Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1990-10-31
1992-05-19
Niebling, John
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204424, 204428, G01N 2726
Patent
active
051145618
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with oxygen probe assemblies which embody solid electrolyte sensors and are used to measure the oxygen potential of gases or molten metals. The invention is particularly concerned with probe assemblies capable of operating accurately at temperatures as low as 300.degree. C.
Measurement of the oxygen potential of gases and molten metals using solid electrolyte sensors is well documented. For example, a sensor designed primarily for determinations in molten copper is described in Australian patent No. 466,251 and in the corresponding patents, U.S. Pat. No. 4,046,661, United Kingdom No. 1,347,937, Canadian No. 952,983, Belgian No. 782,180, Japanese No. 80 17340 and West German Offenlegungsschrift No. 22 18227.0. Modifications, particularly to the electrolyte, to improve the sensor for measurements in gases are described in Australian patent No. 513,552 and its equivalents U.S. Pat. No. 4,193,857, United Kingdom No. 1,575,766, Canadian No. 1,112,438, West German Offenlegungsschrift document No. 27 54522.8 and Japanese patent application No. 146208/77.
The solid electrolyte oxygen sensor uses the fact that when a solid membrane of a material with good oxygen ion conductivity and negligible electronic conductivity, termed a solid electrolyte, is held with its opposite faces in contact with materials having different oxygen potentials, an emf is established across the membrane. If one of the oxygen-containing materials is the gas or molten metal under investigation and the other is a reference material of known oxygen potential, then the emf (E) is given by the Nernst relationship: ##EQU1## where: LR=the gas constant,
This emf is measured using electrodes, reversible to the O.sub.2 /O.sup.2- redox equilibrium, placed in electrical contact with the opposing faces of the solid electrolyte membrane.
Australian patent no.466,251 describes various geometrically distinct forms of solid electrolyte oxygen sensor. The most commonly-used form is that of a tube; either open-ended or closed at one end, made entirely from the solid electrolyte. Other designs use the solid electrolyte as a disc or pellet, sealed in one end of a metal or ceramic supporting tube. In all cases the reference environment, which is generally air, is maintained on one side of the tube (commonly on the inside) and the test environment is exposed to the other side of the tube.
Many solid electrolyte materials are known to be suitable for use in oxygen sensors. Examples include zirconia or hafnia, both fully stabilized or partially stabilized by doping with calcia, magnesia, yttria, scandia or one or a number of rare earth oxides and thoria, also doped with calcia, yttria or a suitable rare earth oxide. Australian patent 513,552 discloses the addition of alumina to these solid electrolyte materials to produce a composite solid electrolyte which is particularly suitable for sealing into the end of an alumina tube, thereby making a rugged and leak-tight sensor useful for demanding industrial applications. Australian patent application No.47828/78 and the corresponding patents or applications U.S. Pat. No. 4,240,891, United Kingdom application No. 79 19671, Canadian application No. 329,100, Japanese application No. 69529/79 and West German application No. 29 22947.8 disclose the use of magnesium aluminate spinel as an alternative to alumina, either for the supporting tube or as the inert diluent in the composite solid electrolyte material.
The electrodes on solid electrolyte oxygen sensors generally consist of porous coatings of noble metals such as platinum, gold, palladium or silver, or alloys of these elements. For measurements in gases using a gaseous reference, an electrode is required on each surface of the solid electrolyte; for measurements in molten metals an electrode is required only on the reference side of the solid electrolyte, and then only if a gaseous reference is used. If a solid reference, e.g., a metal/metal oxide mixture, is used there is no need for a separate noble metal electrode; the solid reference mix
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Bell Bruce F.
Commonwealth Scientific and Industrial Research Organization
Niebling John
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