Gas sensing element

A gas sensing element having a co-fired laminar structure including an electrochemical cell having a planar solid electrolyte body, and at least two electrodes disposed on the planar solid electrolyte body. The sensing element further includes two generally planar heaters which are formed on opposite sides of the electrochemical cell, respectively, and which are spaced apart from each other in a direction perpendicular to the planar solid electrolyte body, and a generally planar thermosensitive portion formed as an integral part of the laminar structure, for detecting a temperature of the laminar structure in the neighborhood of at least one of the electrodes of the electrochemical cell. The thermosensitive portion may partially define a diffusion chamber into which a measurement gas in an external space is introduced under a predetermined diffusion resistance, so that one of the electrodes is exposed to the introduced measurement gas.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an element for sensing gases, 
and more particularly to a gas sensing element which provides 
significantly improved sensing accuracy and which is suitably used as an 
instrument for analyzing gases. 
2. Discussion of the Prior Art 
There has been known a gas sensor which utilizes an ion-conductive solid 
electrolyte, to measure the concentration (partial pressure) of a given 
component in an object atmosphere or measurement gas, according to the 
principle of a concentration cell. For example, an oxygen sensor for 
determining the concentration or partial pressure of oxygen in a 
measurement gas, employs zirconia or similar solid electrolyte materials 
which exhibit relatively high oxygen-ion conductivity at an elevated 
temperature. For detecting the concentration of water (partial pressure of 
water vapor), a sensor uses a solid electrolyte material such as cerium or 
strontium oxides, which are hydrogen-ion conductive at an elevated 
temperature. Also known is a sensor which uses .beta.-alumina (Na.sub.2 
O.11Al.sub.2 O.sub.3) or similar solid electrolyte materials exhibiting 
sodium-ion conductivity at a high temperature, for measuring the 
concentration or partial pressure of sulfur dioxide, by utilizing an 
electrochemical reaction between sodium and sulfur. 
Such electrochemically operating gas sensors utilizing a solid electrolyte 
are used, for example, as oxygen sensors for determining the oxygen 
concentration in exhaust gases emitted from an internal combustion engine 
of a motor vehicle, or from industrial furnaces, boilers and similar 
equipment. The solid electrolyte of such oxygen sensors conventionally 
used is generally formed as a tubular body which is closed at its one end. 
In recent years, there has been an increasing trend toward using an 
elongate planar solid electrolyte body, for easier manufacture, reduced 
production cost, and increased compactness of the sensors. Such oxygen 
sensors are formed as a laminar structure having electrodes suitably 
disposed in contact with the planar solid electrolyte body, such that the 
electrodes and the solid electrolyte cooperate to constitute an 
electrochemical cell. 
For accurate and reliable operation of the gas sensors even while the 
temperature of a gas to be measured is relatively low, it is necessary to 
use a suitable heater for holding the electrodes and solid electrolyte 
body of the electrochemical cell at a relatively high operating 
temperature. For example, a heater is disposed in the neighborhood of the 
electrochemical cell, so as to heat the cell via another solid electrolyte 
layer or other layer, or disposed in the same plane as, but spaced apart 
from, the electrodes. Another heater arrangement is disclosed in Japanese 
Patent Application No. 57-6846 (corresponding to Japanese Laid-Open 
Publication No. 58-124943 and U.S. Pat. No. 4,510,036), wherein a heater 
layer is formed over the electrodes of the electrochemical cell via a 
suitable electrically insulating layer, to heat the electrodes and the 
underlying portion of the solid electrolyte body. This type of heater 
arrangement permits reduced size and electric power consumption of the 
sensor, and relatively rapid heating of the cell. 
In the gas sensors discussed above, the concentration of a given component 
in the object measurement gas is generally determined based on an 
electromotive force detected by the sensing element, according to the 
Nernst equation. To assure accurate measurement of the concentration of 
the desired component, the temperature of the gas sensor must be known. In 
other words, the sensor must be heated to a known temperature. 
Conventionally, the temperature of the sensor is considered to be equal to 
the temperature of the measurement gas, which is detected by a suitable 
temperature detector such as a thermocouple or thermoelectric thermometer. 
Such a temperature detector or thermometer is disposed separately from the 
sensor, more precisely, from its sensing element having the electrochmical 
cell. 
An extensive study and experiment by the inventors revealed a difference 
between the temerature of the gas sensor and the temperature of the 
measurement gas. Namely, there may arise an appreciable difference between 
these temperatures, due to rapid changes in the temperature, flow rate and 
direction of the measurement gas, and other parameters. The sensor tends 
to have a temperature gradient in the direction of thickness of the 
sensor, i.e., in the direction in which the constituent layers are 
superposed on each other. Therefore, the temperature of the measurement 
gas detected by a temperature detector positioned near the sensor does not 
precisely represent the operating temperature of the sensing element of 
the gas sensor. Accordingly, it is difficult to adequately control the 
heater based on the temperature measured by such a temperature detector. A 
research by the inventors showed a temperature difference as large as 
between +3.5% and -3.5%. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a gas 
sensing element having a heater which is adequately controlled so as to 
permit accurate measurement of a gas, without being influenced by external 
parameters. 
The above object may be achieved by a gas sensing element of a co-fired 
laminar structure constructed according to the present invention, which 
comprises (a) at least one electrochemical cell, each of which includes a 
planar solid electrolyte body, and at least two electrodes disposed on the 
planar solid electrolyte body, (b) a first and a second, generally planar 
heater for heating the electrochemical cell or cells, and (c) generally 
planar thermosensitive means incorporated in the laminar structure. The 
first and second heaters are formed on opposite sides of the 
electrochemical cell or cells, respectively, and are spaced apart from 
each other in a direction perpendicular to a plane of the planar solid 
electrolyte body. The generally planar thermosensitive means are adapted 
to detect a temperature of the laminar structure in the vicinity of the 
electrochemical cell or cells. 
In the gas sensing element of the present invention constructed as 
described above, the thermosensitive means having a generally planar 
configuration incorporated in the laminar structure of the sensing element 
is adapted to detect the temperature of the laminar structure in the 
vicinity of at least one of the electrodes of the electrochemical cell or 
cells of the sensing element, which is heated by the first and second 
generally planar heaters that are provided on both sides of the cell or a 
laminar assembly of the cells. In this arrangement, the temperature of the 
sensing element, more precisely, the temperature of the electrochemical 
cell or cells, can be exactly measured by the thermosensitive means, 
whereby the heating by the two heaters can be controlled based on the 
detected temperature, so as to maintain a desired temperature throughout 
the sensing element, in the direction of lamination of the laminar 
structure. Thus, the sensing accuracy of the present gas sensing element 
is remarkably improved. 
In particular, the thermosensitive means if positioned near the electrodes 
of the electrochemical cell permits the temperature of the electrodes to 
be precisely regulated according to the detected temperature. In this 
case, the maximum measuring error of the sensing element is lowered to 
plus or minus 1%, and the sensing element may be used as a precision 
analyzing instrument. 
According to one advantageous feature of the invention, the sensing element 
further comprises means for defining a diffusion chamber into which the 
measurement gas diffuses from the external space under a predetermined 
diffusion resistance, so that one of the electrodes of the sensing element 
is substantially exposed to the introduced measurement gas within the 
diffusion chamber. The diffusion chamber may or may not be filled with a 
porous layer or layers, either entirely or partially. In one form of this 
feature of the invention, the generally planar thermosensitive means 
partially defines the diffusion chamber. 
In the case where the sensing element having such a diffusion chamber is 
used to measure exhaust gases which are produced as a result of combustion 
of a fuel-rich air-fuel mixture, unburned components or incombustibles 
contained in the exhaust gases react with oxygen which has been introduced 
into the diffusion chamber by an oxygen pumping action of the 
electrochemical cell. As a result of the reaction, the temperature in the 
diffusion chamber rises, resulting in adversely changing the temperature 
distribution in the sensing element if the thermosensitive means were not 
provided. In other words, this inconvenience is effectively eliminated or 
avoided according to the invention wherein the heaters are controlled 
based on the temperatures monitored by the generally planar heat-sensitve 
means which is disposed within the diffusion chamber or near this chamber, 
and in the vinicity of the electrode or electrodes of the cell. Thus, the 
provision of the thermosensitive means assures a substantially and 
constant temperature distribution of the sensing element. 
It is preferred that the thermosensitive means be provided in the form of a 
layer covering a relatively large area in the plane parallel to the plane 
of the planar solid electrolyte body, for more uniform temperature 
distribution of the diffusion chamber, and the detecting portion of the 
sensing element which constitutes the electrochemical cell or cells. This 
arrangement makes it possible to reduce an offset amount of the sensing 
element as a concentration cell, and improve the temperature distribution 
within the diffusion chamber, thereby maintaining a substantially constant 
rate of diffusion of the measurement gas through the diffusion chamber, 
without being affected by the ambient temperature, and consequently 
enhancing the measuring accuracy of the sensing element. 
Unlike the conventionally used thermocouple or other detectors which 
measures the temperature at a selected point, the generally planar 
thermosensitive means of the instant sensing element is adapted to detect 
the temperature over a relatively wide area, for increased detecting 
accuracy. Further, where the thermosensitive means is provided in or near 
the diffusion chamber, the thermosensitive means senses the temperature of 
the sensing element between the first and second heaters. This feature is 
effectively combined with the generally planar configuration of the 
thermosensitive means, contributing to further improvements in the 
temperature distribution and measuring accuracy. 
In one preferred form of the above feature of the invention wherein the 
diffusion chamber is provided, the sensing element includes two 
electrochemical cells, that is, an electrochemical pumping cell disposed 
on one side of the diffusion chamber, and an electrochemical sensing cell 
disposed on the other side of the diffusion chamber. The pumping cell is 
adapted to perform an oxygen pumping action for controlling an atmosphere 
within the diffusion chamber, while the sensing cell is adapted to induce 
an electromotive force representative of a difference in oxygen 
concentration between the controlled atmosphere within the diffusion 
chamber, and a reference gas, according to the principle of an oxygen 
concentration cell. 
According to another advantageous feature of the invention, each of the 
first and second generally planar heaters is a multi-layered structure 
including a heat generating element, and an electrically insulating layer 
in which the heat generating element is embedded. In this arrangement, the 
heat generating element is electrically insulated from the planar solid 
electrolyte body by the electrically insulating layer. 
In one form of the above feature of the invention, the multi-layered 
structure of each heater further includes a gas-tight layer which covers 
the electrically insulating layer and thereby isolates the heat generating 
element from the external measurement gas. This gas-tight layer is 
provided since the electrically insulating layer is generally 
gas-permeable. 
In accordance with a further feature of the invention, the generally planar 
heat-sensitive means is a multi-layered structure including an 
electrically resistive thermosensitive element, and an electrically 
insulating layer in which the thermosensitive element is substantially 
embedded. In this case, the electrically resistive thermosensitive element 
as a temperature detector element, is electrically insulated from the 
planar solid electrolyte body by the electrically insulating layer. 
In connection with the electrically insulating layers indicated above, it 
is noted that the solid electrolyte body becomes a semi-conductor and its 
electrical insulating property is reduced, when its temperature reaches 
450.degree. C. In this sense, the electrically insulating layer which 
separates the heat generating or thermosensitive element from the solid 
electrolyte body is useful for avoiding otherwise possible 
short-circuiting of the patterns of the heating generating or 
thermosensitive element, which occur via the solid electrolyte body in a 
semi-conductive state at an elevated temperature. The electrically 
insulating layer may be a thin layer which principally consists of alumina 
or similar insulating materials. 
In one form of the above feature of the invention, the multi-layered 
heat-sensitive structure further includes a gas-tight layer which covers 
the electrically insulating layer and thereby isolates the electrically 
resistive element from the external measurement gas. The provision of this 
gas-tight layer is significant, since electrically insulating materials 
are generally gas-permeable. Preferably, the gas-tight layer is formed of 
a ceramic material which has substantially the same chemical composition 
as that of the solid electrolyte body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To further clarify the concept of the present invention, some preferred 
embodiments of the invention will be described in detail, by reference to 
the accompanying drawings. 
Referring first to FIG. 1, there is shown in transverse cross section an 
oxygen detecting portion of an oxygen sensing element 2, which is one form 
of a gas sensing element constructed according to the invention. This 
oxygen sensing element 2 has a generally elongate planar configuration 
having a generally rectangular transverse cross sectional shape. On one of 
opposite longitudinal ends of the element 2, there is formed the oxygen 
detecting portion having an electrochemical cell which is operable 
according to the principle of an oxygen concentration cell. 
As indicated in the figure, the oxygen detecting portion of the sensing 
element 2 has a co-fired laminar structure, which includes an 
electrochemical pumping cell and an electrochemical sensing cell. The 
pumping cell consists of a first planar solid electrolyte body 4 made of 
stabilized zirconia that exhibits oxygen-ion conductivity at a high 
temperature, and an inner and an outer porous pumping electrode 6, 8 which 
are formed on the opposite major surfaces of the solid electrolyte body 4. 
Similarly, the sensing cell consists of a second planar solid electrolyte 
body 10, and porous measuring and reference electrodes 12, 14 which are 
formed on the opposite major surfaces of the second solid electrolyte body 
10. Between these electrochemical pumping and sensing cells, there is 
formed a diffusion chamber 16 which will be described. On one side of the 
sensing cell on which the reference electrode 14 is disposed, third and 
fourth planar solid electrolyte bodies 17, 18 are superposed, such that 
these two bodies 17, 18 cooperative with the second solid electrolyte 
bodies 10 to define an air passage 20 in which the reference electrode 14 
is accommodated. This air passage 20 is open to the ambient air, at its 
end remote from the oxygen detecting portion, for introducing the air as a 
reference gas to which the reference electrode 14 is exposed. 
Stabilized zirconia of the solid electrolyte bodies 4, 10, 17 and 18 is 
obtained by doping zirconium oxide with yttrium oxide or calcium oxide, as 
well known in the art. The electrodes 6, 8, 12 and 14 are formed of 
platinum or similar metals. 
On the outer surface of the fourth solid electrolyte body 18 remote from 
the air passage 20, there is formed a first heater 22 in the form of a 
planar multi-layered structure. Further, a second heater 24 also in the 
form of a planar multi-layered structure is formed on the outer surface of 
the pumping cell, so as to surround the outer pumping electrode 8 in the 
plane of FIG. 1. Thus, the two generally planar heaters 22 and 24 are 
provided on the opposite sides of a laminar assembly of the pumping and 
sensing cells, such that the two heaters 22, 24 are spaced from each other 
in the direction perpendicular to the plane of the solid electrolyte 
bodies 4, 10. These heaters 22, 24, which are integral parts of the 
laminar structure of the sensing element 2, are operated to heat the two 
electrochemical cells to a predetermined optimum operating temperature. 
Each of the first and second heaters 22, 24 includes a heat generating 
element 26, 28, an electrically insulating porous layer 30, 32 formed of 
alumina or similar ceramic material so as to surround the heat generating 
element 26, 28, and a gas-tight layer 34, 36 formed of zirconia or other 
solid electrolyte material. The gas-tight layer 34, 36 cooperates with the 
solid electrolyte body 18, 4 to enclose the heat generating element 26, 28 
and the insulating layer 30, 32, and thereby isolates the heat generating 
element 26, 28 from an external atmosphere to be measured by the sensing 
element 2. The heat generating elements 26, 28 of the first and second 
heaters 22, 24 are formed, for example, from a cermet film whose major 
components consist of a powdered mixture of alumina and platinum. 
The inner pumping electrode 6 and the measuring electrode 12 are exposed to 
the atmosphere within the diffusion chamber 16 through respective porous 
ceramic layers 38, 40 made of alumina or similar ceramic materials. The 
porous ceramic layer 38 is exposed at its lateral end faces to the 
external measurement gas. In this arrangement, the measurement gas 
diffuses into the diffusion chamber 16 through the porous ceramic layer 
38, under a predetermined diffusion resistance of the layer 38, and the 
inner pumping electrode 6 and the measuring electrode 12 are exposed to 
the introduced measurement gas within the diffusion chamber 16. A similar 
porous ceramic layer 41 is formed so as to cover the outer pumping 
electrode 8. 
Between the porous ceramic layer 38 and the second planar solid electrolyte 
body 10, there is disposed a generally planar multi-layered 
thermosensitive means 42, such that the thermosensitive means cooperates 
with the first and second solid electrolyte bodies 4, 10 to define the 
diffusion chamber 16 which accommodates the ceramic layers 38, 40. More 
specifically, the thermosensitive means 42 is formed so as to surround the 
measuring electrode 12, and positioned adjacent to the inner pumping 
electrode 6. The thermosensitive means 42 includes an electrically 
resistive thermosensitive element 44 in the form of a thermistor whose 
resistance decreases as the temperature increases. The multi-layered 
structure of the thermosensitive means 42 further includes an electrically 
insulating porous layer 46 made of alumina or similar ceramic material. 
The electrically resistive thermosensitive element 44 is embedded in the 
electrically insulating porous layer 46, and is thereby electrically 
insulated from the solid electrolyte body 10. The thermosensitive element 
44 is isolated or protected from the external measurement gas, and from 
the internal measurement gas within the diffusion chamber 16, by a 
gas-tight layer 48 made of zirconia or other solid electrolyte material 
similar to the solid electrolyte body (10). The electrically resistive 
thermosensitive element 44 which is formed in a film, is made for example, 
of a cermet principally consisting of a powdered mixture of a ceramic 
material such as zirconia or alumina, and platinum. Alternatively, the 
resisitive element 44 may be made of a cermet containing 0.1-0.5% of 
TiO.sub.2 (titania), or other materials which positively give the 
thermosensitive film 44 a high negative temperature coefficient of 
resistance. 
The laminar structure of the oxygen sensing element 2 thus constructed is 
prepared by co-firing the successively superposed unfired layers of the 
constituent protions, that is, first heater 22, solid electrolyte bodies 
18, 17, sensing cell (10, 12, 14), thermosensitive means 42, portions 
defining the diffusion chamber 16, pumping cell (4, 6, 8) and second 
heater 24. 
Referring next to FIG. 2, there is illustrated an oxygen sensing element 2 
according to another embodiment of the present invention. This sensing 
element 2 is different from the element 2 of FIG. 1, in the arrangement 
defining the diffusion chamber 16, and the arrangement and position of the 
thermosensitive means 42. Described more specifically, the diffusion 
chamber 16 has a predetermined diffusion resistance due to its 
comparatively small thickness, rather than the porosity of the porous 
ceramic layer 38 used in the preceding embodiment. That is, the diffusion 
chamber 16 in the present embodiment is a thin flat space between the 
first and second planar solid electrolyte bodies 4, 10. Further, the 
pumping cell (first solid electrolyte body 4) has a gas-inlet aperture 50 
which communicates with the central part of the diffusion chamber 16, so 
that the external measurement gas enters the diffusion chamber 16 through 
the aperture 50. The thermosensitive means 42 consists of an electrically 
resistive thermosensitive element 44, and an electrically insulating 
porous layer 46 surrounding the thermosensitive element 44. The 
thermosensitive means 42 is located within the air passage 20, adjacent to 
the reference electrode 14 of the sensing cell, in order to detect the 
temperature in the vicinity of the reference electrode 14. Since the 
thermosensitive means 42 provided within the air passage 20 in this 
embodiment is not influenced by the external measurement gas, the 
thermosensitive means 42 is not covered by a gas-tight layer (as indicated 
at 48 in FIG. 1) for isolating the thermosensitive element 44 from the 
external measurement gas. 
In the oxygen sensing elements 2 illustrated above, an electric current is 
applied between the inner and outer pumping electrodes 6, 8 of the pumping 
cell, according to the concentration of a given component (oxygen) in the 
measurement gas which is introduced into the diffusion chamber 16 under a 
predetermined diffusion resistance. As a result, a well known oxygen 
pumping action is carried out by the pumping cell, to control the 
atmosphere in the diffusion chamber 16, adjacent to the inner pumping 
electrode 6. At the same time, an electromotive force is induced between 
the measuring and reference electrodes 12, 14 of the sensing cell, based 
on a difference in the oxygen concentration between the controlled 
atmosphere within the diffusion chamber 16 and the reference gas (air) 
within the air passage 20, according to the principle of an oxygen 
concentration cell. During this operation, the temperature of the sensing 
element 2, more specifically, the temperature near the inner pumping 
electrode 6 and the measuring electrode 12 within the diffusion chamber 16 
is precisely detected by the electrically resistive thermosensitive 
element 44 of the thermosensitive means 42. 
The heating operations of the first and second heaters 22, 24 disposed on 
the opposite sides of the sensing element 2 can be regulated based on the 
temperature of the sensing element 2, in particular, the temperature near 
the sensing cell, which is detected by the built-in generally planar 
heat-sensitive means 42. The generally planar configuration of the 
thermosensitive means 42 (electrically resistive element 44) permits more 
accurate detection of the temperature in the diffusion chamber 16 or of 
the electrodes 12, 14 of the oxygen concentration sensing cell, than a 
conventional temperature detector which is configured to measure the 
temperature at a selected point, rather than an area. Thus, the 
thermosensitive means 42 provides for improved precision of the 
temperature detection, and accordingly improved temperature distribution 
by the heating of the heaters 22, 24. In other words, the generally planar 
thermosensitive means 42 makes it possible to reduce the amount of offset 
of the concentration cell, improve the temperature distribution within the 
diffusion chamber 16, maintain a substantially constant rate of diffusion 
of the measurement gas into the diffusion chamber 16 without being 
affected by the ambient temperature, and thereby enhance the measuring 
accuracy of the sensing element 2. 
Unlike the conventional temperature detector separate from the sensing 
element itself, the instant thermosensitive means 42 takes the form of a 
layer covering a relatively wide area in the plane of the sensing element 
2, and monitors the temperature of that wide area, rather than the 
temperature at a single point. In this context, the present arrangement is 
advantageous for exact monitoring of the operating temperature of the 
sensing element which is heated by the first and second heaters 22, 24. 
Further, the location of the thermosensitive means 42 between the two 
heaters 22, 24 is also conducive to accurate control of the temperature of 
the sensing element 2, so as to maintain a uniform temperature 
distribution for more accurate measurement of the object gas. 
In particular, the instant thermosensitive means 42 is effective, where the 
gas to be measured is an exhaust gas which is produced in combustion of an 
air-fuel mixture having an air-fuel ratio lower than the stoichiometric 
level and which contains unburned components. In this case, the unburned 
components introduced into the diffusion chamber 16 are burned by oxygen 
which has been pumped into the diffusion chamber 16 by means of an oxygen 
pumping action by the pumping cell. The burning of the unburned components 
will raise the temperature within the diffusion chamber 16. This rise of 
the temperature can be exactly and rapidly sensed as a change in the 
resistance of the electrically resistive thermosensitive element 44 of the 
thermosensitive means 42 disposed within or in the neighborhood of the 
diffusion chamber 16. Consequently, the heaters 22, 24 can be precisely 
controlled so as to maintain the sensing element 2 at an optimum operating 
temperature. Further, the thermosensitive means 42 makes it possible to 
deal with different kinds of measurement gases, which affects the 
operating temperature of the sensing element 2. 
The inner pumping and measuring electrodes 6, 12 exposed to the atmosphere 
within the diffusion chamber 16, and the outer pumping electrode 8 
directly exposed to the external measurement gas, are covered by ceramic 
filters in the form of the porous ceramic layers 38, 40, 41 having 
suitable thicknesses, whereby the durability of these electrodes 6, 8, 12 
is effectively improved. Preferably, these protective porous ceramic 
layers 38, 40, 41 are made of a ceramic material which has a lower 
coefficient of thermal expansion than the solid electrolyte bodies. The 
porosity of the protective layers is preferably within a range of 30-50%, 
and the average particle size is preferably within a range of 1-5 microns. 
As previously indicated, the heat generating elements 26, 28 of the heaters 
22, 24, and the electrically resistive thermosensitive element 44 of the 
thermosensitive means 44, are embedded within or enclosed by the 
electrically insulating porous layers 30, 32, 46. These insulating layers 
protect the heat generating and thermosensitive elements 26, 28, 44 
against otherwise possible short-circuiting with respect to the solid 
electrolyte bodies 4, 10, 18. Described in more detail, the solid 
electrolyte bodies become semiconductive at temperatures higher than 
450.degree. C., with their electrical insulation property being reduced. 
In this condition, carbon and other substances contained in the 
measurement gas (exhaust gases) tend to be easily deposited on the surface 
of the solid electrolyte, causing a short-circuiting if the electrical 
insulation of the heat generating and/or thermosensitive elements 26, 28, 
44 is not sufficient. This undesirable phenomenon is effectively 
eliminated by the provision of the insulating layers 30, 32, 46, and the 
gas-tight layers 34, 36, 48. 
Reference is now made to FIG. 3, which shows in transverse cross section a 
further modified form of the oxygen sensing element according to the 
invention. This sensing element 2 is different from the sensing elements 2 
of FIGS. 1 and 2, in that the outer pumping electrode 8 of the pumping 
cell is not exposed to the external measurement gas, but exposed to the 
atmosphere within a second air passage 56 similar to the air passage 20. 
The second air passage 56 is defined by the first solid electrolyte body 
4, and two other solid electrolyte bodies 52, 52 superposed on the first 
solid electrolyte body 4. This air passage 56 is open to the ambient air, 
at its end remote from the oxygen detecting portion of the sensing element 
2. In the present embodiment, the second multi-layered heater 24 also 
consisting of a heat generating element 28, a porous layer 32 and a 
gas-tight layer 36, is formed on the outer surface of the solid 
electrolyte body 54. 
Further, the diffusion chamber 16 formed between the pumping cell (4, 6, 8) 
and the sensing cell (10, 12, 14) is entirely filled with a porous ceramic 
layer 58. This porous ceramic layer 58 has a predetermined diffusion 
resistance, so that the external measurement gas is introduced into the 
diffusion chamber 16 under the predetermined diffusion resistance, whereby 
the inner pumping electrode 6 and the measuring electrode 12 are exposed 
to the introduced measurement gas. The thermosensitive means 42 also 
consisting of an electrically resistive thermosensitive element 44, an 
electrically insulating layer 46 and a gas-tight layer 48, is disposed 
near the measuring electrode 12 so as to surround the same, and so as to 
partially define the diffusion chamber 16. 
In the present oxygen sensing element 2, the oxygen pumping by the pumping 
cell consisting of the solid electrolyte body 4 and the pumping electrodes 
6, 8 occurs between the second air passage 56 (in which the ambient air is 
introduced), and the diffusion chamber 16 (into which the measurement gas 
diffuses through the porous ceramic layer 58), so that the atmosphere 
adjacent to the measuring electrode 12 in the diffusion chamber 16 is 
controlled by the pumping action. In the meantime, an electromotive force 
is induced between the measuring and reference electrodes 12, 14 of the 
sensing cell, according to the principle of a concentration cell. During 
this operation of the sensing element 2 while being heated by the first 
and second heaters 22, 24, the temperature of the sensing element 2 is 
precisely monitored by the generally planar multi-layered thermosensitive 
means 42 disposed within the diffusion chamber 16. Thus, the operations of 
the two heaters 22, 24 are controlled based on the detected temperature, 
to establish a uniform temperature distribution in the direction 
perpendicular to the plane of the solid electrolyte bodies, whereby 
relatively high measuring accuracy of the sensing element 2 is assured. 
There is shown in FIG. 4 a further modified form of an oxygen sensing 
element 2 also according to the present invention, wherein the oxygen 
concentration is measured according to the principle of a polarographic 
cell. The sensing element 2 includes an electrochemical cell which 
consists of a first planar solid electrolyte body 60, and a first and a 
second porous electrode 62, 64 that are formed on the opposite major 
surfaces of the solid electrolyte body 60. On one side of this 
electrochemical cell, there is formed a multi-layered, generally planar 
first heater 22. On the other side of the cell, there are successively 
superposed one on another a porous ceramic layer 66 whose porous structure 
provides a diffusion chamber 16, a multi-layered, generally planar 
thermosensitive means 42, a second planar solid electrolyte body 68, and a 
multi-layered, generally planar second heater 24. Thus, the oxygen sensing 
element 2 is formed as an integral laminar structure. The second electrode 
64 is is covered by a porous ceramic layer 70, so that the electrode 64 is 
exposed to the external measurement gas through the porous structure of 
the layer 70. 
The thus constructed oxygen sensing element 2 is fabricated in the 
following manner, for example. Initially, unfired layers for the 
electrically insulating porous layer 32, heat generating element 28 and 
gas-tight layer 36 are formed by printing on one surface of a prepared 
unfired layer of the second solid electrolyte body 68. Then, unfired 
layers for the electrically insulating porous layer 46, electrically 
resistive thermosensitive element 44 and gas-tight layer 48 are formed by 
printing on the other surface of the unfired layer of the solid 
electrolyte body 68. 
On one surface of a prepared unfired layer of the first solid electrolyte 
body 60, there is formed by printing an unfired layer for the first 
electrode 62, which serves as an inner pumping electrode. Then, an unfired 
layer for the porous ceramic layer 66 is formed on the same surface of the 
unfired solid electrolyte body 60, so as to cover the unfired inner 
pumping electrode 62. On the other surface of the unfired solid 
electrolyte body 60, there is formed by printing an unfired layer for the 
second electrode 64, which serves as an outer pumping electrode. 
Subsequently, unfired layers for the electrically insulating porous layer 
30, heat generating element 28 and gas-tight layer 34 are formed by 
printing, so as to surround the unfired outer pumping electrode 64. 
The thus prepared two unfired assemblies with the various printed unfired 
layers, that is, a first assembly including the unfired first solid 
electrolyte body 60, and a second assembly including the unfired second 
solid electrolyte body 68, are butted together such that the unfired layer 
for the porous ceramic layer 66 on the first assembly contacts the unfired 
layer for the gas-tight layer 48 on the second assembly. The obtained 
unfired laminar structure is then fired. Thus, the co-fired laminar 
structure of the oxygen sensing element 2 is prepared. 
In the thus prepared oxygen sensing element 2, the porous ceramic layer 66 
formed between the first solid electrolyte body 60 and the thermosensitive 
means 42 (gas-tight layer 48) practically functions as the diffusion 
chamber 16 having a predetermined diffusion resistance. Since the porous 
ceramic layer 66 is exposed to the external measurement gas at its 
periphery, the external measurement gas diffuses into the porous structure 
of the layer 66, that is, the diffusion chamber 16, so that the inner 
pumping electrode 62 is exposed to the atmosphere within the diffusion 
chamber 16. A voltage is applied between the inner and outer pumping 
electrodes 62, 64, to effect an oxygen pumping action. The oxygen 
concentration of the measurement gas is determined by measuring a limit 
current Ip (oxygen pumping current) which passes through the pumping cell, 
as well known in the art. 
According to the above arrangement, the inner pumping electrode 62 is 
exposed to a relatively large amount of the measurement gas within the 
diffusion chamber 16, and an electric signal of up to 0.6 .mu.A/ppm can be 
obtained, for example, where the oxygen concentration of the measurement 
gas is expressed in ppm. This indicates an improvement in the oxygen 
detecting accuracy of the sensing element 2. 
The formation of the porous ceramic layer 66, i.e., the diffusion chamber 
16, by a printing technique, is advantageous for accurate control of the 
temperature of the sensing element 2, in particular, within the diffusion 
chamber 16, namely, for accurate control of the first and second heaters 
22, 24 based on the thermosensitive means 42. In this arrangement, too, 
the temperature of the sensing element 2 can be made substantially 
constant in the direction of thickness, assuring consistently high 
accuracy of measurement, even if the flow rate and temperature of the 
measurement gas are varied. 
While the several specific oxygen sensing elements have been described as 
preferred embodiments of a gas sensing element of the present invention, 
the principle of the invention is applicable to other types of oxygen 
sensing elements, and other gas sensing elements, detectors or controllers 
that are adapted to detect components of the measurement gas, other than 
oxygen, such as nitrogen, carbon dioxide and hydrogen, which are 
associated with an electrode reaction. 
It is to be understood that the invention is not limited to the precise 
details of the illustrated embodiments, but the invention may be embodied 
with various changes, modifications and improvements which may occur to 
those skilled in the art, without departing from the spirit and scope of 
the invention defined in the following claims.