JP2001183331A - Detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a detector comprising a heating element and a detection electrode which eliminated high heat stress caused by local heat generation along with shorter temperature rise time in the detector. SOLUTION: In the detector in which a detection electrode 4 is formed on the external surface of or inside a cylindrical base body 2 while heating element 7 for heating the detection electrode 4 is buried in the perimeter of the detection electrode 4, heating generating sections 7 are arranged in the front and rear of a base body 2 of the detection electrode 4 in the axial direction thereof and the respective heat generating sections 7a and 7b before and after the detection electrode 4 are constructed with heating patterns serially connected having a plurality of bent parts. The front and the rear heat generating sections 7a and 7b are connected in parallel and the length of each of the heating patterns from one parallel connection point 11a to the other parallel connection point 11b are made the same in substance in the front and rear heat generating sections 7a and 7b.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensing element having a cylindrical ceramic substrate and a sensing electrode and a heating element for heating the sensing electrode.

[0002]

2. Description of the Related Art Conventionally, as means for indirectly measuring the air-fuel ratio of an automobile, an oxygen concentration detecting element for measuring the oxygen concentration in exhaust gas has been used. In this oxygen concentration detecting element, it is necessary to maintain the temperature of the detecting element at or above the activation temperature in order to perform the detecting function. At the start of the engine when the exhaust gas temperature is lower than the activation temperature, in order to heat the oxygen concentration detection element to the activation temperature or higher, a hollow inside the sheath-shaped oxygen concentration detection element is inserted into an insulating rod such as ceramics. A method has been adopted in which a rod-shaped heater having a heat-generating element embedded therein is inserted to heat the detection electrode to a predetermined temperature.

However, in the above method, since the heating by the oxygen concentration detecting element by the rod-shaped heater is indirect heating, the heating temperature of the rod-shaped heater is set sufficiently higher than the set temperature of the detection electrode of the oxygen concentration detecting element. Needed. For this reason, there has been a problem that high heat stress is generated in the rod-shaped heater, cracks are generated in the ceramic insulating substrate, and the cracks cause disconnection of the heating element and breakage of the rod-shaped heater itself. In addition, a long time is required for raising the temperature to a temperature at which the operation of the oxygen concentration detecting element is stabilized.

On the other hand, with the recent tightening of exhaust gas regulations,
It became necessary to detect the oxygen concentration immediately after starting the engine. In order to solve such a problem, a sensor / heater integrated oxygen concentration detecting element in which a flat-type resistance heating element is embedded inside a box-shaped oxygen concentration detecting element has been put to practical use.

[0005] A cylindrical heater-integrated type in which a reference electrode and a measurement electrode are provided on the inner and outer surfaces of a cylindrical tube made of a solid electrolyte, and a gas-permeable porous insulating layer is provided on the surface of the measurement electrode. An oxygen concentration detecting element has also been proposed. Unlike the conventional indirect heating type, these heater-integrated detection elements are of a direct heating type, so that the temperature can be rapidly raised, and the time required to heat the detection electrode to a temperature for activating the detection electrode (hereinafter referred to as activation). (Referred to as the aging time).

[0006]

However, since the box-shaped oxygen concentration detecting element described above has a flat plate shape, it has poor thermal stress balance under severe conditions, resulting in durability and heat resistance. Therefore, there is a problem that the sensing element is easily broken during operation.

In order to compensate for such a drawback, the present applicant has formed a detection electrode 22 on the surface of a cylindrical ceramic base 21 as shown in FIG. 5, and provided a heating element around the detection electrode 22. A heater-integrated oxygen concentration detecting element has been proposed in which a heating element 23 is disposed buried, and in particular, a heat generating portion is disposed on a side portion of the detecting electrode 22 which is perpendicular to the cylindrical axis direction of the detecting electrode 22.

However, when the detection electrode 22 is heated by the heating element 23 with such a structure, there is a problem that cracks may occur in the cylindrical ceramic base 21 and malfunction may occur.

That is, since it is necessary to secure the heat generation required for raising the temperature within the limited volume of the cylindrical base,
Depending on the arrangement and pattern of the heating element, local heating is caused, resulting in high thermal stress being partially generated, and the distance between the positive and negative electrodes being shortened and migration of Ca, Si, etc. occurring when voltage is applied. Accordingly, there is a problem that the cylindrical ceramic substrate becomes porous and deteriorates.

Therefore, according to the present invention, in a sensing element having a heating element and a sensing electrode, a high heat stress is not generated due to local heat generation, and a heating time until heating to a predetermined temperature is short. It is an object to provide a sensing element.

[0011]

Means for Solving the Problems In view of the above, the present inventors have found that, in the above-mentioned cylindrical sensing element, the sensing electrode is effectively heated to reduce thermal stress without causing local heat generation. As a result of repeated investigations on the arrangement and pattern of the heating elements that can be used, the heating elements for heating the detection electrodes formed on the outer surface or inside of the cylindrical base were replaced with a detection electrode with a sufficient detection area. By arranging the heating element at the front and back in the axial direction of the cylinder and controlling the effective length of the heating element to a predetermined length, high thermal stress generation due to local heat generation and deterioration of the ceramic base due to migration can be avoided,
The present inventors have found that it is possible to obtain a sensing element capable of rapidly increasing the temperature and capable of detecting with high accuracy, and arrived at the present invention.

That is, the detection element of the present invention has a detection electrode formed on the outer surface or inside of a cylindrical base, and a heating element for heating the detection electrode is buried around the detection electrode. The element is characterized in that the heating element is disposed before and after the detection electrode in the cylindrical axis direction of the base.

Normally, when a detection electrode and a heat generating portion for heating the detection electrode are formed on the cylindrical ceramic substrate, if these are arranged in the circumferential direction of the cylindrical substrate, the area in the circumferential direction is limited. Therefore, each region is easily restricted, and as a result, there arises a problem that an electromotive force required as a sensing electrode cannot be generated or migration occurs due to a potential difference generated between the heating element patterns.

On the other hand, according to the present invention, by forming a heating portion by embedding a heating heater pattern in front and rear in the cylindrical axis direction of the detection electrode, a formation region of the detection electrode and the heating portion is widened. Can be secured.
As a result, a local temperature gradient is less likely to occur during heating, and high-speed temperature rise can be achieved while suppressing generation of thermal stress.

[0015] Further, in such a sensing element, the heating element is constituted by a series-connected heating pattern having a plurality of folded portions as a heating pattern, wherein the folded portion has a sensing electrode side end portion and a sensing electrode side end portion in a cylindrical axis direction. Are provided at the opposite end portions, a heating portion due to the presence of the folded portion can be provided in the vicinity of the detection electrode, so that the heating efficiency of the detection element can be increased.

Further, a front heating section disposed in front of the detection electrode and a rear heating section disposed in rear of the detection electrode are connected in parallel, and are supplied with power from the same power source.
Further, by making the lengths of the heat generating patterns of the front heat generating portion and the rear heat generating portion from the one parallel connection point to the other parallel connection point substantially the same, the temperature gradient of the heat generation portion can be reduced. As a result, the thermal stress generated in the cylindrical ceramic substrate can be reduced.

Furthermore, by forming the effective length of the heat generating portion in the cylindrical axis direction to be 1.2 to 2 times the effective length of the detection electrode, the thermal stress can be reduced. Heating efficiency can be increased.

[0018]

FIG. 1A is a schematic perspective view and FIG. 1B is a schematic cross-sectional view taken along line x1-x1 of FIG. 1, taking an oxygen concentration detecting element as an example of the detecting element of the present invention. 2
FIG. 2 is a schematic plan view for explaining a detection electrode and a heat generation pattern in the detection element of FIG. 1.

The oxygen sensor element 1 shown in FIG. 1 is made of a ceramic solid electrolyte having oxygen ion conductivity, and has a cylindrical ceramic base 2 having a sealed end, that is, an inner surface of a cylindrical tube 2 having a U-shaped cross section. A reference electrode 3 which is in contact with a reference gas such as air is formed on the reference electrode 3 of the cylindrical tube 2.
A measurement electrode 4 that is in contact with a gas to be measured, such as exhaust gas, is formed on the outer surface opposite to the above.

Further, according to the present invention, the ceramic insulating layer 5 is formed on the surface of the measuring electrode 4 formed on the outer surface of the cylindrical tube 2 whose end is sealed or in the vicinity thereof.
A predetermined opening 6 is formed in the ceramic insulating layer 5 so that a part or the whole of the measurement electrode 4 is exposed, and a heating element 7 is provided in the ceramic insulating layer 5 near the opening 6. Is buried. The heating element 7 is connected to a terminal electrode 9 via a lead wire 8, and the terminal electrode 9 is connected to a power supply (not shown). Then, a current is applied to the heating element 7 from the power supply via the terminal electrode 9 and the lead wire 8, and the heating element 7 generates heat, and the measurement electrode 4, and the sensing portion including the cylindrical tube 2 and the reference electrode 3 are moved to a predetermined position. It is configured so that the temperature can be rapidly raised. (Heating Element Pattern) According to the present invention, the heating element 7 is buried before and after in the cylindrical axis direction of the cylindrical tube 2 of the measurement electrode 4, whereby the front heating part 7a and the rear heating part 7b are formed. Make up.

According to the present invention, the effective length L2 of the heat generating portions 7a and 7b in the cylindrical axis direction is equal to the effective length L1 of the measuring electrode 4.
Is preferably 1.2 to 2 times. this is,
If the effective length L2 of the heat generating parts 7a, 7b is shorter than the above,
In some cases, the temperature may be locally high, which tends to cause thermal stress. Further, if the length is longer than the above, heat is dissipated from a wide area, and the activation time of the detection electrode becomes longer.

The front heating section 7a and the rear heating section 7b
As shown in FIGS. 1 and 2, each of the heat generating patterns is formed by a series-connected pattern having a plurality of bent portions 10.
Are present at the end on the measurement electrode 4 side and the end on the opposite side in the cylindrical axis direction, respectively.

According to this configuration, since the amount of heat generated at the bent portion 10 is larger than that of the straight portion, the measurement electrode 4
The heat generation density at the side end can be increased, thereby increasing the heating efficiency of the sensing section including the measurement electrode 4, and as a result,
The time for raising the temperature of the sensing unit, in other words, the activation time can be shortened.

The front heat generating portion 7a and the rear heat generating portion 7b are connected to the parallel connection points 11 on opposite sides of the measuring electrode 4.
a, 11 b are provided, and the parallel connection point 11 is connected to the terminal electrode 9 via the lead wire 8.

As a result, the front heating section 7a and the rear heating section 7b are both supplied from the same power supply.

According to the present invention, in order to equalize the heating of the measurement electrode 4, the heat generation of the front heating section 7a and the rear heating section 7b from the parallel connection point 11a to the other parallel connection point 11b is performed. The lengths of the patterns are set to be substantially the same. As a result, the temperature gradient in the sensing section including the measurement electrode 4 can be reduced, and as a result, the thermal stress generated in the cylindrical ceramic base can be reduced. (Solid Electrolyte) The ceramic solid electrolyte used in the oxygen sensor element of FIG. 1 is made of a ceramic containing ZrO ₂ , specifically, Y ₂ O ₃ and Yb ₂ O ₃ ,
Rare earth oxides such as Sc ₂ O ₃ , Sm ₂ O ₃ , Nd ₂ O ₃ , Dy ₂ O ₃ are 1 to 30 mol%, preferably 3 to 1 mol% in terms of oxides.
Partially stabilized ZrO ₂ or stabilized Z containing 5 mol%
rO ₂ is used.

Further, Zr in ZrO ₂ is 1 to 20 atomic%.
The use of ZrO _{2 in} which is substituted by Ce has the effect of increasing the electron conductivity and further improving the responsiveness. Further, in order to improve sinterability, the above Z
Al ₂ O ₃ or SiO ₂ can be added to and contained in rO ₂ , but if it is contained in a large amount, the creep characteristics at high temperatures deteriorate, so that Al ₂ O ₃ and SiO ₂
Is preferably 5% by weight or less, particularly 2% by weight or less. (Electrode) a reference electrode 3 formed on the surface of the cylindrical tube 2;
The measurement electrodes 4 are all platinum, rhodium, palladium,
One or more alloys selected from the group consisting of ruthenium and gold are used. The ceramic solid electrolyte described above is used for the purpose of preventing grain growth of the metal in the electrode during operation of the sensor and to increase the so-called three-phase interface contact between the metal particles, the solid electrolyte, and the gas related to the response. The components may be mixed in the electrode at a ratio of 1 to 50% by volume, particularly 10 to 30% by volume. (Ceramic insulating layer) On the other hand, as the ceramic insulating layer 5 in which the heating element 7 is embedded, a ceramic material such as alumina, spinel, forsterite, zirconia, or glass is preferably used. At this time, when zirconia is used as the ceramic insulating layer, the zirconia itself is a solid electrolyte, and the zirconia is connected to the cylindrical tube 2 so that the leakage current from the heating element 7 does not affect the oxygen concentration detection. It is desirable to form an intermediate layer of alumina, spinel, forsterite, or the like. Further, glass can be used for the glass insulating layer as the ceramic insulating layer 5, but in this case, BaO, PbO, SrO, Cr
A glass containing at least one of aO and CdO in an amount of 5% by weight or more, particularly, a crystallized glass is desirable.

The ceramic insulating layer 5 is desirably made of dense ceramic having a relative density of 80% or more and an open porosity of 5% or less. This is because the mechanical strength of the oxygen sensor element 1 itself can be increased as a result of the strength of the insulating layer being increased when the ceramic insulating layer 5 is dense. (Heating Element) The heating element 7 embedded in the ceramic insulating layer 5 may be a metal selected from the group consisting of platinum, rhodium, palladium and ruthenium, or
It is desirable to be made of more than one kind of alloy, and it is particularly desirable to select a metal or an alloy having a melting point higher than the firing temperature of the ceramic insulating layer 5 in terms of co-sintering with the ceramic insulating layer 5.

The structure of the heater section in which the heating element 7 is buried inside the ceramic insulating layer 5 is, as shown in the sectional view of FIG. 1B, formed inside the surface of the cylindrical tube 2 made of a solid electrolyte. In addition to the structure in which the ceramic insulating layer 5 having the heating element 7 embedded therein is laminated, the heating element 7 is embedded inside the outer surface of the cylindrical tube 2 as shown in FIG. A ceramic insulating layer 5 of alumina, spinel, forsterite, or the like is formed, and a zirconia layer 10 can be formed on the outer surface of the ceramic insulating layer 5. The zirconia layer 10 relieves stress caused by a difference in thermal expansion and a difference in firing shrinkage between the solid electrolyte and the ceramic insulating layer 5,
To minimize the thermal stress as much as possible,
This is for keeping the heat generated by the heating element 7 buried therein to prevent a sudden change in temperature of the entire oxygen sensor element.

In this configuration, the heating element 7 is
As shown in FIG. 3A, it can be buried inside the ceramic insulating layer 5; as shown in FIG. 3B, it can be buried in the zirconia layer 10, or as shown in FIG. It can be disposed between the layer 5 and the zirconia layer 10.

In any case, the heating element 7 is provided without being in direct contact with the cylindrical tube 2 or the electrode via the ceramic insulating layer 5 having no solid electrolyte performance such as alumina. Preferably, the thickness of the ceramic insulating layer 5 between the cylindrical tube 2 and the heating element 7 is at least 2 μm.
It is desirable that this is the case.

In the present invention, the heating element 7
Is buried in the vicinity of an opening 6 provided in the ceramic insulating layer 5. The shape of the opening 6 is such that the measurement electrode 4 is exposed on the side surface of the cylindrical tube 2 in FIG. As described above, the vertically long opening 6 is provided,
The shape of the opening 6 may be any of a circular shape and a square shape.

Next, a method for manufacturing the sensing element shown in FIG. 1 will be described with reference to FIG. To manufacture the oxygen sensor element of FIG. 1, first, a cylindrical tube 12 as shown in FIG. This cylindrical tube 12 is formed by adding an organic binder for molding to a ceramic solid electrolyte powder having oxygen ion conductivity such as zirconia, etc., as appropriate, for example, extrusion molding, isostatic pressing (rubber pressing), or press forming. To produce a cylindrical molded body having a diameter of 1 to 10 mm, one end of which is sealed by the method described above.

At this time, the solid electrolyte powder used may be a mixed powder obtained by adding the above-mentioned rare earth oxide powder to the zirconia powder at a ratio of 1 to 30 mol% in terms of oxide, or zirconia and the zirconia powder. Coprecipitated raw material powder with a stabilizer is used.

Next, the cylindrical tube 12 made of the solid electrolyte is used.
On the inner and outer surfaces of the substrate, patterns 13 and 14 serving as reference electrodes and measurement electrodes are formed by a slurry dipping method using a conductive paste containing platinum or the like, or by screen printing, pad printing, or roll transfer. At this time, for printing on the inner surface of the cylindrical tube 12, the conductive paste may be filled and discharged, and may be applied and formed on the entire inner surface.

Next, as shown in FIG.
7 and an insulating ceramic green sheet 15 in which a heating element 16 is buried near an opening 17 is formed. The insulating ceramic green sheet 15 is prepared by first using a ceramic powder such as alumina, spinel, forsterite, zirconia, or glass for forming a ceramic insulating layer, and appropriately adding a forming organic binder to prepare a slurry. Using this slurry, a green sheet having a predetermined thickness is produced by a doctor blade method, an extrusion molding method, a pressing method, or the like. The thickness of the green sheet is from 50 to 500 μm, especially from 100 to 3 from the viewpoint of handling of the sheet.
A range of 00 μm is particularly preferred.

Then, a conductive paste containing platinum powder is printed as a heating element 16 on the surface of the formed green sheet on the heating element pattern by a screen printing method, a pad printing method, a roll transfer method, or the like, and another one is further placed thereon. The heating element 16 is embedded by stacking the above-mentioned green sheets or applying a slurry of ceramic powder by a printing method or a transfer method. The opening 17 is formed by punching or the like after or before the formation of the heating element 16.

Next, as shown in FIG. 4C, the insulating ceramic green sheet 15 is wound around the surface of the cylindrical tube 12 to form a cylindrical laminate 18. At this time, in order to wind the insulating ceramic green sheet 15 around the surface of the cylindrical tube 12, an adhesive such as an acrylic resin or an organic solvent is interposed between the insulating ceramic green sheet 15 and the cylindrical tube 12. Glue it,
Alternatively, they can be mechanically bonded while applying pressure with a roller or the like.

At this time, the seams of the wound insulating ceramic green sheet 15 may be overlapped with each other at the end of the sheet or bonded at a predetermined interval in consideration of shrinkage during firing.

Then, the cylindrical laminate 18 is transferred to the cylindrical tube 1.
The solid electrolyte and the insulating ceramic green sheet 15 constituting the two can be integrated by firing simultaneously. A sintered body in which the heater element 15 and the sensor element 12 are integrated is manufactured.

More specifically, when zirconia is used as the solid electrolyte, firing may be performed at 1300 to 1700 ° C. for about 1 to 10 hours in an inert atmosphere such as argon gas or the atmosphere.

In the above manufacturing method, the measurement electrode and the reference electrode are formed on the cylindrical tube 12 before the insulating ceramic green sheet 15 is wound. However, the present invention is not limited to this. The electrode paste is applied to the cylindrical laminate 18 by winding the insulating ceramic green sheet 15 around the surface of the cylindrical tube 12 by a dipping method, screen printing, pad printing, or roll transfer. Opening 1 in inner surface and insulating ceramic green sheet 15
After printing on the surface of the cylindrical tube 12 in 7, it may be fired under the above conditions.

Further, without forming the measurement electrode and the reference electrode, a cylindrical laminated body 18 is formed according to the above-described method, and after firing, the opening 17 in the inner surface of the cylindrical tube 12 and the ceramic insulating layer is formed. The electrode paste may be printed and baked therein, or may be formed by a thin film method such as a sputtering method or a plating method.

[0044]

EXAMPLES The following experiments were performed to confirm the performance of the sensing element of the present invention. Commercially available spinel powder, alumina powder, zirconia powder containing 5 mol% Y ₂ O ₃ , and platinum powder were prepared. First, a polyvinyl alcohol solution is added to zirconia powder containing 5 mol% Y ₂ O ₃ to prepare a kneaded material, and a cylindrical molded body having an outer diameter of about 5 mm and an inner diameter of 3 mm sealed at one end by extrusion molding. did.

A slurry was prepared by adding a polyvinyl alcohol solution to the spinel powder, and a green sheet of about 200 μm was prepared. After forming an opening in a green sheet made of this spinel powder by punching,
A conductor paste containing platinum powder was screen-printed in the vicinity of the opening in the form of a heating element pattern, and then a spinel powder was further applied thereon to produce a heater element in which the heating element was embedded.

Next, the heater element was wound around the surface of the cylindrical sensor element using an acrylic resin as an adhesive to form a cylindrical laminate. Thereafter, the cylindrical laminate is fired in the air at 1500 ° C. for 2 hours,
Fired and integrated.

Thereafter, a porous reference electrode and a measuring electrode made of platinum were formed by electroless plating to a thickness of 2 μm on the surface inside the opening of the cylindrical tube and the entire inner surface of the cylindrical tube.

Then, a ceramic protective layer made of spinel and having a porosity of 30% and having a porosity of 200 μm was formed on the surface of the measurement electrode in the opening by plasma spraying to produce a stoichiometric air-fuel ratio sensor as shown in FIG. .

The oxygen sensor element used in this experimental example was cylindrical with an outer diameter of 3.9 mm and an inner diameter of 1.6 mm. In addition,
The length L1 in the longitudinal direction of the measurement electrode was 4.3 mm, and the width was 7.0 mm. Then, a plurality of oxygen sensor elements having different effective widths within a range of 7.0 mm were prepared before and after the measurement electrode. Further, as a comparative example, as shown in FIG. 5, an oxygen sensor element in which heat generating portions were formed on both side surfaces of the measurement electrode was also manufactured.

A constant voltage of 14 V is applied to the heating element of each oxygen sensor element between the terminal electrodes at room temperature, and the temperature rise time until the temperature of the detecting section reaches 700 ° C., power consumption, and The generated thermal stress was determined by the finite element method, and the results are shown in Table 1.

[0051]

[Table 1]

Sample N in which heating portions were formed on both sides of the electrode
o. In the case of No. 6, a thermal stress of 181 MPa is generated, but it can be seen that the thermal stress can be reduced by forming the heat generating portion before and after the electrode. In particular, it can be seen that the higher the L2 / L1 ratio, the lower the generated thermal stress, but the longer the temperature rise time. The results in Table 1 show that when the L2 / L1 ratio is in the range of 1.2 to 2, the power consumption is 10 W and the thermal stress is 120 M.
Good results were obtained with a temperature of not more than Pa and a temperature rise time of not more than 13 seconds.

[0053]

As described above, according to the heater structure of the sensor element of the present invention, the above heater pattern is arranged in parallel before and after the measuring electrode, so that the sensor element can be heated even during rapid heating. In the state where the intensity is maintained, the temperature uniformity of the sensing unit is maintained, and the oxygen concentration can be accurately detected.

[0054]

[Brief description of the drawings]

FIG. 1 is a schematic perspective view of an oxygen sensor element as an example of a detection element of the present invention.

FIG. 2 is a schematic plan view for explaining a relationship between a measurement electrode and a heating unit in the oxygen sensor element of FIG.

FIG. 3 is an enlarged sectional view of a main part for describing various arrangement structures of heat generating parts in the oxygen sensor element of FIG. 1;

FIG. 4 is a process chart for manufacturing the oxygen sensor element of FIG.

FIG. 5 is a perspective view showing the overall structure of a conventional heater-integrated oxygen sensor element.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Oxygen sensor element (detection element) 2 ... Cylindrical base 3 ... Reference electrode 4 ... Measuring electrode 5 ... Ceramic insulating layer 6 ... Opening 7 ... Heating element 7a Front heating part 7b Rear heating part 8 Lead wire 9 Terminal electrode 10 Folding part 11 Parallel connection point

Claims (5)

[Claims] 1. A sensing element comprising: a detection electrode formed on an outer surface or inside of a cylindrical base; and a heating section having a heating element embedded around the detection electrode for heating the detection electrode. 3. The sensing element according to claim 1, wherein the heat generating portion is disposed before and after the sensing electrode in the cylinder axis direction of the base. 2. A heat generating part before and after the detection electrode,
It is constituted by a series-connected heating pattern having a plurality of folded portions, wherein the folded portions are respectively provided at a detection electrode side end in a cylindrical axis direction and an end opposite to the detection electrode side end. The sensing element according to claim 1, wherein 3. A heat generating portion disposed in front of the detection electrode and a rear heat generating portion disposed in a rear portion of the detection electrode are connected in parallel and supplied with power from the same power supply. Sensing element. 4. The heating pattern of the front heating section and the rear heating section from the one parallel connection point to the other parallel connection point are substantially the same in length. The sensing element as described in the above. 5. The sensing element according to claim 1, wherein the effective length of the heat generating portion in the cylindrical axis direction is 1.2 to 2 times the effective length of the sensing electrode.