WO2011138652A1

WO2011138652A1 - Gas sensor element and gas sensor

Info

Publication number: WO2011138652A1
Application number: PCT/IB2011/000909
Authority: WO
Inventors: Takashi Nomura; Tomohiro Miwa; Takeshi Hirabayashi; Hirotaka Sakakibara
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2010-05-07
Filing date: 2011-04-28
Publication date: 2011-11-10
Also published as: JP2011237222A

Abstract

A gas sensor element (100) has a detector (10) formed by stacking a solid electrolyte body (2) sandwiched between a pair of electrodes (2), and a heating element (3) that includes a heating source (4), and also has a porous protective layer (20) formed around the detector (10). The porous protective layer (20) is formed of a single material which is silicon carbide or aluminum nitride, or is formed of a mixture of one of silicon carbide and aluminum nitride, and another ceramic material.

Description

GAS SENSOR ELEMENT AND GAS SENSOR

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to a gas sensor which is installed in, for example, a vehicle and detects the oxygen concentration in exhaust emitted by the vehicle. The invention further relates to a gas sensor element used in the gas sensor. 2. Description of the Related Art ,

[0002] A variety of endeavors aimed at mitigating environmental impacts are being carried out on a global scale in various industrial fields. In the automotive industry, steady progress is being made in development efforts targeted both at the increased use of gasoline engine cars with outstanding fuel performance and also "eco cars" such as hybrid vehicles and electric cars, as well as further performance improvements in such vehicles.

[0003] Fuel efficiency performance in vehicles is measured by detecting the oxygen concentration in vehicular exhaust or the like with a gas sensor and determining the difference with the concentration of oxygen in air as the reference gas.

[0004] Because such a gas sensor detects the oxygen concentration within exhaust in a high-temperature atmosphere at 700°C or above, thermal shock due to partial quenching occurs when drops of water within this exhaust strike the gas sensor element in the gas sensor, causing cracking of the sensor due to changes in volume associated with the change in temperature, which in turn impairs the sensing function.

[0005] In the gas sensor element disclosed in Japanese Patent Application

Publication No. 2009-80110 (JP-A 2009-80110), for example, this problem is addressed by surrounding the periphery of the element with a porous protective layer made of aluminum to keep drops of water from colliding with the element. [0006] By thus providing a porous protective layer around the gas sensor element, the element has an increased resistance to splashing with water. However, such a protective layer also ends up increasing the sensor activation time of the element, meaning that the time it takes to precisely detect the oxygen concentration increases. This gives rise to a new problem in that it leads to an increase in the emissions level.

[0007] In particular, because the porous protective layer around the gas sensor element disclosed in JP-A 2009-80110 is formed of alumina, the inventors have found that it is necessary to secure a thickness of at least 200 μπι for this protective layer in order to keep the above-described drops of water from penetrating to the element. Yet, when the porous protective film reaches a thickness of about 200 μπι, unburned fuel adheres thereto; the detachment of such fuel from the film when the sensor starts up causes the sensor to shift rich, increasing the sensor activation time. Moreover, the inventors have also found that the above rise in emissions tends to occur with an increase in the sensor activation time.

[0008] However, every country around the world has its own particular environmental safety regulation standards. In the case of the European Union (EU), for example, the inventors have determined that current gas sensor elements are inadequate for achieving a shorter sensor activation time than what is required to meet the Euro VI emission standards which are to be applied starting in 2014.

[0009] Accordingly, there exists today a strong desire in pertinent technical fields for the development of gas sensor elements which are capable of shortening even further the sensor activation time while suppressing such cracking, and for the development of gas sensors equipped with the same. SUMMARY OF THE INVENTION

[0010] It is therefore an object of the invention to provide a gas sensor element which is capable of shortening the sensor activation time while also suppressing such cracking. A further object is to provide a gas sensor equipped with such a gas sensor element.

[0011] In a first aspect, the invention relates to a gas sensor element having a detector formed by stacking a solid electrolyte body sandwiched between a pair of electrodes, and a heating element that includes a heating source, and having a porous protective layer formed around the detector. The porous protective layer is formed of a single material which is silicon carbide or aluminum nitride.

[0012] Instead of the porous protective layers in the related art which are formed of alumina, in the gas sensor according to this aspect of the invention, the porous protective layer surrounding the detector is formed of a. single material which is silicon carbide or aluminum nitride.

[0013] According to investigations by the inventors, it has been demonstrated that by employing a porous protective layer formed of silicon carbide or aluminum nitride, the layer has a thickness which keeps moisture within the exhaust from penetrating and causing cracking of the gas sensor element, in addition to which the sensor activation time can be markedly shortened compared with gas sensor elements in the related art.

[0014] Alternatively, the porous protective layer may be formed of a mixture of one of silicon carbide and aluminum nitride, and another ceramic material.

[0015] In cases where the porous protective layer is formed using either silicon carbide or aluminum nitride alone, and also in cases where the porous protective layer is formed of a mixture of at least one of these materials in combination with another ceramic material such as alumina or silicon nitride, the inventors have demonstrated that the porous protective layer has a thickness which keeps the gas sensor element from incurring cracking and moreover is able to shorten the sensor activation time relative to gas sensor elements in the related art.

[0016] Here, the porous protective layer thickness which ensures a sensor activation time no longer than the time that satisfies the existing rules for emission levels may be 80 μηι or less. [0017] In order for heat transfer to be effectively carried out from the heating source (such as a heater) in the detector to the surface of the porous protective layer in the above thickness range, the porous protective layer may have a thermal conductivity of at least 40 W/mK.

[0018] For example, in cases where the thickness of the porous protective layer has been set to 80 μηι or below, so long as heat from the heating source within this detector can be rapidly transferred to the porous protective layer and moisture in the ; exhaust gases evaporated or otherwise dispersed within the porous protective layer before the moisture penetrates into this porous protective layer and reaches the detector of the gas sensor element, cracking of the detector can be suppressed. The abovementioned range can be exemplified as a thermal conductivity of the porous protective layer that satisfies the suppression.

[0019] The porous protective layer may have a thermal conductivity within the above-indicated range, both in cases where the porous protective layer is formed of a single material which is silicon carbide or aluminum nitride* and also in cases where the porous protective layer is formed of a mixture of either of these in combination with alumina, silicon nitride or the like,.

[0020] The thickness range for the porous protective layer, aside from being set in such a way as to satisfy the desired sensor activation time, may also be set based on the relationship between the thermal conductivity of the porous protective layer and the depth of penetration. Hence, the sensor activation time, the thermal conductivity of the porous protective layer and the penetration depth are intimately related.

[0021] Moreover, in the above aspect of the invention, the material of which the porous protective layer is formed may be in the form of particles having a specific surface area of 20 m /g or less.

[0022] The specific surface area of the particles of material which form the porous protective layer have been set in the above numerical range in order to reduce the depth (distance) of penetration by the maximum water droplet size of 0.3 μL· (microliter) that is generally capable of arising in a normal service environment, thereby controlling such penetration to 80 μηι or less, which is the above-specified thickness of the porous protective layer, and thus enhance both thermal shock mitigation and shortening of the activation time within the gas sensor element.

[0023] With gas sensors containing gas sensor elements according to the above aspects of the invention, it will be possible to fully satisfy not only Euro IV emission standards, but also environmental standards in all countries or regions where such standards are expected to become increasingly stringent. This will make it possible to mitigate the environmental impact of automotive emissions and will be helpful in increasing the supply of environmental friendly vehicles which is awaited throughout the world.

[0024] As can be appreciated from the above, in the gas sensor device of the invention and in gas sensors equipped with the same, by introducing improvements in the material that forms the porous protective layer surrounding the detector, the thickness of the porous protective layer can be made as thin as possible and the sensor activation time can be greatly shortened, in addition to which cracking of the detector that arises from the penetration of moisture within the exhaust gases can be effectively suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a schematic diagram showing a gas sensor element according to the invention;

FIG. 2 is a graph showing experimental results concerning the relationship between the thermal conductivity of a porous protective layer and the depth of penetration by moisture; and FIG. 3 is a graph showing experimental results concerning the relationship between the specific surface area of the porous protective layer and the depth of penetration by moisture. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Embodiments of the invention are described below in conjunction with the diagrams. FIG. 1 is a schematic diagram showing a cross-sectional view of a gas sensor element according to the invention.

[0027] The gas sensor element 100 shown in FIG. 1 is substantially composed of a detector 10 which detects the oxygen concentration in exhaust gases, and a porous protective layer 20 which protects the periphery of the detector 10 from moisture in the exhaust gases, thereby preventing such moisture from reaching the detector 10 and causing cracking of the detector 10.

[0028] The detector 10 is substantially composed of a solid electrolyte body 2 sandwiched between a pair of electrodes (not shown), a porous diffusion resistor 1 which is disposed on a first side of the solid electrolyte body 2 and allows a gas that is to be measured to pass therethrough to one of the electrodes, and a heating element 3 which is disposed on a second side of the solid electrolyte body 2 and is made of ceramic.

[0029] The heating element 3 has at the interior thereof a heater 4 which serves as a heating source, and forms a heating region in the gas sensor element 100.

[0030] The detector 10, in the illustrate cross-sectional shape, has corners which are cut in a tapered manner. Such cuts ensure the thickness of the porous protective layer 20 at those places on the detector 10.

[0031] A voltage having a linear correlation between the oxygen concentration difference and the current is applied to the pair of electrodes (not shown), the measurement gas is brought into contact with one electrode and a reference gas such as air is brought into contact with the other electrode, and the current value that arises between the electrodes due to the oxygen concentration difference therebetween is measured, thereby enabling the air-fuel ratio of a vehicle/engine to be specified based on the measured current.

[0032] The porous protective layer 20 is a layer of ceramic material having numerous pores. In the illustrated example, the thicknesses tl to t5 at each place differ.

[0033] The gas sensor element 100 shown in the diagram is fixed within a housing via, for example, an insulator made of an insulating material. An element cover is provided at the end of this housing, thereby forming a gas sensor.

[0034] Here, the porous protective layer 20 may be formed ofi a single material which is either silicon carbide or aluminum nitride, or it may be formed of a mixture of one of silicon carbide and aluminum nitride, and another ceramic material. By using these materials to form the porous protective layer 20, the thermal conductivity of the 20 may be set to at least 40 W/mK.

[0035] Also, when the thickness of the porous protective layer 20 is 80 μπι or less, it is possible for the sensor activation time of the gas sensor element 100 to be shorter than the sensor activation time that satisfies the rules relating to emission levels in the Euro VI standards, which are regarded as the most stringent emission standards in the world. That is, in FIG. I, the porous protective layer 20 is formed so that the greatest thickness among thicknesses tl to t5 in FIG. 1 is 80 μιη.

[0036] Here, by ensuring that the thermal conductivity of the porous protective layer 20 is at least 40 W/mK, even when the thickness of the porous protective layer 20 has been set to 80 μπι or less, the heat generated by the heater 4 serving as the heating source can be rapidly transferred into the porous protective layer 20 before the moisture within the exhaust gases penetrates the thin porous protective layer 20 and reaches the detector 10. Moreover, by evaporating or otherwise dispersing moisture with this heat, the moisture is prevented from reaching the detector 10, thereby effectively eliminating cracking that can arise due to the arrival of moisture.

[0037] Hence, the thickness of the porous protective layer 20 for ensuring the desired sensor activation time (which layer is thinner than in constructions according to the related art) and the thermal conductivity of the porous protective layer 20 which does not give rise to cracking in the detector 10 even when the porous protective layer 20 is this thin layer, are important features which enable both the desired sensor activation time to be attained and cracking to be eliminated.

[0038] In addition, by setting the specific surface area of the porous protective layer 20 to 20 m²/g or less, the depth of penetration by moisture within the exhaust gases can be held to 80 μπι or less. This conforms with the porous protective layer 20 thickness of 80 μτη or less for ensuring the desired sensor activation time.

[0039] Experiment on Relationship Between Thermal Conductivity of Porous Protective Layer and Depth of Penetration by Moisture, and Results Therefrom

The inventors conducted an experiment to verify the layer-forming materials and thickness for a porous protective layer which does not undergo cracking and which satisfies the desired activation time. In the experiment, the inventors varied the porous protective layer material and the thickness thereof, determined the moisture penetration depth and presence or absence of cracking in the respective test pieces, and measured the activation time.

[0040] Table 1 below shows the results obtained from the following measurements and evaluations carried out for each ceramic material used in the porous protective layer and for each thickness thereof: penetration depth, presence/absence of cracking, activation time, and rating as to whether cracking and activation time requirements were both satisfied. The presence/absence of cracking is indicated in the table as either "yes" or "no," respectively; and the activation times are indicated as a ratio of the activation time for a particular test piece relative to a normalized value of 1 for the activation time of the test piece in the topmost row (alumina having a film thickness of 220 μπι). Cases in which this activation time was at or below the sensor activation time which satisfies the rules relating to emission levels in the Euro IV standards are denoted as "Pass," and cases in which it exceeded the sensor activation time which satisfies the rules are denoted as "Fail." Moreover, in the General Assessment, which represents the results of evaluations as to whether cracking and activation time requirements were both satisfied, test pieces which satisfy both of these criteria are rated as "Good," and test pieces which do not satisfy at least one of these criteria are rated as "NG."

Table 1

[0041] From Table 1, it was demonstrated that test pieces in which silicon carbide was used as the porous protective film material and which were set to a thickness of 80 μπι or less, test pieces in which aluminum nitride was used as the material and which were set to a thickness of 80 μπι or less, and test pieces in which a mixture of equal amounts of silicon carbide and alumina was used as the material and which were set to a thickness of 80 μπι did not give rise to cracking and also satisfied the criteria for the active time.

[0042] In addition to the results in Table 1, a graph of results for other test pieces is shown in FIG 2.

[0043] In this graph, "EX" stands for working example of the invention, and "CE" stands for comparative example. Here, EX 1 is an example of the invention in which the porous protective layer is made of silicon carbide, EX 2 is an example of the invention in which the porous protective layer is made of aluminum nitride, EX 3, 4 and 5 are examples of the invention in which the porous protective layer is made of, respectively, a mixture of aluminum and silicon carbide, a mixture of silicon nitride and silicon carbide, and a mixture of silicon nitride and aluminum nitride. CE 1 and 2 are comparative examples in which the porous protective layer is made of, respectively, alumina and silicon nitride.

[0044] The experimental results in Table 1 and FIG 2 demonstrate the suitability of the above-indicated porous protective layer materials (the thermal conductivity being determined by the material) and thickness ranges. By fabricating gas sensor elements having a porous protective layer composed of the materials shown above and a thickness set to 80 μπι or less, producing gas sensors equipped with such gas sensor elements and installing such gas sensors in vehicles, it is possible to make significant contributions, in any and all countries or regions throughout the world with their widely varying environmental standards, toward the supply of vehicles capable of meeting the standards relating to such environmental impacts.

[0045] Experiment on Relationship Between Specific Surface Area of Porous Protective Layer and Depth of Penetration by Moisture, and Results Therefrom

The inventors produced test pieces with porous protective layers having various differing specific surface area, determined whether cracking occurred in the respective test pieces, measured the active time, and verified whether the test pieces satisfied the requirements for both cracking and active time. Those results are shown below in Table 2. As in Table 1, the presence/absence of cracking was indicated here as either "yes" or "no," respectively; and the active times are indicated as a ratio of the active time for a particular test piece relative to a normalized value of 1 for the active time of the test piece in the topmost row (trimmed film thickness, 250 μπι; penetration depth, 240 μπι). Cases in which this active time was at or below the sensor active time which satisfies the rules relating to emission levels in the Euro IV standards are denoted as "Pass," and cases in which it exceeded the sensor active time which satisfies the rules are denoted as "Fail." Moreover, in the General Assessment, which represents the results of evaluations as to whether cracking and active time requirements were both satisfied, test pieces which satisfy both of these criteria are rated as "Good," and test pieces which do not satisfy at least one of these criteria are rated as "NG."

Table 2

[0046] The results in Table 2 demonstrate that, in gas sensor elements where the porous protective layer has a specific surface area of 20 m /g or less and a thickness of 80 μπι or less, cracking does not arise and the active time criteria are also met.

[0047] FIG. 3 is a graph of the correlation between the specific surface area of the porous protective layer and the depth of penetration by moisture.

[0048] From this graph, the penetration depth at a specific surface area of 20 m²/g was about 80 μπι, which was the same value as the porous protective layer thickness of 80 μηι required to satisfy the desired active time. Based on this, by setting the specific surface area of the particles of the material which form the porous protective layer to 20 m /g or below, it is possible to reduce the moisture penetration depth and control it to 80 μηι or below, thus enabling cracking of the detector to be suppressed even when the thickness of the porous protective film is lowered to about 80 μπι.

[0049] In the foregoing embodiment, as shown in FIG. 1, the thickness of the porous protective layer 20 differs from place to place (tl to t5). However, the invention is not limited in this regard; that is, the porous protective layer 20 may be formed so that the thickness thereof is as uniform as possible. In such a case, the tapered cuts at the corners may be further increased.

[0050] Also, in the above embodiment, the porous protective layer 20 has been formed in such a way that, of the thicknesses at various places (tl to t5), the greatest thickness is 80 μπι. In this case, of thicknesses tl to t5, places which are thinner than 80 μηι may be positioned on the side of the detector 10 where the heating element 3 is disposed. In this way, the moisture in the exhaust gases is evaporated or otherwise dispersed by the heat generated by the heater 4 serving as the heating source before this moisture has a chance to penetrate the porous protective layer 20 and reach the detector 10, thus preventing moisture from reaching the detector 10.

[0051] Embodiments of the invention have been described above in detail with reference made to the drawings. However, the specific structure of the invention is not limited to these embodiments and various design modifications are possible insofar as they do not depart from the gist of the invention.

Claims

1. A gas sensor element characterized, by comprising:

a detector formed by stacking a solid electrolyte body sandwiched between a pair of electrodes, and a heating element that includes a heating source; and a porous protective layer formed around the detector, wherein the porous protective layer is formed of a single material which is silicon carbide or aluminum nitride.

2. A gas sensor element characterized by comprising:

a detector formed by stacking a solid electrolyte body sandwiched between a pair of electrodes, and a heating element that includes a heating source; and a porous protective layer formed around the detector, wherein the porous protective layer is formed of a mixture of one of silicon carbide and aluminum nitride, and another ceramic material.

3. The gas sensor element according to claim 1 or 2, wherein the porous protective layer has a thickness of 80 μπι or less.

4. The gas sensor element according to any one of claims 1 to 3, wherein the porous protective layer situated on a side where the heating element is stacked in the detector has a thickness of approximately 80 μπι.

5. The gas sensor element according to any one of claims 1 to 4, wherein the porous protective layer has a thermal conductivity of at least 40 W/mK.

6. The gas sensor element according to any one of claims 1 to 5, wherein a specific surface area of material forming the porous protective layer is in the form of particles is 20 m²/g or less.

7. The gas sensor element according to any one of claims 1 to 6, wherein the detector has a corner cut in a tapered manner.

8. A gas sensor characterized by comprising the gas sensor element according to any one of claims 1 to 7.