Electrical Conduction Mechanism and Improvement of Odor Discrimination Ability in Chemisorption-Type Solid-State Sensors

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(1) The transient response of SnOィイD22ィエD2 sensors to ethanol vapor was measured as function of temperature. The rise time decreased rapidly above 200℃. The fall time, on the other hand, began to increase at about 200℃ and then decreased after passing a maximum at a temperature between 250 and 300℃. This complicated behavior of the response time suggests the competing process between oxygen chemisorption and reduction at the oxygen-adsorped surface of SnOィイD22ィエD2. Moreover, this result indicates the recommended working temperature above 300℃.(2) The electrical response of n-type SnOィイD22ィエD2 and p-type CuィイD22ィエD2O ceramic sensors for ethanol vapor at temperatures above 200℃ has been studied. It was confirmed that the electrical resistance of these two sensors changed to opposite direction in accordance with the consideration in which the reduction-oxidation reaction at the surface with adsorped oxygen atoms is the main factor controlling the electrical resistance.(3) The electrical r … More esistance of SnOィイD22ィエD2 sensors in dry air showed a peculiar temperature dependence with a positive temperature coefficient in the range above 200℃. However, it was found that the temperature coefficient changed reversibly from positive to negative or vice versa with exposure to ethanol vapor and dry air ; suggesting a close relationship between the positive temperature coefficient and the oxygen chemisorption.(4) 15 kinds of oxide sensors, SnOィイD22ィエD2-ZnO (1〜10 at%), ZnO, CuィイD22ィエD2O, CuィイD22ィエD2O-NiO (1〜10 at%), NiO, SbィイD22ィエD2OィイD23ィエD2, FeィイD22ィエD2OィイD23ィエD2, TaィイD22ィエD2OィイD25ィエD2, CrィイD22ィエD2OィイD23ィエD2 and WOィイD23ィエD2, has been prepared. The activation energies of the electrical resistance in dry air ranged form 0.2 to 0.7 eV, indicating electrically extrinsic nature in these materials. These 15 sensors heated at 300℃ had different detection sensitivities from each other for the tested 5 species of odor gases, α-pinen, 2-heptanone, benzoic ethylester, formic ethylester and ethanol. When 6 sensors were properly selected among 15, the sensitivity patterns scaled on the 6 axes radar-chart were found to be clearly distinguishable each other at least for 5 kinds of odor gases. This result suggests that the composite odor sensors can be constructed by combining several sensors made of different kinds of materials. Because there are a number of combinations of oxide semiconductors and their mixtures, and possible applications of catalytic metals, one can construct composite sensors by best choice of oxide materials which have appropriately sensitive to odor molecules to be identified. Less