GB2190200A

GB2190200A - Solid electrolyte oxygen sensor

Info

Publication number: GB2190200A
Application number: GB08706866A
Authority: GB
Inventors: Toshihiko Aoyama; Haruhisa Shiomi; Ken Minoha
Original assignee: NGK Spark Plug Co Ltd
Current assignee: Niterra Co Ltd
Priority date: 1986-03-24
Filing date: 1987-03-23
Publication date: 1987-11-11
Also published as: JPS62222159A; GB8706866D0; JPH0676988B2; GB2190200B; DE3709516A1; DE3709516C2

Description

GB2190200A 1 SPECIFICATION capacity of the oxygen sensor. In order to hold

the sensor at a temperature that is suit Oxygen sensor able for the intended measurement, a heater for the electrodes must have a high heat gen

BACKGROUND OF THE INVENTION 70 eration rating but then the power consumption

Field of the Invention of the heater is increased while its duranility if

The present invention relates to an oxygen reduced.

sensor that is capable of measuring the con centration of oxygen in, for example, the ex- SUMMARY OF THE INVENTION haust from an internal combustion engine. 75 An object, therefore, of the present inven tion is to provide an oxygen sensor that is Background of the Invention easy to fabricate and has a small heat capa-

With a view to improving fuel economy and city.

reducing emissions, the feedback control of The oxygen sensor of the invention can be air/fuel ratio for internal combustion engines is 80 summarized as a hollow cylindrical member currently performed on the basis of measure- closed at one end but open at the other to ments of the oxygen partial pressure of the admit a reference gas. There is at least one exhaust from the engine. The oxygen partial radial through hole in the hollow cylindrical pressure of the exhaust gas is measured with member. An oxygen-ion conductive solid elec an oxygen sensor that employs a layer of an 85 trolyte layer has at least one pair of elec oxygen-ion conductive electrolyte such as a trodes on its opposite sides. The electrolyte solid solution compound of zirconia or yttria. layer is wrapped around the hollow cylindrical An example of this type of oxygen sensor is member with the electrode on its inner side proposed in Unexamined Published Japanese facing the through hole.

Utility Model Application No.100658/1985. 90 The sensor shown in this patent has an oxy- BRIEF DESCRIPTION OF THE DRAWINGS gen-ion conductive solid electrolyte layer dis- Fig. 1 is a perspective view of an oxygen posed in the gap between an inner and an sensor in accordance with a first embodiment outer electrode that are concentrically arranged of the present invention.

in a cylindrical form. This sensor may be con- 95 Fig. 2 is a cross section of Fig. 1 taken on sidered as an oxygen concentration sensor the line A-A.

which measures a gas to be analyzed on the Fig. 3 is a partial fragmentary view of the outer surface of the solid electrolyte layer. hollow cylindrical member used in the oxygen That is, the sensor constitutes a measuring sensor of Fig. 1.

system without introducing a reference atmos- 100 Fig. 4 shows the sensor of Fig. 1 in an phere. unassembled state.

In order to ensure that the atmospheric air Fig. 5 is a partial fragmentary view of an used as a reference oxygen source in the senoxygen detecting probe that uses the sensor sor described above is introduced to make of Fig. 1.

contact with the inner surface of the solid 105 Fig. 6 shows an oxygen sensor according to electrolyte layer without mixing with the gas a second embodiment of the present invention to be analyzed, a green sheet is used. The in an unassembled state.

green sheet has a slit that provides an air Fig. 7 shows an oxygen sensor according to introducing channel and must be wrapped a third embodiment of the present invention in around a core material, which may be in a bar 110 an unassembled state.

form and which is covered with the solid elec trolyte layer. The assembly of the core and DETAILED DESCRIPTION OF THE PREFERRED the green sheet is sintered to make a product EMBODIMENTS having a channel for introducing the reference The oxygen sensor of the present invention atmosphere. In other words, at least two 115 has been accomplished in order to solve the layers, one being a green sheet of the solid aforementioned problems of the conventional electrolyte layer for effecting measurement of product. It includes a hollow cylindrical mem the oxygen partial pressure and the other be- ber that is open at one end and is provided ing a green sheet that serves as a spacer and with a closing wall at the other end and which is provided with the slit described above, 120 has a through-hole in the side wall. An oxy must be sintered after they are concentrically gen-ion conductive solid electrolyte layer wound. However, it involves considerable diffi- shrouds the hollow cylindrical member. The culty in producing a concentric assembly of hollow cylindrical member has at least one more than one green sheet without introducing pair of electrodes on opposite sides. The elec any distortion. Also the required increase in 125 trode on the inner surface of the solid electro the number of steps in the manufacturing pro- lyte layer is disposed in an area corresponding cess leads to a lower production rate. to the throughhole in the hollow cylindrical Another problem with the technique de- member.

scribed above is that the use of a bar for the The hollow cylindrical member in the oxygen core material causes an increase in the heat 130 sensor of the present invention provides a ref- 2 GB2190200A 2 erence gas introducing channel in that the face contacts the gas to be analyzed, the par atmospheric air serving as a reference oxygen tial pressure of oxygen in the gas to be ana source enters the member at the open end lyzed can be determined by measuring the and exits by way of the through-hole to be current flowing between the electrodes on op introduced into an oxygen concentration elec- 70 posite sides of the electrolyte layer. If this trochemical cell which is composed of the construction is employed, an oxygen sensor solid electrolyte layer and a pair of electrodes. can be fabricated without using any green The hollow cylindrical member may be fabri- sheet having a slit for providing a reference cated by a suitable technique such as pressing gas introducing channel. In addition the use of with dies or extrusion molding. The material 75 a hollow cylindrical member contributes to the of the hollow cylindrical member may be semaking an oxygen sensor that is simple in lected from among ceramics, metals and other construction and which has a small heat capa materials that do not have any significant dif- city.

ference in thermal expansion coefficient from A specific first embodiment of the present the solid electrolyte layer. If the gas to be 80 invention is hereinafter described with refer analyzed is the exhaust from an internal com- ence to Figs. 1 to 5, wherein the components bustion engine that will become as hot as are not shown to scale for the sake of clarity.

60WC or higher, ceramics are advantageously As shown in Fig. 1, an oxygen sensor ac used. If a metal is used, the hollow cylindrical cording to the first embodiment of the present member must be electrically insulated from the 85 invention is generally indicated at 1 and is electrode on the inner surface of the solid composed of a hollow cylindrical member 4 electrolyte layer. which is wrapped with a solid electrolyte layer The solid electrolyte layer may be formed of 8. The hollow cylindrical member 4 is made any material that has oxygen-ion conductivity, of ZrO, and has through- holes 2 and 3 in its such as Y,0JrO, or CaO-W2. 90 outer surface. The solid electrolyte layer 4 is The electrodes provided on opposite sides provided on the inside with a reference elec- of the solid electrolyte layer may be formed of trode 5, and on the outside with a measuring a noble metal such as platinum or a gas-per- electrode 6 and two heating elements 7. The meable material that is made of a mixture of a two heating elements 7 shown in Fig. 1 are noble metal and a ceramic powder. 95 separately shown as being covered by the The components described above may be solid electrolyte layer 8 and in a partially ex assembled into an oxygen sensor by a variety posed section in which the undulatory struc of methods. In one method, a green sheet of ture of the heater elements 7 is evident.

solid electrolyte that has electrodes printed on A cross section of Fig. 1 taken on the line opposite sides by a thick-film deposition pro- 100 A-A is shown in Fig. 2, from which one can cess is wrapped around the hollow cylindrical see that the reference electrode 5 is disposed member in such a manner that the electrode in a selected area of the inner surface of the on the inner surface of the sheet is located in solid electrolyte layer 8 in contact with the an area that corresponds to the through-hole hollow cylindrical member 4. This selected in the hollow cylindrical member. The as- 105 area corresponds to the through-holes 2 and sembly is then fixed in a tubular form with a 3. The measuring electrode 6 is disposed in a suitable jig and sintered to form a unitary sys- selected area of the outer surface of the solid tem of oxygen sensor system. electrolyte layer 8 which is opposite to and A heating element may be printed in the corresponds to the reference electrode 5. The neighborhood of the electrode on the inner 110 heating elements 7 are provided on the outer surface of the solid electrolyte layer by a surface of the solid electrolyte layer 8 in such thick-film deposition process. Alternatively, a a way that the measuring electrode 6 lies be heating element may be printed in the neigh- tween them.

borhood of the electrode on the outer surface The inside reference electrode 5 is con- of the solid electrolyte layer by a thick-film 115 nected to a terminal 11 (see Fig. 1) by way of deposition process. An advantage of providing a through-hole in the solid electrolyte layer 8.

a heating element is that an area of the solid The measuring electrode 6 is connected to a electrolyte layer in the neighborhood of either terminal 12 and the heating elements 7 are electrode can be heated to a temperature that connected to two terminals 13.

is suitable for the intended measurement. 120 Fig. 4 shows the oxygen sensor of the first The oxygen sensor of the present invention embodiment an unassembled state and the has a reference gas introducing channel that hollow cylindrical member 4 shown in the bot extends through the hollow cylindrical member tom of Fig. 4 is depicted also in Fig. 3 in for receiving the atmospheric air at the open cross section taken along the line 13-13. As end and letting it out of the member by way 125 shown, the hollow cylindrical member 4 has of the through-hole so that the air contacts an outside diameter of 3.2 mm and an inside the electrode on the inner surface of the solid diameter of 1.5 mm and is open at one end electrolyte layer. Since then reference gas is 14 while it is closed at the other end with a brought into contact with the inner surface of wall 15. Two through-holes 2 and 3 each the solid electrolyte layer while its outer sur 130 having a diameter of 1 mm are made in a 3 GB2190200A 3 selected area of the side wall which is in the drical member 4. The wrapped green sheet Ela neighborhood of the closing wall 15. The end is securely held against the hollow cylindrical opening 14, the hollow portion 16 and the member 4 by rubber pressing under vacuum, throughholes 2 and 3 of the hollow cylindrical and then sintered in the atmosphere to pro member 4 form a reference gas introducing 70 duce an oxygen sensor which is generally indi channel in that the atmospheric air which encated at 1 in Fig. 1.

ters at the opening 14 flows through the hol- The oxygen sensor 1 thus fabricated is fixed low portion 16 and exits by way of the in holder 32, shown in Fig. 5, with the aid of through-holes 2 and 3. The hollow cylindrical a filler powder 33 of carbon black, talc, etc., member 4 having the construction described 75 a packing 34 and a caulking ring 35. A termi above can be readily produced by pressing nal compressing metal fixture 36 is brazed to with dies or by extrusion molding. each of the terminals 11, 12 and 13 and a As shown in Fig. 4, the solid electrolyte separate lead wire 37 is compressed against layer 8 is formed from a green sheet 8a that each of these metal fixtures. Subsequent in- is made of a powder of a solid solution com- 80 stallation of a metal frame 38, a protective pound Of Y20, and Zr02 which is mixed with casing 39, a grommet 40 and a protector 41, a conventional binder. A through-hole 21 allowing gas flow to the measuring electrode through which the reference electrode 5 is 6, will complete the assembly of an oxygen connected to the terminal 11 is made at a detecting probe 42.

corner of the green sheet 8a. 85 In the oxygen sensor 1 of the first embodi The reverse side of the green sheet 8a ment, the atmospheric air is introduced into which will serve as the inner surface of the the hollow cylindrical member 4 at the open solid electrolyte layer 8 is provided with the end 14, from the upper side of Fig. 5, and reference electrode 5 that is formed by print- flows out by way of the through-holes 2 and ing a zirconia-containing platimum layer in a 90 3 to make contact with the reference elec thickness of 10 urn by a thick-film deposition trode 5. Since the necessary reference gas technique. introducing channel is provided without em The obverse side of the green sheet 8a ploying any slotted green sheet, the oxygen which will serve as the outer surface of the sensor of the present inversion can be manu solid electrolyte layer 8 is provided with the 95 factured at high production rate while involving following components that are formed by a a reduced number of steps.

thick-film printing process. First, the reference In addition, the use of the hollow cylindrical electrode terminal 11, the measuring electrode member 4 contributes to a reduction in the 6 and the measuring electrode terminal 12 are heat capacity of the sensor and this leads to formed from zirconia-containing platimum in a 100 an improvement in the thermal efficiency of urn thick layer. Then, a protective layer 22 the heating elements 7 and, hence, to a smal that is made of platinum-containing alumina ler power consumption. As a further advan and which has a thickness of 20 urn is tage, the heating elements 7 are printed on formed on the surface of the measuring elec- the surface of the green sheet 8a by a thick trode 6. In the next place, an insulating layer 105 film deposition process and this provides for 23 that is made of alumina and which has a very easy fabrication of an oxygen sensor.

thickness of 30 urn is formed on the entire A second embodiment of the present inven surface of the green sheet 8a except for an tion is hereinafter described wit reference to area 24 covered by the terminals 11 and 12 Fig. 6. The most important feature of this em- and an area 25 occupied by the measuring 110 bodiment is that heating elements are pro electrode 6. Subsequently, the two heating vided on the inner surface of a solid electro elements 7 and heater terminals 13, each of lyte layer.

which is made of alumina-containing platinum As shown in Fig. 6, the reverse side of a and has a thickness of 10 urn, are formed on green sheet 108a which will serve as the in the surface of the insulating layer 23. Finally, 115 ner surface of the solid electrolyte layer is an insulation layer 26 that is made of silica- provided by a thick-film printing process with containing alumina and which has a thickness a reference electrode 105, an insulating layer of 20 pm is formed on the entire surface of 152, heating elements 107 and an insulating the insulating layer 23 except for an area 27 layer 151, in the stated order. The obverse covered by the heater terminals 13 and an 120 side of the green sheet 108 which will served area 28 occupied by the measuring electrode as the outer surface of the solid electrolyte 6. layer is provided by a thick-film printing pro The green sheet 8a that has been provided cess with a measuring electrode 106, a pro with the necessary parts by a thick-film print- tective layer 122, an insulating layer 153, a ing process is then coated with a layer of 125 reference electrode terminal 111, a measuring zirconia paste on its reverse, or inner, side electrode terminal 112 and heater terminals and is wrapped around the hollow cylindrical 113, in the stated order. The green sheet member 4 such that the reference electrode 5 108a thus provided with the necessary com will be situated in an area that corresponds to ponents is wrapped around a hollow cylindri the through-holes 2 and 3 in the hollow cylin- 130 cal member 104 in such a manner that the 4 GB2190200A 4 reference electrode 105 on the green sheet A further advantage of the sensor fabricated 108a will be situated in an area that corre- in accordance with the third embodiment is sponds to a through-hole 102 made in the that using the simple construction described side wall of the hollow cylindrical member above, an oxygen concentration electrochemi 104. Thereafter, the assembly is sintered as 70 cal cell element can be combined with an oxy in the first embodiment to thereby produce an gen pump element such that an air/fuel ratio oxygen sensor. The components of the sensor signal can be detected on the basis of a mea are made of the same materials as what are sured pump current.

employed in the first embodiment. While the three embodiments of the present In the second embodiment described above, 75 invention are described above, it should be the heating elements 107 are formed on the noted that the invention is in no way limited same side of the green sheet 108a as where to these particular embodiments and that vari the reference electrode 105 is provided and ous modifications can be made without de this enables the reference electrode to be parting from the spirit of the invention.

heated with a very high efficiency to a tem- 80 The oxygen sensor of the present invention perature that is suitable for the intended mea- has a simple construction in that a hollow surement. Even if the gas to be analyzed has cylindrical member that is open at one end a low temperature, the partial pressure of oxy- and which has a through- hole in the side wall gen in that gas can be readily measured since is used as a reference gas introducing channel the measuring zone will not become too cold 85 and that this hollow cylindrical member is to achieve the measurement. An obvious ad- wrapped with a solid electrolyte layer. Since vantage resulting from this feature is that if only one solid electrolyte layer needs to be the sensor is used for measurement of the wrapped around the hollow cylindrical mem oxygen partial pressure of the exhaust from an ber, the desired oxygen sensor can be manu internal combustion engine, the intended mea- 90 factured at high production rate involving a surement can be started immediately after the reduced number of steps. In addition, the use engine is started. of the hollow cylindrical member is effective in A third embodiment of the present invention reducing the heat capacity of the sensor and is hereinafter described with reference to Fig. not only does this improve the thermal effici 7. The main thrust of this embodiment lies in 95 ency of a heating element used to activate the using the hollow cylindrical member of the sensor but also the power consumption of the first embodiment as a component of an air/- heater is reduced.

fuel ratio sensor. The oxygen sensor of the invention offers As shown in Fig. 7, the reverse side of a several advantages. It is not subject to varia- green sheet 208a which will serve as the in- 100 tions by a changing gas flow. Little strain is ner surface of the solid electrolyte layer is produced by the differential thermal expansion provided with by a thick-film printing process of the structural materials. Its structure is sim with a second pump electrode 262, a diffu- ple so that its manufacturing cost is low. No sion limiting layer 261, a measuring electrode external electric source is required.

206, another electrolyte layer 263 and a refer-

Claims

ence electrode 205, in the stated order. The CLAIMS obverse side of the

green sheet 208a which 1. An oxygen sensor, comprising:

will serve as the outer surface of the solid a hollow cylindrical member opened on a electrolyte layer is provided by a thick-film first axial end, closed on a second axial end printing process with a first pump electrode 110 and having a through- hole in a side wall; and 264, an insulating layer 226, heating elements an oxygen-ion conductive solid electrolyte 207 and an insulating layer 229, in the stated layer fixed to said hollow cylindrical member order. The green sheet 208a thus provided and having formed on opposing principal sides with the necessary components is wrapped at least two electrodes, one of said electrodes around a hollow cylindrical member 204 as in 115 facing said hollow cylindrical member being the first embodiment and subsequently sin- disposed in an area of said through-hole.

tered in the atmosphere to produce an air/fuel
2. An oxygen sensor as recited in Claim 1, ratio sensor. wherein said solid electrolyte layer has a heat In the third embodiment described above, ing element on its principal surface facing said the heating elements 207 are formed on the 120 hollow cylindrical member.

surface of a single layer of green sheet 208a
3. An oxygen sensor as recited in Claim 2, and this provides great convenience for the further comprising an insulating layer between manufacture of an air/fuel ratio sensor. Since said heating element and said solid electrolyte the hollow cylindrical member 204 and the layer.

single layer of green sheet 208a are the two 125
4. An oxygen sensor as recited in Claim 1, principal components, the sensor has a re- wherein said solid electrolyte layer has a heat duced heat capacity and this is effective in ing element on its principal surface facing decreasing the power consumption of the away from said hollow cylindrical member.

heating elements 207 while increasing their
5. An oxygen sensor as recited in Claim 4, durability. 130 further comprising an insulating layer between GB2190200A 5 said heating element and said solid electrolyte layer.
6. An oxygen sensor as recited in Claim 1, wherein said first axial end opens into a refer- ence gas and said one electrode facing away from said hollow cylindrical member is exposed to a gas to be measured.
7. An oxygen sensor as recited in Claim 6, wherein said two electrodes are both formed in said area of said through-hole.
8. An oxygen sensor as recited in Claim 1, wherein said electrodes are formed by a thick film printed on said solid electrolyte layer.
9. An oxygen sensor substantially as de- scribed with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 'I AY, from which copies may be obtained.