Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to resistance-type
oxygen sensors. Gas sensors are used in internal
combustion exhaust systems for determining and
controlling the air/fuel ratio of the engine system.
Generally, such sensors are of two types, a solid
electrolyte type, such as zirconium dioxide, where
a solid electrolyte is used to generate a voltage
and a resistance type, such as titanium dioxide,
where a change in ohmic resistance is monitored.
Prior Art
In zirconium dioxide type gas sensors, a
solid electrolyte is used in which the sensor operates
; on the principle of an oxygen concentration cell
with ionic conductivity effected through the solid
electrolyte with conductors on both sides of the
cell used to measure cell voltage which is related
to the partial pressure of the oxygen and thus
determine the oxygen content of exhaust gas as
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compared to a reference gas, such as the atmosphere.
i 20 Various conductors have been proposed for use on
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both sides of the solid electrolyte, with platinum
one of the preferred conductors. When platinum is
used as a conductor on the exhaust gas side of a
solid electrolyte, it has been found to exhibit
catalytic activity to effect oxidation of carbon
monoxide and residual hydrocarbons in the exhaust gas,
as well as act as a conductor. Illustrative of such
zirconium dioxide sensors and the use of platinum
as a conductive means on the external surface of the
sensor are the sensors described in the following
U.S. Patents:
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1088626
U.S. Patent No. _ssue Date Patentee
3,645,875 February 29, 1972 Record et al.
3,978,006 August 31j 1976 Robert Bosch
G.m.b.H.
3,99~,375 December 21, 1976 Rudd.
These sensors teach the need for a conductive coating
or film of platinum on the external surface of the
sensor and various means of forming such coatings and
protecting the coatings from the harsh environment to
which they are subjected so as to ensure the conductive
nature of the coating. Without such conductivity,
the sensor would be inoperative. The teachings of
U.S. Patent No. 3,941,673, issued March 2, 1976 to
Nissan Motor Co., also relate to zirconium type oxygen
sensors where a conductive layer or second electrode
is used with a noncatalytic material used for the
electrode such as gold, silver or a platinum electrode
containing a substance such as lead, sulfur, phosphorous,
arsenic or their compounds which function as a
catalyst poison. In such a system, the layer serves
as a conductor only, without evidencing catalytic
activity.
In titanium oxide type sensors the sensor
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exhibits an electrical resistance which, at elevated
temperatures, varies as a function of the partial
pressure of oxygen in the exhaust. Such titanium
dioxide sensors often use heating elements either
integral with the sensor or associated therewith in
order to heat the sensor to an operating temperature
where the resistance can be readily monitored. -
Generally, such titanium oxide type sensors must be
heated to a temperature of about 400-450 C before
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they begin to function and, unless heating is provided,
such as by a heating element, the sensor would not be
operative in an exhaust gas system until the exhaust
gases are at or above that elevated temperature to
heat the sensor. Warm up time is then needed before
readings can accurately be made of the composition
of the exhaust gas. The titanium oxide type sensors
are exemplified by the disclosures of the following
U.S. Patents all issued to the Ford Motor Co.: U.S.
Patent No. 3,886,785, issued June 3, 1975, which
teaches a sintered ceramic body of such material,
U.S. Patent No. 3,868,846, issued March 4, 1975, which
uses a sensor in connection with a heating element
to heat the sensor to a most efficient operating
tempe,rature of about 600 to 900 C, U.S. Patent No.
~; 3,936,794, issued February 3, 1976, which employs a -
heater wire in association with the sensor in a probe
, design, and U.S. Patent No. 3,933,028, issued
January 20, 1976, which teaches a cobalt monoxide
ceramic resistance material in connection with a
Z heating coil to raise the temperature of the sensor
to operating temperature.
As discussed in the article entitled "TiO2 as
an Air-to-Fuel Ratio Sensor for Automobile Exhausts"
by T. Y. Tien, H. L. Stadler, E. F. Gibbons and P. J.
Zacmanidis, Ceramic Bulletin 54(3), p. 280 (1975) at
page 281 thereof, the titanium dioxide, itself, when
used in an oxygen sensor, acts as a catalyst to
; catalyze the reaction between carbon monoxide and
oxygen at the solid-gas interface thereof. ~ith such -
: catalysis effected by titanium dioxide, there is no
need for platinum or other catalysis for such a purpose.
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We have found, however, that by applying a
nonconductive catalyst layer, such as platinum, on
the titanium dioxide sensor, the temperature range
of operation of such a sensor is extended and that
the sensor will begin operation at temperatures as
low as 30QC, as compared with normal temperatures of
400-450 C for such sensors, and that the heating
elements generally associated with such sensors can
be eliminated.
Summary of the Invention
The present invention provides a resistive-
type oxygen sensing element and a sensor for determining
the partial pressure of oxygen within a combustion
gas mixture that is operative at temperatures below
that normally required for resistive-type sensors
and does not require a heating element as is normally
found with such sensors, and a method of forming
- such sensors.
In accordance with the invention there is
provided a sensor element for use in an oxygen sensor
of the resistive-type for measuring the partial
pressure of oxygen gas in combustion gas mixture
comprising a body of resistance-type ceramic sensor
material having conductive leads therein, a surface
; of the body having thereon a nonconductive layer of
a catalyst, the catalyst having a higher catalytic
activity than the resistance-type ceramic sensor
material for oxidation of carbon monoxide and hydro-
carbons in a combustion gas.
The invention also relates to an improvement
in a method for forming oxygen sensors of the
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10886Z6
resistive-type for sensing the partial pressure of
oxygen gas in a combustion gas mixture wherein a
sensor element is supported by a ceramic insulator
and conductive leads provided from the sensor
element to means for recording resistance changes
in said sensor element upon contact of a combustion
gas mixture with a surface of said element. The --invention comprises applying a nonconductive layer
of a catalyst to the sensor element surface, the
catalyst having a hlgher catalytic activity than
the sensor element for oxidation of carbon monoxide
and hydrocarbons in a combustion gas.
Brief Description of the Drawings
- Figure 1 is a cross-sectional view of an
oxygen sensor of the resistive-~ype of the present
invention and
Figure 2 is a graph illustrating the lower
temperature activation of sensors of the present
invention as compared with titanium dioxide sensor
~ 20 elements not having a catalyst layer thereon, both
; in the lean and rich air-to-fuel regions.
~; Detailed Description of the Preferred Embodiment
Referring to the drawing, an oxygen sensor 1
of the resistive-type for sensing the partial pressure
of oxygen in a combustion gas mixture is illustrated.
The sensor 1 comprises a resistance-type sensor
element 3 which is preferably a titanium dioxide element
. but which may also be composed of other resistive-
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1088626
type ceramic material, such as cobalt monoxide, which
resistive-type ceramic materials are known in the art.
The sensor element 3 is supported by a ceramic
insulator S which is formed of an insulating ceramic
material, such as mullite, a spinel, or preferably
alumina, A1203. The sensor element has on the surface 7
thereof, which is to be in contact with the combustion
gas, a nonconductive catalytic layer 9. The catalytic
layer is preferably platinum but may be of other catalytic
; 10 composition which is known to have a higher catalytic
activity than the resistance type ceramic sensor material
to effect oxidation of carbon monoxide and residual
hydrocarbons in combustion gases, such as a palladium
catalyst or the like.
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Conductive leads 11, such as thin platinum wires,
are disposed in the sensor element 3 which pass through
the ceramic insulator 5 and are conductively coupled to
terminals 13 through a conductive seal 15, such as a
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conductive glass seal, for example, the type of borosili-
cate glass seal described in U. S. Patents 3,959,765,
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and~4,001,758, issued May 25, 1976 and January 4, 1977
to the Ford Motor Co., which is provided to seal the
ceramic insulator 5 and the ceramic housing 17, which
ceramic housing is of the same or similar insulative
ceramic as the ceramic insulator 5. The ceramic housing
has a center bore 27 and an enlarged bore 29 in which
the ceramic insulator 5 is situated. Terminals 13 are
then, as is conventional, electrically coupled to a
resistance monitoring device (not shown) to record
- 30 resistance changes in the sensor element 3 and thus
monitor the partial pressure of oxygen in the combustion
gas to which the surface 7 of the sensor element is exposed.
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1~8862G -
To complete the sensor unit a metal housing 19 is
provided to encase a portion of the ceramlc housing 17, with a
hot press section 21 and a gasket seal 23 provided, which
sealing means are known in the art for encasing such ceramic
sections in metallic housings. A threaded portion 25 is pro-
vided for securing the sensor into the wall of a combustion
gas conduit,
As illustrated in Figure 2, sensor elements composed
- of titanium dioxide were compared with sensor elementsof the
present invention wherein a titanium dioxide sensor has a non-
conductive catalyst layer thereon. The sensor elements were
composed of titanium dioxide, unlayered sensors designated as
"L" and "R" and sensors with a nonconductive platinum catalyst
; layer thereon designated as "L-P" and "R-P." As illustrated,
both in the lean range, i.e a combustion gas composition
having a fuel lean value with respect to stoichiometry, and in
the rich range, i.e. a combustion gas composition having a fuel
rich value with respect to stoichiometry, the unlayered sensors
"L" and "R" did not exhibit resistance readings until the tem-
perature was in the range of about 400C. In the sensors of
the present invention, however, as illustrated, resistance
readings were exhibited at temperatures below 300C, indicating
the effect of the catalytic layer upon the sensor activity.
In a method for forming a resistive-type oxygen sensor
for sensing the partial pressure of oxygen in a combustion gas
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mixture, a prefired ceramic insulator 5 containing
platinum lead wires 11 is fabricated and the combination
is refired into a ceramic housing 17 to form a ceramic
subassembly.
The housing 17 is formed with a center bore 27
through which the ceramic insulator passes, with a
portion there~f extending from the insulator, the bore
having an enlarged section 29 for insertion of terminals
as hereinafter described. When the ceramic subassembly
has been refired, a conductive sealant such as a conductive
glass seal 15 is provided and the sealant is heat softened
while terminals 13 are pressed therein to provide a
conductive coupling for the platinum wires 11 and terminals
13 as well as a seal between the ceramic insulator 5 and .
the ceramic housing 17. To the end of the ceramic
insulator S there is then provided the sensing element 3.
~i~ ( The sensing element 3 of a resistive-type compound,
. such as titanium dioxide, may be affixed to the ceramic
insulator 5 such as by plasma or flame spraying or other
deposition methods. Or, preformed resistive-type ceramic
sensors can be used, such as those described in the
. Ford Motor Co. U.S. Patents Nos. 3,932,246 and 3,893,230,
i issued January 13, 1976 and July 8, 1975, respectively,
wherein the platinum wires 11 would be sandwiched between
titanium dioxide tapes and the sensor fired into a chip
or wafer which would then be affixed to the ceramic
.: insulator.
` The sensor element 3 has applied to the surface 7
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thereof, which is the surface that is to be in contact
with the combustion gas mixture, a nonconductive layer 9
of a catalyst
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10886Z6 ::
which has a higher catalytic activity than the resistance-type
ceramic sensor material to catalyze the oxidation of carbon
monoxide and hydrocarbons in the combustion gas mixture. Such
catalysts are well known, such as platinum or palladium, with
platinum being preferred. The nonconductive catalyst layer
can be applied to the sensor chip by various known methods for
applying such catalysts, such as by vacuum deposition, by
painting with a platinum paste, or by depositing a chloro-
platinic acid solution on the sensor surface and heating the
surface to affix the platinum catalyst thereto. Where a pre-
formed chip or wafer of resistive-type ceramic material is used
as a sensor element, the platinum catalyst may be applied prior
to or after affixing of the chip to the ceramic insulator.
The ceramic subassembly, with the ~ensor affixed and
the catalyst layer applied thereto, is then secured within a
metal housing 19 by hot pressing section 21 and the unit sealed
by placement of gaskets 23 therebetween. The completed unit
is easily placed into a threaded orifice in a combustion gas
conduit wall by use of threaded section 25, while the terminals
13 are easily electrically coupled to a monitoring device for
determination of the oxygen content of combustion gases.
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