JP2011242237A - Gas sensor element and gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor element with which the occurrence of crack which can be generated in a protection layer due to water crack is allowed while the progress of the crack into the element main body can be inhibited and to provide a gas sensor including the gas sensor element.SOLUTION: In a gas sensor element 100, a detection device 3 which is composed of a solid electrolyte 2 and a pair of electrodes provided on the either side of the solid electrolyte 2, and a heating part 6 obtained by embedding a heat generating source 5 in the inside of the heat generating body 4 are laminated to form an element main body 10, and the surrounding of the element main body 10 is provided with a protection layer 20. A buffer layer 30 is interposed between the element main body 10 and the protection layer 20. The Young's ratio of the buffer layer 30 is less than 1/4 of the Young's ratio of the element main body 10.

Description

  The present invention relates to a gas sensor that is mounted on a vehicle and detects an oxygen concentration in exhaust gas, for example, and a gas sensor element constituting the gas sensor.

  Various industries are making various efforts to reduce environmental impact on a global scale. Among them, in the automobile industry, not only gasoline engine cars with excellent fuel efficiency, but also hybrid cars and electric cars. The development of the so-called eco-cars such as the above and the further improvement of their performance is being carried out every day.

  The measurement of the fuel consumption performance of a vehicle is performed by detecting the oxygen concentration in a gas to be measured such as exhaust gas with a gas sensor and obtaining the difference from this oxygen concentration using oxygen in the atmosphere as a reference gas.

  Since this gas sensor detects the oxygen concentration in the exhaust gas in a high temperature atmosphere of 700 ° C. or higher, if a water droplet in the exhaust gas collides with the element body of the gas sensor element constituting the gas sensor, a thermal shock due to partial quenching occurs. Further, there is a problem that the element body is subject to water cracking due to the volume change accompanying the temperature change, and the sensing function is impaired.

  In response to this problem, for example, in the gas sensor element disclosed in Patent Document 1, a porous protective layer made of alumina surrounds the periphery of the element to suppress the collision of water droplets.

  Although the water resistance is improved by providing a porous protective layer around the gas sensor element in this way, according to the present inventors, once the porous protective layer is cracked due to water cracking, It has been specified that this crack easily propagates into the element body without changing its traveling direction.

  Therefore, by applying the technique disclosed in Patent Document 2, an intermediate layer having an elastic modulus smaller than the elastic modulus (Young's modulus) of the element body is interposed between the element body and the protective layer, so that cracks propagate to the element body. An approach that deters this is possible.

  However, simply by providing an intermediate layer having a relatively low elastic modulus with respect to the element body as in this approach, the crack cannot be retained in the intermediate layer and the progress to the element body cannot be reliably suppressed. These findings have been obtained by the present inventors.

  Accordingly, development of a gas sensor element and a gas sensor having the gas sensor element that can surely suppress the progress of the crack to the element body while allowing the generation of cracks that may occur in the protective layer due to the water cracking is anxious in the art. Has been.

JP 2009-80110 A JP 2007-231420 A

  The present invention has been made in view of the above problems, and a gas sensor element capable of suppressing the progress of cracks to the element body while allowing the generation of cracks that may occur in the protective layer due to water cracking. An object of the present invention is to provide a gas sensor provided with this.

  In order to achieve the above-mentioned object, a gas sensor element according to the present invention includes a detection unit composed of a solid electrolyte body and a pair of electrodes on both sides thereof, and a heating unit in which a heat generation source is embedded in the heating element. A gas sensor element in which an element body is formed and a protective layer is formed around the element body, wherein a buffer layer is interposed between the element body and the protective layer, and the Young's modulus of the buffer layer Is less than 1/4 of the Young's modulus of the element body.

  In the gas sensor element of the present invention, a buffer layer having a Young's modulus equal to or less than 1/4 of the Young's modulus of the element body is interposed between the element body and the protective layer. By changing the direction of the crack that has propagated inside the buffer layer, the progress of the crack in the element body can be avoided by inducing the progress of the crack in a direction different from the direction toward the element body. Is.

  The element body constituting the gas sensor element and the protective layer around it, more specifically, the detection part and the heating part constituting the element body, and the protective layer around it are almost all except for the electrode of the detection part. The constituent member is made of ceramic, and this ceramic is generally alumina.

  Therefore, for example, it can be assumed that the constituent members made of ceramics of the element body are all made of alumina, and a buffer layer having a Young's modulus ¼ of the Young's modulus of this alumina is interposed between the element body and the protective layer. It will be good.

  When the material of each constituent member forming the element body cannot be represented by alumina, that is, when each constituent member is formed from various ceramic materials, the detection unit and the heating unit are formed. The lowest Young's modulus of each ceramic material is defined as the Young's modulus of the element body, and a buffer layer having a Young's modulus ¼ of the defined Young's modulus may be provided.

  Here, by setting the Young's modulus of the buffer layer to ¼ or less of the Young's modulus of the element body, the progress direction of the crack that has propagated from the protective layer into the buffer is changed to a direction different from the direction toward the element body. The reason why the traveling direction can be induced so that cracks remain in the buffer layer will be described below.

  For example, cracks in ceramics tend to progress in a direction in which the energy release rate G (the amount of strain energy released when the crack progresses for a certain length) is maximized.

  The energy release rate G at this time can be generally expressed by the following two equations that are functions of the Young's modulus E, the stress intensity factor K, and the angle θ formed by the progress of the crack.

  From the above formulas 1 and 2, the energy release rate G changes with the advance angle θ, and when θ is 0 degree, that is, when going straight from the buffer layer to the element body, G becomes maximum, and G decreases as θ increases. (Thus, since the energy release rate becomes the largest when the angle θ formed is 90 degrees, the crack is difficult to progress in this direction).

  Therefore, in order to guide the progress of the crack in the direction of diverting from the element main body, when the crack has progressed perpendicular to the element main body in the buffer layer (in the case where θ is 0 degree, it proceeds straight to the element main body. Direction) and at this time, the case where the progress direction of the crack is guided to a direction of 90 degrees is the most severe condition for the control of the traveling direction. This is because when the crack has progressed at an arbitrary inclination angle with respect to the element body, the angle required to divert the advancing direction from the element body direction is less than 90 degrees.

From the above equations 1 and 2, the energy release rate G is inversely proportional to the Young's modulus E. Then, it can be seen that G when the angle θ formed is 90 degrees is 1/4 of G when θ is 0 degrees (by substituting 0 degrees and 90 degrees for θ in Equation 2, h ₁ ( 0), h ₂ (90), and substitute these into equation 1).

  Therefore, by setting the Young's modulus of the buffer layer to ¼ or less of the Young's modulus of the element body, the progressing direction of the crack traveling in the buffer layer can be assumed as a direction toward the element body of 0 ° to 90 °. In all the traveling directions, a condition that always maximizes G can be set.

  Further, according to the verification of the present invention, when the Young's modulus of the element body is defined by the Young's modulus of alumina, the buffer layer is formed of either one of boron nitride or a mixture of alumina and silica. In this case, it has been demonstrated that the Young's modulus ratio is satisfied between the element body and the buffer layer.

  And in this verification, in the dripping amount in the range up to 1 μL (microliter) which is the maximum amount of water droplets in the exhaust gas dripped to the gas sensor element, water cracking occurs in the protective layer and the inside It has been demonstrated that the crack that has developed and further propagated in the buffer layer does not reach the element body.

  According to the gas sensor including the gas sensor element of the present invention described above, since the element main body of the gas sensor element is effectively suppressed from being subjected to water cracking due to water droplets in the exhaust gas, the gas sensor having excellent durability and Become.

  As can be understood from the above description, according to the gas sensor element of the present invention and the gas sensor including the gas sensor element, the buffer layer having a Young's modulus equal to or less than 1/4 of the Young's modulus of the element body between the element body and the protective layer. By interposing, it can be guided in the protective layer by water cracking, and further guide the direction of crack progressing in the buffer layer away from the direction toward the element body so as not to reach the element body, Therefore, since the maintenance of the sensing function of the element body can be guaranteed, the gas sensor is excellent in durability.

It is the schematic diagram explaining the gas sensor element of this invention. (A) is the schematic diagram explaining the state which the crack produced in the protective layer by the water-proof crack has progressed perpendicularly | vertically with respect to the element main body, (b) is a 90 degree direction within a buffer layer. It is the schematic diagram explaining the state which is induced | guided | derived to and is developing. (A) is a diagram simulating the angle θ formed by the direction of travel of the induced crack, and (b) is a graph showing the relationship between the angle formed by the direction of travel and the apparent toughness ratio (ratio of cracking difficulty). It is. It is a graph which shows the experimental result which calculated | required the dripping amount and crack probability in the case of the presence or absence of a crack of an element body, and a crack.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic view illustrating a gas sensor element of the present invention, and shows a cross section thereof.

  A gas sensor element 100 shown in FIG. 1 protects an element body 10 that detects the oxygen concentration in exhaust gas and the surroundings of the element body 10 from moisture in the exhaust gas, and this moisture (water droplet) is applied to the element body 10. The protective layer 20 that suppresses water cracking from reaching the element main body 10, and the progress direction of the crack that has progressed in the protective layer 20 interposed between the element main body 10 and the protective layer 20 is diverted from the element main body 10 side. The buffer layer 30 for guiding is generally constituted.

  The element body 10 includes a detection unit 3 and a heating unit 6. The detection unit 3 is disposed on one side of the solid electrolyte body 2 sandwiched between a pair of electrodes (not shown) and covered on one electrode. The porous diffusion resistor 1 that allows measurement gas to pass therethrough, and the heating unit 6 includes a heating element 4 made of ceramics and a heater 5 that is a heat source inside the heating element 4.

  In addition, the element main body 10 has a corner portion that is notched in a tapered shape in the illustrated cross-sectional shape, and the thickness of the protective layer 20 at the corresponding portion of the element main body 10 is guaranteed by this notch.

  A voltage having an oxygen concentration difference and a linear phase is applied to a pair of electrodes (not shown), a measurement gas is brought into contact with one electrode, and a reference gas such as the atmosphere is brought into contact with the other electrode, A current value generated between the electrodes can be measured according to the difference in oxygen concentration between the two, and the air-fuel ratio of the vehicle engine can be specified based on the measured current.

  A gas sensor element 100 shown in the figure is fixed in a housing through an insulator made of an insulating material, for example, and an element cover is provided at the tip of the housing to form a gas sensor.

  Here, the solid electrolyte body 2, the diffusion resistor 1, and the heating element 4 that configure the heating unit 6 that constitute the detection unit 3 are generally made of alumina.

  On the other hand, the buffer layer 30 has a Young's modulus of the element body 10, that is, when almost all of the constituent members of the ceramic material are made of alumina, the Young's modulus of 1/4 or less of the Young's modulus (approximately 400 GPa) of the alumina. It is formed from the ceramic which has this. Specifically, it can be formed from boron nitride (Young's modulus is about 50 GPa) or a mixture of alumina and silica (Young's modulus varies depending on the mixing ratio of both, but in the range of 40 to 100 GPa).

  Here, the crack progressing in the protective layer is directed toward the element body without changing the traveling direction if the buffer layer has a Young's modulus of the same size or larger than that of the element body. Will progress.

  On the other hand, when the buffer layer 30 has a Young's modulus equal to or less than ¼ of the Young's modulus of the element body 10, the buffer layer 30 proceeds in the protective layer 20 as shown in FIG. As shown in FIG. 2B, the crack K that has progressed toward the head is guided so as to change the travel direction: θ in the buffer layer 30 by 90 degrees without colliding with the element body 10. It proceeds in the buffer layer 30 (traveling direction: X2).

  In this way, by guiding the progress direction of the crack in the buffer layer 30 to be diverted from the direction toward the element body 10, water cracking occurs in the protective layer 20 and the crack progresses into the buffer layer 30. Even in this case, the crack can be prevented from reaching the element body, and at least the sensing function of the gas sensor element 100 can be maintained.

  Here, the relationship between the Young's modulus of the element body 10 and the buffer layer 30 will be described below with reference to FIG. Here, FIG. 3a is a diagram simulating the angle θ formed by the direction of travel of the induced crack, and FIG. 3b shows the relationship between the angle formed by the direction of travel and the apparent toughness ratio (ratio of cracking difficulty). It is a graph.

  With respect to a crack traveling in the buffer layer, the angle θ formed by the subsequent traveling direction of the crack determines the difficulty of traveling in that direction. For example, as shown in FIG. 3a, for a crack K that has progressed in the X1 direction in the protective layer and the buffer layer, as shown in FIG. 3b, the angle formed by the subsequent progress direction of the crack is 0 °. That is, the crack is likely to proceed in the direction toward the element body in the traveling direction as it is (in FIG. 3b, the cracking difficulty is 1).

  On the other hand, as shown in FIG. 2b, in order for the progressing crack to progress in the buffer layer by changing its traveling direction to the 90 degree direction, referring to FIG. It can be seen that it is difficult to travel in this direction twice as much.

  Since the crack progresses in the direction that is most likely to crack, if the material and structure of the buffer layer upstream and the element body downstream are uniform, the crack advances straight from the buffer layer to the element body. (Because the angle formed by the above-mentioned traveling direction is 0 degree, it is most likely to proceed).

On the other hand, in order to bend and induce the crack propagation direction by 90 degrees in the buffer layer, it is only necessary to form a state in which the 90 degree direction is most likely to proceed for the crack. Therefore, the toughness (hardness of cracking) of this buffer layer may be adjusted to ½ of the toughness of the element body.
Here, the formula of the energy release rate mentioned above is described below.

  On the other hand, the graph showing the apparent toughness ratio shown in FIG. 3B is based on the following formula 3, and the toughness (hardness to crack) can be expressed by the following formula 4.

Here, a direction of toughness K _AishitaC release angle θ of a toughness K _I speak angle of 0 degrees (straight), E is the Young's modulus.

  As described above, since the apparent toughness ratio when the angle θ formed by the traveling direction in the buffer layer is 90 degrees is approximately 2, in order to facilitate the progress of cracks in the buffer layer in the direction of 90 degrees. Therefore, the Young's modulus of the buffer layer may be ½ of the element body as described above.

  On the other hand, from the above equation 4, considering that the toughness is proportional to the square root of the Young's modulus, if the Young's modulus of the buffer layer is 1/4 or less of that of the element body, the crack is oriented in the 90 ° direction in the buffer layer. It will be able to be guided to.

  As described above, when the crack progresses perpendicular to the element body 10 in the protective layer 20 as shown in FIG. 2, the crack is induced in the direction of 90 degrees in the buffer layer 30. Therefore, by setting the Young's modulus of the buffer layer to ¼ or less of that of the element body, even if the crack has progressed in any direction, the direction of crack propagation in this buffer layer Can be guided in a direction different from the direction toward the element body.

[Experiment and results of determining the presence of cracks in the element body, the amount of dripping and the probability of cracking when cracked]
The present inventors dropped water droplets on a gas sensor element having a driving temperature of approximately 700 ° C., and increased the amount of water droplets if no cracking occurred. This experiment was repeated in a large amount up to 1 μL (microliter).

Here, the Young's modulus of each component of the element body and the protective layer is set to 400 GPa of alumina, and the conventional structure having no buffer layer is Comparative Example 1, and the buffer layer is made of alumina (Young's modulus is 400 GPa). Comparative Example 3 having a buffer layer made of zirconia (Young's modulus is 210 GPa and greater than 1/4 of the Young's modulus of the element body). The buffer layer is made of boron nitride (Young's modulus is 50 GPa and the Young's modulus of the element body is ¼ or less) of Example 1, and the buffer layer is made of a mixture of alumina and silica (Young's modulus is 40 to 40). The above experiment was conducted on 10 test pieces in each of the comparative examples and examples, with Example 2 having 100 GPa and less than 1/4 of the Young's modulus of the element body.
The experimental results are shown in Table 1 below and FIG. The numbers in the table are the number of test pieces.

  From Table 1 and FIG. 4, each of the test pieces of Comparative Examples 1 to 3 has cracks reaching the element body in the drop amount range of 0.1 to 0.7, while Examples 1 and 2 are both gas sensors. It has been demonstrated that cracks do not reach the element body in the range up to 1 μL (microliter), which is the maximum amount of water droplets in the exhaust gas dripped onto the element.

  In addition, when the element body is made of alumina, zirconia (Young's modulus is 210 GPa), or silicon carbide (Young's modulus is 380 GPa), boron nitride or silica (Young's modulus is less than 1/4 of the Young's modulus). 80 GPa), a mixed material of silica and alumina, a buffer layer made of a material in which these inorganic compounds are dispersed and precipitated in glass, or the like may be formed.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

DESCRIPTION OF SYMBOLS 1 ... Diffusion resistor, 2 ... Solid electrolyte body, 3 ... Detection part, 4 ... Heat generating body, 5 ... Heat generation source (heater), 6 ... Heating part, 10 ... Element main body, 20 ... Protective layer, 30 ... Buffer layer, DESCRIPTION OF SYMBOLS 100 ... Gas sensor element, K ... Crack which advances a protective layer and a buffer layer, K '... Crack after the advancing direction was changed within a buffer layer

Claims (4)

A detection unit composed of a solid electrolyte body and a pair of electrodes on both sides of the solid electrolyte body and a heating unit in which a heat source is embedded inside the heat generation body are laminated to form an element body, and a protection is provided around the element body A gas sensor element in which a layer is formed,
A gas sensor element in which a buffer layer is interposed between the element body and the protective layer, and the Young's modulus of the buffer layer is ¼ or less of the Young's modulus of the element body.   2. The gas sensor element according to claim 1, wherein a Young's modulus of the element body is a lower Young's modulus among the respective ceramic materials forming the detection unit and the heating unit.   The gas sensor element according to claim 1, wherein the Young's modulus of the element body is defined by the Young's modulus of alumina, and the buffer layer is formed of any one of boron nitride or a mixed material of alumina and silica.   A gas sensor comprising the gas sensor element according to claim 1.

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