Oxygen sensor for and method of determining concentration of oxygen

An oxygen sensor for determining the partial pressure of oxygen in a monitored gas environment includes a diffusion housing of zirconia based material having a gas diffusion aperture, an oxygen ion conductive plate of zirconia based material, a pair of electrode layers mounted on opposite sides of the conductive plate, and a sealing glass material sealingly bonding the conductive plate at one side to the housing to provide a diffusion chamber defined by the diffusion housing and the conductive plate. The electrodes are connectable to a power source for being supplied with an electric potential to pump oxygen ion out of the diffusion chamber through the conductive plate to flow a current through the electrodes which current is indicative of the partial pressure of the oxygen in the monitored gas environment. The sealing glass material contains SiO.sub.2, BaO, Na.sub.2 O and ZrO.sub.2.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an oxygen sensor for and a method of determining 
the concentration of oxygen (i.e., the partial pressure of oxygen) in a 
monitored gas environment. 
2. Prior Art 
One conventional oxygen sensor 10 shown in FIG. 1 comprises a diffusion 
housing 12, an oxygen ion conductive plate 14 of solid electrolyte 
hermetically bonded at one surface to the diffusion housing 12 by sealing 
glass 16, and a pair of circular electrode layers 18 and 18 secured to the 
opposite sides of the conductive plate 14, respectively, the electrodes 18 
and 18 being connected to a DC power source 20. For example, the oxygen 
ion conductive plate 14 comprises a solid solution containing ZrO.sub.2, 
Y.sub.2 O.sub.3, MgO and CaO. Each of electrode layers 18 is porous and is 
made, for example, of platinum. The diffusion housing 12 has a gas 
diffusion aperture 12a of a small diameter formed through a wall 12b. The 
diffusion of oxygen from the monitored gas environment G through the gas 
diffusion aperture 12a into a chamber 12c of the diffusion housing 12 is 
effected by the application of a DC potential from the power source 20 
across the two electrodes 18 and 18 to pump oxygen present in the chamber 
12c through the oxygen ion conductive plate 14. As the potential across 
the two electrodes 18 is increased, electrical current flowing through the 
two electrodes 18 is changed as indicated by a curve A in FIG. 2. The 
curve A is divided into three regions X, Y and Z. The region X is a 
transition region at which the above-mentioned pumping of the oxygen out 
of the chamber 12c is in the process of reaching a constant rate. At the 
region Y, the amount of the molecules of the oxygen flowing into the 
chamber 12c through the gas diffusion aperture 12a is equal to the amount 
of the molecules of the oxygen flowing out of the chamber 12c through the 
oxygen ion conductive plate 14. At this region, the current limited by the 
oxygen diffusion is rendered stable so that a stable diffusion limited 
current value Ip is obtained. At the region Z, the current flowing through 
the two electrodes 18 is abruptly increased because the current contains 
components other than the ion current, such as current caused by the 
electronic conduction. The diffusion limited current value Ip is 
proportional to the concentration of oxygen in the monitored gas 
environment G, and therefore the oxygen concentration is detected by 
measuring the diffusion limited current value Ip through an ammeter A. 
Another conventional oxygen sensor 10a shown in FIG. 3 differs from the 
oxygen sensor 10 of FIG. 1 in that a diffusion housing 12 made of an 
open-cell porous structure is used with the gas diffusion aperture 12a 
being omitted. With this construction, the oxygen diffuses into the 
housing chamber 12c through the porous housing 12, and the pumping of 
oxygen out of the chamber 12c is effected as described above for the 
oxygen sensor of FIG. 1. 
The sealing glass 16 sealingly bonding the oxygen ion conductive plate 14 
to the diffusion housing 12 comprises amorphous glass containing 
SiO.sub.2, BaO and Na.sub.2 O. The bond between the oxygen ion conductive 
plate 14 and the diffusion housing 12 must be maintained heat-resistant, 
airtight and thermal shock-resistant. The oxygen ion conductive plate 14 
is made of zirconia, and the diffusion housing 12 is usually made of 
zirconia-based material so that there will not be provided a substantial 
difference in thermal expansion coefficient between the conductive plate 
14 and the diffusion housing 12. Therefore, the diffusion housing 12 and 
the oxygen ion conductive plate 14 have a relatively high thermal 
expansion coefficient, for example, of 100.times.10.sup.-7 /.degree. C., 
and must have such a thermal resistance as to withstand up to 800.degree. 
C. For these reasons, the bond between the diffusion housing 12 and the 
oxygen ion conductive plate 14 must be made through glass. However, it is 
rather difficult to manufacture such glass as to meet these requirements. 
Glass containing SiO.sub.2, BaO and Na.sub.2 O is the only glass which is 
commercially available, partially amorphous and somewhat analoguous in 
physical properties to the above-mentioned desired glass. This amorphous 
glass has a poor thermal shock resistance, and a crack is liable to 
develop in it. 
SUMMARY OF THE INVENTION 
It is therefore an object of this invention to provide an oxygen sensor of 
the type in which the diffusion housing and the oxygen ion conductive 
plate are sealingly bonded together by amorphous glass having a high 
thermal expansion coefficient and excellent thermal resistance and thermal 
shock resistance. 
According to the present invention, there is provided an oxygen sensor for 
determining the partial pressure of oxygen in a monitored gas environment 
which comprises a diffusion housing of zirconia based material having a 
gas diffusion aperture, an oxygen ion conductive plate of zirconia based 
material, a pair of electrode layers mounted on opposite sides of the 
conductive plate, and a sealing glass material sealingly bonding the 
conductive plate at one side to the housing to provide a diffusion chamber 
defined by the diffusion housing and the conductive plate, the electrodes 
being connectable to a power source for being supplied with an electric 
potential to pump oxygen ion out of the diffusion chamber through the 
conductive plate to flow a current through the electrodes which current is 
indicative of the partial pressure of the oxygen in the monitored gas 
environment, and the sealing glass material containing SiO.sub.2, BaO, 
Na.sub.2 O and ZrO.sub.2.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
An oxygen sensor 10b shown in FIG. 4 is identical in construction to the 
oxygen sensor 10 of FIG. 1, and corresponding parts are designated by like 
reference numerals. The oxygen sensor 10b comprises electric heating means 
22 for heating the oxygen sensor to a predetermined temperature during an 
operation thereof, the heating means 22 being in the form of a pattern of 
a continuous resistance heating element formed on the outer surface of the 
housing 12 facing away from the oxygen ion conductive plate 14. The 
heating means 22 is connected to a power source (not shown) for being 
energized. 
A sealing material 16 which sealingly bonds an oxygen ion conductive plate 
14 to a diffusion housing 12 is made of glass containing SiO.sub.2, BaO, 
Na.sub.2 O and ZrO.sub.2. The sealing glass 16 is prepared by adding 
zirconia powder to amorphous glass containing SiO.sub.2, BaO and Na.sub.2 
O, the amorphous glass having an operating temperature of about 
1000.degree. C. and having a relatively high thermal expansion coefficient 
of 90.times.10.sup.-7 /.degree. C. which is relatively close to that of 
zirconia. A typical example of such amorphous glass commercially available 
is one sold by Iwaki Glass Co., Ltd. (Japan) under the tradename of #9010 
of which composition is represented in Table 1 in terms of both elements 
and oxides: 
TABLE 1 
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Si: 33.0% SiO.sub.2 : 
70.6% 
Ba: 9.6% BaO: 10.7% 
Na: 5.4% Na.sub.2 O: 
7.3% 
K: 4.7% K.sub.2 O: 
5.7% 
Al: 1.5% Al.sub.2 O.sub.3 
2.8% 
Li 0.7% Li.sub.2 O: 
1.5% 
Total: 98.6% 
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The above amorphous glass has the following characteristics: 
TABLE 2 
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Thermal expansion coefficient 
96 
(10.sup.-7 /in/.degree.C. (50 to 100.degree. C.) 
Operating temperature (.degree.C.) 
1000 
Softening point (.degree.C.) 
665 
Annealing point (.degree.C.) 
445 
Distortion point (.degree.C.) 
410 
Density (gr/cc) 2.60 
Volume resistance (log.sub.10 .rho.) 
25.degree. 
C. 17 
(.OMEGA..sup.-cm) 150.degree. 
C. 12 
350.degree. 
C. 7 
Dielectric constant 
1 MC 7 
3 MC 7 
tan .delta. .times. 10.sup.4 
1 MC 20 
3 MC 50 
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The amount of addition of zirconia to such amorphous glass to form the 
sealing glass material 16 should be 1 to 30% of the total weight of the 
glass and the zirconia. If the zirconia content is less than 1%, desired 
results can not be achieved. On the other hand, if this content exceeds 
30%, the vitrification of the mixture of the amorphous glass and zirconia 
is adversely affected. Preferably, the zirconia powder to be added to the 
amorphous glass should have a particle size of not more than 3 .mu.m. 
The zirconia powder added to the amorphous glass containing SiO.sub.2, BaO 
and Na.sub.2 O is dispersed therein to serve as cushioning means for 
absorbing shock applied to the resultant sealing glass material. Thus, the 
zirconia powder so added serves to reinforce the sealing glass material. 
The invention will now be illustrated by way of the following Example: 
EXAMPLE 1 
10% by weight of zirconia powder was added to amorphous glass shown in 
Tables 1 and 2 to form a sample glass material A of this invention. 
According to the same procedure, a sample glass material B of this 
invention having a zirconia content of 15% and a sample glass material C 
of this invention having a zirconia content of 20% were prepared. The 
above amorphous glass with no zirconia content was used as a comparative 
sealing glass material D. 
Each of the sealing glass materials A, B, C and D was used to provide an 
oxygen sensor similar to that shown in FIG. 4. The diffusion housing was 
made of zirconia solid solution partially stabilized by 3 mol. % of 
Y.sub.2 O.sub.3 having a thermal expansion coefficient of 
92.times.10.sup.-7 /.degree. C. The oxygen ion conductive plate was made 
of solid electrolyte of zirconia stabilized into a generally cubic crystal 
structure by 8 mol. % of Y.sub.2 O.sub.3 having a thermal expansion 
coefficient of 100.times.10.sup.-7 /.degree. C. 
10 samples were tested with respect to each of the sealing glass materials 
A to D to determine how many samples out of ten were subjected to a crack 
both at the time when the sealing glass material was initially heated from 
room temperatures to 400.degree. C. and at the time when the sealing glass 
was subjected to 100 cycles of heat shock from 400.degree. C. to room 
temperatures. The results obtained are shown in Table 3 below. 
TABLE 3 
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Sample 
(room temp. to 400.degree. C.)Initial temp. rise 
##STR1## 
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A 0/10 0/10 
B 0/10 0/10 
C 0/10 0/10 
D 3/10 7/10 
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As can be seen from Table 3, none of the sample glass materials A, B and C 
were subjected to a crack both at the initial temperature rise from room 
temperatures to 400.degree. C. and during 100 cycles of heat shock from 
400.degree. C. to room temperatures. On the other hand, three out of ten 
comparative sample glass material D were subjected to a crack at the 
initial temperature rise, and seven out of ten comparative glass material 
D underwent a crack during 100 cycles of heat shock. 
One example of the amorphous glass containing SiO.sub.2, BaO and Na.sub.2 O 
has been shown in Table 1, but the content of each component of this glass 
can be varied in the range of .+-.5%. 
SiO.sub.2 is a major component of the amorphous glass and cooperates with 
BaO to promote the solidification rate. Na.sub.2 O renders the glass soft 
so that the glass can be processed easily, but it has a relatively high 
thermal expansion coefficient. However, the addition of K.sub.2 O lowers 
the thermal expansion coefficient. Al.sub.2 O.sub.3 also lowers a thermal 
expansion coefficient and increases a weathering resistance and a 
resistance to acids and alkalis. LiO.sub.2 tends to enhance crystallinity 
and therefore improves a heat resistance. 
According to the present invention, zirconia (ZrO.sub.2) is added to the 
above amorphous glass to provide the sealing glass material. The diffusion 
housing and the oxygen ion conductive plate which are bonded together 
through the sealing glass material are both made of zirconia-based 
material, and therefore the sealing glass material provides for higher 
bonding ability and heat resistance. 
Referring again to FIG. 4, the oxygen ion conductive plate 14 has a 
thickness H of 0.08 to 0.8 mm. Each of the electrodes 18 has a diameter Dd 
of not more than 7 mm. The gas diffusion aperture 12a has a diameter Dh of 
10 to 50 .mu.m. The oxygen sensor 10b is maintained at temperatures of 
about 350.degree. to 450.degree. C. by the heating means 22 during the 
operation thereof. 
The thickness H of the oxygen ion conductive plate 14 is linearly increased 
with the increase of power consumption W as indicated in a dash-and-dot 
line in FIG. 5, but the minimum operative temperature T becomes generally 
constant when the thickness H exceeds 0.8 mm as indicated in a solid line 
in FIG. 5. The minimum operative temperature T means the lower limit of 
the temperature range in which the diffusion limited current can be probed 
in the atmosphere (in which the oxygen concentration is almost constant). 
Therefore, in view of heating efficiency, the thickness H should not be 
more than 0.8 mm. More preferably, the thickness H should be about 0.5 mm 
in view of the strength and fabrication of the oxygen ion conductive plate 
14. The thickness H should not be less than 0.08 mm. The oxygen ion 
conductive plate 14 is made of sintered ceramics, and if the thickness H 
is below 0.08 mm, the plate 14 is rendered impractical because of the 
presence of the voids or pores between the particles of the sintered 
ceramics. 
When the oxygen sensor 10b operates at temperatures of around 500.degree. 
C., the electrodes 18 of platinum and the oxygen ion conductive plate 14 
of solid electrolyte are much deteriorated, so that the oxygen sensor 10b 
can not be used for a long period of time. It has now been found that such 
deterioration will not occur when the oxygen sensor 10b operates at 
temperatures of not more than 450.degree. C. Also, it has been found that 
the minimum operating temperature of the oxygen sensor 10b is 300.degree. 
C. If the operating temperature is less than 300.degree. C., the oxygen 
ion conductive ability is lowered to such an extent that the oxygen sensor 
10b does not function properly. 
More specifically, the practical operating temperature of the oxygen sensor 
10b is seen from FIGS. 6 and 7. The oxygen sensor 10b must be heated to 
the above minimum operating temperature in order to achieve a satisfactory 
response. The satisfactory response is achieved when the change of oxygen 
concentration by 90% is indicated within 10 seconds. 
FIGS. 6 and 7 show the relation between the response and operating 
temperature of the oxygen sensor 10b. 
FIG. 6 shows the relation between response time t.sub.90 (i.e., the time 
required before the oxygen sensor indicates the occurrence of the change 
of oxygen concentration by 90%.) and the value of diffusion limited 
current. The depth or length of the gas diffusion aperture 12a is 
predetermined, that is to say, 1 mm, and therefore the value of the 
diffusion limited current is determined in accordance with the diameter Dh 
of the gas diffusion aperture 12a. The response time is determined solely 
in accordance with the operating temperature of the oxygen sensor 12a when 
the diameter Dh of the gas diffusion aperture 12a is more than a 
predetermined value which is about 10 .mu.m. Therefore, it is necessary 
that the operating temperature should not be less than 370.degree. C. to 
achieve the satisfactory practical response. 
FIG. 7 also shows the relation between the response time and the operating 
temperature (i.e., the temperature of the oxygen sensor 10b). In FIG. 7, a 
broken line indicates the relation when the diameter Dh of the gas 
diffusion aperture 12a is 100 .mu.m, and a solid line indicates the 
relation when the diameter Dh is 30 .mu.m. As can be seen from FIG. 7, the 
response time will not be changed substantially even if the value of the 
diameter Dh is varied substantially. This indicates that the essential 
factor in determining the response time is not the diameter Dh of the gas 
diffusion aperture 12a which corresponds to a diffusion resistance but the 
operating temperature of the oxygen sensor 10b. As can be seen from FIG. 
7, in order to achieve the satisfactory practical response, the operating 
temperature should be more than about 370.degree. C. as is the case with 
the relation shown in FIG. 6. Actually, it has been found that the 
operating temperature of not less than 350.degree. C. can achieve 
substantially the same response. 
FIG. 8 shows the relation between the minimum operative temperature and the 
diameter Dh of the gas diffusion aperture 12a, with the length of the 
aperture 12a being 1 mm. The upper limit of the operating temperature is 
450.degree. C. as described above. In order that 450.degree. C. may be the 
minimum operative temperature, the diameter Dh is not more than about 50 
.mu.m. With respect to the lower limit of the operating temperature, the 
diameter Dh is about 10 .mu.m as described above for the response. 
Therefore, the diameter Dh should be about 10 to about 50 .mu.m. 
FIG. 9 shows the relation between the minimum operative temperature and the 
diameter Dd of each electrode 18 (solid line), the power consumption being 
indicated in a dash-and-dot line. As the electrode diameter Dd increases, 
the power consumption required increases. However, when the electrode 
diameter Dd exceeds about 7 mm, the minimum operative temperature becomes 
generally constant and will not be further lowered. Therefore, the 
electrode diameter Dd should not be more than about 7 mm.