JP2011225434A - Method for producing alumina porous body - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an alumina porous body, which can control an alumina porous body so as to have desired porous properties when producing the alumina porous body.SOLUTION: When producing the alumina porous body, alumina powder and a pore forming agent are mixed at a predetermined fixed mixing ratio so as to be a mixed raw material, the mixed raw material is molded into a molded body, and the molded body is fired at a prescribed firing temperature while performing adjustment in such a manner that predetermined fixed water vapor partial pressure is obtained in an air atmosphere.

Description

  The present invention relates to a method for producing a porous alumina body.

  Conventionally, a method for producing a porous alumina body by mixing alumina powder and a pore-forming agent and sintering the formed compact is known (see, for example, Patent Document 1). Moreover, the alumina porous body is used as a material used in an environment where heat resistance, corrosion resistance, and wear resistance are required. One example is an electrode protective film that covers an electrode of a gas sensor that measures a combustible gas component. In such a gas sensor, the gas diffusion resistance when the gas to be measured passes through the electrode protective film is an important factor for stabilizing the output. For this reason, it is desired to establish a technique for manufacturing an electrode protective film having a desired gas diffusion resistance.

Japanese Patent Laid-Open No. 2003-2760

  However, when an alumina porous body is used as an electrode protective film, it is not easy to control the gas diffusion resistance of the electrode protective film to a desired value. Specifically, when alumina powder and a pore-forming agent are mixed and the formed molded body is sintered, sintering is performed at a predetermined sintering temperature while injecting the atmosphere. Even if the mixing ratio is constant, the gas diffusion resistance may vary greatly if the production lot is different.

  The present invention has been made in order to solve such problems, and it is a main object of the present invention to make it possible to control the alumina porous body so as to have a desired porous property in producing the alumina porous body. To do.

  When the inventors of the present invention mix alumina powder and a pore former to form a mixed raw material and sinter the molded body obtained by molding the mixed raw material at a predetermined sintering temperature while driving the atmosphere, the resulting porous alumina body is obtained. It has been found that the gas diffusion resistance depends on the amount of pore-forming agent used and the partial pressure of water vapor contained in the atmosphere to be injected, and the present invention has been completed.

That is, the manufacturing method of the 1st alumina porous body of this invention is the following.
Alumina powder and pore former are mixed at a predetermined mixing ratio to form a mixed raw material, and the molded body obtained by molding the mixed raw material is adjusted in a firing atmosphere so as to have a constant water vapor partial pressure in the air atmosphere. Firing while realizing the desired porous properties,
Is.

The method for producing the second porous alumina body of the present invention is as follows.
The three-way relationship between the water vapor partial pressure during firing, the mixing ratio of the alumina powder and the pore forming agent, and the gas diffusion resistance of the resulting alumina porous body is determined, and the gas diffusion resistance of the alumina porous body to be manufactured is Determine the water vapor partial pressure during firing and the mixing ratio of the alumina powder and the pore former from the above relationship so that the desired value is obtained, and then mix the alumina powder and the pore former at the determined mixing ratio. Firing at a predetermined calcining temperature while adjusting the molded body obtained by molding the mixed raw material to the determined steam partial pressure in the air atmosphere as a mixed raw material,
Is.

  According to the first and second alumina porous body manufacturing methods of the present invention, when the alumina porous body is manufactured, the alumina porous body is controlled to have a desired porous property (for example, gas diffusion resistance). Can do. The inventors mixed alumina powder and a pore former to form a mixed raw material, and when the molded body obtained by molding the mixed raw material was sintered at a predetermined sintering temperature in an air atmosphere, they were mixed at the same ratio. Despite the use of mixed raw materials, the inventors have noticed that the gas diffusion resistance of the resulting alumina porous body varies greatly depending on the season. And we investigated why gas diffusion resistance fluctuates greatly depending on the season. As a result, the knowledge that when the amount of moisture contained in the air is large, the sinterability of alumina is reduced and the gaps are increased, so that the gas diffusion resistance is reduced, and conversely, the gas diffusion resistance is increased when the amount of moisture is small. It came to. Based on this knowledge, the alumina powder and the pore former are mixed at a predetermined mixing ratio to obtain a mixed raw material. When it was fired while adjusting the firing atmosphere to achieve a pressure, it was found that an alumina porous body having a certain gas diffusion resistance could be obtained regardless of the season or production lot. In addition, the three-way relationship between the partial pressure of water vapor at the time of firing, the mixing ratio of the alumina powder and the pore former, and the gas diffusion resistance of the resulting porous alumina body is obtained, and the porous alumina body to be manufactured From the above relationship, the water vapor partial pressure during firing and the mixing ratio of the alumina powder and the pore former are determined from the above relationship so that the gas diffusion resistance becomes a desired value, and then the alumina powder and the pore former are mixed as determined above. When the mixture obtained by mixing at a ratio to obtain a mixed raw material and calcined at a predetermined calcining temperature while adjusting the water vapor partial pressure determined in the air atmosphere to obtain the desired gas diffusion resistance It was found that an alumina porous body having the following can be obtained.

  In the first and second methods for producing a porous alumina body of the present invention, alumina powders in any crystal form of α, γ, δ, θ can be used as the alumina powder, but γ-alumina is a porous alumina body. Is suitable for use as a catalyst carrier having a high specific surface area, and α-alumina is suitable for use as a structural sintered body.

  In the first and second alumina porous body production methods of the present invention, the pore-forming agent is not particularly limited. For example, carbon materials such as graphite; low molecular organic materials such as melamine; polyethylene, polypropylene, and melamine Resin, high molecular organic materials such as polymethyl methacrylate; and plant-based materials such as starch.

  In the first and second alumina porous body production methods of the present invention, the mixed raw material may contain materials other than alumina powder and pore former, for example, appropriately containing a plasticizer, an organic solvent, and a binder. May be. In mixing the raw materials, for example, a slurry may be obtained by wet mixing in an organic solvent, and the slurry may be dried and granulated to obtain a mixed powder. When performing wet mixing, a mixing and grinding machine such as a pot mill, a trommel, an attrition mill, or the like may be used. Further, dry mixing may be performed instead of wet mixing. In forming the mixed raw material, when the molded body is a block body, press molding may be performed, but when the molded body is a thin film such as an electrode protective film, the slurry may be tape-molded or screen-printed.

  In the first and second alumina porous body production methods of the present invention, when the molded body is fired in an air atmosphere while adjusting the firing atmosphere so as to have a constant steam partial pressure, the adjustment of the steam partial pressure is For example, by measuring the dew point of the atmosphere discharged from the firing furnace where the firing is performed, the actual value of the steam partial pressure in the furnace is obtained, and the firing is performed so that the measured value matches the predetermined steam partial pressure. The amount of water vapor supplied to the furnace may be adjusted. The ability of such a firing furnace can be controlled in the range of the water vapor partial pressure in the range of 0.5 hPa to 40 hPa, and the water vapor variation at the maximum firing temperature holding time is plus or minus 3 hPa. For example, the firing temperature is preferably set in the range of 1000 to 1600 ° C, more preferably in the range of 1300 to 1400 ° C. Such adjustment of water vapor is preferably performed at least after the temperature rise to the firing temperature is completed until the period for holding at the firing temperature is completed, and after the degreasing is completed, the period for holding at the firing temperature is completed. It is more preferable to carry out until the cooling period is started after the period of holding at the firing temperature from the start of temperature rise ends.

In the first and second alumina porous body manufacturing methods of the present invention, gas diffusion resistance, which is one of the porous properties, is provided with electrodes on both surfaces of oxygen ion conductive ceramics, and one electrode is used as the alumina porous body of this time. After covering with a protective film made of, a voltage is applied between both electrodes, a limit current is measured in an air atmosphere, and it can be obtained from the relational expression at that time. That is, when the gas diffusion resistance Rd is defined as diffusion path length / diffusion path cross-sectional area, it is known that the following expression (1) is established, and can be obtained from this expression (1).
Rd = 4FD (Po-Pe) / Ip · R · T (1)
(F is the Faraday constant (96490 C / mol), D is the diffusion constant of oxygen molecules (1.68 cm ² / sec), Po is the oxygen partial pressure of the gas to be measured (in the atmosphere, 0.21 atm), Pe is the protective film Oxygen partial pressure in the vicinity of the inner measurement electrode (zero to determine the limit current), Ip is the limit current value, R is the gas constant (82.05 atm · cm ³ / mol), T is the ambient temperature)

  In the second method for producing a porous alumina body of the present invention, a three-way relationship between the water vapor partial pressure during firing, the mixing ratio of the alumina powder and the pore former, and the gas diffusion resistance of the resulting alumina porous body is obtained. . The relationship between the three is, for example, that a plurality of ratios r1, r2,... Are defined as a mixing ratio of alumina powder and pore former, and a plurality of pressures Ps1, Ps2,. Sintering is sequentially performed so that the water vapor partial pressure during sintering becomes the pressure Ps1, Ps2,... Per ratio, and the gas diffusion resistance Rd of the obtained alumina porous body is measured.

In the first and second alumina porous body production methods of the present invention, the resulting alumina porous body is not particularly limited, but can be used as an electrode protective film of a gas sensor. Examples of the gas sensor include a sensor that measures hydrocarbon (HC), hydrogen (H ₂ ), carbon monoxide (CO), ammonia (NH ₃ ), nitrogen oxide (NOx), and the like.

  You may employ | adopt the manufacturing method of the 1st and 2nd alumina porous body of this invention for the interparticle embedding method. The interparticle embedding method includes alumina particles functioning as an aggregate (referred to as alumina aggregate powder) and alumina particles functioning as a pore-forming agent with a finer particle size than the alumina aggregate powder (referred to as alumina pore-forming agent). Are mixed and sintered in a state where a plurality of alumina pore-forming agents are embedded in the gaps between the alumina aggregate powders, thereby obtaining a gas-permeable porous alumina body having a plurality of continuous air holes. . The alumina aggregate powder has a particle size of 0.1 to 0.7 μm, preferably 0.2 to 0.5 μm, and a sphericity of 0.7 to 1.0, preferably 0.8 to 1.0. The alumina particles are used. As the alumina pore-forming agent, alumina particles having a particle size of 0.01 to 0.1 μm, preferably 0.01 to 0.05 μm are used. The firing temperature is 1200 to 1400 ° C, preferably 1300 to 1380 ° C. When these conditions are satisfied, it is possible to obtain an alumina porous body having a pore diameter of 400 to 1100 mm, preferably 500 to 900 mm, and a porosity of 4 to 16%, preferably 9 to 14%. it can. Even in such an interparticle embedding method, the porous property (for example, gas diffusion resistance) may vary greatly if the production lot is different. Therefore, when the first and second porous alumina manufacturing methods of the present invention were employed in the interparticle embedding method, such variations in porous properties could be suppressed.

  In such an interparticle embedding method, when the particle size of the alumina aggregate powder is out of the range of 0.1 to 0.7 μm and the sphericity is out of the range of 0.7 to 1.0, the alumina aggregate powder There is a possibility that the cohesiveness is increased or the degree of sintering becomes sweet and the pore diameter and the porosity of the alumina porous body obtained by sintering may vary. Further, if the particle size of the alumina pore-forming agent is out of the range of 0.01 to 0.1 μm, the degree of sinterability at the time of sintering the alumina pore-forming agent is out of the preferred range, which is obtained by sintering. There is a risk of causing variations in the pore diameter and porosity characteristics of the resulting alumina porous body. In this sense, the alumina pore-forming agent also preferably has a sphericity of 0.7 to 1.0, and more preferably 0.8 to 1.0. On the other hand, if the firing temperature is outside the range of 1200 to 1400 ° C., the necessary pore characteristics cannot be obtained due to insufficient firing or excessive firing. Furthermore, the pore distribution (pore diameter distribution width) of the obtained alumina porous body is preferably 300 to 1100 、, and more preferably 500 to 1100 Å. Outside this range, the function of trapping pollutants (such as gasoline additives) may be deteriorated, and the gas permeability may be reduced. The alumina aggregate powder and the alumina pore-forming agent are not particularly limited as long as they satisfy the above-mentioned range of particle size and sphericity and have a uniform particle size and a uniform shape. For example, α-alumina, γ -Alumina etc. can be mentioned. As a specific method for making a plurality of alumina pore-forming agents embedded in the gaps between the alumina aggregate powders, for example, mixed alumina obtained by mixing alumina aggregate powder and alumina pore-forming agent is used as an organic binder liquid. Examples of the method include applying the slurry dispersed in the mixture and compacting the mixed alumina. In this case, the mixing ratio of the alumina aggregate powder and the alumina pore-forming agent is preferably such that the alumina pore-forming agent is mixed in an amount of 3.4 to 5.6% by weight with respect to the alumina aggregate powder. More preferably, 7% by weight is mixed. The alumina pore-forming agent may be alumina as described above, but may be an alumina precursor (which changes to alumina in the firing stage).

1 is a cross-sectional view of a gas sensor 100. FIG. 3 is a flowchart showing a flow of processing when manufacturing the gas sensor 100. 3 is a cross-sectional view schematically showing a configuration of a tip portion of a gas sensor 200 including a sensor element 201. FIG. 4 is a graph showing the relationship between the mixing ratio of a pore forming agent to alumina powder and gas diffusion resistance in Comparative Example 1. It is a graph which shows the relationship between the mixing ratio of the pore making material with respect to alumina powder in Example 1, the water vapor partial pressure at the time of baking, and gas diffusion resistance. It is a graph which shows the relationship between the HC density | concentration of Comparative Example 2, and detection electric current. It is a graph which shows the relationship between HC density | concentration of Example 2 and a detection electric current. It is a graph which shows the relationship between the mixing rate of the pore making material with respect to an alumina powder, and gas diffusion resistance in the comparative example 3. It is a graph which shows the relationship between the mixing ratio of the pore making material with respect to the alumina powder in Example 3, the water vapor partial pressure at the time of baking, and gas diffusion resistance. It is a graph which shows the variation in the detection current of the comparative example 4 and Example 4. FIG.

[First Embodiment]
Next, preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view of a gas sensor 100 of the present embodiment.

  The gas sensor 100 can detect a combustible gas component, and is formed in a plate shape by laminating first to sixth solid electrolyte layers 104a to 104f having oxygen ion conductivity. The first and third solid electrolyte layers 104 a and 104 c are uniform flat plates without voids, but the second solid electrolyte layer 104 b has voids 106. A gas to be measured can flow into the void 106 from the outside through a diffusion-controlled passage 112 made of a ceramic porous body. The fourth solid electrolyte layer 104d has a reference passage 110 into which air is introduced. The fifth and sixth solid electrolyte layers 104e and 104f are uniform flat plates without voids, but a heater 142 is sandwiched between them. The heater 142 is provided over the entire space 106, and power is supplied through a terminal (not shown). Further, although not shown, ceramic layers such as alumina are provided on the upper and lower surfaces of the heater 142 in order to ensure electrical insulation from the solid electrolyte layer.

The gas sensor 100 includes a first pump cell P1 and a first sensor cell S1. The first pump cell P1 includes a first inner pump electrode 116 provided on the back surface of the first solid electrolyte layer 104a on the upstream side of the void 106, and a first outer pump provided on the surface of the first solid electrolyte layer 104a. The electrode 118, the first solid electrolyte layer 104 a sandwiched between the electrodes 116, 118, and the variable power source 120 that applies a voltage between the electrodes 116, 118 are configured. The first outer pump electrode 118 is covered with an electrode protective film 119. This electrode protective film 119 is produced by the method for producing a porous alumina body of the present invention so as to have a predetermined gas diffusion resistance designed in advance. When the voltage of the variable power source 120 is applied to both electrodes 116 and 118 of the first pump cell P1 so that a current flows from the first outer pump electrode 118 to the first inner pump electrode 116, oxygen in the void 106 becomes oxygen ions. Thus, the liquid is pumped from the first inner pump electrode 116 to the first outer pump electrode 118 through the first solid electrolyte layer 104a. On the other hand, the first sensor cell S1 is the measurement electrode 122 provided on the surface of the third solid electrolyte layer 104c on the downstream side of the inner pump electrode 116 of the void 106, and the back surface of the third solid electrolyte layer 104c. The reference electrode 124 exposed in the passage 110, the third solid electrolyte layer 104 c sandwiched between the electrodes 122 and 124, and a potentiometer 126 that measures the potential difference between the electrodes 122 and 124. When a difference in oxygen concentration occurs between the void 106 and the reference passage 110, an electromotive force is generated between the measurement electrode 122 and the reference electrode 124 of the first sensor cell S1. By measuring this electromotive force with the potentiometer 126, the oxygen partial pressure in the void 106 can be detected. Then, by controlling the voltage of the variable power source 120 based on the value of the oxygen partial pressure, the oxygen partial pressure in the space 106 is maintained at a predetermined low value (for example, 10 ⁻²⁰ atm). In addition, as each electrode 116,118,122,124, the cermet electrode which consists of platinum as an electrode metal and zirconia as ceramics can be used, for example.

  Further, the gas sensor 100 includes a second pump cell P2. The second pump cell P2 is exposed to the reference passage 110 at the second inner pump electrode 128 provided on the surface of the third solid electrolyte layer 104c on the most downstream side of the void 106 and on the back surface of the third solid electrolyte layer 104c. The second outer pump electrode 130, the third solid electrolyte layer 104c sandwiched between the electrodes 128, 130, the DC power source 132 for applying a voltage between the electrodes 128, 130, and the space between the electrodes 128, 130. And an ammeter 134 for measuring the pump current flowing through the. When a voltage is applied to both electrodes 128 and 130 of the second pump cell P2 such that a current flows from the second inner pump electrode 128 to the second outer pump electrode 130 of the second pump cell P2, oxygen in the reference passage 110 is oxygen ions. Thus, the air is pumped from the second outer pump electrode 130 to the void 106 through the third solid electrolyte layer 104 c and the second inner pump electrode 128. The oxygen pumped into the void 106 is consumed by the combustion reaction of the combustible gas in the measurement gas. As each of the electrodes 128 and 130, the cermet electrode described above can be used.

  Since a gas sensor similar to the gas sensor 100 is described in detail in Japanese Patent No. 3450084, refer to this publication as necessary.

Next, the principle of detecting the concentration of combustible gas contained in the gas to be measured using the gas sensor 100 will be described. For convenience, the gas to be measured will be described as a gas in which nitrogen gas is mainly used and oxygen gas and propane gas (C ₃ H ₈ ), which is a kind of hydrocarbon gas, are mixed therewith.

First, the gas sensor 100 is placed in the gas to be measured, and the gas sensor 100 is heated to 600 ° C. by the heater 142. Then, the gas to be measured flows into the void 106 through the diffusion-controlled passage 112 of the gas sensor 100. The voltage of the variable power source 120 is applied to both electrodes 116 and 118 of the first pump cell P1. Specifically, the value of the potentiometer 126 between the measurement electrode 122 and the reference electrode 124 of the first sensor cell S1 is set to a predetermined low voltage, that is, the oxygen partial pressure of the space 106 is set to a predetermined low value. The voltage of the variable power source 120 is applied to both electrodes 116 and 118 of the first pump cell P1 so as to be (for example, 10 ⁻²⁰ atm). As a result, the oxygen in the void 106 becomes oxygen ions and is pumped from the first inner pump electrode 116 to the first outer pump electrode 118 through the first solid electrolyte layer 104a. The predetermined low voltage at this time is set so that the oxygen partial pressure is so low that the propane gas in the gas to be measured cannot be combusted. As a result, the gas to be measured in the void 106 becomes substantially free of oxygen gas.

  Next, the gas to be measured that has been processed so as not to substantially contain oxygen gas reaches the second inner pump electrode 128 of the second pump cell P <b> 2 in the void 106. A constant voltage of the DC power source 132 is applied to both electrodes 128 and 130 of the second pump cell P2 in a direction in which oxygen is pumped from the reference passage 110 into the void 106. As a result, oxygen is supplied to the second inner pump electrode 128. The combustible gas contained in the gas to be measured is burned by the oxygen supplied in this way, and oxygen is consumed. Since the pump current according to the oxygen consumption at this time flows through both electrodes 128 and 130 of the second pump cell P2, the oxygen consumption is obtained from the pump current value of the ammeter 134, and combustion is performed from the oxygen consumption. The amount of combustible gas thus obtained can be obtained, and finally the concentration of combustible gas in the gas to be measured can be obtained.

  Next, an example of a manufacturing method of such a gas sensor 100 will be described below. FIG. 2 is a flowchart showing a processing flow when the gas sensor 100 is manufactured.

  When producing the gas sensor 100, first, a blank sheet which is a green sheet on which no pattern is formed is prepared (step S10). Since the gas sensor 100 includes six layers of the first to sixth solid electrolyte layers 104a to 104f, six blank sheets are prepared corresponding to each layer. The blank sheet is provided with a plurality of sheet holes used for positioning during printing or lamination. The sheet hole is formed in advance by a punching process using a punching device. In the blank sheet, when the corresponding layer has an internal space (the void 106 or the reference passage 110), the blank sheet is also provided with a space corresponding to the internal space.

  Subsequently, pattern printing for forming various patterns on each blank sheet and subsequent drying processing are performed (step S20). Specifically, electrode patterns such as the first inner pump electrode 116, the first outer pump electrode 118, the measurement electrode 122, the reference electrode 124, the second inner pump electrode 128, and the second outer pump electrode 130 are illustrated by pattern printing. An internal wiring or the like is omitted. The blank sheet corresponding to the first solid electrolyte layer 104a is also printed with a cut mark that serves as a reference for the cutting position when the laminated body is cut in a subsequent process. Each pattern is printed by applying a pattern-forming paste prepared according to the characteristics required for each object to be formed to a blank sheet using a known screen printing technique. About the drying process after printing, a well-known drying technique can be utilized, for example, it is common to carry out at the temperature of 75-90 degreeC in an atmospheric condition.

  Here, in the blank sheet corresponding to the first solid electrolyte layer 104a, the slurry that becomes the electrode protective film 119 is printed on the printed portion that becomes the first outer pump electrode 118. This is a feature of the present invention. Therefore, it will be described in detail below. First, a slurry for preparing the electrode protective film 119 is prepared. This slurry is a mixed raw material obtained by mixing alumina powder having an average particle diameter of 0.1 to 1.0 μm and a pore-forming agent mainly composed of melamine or methyl methacrylate at a predetermined predetermined mixing ratio. And 50 parts by weight of the mixed raw material, 10 parts by weight of the organic binder, 35 parts by weight of an organic solvent having a boiling point of 200 ° C. or higher, and 5 parts by weight of a plasticizer. As the binder, an ethylcellulose-based or butyral-based organic compound is used. As the organic solvent, for example, terpineol, butyl carbitol, octanol or the like can be used depending on the solubility compatibility of the binder. Moreover, as a plasticizer, organic compounds, such as a phthalic acid ester and adipic acid ester, can be used, for example. In addition, the mixing ratio of the alumina powder of the slurry for producing the electrode protective film 119 and the pore former is determined according to the design value of the gas diffusion resistance of the electrode protective film 119. For example, the three-way relationship between the partial pressure of water vapor at the time of firing, the mixing ratio of alumina powder and pore former, and the gas diffusion resistance of the resulting porous alumina body is determined by preliminary experiments, and this time the production is attempted. The water vapor partial pressure during firing corresponding to the gas diffusion resistance of the electrode protective film 119 and the mixing ratio of the alumina powder and the pore former are read out from the above three relationships, and the read out mixing ratio is adopted. Then, this slurry is screen-printed on the printed portion of the first outer pump electrode 118 so as to have a thickness of several tens of μm.

  Next, a printing / drying process of an adhesive paste for laminating and bonding the sheets after the process of step S20 corresponding to each layer is performed (step S30). A known screen printing technique can be used for printing the adhesive paste, and a known drying technique can also be used for the subsequent drying treatment.

  Next, the sheets on which the adhesive paste is printed are stacked in a predetermined order and pressed by applying a predetermined pressure at a predetermined temperature to perform a pressing process to form a single laminate (step S40). Specifically, by stacking and holding the sheets to be stacked on a predetermined stacking jig (not shown) while positioning them with sheet holes, and heating and pressurizing the entire stacking jig with a stacking machine such as a known hydraulic press machine Do. The pressure, temperature, and time for performing heating and pressurization depend on the laminating machine to be used, but appropriate conditions may be set so that good lamination can be realized.

  When the laminated body is obtained as described above, the laminated body is cut with reference to the cut marks and cut into small pieces having the size of the gas sensor 100 (step S50). And the gas sensor 100 is obtained by baking the cut-out small piece on predetermined conditions (step S60). Firing is performed at a firing temperature of 1350 to 1400 ° C. while adjusting the water vapor partial pressure to be a predetermined predetermined pressure in an air atmosphere. The water vapor partial pressure at this time corresponds to the gas diffusion resistance of the electrode protective film 119 to be manufactured this time when determining the mixing ratio of the alumina powder of the slurry for producing the electrode protective film 119 and the pore former. The water vapor partial pressure during firing and the mixing ratio of the alumina powder and the pore forming agent were read out from the above three relationships, and the read out water vapor partial pressure is adopted. In addition, degreasing is not carried out as a separate step, but is also performed during firing. For this reason, in the firing step, first, degreasing is completed from the start of the temperature rise to the predetermined firing temperature, and then the alumina is fired while being held at the predetermined firing temperature.

  According to the gas sensor 100 described in detail above, since the electrode protective film 119 has the gas diffusion resistance as designed, the combustible gas concentration in the gas to be measured can be accurately detected regardless of the production lot. . That is, if the gas diffusion resistance varies widely for each gas sensor 100, the performance of the first pump cell P1 varies, and consequently the combustible gas concentration in the gas to be measured varies, which is not preferable. However, in the present embodiment, when producing the electrode protective film 119, the alumina powder and the pore former are mixed at a predetermined fixed mixing ratio as a mixed raw material, and a molded body obtained by molding the mixed raw material is obtained. Since the firing is performed at a predetermined firing temperature while adjusting to a predetermined constant water vapor partial pressure in the air atmosphere, the electrode protective film 119 is equivalent even if the production lots of the gas sensor 100 are different. It has gas diffusion resistance. For this reason, the performance of the first pump cell P1 does not vary, and as a result, the variation of the combustible gas concentration in the gas to be measured can be suppressed to be small. This variation suppressing effect is thought to be due to the fact that the sintering behavior near the surface of the electrode protective film 119 can be made constant by making the water vapor partial pressure during firing constant.

[Second Embodiment]
FIG. 3 is a schematic cross-sectional view schematically showing an example of the configuration of the gas sensor 200. The gas sensor 200 is for detecting a predetermined nitrogen oxide (NOx) in the gas to be measured (measurement gas) and measuring the concentration thereof.

The sensor element 201 includes a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, and a first solid electrolyte layer 4 each made of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO ₂ ). The spacer layer 5 and the second solid electrolyte layer 6 are elements having a structure in which the layers are laminated in this order from the bottom in the drawing. The solid electrolyte forming these six layers is dense and airtight. The sensor element 201 is manufactured, for example, by performing predetermined processing and circuit pattern printing on a ceramic green sheet corresponding to each layer, laminating them, and firing and integrating them.

  One end of the sensor element 201, and between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4, is a gas inlet 10, a first diffusion rate-limiting unit 11, and a buffer space. 12, the second diffusion rate limiting part 13, the first internal space 20, the third diffusion rate limiting part 30, and the second internal space 40 are adjacently formed in such a manner that they communicate in this order.

  The gas introduction port 10, the buffer space 12, the first internal space 20, and the second internal space 40 are provided on the lower surface of the second solid electrolyte layer 6 with the upper portion provided in a state in which the spacer layer 5 is cut out. Thus, the space inside the sensor element 201 is defined by the lower part being the upper surface of the first solid electrolyte layer 4 and the side parts being the side surfaces of the spacer layer 5.

  Each of the first diffusion rate controlling unit 11, the second diffusion rate controlling unit 13, and the third diffusion rate controlling unit 30 is provided as two horizontally long slits (the opening has a longitudinal direction in a direction perpendicular to the drawing). . In addition, the site | part from the gas inlet 10 to the 2nd internal space 40 is also called a gas distribution part.

  Further, at a position farther from the front end side than the gas circulation part, the side part is partitioned by the side surface of the first solid electrolyte layer 4 between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5. The reference gas introduction space 43 is provided at the position. For example, the atmosphere is introduced into the reference gas introduction space 43 as a reference gas when measuring the NOx concentration.

  The atmosphere introduction layer 48 is a layer made of porous alumina, and the reference gas is introduced into the atmosphere introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is formed so as to cover the reference electrode 42.

  The reference electrode 42 is an electrode formed in such a manner that it is sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4. As described above, the reference electrode 42 leads to the reference gas introduction space 43. An air introduction layer 48 is provided. Further, as will be described later, it is possible to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20 and the second internal space 40 using the reference electrode 42.

  In the gas circulation part, the gas inlet 10 is a part opened to the external space, and the gas to be measured is taken into the sensor element 201 from the external space through the gas inlet 10.

  The first diffusion control unit 11 is a part that provides a predetermined diffusion resistance to the gas to be measured taken from the gas inlet 10.

  The buffer space 12 is a space provided to guide the gas to be measured introduced from the first diffusion rate controlling unit 11 to the second diffusion rate controlling unit 13.

  The second diffusion rate limiting unit 13 is a part that imparts a predetermined diffusion resistance to the gas to be measured introduced from the buffer space 12 into the first internal space 20.

  When the gas to be measured is introduced from the outside of the sensor element 201 to the first internal space 20, the pressure fluctuation of the gas to be measured in the external space (if the gas to be measured is an automobile exhaust gas, the pulsation of the exhaust pressure) ), The gas to be measured that is suddenly taken into the sensor element 201 from the gas inlet 10 is not directly introduced into the first internal space 20, but the first diffusion control unit 11, the buffer space 12, and the second After the concentration variation of the gas to be measured is canceled through the diffusion control unit 13, the gas is introduced into the first internal space 20. As a result, the concentration fluctuation of the gas to be measured introduced into the first internal space is almost negligible.

  The first internal space 20 is provided as a space for adjusting the partial pressure of oxygen in the gas to be measured introduced through the second diffusion rate limiting unit 13. The oxygen partial pressure is adjusted by the operation of the main pump cell 21.

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

  The inner pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first inner space 20, and the spacer layer 5 that provides side walls. Yes. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom portion is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Spacer layers in which the electrode portions 22b are formed and the side electrode portions (not shown) constitute both side walls of the first internal space 20 so as to connect the ceiling electrode portions 22a and the bottom electrode portions 22b. 5 is formed on the side wall surface (inner surface), and is disposed in a tunnel-shaped structure at the portion where the side electrode portion is disposed.

The inner pump electrode 22 and the outer pump electrode 23 are formed as a porous cermet electrode (for example, a cermet electrode of Pt and ZrO ₂ containing 1% of Au). The inner pump electrode 22 in contact with the gas to be measured is formed using a material that has a reduced reduction ability for the NOx component in the gas to be measured.

  In the main pump cell 21, a desired pump voltage Vp 0 is applied between the inner pump electrode 22 and the outer pump electrode 23, and the pump current is positive or negative between the inner pump electrode 22 and the outer pump electrode 23. By flowing Ip0, oxygen in the first internal space 20 can be pumped into the external space, or oxygen in the external space can be pumped into the first internal space 20.

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

  By measuring the electromotive force V0 in the main pump control oxygen partial pressure detection sensor cell 80, the oxygen concentration (oxygen partial pressure) in the first internal space 20 can be known. Further, the pump current Ip0 is controlled by feedback control of Vp0 so that the electromotive force V0 is constant. Thereby, the oxygen concentration in the first internal space 20 can be kept at a predetermined constant value.

  The third diffusion control unit 30 provides a predetermined diffusion resistance to the gas under measurement whose oxygen concentration (oxygen partial pressure) is controlled by the operation of the main pump cell 21 in the first internal space 20, and the gas under measurement is supplied to the gas under measurement. This is the part that leads to the second internal space 40.

  The second internal space 40 is provided as a space for performing a process related to the measurement of the nitrogen oxide (NOx) concentration in the gas to be measured introduced through the third diffusion control unit 30. The NOx concentration is measured mainly in the second internal space 40 in which the oxygen concentration is adjusted by the auxiliary pump cell 50, and further by measuring the pump cell 41 for measurement.

  In the second internal space 40, after the oxygen concentration (oxygen partial pressure) is adjusted in the first internal space 20 in advance, the auxiliary pump cell 50 further supplies the gas to be measured introduced through the third diffusion control unit. The oxygen partial pressure is adjusted. Thereby, since the oxygen concentration in the second internal space 40 can be kept constant with high accuracy, the gas sensor 200 can measure the NOx concentration with high accuracy.

  The auxiliary pump cell 50 includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, and an outer pump electrode 23 (outer pump electrode 23). The auxiliary electrochemical pump cell is configured by the second solid electrolyte layer 6 and the sensor element 201 and a suitable external electrode).

  The auxiliary pump electrode 51 is disposed in the second internal space 40 in the same tunnel configuration as the inner pump electrode 22 provided in the first internal space 20. That is, the ceiling electrode portion 51 a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40 is formed on the first solid electrolyte layer 4. The bottom electrode part 51b is formed, and the side electrode part (not shown) connecting the ceiling electrode part 51a and the bottom electrode part 51b is provided on the spacer layer 5 that provides the side wall of the second internal space 40. It has a tunnel-type structure formed on both wall surfaces.

  Note that the auxiliary pump electrode 51 is also formed using a material having a reduced reducing ability with respect to the NOx component in the gas to be measured, like the inner pump electrode 22.

  In the auxiliary pump cell 50, by applying a desired voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23, oxygen in the atmosphere in the second internal space 40 is pumped to the external space, or It is possible to pump into the second internal space 40 from the space.

  Further, in order to control the oxygen partial pressure in the atmosphere in the second internal space 40, the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte. The layer 4 and the third substrate layer 3 constitute an electrochemical sensor cell, that is, an auxiliary pump control oxygen partial pressure detection sensor cell 81.

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

  At the same time, the pump current Ip1 is used to control the electromotive force of the oxygen partial pressure detection sensor cell 80 for main pump control. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and the electromotive force V0 is controlled, so that the third diffusion rate limiting unit 30 controls the second internal space. The gradient of the oxygen partial pressure in the gas to be measured introduced into the gas 40 is controlled so as to be always constant. When used as a NOx sensor, the oxygen concentration in the second internal space 40 is maintained at a constant value of about 0.001 ppm by the action of the main pump cell 21 and the auxiliary pump cell 50.

  The measurement pump cell 41 measures the NOx concentration in the gas to be measured in the second internal space 40. The measurement pump cell 41 includes a measurement electrode 44 provided on a top surface of the first solid electrolyte layer 4 facing the second internal space 40 and spaced from the third diffusion rate-determining portion 30, an outer pump electrode 23, The electrochemical pump cell is constituted by the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

  The measurement electrode 44 is a porous cermet electrode. The measurement electrode 44 also functions as a NOx reduction catalyst that reduces NOx present in the atmosphere in the second internal space 40. Further, the measurement electrode 44 is covered with an electrode protective film 45.

The electrode protective film 45 is a film composed of a porous body mainly composed of alumina (Al ₂ O ₃ ). The electrode protective film 45 serves to limit the amount of NOx flowing into the measurement electrode 44 and also functions as a protective film for the measurement electrode 44. In the measurement pump cell 41, oxygen generated by the decomposition of nitrogen oxides in the atmosphere around the measurement electrode 44 can be pumped out, and the generated amount can be detected as the pump current Ip2. The electrode protective film 45 is produced by the method for producing a porous alumina body of the present invention so as to have a predetermined gas diffusion resistance designed in advance.

  Further, in order to detect the oxygen partial pressure around the measurement electrode 44, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, An electrochemical sensor cell, that is, an oxygen partial pressure detecting sensor cell 82 for controlling a measurement pump is constituted by the reference electrode 42. The variable power supply 46 is controlled on the basis of the electromotive force V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82.

The gas to be measured introduced into the second internal space 40 reaches the measurement electrode 44 through the electrode protective film 45 under a condition in which the oxygen partial pressure is controlled. Nitrogen oxide in the gas to be measured around the measurement electrode 44 is reduced (2NO → N ₂ + O ₂ ) to generate oxygen. The generated oxygen is pumped by the measurement pump cell 41. At this time, the variable power source is set so that the control voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 becomes constant. The voltage Vp2 is controlled. Since the amount of oxygen generated around the measurement electrode 44 is proportional to the concentration of nitrogen oxide in the gas to be measured, the nitrogen oxide in the gas to be measured using the pump current Ip2 in the measurement pump cell 41. The concentration will be calculated.

  In addition, if the measurement electrode 44, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42 are combined to form an oxygen partial pressure detecting means as an electrochemical sensor cell, The electromotive force according to the difference between the amount of oxygen generated by the reduction of the NOx component in the surrounding atmosphere and the amount of oxygen contained in the reference atmosphere can be detected, thereby the concentration of the NOx component in the gas to be measured Is also possible.

  The second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83. The oxygen partial pressure in the gas to be measured outside the sensor can be detected by the electromotive force Vref obtained by the sensor cell 83.

  In the gas sensor 200 having such a configuration, by operating the main pump cell 21 and the auxiliary pump cell 50, the oxygen partial pressure is always kept at a constant low value (a value that does not substantially affect the measurement of NOx). A gas to be measured is supplied to the measurement pump cell 41. Therefore, the NOx concentration in the measurement gas is determined based on the pump current Ip2 that flows when oxygen generated by the reduction of NOx is pumped out of the measurement pump cell 41 in proportion to the NOx concentration in the measurement gas. You can know.

  Further, the sensor element 201 includes a heater unit 70 that plays a role of temperature adjustment for heating and maintaining the sensor element 201 in order to increase the oxygen ion conductivity of the solid electrolyte. The heater unit 70 includes a heater electrode 71, a heater 72, a through hole 73, a heater insulating layer 74, and a pressure dissipation hole 75.

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

  The heater 72 is an electric resistor formed in a form sandwiched between the second substrate layer 2 and the third substrate layer 3 from above and below. The heater 72 is connected to the heater electrode 71 through the through hole 73, generates heat by being supplied with power from the outside through the heater electrode 71, and heats and keeps the solid electrolyte forming the sensor element 201.

  The heater 72 is embedded over the entire area from the first internal space 20 to the second internal space 40, and the entire sensor element 201 can be adjusted to a temperature at which the solid electrolyte is activated. ing.

  The heater insulating layer 74 is an insulating layer formed on the upper and lower surfaces of the heater 72 by an insulator such as alumina. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and electrical insulation between the third substrate layer 3 and the heater 72.

  The pressure dissipating hole 75 is a portion that is provided so as to penetrate the third substrate layer 3 and communicate with the reference gas introduction space 43, and is for the purpose of alleviating the increase in internal pressure accompanying the temperature increase in the heater insulating layer 74. Formed.

  Next, an example of a manufacturing method of such a gas sensor 200 will be described below. The gas sensor 200 can be basically manufactured in the same manner as the manufacturing method of the gas sensor 100 according to the flowchart shown in FIG. However, the electrode protective film 45 is produced as follows.

  First, a slurry for preparing the electrode protective film 45 is prepared. This slurry is prepared in the same manner as the slurry for producing the electrode protective film 119 of the gas sensor 100. However, the mixing ratio of the alumina powder of the slurry for producing the electrode protective film 45 and the pore forming agent is determined according to the design value of the gas diffusion resistance of the electrode protective film 45. For example, the three-way relationship between the partial pressure of water vapor at the time of firing, the mixing ratio of alumina powder and pore former, and the gas diffusion resistance of the resulting porous alumina body is determined by preliminary experiments, and this time the production is attempted. The water vapor partial pressure during firing corresponding to the gas diffusion resistance of the electrode protective film 45 and the mixing ratio of the alumina powder and the pore-forming agent are read from the above three relationships, and the read mixing ratio is adopted.

  On the other hand, a pattern to be the measurement electrode 44 is printed on the green sheet to be the first solid electrolyte layer 4 so as to have a thickness of about 20 to 30 μm and dried. Printing of the slurry prepared earlier and subsequent drying are repeated on the dried pattern. By doing so, a paste film to be the electrode protection film 45 having a thickness of 20 to 50 μm is formed on the paste film to be the measurement electrode 44. Then, in the subsequent firing step (step S60 in FIG. 2), the pore-forming agent in the paste film that becomes the electrode protective film 45 disappears, whereby the electrode protective film 45 is formed. Firing is performed at a firing temperature of 1350 to 1400 ° C. while adjusting the water vapor partial pressure to be a predetermined predetermined pressure in an air atmosphere. The water vapor partial pressure at this time corresponds to the gas diffusion resistance of the electrode protective film 45 to be manufactured this time when determining the mixing ratio of the alumina powder of the slurry for producing the electrode protective film 45 and the pore former. The water vapor partial pressure during firing and the mixing ratio of the alumina powder and the pore forming agent were read out from the above three relationships, and the read out water vapor partial pressure is adopted. In addition, degreasing is not carried out as a separate step, but is also performed during firing. For this reason, in the firing step, first, degreasing is completed from the start of the temperature rise to the predetermined firing temperature, and then the alumina is fired while being held at the predetermined firing temperature.

  According to the gas sensor 200 described in detail above, the electrode protective film 45 has the gas diffusion resistance as designed, so that the combustible gas concentration in the gas to be measured can be accurately detected regardless of the production lot. . That is, in this embodiment, when producing the electrode protective film 45, the alumina powder and the pore former are mixed at a predetermined constant mixing ratio to obtain a mixed raw material, and a molded body obtained by molding the mixed raw material is obtained. Since the firing is performed at a predetermined firing temperature while adjusting to a predetermined constant water vapor partial pressure in the air atmosphere, the electrode protective film 45 is equivalent even if the production lot of the gas sensor 200 is different. It has gas diffusion resistance. For this reason, variation in the NOx concentration in the gas to be measured can be kept small.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  For example, in the first embodiment described above, the electrode protection film 119 is formed only on the first outer pump electrode 118 of the first pump cell P1, but the same applies to at least one of the other electrodes 116, 122, 124, 128, and 130. An electrode protective film may be formed. The same applies to the second embodiment.

  In the first embodiment described above, the weight parts of the respective raw materials are indicated by numerical values as the slurry for producing the electrode protective film 119, but these numerical values are merely an example. For this reason, you may change suitably as needed. The same applies to the second embodiment.

In the first and second embodiments described above, the HC sensor and the NOx sensor are exemplified, but the electrode protection of the sensor capable of detecting the concentration of hydrogen (H ₂ ), carbon monoxide (CO), ammonia (NH ₃ ), or the like. You may apply the alumina porous body of this invention to a film | membrane.

[Comparative Example 1]
A plurality of gas sensors 100 were produced in accordance with the manufacturing method of the gas sensor 100 of the first embodiment described above. Specifically, when preparing the slurry for producing the electrode protective film 119, the weight ratio of the pore former to the alumina powder is 14% by weight, 16% by weight, and 20% by weight. Manufactured. The firing conditions were 1350 ° C. at a firing temperature without adjusting the water vapor partial pressure in an air atmosphere. The thickness of the electrode protective film 119 was 27 μm. For the obtained gas sensor 100, the first pump cell P1 when the variable power source 120 of the first pump cell P1 is controlled so that the voltage of the potentiometer 126 of the first sensor cell S1 becomes constant at a predetermined low voltage in the atmosphere. The pump current (limit current) Ip flowing through both electrodes 116 and 118 was measured, and the gas diffusion resistance Rd was obtained from the following equation (1). The result is shown in FIG. As is apparent from FIG. 4, even when the weight ratio of the pore forming agent to the alumina powder was the same, the gas diffusion resistance Rd varied greatly when the production lots were different.
Rd = 4FD (Po-Pe) / Ip · R · T (1)
(F is the Faraday constant (96490 C / mol), D is the diffusion constant of oxygen molecules (1.68 cm ² / sec), Po is the oxygen partial pressure of the gas to be measured (in the atmosphere, 0.21 atm), Pe is the protective film Oxygen partial pressure in the vicinity of the inner measurement electrode (zero to determine the limit current), Ip is the limit current value, R is the gas constant (82.05 atm · cm ³ / mol), and T is the ambient temperature (873K))

[Example 1]
A plurality of gas sensors 100 were produced in accordance with the manufacturing method of the gas sensor 100 of the first embodiment described above. Specifically, when preparing the slurry for producing the electrode protective film 119, the weight ratio of the pore former to the alumina powder is 14% by weight, 16% by weight, and 20% by weight. Manufactured. Further, the firing conditions were performed at a firing temperature of 1350 ° C. while adjusting so as to have a predetermined water vapor partial pressure in an air atmosphere. The thickness of the electrode protective film 119 was 27 μm. Here, the steam partial pressure in the firing furnace is the steam partial pressure at the exit of the firing furnace, and the steam partial pressure in the firing furnace is adjusted for the amount of added water by feedback control using the measured steam partial pressure at the exit of the firing furnace. I managed it. For the obtained gas sensor 2, the gas diffusion resistance Rd was determined in the same manner as in Comparative Example 1. The result is shown in FIG. As is apparent from FIG. 5, even when the weight ratio of the pore forming agent to the alumina powder is the same, the gas diffusion resistance Rd is different if the water vapor partial pressure during firing is different. It was also found that when the weight ratio of the pore forming agent to the alumina powder and the water vapor partial pressure during firing were the same, the gas diffusion resistance Rd had the same value.

[Comparative Example 2]
A plurality of gas sensors 100 were produced in accordance with the manufacturing method of the gas sensor 100 of the first embodiment described above. Specifically, when preparing the slurry for producing the electrode protective film 119, 10 lots were manufactured in which the weight ratio of the pore former to the alumina powder was 16% by weight. The firing conditions at this time were 1350 ° C. firing temperature without adjusting the water vapor partial pressure in the air atmosphere. The thickness of the electrode protective film 119 was 27 μm. For the obtained gas sensor 2, a model gas prepared by mixing 1000-5000 ppm of propane gas as hydrocarbon (HC) gas with nitrogen gas is prepared, and the relationship between the HC gas concentration and the detected current of the ammeter 134 of the gas sensor 100 Are summarized in a table and a graph (see Table 1 and FIG. 6). The numerical values in Table 1 indicate the detected current (μA). In FIG. 6, the variation in the detected current is indicated by a white band. As is apparent from Table 1 and FIG. 6, even if the weight ratio of the pore forming agent to the alumina powder was the same, the detection current for a predetermined HC gas concentration varied greatly when the production lots were different.

[Example 2]
A plurality of gas sensors 100 were produced in accordance with the manufacturing method of the gas sensor 100 of the first embodiment described above. Specifically, when preparing the slurry for producing the electrode protective film 119, 10 lots were manufactured in which the weight ratio of the pore former to the alumina powder was 16% by weight. Firing conditions at this time were performed at a firing temperature of 1350 ° C. while adjusting to a predetermined water vapor partial pressure (10 hPa) in an air atmosphere. The thickness of the electrode protective film 119 was 27 μm. Here, the water vapor partial pressure in the baking furnace was set to the water vapor partial pressure at the baking furnace outlet, and was managed by adjusting the amount of added water by feedback control using the measured value of the water vapor partial pressure at the baking furnace outlet. About the obtained gas sensor 100, the table | surface and the graph were created like the comparative example 2 (refer Table 2 and FIG. 7). As is apparent from FIG. 7, if the weight ratio of the pore former to the alumina powder is the same and the water vapor partial pressure at the time of manufacture is the same, the detected current for a given HC gas concentration is the same even if the production lot is different. It became almost the same value. That is, it can be seen that the detection current for a predetermined HC gas concentration can be determined almost uniquely by determining the weight ratio of the pore former to the alumina powder and the water vapor partial pressure during firing.

[Comparative Example 3]
The gas sensor 200 was manufactured using the interparticle embedding method according to the manufacturing method of the gas sensor 200 of the second embodiment described above. That is, alumina particles having a particle size of 0.25 μm and a sphericity of 0.90 were used as the alumina aggregate powder, and alumina particles having a particle size of 0.03 μm and a sphericity of 0.87 were used as the alumina pore former. The gas sensor 200 was produced according to the manufacturing method of the gas sensor 200 of the second embodiment described above. Specifically, when preparing the slurry for producing the electrode protective film 45, the weight ratio of the alumina pore forming agent to the alumina aggregate powder is 3.4 wt%, 3.8 wt%, 4.7 wt%. A 5.8 wt% slurry was prepared. Moreover, the baking temperature was 1350-1400 degreeC, without adjusting water vapor partial pressure in an atmospheric condition. The thickness of the electrode protective film 45 was 27 μm. In this way, eight lots of gas sensors 200 were produced for each weight%. For the obtained gas sensor 200, the gas diffusion resistance Rd was obtained in the same manner as in Comparative Example 1. The result is shown in FIG. As is apparent from FIG. 8, even when the weight ratio of the alumina pore-forming agent to the alumina aggregate powder is the same, the gas diffusion resistance Rd varies greatly when the production lots are different. In particular, the range of variation was widened at 3.8% by weight and 4.7% by weight.

[Example 3]
Using the same alumina aggregate powder and alumina pore former as in Comparative Example 3, a gas sensor 200 was produced according to the method for producing the gas sensor 200 of the second embodiment described above. Specifically, when preparing the slurry for producing the electrode protective film 45, the weight ratio of the alumina pore forming agent to the alumina aggregate powder is 3.4 wt%, 3.8 wt%, 4.7 wt%. A 5.8 wt% slurry was prepared. In addition, for each weight percent, the water vapor partial pressure was adjusted to 6 levels of 1 hPa, 5 hPa, 13 hPa, 20 hPa, 30 hPa, and 35 hPa in an air atmosphere, and the firing temperature was 1350 to 1400 ° C. The thickness of the electrode protective film 45 was 27 μm. For 3.4 wt% and 5.6 wt%, firing at each water vapor partial pressure was performed once, and for 3.8 wt% and 4.7 wt%, firing at each water vapor partial pressure was 2 I went there once. For the obtained gas sensor 200, the gas diffusion resistance Rd was obtained in the same manner as in Comparative Example 1. The result is shown in FIG. As can be seen from FIG. 9, even when the weight ratio of the alumina pore-forming agent to the alumina aggregate powder is the same, the gas diffusion resistance Rd is different if the water vapor partial pressure during firing is different. It was also found that the gas diffusion resistance Rd was uniquely determined if the weight ratio of the pore former to the alumina powder and the water vapor partial pressure during firing were determined.

Therefore, in order to obtain a desired gas diffusion resistance Rd (for example, a value in the range of 130 to 1000 cm ⁻¹ ), the weight ratio of the alumina pore forming agent to the alumina aggregate powder and the water vapor partial pressure during firing are set. I understood that I should do. In addition, it is preferable to set water vapor partial pressure in the range of 1-30 hPa, and it is more preferable to set in the range of 5-20 hPa. Setting to a value of less than 1 hPa is not preferable because it is difficult to configure the firing furnace. Setting a value exceeding 30 hPa is not very meaningful because the value of the diffusion resistance Rd becomes almost constant when it exceeds 30 hPa. If it is 5-20 hPa, on the structure of a baking furnace, since water vapor partial pressure can be adjusted stably, it is preferable.

[Example 4, Comparative Example 4]
Using the same alumina aggregate powder and alumina pore former as in Comparative Example 3, a gas sensor 200 was produced according to the method for producing the gas sensor 200 of the second embodiment described above. Specifically, in Example 4, when preparing the slurry for producing the electrode protective film 45, a plurality of alumina pore formers with a weight ratio of 4.3% by weight to the alumina aggregate powder were prepared. Moreover, the baking conditions adjusted the water vapor partial pressure to 12 hPa in air | atmosphere, and set the baking temperature to 1350-1400 degreeC. On the other hand, in Comparative Example 4, a plurality of the same 4.3% by weight were prepared. The firing conditions were such that the partial pressure of water vapor was not adjusted in the air atmosphere, and the firing temperature was 1350 to 1400 ° C. In any case, the thickness of the electrode protective film 45 was 27 μm. For each of the gas sensors 200 obtained in Example 4 and Comparative Example 4, a model gas prepared by mixing 500 ppm of NO gas with nitrogen gas was prepared, and the relationship with the current Ip2 of the gas sensor 200 was summarized in a graph (FIG. 10). As apparent from FIG. 10, in Comparative Example 4, the detected current with respect to a predetermined NO gas concentration varied greatly when the production lots were different. On the other hand, in Example 4, even if the production lots were different, the detected current with respect to a predetermined NO gas concentration hardly varied. That is, it can be seen that the detection current for a predetermined NO gas concentration can be determined almost uniquely by determining the weight ratio of the pore-forming agent to the alumina powder and the water vapor partial pressure during firing.

DESCRIPTION OF SYMBOLS 1 1st board | substrate layer, 2nd board | substrate layer, 3rd board | substrate layer, 4th 1st solid electrolyte layer, 5 spacer layer, 6 2nd solid electrolyte layer, 10 gas inlet, 11 1st diffusion control part, 12 buffer Space, 13 Second diffusion limiting part, 20 First internal space, 21 Main pump cell, 22 Inner pump electrode, 22a Ceiling electrode part, 22b Bottom electrode part, 23 Outer pump electrode, 30 Third diffusion limiting part, 40 Second Internal space, 41 Measurement pump cell, 42 Reference electrode, 43 Reference gas introduction space, 44 Measurement electrode, 45 Electrode protective film, 46 Variable power supply, 48 Air introduction layer, 50 Auxiliary pump cell, 51 Auxiliary pump electrode, 51a Ceiling electrode part 51b Bottom electrode part, 52 Variable power supply, 70 Heater part, 71 Heater electrode, 72 Heater, 73 Through hole, 74 Heater insulating layer, 75 Pressure dissipation hole, 0 oxygen partial pressure detection sensor cell for main pump control, 81 oxygen partial pressure detection sensor cell for auxiliary pump control, 82 oxygen partial pressure detection sensor cell for measurement pump control, 83 sensor cell, 100 gas sensor, 104a to 104f 1st to 6th solid electrolyte Layer, 106 cavity, 110 reference passage, 112 diffusion-controlled passage, 116 first inner pump electrode, 118 first outer pump electrode, 119 electrode protective film, 120 variable power supply, 122 measurement electrode, 124 reference electrode, 126 potentiometer, 128 Second inner pump electrode, 130 Second outer pump electrode, 132 DC power source, 134 Ammeter, 142 Heater, P1 First pump cell, P2 Second pump cell, Rd Gas diffusion resistance, S1 First sensor cell, Gas sensor 200, Sensor element 101.

Claims (6)

Alumina powder and pore former are mixed at a predetermined mixing ratio to form a mixed raw material, and the molded body obtained by molding the mixed raw material is adjusted in a firing atmosphere so as to have a constant water vapor partial pressure in the air atmosphere. Firing while realizing the desired porous properties,
A method for producing porous alumina. The three-way relationship between the water vapor partial pressure during firing, the mixing ratio of the alumina powder and the pore forming agent, and the gas diffusion resistance of the resulting alumina porous body is determined, and the gas diffusion resistance of the alumina porous body to be manufactured is Determine the water vapor partial pressure during firing and the mixing ratio of the alumina powder and the pore former from the above relationship so that the desired value is obtained, and then mix the alumina powder and the pore former at the determined mixing ratio. Firing at a predetermined calcining temperature while adjusting the molded body obtained by molding the mixed raw material to the determined steam partial pressure in the air atmosphere as a mixed raw material,
A method for producing porous alumina. The alumina porous body is for an electrode protective film of a gas sensor,
The manufacturing method of the alumina porous body of Claim 1 or 2. The alumina powder is alumina particles functioning as an aggregate, and the pore-forming agent is alumina particles having a particle diameter smaller than that of the alumina powder, and a plurality of the pore-forming agents are provided in the gaps between the alumina powders. Is used as the molded body,
The manufacturing method of the alumina porous body of any one of Claims 1-3. The alumina powder is an alumina particle having a particle size of 0.1 to 0.7 μm and a sphericity of 0.7 to 1.0, and the pore former is an alumina having a particle size of 0.01 to 0.1 μm. Particles, fired at 1200-1400 ° C.,
The manufacturing method of the alumina porous body of Claim 4. The mixing ratio of the alumina powder and the pore former is set in the range of 3.4 to 5.6% by weight of the pore former with respect to the alumina powder.
The manufacturing method of the alumina porous body of Claim 4 or 5.