AU513552B2 - Oxygen sensor - Google Patents

Description

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:r~j i i COMMONWEALTH OF AUSTRALIA at:: :6 iii 31250m Application.......... THE PATENTS ACT 1952-1973 Declaration

Specificatlon.. D S Calings....~T^- APPLICATION FOR A PATENT

We, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANIZATION,

of Limestone Avenue, Campbell, hereby apply for the

grant of a Patent for an invention entitled:

"COMPOSITE CERAMIC/SOLID ELECTROLYTE TUBES FOR OXYGEN SENSORS"

which is described in the accompanying provisional specification.

Our address for service is care of CSIRO Head Office,

Limestone Avenue, Campbell, A.C.T.

DATED this 18th day of AUGUST 1977.

COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANIZATION

ByRegisofy Registered Pecnt A toTTr7 1

TO: THE COMMISSIONER OF PATENTS ,Ar,,

Kb: i;i W

B' :r

COMMONWEALTH OF AUSTRALIA

THE PATENTS ACT 1952-1973

DECLARATION IN SUPPOR FA APPLICATION FOR A PATENT

In support of the Application No. PC 8395 'nd PD 1302

made by Commonwealth Scientific ana Industrial Research s' Organization (CSIRO), a body corporate constituted and F existing under the Science and Industry Research Act 1949

(as amended) of the Commonwealth, for a patent for an

invention entitled: "OXYGEN SENSORS" ii*' I, ALAN DAVID DUNCAN, Scientific Services Officer, CSIRO, of Limestone Avenue, Campbell, in the Australian Capital r- Territory, Commonwealth of Australia, do solemnly and sincerely declare:- that I am authorised by a delegation under Section 14 of the Science and Industry Research Act 1949 (as amended) to make this declaration on behalf of CSIRO;

(ii) MICHAEL JOHN BANNISTER, of 7 Montclair Avenue, Glen Waverley; NEIL ALEXANDER McKINNON, of 22 Stephens Street, North Balwyn; and ROBERT RANDOLF HUGHAN, of 32 Bella Vista Road, Glen Iris; all in the State of Victoria, Commonwealth of Australia; .w/'are the actual inventor(s) of the invention and the facts upon which the applicant is entitled to make the application are as follows;-

The actual inventor(s) i7are an officer(s) of CSIRO and the I.' invention was made in the course of- hihe::/their official

duties with CSIRO; the applicant is therefore entitled to 'I:

apply for Letters Patent by virtue of Section 27 of the

Science and Industry Research Act 1949 (as amended).

Declared at CANBERRA, this f dy o. 444vL 4 9 1977

r cay itc-pAU/i, i'l

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:I ~:iP (11) AU-B1-31,250/77

(12) PATENT SPECIFICATION ABRIDGEMENT (19) AU

(21) 31,250/77 513,552 (22) 7.12.76 (23) 6.12.77 (24) 7.12.76 (43) 14.6.80 (44) 11.12.80 (51) 2 G01N 27/30 G01N 27/56 G01N 27/58 (54) Oxygen sensor PC 8395, FD 1302 (71) Commonwealth Scientific and Industrial Research Organization (72) Bannister, McKinnon, N.A. and Hughan, R.R. (74) DM (56) 83,369/75 492,277 G01N 72,732/74 470,710 56.62, 21.5, 19.2 51,272/73 473,879 21.5, 56.62 (57) Claim 1. An oxygen sensor comprising a ceramic hollow body composed of or containing a composite solid electrolyte material and a pair of electrodes, one of the electrodes being located within said hollow body and the other being located outside said hollow body, said elec- trodes being separated by said composite solid electrolyte material, and said composite solid electrolyte material comprising a mixture of at least one non-electrolyte con- stituent or phase of alumina, aluminous porcelain or mullite, and at least one constituent or phase which is a good oxygen ion conductor, the microstructure of the com- posite material consisting of an intimate mixture of fine grains of the oxygen ion-conducting and the non-electro- lyte constitue.-ts, the oxygen ion-conducting constituent grains of which comprise from 25 to 75 percent by volume of the mixture.

47:1

51 35 5 2 COMPLETE SPECIFICAT3

(ORIGINAL) FOR OFFICE USE

Class: 31 2501 Int. Cl:

Application Number:

Lodged: fo

Complete Specification-Lodged: .0 S Accepted: Published: S0 0O S S 0 OSO Priority: Tu:.L~ 00 0

S* S

0e5006 S Related Art: OS S S

0

TO BE COMPLETED BY APPLICANT

Name of Applicant: COMMONWEALTH SCIENTIFIC AND INDUSTRIAL :0, RESEARCH ORGANIZATION a g*

Address of Applicant: Limestone Avenue, Campbell, A.C.T. 2601

@0 0@ SS Actual Inventors: MICHAEL JOHN BANNISTER 0 NEIL ALEXANDER McKINNON 0@SS0S 0 ROBERT RANDOLPH HUGHAN 0S 0 SeeS Address for Service: The Secretary (Research), CSIRO Head Office P.O. Box 225, DICKSON, 2602

Complete Specification for the invention entitled:

"OXYGEN SENSORS"

(A COMPLETE SLECIFICATION IN RESPECT OF FORMER PROVISIONAL APPLICATIONS NOS PC 8395, FILED 7TH DECEMBER 1976 AND PD 1302, FILED 18TH AUGUST, 1977)

The following statement is a full description of -this invention, including the best method of performing it known to us:

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This invention concerns probes incorporating -solid

electrolyte materials which are used to measure the oxygen

potential in high temperature fluids such as gases, molten

metals and glass melts.

Measurement of the oxygen potential of fluids using

a solid electrolyte is a well-documented technique. It is

outlined, for example, in the specification of Australian

patent No. 466,251 (and in the specifications of the

corresponding U.S. patent No. 4,046,661, UniLed Kingdom

patent No. 1,347,937, Canad'an patent No. 952,983, West

4, 5 German Offenlegungsschrift document No. 2,218,227 and 0. Japanese patent application No. 36,E98/72). Those

specifications also describe an oxygen probe for use in the

measurement of the oxygen potential in molten copper and in S

other molten metals (and also in hot gases).

Briefly, the oxygen potential measurement technique

relies upon the fact that when a body of a solid material 5* having good oxygen ion conductivity, termed a solid

electrolyte, is held with opposing faces of the body in 4

20 contact with materials possessing different oxygen partial S

pressures, an e.m.f. is established across the body. If S

55 one of the oxygen-containing materials is a fluid under 5* S S

investigation and the other is a material of known oxygen

concentration, then the E, is given by the relationship

E RT .n fP0 2 (reference material)' nF "p02 (test fluid)

where

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R gas constant,

T the absolute temperature,

n 4 (the number of electrons transferred per oxygen

molecule),

F the value of the Faraday constant, and

pQ2 the oxygen partial pressure.

This e.m.f. can be measured using current collectors or

electrodes placed in electrical contact with the opposing

faces of the solid electrolyte body.

When measuring the oxygen potential of hot fluids, the solid electrolyte must also be resistant to temperatures

I. at least as high as those of the fluids being investigated

and also, of course, to chemical reactions with those fluids.

This has meant that, for molten metal and furnace gas testing,

S 15 the electrolytes have been ceramic materials such as thoria

(ThO2) doped with either calcia (Ca0) or yttria (Y 2 0 3 and

zirconia (Zr02) or hafnia (Hf0 2 which have been stabilised 2

or partially stabilised with, typically, calcia, yttria,

magnesia (MgO) or scandia (Sc203. 000

20 To perform the actual measurement of oxygen potential,

a variety of devices have been developed. When testing hot

gases, a sample of the hot gas may be withdrawn from the

major flow path, cleaned (if necessary) of any particulate

material which might affect the

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accuracy of measurement either by mechanical obstruction

or by chemical reaction with the solid electrolyte or the

exposed electrode, and then presented to one face of the

solid electrolyte. Alternatively, and this is the form of

device used with liquid metals, the solid electrolyte may be

O incorporated into a probe which is inserted directly into U the hot fluid, an "in situ" probe. The general 4 construction of sensors used in most in situ probes falls into one of three categories, namely;

a tube having a closed end, formed entirely

from the solid electrolyte material (sometimes lengthened,

for access to the te3t region, and/or for cost reasons,

S.I by joining to an open ended t be of another material);

a solid electrolyte coating around the end

15 of a refractory tube plugged with a pellet of a reference material, e.g. nickel/nickel oxide;

a pellet or disc of the solid electrolyte

material held mechanically in a fused quartz tube or

other type of non-conducting ceramic tube, brazed or 20 cemented into a metal tube, or bonded by a coment or sealed by fusion in or to a non-conducting ceramic tube.

It should be noted that, although sensors having a

tubalar structure have been referred to in these categories, the tubular body of the sensor need not be of uniform

or circular cross-section, nor need the wall or walls of the tube be of uniform thickness. Indeed, the shapes of sensors are increasing in their variety as oxygen probes are constructed for specific applications. For example,

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generally conical or thimble-shaped sensors and also-discs

have been used in automobile exhaust gas analysers. In

practice, the body of the sensor must ultimately be of a

generally hollow, but internally accessible, configuration.

The term "hollow body" will be used in this specification

on the understanding that it has this more specific

connotation.

In part, it was to overcome problems associated with

an oxygen sensor of the fusion-sealed type of the third category above that the present work was carried out and an

improved form of in situ oxygen sensor of this type was S*

realised.

The aforementioned specification of Australian

patent No. 466,251 describes an oxygen sensor formed by the fusion sealing of a pellet of a zirconia-based solid electro-

lyte into the end of an alumina, mullite or aluminous

porcelain tube, the electrodes being mounted to measure the

electrical potential developed across the electrolyte pellet.

S" That sensor, as already noted (and particularly that form

.*20 *see of sensor fabricated with an aluminous porcelain tube), has

been found especially useful in the measurement of the

oxygen potential of molten copperA due to its satisfactory

performance and low cost.

The sensorsmade in accordance with the fusion sealing technique of Australian patent No. 466,251 have electrolyte

pellets which are extensively microcracked. These cracks

are of no concern for measurements in molten copper, since

they are too fine to be easily penetrated by the copper.

However, gases can readily leak through the cracks, causing errors when the sensors are used for gas

A analysis, particularly the analysis of gases with low oxygen content. Of the three tubing materials alumina, aluminous porcelain and mullite, the least microcracking, and thus the most leak-tight assembly, is obtained with alumina. However, with alumina tubes a fair degree of skill is needed to make the seal described in afore- mentioned Australian specification No. 466,251, and even

V with skilled operators about 50 per cent of the seals so

Smade are incomplete or otherwise faulty. Furthermore, the

microcracking in the electrolyte makes the pellet

susceptible to spalling under thermal cycling or thermal

1 shock conditions, and thus the/-preee&have restricted

lives under severe in situ gas measurement conditions. Detailed investigation of this problem of

pellet cracking during the fusion sealing operation

has shown that the cracking is, to a. considerable

degree, a consequence of the heating and cooling cycle

involved in the sealing operation. Tensile stresses

generated on cooling from the sealing temperature, due

S 20 to the different linear expansion coefficients of the

S* zirconia electrolyte and the alumina tubing, cause

o* cracking of the electrolyte. The coefficients are 1 3 x 10 -6 deg C -1 (average value from ambient up to

sealing temperature) for the electrolyte and (over the -6 -l same temperature range) about 10 x 10 deg C

for alumina. Aluminous porcelain and mullite have

even lower coefficients of linear expansion. When the

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tensile stresses in the electrolyte exceed the fracture

stress, cracks are formed in the electrolyte. Cracks do

not occur in the alumina tube, for ceramics are generally

stronger in compression than under tension.

Various mechanisms for reducing stresses in the

electrolyte were tried. One approach was to reduce the wall

thickness of the tubing to lower the stresses in the

electrolyte and increase those in the tubing. Others were

to replace the zirconia-based electrolyte with a hafnia-

based electrolyte having a slightly lower expansion coefficient

and a partially stabilised zirconia (PSZ) electrolyte e having a composition chosen so that it had an average

coefficient of linear expansion close to that of the tubing. (PSZ electrolytes, it may be noted, are limited in their

15 maximum use temperature by consequent changes in microstructure

and ensuing degradation). All the methods of reducing stresses

met with varying quantitative degrees of success but could

not be described as totally satisfactory.

One objective of the present invention is the provision

20 of an oxygen sensor which successfully avoids the cracking

problem associdted with the production of oxygen sensors

having a pellet of electrolyte material fusion sealed into *5

the end of a non-electrolyte ceramic tube. A second objective

is the production of an oxygen sensor which is che-per to make,

stronger and more resistant to thermal shock than existing

fusion-sealed oxygen sensors. A third objective is the

7

ii E-YI i' ii ii C~p_ 511~41~ ~1 production of an oxygen sensor having a solid electrolyte tip at or near the end of a non-electrolyte ceramic tube or other hollow body, by conventional ceramic fabrication techniques not involving fusion sealing.

The present invention provides an oxygen sensor

comprising a ceramic hollow body composed of or containing

a composite solid electrolyte material and a pair of

electrodes, one of the electrodes being located wi:hin said

hollow body and the other being located outside said hollow

body, said electrodes being separated by said composite solid

**O electrolyte material, and said composite solid electrolyte

material comprising a mixture of at least one non-electrolyte

S. constituent or phase of alumina, aluminous porcelain or

0* mullite, and at least one constituent or phase which is a

15 good oxygen ion conductor, the microstructure of the composite

material consisting of an intimate mixture of fine grains of

the oxygen ion-conducting and the non-electrolyte constituents,

the oxygen ion-conducting constituent grains of which comprise

from 25 to 75 percent by volume of the mixture.

20 Some or all of the above-mentioned objectives are

realised, to various extents, by the present invention, as

hereinafter explained.

In a preferred form of the invention the sensor is of the type having a solid electrolyte pellet or disc

8

<'1 r -ur IL and this pellet or disc is made of the mixture of electrolyte and nop-electroly.e materials, the non-electrolyte ceramic material being the same kind (alumina, aluminous porcelain or mullite) which forms the hollow body of the oxygen sensor. If the proportions of the electrolyte and non- electrolyte phases or constituents are chosen appropriately, a pellet or disc is obtained having both satisfactory electrolyte properties and a thermal expansion coefficient close to that of the non-electrolyte ceramic body of the oxygen sensor.

In the above description a "pellet or disc" of the

sensor is referred to and normally provides the tip of the

sensor. Of course, the same principle applies to any small

region formed in the hollow body, this small region

15 separating the two electrodes.

It is also within the scope of this invention to

"dilute" the non-electrolyte ceramic body of the oxygen

sensor with constituents of the oxygen ion conducting phase

S. (for example with pure or doped thoria, or with stabilised

or unstabilised zirconia or hafnia), the amounts of these

additions generally being insufficient to turn the body of

the sensor into a good oxygen ion conductor, in order to

strengthen or toughen the body of the sensor and/or to

effect an even closer thermal expansion match between the

non-electrolyte body and the solid electrolyte tip of the

sensor.

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The mixture of electrolyte and non-electrolyte

phases or constituents (hereinafter referred to as a

composite electrolyte) has permitted the construction, by

the fusion-sealing technique, of leak-tight "pellet-in-tube"

sensors which are suitable for use in measuring the oxygen

potential or oxygen content of molten metals and hot gases.

Furthermore, it has enabled oxygen sensors having a solid

electrolyte tip and a non-electrolyte body to be fabricated

by a technique previously unavailable for this purpose,

namely, the construction of a sensor in the "green"

condition by conventional ceramic forming methods, followed 000* oy firing to yield the sensor for actual use. If this "green"

*eforming and firing approach is adopted with the materials S. hitherto used for solid electrolytes and ceramic tubes, the S 15 difference in coefficient of thermal expansion between the electrolyte tip and the tube body causes severe cracking in

the electrolyte tip and at the electrolyte tube boundary as

*5eS the fired sensor is cooled to ambient temperature. In such 5

o a case, the tip generally breaks away, but if it does remain

20 intact the sensor thus formed is sc fragile and prone to

leaks that it is useless.

S" A further advantage obtained in fabricating the S.

sensors of the present i.nvention is that, in part by the S.0 dilution of the electrolyte pellet or disc with a non-

electrolyte material and in part by the ability to fabricate

a sensor having only an active tip containing the solid

10

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electrolyte, sensors may be constructed using limite-d or

minimal quantities of solid electrolyte materials. Since

the electrolyte is considerably more expensive than the

supporting ceramic tube or hollow body, this meanF that

electrolytes previously rejected on cost grounds for use in

probe assemblies for example, with a rare earth oxide or

with a mixture of rare earth oxides (with or without one or

more of the commoner dopants or stabilising compounds being

present) can now be used for oxygen sensors without the

probe being prohibitivel expensive. The use of such less

common dopants and/or electrolytes is of particular value in Do:: the construction of low temperature gas analysis probes, such

as those required for use in the monitoring of automobile Doe

exhaust gases. 0@

S 9000 S 15 Of course, the composite electrolyte used in the S

000. pellets or discs of sensors of the present invention may

also be used to fabricate oxygen sensors comprising or *00 S containing complete tubes of composite electrolyte, either 5600

*0 S S. open or closed-end, the coatings or discs used in prior art 0

S.. 20 sensors, and even pellets or discs having only a central core 0 0Or

*0S*O of composite electrolyte material extending between the

opposing faces of the pellet or disc and surrounded by an

annulus of the non-electrolyte ceramic material. S Preferably, the content of the oxygen ion conducting

S 25 constituent(s) in the composite solid electrolyte material is

from 30 to 60 per cent by volume. More preferably, it is

11

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from 30 to 50 per cent by volume.

The non-electrolyte material used as the non-conducting

constituent of the composite solid electrolyte is alumina,

aluminous porcelain or mullite. Thus, necessarily in the

a-- 5 above embodiments in which the hollow body is of the same

i a.L' non-conducting material, and as a matter of general preference

in all embodiments of the invention, the hollow body of the

sensor is also of alumina,aluminous porcelain or mullite.

Incidentally, it is recognised that materials described

in this specification as "non-electrolytes" or "non-conducting" K S ceramic materials (such as alunina, mullite and aluminous a6- K 4 porcelain) are, in fact, ionic conductors under conditions

*b 0 of high temperature. However, their conductivity is very

0 small compared with that of the materials described as solid S ,S00S a 15 electrolytes in this specification and thus use of the terms

"solid electrolyte" and "non-electrolyte" will not be

confusing to persons skilled in this art.

The oxygen ion conducting constituent used in the 'See I. sensors may be thoria doped with calcia, with yttria or with be

various rare earth metal oxides, or zirconia or hafnia e0e ,060 stabilised or partially stabilised with calcia, ;-ith magnesia, *0 00S. be S. with yttria or with other rare earth metal oxides, either .44 singly or in combination, provided that no adverse reactions

occur between the electrolyte and non-electrolyte phases or

constituents, leading to a loss of ionic conductivity or

unusual Ehermal expansion behaviour.

In a preferred embodiment of this invention suitable

for use in severe oxygen sensing situations (examples

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are given below), the non-electrolyte material used for the

body of the sensor, and as the non-conducting diluent in the composite solid electrolyte, is alumina, and the oxygen ion conducting constituent is zirconia stabilised or partially 5 stabilised with yttria, the yttria content of the zirconia- yttria material being in the range of from 4 to 25 mole per cent Y 2 0 3 (7 to 38 per cent by weight). A more restricted range of yttria content, namely from 6 to 15 mole per cent

Y203 (10.5 to 24.5 per cent by weight), is preferred since in this range of yttria there is no reaction between the yttria and the alumina diluent, yet the zirconia is fully stabilised; however, minor amounts of unstabilised zirconia (found with S. an yttria content below 6 mole per cent) or the compound

3Y203.5A 1 203 (formed during firing if the yttria content is

above 15 mole per cent) may be tolerp-.ed, provided the fired a

a body contains from 25 to 75 per cent by volume of the

stabilised zirconia phase.

An oxygen sensor made in accordance with the present

4; 5 55 invention with a pellet or disc, providing a small tip region 20 of composite electrolyte fusion-sealed to, or otherwise bonded to or formed in, the end of a hollow body of alumina, aluminous Sb

porcelain or mullite, will normally have electrodes mounted F' a. to enable the electrical potential across the pellet or disc i) S S to be measured. The alternative sensor construction, in which

the complete hollow body is fabricated from composite electrolyte material, will again have, usually, the

13

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I I I i i conventional construction of one electrode mounted i-n electrical contact with the inner surface of the sensor, and the other electrode in electrical contact with the outer surface of the sensor. A protective sheath may be provided around each form of sensor, with apertures to allow the fluid under investigation to contact the outer surface of the composite electrolyte. Such a protective sheath may, in some cases, be the outer or forwarC electrode tjf the sensor.

For a better understanding of the present invention,

examples of oxygen sensors incorporating composite solid

electrolyte materials will now be given, after a discussion

*too" of the construction of oxygen probes. Reference will be made

to the accompanying drawings, of which Figure 1 is a sectional

view of the sensing head of a "pellet-in-tube" oxygen probe,

S 15 and Figures 2 to 7 are sectional views which illustrate

alternative constructions of sensors formed in the "gre'n"

state.

In Figure 1, a non-conducting ce: amic tube 10 has a

pellet or disc 11 of composite electrolyte material fusion

sealed or otherwise bonded into the end of it. Various

alternative electrode arrargements are then possible,

Sincluding the illustrated tubular inner electrode 12 and

the outer electrode assembly consisting of electrode pad 14 supported in position in contact with the pellet 11 of

composite electrolyte material by the electrically conducting

protective sheath 13. Among these alternatives are binding

a wire electrode material around grooves cut into the free

(outer; end of the composite electrolyte pellet, and

14

N- r. P- -L~C -_-CIR--IIIB~I~C Y constructing the protective sheath of a suitable electrode material, thus avoiding the need for electrode pad 14.

A further alternative is to coat both the outer and

inner faces of the composite electrolyte pellet with a porous

layer of a suitable electrode material, for example platinum,

and to make electrical connections to these layers (or to

extensions of the layers along the non-conductir

away from the composite electrolyte pellet) us'' .cting

wires, tubes or rods. A further alternative again 's to

use wires sintered, embedded or otherwise rixed into holes

at either end or both ends of the composite electrolyte

"*4900 pellet. 9.

S.The inner elec-trode (a conducting tube 12 in the

illustrated embodiment) is pressed or biassed (generally

with a spring) against the inside surface of composite

electrolyte pellet or disc 11 providing a tip. If the

illustrated tubular inner electrode is used, it will generally

be provided with an aperture 15 near the end 16 of the tube in *fee contact with the upper surface of its tip, and a refercnce gas

is passed down the tube 12, through aperture 15, to leave the

*probe after passage up (or along), depending on the orientati.on

of the probe when in use) the ceramic tube 10. Reference gas

flow in the opposite direction (for example, through the

*annular gap between tubes 10 and 12 and out through tube 12)

is also possible. A further alternative is to maintain the

reference gas on the outside of non-conducting ceramic tube

and to convey the gas under examination to the inner

electrode.

15

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If a protective sheath 13 is provided and the probe

is to be used for gas analysis with the gas to be analysed

on the outside of the sensor, a plurality of apertures 17 are

provided in the sheath to enable the gas being analysed to

flow past the pellet 11 of electrolyte material.

As noted above, other constructions of sensor ara

possible, including a sensor formed entirely in the

"green" state, then fired (these are discussed in more detail

later), a single, closed-end tube of composite electrolyte

material, and a sensor of category type referred to

in the introductory part of this specification.

o* The materials that may be used for electrodes will S depend on the use to which the probe is to be put. Platinum

has often been used in the past, but in atmospheres which

15 attack platinum, other noble metals such as gold, or ailoys e *f

of chromium with nickel and/or iron, such as "Inconel",

"Incoloy", "Nichrome" (all Registered Trade Marks) or stainless

steel, may be used. If an alloy of chromium with nickel

o *o and/o: iron is used as either electrode, pre-oxidation at 00

high tempe:rtures is needed if spurious e.m.f.s are not to

be observed when the probe is first used. However, such

pre-conditioning of the electrode is not essential; the alloy

electrode will rapidly adopt an inert oxide coating in use, 0 0 and on formation of that coating the spurious e.m.f.s will

not be observed. A chromium-alumina cermet may also be used

as either electrode in configurations such as the pad 14 or

protective sheath 13, shown in this case at the external

electrode.

16

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The invention also provides a method of manufacturing

an oxygen sensor of the kind which is formed in the green

state so that the composite solid electrolyte material forms

a conducting region in a non-electrolyte, ceramic holow

body and then fired. The method comprises slip casting,

extrusion, isostatic pressing, die pressing or throwing to

produce a "green" ceramic hollow body and firing the green

body.

In the preferred embodiment of the above method a

slip casting technique is used. Three preferred sequences

of steps of the slip casting are:- 4 1. depositing a limited quantity of a slip of the S S composite electrolyte material in the bottom of a porous I SI mould. .:060:

filling the mould with a slip of the non-electrolyte 0 0

ceramic material used for the body of the sensor, and

0 0 after allowing the casting to acquire the required

thickness, inverting the mould and draining the excess slip

0 00 therefrom. 0 0 9000

a 0 0O0 *S 20 2. filling a porous mould with a slip of the non- SSSO electrolyte ceramic material used for the body of the sensor, 0 allowing the casting to acquire the required thickness, and 0..00: S draining the excess slip therefrom,

depositing a small quantity of a slip of the

composite electrolyte material in the bottom of the casting,

16a allowing the slip of composite electrolyte material to harden, then before or after firing the "green" sensor, removing part of the outer casting to expose the outer surface of the hardened slip of electrolyte material.

3. forming, in a porous mould, a casting of the

non-electrolyte ceramic material used for the body of the

sensor, and

injecting through the base of the casting, from the

exterior thereof, a slip of the composite electrolyte

material.

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The following examples illustrate the preparation

and properties of oxygen sensors of this invention, comprised

of or incorporating composite solid electrolyte materials and

made by both the conventional or known techniques and by the

"green" forming route.

EXAMPLE 1

A batch of zirconia containing 6 mole per cent Y203

as stabiliser was prepared by a coprecipitation process.

Coprecipitation gives a very fine, intimate mixture of the

zirconia and yttria, which react to form a stabilised zirconia

solid solution at quite low temperatures. By crushing,

sieving and calcining at about 800 0 C, fine particles of the ee* .solid solution are obtained. The coprecipitated powder was roe O*O

ground in an alumina mortar to less than 200 mesh

15 calcined in air at 800 C for 1 hour, and blended with "Linde A"

alumina powder in quantities calculated to give volume

fractions of electrolyte of 0.50, 0.40, 0.30 and 0.20 in the

sintered body. In calculating these mixtures it was assumed

that the theoretical density of the zirconia- 6 mole per cent 20 Y203 solid solution was 5.97 g/cm and that the mixtures

could be sintered to zero porosity. Total masses of 10g of

powder were prepared at each composition, and each mixture

was blended dry for two hours in a mechanical shaker using plastic containers and two acrylic balls. 5 per cent by

weight of water was then added, and blending continued for

a further 10 minutes. Each mixture was then isostatically

pressed at 30,000 psi to give a cylindrical bar about 6 cm long and 8 mm diameter. These bars were sintered in air at

1700 C for 15 hours, using an alumina boat.

To determine the characteristics of these materials,

the following tests were performed:-

1. The electrical resistance of each specimen was

determined in air, in a laboratory muffle furnace,

over the range 5000 to 900 C using a simple two- 11; c. terminal technique.

2. The rods were machined to a diameter of about

and the densities determined by mercury displacement

and by mensuration. .0 0

006 0 3. Each rod was broken in a four-point bending rig *00

000 and the modulus of rupture calculated.

4. A small slice from the end of each rod was mounted 00 00 and polished for optical microscopy and X-ray

diffraction.

The thermal expansion behaviour of a 1-inch length 0 0* 00* of each rod was determined in a dilatometer

calibrated against vitreous silica to 1000 0 C, and 00 0 previously used to determine the expansion

behaviour of alumina tubing. *oo 0 The densities, bend strengths, and electrical conduct-

ivity results are tabulated below.

Volume Fraction Stabilised Zirconia 0.50 0.40 0.30 0.20 Mass Fraction Stabilised Zirconia 0.60 0.50 0.39 0.27

Density (mercury displacement) g/cm 4.74 4.58 4.42 4.24 0 Density (Mensuration), g/cm 3 4.72 4.54 4.41 4.25

Theoretical density, g/cm3 4.98 4.78 4.58 4.38

Modulus of rupture, psi. 26,400 27,300 32,100 34,500

Activation energy for electrical conductivity, kcal/mol. 23.0+0.3 25.7+1.1 23.3+0.2 23.8+0.7

S *s Conductivity at 9000C, -1 cm 1 3 3 4 1 J- 5.1x10 1.7x10 5.8x10 1.6x10 Si- C

The densities of these specimens all lay in S. the range 95-97 per cent of theoretical. The n)dulus of C 050., rupture was comparable with values previously determined

using an undiluted Zr02-6 mole per cent Y 2 03 electrolyte. 0@ The activation energy for electrical conductivity was S

.e independent of composition and compared well with accepted COBS

values for the undiluted electrolyte, suggesting that

conduction occurred via the yttria-stabilised zirconia

constituent This conclusion was supported by the reduction

in conductivity with decreased zirconia content.

19

:_ii

:ir i: j

Optical microscopy showed all the specimens to be

similar in containing a fine intergrowth of alumina and

yttria-stabilised zirconia, together with isolated areas of

yttria-stabilised zirconia. X-ray diffraction patterns

taken before and after sintering could be completely indexed

in terms of q-Al 2 03 and cubic zirconia with lattice parameter

u 5.13R. There was no additional phase frmed in the sintered

material.

Thermal expansion results over the temperature range

200 to 1000 C showed that the bast match with alumina

occurred for 0.50 volume fraction of electrolyte; however

S53. the match was still good for the other compositions and up

to fusion sealing temperatures (about 1900 C) the better

overall agreement probably occurred with the lower electrolyte

compositions. Fracture strains of ceramics a)re typically

0.1 per cent, so that the differences in fractional thermal -3 expansion should be within 1 x 10 to adequately limit

r. cracking. All the expansion curves agreed with the curve -4 0 for alumina tubing within 5 x 10 up to 1000 C.

20 All of the compositions were readily sealed into

alumina tubing. A slight modification to the technique

recited in aforementioned specification No. 466,251 was

required in that, to avoid melting the composite electrolyte

pellet, the oxy-acetylene flame had. to be directed mainly on

the alumina tubing. Also, compared with previous experience

using the conventional electrolyte pellets (without alumina

present), there was less tendency for the pellet to draw

into the tube as the seal was made. Because of the good thermal expansion match between pellet and tubing it was also found that there was no longer any need to provide a closely controlled minimum clearance prior to sealing.

EXAMPLE 2

Twelve oxygen sensors of the type illustrated in

Figure 1 of the accompanying drawings were made by sealing

short lengths of the composite electrolyte rods described in

Example I ,three sensors of each electrolyte composition) v- into alumina tubing. These sensors were tested by internal

pressurisation with air at 30 psig, the vacuum leak rate was OSS. S determined at temperatures up to 1200 C, and the cell .555

*i ft. a voltage against both oxygen and air references was determined f *0ft ft. in a 50 per cent CO, 50 per cent CO 2 atmosphere at 1100 SO 0@

12000 and 1300 0 C, and in nitrogen at 1100°C. S

15 All twelve sensors gave leak rates under 30 psig air 5*

which were less than those of 'normal' fusion sealed sensors

of the type described and claimed in aforementioned Australian

S. specification No. 466,251, and its equivalents, while S. 0 occasional sensors showed no leaks at all.

All sensors showed vacuum leak rates which were better @0

that the average behaviour of such 'normal' sensors and ft

comparable with the best. With such 'normal' sensors, the 0

leak rate drops rapidly with increasing temperature; with

the present sensors the leak rate was initially much lower

and varied only slightly with temperature.

Sensors were sectioned to expose the junction of the

composite electrolyte and the alumina tube and examined by

21

i i optical microscopy. There was no cracking at all in the composite solid electrolyte pellets and only minor cracking in the alumina tubing.

When the sensors were tested in CO/CO 2 atmospheres and

nitrogen, they all gave cell voltages comparable with those

obtained from 'normal' sensors. In fact, the experimental k sensors gave higher voltages than customary in nitrogen, a

sign that the lower leak rate makes them better suited to

measurements of inert gases. The electrical resistivity was

greater than that of sensors made using undiluted elect-

Srolyte pellets, but only at 0.20 volume fraction of elect-

rolyte in the composite material was it considered to be 9 S.unacceptably high.

EXAMPLE 3

15 A batch of fifty-six "pellet-in-tube" sensors was made

using pellets containing alumina with ).40 volume fraction

ZrO -6 mole per cent Y 2 0 3 The sealing success rate was 82

per cent, as judged by their leak rates and good performance in test gases of nitrogen and 50 per cen- CO, 50 per cent

20 CO 2 (Incidentally, the 18 per cent 'failed' tubes need not

be wasted the host tube is available fo. re-use after

cutting off the composite electrolyte pellet; Samples of the

successfully sealed sensors were tested by continuous use

for periods of up to 6 months in the following industrial applications:-

a reducing atmosphere metal sintering furnace, in

which the furnace gases contained zinc vapour from a zinc stearate pressing aid in the metal powders,

22

2-s~i

~ii4~1 i

a large, coal-fired rotary kiln in which ilmenite nil v- concentrates are reduced to rutile and metallic

iron,

a steel soaking pit,

a reverberatory furnace smelting copper concentrates,

in: which a probe was used as an aid to the control

LM of solid fuel additions, and

V-4 various other gas-fired furnaces and kilns, with a COZ probe used for on-line monitoring and control of

combustion stoichiometry.

Of these applications, and would be regarded

as severe oxygen sensing situations. C S 8 In each case the sensor performed accurately and

0 reliably in conditions needing a rugged, high-integrity,

C 15 oxygen potential measuring device, despite, in some instances, S. Se 5 temperatures as high as 1600 C, contaminants such as sulphur

and zinc vapour in the gases, and direct contact with

materials such as partly-reduced ilmenite. In evaluations S. C such as it was found that the probes could be thrust

S 20 repeatedly into gas flames at temperatures of up to at least ge*

C. S 1600 0 C without spalling or failure of the sensor. Similar C Ci

treatment given to sensors made using pellets of the undiluted

Ii C s C electrolyte caused spalling of the electrolyte pellet.

better performance of the sensors made .in.g the The

omp..e.it

electrolyte materia1 of the present invention is indicative

of an increased resistance to thermal shock, and is consistent :a *e7~

with the absence of cracking in the pell.ets after sealing.

In fact, the sensors were so free of cracking, and therefore

of leaks, that flowing reference air was often no needed;

Ai7i diffusion from the open end of the sensor was adequate.

EXAMPLE 4

.I ii:C Composite solid electrolyte pellets for use in oxygen

sensors were made from mixtures of "Linde A" alumina powder 1 and the following alternatives to the yttria-stabilised

zirconia coprecipitate used in Examples 1, 2 and 3. (a ea/-c^ cc lneink) "Zyttrite"/ an alkoxy-derived stabilised zirconia

powder containing 6.9 mole per cent Y 2 0 3 obtained

from Wright-Patterson Air Force Base (powder an yttria-stabilised zirconia powder containing 6

9.. mole per cent Y203 supplied by Magnesium Elektron

Limited (powder and

a mixture of unstabilised zirconia powder purchased

from Ugine Kuhlmann and yttria powder purchased

from the American Potash and Chemical Corporation,

with the proportions of zirconia and yttria chosen

to yield a stabilised zirconia containing 7 mole

per cent Y 2 03 (powder

All the powders were heated in air to form or partly

form the cubic zirconia solid solution prior to mixing with

alumina. For powder one hour at 800 C was sufficient,

whereas for powders and two hours at 1000 C and one

Si.. 9 hour at 1100 0 C, respectively, were required.

Rods of alumina containinq 0.30, 0.40 and 0.50 volume

fraction of yttria-stabilised zirconia were prepared from

powders and using the technique outlined in

Example 1. The fired rods were characterised by X-ray i

4 c diffraction, optical microscopy and the measurement of

density, electrical resistance, modulus of rupture and

thermal expansion as described in Example 1. E' All three alternative sources of stabilised zirconia

gave composite electrolytes having similar properties to

those of Example 1. There were minor differences in grain

size depending on the powder used, and in the magnitude of C 4 the electrical conductivity, but all the compositions were

C) considered suitable for sealing into alumina tubing to form

oxygen sensors. Subsequent trials using two or three

fee* pellets cut from each rod showed that all compositions could S 0

8. be successfully fusion sealed into alumina tubing.

EXAMPLE 0& 0 0 Mixtures of zirconia, yttria and alumina were prepared

by coprecipitation of the hydroxides and by mixing 00.0 0 0 the three oxide powders to give 50 weight per cent alumina K and from 5.7 to 40 mole per cent Y 2 0 3 (10 to 55 weight per

cent) in the zirconia. The various mixtures were pressed *0 o

into pellets and fired at 1700 C in air for two weeks, with

an intermediate cooling to ambient temperature, grinding and

0* repelleting after the first week. X-ray diffraction was

*r used to identify the compounds present in the mixtures after 9

firing.

It was established that a two-phase equilibrium at

17000C exists between alumina and stabilised zirconia for

yttria levels in the zirconia from e ett 6 to about

mole per cent Y 2 03 (10.5 to 24.5 per cent by weight).

Unstabilised zirconia was present with yttria levels below

25

C~ ~k _1

L

this range. With yttria levels above 15 mole per cent Y203'

thL compound 3Y203.5A1203 was produc-d. Extensive formation

of either unstabilised zirconia or 3Y 2 0 3 .5A1 2 0 3 is detrimental

to the oxygen ion conductivity of the solid electrolyte.

Thus zirconia with an Y203 concentration outside the range

6 to 15 mole per cent becomes progressively less useful

in the present invention as the concentration departs from

the range boundaries.

EXAMPLE 6

A mixture of 50 parts by weight of "Linde A" alumina

10 and 50 parts by weight of yttrld-stabilised zirconia

coprecipitate containing 6 mole per cent Y 2 0 3 was dispersed

in 50 par:s by weight of water and five parts by weight of

Dispex A40 (an organic deflocculant) and slip-cast to form

closed-end tubes using gypsum moulds in a conventional

5 manner. Firing at 1800 C for 2 hours in a natural gas-air-

oxygen furnace gave high density, impervious tubes with

sufficient oxygen ion conductivity to be used as oxygen

sensors. The presence of alumina gave these sensors an

m improved resistance to mechanical and thermal shock when S* 20 compared with sensors of similar construction fabricated

from undiluted electrolyte n.tterials.

EXAMPLE 7

An alumina slip (Slip A) was produced by dispersing

"Linde A" alumina in water in the proportions 100 parts by

weight of alumina to 50 parts by weight of water. 1.7 parts

by weight "f Dispex A 40 were used as a deflocculant and

the slip was ball milled for twelve hours. A stabilised

1 26 zirconia slip (Slip B) was also prepared using 100 parts by wei 'it of a zirconia 6 mole per cent Y 2 0 3 co-precipitate, previously calcined in air at 700 C for 30 minutes, 50 parts by weight of water, and 8 parts by weight of Dispex A

In this case the constituents were hand mixed using a mortar h and pestle. Equal masses of the alumina slip and the u3 641 stabilised zirconia slip were then blended together to form

09 Slip C, containing approximately equal proportions by weight

of alumina and yttria-stabilised zirconia.

A closed-end tube, consisting of a composite elect- a a. rolyte composition at L'e closed end and an alumina a

a.. composition for the main body, was then prepared by slip-

a casting in a gypsum mould, as illustrated in Figure 2, in C the following manner. W 15 A small quantity of Slip C (reference 21 in Figure 2a)

was first deposited at the closed end of the mould 20 using

a long syringe. The remainder of the mould cavity was then a. a. gently filled with Slip A (reference 22). After allowing

the time necessary to build up the desired thickness of the

9 20 casting, the mould was carefully inverted and the excess r a. slip was allowed to drain out. The closed-end tube thus *9*a a

a 9* fcrnred, illustrated in Figure 2b, was then fired.

Since it is necessary for the cast-up composite

electrolyte to be exposed internally in the tube after

draining, it is important that an appropriate predetermined

and measured quantity of Slip C is admitted to the mould in

It: 27 the first instance. An insufficiency of Slip C will lead to complete covering of the cast-up co-.posite electrolyte by an overlay of alumina from the main body slip. Excess composite electrolyte slip will cause undesirable contamination of the drained slip, which is normally returned to the batch of Sl'p A (which is generally made up to fabricate a number of closed-end tubes).

Some control over the casting rate of each slip is

necessary to ensure that the relative lengths of the

10 composite electrolyte tip and the main body are within the

desired limits. Such control can be exercised by varying *s

the permeability of the casts, which in turn is achieved by 900 0o

varying the grain size of the powders used and/or the degree

of deflocculation of the slips.

The area of composite electrolyte exposed within the

tube may be varied within limits by allowing for a greater 00 0000g. or lesser amount of uncast composite electrolyte slip at the

stage when draining begins. Gentle rotation of the mould

during draining may be used to effect an even deposition of 600 excess composite electrolyte slip over the inside of the

tube, if so desired.

EXAMPLE 8

Slips A, B and C were prepared as described in Example

7. A gypsum mould 30 (Figure 3a) was then filled with Slip

A, allowed to stand for the time needed to deposit the required thickness of cast and drained in the normal manner.

Immediately after draining had ceased, the mould 30 was

righted and a measured quantity of Slip C was admitted to

the inside base of the cast tube 31, using a long syrinye.

The amount of composite electrolyte slip so admitted was

I predetermined in order to leave an integrally formed button

C 32 of composite electrolyte within the tube. This button

Swas subsequently exposed externally, as shown in Figure 3b.

In separate experiments, successful exposure was achieved by

abrading and by cutting away the layer of alumina at the end of the tube, before and after firing. Successful closed-end

tubes were also prepared by righting the mould and admitting Slip C just before completion of the draining.

EXAMPLE 9

15 Slips A, B and C were prepared as described in Example

7. In this case the gypsum mould 40 (Figure 4a) was

provided with a means of entry to the inside bottom of the PO cavity, namely, a length of narrow tubing 41 cast into the

20 mould (but not extending beyond the inner surface of the i mould).

A closed-end tube 42 of alumina was cast conventionally

in mould 40 using Slip A.

During this, operation the bottom entry tube 41 was

sealed at its external end, and in the process of casting

the inner end of this tube 41 became partly or wholly sealed over with solidified casting slip. Draining of the cavity was then performed by inversion of the mould 40 in the usual way (in fact, draining could have been effected through the bottom entry tube by puncturing the accumulated cast at its inner end). Either just before, or soon after draining was complete, the bottom entry tube 41 was connected

V-Z to a reservoir of Slip C, in the form of a graduated

I. syringe, and a predetermined quantity of Slip C was admitted

to the interior of the cast, using sufficient pressure

10 1 to force its entry. In some instances, an excess of Slip C was used to coat the inside of the body of the alumina tube

42 with a composite electrolyte layer 43, by gently rotating Sthe mould to spread the composite electrolyte. The tubes

formed by this technique were fired.

15 The technique recited in this Example has a possible

advantage over the technique of Examples 7 and 8 since it

provides a small external exposure of composite solid electrolyte material (as small as the diameter of the bottom

access tube) in the finished tube, while retaining a large o.

20 internal exposed composite electrolyte surface.

EXAMPLE

I: An open-end alumina tube 50 (Figure 5a) was extruded in

a conventional manner using a mixture containing 100 parts

by weight of "Linde A" alumina, 30 parts by weight of water,

2 parts by weight of gelatin, 2 parts by weight of glycerol

and 2 parts by weight of poly(ethylene glycol). A solid rod

of composite electrolyte material, with a diameter slightly less than the inside diameter of the extruded alumina tube, was also extruded using a mixture containing 50 parts by weight of "Linde A" alumina, 50 parts by weight of zirconia

6 mole per cent Y 2 03 co-precipitate previously calcined in air at 700C for 30 minutes, 30 parts by weight of water, 2

parts by weight of gelatin, 2 parts by weight of glycerol

U. and 2 parts by weight of poly(ethylene glycol). Cl In other experiments, the glycerol, gelatin nnd

Spoly(ethylene glycol) were used singly or in pairs to effect the desired degree of plasticity for a given size

I of extrusion of both the alumina and the composite

|g electrolyte material.

;A pellet 51 was cut from the composite electrolyte rod

(conveniently a pellet approximately equal in length to the

15 diameter of the rod was used) and was fitted into one end of

the extruded alumina tube 50. A circumferential pressure P was used to ensure a firm and even contact between the

composite electrolyte pellet and the inner surface of the alumina tube. Depending on the ultimate application of the

20 sensor, the pellet was positioned either flush with the end

of the tube or with a portion protruding beyond the end of

the tube (as shown in Figure 5b). The tube was then fired.

EXAMPLE 11

An open-end alumina tube 60 (Figure 6) was extruded as

described -n 7xample 10. One end of the tube was closed i.- r g; ii: with the mixture used in Example 10 for extruding a rod of composite electrolyte material, by pressing a small pellet 61 of the composite electrolyte mixture into the end of the tube and smearing the mixture over the outer wall of the tube, adjacent to the end to be closed, to effect a good bond between the alumina and the composite electrolyte mixture. The tube was then fired.

EXAMPLE 12

A short alumina tube with a composite solid electrolyte

10 tip was made by isostatic pressing in the following manner a a (see Figure 7).

Conventional tooling, comprising a vented plastic tube 9 (a metal tube could have been used) with a rubber bag

liner 71, was used to contain the powder 73 to be formed

15 into the alumina tube. A metal mandrel 72, held concentric

with the vented tube 70 by means of a temporary bridge

across the end of the tooling, vas used to form the central 9 *6

00 hole of the sensor tube. "Linde A" alumina powder, spray-

dried to improve its die-fill properties, was loaded into 6 250 the rubber bag 71 until only the tip of the mandrel remained

uncovered. The remaining space 74 above the mandrel was then 9*

filled wi-h a mixture of 50 parts by weight of "Linde A"

alumina and 50 parts by weight of a zirconia 6 mole per

cent Y203 coprecipitate previously calcined in air at 700 0 C

for 30 min. This yttria-stabilised zirconia plus alumina

mixture was also spray-dried before use to improve its die-

fill properties.

Ir!

The temporary bridge supporting the mandrel was then

removed and the rubber bag was closed with a stopper

The tooling, filled now with ceramic powder, was immersed in

a suitable liquid in a pressure vessel and pressed isostatic-

ally to about 40,000 psi. After release of the pressure the

tooling was taken out of the pressure vessel, the bag was

opened and the pressed tube and mandrel were removed

together. The mandrel was carefully extracted from the

pressed body, leaving a closed end tube of alumina with a

composite solid electrolyte tip. This tube was subsequently

densified by firing at high temperatures.

To fire the tubes of Examples 7 to 12, in order to a a convert them to sound, impervious bodies for use as sensors

*0 a in solid electrolyte oxygen probes or analysers, they were a a 15 heated to 17500C and maintained at this temperature for a. 'a. a hours, using a furnace burning natural gas. Firing

temperatures should not exceed 1850 C, otherwise partial

melting will occur due to a eutectic reaction between the

alumina and the yttria-stabilized zirconia. a. From the foregoing, it will be clear to those skilled

a a in this art that by the present invention, ceramic oxygen

sensors can be produced with the following advantages over

prior art sensors. a

a. a

33

Si, j- b:-

i. Sensors comprising a composite electrolyte pellet

fusion-sealed into alumina, aluminous porcelain or mullite

tubing are more easily fabricated. The sealing success rate

in alumina tubing is over 80 per cent compared with only

per cent for prior art sensors of similar construction.

2. The "pellet-in-tube" sensors so made have greater

mechanical integrity than prior art sensors of similar

construction, and thus have lower leak rates and greater

resistance to thermal and/or mechanical shock, factors which

permit operation without a flowing reference air supply,

produce long sensor lifetimes and enable "in situ" oxygen

-r probes to be used in applications found to be beyond the

c capability of prior art sensors. 9** 3. Sensors comprising an oxygen ion conducting tip and

15 a non-conducting body may also be fabricated by a new route a. a. sensor construction in which the sensor is fabricated "green"

and subsequently fired, whereby a sensor is produced with

inherently low cost of manufacture and good mechanical j a 9 properties.

20 Particularly in respect of the "green" sensor route of a..

a construction, it should be noted that the use of a limited

.e volume of composite solid electrolyte material at the end of

.r a body of a ceramic material which is not an electrolyte

enables and permits accurate positional sampling of a gas

stream under test. This is a useful feature where local

variations in oxygen concentrations can exist. In addition,

because the reduction in usage of expensive solid electrolyte

material provides considerable cost advantages over prior i' ?rr.:I I: r art oxygen sensor tubes, oxygen sensing devices produced in this way can incorporate electrolyte materials in which the solid electrolyte oxide is doped, stabilised or partially stabilised by a rare earth metal oxide or a mixture of oxides which includes one or more rare earth metal oxides. LM Yet another feature of sensors formed in the "green"

'-4 state and then fired is that the sensor can be fabricated in

various shapes. For example, some automobile exhaust gas

sensors use a generally conical sensor element, constructed

o 10 entirely of electrolyte material. Sensors of the same

shape, but made of a non-electrolyte ceramic material with S

a tip of composite electrolyte at the point of the cone, can

readily be fabricated by the "green" sensor construction

techniques exemplified above. Such sensors will perform as 0 efficiently as the current sensors in the exhaust gas

monitoring system.

*o S

9S

*a S

S.

Claims (1)

1. ^il

   THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

   1. An oxygen sensor comprising a ceramic hollow body

   composed of or containing a composite solid electrolyte

   material and a pair of electrodes, one of the electrodes

   being located within said hollow body and the other being

   located outside said hollow body, said electrodes being

   separated by said composite solid electrolyte material, and

   said composite solid electrolyte material comprising a

   mixture of at least one non-electrolyte constituent or phase of alumina, aluminous porcelain or mullite, and at least

   one constituent or phase which is a good oxygen ion

   S* condactcr, the microstructure of the composite material

   0 consisting of an intimate mixture of fine grains of the

   oxygen ion-conducting and the non-electrolyte constituents, .the oxygen ion-conducting constituent grains of which

   comprise from 25 to 75 percent by volume of the mixture.

   2. An oxygen sensor as defined in Claim 1, in which

   S the content of oxygen ion-conducting constituent grains

   in the composite solid electrolyte material is from 30 to

   60 per cent by volume.

   0too

   S. 0:cc

   3. An oxygen sensor as defined in claim 2, in which

   the content of oxygen ion conducting constituent grains

   in the composite solid electrolyte material is from

   to 50 per cent by volume.

   4. An oxygen sensor as defined in claim 1, 2 or 3

   in which the oxygen ion-conducting constituent of the

   composite solid electrolyte material is thoria doped

   with calcia, thoria doped with yttria, zirconia

   stabilised or partially stabilised with calcia, zirconia

   stabilised or partially stabilised with magnesia or

   zirconia stabilised or partially stabilised with yttria.

   I* I5. An oxygen sensor as defined in claim 4, in which

   the non-electrolyte constituent of the composite solid

   electrolyte material is alumina and the oxygen ion

   conducting constituent is zirconia stabilised or

   partially stabilised with yttria.

   6. An oxygen sensor as defined in claim 5, in which

   the yttria content of the zirconia-yttria conducting

   constituent of the composite solid electrolyte material

   is from 4 to 25 mole per cent yttria.

   7. An oxygen sensor as defined in claim 6, in which

   the yttria content of the zirconia-yttria oxygen ion

   conducting constituent of the composite solid electrolyte

   material is from 6 to 15 mole per cent yttria.

   0)

   ~1I~C~s 4Ls~gp4)J~qeS

   8. An oxygen sensor as defined in claim 5, 6 or 7

   in which the stabilised or partially stabilised

   zirconia is obtained by the sequential steps of (a)

   coprecipitation of zirconia and yttria, crushing

   and sieving the coprecipitate particles to less than

   200 mesh and calcining the resulting

   powder to form a homogeneous solid solution of yttria

   in zirconia.

   9. An oxygen sensor as defined in claim 5,6, or 7

   in which the composite solid electrolyte material is

   produced by the admixture of stabilised or partially

   stabilised zirconia in finely ground form with alumina

   powder, followed by consolidation of the admixture and

   9. firing to a temperature below 1850 C but sufficiently

   S* high to produce a high density impervious body. 99

   An oxygen sensor as defined in claim 5,6 or 7

   in which the composite solid electrolyte material is

   produced by the admixture of finely ground stabilised

   or partially stabilised zirconia with alumina powder,

   an organic deflocculant and water, followed by slip

   *casting the admixture in gypsum moulds and firing the

   solid material so produced at an elevated temperature

   below 1850°C.

   11. An oxygen sensor as defined in any preceding claim

   in which the composite solid electrolyte material contain-

   ing alumina, aluminous porcelain or mullite is in

   Spellet or disc form, bonded in or to the end of an s' open tube made from alumina, aluminous porcelain or mullite, respectively.

   12. An oxygen sensor as defined in claim 11, in

   which the composite solid electrolyte pellet or disc

   is fusion-sealed to the open tube.

   13. An oxygen sensor as defined in any one of claims

   1 to 10, in which the composite solid electrolyte

   material is in the form of a closed tube.

   14. An oxygen sensor as defined in any one of

   claims 1 to 10 which has been formed in the "green"

   state so that the composite solid electrolyte material lforms a conducting region in a non-electrolyte, ceramic

   hollow body, and then fire.

   An oxygen sensor as defined in claim 14, in which

   the hollow body of the sensor contains a proportion

   of the conducting constituent of the composite solid

   electrolyte material which is less than the proportiov

   present in the composite solid electrolyte material

   forming the conducting region.

   S e*

   16. An oxygen sensor as defined in claim 14 or in which the same non-'lectrolyte ceramic material is

   used for the hollow body and for the non-electrolyte

   constituent in the composite solid electrolyte material.

   17. An oxygen sensor as defined in any preceding

   claim, in which said electrode within said hollow

   body comprises a conducting tube located within the

   ceramic hollow body, one end of the said conducting

   tube being in electrical contact with the composite

   solid electrolyte material, at least one aperture Feing

   formed in the conducting tube in the region of

   said end thereof, whereby a gas may be conveyed through

   the conducting tube before or after passing through

   the aperture or apertures formed therein.

   18. An oxygen sensor as defined in any preceding

   claim, in which said electrode outside said hollow

   fl* S body comprises a conducting sheath around the ceramic 4,e

   hollow body, said sheath being in electrical contact

   S with the composite solid electrolyte material, and 0S S said sheath having a plurality of apertures to enable

   a gas to flow from outside the sheath to within the 5

   sheath and past the composite solid electrolyte material.

   19. An oxygen sensor as defined in any one of claims

   .tSS S 1 to 3 or any one of claims 11 to 18, in which the .55.55 S electrodes are made from platinum, gold, an alloy of

   55 S chromium with nickel, an alloy of chromium with iron, S 555

   t 55 S C a chromium-alumina cermet, or stainless steel. 1> 55

   An oxygen sensor as defined in any one of claims

   4 to 10, wherein said electrodes are made from platinum,

   gold, an alloy of chromium with nickel, an alloy of

   chromium with iron, a chromium-alumina cermet or stainless

   steel. f

   ~I IN-MM

   21. A method of manufacturing an oxygen sensor as

   defined in any one of claims 14,15 and 16, which

   comprises slip casting, extrusion, isostatic pressing

   die pressing or throwing to produce a "green" ceramic

   hollow body and firing the green body.

   22. A method as defined in claim 21, wherein the

   "green" sensor is produced by a slip casting technique

   comprising

   depositing a limited quantity of a slip of the

   composite electrolyte material in the bottom of a

   *0porous mould,

   filling the mould with a slip of the non-electrolyte

   ee. ceramic material used for the body of the sensor, and

   after allowing the casting to acquire the *9 *9

   S. required thickness, inverting the mould and draining

   the excess slip therefrom.

   23. A method as defined in claim 21, wherein the S"green" sensor is produced by a slip casting technique

   0' *006 comprising

   filling a porous mould with a slip of the non-

   electrolyte ceramic material used for the body of the

   sensor, allowing the casting to acquire the required thickness, and draining the excess slip therefrom,

   depositing a small quantity of a slip of the

   composite electrolyte material in the bottom of the

   casting,

   i: A allowing the slip of composite electrolyte material to harden, then before or after firing the "green" sensor, removing part of the outer casting to expose the outer surface of the hardened slip of electrolyte material.

   24. A method as defined in claim 21, wherein the

   "green" sensor is produced by a slip casting technique

   comprising

   forming, in a porous mould, a casting of the

   non-electrolyte ceramic material used for the body of the 0* 0

   0 sensor, and

   injecting through the base of the casting, from

   the exterior thereof, a slip of the composite

   electrolyte material.

   A method as defined in claim 21, in which the

   "green" sensor is produced by an extrusion technique

   comprising the steps of extruding an open ended tube of the non-electrolyte

   ceramic material used for the body of the sensor,

   extruding a rod of the composite electrolyte

   material having cross-sectional shape and dimensions

   to enable the rod to fit within the open end tube, and

   inserting a pellet cut from the rod into one

   end of the extruded tube and applying inwardly directed

   pressure around the circumference of the end of the

   tube to seal the plug therein, or, alternatively,

   ~1

   pressing a small quantity of the extruded composite

   electrolyte into the end of the tube and smearing

   the composite electrolyte over the end and part of the

   outer wall of the tube, near the end to be closed,

   to effect a good bond between the tube and the composite

   electrolyte.

   26. A method as defined in claim 21, in which the "green" sensor is produced by an isostatic pressing

   technique comprising the steps of

   loading powder of the non-electrolyte ceramic

   material used for the body of the sensor into a

   rubber bag liner within a vented outer plastic or metal Sl. tube until a metal mandrel located axially within said

   rubber bag liner is almost covered,

   filling the remaining space above the mandrel

   with powdered composite electrolyte material.

   closing the rubber bag with a stopper and

   subjecting the bag and contents to an external isostatic

   pressure and

   after release of the pressure, removing the *oo0 contents from the rubber bag and extracting the mandrel

   from the pressed tube. C eoS

   27. A method as defined in any one of claims 21 to 26

   in which the "green" sensor is fired at a temperature

   below 1850 C. to densify it.

   28. A method of manufacturing an oxygen sensor,

   substantially as hereinbefore described with reference to any one of the Examples or any Figure of the accompanying drawings.

   An oxygen sensor made by a method claimed

   in any one of claims 21 to 28.

   An oxygen sensor substantially as hereinbefore

   described with reference to any one of the Examples

   or any Figure of the accompanying drawings.

   31. A probe for measuring the oxygen content and

   temperature of a fluid, the probe incorporating a eeu. 6 sensor claimed in claim 29 and 000e

   @0 *s Os@ 0

   0C

   Dated this 23rd day of October 1980. *s S

   COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANIZATION By its Patent Attorneys *000

   so.. DAVIES COLLISON 0

   0* 0 S

   0000

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   S. S

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