Gas Sensors

Abstract

The present invention provides a gas sensor that can accurately measure the gas to be measured even in the presence of gas species that can generate an exothermic reaction. The gas sensor includes: a sensor element in the shape of a long strip; and a bottomed cylindrical protective cover that accommodates a portion of the sensor element including one end, the length La of the sensor element in the longitudinal direction from the one end of the sensor element to the midpoint of the length direction of the gas inlet that passes through the side of the protective cover relative to the length index Lb between the sensor element and the inner surface of the protective cover in a plane that includes the midpoint of the length direction of the gas inlet and is orthogonal to the length direction of the sensor element. The ratio La / Lb is 0.9 or more, and the ratio S / V of the surface area S of the portion of length La in the two main surfaces and two side surfaces of the sensor element to the volume V of the space where the portion of length La exists in the protective cover is 0.15 mm ‑1 above.

Description

Technical Field

[0001] The present invention relates to a gas sensor for detecting a measurement target gas in a measured gas. Background Art

[0002] Gas sensors are used to detect and measure the concentration of target gas components (oxygen O2, nitrogen oxides NOx, ammonia NH3, hydrocarbons HC, carbon dioxide CO2, etc.) in the exhaust gas of automobiles and other measured gases. For example, gas sensors are installed in the exhaust path of gasoline and diesel vehicles to measure the concentration of nitrogen oxides NOx, ammonia NH3, etc. in the exhaust gas.

[0003] As the above-mentioned gas sensor, there is known a gas sensor having a sensor element using a solid electrolyte having oxygen ion conductivity such as zirconium oxide (ZrO2), and a protective cover for protecting the portion of the sensor element that contacts the measured gas. In the above-mentioned gas sensor, protective covers of various structures are adopted for various purposes such as reducing water splashing onto the sensor element and reducing temperature changes of the sensor element caused by the measured gas (for example, Japanese Patent No. 3829026, Japanese Patent No. 4174004, Japanese Patent No. 6998802, Japanese Patent Publication No. 2021-148656, and Japanese Patent No. 4571974).

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent No. 3829026

[0007] Patent Document 2: Japanese Patent No. 4174004

[0008] Patent Document 3: Japanese Patent No. 6998802

[0009] Patent Document 4: Japanese Patent Application Publication No. 2021-148656

[0010] Patent Document 5: Japanese Patent No. 4571974 Summary of the invention

[0011] Not only gasoline and diesel vehicles, but also hydrogen engine vehicles are required to measure the concentrations of nitrogen oxides NOx, ammonia NH3, etc. in exhaust gas.

[0012] In the exhaust path of a hydrogen-powered vehicle, a gas atmosphere containing oxygen and hydrogen is assumed. It is well known that when the concentration of oxygen and hydrogen is within a certain range, hydrogen explosion may occur, or hydrogen may spontaneously ignite when the temperature in the atmosphere reaches 500°C or higher.

[0013] When a gas sensor such as the one described above is used in the exhaust path of a hydrogen-powered vehicle, the following situation may occur, depending on the driving conditions such as the gas atmosphere in the exhaust path (especially the oxygen concentration and hydrogen concentration) and the temperature of the gas sensor. That is, oxygen and hydrogen react rapidly in the gas sensor, causing cracks in the internal structure of the sensor element due to the thermal stress caused by the reaction heat.

[0014] Therefore, an object of the present invention is to provide a gas sensor that can suppress the occurrence of cracks in the sensor element and measure the target gas in the measured gas with good accuracy even when a gas with coexisting gas species that can produce an exothermic reaction, such as a gas with coexisting oxygen and hydrogen, is introduced into the gas sensor as a measured gas.

[0015] The inventors of the present invention have conducted intensive research and found that by defining the positional relationship between a sensor element and a gas inlet provided in a protective cover that accommodates a portion of the sensor element as follows, it is possible to suppress the occurrence of cracks in the sensor element even when a gas in which gas types that can produce an exothermic reaction coexist is introduced into the gas sensor as the measured gas.

[0016] The present invention includes the following inventions.

[0017] (1) A gas sensor for detecting a gas to be measured in a gas to be measured, comprising:

[0018] A sensor element, the sensor element being a long plate-shaped sensor element including an oxygen ion conductive solid electrolyte layer, wherein a measured gas flow cavity is formed at one end side in the longitudinal direction of the sensor element; and

[0019] A bottomed cylindrical protective cover, the protective cover accommodating a portion of the sensor element including the one end,

[0020] in,

[0021] The protective cover has a gas inlet that penetrates a side surface of the protective cover surrounding the sensor element.

[0022] The length of the sensor element in the longitudinal direction from the one end of the sensor element to the midpoint of the gas inlet of the protective cover in the longitudinal direction is represented as length La,

[0023] In a plane of the protective cover that includes the midpoint of the gas inlet in the longitudinal direction and is orthogonal to the longitudinal direction of the sensor element, the longest distances between each of the two main surfaces of the sensor element and the circumference of a circle defined by the inner surface of the protective cover and opposite to the main surfaces are set to length x1 and length x2, respectively, and the longest distances between each of the two side surfaces of the sensor element and the circumference of the circle opposite to the side surfaces are set to length y1 and length y2, respectively, and the index of the length between the sensor element and the circle defined by the inner surface of the protective cover is expressed as length index Lb=(x1+x2+y1+y2) / 4,

[0024] The ratio La / Lb of the length La to the length index Lb is greater than 0.9,

[0025] Furthermore, when the total of the surface areas of the portions of the length La in the longitudinal direction of the two main surfaces and the two side surfaces of the sensor element is represented as a surface area S, and the volume of the space where the portion of the length La in the longitudinal direction in the protective cover exists is represented as a volume V, the ratio S / V of the surface area S to the volume V is 0.15 mm -1 above.

[0026] (2) The gas sensor according to (1) above, wherein:

[0027] The length La in the longitudinal direction is 10 mm or less.

[0028] (3) The gas sensor according to (1) or (2), wherein:

[0029] The length x1 and the length x2 satisfy x1=x2=x, the length y1 and the length y2 satisfy y1=y2=y, and the length index Lb is represented by Lb=(x+y) / 2.

[0030] The above (3) refers to a case where the sensor element is arranged at the center of the protection cover in a plane that includes the midpoint of the gas inlet in the longitudinal direction of the protection cover and is orthogonal to the longitudinal direction of the sensor element.

[0031] (4) The gas sensor according to any one of (1) to (3), wherein:

[0032] The protective cover has a gas outlet at the bottom.

[0033] (5) The gas sensor according to any one of (1) to (4) above, wherein:

[0034] An outer cover for housing the protective cover is also provided on the outer side of the protective cover.

[0035] The outer cover has an outer gas inlet at a position different from the gas inlet of the protection cover in the longitudinal direction of the sensor element.

[0036] (6) The gas sensor according to (5) above, wherein:

[0037] The outer cover has an outer gas outflow port at a position different from the gas inflow port of the protection cover and the outer gas inflow port of the outer cover in the longitudinal direction of the sensor element.

[0038] (7) The gas sensor according to any one of (1) to (6), wherein:

[0039] The measured gas flow cavity of the sensor element has a gas introduction port opened at an end surface of the one end in the longitudinal direction.

[0040] Effects of the Invention

[0041] If the gas sensor of the present invention is used, it is possible to provide a gas sensor that can suppress the generation of cracks in the sensor element and accurately measure the target gas in the measured gas even when a gas in which gas species that can produce an exothermic reaction, such as a gas in which oxygen and hydrogen coexist, is introduced into the gas sensor as the measured gas. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 It is a partially schematic cross-sectional view showing an example of a schematic structure of the gas sensor 100 .

[0043] Figure 2 1 is a schematic vertical cross-sectional view in the longitudinal direction of the sensor element 101 , showing an example of the schematic configuration of the gas sensor 100 .

[0044] Figure 3 1 is a conceptual diagram showing the flow of the gas to be measured in the element protection cover 120 in the gas sensor 100 .

[0045] Figure 4 It is used to explain the length La and Figure 1 The same partial cross-sectional view.

[0046] Figure 5 It is used to explain the length index Lb along Figure 4 This is an enlarged cross-sectional diagram of the V-V line. Figure 5 In the figure, the outer protection cover 140 is omitted.

[0047] Figure 6 It is used to describe the volume V and Figure 1 The same partial cross-sectional view.

[0048] Figure 7 It is used to illustrate the surface area S and volume V along Figure 6 An enlarged cross-sectional diagram of line VII-VII.

[0049] Figure 8 FIG. 1 is a perspective view of the sensor element 101 for explaining the surface area S. FIG.

[0050] Explanation of symbols

[0051] 10…gas inlet; 12…buffer space; 15…measured gas flow cavity; 20…first internal cavity; 40…second internal cavity; 61…third internal cavity; 100…gas sensor; 101…sensor element; 101a…front end face (of sensor element); 101b…rear end face (of sensor element); 101c…upper surface (of sensor element); 101d…lower surface (of sensor element); 101e…left surface (of sensor element); 101f…sensor 120…element protection cover; 130…inner protection cover; 131…first part (of the inner protection cover); 132…second part (of the inner protection cover); 132c…gas diffusion sheet (of the second part of the inner protection cover); 140…outer protection cover; 150…element enclosure; 151…housing; 152…ceramic support; 160…fixing portion; H1…outer gas inlet; H2…inner gas inlet; H3…inner gas outlet; H4…outer gas outlet DETAILED DESCRIPTION

[0052] A gas sensor of the present invention includes: a sensor element; and a bottomed cylindrical protection cover that accommodates a portion of the sensor element including one end.

[0053] The sensor element included in the gas sensor of the present invention is in the shape of a long plate including an oxygen ion conductive solid electrolyte layer, and a measured gas flow cavity is formed at one end side in the longitudinal direction of the sensor element.

[0054] The protective cover included in the gas sensor of the present invention is a bottomed cylindrical cover that accommodates a portion of the sensor element including the one end where the measured gas flow cavity is formed. The protective cover has a gas inlet that penetrates the side surface of the protective cover surrounding the sensor element.

[0055] In the gas sensor of the present invention,

[0056] The length of the sensor element in the longitudinal direction from the one end of the sensor element to the midpoint of the gas inlet of the protective cover in the longitudinal direction is represented as length La,

[0057] In a plane of the protective cover that includes the midpoint of the gas inlet in the longitudinal direction and is orthogonal to the longitudinal direction of the sensor element, the longest distances between each of the two main surfaces of the sensor element and the circumference of a circle defined by the inner surface of the protective cover and opposite to the main surfaces are set to length x1 and length x2, respectively, and the longest distances between each of the two side surfaces of the sensor element and the circumference of the circle opposite to the side surfaces are set to length y1 and length y2, respectively, and the index of the length between the sensor element and the circle defined by the inner surface of the protective cover is expressed as length index Lb=(x1+x2+y1+y2) / 4,

[0058] The ratio La / Lb of the length La to the length index Lb is greater than 0.9,

[0059] Furthermore, when the total of the surface areas of the portions of the length La in the longitudinal direction of the two main surfaces and the two side surfaces of the sensor element is represented as a surface area S, and the volume of the space where the portion of the length La in the longitudinal direction in the protective cover exists is represented as a volume V, the ratio S / V of the surface area S to the volume V is 0.15 mm -1 above.

[0060] [General structure of gas sensor]

[0061] Hereinafter, the gas sensor of the present invention will be described with reference to the drawings. Figure 1 1 is a partially schematic cross-sectional view showing an example of a schematic configuration of a gas sensor 100 as one embodiment of the present invention. Figure 2 FIG. 1 is a schematic vertical cross-sectional view of a sensor element 101 in the longitudinal direction, showing an example of the schematic structure of the gas sensor 100. The internal structure of the sensor element 101 is shown in detail in FIG. Figure 2 ,exist Figure 1 In the figure, a part of the internal structure of the sensor element 101 is omitted. Figure 2 As a benchmark, the so-called up and down Figure 2 Set the upper side of the , set the lower side to lower, and Figure 2 The left side is defined as the front end side, and the right side is defined as the rear end side. Figure 2 As a reference, the front side perpendicular to the paper surface is set as the right, and the back side is set as the left. Figure 1 If it is combined with Figure 2 Correspondingly, Figure 1The front side perpendicular to the paper surface is the top, and the back side is the bottom. Figure 1 The lower side in the figure is the front end side, the upper side is the rear end side, the right side is right, and the left side is left.

[0062] like Figure 1 As shown, the gas sensor 100 includes: a sensor element 101; and an element protection cover 120, which protects the front end side of the sensor element 101. In addition, the gas sensor 100 includes: an element sealing body 150, which seals and fixes the sensor element 101; and a fixing part 160, which is mounted on the element sealing body 150. The gas sensor 100 is mounted on a pipe such as an exhaust pipe of a vehicle, and is used to measure the concentration of a measurement target gas such as NOx, NH3, O2 contained in the exhaust gas as the measured gas. In this embodiment, the gas sensor 100 measures the concentration of NOx as the measurement target gas.

[0063] The element enclosure 150 includes: a metal cylindrical housing 151; and a ceramic support 152, which is sealed in a through hole inside the housing 151. The sensor element 101 is located on the central axis of the element enclosure 150 and penetrates the element enclosure 150 in the front-to-back direction. A powder compact (not shown) made of ceramic powder such as talc and at least one ceramic support (not shown) are arranged on the inner side of the housing 151 at a position closer to the rear end side of the sensor element 101 than the ceramic support 152 in the through hole. The through hole inside the housing 151 is sealed by the powder compact clamped by the ceramic support, thereby ensuring the airtightness between the front end side and the rear end side of the sensor element 101, and fixing the sensor element 101.

[0064] The fixing portion 160 is an annular metal component. The fixing portion 160 is fixed coaxially with the housing 151, and an external thread portion (not shown) is formed on the outer peripheral surface. The gas sensor 100 is threadedly fixed to the piping through the fixing portion 160. The fixing portion 160 can also be configured to achieve fixation by welding or the like. In addition, the fixing portion 160 can be formed integrally with the element enclosure 150. Through the fixing portion 160, the gas sensor 100 is fixed to the piping in a state where the front end of the sensor element 101 in the gas sensor 100 and part of the element protection cover 120 protrude into the piping.

[0065] In addition, the rear end side of the sensor element 101 is housed in a metal outer cylinder (not shown) that is welded and fixed to the housing 151. The internal space of the outer cylinder is filled with a reference gas (e.g., air) for measuring the NOx concentration, so that the rear end side of the sensor element 101 is in contact with the reference gas. Each electrode of the sensor element 101 is electrically connected to the outside (power supply, control device, etc.) via a connector disposed at the rear end of the sensor element 101.

[0066] The element protection cover 120 includes: an inner protection cover 130 having a bottomed cylindrical shape, which accommodates a portion of the sensor element 101 including the front end; and an outer protection cover 140 having a bottomed cylindrical shape, which accommodates the inner protection cover 130. The element protection cover 120 has a plurality of through holes for gas flow, and is configured to supply the measured gas to the front end side of the sensor element 101 through these through holes. The element protection cover 120 will be described in detail below.

[0067] (Sensor element)

[0068] like Figure 2 As shown, the sensor element 101 is a long strip-shaped element, which includes: a substrate portion 102 having a structure obtained by stacking multiple oxygen ion conductive solid electrolyte layers. The long strip-shaped is also called a long plate or a strip. The substrate portion 102 has: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, an isolation layer 5 and a second solid electrolyte layer 6, which are respectively formed by oxygen ion conductive solid electrolyte layers such as zirconium oxide (ZrO2), are stacked in this order from the bottom in the accompanying drawings. The solid electrolyte forming these six layers is a dense and airtight solid electrolyte. The six layers can be all of the same thickness, or they can be different thicknesses for each layer. The layers are bonded to each other by means of an adhesive layer containing a solid electrolyte, and the substrate portion 102 includes the adhesive layer. Figure 2 Although the layer structure including the above six layers is exemplified in FIG. 1 , the layer structure in the present invention is not limited thereto, and any number of layers and layer structure may be adopted.

[0069] The sensor element 101 is manufactured by, for example, subjecting ceramic green sheets corresponding to the respective layers to predetermined processing and printing of circuit patterns, stacking them, and further sintering them to integrate them.

[0070] The sensor element 101 is in the shape of a long strip and has six surfaces: two end surfaces in the length direction (front end surface 101a and rear end surface 101b), two main surfaces along the length direction (upper surface 101c and lower surface 101d), and two side surfaces along the length direction (left surface 101e and right surface 101f). Figure 2 In the figure, the left surface 101e and the right surface 101f are not shown.

[0071] A gas flow cavity 15 is formed at one end side in the longitudinal direction of the sensor element 101. Hereinafter, the end on the side where the gas flow cavity 15 is formed is referred to as the front end. In this embodiment, the gas inlet 10 is open at the front end surface 101a of the sensor element, and the gas to be measured is introduced from the gas inlet 10 into the gas flow cavity 15 inside the sensor element 101.

[0072] Specifically, a gas inlet 10 is formed at the front end portion in the longitudinal direction of the sensor element 101 and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measured gas flow cavity 15, that is, the measured gas flow portion is formed in such a manner that the first diffusion rate limiting portion 11, the buffer space 12, the second diffusion rate limiting portion 13, the first internal cavity 20, the third diffusion rate limiting portion 30, the second internal cavity 40, the fourth diffusion rate limiting portion 60 and the third internal cavity 61 are sequentially connected in the above order in the longitudinal direction starting from the gas inlet 10.

[0073] The gas inlet port 10, the buffer space 12, the first internal cavity 20, the second internal cavity 40 and the third internal cavity 61 are: the internal space of the sensor element 101 arranged in a manner of hollowing out the isolation layer 5, wherein the upper part of the internal space is divided by the lower surface of the second solid electrolyte layer 6, the lower part is divided by the upper surface of the first solid electrolyte layer 4, and the side part is divided by the side surface of the isolation layer 5.

[0074] The first diffusion rate limiting section 11, the second diffusion rate limiting section 13 and the third diffusion rate limiting section 30 are all provided with two horizontally long slits ( Figure 2 In the figure, the direction perpendicular to the drawing constitutes the length direction of the opening. The first diffusion rate limiting section 11, the second diffusion rate limiting section 13, and the third diffusion rate limiting section 30 may be in any form as long as they apply a desired diffusion resistance, and their forms are not limited to the slits.

[0075] The fourth diffusion rate limiting unit 60 has a horizontally long slit ( Figure 2 The fourth diffusion rate limiting portion 60 is provided between the separator 5 and the second solid electrolyte layer 6 in such a manner that the direction perpendicular to the drawing constitutes the longitudinal direction of the opening. The fourth diffusion rate limiting portion 60 may be in any form as long as it applies a desired diffusion resistance, and its form is not limited to the slit.

[0076] In addition, a reference gas introduction space 43 is provided at a position farther from the front end side than the measured gas flow cavity 15, between the upper surface of the third substrate layer 3 and the lower surface of the isolation layer 5, and at a position where the side is partitioned by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. For example, the atmosphere is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

[0077] The atmosphere introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. The atmosphere introduction layer 48 is formed so as to cover the reference electrode 42.

[0078] The reference electrode 42 is an electrode formed by being sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, an atmosphere introduction layer 48 connected to the reference gas introduction space 43 is provided around it. That is, the reference electrode 42 is configured to be in contact with the reference gas by means of the porous atmosphere introduction layer 48 and the reference gas introduction space 43. In addition, as described later, the reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61. The reference electrode 42 is formed as a porous metal ceramic electrode (for example, a metal ceramic electrode of Pt and ZrO2).

[0079] The gas inlet port 10 is a portion of the measured gas flow cavity 15 that is open to the external space, and the measured gas enters the sensor element 101 from the external space through the gas inlet port 10 .

[0080] In this embodiment, the measured gas flow cavity 15 is a form in which the measured gas is introduced from the gas inlet 10 which is open at the front end surface of the sensor element 101, but the present invention is not limited to this form. For example, there may be no concave part of the gas inlet 10 in the measured gas flow cavity 15. In this case, the first diffusion rate limiting part 11 is essentially a gas inlet.

[0081] In addition, for example, the measured gas flow cavity 15 may be configured such that the measured gas is introduced through a porous body.

[0082] The first diffusion rate limiting portion 11 is a portion that applies a predetermined diffusion resistance to the gas to be measured introduced from the gas inlet 10 .

[0083] The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate limiting section 11 to the second diffusion rate limiting section 13 .

[0084] The second diffusion rate limiting portion 13 is a portion that applies a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal cavity 20 .

[0085] As a result, it is sufficient that the amount of the measured gas introduced into the first internal cavity 20 is within a predetermined range. That is, it is sufficient that a predetermined diffusion resistance is applied to the entire area from the front end of the sensor element 101 to the second diffusion rate limiting section 13. For example, the first diffusion rate limiting section 11 may be directly connected to the first internal cavity 20, that is, the buffer space 12 and the second diffusion rate limiting section 13 may not exist.

[0086] The buffer space 12 is a space provided to alleviate the influence of the pressure change on the detection value when the pressure of the gas to be measured changes.

[0087] When the measured gas is introduced from the outside of the sensor element 101 into the first internal cavity 20, it is rapidly introduced into the sensor element 101 from the gas inlet port 10 due to the pressure change of the measured gas in the external space (when the measured gas is automobile exhaust gas, it is the pulsation of exhaust pressure). However, the measured gas is not introduced directly into the first internal cavity 20, but is introduced into the first internal cavity 20 after the pressure change of the measured gas is eliminated by passing through the first diffusion rate limiting section 11, the buffer space 12, and the second diffusion rate limiting section 13. As a result, the pressure change of the measured gas introduced into the first internal space is almost negligible.

[0088] The first internal cavity 20 is provided as a space for adjusting the oxygen partial pressure in the gas to be measured introduced after passing through the second diffusion rate limiting section 13. The main pump unit 21 operates to adjust the oxygen partial pressure.

[0089] The main pump unit 21 is an electrochemical pump unit composed of an inner main pump electrode 22, an outer pump electrode 23 and a second solid electrolyte layer 6 clamped by these electrodes, wherein the inner main pump electrode 22 has: a top electrode portion 22a arranged on the substantially entire surface of the lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, and the outer pump electrode 23 is arranged on the upper surface of the second solid electrolyte layer 6 in an area corresponding to the top electrode portion 22a in a manner exposed in the external space.

[0090] The inner main pump electrode 22 is arranged so as to face the first inner cavity 20 on the inner surface of the measured gas flow cavity 15. That is, the inner main pump electrode 22 is formed across: the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that partition the first inner cavity 20, and the isolation layer 5 that constitutes the side wall. Specifically, a top electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that constitutes the top surface of the first inner cavity 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that constitutes the bottom surface, and a side electrode portion (not shown) is formed on the side wall surface (inner surface) of the isolation layer 5 that constitutes the two side wall portions of the first inner cavity 20 in a manner that connects the top electrode portion 22a and the bottom electrode portion 22b, so that the side electrode portion is arranged in a tunnel-shaped structure.

[0091] The inner main pump electrode 22 and the outer pump electrode 23 are formed as porous metal ceramic electrodes (for example, metal ceramic electrodes containing 1% Au, Pt and ZrO2). In addition, the inner main pump electrode 22 in contact with the measured gas is formed using a material that weakens the reducing ability of the NOx component in the measured gas.

[0092] In the main pump unit 21, a variable power supply 24 is used to apply a desired pump voltage Vp0 between the inner main pump electrode 22 and the outer pump electrode 23, so that the pump current Ip0 flows between the inner main pump electrode 22 and the outer pump electrode 23 in a positive direction or a negative direction. Thus, the oxygen in the first internal cavity 20 can be absorbed into the external space, or the oxygen in the external space can be absorbed into the first internal cavity 20.

[0093] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere of the first internal cavity 20, an electrochemical sensor unit, i.e., an oxygen partial pressure detection sensor unit 80 for main pump control, is formed by the inner main pump electrode 22, the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42.

[0094] By measuring the electromotive force (voltage V0) of the oxygen partial pressure detection sensor unit 80 for main pump control, the oxygen concentration (oxygen partial pressure) in the first internal cavity 20 can be known. In addition, the pump voltage Vp0 is feedback-controlled so that the voltage V0 is constant, thereby controlling the pump current Ip0. Thus, the oxygen concentration in the first internal cavity 20 can be maintained at a predetermined constant value.

[0095] The third diffusion rate limiting unit 30 is a portion that applies a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump unit 21 in the first internal cavity 20 , and guides the gas to be measured to the second internal cavity 40 .

[0096] The second internal cavity 40 is provided as a space for more accurately adjusting the oxygen partial pressure in the gas to be measured introduced through the third diffusion rate limiting section 30. The auxiliary pump unit 50 is operated to adjust the oxygen partial pressure.

[0097] In the second internal cavity 40, the oxygen partial pressure of the measured gas, which has been introduced after the oxygen concentration (oxygen partial pressure) has been adjusted in advance in the first internal cavity 20 and has passed through the third diffusion rate limiting section 30, is further adjusted by the auxiliary pump unit 50. Thus, the oxygen concentration in the second internal cavity 40 can be kept constant with high accuracy, and therefore, in such a gas sensor 100, the NOx concentration can be measured with high accuracy.

[0098] The auxiliary pump unit 50 is an auxiliary electrochemical pump unit consisting of an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, as long as it is a suitable electrode at a position different from the position in the measured gas flow cavity 15, such as the outside of the sensor element 101), and a second solid electrolyte layer 6, wherein the auxiliary pump electrode 51 has: a top electrode portion 51a arranged on the lower surface of the second solid electrolyte layer 6 and facing the second internal cavity 40 in a substantially entirety.

[0099] The auxiliary pump electrode 51 is disposed on the inner surface of the measured gas flow cavity 15 at a position farther from the one end (front end) in the longitudinal direction of the base portion 102 (sensor element 101 ) than the inner main pump electrode 22 .

[0100] Such an auxiliary pump electrode 51 is arranged in the second internal cavity 40 in the same tunnel-shaped structure as the inner main pump electrode 22 previously arranged in the first internal cavity 20. That is, a top electrode portion 51a is formed on the second solid electrolyte layer 6 constituting the top surface of the second internal cavity 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 constituting the bottom surface of the second internal cavity 40, and a side electrode portion (not shown) connecting the top electrode portion 51a and the bottom electrode portion 51b is formed in a tunnel-shaped structure respectively formed on the two wall surfaces of the isolation layer 5 constituting the side wall of the second internal cavity 40.

[0101] In addition, similarly to the inner main pump electrode 22 , the auxiliary pump electrode 51 is also formed using a material having a weakened reducing ability for NOx components in the measured gas.

[0102] In the auxiliary pump unit 50 , the variable power supply 52 is used to apply a desired pump voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 , thereby being able to absorb oxygen in the atmosphere in the second internal cavity 40 into the external space, or to absorb oxygen from the external space into the second internal cavity 40 .

[0103] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal cavity 40, an electrochemical sensor unit, namely, an auxiliary pump control oxygen partial pressure detection sensor unit 81, is formed by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4 and the third substrate layer 3.

[0104] Furthermore, the auxiliary pump unit 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the auxiliary pump control oxygen partial pressure detection sensor unit 81. Thus, the oxygen partial pressure in the atmosphere in the second internal cavity 40 is controlled to a relatively low partial pressure that has substantially no effect on the measurement of NOx.

[0105] At the same time, the pump current Ip1 is used to control the voltage V0 of the main pump control oxygen partial pressure detection sensor unit 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor unit 80, and by controlling the voltage V0, the gradient of the oxygen partial pressure in the measured gas introduced from the third diffusion rate limiting unit 30 into the second internal cavity 40 is controlled to be constant at all times. When used as a NOx sensor, the oxygen concentration in the second internal cavity 40 is maintained at a constant value of about 0.001 ppm due to the action of the main pump unit 21 and the auxiliary pump unit 50.

[0106] The fourth diffusion rate limiting section 60 is a portion that applies a predetermined diffusion resistance to the gas to be measured whose oxygen concentration (oxygen partial pressure) is controlled to be lower by the operation of the auxiliary pump unit 50 in the second internal cavity 40 , and guides the gas to be measured to the third internal cavity 61 .

[0107] The third internal cavity 61 is provided as a space for measuring the concentration of nitrogen oxides (NOx) in the gas to be measured introduced through the fourth diffusion rate limiting section 60. The NOx concentration is measured by the operation of the measurement pump unit 41.

[0108] The measuring pump unit 41 measures the NOx concentration in the measured gas in the third internal cavity 61. The measuring pump unit includes an inner measuring electrode (the measuring electrode 44 in this embodiment) and an outer measuring electrode (the outer pump electrode 23 in this embodiment), wherein the inner measuring electrode is arranged in the measured gas circulation cavity 15 (the inner surface of the measured gas circulation cavity 15), and the outer measuring electrode is arranged at a position of the base part 102 different from the measured gas circulation cavity 15 and corresponds to the inner measuring electrode.

[0109] That is, in the present embodiment, the measuring pump unit 41 is an electrochemical pump unit composed of a measuring electrode 44, an outer pump electrode 23 (not limited to the outer pump electrode 23, as long as it is a suitable electrode at a position different from that in the measured gas flow cavity 15, such as the outside of the sensor element 101), a second solid electrolyte layer 6, an isolation layer 5 and a first solid electrolyte layer 4, wherein the measuring electrode 44 is arranged on the upper surface of the first solid electrolyte layer 4, facing the third internal cavity 61.

[0110] The measuring electrode 44 is disposed on the inner surface of the measured gas flow cavity 15 at a position farther from the one end (front end) in the length direction of the base portion 102 (sensor element 101 ) than the inner main pump electrode 22 and the auxiliary pump electrode 51 .

[0111] The measuring electrode 44 is a porous metal ceramic electrode. The measuring electrode 44 also functions as a NOx reduction catalyst for reducing NOx present in the atmosphere in the third internal cavity 61. For example, in this embodiment, the measuring electrode 44 is a porous metal ceramic electrode of Pt, Rh, and ZrO2.

[0112] The measuring pump cell 41 can absorb oxygen generated by decomposition of nitrogen oxides in the atmosphere around the measuring electrode 44 , and the generated amount can be detected as the pump current Ip2 .

[0113] In order to detect the oxygen partial pressure around the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, the measuring electrode 44 and the reference electrode 42 constitute an electrochemical sensor unit, that is, a measuring pump control oxygen partial pressure detection sensor unit 82. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the measuring pump control oxygen partial pressure detection sensor unit 82.

[0114] The gas to be measured introduced into the second internal cavity 40 passes through the fourth diffusion rate limiting section 60 under the condition that the oxygen partial pressure is controlled and reaches the measuring electrode 44 in the third internal cavity 61. The nitrogen oxides in the gas to be measured around the measuring electrode 44 are reduced (2NO→N2+O2) to generate oxygen. And the generated oxygen is pumped by the measuring pump unit 41. At this time, the pump voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the oxygen partial pressure detection sensor unit 82 for measuring pump control is constant. The amount of oxygen generated around the measuring electrode 44 is proportional to the concentration of nitrogen oxides in the gas to be measured. Therefore, the concentration of nitrogen oxides in the gas to be measured is calculated using the pump current Ip2 in the measuring pump unit 41.

[0115] In addition, if the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3 and the reference electrode 42 are combined to form an oxygen partial pressure detection structure in the form of an electrochemical sensor unit, the electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of the NOx component in the atmosphere around the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere can be detected, thereby also being able to calculate the concentration of the NOx component in the measured gas.

[0116] In addition, the electrochemical sensor unit 83 is composed of the second solid electrolyte layer 6, the isolation layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23 and the reference electrode 42. The sensor unit 83 can be used to obtain an electromotive force (voltage Vref), and the electromotive force can be used to detect the oxygen partial pressure in the measured gas outside the sensor.

[0117] In the gas sensor 100 having such a structure, the main pump unit 21 and the auxiliary pump unit 50 are operated to supply the measured gas whose oxygen partial pressure is always kept at a constant low value (a value that has substantially no effect on the measurement of NOx) to the measurement pump unit 41. Therefore, the NOx concentration in the measured gas can be known based on the pump current Ip2 that is approximately proportional to the NOx concentration in the measured gas and is circulated by oxygen generated by the reduction of NOx being sucked out from the measurement pump unit 41.

[0118] In addition, the sensor element 101 further includes a heater unit 70, which has a temperature adjustment function of heating and keeping the sensor element 101 warm so as to improve the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater electrode 71, a heater 72, a heater conductive portion 76, a through hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0119] The heater electrode 71 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power source as an external power source, power can be supplied to the heater unit 70 from the outside.

[0120] The heater 72 is a resistor formed by being sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via the heater conductive portion 76 and the through hole 73, and is powered from the outside through the heater electrode 71 to generate heat, thereby heating and keeping the solid electrolyte forming the sensor element 101 warm. The heater conductive portion 76 is connected to the heater 72 and extends to the rear end side in the longitudinal direction of the sensor element 101.

[0121] In addition, the heater 72 is buried in the entire area from the first internal cavity 20 to the third internal cavity 61, and the sensor element 101 as a whole can be adjusted to the temperature at which the solid electrolyte is activated. It is sufficient to adjust the temperature in such a way that the main pump unit 21, the auxiliary pump unit 50, and the measurement pump unit 41 can work. It is not necessary to adjust the entire area to the same temperature, and the sensor element 101 can have a temperature distribution. For example, the sensor element 101 is heated so that the temperature of the solid electrolyte and each electrode around the measured gas flow cavity 15 reaches about 750°C to about 900°C.

[0122] In the sensor element 101 of the present embodiment, the heater 72 is embedded in the base portion 102, but the present invention is not limited to this embodiment. The heater 72 may be arranged to heat the base portion 102. That is, the heater 72 may heat the sensor element 101 to such an extent that the oxygen ion conductivity of the main pump unit 21, the auxiliary pump unit 50, and the measurement pump unit 41 described above can work. For example, it may be embedded in the base portion 102 as in the present embodiment. Alternatively, for example, the heater portion 70 may be formed as another heater substrate different from the base portion 102 and arranged at an adjacent position of the base portion 102.

[0123] The heater insulating layer 74 is an insulating layer formed by an insulator such as alumina on the upper and lower surfaces of the heater 72 and the heater conductive portion 76. The heater insulating layer 74 is formed to obtain electrical insulation between the second substrate layer 2 and the heater 72 and the heater conductive portion 76, and to obtain electrical insulation between the third substrate layer 3 and the heater 72 and the heater conductive portion 76.

[0124] The pressure release hole 75 is formed to penetrate the third substrate layer 3 so as to communicate the heater insulating layer 74 with the reference gas introduction space 43. The pressure release hole 75 can alleviate the internal pressure increase accompanying the temperature increase in the heater insulating layer 74. In addition, a configuration without the pressure release hole 75 may be adopted.

[0125] In addition, the surface of the sensor element 101 may be covered by a porous protective layer (not shown) from the front end to a predetermined length in the longitudinal direction. The purpose of forming the porous protective layer is to protect the region where the internal cavity and electrodes exist in the sensor element 101 from thermal shock caused by splashing water, etc. The porous protective layer is made of ceramics such as alumina, and the thickness may be about 10 μm to 2000 μm. In addition, it is preferably formed to be able to withstand a force of about 50 N.

[0126] The sensor element 101 is embedded in the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas. Figure 1 As shown, it is accommodated in the internal space of the element protection cover 120.

[0127] [Use of gas sensors in the presence of oxygen and hydrogen]

[0128] Consider the case where the gas sensor 100 described above is used to measure the target gas (NOx in this embodiment) in a gas to be measured where oxygen and hydrogen coexist. Examples of the gas to be measured include exhaust gas from internal combustion engines such as hydrogen engines and gasoline engines.

[0129] For example, in a hydrogen-powered vehicle, an exhaust gas purification device including an oxidation catalyst is usually provided in the exhaust pipe downstream of the hydrogen engine, and unburned hydrogen in the hydrogen engine is oxidized in the presence of the oxidation catalyst. However, before the oxidation catalyst is preheated during a cold start, it is conceivable that hydrogen is discharged downstream of the oxidation catalyst without being oxidized. Before the oxidation catalyst is preheated during a cold start, even if the gas sensor 100 is installed at a position downstream of the oxidation catalyst for use, it is possible that hydrogen reaches the gas sensor 100 without being oxidized in the presence of the oxidation catalyst.

[0130] When the gas sensor 100 is used, as described above, the sensor element 101 is heated by the heater 72, and the front end portion of the sensor element 101 where the measured gas flow cavity 15 is formed becomes a high temperature (for example, about 800°C). The high-temperature portion is housed in the inner protective cover 130 and is configured in contact with the measured gas. The measured gas reaching the vicinity of the sensor element 101 is heated and becomes a high temperature by passing through the sensor element 101 in a high-temperature state. Therefore, in the case where oxygen and hydrogen coexist in the measured gas, a reaction between oxygen and hydrogen (oxidation reaction of hydrogen) may occur. The reaction between oxygen and hydrogen (oxidation reaction of hydrogen) is an exothermic reaction. It is considered that if a local oxidation reaction of hydrogen occurs near the sensor element 101, thermal stress is generated on the sensor element 101 due to its reaction heat.

[0131] It is considered that since the front end portion of the sensor element 101 where the measured gas flow cavity 15 is formed becomes high temperature due to the heater 72, a rapid hydrogen oxidation reaction is likely to occur particularly at the front end portion of the sensor element 101. If a local hydrogen oxidation reaction rapidly occurs near the sensor element 101, cracks may be generated in the internal structure of the sensor element 101 depending on the magnitude of the thermal stress generated at this time.

[0132] [Protective cover]

[0133] Next, use Figure 1 , Figure 3 to Figure 8 , the structure of the protective cover in the present invention is described in detail. Figure 1 As shown, the gas sensor 100 of this embodiment includes a metal element protection cover 120, which protects the front end side of the sensor element 101, that is, the portion of the sensor element 101 that contacts the measured gas. The element protection cover 120 has a plurality of through holes for gas circulation, and is configured to supply the measured gas to the front end of the sensor element 101 through these through holes.

[0134] In this embodiment, the element protection cover 120 includes: a bottomed cylindrical inner protection cover 130, which accommodates a portion of the sensor element 101 including the one end (front end); and a bottomed cylindrical outer protection cover 140, which is disposed outside the inner protection cover 130 and accommodates the inner protection cover 130. In this embodiment, the inner protection cover 130 is equivalent to the protection cover of the present invention, and the outer protection cover 140 is equivalent to the outer cover of the present invention.

[0135] The inner protective cover 130 is a bottomed cylindrical metal component that accommodates a portion of the sensor element 101 including the one end (front end). The inner protective cover 130 is configured such that the axial direction of the inner protective cover 130 is consistent or approximately consistent with the longitudinal direction of the sensor element 101. The inner protective cover 130 has a bottom at a position closer to the front end side than the front end face 101a of the sensor element 101. There is a space between the sensor element 101 and the inner protective cover 130 so that the measured gas can flow. The inner protective cover 130 accommodates at least the portion of the sensor element 101 to be in contact with the measured gas. For example, at least the portion of the sensor element 101 from the front end face 101a having the gas inlet 10 to the rear end in the longitudinal direction of the sensor element 101 of the outer pump electrode 23 can be accommodated. Alternatively, the portion of the sensor element 101 where the measured gas flow cavity 15 exists can be accommodated. The inner protective cover 130 may further accommodate the reference electrode 42 or a portion behind it, and may be appropriately determined within a range in which the airtightness between the front end side of the sensor element 101 in contact with the measured gas and the rear end side in contact with the reference gas is maintained.

[0136] In this embodiment, the inner protective cover 130 includes a first component 131 and a second component 132. The first component 131 includes: a cylindrical large diameter portion 131a, which is fitted into the outer periphery of the front end side of the housing 151; a cylindrical small diameter portion 131b, whose diameter is smaller than the large diameter portion 131a; and a step portion 131c, which connects the large diameter portion 131a and the small diameter portion 131b. The second component 132 includes: a cylindrical tubular portion 132a, which is fitted into the small diameter portion 131b of the first component 131; and a conical bottom 132b, which is continuous with the front end side of the tubular portion 132a. In addition, the rear end of the tubular portion 132a is deformed into a U-shape to form a gas diffusion sheet 132c. The two main surfaces (upper surface 101c, lower surface 101d) and two side surfaces (left surface 101e and right surface 101f) of the sensor element 101 are surrounded by the side surfaces of the inner protective cover 130 formed by the small diameter portion 131b of the first component 131 and the cylindrical portion 132a of the second component 132. In addition, the inner protective cover 130 may be a component formed integrally with the first component 131 and the second component 132.

[0137] The inner protective cover 130 has an inner gas inlet H2 and an inner gas outlet H3. The inner gas inlet H2 is a hole (e.g., a circular hole) that penetrates the side of the inner protective cover 130 surrounding the sensor element 101. That is, the inner gas inlet H2 is formed on the side of the position where the sensor element 101 is located in the axial direction of the inner protective cover 130 (the length direction of the sensor element 101). Specifically, a plurality of inner gas inlet H2 are formed on the circumference of the small diameter portion 131b of the first component 131 at a position closer to the rear end side than the second component 132.

[0138] The inner gas outlet H3 is a through hole formed at the bottom of the inner protective cover 130. The inner gas outlet H3 is formed at the bottom of the bottom 132b of the second component 132. The inner gas outlet H3 can be arranged to be closer to the front end side than the front end surface 101a of the sensor element 101. For example, the inner gas outlet H3 can also be arranged on the side portion of the tapered shape in the bottom 132b. In addition, the bottom 132b is not limited to a tapered shape such as a cone or a truncated cone, and can be a flat shape or the like.

[0139] The outer protective cover 140 is a bottomed cylindrical metal component that is disposed outside the inner protective cover 130 and accommodates the inner protective cover 130. The outer protective cover 140 is configured so that the axial direction of the outer protective cover 140 is consistent or substantially consistent with the axial direction of the inner protective cover 130. The outer protective cover 140 has a bottom at a position closer to the front end side than the bottom 132b of the inner protective cover 130 (second component 132). There is a space between the inner protective cover 130 and the outer protective cover 140 so that the measured gas can flow. The outer protective cover 140 includes: a cylindrical large diameter portion 140a, which is fitted into the outer periphery of the large diameter portion 131a of the first component 131 of the inner protective cover 130 and the outer periphery of the housing 151; a cylindrical main body 140b, which extends from the large diameter portion 140a to the front end side; a front end portion 140c, which is formed on the front end side of the main body 140b in a bottomed cylindrical shape having a smaller diameter than the main body 140b and a bottom at the front end side; and a step portion 140d, which connects the main body 140b and the front end portion 140c. The front end portion 140c is fitted into the cylindrical portion 132a of the second component 132 of the inner protective cover 130. The space between the main body 140b of the outer protective cover 140 and the inner protective cover 130 and the space between the front end portion 140c of the outer protective cover 140 and the inner protective cover 130 are divided by the fitting portion.

[0140] The outer protective cover 140 has an outer gas inlet H1 and an outer gas outlet H4. The outer gas inlet H1 is a through hole formed at a position different from the inner gas inlet H2 and the inner gas outlet H3 of the inner protective cover 130 in the longitudinal direction of the sensor element 101 (the axial direction of the outer protective cover 140). A plurality of outer gas inlets H1 are formed on the circumference of the main body 140b of the outer protective cover 140 at a position closer to the front end side than the gas diffusion sheet 132c of the second component 132 of the inner protective cover 130. The outer gas outlet H4 is a through hole formed at a position different from the inner gas inlet H2 and the inner gas outlet H3 of the inner protective cover 130 and the outer gas inlet H1 of the outer protective cover 140 in the longitudinal direction of the sensor element 101 (the axial direction of the outer protective cover 140). A plurality of outer gas outlets H4 are formed on the circumference of the side surface of the front end portion 140c of the outer protective cover 140 at a position closer to the front end side than the inner gas outlet H3 of the inner protective cover 130. Alternatively, the outer gas outlets H4 may be formed on the bottom surface of the front end portion 140c of the outer protective cover 140. In this case, when viewed from a plane including the bottom surface of the outer protective cover 140, the outer gas outlets H4 may be arranged at a position different from the inner gas outlet H3 of the inner protective cover 130.

[0141] Figure 3 1 is a schematic diagram showing the flow of the gas to be measured in the element protection cover 120 when the gas sensor 100 is installed in a pipe such as an exhaust pipe of an automobile to measure the gas to be measured such as exhaust gas. Figure 3 , the flow of the measured gas is schematically indicated by arrows in the example of flowing from the left side to the right side in the figure. The measured gas flows into the outer protective cover 140 from the outer gas inlet H1 of the outer protective cover 140. The measured gas flowing into the outer protective cover 140 is diffused by the gas diffusion sheet 132c of the second component 132 of the inner protective cover 130 and flows backward. Then, it flows into the inner protective cover 130 from the inner gas inlet H2 of the inner protective cover 130. The measured gas flowing into the inner protective cover 130 flows toward the front end side along the sensor element 101 and reaches the vicinity of the front end face 101a of the sensor element 101. The measured gas is introduced into the interior of the sensor element 101 from the gas inlet 10 of the front end face 101a, thereby measuring the measurement target gas in the measured gas. The measured gas passes through the inner gas outlet H3 of the inner protective cover 130 and the outer gas outlet H4 of the outer protective cover 140 from the vicinity of the front end surface 101a of the sensor element 101 and is discharged to the outside of the element protective cover 120. In addition, according to the flow of the measured gas in the piping, the measured gas also flows out from the outer gas outlet H4 on the upstream side (for example, Figure 3The measured gas discharged from the inner gas outlet H3 of the inner protective cover 130 flows into the outer gas outlet H4 on the left side of the inner protective cover 130. The measured gas discharged from the inner gas outlet H3 of the inner protective cover 130 flows into the outer gas outlet H4 on the downstream side (for example, Figure 3 In addition, the outer gas inlet H1, the inner gas inlet H2, the inner gas outlet H3, and the outer gas outlet H4 are formed in multiple numbers, so in fact, the measured gas also flows in and / or flows out from the holes other than the holes indicated by the arrows roughly according to the above-mentioned gas flow.

[0142] In the gas sensor of the present invention, the protective cover (the inner protective cover 130 in the present embodiment) has a gas inlet (the inner gas inlet H2 in the present embodiment) that penetrates the side of the protective cover surrounding the sensor element 101, and the gas inlet is configured in such a way that the parameters described later (length ratio La / Lb and oxidation ability α of the gas in the sensor element 101) are within a specified range.

[0143] Reference Figures 4 to 8 , explained in detail. Figure 4 and Figure 5 It is a schematic diagram for explaining the length La and the length index Lb. Figure 4 is with Figure 1 The same partial cross-sectional view. Figure 5 It is along Figure 4 This is an enlarged cross-sectional diagram of the V-V line. Figure 5 In the figure, the outer protection cover 140 is omitted. Figure 6 to Figure 8 It is a schematic diagram used to illustrate the surface area S and volume V. Figure 6 is with Figure 1 The same partial cross-sectional view. Figure 7 It is along Figure 6 The VII-VII cross section is a cross section perpendicular to the longitudinal direction of the sensor element 101 at a position of the inner protective cover 130 (second member 132 ) behind the front end of the sensor element 101 . Figure 8 1 is a perspective view of the sensor element 101. The configuration of the electrodes and the like in the sensor element 101 is omitted from the illustration.

[0144] like Figure 4As shown in FIG. 1 , the length of the sensor element 101 in the longitudinal direction from the one end (front end) of the sensor element 101 to the midpoint of the gas inlet (inner gas inlet H2) of the protective cover (inner protective cover 130) in the longitudinal direction of the sensor element 101 is represented as length La. The gas to be measured that flows into the inner protective cover 130 from the inner gas inlet H2 flows toward the front end side along the sensor element 101 as described above, and reaches the vicinity of the front end face 101a of the sensor element 101. The length La represents the flow path length of the gas to be measured that flows along the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the front end face 101a of the sensor element 101.

[0145] Figure 5 The cross section shown is a cross section perpendicular to the longitudinal direction of the sensor element 101 at the center position of the inner gas inlet H2 of the inner protective cover 130 (first component 131) in the longitudinal direction of the sensor element 101. In this embodiment, the inner gas inlet H2 of the inner protective cover 130 is as shown in FIG. Figure 5 As shown, six inner gas inlets H2 are evenly arranged on the circumference, but the number of the inner gas inlet H2 and the positional relationship with the sensor element 101 are not limited to this. Figure 5 In the cross section, the inner surface of the inner protective cover 130 is circular. The circular shape includes a circle, an ellipse, a circle with at least a portion deformed, etc. In addition, at each position in the axial direction of the inner protective cover 130, each cross section can be the same as Figure 5 The inner protective cover 130 may be of the same shape as the cross section of the sensor element 101, or may be of different shapes. The inner protective cover 130 may be roughly cylindrical as in the present embodiment within the range of the sensor element 101 in the axial direction. Alternatively, it may be of a conical shape or an inverted conical shape, or may have a portion with a locally different inner diameter. Alternatively, there may be a polygonal portion such as a triangle or a rectangle.

[0146] In a plane of the protective cover (inner protective cover 130) that includes the midpoint of the gas inlet (inner gas inlet H2) in the longitudinal direction of the sensor element 101 and is orthogonal to the longitudinal direction of the sensor element 101, that is, Figure 5 In the plane of the paper, the index of the length between the sensor element 101 and the circle defined by the inner surface of the protective cover (inner protective cover 130) is described. First, the longest distance between each of the two main surfaces of the sensor element 101 and the circumference of the circle defined by the inner surface of the inner protective cover 130 (first component 131) facing the respective main surfaces is expressed as length x1 and length x2, respectively. In addition, the longest distance between each of the two side surfaces of the sensor element 101 and the circumference of the circle facing the respective side surfaces is expressed as length y1 and length y2, respectively.

[0147] That is, the length x1 represents the circle defined by the upper surface 101c and the inner surface of the inner protective cover 130 ( Figure 5 The circumference (circle indicated by a dotted line in FIG. 100 ) opposite to the upper surface 101c Figure 5 Here, the longest distance refers to the longest distance among the lengths (distances) of the perpendicular lines extending from each position of the upper surface 101c to the circumference (the same applies below). The length x2 represents the circle defined by the lower surface 101d and the inner surface of the inner protective cover 130 ( Figure 5 The circumference (circle indicated by a dotted line in FIG. 100 ) opposite to the lower surface 101d Figure 5 The length y1 represents the longest distance between the left surface 101e and the inner surface of the inner protective cover 130 ( Figure 5 The circumference (circle indicated by a dotted line in FIG. 100 ) opposite to the left surface 101e Figure 5 The length y2 represents the longest distance between the right surface 101f and the inner surface of the inner protective cover 130 ( Figure 5 The circumference (circle indicated by a dotted line in FIG. 100 ) opposite to the right surface 101f Figure 5 The longest distance between the right side.

[0148] Using the above-mentioned lengths x1, x2, y1, and y2, the index of the length between the sensor element 101 and the circle defined by the inner surface of the inner protective cover 130 is expressed as the length index Lb=(x1+x2+y1+y2) / 4. The length index Lb is an index of the length of the gap between the sensor element 101 and the inner protective cover 130 (the length in the width direction of the sensor element 101) in the flow path of the measured gas flowing along the length direction of the sensor element 101 from the inner gas inlet H2 to the front end surface 101a of the sensor element 101. It is considered to be an index roughly indicating the size of the flow path cross section. The sensor element 101 is a long strip plate that is roughly rectangular, and the inner protective cover 130 is roughly cylindrical in the region of the length La in the length direction of the sensor element 101. Therefore, in this embodiment, it is considered that the size of the flow path cross section is roughly the same in the entire region of the length La.

[0149] In this embodiment, if Figure 5 As shown, the circle ( Figure 5The sensor element 101 is arranged at the center of the circle indicated by the dotted line in FIG. 1 . In this case, the length x1 is equal to the length x2, and the length y1 is equal to the length y2. If it is expressed as x1=x2=x and y1=y2=y, the length index Lb is expressed as length index Lb=(x+y) / 2.

[0150] The ratio of the length La of the sensor element 101 in the longitudinal direction to the length index Lb in the width direction of the sensor element 101 is expressed as the length ratio La / Lb. The length ratio La / Lb is an index indicating the degree to which the measured gas flowing into the inner protective cover 130 contacts the surface of the sensor element 101 or passes through the vicinity of the surface until it reaches the front end surface 101a of the sensor element 101. It is considered that the larger the length ratio La / Lb, the easier it is for the measured gas flowing into the inner protective cover 130 to further contact at least one of the two main surfaces (upper surface 101c, lower surface 101d) and two side surfaces (left surface 101e and right surface 101f) of the sensor element 101.

[0151] The total of the surface areas of the two main surfaces (upper surface 101c and lower surface 101d) and two side surfaces (left surface 101e and right surface 101f) of the sensor element 101 over the length La in the longitudinal direction of the sensor element 101 is represented as the surface area S. The surface area S is the area of ​​the sensor element 101 that exists in the sensor element 101. Figure 6 The surface area of ​​the shaded area in Figure 8 , if the surface areas S of the upper surface 101c, the lower surface 101d, the left surface 101e, and the right surface 101f, which are measured by the length La from the front end surface 101a, are expressed as Sc, Sd, Se, and Sf, respectively, then the surface area S is S=Sc+Sd+Se+Sf. The surface area S is the surface area of ​​the sensor element 101 that can be contacted by the gas to be measured in the flow path of the gas to be measured flowing along the length direction of the sensor element 101 from the inner gas inlet H2 to the front end surface 101a of the sensor element 101.

[0152] like Figure 6 and Figure 7 As shown, the space ( Figure 6 and Figure 7The volume of the shaded space in the figure (the space where the measured gas exists) is represented as volume V. Volume V represents the total volume of the flow path of the measured gas flowing along the length direction of the sensor element 101 from the inner gas inlet H2 to the front end surface 101a of the sensor element 101. In this embodiment, volume V is roughly the value obtained by subtracting the volume of the portion of the length La in the plate-shaped sensor element 101 from the volume of the cylindrical shape of the inner protective cover 130 with the circle defined by the inner surface of the side as the cross section.

[0153] The ratio of the surface area S to the volume V is expressed as the ratio S / V. The ratio S / V indicates the ratio of the surface area of ​​the sensor element 101 that can be contacted by the gas to be measured existing inside the inner protective cover 130 in the flow path of the gas to be measured flowing along the longitudinal direction of the sensor element 101 from the inner gas inlet H2 to the front end surface 101a of the sensor element 101, to the volume of the gas to be measured. As described above, it is considered that the gas to be measured becomes high temperature near the surface of the high-temperature sensor element 101, and hydrogen and oxygen in the gas to be measured react. Therefore, it is considered that the larger the surface area of ​​the sensor element 101 relative to the volume of the gas to be measured, that is, the larger the ratio S / V, the more hydrogen and oxygen in the gas to be measured can react on the surface of the sensor element 101. That is, it is considered that the larger the ratio S / V, the higher the oxidizing power of the gas at the surface of the sensor element 101. This ratio S / V is also called the oxidizing power α of the gas in the sensor element 101. α=S / V.

[0154] As described above, the sensor element 101 becomes high temperature, and when the gas to be measured in which oxygen and hydrogen coexist contacts the surface of the sensor element 101 or passes near the surface, the gas to be measured is heated to a high temperature, and a reaction of hydrogen and oxygen (hydrogen oxidation reaction) occurs rapidly. It is considered that when the gas to be measured is heated to a high temperature near the two main surfaces (upper surface 101c, lower surface 101d) and two side surfaces (left surface 101e and right surface 101f) of the sensor element 101, a reaction of hydrogen and oxygen occurs near the two main surfaces and two side surfaces of the sensor element 101, and hydrogen in the gas to be measured is consumed, and the amount of unreacted hydrogen decreases. And, the gas to be measured after the amount of unreacted hydrogen decreases reaches the gas inlet 10 opened at the front end surface 101a of the sensor element 101.

[0155] In the gas sensor of the present invention, the length ratio La / Lb of the length La to the length index Lb is 0.9 or more, and the ratio S / V of the surface area S to the volume V is 0.15 mm -1Above. If it is such a range, even if the gas to be measured in which oxygen and hydrogen coexist flows into the inner protective cover 130, the surface area of ​​the sensor element 101 for oxidizing the hydrogen in the gas to be measured flowing into the inner protective cover 130 can be ensured, so that the hydrogen can be effectively oxidized on the two main surfaces and two side surfaces of the sensor element 101. It is believed that hydrogen can be evenly oxidized on the part of the length La from the front end surface 101a of the two main surfaces and two side surfaces of the sensor element 101, so that the local oxidation reaction of hydrogen can be suppressed. If the local oxidation reaction of hydrogen is suppressed, the generation of large thermal stress locally can be suppressed. Therefore, it is believed that even if the gas to be measured in which oxygen and hydrogen coexist is measured, cracks will not be generated in the internal structure of the sensor element 101, and the NOx concentration can be measured with good accuracy. In addition, there are no openings on the two main surfaces and two side surfaces of the sensor element 101, and it is believed that the structural strength is relatively high compared with the vicinity of the gas inlet 10 of the sensor element 101. The hydrogen in the measured gas is effectively oxidized on the two main surfaces and two side surfaces of the sensor element 101, thereby making it possible to sufficiently reduce the hydrogen concentration in the measured gas reaching the gas inlet 10 at the front end surface 101a of the sensor element 101. As a result, the oxidation reaction of hydrogen near the gas inlet 10 can be further suppressed, and the thermal stress caused by the reaction heat can be further reduced. Therefore, it is considered that the generation of cracks in the sensor element 101 can be further reduced.

[0156] As described above, the length ratio La / Lb is greater than 0.9. Alternatively, the length ratio La / Lb may be, for example, greater than 1.0, greater than 1.2, greater than 1.5, greater than 2.0, etc. The upper limit is not particularly limited, and may be appropriately set in consideration of the use environment of the gas sensor 100, the purpose of use, the responsiveness described later, and other performances required of the gas sensor 100. For example, it may be less than 7, less than 5, etc.

[0157] As described above, the oxidizing power α (= ratio S / V) of the gas in the sensor element 101 is 0.15 mm -1 Alternatively, the oxidation capacity α (= ratio S / V) may be, for example, 0.18 mm -1 Above, 0.2mm -1 Above, 0.25mm -1 The upper limit is not particularly limited, and may be appropriately set in consideration of the use environment of the gas sensor 100, the purpose of use, the performance required of the gas sensor 100, such as the responsiveness described later, etc. For example, it may be 0.5 mm -1 Below, 0.4mm -1 The following etc.

[0158] Even if the length ratio La / Lb is 0.9 or more, when the oxidation capacity α (= ratio S / V) is extremely small, the surface area of ​​the sensor element 101 is also extremely small relative to the volume of the measured gas, so the hydrogen in the measured gas may not be fully oxidized. -1 As described above, when the length ratio La / Lb is extremely small, the flow path of the measured gas flowing along the length direction of the sensor element 101 from the inner gas inlet H2 to the front end surface 101a of the sensor element 101 is too short, and the measured gas cannot fully contact the surface of the sensor element 101 or pass through the vicinity of the surface, and there is a possibility that the hydrogen in the measured gas cannot be fully oxidized. Therefore, it is preferred that the length ratio La / Lb is 0.9 or more and the oxidation capacity α (= ratio S / V) is 0.15 mm -1 above.

[0159] For example, when the concentration of the measurement target gas in the measured gas changes all the time, as in the case where the gas sensor 100 is used to measure the exhaust gas of a hydrogen-powered vehicle, the concentration output value of the gas sensor 100 is required to have good responsiveness to the change in the NOx concentration in the measured gas. For example, it can be said that the shorter the time required from when the NOx concentration in the measured gas flowing outside the gas sensor 100 changes to when the pump current Ip2 flowing through the measurement pump unit 41 reaches the current value corresponding to the changed NOx concentration, the better the responsiveness. If the measured gas with a changed NOx concentration flows into the element protection cover 120, and then reaches the gas inlet 10 of the sensor element 101, is introduced into the interior of the sensor element 101 and reaches the measurement electrode 44, the pump current Ip2 reaches the current value corresponding to the changed NOx concentration. It is considered that the faster the measured gas in the element protection cover 120 is replaced, the better the responsiveness is obtained.

[0160] As described above, the length La represents the length of the flow path of the measured gas flowing along the length direction of the sensor element 101 from the inner gas inlet H2 to the front end surface 101a of the sensor element 101. There is a tendency that if the flow path length is shorter, the speed of replacing the measured gas in the inner protective cover 130 becomes faster. Although it varies depending on the responsiveness required by the gas sensor 100 and the structure of the gas sensor, the length La can be, for example, less than 10 mm. It is believed that if it is within such a range, good responsiveness can be maintained. Alternatively, it can be, for example, less than 9 mm, less than 7 mm, less than 4 mm, etc. Regarding responsiveness, the lower limit value of the length La is not particularly limited, and the length La can be determined by considering the above-mentioned length ratio La / Lb and oxidation capacity α (= ratio S / V).

[0161] As an example of an embodiment of the present invention, the gas sensor 100 for detecting the NOx concentration in the measured gas is shown above, but the present invention is not limited to this form. The present invention may include various forms of gas sensors as long as the purpose of the present invention is achieved, and the purpose is to suppress the occurrence of cracks in the sensor element and accurately measure the measurement target gas in the measured gas even when a gas that can generate an exothermic reaction such as a gas that coexists with oxygen and hydrogen is introduced into the gas sensor as the measured gas.

[0162] In the above-mentioned embodiment, the gas sensor 100 detects the NOx concentration in the measured gas, but the measured gas is not limited to NOx. The measured gas can be, for example, other oxide gases (such as carbon dioxide CO2, water H2O, etc.) other than oxygen O2 or NOx. Alternatively, it can be a non-oxide gas such as ammonia NH3. In the case where the measured gas is a non-oxide gas, the non-oxide gas is converted into an oxide gas (for example, in the case of ammonia NH3, it is converted into NO), and the measured gas containing the converted oxide gas is introduced into the third internal cavity 61. At the measuring electrode 44, the converted oxide gas in the measured gas is reduced to generate oxygen. The generated oxygen can be obtained in the form of a pump current Ip2 in the measuring pump unit 41, and the measured gas can be detected. By at least one of the inner main pump electrode 22 and the auxiliary pump electrode 51 acting as a catalyst, the non-oxide gas can be converted into an oxide gas.

[0163] In addition, it can be a gas sensor that can measure a variety of measurement target gases. For example, it can be a gas sensor that detects the concentration of nitrogen oxides NOx when the measured gas is a fuel-lean atmosphere, and detects the concentration of ammonia NH3 when the measured gas is a fuel-rich atmosphere. For example, it can be determined whether the measured gas is a fuel-lean atmosphere or a fuel-rich atmosphere based on the electromotive force (voltage Vref) generated in the sensor unit 83, and when the measured gas is a fuel-lean atmosphere, as described above, the NOx concentration is detected based on the pump current flowing through the measurement pump unit 41. In addition, when the measured gas is a fuel-rich atmosphere, ammonia NH3 can be converted into NO at at least one of the inner main pump electrode 22 and the auxiliary pump electrode 51, and the NH3 concentration can be detected based on the pump current flowing through the measurement pump unit 41. This gas sensor is effective when used downstream of the so-called three-way catalyst (TWC).

[0164] In the above-mentioned embodiment, the element protection cover 120 adopts a cover of a double structure including an inner protection cover 130 and an outer protection cover 140, but it is not limited thereto. The inner protection cover 130 disposed at a position closest to the sensor element 101 may be configured to satisfy the length ratio La / Lb and the oxidation capacity α (= ratio S / V) of the present invention. The position, shape, size, and number of the inner gas inlet H2 and the inner gas outlet H3 of the inner protection cover 130 may be appropriately determined. For example, the shape of the inner gas inlet H2 may be circular, elliptical, rectangular, or other various shapes. For example, a rectifying sheet may be formed near the through hole of the inner gas inlet H2. In addition, the presence or absence of the diffusion sheet 132c of the inner protection cover 130 and the shape when formed may also be appropriately determined. The position, shape, size, and number of the outer gas inlet and the outer gas outlet of the outer protection cover 140 may also be appropriately determined. Other shapes, plate thicknesses, etc. of the inner protection cover 130 and the outer protection cover 140 may also be appropriately determined. In addition, the element protection cover 120 may be a single-layer structure (only the protection cover in the present invention), a triple structure (for example, in addition to the protection cover and the outer cover in the present invention, it also has an intermediate cover) or a multiple structure of three or more layers. It can be appropriately determined in consideration of the use environment of the gas sensor (flow rate, temperature, possibility of splashing of condensed water, etc. of the measured gas), the responsiveness and other performance required of the gas sensor, etc.

[0165] In the above-mentioned embodiment, the sensor element 101 is obtained by stacking oxygen ion conductive solid electrolyte layers, and the portion not constituting each pump unit and each sensor unit may not be a solid electrolyte layer but may be other ceramics such as alumina.

[0166] In the above-mentioned embodiment, the sensor element 101 has an opening (gas inlet 10) on the front end face 101a and the measured gas is introduced into the interior of the sensor element 101 from the opening of the front end face 101a, but it is not limited to this. For example, the measured gas flow cavity 15 can be a structure in which the gas is introduced from the vicinity of the front end of the side surface along the length direction of the sensor element 101 (base part 102). For example, it can be a form in which there is an opening on the side surface along the length direction of the base part 102 that is connected to the buffer space 12 or a position near the buffer space 12 of the first internal cavity 20. In this case, the measured gas is introduced from the side surface along the length direction of the base part 102 through the opening.

[0167] It is considered that even when there is an opening near the front end of the side surface of the sensor element, if the positional relationship between the sensor element and the inner protective cover 130 is configured to satisfy the length ratio La / Lb and the ratio S / V of the present invention, hydrogen can be evenly oxidized in the portion of the length La from the front end surface 101a of the two main surfaces and two side surfaces of the sensor element 101, similarly to the above-mentioned embodiment, so that local oxidation reaction of hydrogen can be suppressed. Therefore, it is considered that the occurrence of cracks in the sensor element 101 can be reduced, similarly to the above-mentioned embodiment.

[0168] In the gas sensor 100 of the above-mentioned embodiment, Figure 2 As shown in FIG. 1 , the structure of the sensor element 101 is: it has three internal cavities, namely, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61, and the inner main pump electrode 22, the auxiliary pump electrode 51, and the measuring electrode 44 are respectively arranged in each internal cavity, but it is not limited to this. For example, it may be a structure in which it has two internal cavities, namely, the first internal cavity 20 and the second internal cavity 40, the inner main pump electrode 22 is arranged in the first internal cavity 20, and the auxiliary pump electrode 51 and the measuring electrode 44 are respectively arranged in the second internal cavity 40. In this case, for example, as a diffusion rate limiting portion between the auxiliary pump electrode 51 and the measuring electrode 44, a porous body protective layer covering the measuring electrode 44 may be formed.

[0169] In addition, in addition to the above-mentioned internal cavity structure, various configurations may be adopted for various components of the sensor element 101 such as the measured gas flow cavity 15 and the electrodes depending on the type of the measured gas, the purpose of use of the gas sensor, the use environment, etc.

[0170] The gas sensor 100 of the above-mentioned embodiment is a so-called limiting current type gas sensor that detects the concentration of the gas to be measured based on the pump current Ip2 flowing through the measurement pump unit 41, but the gas sensor is not limited thereto. It may also be a so-called mixed potential type gas sensor that detects the concentration of the gas to be measured based on the potential difference between an electrode in contact with the gas to be measured and a reference electrode in contact with a reference gas.

[0171] Example

[0172] Hereinafter, an example in which a gas sensor is specifically manufactured and tested will be described as an embodiment. In addition, the present invention is not limited to the following embodiments.

[0173] [Examples 1 to 5 and Comparative Examples 1 to 3]

[0174] about Figure 1 and Figure 2The gas sensor 100 shown in FIG. 1 is manufactured by taking into account the length La [mm], the length ratio La / Lb, and the oxidation capacity of the gas in the sensor element 101 α=S / V [mm -1 ] are used as embodiments 1 to 5 and comparative examples 1 to 3.

[0175] The values ​​of the parameters in Examples 1 to 5 and Comparative Examples 1 to 3 are as follows.

[0176] In Example 1, La = 7.1 mm, La / Lb = 3.3, α = 0.3 mm -1 .

[0177] In Example 2, La = 4 mm, La / Lb = 3.3, α = 0.3 mm -1 .

[0178] In Example 3, La = 10 mm, La / Lb = 2.3, α = 0.2 mm -1 .

[0179] In Example 4, La = 10 mm, La / Lb = 0.9, α = 0.15 mm -1 .

[0180] In Example 5, La = 14 mm, La / Lb = 3.3, α = 0.3 mm -1 .

[0181] In Comparative Example 1, La = 0.6 mm, La / Lb = 0.2, α = 0.1 mm -1 .

[0182] In Comparative Example 2, La = 10 mm, La / Lb = 0.6, α = 0.15 mm -1 .

[0183] In Comparative Example 3, La = 10 mm, La / Lb = 0.9, α = 0.1 mm -1 .

[0184] In the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3, the position of the inner gas inlet H2 of the inner protective cover 130 in the sensor element length direction and the inner diameter of the inner protective cover 130 are changed, and the gas sensors are manufactured according to the desired length ratio La / Lb, the oxidation ability of the gas in the sensor element 101 α=S / V, and the length La. In the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3, the dimensions other than the position of the inner gas inlet H2 in the sensor element length direction and the inner diameter of the inner protective cover 130 are the same. In addition, in the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3, the sensor element 101 and the outer protective cover 140 are the same.

[0185] [Evaluation of crack risk]

[0186] For the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3, the difficulty (crack risk) of cracks in the sensor element 101 was evaluated. Specifically, first, the gas sensor 100 was installed in the test piping, and the gas sensor 100 was driven. That is, the heater 72 was energized to heat the sensor element 101, and each pump unit 21, 50, 41 and each sensor unit 80, 81, 82, 83 were operated as described above. The driving temperature of the sensor element 101 was set to 840°C. In this state, a model gas containing oxygen and hydrogen was circulated in the test piping, and the gas sensor 100 in the driven state was exposed to the model gas for 100 seconds. At this time, whether cracks were generated in the sensor element 101 was confirmed. The composition of the model gas is as follows: oxygen O2 = 5%, hydrogen H2 = 1.8%, and the remainder is nitrogen N2. In addition, the units of gas concentration are all based on volume (the same below).

[0187] Twelve gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3 were prepared, and the crack occurrence probability of the sensor element was calculated based on the number of cracks generated in the 12 sensors (= number of cracks generated / number of test sensors × 100) [%]. The crack occurrence probability was determined based on the following criteria.

[0188] A+: The probability of crack generation is less than 5%

[0189] A: The probability of crack generation is greater than 5% and less than 15%.

[0190] A-: The probability of crack generation is greater than 15% and less than 30%

[0191] B: The probability of crack generation is greater than 30% and less than 50%

[0192] [Evaluation of responsiveness]

[0193] The responsiveness of the detection of NOx concentration was evaluated for the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3. Specifically, first, the gas sensor 100 was installed in the test piping, and the gas sensor 100 was driven. That is, the heater 72 was energized to heat the sensor element 101, so that each pump unit 21, 50, 41 and each sensor unit 80, 81, 82, 83 operated as described above. The driving temperature of the sensor element 101 was set to 840°C. When the gas sensor 100 was driven, as described above, the pump current Ip2 flowing through the measurement pump unit 41 flowed in accordance with the NOx concentration in the measured gas.

[0194] In this state, the first model gas with a NOx content of 0 ppm (the remainder is nitrogen N2) is made to flow through the test pipe as the measured gas, and the pump current Ip2 is waited until it stabilizes. After the pump current Ip2 stabilizes, the measured gas flowing through the test pipe is switched to the second model gas with a NOx content of 500 ppm (the remainder is nitrogen N2). Then, the temporal change of the pump current Ip2 when the NOx concentration in the measured gas is changed from 0 ppm to 500 ppm is investigated. The pump current Ip2 immediately before the NOx concentration in the measured gas is changed (immediately before the switch from the first model gas to the second model gas) is set to 0%, the pump current Ip2 when the NOx concentration in the measured gas is changed and the pump current Ip2 is stabilized after the change is set to 100%, and the time from when the value of the pump current Ip2 exceeds 10% to when it exceeds 90% is set to the response time [msec] of the NOx concentration detection. The shorter the response time, the higher the responsiveness of the NOx concentration detection in the sensor element 101.

[0195] The above-mentioned response time was calculated for each of the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3. The response time was determined based on the following criteria.

[0196] A: The response time is less than 1200msec

[0197] B: Response time is longer than 1200msec and less than 2000msec

[0198] C: Response time is longer than 2000msec

[0199] Table 1 shows the parameters and evaluation results of the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3. Table 1 shows the values ​​of the parameters (length La, length ratio La / Lb, and oxidation ability of the gas in the sensor element 101 α=S / V), crack risk evaluation results, responsiveness evaluation results, and comprehensive evaluation in each gas sensor. The comprehensive evaluation is obtained by considering the evaluation results of responsiveness based on the evaluation results of crack risk.

[0200] Table 1

[0201]

[0202] The gas sensors of Examples 1 to 5 were all confirmed to have a lower crack risk than the gas sensors of Comparative Examples 1 to 3. The length ratio La / Lb and the oxidizing power of the gas in the sensor element 101 α=S / V were all confirmed to further reduce the crack risk as their values ​​increased.

[0203] In the gas sensors of Examples 1 to 5 and Comparative Examples 1 to 3, the following trend was confirmed: the shorter the length La, the better the responsiveness. In the gas sensor of Example 5, the length La is the longest, and the responsiveness is slightly slower than that of the gas sensors of Examples 1 to 4 and Comparative Examples 1 to 3. Regarding Example 5, it is considered that due to the longer length La, the time required to replace the gas in the element protection cover 120 is slightly longer than that of other gas sensors. In the case where better responsiveness is required according to the purpose of use and the use environment of the gas sensor 100, the length La can be set to, for example, 10 mm or less.

[0204] Based on the evaluation results of crack risk, a comprehensive evaluation is performed taking into account the evaluation results of responsiveness. All gas sensors can be used in practice, but when oxygen and hydrogen can coexist in the measured gas, a gas sensor with a comprehensive evaluation of B or above can be used. In addition, if a gas sensor with a comprehensive evaluation of A- or above is used, it is more preferred because it can achieve both a reduced crack risk and good responsiveness.

[0205] As described above, according to the present invention, even when a gas in which gas species that can produce an exothermic reaction coexist, such as a gas in which oxygen and hydrogen coexist, is introduced into a gas sensor as a measured gas, the generation of cracks in the sensor element can be suppressed, thereby enabling the measurement target gas in the measured gas to be measured with good accuracy.

Claims

1. A gas sensor for detecting a gas to be measured in a gas to be measured, comprising: A sensor element, the sensor element being a long plate-shaped sensor element including an oxygen ion conductive solid electrolyte layer, wherein a measured gas flow cavity is formed at one end side in the longitudinal direction of the sensor element; as well as A bottomed cylindrical protective cover, the protective cover accommodating a portion of the sensor element including the one end, in, The protective cover has a gas inlet that penetrates a side surface of the protective cover surrounding the sensor element. The length of the sensor element in the longitudinal direction from the one end of the sensor element to the midpoint of the gas inlet of the protective cover in the longitudinal direction is represented as length La, In a plane of the protective cover that includes the midpoint of the gas inlet in the longitudinal direction and is orthogonal to the longitudinal direction of the sensor element, the longest distances between each of the two main surfaces of the sensor element and the circumference of a circle defined by the inner surface of the protective cover and opposite to the main surfaces are set to length x1 and length x2, respectively, and the longest distances between each of the two side surfaces of the sensor element and the circumference of the circle opposite to the side surfaces are set to length y1 and length y2, respectively, and the index of the length between the sensor element and the circle defined by the inner surface of the protective cover is expressed as length index Lb=(x1+x2+y1+y2) / 4, The ratio La / Lb of the length La to the length index Lb is greater than 0.9, Furthermore, when the total of the surface areas of the portions of the length La in the longitudinal direction of the two main surfaces and the two side surfaces of the sensor element is represented as a surface area S, and the volume of the space where the portion of the length La in the longitudinal direction in the protective cover exists is represented as a volume V, the ratio S / V of the surface area S to the volume V is 0.15 mm -1 above.

2. The gas sensor according to claim 1, wherein: The length La in the longitudinal direction is 10 mm or less.

3. The gas sensor according to claim 1, wherein: The length x1 and the length x2 satisfy x1=x2=x, the length y1 and the length y2 satisfy y1=y2=y, and the length index Lb is represented by Lb=(x+y) / 2.

4. The gas sensor according to claim 1, wherein: The protective cover has a gas outlet at the bottom.

5. The gas sensor according to claim 1, wherein: An outer cover for housing the protective cover is also provided on the outer side of the protective cover. The outer cover has an outer gas inlet at a position different from the gas inlet of the protection cover in the longitudinal direction of the sensor element.

6. The gas sensor according to claim 5, wherein: The outer cover has an outer gas outflow port at a position different from the gas inflow port of the protection cover and the outer gas inflow port of the outer cover in the longitudinal direction of the sensor element.

7. The gas sensor according to claim 1, wherein: The measured gas flow cavity of the sensor element has a gas introduction port opened at an end surface of the one end in the longitudinal direction.

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