Study On Preparation And Gas Sensing Properties Of Oxide Semiconductor-catalytic Metal Composite Nanoceramics

Gas detection has been widely used in practical application,particularly in the field of environmental protection and safety monitoring.Gas sensors have developped for several decades,and during the past century the form of gas sensitive element is mainly thick film and thin film materials.In recent years nano-technology was introduced in gas sensors,so the researches of gas sensitive materials have been focused on materials in sizes of nanometer.Traditional gas sensors usually require heating devices in order to work,the form of which includes directly-heated type and in directly-heated type.Their working temperature range from 300 to 800?,which has brought in enormous energy consumption and explosion risk.Nano gas sensitive elements,mainly with the structure of one-dimensional,have greatly decreased the working temperature of the gas sensor even down to room temperature.However,nanomaterials also suffer from the poor mechanical performance,weak stability and bad interference immunity,which seriously hampered the practical application of nanometer gas sensitive element.This thesis chooses ceramic materials as the form of gas sensors.The nano powders of oxide as well as catalytic metal were employed as raw material to fabricate composite ceramics.Two kinds of oxide Ti02 and Sn02 nanoparticles were taken into account as well as catalytic metal Pt,Pd and Au.Mechanical strength,porous structure and appropriate resistance value were obtained through the control of sintering temperature.The target gases were chosen as the common H2 and CO.After materials preparation had been completed,we have characterized their phase structure,microstructure and pore size distribution.It is found that the porous materials with large surface area and sufficient mechanical strength can be prepared by the simple powder sintering process.The experimental results show that the diffusion coefficient of gas in the gap of the materials is very close to that in air.The porosity is the basis for the rapid diffusion of gases and the large amount of contact with the semiconductor surface.We have studied all the samples on gas sensing response to obtain materialswith good performance.Through the carefully study of the real time response behavior of the materials under different conditions,the hydrogen sensing mechanism was established.Specific contents are as follows:1.At room temperature Pd/Au-TiO2 composite nano ceramic is not sensitive to hydrogen,while the sensitivity of Pt-TiO2 composite nano ceramic to 1000 ppm H2in N2of in more than 5000,and the sensitivityto 30 ppm H2 is 200.There is a good corresponding relationship between the sensitivity and concentration.The response time is within 50 s and the recovery time is within 20 s.This experimental resultshows that the bulk gas sensitive elementscan also have room temperature gas sensing properties.When oxygen exists in ambient gas,the relationship between the sensitivity of Pt-TiO2 composite nano ceramic to hydrogen and the oxygen concentration is mutant witha critical value of oxygen concentration that when the hydrogen concentration is about 4.7%,the critical oxygen concentration of about 6.7%.If the oxygen concentration in.the atmosphere is lower than the critical value,the sensitivity is high while the sensitivity decreases sharply when the oxygen concentration is higher than the critical value.2.At room temperature,the sensitive mechanism of Pt-TiO2 composite nano-ceramic to hydrogen is that:the hydrogen atom is adsorbed on the surface of semiconductor and diffusion as dynamic donor impurity,consequently enlarging the carrier concentrationin Ti02.The diffusion capability of hydrogen atom in Tio2 is pvoved by the experiment of the electrolytic treatment of the samples in water.3.At room temperature,the sensitivity of Pt/Pd/Au-Tio2 nanocomposite to CO is very small,while at the temperature of 300?,the sensitivity of the three samples to 700ppm CO in N2 is higher than 1000,which could beattributed to the improvement of catalytic performance caused by high temperature CO.The sensitivity mechanism of Pt/Pd/Au-SnO2 to CO at 300? is the normal depletion layer control mechanism.4.At room temperature,the Pt-SnO2 composite nano-ceramic has a sensitivity of more than 5 to 800 ppm hydrogen in air and a sensitivity of more than 200 to 1%hydrogen in air.The sensitivity of Pt to SnO2in air decreases with the increase of temperature,which is different from the common sense that heating increase the reaction rate.5.At room temperature Pt-SnO2 composites nano ceramicshave the same hydrogen sensitive mechanism as Pt-TiO2composites nano ceramic.But due to the different adsorption energy of TiO2 and SnO2to hydrogen atom,the influence of oxygen to the hydrogen response is of great difference.The sensitivity of Pt-SnO2to hydrogen decreases with the increase of oxygen concentration,but the variation regularityis continuous.The Pt-SnO2composites nano ceramic has a sensitivity of 40 to 1%hydrogenin an atmosphere with 80%oxygen,which is quite different from Pt-TiO2Composites nano ceramic.6.At room temperature,the sensitivity of Pt/Au-SnO2 to CO was low,and the sensitivity of Pd-SnO2 to CO was relative high.The sensitivity of Pd-SnO2 to 1000ppm CO in N2is40,higher thanthat to H2,NO2 and NH3,reflecting that the Pd-SnO2composites nano ceramics havegood selectivity for CO.