SENSOR ELEMENT AND GAS SENSOR

A sensor element for detecting a specific gas concentration in a measurement-object gas, the sensor element includes; an elongate element body that includes a solid electrolyte layer and has a shape including at least one side surface extending in a longitudinal direction; a dense layer that is disposed on the side surface; and an intermediate layer disposed at least between the dense layer and the element body, wherein, when thermal expansion coefficients of the solid electrolyte layer, the dense layer, and the intermediate layer in a temperature range of from 20° C. to 1360° C. are denoted by thermal expansion coefficients Ea, Eb, and Ec, respectively, a ratio Ea/Eb is more than 1.0 and 5.0 or less, and Ea>Ec>Eb is satisfied.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sensor element and a gas sensor.

2. Description of the Related Art

A sensor element that detects the concentration of a specific gas such as NOx in a measurement-object gas such as an exhaust gas of an automobile is a known art (for example, Patent Literature 1). The sensor element in Patent Literature 1 includes: an elongate element body; an outer electrode disposed on the upper surface of the element body; an outer lead portion; a connector electrode; and a porous layer that covers the outer electrode and the outer lead portion. The outer electrode, the outer lead portion, and the connector electrode are connected in this order and are electrically continuous with each other, and the connector electrode is electrically connected to the outside. The sensor element in Patent Literature 1 further includes a dense layer disposed so as to divide the porous layer in the longitudinal direction of the element body. The dense layer covers the outer lead portion. The dense layer does not easily allow moisture to pass therethrough. Therefore, even when moisture in the measurement-object gas moves through the porous layer by capillary action, the presence of the dense layer prevents the moisture from reaching the connector electrode. A method for producing the above sensor element that is described in Patent Literature 1 includes: forming electrodes, green porous layers, and green dense layers on a plurality of green ceramic sheets corresponding to the element body by screen printing; stacking the plurality of green ceramic sheets; and firing the stacked green ceramic sheets.

CITATION LIST

Patent Literature

SUMMARY OF THE INVENTION

As for the sensor element including the dense layer as described in Patent Literature 1, cracking may occur in the sensor element. It is therefore desirable to reduce the occurrence of cracking in the sensor element.

The present invention has been made to solve the foregoing problem, and it is a main object to further reduce the occurrence of cracking in a sensor element.

To achieve the above main object, the present invention employs the following means.

The sensor element of the present invention is a sensor element for detecting a specific gas concentration in a measurement-object gas, the sensor element including: an elongate element body that includes a solid electrolyte layer and has a shape including at least one side surface extending in a longitudinal direction and forward and rear ends that are ends opposite to each other in the longitudinal direction; at least one connector electrode that is disposed on a rear end side of any of the at least one side surface and provided for electrical continuity with the outside of the sensor element; a porous layer that has a porosity of 10% or more and covers at least a forward end side of the side surface on which the connector electrode is disposed; a dense layer that is disposed on the side surface so as to divide the porous layer in the longitudinal direction or to be located rearward of the porous layer, is located forward of the connector electrode, and has a porosity of less than 10%; and an intermediate layer disposed at least between the dense layer and the element body, wherein, when thermal expansion coefficients of the solid electrolyte layer, the dense layer, and the intermediate layer in a temperature range of from 20° C. to 1360° C. are denoted by thermal expansion coefficients Ea, Eb, and Ec, respectively, the ratio Ea/Eb is more than 1.0 and 5.0 or less, and Ea>Ec>Eb is satisfied.

The sensor element includes the solid electrolyte layer, the dense layer, and the intermediate layer. The ratio Ea/Eb of the thermal expansion coefficient Ea of the solid electrolyte layer to the thermal expansion coefficient Eb of the dense layer is more than 1.0 and 5.0 or less, and the thermal expansion coefficient of the solid electrolyte layer is relatively close to the thermal expansion coefficient of the dense layer. Moreover, the intermediate layer is present at least between the dense layer and the solid electrolyte layer, and the thermal expansion coefficient Ec of the intermediate layer satisfies Ea>Ec>Eb. Specifically, the intermediate layer whose thermal expansion coefficient Ec is between the thermal expansion coefficient of the solid electrolyte layer and the thermal expansion coefficient of the dense layer is present between them. Since the solid electrolyte layer, the dense layer, and the intermediate layer satisfy the above positional relation and the relations between the thermal expansion coefficients Ea to Ec, the intermediate layer reduces stress caused by the difference between the thermal expansion coefficient Ea of the solid electrolyte layer and the thermal expansion coefficient Eb of the dense layer when the sensor element is heated during use. When the stress is generated in the sensor element, cracking is likely to occur. However, in the above sensor element, since the stress is reduced, the occurrence of cracking is reduced.

In the sensor element of the present invention, when the mean value of the thermal expansion coefficient Ea and the thermal expansion coefficient Eb is denoted by Ed (=(Ea+Eb)/2), formula (1) below may be satisfied. In this case, the thermal expansion coefficient Ec is relatively close to the median Ed of the thermal expansion coefficients Ea and Eb. Specifically, the thermal expansion coefficient Ec is not excessively close to the thermal expansion coefficient Ea and not excessively close to the thermal expansion coefficient Eb. Therefore, the stress generated when the sensor element is heated is further reduced, and the occurrence of cracking is further reduced.

In the sensor element of the present invention, the ratio Ea/Eb may be 3.0 or less. In this case, the thermal expansion coefficient Ea of the solid electrolyte layer and the thermal expansion coefficient Eb of the dense layer are closer to each other, so that the occurrence of cracking in the sensor element is further reduced.

In the sensor element of the present invention, the intermediate layer may have a thickness T of 1 μm or more. In this case, the effect of the presence of the intermediate layer in reducing the occurrence of cracking in the sensor element is obtained more reliably. The thickness T of the intermediate layer may be 10 μm or less.

In the sensor element of the present invention, the solid electrolyte layer may contain zirconia as a main component, and the dense layer may contain alumina as a main component. The intermediate layer may contain zirconia and alumina. The main component as used herein means a component with the highest content and is specifically a component with the highest volume ratio.

In the sensor element of the present invention, the sensor element may include: a detection portion including a plurality of electrodes disposed on a forward end side of the element body and used to detect the specific gas concentration in the measurement-object gas; and an outer lead portion that is disposed on the side surface on which the connector electrode is disposed and provides electrical continuity between any of the plurality of electrodes and the connector electrode. The porous layer may cover at least part of the outer lead portion. In this case, the porous layer may fully cover a portion of the outer lead portion that is not covered with the dense layer. The sensor element of the present invention may include an outer electrode that is one of the plurality of electrodes included in the detection portion and that is electrically continuous with the connector electrode through the outer lead portion and disposed on the side surface on which the connector electrode is disposed. In this case, the porous layer may cover the outer electrode.

The gas sensor of the present invention includes the sensor element in any of the above modes. Therefore, the gas sensor has the same effect as the effect of the above-described sensor element of the present invention, e.g., the effect of reducing the occurrence of cracking in the sensor element.

DETAILED DESCRIPTION OF THE INVENTION

Next, embodiments of the present invention will be described using the drawings.FIG.1is a vertical cross-sectional view showing the manner of attaching, to a pipe58, a gas sensor10including a sensor element20in an embodiment of the present invention.FIG.2is a perspective view of the sensor element20when it is viewed from the upper right front.FIG.3is a cross-sectional view taken along A-A inFIG.2.FIG.4is a top view of the sensor element20.FIG.5is a bottom view of the sensor element20.FIG.6is an enlarged view around an intermediate layer98inFIG.3.FIG.7is a partial cross-sectional view around the intermediate layer98in a cross-section taken along B-B inFIG.4. In the present embodiment, as shown inFIGS.2and3, the longitudinal direction of an element body60of the sensor element20is defined as a forward-rearward direction (lengthwise direction) of the element body60, and the stacking direction (thickness direction) of the element body60is defined as an upward-downward direction. A direction perpendicular to the forward-rearward direction and the upward-downward direction is defined as a left-right direction (width direction).

As shown inFIG.1, the gas sensor10includes an assembly15, a bolt47, an external cylinder48, a connector50, lead wires55, and a rubber stopper57. The assembly15includes the sensor element20, a protective cover30, and an element-sealing member40. The gas sensor10is attached to the pipe58such as an exhaust gas pipe of a vehicle and used to measure the concentration of a specific gas (a specific gas concentration) such as NOx or O2contained in the exhaust gas used as a measurement-object gas. In the present embodiment, the gas sensor10measures the concentration of NOx as the specific gas concentration. The sensor element20has opposite ends (forward and rear ends) in the longitudinal direction, and the forward end side is the side exposed to the measurement-object gas.

As shown inFIG.1, the protective cover30includes a bottomed cylindrical inner protective cover31that covers the forward end side of the sensor element20and a bottomed cylindrical outer protective cover32that covers the inner protective cover31. A plurality of holes for allowing circulation of the measurement-object gas are formed in each of the inner and outer protective covers31and32. An element chamber33is formed as a space surrounded by the inner protective cover31, and a fifth surface60e(forward end surface) of the sensor element20is disposed inside the element chamber33.

The element-sealing member40is a member for sealing and fixing the sensor element20. The element-sealing member40includes: a cylindrical member41including a main metal fitting42and an inner cylinder43; insulators44ato44c; powder compacts45aand45b; and a metal ring46. The sensor element20is located on the center axis of the element-sealing member40and pierces through the element-sealing member40in the forward-rearward direction.

The main metal fitting42is a cylindrical metallic member. The main metal fitting42has a thick-walled portion42alocated on the forward side and having an inner diameter smaller than that of the rear side. The protective cover30is attached to a portion of the main metal fitting42that is on the same side as the forward end of the sensor element20(i.e., the forward side). The rear end of the main metal fitting42is welded to a flange portion43aof the inner cylinder43. A part of the inner circumferential surface of the thick-walled portion42ais formed as a bottom surface42bthat is a step surface. The bottom surface42bbears the insulator44asuch that the insulator44adoes not protrude forward.

The inner cylinder43is a cylindrical metallic member and has the flange portion43aat its forward end. The inner cylinder43and the main metal fitting42are welded and fixed to each other so as to be coaxial with each other. The inner cylinder43has a reduced diameter portion43cfor pressing the powder compact45bin a direction toward the center axis of the inner cylinder43and a reduced diameter portion43dfor pressing the insulators44ato44cand the powder compacts45aand45bin a downward direction inFIG.1through the metal ring46.

The insulators44ato44cand the powder compacts45aand45bare disposed between the inner circumferential surface of the cylindrical member41and the sensor element20. The insulators44ato44cserve as supporters for the powder compacts45aand45b. Examples of the material of the insulators44ato44cinclude ceramics such as alumina, steatite, zirconia, spinel, cordierite, and mullite and glass. The powder compacts45aand45bare formed, for example, by molding a powder and each serve as a sealing medium. Examples of the material of the powder compacts45aand45binclude talc and ceramic powders such as alumina powder and boron nitride powder, and the powder compacts45aand45bmay each contain at least one of these materials. The powder compact45ais filled between the insulators44aand44b, sandwiched therebetween from opposite sides (forward and rear sides) in the axial direction, and pressed by the insulators44aand44b. The powder compact45bis filled between the insulators44band44c, sandwiched therebetween from opposite sides (forward and rear sides) in the axial direction, and pressed by the insulators44band44c. The insulators44ato44cand the powder compacts45aand45bare sandwiched between the bottom surface42bof the thick-walled portion42aof the main metal fitting42and both the reduced diameter portion43dand the metal ring46and pressed from the forward and rear sides. The pressing force applied by the reduced diameter portions43cand43dcauses the powder compacts45aand45bto be compressed between the cylindrical member41and the sensor element20, and the powder compacts45aand45bclose the communication between the element chamber33in the protective cover30and a space49in the external cylinder48and fix the sensor element20.

The bolt47is fixed to the outer side of the main metal fitting42so as to be coaxial with the main metal fitting42. The bolt47has a male thread portion formed on the outer circumferential surface of the bolt47. The male thread portion is inserted into a fixing member59that is welded to the pipe58and has a female thread portion formed on the inner circumferential surface of the fixing member59. In this manner, the gas sensor10can be fixed to the pipe58with the forward end side of the sensor element20of the gas sensor10and the protective cover30protruding into the pipe58.

The external cylinder48is a cylindrical metallic member and covers the inner cylinder43, the rear end side of the sensor element20, and the connector50. An upper portion of the main metal fitting42is inserted into the external cylinder48. The lower end of the external cylinder48is welded to the main metal fitting42. The plurality of lead wires55connected to the connector50are drawn from the upper end of the external cylinder48to the outside. The connector50is in contact with and electrically connected to upper connector electrodes71and lower connector electrodes72that are disposed on rear end portions of respective surfaces of the sensor element20. The lead wires55are electrically continuous with electrodes64to68and a heater69of the sensor element20through the connector50. The gap between the external cylinder48and the lead wires55is sealed by the rubber stopper57. The space49inside the external cylinder48is filled with a reference gas. A sixth surface60f(rear end surface) of the sensor element20is disposed inside the space49.

As shown inFIGS.2to7, the sensor element20includes the element body60, a detection portion63, the heater69, the upper connector electrodes71, the lower connector electrodes72, a protective layer80, a first dense layer92, a second dense layer95, and the intermediate layer98. The element body60includes a layered body prepared by stacking a plurality of oxygen-ion-conductive solid electrolyte layers. As shown inFIG.3, in the present embodiment, the element body60includes six solid electrolyte layers78ato78f. The solid electrolyte layers78ato78fare made of a ceramic containing zirconia (ZrO2) as a main component. The element body60has an elongate rectangular parallelepiped shape whose longitudinal direction extends in the forward-rearward direction and has first to sixth surfaces60ato60fthat are the upper, lower, left, right, forward, and rear outer surfaces of the element body60. The first to fourth surfaces60ato60dare surfaces extending in the longitudinal direction of the element main body60and correspond to the side surfaces of the element main body60. The fifth surface60eis the forward end surface of the element body60, and the sixth surface60fis the rear end surface of the element body60. As for the dimensions of the element body60, for example, the length may be from 25 mm to 100 mm inclusive. The width may be from 2 mm to 10 mm inclusive, and the thickness may be from 0.5 mm to 5 mm inclusive. The element body60has formed therein: a measurement-object gas inlet61having an opening on the fifth surface60eto introduce the measurement-object gas into the element body60; and a reference gas inlet62having an opening on the sixth surface60fto introduce the reference gas (air in the present embodiment) used as a reference for detection of the specific gas concentration into the element body60.

The detection portion63is used to detect the specific gas concentration in the measurement-object gas. The detection portion63includes a plurality of electrodes disposed on a forward end side of the element body60. In the present embodiment, the detection portion63includes an outer electrode64disposed on the first surface60aand further includes an inner main pump electrode65, an inner auxiliary pump electrode66, a measurement electrode67, and a reference electrode68that are disposed inside the element body60. The inner main pump electrode65and the inner auxiliary pump electrode66are disposed on the inner circumferential surface of an internal space of the element body60and each have a tunnel-like structure.

The principle of the detection of the specific gas concentration in the measurement-object gas by the detection portion63is well known, and its detailed description will be omitted. The detection portion63detects the specific gas concentration, for example, in the following manner. The detection portion63pumps oxygen in the measurement-object gas around the inner main pump electrode65to the outside (the element chamber33) or pumps oxygen from the outside according to a voltage applied between the outer electrode64and the inner main pump electrode65. Moreover, the detection portion63pumps oxygen in the measurement-object gas around the inner auxiliary pump electrode66to the outside (the element chamber33) or pumps oxygen from the outside according to a voltage applied between the outer electrode64and the inner auxiliary pump electrode66. This allows the measurement-object gas whose oxygen concentration has been adjusted to a prescribed value to reach the measurement electrode67. The measurement electrode67functions as a NOx reduction catalyst and reduces the specific gas (NOx) in the measurement-object gas that has reached the measurement electrode67. Then the detection portion63generates an electric signal corresponding to an electromotive force generated between the measurement electrode67and the reference electrode68according to the oxygen concentration in the reduced gas or corresponding to a current flowing between the measurement electrode67and the outer electrode64according to the electromotive force. The electric signal generated by the detection portion63is a signal indicating a value corresponding to the specific gas concentration in the measurement-object gas (a value from which the specific gas concentration can be derived) and corresponds to the detection value detected by the detection portion63.

The heater69is an electric resistor disposed inside the element body60. When electric power is supplied to the heater69from the outside, the heater69generates heat and heats the element body60. The heater69can heat the solid electrolyte layers78ato78fincluded in the element body60, can keep them hot, and can adjust their temperature to the temperature at which the solid electrolyte layers78ato78fare activated (e.g., 800° C.)

The upper connector electrodes71and the lower connector electrodes72are disposed on rear end-side portions of side surfaces of the element body60and are electrodes that allow electrical continuity between the element body60and the outside. The upper and lower connector electrodes71and72are not covered with the protective layer80and are exposed. In the present embodiment, the upper connector electrodes71include four upper connector electrodes71ato71darranged in the left-right direction and disposed on the rear end side of the first surface60a. The lower connector electrodes72include four lower connector electrodes72ato72darranged in the left-right direction and disposed on the rear end side of the second surface60b(lower surface) opposite to the first surface60a(upper surface). Each of the connector electrodes71ato71dand72ato72dis electrically continuous with a corresponding one of the heater69and the plurality of electrodes64to68of the detection portion63. In the present embodiment, the upper connector electrode71ais electrically continuous with the measurement electrode67, and the upper connector electrode71bis electrically continuous with the outer electrode64. The upper connector electrode71cis electrically continuous with the inner auxiliary pump electrode66, and the upper connector electrode71dis electrically continuous with the inner main pump electrode65. The lower connector electrodes72ato72care electrically continuous with the heater69, and the lower connector electrode72dis electrically continuous with the reference electrode68. The upper connector electrode71bis electrically continuous with the outer electrode64through an outer lead wire75disposed on the first surface60a(seeFIGS.3and4). Each of the other connector electrodes is electrically continuous with a corresponding electrode or the heater69through a lead wire disposed inside the element body60, a through hole, etc.

The outer lead wire75is a conductor containing a noble metal such as platinum (Pt) or a high-melting point metal such as tungsten (W) or molybdenum (Mo). Preferably, the outer lead wire75is a cermet conductor containing a noble metal or a high-melting point metal and the oxygen-ion-conductive solid electrolyte contained in the element body60(zirconia in the present embodiment). In the present embodiment, the outer lead wire75is a cermet conductor containing platinum and zirconia. The porosity of the outer lead wire75may be, for example, from 5% to 40% inclusive. The line width of the outer lead wire75(its thickness, i.e., the width in the left-right direction) is, for example, from 0.1 mm to 1.0 mm inclusive. An unillustrated insulating layer for insulation between the outer lead wire75and the solid electrolyte layer78aof the element body60may be disposed between the outer lead wire75and the first surface60aof the element body60.

The protective layer80includes inner porous layers81and an outer porous layer85. The inner porous layers81are porous bodies that cover at least the forward end side of the side surfaces of the element body60on which the upper and lower connector electrodes71and72are disposed, i.e., of the first and second surfaces60aand60b. In the present embodiment, the inner porous layers81cover the first and second surfaces60aand60b. The outer porous layer85is a porous body that covers the forward end side of the element body60. The outer porous layer85is disposed on the outer side of the inner porous layers81.

The inner porous layers81include a first inner porous layer83that covers the first surface60aand a second inner porous layer84that covers the second surface60b. The first inner porous layer83covers the entire region, from the forward end to the rear end, of the first surface60aon which the upper connector electrodes71ato71dare disposed, except for the regions in which the first dense layer92and the upper connector electrodes71are present (seeFIGS.2to4). The width of the first inner porous layer83in the left-right direction is the same as the width of the first surface60ain the left-right direction, and the first inner porous layer83covers the first surface60aso as to extend from the left edge of the first surface60ato its right edge. Since the first dense layer92is present, the first inner porous layer83is divided in the longitudinal direction into a forward end-side portion83alocated forward of the first dense layer92and a rear end-side portion83blocated rearward of the first dense layer92. The first inner porous layer83covers at least partially the outer electrode64and the outer lead wire75. In the present embodiment, as shown inFIGS.3and4, the first inner porous layer83covers the entire outer electrode64and covers the entire portion of the outer lead wire75in which the first dense layer92is not present. The first inner porous layer83protects the outer electrode64and the outer lead wire75from components of the measurement-object gas such as sulfuric acid and plays a role in preventing corrosion of the outer electrode64and the outer lead wire75.

The second inner porous layer84covers the entire region, from the forward end to the rear end, of the second surface60bon which the lower connector electrodes72ato72dare disposed, except for the regions in which the second dense layer95and the lower connector electrodes72are present (seeFIGS.2,3, and5). The width of the second inner porous layer84in the left-right direction is the same as the width of the second surface60bin the left-right direction, and the second inner porous layer84covers the second surface60bso as to extend from the left edge of the second surface60bto its right edge. The presence of the second dense layer95divides the second inner porous layer84into a forward end-side portion84alocated forward of the second dense layer95in the longitudinal direction and a rear end-side portion84blocated rearward of the second dense layer95.

The outer porous layer85covers the first to fifth surfaces60ato60e. The outer porous layer85covers the inner porous layers81to thereby cover the first surface60aand the second surface60b. The length of the outer porous layer85in the forward-rearward direction is shorter than that of the inner porous layers81. Unlike the inner porous layers81, the outer porous layer85covers only the forward end of the element body60and a region around the forward end. In this case, the outer porous layer85covers a portion of the element body60that surrounds the electrodes64to68of the detection portion63, i.e., a portion of the element body60that is disposed inside the element chamber33and exposed to the measurement-object gas. In this manner, the outer porous layer85plays a role in preventing the occurrence of cracking in the element body60that are caused by adhesion of, for example, moisture etc. in the measurement-object gas.

The protective layer80is formed of, for example, a ceramic porous material such as an alumina porous material, a zirconia porous material, a spinel porous material, a cordierite porous material, a titania porous material, or a magnesia porous material. In the present embodiment, the protective layer80is formed of an alumina porous material. The thickness of the first inner porous layer83and the thickness of the second inner porous layer84may be, for example, 5 μm or more and may be 14 μm or more. The thickness of the first inner porous layer83and the thickness of the second inner porous layer84may be 40 μm or less and may be 23 μm or less. The thickness of the outer porous layer85is, for example, from 40 μm to 800 μm inclusive. The porosity of the protective layer80is 10% or more. The protective layer80covers the outer electrode64and the measurement-object gas inlet61. However, when the porosity is 10% or more, the measurement-object gas can pass through the protective layer80. The porosity of the inner porous layers81may be from 10% to 50% inclusive. The porosity of the outer porous layer85may be from 10% to 85% inclusive. The porosity of the outer porous layer85may be higher than the porosity of the inner porous layers81.

The porosity of the inner porous layers81is a value derived as follows using an image (SEM image) obtained by observation using a scanning electron microscope (SEM). First, the sensor element20is cut in the thickness direction of the inner porous layers81, and a cross section of one of the inner porous layers81is used as an observation surface. The cross-section is embedded in a resin and polished to obtain an observation sample. Next, the magnification of the SEM is set to 1000× to 10000×, and an image of the observation surface of the observation sample is captured to thereby obtain an SEM image of the inner porous layer81. Next, the image obtained is subjected to image analysis, and a threshold value is determined by a discriminant analysis method (Otsu's binarization) using a brightness distribution obtained from the brightness data of pixels in the image. Using the determined threshold value, the pixels in the image are binarized and classified into object portions and pore portions, and the area of the object portions and the area of the pore portions are computed. Then the ratio of the area of the pore portions to the total area (the total area of the object portions and the pore portions) is computed as a porosity (unit: %). The porosity of the outer porous layer85and the porosities of the first dense layer92, the second dense layer95, and the intermediate layer98described later are computed in the same manner as described above.

The first dense layer92and the second dense layer95serve as water intrusion preventing portions that prevent capillary action of water in the longitudinal direction of the element body60. The first dense layer92is disposed on the first surface60aon which the upper connector electrodes71and the first inner porous layer83are disposed. The first dense layer92is disposed on the first surface60aso as to divide the first inner porous layer83into forward and rear portions in the longitudinal direction as described above. The first dense layer92is disposed closer to the forward end of the element body60than the upper connector electrodes71, i.e., disposed forward of the upper connector electrodes71. The first dense layer92is disposed rearward of the outer electrode64. The first dense layer92is disposed rearward of all the plurality of electrodes64to68, including the outer electrode64, included in the detection portion63(seeFIG.3). The first dense layer92is disposed at a position that overlaps the insulator44bin the forward-rearward direction (seeFIG.1). In other words, a region extending from the forward end of the first dense layer92to its rear end is located within a region extending from the forward end of the insulator44bto its rear end. The first dense layer92plays a role in preventing moisture moved rearward through the forward end-side portion83aby capillary action from passing through the first dense layer92to thereby prevent the moisture from reaching the upper connector electrodes71. The first dense layer92is a dense layer with a porosity of less than 10%. The width of the first dense layer92in the left-right direction is the same as the width of the first surface60ain the left-right direction, and the first dense layer92covers the first surface60aso as to extend from the left edge of the first surface60ato its right edge. The first dense layer92is adjacent to the rear end of the forward end-side portion83a. The first dense layer92is adjacent to the forward end of the rear end-side portion83b. As shown inFIG.4, the first dense layer92covers part of the outer lead wire75.

The second dense layer95is disposed on the second surface60bon which the lower connector electrodes72and the second inner porous layer84are disposed. The second dense layer95is disposed on the second surface60bso as to divide the second inner porous layer84into forward and rear portions in the longitudinal direction as described above. The second dense layer95is disposed closer to the forward end of the element body60than the lower connector electrodes72, i.e., disposed forward of the lower connector electrodes72. The second dense layer95is disposed rearward of the outer electrode64. The second dense layer95is disposed rearward of all the plurality of electrodes64to68, including the outer electrode64, included in the detection portion63(seeFIG.3). The second dense layer95is disposed at a position that overlaps the insulator44bin the forward-rearward direction (seeFIG.1). In other words, a region extending from the forward end of the second dense layer95to its rear end is located within a region extending from the forward end of the insulator44bto its rear end. The second dense layer95plays a role in preventing moisture moved rearward through the forward end-side portion84aby capillary action from passing through the second dense layer95to thereby prevent the moisture from reaching the lower connector electrodes72. The second dense layer95is a dense layer with a porosity of less than 10%. The width of the second dense layer95in the left-right direction is the same as the width of the second surface60bin the left-right direction, and the second dense layer95covers the second surface60bso as to extend from the left edge of the second surface60bto its right edge. The second dense layer95is adjacent to the rear end of the forward end-side portion84a. The second dense layer95is adjacent to the forward end of the rear end-side portion84b.

The length Le of each of the first dense layer92and the second dense layer95in the longitudinal direction (seeFIGS.4and5) is preferably 0.5 mm or more. When the length Le is 0.5 mm or more, the passage of moisture through the first dense layer92and the second dense layer95can be prevented sufficiently. The length Le may be 5 mm or more. The length Le may be 25 mm or less and may be 20 mm or less. In the present embodiment, the length Le of the first dense layer92and the length Le of the second dense layer95are the same but may be different values.

The first dense layer92and the second dense layer95differ from the protective layer80in that their porosity is less than 10%. However, a ceramic composed of any of the materials exemplified for the protective layer80described above can be used. Specifically, the first dense layer92may be a ceramic porous body containing, as a main component, at least one type of ceramic particles selected from alumina particles, zirconia particles, spinel particles, cordierite particles, titania particles, and magnesia particles. In the present embodiment, the first dense layer92and the second dense layer95are both formed of a ceramic containing alumina as a main component. The thickness of the first dense layer92and the thickness of the second dense layer95may each be, for example, from 1 μm to 40 μm inclusive. The thickness of the first dense layer92and the thickness of the second dense layer95may each be 20 μm or less, may be 9 μm or less, and may be 3 μm or less. The porosity of the first dense layer92and the porosity of the second dense layer95are each preferably 8% or less and more preferably 5% or less. The smaller the porosity, the further the first dense layer92and the second dense layer95can reduce the capillary action of water in the longitudinal direction of the element body60.

As shown inFIGS.1to3,6, and7, the intermediate layer98is disposed between the first dense layer92and the element body60. Although the details will be described later, the intermediate layer98plays a role in reducing the occurrence of cracking in the sensor element20. As shown inFIG.7, the intermediate layer98is located between the first dense layer92and the outer lead wire75and covers the outer lead wire75. Therefore, the first dense layer92covers the outer lead wire75with the intermediate layer98interposed therebetween. As shown inFIG.6, in the present embodiment, the length of the intermediate layer98in the forward-rearward direction is the same as the length Le of the first dense layer92. Specifically, the intermediate layer98is disposed only on the lower side of the first dense layer92and not disposed between the first inner porous layer83and the element body60. As shown inFIG.7, the width of the intermediate layer98in the left-right direction is the same as the width of the first surface60ain the left-right direction. Moreover, the width of the intermediate layer98in the left-right direction is the same as the width of the first dense layer92in the left-right direction. The thickness T of the intermediate layer98is, for example, from 1 μm to 40 μm inclusive. Like the inner porous layer81, the intermediate layer98may have a porosity of 10% or more, i.e., may be a porous body. The porosity of the intermediate layer98may be 50% or less. Like the first dense layer92, the intermediate layer98may have a porosity of less than 10%, i.e., may be dense. The porosity of the intermediate layer98may be 8% or less and may be 5% or less. In the present embodiment, the intermediate layer98is dense. When the thickness T of the intermediate layer98is 1 μm or more, the effect of the presence of the intermediate layer98in reducing the occurrence of cracking in the sensor element20can be obtained more reliably. The thickness T of the intermediate layer98may be 10 μm or less.

The thickness T of the intermediate layer98may be the thickness T1 of the thinnest portion of the intermediate layer98(for example, a portion located directly above the outer lead wire75as shown inFIG.7) or the overall average thickness T2 of the intermediate layer98. Even when the thickness T used is the thickness T1 or the thickness T2, the effect of reducing the occurrence of cracking can be obtained more reliably when the thickness T is 1 μm or more.

The intermediate layer98may be formed, for example, of a ceramic containing, as a main component, at least one type of ceramic particles selected from alumina particles, zirconia particles, spinel particles, cordierite particles, titania particles, and magnesia particles. The intermediate layer98may be formed of a noble metal such as platinum. The intermediate layer98may be formed of cermet containing the above-described ceramic particles and noble metal particles. Preferably, the intermediate layer98contains, as main components, the main component of the solid electrolyte layers78ato78fand the main component of the first dense layer92. In the present embodiment, the intermediate layer98is a ceramic containing, as main components, zirconia used as the main component of the solid electrolyte layers78ato78fand alumina used as the main component of the first dense layer92.

A method for producing the gas sensor10having the above-described structure will be described below. First a method for producing the sensor element20will be described. The method for producing the sensor element20includes a production step of producing a green sensor element that is the sensor element20before firing and a firing step of firing the green sensor element. In the present embodiment, the outer porous layer85is formed by plasma spraying after the firing step. Therefore, the green sensor element produced in the production step does not include a green outer porous layer85, and the sensor element20after the firing step does not include the outer porous layer85.

In the production step, the green sensor element that is the sensor element20before firing is produced. In the production step, first, six ceramic green sheets (green solid electrolyte layers) corresponding to the solid electrolyte layers78ato78fincluded in the element body60are prepared. The ceramic green sheets are produced, for example, by mixing a solvent, a binder, etc. with a raw material powder containing the material of the solid electrolyte layers78ato78f(a zirconia powder in the present embodiment) to obtain a paste containing the material of the raw material powder as a main component and then forming the paste into a sheet shape. If necessary, through holes, grooves, etc. are punched in the ceramic green sheets to form portions that later become inner spaces of the element body60through firing. Next, patterns for green electrodes, green lead wires, green connector electrodes, a green heater, etc. are formed by screen printing on the ceramic green sheets to be used as the solid electrolyte layers78ato78f. The green electrodes later become the above-described electrodes64to68of the detection portion63through firing. The green lead wires later become, through firing, the lead wires that connect the electrodes to the upper connector electrodes71and the lower connector electrodes72. The green lead wires include a lead wire that later becomes the outer lead wire75through firing. The green connector electrodes later become the upper connector electrodes71and the lower connector electrodes72through firing. The green heater later becomes the heater69through firing. Moreover, patterns for a green intermediate layer that later becomes the intermediate layer98through firing, a green first dense layer that later becomes the first dense layer92through firing, and a green first inner porous layer that later becomes the first inner porous layer83through firing are formed by screen printing on a surface of the ceramic green sheet that later becomes the solid electrolyte layer78athrough firing (a surface that later becomes the first surface60aof the element body60). Similarly, patterns for a green second dense layer that later becomes the second dense layer95through firing and a green second inner porous layer that later becomes the second inner porous layer84through firing are formed by screen printing on a surface of the ceramic green sheet that later becomes the solid electrolyte layer78fthrough firing (a surface that later becomes the second surface60bof the element body60). Next, the six ceramic green sheets with the patterns formed thereon are stacked to form a layered body. The layered body is cut into small layer bodies having the same size as the size of the sensor element20. These small layered bodies are green sensor elements. The patterns for the green first inner porous layer, the green second inner porous layer, the green intermediate layer, the green first dense layer, and the green second dense layer may be printed after the production of the layered body described above.

The paste used to form the green first inner porous layer is, for example, a paste that is prepared by mixing a raw material powder composed of the material of the above-described first inner porous layer83(an aluminum powder in the present embodiment), a binder, a solvent, a pore-forming material, etc. and that contains, as a main component, the material of the raw material powder. The paste forming the green second inner porous layer is prepared in the same manner as described above. The paste used to form the green first dense layer is, for example, a paste that is prepared by mixing a raw material powder composed of the material of the above-described first dense layer92(an aluminum powder in the present embodiment), a binder, a solvent, etc. and that contains, as a main component, the material of the raw material powder. To control the porosity of the first dense layer92, a pore-forming material may be added to the paste. The paste for forming the green second dense layer is prepared in the same manner as above. The paste used to form the green intermediate layer is, for example, a paste that is prepared by mixing a raw material powder composed of the materials of the above-described intermediate layer98(an aluminum powder and a zirconia powder in the present embodiment), a binder, a solvent, etc. and that contains, as main components, the materials of the raw material powder. To control the porosity of the intermediate layer98, a pore-forming material may be added to the paste.

The green first inner porous layer and the green second inner porous layer may be formed using the same paste or using pastes prepared using different raw material powders. The green first dense layer and the green second dense layer may also be formed using the same paste or using pastes prepared using different raw material powders.

Next, the firing step of firing the green sensor element obtained in the production step is performed. In the firing step, the green sensor element is fired at a prescribed firing temperature (e.g., 1360° C.±50° C.), and then the temperature is lowered to room temperature (e.g., 20° C.) after firing. In this manner, the six ceramic green sheets become the solid electrolyte layers78ato78f, and the green electrodes become the electrodes64to68. The green lead wires become the plurality of wires including the outer lead wire75, and the green connector electrodes become the upper connector electrodes71and the lower connector electrodes72. Moreover, the green heater becomes the heater69. The green intermediate layer becomes the intermediate layer98, and the green first dense layer becomes the first dense layer92. The green first inner porous layer becomes the first inner porous layer83, and the green second dense layer becomes the second dense layer95. The green second inner porous layer becomes the second inner porous layer84. The sensor element20is obtained through the firing step.

In the present embodiment, after the firing step has been performed to produce the sensor element20, the outer porous layer85is formed by plasma spraying. The plasma spraying can be performed, for example, in the same manner as in plasma spraying described in Japanese Unexamined Patent Application Publication No. 2016-109685. Then the gas sensor10equipped with the sensor element20is produced. First, the sensor element20is caused to pierce axially through the cylindrical member41, and the insulator44a, the powder compact45a, the insulator44b, the powder compact45b, the insulator44c, and the metal ring46are placed in this order between the inner circumferential surface of the cylindrical member41and the sensor element20. Next, the metal ring46is pressed to compress the powder compacts45aand45b. With this state maintained, the reduced diameter portions43cand43dare formed to thereby produce the element-sealing member40, and the gap between the inner circumferential surface of the cylindrical member41and the sensor element20is thereby sealed. Then the protective cover30is welded to the element-sealing member40, and the bolt47is attached to obtain the assembly15. Then the lead wires55piercing through the rubber stopper57and the connector50connected to the lead wires55are prepared, and the connector50is connected to the rear end side of the sensor element20. Then the external cylinder48is welded and fixed to the main metal fitting42to thereby obtain the gas sensor10.

In the sensor element20in the present embodiment, when the thermal expansion coefficients of the solid electrolyte layers78ato78f, the first dense layer92, and the intermediate layer98in the temperature range of from 20° C. to 1360° C. are denoted by thermal expansion coefficients Ea, Eb, and Ec, respectively, the ratio Ea/Eb is more than 1.0 and 5.0 or less, and the relation Ea>Ec>Eb is satisfied. The thermal expansion coefficients Ea to Ec are not volume expansion coefficients but are linear expansion coefficients. Let the median of the thermal expansion coefficient Ea and the thermal expansion coefficient Eb be Ed (=(Ea+Eb)/2). Then it is preferable that the thermal expansion coefficient Ec satisfies formula (1) below. The thermal expansion coefficients Ea, Eb, and Ec are referred to also as thermal expansion coefficients A′, B′, and C′. The median Ed is referred to also as the median D′.

The thermal expansion coefficient Ea of the solid electrolyte layers78ato78fis measured by thermomechanical analysis (TMA) as follows. First, the sensor element20is cut such that a portion including the solid electrolyte layers78ato78fof the element body60is cut out to thereby obtain a measurement piece. Next, the measurement piece is placed in a container, and the expansion coefficient of the measurement piece when the temperature is changed from 20° C. to 1360° C. is measured under an applied load of 1 g. Specifically, a dimension a1′ of the measurement piece at 20° C. is measured. Next, a dimension a2′ of the measurement piece heated to 1360° C. while a load of 1 g is applied is measured. Then the thermal expansion coefficient Ea is computed using the formula: the thermal expansion coefficient Ea[%]=(a2′−a1′)/a1′×100. The dimensions a1′ and a2′ of the measurement piece are measured as dimensions in the forward-rearward direction, i.e., the longitudinal direction, of the sensor element20. Similarly, the thermal expansion coefficient Eb is computed using a measurement piece that is cut from the sensor element20so as to include part of the first dense layer92, and the thermal expansion coefficient Ec is computed using a measurement piece that is cut from the sensor element20so as to include part of the intermediate layer98. When thermal expansion coefficients of the solid electrolyte layers78ato78fare not the same, e.g., when the materials of the solid electrolyte layers78ato78fare not the same, the thermal expansion coefficient of a layer closest to the first dense layer92and the intermediate layer98(the solid electrolyte layer78ain the present embodiment) is used as the thermal expansion coefficient Ea.

The thermal expansion coefficients Ea to Ec of the sensor element20can be controlled as follows. For example, the thermal expansion coefficient Ea of the solid electrolyte layers78ato78fcan be controlled by changing the material of the raw material powder contained in the paste for forming the ceramic green sheets. The thermal expansion coefficient Eb of the first dense layer92can be controlled by chaining the material of the raw material powder contained in the paste for forming the green first dense layer. The thermal expansion coefficient Ec of the intermediate layer98can be controlled by changing the material of the raw material powder contained in the paste for forming the green intermediate layer. Therefore, the ratio Ea/Eb can be set to be more than 1.0 and 5.0 or less by selecting an appropriate combination of the material of the raw material powder of the solid electrolyte layers78ato78fand the material of the raw material powder of the first dense layer92. For example, the thermal expansion coefficient of zirconia at 40° C. to 400° C. is 10.5×10−6/° C., and the thermal expansion coefficient of alumina at 40° C. to 400° C. is 7.2×10−6/° C. Therefore, the thermal expansion coefficient of zirconia is larger than the thermal expansion coefficient of alumina. When the raw material powder of the solid electrolyte layers78ato78fis zirconia and the raw material powder of the first dense layer92is alumina, the thermal expansion coefficient Ea is larger than the thermal expansion coefficient Eb, and the ratio Ea/Eb can be set to be more than 1.0 and 5.0 or less. When cordierite (thermal expansion coefficient: less than 0.1×10−6/° C.) or silicon nitride (thermal expansion coefficient: 2.8×10−6/° C.), which are materials having a smaller thermal expansion coefficient than alumina, is used as the raw material powder of the first dense layer92, the value of the ratio Ea/Eb can be larger than that when alumina is used (for example, a value of about 5.0) while the value of the ratio Ea/Eb is set to be more than 1.0 and 5.0 or less. When a material whose thermal expansion coefficient is between the thermal expansion coefficient of the material of the raw material powder of the solid electrolyte layers78ato78fand the thermal expansion coefficient of the material of the raw material powder of the first dense layer92is appropriately selected as the material of the raw material powder of the intermediate layer98, Ea>Ec>Eb can be satisfied, and formula (1) can be satisfied. Alternatively, the raw material powder of the intermediate layer98may contain both the material of the raw material powder of the solid electrolyte layers78ato78fand the material of the raw material powder of the first dense layer92. In this case also, Ea>Ec>Eb can be satisfied, and formula (1) can be satisfied. By appropriately controlling the volume ratio of the material of the raw material powder of the solid electrolyte layers78ato78fand the volume ratio of the material of the raw material powder of the first dense layer92in the raw material powder of the intermediate layer98, the thermal expansion coefficient Ec can be controlled while Ea>Ec>Eb is satisfied, and this allows formula (1) to be satisfied.

Next, an example of the use of the thus-produced gas sensor10will be described below. When the measurement-object gas flows through the pipe58with the gas sensor10attached to the pipe58as shown inFIG.1, the measurement-object gas flows through the protective cover30and into the element chamber33, and the forward end side of the sensor element20is exposed to the measurement-object gas. Then, with the sensor element20heated by the heater69, the measurement-object gas passes through the protective layer80, reaches the outer electrode64, and also reaches the sensor element20through the measurement-object gas inlet61, and the detection portion63generates an electrical signal corresponding to the NOx concentration in the measurement-object gas as described above. By outputting this electrical signal through the upper and lower connector electrodes71and72, the NOx concentration is detected based on the electrical signal.

In this case, the measurement-object gas may contain moisture, and the moisture may move through the protective layer80by capillary action. When the moisture reaches the exposed upper and lower connector electrodes71and72, rust or corrosion may occur in the upper and lower connector electrodes71and72due to components such as water and sulfuric acid dissolved in water, or a short circuit may occur between adjacent ones of the upper and lower connector electrodes71and72. However, in the present embodiment, even when moisture in the measurement-object gas moves through the protective layer80(in particular, the first inner porous layer83and the second inner porous layer84) toward the rear end of the element body60by capillary action, the moisture reaches the first dense layer92or the second dense layer95before it reaches the upper and lower connector electrodes71and72. Since the porosity of the first dense layer92is less than 10%, the capillary action of water in the longitudinal direction of the element body60is unlikely to occur. In this case, the first dense layer92can prevent moisture from passing through the first dense layer92from the forward end-side portion83aside and reaching the upper connector electrodes71(the upper connector electrodes71ato71d). Therefore, in the sensor element20, the occurrence of the above-described problem caused by water adhering to the upper connector electrodes71can be reduced. Similarly, the second dense layer95can prevent moisture from passing through the second dense layer95from the forward end-side portion84aside and reaching the lower connector electrodes72(the lower connector electrodes72ato72d). Therefore, in the sensor element20, the occurrence of the above-described problem caused by water adhering to the lower connector electrodes72is reduced. Preferably, the length Le of the first dense layer92in the longitudinal direction is 0.5 mm or more because the passage of moisture through the first dense layer92can be reduced sufficiently. Similarly, the length Le of the second dense layer95is 0.5 mm or more.

Moreover, the sensor element20has a ratio Ea/Eb of more than 1.0 and 5.0 or less and satisfies Ea>Ec>Eb as described above. Therefore, the intermediate layer98reduces the stress caused by the difference between the thermal expansion coefficient Ea of the solid electrolyte layers78ato78fand the thermal expansion coefficient Eb of the first dense layer92when the sensor element20is heated by the heater69during use. When stress is generated in the sensor element20, cracking tends to occur. However, in the sensor element20in the present embodiment, the stress generated during heating is reduced, so that the occurrence of cracking in the sensor element20is reduced. Moreover, since formula (1) above is satisfied, the stress during heating of the sensor element20is further reduced, and the occurrence of cracking during heating of the sensor element20is further reduced. The ratio Ea/Eb is preferably 3.0 or less. When the ratio Ea/Eb is 3.0 or less, the thermal expansion coefficient Ea of the solid electrolyte layers78ato78fis closer to the thermal expansion coefficient Eb of the first dense layer92, so that the occurrence of cracking in the sensor element20is further reduced.

When cracking occurs in the sensor element20, particularly in the first dense layer92or the second dense layer95, the function of the first dense layer92or the second dense layer95as the water intrusion preventing portion described above may deteriorate. However, since the occurrence of cracking in the sensor element20in the present embodiment is reduced, the function of the first dense layer92and the second dense layer95as the water intrusion preventing portions is unlikely to deteriorate, and thus the occurrence of the above-described problem caused by water adhering to the upper connector electrodes71and the lower connector electrodes72is reduced.

The correspondences between the components in the present embodiment and the components in the present invention will be clarified. The solid electrolyte layers78ato78fin the present embodiment correspond to the solid electrolyte layer in the present invention, and the element body60corresponds to the element body. The upper connector electrodes71ato71dcorrespond to the connector electrode, and the first surface60acorresponds to the side surface on which the connector electrode is disposed. The first inner porous layer83corresponds to the porous layer, and the first dense layer92corresponds to the dense layer. The intermediate layer98corresponds to the intermediate layer. The detection portion63corresponds to the detection portion, and the outer lead wire75corresponds to the outer lead portion. The outer electrode64corresponds to the outer electrode.

In the gas sensor10in the present embodiment described above in detail, as for the thermal expansion coefficients Ea, Eb, and Ec of the solid electrolyte layers78ato78f, the first dense layer92, and the intermediate layer98of the sensor element20in the temperature range of from 20° C. to 1360° C., the ratio Ea/Eb is more than 1.0 and 5.0 or less, and Ea>Ec>Eb is satisfied. In this case, the intermediate layer98reduces the stress caused by the difference between the thermal expansion coefficient Ea of the solid electrolyte layers78ato78fand the thermal expansion coefficient Eb of the first dense layer92when the sensor element20is heated during use. Therefore, the occurrence of cracking in the sensor element20during heating is reduced. Moreover, since the thermal expansion coefficients Ea to Ec in the sensor element20satisfy formula (1) above, the stress generated when the sensor element20is heated is reduced, and the occurrence of cracking is further reduced. Moreover, since the ratio Ea/Eb in the sensor element20is 3.0 or less, the occurrence of cracking is further reduced. In the sensor element20, since the thickness T of the intermediate layer98is 1 μm or more, the above-described effect of the presence of the intermediate layer98in reducing the occurrence of cracking in the sensor element20is obtained more reliably.

The present invention is not limited to the embodiment described above. It will be appreciated that the present invention can be implemented in various forms so long as they fall within the technical scope of the invention.

For example, in the above embodiment, the length of the intermediate layer98in the forward-rearward direction is the same as the length Le of the first dense layer92as shown inFIG.6, but this is not a limitation. It is only necessary that the intermediate layer98be disposed at least between the first dense layer92and the element body60. For example, as shown inFIG.8, the length of the intermediate layer98may be larger than the length Le, and the intermediate layer98may be present also between the first inner porous layer83and the element body60. InFIG.8, the intermediate layer98extends frontward and rearward from the first dense layer92. Specifically, the intermediate layer98is present between the forward end-side portion83aand the element body60and also between the rear end-side portion83band the element body60. The intermediate layer98may be present over a region extending from the forward end of the first surface60ato its rear end. However, when the intermediate layer98is dense, it is preferable that the intermediate layer98is disposed so as to avoid overlapping the outer electrode64such that the intermediate layer98does not cover the outer electrode64. The length of the intermediate layer98may be smaller than the length Le, and a region in which the intermediate layer98is not present may be present between the first dense layer92and the element body60. In other words, it is only necessary that, in a cross-section taken in the forward-rearward direction, the intermediate layer98be present between at least part of the first dense layer92and the element body60(the solid electrolyte layer78a). However, it is preferable that the intermediate layer98is present between the first dense layer92and the solid electrolyte layer78aat least in a region extending from the forward end of the first dense layer92to its rear end, as shown inFIGS.6and8. In this manner, the occurrence of cracking in the sensor element20can be further reduced. The length of the intermediate layer98in the forward-rearward direction may be, for example, from 0.5 mm to 55 mm inclusive.

In the embodiment described above, as shown inFIG.7, the width of the intermediate layer98in the left-right direction is the same as the width of the first surface60ain the left-right direction and the width of the first dense layer92in the left-right direction, but this is not a limitation. It is only necessary that, in a cross section taken in the left-right direction, the intermediate layer98be present between at least part of the first dense layer92and the element body60(the solid electrolyte layer78a), and the width of the intermediate layer98in the left-right direction may be smaller than that inFIG.7. However, it is preferable that the intermediate layer98is present at least between the element body60and a region of the first dense layer92that is located at the center in the left-right direction and has a width of 50% (this region is hereinafter referred to as a central region). As shown inFIG.9, the central region includes two left and right central regions among four regions obtained by dividing the left-right width We of the first dense layer92into quarters. It is preferable that the intermediate layer98is present at least between the central region of the first dense layer92and the element body60, as shown inFIG.9. In the cross section in the left-right direction in the example inFIG.9, the intermediate layer98is present only between the central region of the first dense layer92and the element body60, and the width Wc of the intermediate layer98in the left-right direction is the same as the width of the central region of the first dense layer92(We/2). Preferably, the width Wc of the intermediate layer98is equal to or larger than the width of the central region (We/2). As shown inFIG.10, the intermediate layer98may be divided into left and right portions and may include an intermediate layer98alocated on the left side and an intermediate layer98blocated on the right side. In this case also, it is preferable that the width Wc of the intermediate layer98(the sum of the width of the intermediate layer98aand the width of the intermediate layer98b) is equal to or more than We/2. InFIG.10, the sum of the width of the intermediate layer98aand the width of the intermediate layer98bis equal to We/2. From the viewpoint of reducing the occurrence of cracking in the sensor element20described above, the intermediate layer98inFIG.9that is present between the central region of the first dense layer92and the element body60is preferred to the intermediate layer98inFIG.10.

In the embodiment described above, the intermediate layer98is present between the element body60and the first dense layer92disposed on the first surface60aside (the upper side) of the element body60, but this is not a limitation. As shown inFIG.11, an intermediate layer99may be present between the element body60and the second dense layer95disposed on the second surface60bside (the lower side) of the element body60. In this case, it is preferable that, when the thermal expansion coefficients of the solid electrolyte layers78ato78f, the second dense layer95, and the intermediate layer99in the temperature range of from 20° C. to 1360° C. are denoted by thermal expansion coefficients Ea, Eb, and Ec, respectively, the ratio Ea/Eb is more than 1.0 and 5.0 or less and Ea>Ec>Eb is satisfied, and it is more preferable that formula (1) above is satisfied. The above-described various modes of the intermediate layer98can be applied to the intermediate layer99. Preferably, the thermal expansion coefficient of the first dense layer92and the thermal expansion coefficient of the second dense layer95are the same, and the thermal expansion coefficient of the intermediate layer98and the thermal expansion coefficient of the intermediate layer99are the same.

In the embodiment described above, the second surface60bof the element body60may have an exposed portion on which the second inner porous layer84and the second dense layer95are not present.FIG.12shows an example in which gap regions96are disposed rearward and forward of the second dense layer95so as to be adjacent thereto. The gap regions96inFIG.12include a forward gap region96adisposed between the forward end-side portion84aand the second dense layer95and a rear gap region96bdisposed between the rear end-side portion84band the second dense layer95. In portions in which the gap regions96are present, the second surface60bis exposed. The gap regions96are spaces in which the second inner porous layer84is not present, so that the capillary action of water in the longitudinal direction of the element body60is unlikely to occur. Therefore, the gap regions96also serve as water intrusion preventing portions that prevent moisture moving in the longitudinal direction of the element body60from reaching the lower connector electrodes72, as does the second dense layer95. As for the gap regions96, only one of the forward gap region96aand the rear gap region96bmay be provided. The length Lg of the gap regions96in the longitudinal direction is preferably 1 mm or less. When the gap regions96include the forward gap region96aand the rear gap region96bas inFIG.12, the length Lg is the sum of the length Lg1of the forward gap region96ain the longitudinal direction and the length Lg2of the rear gap region96bin the longitudinal direction. A gap region may be present on the first surface60aside of the element body60. However, in the embodiment described above, the outer lead wire75is disposed on the first surface60a. It is therefore preferable, from the viewpoint of protecting the outer lead wire75, that no gap region is present on the first surface60aside.

In the embodiment described above, the outer porous layer85is formed by plasma spraying after the firing step, but this is not a limitation. For example, a green outer porous layer that later becomes the outer porous layer85through the firing step may be formed by, for example, dipping in the production step. In this case, the green outer porous layer becomes the outer porous layer85through the firing step. In the embodiment described above, the protective layer80may not include the outer porous layer85.

In the embodiment described above, the first dense layer92divides the first inner porous layer83in the longitudinal direction into the forward end-side portion83aand the rear end-side portion83b, but this is not a limitation. The first dense layer92may be located rearward of the protective layer80. For example, in the embodiment described above, the first inner porous layer83may not include the rear end-side portion83b. Similarly, the second dense layer95may not divide the second inner porous layer84and may be located rearward of the protective layer80. However, when the first inner porous layer83does not include the rear end-side portion83b, part of the outer lead wire75is exposed. It is therefore preferable that the first inner porous layer83includes the rear end-side portion83b.

In the embodiment described above, the first dense layer92and the second dense layer95are disposed at respective positions overlapping the insulator44bin the forward-rearward direction, but this is not a limitation. For example, the first dense layer92and the second dense layer95may be disposed at positions overlapping the insulator44aor the insulator44cin the forward-rearward direction or may be disposed rearward of the metal ring46. In the embodiment described above, the first dense layer92and the second dense layer95are disposed at positions that are not exposed to the element chamber33. However, at least one of the first dense layer92and the second dense layer95may be disposed at a position exposed to the element chamber33, i.e., a position exposed to the measurement-object gas. For example, at least one of the first dense layer92and the second dense layer95may be disposed at a position located rearward of the outer porous layer85and exposed to the element chamber33.

In the embodiment described above, the sensor element20may not include the second inner porous layer84, and the second surface60bmay not be covered with the second inner porous layer84. In this case, the sensor element20may not include the second dense layer95. It is only necessary that a dense layer be disposed on at least one side surface on which connector electrodes and a porous layer are disposed (the first and second surfaces60aand60bin the embodiment described above) among the side surface of the element body (the first to fourth surfaces60ato60din the embodiment described above). In this case, moisture is prevented from reaching the connector electrodes at least on the side surfaces on which the dense layer is disposed. Moreover, it is only necessary that the intermediate layer be disposed between the element body and the dense layer.

In the embodiment described above, the first inner porous layer83covers a region of the first surface60athat extends from the forward end of the first surface60ato its rear end except for the regions in which the first dense layer92and the upper connector electrodes71are present, but this is not a limitation. For example, the first inner porous layer83may cover a region extending from forward end of the first surface60ato the forward ends of the upper connector electrodes71ato71dexcept for the region in which the first dense layer92is present. Alternatively, the first inner porous layer83may cover at least a region extending from the forward end of the first surface60ato a position rearward of the first dense layer92except for the region in which the first dense layer92is present. The same applies to the second inner porous layer84.

In the embodiment described above, the element body60has a rectangular parallelepiped shape, but this is not a limitation. For example, the element body60may be cylindrical or columnar. In this case, the element body60has one side surface.

In the embodiment described above, the gas sensor10detects the NOx concentration as the specific gas concentration, but this is not a limitation. The concentration of a different oxide may be used as the specific gas concentration. In the case where the specific gas is an oxide, when the specific gas itself is reduced near the measurement electrode67, oxygen is generated, as in the embodiment described above. Therefore, the specific gas concentration can be detected based on the value that is detected by the detection portion63and corresponds to the oxygen. The specific gas may be a non-oxide such as ammonia. When the specific gas is a non-oxide, the specific gas is converted to an oxide, for example, near the inner main pump electrode65(for example, ammonia is oxidized and converted to NO). When the oxide produced by conversion is reduced near the measurement electrode67, oxygen is generated, and the specific gas concentration can be detected based on the value that is detected by the detection portion63and corresponds to the oxygen. As described above, even when the specific gas is an oxide or a non-oxide, the gas sensor10can detect the specific gas concentration based on oxygen derived from the specific gas and generated near the measurement electrode67.

Examples will next be described. In each Example, sensor elements were actually produced. However, the present invention is not limited to the following Examples.

A sensor element that was the same as the sensor element20shown inFIGS.2to4,6, and7was produced, except that the gap regions96(the forward gap region96aand the rear gap region96b) were present on the second surface60bside of the element body60as shown inFIG.12and that the protective layer80did not include the outer porous layer85. The sensor element produced was used as Example 1. The sensor element20in Example 1 was produced as follows. First, the production step was performed as follows. Zirconia particles containing 4 mol % of yttria serving as a stabilizer, an organic binder, and an organic solvent were mixed, and the mixture was used to prepare six ceramic green sheets by tape molding. The green sheets were subjected to punching processing as needed, and patterns for the green electrodes, the green lead wires, the green connector electrodes, the green heater, the green intermediate layer, the green first dense layer, the green first inner porous layer, the green second dense layer, and the green second inner porous layer were formed by screen printing. The green first inner porous layer and the green second inner porous layer were formed using a paste prepared by mixing a raw material powder (alumina powder), a binder solution (polyvinyl acetal and butyl carbitol), a solvent (acetone), and a pore-forming material. A paste for the green first dense layer and the green second dense layer was prepared such that the porosities of the first and second dense layers92and95were 0%. Specifically, the same paste as the paste for the green first inner porous layer except that no pore-forming material was added and the amount of the solvent added was changed to adjust the viscosity was used as the paste for the green first dense layer and the green second dense layer. The green intermediate layer was formed using a paste prepared by mixing raw material powders (the alumina powder and the zirconia powder), a binder solution (polyvinyl acetal and butyl carbitol), and a solvent (acetone). No pore-forming material was added to the paste for the green intermediate layer, so that the porosity of the intermediate layer98was 0%. When screen printing was performed, the green first dense layer was formed by printing after the formation of the green intermediate layer such that the green intermediate layer was present between the green first dense layer and the ceramic green sheet that later became the solid electrolyte layer78a. Then the six green sheets were stacked to obtain a layered body, and the layered body was cut to obtain small layered bodies, i.e., green sensor elements. Next, the firing step was performed in which each green sensor element was fired at 1360° C.±50° C. and the fired sensor element was cooled to room temperature (20° C.). A sensor element20including the element body60, the first dense layer92, the intermediate layer98, etc. was thereby produced and used as a sensor element20in Example 1. The thickness T of the intermediate layer98was 5 The value of the thickness T used was the thickness T1 measured at the thinnest portion of the intermediate layer98(the portion located directly above the outer lead wire75).

Examples 2 to 10 and Comparative Examples 1 to 3

Sensor elements20with the relations between the thermal expansion coefficients Ea, Eb, and Ec different from those in Example 1 were produced and used as Examples 2 to 10 and Comparative Examples 1 to 3. In Examples 2 to 10 and Comparative Examples 1 to 3, the solid electrolyte layers78ato78fwere the same as those in Example 1.

Specifically, the value of the thermal expansion coefficient Ea was the same for Examples 1 to 10 and Comparative Examples 1 to 3. In Examples 2 to 10 and Comparative Examples 1 to 3, the thermal expansion coefficient Eb of the first dense layer92was changed by selecting the raw material powder of the green dense layer from alumina, cordierite, and silicon nitride. In Examples 2 to 10 and Comparative Examples 1 to 3, the thermal expansion coefficient Ec of the intermediate layer98was changed by adjusting the volume ratio of the material of the raw material powder of the solid electrolyte layers78ato78fcontained in the green intermediate layer and the volume ratio of the material of the raw material powder of the first dense layer92contained in the green intermediate layer. In Examples 2 to 8 and Comparative Examples 1 to 3, the thickness T of the intermediate layer98was set to 5 μm, which was the same as the thickness in Example 1. In Example 9, the thickness T of the intermediate layer98was set to 10 μm. In Example 10, the thickness T of the intermediate layer98was set to 1 μm. The thickness T of the intermediate layer98was controlled by changing the amount of the solvent contained in the paste for the green intermediate layer to thereby adjust the viscosity or adjusting the number of screen printing operations for printing the green intermediate layer.

[Measurement of Thermal Expansion Coefficients Ea to Ec]

The thermal expansion coefficients Ea to Ec in each of Examples 1 to 10 and Comparative Examples 1 to 3 were measured using the method described above. A thermomechanical analysis apparatus (type: TMA4000SA) manufactured by NETZSCH was used for the measurement.

For each of Examples 1 to 10 and Comparative Examples 1 to 3, ten sensor elements20were produced, and each sensor element20was subjected to a test in which the sensor element20was exposed to high-temperature high-pressure vapor for a prescribed time to thereby evaluate cracking resistance. This test was performed using a method according to JIS A 1509-8:2014. First, the sensor element20was placed in an autoclave. Then the pressure inside the autoclave was gradually increased so as to reach 1 MPa or higher over about 1 hour, and the increased pressure was maintained for 1 hour or longer. Then the pressure was reduced to normal pressure as fast as possible, and the sensor element20was left to cool. The cooled sensor element20was visually checked to determine whether or not cracking occurred in the first dense layer92. When the number of cracked sensor elements20out of the ten sensor element20was zero, the cracking resistance was rated “excellent (A).” When the number of cracked sensor elements20was one, the cracking resistance was rated “good (B).” When the number of cracked sensor elements20was two or more, the cracking resistance was rated “fail (F).” The high-temperature high-pressure state in the autoclave during the test is severer than a normal use environment of the sensor element20attached to a vehicle.

Table 1 summarizes the thermal expansion coefficients Ea to Ec in each of Examples 1 to 10 and Comparative Examples 1 to 3 and the results of the evaluation of the cracking resistance of the sensor elements20. In Table 1, the values of the thermal expansion coefficients Ea to Ec are the ratios with respect to the thermal expansion coefficient Ea with the value of the thermal expansion coefficient Ea used as a reference (value: 1). Table 1 also shows the ratio Ea/Eb, the magnitude relation between Ea to Ec, the median Ed of the thermal expansion coefficient Ea and the thermal expansion coefficient Eb, the value of the left-hand side of formula (1), i.e., (Ed−0.8×(Ed−Eb)), the value of the right-hand side of formula (1), i.e., (Ed+0.8×(Ea−Ed)), whether the thermal expansion coefficients Ea to Ec satisfy formula (1), and the thickness T of the intermediate layer98.

As can be seen from Table 1, in Examples 1 to 10 in which the ratio Ea/Eb was more than 1.0 and 5.0 or less and Ea>Ec>Eb was satisfied, the evaluation of the cracking resistance of the sensor elements20was “excellent (A)” or “good (B),” and the occurrence of cracking was reduced. However, in Comparative Examples 1 and 2 in which Ea>Ec>Eb was not satisfied and in Comparative Example 3 in which the ratio Ea/Eb was more than 5.0, the evaluation of the cracking resistance of the sensor elements20was “fail (F).” This confirms that, when the ratio Ea/Eb is more than 1.0 and 5.0 or less and Ea>Ec>Eb is satisfied, the occurrence of cracking in the sensor element20can be reduced. In Examples 2 and 3 in which formula (1) was not satisfied, the evaluation of the cracking resistance was “good (B).” However, in Examples 1, 4 to 6, and 8 to 10 in which formula (1) was satisfied, the evaluation of the cracking resistance was “excellent (A).” This confirms that, when formula (1) is satisfied, the occurrence of cracking in the sensor element20can be further reduced. In Example 7 in which the ratio Ea/Eb was more than 3.0, the evaluation of the cracking resistance was “good (B).” However, in Examples 1, 4 to 6, and 8 to 10 in which the ratio Ea/Eb was 3.0 or less, the evaluation of the cracking resistance was “excellent (A).” This confirms that, when the ratio Ea/Eb is 3.0 or less, the occurrence of cracking in the sensor element20can be further reduced. The results in Example 10 confirm that the effect of reducing the occurrence of cracking is obtained when the thickness T of the intermediate layer98is in the range of 1 μm or more. The results in Example 9 confirm that the effect of reducing the occurrence of cracking is obtained when the thickness T of the intermediate layer98is in the range of 10 μm or less.

The present application claims priority based on U.S. Patent Application No. 63/211,665 filed on Jun. 17, 2021, and the entire contents of which are incorporated herein by reference.