Electrical resistors – Resistance value responsive to a condition – Gas – vapor – or moisture absorbing or collecting
Patent
1980-09-30
1982-08-31
Albritton, C. L.
Electrical resistors
Resistance value responsive to a condition
Gas, vapor, or moisture absorbing or collecting
252518, 252521, H01L 700
Patent
active
043474950
DESCRIPTION:
BRIEF SUMMARY
The present invention is concerned with detecting the concentration of oxygen in a gas mixture. It is known to detect oxygen concentration using an oxygen sensor comprising an electrically conducting oxide material the resistance of which depends on the concentration of oxygen in the ambient atmosphere of the sensor. Known oxygen responsive materials for such sensors are based on titanium oxide, zinc oxide, and cobalt oxide. In order to use the sensor, the material is heated to an elevated temperature at which the resistance of the material becomes responsive to the oxygen concentration in the atmosphere, and then the resistance of the material is monitored and provides an indication of the oxygen concentration. Devices of this kind, have various drawbacks. The temperature at which the oxygen responsive materials become effectively responsive to oxygen concentration to be relatively high and difficulty is experienced in keeping the resistance values of the materials stable. Often the resistances of the materials when in a constant oxygen concentration tend to drift upwards in value when the materials are held at the elevated operating temperature for an extended period of time. Also, phase changes can occur in the materials used which degrade the sensitivity of the materials.
According to the present invention, there is provided a sensing element for an oxygen detector, comprising a body of chromium-rich zinc chromite, and means for making electrical contact with the body of zinc chromite to sense its electrical resistance.
The invention also provides a method of detecting oxygen concentration comprising providing a body of chromium rich zinc chromite, heating the body to a temperature at which the resistance of the material of the body is responsive to variations in oxygen concentration in the ambient atmosphere of the body, and measuring the resistance of the material.
Stoichiometric zinc chromite (Zn Cr.sub.2 O.sub.4) has a characteristic brown colour, and its electrical resistance does not respond to changes in ambient oxygen concentration. Zinc chromite can also be prepared in which the molar ratio of chromium to zinc is greater than 2. Even very small amounts of excess chromium, for example as little as 0.1% molar excess or less, confer on the compound a characteristic green colour, and the property that the compound's electrical resistance at elevated temperature is responsive to oxygen concentration.
It has been found that chromium rich zinc chromite exhibits useful variations in resistance with oxygen concentration at temperatures as low as 400.degree. C.
In a preferred form of the invention, the chromium-rich zinc chromite of the body has an amount of excess chromium above the solid solution limit of the various oxides of chromium produced in the zinc chromite lattice. Good results have been obtained with zinc chromite having approximately 0.2% molar excess of chromium. Zinc chromite may be prepared by a variety of known reactions, of which the preferred is the reaction of +2NH.sub.4 Cl+1.50.sub.2.
The desired stoichiometry of the product may be ensured by appropriate adjustment of the ratio of the starting materials, so that there is a slight excess of Chromium over that which would be required for the theoretical reaction.
Pure chromium rich zinc chromite has been found to suffer from some instability of resistance and the resistance at constant oxygen concentration has been found to rise progressively over a period of days if the material is kept at an elevated temperature.
Preferably, the material includes up to 25 mole percent of a rare earth oxide. The addition of the rare earth dopant was found to enhance greatly the stability of resistance of the doped material with time and also, at least in some cases, to improve the responsiveness or sensitivity of the material to changes in oxygen concentration. Oxides of Gadolinium Neodymium, Yttrium and Lanthanum all exhibited improved stability, but Lanthanum is preferred, providing not only substantial stability of resistance over a period of at l
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patent: 3695848 (1972-10-01), Taguchi
patent: 3932246 (1976-01-01), Stadler et al.
patent: 4007435 (1977-02-01), Tien
patent: 4045764 (1977-08-01), Ichinose et al.
patent: 4086556 (1978-04-01), Nitta et al.
Brook Richard J.
Hunter Donald N.
Albritton C. L.
Rosemount Engineering Company Limited
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