Apparatus for determining the oxygen content in gases

An apparatus for determining oxygen concentration in exhaust gases from an automotive engine is provided which includes a solid electrolytic member of oxygen ion conductive material disposed between a pair of first and second electrodes. In this apparatus, a porous insulating layer is formed on a cylindrical heater made of ceramics and a first electrode is formed on the outer periphery of the porous insulating layer. A solid electrolytic member is formed as a thin film form on the outer periphery of the first electrode layer and a second electrode is formed on the outer periphery of the solid electrolytic film member. Since the solid electrolytic film is formed such that it overlies the outer periphery of the cylindrical heater, it has an adequate structural strength even if formed to be a thinner film.

BACKGROUND OF THE INVENTION 
This invention relates to an apparatus for determining the oxygen content 
in gases and, in particular, to an apparatus used in an automotive engine 
mechanism and measuring a saturated current level which corresponds to the 
oxygen concentration in the exhaust gas from the engine, for an electronic 
control of the engine. 
An apparatus for determining the oxygen concentration in gases by measuring 
a saturated current level is well known in U.S. Pat. No. 4,282,080; 
Japanese Patent Disclosure (KOKAI) No. 55-116248; Japanese Patent 
Disclosure (KOKAI) No. 57-166554 and Japanese Utility Model Disclosure 
(KOKAI) No. 57-92159. 
An oxygen concentration detecting element of the oxygen content determining 
apparatus includes a plate-like, solid electrolytic member made of oxygen 
ion conductive metal oxide and a pair of electrodes attached one at each 
side of the electrolytic member. 
When a voltage is applied between the pair of electrodes, oxygen in an 
atmosphere to be measured is migrated as ions in the solid electrolytic 
member from one electrode toward the other for diffusion. 
The voltage-current characteristic with respect to the above electrode pair 
exhibits a constant current characteristic within a certain voltage range. 
Hereinafter, a current corresponding to such a voltage range of the 
constant current characteristic is referred to as a saturated current 
level. 
The saturated current level corresponds to the amount of oxygen ions 
diffusing in the solid electrolytic member. Thus it is possible to know 
the oxygen concentration in gases to be measured, by applying such a 
voltage between the pair of electrodes that the current is at a saturated 
current level, and measuring the saturated current level at that time. 
The oxygen content determining apparatus is mounted on an automotive 
exhaust pipe in order to determine the oxygen concentration in the exhaust 
gases. In this case it is required that the solid electrolytic member has 
an enough structural strength to withstand severe conditions, such as 
intense vibrations, to which it is subjected when incorporated into an 
automotive vehicle. 
Conventionally in oxygen content determining apparatus of such a type, the 
solid electrolytic member is formed of a plate-like or a cup-like sintered 
ceramics, and a pair of electrodes are formed at the surfaces of the 
member. The solid electrolytic member shows a withstand characteristic 
against the above-mentioned severe conditions, only when it is formed to a 
thick shape. 
It is preferred that the solid electrolytic member be formed with a thin 
shape, because the thinner member provides a smaller internal resistance 
and permits the temperature for operating the detecting element to be 
lower. It is difficult, however, to form the thin solid electrolytic 
member from the standpoint of securing the above-mentioned adequate 
structural strength. It is therefore necessary to set the operating 
temperature for detection of oxygen concentration at a high level, for 
example, above 700.degree. C. As a result, it is necessary to enhance the 
capacity of the heater. This results in complicating the structure and 
increasing power consumption of the heater. 
SUMMARY OF THE INVENTION 
It is accordingly one object of this invention to provide an apparatus for 
determining the oxygen content under the electronic control of an 
automotive engine by measuring a saturated current level which corresponds 
to the oxygen concentration in exhaust gases from the engine, while 
maintaining a sufficient structural strength for the solid electrolytic 
member of the oxygen concentration detecting element regardless of a small 
thickness of the electrolytic member. 
Another object of this invention is to provide an apparatus which can lower 
a temperature at which it is operated. 
Another object of this invention is to provide an apparatus which can 
simplify the structure of a heater and thus assuring the ready manufacture 
of the heater. 
Further objects and advantages of this invention can be seen in the 
following description. 
An oxygen content determining apparatus according to this invention 
comprises a bar shaped ceramics heater, a porous insulating layer formed 
on the outer periphery of the ceramics heater, a first electrode formed on 
the outer periphery of the porous insulating layer, a solid electrolytic 
film formed on a part of the outer periphery of the first electrode, a 
second electrode formed on the outer periphery of the solid electrolytic 
film in a manner insulated from the first electrode by an insulating 
layer, and an oxygen diffusion resistant layer formed on the outer 
periphery of the second electrode. The apparatus is mounted in an 
automotive engine mechanism, with an area incorporating the solid 
electrolytic film being exposed to the exhaust gases from the automotive 
engine. The oxygen concentration can be detected by applying a given 
voltage between the first and second electrodes and measuring the level of 
a current. 
In the apparatus according to this invention, since the solid electrolytic 
film is formed on the outer periphery of the cylindrical ceramic heater 
via the porous insulating layer, it assures an adequate structural 
strength, even if it is thinner. In consequence, the internal resistance 
of the solid electrolytic film can be adequately decreased to permit 
improving the sensitivity of detection of the oxygen concentration and 
thus enhancing the response to the electronic control of the engine. It is 
also possible to effectively lower the operating temperature of the oxygen 
concentration detecting element. Furthermore, the heater can be made 
compact due to the provision of the adequately-thin, solid electrolytic 
film and thus the apparatus involving the detecting element and heater can 
also be made compact and assures a ready attachment to the engine 
mechanism. Since the solid electrolytic film is located substantially in 
direct contact with the surface of the heater, an enhanced heating 
efficiency of the heater is assured with respect to the solid electrolytic 
film, thus saving a power consumption of the heater and, in addition, 
positively improving the durability of the heater.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows an oxygen concentration detecting element 10 which composes a 
part of an apparatus according to this invention. The oxygen concentration 
detecting element 10 has a cylindrical heater 11 made of ceramics. 
The ceramic heater 11 includes an unbaked alumina sheet 111 shown in FIG. 
2A. As shown in this figure, the paste of heat-resistant metal, such as 
Pt, is screen-printed on the surface of the alumina sheet 111 to provide a 
heater pattern 112. The alumina sheet 111 having the heater pattern 112 is 
wrapped on the outer periphery of a core rod 113 of unbaked alumina, as 
shown in FIG. 2C, with the pattern-printed surface inside. The resultant 
structure is baked integrally to provide a ceramic heater 11. 
A pair of grooves 114a, 115a are formed on the outer periphery of the 
proximal end of the core rod 113 as shown in FIG. 2B. Into the grooves 
114a, 115a, a pair of leads 114, 115 are fitted as shown in FIG. 2C and 
brazed there. Then the pattern-printed alumina sheet 111 is wrapped on the 
outer periphery of the core rod 113, having the leads 114, 115, with the 
result that the leads 114, 115 for the heater pattern 112 extend outward. 
As shown in FIG. 1, a porous insulating layer 12 made of insulating metal 
oxide and having a thickness of 100 to 200 .mu.m is formed on the outer 
periphery of the cylindrical ceramic heater 11. The porosity of insulating 
layer 12 is 4 to 20% in order that oxygen which is drawn in from a gas 
atmosphere to be measured can pass therethrough. The porous insulating 
layer 12 may be formed on the surface of the heater 11 by plasma flame 
spraying with powder of an electrically insulating material, such as 
alumina (Al.sub.2 O.sub.3) or magnesia-alumina (MgO-Al.sub.2 O.sub.3) 
spinel, having a particle size of 50 to 100 .mu.m. 
A first electrode 13 of Pt which is used as positive electrode is formed by 
a chemically plating method on the outer periphery of the porous 
insulating layer 12. The electrode 13 is a porous thin film to permit the 
gas to freely pass therethrough. 
A solid electrolytic film 14 is formed on the outer periphery of the distal 
end of the first electrode 13 and is a dense film of about 100 .mu.m in 
thickness. The film 14 is deposited on the surface of the first electrode 
13 by following process; material consisting of 90-95 mole % of ZrO.sub.2 
and 5-10 mole % of Y.sub.2 O.sub.3 is mixed, crushed, granulated and 
preheated yielding powder having a particle size of 1.about.20 .mu.m, and 
then the resultant powder is plasma frame sprayed at a high energy level 
of over 50 kw. 
An insulating layer 15 is formed on the surface of the resultant structure. 
The insulating layer is made of, for example, MgO-Al.sub.2 O.sub.3 spinel 
and has a thickness of about 50 .mu.m. In more detail, the insulating 
layer 15 is formed on the surface of the electrode 13 such that it partly 
or never, covers the surface of the solid electrolytic film 14. FIG. 1 
shows the former case where the insulating film 15 covers a part of the 
surface of the solid electrolytic film 14, noting that an area of the 
solid electrolytic film 14 which is not covered by the insulating film 15 
serves as an oxygen concentration detecting area 16. The insulating layer 
15 terminates short of the proximal end (upper end) of the electrode 13. 
A second electrode 17, the negative electrode, is formed on the surface of 
the resultant structure including the surface of detecting area 16. The 
electrode 17 is porous to permit oxygen to pass therethrough. The 
electrode 17 can be formed by the chemical plating method as in the case 
of the electrodes 13, and it may terminate short of the insulating layer 
15 as shown in FIG. 1. 
An oxygen diffusion resistant layer 18 is formed on the surface of the 
outer periphery of the second electrode 17. The layer 18 has a thickness 
of 200 to 600 .mu.m and a porosity of 3 to 8%, and is formed by virtue of 
plasma flame spraying with a chemically and thermally stable material, 
such as the MgO-Al.sub.2 O.sub.3 spinel. The body of the oxygen 
concentration detecting element 10 is formed as set out above. 
In the selection of the materials for these films or layers of the body of 
the detecting element 10 it is better that the porous insulating layer 12 
consists of a material whose thermal expansion coefficient is within a 
range between the thermal expansion coefficient of the material for the 
solid electrolytic film 14 and that of the material for the sheet 111 of 
the ceramic heater 11, because a separation and fall of the solid 
electrolytic film 14 from the adjacent film or layer is prevented. For 
example, when the sheet 111 is made of Al.sub.2 O.sub.3 with a thermal 
expansion coefficient of 8.5.times.10.sup.-6 /.degree.C. and the solid 
electrolytic film 14 is made of full stabilized ZrO.sub.2 -Y.sub.2 O.sub.3 
with a thermal expansion coefficient of 10.5.times.10.sup.-6 /.degree.C., 
then ZrO.sub.2 -Y.sub.2 O.sub.3 -Al.sub.2 O.sub.3 with a thermal expansion 
coefficient of 9.5.times.10.sup.-6 /.degree.C. is preferably selected as 
the material of the porous insulating layer 12. Further, in this 
combination, the insulating layer 15 preferably consists of MgO-Al.sub.2 
O.sub.3, having a thermal expansion coefficient of 8.5.times.10.sup.-6 
/.degree.C. and a thickness of about 50 .mu.m, and the oxygen diffusion 
resistant layer 18 is made preferably of ZrO.sub.2 -Y.sub.2 O.sub.3 
-Al.sub.2 O.sub.3 with a thermal expansion coefficient of 
10.times.10.sup.-6 /.degree.C. 
As shown in FIG. 1, a metal stem 19 is brazed to that proximal end of the 
body of the detecting element 10. The stem 19 is fitted over the outer 
periphery of the first electrode 13 and acts as an outer connection 
terminal of the electrode 13. 
A metal ring 20 which acts as an outer connection terminal of the second 
electrode 17 is fitted over the outer periphery of the electrode 17. The 
ring 20 is connected to a lead 21, which extends up through the stem 19, 
as shown in FIG. 1, in a manner protected by an insulator 22. 
In this way, the oxygen concentration detecting element 10 is provided 
which has the leads 114, 115 for the heater 11, the outer connection 
terminal (stem 19) for the first electrode 13, and the outer connection 
lead 21 for the second electrode 17. 
FIG. 3 shows an oxygen content determining apparatus incorporating the 
oxygen concentration detecting element 10 therein. Within a cylindrical 
housing 30 the stem 19 overlies a stepped section 31 of the housing 30 
with a ceramics insulating plate 33 therebetween. The housing 30 has a 
metal flange 34 integrally which serves as a part for attaching the 
housing to an exhaust pipe of an engine not shown. 
A spring 35 which pushes the stem 19 of the detecting element 10 against 
the stepped section 31 is set in the housing 30. The spring 35 is 
compressed by a metal pipe 36 with the result that the spring 35 and pipe 
36 are electrically connected to the stem 19 of the detecting element 10. 
Thus spring 35 provides the outer connection terminal of the first 
electrode 13. 
An insulating pipe 37 made of ceramics, such as alumina, is fitted over the 
outer periphery of the metal pipe 36 to provide an integral structure. The 
insulating pipe 37 is located within an outer protection tube 38 which is 
jointed to the housing 30 by caulking and thus the detecting element 10 is 
fixedly jointed to the housing 30 at a position where the stem 19 is 
located. In this case, the leads 21, 114 and 115 are brought to the 
outside through the metal pipe 36, and the lead 39 is welded to the metal 
pipe 36 which is connected to the electrode 13. The leads 21, 114, 115 and 
39 are held in place by a rubber packing 40 which is attached to the metal 
pipe 36. The portions of these leads which extend out of the metal pipe 36 
are used as assemblies to be connected to outer connectors. 
The detecting element 10, except for the stem 19, extends beyond the 
housing 30 to be surrounded with a cover 41 having a number of through 
holes at a location facing the detecting section 16. When the apparatus of 
this invention is attached to an engine's exhaust pipe by use of the 
flange 34, the cover 41 is exposed to an inner space of the exhaust pipe 
and thus exhaust gas acts upon the oxygen concentration detecting element 
10. 
In the oxygen concentration detecting element 10 which is used as one 
element of the oxygen content determining apparatus, when voltage is 
applied across the first and second electrodes 13 and 17, a current flows 
from the electrode 13 into the electrode 17. This current is based on the 
following phenomenon: oxygen molecules which have passed through the 
oxygen diffusion resistant layer 18 are ionized to oxygen ions and these 
ions are migrated in the solid electrolytic film 14 made of oxygen ion 
conductive electrolytic material toward the first electrode 13. In the 
first electrode 13, the oxygen ions turn into oxygen molecules in 
accordance with the equation: 
EQU 20.sup.2- -4e.sup.- .fwdarw.O.sub.2, 
noting that the oxygen molecules thus generated are expelled toward the 
outside of the detecting element 10 through the porous insulating layer 
12. 
When the voltage between the electrodes 13 and 17 is sequentially 
increased, a voltage range exists in which range current does not increase 
because the passage of the oxygen molecules is restricted by the diffusion 
resistant layer 18. The level of a constant current present within the 
voltage range is referred to as a saturated current "Il" which is a signal 
for detecting oxygen concentration. Il is given by: 
EQU Il=(4FDo.sub.2 /RT)S/l.multidot.Po.sub.2 (1) 
where 
F: Farady constant 
R: gas constant 
Do.sub.2 : diffusion coefficient of oxygen 
T: absolute temperature 
S: area of electrode 
l: effective diffusion distance of the diffusion resistant layer 
Po.sub.2 : partial pressure of oxygen. 
The saturated current level varies in accordance with the oxygen 
concentration of the gas to be measured. When a specific voltage is 
applied across the electrodes 13 and 17 and a saturated current level at 
that time is measured, then it is possible to know the oxygen 
concentration of the gas. 
FIGS. 4 and 5 show the results of tests carried out on the oxygen 
concentration detecting element 10 in a model gas of O.sub.2 -N.sub.2 at 
600.degree. C. As seen from FIG. 4, within a voltage range of about 0.4 V 
to 1.2 V the characteristic curves in the voltage-current became flat at 
the different level respectively according to each oxygen concentration, 
and the currents again increased with an increasing voltage. 
Here, the constant current level involved is a saturated current level. The 
saturated current level varies based on Equation (1), depending upon the 
oxygen concentration. 
FIG. 5 shows a relation between the oxygen concentration level and the 
saturated current level when a predetermined voltage of 0.7 V is applied. 
As seen from FIG. 5, the saturated current level varies in exact 
proportion to the oxygen concentration level. The air/fuel ratio of, for 
example, the automotive internal engine can be controlled by this current 
level. In practical application, since the temperature of the atmosphere 
gas varies and thus the saturated current level varies, the detection 
element is maintained at a constant temperature level by carrying current 
through the leads 114 and 115. 
It is important to note that the porous insulating layer 12 plays an 
important part in the above-mentioned operation of the detecting element. 
As set out above, oxygen molecules are generated at the first electrode 13 
due to a reaction of: 
EQU 20.sup.2- -40e.sup.- .fwdarw.O.sub.2 
The porous insulating layer 12 serves to expel the oxygen molecules toward 
the outside of the detecting element 10. Now suppose that there is no 
porous insulating layer 12 and thus the generated oxygen molecules are not 
smoothly expelled beyond the porous insulating layer 12. Then, an over 
voltage is generated, exerting an adverse influence over the diffusion 
rate of O.sub.2 which is determined by the oxygen diffusion resistant 
layer 18. As a result, the detecting element without the porous insulating 
layer 12 shows a lower saturated current level than the detecting element 
with the porous insulating layer 12 under the same oxygen concentration, 
failing to provide any stable saturated current characteristic. 
FIG. 6A shows such a state as mentioned, i.e., a result of measurements of 
the voltage-current characteristic of the detecting element (the curve A) 
with the porous insulating layer 12 and detecting element (the curve B) 
without the porous insulating layer 12. Here, the ordinate and abscissa in 
FIG. 6A are substantially the same as those of FIG. 4, noting that in FIG. 
6A the measurements were conducted at the oxygen concentration of 10%. As 
seen from FIG. 6A, the current level of the curve B (the element without 
the porous insulating layer 12) is lower than that of the curve A and the 
saturated current characteristic of the curve B is not so stable as that 
of the curve A. 
Furthermore, where no porous insulating layer 12 exists, a response of the 
detecting element to the variation of the oxygen concentration is lowered. 
FIG. 6B shows a time-sequential variation in the output currents of the 
detecting element (the curve A) with the porous insulating layer and the 
detecting element (the curve B) without the porous insulating layer when 
the oxygen concentration of a gas to be measured varies from 10% down to 
3% at a voltage of 0.7 V. As seen from FIG. 6B, the curve A of the 
detecting element 10 with the porous insulating layer 12 shows a quick 
response to the O.sub.2 concentration variation and thus an abrupt output 
drop, while the curve B of the detecting element without the porous 
insulating layer shows a slow response to the variation of oxygen 
concentration. 
In each detecting element used in the above-mentioned experiments, the 
porous insulating layer (curve A only) has a porosity of 10% and a 
thickness of 100 .mu.m, the oxygen diffusion resistant layer has a 
porosity of 6% and a thickness of 400 .mu.m, the electrode is made of Pt, 
and the solid electrolytic film has a composition of ZrO.sub.2 
(90%)-Y.sub.2 O.sub.3 (10%) and a thickness of 100 .mu.m. 
In the oxygen concentration detecting element 10 of the above-mentioned 
embodiment the diffusion resistant layer 18 may be extended still farther 
than shown in FIG. 1 to serve as a protective layer for the second 
electrode 17. Further, the porous insulating layer 12 and insulating layer 
15 may be formed not only by plasma frame spraying but also by other 
process, for example, by a dipping method. Furthermore, the first and 
second electrodes 13 and 17 can be formed by, for example, a chemical 
plating, electric plating, spattering and paste baking method. 
Further, the solid electrolytic film 14 may be formed of not only the 
material as set out above but also, for example, ZrO.sub.2 -YbO.sub.3 
-CaO, or the other oxygen ion conductive metal oxide.