil fuels,the content of CO₂ in the atmosphere is nearly doubled that before the first industrial revolution,which has caused serious environmental problems such as the greenhouse effect,imbalance of carbon cycle,and seawater acidification.Meanwhile,it also have serious influences on human body,if someone stay in high content of CO₂.Therefore,CO₂detection technology has become one hot topic recently.Semiconductor gas sensors have the advantages of simple preparation process,low cost,small size,high sensitivity,and fast response and have become one of the most important sectors of gas sensors.In this thesis,SnO₂was selected as the gas-sensitive material and screen printing method was used to prepare the CO₂ gas sensor.Firstly,the effects of each component in the SnO₂ paste were thoroughly studied.The volatility,viscosity,thixotropy,and dispersion properties of the paste were analyzed.It found that when the mass ratio of terpineol and butyl carbitol acetate in the solvent was 7:3,the solvent volatilized slowly at low temperatures（below 100°C）,and became more volatile at high temperatures（above 150°C）.This composition is more suitable for the tape casting process.When the mass ratio of ethyl cellulose and plasticizer was 3:1,and the mass ratio of dibutyl phthalate（DBP）and dioctyl phthalate（DOP）in the plasticizer was 2:1,the organic vehicle possessed the best thixotropy behavior.When the content of the surfactant triethanolamine was 1.5 wt%of SnO₂,the paste has the lowest viscosity,indicating that the SnO₂ particles were well dispersed.When the mass ratio of organic carrier and SnO₂was 5:4,the paste with 5 wt%ethyl cellulose will not cause the phenomenon of flow edge or cannot completely pass through the screen during printing,and it had the best printing properties.The CO₂ gas sensor was prepared by screen printing technology.In order to improve the response of sensor,palladium nanoparticles were added into the SnO₂ceramics.It found that when the Pd content was 1 wt%（base:SnO₂）,the thick-film gas sensor had the best response at 198°C,and the value is 1.36.The prepared CO₂sensor can detect CO₂ with a concentration of higher than 1000 ppm.The SnO₂ thick film sensor with Pd content of 1 wt%show the highest response to CO₂,when the operating temperature is 258°C.The operating temperature of the semiconductor gas sensors is usually high,which limits its applications.By adding photothermal materials to semiconductor gas-sensitive materials,one can use light illumination instead of traditional heating methods to activate the gas sensor.Finally,a flexible photothermal nanocomposite film based on Ti₂O₃-PVA has been demonstrated for converting sunlight into thermal energy.Ti₂O₃ nanoparticles were surfaced-modified by four kinds of different surfactants（γ–aminopropyltriethoxy Silane,Hexadecyl Trimethyl Ammonium Bromide,Stearic Acid,Polysorbate-80）,respectively,and then the nanocomposites were prepared by a simple solution casting method.The flexible composite shows strong sunlight absorption in a wide spectrum range（300 nm-2000 nm）.Under illumination of 5 k W/m² simulated sunlight,the nanocomposite can be quickly heated up to 64.1°C.The nanocomposite presented stable performances after tens of bending tests,indicating good flexibility.Our results show that the nanocomposite with Ti₂O₃nanoparticles modified with Stearic Acid exhibits the strongest light absorption capability and thus the best photothermal effect.This excellent photothermal material can be used to enhance the performance of the CO₂ gas-sensitive materials.For example,a low-power comsumption CO₂ gas sensor can be realized,which is activated by light illumination at room temperature.Meanwhile,the effects of Ti₂O₃content and different wavelengths of light on the gas sensing performance of the sensors could be studied（due to the new corona virus epidemic,this part of work is not completed yet）.