CA1106446A - Probe for an electrochemical oxygen measurement pickup - Google Patents

Abstract

F-6449a PROBE FOR AN ELECTROCHEMICAL OXYGEN MEASUREMENT PICKUP

Abstract of the Disclosure A probe for an electrochemical oxygen measurement pickup having an oxygen ion-conducting solid electrolyte with electrodes and electrical contact points connected by conduc-tors to the electrodes, in which the probe has an electro-chemically acive region provided with the electrodes and an electrochemically passive region provided with the contact points as well as with their electrical connections to the electrodes. This minimizes or avoids false readings resulting from temperature gradient and changes in chemical equilibrium.

Description

` 1106~46 ) Background of the Invention ¦ .

Field o~ the Invention This invention relates to a probe ~or an electrochemical oxygen measurement pickup having a solid3 oxygen ion-conducting electrolyte with electrodes, and contact points at the end, ..
which are connected to the electrodes in an electron-conducting manner.

Description of the Prior Art Probes (sensor elements) for oxygen measurement pickups often consist of an ion-conducting solid-electrolyte tube which is closed on one end and the inner and outer surfaces of which .are provided with ~lectrodesO These electrodes extend up to .
the open end of khe solid-electrolyte tube and form there the electrical contact points, from which the electric probe voltage is taken and is conducted ~o exter~al terminals. If su~h .
mea~uring pick~lps are used~ there is danger of fals~ measure-men~s~ as the solid-elec~rolyte tube which usually protrude~ !
;~ ~ransver~ely into ~he hot gas stream has a temperature gradient :; ~rom it~ tip to its end and there~ore has dlfferent ion '~.
conductivity. As a ~esult; a voltage which decreases from the : prob~ tip toward the end is delivered and the resultant electric v41~age whlch i~ taken off at the contact points is thereby inaccurate and~cannot be related unequivocally to a definite ox~gen content o~ the gas ko be measuredO

in addltlon3 the electrode exposed to the gas to be measured must be catalytically effective and must bring the gas compo-, nents into chemical equilibrium, ~hen the chemical equilibrium ' .
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~ - 1106446 ¦ is adjusted differently alDng the probe in accordance with the ., temperature gradient. Fr~m this, dif~erent gas concentrations result~and therefore~ different electric electrode voltages (mixed potentials), which additionally falsify the electrical signal of the probe.

Summary o~ the Invention An object of the invention is to provide a probe ~or oxygen measure-ment pickups of the type mentioned which will deliver a more accurate voltage measurement, i.e. a measurement substantially free of the errors resulting ~rom temperature gradient and vari-ances in chemical equilibrium.

With the ~oregoing and other obJects in ~iew~ there is provided in accordance with the invention a probe ~or an electrochemical oxygen measurement pickup having an oxygen ion-conducting solid electrolyte with electrodes, and electrical contact points con-nected to the electrodes, the combination therewith of a tubular-shaped probe having an electrochemically active region o~ the probe which has the electrodes and the oxygen ion-conducting solid electrolyte for passage of oxygen ions through the solid electro-lyte, and an electrochemically passive region of the probe which .
carries the contact points as well as their electrical connectionsto the electrodes, with the passive region having a solid electro-lyte on which the contact points and electrical connections are arranged, and with the contact points spaced from each other as

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1~ 446 well as the electrical connections consisting of conductor runs to the electrodes spaced from each other, with the conductor runs arranged diametrically opposite to each other on the tubular probe to require ions moving ~rom one contact point to another and one connection to another to t-ravel a long path thereby reducing an interfering current.

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11~64~6 Brie~ Description of ~he ~rawings ~he invention, howeverJ together with additional ob~ects .
and advantages thereo~ will be best understood from the follo~ng description when read in connection wlth the accom- i panying drawings, in which:

- ~IG. 1 shows an axial longitudinal cross section through a tubular probe, closed on one side, in accordance with the invention; and .

~IGc 2 i~ a cross section through the tubular probe along llne I~-~X o~ ~XG~ l; and ~X~ 3 is a cross section through the tubular probe along line III-XII o~ ~G. l; and FIG. 4 i~ a variant of the tubular probe shown in ~XG. l; and ~G. 5 ls a~ a~lal lo~gitudinal sectlon o~ another variant of ~he tubular probe ~hown ~n ~IG. l; and ~XG~ 6 is the ~ubular probe of ~IG. 5 with another des1gn o~
~he lnsula~ing body.

. ~ led_Descrlptlon Or the Invention ~o r~he probe has ~ least one electrochemlcally active region prov.ld~d wlkh ~he electrodes a~d an electrochemically passive~
or at lPast largely passive reg~on which is provided with the contact points as well as wi~h their electrical co~nections to the electrodes. The active regio~ can therefore be chosen independently of the total length of the probe ~nd be adapted !

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11064A.6 specially for the intended application. Here, di~ferent temperatures of the passive part are without, or at least wlthout appreciable effect on the voltage delivered by the probe.

A m~nimum o~ structural means is required if both regions of the probe have a solid electrolyte, on which the contact points as well as the connections are arranged with a large mutual distance for forming the passive region. Although in this embodiment, the contact points as well as the connec-~ion~ are arranged on the solid electrolyte, their ef~ect as active electrodes is very small, as the path which i~ very long as compared to ~he active reglon, for the ions in the ~olld electro~yte of the passive region results in a high lnternal resistance and thus in a small delivered current, whlch hardly causes inter~erence. The active region e~fects the voltage delivered by the probe. The passive reglon is wl~houk appreciable e~rect on the voltage delivered by the probeO, I

In ~rde~ to reduce the influence of the passlve part further, ~n insulatlng layer which does not conduct ion~ or electrons i~ in~erted bet~een ~he two regions. This elimlnates part- .
lcularly the effects of the junctions rrom the electrodes to ~he connec~lons. The in~luence of the pass1ve part on the measureme~t is p~actically completely eliminated by forming - the passi~e region with an insulatlng part~ preferably of eeramicD which does not conduct electrons or ions.
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The connections consist advantageously of conductor runsO I~
the probe is of tubular design, it is advisable to arrange the 644~

i conductor runs approximately diametrically. For a good compromise between the requirement o~ an active region as large as possible and the requirement of a measurement s~b-~tantlally una~ected by in~luence of temperature gradient and chemical reaction, it is advisable to have the passive region o~ the probe disposed in that part of the probe which is to extend in a probe housing.

~n the drawings, identical parts in the individual f~gures have the same reference symbols.

~e~erring to FIG~ 1, the probe comprises a solid-electrolyte ~ube 10 with a closed le~t and an open right end. Zirconium diox~de serves as the oxygen ion-conducting solid electrolyte.
~n ~he actLve region 11 of the probe which is provided ror del~vering the measurement signal a~d which extends from khe t~p Or the solid-electrolyte tube 10 in the axial dlrection~
the entire outer ~urface ls pre~erably provlded with an outer electrode 13 and ~he entire inside surface with an inner e~ectrode 14. In so~e cases it ma~ also be su~flcient to make one or both electrodes as mutually opposite stripsO The act~ve reglon 11 i~ deslgned as to its electrode areas and the e^ectro l~te thickness so that at the operating temperature a suf~icient-ly ~.OW inte~nal electric resistance is obtalned5 ~or instance, be~ een :lO and 10~000 K-Q3 and preferably 10 to 100 K~. The passive region 1~ formed with the same solid electrolyte carrles an outer connection 15 connected to the outer elec~rode 13 and an lnner connect~on 16 connected to the inner electrode 14, the ends of which form the c~ntact points 17 and 18.

The connections are electron-conducting and, as can be seen clearly ~rom FIG. 3, co~sist of conductor runs~ the width _5'_' ~ I

llQ6~A~fi which is between l and lO mm, depending on the probe diame~er, but preferably between 2 and 4 mm. It is important here that the path for the ions from the one to the othe~
connectlon through the solid-electrolyte tube i5 very long, `
as thereby ~he ln~ernal electric resis~ance is increased considerably and the passive region 12 cannot therefore deliver an appreciably inte~fering current and thus, for all practlcal purposes, cannot falsify the measurement voltage. ~he connec-tlons 15 and 16 may be made of any electron-conducti~g, corrosiorl-resistant materlal but preferably, they conslst of extensions of the respective electrodes. In order to ensure good contact with the potential leads which go on from there, the conductors can be widened at the contact polnts.

; The construction of the probe according to FIG. 4 is slmilar to that of the probe of FIG. 1. The difference between the probe o~ FIG. 1 and the probe Or FIG. 4 consists merely in the ~olid electrolyte Or the active region ll being separated ~rom the solid electrolyte of the passive region 12 by an . ~ :
1nterposed clrcular insulatlng layer 19 (FIG. 4). This o~ measure creates a clean separation Or the two regions.

; The 1nsula~ing layer consists preferably o~ gas~tightJ sintered glass ceramic which does not conduct ~ons or electrons, such as ceramic Or the ~ollowlng composition: SiO2, 35 to 50%;
MgOg 50 to 30~, and A1203, 15 to 20~. The insulating layer .s conneGted b~ a sintering or fusing process to the tw~ parts o~ ;the tube~ which, incidentally, may also have dirferent outside diameters, as shown in FIG. 6.
e probe shown in FIG. 5 corresponds generally to the probe :.
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~ IG. l. The probe shown in FIG. 5 corresponds generallyto the probe of ~G. 1. The difference between the probes is tha~ the passive region 12 is bullt up on an insulating part of kubular shape 20 (FIGo 5) which ls fastened to ~he solid-electrolyte tube ~0, and the outside and inside d~ameter o~
the insulating part 20 corresponds approximately to the diameters o~ the solid-electrolyte tube lO. As this insulating part 20 ..
consists of a material that does not conduct ions or electrons, there is practically no danger that the measurement result is influenced in any way by the passive region. This would not he the case i~ the connections 15 and 1~ did not . .
consist of narrow c~nductor runs but were simply ~ormed by an ex~enslon o~ the electrodes 13 and 14 covering the entlre lnside and outside sur~aces. The insulating part 20 preferably cons~sts of magnesium silicate Mg2(SiO4), magnesium-aluminum spinel MgO-A1203 or the sintered-glass ceramic mentioned above.
The elect;x~olyte tu~e is ~astened to the lnsulating part either b~ dlrec~l~ sintering together the two par~s or by inserting a co.nnec~ng layer of glass solder or sintered~glas~ ceram~c.

20 The ba~ic design of the probe shown in FIG. 6 corresponds to thak o~ ~IG. 5~ However9 the insulat~ng part 20.o~ FIGo 6 ls made ln the shape o~ a circular flange, the outside diameter of which i~ larger than the corresponding diameker o~ ~he so~.id-electrolyte tube lO. In order to center this tube with ~espect to the f`lange, it is inserted into a step-l~ke expansion of ~he flange opening and fastened there. me free end o~ the flange has likewise an lnner s~ep ~or forming the shoulder ~l, on which the connec~ions 15 and 16 end and form the contact points 17 and 18. To this end, the outer connection 15 is 30. brou~sht on the ~lange on the outside up to the shoulder 21, ~ 7 --- 1106~46 -~ as can be seen in ~IG. 6, and the inner connection 15 runs on the inner wall of the flange up to the shoulder 21. However, i~ would also be possible to also arrange the connection 15 on the inside wall Or the flange approximately diametrically oppo-- s~te to the connectlon 16 and to let it end on the shoulder 21.
To this end it is necessary to bring the connection 15 at the ~oint o~ the tube and the flange from the outside to the inside, which can be done easily part~cularly if a connecting layer is in~erposed at this pointO In order to avold shor~ clrcults, the ~nner electrode 14 should not quite extend to the end Or the solid-electrolgte tube 10 at this feedthrough poi~t. The inside diameters of the solid-electrolyte tube and the flange are app~o~imately equal and the ratio o~ the outside diameters ls aboul; 1 0 1~5 to 1: 2. The choice o~ the material ~or the ~lange may be the same as those made in connection wi~h FIG. 5.

For determining the size and length relations of the active to the passlve regionJ the rule generally applies that the active reglon is ma~e only large eno~gh so that a suf~iciently low lnternal resistance of the probe is provided. The remaining part o~ the probe is'made passive. me slze will vary dependlng on ~he glven measuremen~ problem. As a minimumj however, at least the region o~ the probe which is intended to be inserted in a probe houslng, should be designed as passive.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

Claim 1. In a probe for an electrochemical oxygen measurement pickup having an oxygen ion-conducting solid electrolyte with electrodes, and electrical contact points connected to the elec-trodes, the combination therewith of a tubular-shaped probe having an electrochemically active region of the probe which has the electrodes and the oxygen ion-conducting solid electrolyte for passage of oxygen ions through the solid electrolyte, and an elec-trochemically passive region of the probe which carries the con-tact points as well as their electrical connections to the electrodes with the passive region having a solid electrolyte on which the contact points and electrical connections are arranged, and with the contact points spaced from each other as well as the electrical connections consisting of conductor runs to the electrodes spaced from each other, with the conductor runs arranged diametrically opposite to each other on the tubular probe to require ions moving from one contact point to another and one connection to another to travel a long path thereby reducing an interfering current.