JP2019105620A - Gas sensor - Google Patents

Abstract

To provide a gas sensor that suppresses a damage of both joints even if a thermal expansion coefficient of a constituent material of a protector is higher than the thermal expansion coefficient of the constituent material of the main metal fitting.SOLUTION: The gas sensor includes a sensor element, a metal tubular main bracket surrounding the sensor element, and a metal protector fixed to the main bracket, and a joint W is formed by joining the rear end 140b of the protector to the outer surface of the main bracket, the thermal expansion coefficient at 800°C of the constituent material of the protector is higher than the thermal expansion coefficient at 800°C of the constituent material of the main metal fittings, and the minimum distance t1 from the outer surface of the subject metal fitting to the outer surface of the protector and the minimum distance t2 from the outer surface of the subject bracket to the inner surface of the subject bracket satisfy the relationship of 0.6≤(t1/t2)≤2.0.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a gas sensor including a sensor element which is exposed to a detection gas and detects a specific gas component in the gas.

BACKGROUND ART Oxygen sensors and air-fuel ratio sensors that detect oxygen concentration in exhaust gas are known as gas sensors that perform fuel efficiency improvement and combustion control of internal combustion engines such as automobile engines.
As such a gas sensor, there is a structure in which the main metal fitting holds a sensor element having a detection unit for detecting the concentration of a specific gas at the tip end side, and covers the detection part protruding at the tip of the main fitting with a metal protector. It is known (for example, refer to patent documents 1).
However, such a gas sensor is exposed to high temperature exhaust gas, and in particular, the weld joint between the protector and the metal shell at the tip of the gas sensor becomes high temperature, and repeated thermal stress cycles occur after cooling. There is a risk of breakage of parts. So, in the gas sensor of patent document 1, the difference of the thermal expansion coefficient of a protector and a main metal fitting is made small, stress is reduced repeatedly, and the fracture | rupture of a welding connection part is suppressed.

Patent No. 3932881 (FIG. 1)

However, in recent years, the demand for raising the heat-resistant temperature of automobile parts has become severe, and it has become necessary to change the constituent material of the parts to a material having high heat resistance. In particular, it is conceivable to change the protector to a heat-resistant material (Inconel (registered trademark) alloy) since the protector located at the outermost part of the tip of the gas sensor is most required to have heat resistance, but there is a problem of high cost.
From such a thing, using an austenitic stainless steel which added Nb for example as a protector is mentioned as a protector which is excellent in heat resistance and cheap, for example, but this material has a thermal expansion coefficient of a metal shell (for example, SUS430) Higher than the coefficient of thermal expansion, the above-described repeated stress causes the protector to expand more than the metal shell, resulting in the problem of promoting fracture of the welded joint.

  The present invention has been made in view of the present situation, and even when the thermal expansion coefficient of the constituent material of the protector is higher than the thermal expansion coefficient of the constituent material of the metal shell, breakage of the joint portion between the two etc. It is an object of the present invention to provide a gas sensor in which

  The gas sensor according to the present invention includes a sensor element extending in an axial direction and having a detection unit on the tip end, and a metal cylindrical metal shell surrounding the periphery of the sensor element while causing the detection unit to protrude from its tip. A gas sensor comprising: a metal protector that accommodates the detection unit and is fixed to the metal shell, wherein a rear end portion of the protector is joined to an outer surface of the metal shell to form a joint. The thermal expansion coefficient at 800 ° C. of the constituent material of the protector is higher than the thermal expansion coefficient at 800 ° C. of the constituent material of the metal shell, and viewed from the outer surface of the metal shell from the outer surface of the metal shell The minimum distance t1 up to and the minimum distance t2 from the outer surface of the metal shell to the inner surface of the metal shell satisfy the relationship 0.6 ≦ (t1 / t2) ≦ 2.0.

According to this gas sensor, the ratio (t1 / t2) = 1 even if the thermal sensor that uses a material whose thermal expansion coefficient at 800 ° C. is higher than the thermal expansion coefficient of the metal shell for the protector is subjected to repeated heating and cooling cycles. Thus, the strength balance between the rear end portion of the protector and the metal shell at the joint portion can be appropriately held, and repeated stress and deformation of the metal shell can be reduced to suppress breakage of the joint portion and the like.
When the ratio (t1 / t2) is less than 0.6, the thickness of the rear end portion becomes thinner than the thickness of the metal shell, the strength thereof is reduced, and the repeated stress is increased. On the contrary, when the ratio (t1 / t2) exceeds 2.0, the thickness of the metal shell becomes thinner than the thickness of the rear end portion, and the strength thereof decreases, and the deformation of the metal shell and the breakage of the joint etc. Becomes noticeable.

The protector includes an inner protector arranged with a gap between the detection unit and the outer protector, and an outer protector arranged with a gap between the inner protector and the clearance, and the joint portion is formed behind the inner protector and the outer protector. The end portions may be formed in overlapping rear end joints.
According to this gas sensor, the present invention can be applied not only to a single protector but also to a dual protector having an inner protector and an outer protector.

In the gas sensor according to the present invention, the first inner surface of the metal shell in a region of the outer surface of the metal shell projected onto the inner surface of the metal shell is the rear of the metal shell connected to the rear end side of the first inner surface. It may extend radially outward from the end inner surface.
According to this gas sensor, compared to the case where the first inner surface of the metallic shell having the joint portion on the outer side is parallel (coplanar) with the inner surface on the rear end side, the sensor element is surrounded by the entire inner surface of the metallic shell. The space in the vicinity of the detection unit is increased, and the gas to be detected easily enters and leaves the detection unit, and the detection accuracy is improved. In addition, the protector can be made larger in diameter (away from the detection portion) as compared with the case where the first inner surface of the metal shell having the joint portion on the outer side is parallel to the rear end inner surface in the axial direction. Water can be further suppressed from the outer surface of the protector) to the sensor element.

In the gas sensor of the present invention, the tensile strength specified in JIS-G0567 at 800 ° C. of the constituent material of the protector may be higher than the tensile strength of the constituent material of the metal shell.
According to this gas sensor, the heat resistance of the protector located on the outermost side of the tip of the gas sensor is further improved, so the heat resistance of the entire gas sensor can be further improved.

In the gas sensor of the present invention, the constituent material of the protector may be austenitic stainless steel, and the constituent material of the metal shell may be ferritic stainless steel.
According to this gas sensor, it is possible to suppress damage or the like of the bonding portion while inexpensively improving the heat resistance of the entire gas sensor.

  According to the present invention, even when the thermal expansion coefficient of the constituent material of the protector is higher than the thermal expansion coefficient of the constituent material of the metal shell, a gas sensor can be obtained in which breakage or the like of the joint portion between the two is suppressed.

It is a sectional view which meets a longitudinal direction of a gas sensor concerning an embodiment of the present invention. It is the elements on larger scale of FIG. 1 in the circumference | surroundings of a junction part. It is a schematic diagram which shows the influence on various characteristics when changing ratio (t1 / t2) of the distance of the back end joint part in a junction part, and a main metal fitting. It is the figure which evaluated the failure | damage etc. of a junction when changing ratio (t1 / t2) of the distance of the back end junction part in an actual junction, and a metallic shell.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 is an overall cross-sectional view along the longitudinal direction of a gas sensor (oxygen sensor) 200 according to an embodiment of the present invention, and FIG. 2 is a partial enlarged view of FIG.
The gas sensor 200 is an oxygen sensor (full-range air-fuel ratio gas sensor) that detects the oxygen concentration in the exhaust gas of a car or various internal combustion engines.
The gas sensor 200 is an example in which a protector 140 described later forms a dual protector of the inner protector 143 and the outer protector 142.

In FIG. 1, the gas sensor 200 has a cylindrical metal shell 138 formed on the outer surface with a screw portion 139 to be fixed to the exhaust pipe, and an axial O direction (longitudinal direction of the gas sensor 200: vertical direction in the figure) The sensor element 10 in the form of an elongated plate, the cylindrical ceramic sleeve 106 and the ceramic holder 151 disposed so as to surround the radial direction on the rear end side of the sensor element 10, and the insertion hole 168 penetrating in the axial direction A ceramic separator 166 is disposed inside the front end side so as to surround the rear end of the sensor element 10, and five terminal fittings 21 disposed between the sensor element 10 and the separator 166 (see FIG. Only three are shown in 1), and the dual protector 140 fixed to the front end side of the metal shell 138 is provided.
Further, the detection unit 10 a at the tip of the sensor element 10 is covered with a porous protective layer 20 such as alumina.

  The metal shell 138 is made of stainless steel, has a through hole 154 penetrating in the axial direction, and is formed in a substantially cylindrical shape having a shelf 152 projecting inward in the radial direction of the through hole 154. The sensor element 10 is disposed in the through hole 154 so that the tip end portion including the detection unit 10 a of the sensor element 10 protrudes from the tip end of the sensor element 10. Furthermore, the shelf 152 is formed as an inward tapered surface having an inclination with respect to a plane perpendicular to the axial direction.

An annular ceramic holder 151 made of alumina, a powder-filled layer 153 (hereinafter also referred to as a talc ring 153), and a ring-shaped ceramic holder 151 surrounding the periphery of the sensor element 10 in the radial direction The above-mentioned ceramic sleeve 106 is laminated in this order from the front end side to the rear end side.
Further, a caulking packing 157 is disposed between the ceramic sleeve 106 and the rear end portion 158 of the metal shell 138. Then, the rear end portion 158 of the metal shell 138 is crimped so as to press the ceramic sleeve 106 toward the tip end via the crimp packing 157.

Further, the ceramic holder 151 is made of insulating ceramic (for example, alumina), is formed in a substantially short cylindrical shape, and has a tip-facing surface 151a formed in a tapered shape toward the tip. Then, the ceramic holder 151 is held in the metal shell 138 by pressing the ceramic holder 151 from the rear end side with the talc ring 153 while the portion near the outer periphery of the front end face 151 a is locked to the shelf 152 of the metal shell 138. Are positioned and clearance fit.
Further, on the front end side of the ceramic holder 151, there is formed a recess 151h which is recessed backward by surrounding the insertion hole of the sensor element 10.

On the other hand, as shown in FIG. 1, on the outer periphery of the tip end portion 138s (lower side in FIG. 1) of the metal shell 138, the tip portion (including the detection unit 10a) of the sensor element 10 protruding from the metal shell 138 is covered. A protector 140 which is a metal cylindrical double protector having a plurality of holes is attached by welding or the like.
The dual protector 140 includes a bottomed cylindrical inner protector 143 disposed between the detection unit 10a and the gap, and a bottomed cylindrical outer protector 142 disposed between the inner protector 143 and the gap. The rear end portions which become the opening portions of the inner protector 143 and the outer protector 142 overlap to form the rear end joint portion 140b, and both are connected.
Further, the rear end joint portion 140b is joined to the outer surface of the front end portion 138s of the metal shell 138 by welding (in the present embodiment, welding all around) to form a joint portion W.

  An outer cylinder 144 is fixed to the outer periphery of the metal shell 138 on the rear end side. The opening on the rear end side (upper side in FIG. 1) of the outer cylinder 144 is electrically connected to the five terminal fittings 21 (only three are shown in FIG. 1) of the sensor element 10 A rubber grommet (seal member) 170 having a lead wire insertion hole 170h through which five lead wires 146 (only three are shown in FIG. 1) are inserted is disposed.

  A separator 166 is disposed on the rear end side (upper side in FIG. 1) of the sensor element 10 protruding from the rear end portion 158 of the metal shell 138 so as to be separated from the metal shell 138. The separator 166 is disposed around a total of five electrode pads (not shown) formed on the main surface on the rear end side of the sensor element 10. The separator 166 is formed in a cylindrical shape having an insertion hole 168 penetrating in the axial direction, and is provided with a flange portion 167 projecting radially outward from the outer surface. The separator 166 is held inside the outer cylinder 144 by bringing the flange portion 167 into contact with the step portion of the outer cylinder 144 and caulking the outer cylinder 144 via the holding member 169.

  FIG. 2 shows a partially enlarged view of FIG. Here, the joint portion W is not only in contact with the outer surface of the metal shell 138 (the tip portion 138s thereof) and the rear end joint portion 140b, but also the outer surface of the metal shell 138 (the tip portion 138s) and the rear It refers to a portion where the end coupling portion 140b is integrated. Specifically, in the enlarged image of the cross section in the direction of the axis O of the portion where the outer surface of the metal shell 138 is in contact with the rear end coupling portion 140b, the boundary line B1 of the inner protector 143 and the outer protector 142 and the inner protector The part P where the boundary line B2 of 143 and the metal shell 138 (the tip part 138s of) has disappeared altogether is said. This is because portions where the materials of the inner protector 143, the outer protector 142, and the metal shell 138 are not integral separate from each other and thermally expand, so that repeated stress hardly occurs.

When viewed in a cross section including the joint W, the minimum distance from the outer surface of the metal shell 138 to the outer surface of the protector 140 is t1, and the minimum distance from the outer surface of the metal shell 138 to the inner surface of the metal shell 138 is t2.
Here, the outer surface of the protector 140 is the outermost surface of the entire protector 140, that is, the outer surface of the outer protector 142 if it is a dual protector.
Since the calculation of the distances t1 and t2 is based on the cross section including the joint W, the outer surface and the inner surface of the tip portion 138s of the metal shell 138 at the joint W serve as a reference for calculation of t2.
Further, in the joint portion W, since the rear end joint portion 140b and the metal shell 138 are integrated, an imaginary straight line connecting boundary lines B2 in contact with the front end side and the rear end side of the joint portion W I assume. Therefore, the distance t2 is the distance between the inner surface of the metal shell 138 at the joint W and the imaginary straight line. The distance t1 is the distance between the outer surface of the rear end joint 140b at the joint W and the imaginary straight line.

Here, in the present invention, the thermal expansion coefficient at 800 ° C. of the constituent material of the protector 140 (the inner protector 143 and the outer protector 142) is larger than the thermal expansion coefficient at 800 ° C. of the constituent material of the metal shell 138 . For example, in this example, the protector 140 is made of austenitic stainless steel to which Nb is added, and the metal shell 138 is made of SUS430LX (JIS standard) which is a ferritic stainless steel.
For this reason, when the gas sensor 200 is exposed to high temperature exhaust gas and the like and repeated stress is generated after cooling, combined with the protector 140 being located at the outermost end of the tip of the gas sensor 200, the protector 140 becomes a metallic shell. As described above, it expands more than 138 and promotes breakage or the like of the welded joint (joint W).

Therefore, in the present invention, the damage or the like of the joint portion W between the protector 140 and the metal shell 138 can be suppressed by controlling 0.6 ≦ (t1 / t2) ≦ 2.0. Furthermore, heat resistance of the entire gas sensor 200 can be improved by using a material having excellent heat resistance and being inexpensive but having a thermal expansion coefficient at 800 ° C. higher than that at 800 ° C. of the metal shell for the protector 140. Can.
The reason why the damage or the like of the joint W can be suppressed by managing the ratio (t1 / t2) will be described with reference to FIG.

FIG. 3 is a schematic view showing the influence on various characteristics when the ratio (t1 / t2) of the distance between the rear end joint portion 140b at the joint portion W and the front end portion 138s of the metal shell 138 is changed.
As a premise, in the present invention, t2 is smaller than that of the conventional metal shell, and is at most about 1.7 times t1 ((t1 / t2) = 0.6) at the maximum. For this reason, by changing the ratio of t2 and t1, the influence of the cyclic stress can be adjusted. On the other hand, in the conventional configuration in which t2 is significantly larger than t1 (for example, (t1 / t2) is 0.2 or less), the degree of deformation of the metal shell almost changes even if (t1 / t2) is changed Without adjustment, it is difficult to adjust the influence of repeated stress.

First, as shown by the solid line in FIG. 3, in the region sandwiching the ratio (t1 / t2) = 1, the distance t1 as the value of the ratio increases, that is, the thickness of the rear end joint portion 140b increases and the strength increases. To the strength of the end portion 138s of the metal shell 138. For this reason, the repeated stress of the bonding portion W due to the cooling and heating cycle is reduced, and breakage or the like of the bonding portion W is suppressed. Therefore, as far as focusing on the rear end coupling portion 140b, the larger the value of the ratio, the better.
On the other hand, the larger the ratio value, the thinner the distance t2, that is, the thickness of the tip portion 138s of the metallic shell 138, and the strength of the metallic shell 138 (tip portion 138s) decreases as shown by the broken line in FIG. The deformation of the metal shell 138 due to the stress, and hence the breakage of the joint portion W and the like increase. Therefore, as far as focusing on the metal shell 138, the smaller the ratio value, the better.
From such a thing, a ratio (t1 / t2) will have a suitable range on both sides of 1, and the concrete range of ratio was calculated | required by the experiment mentioned later.

When the ratio (t1 / t2) is less than 0.6, the thickness of the rear end joint portion 140b becomes thinner at the joint portion W compared with the thickness of the metal shell 138 and the strength thereof is reduced, and the repeated stress becomes large. Damage or the like of the joint W becomes remarkable.
Conversely, when the ratio (t1 / t2) exceeds 2.0, the thickness of the metal shell 138 becomes thinner at the joint W compared to the thickness of the rear end joint 140b, and the strength thereof decreases. The deformation and thus the breakage of the joint W and the like become remarkable.
The ratio (t1 / t2) is preferably 0.6 to 1.6, and more preferably 0.8 to 1.6. Further, the ratio (t1 / t2) is preferably 0.6 to 1.5, and more preferably 0.8 to 1.5. Further, the ratio (t1 / t2) is preferably 0.6 to 1.3, and more preferably 0.8 to 1.3.

In the present invention, as shown in FIG. 2, a region of the outer surface of the metal shell projected radially inward toward the inner surface of the metal shell 138 is R, and the metal shell 138 in the region R (tip portion 138s The inner surface of) is referred to as a first inner surface 138i1. At this time, the first inner surface 138i1 extends radially outward more than the rear end side inner surface 138i2 of the metal shell 138 connected to the rear end side of the first inner surface 138i1.
In this case, the space in the vicinity of the detection portion 10a of the sensor element 10 surrounded by the inner surface of the metal shell 138 is increased compared to the case where the inner surface 138i1 and the rear end inner surface 138i2 are parallel (coplanar) with the axis O direction. The gas to be detected easily enters and leaves the detection unit 10a, and the detection accuracy is improved. In addition, the protector 140 can be made larger in diameter (away from the detection unit 10a) as compared with the case where the inner surface 138i1 and the rear end side inner surface 138i2 are parallel (coplanar) with the axis O direction. The water from the outer surface of the sensor element 10 to the sensor element 10 can be further suppressed.
The inner surface of the metal shell 138 in the region R is an inner surface including the region R in the direction of the axis O.

  In the present invention, if the tensile strength specified in JIS-G0567 at 800 ° C. of the constituent material of the protector 140 is higher than the tensile strength of the constituent material of the metal shell 138, the outermost end of the gas sensor 200 Since the heat resistance of the protector 140 located in the above further improves, the heat resistance of the entire gas sensor 200 can be further improved.

It is needless to say that the present invention is not limited to the above embodiments, but extends to various modifications and equivalents included in the spirit and scope of the present invention.
The shape of the protector or the metal shell to which the protector is joined is not limited to the above. For example, in the above embodiment, the protector is a dual protector of an inner protector and an outer protector, but may be a single (single) protector.
The joint is not limited to welding, but welding is preferred.
Moreover, as a gas sensor, in addition to an oxygen sensor and a full range gas sensor, a NOx sensor may be mentioned. It may be a cylindrical sensor element.
Preferably, the constituent material of the metal shell is higher in strength than the constituent material of the protector.

The protector 140 was manufactured from austenitic stainless steel to which Nb was added, the metal shell 138 was manufactured from SUS430LX (JIS standard), the ratio (t1 / t2) was changed, and the gas sensor 200 shown in FIG. 1 was assembled. The protector 140 was fixed to the outer surface of the tip portion 138s of the metal shell 138 by welding all around.
The gas sensor 200 was tested under the thermal cycle conditions of 500 cycles where one cycle is 900 ° C. × 20 minutes to 200 ° C. × 20 minutes, and the presence or absence of breakage or deformation of the joint W after the test was visually evaluated.
Evaluation :: peeling of the rear end joint portion 140b of the protector 140 from the joint portion W or no deformation of the metal shell 138 Evaluation x: peeling of the rear end joint portion 140b or deformation of the metal shell 138 thing

The obtained result is shown in FIG.
As shown in FIG. 4, in the case of 0.6 ≦ (t1 / t2) ≦ 2.0, there is neither peeling of the rear end joint portion 140b of the protector 140 from the joint portion W nor deformation of the metal shell 138, It was possible to suppress breakage or the like of the joint W.
On the other hand, in the case of (t1 / t2) <0.6, the rear end joint portion 140b of the protector 140 was peeled from the joint portion W. In the case of (t1 / t2)> 2.0, the metal shell 138 was deformed.

DESCRIPTION OF SYMBOLS 10 sensor element 10a detection part 138 metal shell 138i1 1st inner surface of metal shell 138i2 rear end side inner surface of metal shell 140 protector 140b rear end portion (rear end joint portion)
142 Outer protector 143 Inner protector 200 Gas sensor O Axis W Joint t1 Minimum distance from the outer surface of the metal shell to the outer surface of the protector t2 Minimum distance from the outer surface of the metal shell to the inner surface of the metal shell R Joint projected onto the inner surface of the metal shell Area

Claims (5)

A sensor element extending in the axial direction and having a detection unit on the tip end side;
A metallic cylindrical metal shell surrounding the periphery of the sensor element while causing the detection unit to project from its tip;
A metal protector that accommodates the detection unit and is fixed to the metal shell;
A gas sensor comprising
The rear end of the protector is joined to the outer surface of the metal shell to form a joint.
The coefficient of thermal expansion at 800 ° C. of the component material of the protector is higher than the coefficient of thermal expansion at 800 ° C. of the component material of the metal shell,
The minimum distance t1 from the outer surface of the metal shell to the outer surface of the protector and the minimum distance t2 from the outer surface of the metal shell to the inner surface of the metal shell when viewed in a cross section including the joint portion are 0.6 ≦ (t1 / T2) A gas sensor satisfying the relationship of ≦ 2.0. The protector includes an inner protector disposed between the detection unit and the gap, and an outer protector disposed between the inner protector and the gap.
The gas sensor according to claim 1, wherein the joint portion is formed at a rear end joint portion in which rear end portions of the inner protector and the outer protector overlap.   The first inner surface of the metal shell in a region of the outer surface of the metal shell projecting the joint on the inner surface of the metal shell is larger in diameter than the rear surface of the metal shell connected to the rear end side of the first inner surface. The gas sensor according to claim 1, wherein the gas sensor extends outward in the direction.   The gas sensor according to any one of claims 1 to 3, wherein a tensile strength defined in JIS-G0567 at 800 ° C of a constituent material of the protector is higher than that of the constituent material of the metal shell.   The gas sensor according to any one of claims 1 to 4, wherein a constituent material of the protector is austenitic stainless steel, and a constituent material of the metal shell is a ferritic stainless steel.

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