Oxygen sensing element

Low temperature operability of an oxygen sensing element, which comprises a solid electrolyte member composed of sintered material, and a means for providing a reference oxygen partial pressure, composed of a sintered product of a finely divided metal or metal-metal oxide mixture powder and being completely embedded within the solid electrolyte member, is improved by the incorporation of from 0.5 to 10% by weight, based on the total weight of the sintered product, of a finely divided platinum group metal powder in the sintered product of the reference oxygen partial pressure-providing means.

BACKGROUND OF THE INVENTION 
This invention relates to an oxygen sensing element capable of measuring 
partial pressures of oxygen in sample gases. More particularly, it relates 
to an oxygensensing element suitable for use in an exhaust gas purifying 
system wherein the contact of oxygen in an exhaust gas from an automobile 
internal combustion engine is measured, thereby to determine the content 
of unburnt hydrocarbons, carbon monoxide and nitrogen oxides in the 
exhaust gas and, based on the measurement results, the air-fuel ratio is 
appropriately adjusted so that the efficiency of a catalyst for purifying 
the exhaust gas is enhanced; or suitable for use in a device for measuring 
the concentration of oxygen in a molten metal in the course of metal 
refining. 
An oxygen sensor is an oxygen concentration cell having a structure such 
that electrodes are mounted on the opposite sides of a solid electrolyte 
composed of a sintered ceramic material capable of conducting an oxygen 
ion. An electromotive force is produced across the solid electrolyte by 
the difference between the partial pressures of oxygen in reference and 
sample gases contacting opposite sides of the solid electrolyte. The 
concentration of oxygen in the sample gas can be determined by measuring 
the electromotive force so produced. That is, as is well known, assuming 
that the partial pressures of oxygen in the reference and sample gases are 
PO.sub.2 (1) and PO.sub.2 (2), respectively, the electromotive force E 
produced between the electrodes on the opposite sides of the solid 
electrolyte is expressed by the following equation. 
##EQU1## 
wherein R is gas constant, T is absolute temperature and F is Faraday's 
constant. Thus, if the partial pressure of oxygen PO.sub.2 (1) in the 
reference gas is known, the partial pressure of oxygen PO.sub.2 (2) in the 
sample gas can be determined from the above-mentioned equation by 
measuring the electromotive force E. Conventionally, air is used as the 
reference gas. The reference gas may also be generated chemically by using 
a mixture of a metal and its oxide which produces an equilibrium partial 
pressure of oxygen. This reference gas-generating metal-metal oxide 
mixture is hereinafter referred to as "reference solid electrode" for 
brevity. 
However, the conventional oxygen sensors, wherein the reference solid 
electrode of a metal-metal oxide mixture is employed, are not advantageous 
compared with the oxygen sensors wherein air is used as the reference gas. 
This is because the former oxygen sensors do not successfully operate at a 
low temperature. That is, at a temperature lower than about 400.degree. 
C., the former oxygen sensors generate little or no electromotive force 
and the internal impedance thereof is undesirably increased together with 
an apparent reduction of the electromotive force. In order to overcome 
this defect, it has been proposed to provide an electrode layer on the 
interface between the metal-metal oxide mixture reference solid electrode 
and the solid electrolyte, which electrode layer is composed of an 
electrochemically active metal such as platinum. The electrode layer 
accelerates the conversion of oxygen ions to molecular or atomic oxygen 
according to the following formula and, thus, reduces the polarization 
occurring in the metal-metal oxide mixture. 
EQU 20-.fwdarw.02(or 20) +4e- 
Such as electrode layer is formed by chemical or electrical plating, 
ion-plating or the like. However, the formation of such an electrode layer 
is complicated, and it is difficult to avoid a variability of some 
performances such as the operating temperature, the response time and the 
internal resistance among the resulting oxygen sensors. 
Japanese Patent Publication (KOKAI) No. 9497/1976 discloses an oxygen 
sensing electrochemical cell having a structure such that a reference 
solid electrode of a metal-metal oxide mixture is completely enclosed 
within a solid electrolyte member having an electrode mounted on the 
exterior surface thereof. This oxygen sensing electrochemical cell does 
not have such a defect as is encountered in the above-mentioned oxygen 
sensing cell provided with an electrochemically active metal electrode 
layer on the boundary between the metal-metal oxide mixture reference 
medium and the solid electrolyte. This cell is, however, still not 
satisfactory in its operability at a low temperature. 
SUMMARY OF THE INVENTION 
The main object of the present invention is to provide improved oxygen 
sensing elements which exhibit a reduced internal impedance and a 
satisfactory low temperature operability and are capable of being 
manufactured without a substantial variability of performances among the 
resulting oxygen sensing elements. 
Other objects and advantages of the present invention will be apparent from 
the following description. 
In accordance with the present invention, there is provided an improvement 
in an oxygen sensing element which comprises a solid electrolyte member 
composed of sintered material, and a means for providing a reference 
partial pressure of oxygen, composed of a sintered product of a finely 
divided metal or metal-metal oxide mixture powder; the reference oxygen 
partial pressure-providing means being completely embedded within the 
solid electrolyte member and having a lead-out wire connected thereto, and 
the solid electrolyte member having an electrode or electrodes mounted on 
the exterior surface thereof. The improvement of the present invention 
resides in the fact that the sintered product of the reference oxygen 
partial pressure-providing means has dispersed therein a minor amount of a 
finely divided platinum group metal powder.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1 there is illustrated a schematic section of an 
embodiment of the oxygen sensing element of the invention. The oxygen 
sensing element 1 has a means 3 for providing a reference partial pressure 
of oxygen, composed of a sintered body of a finely divided metal or 
metal-metal oxide mixture powder, i.e. a reference solid electrode. The 
reference solid electrode 3 is totally embedded within a solid electrolyte 
member 4, and has an electrode lead-out wire 2 connected thereto, made of 
a thermally resistant electroconductive metal such as platinum or a 
platinum-rhodium alloy. The solid electrolyte member 4 has metal 
electrodes 5 and 6 on the exterior surface thereof. The electrode 5 is an 
auxiliary electrode for transmitting therethrough an output signal from 
the reference solid electrode 3 to an electrical measuring circuit (not 
shown in FIG. 1). The electrode 6 is a porous metal electrode to be 
exposed to a sample gas. Electrode lead 7-8 is intended to transmit 
therethrough an output signal from the electrode 6 to the electrical 
measuring circuit. The oxygen sensing element 1 may be of any desired 
shape, such as, for example, a disc, column, sphere or parallelopiped. Of 
these, a disc and column are desirable. 
The solid electrolyte member 4 may be composed of a solid electrolyte 
material conventionally used in oxygen concentration cells, such as 
zirconia (ZrO.sub.2). The solid electrolyte material is preferably a solid 
solution prepared by incorporating a minor amount of Y.sub.2 O.sub.3, CaO 
or MgO followed by sintering. An optimum solid electrolyte material is 
comprised of a sintered zirconia composition having incorporated therein 5 
to 10% by mole of Y.sub.2 O.sub.3. 
The reference solid electrode 3 is a sintered product of a finely divided 
metal or metal-metal oxide mixture powder. Even when the reference solid 
electrode is not made of a metal-metal oxide mixture but only metal, it 
can provide a reference partial pressure of oxygen, because the reference 
solid electrode accepts oxygen ions transmitted through the solid 
electroyte material during the operation of the oxygen sensing element 
and, thus, the metal is partially converted into metal oxide. The metal 
ingredients used for the preparation of the reference solid electrode 
include, for example, iron, molybdenum, chromium, tungsten, nickel, 
cobalt, silicon and manganese. 
The reference solid electrode 3 employed in the oxygen sensing element of 
the invention is characterized as being composed of the sintered product 
of a finely divided metal or metal-metal oxide mixture composition having 
incorporated therein a finely divided platinum group metal in an amount of 
from 0.5 to 10% by weight, preferably from 1.0 to 5.0% by weight, based on 
the total weight of the sintered product. When the amount of the platinum 
group metal is less than about 0.5% by weight, the intended purpose cannot 
be achieved. In contrast, when the amount of the platinum group metal 
exceeds about 10% by weight, the manufacturing cost increases, and both 
the reduction of the internal impedance and the improvement of the low 
temperature operability are not in proportion to the increase in the 
amount of the incorporated platinum group metal. 
The platinum group metal includes, for example, platinum, rhodium, 
palladium and iridium. These metals may be used either alone or in 
combination. Of these metals platinum is preferable. A mixture of from 1.0 
to 5.0% by weight, based on the weight of the reference solid electrode, 
of platinum, and not more than 2.0% by weight, particularly from 0.1 to 
0.5% by weight, based on the weight of the reference solid electrode, of 
rhodium is more preferable. 
It is presumed that the platinum group metal dispersed in the reference 
solid electrode catalytically accelerates the electrode reaction, i.e., 
the conversion of oxygen ions, transmitted through the solid electrolyte, 
into molecular or atomic oxygen due to the oxidative effect of the metal 
constituting the reference solid electrode. Thus, the oxygen sensing 
element of the present invention exhibits a reduced internal impedance and 
a good low temperature operability, which are comparable with ormore 
satisfactory than those of the conventional oxygen sensing element having 
an electrode layer of an electrochemically active metal on the interface 
between the reference solid electrode and the solid electrolyte. 
Furthermore, the oxygen sensing element of the invention has a simple 
structure and exhibits little or no variability of performances such as 
the operating temperature, the response time and the internal resistance. 
Referring to FIG. 2, there is disclosed a graph showing the dependence of 
the electromotive force upon the temperature of a sample gas. The ordinate 
and the abscissa represent the electromotive force in volts and the 
temperature of a sample gas in .degree.C., respectively. In FIG. 2, curves 
A and B correspond to the oxygen sensing elements of the invention (the 
reference solid electrodes of A and B have dispersed therein, 
respectively, 1.0% by weight of platinum and a mixture of 1.0% by weight 
of platinum and 0.1% by weight of rhodium). Curves C and D correspond to, 
respectively, an oxygen sensing element having a reference solid electrode 
having no platinum group metal dispersed therein and an oxygen sensing 
element utilizing air as the reference gas. Apparently, the oxygen sensing 
elements of the invention (curves A and B) are advantageous over the 
conventional ones (curves C and D) in operability in a low temperature 
region. 
FIG. 3 is a graph showing the relationship of load current to terminal 
voltage of oxygen sensing elements. The ordinate and the abscissa 
represent the terminal voltage (volt) and the load current 
(.times.10.sup.-3 mA), respectively. Curves A, B, C and D correspond to 
oxygen sensing elements identical to those mentioned with reference to 
FIG. 2. These curves show that the terminal voltage of the oxygen sensing 
elements of the invention (curves A and B) decrease to a lesser extent as 
the load current increases, than the conventional, oxygen sensing elements 
(curves C and D) decrease. 
The results shown in FIGS. 2 and 3 were obtained on oxygen sensing elements 
each having a reference solid electrode made of a iron-iron oxide mixture. 
However, approximately similar tendencies were observed on oxygen sensing 
elements each having a reference solid electrode made of another 
metal-metal oxide mixture. 
The reference solid electrode may preferably contain, in addition to the 
metal or metal-metal oxide mixture ingredient and the platinum group metal 
ingredient, an appropriate amount of an antisintering material. The 
antisintering material used includes, for example, stabilized zirconia, 
which is usually identical to that used for the solid electrolyte 
material, and alumina, alumina-magnesia, silica and alumina-silica. These 
antisintering materials may be used either alone or in combination. The 
amount of the antisintering material may be in the range of from 5 to 70% 
by weight based on the total weight of the reference solid electrode. By 
the incorporation of the antisintering material, it can be avoided that 
the reference solid electrode is sintered to an excessive degree in the 
sintering step, and the thermal shrinkage of the reference solid electrode 
can be made to be the same as that of the solid electrolyte. Thus, the 
distortion of the oxygen sensing element and the separation of electrodes 
therefrom can be completely avoided. 
The oxygen sensing element of the present invention is manufactured in 
various ways. For example, a finely divided metal or metal-metal oxide 
mixture powder having incorporated therein predetermined amounts of a 
platinum group metal and optional, other additives is press-molded to form 
a reference solid electrode. Then, the reference solid electrode is 
encapsulated within a solid electrolyte member by forming the solid 
electrolyte member on the exterior surface of the reference solid 
electrode by vapor deposition, ion plating, sintering and sputtering, as 
disclosed in Japanese Patent Publication (KOKAI) No. 9497/1976. Finally, 
the so formed product is sintered. 
In another more preferable technique, a part of the amount of a finely 
divided solid electrolyte material, required for the formation of the 
solid electrolyte member is press-molded to form a provisional solid 
electrolyte member having a hole in which a reference solid electrode is 
to be formed. Then, a predetermined amount of a finely divided metal or 
metal-metal oxide mixture composition having incorporated therein a 
platinum group metal and optional other additives is charged in the hole 
of the provisional solid electrolyte member followed by pressing the 
charged composition. In the recess of the provisional solid electrolyte 
member, formed by the pressing of the charged composition, the remaining 
part of the finely divided solid electrolyte material is heaped up. Then 
the heaped-up material is pressed to obtain a structure such that the 
reference solid electrode is completely encapsulated or embedded within 
the solid electrolyte member. Finally, the obtained structure is sintered. 
The mounting of the external electrode or electrodes on the exterior 
surface of the solid electrolyte member may be carried out by a 
conventional technique such as paste coating and baking, electrical or 
chemical plating or ion plating. 
It is preferable that the porous external electrode 6 (FIG. 1) to be 
exposed to a sample gas be coated with a porous layer having a magnesium 
spinel structure or another spinel structure composed of a thermal 
resistant metal oxide. Such a porous layer minimizes the deterioration of 
the porous external electrode caused by the phosphorus, lead and sulfur 
present in the exhaust gas from an automobile. 
The oxygen sensing element of the invention is advantageously used for 
measuring the content of oxygen, for example, in an exhaust gas from an 
automobile internal combustion engine or in a molten metal in the course 
of metal refining. It is particularly suitable for use in an exhaust gas 
purifying system wherein the content of oxygen in an exhaust gas from an 
automobile internal combustion engine is measured, thereby to determine 
the content of unburnt hydrocarbons, carbon monoxide and nitrogen oxides 
in the exhaust gas, and based on the measurement results, the air-fuel 
ratio is appropriately adjusted so that the efficiency of a catalyst for 
purifying the exhaust gas is enhanced. 
Referring to FIGS. 4A and 4B there is disclosed an embodiment of the oxygen 
sensor device useful for measuring the content of oxygen in an exhaust gas 
from an automobile internal combustion engine. The oxygen sensor device is 
fitted to the exhaust manifold in a manner such that the external platinum 
electrode 6 of an oxygen sensing element 1 is exposed to the exhaust gas. 
A casing 9 for protecting the oxygen sensing element 1 has a plurality of 
perforations through which the exhaust gas is allowed to flow. The output 
signals are transmitted from the respective electrodes through lead-out 
wires such as a platinum lead and to an electrical measuring circuit (not 
shown in FIGS. 4A and 4B). The output signal-taking out mechanism is 
electrically protected by an alumina tube 13, a Teflon tube 16 and an 
insulative tube 12 and is mechanically protected by metallic tubular 
members 14 and 15. 
The invention will be further illustrated by way of the following examples. 
EXAMPLE 1 
A mixture comprised of 70% by weight of a commercially available carbonyl 
iron powder and 30% by weight of an alpha-A1203 powder was uniformly 
blended with 1% by weight, based on the weight of the mixture, of a 
platinum powder of 325 mesh in particle size. One end of a platinum 
lead-out wire having a diameter of 0.5 mm was inserted into a mass of the 
platinum-added blend. Then, the blend was compression molded into a pellet 
of a columnar shape by using a hand press. The pellet was encapsulated 
with a ZrO2 powder having incorporated therein 8% by mole of Y203. The 
ZrO2 encapsulated pellet was pressed into a pellet of columnar shape by 
using a hand press. The so obtained pellet was sintered in an electric 
oven at a temperature of 1,400.degree. C. for three hours while a hydrogen 
(1% by volume)-argon (99% by volume) gaseous mixture was introduced in the 
oven at a rate of 1 liter/min. The upper and lower flat surfaces of the 
sintered columnar pellet were abraded by using a number 250 abrasive paper 
and degreased, and then, coated with a platinum paste, as illustrated in 
FIG. 1. The platinum paste-coated pellet was baked in an electric oven at 
a temperature of 600.degree. C. for 10 minutes to obtain an oxygen sensing 
element having external platinum electrodes mounted on the exterior 
surface thereof. 
Electromotive force characteristics of the oxygen sensing element were 
evaluated as follows. The oxygen sensing element was fitted to a tube in a 
manner such that one of the external electrodes of the element was exposed 
to the inner atmosphere of the tube. A gaseous mixture of 1% by volume of 
oxygen and 99% by volume of nitrogen was introduced into the tube at a 
rate of 2,000 ml/min. and a load of 1M ohm was imparted to the electrode, 
while the temperature of the gaseous mixture was elevated at a rate of 
10.degree. C./min. The dependence of the electromotive force of the oxygen 
sensing element upon the temperature of the element was determined by 
using a DC voltmeter having an input impedance of 1,000 M ohm. The results 
are shown in FIG. 2 (curve A) and Table I, below. 
EXAMPLE 2 
By a procedure similar to that mentioned in Example 1, an oxygen sensing 
element was manufactured and its electromotive force characteristics were 
evaluated. In this procedure, a mixture of 1% by weight of a platinum 
powder and 0.1% by weight of a rhodium powder was employed instead of 1% 
by weight of a platinum powder. The results are shown in FIG. 2 (curve B) 
and Table I, below. 
COMATIVE EXAMPLE 1 
By a procedure similar to that mentioned in Example 1, an oxygen sensing 
element was manufactured and its electromotive force characteristics were 
evaluated. In this procedure, no platinum powder was added to the carbonyl 
iron-alpha-alumina mixture. The results are shown in FIG. 2 (curve C) and 
Table I, below. 
COMATIVE EXAMPLE 2 
A zirconia powder stabilized with 8% by mole of Y.sub.2 O.sub.3 was molded 
to obtain a cup-shaped solid electrolyte member. Two platinum electrode 
layers of about one micron in thickness were formed on both surfaces of 
the solid electrolyte member by chemical plating and then electrical 
plating. A porous spinel (Al.sub.2 O.sub.3.MgO) coating layer of about 80 
microns in thickness was formed by a conventional procedure on one of the 
platinum electrode layers to be exposed to a sample gas. 
The electromotive force characteristics of the so obtained oxygen sensing 
element were evaluated by a procedure similar to that mentioned in Example 
1, wherein a gaseous mixture of 1% by volume of hydrogen and 99% by volume 
of nitrogen was employed as a sample gas. The results are shown in FIG. 2 
(curve D) and Table I, below. 
EXAMPLES 3 through 16 and Comparative Examples 3 through 9 
By a procedure similar to that mentioned in Example 1, oxygen sensing 
elements were manufactured and their electromotive force characteristics 
were evaluated. In this procedure, various mixtures comprised of 70% by 
weight of a metal selected from molybdenum, chromium, tungsten, nickel, 
cobalt, silicon and manganese and 30% by weight of alpha-alumina were 
employed instead of the carbonyl iron-alpha-alumina mixture. The results 
are shown in Table I, below. 
Table I 
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Ex. Temperature 
and Composition of reference solid electrode 
off 
Com. Anti- Platinum group 
element 
Ex. Metal sintering 
metal (wt. %) 
(.degree.C.) 
*1 *2 material Pt Rh *3 
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C 2 -*4 -- -- -- 437 
C 1 Fe .alpha.-Al.sub.2 O.sub.3 
-- -- 395 
30 wt% 
E 1 Fe .alpha.-Al.sub.2 O.sub.3 
1.0 -- 362 
30 wt% 
E 2 Fe .alpha.-Al.sub.2 O.sub.3 
0.1 341 
30 wt% 
C 3 Mo .alpha.-Al.sub.2 O.sub.3 
-- -- 392 
30 wt% 
E 3 Mo .alpha.-Al.sub.2 O.sub.3 
1.0 -- 368 
30 wt% 
E 4 Mo .alpha.-Al.sub.2 O.sub.3 
0.1 345 
30 wt% 
C 4 Cr .alpha.-Al.sub.2 O.sub.3 
-- -- 387 
30 wt% 
E 5 Cr .alpha.-Al.sub.2 O.sub.3 
1.0 -- 358 
30 wt% 
E 6 Cr .alpha.-Al.sub.2 O.sub.3 
0.1 342 
30 wt% 
C 5 W .alpha.-Al.sub.2 O.sub.3 
-- -- 386 
30 wt% 
E 7 W .alpha.-Al.sub.2 O.sub.3 
1.0 -- 361 
30 wt% 
E 8 W .alpha.-Al.sub.2 O.sub.3 
0.1 350 
30 wt% 
C 6 Ni .alpha.-Al.sub.2 O.sub.3 
-- -- 402 
30 wt% 
E 9 Ni .alpha.-Al.sub.2 O.sub.3 
1.0 -- 385 
30 wt% 
E10 Ni .alpha.-Al.sub.2 O.sub.3 
0.1 364 
30 wt% 
C 7 Co .alpha.-Al.sub.2 O.sub.3 
-- -- 395 
30 wt% 
E11 Co .alpha.-Al.sub.2 O.sub.3 
1.0 -- 366 
30 wt% 
E12 Co .alpha.-Al.sub.2 O.sub.3 
0.1 345 
30 wt% 
C 8 Si .alpha.-Al.sub.2 O.sub.3 
-- -- 410 
30 wt% 
E13 Si .alpha.-Al.sub.2 O.sub.3 
1.0 -- 387 
30 wt% 
E14 Si .alpha.-Al.sub.2 O.sub.3 
0.1 363 
30 wt% 
C 9 Mn .alpha.-Al.sub.2 O.sub.3 
-- -- 386 
30 wt% 
E15 Mn .alpha.-Al.sub.2 O.sub.3 
1.0 -- 367 
30 wt% 
E16 Mn .alpha.-Al.sub.2 O.sub.3 
0.1 351 
30 wt% 
______________________________________ 
*1 C: Comparative Example, E: Example 
*2 Metal used in the metalmetal oxide mixture 
*3 Temperature of oxygen sensing element when the oxygen sensing element 
generated an electromotive force equal to one half of the maximum 
electromotive force. 
*4 Reference gas is air 
It will be apparent from Table I and FIG. 2 that the oxygen sensing element 
of the invention exhibits improved low temperature operability, i.e., can 
produce a normal signal even in a low temperature region. Thus, when the 
oxygen sensing element of the invention is used in an exhaust gas 
purification system of an automobile internal combustion engine, the 
exhaust gas purification system starts the normal operation with a high 
accuracy immediately after the automobile engine starts to be driven. 
EXAMPLE 17 
This example illustrates the effect of an antisintering agent 
(alpha-Al.sub.2 O.sub.3) upon the high temperature durability of the 
oxygen sensing element. 
By a procedure similar to that mentioned in Example 1, oxygen sensing 
elements were manufactured wherein the following four compositions were 
separately employed for the preparation of reference solid electrodes. The 
weight of each composition was about 0.5 g. 
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COMPOSITION OF REFERENCE SOLID ELECTRODE 
(% by weight) 
Specimen No. 
Carbonyl iron 
.alpha.-Al.sub.2 O.sub.3 
Platinum 
______________________________________ 
17-1 99 0 1 
17-2 89 10 1 
17-3 69 30 1 
17-4 49 50 1 
______________________________________ 
Each oxygen sensing element was given a load of 1K-ohm and maintained at a 
temperature of 800.degree. C. in an air atmosphere by using an electric 
oven, and under such conditions, its terminal voltage was measured. The 
results are shown in FIG. 5. The period of time for which the terminal 
voltage was reduced to 0.5 volt was as follows. 
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HIGH TEMPERATURE DURABILITY 
Specimen No. Time period (hours) 
______________________________________ 
17-1 5 
17-2 27 
17-3 Longer than 30 
17-4 25 
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