Oxygen sensor

An oxygen sensor 3 having a heater 2 capable of providing an improved temperature increasing characteristic, while maintaining an increased amount of introduced air and a high production efficiency. The sensor 3 includes a sensor element 1 having an air chamber 100 in which a heater 2 is arranged. The heater 2 has a rectangular cross sectional shape. In order to produce the heater 2, onto a green ceramic sheet, a heater of a predetermined pattern including a plurality of heater sections is printed. Then, to the printed sheet, different ceramic green sheets are applied so as to obtain a laminated body. The laminated body is subjected to a cutting so as to produce an intermediate body in which a heater section is included. The intermediate body is subjected to a firing to obtain a completed heater.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an oxygen sensor for use in a system, for 
an internal combustion engine, such as an air fuel ratio control system, 
and a method for producing such an oxygen sensor. 
2. Description of Related Art 
In a known air fuel ratio control system for an internal combustion engine, 
an oxygen sensor is arranged in an exhaust system of the internal 
combustion engine to detect an air fuel ratio and combustion control is 
done in accordance with the air fuel ratio detected by the oxygen sensor, 
so that an increase in a purification efficiency of an exhaust gas is 
obtained in a three way catalytic converter arranged in an exhaust system 
of the internal combustion engine. 
Such an oxygen sensor includes a detecting element (an oxygen sensor 
element), which is constructed as a body of a solid electrolyte having 
oxygen ion conductivity. Namely, the solid electrolyte body is formed as a 
cup shape, in which an air chamber is formed. Furthermore, the solid 
electrolyte body forms, at its outer surface, an outer electrode and, at 
its inner surface, an inner electrode, which is in contact with the air in 
the air chamber. Furthermore, inside the air chamber, a heater is arranged 
so that the oxygen sensor element is heated to an activating temperature. 
In the prior art structure of the oxygen sensor, as shown in FIG. 16, a 
heater 9 as rod shape of a circular cross sectional shape is used. The 
heater 9 is constructed by a heat generating part 21 in which a heat 
generating element is housed and a supporting section 22 for supporting 
the heat generating part 21 and for housing lead members 22 electrically 
connected to the heat generating element in the heat generating part 21. 
Now, a method for producing the above construction of the heater 9 will be 
explained. Namely, as shown in FIG. 17, a ceramic green sheet 90 is 
subjected to coating with a layer of a desired pattern 200 to be formed 
into a heat generating element 210 and lead elements 220 by being 
subjected to a later firing process. Then, on the sheet, a organic binder, 
which is an ethyl cellulose dissolved in an organic solvent, is painted. 
Then, the sheet 90 is wrapped around a core rod 900 made of a ceramic 
material. Then, the core rod 900 together with the sheet 90 wrapped around 
the rod 900 is subjected to a firing in a furnace of a high temperature in 
a range between 1400 to 1500.degree. C. Then, at an end of the heater 9, 
terminal elements 290 are formed so that they are in an electric 
connection with the lead element 220. Then, lead wires 29 are connected to 
the terminal elements 290, respectively, by means of a soldering using a 
soldering material such as AuCu in a vacuum condition at a high 
temperature in a range between 950 to 1000.degree. C. Finally, the lead 
wires 29 are connected to an outside electric power source, so that the 
heat generating element 210 is fed with electric power via the electrode 
element 220. 
In the above mentioned structure in the prior art in FIG. 16, the heater 9 
is formed as the rod of a circular cross sectional shape, which, on one 
hand, causes the value of the cross sectional area to be increased and, on 
the other hand, causes a loss of the heat to be increased in the upward 
direction. As a result, the prior art structure of the rod of the circular 
cross sectional shape is defective in that the speed of the increase in 
the temperature of the oxygen sensor element is reduced. Furthermore, the 
circular cross sectional shape of the rod causes the clearance to be small 
with respect to the faced inner wall of the air chamber, which results in 
a reduced amount of air introduced into the air chamber, which causes the 
precision to be reduced in the detection of the oxygen density by the 
oxygen sensor. 
Furthermore, according to the method for producing the heater 9 as briefly 
explained with reference to FIG. 17, a production efficiency is low due to 
the fact that a formation of the heater pattern is done on a rod by rod 
basis. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an oxygen sensor having a 
heater of an improved temperature increasing characteristic. 
Another object of the present invention is to provide an oxygen sensor 
having a heater which allows an increased amount of air to be introduced 
into the air chamber. 
Still another object of the present invention is to provide a method of 
producing a heater for an oxygen sensor, capable of a high efficiency. 
According to the present invention, an oxygen sensor is provided, 
comprising: 
a cup shaped body made of a solid electrolyte body; 
an air chamber inside said solid electrolyte body, which is opened to 
atmospheric air; 
an outer electrode on an outer surface of said solid electrolyte body; 
an inner electrode on an inner surface of said solid electrolyte body, and; 
a heater including a heat generating part in which a heat generating 
element is stored and a supporting part for supporting the heat generating 
portion, the supporting part having a lead element which is in an electric 
connection with the heat generating element; 
a relationship between the cross sectional area Sh of the heat generating 
part and the cross sectional area Ss of said supporting part being such 
that Sh.gtoreq.Ss; 
a relationship between the length a of the long side and the length b of 
the short side in the rectangular cross sectional shape of the heater 
being such that 0.3.ltoreq.b/a.ltoreq.0.7. 
When the relationship between Ss and Sh is such that Sh&lt;Ss, i.e, the cross 
sectional area of the supporting part is larger than that of the heat 
generating part, it may cause a loss of heat to the upper part of the heat 
onto be increased. 
The cross sectional area Sh implies a cross sectional area of a portion of 
the heater where the heat generating element is embedded as shown in FIGS. 
2 and 3. Furthermore, the cross sectional area Ss implies a cross 
sectional area of a portion of the heater where the lead element is 
embedded as shown in FIGS. 2 and 3. 
A value of b/a larger than 0.7 may cause the temperature increase speed 
characteristic of the oxygen sensor to be worsened and a clearance between 
the inner wall of the air chamber of the oxygen sensor and the heater to 
be reduced, which may cause the amount of air introduced into the air 
chamber to be too small. 
On the other hand, a value of b/a smaller than 0.3 may cause a clearance 
between the inner wall of the air chamber of the oxygen sensor and the 
heater to be unnecessarily increased, which causes the size of the oxygen 
sensor to be increased, which makes it difficult to install the sensor in 
a small space. Furthermore, the diameter of the heater is reduced, 
resulting in a reduction in the mechanical strength as well as the 
durability of the heater. As to practical shapes of the heater, i.e., the 
arrangement of the long side a and the short side b, refer to FIGS. 4 and 
11. 
In the construction of the present invention, between the cross sectional 
area Sh of the heat generating part and the cross sectional area Ss of the 
supporting part, a relationship Sh.gtoreq.Ss is obtained. As a result, a 
loss of heat in the upper direction of the heater is reduced. 
Furthermore, in the heater according to present invention, the cross 
sectional shape of the heater is rectangular. As a result, as shown in 
FIG. 5, the cross sectional area of the heater insertable into the air 
chamber of the same diameter is reduced over the case where the cross 
sectional shape of the heater is of a circular shape. As a result, the 
amount of the air which is in contact with the heater is reduced, so that 
a loss of heat, from the upper end of the heater to the atmosphere, is 
reduced, which results in a more effective heating of the oxygen sensor. 
Due to the relationship between the length a of the long side and the 
length b of the short side in the rectangular cross sectional shape of the 
heater, which is such that 0.3.ltoreq.b/a&lt;0.7, a desirable clearance is 
maintained in the air chamber, thereby keeping a sufficient amount of the 
air introduced into the air chamber. As a result, if a large amount of a 
pumping of the oxygen in the air chamber via the solid electrolyte body 
occurs, a lack of oxygen in the air chamber does not occur, which makes it 
always possible to obtain an oxygen measurement of an increased precision. 
Due to the reduction in the amount of air contacting the heater, the amount 
of the heat discharged to the atmosphere from the upper end of the heater 
is reduced, which allows the oxygen sensor to be more effectively heated. 
As a result, production of an oxygen sensor of an improved temperature 
increasing speed characteristic is possible. 
In short, according to present invention, the production of an oxygen 
sensor, having an improving temperature increasing characteristic while 
keeping a sufficient amount of air introduced into the air chamber and 
providing an increased production efficiency, is possible. 
Preferably, said heater is beveled at its corners of the rectangular cross 
section. Such a beveling is advantageous in that an occurrence of cracking 
or damage at the corner portions of the heater is prevented when the 
heater is assembled to the oxygen sensor. 
Preferably, said heater has, at its outer surface, a coating of a high 
radiation film made of at least one of materials selected from Fe.sub.2 
O.sub.3, NiO, Y.sub.2 O.sub.3 and Si.sub.3 N.sub.4. Due to the provision 
of such a high radiation film, the heater is, at the corner portions, 
formed as a curved shape, which is also advantageous in that an occurrence 
of cracking or damage at the corner portions of the heater is prevented 
when the heater is assembled to the oxygen sensor. 
Such a high radiation film is made of a material which has a radiation 
(adsorption) rate which is almost equal to 1.0, which allows the heat to 
be easily emitted or absorbed. Namely, a provision of such a high 
radiation film allows the heat of the heater to be effectively absorbed, 
which results in an effective emission of a heat toward the inner surface 
of the air chamber of the oxygen sensor. As a result, an oxygen sensor of 
an improved temperature increasing speed is obtained. 
Preferably, in a gap between an inner wall of the air chamber facing the 
heater and the heat generating part of the heater, a relationship between 
the distance L1 of said inner wall said from the long side of the 
rectangular cross section and the distance L2 of said inner wall said from 
the short side of the rectangular cross section is such that 
1.5.ltoreq.L1/L2.ltoreq.2.5. 
As a result, an oxygen sensor with an improved speed of the temperature 
increase is obtained. In the case where the ratio L1/L2 is lower than 1.5, 
it may be possible that the temperature increasing speed is insufficient. 
Furthermore, a clearance between the inner surface of the air chamber and 
the heater is reduced, resulting in a lack of introduced air. 
Contrary to this, a ratio L1/L2 larger than 2.5 may means that the 
clearance between the inner wall of the air chamber and the heater is 
unnecessarily increased, which causes the size of the oxygen sensor to be 
increased, which makes it difficult for the sensor to be mounted in a 
limited space. Furthermore, the shape of the heater is thinner, which 
makes it possible that the mechanical strength or durability of the sensor 
is reduced. 
It should be noted that the distance L1 and L2 are distances on the inner 
wall of the air chamber from the long side length and the short side 
length respectively, of a rectangular shape in the cross section of the 
heater. 
Preferably, a relationship between a cross sectional area S1 of the space 
in the air chamber in which the heater is inserted and the cross sectional 
area S2 of the heat generating part of the heater is such that S2 
/S1.ltoreq.0.5. 
By this arrangement, an oxygen sensor with an improved temperature 
increasing speed characteristic is obtained. Namely, when the value of S2 
/S1 is smaller than 0.5, the temperature increasing speed characteristic 
of the oxygen sensor is worsened. In this regard, the space area in the 
air chamber is the cross sectional area of the air chamber minus the cross 
sectional area of the heat generating part of the heater, which 
corresponds to the clearance between the inner wall of the air chamber and 
the heater. 
According to the present invention, a process is provided for producing a 
heater for an oxygen sensor, said heater including a heat generating part 
in which a heat generating element is stored and a supporting part for 
supporting the heat generating portion, said process comprising the steps 
of: 
providing a green ceramic sheet; 
printing, on said green sheet, a heater material at a predetermined pattern 
including a plurality of heater sections; 
applying a different green ceramic sheet to the ceramic sheet on which said 
pattern is formed, so as to obtain a laminated sheet body; 
cutting the laminated sheet body so as to separate an intermediate body 
including a heater section, and; 
firing the intermediate body. 
According to this method, the production efficiency is increased due to the 
fact that a number of heaters are simultaneously produced. In this regard, 
the heater pattern corresponds to screen printed portions of 
electro-conductive material, such as Pt or Pd, which become a heat 
generating element and a lead after the firing.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Now, embodiments of the present invention will be explained with reference 
to the attached drawings. 
FIG. 14 is related to a specific embodiment of the present invention 
directed to supporting a heater as will explained later. However, the 
remaining general structure as an oxygen sensor is similar to all of the 
embodiments of the present invention. Thus, a general structure of an 
oxygen sensor 3 will first be explained with reference to FIG. 14. Namely, 
the oxygen sensor 3 includes an oxygen sensor element 1 and a heater 2 
with lead wires 29. The oxygen sensor element 1 is fixed to a housing 30, 
which is adapted to be connected to a desired location, such as an exhaust 
manifold (not shown) of an internal combustion engine (not shown) in an 
engine compartment of a vehicle. 
The oxygen sensor element 1 together with the heater 2 is inserted to the 
housing 30. A lower cover 311 is fixed to an upper end of the housing 30 
via a layer made of talc 301, a ring shaped packing member 302 and an 
insulator 303. An intermediate cover 312 is inserted to a top end of the 
lower cover 311, while an insulator 330 is nipped between a top edge of 
the cover 311 and an inner shoulder portion of the intermediate cover 312. 
The lead wires 29 of the heater 2 are passed through corresponding holes 
in the insulator 330 and are connected to ends of outside connecting wires 
342 by means of respective connectors 341. The outside connecting wires 
342 are passed through respective holes in a rubber bushing 339 fitted to 
an upper end of the intermediate cover 312. An upper cover 313 is inserted 
into the intermediate cover 312 and is, at a location 313-1, crimped so 
that the covers 312 and 313 are connected with each other while the rubber 
bushing 339 is elastically deformed. A water repellent filter 318 is 
arranged in an annular space between the intermediate cover 312 and the 
outer cover 313, which has vent holes 319 opened to the water repellent 
filter 318. 
First Embodiment 
FIG. 1 shows a detail of an oxygen sensor. A sensor element 1 is 
constructed by a cup shaped body 10, of a solid electrolyte, in which an 
air chamber 100 is formed. Furthermore, the solid electrolyte body 10 
forms, at its outer surface 101, an outer electrode 11 and, at its inner 
surface 102, an inner electrode 12, which is in contact with the air in 
the air chamber 100. Furthermore, inside the air chamber 100, a heater 2 
is arranged, so that the oxygen sensor element 1 is subjected to heating 
to an activating temperature. 
An outer protection layer 191 is applied to an outer surface of the solid 
electrolyte body 10 so that the outside electrode 11 is covered by the 
protection layer 191. Finally, as the outermost layer, a toxic material 
trapping layer 192 is applied. 
As shown in FIGS. 2 and 3, the heater 2 is constructed by a heat generating 
part 21 in which a heat generating element 210 and a supporting part 22 
for supporting the heat generating element 210 are housed. A lead element 
220, which is in an electrical connection with the heat generating element 
210, is formed on the supporting part 22. The heater 2 is of an 
rectangular cross sectional shape. Furthermore, the heater 2 has the same 
value of the cross sectional area between the heat generating part 21 and 
the supporting part 22. In other words, the cross sectional area Sh of the 
heater 2 at the heat generating part 21 and the cross sectional area Ss of 
the heater 2 at the supporting part 22 have the same value. Furthermore, 
as shown in FIG. 4, in the rectangular shape at the cross section of the 
heater 2, between the length a and the width b, a relationship of b/a=0.6 
is obtained. 
Next, a method for producing the heater 2 will be explained with reference 
to FIGS. 6 and 7. First, a green sheet 293 of a ceramic material is 
prepared. Then, a heater layer of a predetermined pattern 280 of equally 
spaced elongated parallel sections, is printed on the ceramic green sheet 
293. Then, to the ceramic sheet 293 on which the pattern 280 is printed, 
from the opposite sides thereof, further ceramic green sheets 291 and 292 
are applied so that a laminated body 271 (FIG. 7(a))is obtained. 
Then, as shown in FIG. 7(a), the laminated body 271 is subjected, along a 
line parallel to the length of a section of the pattern 280 in FIG. 6, to 
a cutting so that an elongated intermediate body 272 including a section 
of the heater pattern 280 is obtained. Then, the intermediate body 272 is 
subjected to a firing process, which is followed by an application of lead 
mounting parts 290. Finally, lead wires 29 are connected to the lead 
mounting parts 290, thereby providing a completed heater 2. 
The heater 2 as obtained above is housed in the air chamber 100 in FIG. 1. 
As already explained with reference to FIGS. 1 to 4, the heater is 
constructed by the heat generating element 210 which, when energized, 
generates a heat and a lead element 220 which applies an electric voltage 
to the heat generating element 210. The portion in which the heat 
generating element is housed is a heat generating portion 21. The 
arrangement of the heater 2 in the air chamber 100 is such that the heat 
generating part 21 faces the inner electrode 12. 
As shown in FIG. 3, the heater 2 is constructed as a three layer structure, 
which is constructed by two ceramic plates 231 and 232 and a heater plate 
233, which is arranged between the ceramic plates 231 and 232 and which is 
provided with a patterned heater 200 constructed by the heat generating 
element 210 and the lead element 220. The ceramic plate 231 and 232 and 
the heater plate 233 are made of Al.sub.2 O.sub.3. The patterned heater 
200, which is constructed by the heat generating element 210 and the lead 
element 220, is made from tungsten-rhenium (W--Re). 
As shown in FIG. 4, the rectangular shape of the heat generating part 21 of 
the heater 2 has a long side a of a length of 2.8 mm and a short side b of 
a length of 1.6 mm. Furthermore, in a state that the heater 2 is arranged 
in the air chamber 100, a distance L.sub.1 between the long side and the 
inner surface 102 of the air chamber 100 is 0.8 mm and a distance L.sub.2 
between the short side and the inner surface 102 of the air chamber 100 is 
0.4 mm. Furthermore, the cross sectional area of the heater 2 is identical 
between the heat generating part 21 and the supporting part 22 and is 
equal to 4.48 mm.sup.2. 
The method for producing the heater according to present invention will 
explained in more detail. As the first stage, ceramic green sheets 291, 
292 and 293 (FIG. 7) made of Al.sub.2 O.sub.3 are prepared. Then, to the 
green sheet 293, a layer of an electric conductive paste made of a 
powdered tungsten is printed at a predetermined pattern 280 of five 
equally spaced elongated heater sections as shown in FIG. 6. Then, to the 
green sheet 293 on which the pattern 280 is printed, from the opposite 
sides, the green sheets 291 and 292 are applied, so that the laminated 
body 271 as shown in FIG. 7(a) is obtained. 
Then, the laminated body 271 is subjected to a drying process. Then, as 
shown in FIG. 7(b), the laminated body 271 is subjected to cutting so that 
an intermediate body 272 is separated in a manner that the intermediate 
body 272 includes only one heater pattern 280. 
Then, the intermediate body 272 is subjected to a firing process at a 
temperature in a range between 1400 to 1500.degree. C. 
After the completion of the firing process, to the sides of the 
intermediate body 272 where the heater pattern 280 is exposed, a paste of 
tungsten is coated and is fired. Then, soldering of the lead wires 29 is 
done in a high temperature furnace of a range of a temperature between 950 
to 1000.degree. C. while using a soldering material such as the one based 
on Au-Cu. 
As a result, the heater according to present invention as shown in FIG. 
7(c) is obtained. 
Now, the advantages of the heater 2 according to present invention will be 
explained. In the heater arranged in the air chamber 100 of the oxygen 
sensor element 1, the cross sectional area Sh of the heat generating part 
21 and the cross sectional area Ss of the supporting part 22 are 
identical. On the other hand, introduction of the air into the air chamber 
100 is done at the upper end of the chamber 100. According to the present 
invention, a quick introduction of the air is realized when the heater 2 
is arranged in the air chamber 100 due to the fact that the cross 
sectional area of the supporting part 22 located upstream from the heat 
generating part 21 is equal to the cross sectional area of the heat 
generating part 21. As a result, even in a situation that pumping of the 
oxygen at an increased rate via the solid electrolyte body 10 is needed 
due to an especially reduced density of the oxygen, the air chamber 100 is 
prevented from lacking in oxygen. Thus, the sensor element 1 according to 
present invention allows the oxygen density to be correctly detected. 
Furthermore, according to the present invention, as shown by the shaded 
area in FIG. 5(b), the cross sectional shape of the heater 2 is a 
rectangular shape, which is compared with the prior art where the cross 
sectional shape of the heater 9 is a circular shape, as shown by a shaded 
area in FIG. 5(a). In order to allow the heater to be inserted to the air 
chamber 100 of a fixed inner diameter, the area of a rectangular cross 
sectional shape of the heater in the present invention is smaller than the 
area of a circular cross sectional shape of the heater in the prior art. 
As a result, an amount of the air, which contacts with the upper end of 
the heater 2, is reduced, thereby reducing a loss of heat to the 
atmosphere. Thus, the heater 2 can effectively heat the oxygen sensor 
element 1. Thus, the oxygen sensor element 1 according to present 
invention can produce an increased speed of heating of the sensor 2. 
Furthermore, according to present invention, in the rectangular shaped 
cross section of the heater, between the length of the long side a and the 
short side b, a relationship of b/a=0.57 is obtained. Thus, a desired 
clearance is maintained between the outer surface of the heater and the 
inner surface of the air chamber 100, which allows a desired amount of air 
to be introduced into the chamber 100. Furthermore, according to the 
construction of the heater 2 of the present invention, a loss of heat to 
the atmospheric air is reduced, which also assists in increasing the speed 
of the increase in the temperature of the sensor. Furthermore, in the 
method for producing the sensor according to present invention, a large 
number of heaters 2 can be simultaneously produced, thereby increasing the 
production efficiency as well as reducing the production cost. 
In short, according to the first embodiment of the resent invention, it is 
possible to provide an oxygen sensor element having a heater with an 
increased speed of a temperature increase as well as providing a 
sufficient amount of introduced air into an air chamber and a method for 
producing such a heater at an increased efficiency. 
Second Embodiment 
A second embodiment of the present invention shown in FIG. 8 is related to 
a relationship between the aspect ratio b/a (ratio of the length of the 
short side to the length of the long side) and the temperature increase 
speed characteristic of the oxygen sensor element 1. The construction of 
the oxygen sensor element 1 and the method of producing it are identical 
with those in the first embodiment. However, the range of the values of a 
and b in the shape of the heater is different. 
A DC voltage of 8 V is applied to the leads of the heater of the sensor in 
atmospheric air at room temperature, while a measurement of the speed of 
the increase in the temperature of the heater is done. The result is shown 
in FIG. 8. In FIG. 8, an ordinate in the left-handed side shows a time for 
causing the heater to obtain a temperature of 300.degree. C. from the 
commencement of the heating by the heater. An ordinate in the right-handed 
side is the cross sectional area of the air chamber 100 of the oxygen 
sensor element minus the cross sectional area of the heat at the heat 
generating part 21, which corresponds to the cross sectional area of the 
air gap in the air chamber 100. The cross sectional area of the air 
chamber is measured at a location faced with the heat generating part 21 
of the oxygen sensor element 1. The abscissa shows a value of the aspect 
ratio b/a. 
In FIG. 8, when the aspect ratio b/a is increased to a value of around 0.7, 
the time for causing the temperature of the heater to be increased to 
300.degree. C. is prolonged. 
On the other hand, the smaller the value of the ratio b/a, the shorter is 
the time for increasing the temperature of the heater to 300.degree. C. 
However, the smaller e value of the aspect ratio, the larger is the 
clearance between the inner surface of the air chamber of the oxygen 
sensor element 1 and the heater, which causes the size of the oxygen 
sensor element 1 to be increased, which makes it difficult for the sensor 
to be arranged in a limited space. Furthermore, the smaller value of the 
aspect ratio causes the oxygen sensor element to be in the shape of a thin 
plate, which may cause the mechanical strength of the heater to be 
decreased, which makes the sensor not practically applicable. 
In view of the above, the inventor has found that the value of the aspect 
ration b/a should be in a range between 0.3 and 0.7 
(0.3.ltoreq.b/a.ltoreq.0.7). 
Third Embodiment 
The embodiment is concerned with a desired range of a ratio L1 to L2 with 
respect to a heat increasing characteristic of the oxygen sensor, where L1 
is the distance of the long side a of the heat generating part 21 from the 
inner surface of the air chamber 100 and L2 is the distance of the short 
side b of the heat generating part 21 from the inner surface of the air 
chamber 100. Such a relationship between the distance ratio L1/L2 and the 
heat increasing characteristic is shown in FIG. 9. 
According to this embodiment, the construction of the oxygen sensor and the 
method for producing the same are identical to those explained in the 
first embodiment. The only a difference is the values of L1 and L2, which 
are distances to the inner surface of the air chamber along the long side 
a and the short side b, respectively, of the heat generating part. 
In FIG. 9, the ordinate on the left-handed side shows a time to an increase 
to a temperature of 300.degree. C. of the oxygen sensor element, while the 
ordinate on the right-handed side shows an area of the space in the air 
chamber. The abscissa shows a value of the distance ratio L1/L2. 
As will be clear from FIG. 9, when the value of the distance ratio L1/L2 is 
smaller than 1.5, the temperature increasing speed is too slow, which 
causes the oxygen sensor to be impractical. 
As will also be clear from FIG. 9, larger the value of the distance ratio 
L1/L2, shorter is the time to the temperature of 300.degree. C. The larger 
value of the distance ratio L1/L2, however, causes the clearance to be 
unnecessarily increased between the inner surface of the air chamber of 
the oxygen sensor element and the heater, which makes the size of the 
sensor to be intolerably increased, which makes it difficult for the 
sensor to be arranged in a limited space. Furthermore, a larger value of 
the distance ratio L1/L2 causes the shape of the oxygen sensor to be thin 
plate shape, which may cause its mechanical strength to be reduced, which 
makes the sensor to be unsuitable in practical use. 
In view of the above, it is desirable that the value of the distance ratio 
L1/L2 is in a range between 1.5 and 2.5. 
Fourth Embodiment 
The fourth embodiment is concerned with a desired relationship between the 
cross sectional area ratio S2 /S1 and the temperature increasing 
characteristic of the oxygen sensor element, where S1 designates the cross 
sectional area of the gap between the inner surface of the air chamber 100 
and the outer surface of the heater 2 when the heater 2 is inserted into 
the air chamber 100 and S2 designates the cross sectional area of the 
heater 2 at the heat generating part 21. 
According to this embodiment, the construction of the oxygen sensor and the 
method for producing the same are identical to those explained in the 
first embodiment. The only difference is the values of S1 and S2. As to 
the sensor element, tests as to the temperature increasing characteristic 
were performed in a similar way to those in the second embodiment and the 
results are shown in FIG. 10. 
In FIG. 10, an ordinate shows a time up to the increase in the temperature 
of the sensor element to 300.degree. C., while the abscissa is the value 
of the ratio S2 /S1. 
In FIG. 10, when the value of the S2 /S1 is increased to a value higher 
than 0.5, the temperature increase characteristic is rapidly worsened. 
Thus, it can be concluded that the value of the ratio S2 /S1 should be in 
a range equal to or larger than 0.5 (S2 /S1.ltoreq.0.5). 
Modified Embodiments 
FIGS. 11(a) to 11(d) show various shapes of the transverse cross section of 
the heater according to the present invention. 
FIG. 11(a) shows a rectangular cross sectional shape with acute corners. 
FIG. 11(b) shows a rectangular cross sectional shape with beveled corners. 
FIG. 11(c) shows a rectangular cross sectional shape with rounded corners. 
FIG. 11(d) shows a elliptic cross sectional shape. 
In all of the cross sectional shapes in FIGS. 11(a) to (d), between the 
length of the long side a and the length of the short side b, a 
relationship of 0.3.ltoreq.b/a .ltoreq.0.7 is maintained. 
Furthermore, in order to produce the heater in each of FIGS. 11(b) to (d), 
the heater element is, first, produced by the same method as described 
with reference to the first embodiment, which is followed by a coating of 
a high radiation film of a thickness of 20 .mu.m on the outer surface of 
the heater and a beveling at the corner portions. 
The formation of the high radiation film is as follows. Namely, a slurry of 
ceramic powder made of Fe.sub.2 O.sub.3 mixed with Al.sub.2 O.sub.3 is 
first prepared. Then, the heater is dipped into the slurry produced by the 
method which is the same as that explained with reference to the first 
embodiment, while the heater is not yet equipped with lead wires, so that 
the ceramic powder is applied to the entire surface of the heater. Then, 
the heater with the ceramic coating is subjected to a heating and a firing 
at a temperature in a range 500 to 1000.degree. C., so that a high 
radiation film of the ceramic is created on the surface of the heater. 
The construction of the heater is identical to that in the first 
embodiment. 
The embodiment in FIGS. 11(b) to (d) has advantages in that the heater 2 is 
prevented from being damaged or cracked at the corners 209 when 
installation of the heater 2 to the housing is performed. Furthermore, the 
provision of the beveled portions 209 at the corners is done by the 
formation of the high radiation film. Thus, as an additional advantage, 
the efficiency of the heat emitted to the air chamber is increased, 
thereby improving the oxygen sensor element in its temperature increasing 
speed characteristic. 
In addition, the advantages as explained with reference to the first 
embodiment are maintained. 
Further Embodiment 
A further embodiment will, now, be explained with reference to FIGS. 12 to 
15. 
In the heater 2 shown in FIG. 12, the heater is provided with a laminated 
structure of ceramic plates of the same structure as explained with 
reference to FIGS. 1 and 3 which has a cut-out portion 291 for positioning 
a lead wire. In the cut-out portion 291, the lead connecting parts 290 and 
lead wires are arranged. 
In the embodiment shown in FIG. 13, the heater 2 is, at its top end, formed 
with a groove 292 of a V-cross sectional shape for a positive positioning 
of the heater when the oxygen sensor element is used in the oxygen sensor. 
In more detail, as shown in FIG. 14, the sensor 3 is provided with a 
retaining rod 349, which is passed through an axial hole in the insulator 
330. The retaining rod 349 has a top end fitted to the rubber bushing 339 
and a bottom end of a V-cross sectional shape which is in an engagement 
with the V-cross sectional shaped groove 292 at the top end of the heater. 
As a result, a positive and a reliable positioning of the heater 2 is 
realized. 
FIG. 15 illustrates an embodiment of the heater 2, where the cross 
sectional area of the heat generating part 21, in which the heat 
generating element 210 is stored, is larger than that of the supporting 
part 22, in which a lead element 210 is arranged. Other constructions are 
identical to those in the first embodiment. 
The heater 2 in FIG. 12 is advantageous in that work to connect the lead 
wire 29 to the heater is easy, which increases the production efficiency. 
The heater 2 in FIG. 13 is advantageous in that the positioning of the 
heater is easy. 
Finally, the heater 2 in FIG. 15 is advantageous in that an increased 
clearance is obtained between the inner wall of the air chamber 100 and 
the heater 2, which allows the amount of introduced air to be increased.