gas sensor element

Abstract

To provide problems that a gas sensor element is subjected to thermal shock due to repetition of heating / cooling and adhesion of moisture, and the peeling or detachment of a porous coating from an element body due to thermal shock is required to be suppressed and in addition, the moisture, which adheres to a porous film and has alkali metal dissolved therein, permeates into the film and the measurement sensitivity of the gas sensor element is reduced when the noble metal contained in an electrode reacts with the alkali metal in the moisture.SOLUTION: A gas sensor element includes: an element body and a porous protective layer covering a part of the element body. In the gas sensor element, the content of alkali metals in a porous protective layer is 150 ppm-3500 ppm.SELECTED DRAWING: Figure 4

Description

Technical Field

[0005] , , ,

[0001] The present invention relates to a gas sensor element.

Background Art

[0002] A gas sensor element detects, for example, a predetermined gas component contained in exhaust gas (measured gas) of an internal combustion engine. The gas sensor element measures the concentration of a predetermined gas component in a state of being driven at a high temperature. When moisture in the measured gas adheres to the element body constituting the gas sensor element during driving of the gas sensor element, the element body is rapidly cooled. When the element body is rapidly cooled, cracks occur in the element body. In order to avoid the occurrence of cracks in the element body, a part of the element body is covered with a porous protective layer (porous film).

[0003] An electrode for measuring the concentration of a predetermined gas component is provided on the surface of the element body. When the electrode is covered with a porous film, the measurement error of the gas component concentration increases. Patent Document 1 discloses that in order to reduce the measurement error, the content of an alkali metal contained in the porous film is made less than 150 [ppm].

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The gas sensor element is heated up by driving and cooled down (cooled) by stopping driving. The gas sensor element is frequently subjected to thermal shock by repeating heating up and cooling down. In addition, the gas sensor element is subjected to thermal shock due to the adhesion of moisture contained in the exhaust gas. In order to stably drive the gas sensor element for a long time, it is necessary to suppress the peeling or detachment of the porous film from the element body due to thermal shock.

[0006] Furthermore, when moisture adheres to the porous coating, alkali metals within the coating may dissolve into the moisture. The moisture containing dissolved alkali metals penetrates into the porous coating. The moisture that penetrates into the porous coating reaches the electrodes provided on the surface of the element body. If the precious metals contained in the electrodes react with the alkali metals in the moisture, the measurement sensitivity of the gas sensor element will decrease.

[0007] The present invention aims to solve the problems described above. [Means for solving the problem]

[0008] Examples of aspects of the present invention are given below. [Item 1] A gas sensor element comprising an element body and a porous coating covering a part of the element body, wherein the alkali metal content in the porous coating is 150 ppm to 3500 ppm. [Item 2] A gas sensor element as described in item 1, wherein the alkali metal content is 200 ppm to 2000 ppm. [Item 3] A gas sensor element according to item 1 or 2, wherein the porous coating comprises at least two or more layers. [Item 4] A gas sensor element as described in item 3, wherein the thickness of the layer closest to the element body of the porous coating is 170 [μm] to 900 [μm], and the porosity of the layer closest to the element body is 20 [%] to 70 [%]. [Item 5] A gas sensor element according to item 3 or 4, wherein the thickness of the outermost layer of the porous coating, furthest from the element body, is 30 [μm] to 400 [μm], and the porosity of the outermost layer, furthest from the element body, is 10 [%] to 60 [%]. [Item 6] A gas sensor element according to any one of items 3 to 5, wherein the alkali metal content in each layer of the porous coating is 150 ppm to 3500 ppm. [Item 7] A gas sensor element according to any one of items 3 to 6, wherein the alkali metal content in each layer of the porous coating is 200 ppm to 2000 ppm. [Item 8] A gas sensor element according to any one of items 1 to 7, wherein the alkali metal is Na. [Item 9] A gas sensor element according to any one of items 1 to 8, wherein the Si content in the porous coating is 1500 ppm or less in terms of SiO2. [Item 10] A gas sensor element according to any one of items 1 to 9, wherein the Fe content in the porous coating is 500 ppm or less in terms of Fe2O3. [Item 11] A gas sensor element according to any one of items 1 to 10, wherein the porous coating is formed by plasma spraying. [Item 12] A gas sensor element according to any one of items 1 to 11, wherein the porous coating comprises at least one material from alumina, spinel, mullite, zirconia, yttria, and gray alumina. [Effects of the Invention]

[0009] According to the present invention, the bonding strength between the element body and the porous coating is improved. Furthermore, a decrease in the measurement sensitivity of the gas sensor element can be suppressed. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a cross-sectional view of the gas sensor. [Figure 2]FIG. 2 is a perspective view of the gas sensor element. [Figure 3] FIG. 3 is a cross-sectional view taken along line III-III of FIG. 2. [Figure 4] FIG. 4 is a table showing the evaluation results of thermal shock resistance and NOx sensitivity. [Figure 5] FIG. 5 is a table showing the evaluation results of thermal shock resistance and NOx sensitivity.

Mode for Carrying Out the Invention

[0011] FIG. 1 is a cross-sectional view of a gas sensor 12 including a gas sensor element 10 according to the present embodiment. FIG. 2 is a perspective view of the gas sensor element 10. FIG. 3 is a cross-sectional view of the gas sensor element 10.

[0012] As shown in FIGS. 1 and 2, the gas sensor element 10 has a long rectangular parallelepiped shape. In the following description, the longitudinal direction of the gas sensor element 10 is the front-rear direction of the gas sensor element 10 and the gas sensor 12. The thickness direction of the gas sensor element 10 is the up-down direction of the gas sensor element 10 and the gas sensor 12. Further, the width direction (the direction perpendicular to the front-rear direction and the up-down direction) of the gas sensor element 10 is the left-right direction of the gas sensor element 10 and the gas sensor 12.

[0013] The gas sensor 12 includes a gas sensor element �0, a protective cover 14, and an element sealing body 16.

[0014] As shown in FIG. 2, the gas sensor element 10 includes an element main body 20 and a porous protective layer 22 (porous film). The element main body 20 has a long rectangular parallelepiped shape. The element main body 20 extends in the front-rear direction. The porous protective layer 22 covers the front end portion of the element main body 20. Note that the front end portion of the element main body 20 is covered with a protective cover 14 (see FIG. 1). The element main body 20 is sealed inside the gas sensor 12 by an element sealing body 16.

[0015] The gas sensor 12 measures the concentration of a specific gas contained in the exhaust gas (the gas to be measured). The specific gas is NOx, O2, etc. In this embodiment, the gas sensor 12 measures the NOx concentration.

[0016] As shown in Figure 3, the gas sensor element 10 has a laminate in which, for example, six layers are stacked sequentially from bottom to top. The six layers, in order from bottom to top, are a first substrate layer 42, a second substrate layer 44, a third substrate layer 46, a first solid electrolyte layer 48, a spacer layer 50, and a second solid electrolyte layer 52. The six layers are, for example, oxygen ion conductive solid electrolyte layers such as zirconia (ZrO2). The solid electrolytes forming the six layers are dense and airtight. The gas sensor element 10 is manufactured as follows: For example, a predetermined processing and printing of a circuit pattern are performed on a ceramic green sheet corresponding to each layer. Next, each ceramic green sheet is stacked to form a laminate. Next, the laminate is fired to integrate it. Note that not all of the above six layers need to be oxygen ion conductive solid electrolyte layers. For example, some of the six layers may be alumina layers.

[0017] The gas sensor element 10 has a plurality of diffusion-limiting sections and a plurality of internal cavities. The plurality of diffusion-limiting sections and the plurality of internal cavities are provided between the lower surface of the second solid electrolyte layer 52 and the upper surface of the first solid electrolyte layer 48. The gas sensor element 10 has a gas inlet 54, a first diffusion-limiting section 56, a buffer space 58, a second diffusion-limiting section 60, a first internal cavity 62, a third diffusion-limiting section 64, and a second internal cavity 66.

[0018] The gas inlet 54, the buffer space 58, the first internal cavity 62, and the second internal cavity 66 are provided by cutting out the spacer layer 50. These spaces are the internal spaces of the gas sensor element 10. These internal spaces are partitioned by the lower surface of the second solid electrolyte layer 52, the upper surface of the first solid electrolyte layer 48, and the side surface of the spacer layer 50.

[0019] The first diffusion rate-limiting section 56, the second diffusion rate-limiting section 60, and the third diffusion rate-limiting section 64 are all two horizontally elongated slits. The longitudinal direction of these slits is perpendicular to the plane of the paper in Figure 3. The section from the gas inlet 54 to the second internal cavity 66 is also referred to as the gas flow section under measurement.

[0020] In the gas sensor element 10, a reference gas introduction space 68 is provided at a location away from the gas flow section to be measured. The reference gas introduction space 68 is partitioned by the upper surface of the third substrate layer 46, the lower surface of the spacer layer 50, and the side surface of the first solid electrolyte layer 48. A reference gas used when measuring NOx concentration is introduced into the reference gas introduction space 68. The reference gas is, for example, air.

[0021] The atmospheric introduction layer 70 is exposed in the reference gas introduction space 68. The atmospheric introduction layer 70 is made of porous ceramics. The reference gas is introduced into the atmospheric introduction layer 70 through the reference gas introduction space 68. The atmospheric introduction layer 70 covers the reference electrode 72.

[0022] The gas inlet 54 is open to the external space. The gas inlet 54 takes in the gas to be measured into the gas sensor element 10 from the external space. The first diffusion rate-limiting unit 56 imparts a predetermined diffusion resistance to the gas to be measured taken in from the gas inlet 54. The buffer space 58 guides the gas to be measured from the first diffusion rate-limiting unit 56 to the second diffusion rate-limiting unit 60. The second diffusion rate-limiting unit 60 imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 58 into the first internal space 62.

[0023] The oxygen concentration (partial pressure of oxygen) in the atmosphere of the gas to be measured, introduced into the first internal cavity 62, is adjusted by the operation of the main pump cell 74. The main pump cell 74 is an electrochemical pump cell. The main pump cell 74 has an inner pump electrode 76, an outer pump electrode 78, and a second solid electrolyte layer 52. The inner pump electrode 76 is provided on the inner surface of the first internal cavity 62. The outer pump electrode 78 is provided on the upper surface of the second solid electrolyte layer 52 in a region corresponding to the inner pump electrode 76. The outer pump electrode 78 is exposed to the external space in the region corresponding to the inner pump electrode 76. The second solid electrolyte layer 52 is sandwiched between the inner pump electrode 76 and the outer pump electrode 78.

[0024] The inner pump electrode 76 is formed across the upper and lower solid electrolyte layers (second solid electrolyte layer 52 and first solid electrolyte layer 48) that partition the first internal cavity 62, and the spacer layer 50 that constitutes the side wall. The ceiling electrode portion 80 of the inner pump electrode 76 is formed on the lower surface of the second solid electrolyte layer 52. The lower surface of the second solid electrolyte layer 52 constitutes the ceiling surface of the first internal cavity 62. The bottom electrode portion 82 is directly formed on the upper surface of the first solid electrolyte layer 48. The upper surface of the first solid electrolyte layer 48 constitutes the bottom surface of the first internal cavity 62. The ceiling electrode portion 80 and the bottom electrode portion 82 are connected via a side electrode portion (not shown). The side electrode portion is formed on the side wall surface (inner surface) of the spacer layer 50 that constitutes the wall of the first internal cavity 62. The inner pump electrode 76 is arranged as a tunnel-shaped structure.

[0025] The inner pump electrode 76 and the outer pump electrode 78 are porous cermet electrodes. A porous cermet electrode is, for example, a cermet electrode made of Pt containing 1% Au and ZrO2. The inner pump electrode 76, which comes into contact with the gas to be measured, is formed using a material with weakened or no reducing ability for NOx components in the gas to be measured.

[0026] The main pump cell 74 applies a desired pump voltage Vp0 between the inner pump electrode 76 and the outer pump electrode 78. When the pump voltage Vp0 is applied, a pump current Ip0 flows between the inner pump electrode 76 and the outer pump electrode 78 in either the positive or negative direction. When the pump current Ip0 flows, the main pump cell 74 can pump oxygen from the first internal cavity 62 to the outside space. Also, when the pump current Ip0 flows, the main pump cell 74 can pump oxygen from the outside space into the first internal cavity 62.

[0027] The gas sensor element 10 has an oxygen partial pressure detection sensor cell 84 for controlling the main pump. Hereinafter, the oxygen partial pressure detection sensor cell 84 for controlling the main pump will be referred to as the main pump sensor cell 84. The main pump sensor cell 84 detects the oxygen concentration (oxygen partial pressure) in the atmosphere within the first internal cavity 62. The main pump sensor cell 84 is an electrochemical sensor cell. The main pump sensor cell 84 has an inner pump electrode 76, a second solid electrolyte layer 52, a spacer layer 50, a first solid electrolyte layer 48, and a reference electrode 72.

[0028] The gas sensor element 10 detects the oxygen concentration (partial pressure of oxygen) in the first internal cavity 62 by measuring the electromotive force V0 of the main pump sensor cell 84. The gas sensor element 10 controls the pump current Ip0 by feedback-controlling the pump voltage Vp0 of the variable power supply 86 so that the electromotive force V0 remains constant. By controlling the pump current Ip0, the oxygen concentration in the first internal cavity 62 is maintained at a predetermined constant value.

[0029] The third diffusion rate-limiting unit 64 imparts a predetermined diffusion resistance to the gas to be measured, whose oxygen concentration (partial pressure of oxygen) is controlled within the first internal cavity 62. The third diffusion rate-limiting unit 64 then guides the gas to be measured, to which the predetermined diffusion resistance has been applied, into the second internal cavity 66.

[0030] The second internal cavity 66 is a space for further adjusting the oxygen partial pressure of the gas to be measured, which is introduced through the third diffusion rate-limiting unit 64, using an auxiliary pump cell 88. By adjusting the oxygen partial pressure, the oxygen concentration in the second internal cavity 66 is kept constant with high precision.

[0031] The auxiliary pump cell 88 is an auxiliary electrochemical pump cell. The auxiliary pump cell 88 has an auxiliary pump electrode 90, an outer pump electrode 78, and a second solid electrolyte layer 52. The auxiliary pump electrode 90 is provided on the inner surface of the second internal cavity 66. The outer pump electrode 78 can be any suitable electrode on the outside of the gas sensor element 10.

[0032] The auxiliary pump electrode 90 is disposed within the second internal cavity 66. Similar to the inner pump electrode 76 provided within the first internal cavity 62, the auxiliary pump electrode 90 has a tunnel-shaped structure. The auxiliary pump electrode 90 has a ceiling electrode portion 92 formed in the second solid electrolyte layer 52. The second solid electrolyte layer 52 constitutes the ceiling surface of the second internal cavity 66. The auxiliary pump electrode 90 further has a bottom electrode portion 94 directly formed on the upper surface of the first solid electrolyte layer 48. The upper surface of the first solid electrolyte layer 48 constitutes the bottom surface of the second internal cavity 66. The ceiling electrode portion 92 and the bottom electrode portion 94 are connected via side electrode portions (not shown). The side electrode portions are formed on both wall surfaces of the spacer layer 50 that constitutes the side wall of the second internal cavity 66. Similar to the inner pump electrode 76, the auxiliary pump electrode 90 is formed using a material with reduced or no reducing ability for NOx components in the gas being measured.

[0033] The auxiliary pump cell 88 applies a desired voltage Vp1 between the auxiliary pump electrode 90 and the outer pump electrode 78. The application of voltage Vp1 enables the auxiliary pump cell 88 to pump oxygen from the atmosphere within the second internal cavity 66 into the outside space. Furthermore, the application of voltage Vp1 enables the auxiliary pump cell 88 to pump oxygen from the outside space into the second internal cavity 66.

[0034] The oxygen partial pressure detection sensor cell 96 for auxiliary pump control controls the oxygen partial pressure in the atmosphere within the second internal cavity 66. Hereinafter, the oxygen partial pressure detection sensor cell 96 for auxiliary pump control will be referred to as the auxiliary pump sensor cell 96. The auxiliary pump sensor cell 96 is an electrochemical sensor cell. The auxiliary pump sensor cell 96 includes an auxiliary pump electrode 90, a reference electrode 72, a second solid electrolyte layer 52, a spacer layer 50, and a first solid electrolyte layer 48.

[0035] The auxiliary pump sensor cell 96 detects the electromotive force V1. The variable power supply 98 is voltage-controlled based on the detected electromotive force V1. The auxiliary pump cell 88 performs pumping using the variable power supply 98. By pumping, the partial pressure of oxygen in the atmosphere within the second internal cavity 66 is controlled to a low partial pressure that does not substantially affect the measurement of NOx.

[0036] The pump current Ip1 is used to control the electromotive force V0 of the main pump sensor cell 84. The pump current Ip1 is input to the main pump sensor cell 84 as a control signal, thereby controlling the electromotive force V0. By controlling the electromotive force V0, the gradient of the oxygen partial pressure in the gas being measured, introduced from the third diffusion rate-limiting unit 64 into the second internal cavity 66, is controlled to remain constant. When the gas sensor 12 (see Figure 1) is used as a NOx sensor, the oxygen concentration in the second internal cavity 66 is maintained at a constant value of approximately 0.001 [ppm] through the action of the main pump cell 74 and the auxiliary pump cell 88.

[0037] The measuring pump cell 100 measures the NOx concentration in the gas to be measured within the second internal cavity 66. The measuring pump cell 100 is an electrochemical pump cell. The measuring pump cell 100 has a measuring electrode 102, an outer pump electrode 78, a second solid electrolyte layer 52, a spacer layer 50, and a first solid electrolyte layer 48. The measuring electrode 102 is formed directly on the upper surface of the first solid electrolyte layer 48 facing the second internal cavity 66. The measuring electrode 102 is provided on the upper surface of the first solid electrolyte layer 48, spaced apart from the third diffusion rate-limiting section 64. The measuring electrode 102 is a porous cermet electrode. The measuring electrode 102 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere within the second internal cavity 66. The measuring electrode 102 is covered by a fourth diffusion rate-limiting section 104.

[0038] The fourth diffusion rate-limiting section 104 is a film made of a porous ceramic material. The fourth diffusion rate-limiting section 104 plays a role in limiting the amount of NOx flowing into the measuring electrode 102. The fourth diffusion rate-limiting section 104 also functions as a protective film for the measuring electrode 102. The measuring pump cell 100 pumps out oxygen generated by the decomposition of nitrogen oxides in the atmosphere surrounding the measuring electrode 102 and detects the amount generated as the pump current Ip2.

[0039] The oxygen partial pressure detection sensor cell 106 for controlling the measuring pump detects the oxygen partial pressure around the measuring electrode 102. Hereinafter, the oxygen partial pressure detection sensor cell 106 for controlling the measuring pump will be referred to as the measuring pump sensor cell 106. The measuring pump sensor cell 106 is an electrochemical sensor cell. The measuring pump sensor cell 106 has a first solid electrolyte layer 48, a measuring electrode 102, and a reference electrode 72. The variable power supply 108 is controlled based on the electromotive force V2 detected by the measuring pump sensor cell 106.

[0040] The gas to be measured, introduced into the second internal cavity 66, reaches the measuring electrode 102 through the fourth diffusion rate-limiting unit 104 under controlled conditions of oxygen partial pressure. Nitrogen oxides in the gas to be measured surrounding the measuring electrode 102 are reduced to generate oxygen (2NO → N2 + O2). The generated oxygen is pumped by the measuring pump cell 100. The voltage Vp2 of the variable power supply 108 is controlled so that the electromotive force V2 detected by the measuring pump sensor cell 106 remains constant. The amount of oxygen generated around the measuring electrode 102 is proportional to the concentration of nitrogen oxides in the gas to be measured. Therefore, the nitrogen oxide concentration in the gas to be measured is calculated using the pump current Ip2 of the measuring pump cell 100.

[0041] The sensor cell 110 is an electrochemical sensor cell. The sensor cell 110 includes a second solid electrolyte layer 52, a spacer layer 50, a first solid electrolyte layer 48, a third substrate layer 46, an outer pump electrode 78, and a reference electrode 72. The electromotive force Vref obtained by the sensor cell 110 allows detection of the partial pressure of oxygen in the gas to be measured outside the gas sensor 12.

[0042] The measuring electrode 102, the first solid electrolyte layer 48, the third substrate layer 46, and the reference electrode 72 may be combined to form an oxygen partial pressure detection unit as an electrochemical sensor cell. This makes it possible to detect the electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of NOx components in the atmosphere around the measuring electrode 102 and the amount of oxygen contained in the reference atmosphere. As a result, it is possible to determine the concentration of NOx components in the gas being measured.

[0043] The gas sensor 12 operates the main pump cell 74 and the auxiliary pump cell 88 to supply the gas to be measured to the measuring pump cell 100, where the oxygen partial pressure is always kept at a constant low value. A constant low value for oxygen partial pressure refers to a value that does not substantially affect the measurement of NOx. Therefore, the NOx concentration in the gas to be measured can be determined based on the pump current Ip2 described above. The pump current Ip2 flows as oxygen generated by the reduction of NOx is pumped out of the measuring pump cell 100, and is approximately proportional to the NOx concentration in the gas to be measured.

[0044] The gas sensor element 10 includes a heater section 112. The heater section 112 plays a role in temperature control, heating and maintaining the temperature of the gas sensor element 10. This increases the oxygen ion conductivity of the solid electrolyte. The heater section 112 includes a heater connector electrode 114, a heater 116, a through-hole 118, a heater insulating layer 120, and a pressure relief hole 122.

[0045] The heater connector electrode 114 is formed on the lower surface of the first substrate layer 42. The heater connector electrode 114 is connected to an external power supply (not shown). The heater connector electrode 114 supplies power from the external power supply to the heater unit 112.

[0046] The heater 116 is sandwiched from above and below by the second substrate layer 44 and the third substrate layer 46. The heater 116 is an electrical resistor. The heater 116 is connected to the heater connector electrode 114 via a through-hole 118. The heater 116 generates heat through power supplied from the heater connector electrode 114. By generating heat, the heater 116 heats and maintains the temperature of the solid electrolyte forming the gas sensor element 10.

[0047] The heater 116 is embedded throughout the entire area from the first internal cavity 62 to the second internal cavity 66. The heater 116 can adjust the temperature of the entire gas sensor element 10 to the temperature at which the solid electrolyte is activated.

[0048] The heater insulating layer 120 is formed on the upper and lower surfaces of the heater 116 using an insulator such as alumina. The heater insulating layer 120 is formed to provide electrical insulation between the second substrate layer 44 and the heater 116, and between the third substrate layer 46 and the heater 116.

[0049] The pressure relief holes 122 penetrate the third substrate layer 46 and are provided to communicate with the reference gas introduction space 68. The pressure relief holes 122 are formed to mitigate the rise in internal pressure due to the rise in temperature within the heater insulating layer 120.

[0050] The gas sensor element 10 has a coating layer 124. The coating layer 124 comprises an upper coating layer 126 that covers the upper surface of the element body 20 and a lower coating layer 128 that covers the lower surface of the element body 20. The upper coating layer 126 also covers the outer pump electrode 78. The coating layer 124 is made of porous ceramics.

[0051] The porous protective layer 22 covers the outer surface (top, bottom, front, and left and right sides) of the front end of the element body 20. The porous protective layer 22 covers a portion of the upper coating layer 126, a portion of the lower coating layer 128, and the outer pump electrode 78. The porous protective layer 22 is made of porous ceramics containing alkali metals. The alkali metals are Na, K, Li, Rb, or Cs. The alkali metal content in the porous protective layer 22 is preferably in the range of 150 ppm to 3500 ppm. More preferably, the alkali metal content in the porous protective layer 22 is in the range of 200 ppm to 2000 ppm. In this embodiment, the alkali metal content includes not only the alkali metal content but also the alkali metal oxide content.

[0052] Furthermore, the porous protective layer 22 may contain Si or Fe. In this case, Si or Fe may be, for example, an additive to the alkali metal in the porous protective layer 22, or an impurity in the porous protective layer 22. The Si content in the porous protective layer 22 is preferably 1500 ppm or less in terms of SiO2. The Fe content in the porous protective layer 22 is preferably 500 ppm or less in terms of Fe2O3.

[0053] The porous protective layer 22 is formed on the front end of the element body 20, for example, by plasma spraying. Specifically, a plasma-state gas is injected from a plasma gun (not shown) toward the front end of the element body 20. At this time, the powder spraying material, which is the material for forming the porous protective layer 22, is supplied to the front end of the element body 20. The powder spraying material is sprayed toward the front end of the element body 20 together with the plasma-state gas. The powder spraying material is heated and melted by the plasma-state gas and collides with the front end of the element body 20. The porous protective layer 22 is formed as the powder spraying material that collides with the front end of the element body 20 rapidly solidifies.

[0054] The powder spraying material is a powder that forms the porous protective layer 22. For example, the powder spraying material is alumina powder. However, the powder spraying material is not limited to alumina powder. The powder spraying material may be spinel, mullite, zirconia, yttria, gray alumina, or a mixture containing one or more of these. Note that mullite contains silica as a constituent material. Therefore, when the powder spraying material is mullite, or a mixture containing one or more mullites, there are no restrictions on the Si content in the powder spraying material.

[0055] The porous protective layer 22 may be formed on the front end of the element body 20 by a method other than plasma spraying, such as a dip method or a spray method. For example, in the dip method, a slurry containing ceramic particles is applied to the front end of the element body 20. Next, the slurry is fired to form the porous protective layer 22.

[0056] Figure 3 illustrates a case where one layer of porous protective layer 22 covers the front end of the element body 20. In this case, the thickness of the one layer of porous protective layer 22 is preferably in the range of 100 [μm] to 1000 [μm]. Furthermore, the porosity of the one layer of porous protective layer 22 is preferably in the range of 10 [%] to 40 [%].

[0057] The porous protective layer 22 may have a two-layer structure consisting of an inner layer and an outer layer. Alternatively, the porous protective layer 22 may have a structure of three or more layers.

[0058] If the porous protective layer 22 comprises two or more layers, the thickness of the layer closest to the element body 20 (the innermost layer) is preferably in the range of 170 [μm] to 900 [μm]. Furthermore, the porosity of the layer closest to the element body 20 is preferably in the range of 20 [%] to 70 [%].

[0059] Furthermore, if the porous protective layer 22 comprises two or more layers, the thickness of the outermost layer furthest from the element body 20 is preferably in the range of 30 [μm] to 400 [μm]. In addition, the porosity of the outermost layer furthest from the element body 20 is preferably in the range of 10 [%] to 60 [%].

[0060] Furthermore, if the porous protective layer 22 comprises two or more layers, the alkali metal content in each layer is preferably in the range of 150 ppm to 3500 ppm. More preferably, the alkali metal content in each layer is in the range of 200 ppm to 2000 ppm. When each layer is formed using a powder spray material other than mullite or a mixture containing one or more types of mullite, the Si content in each layer is preferably 1500 ppm or less in terms of SiO2. Also, the Fe content in each layer is preferably 500 ppm or less in terms of Fe2O3.

[0061] Next, evaluation tests for the thermal shock resistance and measurement sensitivity of the gas sensor element 10 will be described. In the evaluation tests, thermal shock resistance and measurement sensitivity were evaluated for multiple gas sensor elements 10 (Examples 1 to 6, Comparative Examples 1 to 4) having different alkali metal content in one or two porous protective layers 22. In addition, in the evaluation tests, thermal shock resistance and measurement sensitivity were evaluated for multiple gas sensor elements 10 (Examples 7 to 10) having different Si or Fe content in one porous protective layer 22.

[0062] The alkali metal content in the porous protective layer 22 was measured using the following method. Note that the following measurement method is for measuring the sodium oxide content. Other alkali metals can be measured similarly. The Si and Fe content can also be measured using the same method.

[0063] First, the porous protective layer 22 is peeled off the gas sensor element 10. Next, the peeled porous protective layer 22 is crushed in a mortar made of high-purity alumina. After weighing 0.5 g of the crushed porous protective layer 22 powder, the weighed powder is placed into the first container made of PTFE.

[0064] Next, add 7.5 ml of a sulfuric acid solution (water:sulfuric acid = 1:3) to the first container. Then, place the first container into the second stainless steel container. With the first container inside, close the lid and seal the second container.

[0065] Next, the sealed second container is placed in a 230°C constant temperature bath and heated for 24 hours. During this heating process, the temperature is maintained at 230°C for 24 hours, excluding the heating and cooling times within the constant temperature bath. The heating process dissolves the porous protective layer 22 powder in the sulfuric acid solution. As a result, a solution of the porous protective layer 22 powder is obtained.

[0066] Next, remove the second container from the constant temperature bath and remove the lid from the second container. Then, add water to the dissolving solution to make a 50 ml sample.

[0067] Next, a qualitative analysis of the sample was performed using ICP emission spectroscopy (ICP-AES). This qualitative analysis recorded the test concentration value of Na. In this qualitative analysis, no alkali metals other than Na were detected in the sample.

[0068] The actual Na concentration is determined using the Na test concentration values ​​obtained from the qualitative analysis described above and the Na calibration curve that was determined in advance.

[0069] The calculated actual Na concentration value is converted to a Na oxide concentration value. This allows the Na oxide content, which is the Na oxide concentration (Na2O concentration), to be calculated.

[0070] In the evaluation test of the thermal shock resistance of the porous protective layer 22, the following thermal cycling test was performed. In the thermal cycling test, the gas sensor element 10 was repeatedly heated by driving the heater unit 112 and cooled by stopping the heater unit 112. Specifically, in the thermal cycling test, the following temperature profile was used for the gas sensor element 10, with one heating / cooling cycle being 600 cycles.

[0071] Specifically, one cycle consisted of (1) 5 minutes in a temperature environment of 950 [°C] and (2) 5 minutes in a temperature environment of 300 [°C]. In case (1) above, the gas sensor element 10 was heated in an exhaust gas atmosphere with an air-fuel ratio of 1.1. In case (2) above, the gas sensor element 10 was heated in air. After the thermal cycle test, the presence or absence of delamination of the porous protective layer 22 from the element body 20 and the presence or absence of cracks in the porous protective layer 22 were checked. X-ray CT was used to check for delamination and cracks in the porous protective layer 22.

[0072] The evaluation test for the measurement sensitivity of NOx concentration was performed using the following method. In this evaluation test, the NOx concentration was measured using a measurement pump cell 100, etc.

[0073] Specifically, the sensitivity of the gas sensor element 10 to NOx concentration was measured in a model gas containing 500 ppm of NOx. This sensitivity was used as the initial sensitivity for measuring NOx concentration.

[0074] Next, the process of dropping water onto the porous protective layer 22 and operating the gas sensor element 10 at a high temperature was repeated 100 times, with each cycle being the same. Specifically, in one cycle, (1) 1 μl of water was dropped onto the porous protective layer 22 formed on the gas sensor element 10, and after standing for 1 minute, (2) the gas sensor 12 was operated at 800°C for 10 minutes. Thus, by repeating this 100 times, a total of 100 μl of water was dropped onto the porous protective layer 22.

[0075] Next, the sensitivity of the NOx concentration in the model gas was measured again using the gas sensor element 10. By comparing the measured NOx concentration sensitivity with the initial NOx concentration measurement sensitivity, the rate of decrease in measurement sensitivity was calculated.

[0076] Figure 4 is a table showing the evaluation results (Examples 1 to 6) of the gas sensor element 10 according to this embodiment (see Figures 1 to 3) and the evaluation results (Comparative Examples 1 to 4) of the gas sensor element 10 of the comparative examples. Examples 1 to 6 show cases where the alkali metal content (Na2O content) in the one or two porous protective layers 22 is in the range of 150 [ppm] to 3500 [ppm]. Comparative Examples 1 to 4 show cases where the Na2O content in the one or two porous protective layers 22 is less than 150 [ppm] or more than 3500 [ppm].

[0077] Figure 5 is a table showing the evaluation results (Examples 7 to 10) of the gas sensor element 10 (see Figures 1 to 3) according to this embodiment. Examples 7 to 10 show the case where the alkali metal content (Na2O content) in one porous protective layer 22 is in the range of 500 [ppm] to 700 [ppm]. Examples 7 to 10 also show the case where the Si content in one porous protective layer 22 is in the range of 900 [ppm] to 1900 [ppm] in terms of SiO2, and the Fe content in one porous protective layer 22 is in the range of 400 [ppm] to 700 [ppm] in terms of Fe2O3.

[0078] The evaluation criteria are as follows: In evaluating thermal shock resistance, X-ray CT was used, and if there was no delamination or cracking of the porous protective layer 22, it was judged as "○" (good thermal shock resistance). If there were small cracks in the porous protective layer 22, it was judged as "△" (slightly reduced thermal shock resistance). If both delamination and cracking of the porous protective layer 22 were present, it was judged as "X" (reduced thermal shock resistance).

[0079] In evaluating the measurement sensitivity of NOx concentration, a decrease of less than 1% from the initial measurement sensitivity was judged as "◎" (excellent measurement sensitivity). A decrease of less than 2% from the initial measurement sensitivity was judged as "○" (good measurement sensitivity). A decrease of less than 5% from the initial measurement sensitivity was judged as "△" (slightly decreased measurement sensitivity). A decrease of 5% or more from the initial measurement sensitivity was judged as "X" (decreased measurement sensitivity).

[0080] As shown in Figure 4, in Examples 1 to 6, the evaluation results for thermal shock resistance and measurement sensitivity were "○" or "△". In other words, by keeping the alkali metal content in the porous protective layer 22 (see Figures 1 to 3) within the range of 150 [ppm] to 3500 [ppm], it is possible to suppress the decrease in thermal shock resistance and the decrease in the measurement sensitivity of NOx concentration.

[0081] In particular, in Examples 2 to 5, the evaluation results for thermal shock resistance and measurement sensitivity were all "○". That is, if the alkali metal content in the porous protective layer 22 is within the range of 200 [ppm] to 2000 [ppm], the decrease in thermal shock resistance and the decrease in NOx concentration measurement sensitivity are further suppressed. As a result, both thermal shock resistance and NOx concentration measurement sensitivity can be improved.

[0082] The alkali metals in the porous protective layer 22 form a glass phase at the grain boundaries. The formation of the glass phase increases the bonding strength between particles in the porous protective layer 22. If the alkali metal content in the porous protective layer 22 is high, peeling or detachment of the porous protective layer 22 from the element body 20 is suppressed.

[0083] Furthermore, as shown in Figure 4, both Example 3, which has two porous protective layers 22 (see Figures 1 to 3), and Example 4, which has one porous protective layer 22, show a "○" evaluation result for thermal shock resistance and measurement sensitivity. However, in this embodiment, the higher the porosity of the porous protective layer 22, that is, the greater the number of porous protective layers 22, the greater the suppression effect against peeling or detachment of the porous protective layer 22 from the element body 20 and against cracking of the porous protective layer 22. In other words, in the above embodiments, Example 3 with two porous protective layers 22 shows a more pronounced suppression effect than Example 4 with one porous protective layer 22.

[0084] As shown in Figure 5, in Examples 7 and 8, the evaluation results for thermal shock resistance and measurement sensitivity were "○". In other words, when the alkali metal content in the porous protective layer 22 (see Figures 1 to 3) is in the range of 150 [ppm] to 3500 [ppm], it is possible to suppress the decrease in thermal shock resistance and the decrease in NOx concentration measurement sensitivity even if Si or Fe is contained in the porous protective layer 22.

[0085] Furthermore, in Examples 9 and 10, the evaluation result for thermal shock resistance was "○" and the evaluation result for measurement sensitivity was "◎". In other words, when the alkali metal content in the porous protective layer 22 is in the range of 150 [ppm] to 3500 [ppm], if the Si content is 1500 [ppm] or less in terms of SiO2, or if the Fe content is 500 [ppm] or less in terms of Fe2O3, it is possible to suppress the decrease in measurement sensitivity of NOx concentration while suppressing the decrease in thermal shock resistance.

[0086] Furthermore, when moisture adheres to the porous protective layer 22, alkali metals, Si, or Fe in the porous protective layer 22 may dissolve into the moisture. The moisture containing dissolved alkali metals, Si, or Fe penetrates into the porous protective layer 22 and reaches the surface of the element body 20. An outer pump electrode 78 is provided on the upper surface of the element body 20. The outer pump electrode 78 is covered with the porous protective layer 22. When the above-mentioned moisture reaches the outer pump electrode 78, the noble metal in the outer pump electrode 78 reacts with the alkali metals, Si, or Fe in the moisture. This reaction between the noble metal and the alkali metals, Si, or Fe may reduce the sensitivity of the NOx concentration measurement.

[0087] In this embodiment, by setting the alkali metal content in the porous protective layer 22 within the above range, the bonding strength between the porous protective layer 22 and the element body 20 is increased while suppressing a decrease in the measurement sensitivity of the gas sensor element 10. In other words, by setting the alkali metal content within the above range, both improvement in the adhesion strength of the porous protective layer 22 and reduction in the decrease in sensitivity of the gas sensor element 10 are achieved.

[0088] Furthermore, in this embodiment, by setting the content of Si and Fe in the porous protective layer 22 to the above range, in addition to the above effects, the measurement sensitivity of the gas sensor element 10 is further improved.

[0089] In contrast, the thermal shock resistance evaluation result for Comparative Examples 1 and 2 is "X". In Comparative Examples 1 and 2, the alkali metal content in the porous protective layer 22 is low, making it impossible to suppress peeling or detachment of the porous protective layer 22. In Comparative Examples 3 and 4, the NOx sensitivity evaluation result is "X". In Comparative Examples 3 and 4, the alkali metal content in the porous protective layer 22 is too high, causing a reaction between the precious metal in the outer pump electrode 78 and the alkali metal in the water, reducing the sensitivity of NOx concentration measurement.

[0090] The inventions that can be understood from the above embodiments are described below.

[0091] An aspect of the present invention is a gas sensor element (10) comprising an element body (20) and a porous coating (22) covering a part of the element body, wherein the alkali metal content in the porous coating is 150 [ppm] to 3500 [ppm].

[0092] According to the present invention, the bonding strength between the surface of the element body and the porous coating is improved. Furthermore, a decrease in the measurement sensitivity of the gas sensor element can be suppressed. In other words, in the present invention, by setting the alkali metal content in the porous coating within the above range, it is possible to achieve both improved adhesion strength of the porous coating and suppression of a decrease in the measurement sensitivity of the gas sensor element.

[0093] In aspects of the present invention, the alkali metal content may be 200 ppm to 2000 ppm.

[0094] According to the present invention, the bonding strength between the surface of the element body and the porous coating is further improved, and the decrease in the measurement sensitivity of the gas sensor element can be further suppressed.

[0095] In one aspect of the present invention, the porous coating may comprise at least two or more layers.

[0096] According to the present invention, it is possible to set the alkali metal content for each layer. Furthermore, by increasing the number of porous coating layers, the effect of suppressing peeling or detachment of the porous coating from the element body and cracking of the porous coating is greatly enhanced.

[0097] In aspects of the present invention, the thickness of the layer closest to the element body among the porous coatings may be 170 [μm] to 900 [μm], and the porosity of the layer closest to the element body may be 20 [%] to 70 [%].

[0098] According to the present invention, it is possible to improve the bonding strength between the surface of the element body and the porous coating while suppressing a decrease in the measurement sensitivity of the gas sensor element.

[0099] In aspects of the present invention, the thickness of the outermost layer of the porous coating, furthest from the element body, may be 30 [μm] to 400 [μm], and the porosity of the outermost layer, furthest from the element body, may be 10 [%] to 60 [%].

[0100] According to the present invention, it is possible to suppress the decrease in the measurement sensitivity of the gas sensor element.

[0101] In aspects of the present invention, the alkali metal content in each layer of the porous film may be 150 ppm to 3500 ppm.

[0102] According to the present invention, it is possible to improve the bonding strength between the surface of the element body and the porous coating, and to suppress the decrease in the measurement sensitivity of the gas sensor element.

[0103] In aspects of the present invention, the alkali metal content in each layer of the porous film may be 200 ppm to 2000 ppm.

[0104] According to the present invention, the bonding strength between the surface of the element body and the porous coating can be further improved, and the decrease in the measurement sensitivity of the gas sensor element can be further suppressed.

[0105] In aspects of the present invention, the alkali metal may be Na.

[0106] According to the present invention, the above effects can be easily achieved.

[0107] In aspects of the present invention, the Si content in the porous film may be 1500 ppm or less in terms of SiO2.

[0108] According to the present invention, the measurement sensitivity of the gas sensor element can be further improved.

[0109] In aspects of the present invention, the Fe content in the porous film may be 500 ppm or less in terms of Fe2O3.

[0110] According to the present invention, the measurement sensitivity of the gas sensor element can be further improved.

[0111] In aspects of the present invention, the porous coating may be formed by plasma spraying.

[0112] According to the present invention, the above-mentioned porous coating can be easily formed.

[0113] In aspects of the present invention, the porous coating may include at least one material selected from alumina, spinel, mullite, zirconia, yttria, and gray alumina.

[0114] According to the present invention, the above-mentioned porous coating can be easily obtained.

[0115] Furthermore, the present invention is not limited to the disclosure described above, and can take various configurations without departing from the spirit of the invention. [Explanation of Symbols]

[0116] 10...Gas sensor element 20... Element body 22...Porous protective layer (porous coating)

Claims

1. A gas sensor element comprising an element body and a porous coating covering a part of the element body, The alkali metal content in the porous coating is 150 ppm to 3500 ppm. The porous coating comprises at least two or more layers, Of the porous coating, the thickness of the layer closest to the element body is 170 [μm] to 900 [μm]. A gas sensor element wherein the porosity of the layer closest to the element body is 20% to 70%.

2. A gas sensor element comprising an element body and a porous coating covering a part of the element body, The alkali metal content in the porous coating is 150 ppm to 3500 ppm. The porous coating comprises at least two or more layers, Of the porous coating, the thickness of the outermost layer furthest from the element body is 30 [μm] to 400 [μm]. A gas sensor element in which the porosity of the outermost layer furthest from the element body is 10% to 60%.

3. A gas sensor element comprising an element body and a porous coating covering a part of the element body, The alkali metal content in the porous coating is 150 ppm to 3500 ppm. The porous coating comprises at least two or more layers, A gas sensor element in which the alkali metal content in each layer of the porous coating is 150 ppm to 3500 ppm.

4. A gas sensor element comprising an element body and a porous coating covering a part of the element body, The alkali metal content in the porous coating is 150 ppm to 3500 ppm. A gas sensor element wherein the Si content in the porous film is 1500 ppm or less in terms of SiO₂.

5. A gas sensor element comprising an element body and a porous coating covering a part of the element body, The alkali metal content in the porous coating is 150 ppm to 3500 ppm. A gas sensor element wherein the Fe content in the porous coating is 500 ppm or less in terms of Fe₂O₃.

6. In the gas sensor element according to any one of claims 1 to 5, A gas sensor element having an alkali metal content of 200 ppm to 2000 ppm.

7. In the gas sensor element according to claim 4 or 5, The porous coating comprises at least two layers, comprising a gas sensor element.

8. In the gas sensor element according to claim 7, Of the porous coating, the thickness of the layer closest to the element body is 170 [μm] to 900 [μm]. A gas sensor element wherein the porosity of the layer closest to the element body is 20% to 70%.

9. In the gas sensor element according to claim 7, Of the porous coating, the thickness of the outermost layer furthest from the element body is 30 [μm] to 400 [μm]. A gas sensor element in which the porosity of the outermost layer furthest from the element body is 10% to 60%.

10. In the gas sensor element according to claim 7, A gas sensor element in which the alkali metal content in each layer of the porous coating is 150 ppm to 3500 ppm.

11. In the gas sensor element according to claim 7, A gas sensor element in which the alkali metal content in each layer of the porous coating is 200 ppm to 2000 ppm.

12. In the gas sensor element according to any one of claims 1 to 5, The alkali metal is Na in the gas sensor element.

13. In the gas sensor element according to any one of claims 1 to 3, 5, The Si content in the porous coating is SiO ２ A gas sensor element with a concentration of 1500 ppm or less (converted).

14. In the gas sensor element according to any one of claims 1 to 4, The Fe content in the porous coating is ２ O ３ A gas sensor element with a concentration of 500 ppm or less (converted).

15. In the gas sensor element according to any one of claims 1 to 5, The porous coating is a gas sensor element formed by plasma spraying.

16. In the gas sensor element according to any one of claims 1 to 5, The porous coating comprises at least one material from alumina, spinel, mullite, zirconia, yttria, and gray alumina, and is used as a gas sensor element.

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