Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1998-06-24
2000-07-18
Tung, T.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204424, 204291, 204292, 205787, 264104, 4271263, 4271264, 4271266, 429 33, 4292181, G01N 27407
Patent
active
060902493
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a novel electrode material for hydrocarbon sensors, as well as a novel sensor and a procedure for its manufacture.
As is known, the concentration of nonburned fuels in oxygenous gases can be determined directly by means of sensors in the combustion gas flow which feature two electrodes on a solid electrolyte, e.g., yttrium-stabilized zirconium dioxide, which respond in different ways to the test gas. As the potential of one of the electrodes is largely determined by the equilibrium oxygen partial pressure of the gas, a voltage level between the electrodes in the same gas can be determined which is dependent on the hydrocarbon concentration. Preferably, gold and alloys of gold and platinum are used as CHx-sensitive electrodes (c.g., A. Vogel, G. Baier, V. Schule, Sensors and Actuators 15-16 (1993) 147-150).
A disadvantage of these types of arrangements is that gold electrodes are morphologically unstable over time at the relatively high operating temperatures of the cells (.gtoreq.700.degree. C.) and that, consequently, the developing potential is subject to changers over time. Another disadvantage is that a potential jump generally takes place with these types of electrodes when the stoichiometric ratio (of .lambda.=1) is exceeded. In addition, the potential of these types of electrodes is dependent upon pre-treatment with respect to gas admission and temperature, so that memory effects occur, which must be eliminated by constant calibration during the use of electrodes in sensors.
As is known, the electrode material may consist of mixed oxides of the perovskite type, which are generally well-known and well-researched as an oxygen electrode, and are used as materials for electrodes which, preferably, only convert oxygen electrochemically. These types of electrodes are not known as burnable gas sensitive electrodes.
The object of the invention is to provide an electrode material for a sensor which is stable over long periods of time, as well as a sensor which is stable over long periods of time.
This object is solved by an electrode material of the invention.
Because a material with the chemical composition Ln.sub.1-z A.sub.1-x B.sub.x O.sub.3 is provided as the electrode material for potentiometric or amperometric, electrochemical sensors, where Ln is at least lanthanide cation or a mixture of rare earth cations, A is at least a trivalent transitional material, and B is at least a trivalent or bivalent redox-stable cation, an electrode with a perovskite structure can be provided for an electrochemical sensor which is stable over long periods of time, even is aggressive high-temperature environments, once it has been sinter-fused onto a solid electrolyte or a ceramic base material.
In this regard, particularly favorable burnable gas sensitivity is achieved (with burnable gas being generally defined as a component which is gaseous or oxidizable under the operating conditions of the sensor) if the element or the element mixture A and/or the element or the element mixture B exhibits low catalytic activity.
In this regard, it is particularly advantageous if A consists of an element or a mixture of elements from the group comprising manganese, chromium, cobalt, iron and titanium. Preferably, the element or the element mixture B is selected from the group comprising gallium, aluminum, magnesium, calcium, gadolinium, and other redox-stable rare earth elements.
In particular, it is advantageous if A is manganese or chromium or a mixture of the two and if B is an element or a mixture from the group comprising gallium, aluminum, and magnesium. An electrode material in which A is chromium and B is gallium has proven to be particularly advantageous.
Furthermore, it is advantageous if Ln is a lanthanide or a mixture of lanthanides, particularly lanthanum itself, which exhibits especially advantageous properties is relation to this invention.
The parameter x ranges from 0.001 to 0.99, particularly 0.1 to 0.9, preferable 0.1 to 0.8 or 0.2 to 0.5, with a range of 0.15 to 0.25 and, particularly, 0.1
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patent: 5306411 (1994-04-01), Mazanec et al.
Alcock et al, "Perovskite Electrodes for Sensors", Solid State Ionics, vol. 51, No. 3/4, Apr., 1992, pp. 281-289.
Database WPI, Section Ch, Week 9308, Derwent Publications Ltd., London.
Heraeus Electro-Nite International
Tung T.
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