JPH01203963A - Oxygen sensor element - Google Patents

Abstract

PURPOSE:To obtain a sensor which is superior in durability, lambda point deviation, and responsiveness and performs accurate air fuel ratio control continuously for a long period by coupling a solid-state electrolyte body with a protection layer tightly by the presence of spherical projection parts. CONSTITUTION:The element 1 consists of the solid-state electrolyte body 2 which generates an oxygen concentration difference between a reference gas and a gas to be measured, a couple of porous electrodes (internal electrode) 3 and (external electrode 4) formed on the internal and external surface of the solid-state electrolyte body 2, the porous protection layer 5 which covers the external electrode 4, a noble metal catalyst 6 dispersed uniformly in the protection layer 5.... Further, the solid-state electrolyte body 4 consists of a base part 2a and spherical projection parts 2b and the external electrode 4 and protection layer 5 are formed in conformity with the shapes of the spherical projection parts 2b. Further, the protection layer 5 consists of a 1st protection layer 5a which is positioned more inside and covers the external electrode 4 directly and a 2nd protection layer 5b which is positioned outside and covers the external electrode 4 directly. The protection layers 5a and 5b carry catalyst 6.... Consequently, the sensor which is superior in durability and responsiveness is obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an oxygen sensor element for detecting oxygen concentration in various combustion equipment, particularly for air-fuel ratio control used to purify exhaust gas from an internal combustion engine. The present invention relates to an oxygen sensor element and a manufacturing method thereof.

[Prior art and problems] An oxygen sensor element for air-fuel ratio control consists of an oxygen ion-conducting solid electrolyte body and a pair of electrodes (reference electrode, measurement electrode) provided on the inner and outer surfaces of the solid electrolyte body, the measurement electrode being in contact with exhaust gas. It is common to coat the material with a porous protective layer to protect it from exhaust gas. However, in this type of sensor element, the excess air ratio (
λ) shifts, a so-called λ point shift occurs, and the detection accuracy decreases. For this reason, various studies and proposals have been made.

For example, an oxygen sensor element in which a noble metal catalyst is supported in a protective layer has been proposed (Japanese Patent Laid-Open No. 53-50888.
50-14396. 54-89898). but,
This type of oxygen sensor element has a problem in durability during use. In other words, the unburned component (Co) in the exhaust gas
etc.) adsorbs or reacts with the supported catalyst and the catalyst expands in volume, causing the protective layer to crack, and in the worst case, the protective layer peels off and the electrode sublimates. Furthermore, if the amount of catalyst supported is too large, clogging will occur and the response will deteriorate, while if it is too small, the catalyst will scatter and the effect will disappear.

Furthermore, an oxygen sensor element has been proposed in which the protective layer is made up of two layers and only the outer layer carries a catalyst (
Japanese Patent Publication No. 53-72688. 55-13828). However, in this type of oxygen sensor element, the catalyst supporting layer itself tends to peel off, and the catalytic action cannot be effectively utilized.

Furthermore, an oxygen sensor element composed of a substance that absorbs and releases oxygen as a protective layer has also been proposed (Japanese Patent Application Laid-Open No. 2002-02).
245148). However, as in the above technology, the protective layer is likely to peel off, and there is still concern about deterioration of durability.

The present invention solves these problems, that is, it has excellent durability, makes effective use of precious metal catalysts, and can maintain accurate air-fuel ratio control stably for a long period of time without causing λ point deviation or decrease in response. The purpose is to develop oxygen sensor elements.

Another object of the present invention is to develop a manufacturing method that can easily mass-produce such oxygen sensor elements.

[Means for solving the problem] As a result of extensive research in view of the above, the present invention has been completed in the same way as Japanese Patent Application No. 62-311278 filed earlier by the same applicant. The present invention solves the above-mentioned problems by the following means.

(1) In an oxygen sensor element that includes a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side, and detects the oxygen concentration of a gas to be measured.

The solid electrolyte body comprises a base and a spherical protrusion directly connected to the base, and a measurement electrode is provided at a position including at least one spherical protrusion.

The measurement electrode is covered with a porous first protective layer, and the first protective layer is covered with a porous second protective layer, and each of the first and second protective layers is made of a noble metal that promotes the oxidation reaction of the gas to be measured. Supports catalyst.

The first protective layer is made of a metal oxide that is chemically stable to the gas to be measured.

The second protective layer comprises a non-stoichiometric transition metal oxide.

Oxygen sensor element.

(2) An oxygen sensor element consisting of a solid electrolyte body and a spherical protrusion coupled to the base via a measurement electrode (other configurations are the same as in (1) above).

(3) In an oxygen sensor element that includes a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side, and detects the oxygen concentration of a gas to be measured.

The solid electrolyte body comprises a base and a spherical protrusion directly connected to the base, and a measurement electrode is provided at a position including at least one spherical protrusion.

The measurement electrode is covered with a porous protective layer, at least a portion of which is made of a non-stoichiometric transition metal oxide and supports a noble metal catalyst that promotes the oxidation reaction of the gas to be measured.

Oxygen sensor element.

(4) An oxygen sensor element in which the solid electrolyte body has the configuration described in (2) above (other configurations are the same as in (3) above).

(5) In a method for manufacturing an oxygen sensor element that includes a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side and detects the oxygen concentration of a gas to be measured.

Regarding the treatment of one side of the base material of the solid electrolyte body.

At least each of the following steps: (a) Step of attaching spherical particles made of solid electrolyte.

(b) Step of folding the electrode material.

(C) A step of thermally spraying a metal oxide component.

(d) a step of immersing j14 in a metal salt solution; and (e) a step of coating it with a paste-like material made by blending a non-stoichiometric transition metal oxide component and a noble metal component, and then firing it.

It consists of including. A method for manufacturing an oxygen sensor element.

[Preferred Embodiment] The solid electrolyte body may have a bag-like, plate-like, or cylindrical shape as long as it has a closed end and an open rear end, and the solid electrolyte material may be stable to ZrO, for example. It is preferable to use one to which YO, CaO, etc. are added as a curing agent. Both the reference electrode and the measurement electrode (layered) are porous and Pt
Alternatively, a noble metal such as pt containing Rh of about 296 or less may be used.

The other surface of the solid electrolyte body (the surface forming the measurement electrode) is a spherical protrusion made of solid electrolyte. This is because the spherical protrusion penetrates into the measurement electrode and further into the protective layer in a wedge shape, thereby firmly physically bonding the solid electrolyte body and the protective layer. Due to the presence of the spherical protrusions, even if the unfired component adsorbs or reacts with the catalyst in the protective layer and expands in volume, the protective layer is difficult to peel off from the solid electrolyte body, and the durability of the element is increased.

The spherical protrusion may be located outside the measurement electrode.

The spherical protrusion is composed of an aggregate of granulated particles, and the granulated particles are preferably formed in a single layer or in multiple layers on the base surface of the solid electrolyte body. Also, the granulated particles have a particle size of 40 to 100-1, preferably 5
It is preferable to set the thickness to 0 to 80 um. This is to form wedge-shaped unevenness to obtain a strong bond with the protective layer, and if it is less than 40 μl, it will not function well as a wedge, and if it exceeds 100 μl, the adhesion to the base material will be weakened. . The spherical protrusion is
It is preferable to distribute the granulated particles so as to leave concave portions between each granulated particle. In addition to further increasing the bonding force with the protective layer! It can also contribute to expanding the surface performance.

The material of the spherical protrusion is preferably the same as that of the base of the solid electrolyte body, but any solid electrolyte may be used. For example, if the base is Z"02-Y2o3 system 1, the spherical protrusion is ZrO2-
(Cab, Mg0) system, and the base is Zr02-Y203 system 1 spherical protrusion is ZrO with a different Y2O3 content m from the base
It may also be 2-Y2O3-based.

The measurement electrode is covered with a porous first protective layer as described above, and this first protective layer must be covered with a porous second protective layer (configuration (1) above).

The first protective layer protects the unburned components (CO) of exhaust gas during use.
etc.) adsorbs or reacts with the measuring electrode (noble metal), thereby preventing the measuring electrode from expanding in volume and peeling off from the solid electrolyte body. The first protective layer is preferably composed of a ceramic such as A(0, spinel, Bed, ZrO2, etc.) or a mixture thereof, and is particularly preferably composed of spinel as a main component.The porosity is about 5 to 20%, and the thickness is about 100%. ~180 μl, preferably about 150 μl.The thickness of the first protective layer at the tip of the element is larger than the thickness at the rear (for example, 372~2
It is recommended to increase the amount (double). Sensor output becomes irregular when used at low temperatures. This is to suppress the occurrence of so-called "chemical noise" phenomenon and to perform more accurate control even when used at low temperatures. The axial length of the tip portion is preferably selected from a range of 115 to 1/2 of the axial length from the element tip to the element mounting portion. The thicker portion may be made of different materials.

The noble metal catalyst supported on the first protective layer and promoting the oxidation of the unburned components of the gas to be measured is particularly preferably one containing platinum (Pt) as a main component, for example, a catalyst containing Pt of 80wt1% or more. The amount supported is preferably in the range of 0.01 to 5 vt1% based on the total amount of the constituent materials of the first protective layer. There is no effect at less than 0.01 wt%.

This is because if it exceeds 5 vt%, clogging may occur. However, under conditions of exposure to rich exhaust gas, it is preferably lvt% or less.

This is because if the lvt% is exceeded, the unburned components present in large amounts will be adsorbed or reacted with the noble metal catalyst, causing the protective layer to crack. This catalyst can be dispersed uniformly or non-uniformly over the entire area of the protective layer. For example, the content of the noble metal may be increased in the front part of the element where the unburned components of the exhaust gas are large. Further, the material of the catalyst may be made different for each part.

The second protective layer must be composed of a non-stoichiometric transition metal oxide as described above. This is to prevent the noble metal catalyst supported on the first protective layer from scattering during use, causing a λ point shift and a decrease in output. In addition, the second protective layer itself further promotes the oxidation of the unburned components of the exhaust gas through the catalytic action peculiar to the transition metal and the action of the supported catalyst, and its non-stoichiometric nature allows it to oxidize electrons or electrons depending on the amount of oxygen. This is to prevent unburned components from excessively adsorbing to the supported catalyst due to the change in pores, and to maintain stable operation of the supported catalyst over a long period of time. As transition metal oxides,
Any transition metal oxide from Group 3A to Group 8 can be selected as long as it can exhibit the above effects, but Group 4A 2, for example, titanium (
TL), group 8 and others such as cobalt (Co), nickel (N
The oxide i) is preferred. Especially TiOx (x-1
, 8 or more and less than 2, preferably 1°95 or more and less than 2). This is because it can efficiently exhibit the above effects and has excellent heat resistance. The content of titania (T i Ox ) is preferably 50 wt % or more, preferably 70 wt % or more based on the total amount of the constituent materials (excluding the supported catalyst) of the second protective layer. In this case, the remainder may be made of another non-stoichiometric transition metal oxide, but may also be made of a stoichiometric transition metal oxide or the same ceramic material as the first protective layer. The porosity of the second protective layer is preferably greater than that of the first protective layer. This is to prevent deterioration of the permeability of the gas to be measured and the sensor response. For example, it may be set at 8% to 15%, and may be present as open pores (passing pores). Also, from a similar standpoint, the thickness of the second protective layer is preferably thinner than that of the first protective layer. For example, it is preferable to set the thickness to 10μ11 to 50μm.

The amount of noble metal catalyst supported in the second protective layer may be smaller than that in the first layer. This is because the transition metal oxide also has a catalytic effect, and by reducing the supported amount, clogging caused by the catalyst can be prevented and the precious metal can be used effectively.

In particular, if the amount supported is 0.02 to 5 mo1% with respect to the total amount of the second protective layer, the catalytic action can be effectively exhibited and layer peeling due to volume expansion caused by the reaction between the catalyst and the unburned gas can be prevented. Preferably it is 0.1-2 a+o1%.

This catalyst may also be dispersed uniformly or non-uniformly throughout the second protective layer.

Further, when at least a part (specific part) of the protective layer is made of a non-stoichiometric transition metal oxide (the above means (3),
(4)), that is, even if the two layers are not clearly distinguished, the structure may be the same as that of the second protective layer described above. The remaining structure may be the same as that of the first protective layer described above.

The entire protective layer may consist of a non-stoichiometric transition metal oxide. It is preferable that the noble metal catalyst is present in both the specific part and the remainder.

Next, regarding the manufacturing method of the present invention, particularly the treatment step of the other side of the solid electrolyte base material (the side on which the measurement electrode is to be formed),
Preferred aspects and effects will be described.

The solid electrolyte base material is made by mixing raw material powder, calcining it, and then pulverizing it (
2.5 μm or less) and then spray drying to form secondary particles (20 to 150 μm) and forming them into a predetermined shape.

The spherical projections may be formed by attaching spherical particles with an average particle size of 50 to 100 μl to the surface of the solid electrolyte base material, and then firing. This is to ensure that the wedge-shaped unevenness remains sufficiently even after the electrode deposition treatment in the subsequent step, and to obtain a strong bond with the first protective layer. That is, if the spherical particles after firing are less than 40 μm, they cannot function as a wedge sufficiently, and if they exceed 100 μm, the adhesion to the base material becomes weak. More preferably 50-8
It is best to set it to 0μI. Further, finer particles of 10 μm or less may be mixed to further improve the strength. The solid electrolyte base material and the spherical particles may be fired simultaneously. This is to increase the adhesion strength of both. The firing temperature is 140 (1
It is preferable to set the temperature to ~1500°C. Alternatively, one spherical particle may be formed after the electrode deposition treatment. This is particularly effective when a measurement electrode cannot be formed when one spherical particle is deposited first. This ensures reliable protection of the measurement electrode and further enhances the bonding with the protective layer.

In addition to conventional plating processes such as electroplating and chemical plating, the electrodes may be formed by a conventional vapor deposition method such as sputtering, vapor deposition, or screen printing.

The first protective layer can be formed by a number of methods such as brush coating, dipping, spraying, and subsequent baking of a solution or powder of the material, but plasma spraying is particularly preferred. The adhesion strength of the thermal spray powders is strong, and by changing the conditions appropriately, any porosity can be achieved.

This is because the pore diameter can be adjusted.

The catalyst may be supported on the first protective layer by immersion treatment in a noble metal salt solution, followed by drying and baking. The concentration of the solution is preferably determined from the standpoint of sufficiently dispersing the catalyst and preventing clogging in terms of impregnation. For example, when the catalyst is Pt, a solution in which Pt is sufficiently dispersed is H2
PtC,g. There is a solution whose pt concentration is 0. O1~5
g/j! It is better to make it . PT concentration is 0.01g#! If it is less than 5g#, the catalytic action will be insufficient. 1st
This is because the pores in the protective layer become clogged, resulting in poor sensor response. The immersion treatment is preferably carried out under reduced pressure or increased pressure. The noble metal-containing salt solution is immersed deep into the first protective layer,
Therefore, the noble metal catalyst can be uniformly dispersed within the first protective layer. The firing temperature is preferably 400 to 700°C.

The second protective layer must be formed by covering the first protective layer with a paste made of a protective layer material and a noble metal component, and then firing. Maintenance: By forming the J layer and supporting the catalyst at the same time, the catalyst is more strongly supported, preventing scattering during use, and stably exhibiting the catalytic action over a long period of time. Moreover, by forming the paste into a paste-like material, the binder and the like are scattered during firing, making it possible to easily obtain the desired porosity and pore diameter. A paste-like material is obtained by blending a binder, a solvent, etc. in the usual manner. Coating methods include brush application,
Any method such as dipping or spraying may be used. However, plasma spraying is not suitable. This is because sintering of the protective layer material progresses during thermal spraying, making it impossible to obtain pores in a desired state (particularly with high porosity). Further, the combination of the protective layer material and the supported catalyst is preferably carried out by impregnating the protective layer material powder with a noble metal salt solution. This is to ensure homogeneous blending. In addition to transition metal oxides, protective layer materials include
It may also be a compound that can form the transition metal oxide by thermal decomposition, such as a hydroxide or a salt. The powder particle size is preferably 2 μm or less. This is because the sinterability is improved and the adhesion strength is increased, so that the second protective layer becomes difficult to peel off during use. Preferably it is 0.3 to 1.5 μm. The heat treatment temperature is preferably 700 to 900°C in a non-oxidizing atmosphere.

[Example] Two embodiments of the present invention will be described below.

1 to 3 show one embodiment, and in each figure, numeral 1 represents an oxygen sensor element.

This element 1 includes a solid electrolyte body 2 that can cause a difference in oxygen concentration between a reference gas and a gas to be measured (exhaust gas), and a pair of porous electrodes (inner electrodes) formed on the inner and outer surfaces of the solid electrolyte body 2.3. (Outer electrode) 4.

It is composed of a porous protective layer 5 covering the outer electrode 4, and a noble metal catalyst 6 supported on the protective layer S in a uniformly dispersed manner. Here, the solid electrolyte body 2 is made of Z r O2 with Y2O3 added, the electrodes 3.4 are both PT main electrodes, and the noble metal catalysts 6 are made of PT particles.

The solid electrolyte body 2 consists of a base 2a and a spherical protrusion 2b located on the outer surface thereof, and an outer electrode 4. Furthermore, a protective layer 5 is formed. Further, the protective layer 5 includes a first protective layer 5a located further inside and directly covering the outer electrode 4, and a second protective layer 5b located further outside and exposed to exhaust gas. Both protective layers 5a,
5b both support a pt catalyst 6... here.

The first protective layer 5a is made of spinel, and the second protective layer 5b is made of titania.

In FIG. 1, 7 is a housing, 8 is a caulking ring, 9 is a filler, and 10 is a protective tube.

FIG. 4 shows another embodiment, that is, an example of a plate-shaped oxygen sensor element, and the rest is the same as the previous embodiment.
Identical components are given the same reference numerals and their explanations will be omitted.

Next, an example of manufacturing the oxygen sensor element of the present invention will be described. Perform each of the following steps in sequence.

Step 1: Z r 02 with a purity of 99% or more and a purity of 99.9
After adding 511% of Y2O3 and mixing, 1
Calculate at 300℃ for 2 hours.

Step 2: Add water and mill the particles wet in a ball mill.
% is ground to a particle size of 2.5 μm or less.

Step 3: Add a water-soluble binder and spray dry to obtain spherical granulated particles with an average particle size of 70M.
- (1) 3 Step 4: The powder obtained in (1) 3 is rubber pressed to form a desired tube shape (test tube shape), dried, and then ground into a predetermined shape using a grindstone.

Step 5: A slurry prepared by adding a water-soluble binder cellulose sodium glycolate and a solvent to the granulated particles obtained in (1) 3 is attached to the outer surface.

Step 6: After drying, it is fired at 1500°C x 2 firs.

The axial length of the part corresponding to the detection part is 25 mm.
The outer diameter was about 5 mm and the inner diameter was about 3 mm.

Step 7: Apply a PT layer to the inner and outer surfaces to a thickness of 0 using chemical plating.
．． The plate was precipitated to 9 μι, and then baked at 1000°C.

Step 8: M g O-A I! 203 (spinel) powder is plasma sprayed to form a first protective layer having a thickness of about 150 mm.

Step 9: Pt is 0.05g/, i! of HPtC, gB solution and leave it for about 5 minutes under reduced pressure of 50 to 100 ml.

Step 1O: Dry After drying, gold noble metal-containing titania paste 1
By applying it to the surface of the protective layer and baking it in a reducing atmosphere at 800°C, it forms a layer with a thickness of about 25μ with pores of about 2μ.
(2) Form the second protective layer. The above paste is made by soaking titania powder in H2PtC, C6 liquid or Pt black.

It is obtained by drying and impregnating it while stirring, and then adding an organic binder and a solvent (butylgarbidol).

Furthermore, using the oxygen sensor element l manufactured in this way,
Oxygen sensor A was obtained through the following steps.

Step 11: After inserting the element 1 into the housing 7.

Insert the caulking ring 8 and filler 9 such as talc, and make the base T.
-1 is fixed in the housing 7.

Step 12: Placing a protective layer 10 covering the tip of the element,
The tip of the housing 7 and the rear end of the protection tube 10 are welded.

Step 13: Connect terminals and lead wires (not shown) to the ns pole, and cover with an outer cylinder (not shown) to obtain an oxygen sensor.

[Test Example 1 The following tests were conducted based on the oxygen sensor element of the present invention according to the above-mentioned example, and each evaluation item was investigated. Comparative examples were also examined in the same manner.

Tests 1 and 2 Oxygen sensor elements formed by varying the noble metal catalyst source used in step IO and the amount of catalyst supported on the second protective layer were subjected to a durability test using a Bunsen burner. In test 1, the T19 section of each oxygen sensor element (
In test 2, air was introduced and the T19 part was heated to approximately 850"C1 in an almost complete combustion state.
,: Heat and make it durable for 50011rs.

Evaluation item A: An oxygen sensor comprising the above-mentioned heated oxygen sensor element is attached to a combustion tube (inner diameter width 43), and burner flame is blown from a position 1 m away to evaluate sensor responsiveness.

Evaluation item B: Similarly, after heating, the oxygen sensor is installed at a predetermined position on the actual engine vehicle, and the sensor is controlled to determine the temperature of λ located further downstream.
Check the scan value (control A/F average value) and evaluate the λ characteristic.

Evaluation item C: Evaluate the condition of the element surface by visual inspection.

These results are shown in Table 1.

Table 1 This Comparative Example As is clear from Table 1, the oxygen sensor element (and oxygen sensor) according to the example had a higher performance in each test 1 than the comparative example.
, 2 showed excellent results for each evaluation item A, B, and C. In addition, the amount of catalyst supported on the second protective layer was 8.0I.
It can be recognized that lo is preferably less than 1%.

Test 3 Using an oxygen sensor element in which the amount of each catalyst supported in the first and second protective layers is varied, the frequency (Hz) during sensor control is measured as its initial characteristic (before durability) [Evaluation item D] .

The oxygen sensor related to the above element was installed in a predetermined position on an actual engine vehicle, and was subjected to durability for 200 hours at A/FIO (rich atmosphere) and exhaust gas temperature of 700°C, and the above evaluation item B
, C was also investigated.

Table 2 Table 2 also supports the excellent results of the two Examples.

Furthermore, it can be recognized that under conditions of exposure to such rich exhaust gas, the amount of catalyst supported in the first protective layer is preferably less than 1 νt%.

[Effects] As described above, according to the present invention, the solid electrolyte body and the protective layer are firmly bonded due to the presence of one spherical protrusion, so peeling of the protective layer can be prevented and the durability is excellent. In addition, excessive adsorption and reaction of unfired components to precious metals can be suppressed by the second protective layer (or at least a part of the protective layer, the same applies hereinafter), resulting in excellent sensor response and λ characteristics, and high precision. Air-fuel ratio control can be maintained. Since the second protective layer prevents the catalyst supported on the first protective layer from scattering, the amount of catalyst supported on the first protective layer can be increased, and its catalytic action can maintain even more precise air-fuel ratio control. Furthermore, in addition to the supported noble metal catalyst in the first and second protective layers, the second protective layer itself also has a catalytic effect that promotes the oxidation reaction of unburned components in the gas to be measured, so the entire protective layer shares the catalytic effect. This makes it possible to suppress volume expansion due to the reaction between the catalyst and the unburned components as much as possible, which also contributes to improved durability. Moreover,
Even if the noble metal in the second protective layer sublimes, the supported catalyst in the first protective layer and the second protective layer itself can continue to exert sufficient catalytic action.

Therefore, the present invention makes effective use of expensive precious metals and can efficiently oxidize the unburned components in the gas to be measured, making it possible to stably maintain high-precision sensor control, thus making it suitable for use in the field of oxygen sensors. It is extremely useful.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an embodiment of the oxygen sensor element and oxygen sensor of the present invention. FIG. 2 is an enlarged sectional view of FIG. 1. 3 shows an enlarged sectional view of FIG. 2, and FIG. 4 shows a half sectional view showing another embodiment of the oxygen sensor element of the present invention. A...Oxygen sensor 1...Oxygen sensor element 2...
・Solid electrolyte body 2a... Base 2b... Spherical protrusion 3... Reference electrode 4... Measuring electrode 5... Protective layer 5a... First protective layer 5b... Second protective layer 6.
...Catalyst applicant: NGK Spark Plug Co., Ltd. Agent: Patent attorney Asamichi Kato (1 other person) Figure 1 Figure 2 Figure 3 Above the circle / Procedural amendment (voice) March 15, 1988

Claims (9)

[Claims] (1) In an oxygen sensor element that has a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side and detects the oxygen concentration of a gas to be measured, the solid electrolyte body has a base and a spherical protrusion directly connected to the base. comprising a measuring electrode at a position including at least the spherical protrusion, the measuring electrode is covered with a porous first protective layer, the first protective layer is covered with a porous second protective layer, and the first protective layer is covered with a porous second protective layer; , each of the two protective layers supports a noble metal catalyst that promotes the oxidation reaction of the gas to be measured, the first protective layer is made of a metal oxide that is chemically stable to the gas to be measured, and the second protective layer is made of a non-chemically Oxygen sensor element made of stoichiometric transition metal oxide. (2) In an oxygen sensor element that has a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side and detects the oxygen concentration of a gas to be measured, the solid electrolyte body is connected to the base and the base through the measurement electrode. The measurement electrode is covered with a porous first protective layer, the first protective layer is covered with a porous second protective layer, and the first and second protective layers each contain a gas to be measured. The first protective layer is made of a metal oxide that is chemically stable against the gas to be measured, and the second protective layer is made of a non-stoichiometric transition metal oxide. An oxygen sensor element. (3) In an oxygen sensor element that has a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side and detects the oxygen concentration of a gas to be measured, the solid electrolyte body has a base and a spherical protrusion directly connected to the base. comprising a measuring electrode at a position including at least the spherical protrusion, the measuring electrode is covered with a porous protective layer, at least a part of the protective layer is made of a non-stoichiometric transition metal oxide, and the measuring electrode is covered with a porous protective layer; An oxygen sensor element that supports a noble metal catalyst that promotes the oxidation reaction of the measurement gas. (4) In an oxygen sensor element that has a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side and detects the oxygen concentration of a gas to be measured, the solid electrolyte body is connected to the base and the base via the measurement electrode. The measuring electrode is covered with a porous protective layer, at least a part of which is made of a non-stoichiometric transition metal oxide and a noble metal that promotes the oxidation reaction of the gas to be measured. An oxygen sensor element that supports a catalyst. (5) The oxygen sensor element according to any one of claims 1 to 4, wherein the transition metal oxide is titania. (6) The oxygen sensor element according to any one of claims 1 and 2, wherein the amount of the catalyst supported on the second protective layer is 0.02 to 5 mol% (calculated as noble metal) based on the material of the second protective layer. . (7) In a method for manufacturing an oxygen sensor element that includes a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side and detects the oxygen concentration of a gas to be measured, one side of the base material of the solid electrolyte body is Regarding the treatment, at least the following steps: (a) A step of attaching spherical particles made of a solid electrolyte. (b) a step of folding the electrode material; (c) a step of thermal spraying the metal oxide component; (d) a step of immersion in a noble metal salt solution; and (e) a non-stoichiometric transition metal oxide component. 1. A method for manufacturing an oxygen sensor element, comprising the steps of: coating with a paste made of a mixture of and a noble metal component, and then firing. (8) The manufacturing method according to claim 7, wherein in step (a), the solid electrolyte base material and the spherical particles are co-fired. (9) The manufacturing method according to claim 7, wherein in the step (d), the immersion is performed under reduced pressure or increased pressure.