Gas sensor

Abstract

Gas sensor, showing: a plate-like solid electrolyte body (2) which exhibits oxygen ion conductivity; a pump electrode (21) which is placed on a first surface (201) of the solid electrolyte body (2) which is exposed to the gas (G) containing oxygen, wherein the pump electrode (21) is used to regulate an oxygen concentration in the gas (G); a sensor electrode (22) which is placed on the first surface (201) of the solid electrolyte body (2) closer to a base end face than the pump electrode (21), wherein the sensor electrode (22) is used to measure a concentration of a specific gas component of the gas (G) whose oxygen concentration has already been regulated by the pump electrode (21); and a plate-like heating device (3) which is arranged such that it is positioned opposite the solid electrolyte body (2) and which serves to heat the solid electrolyte body (2), characterized by the fact that the gas sensor has a front end face in its longitudinal direction (L) which is exposed to the gas (G), and also has a base end face in the longitudinal direction (L) which is maintained by an insulator (6), the heating device (3) is composed of a heating base (31) and a conductive layer (32) which is placed in the heating base (31), the conductive layer (32) includes a pair of connecting leads (40) arranged on the base end face and a heating element (4), wherein the heating element (4) is arranged closer to the front end face than the connecting leads (40), is connected to the connecting leads (40) and has a cross-sectional area smaller than that of the connecting leads (40), and an entire area (R) of a front end section (11) of the heating base (31), in which the heating element (4) is installed, is divided into three areas: a base end region (R1), an intermediate region (R2) and a front end region (R3) arranged in the preceding longitudinal direction (L), wherein the intermediate region (R2) is located between a front end (222) and a base end (221) of the sensor electrode (22), the front end region (R3) is located closer to the front end than to the intermediate region (R2), and the base end region (R1) is located closer to the base end than to the intermediate region (R2), wherein resistance values ​​per unit area of ​​the heating element (4) in the base end region (R1) and the front end region (R3) are higher than those in the intermediate region (R2).

Description

Technical area

[0001] The present invention generally relates to a gas sensor that measures the concentration of a specific gas component contained in oxygen-containing gas. State of the art

[0002] Gas sensors designed to include a plate-like heater stacked on a plate-like solid electrolyte body are equipped with a plurality of types of cells through which electric current flows via a portion of the solid electrolyte body and a pair of electrodes placed on a portion of the solid electrolyte body. The cells are heated by a heater to a suitable temperature at which the electrodes exhibit catalytic activity.

[0003] For example, a gas sensor as taught in Patent Literature 1 includes electrodes placed on a solid electrolyte body to form a first pump cell and a second pump cell, and also includes a heater stacked on the solid electrolyte body. The first pump cell serves to control the oxygen partial pressure. The second pump cell serves to measure a specific gas component included in the measurement gas. The heater serves to heat the first pump cell and the second pump cell. A resistance value of a resistance portion of the heater disposed opposite to the first pump cell is selected to be higher than that of a resistance portion of the heater disposed opposite to the second pump cell.This lowers the temperature of the second pump cell to eliminate a change in an offset current, which will be detected when the concentration of the particular gas component is zero. State of the art documentPatent literature

[0004] Patent Literature 1: Japanese Patent First Publication No. JP 2009 - 265 085 A Further prior art is disclosed in the following documents.

[0005] DE 195 39 357 B4 discloses an air-fuel ratio detection device. The air-fuel ratio detection device comprises a solid electrolyte layer, a heating element layer including two ceramic insulation layers and an electric heating part arranged between the ceramic insulation layers and opposite the solid electrolyte layer, a layered ceramic insulating spacer arranged between the heating element layer and the solid electrolyte layer to form an internal space between the heating element layer and the solid electrolyte layer, which space communicates with the gas to be measured, and to form a reference oxygen space insulated from the internal space and in which a substance containing reference oxygen is present, a pair of electrodes arranged on the solid electrolyte layer, the electrodes being arranged separately from each other, and an oxygen pumping part.consisting of the solid electrolyte layer and the pair of electrodes, one of the electrodes being exposed to the inner space.,

[0006] US 6,287,439 B1 discloses a gas sensor. The gas sensor reduces the offset value to such an extent that no disturbances occur in the measurement without causing a decrease in NOx, thus improving the measurement accuracy of NOx. In the gas sensor, NOx contained in a measurement gas introduced into a second chamber is decomposed by catalytic action and / or electrolysis into pump oxygen generated by the decomposition, so that NOx is measured based on a pump current flowing through a measurement pump cell. The gas sensor has the following pattern of a heater. A pattern at a portion corresponding to the front end of a sensor element is narrow, a pattern at a middle portion is widened, and there is no pattern at a portion corresponding to the rear end of the sensor element.In this way, it is possible for the measuring pump cell, after conversion into a resistance value, to have an electronic line resistance of not less than 1 MΩ. Summary of the inventionProblem to be solved by the invention

[0007] However, the gas sensor is held by a housing via an insulator, so heat from the gas sensor will be dissipated or transferred to the insulator. Such heat dissipation, however, results in a significant negative impact on the gas sensor, which typically changes with a change in the gas temperature. Therefore, techniques are required to minimize the temperature change of each cell (i.e., each electrode) of the gas sensor when the gas temperature changes.

[0008] The invention was made in view of the above problem. It is an object of the invention to provide a gas sensor capable of maintaining the temperature of a region around a sensor electrode at a desired value when the temperature of the gas changes. Means of solving the problem

[0009] According to one aspect of the invention, there is provided a gas sensor comprising: (a) a plate-like solid electrolyte body (2) exhibiting oxygen ion conductivity; (b) a pump electrode (21) placed on a first surface (201) of the solid electrolyte body exposed to the gas (G) containing oxygen, the pump electrode being used to regulate an oxygen concentration in the gas; (c) a sensor electrode (22) placed on the first surface of the solid electrolyte body closer to a base end side than the pump electrode, the sensor electrode being used to measure a concentration of a specific gas component of the gas whose oxygen concentration has already been regulated by the pump electrode;and (d) a plate-like heating device (3) which is arranged so as to be opposite to the solid electrolyte body and which serves to heat the solid electrolyte body;

[0010] The gas sensor has a front end side in its longitudinal direction (L) which is exposed to the gas, and also has a base end side in the longitudinal direction which is held by an insulator (6).

[0011] The heating device is constructed from a heating base (31) and a conductive layer (32) placed in the heating base.

[0012] The conductive layer includes a pair of lead wires (40) arranged on the base end side, and a heating element (4). The heating element is arranged closer to the front end side than the lead wires, is connected to the lead wires, and has a cross-sectional area smaller than that of the lead wires.

[0013] An entire area (R) of a front end portion (11) of the heater base in which the heating element is installed is divided into three areas: a base end area (R1), an intermediate area (R2), and a front end area (R3), which are arranged in the above longitudinal direction. The intermediate area (R2) is located between a front end (222) and a base end (221) of the sensor electrode. The front end area (R3) is closer to the front end side than the intermediate area. The base end area (R1) is closer to the base end side than the intermediate area. The resistance values ​​per unit area of ​​the heating element in the base end area and the front end area are higher than that in the intermediate area. Favorable effects

[0014] In the above gas sensor, how the heating element of the conductive layer of the heater should be formed is designed.

[0015] Specifically, when the area of ​​the entire front end portion of the heating base of the heating device in which the heating element is disposed is divided into three regions: the intermediate region, the front end region, and the base end region, the resistance value per unit area of ​​the heating element in the base end region becomes higher than that in the intermediate region. This allows a portion of the solid electrolyte body opposite the base end region, which is closest to the insulator, to be heated more than a portion of the solid electrolyte body opposite the intermediate region and a region around the sensor electrode.

[0016] The strong heating of the portion of the solid electrolyte body located close to the insulator minimizes a risk that the sensor electrode is adversely affected by the dissipation or transfer of heat to the base end side where the insulator is placed when the temperature of the gas is low or falling.

[0017] The resistance value per unit area of ​​the heating element located in the front end region is selected to be higher than that of the heating element located in the intermediate region. This allows a portion of the solid electrolyte body opposite the front end region and an area around the pump electrode to be heated more than the portion of the solid electrolyte body opposite the intermediate region and the area around the sensor electrode, making it easier to bring the temperature of an area around the pump electrode to a desired temperature that creates catalytic activity.

[0018] This allows the gas sensor to maintain the temperature of the area around the sensor electrode at a suitable level or value even when the gas temperature changes. This ensures high accuracy when measuring the concentration of the specific gas component using the sensor electrode. Short description of the drawings

[0019] It shows / it shows: Fig. 1 is a sectional view illustrating a gas sensor according to the first embodiment. Fig. 2 is a plan view illustrating the layout of a heating element in a heating base of a gas sensor according to the first embodiment. Fig. 3 is a graph illustrating a relationship between a distance from a front end of a gas sensor and a corresponding temperature of the gas sensor according to the first embodiment. Fig. 4 is a plan view illustrating the layout of a heating element in a heating base of a gas sensor according to the third embodiment. Fig. 5 is a plan view illustrating the layout of a heating element in a heating base of another gas sensor according to the third embodiment. Fig. 6 is a plan view illustrating the layout of a heating element in a heating base of another gas sensor according to the third embodiment. Fig. 7 is a plan view illustrating the layout of a heating element in a heating base of another gas sensor according to the third embodiment. Fig. 8 is a plan view illustrating the layout of a heating element in a heating base of another gas sensor according to the third embodiment. Fig. 9 is a plan view showing the layout of a heating element in a heating base of a comparative example of a gas sensor. Embodiment for carrying out the invention

[0020] Preferred embodiments of the gas sensor described above will be discussed below.

[0021] "A resistance value per unit area," as referred to in this disclosure, represents a value derived by dividing a resistance value of a heating element located in each of a plurality of regions of the gas sensor by an area of ​​a corresponding one of the regions. "The resistance value" represents a value derived by measuring an electric current flowing through the heating element when a specific voltage is applied to the heating element located in each of the regions, dividing the specific voltage by the measured electric current.

[0022] An intermediate region represents a region of the heater base located between a front end of a sensor electrode and a base end of the sensor electrode, which is opposite to the front end. A front end region represents an entire region of the heater base that is closer to the front end side of the heater base than the front end of the sensor electrode. A base end region represents an entire region of a front end portion of the heater base that is closer to the base end side of the heater base than the base end of the sensor electrode. The front end portion has the heating element placed therein. The front end portion of the heater base occupies an entire region of the heater base that is closer to the front end side of the heater base than an end of the heater base to which the lead wires are connected.

[0023] The entire heating element has a constant cross-sectional area defined in a width direction of the heating element. A length per unit area of ​​the heating element located in the base end region and a length per unit area of ​​the heating element located in the front end region can be increased to be greater than a length per unit area of ​​the heating element located in the intermediate region.

[0024] The increased length of the heating element located in the base end region results in an increased resistance value per unit area of ​​the heating element in the base end region, which is higher than the resistance value per unit area of ​​the heating element located in the intermediate region. Similarly, the increased length of the heating element located in the front end region results in an increased resistance value per unit area of ​​the heating element in the front end region, which is higher than the resistance value per unit area of ​​the heating element located in the intermediate region.

[0025] “The length per unit area” represents a value derived by dividing a total length of a portion of the heating element located in each region by an area of ​​that region.

[0026] An average value of a transverse sectional area per unit length of the heating element located in the base end region and an average value of a transverse sectional area per unit length of the heating element located in the front end region may be reduced to be smaller than an average value of a transverse sectional area per unit length of the heating element located in the intermediate region.

[0027] The reduced cross-sectional area of ​​the heating element located in the base end region results in an increased resistance value per unit area of ​​the heating element in the base end region, which is higher than the resistance value per unit area of ​​the heating element located in the intermediate region. Similarly, the reduced cross-sectional area of ​​the heating element located in the front end region results in an increased resistance value per unit area of ​​the heating element in the front end region, which is higher than the resistance value per unit area of ​​the heating element located in the intermediate region.

[0028] "The average cross-sectional area per unit area" represents a value derived by dividing the average value of a transverse cross-sectional area, as defined in the width direction, of the heating element located in each region by the area of ​​that region. "The constant cross-sectional area of ​​the heating element" may contain an error of ±10%. Embodiment

[0029] A gas sensor according to the embodiments will be described below with reference to the drawings. First embodiment

[0030] The gas sensor 1 is, as shown in the Fig. 1 and Fig. 2, is equipped with the solid electrolyte body 2, the pump electrode 21, the sensor electrode 22 and the heating device 3.

[0031] The solid electrolyte body 2 exhibits oxygen ion conductivity and is formed in the shape of a plate. The pump electrode 21 is placed on the first surface 201 of the solid electrolyte body 2, which is exposed to the gas G containing oxygen. The pump electrode 21 is used to regulate the oxygen concentration of the gas G. The sensor electrode 22 is placed on the first surface 201 of the solid electrolyte body 2, which is located closer to a base end side of the solid electrolyte body 2 than the pump electrode 21. The sensor electrode 22 is used to measure the concentration of the specific gas component in the gas G, whose oxygen concentration has already been regulated by the pump electrode 21. The heater 3 is shaped in the shape of a plate and is arranged opposite the solid electrolyte body 2 to heat the solid electrolyte body 2 and the electrodes 21 and 22.

[0032] The gas sensor 1 has a certain length, with two ends opposite to each other in the longitudinal direction thereof. In this disclosure, one of the ends of the gas sensor 1 exposed to the gas G will be referred to as a front end side, while the other end held by the insulator 6 will also be referred to as a base end side. The gas sensor 1 is shaped to be long as described above. The front end side of the gas sensor 1 is also a free end. The base end side of the gas sensor 1, as described above, is opposite to the front end side in the longitudinal direction L and is held by the insulator 6.

[0033] The heating device 3 is, as shown in the Fig. 1 and Fig. 2, is made of the insulating heater bases 31 and the conductive layer 32 placed between the heater bases 31. The conductive layer 32 includes a pair of lead wires 40 located on the base end side thereof, and the heating element 4 located closer to the front end side than the lead wires 40. The heating element 4 connects the lead wires 40 to each other and is smaller in transverse sectional area than the lead wires 40.

[0034] The heating bases 31 of the gas sensor 1 have, as shown in Fig. 2, has a portion in which the heating element 4 is placed near the front end of the gas sensor 1. In the following discussion, such a portion will be referred to as a front end portion 11. An entire area of ​​the front end portion 11 will be referred to as an area R, which, as seen from the drawing, is divided into three areas: an area R1, an area R2, and an area R3, arranged in the longitudinal direction L. The area R2 is located between the front end 222 of the sensor electrode 22 and the base end 221 of the sensor electrode 22 and will also be referred to as an intermediate area R2 hereinafter. The area R3 is located closer to the front end side than the intermediate area R2 and will also be referred to as a front end area R3 hereinafter.The region R1 is located closer to the base end side than the intermediate region R2 and will also be referred to as a base end region R1 hereinafter. The pattern or layout of the heating element 4 is selected such that the resistance values ​​per unit area of ​​the heating element 41 in the base end region R1 and the heating element 43 in the front end region R3 are higher than that of the heating element 42 in the intermediate region R2. In other words, the heating element 4 is made of three sections 41, 42, and 43. Section 41 is located in the region R1. Section 42 is located in the region R2. Section 43 is located in the region R3. The resistance value per unit area is expressed by a value derived by dividing a value of resistance of each of the heating elements 41, 42, and 43 located in the regions R1, R2, and R3 by an area of ​​a corresponding one of the regions R1, R2, and R3.The front end portion 11 of the heating bases 31, in which the heating element 4 is placed, occupies the entire area R, which is closer to the front end side than the ends 401 of the heating element 4, which are connected to the connecting lines 40 in the heating bases 31.

[0035] The gas sensor 1 is described with reference to Fig. 1 to 3 are described in detail.

[0036] In use, the gas sensor 1 is installed in an exhaust pipe of an internal combustion engine. The gas G contains oxygen and is exhaust gas flowing through the exhaust pipe extending from the internal combustion engine. The specific gas component is NOx (nitrogen oxide) contained in the exhaust gas. The gas sensor 1 is held by a housing using the insulator 6. The housing is fixed to the exhaust pipe. The gas sensor 1 has a front end portion extending outside the insulator 6. The front end portion is covered with a protective cover having gas holes through which the gas G passes.

[0037] The solid electrolyte body 2 has, as in Fig. 1, the solid electrolyte body 2 has the first surface 201 and the second surface 202 opposite each other in a thickness direction of the solid electrolyte body 2. The reference electrode 24 is placed on the second surface 202, which is exposed to atmospheric air as a reference gas A. The reference electrode 24 is arranged on a portion of the second surface 202 that coincides with or overlaps the pump electrode 21 and the sensor electrode 22 placed on the first surface 201 of the solid electrolyte body 2 in the thickness direction of the solid electrolyte body 2. The reference electrode 24 may be made of a single electrode whose size completely overlaps with the pump electrode 21 and the sensor electrode 22.Alternatively, the reference electrode 24 may be made from a combination of a plurality of discrete electrodes or two electrodes, one for both the pump electrode 21 and the sensor electrode 22.

[0038] The pump electrode 21, the sensor electrode 22, and the reference electrode 24 are provided on the single solid electrolyte body 2. The plate-like insulator 52 is stacked on the first surface 201 of the solid electrolyte body 2 through the spacer 51. The gas chamber 501, into which the gas G is introduced, is formed by the solid electrolyte body 2, the spacer 51, and the insulator 52 on the first surface 201 of the solid electrolyte body 2. The diffusion resistance layer 511, through which the gas G is introduced into the gas chamber 501 while subjected to a certain diffusion resistance, is placed in a hole formed in the spacer 51. The heater 3 is stacked on the second surface 202 of the solid electrolyte body 2 through the spacer 53.The reference gas chamber 502 into which the reference gas A is introduced is formed by the solid electrolyte body 2, the spacer 53 and the heater 3 on the second surface 202 of the solid electrolyte body 2.

[0039] The pump electrode 21 and the reference electrode 24 are made of a material such as platinum or gold, which is catalytically active toward oxygen. The sensor electrode 22 is made of a material in which rhodium, which is catalytically active toward NOx, is added to platinum.

[0040] In the gas sensor 1, a pump cell is formed by the pump electrode 21, the reference electrode 24 (i.e., a portion of the reference electrode 24 in this embodiment), and a portion of the solid electrolyte body 2 sandwiched between the pump electrode 21 and the reference electrode 24. The pump cell is configured to apply a voltage between the pump electrode 21 and the reference electrode 24 to generate a flow of oxygen ions between the pump electrode 21 and the reference electrode 24, thereby removing oxygen from the gas G.

[0041] In the gas sensor 1, a sensor cell is formed by the sensor electrode 22, the reference electrode 24 (i.e., a portion of the reference electrode 24 in this embodiment), and a portion of the solid electrolyte body 2 sandwiched between the sensor electrode 22 and the reference electrode 24. The sensor cell is configured to measure the oxygen ion current flowing between the sensor electrode 22 and the reference electrode 24 when a voltage is applied between the sensor electrode 22 and the reference electrode 24 to calculate the concentration of NOx contained in the gas G as a function of the level of the oxygen ion current.

[0042] The heating bases 31, the insulator 52, and the spacers 51 and 53 are made of ceramics such as alumina. The conductive layer 32 is made of a conductive material having a constant thickness and placed in the heating base 31. The conductive layer 32 is firmly held between the two heating bases 31. The two lead wires 40 of the conductive layer 32 extend parallel to each other in a base end portion of the heating bases 31. The heating element 4 of the conductive layer 32 is smaller in the transverse sectional area than the lead wires 40, causing the heating element 4 to generate a larger amount of Joule heat than the lead wires 40 when the electric current is supplied to the lead wires 40.

[0043] The heating element 4 has, as shown in Fig. 2, has a constant width along its entire length. In addition, the heating element 4 has a constant transverse sectional area along its entire length. The pattern or layout of the heating element 4 changes between the base end region R1, the intermediate region R2, and the front end region R3. The change in the layout of the heating element 4 results in a difference in the length per unit area between the heating elements 41, 42, and 43 in the regions R1, R2, and R3. The length per unit area, as referred to herein, is derived by dividing a total length of each of the heating elements 41, 42, and 43 placed in the regions R1, R2, and R3 by an area of ​​a corresponding one of the regions R1, R2, and R3.

[0044] Specifically, the length per unit area of ​​the heating element 41 in the base end region R1 and the length per unit area of ​​the heating element 43 in the front end region R3 are larger than those of the heating element 42 in the intermediate region R2. The length per unit area of ​​the heating element 41 in the base end region R1 is larger than that of the heating element 43 in the front end region R3.

[0045] The heating element 41 in the base end region R1 is made of portions extending in parallel in the longitudinal direction L of the heater 3 and portions extending in parallel in the width direction W, which is perpendicular to the longitudinal direction L and meanders in the width direction W. The heating element 41 in the base end region R1 is made of two conductors extending symmetrically in the width direction W. The heating element 42 in the intermediate region R2 is made of two conductors extending in parallel in the longitudinal direction L and symmetrically in the width direction W. The conductors of the heating element 42 are located outside a portion of the intermediate region R2 in the width direction W that coincides with or overlaps the sensor electrode 22 in the thickness direction of the heater base 31.The heating element 43 in the front end portion R3 is made of sections extending in parallel in the longitudinal direction L and sections extending in parallel in the width direction, and meanders in the longitudinal direction L. In other words, the heating element 43 in the front end portion R3 is made of two conductors that are symmetrical in the width direction W. The heating element 43 in the front end portion R3 includes two outer sections 431 extending in parallel in the longitudinal direction L and two inner sections 432 extending in parallel in the longitudinal direction L, which are connected to each other on the front end side. The inner sections 432 are connected to each other on the base end side.

[0046] As described above, the gas sensor 1 includes the entire area R of the front end portion 11 of the heating bases 31, in which the heating element 4 is placed. The entire area R is classified into three areas arranged in the longitudinal direction L of the gas sensor 1. The heating characteristics of the heating element 4 (i.e., the amount of heat generated by the heating element 4) differ among the three areas, which are the front end area R3 opposite the pump electrode 21, the intermediate area R2 located between the front end 222 and the base end 221 of the sensor electrode 22, and the base end area R1 located closer to the base end side than the sensor electrode 22.

[0047] The resistance values ​​per unit area of ​​the heating element 41 in the base end region R1 and the heating element 43 in the front end region R3 are selected to be higher than those of the heating element 42 in the intermediate region R2, whereby the amount of heat generated in the base end region R1 and the front end region R3 is increased to be larger than that in the intermediate region R2.

[0048] The above structure is capable of heating a portion of the solid electrolyte body 2 opposite to the base end region R1, which is closest to the insulator 6 among the three regions R1 to R3, in the thickness direction of the heating bases 31, more than a portion of the solid electrolyte body 2 opposite to the intermediate region R2 and a region around the sensor electrode 22.

[0049] The resistance value per unit area of ​​either the heating element 41 in the base end region R1 or the heating element 43 in the front end region R3 may be selected to be higher.

[0050] The front end portion R3 is less subject to heat dissipation (i.e., heat transfer) toward the base end side where the insulator 6 is located, so that a region around the pump electrode 21, which is located opposite to the front end portion R3, will have the highest temperature in the longitudinal direction L of the gas sensor 1. Conversely, the base end portion R1 is highly subject to heat dissipation toward the base end side where the insulator 6 is located. For these reasons, the gas sensor 1 is designed to have the resistance value per unit area of ​​the heating element 41 in the base end portion R1 higher than that of the heating element 43 in the front end portion R3, causing the portion of the solid electrolyte body 2 opposite to the base end portion R1 to be heated more.Therefore, the region around the sensor electrode 22, which is opposite to the intermediate region R2, and a portion located closer to the base end side than the sensor electrode 22 are maintained at a desired temperature lower than that of a region around the pump electrode 21. The temperature of the region around the pump electrode 21 is maintained at a temperature suitable for exhibiting the catalytic activity.

[0051] Usually, the temperature of the gas G (ie, the exhaust gas) discharged from the internal combustion engine is lower than a target temperature at which the heater 3 heats the solid electrolyte body 2.

[0052] The temperature of the gas G (ie, the exhaust gas) may be greatly lower than the target temperature at which the solid electrolyte body 2 is heated when the internal combustion engine is operating in lean combustion. This raises a problem in dissipating or transferring heat from the gas sensor 1 to the base end side where the insulator 6 is located.

[0053] The temperature of the gas G (i.e., the exhaust gas) flowing through the exhaust pipe of the internal combustion engine in which the gas sensor 1 is installed usually rises and falls cyclically in response to combustion cycles in the internal combustion engine. When the temperature of the gas G drops, a problem arises regarding the dissipation or transfer of heat from the gas sensor 1 to the base end side where the insulator 6 is located.

[0054] For the reasons described above, the gas sensor 1 is designed to strongly heat the solid electrolyte body 2 near the insulator 6 in order to minimize the adverse effects of the sensor electrode 22 resulting from the dissipation or transfer of heat to the base end side where the insulator 6 is located when the temperature of the gas G drops.

[0055] The gas sensor 1 therefore serves to maintain the temperature around the sensor electrode 22 at a desired level or value even when the temperature of the gas G changes, ensuring the stability of the accuracy in measuring the concentration of the specific gas component using the sensor electrode 22.

[0056] Fig. Fig. 3 demonstrates a relationship between the distance (mm) from the front end of the gas sensor 1 and the temperature of a portion of the gas sensor 1 located at a corresponding distance from the front end of the gas sensor 1 in Fig. 2 is located. Fig. 3 also demonstrates such a relationship of a comparative example of a conventional gas sensor 9 in Fig. 9. The conventional gas sensor 9 is, as in Fig. 9, is equipped with the heating device 93. The heating element 94 of the heating device 93 does not have the heating element 41 in the base end region R1 of the heating bases 31. The graph of Fig. Figure 3 shows results of simulations of the temperature of gas sensors 1 and 9.

[0057] In Fig. 3, a temperature change of gas sensor 1 is represented by "E1" when the temperature of gas G is 500 °C. A temperature change of gas sensor 1 is represented by "E2" when the temperature of gas G is 200 °C. A temperature change of gas sensor 9 is represented by "F1" when the temperature of gas G is 500 °C. A temperature change of gas sensor 9 is represented by "F2" when the temperature of gas G is 200 °C.

[0058] In either the gas sensor 1 of this embodiment or the conventional gas sensor 9, the temperature around the center of the pump electrode 21 (i.e., the pump cell) in the longitudinal direction L has a peak. The temperature around the sensor electrode 22 (i.e., the sensor cell) is lower than that around the pump electrode 21.

[0059] In the conventional gas sensor 9, when the temperature of the gas G drops from 500°C to 200°C, the temperature around the sensor electrode 22 drops significantly. Such a drop is caused by the dissipation or transfer of heat to the base end of the gas sensor 9.

[0060] In the gas sensor 1 of this embodiment, when the temperature of the gas G drops from 500°C to 200°C, the temperature around the sensor electrode 22 hardly changes. Such beneficial effects for minimizing temperature change around the sensor electrode 22 are obtained by heating the portion of the solid electrolyte body 6 opposite the base end region R1, which is located closest to the insulator 6, more than the portion of the solid electrolyte body 2 opposite the intermediate region R2 and the region around the sensor electrode 22. The gas sensor 1 serves to maintain the temperature around the sensor electrode 22 at a desired level even when the temperature of the gas G changes.

[0061] In the case where the gas sensor 1 is provided with a monitor electrode 23 (ie, a monitor cell) which will be described later in the second embodiment, the temperature of the monitor cell 23 will be identical to that of the sensor electrode 22. Second embodiment

[0062] This embodiment is an example (see Fig. 1), in which the monitoring electrode 23 is arranged adjacent to the sensor electrode 22 in the width direction W and is located closer to the base end side than the pump electrode 21 on the first surface 201 of the solid electrolyte body 2.

[0063] The monitoring electrode 23 is used to measure the oxygen concentration of the gas G, whose oxygen concentration has already been regulated by the pump electrode 21. The distance between the center of the pump electrode 21 and the center of the sensor electrode 22 is substantially identical to that between the center of the pump electrode 21 and the center of the monitoring electrode 23.

[0064] The monitoring electrode 23 is made of a material such as platinum or gold, which is catalytically active toward oxygen. The reference electrode 24 is placed on the second surface 202 of the solid electrolyte body 2 and is arranged opposite the monitoring electrode 23 in the thickness direction of the solid electrolyte body 2. The gas sensor 1 has a monitoring cell defined by the monitoring electrode 23, the reference electrode 24 (i.e., a portion of the reference electrode 24 in this embodiment), and a portion of the solid electrolyte body 2 sandwiched between the monitoring electrode 23 and the reference electrode 24. The monitoring cell is configured to measure an oxygen ion current flowing between the monitoring electrode 23 and the reference electrode 24 when the voltage is applied between the monitoring electrode 23 and the reference electrode 24.

[0065] The sensor cell develops a flow of oxygen ions resulting from NOx and residual oxygen. The monitoring cell develops a flow of oxygen ions resulting from the residual oxygen. The concentration of NOx contained in the gas G is therefore derived by subtracting a value of the oxygen ion current in the monitoring cell from that of the oxygen ion current in the sensor cell.

[0066] The pump electrode 21, the sensor electrode 22, the monitoring electrode 23 and the reference electrode 24 are formed by the single solid electrolyte body 2.

[0067] The remaining arrangements of the gas sensor 1 of this embodiment are identical to those of the first embodiment. The same reference numerals as those used in the first embodiment will denote the same parts. The same beneficial effects as those of the first embodiment are provided. Third embodiment

[0068] This embodiment differs from the first embodiment in the layout of the heating element 4 in the heating bases 31. The heating element 4 in the base end region R1 has, as shown in Fig. 4, the heating element 42 has the central portion 415 extending from the inner portion 422 of the heating element 42 in the intermediate region R2. The volume of the heating element 42 in the intermediate region R2 is larger than that in the first embodiment, thereby enabling the area around the sensor electrode 22 to be heated to a high degree compared to the first embodiment.

[0069] The heating element 43 in the front end region R3 can, as in Fig. 5, be designed such that, like the heating element 41 in the base end region R1, it includes sections that extend parallel in the longitudinal direction L and sections that extend parallel in the width direction W. The sections extend continuously and meander in the width direction W. The heating element 42 in the intermediate region R2 lies outside ends of a region in the width direction W that is arranged opposite the sensor electrode 22 in the thickness direction of the solid electrolyte body 2. The heating element 41 in the base end region R1 can, as in Fig. 6, be designed such that it includes sections which extend parallel in the longitudinal direction L and sections which extend parallel in the width direction W and meander in the longitudinal direction L.

[0070] The heating element 42 in the intermediate region R2 can, as shown in the Fig. 7 and Fig. 8, have at least one section whose width is greater than that of the heating element 4 in the base end region R1 and the front end region R3. In this case, the heating element 4 may include two outer sections 411 and two inner sections 412, which extend in the longitudinal direction L over the entire base end region R1, the intermediate region R2 and the front end region R3, and meander in the longitudinal direction L. The two outer sections 411, which are located in the intermediate region R2, may, as shown in Fig. 7, have a width which is greater than that of another section of the heating element 4. The two inner sections 412 which lie in the intermediate region R2 may alternatively, as in Fig. 8, be designed such that they have a width which is greater than that of another portion of the heating element 4.

[0071] In the cases in the Fig. 7 and Fig. 8, an average value of a cross-sectional area per unit length of the heating element 41 located in the base end region R1 and an average value of a cross-sectional area per unit length of the heating element 43 located in the front end region R3 are smaller than an average value of a cross-sectional area per unit length of the heating element 42 located in the intermediate region R2. The average value of a cross-sectional area per unit area, as referred to herein, is derived by dividing an average value of a transverse sectional area of ​​each of the heating elements 41, 42, and 43 in the regions R1, R2, and R3 by an area of ​​a corresponding one of the regions R1, R2, and R3.

[0072] This provides the resistance value per unit area of ​​the heating element 41 in the base end region R1 and the resistance value per unit area of ​​the heating element 43 in the front end region R3 to be larger than the resistance value per unit area of ​​the heating element 42 in the intermediate region R2. This allows the amount of heat generated in the base end region R1 and the front end region R3 to be larger than that in the intermediate region R2.

[0073] The remaining arrangements of the gas sensor 1 of this embodiment are identical to those of the first embodiment. The same reference numerals as those used in the first embodiment will denote the same parts. The same beneficial effects as those of the first embodiment are provided.

Claims

[1] Gas sensor, comprising: a plate-like solid electrolyte body (2) which has oxygen ion conductivity; a pump electrode (21) placed on a first surface (201) of the solid electrolyte body (2) exposed to the gas (G) containing oxygen, the pump electrode (21) being used to regulate an oxygen concentration in the gas (G); a sensor electrode (22) placed on the first surface (201) of the solid electrolyte body (2) closer to a base end side than the pump electrode (21), the sensor electrode (22) being used to measure a concentration of a specific gas component of the gas (G) whose oxygen concentration has already been regulated by the pump electrode (21); and a plate-like heating device (3) which is arranged such that it is arranged opposite the solid electrolyte body (2) and which serves to heat the solid electrolyte body (2), characterized by , that the gas sensor has a front end side in its longitudinal direction (L) which is exposed to the gas (G), and also has a base end side in the longitudinal direction (L) which is maintained by an insulator (6), the heating device (3) is constructed from a heating base (31) and a conductive layer (32) placed in the heating base (31), the conductive layer (32) includes a pair of lead wires (40) arranged on the base end side and a heating element (4), wherein the heating element (4) is arranged closer to the front end side than the lead wires (40), is connected to the lead wires (40) and has a cross-sectional area smaller than that of the lead wires (40), and an entire area (R) of a front end portion (11) of the heating base (31) in which the heating element (4) is installed is divided into three areas: a base end region (R1), an intermediate region (R2), and a front end region (R3) arranged in the above longitudinal direction (L), wherein the intermediate region (R2) is located between a front end (222) and a base end (221) of the sensor electrode (22), the front end region (R3) is closer to the front end side than the intermediate region (R2), and the base end region (R1) is closer to the base end side than the intermediate region (R2), wherein resistance values ​​per unit area of ​​the heating element (4) in the base end region (R1) and the front end region (R3) are higher than those in the intermediate region (R2). [2] Gas sensor according to claim 1, characterized bythat the entire heating element (4) has a constant cross-sectional area, and that a length per unit area of ​​the heating element (4) located in the base end region (R1) and a length per unit area of ​​the heating element (4) located in the front end region (R3) are greater than those of the heating element (4) located in the intermediate region (R2). [3] Gas sensor according to claim 1 or claim 2, characterized by that an average value of a cross-sectional area per unit length of the heating element (4) placed in the base end region (R1) and an average value of a cross-sectional area per unit length of the heating element (4) placed in the front end region (R3) are smaller than that of the heating element (4) placed in the intermediate region (R2). [4] The gas sensor according to any one of claims 1 to 3, wherein the heating element (4) located in the base end portion (R1) has portions extending in parallel in the longitudinal direction (L) and portions extending in parallel in a width direction (W) perpendicular to the longitudinal direction (L) and meandering in the width direction (W). [5] The gas sensor according to any one of claims 1 to 3, wherein the heating element (4) located in the base end portion (R1) has portions extending in parallel in the longitudinal direction (L) and portions extending in parallel in the width direction (W) perpendicular to the longitudinal direction (L) and meandering in the longitudinal direction (L). [6] A gas sensor according to claim 4 or claim 5, wherein the heating element (4) located in the base end portion (R1) is also formed in a center area located inside the inner ends of the lead wires (40) in the width direction (W). [7] A gas sensor according to any one of claims 1 to 6, wherein the heating base (31) has a portion opposite to the sensor electrode (22), and the heating element (4) located in the intermediate region (R2) is located outside ends of this portion of the heating base (31) which are opposite to each other in the width direction (W) of the heating base (31). [8] A gas sensor according to any one of claims 1 to 6, wherein the heating element (4) located in the intermediate region (R2) includes a pair of outer portions (411) extending in parallel in the longitudinal direction (L) and a pair of inner portions (412) extending in parallel in the longitudinal direction (L), the inner portions (412) being located inside the outer portions (411) in the width direction (W). [9] A gas sensor according to claim 7 or claim 8, wherein the heating element (4) located in the intermediate region (R2) has at least a portion whose width is larger than that of the heating element (4) in the base end region (R1) and the front end region (R3). [10] The gas sensor according to any one of claims 1 to 9, wherein the heating element (4) located in the front end portion (R3) has portions extending in parallel in the longitudinal direction (L) and portions extending in parallel in the width direction (W) perpendicular to the longitudinal direction (L) and meandering in the longitudinal direction (L), and wherein the portions extending in the longitudinal direction (L) include a pair of outer portions (411) and a pair of inner portions (412) located inside the outer portions (411) in the width direction (W). [11] The gas sensor according to any one of claims 1 to 9, wherein the heating element (4) located in the front end portion (R3) has portions extending in parallel in the longitudinal direction (L) and portions extending in parallel in a width direction (W) perpendicular to the longitudinal direction (L) and meandering in the width direction (W).

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