AU479218B2 - An electrochemical measuring sensor - Google Patents

Description

53, 14 9 73

The invention relates to an electro-chen.ical measuring

sensor for detcrmining the ox-ygen content of exhaust gases,

such as the exhaust gases of internal combustion engines, by neans of an oxygen concentration cell having an ion-conducting

solid electrolyte.

The exhaust gas of motcr vehicle internal combustion

engines contains, inter alia, carbon monoxide, nitrogen oxides and unburnt or zartially burnt hydrocarbons which contribute to

the pollution of the air. In order to reduce to a minimum the air pollution caused by these substances, it is necessary, as far as rossible, to free the exhaust gases of motor vehicle

internal combustion engines from these substances. This means that carbon monoxide and hydrocarbons have to be converted as

fully as possible into their highest stage of oxidation, i.e. carbon dioxide and, in the case of the hydrocarbons, water, and that nitrogen oxides have to be converted into elementary

nitrogen.

By way of example, the injurious constituents of the

exhaust gas can be converted into the harmless compounds, namely

carbon dioxide, nitrogen and water, by subjecting the exhaust

gases to secondary burning by conducting them over catalysts at

temperatures in excess of about 600 0 C. However, a prerequisite

for the success of this method is that the composition of the

exhaust gas has to be adjusted such that it is possible to

convert the injurious substances in the exhaust gas virtually

fully into the harmless compounds, i.e. the ratio of air to

fuel has to be substantially stoichiometric, this commonly being

-2- 479.2 1 8 l3,149ZI3 characterised by an air number in the vicinity of 1. The air number represents the mass ratio of fuel to air in the mixture fed to the engine and is chosen so that I when the mixture is stoichiometric. W:ith regard to the oxygen content of the exhaust gas, this means that, when 1 no "surplus" oxygen in excess of the equilibrium quantity of the different possible reactions is present, while "surplus" oxygen is present in the mixture when 1. Thus, when 1I the exhaust gas passes from the reducing to the oxidizing state.

In order to maintain a A value of substantially 1, it is necessary to introduce into the path of the exhaust gas a

measuring sensor which determines, for example, the oxygen

content and correctly adjusts the composition of the exhaust

gas by way of a regulating device.

Australian patent application No. 51272/73 describes

a measuring sensor based on the principle of the oxygen

concentration cell having an ion-conducting solid electrolyte

in the form of a tube closed at one end. Such a sensor is

fitted into the wall of the exhaust gas outlet. However, it

should be possible to exchange the measuring sensor, since it

is possible for the measuring sensor to become defective or to

be less 'durable than the exhaust gas outlet into which the

measuring sensor is fitted.

A feature of the invention is to provide an electro-

chemical measuring sensor for determining the oxygen content of

exhaust gases and which is so cornructed that it may be exchanged

in a siImple manner, it also being a simple matter to repair the

electrical connections required for operating the measuring sensor. 479,218

,53, 14 9 i3

The outer surface of the solid electrolyte tube bears an

electron- conducting layer which serves as an outer electrode and

which is able to catalyze the adjustment of the gas equilibrium,

while an inner electrode connected to the solid electrolyte tube

is located within the cavity in the latter.

In accordance with the invention, an electro-chemical

measuring sensor comprises an oxygen concentration cell having

an ion-conducting solid electrolyte in the form of a tube closed

at one end, an electron-conducting layer being located on the

outer surface of the tube and an inner electrode being located

in the cavity in the tube, the solid electrolyte tube having A

shoulder adjacent its open end with the electron-conducting

layer, or a conductor path electrically connected thereto, extend-

ing up to the shoulder, the inner electrode extending into the

region of the shoulder, and a metal holde r in which the solid

electrolyte tube is located, the metal holder being adapted

to the shape of the shoulder on said tube and having a screw-in thread, a free space between the shoulder and the holder being

filled with an electrically conductive sealing composition which

is resistant to the temperatures occurring at this location, so

that an electrical contact exists between the electron-conducting layer and the holder.

The electrically conductive sealing composition is 0 preferably resistant to temperatures of 350 to 450 C.

Furthermore, it is advantageous if the inner electrode

also extends into the region of the shoulder, and if a contact

tag, provided with at least one bore or a slot, is located in the

open portion of the solid electrolyte tube, the free space

between the contact tag and the inner wall of the shoulder

bein7 filled -a.ith 4 479.218

53, 14 9;-

an electrically conductive sealing composition which is resistant

to the temperatures of 350 to 4500C cccurring at this location,

so that an electrical contact exists between the inner electrode

and the contact tag.

Eetal rings serving as compression stamps are preferably

located above the electrically conductive sealing composition be-

tween the solid electrolyte and the metal holder and between the

solid electrolyte and the contact tag, which metal rings, if

required, are mechanically connected to the outer metal holder or

to the contact tag by flanging or screwing.

The electrically conductive sealing composition may comprise

an electrically conductive melt or a mixture of plastic siliceous

raw materials such as talc and/or kaolin and metal powder. The

said electrically conductive melt comprises a mixture of two

components, one of the components being glass, while the other

component constitutes electron-conducting materials such as metals,

electron-conductive oxides, hard materials or carbon and mixtures

thereof. Copper, iron, nickel, a mixture of iron and graphite,

silicon, silicon carbide, carbon black, Fe2 0 3 /Fe 04 and Cu20.Cr2 0 3 have given particularly satisfactory results. These materials

are present in the electrically conductive fused-in glass in a

quantity of from 10 to 50 percent by volume, preferably 15 to

percent by volume. The transformation point of the glass 0 phase is at about 400 to 5S0 C.

When using plastic siliceous raw materials such as talc

and/or kaolin and metal powder as an electrically conductive

sealing composition, the metal, preferably aluminium, is present

47 9.21

53,14113

in a quantity of from 10 to 50 percent by volume, preferably to 25 percent by volume. The invention will be further described, by way of exa=ple, with reference to the accompanying drawing which is a longitudinal section through a measuring sensor in accordance with the invention. The measuring sensor includes a solid electrolyte tube which is closed at one end and which has a shoulder 1.1 at its oren end. The solid electrolyte tube comprises stabilized cubic zirconium dioxide. A platinum layer 12 of about 10 p'm thickness is located on the outer surface of the solid electrolyte tube This platinum layer also substantially covers the surface of the shoulder 11. The inner electrode 13 in the form of a conductor path is mounted in the cavity in the solid electrolyte tube The inner electrode 13 also comprises platinum and is produced by painting a platinum suspension onto the inner surface of the tube, and subsequent stoving. The inner electrode 13 also extends into the region of the shoulder 11. The solid electrolyte tube is mounted by means of its shoulder 11 in a steel holder 14Jof complementary configuration. The holder 14Ihas an external screw thread 15 for the screwing-in of the measuring sensor. An electrically conductive melt 16 is located between the shoulder 11 of the tube 10 and the holder 14. This melt serves to mechanicall:y connect the solid electrolyte tube 10 to the holder 14 and at the san.e time ensures electrical contact between the outer platinum layer 12 and the holder 14. -Furthermore, the melt ensures sealing between the solid electrolyte tube and the holder. A gas-tight 479,218

53, 1 4 973

connection of this kind between the two members is important,

since exhaust gases entering the interior of the solid electrolyte

tube 10 lead to a change in the partial pressure of the oxygen

on the inner electrode 13, so that mixed potentials are measured.

A contact tag 18 is located in the upper portion of the

cavity in the solid electrolyte tube 10 and is mechanically fixed

by the sane electrically conductive melt 16 which at the same

time establishes the electrical contact between the inner

electrode 13 and the contact tag 18. Respective steel rings 17

and 19 are located above the electrically conductive melt 16 and

serve as compression stamps for compressing the electrically

conductive melt. The contact tag 18 is provided with a bore

which enables the atmospheric air, serving as a reference

parameter, to enter the cavity in the tube 10 in an unobstructed

manner.

To protect the platinum layer 12 against mechanical and

chemical damage, the platinum, layer 12 is provided with a porous

coating 21 of about 200 om thickness made from lanthanum cobalt

oxide doped with strontium oxide.

The electrically conductive melt 16 comprises 60 percent

by weight of copper grindings and 40 percent by weight of glass

which has a grain size of 60 im and which has the following

composition: 60.9 percent by weight of Si02 28.4 percent by

weight of B 2 03 9.2 percent by weight of Na20 and 0.8 percent

by weight of A1203.

The components are mixed dry aid, to obtain an improved

metering ability, are granulated in accordance with known methods

such as adding organic binding agents. 7 47 9 21

3, 1 4 913

14, The solid electrolyte tube 10 is inserted into the holder into the bore provided and the tubular contact tag 18 is inserted Rings, which have for this purpose in the solid electrolyte tube. above-described granulated previously been press-formed from the kp/cm2 mixture of glass and copper at a pressure of about 600 gaps between the solid are subseouently inserted into the annular the solid electrolyte tube 10 and the holder 14 and between metal rings 17 and electrolyte tube 10 and the contact tag 18, the gaps. 19 then being inserted into the said annular be metered Alternatively, the granulatad material may by pressing-in. directly into the annular gaps and precompressed that it is difficult However, this rethod has the disadvantages into the gaps which to meter the granulated material uniformly

are only about 2mm in width, and only relatively low pressures

can be used during precompressing, since the solid electrolyte

tube would otherwise be destroyed. granulated material The precompressed rings made from the gaps to about 1 to of glass and copper should fill the annular solid electrolyte 2 mm below the top edge of the holder and the placed thereon project tube, so that the steel rings ?1and 19 the holder. about 4 to 5 mm above the top edge of is arranged The measuring sensor preassembled in this manner a non-scaling material and is in a corresponding holder made from 0 after the heated to about 820 C in a furnace. Imediately the furnace, the two metal measuring sensor has been removed from the hot state under a pressure rings 17 and 19 are pressed-in in is of about 50 kp/cm 2 Thus, the softened granulated material the cavities, so that the two compressed and is pressed into all 419.218

53, 14 9P,

annular gaps are satisfactorily filled with the electrically

conductive melt 16. After the pressing-in operation, the

pressure is mai-tained until the glass melt has been cooled to 0 a temperature below its transformation temperature of about 500 C.

The metal ring 17 can be attached to the holder 14 by flanging

or screwing.

479.21 8

Claims (1)

1. 53, 14 9/73

   THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

   1. An electro-chemical measuring sensor for determining

   the oxygen content of exhaust gases, comprising an oxygen

   concentration cell having an ion-conducting solid electrolyte

   in the form of a tube closed at one end, an electron-conducting

   layer being located on the outer surface of the tube and an

   inner electrode being located in the cavity in the tube, the

   solid electrolyte tube having a shoulder adjacent its open end

   with the electron-conducting layer, or a conductor path electrical-

   ly connected thereto, extending up to the shoulder, the inner

   electrode extending into the region of the shoulder, and a metal holder in which the solid electrolyte tube is located, the metal

   holder being adapted to the shape of the shoulder on said tube

   and having a screw-in thread, a free space between the shoulder

   and the holder being filled with an electrically conductive

   sealing composition which is resistant to the temperatures

   occurriding at this location, so that an electrical contact

   exists between the electron-conducting layer and the holder. 2. An electro-chemical measuring sensor as claimed

   in claim i, in which the inner electrode extends into the region of the shoudler, and a contact tag, provided with at least one

   bore or a slot, is located in the open end of the solid electrol- yte tube, the free space between the contact tag and the inner wall of the tube at the shoulder being filled with an

   electrically conductive sealing composition which is resistant

   to the temperatures occurring at this location, so that

   electrical contact exists between the inner electrode and the

   contact tag.

   479.218

   3, 14 9/3

   3. A measuring sensor as claimed in claim 1 or 2, in

   which at least one metal ring is located above the electrically

   conductive sealing composition.

   4. A measuring sensor as claimed in claim 3 in which

   the metal ring is mechanically connected to the metal holder

   by flanging or screwing.

   A measuring sensor as claimed in any of claims 1

   to 4, in which the electrically conductive sealing composition comprises an electrically conductive melt.

   6. A measuring sensor as claimed in any of claims 1

   to 4 in which the electrically conductive sealing composition

   comprises a mixture of plastic siliceous raw materials, such

   as talc and/or kaolin, and metal powder. which 7. A measuring sensor as claimed in claim 5, in

   the electrically conductive melt comprises a mixture of glass

   and electron-conducting materials.

   8. A.measuring sensor as claimed in claim 7 in which

   the electron-conducting material comprises a metal, an .electron- thereof. conductive oxide, a hard material or carbon or a mixture

   9. A measuring sensor as claimed in claim 8, in

   which the electron-conducting material comprises copper, iron,

   nickel, a mixture of iron and graphite, silicon, silicon carbide,

   carbon black, Fe 2 0 /?e304 or Cu 2 0.Cr 2 0 3 A measuring sensor as claimed in claim 7, 8 or 9 in the in which the content of the electron-conducting material by volume. electrically conductive melt is from 10 to 50 percent -11- 419.218

   3, 1 4 /3

   11. A measuring sensor as claimed in claim 10 in which the electron-ccaduct-in-g material content is 15 to 40 percent by volume. 12. A measuring sensor as claimed in claim 6 in T:hich the electrically conductive sealing compound comprises talc and 10 to percent by volume of a metal powder. 13. A reasur-in- sensor as claimed in claim 12 in -whnich the vollume of -etal o;:wder is 20 to 25 percent. 14. A neasurin sensor as claimed in claim 12 or 13 in ,hich the metal ow.-der is aluminium. A =easuri2n sensor as claimed in claim 12, 13 or 14 in which the electrically conductive sealing compound further conprises kaolin or other plastic siliceous raw materials. 16. An electro-chemical measuring sensor constructed substantially as herein described with reference to and as illustrated in the acccmpanying drawing.

   DATE D this 8th day of March, 1973.

   RO ERT BOSCH GmbH By its Patent Attorneys: CALLINA AND YEWTON.

   -12- 479.2 18

   53, 14 9/73

   4179,21 8