Multilayered air-fuel ratio sensing element

Details Multilayered air-fuel ratio sensing element Multilayered air-fuel ratio sensing element

1999-01-27

2001-07-10

Warden, Sr., Robert J. (Department: 1744)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C501S103000, C501S134000, C501S152000

Reexamination Certificate

active

06258233

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a multilayered air-fuel ratio sensing element preferably used for the air-fuel ratio control of internal combustion engines for automotive vehicles.
From the recent trend toward shortened sensor activation time and the positional restriction in installing the sensor (for example, installation to the exhaust gas pipe under a vehicle floor panel), improvement of the sensor warmup ability as well as downsizing of the sensor body are important goals to be attained.
Multilayered air-fuel ratio sensing elements, including united sensing and heating portions, have prospective properties to satisfy these requirements.
From the view point of electric insulation and heat transfer, conventionally proposed multilayered air-fuel ratio sensing elements generally comprise a heater-equipped alumina substrate and an oxygen ion conductive solid electrolytic body which are laminated integrally and sintered together. As having sufficient strength and excellent oxygen ionic conductivity, the partially stabilized zirconia is generally used as the oxygen ion conductive solid electrolytic body.
However, the multilayered air-fuel sensing elements have the following drawbacks because of their structural features including the different members (i.e., alumina and partially stabilized zirconia). When the sensing element is sintered in the manufacturing process or heated in the actual operating environment, a significant amount of thermal stress concentrates at the boundary between the alumina and the partially stabilized zirconia due to thermal expansion difference between them. This thermal stress triggers the cracks.
Enhancing the composition of the partially stabilized zirconia as well as increasing the strength and controlling the thickness of the alumina substrate will be effective to suppress the cracks from generating during the sintering step for manufacturing the sensing element from laminated green sheets of the alumina substrate and the solid electrolytic body (refer to the U.S. Pat. No. 5,447,618).
However, when the multilayered air-fuel ratio sensing element is installed in the internal combustion engine of an automotive vehicle, cracks may appear by the following mechanism.
The partially stabilized zirconic solid electrolytic body has a mixed phase structure including three different crystal structures referred to as a cubic (C) phase, a monoclinic (M) phase and a tetragonal (T) phase, with a small amount of additives. According to this phase structure, the T phase can transform into the M phase through the isothermal martensitic transformation (refer to T→M transformation).
The T→M transformation progresses rapidly when the partially stabilized zirconia is exposed to an atmosphere of approximately 200° C. Presence of water (e.g., moisture or vapor) promotes the T→M transformation. Furthermore, the T→M transformation causes a volumetric change.
The operating environment of the air-fuel ratio sensing element incorporated in the automotive internal combustion engine can be regarded as repetitive heating and cooling cycles in a temperature range from the room temperature (20° C.) to the exhaust gas temperature (1,000° C.). The exhaust gas contains a large amount of vapor. Under such environment, the T→M transformation progresses smoothly.
When the T→M transformation occurs in the solid electrolytic body, cracks will appear along the boundary between the solid electrolytic body and the alumina substrate or along the surface of the solid electrolytic body.
SUMMARY OF THE INVENTION
In view of the foregoing problems encountered in the prior art, the present invention has an object to provide a multilayered air-fuel ratio sensing element causing no cracks even when it is subjected to severe heating and cooling cycles under a high humid environment.
In order to accomplish the above-described and other related objects, the present invention provides a multilayered air-fuel ratio sensing element comprising a zirconic solid electrolytic body and a heat-generating portion, wherein the zirconic solid electrolytic body is made of a partially stabilized zirconia containing 5~7 mol % yttria and having a mixed phase structure including a cubic (C) phase, a monoclinic (M) phase and a tetragonal (T) phase. The zirconic solid electrolytic body has a relative density of 94~100%, with a mean sintered grain size R
ZR
of 0.5~3.0 &mgr;m. The heat-generating portion includes an alumina substrate which is located adjacent to the zirconic solid electrolytic body and has a relative density of 95~100% with a mean sintered grain size R
AL
of 0.5~4.0 &mgr;m. And, the partially stabilized zirconia has an M/C ratio in a range from 0.05 to 0.25. The M/C ratio is defined by the following equation:
M
C
=
M
⁡
(
111
)
+
M
⁡
(
11
⁢
 
⁢
1
_
)
M
⁡
(
111
)
+
M
⁡
(
11
⁢
 
⁢
1
_
)
+
C
⁡
(
111
)
wherein M(11{overscore (1)}) represents a reflective integrated intensity of a monoclinic phase(11{overscore (1)}); M(111)represents a reflective integrated intensity of a monoclinic phase (111); and C(111) represents a reflective integrated intensity of a cubic phase (111).
The zirconic solid electrolytic body is made of the partially stabilized zirconia. When the yttria content in the partially stabilized zirconia is out of the range of 5~7 mol %, the thermal expansion difference between the zirconic solid electrolytic body and the alumina substrate increases, while causing a stress acting on the alumina substrate. Thus, cracks appear on the alumina substrate.
When the relative density of the zirconic solid electrolytic body is in a range from 0 to 94%, the zirconic solid electrolytic body may loose gas tightness (i.e., may have poor gas permeability).
As described later, to detect an air-fuel ratio of the measuring gas, the zirconic solid electrolytic body is provided with at least a pair of electrodes. One of the paired electrodes is exposed to the measuring gas, while the other electrode is exposed to the reference gas. When the solid electrolytic body is not gas tight, the measuring gas may mix with the reference gas. In this case, the air-fuel ratio cannot be measured accurately.
Furthermore, the solid electrolytic body will be deteriorated in strength.
In view of strength and ionic conductivity, it is preferable that the allowable upper limit of the relative density is 100%.
When the mean sintered grain size R
ZR
of the zirconic solid electrolytic body is in a range from 0 to 0.5 &mgr;m, it is difficult in the manufacturing of the zirconic solid electrolytic body to attain the relative density of 94% or more even if the industrially obtainable finest material is used. Thus, a gas tight and strong zirconic solid electrolytic body cannot be obtained.
When the mean sintered grain size R
ZR
exceeds 3.0 &mgr;m, a large volumetric change occurs in accordance with the T→M transformation of the T-phase crystal particles in the sintered body. The produced internal stress may concentrate at the grain boundary, causing cracks in the solid electrolytic body.
When the relative density of the alumina substrate is in a range from 0 to 95%, the alumina substrate will be deteriorated in strength. A thermal stress derived from the thermal expansion difference between the alumina substrate and the zirconic solid electrolytic body will cause cracks.
In view of electric insulation, it is preferable that the allowable upper limit of the relative density is 100%.
When the mean sintered grain size R
AL
of the alumina substrate exceeds 4.0 &mgr;m, cracks may appear when the alumina substrate is subjected to repetitive heating and cooling cycles.
Although the reasons are not clear, it is generally assumed that a thermal stress derived from the thermal expansion difference concentrates at the boundary between the zirconia and the alumina when there is a large difference in the mean sintered grain size between the zirconia and the alumina. This develops fine cracks.
When the mean sintered grain size R
AL
is in a range from 0 t

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