Thick film organic vapour sensors based on binary mixtures of metal oxides

Previously, composite sensors incorporating differing proportions of tin dioxide and zinc oxide were reported which exhibited high sensitivity to a range of organic vapours under dry conditions (0% relative humidity, RH). Most importantly, the composite sensors exhibited significantly higher sensitivity than sensors constructed solely from tin dioxide or zinc oxide when tested under identical experimental conditions. Work is reported which was carried out on an extended range of composite materials, and involved testing these composites to a range of vapours that have been identified as being associated with the microbial spoilage of cereal grains in storage. Both previously and in this investigation, tests at 0% RH showed that the tin dioxide/zinc oxide sensor elucidated the highest sensitivity to alcohol vapours. However, for the purpose of a practical device it would be necessary to operate the sensors under a flow of high humidity in order to nullify the effects of variations in grain moisture levels. The sensitivity of the composite sensors to known concentrations of volatile organic compounds was measured over a range of relative humidities (0-100%) at a constant temperature of 20 degreesC. A composite sensor comprising 50:50 (w/w) tin dioxide/indium oxide was found to give the highest sensitivity when tested to alcohol vapours at 100% relative humidity. The work also highlighted a difference in the response of sensors to classes of organic compounds when operated at differing humidities. For example, the response to alcohols was reduced significantly when operated at high humidity, whereas the response to carbonyl containing compounds particularly ketones was relatively unaffected. (C) 2003 Elsevier Science B.V. All rights reserved.