JP7733592B2 - Sensor - Google Patents

Description

The present invention relates to a sensor.

Some sensors (such as the gas sensor disclosed in Patent Document 1) are composed of a sensor element. To reduce the effects of temperature, a heater may be built into the sensor element. For example, in the invention proposed in Patent Document 1, a heater is built into the sensor element to activate the solid electrolyte that makes up the sensor element.

Sensors may also be used in locations exposed to environmental influences such as weather. For example, a gas sensor is placed inside a vehicle's exhaust pipe to monitor the vehicle's exhaust gases. In such cases, if the sensor element (particularly the heater portion) becomes wet (i.e., becomes wet) due to, for example, humidity, condensation, or rain, the efficiency of heating by the heater may decrease, which could have adverse effects such as the risk of the sensor element cracking.

In response to this, Patent Documents 2 and 3 propose protecting the sensor element with a cover member such as a protective cover or housing. The cover members proposed in Patent Documents 2 and 3 reduce the probability of the sensor element (particularly the heater portion) becoming wet, thereby reducing the likelihood of the above-mentioned adverse effects occurring.

Japanese Patent Application Laid-Open No. 2020-180985 Japanese Patent Application Laid-Open No. 2017-223619 Special table 2018-536860 publication

The inventors of this application have discovered the following problem with conventional cover members. Specifically, the housing proposed in Patent Document 3 has an inner wall with a wider diameter toward the tip, which increases the distance between the sensor element and the inner wall of the housing toward the tip. This reduces the likelihood of the sensor element becoming wet, even if water forms on the inner wall of the housing due to condensation or other reasons. However, this housing shape alone may not be sufficient to prevent water from entering the housing. For example, water generated by condensation or other reasons may flow along the inner wall of the housing to the rear end of the widening diameter (the starting point of the widening diameter), potentially dripping from this point onto the sensor element. This dripping water may land near the heat-generating portion of the heater in the sensor element, potentially causing the adverse effects described above.

In one aspect, the present invention was made in consideration of these circumstances, and its purpose is to provide a sensor in which the heater portion of the sensor element is less likely to become wet.

To solve the above-mentioned problems, the present invention adopts the following configuration.

A sensor according to one aspect of the present invention comprises a sensor element that incorporates a heater and extends in the longitudinal direction, and a housing configured to surround the sensor element along the longitudinal direction. The sensor element has a front end and a rear end. The heater comprises a heat generating portion and a lead portion. The heat generating portion has a front end and a rear end and is located on the front end side of the sensor element. The housing comprises an expanded diameter portion configured so that, in a cross section parallel to the longitudinal axis, the diameter of the inner wall of the housing expands toward the front end of the sensor. The expanded diameter portion has a front end and a rear end. The rear end of the heat generating portion is located closer to the front end of the sensor than the rear end of the expanded diameter portion.

In this configuration, the housing has an expanded diameter section near the heat-generating portion of the heater of the sensor element. This expanded diameter section is configured so that the distance between the inner wall of the housing and the sensor element increases toward the tip of the sensor. In addition, the rear end of the expanded diameter section is positioned closer to the rear end of the sensor than the rear end of the heater's heat-generating portion. As a result, even if water flows along the inner wall of the housing and drips from the rear end of the expanded diameter section, the heater's heat-generating portion is positioned further forward than this rear end, preventing water dripping from the inner wall of the housing (rear end of the expanded diameter section) from contacting the heater's heat-generating portion. Therefore, a sensor with this configuration can reduce the likelihood of the heater portion of the sensor element becoming wet.

In the sensor according to the above aspect, the cross-sectional shape of the expanded diameter portion may be configured to have a taper. The taper angle may be 10 degrees or greater and less than 90 degrees. The taper angle may be 20 degrees or greater and less than 90 degrees. Alternatively, the taper angle may be 30 degrees or greater and less than 90 degrees. These configurations effectively reduce the probability of the heater portion of the sensor element becoming wet.

In the sensor according to the above aspect, the cross-sectional shape of the expanded diameter portion may be configured as an R-shape. This configuration effectively reduces the probability of the heater portion of the sensor element becoming wet.

The sensor according to the above aspect may further include a protective cover configured to surround at least a portion of the enlarged diameter portion of the housing along the longitudinal direction and extend beyond the tip of the sensor element. With this configuration, the protective cover can reduce the likelihood of the housing and sensor element becoming wet.

In the sensor according to the above aspect, the sensor element may have a porous protective layer that covers at least a portion of the sensor element. According to this configuration, the porous protective layer can reduce the effects of water on the sensor element and improve the strength of the sensor element.

In the sensor according to the above aspect, the sensor element may be a gas sensor element. This configuration makes it possible to provide a gas sensor in which the probability of the heater portion becoming wet is reduced.

This invention provides a sensor that reduces the probability of the heater portion of the sensor element becoming wet.

FIG. 1 is a schematic cross-sectional view illustrating an example of the configuration of a sensor according to an embodiment. FIG. 2 is a cross-sectional view schematically illustrating an example of the configuration of the sensor element according to the embodiment. FIG. 3 is a schematic cross-sectional view illustrating an example of the configuration of a sensor according to a modified example. FIG. 4 is a schematic cross-sectional view illustrating an example of the configuration of a sensor according to a modified example. FIG. 5 is a schematic cross-sectional view illustrating an example of the configuration of a sensor element according to a modified example. FIG. 6 is a schematic cross-sectional view illustrating an example of the configuration of a sensor according to a modified example. FIG. 7 is a schematic cross-sectional view illustrating an example of the configuration of a sensor according to a modified example. FIG. 8 is a schematic cross-sectional view illustrating an example of the configuration of a sensor according to a modified example.

An embodiment of one aspect of the present invention (hereinafter also referred to as "the present embodiment") will be described below with reference to the drawings. However, the present embodiment described below is merely an example of the present invention in all respects. Needless to say, various improvements and modifications can be made without departing from the scope of the present invention. In other words, specific configurations corresponding to the embodiment may be adopted as appropriate when implementing the present invention.

[Configuration example]
FIG. 1 is a cross-sectional view schematically illustrating an example of the configuration of a sensor S according to this embodiment. The sensor S has an axis and is configured to extend along the longitudinal direction (axial direction). FIG. 1 schematically illustrates the configuration of a cross section parallel to and tangent to the longitudinal axis (a line extending in the vertical direction of the figure). The sensor S has a front end and a rear end as its respective longitudinal ends. One longitudinal end is the front end, and the other longitudinal end is the rear end. In the example of FIG. 1, the sensor S is disposed so that the front end of the sensor S faces downward and the rear end of the sensor S faces upward. In other words, the vertical direction in FIG. 1 corresponds to the longitudinal direction (axial direction). In this embodiment, the sensor S includes a sensor element 100, a housing 200, and a protective cover 300.

(sensor element)
The sensor element 100 incorporates a heater 70 and is configured to extend in the longitudinal direction (the vertical direction in FIG. 1 ). The sensor element 100 has a leading end 110 and a trailing end 120 as its respective longitudinal ends. The sensor element 100 is arranged so that the leading end 110 faces the leading end of the sensor S. The heater 70 includes a heat generating portion 72 and a lead portion 73. The heat generating portion 72 has a leading end 721 and a trailing end 722 as its respective longitudinal ends. The heat generating portion 72 is arranged on the leading end 110 side of the sensor element 100 with the leading end 721 of the heat generating portion 72 facing the leading end 110 of the sensor element 100. The lead portion 73 is connected to the trailing end 722 of the heat generating portion 72 and is arranged on the trailing end 120 side of the sensor element 100.

In the example of FIG. 1, the sensor element 100 is formed in a rectangular parallelepiped shape, but the shape of the sensor element 100 need not be limited to this example. The shape of the sensor element 100 may be selected appropriately depending on the embodiment. Furthermore, the type of sensor element 100 is not particularly limited and may be selected appropriately depending on the embodiment. In one example, the sensor element 100 may be a gas sensor element, and the sensor S may be configured to measure the concentration of any gas component. An example of the configuration of the sensor element 100 when a gas sensor element is used as the sensor element 100 will be described in detail below.

(housing)
The housing 200 is configured to surround the sensor element 100 along the longitudinal direction. In the example shown in FIG. 1 , the housing 200 is formed in a cylindrical shape. As a result, the housing 200 is configured to surround the sensor element 100 except for a portion on the tip portion 110 side by accommodating the sensor element 100 in the internal space of the housing 200. However, the shape of the housing 200 is not limited to this example. As shown in FIG. 1 , the housing 200 may be configured to surround a portion of the sensor element 100, or may be configured to completely surround the sensor element 100 along the longitudinal direction. As long as the housing 200 is configured to surround at least a portion of the sensor element 100, the shape of the housing 200 is not particularly limited and may be selected appropriately depending on the embodiment.

The housing 200 has a front end and a rear end as its respective longitudinal ends, and is positioned so that the front end of the housing 200 faces the front end of the sensor S. In a cross section parallel to the longitudinal axis shown in FIG. 1, the housing 200 has an expanding diameter portion 210 configured so that the diameter of the inner wall 215 of the housing 200 expands toward the front end of the sensor S. The inner wall 215 of the housing 200 is the inner wall of the housing 200 that faces the sensor element 100 housed in the internal space of the housing 200.

The expanded diameter section 210 is disposed on the front end side of the housing 200. The expanded diameter section 210 has a front end 211 and a rear end 212 as its longitudinal ends, and is disposed so that the front end 211 of the expanded diameter section 210 faces the front end of the sensor S. When viewing the expanded diameter section 210 from the rear end to the front end of the sensor S, the rear end 212 of the expanded diameter section 210 is the position where the diameter of the inner wall 215 begins to expand. Therefore, the rear end 212 may also be referred to as the starting point of the expanded diameter section 210. In this embodiment, the rear end 212 of the expanded diameter section 210 is disposed closer to the rear end of the sensor S than the rear end 722 of the heat generating section 72 of the heater 70. In other words, the rear end 722 of the heat generating section 72 of the heater 70 is disposed closer to the front end of the sensor S than the rear end 212 of the expanded diameter section 210.

The shape of the expanded diameter section 210 may be determined appropriately depending on the embodiment, as long as the diameter of the inner wall 215 increases toward the tip of the sensor S, i.e., the distance between the sensor element 100 and the inner wall 215 in the direction perpendicular to the axis (the left-right direction in FIG. 1) is greater at the tip end 211 than at the rear end 212. As an example, as shown in FIG. 1, the shape of the expanded diameter section 210 in the cross section of FIG. 1 may be tapered. The taper angle 220 may be determined appropriately depending on the embodiment. As an example, the taper angle 220 may be 10 degrees or greater and less than 90 degrees. As another example, the taper angle 220 may be 20 degrees or greater and less than 90 degrees. As yet another example, the taper angle 220 may be 30 degrees or greater and less than 90 degrees. If it is assumed that the shape of the rear end of the sensor S continues in a straight line from the starting point of the enlarged diameter section 210 (i.e., the rear end 212) toward the tip end in the cross section of Figure 1, the taper angle 220 may be determined by the angle between this imaginary straight line and the inner wall 215 of the enlarged diameter section 210.

In the example shown in Figure 1, the inner wall 215 in the area other than the expanded diameter section 210 has a portion that is continuous with the rear end 212 of the expanded diameter section 210 and has the same diameter as the rear end 212, and the portion further rearward of the sensor S than that portion has a slightly wider shape to accommodate the support member of the sensor element 100. However, the shape and diameter of the inner wall 215 in the area other than the expanded diameter section 210 are not limited to this example and may be determined appropriately depending on the embodiment.

The housing 200 may be made of a metal material such as stainless steel (e.g., SUS). The housing 200 may be manufactured by appropriately shaping the metal material.

(protective cover)
The protective cover 300 is configured to surround at least a portion of the enlarged diameter portion 210 of the housing 200 along the longitudinal direction and extend beyond the tip portion 110 of the sensor element 100. In the example of Fig. 1, the protective cover 300 is formed in a roughly cylindrical shape and is configured to surround the sensor element 100 and a portion of the tip side of the housing 200 around the axis. The protective cover 300 has a front end and a rear end as respective ends in the longitudinal direction, and the front end of the protective cover is positioned closer to the tip side of the sensor S than the tip portion 110 of the sensor element 100.

As an example, the protective cover 300 may be configured as proposed in Japanese Patent Application Laid-Open No. 2017-223619. Specifically, in the example shown in FIG. 1, the protective cover 300 includes an inner cover 310 and an outer cover 320. The inner cover 310 includes a first member 311 and a second member 315, and is configured to cover the periphery of the sensor element 100 and the tip end of the housing 200. The first member 311 extends longitudinally from the outer wall of the tip end of the housing 200, reduces in diameter in a direction perpendicular to the longitudinal direction after passing the tip of the housing 200, and then continues to extend longitudinally. The second member 315 is configured to cover the periphery of a portion of the tip end of the first member 311. The outer cover 320 is configured to cover the periphery of the inner cover 310. Openings are appropriately provided in the first member 311, second member 315, and outer cover 320 of the inner cover 310, thereby connecting the space at the tip portion 110 of the sensor element 100 with the space outside the protective cover 300. However, the configuration and shape of the protective cover 300 are not limited to this example. The configuration and shape of the protective cover 300 may be determined appropriately depending on the embodiment.

The protective cover 300 may be made of a metal material such as stainless steel (e.g., SUS). The protective cover 300 may be manufactured by appropriately shaping the metal material. Note that the protective cover 300 may be omitted from the configuration of the sensor S.

(Example of dimensions)
The dimensions of the sensor element 100, the housing 200, and the protective cover 300 may be determined appropriately depending on the embodiment. As an example, a first distance (length in the left-right direction in FIG. 1 / distance between an inner wall 215 of the rear end 212 and an outer wall of the sensor element 100) between the rear end 212 of the enlarged diameter portion 210 of the housing 200 and the sensor element 100 may be 1 mm to 4 mm. Furthermore, a second distance (length in the left-right direction in FIG. 1 / distance between an inner wall 215 of the front end 211 and an outer wall of the sensor element 100) between the front end 211 of the enlarged diameter portion 210 of the housing 200 and the sensor element 100 may be 2 mm to 6 mm, which is longer than the first distance.

(Example of usage)
The sensor S may be attached as appropriate depending on the embodiment. As an example, when a gas sensor element is used as the sensor element 100, the sensor S may be attached to a pipe 410, which is an exhaust path from a vehicle engine. In the example of FIG. 1 , the sensor S further includes a metal nut 400 having a male thread on its outer circumferential surface. The nut 400 is fixed to the outer circumferential surface of the housing 200. The housing 200 is welded to the pipe 410 and is inserted, together with the nut 400, into a fixing member 412 having a female thread on its inner circumferential surface. The fixing member 412 is fixed to the pipe 410. The male thread of the nut 400 is then screwed into the female thread of the fixing member 412. In this way, the sensor S is fixed to the pipe 410 via the housing 200 and the fixing member 412.

In one example, the sensor element 100 configured as a gas sensor element may measure the concentrations of components such as NO _x and O ₂ contained in exhaust gas (measurement gas) emitted from an engine. In the example of FIG. 1 , the sensor S is fixed to the pipe 410 with the axis of the sensor S perpendicular to the flow of exhaust gas in the pipe 410 and the tip 110 of the sensor element 100 facing downward. The mounting angle of the sensor S with respect to the pipe 410 is not limited to this example. The sensor S may be fixed to the pipe 410 with the axis of the sensor S inclined at any angle with respect to the flow of exhaust gas. The mounting angle of the sensor S may be configured, for example, so that the tip 110 of the sensor element 100 faces an angle of 0 to 80 degrees with respect to the vertically downward direction. A similar mounting angle may be adopted in cases where the sensor element 100 is mounted other than on the pipe 410 or in cases where the sensor element 100 is configured as a non-gas sensor element.

(Configuration of sensor element)
FIG. 2 is a cross-sectional view illustrating an example of the configuration of the sensor element 100 when a gas sensor element is used as the sensor element 100. The sensor element 100 has a structure in which six layers are stacked in order from bottom _to top in the cross-sectional view of FIG. 2: a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is composed of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO 2 ). The solid electrolyte forming these six layers may be dense and airtight. Note that "dense and airtight" refers to a porosity of 5% or less. The sensor element 100 is manufactured, for example, by stacking ceramic green sheets corresponding to each layer after performing processes such as predetermined processing and printing wiring patterns on them, and then sintering the stacked layers to integrate them. As an example, the sensor element 100 is a laminate of multiple ceramic layers.

At one tip of the sensor element 100, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, the gas inlet 10, first diffusion-controlling section 11, buffer space 12, second diffusion-controlling section 13, first internal space 20, third diffusion-controlling section 30, and second internal space 40 are formed adjacent to each other and communicate with each other in this order.

The gas inlet 10, buffer space 12, first internal cavity 20, and second internal cavity 40 are spaces within the sensor element 100 that are defined by hollowing out the spacer layer 5, with the upper portion defined by the underside of the second solid electrolyte layer 6, the lower portion defined by the upper surface of the first solid electrolyte layer 4, and the sides defined by the side surfaces of the spacer layer 5.

The first diffusion-controlling section 11 is formed as two horizontally elongated slits (with the opening's long side oriented perpendicular to the plane of the drawing). The second diffusion-controlling section 13 and the third diffusion-controlling section 30 are formed as holes whose lengths extending perpendicular to the plane of the drawing are shorter than those of the first internal space 20 and the second internal space 40, respectively. The second diffusion-controlling section 13 and the third diffusion-controlling section 30 will be described in detail below. The section extending from the gas inlet 10 to the second internal space 40 is also referred to as the gas flow section.

A reference gas introduction space 43 is provided at a position farther from the tip side than the gas flow section, between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, with the side portion defined by the side surface of the first solid electrolyte layer 4. A reference gas, such as the atmosphere, is introduced into the reference gas introduction space 43. However, the configuration of the sensor element 100 is not limited to this example. As another example, the first solid electrolyte layer 4 may be configured to extend to the rear end of the sensor element 100, and the reference gas introduction space 43 may be omitted. In this case, the atmosphere introduction layer 48 may be configured to extend to the rear end of the sensor element 100.

The air introduction layer 48 is made of porous alumina and is configured to allow the reference gas to be introduced through the reference gas introduction space 43. In addition, the air introduction layer 48 is formed to cover the reference electrode 42.

The reference electrode 42 is formed so as to be sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and is surrounded by an air introduction layer 48 that connects to the reference gas introduction space 43. The reference electrode 42 is used to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40. Details will be described later.

The gas inlet 10 is a portion of the gas flow section that opens to the external space. The sensor element 100 is configured to take in the gas to be measured from the external space through the gas inlet 10.

The first diffusion-controlling section 11 is a section that imparts a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10.

The buffer space 12 is a space provided to guide the measurement gas introduced from the first diffusion-controlling section 11 to the second diffusion-controlling section 13.

The second diffusion rate-controlling section 13 is a section that provides a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20.

When the measurement gas is introduced from outside the sensor element 100 into the first internal space 20, pressure fluctuations in the measurement gas in the external space (exhaust pressure pulsations, if the measurement gas is automobile exhaust gas) cause the measurement gas to be suddenly drawn into the sensor element 100 from the gas inlet 10. Instead of being introduced directly into the first internal space 20, the measurement gas passes through the first diffusion-controlling section 11, buffer space 12, and second diffusion-controlling section 13, where the concentration fluctuations are canceled out, before being introduced into the first internal space 20. This makes the concentration fluctuations of the measurement gas introduced into the first internal space almost negligible.

The first internal space 20 is provided as a space for adjusting the oxygen partial pressure in the measurement gas introduced through the second diffusion-controlling section 13. This oxygen partial pressure is adjusted by the operation of the main pump cell 21.

The main pump cell 21 is an electrochemical pump cell comprising an inner pump electrode 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal cavity 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a and exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (second solid electrolyte layer 6 and first solid electrolyte layer 4) that define the first internal space 20, and the spacer layer 5 that provides the sidewalls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6, which provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4, which provides the bottom surface. Side electrode portions (not shown) are formed on the sidewall surfaces (inner surfaces) of the spacer layer 5 that form both sidewalls of the first internal space 20, connecting to the ceiling electrode portion 22a and bottom electrode portion 22b. The inner pump electrode 22 is arranged in a tunnel-like structure at the locations where the side electrode portions are located.

The inner pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (for example, cermet electrodes made of Pt containing 1% Au and _ZrO2 ). The inner pump electrode 22, which comes into contact with the measurement gas, is made of a material with a weakened ability to reduce nitrogen oxide ( _NOx ) components in the measurement gas.

The sensor element 100 is configured to pump oxygen from the first internal space 20 to the external space, or pump oxygen from the external space into the first internal space 20, by applying a desired pump voltage Vp0 between the inner pump electrode 22 and the outer pump electrode 23 in the main pump cell 21 and flowing a pump current Ip0 in either the positive or negative direction between the inner pump electrode 22 and the outer pump electrode 23.

In addition, to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, the inner pump electrode 22, second solid electrolyte layer 6, spacer layer 5, first solid electrolyte layer 4, third substrate layer 3, and reference electrode 42 constitute an oxygen partial pressure detection sensor cell 80 (i.e., an electrochemical sensor cell) for controlling the main pump.

The sensor element 100 is configured to be able to determine the oxygen concentration (oxygen partial pressure) within the first internal space 20 by measuring the electromotive force V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Furthermore, the pump current Ip0 is controlled by feedback controlling Vp0 so that the electromotive force V0 remains constant. This allows the oxygen concentration within the first internal space 20 to be maintained at a predetermined constant value.

The third diffusion rate-controlling section 30 applies a predetermined diffusion resistance to the measurement gas, the oxygen concentration (oxygen partial pressure) of which has been controlled by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas into the second internal space 40.

The second internal space 40 is provided as a space for performing processing related to the measurement of the nitrogen oxide concentration in the measurement gas introduced through the third diffusion-controlling part 30. The _NOx concentration is mainly measured by the operation of the measuring pump cell 41 in the second internal space 40, where the oxygen concentration has been adjusted by the auxiliary pump cell 50.

In the second internal space 40, the sensor element 100 is configured so that, after the oxygen concentration (oxygen partial pressure) has been adjusted in advance in the first internal space 20, the oxygen partial pressure of the measurement gas introduced through the third diffusion-controlling part is further adjusted by the auxiliary pump cell 50. This makes it possible to keep the oxygen concentration in the second internal space 40 constant with high precision, thereby enabling the sensor element 100 to measure the _NOx concentration with high precision.

The auxiliary pump cell 50 is an auxiliary electrochemical pump cell composed of an auxiliary pump electrode 51, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any suitable electrode on the outside of the sensor element 100 will suffice), and a second solid electrolyte layer 6. The auxiliary pump electrode 51 has a ceiling electrode portion 51a provided over substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal cavity 40.

The auxiliary pump electrode 51 is disposed in the second internal space 40 with a tunnel-shaped structure similar to the inner pump electrode 22 disposed in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6, which provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4, which provides the bottom surface of the second internal space 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5, which provides the side walls of the second internal space 40. As a result, the auxiliary pump electrode 51 has a tunnel-shaped structure.

Like the inner pump electrode 22, the auxiliary pump electrode 51 is also made of a material with reduced reduction ability for nitrogen oxide components in the measured gas.

The sensor element 100 is configured so that oxygen in the atmosphere within the second internal space 40 can be pumped out to the external space or pumped into the second internal space 40 from the external space by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 in the auxiliary pump cell 50.

In addition, to control the oxygen partial pressure in the atmosphere within the second internal space 40, the auxiliary pump electrode 51, reference electrode 42, second solid electrolyte layer 6, spacer layer 5, first solid electrolyte layer 4, and third substrate layer 3 constitute an auxiliary pump control oxygen partial pressure detection sensor cell 81 (i.e., an electrochemical sensor cell).

The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force V1 detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. As a result, the oxygen partial pressure in the atmosphere within the second internal space 40 is controlled to a low partial pressure that does not substantially affect the measurement of _NOx .

At the same time, the pump current Ip1 is used to control the electromotive force of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and by controlling the electromotive force V0, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-controlling part 30 into the second internal space 40 is controlled to be always constant. When used as a NO _x sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of approximately 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

The measurement pump cell 41 measures the nitrogen oxide concentration in the measurement gas within the second internal space 40. The measurement pump cell 41 is an electrochemical pump cell composed of a measurement electrode 44, an outer pump electrode 23, a second solid electrolyte layer 6, a spacer layer 5, and a first solid electrolyte layer 4. The measurement electrode 44 is provided on the upper surface of the first solid electrolyte layer 4 facing the second internal space 40, at a position spaced apart from the third diffusion-controlling section 30.

The measuring electrode 44 is a porous cermet electrode. The measuring electrode 44 also functions as a _NOx reduction catalyst that reduces _NOx present in the atmosphere inside the second internal space 40. Furthermore, the measuring electrode 44 is covered with a fourth diffusion-controlling part 45.

The fourth diffusion-controlling part 45 is a film made of a porous body mainly composed of alumina ( _Al2O3 ). The fourth diffusion-controlling part 45 serves to limit the amount of _NOx that flows into _the measuring electrode 44 and also acts as a protective film for the measuring electrode 44.

The sensor element 100 is configured to pump out oxygen produced by the decomposition of nitrogen oxides in the atmosphere surrounding the measurement electrode 44 in the measurement pump cell 41, and detect the amount of oxygen produced as a pump current Ip2.

In addition, to detect the oxygen partial pressure around the measurement electrode 44, the second solid electrolyte layer 6, spacer layer 5, first solid electrolyte layer 4, third substrate layer 3, measurement electrode 44, and reference electrode 42 constitute a measurement pump control oxygen partial pressure detection sensor cell 82 (i.e., an electrochemical sensor cell). The variable power supply 46 is controlled based on the voltage (electromotive force) V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82.

The measurement gas introduced into the second internal space 40 reaches the measurement electrode 44 through the fourth diffusion-controlling part 45 under conditions where the oxygen partial pressure is controlled. Nitrogen oxides in the measurement gas around the measurement electrode 44 are reduced (2NO → _N2 + _O2 ) to generate oxygen. This generated oxygen is then pumped by the measurement pump cell 41, and the voltage Vp2 of the variable power supply is controlled so that the control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 remains constant. Because the amount of oxygen generated around the measurement electrode 44 is proportional to the nitrogen oxide concentration in the measurement gas, the pump current Ip2 in the measurement pump cell 41 can be used to calculate the nitrogen oxide concentration in the measurement gas.

Furthermore, by combining the measuring electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 to form an electrochemical sensor cell that serves as an oxygen partial pressure detection means, it is possible to detect an electromotive force corresponding to the difference between the amount of oxygen generated by the reduction of _NOx components in the atmosphere around the measuring electrode 44 and the amount of oxygen contained in the reference atmosphere. This makes it possible to determine the concentration of nitrogen oxide components in the measurement gas.

The second solid electrolyte layer 6, spacer layer 5, first solid electrolyte layer 4, third substrate layer 3, outer pump electrode 23, and reference electrode 42 form an electrochemical sensor cell 83. The sensor element 100 is configured to be able to detect the oxygen partial pressure in the measurement gas outside the sensor using the electromotive force Vref obtained by this sensor cell 83.

In the sensor element 100 having the above configuration, by operating the main pump cell 21 and the auxiliary pump cell 50, it is possible to supply measurement gas whose oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NO _x ) to the measurement pump cell 41. Therefore, the sensor element 100 is configured to be able to determine the concentration of nitrogen oxides in the measurement gas based on the pump current Ip2 that flows when oxygen generated by the reduction of NO _x is pumped out of the measurement pump cell 41, in proportion to the concentration of nitrogen oxides in the measurement gas.

Furthermore, the sensor element 100 is equipped with a heater 70 that adjusts the temperature by heating and keeping the sensor element 100 warm in order to increase the oxygen ion conductivity of the solid electrolyte. In the example of FIG. 2, the heater 70 further includes a heater electrode 71, a heater insulating layer 74, and a pressure release hole 75 in addition to the heat generating portion 72 and lead portion 73. The lead portion 73 may be configured as a through-hole. The heater 70 is disposed in a position closer to the bottom surface of the sensor element 100 than to the top surface of the sensor element 100 in the thickness direction of the sensor element 100. The top surface of the sensor element 100 is the top surface of the second solid electrolyte layer 6, and the bottom surface of the sensor element 100 is the bottom surface of the first substrate layer 1.

The heater electrode 71 is an electrode formed in contact with the underside of the first substrate layer 1. By connecting the heater electrode 71 to an external power source, power can be supplied to the heater 70 from outside.

The heating element 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heating element 72 is connected to the heater electrode 71 via the lead portion 73, and generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 100.

The heat generating portion 72 is embedded throughout the entire area from the first internal space 20 to the second internal space 40, making it possible to adjust the temperature of the entire sensor element 100 to a temperature at which the solid electrolyte is activated.

The heater insulating layer 74 is an insulating layer made of an insulating material such as alumina and formed on the upper and lower surfaces of the heating portion 72. The heater insulating layer 74 is formed for the purpose of achieving electrical insulation between the second substrate layer 2 and the heating portion 72, and between the third substrate layer 3 and the heating portion 72.

The pressure relief hole 75 is a portion that penetrates the third substrate layer 3 and is provided to communicate with the reference gas introduction space 43. It is formed for the purpose of mitigating the increase in internal pressure that accompanies a rise in temperature within the heater insulating layer 74.

(Features)
As described above, in the sensor S according to this embodiment, the housing 200 is provided with the expanded diameter portion 210 near the heat generating portion 72 of the heater 70 of the sensor element 100. Due to this expanded diameter portion 210, the distance between the inner wall 215 of the housing 200 and the sensor element 100 increases toward the tip of the sensor S (i.e., the distance from the sensor element 100 increases). In addition, the rear end 212 of the expanded diameter portion 210 is located closer to the rear end of the sensor S than the rear end 722 of the heat generating portion 72 of the heater 70. As a result, even if water flows on the inner wall 215 of the housing 200 and drips from the rear end 212 of the expanded diameter portion 210, the heat generating portion 72 of the heater 70 is located closer to the tip of the sensor S than the rear end 212, so that the water dripping from the inner wall 215 of the housing 200 (the rear end 212 of the expanded diameter portion 210) can be prevented from splashing on the heat generating portion 72 of the heater 70. Therefore, the sensor S according to this embodiment can reduce the probability that the heat generating portion 72 of the heater 70 of the sensor element 100 will be exposed to water.

Furthermore, as shown in FIG. 1, the cross-sectional shape of the expanded diameter portion 210 may be configured to have a taper. The taper angle 220 may be 10 degrees or more and less than 90 degrees, 20 degrees or more and less than 90 degrees, or 30 degrees or more and less than 90 degrees. This effectively reduces the probability of the heat generating portion 72 of the sensor element 100 becoming wet, as will be shown in the examples described below. In the sensor S according to this embodiment, the protective cover 300 reduces the probability of the housing 200 and sensor element 100 becoming wet. Furthermore, by adopting the gas sensor element illustrated in FIG. 2 as the sensor element 100, a gas sensor can be provided in which the heat generating portion 72 of the heater 70 becomes less likely to become wet.

[Modification]
Although the embodiments of the present invention have been described above, the above-described embodiments are merely illustrative of the present invention in every respect. Various improvements and modifications may be made to the above-described embodiments. Components of the above-described embodiments may be omitted, replaced, or added as appropriate. Furthermore, the shape and dimensions of each component of the above-described embodiments may be modified as appropriate depending on the embodiment. For example, the following modifications are possible. Note that, below, the same reference numerals are used for components similar to those of the above-described embodiment, and descriptions of similar points to those of the above-described embodiment are omitted as appropriate. The following modifications may be combined as appropriate.

(I) Shape of the Expanded Diameter Portion In the example of Fig. 1, the expanded diameter portion 210 of the housing 200 is configured to have a taper. However, the shape in which the diameter of the inner wall 215 expands toward the tip of the sensor S is not limited to this example. In another example, the cross-sectional shape of the expanded diameter portion may be configured to be R-shaped.

Figures 3 and 4 are cross-sectional schematic diagrams illustrating an example of the configuration of sensors (SA, SB) according to this modified example, in which the expanded diameter portion is formed in an R-shape. Figures 6 to 8 are cross-sectional schematic diagrams illustrating an example of the configuration of sensors (SP, SQ, and SR) according to this modified example, in which the inner wall of the expanded diameter portion has multiple straight segments, a straight segment and an inclined segment, or multiple inclined segments. Similar to Figure 1, Figures 3, 4, and 6 to 8 illustrate the configuration of a cross section parallel to and tangent to the longitudinal axis. In sensor SA shown in Figure 3, the expanded diameter portion 210A of housing 200A is formed so that the R-shaped convexity faces inward. In other words, when comparing the amount of expansion of the diameter of inner wall 215A along the axial direction, the amount of expansion of the diameter of inner wall 215A is greater at the front end 211A of expanded diameter portion 210A than at the rear end 212A of expanded diameter portion 210A. The dimensions of the R-shape may be determined arbitrarily. For example, if the inner wall 215A is considered to be an arc, the radius of curvature of the inner wall 215A at the expanded diameter portion 210A in the cross section of FIG. 3 may be 1 mm to 4 mm. On the other hand, in the sensor SB shown in FIG. 4, the expanded diameter portion 210B of the housing 200B is formed so that the convex portion of the R faces outward. That is, when comparing the amount of expansion of the diameter of the inner wall 215B along the axial direction, the amount of expansion of the diameter of the inner wall 215B is greater at the rear end 212B side of the expanded diameter portion 210B than at the front end 211B side. In other words, in the cross section of FIG. 4, the expanded diameter portion 210B has a shape in which an arc has been cut out. The dimensions of this cut-out arc may be determined arbitrarily. For example, the radius of curvature of the cut-out arc in the cross section of FIG. 4 may be 1 mm to 4 mm. Apart from these points, the configuration of each sensor (SA, SB) may be the same as that of the above-described sensor S. In the sensors (SA, SB) of this modified example, the expanded diameter sections (210A, 210B) are formed in an R-shape, which effectively reduces the probability that the heat generating section 72 of the sensor element 100 will become wet.

The shape in which the diameter of the inner wall increases toward the tip may be a shape in which the diameter of the inner wall increases in stages, i.e., in stages, from the rear end to the tip. For example, the inner wall may be formed with multiple straight sections where the diameter of the inner wall is constant, as long as the diameter of the inner wall in the straight sections toward the tip is wider (larger) than the diameter of the inner wall in the straight sections toward the rear end.

In the sensor SP illustrated in FIG. 6 , the expanded diameter portion 210P of the housing 200P is formed so that the diameter of the inner wall 215P expands in stages (i.e., in a stepwise manner) from the rear end 212P side to the front end 211P side. Specifically, in the expanded diameter portion 210P, the inner wall 215P includes a straight portion 2011 and a straight portion 2013, each of which has a constant diameter. That is, the diameter of the inner wall 215P is constant in the straight portion 2011, and the diameter of the inner wall 215P is constant in the straight portion 2013. The straight portion 2013 is positioned closer to the rear end 212P of the expanded diameter portion 210P than the straight portion 2011. In other words, the straight portion 2011 is positioned closer to the front end 211P of the expanded diameter portion 210P than the straight portion 2013. The diameter of inner wall 215P at straight portion 2011 located on the tip end 211P side is wider (larger) than the diameter of inner wall 215P at straight portion 2013 located on the rear end 212P side. In other words, when comparing the diameters of inner wall 215P along the axial direction, the diameter of inner wall 215P at straight portion 2011 is larger than that at straight portion 2013. Therefore, when comparing the amount by which the diameter of inner wall 215P expands along the axial direction, the amount by which the diameter of inner wall 215P expands is greater on the tip end 211P side of expanded diameter portion 210P than on the rear end 212P side.

6, intermediate portion 2012 connecting straight portion 2011 and straight portion 2013 is perpendicular to both straight portion 2011 and straight portion 2013. More precisely, inner wall 215P in intermediate portion 2012 is perpendicular to both inner wall 215P in straight portion 2011 and inner wall 215P in straight portion 2013. However, it is not necessary for intermediate portion 2012 to be perpendicular to both straight portion 2011 and straight portion 2013. For example, intermediate portion 2012 may be inclined with respect to both straight portion 2011 and straight portion 2013 so that the diameter of inner wall 215P widens toward tip portion 211P. Specifically, in a cross section parallel to and tangent to the axis, the intermediate portion 2012 (inner wall 215P in the intermediate portion 2012) may be configured as "a straight line or curve inclined relative to the axial direction so that the diameter of the inner wall 215P increases toward the tip end 211P."

Figure 6 shows an example of an expanded diameter section in which the inner wall has multiple straight sections with a constant inner diameter, with the diameter of the inner wall at the straight sections toward the tip being wider (larger) than the diameter of the inner wall at the straight sections toward the rear end. However, as a shape of an expanded diameter section in which the inner wall diameter widens toward the tip, the example in which "the inner wall in the expanded diameter section includes multiple straight sections" is just one example, and the inner wall in the expanded diameter section may also include an inclined section. That is, the inner wall in the expanded diameter section may include an inclined section in which the inner wall diameter widens toward the tip. In other words, in a cross section parallel to and tangent to the axis, the inclined section may be represented as "a straight line or curve inclined with respect to the axial direction so that the inner wall diameter widens toward the tip." In addition to the inclined section, the inner wall in the expanded diameter section may also include a straight section as shown in Figure 6, i.e., a straight section in which the inner wall diameter is constant. The inner wall may include one or more straight sections and one or more inclined sections. The inner wall may include multiple straight portions and/or inclined portions. If the inner wall includes straight portions and inclined portions, the straight portions may be located closer to the rear end than the inclined portions, or closer to the front end than the inclined portions. If the inner wall includes multiple inclined portions, the inclination of each of the multiple inclined portions relative to the axial direction may be the same or different from each other.

In the sensor SQ shown in FIG. 7, the inner wall 215Q in the expanded diameter portion 210Q of the housing 200Q includes an inclined portion 2022 where the diameter of the inner wall 215Q increases toward the tip 211Q, and a straight portion 2021 where the diameter of the inner wall 215Q is constant. The inclined portion 2022 is located closer to the rear end 212Q of the expanded diameter portion 210Q than the straight portion 2021. In other words, the straight portion 2021 is located closer to the tip 211Q of the expanded diameter portion 210Q than the inclined portion 2022. In the example shown in FIG. 7, the inclined portion 2022 is tangent to the rear end 212Q of the expanded diameter portion 210Q, and the straight portion 2021 is tangent to the tip 211Q of the expanded diameter portion 210Q. In the inclined portion 2022, the diameter of the inner wall 215Q increases toward the tip 211Q. Therefore, when comparing the amount of expansion of the diameter of the inner wall 215Q along the axial direction, the amount of expansion of the diameter of the inner wall 215Q is greater on the front end 211Q side than on the rear end 212Q side of the expanded diameter section 210Q.

In the example shown in Figure 7, the inclined portion 2022 is represented in a cross section parallel to and tangent to the axis by a straight line inclined with respect to the axial direction so that the diameter of the inner wall 215Q increases toward the tip 211Q. However, the inclined portion 2022 may also have a shape represented in a cross section parallel to and tangent to the axis by a curve inclined with respect to the axial direction so that the diameter of the inner wall 215Q increases toward the tip 211Q. The "inclined portion" in Figures 7 and 8 may be represented in a cross section parallel to and tangent to the axis by a straight line or curve inclined with respect to the axial direction so that the diameter of the inner wall increases toward the tip.

In addition, in the example shown in Figure 7, the inclined portion 2022 is located on the rear end 212Q side and the straight portion 2021 is located on the front end 211Q side, but the inclined portion 2022 may be located on the front end 211Q side and the straight portion 2021 may be located on the rear end 212Q side.

Furthermore, it is not essential that the inclined portion 2022 contact the rear end 212Q of the enlarged diameter portion 210Q; the inclined portion 2022 does not have to contact the rear end 212Q of the enlarged diameter portion 210Q, or the inclined portion 2022 may contact the front end 211Q of the enlarged diameter portion 210Q. Similarly, it is not essential that the straight portion 2021 contact the front end 211Q of the enlarged diameter portion 210Q; the straight portion 2021 does not have to contact the front end 211Q of the enlarged diameter portion 210Q, or the straight portion 2021 may contact the rear end 212Q of the enlarged diameter portion 210Q.

As shown in Figure 7, the inner wall of the expanded diameter section may include one or more straight sections (with a constant diameter) in addition to the inclined section. The inner wall may also have a straight section located at least on either the leading end or trailing end of the inclined section.

In the sensor SR shown in FIG. 8, the inner wall 215R in the expanded diameter portion 210R of the housing 200R includes multiple inclined portions where the diameter of the inner wall 215R increases toward the tip end 211R, specifically, inclined portions 2031 and 2033. Inclined portion 2031 is positioned closer to the tip end 211R of the expanded diameter portion 210R than inclined portion 2033; in other words, inclined portion 2033 is positioned closer to the rear end 212R of the expanded diameter portion 210R than inclined portion 2031. In the example shown in FIG. 8, inclined portion 2031 contacts the tip end 211R of the expanded diameter portion 210R, and inclined portion 2033 contacts the rear end 212R of the expanded diameter portion 210R. Furthermore, inclined portion 2031, the diameter of the inner wall 215R increases toward the tip end 211R. Additionally, in the inclined portion 2033, the diameter of the inner wall 215R increases toward the tip 211R. Therefore, when comparing the amount of expansion of the diameter of the inner wall 215R along the axial direction, the amount of expansion of the diameter of the inner wall 215R is greater toward the tip 211R than toward the rear end 212R of the expanded diameter portion 210R.

In the example shown in FIG. 8, the diameter of inner wall 215R is constant in intermediate portion 2032 connecting inclined portion 2031 and inclined portion 2033. That is, intermediate portion 2032 configured as a "straight portion with a constant inner wall diameter" is disposed between inclined portion 2031 and inclined portion 2033. However, it is not necessary to configure intermediate portion 2032 as a straight portion, and intermediate portion 2032 may be configured as a "sloped portion with an inner wall diameter that widens toward the tip." That is, in intermediate portion 2032, the diameter of inner wall 215R may widen toward tip portion 211R.

In the example shown in Figure 8, the inclination of inclined portion 2031 relative to the axial direction is different from the inclination of inclined portion 2033 relative to the axial direction. However, the inclination of inclined portion 2031 relative to the axial direction and the inclination of inclined portion 2033 relative to the axial direction may be the same. Furthermore, if the inclinations are different, it is optional to determine which inclination is greater.

Furthermore, if the inclination of inclined portion 2031 relative to the axial direction is different from the inclination of inclined portion 2033 relative to the axial direction, inclined portion 2031 and inclined portion 2033 may be directly connected without disposing intermediate portion 2032.

As explained above, the shape of the expanding diameter section, in which the diameter of the inner wall increases toward the tip, does not have to be represented by "a single straight line or a single curve inclined relative to the axial direction so that the diameter of the inner wall increases toward the tip" in a cross section parallel to and tangent to the axis. The inner wall in the expanding diameter section may include multiple straight line segments, as long as the diameter of the inner wall in the straight line segment located toward the tip is wider (larger) than the diameter of the inner wall in the straight line segment located toward the rear end (Figure 6). Furthermore, the inner wall in the expanding diameter section may include one or more inclined portions represented by "a straight line or a curve inclined relative to the axial direction so that the diameter of the inner wall increases toward the tip" in a cross section parallel to and tangent to the axis. If the inner wall in the expanding diameter section includes a single inclined portion, a straight line may be located at least on the tip or rear end of the inclined portion (Figure 7). If the inner wall of the expanded diameter section includes multiple inclined sections, straight sections may be arranged between the multiple inclined sections (Figure 8), or the inclined sections may be directly connected to each other without any straight sections. If the inner wall of the expanded diameter section includes multiple inclined sections, the inclination (angle) of each of the multiple inclined sections relative to the axial direction may be different or the same. However, if two inclined sections are directly connected without any straight sections, the inclination (angle) of each of the two inclined sections relative to the axial direction must be different from each other.

(II) Configuration of the Sensor Element In the above embodiment, components may be omitted, replaced, or added as appropriate for each component of the sensor element 100. As another example, the sensor element may be configured to have a porous protective layer that covers at least a portion of the sensor element.

FIG. 5 is a cross-sectional schematic diagram illustrating an example of the configuration of a sensor element 100C according to this modified example. The sensor element 100C is configured similarly to the sensor element 100 described above, except that it further includes a porous protective layer 91. In the example of FIG. 5, the porous protective layer 91 covers a portion of the tip end of the upper surface (upper surface of the second solid electrolyte layer 6) of the sensor element 100C, the tip end surface (the surface on the left side of FIG. 5), and a portion of the tip end of the lower surface (lower surface of the first substrate layer 1). As long as at least a portion of the sensor element 100C is covered by the porous protective layer 91, the area covered by the porous protective layer 91 is not limited to this example and may be determined appropriately depending on the embodiment. The porous protective layer 91 may be composed of a porous material such as porous alumina, porous zirconia, porous spinel, porous cordierite, porous titania, or porous magnesia. According to this modification, the porous protective layer 91 reduces the effects of water on the sensor element 100C and improves the strength of the sensor element 100C.

[Example]
In order to verify the effects of the present invention, sensors according to the following examples and comparative examples were produced, although the present invention is not limited to the following examples.

The sensor of Example 1 was fabricated by adopting the configuration shown in Figure 1 above for the sensor configuration and the configuration shown in Figure 2 above for the sensor element configuration. In the sensor of Example 1, as in the above embodiment, the sensor element was positioned within the housing so that the rear end (rearmost end) of the heater's heat generating portion was located closer to the tip of the sensor than the rear end (starting point) of the enlarged diameter portion. The enlarged diameter portion of the housing was configured to have a 20-degree taper. The distance between the rear end of the enlarged diameter portion of the housing and the sensor element was 2 mm. The distance between the tip of the enlarged diameter portion of the housing and the sensor element was 3 mm.

The sensor according to the second embodiment was fabricated by changing the taper angle of the expanded diameter portion of the first embodiment to 30 degrees. The sensor according to the third embodiment was fabricated by changing the taper angle of the expanded diameter portion of the first embodiment to 40 degrees. The sensor according to the fourth embodiment was fabricated by changing the taper angle of the expanded diameter portion of the first embodiment to 60 degrees. The sensor according to the fifth embodiment was fabricated by changing the taper angle of the expanded diameter portion of the first embodiment to 10 degrees. The sensor according to the sixth embodiment was fabricated by changing the shape of the expanded diameter portion of the first embodiment to the R-shape shown in Figure 3. The radius of curvature of the inner wall of the expanded diameter portion in the sixth embodiment was 2 mm. The sensor according to the first comparative example was fabricated by omitting the expanded diameter portion of the first embodiment and replacing it with a linear portion of the inner wall with a constant diameter. The sensor according to the second comparative example was fabricated by changing the position of the sensor element in the first embodiment so that the rear end of the heater's heat-generating portion was located closer to the rear end of the sensor than the rear end (starting point) of the expanded diameter portion. Other conditions for Examples 2 to 6 and Comparative Examples 1 to 2 are the same as those for Example 1.

The water exposure test device described in JP 2019-185615 A was used to evaluate the amount of water exposure for the sensors in Examples 1 to 6 and Comparative Examples 1 to 2. This water exposure test device includes a horizontal, linear pipe with an internal gas flow path, a blower installed upstream of the pipe, a pressure fluctuation generator installed downstream of the pipe, and a chamber, which is part of the pipe between the blower and the pressure fluctuation generator and in which the sensors of each Example and Comparative Example are attached. A vibrator that applies vibrations to the chamber is connected to the chamber. This water exposure test device can scatter water toward the sensor using gas simulating engine exhaust gas.

In the water exposure test, the sensor was first placed in the chamber of the water exposure test device with its central axis perpendicular to the axis of the piping and tilted 10 degrees from the horizontal. A predetermined amount of moisture was then supplied to the piping between the blower and the chamber. Subsequently, gas (atmospheric air) was supplied to the piping using the blower, and the gas pressure was fluctuated using a pressure fluctuation generator, while the chamber was vibrated using a vibrator. As a result, the moisture supplied to the piping was dispersed toward the sensor placed in the chamber by the pressure-fluctuating gas. In this state, the heater built into the sensor element was driven, and the heater power was controlled so that the temperature of the sensor element reached a predetermined target value between 100 and 200 degrees Celsius. The heater power control value at this time was applied to the previously derived relationship between heater power and water exposure, and the amount of water exposure for the sensor element in each example and comparative example was calculated. The amount of water exposure for each example and comparative example was evaluated using the following criteria: "A (excellent)" for an amount of water exposure of 10 μL (microliters) or less, "B (good)" for more than 10 μL but not more than 20 μL, "C (passable)" for more than 20 μL but not more than 30 μL, and "F (unacceptable)" for more than 30 μL. Table 1 below shows the evaluation results for each example and comparative example.

As shown in Table 1, in the first and second comparative examples, the amount of water exposure on the sensor element exceeded 30 μL, whereas in all examples, the amount of water exposure on the sensor element was kept to 30 μL or less. These results demonstrate that the present invention can reduce the amount of water exposure on the sensor element, particularly on the heat-generating portion. Furthermore, the evaluation results for Examples 1 to 5 also demonstrate that when the expanded diameter portion is tapered, increasing the taper angle can effectively reduce the amount of water exposure on the sensor element. In particular, setting the taper angle to 20 degrees or more, 30 degrees or more, or 40 degrees or more can effectively reduce the amount of water exposure on the sensor element. Furthermore, the evaluation results for Example 6 demonstrate that forming the expanded diameter portion into an R-shape can effectively reduce the amount of water exposure on the sensor element.

S...sensor,
100: sensor element, 110: front end portion, 120: rear end portion, 70: heater, 72: heat generating portion, 721: front end portion, 722: rear end portion,
73...lead portion,
200...Housing,
210... Expanded diameter part, 211... Tip part, 212... Rear end part,
215...Inner wall

Claims (9)

a sensor element incorporating a heater and extending in a longitudinal direction; and a housing configured to surround the sensor element along the longitudinal direction.
A sensor comprising:
the sensor element has a leading end and a trailing end;
the heater includes a heat generating portion and a lead portion,
the heat generating portion has a front end and a rear end, and is disposed on the front end side of the sensor element;
the housing includes an expanding diameter portion configured such that the diameter of the inner wall of the housing expands toward the tip end of the sensor in a cross section parallel to the longitudinal axis,
the enlarged diameter portion has a leading end and a trailing end,
a rear end of the heat generating portion is disposed closer to the front end of the sensor than a rear end of the enlarged diameter portion , and closer to the rear end of the sensor than the front end of the enlarged diameter portion ;
a tip of the heat generating portion is disposed closer to the tip of the sensor than the tip of the enlarged diameter portion;
Sensor. The cross-sectional shape of the expanded diameter portion is configured to have a taper.
The sensor of claim 1 . The taper angle is equal to or greater than 10 degrees and less than 90 degrees.
The sensor of claim 2 . The taper angle is equal to or greater than 20 degrees and less than 90 degrees.
The sensor of claim 2 . The taper angle is equal to or greater than 30 degrees and less than 90 degrees.
The sensor of claim 2 . The cross-sectional shape of the expanded diameter portion is configured to be R-shaped.
The sensor of claim 1 . a protective cover configured to surround at least a portion of the enlarged diameter portion of the housing along the longitudinal direction and extend beyond the tip of the sensor element;
The sensor according to any one of claims 1 to 6. the sensor element has a porous protective layer covering at least a portion of the sensor element;
A sensor according to any one of claims 1 to 7. The sensor element is a gas sensor element.
A sensor according to any one of claims 1 to 8.