IE903288A1

IE903288A1 - Solid oxygen sensor

Info

Publication number: IE903288A1
Application number: IE328890A
Authority: IE
Inventors: Livio Manes; Claudio Maria Mari
Original assignee: Europ Economic Community
Priority date: 1989-09-25
Filing date: 1990-09-10
Publication date: 1991-04-10
Also published as: IE64606B1; PT95406B; DK0420107T3; EP0420107B1; AU6415590A; JPH06502910A; DE59004598D1; CA2066273C; ES2049882T5; LU87596A1; ES2049882T3; EP0420107A1; GR3025334T3; ATE101719T1; PT95406A; WO1991004485A1; CA2066273A1; EP0420107B2; AU637147B2; JP3010376B2

Abstract

The invention concerns a solid-state oxygen sensor in the form of an electrolytic cell with a first electrode exposed to the atmosphere containing the oxygen whose partial pressure is to be measured, a second electrode exposed to a reference atmosphere, and a solid electrolyte in surface contact with both electrodes. The electrolyte material is a ceramic oxide or glassy phase exhibiting pure ionic conduction, and at least the first electrode contains a non-stoichiometric ceramic oxide. The invention calls for the first electrode to be made of a mixture of at least one non-stoichiometric ceramic oxide and a metal, the latter being gold, platinum, nickel or chromium. A sensor of this kind can determine the oxygen partial pressure over a wide range of pressures and is sensitive to poisoning of the electrodes by carbon monoxide or lead.

Description

The invention relates to a solid oxygen sensor consisting of an electrolytical cell, with a first electrode exposed to the atmosphere whose partial oxygen pressure is to be measured, a second electrode exposed to a reference atmosphere and a solid electrolyte in surface contact with both electrodes, the material of the electrolyte being a ceramic oxide or a vitreous phase with pure ion conduction and at least the first electrode containing a non-stoechiometric ceramic oxide. A non-stoechiometric oxide is a thermally stable system which is clearly different from the stoechiomet ric composition. Hereinafter, this will mean both sub-stoechiometric and over-stoechiometric systems.

Such an oxygen sensor is known from DE-32 29 931 Al.

The two electrodes of this sensor are constituted by nonstoechiometric oxides or mixtures of such compounds, for example PrO_ , , CeO_ , TbC> . The solid electrolyte is a 2+x 2-x 2-x ceramic oxide or a glass phase with pure ion conductivity.

Such sensors (PrO2+x is used as electrode material in oxidizing and CeO_ or TbO_ is used in reducing atmosphere) are, ύ X ώ X however, only applicable in restricted measuring ranges.

Also, one has to take into account a poisoning of the electrodes in particular by carbon monoxide or lead, so that such a sensor, when used for the control of atmospheres containing carbon monoxide or of exhaust gas emissions, has only a limited operating time due to poisoning phenomena.

It is thus the aim of the invention to conceive a solid oxygen sensor which permits the measurement of a wide range of partial oxygen pressures, and this as well in a very reducing as in a very oxidizing atmosphere and without an important loss in sensibility or an unacceptable increase in the response delay. The extension of operating time signifies - 2 a very high economic gain, since it is thus possible to use one electrode for the measurement of suddenly changing partial oxygen pressures - such as it happens in operation or by accident.

According to the invention, this aim is attained by the fact that the first electrode consists of a mixture of at least one non-stoechiometric ceramic oxide and a metal, the latter being chosen from the group constituted by gold, platinum, nickel and chromium.

Preferably, the metal weight in the first electrode lies between 10 and 30%, in particular 20% of the total weight.

The invention will now be described more in detail by means of some embodiments.

A solid oxygen sensor essentially consists in two electrodes and a solid electrolyte therebetween. The electrodes are connected to a measuring circuit, the voltage difference between the two electrodes defining the difference of the partial oxygen pressure affecting the two electrodes respectively.

Often, the sensor is in the shape of a tube closed on one side, the inner wall of which constitutes the reference electrode and the outer wall of which constitutes the measuring electrode. The area between these walls is at least par25 tially filled by the solid electrolyte. Inside the tube there is a reference gas of given composition, for example pure oxygen, air or a known gas mixture, such as CO/CO2 or The reference atmosphere can also be constituted by a mixture of metals and metal oxides such as Fe/FeO or Ni/NiO.

The outer side of the tube is submitted to the atmosphere the partial oxygen pressure of which is to be measured.

Of course, the invention is not limited to this shape of a sensor. Thus, the sensor could also be an even platelet, which constitutes the partition between two chambers, i.e. a reference chamber and a measurement chamber. - 3 The reference electrode can, like in the case of the above mentioned document, be constituted by a noble metal or a ceramic oxide or by a mixture thereof.

The electrode facing the atmosphere to be analysed is 5 constituted, according to the invention, by a mixture of at least one non-stoechiometric ceramic oxide and a metal, the latter being chosen from the group constituted by gold, platinum, nickel and chromium, and the ceramic oxide being chosen from the group containing the following compounds: Pr°2+X' CeO- , TbO„, , LaCrO-,, La. Sr CrO.. It is also possible to admix two different ceramic oxides to the metals, one being over-stoechiometric and the other sub-stoechiometric, for example Ce0o_ and PrO. . This takes into account a highly reducing as well as a highly oxidizing atmosphere. Further, it is also possible to use solid solutions of these oxides, for example Ce Pr. 0., . For x there is chosen a value between 0 y 1-y 2+x and 0,2. A mixture of ceramics and metals is called cermet. The oxygen sensor according to the invention has a wide field of application in the supervision of metallurgical processes, in the operation of vapour turbines and ovens, in the exhaust gases control of vehicles and in air supervision. The measuring gas for the partial oxygen pressure can lie between 10 and 1 atm.

Claims

1. A solid oxygen sensor consisting of an electrolytical cell, with a first electrode exposed to the atmosphere whose partial oxygen pressure is to be measured, a second electrode exposed 5 to a reference atmosphere and a solid electrolyte in surface contact with both electrodes, the material of the electrolyte being a ceramic oxide or a vitreous phase with pure ion conduction and at least the first electrode containing a nonstoechiometric ceramic oxide, wherein the first electrode 10 consists of a mixture (CERMET) of at least one nonstoechiometric ceramic oxide and a metal, the latter being chosen from the group constituted by gold, platinum, nickel and chromium.

2. A sold oxygen sensor according to claim 1, wherein the 15 ceramic oxide is chosen from the group containing the following compounds: PrO 2+x , Tb0 2+x' Ce °2- X ' LaCr0 3> La. Sr CrO-, with 0 < x < 0,2, or mechanical mixtures of a 1-x x

3. Sub- and an over-stoechiometric oxide or a solid solution of both . 20 3. A solid oxygen sensor according to one of claims 1 to 2, wherein the metal weight in the first electrode lies between 10 and 30% of the total weight.

4. A solid oxygen sensor as claimed in claim 1 substantially as hereinbefore described.

5. A solid oxygen sensor according to any preceding claim, substantially as hereinbefore described and exemplified.