Transition Metal Doped Lamo <sub> 3 </ Sub> (m = Cr, Ni) Composite Oxide Synthesis And Gas Sensing Properties

ABO₃ perovskite-type compounds possess high thermal stability and crystal structure stability, they also can exhibit high sensitivity and good selectivity to some gases, and the gas-sensing property of this kind of materials can be improved by doping suitable elements in their A and/or B-sites. Therefore ABO₃ perovskite-type compounds have received much attention during recent years.LaCr_1-xM_xO₃(M=Co, Fe, Ni, Sn, Ti)and LaNi_1-xTi_xO₃ compounds were synthesized by sol-gel technique in this paper. The thick film based sensors were fabricated and their gas-sensing properties were tested in the acetone, ethanol, normal propyl alcohol, carbinol, formaldehyde gases etc. LaNi_0.5Ti_0.5O₃ thin film was deposited by RF-magnetron sputtering on the Al₂O₃ substrate, and its gas-sensing capability was tested. Following are summary of our works:(1)The powders of LaCr_0.9Co_0.1O₃, LaCr_0.9Fe_0.1O₃, LaCr_0.9Ni_0.1O₃, LaCr_0.95Sn_0.05O₃ and LaCr_1-xTi_xO₃(x=0.0-0.3) were single perovskite phase with orthorhombic structure. The gas-sensing properties of their thick film sensors were tested in the acetone, ethanol, normal propyl alcohol, carbinol and formaldehyde organic gases. The sensitivity of the sensors can not be enhanced effectively by doping Co,Fe,Ni,Sn elements in B-site of LaCrO₃. Ti dopant with proper concentration can greatly improve the gas-sensing properties, which make the sensor display higher sensitivity and quicker response-recovery characteristics.(2)When 0.0≤x≤0.7, LaNi_1-xTi_xO₃ compounds are perovskite-type single phases; then La₂Ti₂O₇ phase appears for x=0.8; and it changes into the structure of La₂Ti₂O₇ completely when x is greater than 0.8. Ti inclusion in the LaNiO₃ has significant influence on the crystal structure, conductivity and activation energy. The sensitivity and selectivity of the material to acetone can be increased by adjusting Ti content in the complex. The sensor exhibits an excellent sensing property to acetone when x = 0.5. In comparison with LaCr_1-xTi_xO₃, the initial resistance, sensitivity and selectivity of LaNi_0.5Ti_0.5O₃ are all increased and improved. The origin of the gas response is attributed to the processes as following: a reaction may occur between acetone and adsorbed oxygen species forming an intermediate {acetone-oxygen}, which decomposes and gets oxidized immediately. Finally it generates CH₃COOH, which desorbs from the surface of sensing material at 350℃.(3)LaNi_0.5Ti_0.5O₃ thin film was prepared by the RF-magnetron sputtering method and heated at 700℃for 3h. The XRD patterns, EDAX and XPS analysis indicated that the composition and surface state of LaNi_0.5Ti_0.5O₃ thin film are basically similar to those of the powder. Comparing with the LaNi_0.5Ti_0.5O₃ thick film, the selectivity to acetone and conductivity of the thin film get further improved, but the sensitivity, response-recovery characteristics get some decreased. Better parameters of sputtering in film deposition should be chosen in the future research.