Study On The Gas And Humidity Sensing Properties Of Bismuth Titanate Based Perovskite Composite Oxides

Because of potential application in chemical sensors field,low-dimensional sensitive functional materials are fabricated intolow-dimensional micro （nano） device. The low-dimensional sensitivefunctional materials, which are of special physics properties, chemicalproperties and the great potential application in the filed of micro （nano）device, have attracted a great attention. For the bismuth titanate based powderand thin film, we firstly introduced the research progress and applicationbackground of gas and humidity sensing properties for ABO₃-type perovskitefunctional materials, and the review focuses on the preparation andcharacterization methods of sensitive functional materials and the gas sensingmechanism and humidity sensing mechanism of sensitive functional materials.Then, according to the idea of preparation-characterization-measurement-application, the gas and humidity sensing properties of bismuth titanate basedperovskite composite oxides, which are of large specific surface, areinvestigated systematically by the experimental methods, such as thefabrication, sensitive properties, response and recovery properties, hysteresisproperties, selectivity properties and stability properties. The main results aresummarized as follows.1. KBT powders with large specific surface were synthesized by metalorganic decomposition method （MOD）, and characterized by XRD, FE-SEM,TEM and EDS. The humidity sensing properties of KBT powders areinvestigated at different relative humidity （RH） by using the LCR analyzer andhumidity generation sources. By using the complex impedance plots, wediscuss the humidity sensing mechanism based on the KBT powder humiditysensor. The results show:（a） The morphologie of KBT powders is composed ofcoral-like structure, and the crystalline structure of KBT powders is tetragonalperovskite structure.（b） The impedance of KBT powders is greatly affected bythe working frequency at different RH, and the best working frequency is100Hz. At the frequency of100Hz, the impedance changes about four ordersof magnitude within the whole humidity range from11%to95%relativehumidity （RH）, and the impedance versus RH curve have the good linearity.（c） At the frequency of100Hz, the response and recovery time are12s and25s,and the maximum hysteresis is around3%RH at the range11–95%RH.（d）According to Kulwicki theory, the good humidity sensing properties can beexplained for the humidity sensor based KBT powders. The coral-like specialmicrostructures are helpful for the increase of the specific surface, andincrease the adsorption of water molecule, which is beneficial for their sensingproperties.2. NKBT powders were synthesized by MOD, and characterized by XRD,FE-SEM, TEM and EDS. The humidity sensing properties of NKBT powdersare investigated at different relative humidity （RH） by using the LCR analyzerand static gas distribution system. Based on investigation for the compleximpedance plots, the morphologies of NKBT powders and the electricalproperties of doping ions, the effects of doping ions on the humidity sensingproperties are systematically studied for NKBT powders. The results show:（a）The morphologie of NKBT powders is of homogeneous sphericalmicrostructure with diameters of0.5-2μm, and is also featured by a highporosity. The crystalline structure of NKBT powders is tetragonal perovskitestructure.（b） The best working frequency of NKBT humidity sensor is100Hz,and the impedance changes more than four orders of magnitude within thewhole humidity range from11%to95%RH. The response value of NKBTpowders is nearly three times larger than it of KBT powders.（c） At thefrequency of100Hz, the response and recovery time are20s and60s, and themaximum hysteresis is around4%RH at the range11–95%RH.（d） We canfound that the humidity sensing improvement brought about by Na doping canbe explained by the high solubility of Na+ions combining with its goodconductivity in water.3. Zr-NKBT powders were prepared by MOD, and XRD, FE-SEM, TEMand EDS were used to characterize the morphologie and crystalline structureof Zr-NKBT powders. By studying the humidity sensing properties ofZr-NKBT powders at different relative humidity （RH）, the influencemechanism of Zr-doping in enhancing the humidity sensing properties isinvestigated for Zr-NKBT powders. The results show:（a） The morphologie ofZr-NKBT powders is of homogeneous spherical micro-structure, and thespherical micro-structure size of Zr-NKBT powders is larger than the one of NKBT powders. The crystalline structure of Zr-NKBT powders is tetragonalperovskite structure.（b） The best working frequency of Zr-NKBT humiditysensor is100Hz, and the impedance changes about four orders of magnitudewithin the whole humidity range from11%to95%RH. At the same RH, theimpedance of Zr-NKBT powders is much lower than it of NKBT powders.（c）At the frequency of100Hz, the response and recovery time are8s and130s.（d） Comparing with the grain size of NKBT powders, Zr-NKBT powders are oflarger grain size due to the Zr-doping, and it make the grain boundary surfacedecreased which normally account for low resistivity of a polycrystallinematerial.4. The side-heated gas sensors based on KBT powders were fabricated byusing the process of sensor fabrication, and the gas sensing properties of KBTgas sensors were measured by using intelligent gas sensing analyzer and staticvolumetric method. The sensitivity vs. temperature curve, sensitivity vs.concentration curve, response and recovery curve and selectivity curve wasobtained by comparing the resistance of the gas sensor in air with that in thetarget gas. The results show:（a） The sensitivity of the gas sensor based oncoral-like KBT powders is greatly affected by the working temperature, andthe best working temperature of KBT gas sensor is360°C. The sensitivity ofKBT gas sensor to500ppm ethanol is10at the temperature of360°C.（b） Thesensitivity vs. concentration curve is of the good linearity at the ethanolconcentration range of100ppm-1000ppm, and the response and recovery timeare8s and12s.（c） The KBT gas sensor is of good selectivity to gaseousethanol （S=10）, and less sensitive to CH3OH, and totally insensitive to H2, CO,NH3and C2H2. The sensitivities are about3,1,1,1and1to CH3OH, H2, CO,NH3and C2H2, respectively.（d） The gas sensing mechanism for ethanol gas isexplained based on the surface reaction mechanism and oxygen-adsorptiontheory.5. BNdT thin film with large specific surface was prepared by MOD andSpin-coating technology at the different annealed conditions, and themorphologie and crystalline structure of BNdT thin film were characterized byXRD, FE-SEM and EDS. Based on dissection method, the plane gas sensorbased on BNdT thin film was fabricated by magnetron sputtering andlithograph technology. The gas sensing properties of BNdT thin film, such as sensitivity characterization, response and recovery behaviors etc., weremeasured by intelligent gas sensing analyzer and dynamic volumetric method.The results show:（a） Compared with standard card, BNdT thin film consistingof bismuth-layered perovskite structure is polycrystalline, without a preferredorientation. The morphologie of BNdT thin film which is annealed at airpresents a state of loose and porous, and the morphologie of BNdT thin filmwhich is annealed at O2forms a compact structure.（b） Compared with theBNdT thin film annealed at O2, the BNdT thin film annealed at air is of bettergas sensing properties. The best working temperature of BNdT thin film is100°C, which is lower than the temperature of other gas sensing materials（about300°C）, and the sensitivity to1ppm ethanol is6. This means that BNdTthin film has potential application in fabricating low power consumptionsensors.（c） BNdT thin film is sensitive to gaseous ethanol at concentrationdown to0.1ppm, and the corresponding sensitivity is3. The response andrecovery time are6s and10s for the BNdT thin film.（d） The ethanol sensingmechanism for ethanol can be explained by the surface depletion and the targetgas chemisorption and desorption on the materials surface, and the goodethanol sensing properties are mainly caused by the large surface-to-volumeratio of BNdT thin film and plane structure of the gas sensor.