Measuring and testing – Gas analysis – Gas chromatography
Patent
1986-06-16
1988-01-26
Noland, Tom
Measuring and testing
Gas analysis
Gas chromatography
73 27R, 324 715, 338 34, G01N 2712, G01N 2714
Patent
active
047209933
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Technical Field
This invention concerns semiconducting nonstoichiometric oxide materials to be used as oxygen sensors to measure oxygen partial pressures in hot gases.
2. Description of the Related Art
Oxygen sensors based on the measurement of electrical resistivity of semiconducting oxides have been proposed. For example D. E. Williams and P. McGeehin in their article "Solid State Gas Sensors and Monitors" published in Electrochemistry, Vol 9, p 246-290 (1984) have discussed the characteristics of this and other types of sensors. Such devices are considered as alternatives to Nernst sensors which are currently used for measuring oxygen partial pressure in combustion products of boilers and car engines. An International patent application No. PCT/AU84/00013 describes one such Nernst sensor.
In the Nernst sensor, a solid electrolyte membrane wirh good oxygen ion conductivity and negligible electronic conductivity separates a reference gas from the test gas. When two electrodes reversible to the O.sub.2 /O.sup.2- redox equilibrium are placed in contact with the opposite faces of the membrane, an emf is established which is proportional to the oxygen concentration difference across the membrane. To avoid mixing of reference gas with the test gas, long impervious tubes of the solid electrolyte need to be used. Alternatively, hermetic seals are formed between a solid electrolyte membrane and a more robust tube of a ceramic or a metal.
On the other hand, the construction of a sensor based on measurement of electrical resistivity of a semiconducting nonstoichiometric oxide material is very simple and no hermetic seals are required. In its simplest form, the nonstoichiometric semiconducting oxide sensor consists of a porous disc of the material or a thin layer of the material painted on an insulator substrate such as alumina and two separate electrical contacts to measure the resistance. The principle of operation of a resistance measuring oxygen sensor is that the resistance of a semiconducting nonstoichiometric oxide material is represented by the expression:
SUMMARY OF THE INVENTION
The value of n varies according to the defect chemistry of the nonstoichiometric oxide. The value of n for TiO.sub.2, ZnO, SnO.sub.2 or CoO based resistance sensors varies between 0.1 to 0.25. The sensitivity of the sensor to oxygen partial pressure changes increases and the relative error in oxygen partial pressure measure ment decreases with increasing values of n. If the value of E, the activation energy for conduction, is high then the sensor is subject to large errors due to fluctuations in the ambient temperature As an example, for an activation energy of leV, the temperature coefficient of resistance is 2.5%/.degree.C. at 400.degree. C. and 1.5%/.degree.C. at 600.degree. C. This, for a value of n of 0.200, will lead to about .+-.13% and .+-.7.5% errors respectively in oxygen partial pressure measurements for .+-.1.degree. C. change in temperature. These errors are rather large in comparison with the Nernst sensor. Thus it is vital to measure the temperature of the sensing material accurately. Since this type of sensor can be of very small size, the accurate measurement, and control by an external heater, of temperature is relatively easy. Alternatively, from the known value of activation energy, correction for temperature deviation from the mean value will significantly reduce errors in oxygen partial pressure measurements.
Oxygen sensing devices based on measurement of resistance of semiconducting nonstoichiometric oxides in general have several advantages which may be summarized as: be prepared in any shape and size; accurate and uniform temperature control; sensor can be increased by simply increasing the current; films of the sensing material.
In principle, a large number of nonstoichiometric oxides can be used for measuring oxygen partial pressures. Limitations are imposed by: geometry and other factors; pressure over the useful oxygen concentration range (0.1-10% O.sub.2); flue or oth
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Commonwealth Scientific & Industrial Research Organization
Noland Tom
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