JP7351783B2 - Manufacturing method of gas sensor element - Google Patents

Description

The present invention relates to a method for manufacturing a gas sensor element that measures the concentration of gas.

Methods for manufacturing gas sensor elements have been developed. Patent Document 1 discloses that a plurality of ceramic sheets are laminated and fired to produce an element body, and then a paste-like tip protection layer forming material is applied to the element body and fired to form the tip protection layer. A method of forming is disclosed.

Patent No. 5387550

However, such a method requires multiple firings, making the process complicated. Further, the adhesion between the tip protection layer and the element main body is not necessarily good.

An object of the present invention is to provide a method for manufacturing a gas sensor element that can simplify the process and improve the adhesion of the tip protective layer.

A method for manufacturing a gas sensor element according to one aspect includes: an element substrate having a solid electrolyte; a tip protection layer covering at least a portion of the element substrate; and a buffer layer disposed between the element substrate and the tip protection layer. , a step of laminating sheets containing the solid electrolyte to form a laminate, and a step of forming a buffer layer pattern containing a buffer layer forming substance on the laminate. , a step of firing the laminate on which the buffer layer pattern is formed to produce a fired body with a buffer layer formed thereon, and a step of forming a tip protection layer on the buffer layer of the fired body.

According to the present invention, it is possible to provide a method for manufacturing a gas sensor element that can simplify the process and improve the adhesion between the tip protection layer and the element body (element base).

FIG. 3 is a vertical cross-sectional view along the longitudinal direction of the gas sensor element. FIG. 3 is a schematic sectional view of a cross section perpendicular to the longitudinal direction of the gas sensor element. It is a figure showing the manufacturing method of the gas sensor element concerning an embodiment. 7 is a diagram showing a method for manufacturing a gas sensor element according to comparative example A. FIG. It is a graph showing decomposition temperature. 3 is a SEM photograph of the surface of a buffer layer of a gas sensor element according to an example and a comparative example. 3 is a cross-sectional SEM photograph of the vicinity of the buffer layer (before formation of a tip protection layer) of gas sensor elements according to Examples and Comparative Examples. 3 is a cross-sectional SEM photograph of the vicinity of the buffer layer (after formation of the tip protection layer) of the gas sensor elements according to Examples and Comparative Examples.

<Overview of sensor element>
FIG. 1 schematically shows the structure of a gas sensor element (hereinafter also simply referred to as a sensor element) 100 according to this embodiment. Here, a vertical cross section along the longitudinal direction of the sensor element 100 is shown. The sensor element 100 is a gas sensor element, and is a main component of a gas sensor (not shown) for detecting a predetermined gas component in a gas to be measured and measuring its concentration.

In the sensor element 100 shown in FIG. 1, various components are provided on the outside and inside of the long plate-shaped element base 10. The element substrate 10 is formed by laminating a plurality of solid electrolyte layers (for example, zirconia which is an oxygen ion conductive solid electrolyte; for example, a ceramic whose main component is yttrium-stabilized zirconia). The solid electrolyte preferably contains 50 wt% or more of zirconia (that is, contains zirconia as a main component), and more preferably contains 70 wt% or more. Note that the configuration of the sensor element 100 shown in FIG. 1 is an example, and the specific configuration is not limited to this.

The sensor element 100 shown in FIG. 1 has internal cavities 102, 103, and 104, which are internal spaces provided inside the element base 10 (so-called series three-chamber structure type). Gas inlet 105 opens to the outside on the end E1 side of ceramic body 101 (strictly speaking, communicates with the outside via tip protection layer 180). The internal cavity 102 communicates with the gas inlet 105 through diffusion control sections 110 and 120. The internal cavity 103 communicates with the internal cavity 102 through the diffusion control section 130. The internal cavity 104 communicates with the internal cavity 103 through the diffusion control section 140.

Note that the path from the gas inlet 105 to the internal cavity 104 is referred to as a gas flow section. In the sensor element 100 according to the present embodiment, the gas flow section is provided in a straight line along the longitudinal direction of the element base 10.

Each of the diffusion rate controlling parts 110, 120, 130, and 140 is provided as two slits, upper and lower, as viewed in the drawing, and provides a predetermined diffusion resistance to the gas to be measured passing therethrough. Note that a buffer space 115 is provided between the diffusion rate controlling parts 110 and 120 to buffer the pulsation of the gas to be measured.

An outer pump electrode 141 is provided on the outer surface of the element substrate 10, and an inner pump electrode 142 is provided in the internal cavity 102. The internal cavity 103 is provided with an auxiliary pump electrode 143. A reference gas inlet 106 is provided on the end E2 side of the element substrate 10 to communicate with the outside and introduce a reference gas. A reference electrode 147 is provided within the reference gas inlet 106 in the element substrate 10 .

For example, when the measurement target of the sensor element 100 is NOx, the NOx gas concentration in the gas to be measured is calculated by the following process.

The gas to be measured introduced into the internal cavity 102 is adjusted to have an approximately constant oxygen concentration by the pumping action (pumping in or out of oxygen) of the main pump cell P1, and then introduced into the internal cavity 103. The main pump cell P1 is an electrochemical pump cell composed of an outer pump electrode 141, an inner pump electrode 142, and a ceramic layer 101a (a part of the ceramic body 101 existing between both electrodes). In the internal chamber 103, oxygen in the gas to be measured is pumped out to the outside of the element by the pumping action of the auxiliary pump cell P2, which is also an electrochemical pump cell, resulting in a low oxygen partial pressure state. The auxiliary pump cell P2 includes an outer pump electrode 141, an auxiliary pump electrode 143, and a ceramic layer 101b (a part of the ceramic body 101 existing between both electrodes).

The outer pump electrode 141, the inner pump electrode 142, and the auxiliary pump electrode 143 are formed as porous cermet electrodes (for example, cermet electrodes of Pt and ZrO ₂ containing 1% Au). Note that the inner pump electrode 142 and the auxiliary pump electrode 143 that come into contact with the gas to be measured are formed using a material that has a weakened ability to reduce NOx components or has no ability to reduce NOx components.

NOx in the gas to be measured, which has been brought into a low oxygen partial pressure state by the auxiliary pump cell P2, is reduced or decomposed at the measurement electrode 145 in the internal cavity 103. The measurement electrode 145 is a porous cermet electrode and functions as a catalyst for reducing NOx. During the reduction or decomposition, the potential difference between the measuring electrode 145 and the reference electrode 147 is kept constant.

Oxygen ions generated by reduction or decomposition are pumped out of the element by the measurement pump cell P3. The measurement pump cell P3 includes an outer pump electrode 141, a measurement electrode 145, and a ceramic layer 101c (a part of the ceramic body 101 existing between both electrodes). The measurement pump cell P3 pumps out oxygen generated by decomposition of NOx.

The sensor element 100 detects the pump current Ip2 flowing between the measurement electrode 145 and the outer pump electrode 141 depending on the amount of oxygen pumped out. The NOx concentration is determined based on the linear relationship between the current value of the pump current Ip2 (NOx signal) and the NOx concentration.

Pumping (pumping in or out of oxygen) in the main pump cell P1, the auxiliary pump cell P2, and the measurement pump cell P3 is realized in the same way as in the sensor element 100. That is, a voltage is applied between electrodes provided in each pump cell by a variable power source (not shown) that is a component of the gas sensor. In the case of the measurement pump cell P3, a voltage is applied between the outer pump electrode 141 and the measurement electrode 145 so that the potential difference between the measurement electrode 145 and the reference electrode 147 is maintained appropriately. A variable power supply is typically provided for each pump cell.

A heater 150 is embedded inside the element base 10. The heater 150 is provided on the lower side of the gas flow section in FIG. 1 in the drawing view, in a range from near the end E1 to near the formation positions of the measurement electrode 145 and the reference electrode 147. Heater 150 heats sensor element 100 during use, increasing the oxygen ion conductivity of the solid electrolyte. At this time, the temperature near the internal cavity 102 is, for example, about 800°C to 850°C. The heater 150 is a resistance heating element made of platinum, for example, and is covered with an insulating layer 151.

Hereinafter, of the two main surfaces (outer surfaces) of the element substrate 10, the upper main surface and the lower main surface in the drawing in FIG. 1 may be referred to as a pump surface and a heater surface, respectively. The pump surface includes a main pump cell P1, an auxiliary pump cell P2, and a measuring pump cell P3, and is adjacent to the gas inlet 105, the internal cavities 102-104, and each pump cell. The heater surface includes a heater 150.

A plurality of electrode terminals 160 are formed on the main surface of the element base 10 on the end E2 side to electrically connect the sensor element 100 to the outside. Four electrode terminals 160 (160a to 160d) are provided on the pump surface side, and four electrode terminals 160 (160e to 160h) are provided on the heater surface side. These electrode terminals 160 are electrically connected to the five electrodes described above, both ends of the heater 150, and a lead wire for detecting heater resistance (not shown) through lead wires (not shown) in the element substrate 10. Application of voltage to each pump cell in the sensor element 100 and heating of the heater 150 are performed through the electrode terminal 160.

In the sensor element 100, the element base 10 including the internal space (internal cavity, reference gas space, etc.), the various electrodes and lead wires in the element base 10, the electrode terminal 160, the heater 150, and the insulating layer 151 are connected to the sensor element. It is called the main part.

The pump surface and heater surface of the element substrate 10 are provided with outer (pump) electrode protective layers 170 (170a, 170b). The outer (pump) electrode protective layer 170 is made of alumina, has a thickness of about 5 μm to 30 μm, and is a relatively dense layer with a lower porosity than the buffer layer 190. The outer (pump) electrode protective layer 170 prevents foreign matter and poisonous substances from adhering to the surface of the element substrate 10 and the outer pump electrode 141 provided on the pump surface side.

In FIG. 1, an outer (pump) electrode protective layer 170 is provided on substantially the entire surface of the pump surface and the heater surface, excluding a portion of the electrode terminal 160. This is just an example, and it may be unevenly distributed on the end E1 side rather than in FIG. Further, the formation of the outer (pump) electrode protective layer 170b on the heater surface side may be omitted.

A tip protection layer 180 is provided at the outermost periphery of the element base 10 in a predetermined range from the end E1. A buffer layer 190 is interposed between the outer (pump) electrode protection layer 170 and the tip protection layer 180.

FIG. 2 is a schematic cross-sectional view perpendicular to the longitudinal direction of the sensor element 100, showing the arrangement of the element base 10, the outer (pump) electrode protective layer 170, the tip protective layer 180, and the buffer layer 190. However, in FIG. 2, the electrodes and internal spaces are not shown.

As can be seen from FIGS. 1 and 2, the tip protection layer 180 covers the entire end E1 of the element base 10 and a part of the side surface of the element base 10 (a predetermined range in the longitudinal direction of the element). It is not limited to the pump side and the heater side.

As shown in FIGS. 1 and 2, the tip protection layer 180 can have a two-layer structure of tip protection layers 181 and 182. It is also possible for the tip protection layer 180 to have a single layer structure, and the case where the tip protection layer 180 has a single layer structure will be described first.

In the case of a single layer structure, the tip protection layer 180 is, for example, a porous layer made of alumina with a purity of 99.0% or more, and prevents so-called water cracking of the sensor element 100. Water cracking refers to cracks that occur when water droplets adhere to the sensor element 100 during use (for example, water vapor in exhaust gas condenses and becomes water droplets). Since the sensor element 100 is heated to a high temperature by the heater 150, a thermal shock occurs due to adhesion of water droplets. As a result, there is a possibility that cracks will occur in the sensor element 100, particularly in the element base 10 (the element base 10 will break).

It is sufficient that the tip protection layer 180 is provided not over the entire sensor element 100 but within a predetermined range from the end E1 where water droplets may adhere. The tip protection layer 180 is formed, for example, in a range of about 0 mm to 25 mm from the tip in the longitudinal direction of the element. Depending on the configuration of the sensor element 100, the tip protection layer 180 may be formed closer to the end E2.

The tip protection layer 180 has a thickness of 200 μm or more and 1800 μm or less. If the thickness is less than 200 μm, the strength of the tip protection layer 180 itself cannot be sufficiently ensured. There is also a possibility that the pores in the tip protection layer 180 are connected and penetrate the tip protection layer 180, and water vapor in the gas to be measured reaches the outer (pump) electrode protection layer 170 (and further, the element substrate 10). On the other hand, if the thickness is greater than 1800 μm, it becomes difficult for the gas to be measured to pass through the tip protection layer 180 and reach the gas inlet 105, resulting in poor responsiveness of the gas sensor element 100. It is also disadvantageous from a cost standpoint.

The porosity of the tip protection layer 180 is preferably about 15% to 80%. This is because it does not affect the ease and uniformity of manufacturing, and also the intake of the gas to be measured from the gas inlet 105 into the element substrate 10 . However, it may be outside this range as long as water cracking is suitably suppressed and the responsiveness of the sensor element 100 is not affected.

Hereinafter, a case where the tip protection layer 180 has a two-layer structure of tip protection layers 181 and 182 will be described.

In this case, the tip protection layers 181 and 182 are provided in order from the inside so as to cover the tip surface 101e on the end E1 side of the element base 10 and the four side surfaces (on the outer periphery of the element base 10 on the end E1 side). It will be done. Of the tip protection layer 181, the portion on the tip surface 101e side is particularly referred to as the tip portion 221, and the portions on the pump surface side and the heater surface side are particularly referred to as the main surface portion 222. Similarly, the portion of the tip protection layer 182 on the tip surface 101e side is particularly referred to as the tip portion 231, and the portions on the pump surface side and the heater surface side are particularly referred to as the main surface portion 232.

The tip protection layer 181 is made of alumina, has a porosity of 30% to 80%, and has a thickness 30 to 50 times the thickness of the buffer layer 190. The tip protection layer 182 is made of alumina, has a porosity of 15% to 30%, and is 5 to 10 times thicker than the buffer layer 190. A tip protection layer 181 with low thermal conductivity is coated on a tip protection layer 182 with low porosity. Note that the sum of the thickness of the distal end portion 221 of the distal end protection layer 181 and the thickness of the distal end portion 231 of the distal end protection layer 182 is the sum of the thickness of the main surface portion 222 of the distal end protection layer 181 and the thickness of the main surface portion 232 of the distal end protection layer 182. larger than

The tip protection layer 181 has a relatively low thermal conductivity and suppresses heat conduction from the outside to the element substrate 10. On the other hand, the tip protection layer 182 maintains the overall strength and prevents water from entering inside. As a result, even if water adheres to the surface of the tip protective layer 180 (tip protective layer 182) of the high-temperature sensor element 100 (when in use), water is prevented from entering the interior, and the cold heat (quenching) caused by the adhesion is suppressed. It becomes difficult for the light to be transmitted to the element substrate 10. That is, the tip protection layer 180 has excellent thermal shock resistance, and the sensor element 100 is less susceptible to water cracking and has excellent water resistance.

Here, the thickness of the tip protection layer 181 is 30 to 50 times the thickness of the buffer layer 190, and the thickness of the tip protection layer 182 is 5 to 10 times the thickness of the buffer layer 190.

The larger the thickness of the tip protection layers 181 and 182, the more difficult it is for thermal shock to be transmitted to the element substrate 10, and the water resistance of the sensor element 100 is improved. However, if this thickness is made too large, when the heater 150 heats the entire sensor element 100 including the tip protection layer 180, the thermal load on the heater 150 becomes large, which may cause the sensor element 100 to crack. For this reason, the thickness of the tip protection layer 181 is preferably 1500 μm or less, and the thickness of the tip protection layer 182 is preferably 300 μm or less.

The tip protection layers 181 and 182 are formed, for example, by sequentially thermal spraying (plasma spraying) the respective constituent materials onto the element substrate 10 on which the buffer layer 190 is formed. An anchor effect is created between the buffer layer 190 and the tip protection layer 181, and the adhesion (adhesion) of the tip protection layer 181 (including the tip protection layer 182 formed on the outside) to the buffer layer 190 is ensured. . The buffer layer 190 ensures adhesiveness (adhesion) with the tip protection layer 181. By ensuring adhesiveness (adhesion), peeling of the tip protective layer 180 from the element substrate 10 due to thermal shock due to adhesion of water droplets is suppressed.

Note that the tip protection layers 181 and 182 do not cover the entire buffer layer 190 (190a, 190b). The tip protection layers 181 and 182 expose an end of the buffer layer 190 on the side opposite to the end E1 side in the longitudinal direction of the sensor element 100. This is to more reliably ensure the adhesion (adhesion) of the tip protection layer 181 (including the tip protection layer 182 formed on the outside) to the buffer layer 190.

The buffer layer 190 prevents the tip protection layer 180 from peeling or breaking when the sensor element 100 is used due to the difference in coefficient of thermal expansion between the alumina that makes up the tip protection layer 180 and the zirconia that makes up the element substrate 10. prevent. The buffer layer 190 ensures adhesion (adhesion) with the tip protection layer 180 formed thereon.

When heated by the heater 150, the element substrate 10 (made of zirconia with a large coefficient of thermal expansion) thermally expands, and tensile stress is applied to the tip protection layer 180. If the buffer layer 190 is not provided, the tip protection layer 180 may be destroyed by this tensile stress. The buffer layer 190 alleviates the difference in thermal expansion between the element substrate 10 and the tip protection layer 180, and suppresses breakage of the tip protection layer 180.

Like the outer (pump) electrode protection layer 170, the buffer layer 190 is formed on the pump surface side and the heater surface side of the element substrate 10 in an area slightly wider than the area where the tip protection layer 180 exists. That is, the buffer layer 190 includes a buffer layer 190a on the pump surface side and a buffer layer 190b on the heater surface side. In order to reduce the difference in thermal expansion between the element substrate 10 and the tip protection layer 180, a buffer layer 190 may be formed in correspondence with the tip protection layer 180. That is, the buffer layer 190 covers a part of the side surface of the element substrate 10 (a predetermined range in the longitudinal direction of the element), and is not limited to the pump surface side and the heater surface side of the element substrate 10.

In addition to (1) alleviating the difference in thermal expansion between the element substrate 10 and the tip protection layer 180, the buffer layer 190 is also required to have (2) adhesion to the tip protection layer 180, and (3) gas permeability. It will be done. As described below, these (1) to (3) can be improved by using a material with a lower decomposition temperature Td than the organic binder in the paste or sheet as the pore-forming material in the paste for the buffer layer 190. .

Like the tip protection layer 180, the buffer layer 190 is a porous layer made of alumina or the like, and has a thickness of 8 μm to 50 μm.

<Sensor element manufacturing process>
A process for manufacturing the sensor element 100 will be explained. FIG. 3 shows the flow of processing when manufacturing such a sensor element 100.

1. Preparation of laminate (steps S1 to S4)
Hereinafter, a laminate is produced according to the steps (a) to (d).

(a) Six or more blank sheets (green sheets on which no pattern is formed, not shown) are prepared (step S1). Here, the element substrate 10 of the sensor element 100 has a structure in which a plurality of solid electrolyte layers are stacked. For example, when producing the element substrate 10 consisting of six solid electrolyte layers, six blank sheets are prepared corresponding to each layer. Note that the thickness of the blank sheets does not need to be all the same.

A blank sheet is provided with a plurality of sheet holes used for positioning during printing and lamination. The sheet holes are formed in advance by punching using a punching device or the like in the blank sheet prior to pattern formation. In addition, in the case of a green sheet in which the corresponding layer has an internal space, a through part corresponding to the internal space is also provided in advance by a similar punching process or the like.

(b) Print various patterns on each blank sheet and dry them (Step S2). These patterns include, for example, various pump electrodes, measurement electrodes 145, reference electrodes 147, and other electrode patterns, patterns for the outer (pump) electrode protective layer 170, patterns for the heater 150 and the insulating layer 151 that covers its surroundings, and illustrations are omitted. This is the internal wiring pattern.

At the timing of pattern printing, a sublimable material is also applied or arranged to form the diffusion-limiting section 110, the diffusion-limiting section 120, and the diffusion-limiting section 130.

A pattern-forming paste is applied to a blank sheet using screen printing technology (pattern printing). Thereafter, it can be dried using an appropriate drying means.

(c) Adhesive paste (adhesive) is printed on each green sheet and dried (step S3). Adhesive paste is applied to the green sheet using screen printing technology (adhesive printing). Thereafter, it can be dried using an appropriate drying means.

(d) The green sheets coated with adhesive are laminated and pressed together to form one laminate (step S4). For example, green sheets are stacked and held in a jig (a stacking jig), with the green sheets positioned using sheet holes, and the jig is heated and pressurized using a hydraulic press or the like. Appropriate conditions are determined for the pressure, temperature, and time of heating and pressurization so that good lamination can be achieved.

2. Formation of buffer layer pattern (step S5)
A paste for forming the pattern of the buffer layer 190 (buffer layer paste) is printed on a predetermined range on the upper and lower surfaces of the laminate and dried (step S5).

The buffer layer paste is a mixture of at least a ceramic powder, a pore-forming material, an organic binder, and a solvent. The buffer layer paste may contain at least one of a plasticizer and a dispersion aid.

The ceramic powder is a material for forming the buffer layer 190 of the sensor element 100 finally obtained. The material of the ceramic powder is not particularly limited, and examples thereof include zirconium, aluminum, yttrium, magnesium, silicon oxides, nitrides, carbides, composite oxides, and mixtures thereof. The buffer layer forming material (or the fabricated buffer layer 190) preferably contains 50 wt% or more of alumina (that is, has alumina as a main component), and more preferably 80 wt% or more.

The pore-forming material is a resin material that disappears by firing and forms pores in the buffer layer 190. As the pore-forming material, it is preferable to use spherical powder (particles) whose diameter (D50) is about twice or less the printed film thickness of the buffer layer pattern before firing. Note that the structure of the particles may be hollow or solid. Note that the diameter (D50) is a median diameter, and means a particle diameter (diameter) at which the cumulative frequency of particle diameters in the powder (pore-forming material) is 50%.

The pore-forming material may be any component as long as it is burned out during firing, and includes polymeric materials (acrylic, butyral, epoxy, polyester, etc.).

As the organic binder, for example, acrylic, butyral, or epoxy binders can be used alone or in a mixed state. Further, polyols such as isocyanates and ethylene glycol can also be used.

As will be described later, the decomposition temperature Td of the pore-forming material and the organic binder is preferably lower than that of the latter.

An example of such a combination is a combination in which the pore-forming material is an acrylic-based material and the organic binder is a butyral-based material.

The solvent is not particularly limited as long as it dissolves the organic binder, but includes ethers (ethylene glycol monoethyl ether, butyl carbitol, butyl carbitol acetate, etc.), alcohols (isopropanol, 1-butanol, ethanol, 2-butyl carbitol, etc.). Ethylhexanol, terpineol, ethylene glycol, glycerin, etc.), ketones (acetone, methyl ethyl ketone, etc.), esters (butyl acetate, dimethyl glutarate, triacetin, etc.), polybasic acids (glutaric acid, etc.) may be used.

The plasticizer is not particularly limited as long as it imparts plasticity to the organic binder used, but phthalate esters, adipate esters, polyesters, and the like can be used.

The dispersion aid is not particularly limited as long as it can uniformly disperse the ceramic powder in the organic solvent, but examples include polycarboxylic acid copolymers, polycarboxylic acid salts, sorbitan fatty acid esters, and polyglycerin fatty acid esters. , a phosphate ester salt copolymer, a sulfonate salt copolymer, a polyurethane polyester copolymer having a tertiary amine, and the like can be used.

Whether or not the plasticizer and dispersion aid should be added and the amount to be added may be determined depending on the balance between their function and formability with the ceramic powder and organic binder.

3. Firing (steps S6, S7)
The element body is cut out (step S6) and fired at a firing temperature of about 1300° C. to 1500° C. (step S7). The solid electrolyte layer, electrodes, and buffer layer 190 are fired. Since there is no step of separately firing the solid electrolyte layer, the manufacturing process is simplified. Further, by integrally firing, the adhesion strength of the buffer layer 190 of the sensor element 100 can be ensured.

4. Formation of tip protection layer 180 (step S8)
A tip protection layer 180 is formed on the cut out element body (step S8). The tip protection layer 180 is formed by a method such as plasma spraying, spray coating, gel casting, or dipping. In either case, the thickness (film thickness) of the tip protection layer 180 can be easily controlled. Note that in spray coating, gel casting, and dipping, baking is performed as necessary. Plasma spraying does not require baking.

The obtained sensor element 100 is housed in a predetermined housing and incorporated into a main body (not shown) of a gas sensor.

In the case of plasma spraying, the sprayed film that is formed contains pores, and the porosity can be controlled by adjusting output power, irradiation angle, properties of powder raw materials, etc. Furthermore, in the case of gel casting and dipping that use alumina slurry as a raw material, the porosity can be controlled by adjusting the conditions of the pore-forming material added to the slurry.

(Comparative example A)
A comparative example will be explained below. FIG. 4 shows a process flow in a method for manufacturing the sensor element 100 according to Comparative Example A. In Comparative Example A, before forming the pattern of the buffer layer 190, the element body is cut out (step S6) and fired (step S11). After that, a pattern of the buffer layer 190 is formed (step S12), and baked (step S13), and the tip protection layer 180 is formed (step S14). That is, in Comparative Example A, firing was performed twice (firing of the element body without the pattern of the buffer layer 190 and firing of the pattern of the buffer layer 190).

In contrast, in the embodiment, the pattern of the buffer layer 190 is formed at the stage of the element body (green body) before firing, and the buffer layer 190 is produced by firing. That is, the element body and the pattern of the buffer layer 190 are fired at the same time. As a result, in the embodiment, compared to Comparative Example A, (a) the number of steps can be reduced, and (b) the adhesion between the buffer layer 190 and the element substrate 10 is improved.

As described above, the embodiment can reduce the firing process compared to Comparative Example A. Furthermore, since the buffer layer 190 and the element substrate 10 are in contact with each other in an uncured state before firing, they have good affinity, and it is easy to ensure the adhesion between the buffer layer 190 and the element substrate 10 after firing. In addition, stress concentration on the interface of the buffer layer 190 (and even the element substrate 10) during firing is alleviated. In Comparative Example A, the green body of the buffer layer 190 is formed and fired on the element substrate 10 that has been hardened by the first firing, so a large stress can be applied (stress concentration) during this second firing. There is sex.

(Comparative example B)
As described above, in the embodiment, a material having a lower decomposition temperature Td than the organic binder is used as the pore-forming material. In Comparative Example B, as in the embodiment, a pattern of the tip protection layer 180 is formed on the element body before firing and fired, but a material having a higher decomposition temperature Td than the organic binder is used as the pore-forming material.

As shown in the following example, due to the magnitude relationship between the decomposition temperatures Td of the pore-forming material and the organic binder, the buffer layer 190 of the embodiment and comparative example B was subjected to a heat load with respect to the tip protection layer 180. Differences were observed in adhesion and gas permeability.

The results of comparing the experimental example with comparative example B are shown below. A pattern of a buffer layer 190 was formed on the element body (green body) before firing, and pressure was applied. After that, a tip protection layer 180 was formed and then fired. To form the buffer layer 190, a buffer layer paste containing a mixture of ceramic powder, a pore-forming material, an organic binder, a plasticizer, and a solvent was used.

Here, sensor elements 100 of Examples and Comparative Examples were manufactured using pore formers of types A and B. The type A pore-forming material is made of an acrylic polymer material and has a decomposition temperature Td of about 220°C. The type B pore-forming material is made of a polyester polymer material and has a decomposition temperature Td of about 360°C. Compared to the decomposition temperature Td of the organic binder described below, the decomposition temperature Td of type A is lower and the decomposition temperature Td of type B is higher.

As shown in FIG. 5, the decomposition temperature Td is the temperature at which the weight of the object decreases by 50%.

The diameter (D50) of both types A and B of the pore-forming materials was about 15 μm.

The produced buffer layer 190 was evaluated.

A. SEM Observation FIGS. 6A and 6B are SEM (Scanning Electron Microscope) photographs of the surfaces of the buffer layers 190 of Examples and Comparative Examples, respectively, in which firing was performed without forming the tip protective layer 180. Indicates the condition. 7A and 7B are SEM photographs of cross sections of samples according to Examples and Comparative Examples. FIGS. 8A and 8B are SEM photographs of cross sections of samples of Examples and Comparative Examples that were fired without forming the tip protection layer 180.

As shown in these figures, the structure (and porosity) of the buffer layer 190 is different between the example and the comparative example. In the comparative example, relatively large pores (coarse pores) are clearly observed in the buffer layer 190, and other regions tend to be dense (see FIGS. 6B to 8B). On the other hand, in the example, relatively large pores (coarse pores) are not clearly observed in the buffer layer 190 (see FIGS. 6A to 8A). That is, it is considered that various pores are arranged relatively uniformly.

This difference in structure also affects the porosity of the buffer layer 190, and the porosity of the example is larger than that of the comparative example. This difference in structure can be explained by the difference in decomposition temperature Td between the pore-forming material and the organic binder.

In the example, during firing, decomposition progresses in the order of the pore-forming material and the organic binder. That is, even after the pore-forming material is decomposed, the shape of the ceramic powder and the distance between the powders are maintained by the organic binder (the organic binder connects the ceramic particles). Thereafter, even if the organic binder decomposes, the distance between the ceramic particles, that is, the voids are maintained. As a result, the pores are arranged relatively uniformly, and the porosity is considered to be high.

On the other hand, in the comparative example, decomposition proceeds in the order of the organic binder and the pore-forming material during firing. Therefore, after the organic binder is decomposed, the ceramic particles enter between the particles of the pore-forming material, and the distance between the ceramic particles becomes shorter. Even after the pore-forming material is decomposed, the distance between the ceramic particles remains close, and the ceramic particles become dense, except for large pores remaining where the pore-forming material was. As a result, coarse pores are scattered in a dense region (substantially no pores), and the porosity is considered to decrease as a whole.

B. Gas Permeability Gas permeability of Examples and Comparative Examples was evaluated. The gas permeability was good in the examples, but the gas permeability was not good in the comparative examples. This result corresponds to the porosity of the buffer layer 190 of Examples and Comparative Examples, and it can be seen that the gas permeability changes depending on the magnitude relationship between the decomposition temperature Td of the pore-forming material and the organic binder.

C. Adhesion Strength The adhesion strength of the sensor elements 100 of Examples and Comparative Examples was measured.

First, the adhesion strength at room temperature was measured using the Sebastian method. That is, a pin was adhered to the surface of the tip protection layer 180 using an adhesive, a tensile force was applied to the pin, and the tensile force (adhesion strength) when the tip protection layer 180 was peeled off was measured. As a result, no significant difference was observed in the adhesion strength between Examples and Comparative Examples at room temperature.

Furthermore, the adhesion strength at high temperatures was measured by a pull-out method. The sensor element 100 is heated to a high temperature (1000° C.) higher than when it is driven, the vicinity of the base of the tip protection layer 180 is grasped, and the sensor element 100 is pulled in the longitudinal direction to obtain the tensile force (adhesion strength) when the tip protection layer 180 is peeled off. was measured. The average tensile force was calculated using five samples.

In the examples and comparative examples, average tensile forces of 532 and 432 [N] were obtained, respectively. That is, it can be seen that the adhesion strength at high temperatures changes depending on the magnitude relationship between the decomposition temperature Td of the pore-forming material and the organic binder.

In embodiments, the pores are formed relatively uniformly in the buffer layer 190, and no sharp boundaries occur in the buffer layer 190. Therefore, concentration of thermal stress is avoided even under high temperatures. On the other hand, in the comparative example, large pores are formed in isolation in the buffer layer 190, other parts are in a dense state, and boundaries exist between them. Therefore, stress tends to concentrate at the boundaries at high temperatures.

Note that cracks generated during firing may also affect the adhesion strength. In the example, shrinkage cracks during pore formation can be suppressed by removing the pore-forming material while the organic binder that holds the spaces between the ceramic powders remains (before the organic binder disappears). reduction).

[Invention obtained from this embodiment]
The invention that can be understood from the above embodiments will be described below.

[1] The method for manufacturing a gas sensor element (100) according to the present embodiment includes: an element base (10) having a solid electrolyte; a tip protection layer (180) covering at least a portion of the element base; A buffer layer (190) disposed between the tip protection layer and the buffer layer (190) is manufactured, and a step of laminating sheets (green sheets) containing the solid electrolyte to form a laminate; , forming a buffer layer pattern containing a buffer layer forming substance on the laminate; firing the laminate on which the buffer layer pattern is formed to produce a fired body on which the buffer layer is formed; forming the tip protection layer on the buffer layer of the fired body.

Since the laminate is fired after the buffer layer pattern is formed, the number of firings can be reduced and the process can be simplified. Furthermore, the adhesion between the buffer layer and the tip protection layer can be improved.

[2] In the present embodiment, the step of forming the buffer layer pattern includes the step of printing a paste (buffer layer paste) containing the buffer layer forming substance, a pore-forming material, and an organic binder, and The decomposition temperature Td of the pore-forming material is lower than the decomposition temperature Td of the organic binder.

As a result, the pore-forming material disappears before the organic binder during firing. As a result, the shape of the ceramic powder is maintained during decomposition of the pore-forming material, making it possible to form relatively uniform pores. Adhesion can be improved due to relatively uniform pores.

[3] In the present embodiment, the diameter (D50) of the pore-forming material is not more than twice the thickness of the buffer layer pattern. This makes it possible to form pores that contribute to improved adhesion.

[4] In this embodiment, the solid electrolyte contains zirconia as a main component, and the buffer layer forming material contains alumina as a main component. Thereby, a gas sensor element using a solid electrolyte can be manufactured.

In carrying out the present invention, various means for improving reliability as an automobile part may be added to the extent that the idea of the present invention is not impaired.

DESCRIPTION OF SYMBOLS 10...Element base body 100...Gas sensor element 101a-101c...Ceramics layer 102-104...Internal cavity 105...Gas inlet 110, 120, 130...Diffusion control part 115...Buffer space 141...Outer pump electrode 142...Inner pump electrode 143 ...Auxiliary pump electrode 145...Measurement electrode 147...Reference electrode
150... Heater 151... Insulating layer
160... Electrode terminals 170, 170a, 170b... Outer (pump) electrode protective layer 180... Tip protective layer 190... Buffer layer

Claims (3)

an element substrate having a solid electrolyte;
a tip protection layer covering at least a portion of the element substrate;
A method for manufacturing a gas sensor element, comprising: a buffer layer disposed between the element base and the tip protection layer,
a step of laminating sheets containing the solid electrolyte to form a laminate;
forming a buffer layer pattern containing a buffer layer forming substance on the laminate;
firing the laminate on which the buffer layer pattern is formed to produce a fired body on which the buffer layer is formed;
forming the tip protection layer on the buffer layer of the fired body;
Equipped with
The step of forming the buffer layer pattern includes the step of applying a paste containing the buffer layer forming substance, a pore-forming material, and an organic binder,
A method for manufacturing a gas sensor element , wherein the decomposition temperature of the pore-forming material is lower than the decomposition temperature of the organic binder . A method for manufacturing a gas sensor element according to claim 1 , comprising:
The method for manufacturing a gas sensor element, wherein the diameter (D50) of the pore-forming material is not more than twice the thickness of the buffer layer pattern. A method for manufacturing a gas sensor element according to claim 1 or 2 , comprising:
The solid electrolyte mainly contains zirconia,
A method for manufacturing a gas sensor element, wherein the buffer layer forming substance contains alumina as a main component.