Detection element for determining oxygen content and detector for determining oxygen content using the same

This invention discloses a crack-free and low-cost detection element for determining an oxygen content and a detector for determining an oxygen content using the element. The element comprises a ceramic tubular body, an ion-conductive ceramic bowllike closing member fixed to the opening end of the tubular body in such manner that the opening end of the closing member may not be inserted into the opening of the tubular body, and an inner and an outer electrode respectively mounted on the inside and the outside surface of the bowllike closing member.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a detection element for determining an oxygen 
content, and more particularly, to a detection element adapted for 
determining an oxygen content of the hot exhaust gas discharged from an 
internal combustion engine such as an automobile and to a detector for 
determining the oxygen content using the element. 
2. Prior Art 
It is a general practice of the automotive industry to reduce exhaust gas 
pollution by mounting an exhaust pipe with a detector for determining an 
oxygen content, and by regulating and maintaining an air fuel ratio at a 
value of about one. In the conventional detectors of this type it is 
typical to use a solid electrolyte tubular body as a detection element. 
Such elements have a tubular body being closed at one end and formed 
integrally with an ion-conductive ceramic material such as stabilized 
zirconium oxide. Such elements have serious problems in that 
ion-conductive ceramic materials, such as zirconium oxide are very high in 
price. 
In contrast thereto, it is proposed in the Japanese Patent Application No. 
81027/1975 to provide a detection element whose production cost is reduced 
by fixing a plate-like stabilized zirconium oxide to the concentric hole 
formed at the end of a tubular body. The tubular body is formed of an 
ion-nonconductive ceramic material such as folsterite which is 
inexpensive. The detection element described that the Japanese Patent 
Application has the following disadvantages. Namely, (a) the end portion 
of the element is quickly heated by the hot exhaust gas discharged and a 
sharp temperature gradient is produced immediately after the start of an 
internal combustion engine. Because folsterite is inferior in thermal 
shock resistance to zirconium oxide, the detection element is liable to 
crack at the end of the folsterite tubular body; (b) the flat zirconium 
oxide plate fixed to the concentric hole at the end of the folsterite 
tubular body is greater in the percentage of a catalyzing layer such as 
platinum on the inside and outside surfaces of the plate than the 
folsterite tubular body. This plate quickly expanded, with the result that 
the end portion of the folsterite tubular body is subjected by such 
expansion to centrifugal depression which, in turn, causes more cracks to 
occur. Above all, when, as in the previous application, the outer 
circumferential surface of the flat zirconium oxide plate is mounted on 
the inner circumference of the opening end of the folsterite tubular body, 
the likelihood of cracking is great. 
OBJECT OF THE INVENTION 
A primary object of the invention is to provide a detection element which 
retains the economical advantages of the detection element disclosed in 
the above Japanese Patent Application No. 81027/1975 and which can prevent 
as much as possible the cracking considered as a disadvantage inherent in 
the element and to provide a detector using the element. 
Other objects and advantages of the invention will become more apparent to 
those skilled in the art from a description given below in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The detection element shown in FIG. 1 is formed such that the opening end 
of a bowllike closing member 2 is brought into abutment against the 
opening end of a tubular body having a flange 12 for fixing the element. 
The element is fixed thereto with suitable sealing glass. According to the 
embodiment shown in FIG. 2, a tubular body 1 slightly thick-walled and 
having a similar flange 12 is used and is formed at the outer 
circumferential edge of the opening end thereof with a stepped groove. A 
fixing opening 21 of the bowllike closing member 2 is fitted into the 
groove 21 and is sealed with similar sealing glass. The detection element 
shown in FIG. 3 is formed of a slightly thick-walled tubular body 1 having 
generally in the middle thereof an expanded portion 11 for fixing the 
element. The detection element of FIG. 3 is formed with a stepped groove 
into which the bowllike closing member 2 is fitted and sealed in the same 
manner as the embodiment in FIG. 2. FIG. 4 shows a detection element 
formed such that a stepped groove is formed at the outer circumferential 
edge of the opening end of a slightly thick-walled tubular body 1 having a 
flange 12. A stepped groove engaging with the above stepped groove is 
formed at the inner circumferential edge of the opening 21 of the bowllike 
closing member 2 so as to connect and seal both openings with sealing 
glass. In all embodiments illustrated, the inner and outer electrodes 3 
and 4 are extended, respectively, to the inside and outside surfaces of 
the tubular body to facilitate electrical connection, but, in the event 
that the electrodes 3 and 4 are mounted only on the inside or the outside 
surface of the member 2, recourse may be had to other suitable means for 
electrical connection. 
The bowllike closing member 2 is made, for example, of an ion-conductive 
ceramic material such as stabilized zirconium oxide, and the tubular body 
1 is made of a ceramic material such as folsterite, alumina, beryllia, 
having a coefficient of thermal expansion approximate to the above ceramic 
material making the member 2. The inner electrode 3 and the outer 
electrode 4 are of a thin layer strip of non-oxidizable metal such as 
platinum. 
As is understood from the embodiments shown in FIGS. 1 through 4, the 
detection element according to the present invention consists of a tubular 
body 1 of ceramics, a bowllike closing member 2 of ion-conductive ceramics 
fixed to the opening end of the tubular body 1, and an inner electrode 3 
and an outer electrode 4 mounted to the member 2. The main characteristics 
of the element are that the bowllike closing member 2 is used and that the 
fixable opening 21 of the member 2 is fixed in abutment against the 
opening end of the tubular body 1 without being fitted into the body 1. 
When, instead of the platelike closing member used in the prior art 
element, the bowllike closing member 2 is fixed in abutment against the 
opening end of the tubular member 1 as proposed by the present invention, 
the cracking of the tubular body 2 considered as a disadvantage inherent 
in the prior art element is substantially eliminated. This is believed to 
be because the end portion of the detection element, which is subjected to 
rapid heating by exhaust gas to thereby produce a sharp thermal gradient 
in starting time of an internal combustion engine is made of the bowllike 
closing member 2 of zirconium oxide. This material is high in thermal 
shock resistance. Cracking is also prevented because the tubular body 1 is 
made of a material low in thermal shock resistance such as folsterite 
which is gradually heated by heat conduction from the member 2. 
Furthermore, a means for bringing about a synergic effect in preventing 
cracking has been disclosed which relates to the way member 2 is fixed to 
the body 1. The bowllike closing member 2 is fixed, not by inserting the 
member 2 into the opening of the tubular body 1, but by fixing the former 
to the latter in abutment against the latter. In this manner even if the 
member 2 is quickly heated and expanded at the time the engine is started, 
the tubular body 1 is not heavily subjected to centrifugal depression by 
the expansion of the member 2. Moreover, since the bowllike closing member 
2 made of the more expensive zirconium oxide is very small in percentage 
member 2 occupies out of the whole detection element, the element of the 
invention retains the advantage possessed by the prior art. 
As seen in FIGS. 2 and 4, when a stepped groove is formed at the 
circumferential edge of the opening end of the tubular body 1 and the 
bowllike closing member 2 is fitted in the groove and fixed thereto, there 
is additional advantage that coaxial fixing of both the member 2 and the 
body 1 improves workability. Furthermore, the element of the invention has 
another advantage in that the provision of the stepped groove at the outer 
circumferential edge of the end portion reduces the area which is exposed 
to the heat of the glass sealed portion which is low in heat resistance. 
Accordingly, the groove structurally protects the sealed portion against 
thermal damage. 
Thus, the detection element of the invention is an improved invention 
designed to prevent the cracking of the element while it retains the 
economic advantages of the prior art detection element. It is believed to 
be very useful in the automotive industry and other related industries. 
A description will now be given of a detector for determining an oxygen 
content using the detection element of the invention described above with 
reference to FIG. 5. The reference character A in the figure designates a 
detection element of the type shown in FIG. 3. The detection element A is 
supported on its expanded portion 11 from below by a sealing member 6 for 
the prevention of gas leakage and is depressed from above and retained by 
a fixing means 5. Fixing means 5 is made up of a ringlike washer 51 and a 
compression spring 52 and is incorporated in a casing 8. Electrical 
connection between the inner electrode 3 of the detection element A and 
the outside is secured by an electrical connection means 7 made up of a 
metal lead cap 71 fitted over the upper end opening of the element A and a 
cylindrical metal lead terminal 72 mounted to the cap 71. 
The outer electrode 4 is conducted to the metal casing 8 by the sealing 
member 6. Sealing member 6 has a dual function in that it acts as the 
electrical connection means 7. It is electrically grounded by the casing 8 
and maintained at zero potential. 
The casing 8 is made up of an upper casing 81 and a lower casing 82 having 
opposing flanges 811 and 821, respectively. Both casings are integrally 
connected in the flanges 811 and 821 by suitable fasteners. Furthermore, 
an insulating bush 9 is fitted in a hole in the upper end face of the 
upper casing 81 for inserting a lead terminal through the hole so as to 
prevent the lead terminal 9 and the upper casing 81 from coming into 
contact with each other thereby causing a short-circuit. In addition 
thereto, in the detector illustrated, a heat-resisting member 10 such as 
heat-proof wool is stuffed between the lower casing 82 and the tubular 
body 1 of the detection element. This helps prevent the tubular body 1 of 
the element which is low in thermal shock resistance from coming into 
direct contact with hot exhaust gas flowing in from gas vent pores 822 of 
the lower casing 82, and from being quickly heated by direct contact with 
the lower casing. Such configuration also gives heat screening effect to 
the glass sealed portion between the tubular body 1 and the bowllike 
closing member 2. 
Since the detector of the construction described above uses the described 
type detection element of the invention as a detection element and, if 
necessary, the heat resisting member 10, the detector is believed to be 
free from problems such as cracking. It is also easy to reduce the 
production cost. In addition to such marked effects, the detector of the 
invention provides advantages such as (a) vibration resistance is greatly 
increased by the employment of the ringlike washer 51 and compression 
spring 52 as a fixing means 5 for the detection element A; and (b) the 
electrically conductive members heretofore incorporated in large numbers 
into the detection element are eliminated by the use of the lead cap 71, 
lead terminal 72 and electrically conductive sealing member 6. Thus, the 
detector structurally is simplified and reduced in the number of 
manufacturing steps. 
As will be understood from the embodiment illustrated in FIG. 5, the 
detector of the invention is of simple construction in that it comprises 
detection element A of the invention having an inner electrode 3 and an 
outer electrode 4, a fixing means 5 for the element A, preferably a gas 
leak preventing means 6, an electrical connection means for the electrodes 
3 and 4, and a casing 8 for containing the preceding members and means 
therein, and preferably the aforementioned heat-resisting member 10. Thus, 
the detector obviates those problems, due to the detection element itself 
and to shock, and makes it easy to reduce the production cost of the 
detector, a great contribution to the automotive industry. 
It should be understood that the detection element and the detector of this 
invention are not limited to the embodiments described and illustrated, 
and that various modifications may be made without departing from the 
scope of the invention.