Novel strategies for design of high temperature titania-based gas sensors for combustion process monitoring
TiO 2 -based sensors were developed and characterized for improvement of selectivity and sensitivity towards CO gas at high temperatures. Selectivity was improved by combining several sensors into an array, while sensitivity improvement was studied by examination of the effects of various metal and metal oxide additives and reactive sputtering film preparation. TiO 2 -based sensors containing La 2 O 3 and La 2 O 3 /CuO were combined into an array for testing of CO and O 2 gas mixtures. Nonlinear regression analysis was developed to determine the concentrations of gases in the mixture. The regression technique can be used to determine the prediction ability of gas sensor combinations for given gas concentration ranges using a defined orthogonality index, as well as to determine the amount of each gas in the mixture based on the responses of a set of sensors. The prediction ability for a La 2 O 3 -containing sensor combined with La 2 O 3 /CuO-containing sensors with varying levels of CuO are compared for a range of CO and O 2 concentrations. Concentrations of CO and O 2 in gas mixtures are predicted using the La 2 O 3 - and La 2 O 3 /CuO-containing sensors and the regression analysis. Addition of various metals and metal oxides, including Au, CuO and La 2 O 3 to TiO 2 -based gas sensors has been examined to determine the effect of additives on the gas-surface reactivity and electrical response of the sensors. Electrical measurements show that sensor sensitivity is decreased upon addition of La 2 O 3 with only a slight increase in sensitivity when CuO is added to La 2 O 3 -containing sensors. Au, however, was found to increase the sensitivity of the TiO 2 -based sensors with and without La 2 O 3 . Gas chromatography was used to determine the catalytic conversion of CO for the various sensor materials at different temperatures for comparison with electrical measurements. General guidelines for additive selection have been proposed based on the results. Reactively sputtered thin film TiO 2 gas sensors have been fabricated and characterized for improved sensitivity to CO gas. Sensitivity of optimized thin film sensors to CO was found to be much higher than for traditional thick film TiO 2 -based sensors. Characterization of the phase and morphology of the sensors using XRD, TEM and AFM show that the film consists of a dense layer of TiO 2 in the rutile phase, with surface roughness changing with film thickness. Sensitivity of the thin film sensor could be varied by changing the film thickness, with a film thickness of 240 nm showing the best sensitivity to CO gas, although response and recovery times were slow.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:titania gas sensor carbon monoxide combustion process monitoring
Date of Publication:01/01/2003