Honeycomb structure and electrically heated carrier using the honeycomb structure

Abstract

To provide a honey-comb structure excellent in balance between oxidation resistance and thermal shock resistance under a high temperature environment.SOLUTION: A honey-comb structure according to an embodiment of the present invention comprises a partition wall defining a cell forming a flow path of a fluid by extending from a first end surface to a second end surface and an outer periphery wall, and the partition wall and the outer periphery wall are composed of ceramic containing silicon carbide and silicon. In one embodiment, on a surface of silicon, an oxide film having a thickness of 0.1 μm to 5.0 μm is formed. In another embodiment, the honey-comb structure contains cristobalite of 1.0 mass% or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a honeycomb structure and an electrically heated carrier using the honeycomb structure. [Background technology]

[0002] Honeycomb structures made of cordierite or silicon carbide, on which catalysts are supported, are used to treat harmful substances in exhaust gases emitted from automobile engines. A typical example of such a honeycomb structure is a columnar honeycomb structure with partition walls that extend from the first end face to the second end face, dividing multiple cells that form exhaust gas flow paths. When treating exhaust gas with a catalyst supported on a honeycomb structure, it is necessary to raise the catalyst to a predetermined temperature, but there is a problem that the exhaust gas is not sufficiently purified because the catalyst temperature is low when the engine is started. To solve this problem, development is underway on a system called an electroheated catalyst (EHC), which uses a honeycomb structure made of conductive ceramic with electrodes placed on it, and heats up the honeycomb structure itself by applying an electric current, thereby raising the catalyst supported on the honeycomb structure to its activation temperature before or during engine start-up. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Patent No. 5735428 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] During use (such as when a vehicle is running), EHCs may be exposed to high-temperature oxidizing atmospheres, which can cause disruption of the conductive path and / or a decrease in conductivity, leading to a significant increase in resistance. Furthermore, EHCs may have insufficient thermal shock resistance at high temperatures. The main objective of the present invention is to provide a honeycomb structure that offers an excellent balance between oxidation resistance and thermal shock resistance in high-temperature environments, and an electrically heated carrier using such a honeycomb structure. [Means for solving the problem]

[0005] A honeycomb structure according to an embodiment of the present invention has a partition wall that defines cells extending from a first end face to a second end face and forming fluid flow paths, and an outer peripheral wall, wherein the partition wall and the outer peripheral wall are made of ceramics containing silicon carbide and silicon. In one embodiment, an oxide film with a thickness of 0.1 μm to 5.0 μm is formed on the silicon surface. In another embodiment, the honeycomb structure contains 1.0 mass% or more of cristobalite. The thermal expansion coefficient of the above honeycomb structure is, for example, 4.00 ppm / K to 5.30 ppm / K, or for example, 4.00 ppm / K to 4.60 ppm / K, or for example, 4.20 ppm / K to 4.35 ppm / K. In one embodiment, the thickness of the oxide film is 0.1 μm to 0.2 μm, and the thermal expansion coefficient of the honeycomb structure is 4.20 ppm / K to 4.35 ppm / K. In one embodiment, the cristobalite content is 1.5% by mass to 3.5% by mass, and the thermal expansion coefficient of the honeycomb structure is 4.20 ppm / K to 4.35 ppm / K. According to another aspect of the present invention, an electrically heated carrier is provided. The electrically heated carrier comprises the honeycomb structure described above, a pair of electrode layers disposed on the outer surface of the outer peripheral wall of the honeycomb structure, straddling the central axis of the honeycomb structure, and a pair of metal terminals connected to the pair of electrode layers. [Effects of the Invention]

[0006] According to embodiments of the present invention, a honeycomb structure with an excellent balance between oxidation resistance and thermal shock resistance in high-temperature environments can be realized. [Brief explanation of the drawing]

[0007] [Figure 1] This is a schematic perspective view of an electrically heated carrier including a honeycomb structure according to one embodiment of the present invention. [Figure 2] Figure 1 is a schematic cross-sectional view of the electrically heated carrier in a direction parallel to the flow path direction of the exhaust gas. [Modes for carrying out the invention]

[0008] Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments.

[0009] Figure 1 is a schematic perspective view of an electrically heated carrier including a honeycomb structure according to one embodiment of the present invention, and Figure 2 is a schematic cross-sectional view of the electrically heated carrier of Figure 1 in a direction parallel to the flow path direction of the exhaust gas. First, the honeycomb structure will be described, and the electrically heated carrier will be described later in section B.

[0010] A. Honeycomb structure A-1. Structure of the honeycomb structure The honeycomb structure 100 in the illustrated example has a partition wall 30 that defines a cell 20 extending from a first end face 10a to a second end face 10b and forming a fluid flow path, and an outer peripheral wall 40. In Figure 2, the fluid can flow in either the left or right direction of the paper. The fluid can be any suitable liquid or gas depending on the purpose. For example, when the honeycomb structure is used in an electrically heated carrier described later, the fluid is preferably exhaust gas.

[0011] The partition wall 30 and the outer peripheral wall 40 are made of ceramics containing silicon carbide and silicon (hereinafter sometimes referred to as silicon carbide-silicon composite material). The ceramics contain, for example, 90% or more by mass of silicon carbide and silicon in total, or for example, 95% or more by mass. With such a configuration, the volume resistivity of the honeycomb structure at 25°C can be set to a predetermined range (for example, 0.1 Ω·cm to 200 Ω·cm, or for example, 1 Ω·cm to 200 Ω·cm, and further for example, 10 Ω·cm to 100 Ω·cm). As a result, even when the honeycomb structure is energized with a high-voltage power supply of, for example, 200V or more, excessive current flow can be suppressed, and the desired heat generation can be achieved by the flow of an appropriate current. The ceramics may also contain substances other than silicon carbide-silicon composite material. Examples of such substances include aluminum and strontium.

[0012] A silicon carbide-silicon composite typically includes silicon carbide particles as aggregate and silicon as a binder to bind the silicon carbide particles together. In a silicon carbide-silicon composite, for example, multiple silicon carbide particles are bound together by silicon such that pores are formed between them. That is, the partition wall 30 and the outer periphery wall 40 containing the silicon carbide-silicon composite may be, for example, porous.

[0013] In embodiments of the present invention, an oxide film with a thickness of 0.1 μm to 5.0 μm is formed on the silicon surface. By forming such an oxide film on the silicon surface, a honeycomb structure with an excellent balance of oxidation resistance and thermal shock resistance in high-temperature environments can be realized. More specifically, the increase in resistance of the honeycomb structure in a high-temperature oxidizing atmosphere can be suppressed, and the coefficient of thermal expansion of the honeycomb structure can be reduced. The thickness of the oxide film is preferably 0.1 μm to 2.0 μm, more preferably 0.1 μm to 1.0 μm, even more preferably 0.1 μm to 0.5 μm, and particularly preferably 0.1 μm to 0.2 μm. By setting the thickness of the oxide film within this range, the increase in resistance in a high-temperature oxidizing atmosphere can be further suppressed, and the coefficient of thermal expansion can be further reduced. The thickness of the oxide film can be determined, for example, from an image taken with a scanning electron microscope (SEM).

[0014] The oxide film can be formed, for example, by heat-treating the honeycomb structure (as described later, a predetermined amount or more of cristobalite can also be formed by this heat treatment). The heating temperature in the heat treatment is, for example, 1300°C or less, 1200°C or less, 1150°C or less, 1100°C or less, 1050°C or less, 1000°C or less, and 950°C or less. On the other hand, the heating temperature is, for example, 750°C or higher, and 800°C or higher. If the heating temperature is within this range, an oxide film of the predetermined thickness can be formed. As a result, the increase in resistance of the honeycomb structure under a high-temperature oxidizing atmosphere can be suppressed, and the thermal expansion coefficient of the honeycomb structure can be reduced. Preferably, by setting the heating temperature to 1150°C or lower, these effects become more pronounced. The heating time can be varied according to the heating temperature. For example, when the heating temperature is 1200°C or higher, the heating time is preferably 20 minutes to 100 hours, more preferably 30 minutes to 80 hours, even more preferably 30 minutes to 40 hours, and particularly preferably 5 hours to 10 hours. For example, when the heating temperature is 1150°C or lower, the heating time is preferably 1 hour or more, more preferably 5 hours or more, even more preferably 10 hours or more, and particularly preferably 20 hours to 70 hours. If the heating time is too long, the coefficient of thermal expansion will increase, which may lead to a problem of reduced thermal shock resistance. If the heating time is too short, the oxide film (and / or cristobalite described later) may not be sufficiently formed. The heat treatment may be carried out in an atmospheric environment or in a water vapor atmosphere (for example, including supplying a gas with a nitrogen base and a water vapor content adjusted to 10% to 30% by volume during the heat treatment). Under the same heating conditions, performing the heat treatment in a steam atmosphere allows for the formation of an oxide film of a more desirable thickness (and / or a more desirable amount of cristobalite, as described later), further promoting the effect of suppressing resistance increase and reducing the coefficient of thermal expansion.

[0015] The oxide film is substantially composed of silicon oxide. The oxide film may be formed on the silicon surface as the binder as described above. Therefore, the oxide film may be formed not only on the silicon surface but also on the surface of silicon carbide particles or other parts within the structure of the partition walls and the outer peripheral wall. Substantially, oxidation of silicon under high-temperature environment mainly causes an increase in resistance. Therefore, by forming a predetermined oxide film on the silicon surface of the honeycomb structure in advance, the effect of suppressing the increase in resistance and the effect of reducing the thermal expansion coefficient can be efficiently obtained.

[0016] In an embodiment of the present invention, further / or, the honeycomb structure contains cristobalite of 1.0% by mass or more based on the total mass of the honeycomb structure (substantially the partition walls and the outer peripheral wall). By including cristobalite in the honeycomb structure, a honeycomb structure excellent in the balance between oxidation resistance and thermal shock resistance under a high-temperature environment can be realized, similar to the case where an oxide film is formed on the silicon surface. More specifically, an increase in resistance of the honeycomb structure under a high-temperature oxidation atmosphere can be suppressed, and the thermal expansion coefficient of the honeycomb structure can be reduced. The content of cristobalite is preferably 1.0% by mass to 7.5% by mass, more preferably 1.0% by mass to 6.0% by mass, still more preferably 1.2% by mass to 4.0% by mass, and particularly preferably 1.5% by mass to 3.5% by mass. By setting the content of cristobalite within such a range, an increase in resistance under a high-temperature oxidation atmosphere can be further suppressed, and the thermal expansion coefficient can be further reduced. Cristobalite can typically be formed in the oxide film formed by the above heat treatment. Substantially, oxidation of silicon under high-temperature environment mainly causes an increase in resistance. Therefore, by forming an oxide film containing cristobalite on the silicon surface of the honeycomb structure in advance, the effect of suppressing the increase in resistance and the effect of reducing the thermal expansion coefficient can be efficiently obtained. The content of cristobalite can be measured, for example, by X-ray diffraction method.

[0017] The thermal expansion coefficient of the honeycomb structure is, for example, 4.00 ppm / K to 5.30 ppm / K, preferably 4.00 ppm / K to 4.75 ppm / K, more preferably 4.00 ppm / K to 4.60 ppm / K, still more preferably 4.10 ppm / K to 4.50 ppm / K, and particularly preferably 4.20 ppm / K to 4.35 ppm / K. According to an embodiment of the present invention, such a thermal expansion coefficient can be achieved by forming an oxide film on the silicon surface and / or by the honeycomb structure containing a predetermined amount or more of cristobalite. As a result, a honeycomb structure having excellent thermal shock resistance can be realized. For example, in a heating and cooling durability test (replaced with a specified temperature of 825°C or higher and an environment of 100°C), a honeycomb structure with high thermal shock resistance can be realized.

[0018] The strength / Young's modulus ratio σ / E of the honeycomb structure is preferably 0.40 or more, more preferably 0.45 to 0.90, and still more preferably 0.65 to 0.85. According to an embodiment of the present invention, such a strength / Young's modulus ratio σ / E can be achieved by forming an oxide film on the silicon surface and / or by the honeycomb structure containing a predetermined amount or more of cristobalite. As a result, a honeycomb structure having excellent thermal shock resistance can be realized. The Young's modulus E can be measured in accordance with JIS R1602, and the strength σ is typically the flexural strength at four points that can be measured in accordance with JIS R1601.

[0019] Hereinafter, a typical configuration of the honeycomb structure will be described.

[0020] The shape of the honeycomb structure can be appropriately designed according to the purpose. The honeycomb structure 100 in the illustrated example is cylindrical (the cross-sectional shape perpendicular to the direction in which the cells extend is circular), but the honeycomb structure may also be columnar with a cross-sectional shape that is, for example, elliptical or polygonal (e.g., quadrilateral, pentagonal, hexagonal, heptagonal, or octagonal). The length of the honeycomb structure can be appropriately set according to the purpose. The length of the honeycomb structure may be, for example, 5 mm to 250 mm, or 10 mm to 150 mm, or 20 mm to 100 mm. The diameter of the honeycomb structure can be appropriately set according to the purpose. The diameter of the honeycomb structure may be, for example, 20 mm to 200 mm, or 30 mm to 100 mm. If the cross-sectional shape of the honeycomb structure is not circular, the diameter of the honeycomb structure can be the diameter of the largest inscribed circle inscribed in the cross-sectional shape of the honeycomb structure (e.g., polygonal).

[0021] The partition wall 30 and the outer peripheral wall 40 may be porous bodies containing silicon carbide-silicon composite material, as described above. The silicon content in the silicon carbide-silicon composite material is preferably 10% to 40% by mass, and more preferably 15% to 35% by mass. If the silicon content is 10% by mass or more, the strength of the honeycomb structure will be sufficient. If the silicon content is 40% by mass or less, the honeycomb structure can maintain its shape accurately during firing.

[0022] The average particle size of the silicon carbide particles is preferably 3 μm to 50 μm, more preferably 3 μm to 40 μm, and even more preferably 10 μm to 35 μm. If the average particle size of the silicon carbide particles is within this range, the volume resistivity of the honeycomb structure can be set to the appropriate range described above. The average particle size of the silicon carbide particles can be measured, for example, by laser diffraction.

[0023] The average pore diameter of the partition wall 30 and the outer peripheral wall 40 is preferably 2 μm to 20 μm, more preferably 2 μm to 15 μm, and even more preferably 4 μm to 8 μm. If the average pore diameter of the partition wall is within this range, the volume resistivity can be within the appropriate range described above. The average pore diameter can be measured, for example, by a mercury porosimeter.

[0024] The porosity of the partition walls 30 and the outer periphery walls 40 is preferably 30% to 60%, and more preferably 35% to 45%. If the porosity is 30% or higher, deformation of the honeycomb structure during firing can be sufficiently suppressed. If the porosity is 60% or lower, the strength of the honeycomb structure will be sufficient. Porosity can be measured, for example, by a mercury porosimeter.

[0025] The thickness of the partition wall 30 can be appropriately set according to the purpose. The thickness of the partition wall 30 can be, for example, 50 μm to 0.3 mm, or for example, 150 μm to 250 μm. If the thickness of the partition wall is within this range, the mechanical strength of the honeycomb structure can be made sufficient, and the opening area (total area of ​​cells in the cross-section) can be made sufficient, thereby suppressing pressure loss when exhaust gas is passed through when the honeycomb structure is used as a catalyst support.

[0026] The density of the partition wall 30 can be appropriately set depending on the purpose. For example, the density of the partition wall 30 may be 0.5 g / cm³. 3 ~5.0g / cm 3 This is possible. If the density of the partitions is within this range, the honeycomb structure can be made lighter while maintaining sufficient mechanical strength. The density can be measured, for example, by the Archimedes method.

[0027] In one embodiment, the thickness of the outer peripheral wall 40 is greater than the thickness of the partition wall 30. With such a configuration, it is possible to suppress the destruction, cracking, and other damage to the outer peripheral wall caused by external forces (e.g., external impact, thermal stress due to the temperature difference between the exhaust gas and the outside). The thickness of the outer peripheral wall 40 is, for example, 0.05 mm or more, preferably 0.1 mm or more, and more preferably 0.15 mm or more. However, if the outer peripheral wall is made too thick, the heat capacity increases, the temperature difference between the inner peripheral side of the outer peripheral wall and the partition wall on the inner peripheral side becomes large, and the thermal shock resistance decreases. Therefore, the thickness of the outer peripheral wall is preferably 1.0 mm or less, more preferably 0.7 mm or less, and even more preferably 0.5 mm or less.

[0028] Cell 20 has any suitable cross-sectional shape in a direction perpendicular to the direction in which the cell extends. In the illustrated example, the partition walls 30 defining the cell are perpendicular to each other, and the cell 20 is defined to have a rectangular (square) cross-sectional shape except for the part that is in contact with the outer peripheral wall 40. The cross-sectional shape of cell 20 may be other than a square, such as a triangle, pentagon, or a polygon with hexagons or more. The cross-sectional shape of the cell is preferably a rectangle or a hexagon. With such a configuration, there is an advantage that the pressure loss when exhaust gas is flowed is small and the purification performance is excellent.

[0029] The cell density in the direction perpendicular to the direction in which the cells 20 extend (i.e., the number of cells 20 per unit area) can be appropriately set depending on the purpose. The cell density is preferably 4 cells / cm². 2 ~150 cells / cm 2 More preferably, 50 cells / cm 2 ~150 cells / cm 2 And more preferably 70 cells / cm 2 ~100 cells / cm 2 Therefore, if the cell density is within this range, it is possible to ensure sufficient strength and effective GSA (geometric surface area, i.e., catalyst support area) of the honeycomb structure, while suppressing pressure loss when exhaust gas is passed through it.

[0030] A-2. Method for manufacturing a honeycomb structure Honeycomb structures can be manufactured by any suitable method. Representative examples are described below.

[0031] First, a molding raw material is prepared by adding metallic silicon powder, a binder, a surfactant, a pore-forming agent, water, etc., to silicon carbide powder. As described in item A-1 above, the metallic silicon powder can be blended in an amount of preferably 10% to 40% by mass relative to the total mass of silicon carbide powder and metallic silicon powder. The average particle size of silicon carbide particles in the silicon carbide powder is preferably 3 μm to 50 μm, as described in item A-1 above. The average particle size of metallic silicon particles in the metallic silicon powder is preferably 2 μm to 35 μm. If the average particle size of the metallic silicon particles is too small, the volume resistivity of the resulting honeycomb structure may become excessively small. If the average particle size of the metallic silicon particles is too large, the volume resistivity of the resulting honeycomb structure may become excessively large. The total content of silicon carbide powder and metallic silicon powder can be appropriately set according to the desired configuration of the resulting honeycomb structure. The total content is preferably 30% to 78% by mass relative to the total mass of the molding material. The average particle size of the metallic silicon particles can be measured, for example, by laser diffraction.

[0032] Examples of binders include methylcellulose, hydroxypropoxylcellulose, hydroxyethylcellulose, carboxymethylcellulose, and polyvinyl alcohol. Among these, it is preferable to use methylcellulose and hydroxypropoxylcellulose in combination. The binder content can also be appropriately set according to the desired configuration of the resulting honeycomb structure. The binder content is preferably 2 to 10 parts by mass, when the total mass of silicon carbide powder and metallic silicon powder is 100 parts by mass.

[0033] Examples of surfactants include ethylene glycol, dextrin, fatty acid soap, and polyalcohol. These may be used individually or in combination of two or more. The surfactant content can also be appropriately set according to the desired configuration of the resulting honeycomb structure. The surfactant content is preferably 0.1 to 2 parts by mass, when the total mass of silicon carbide powder and metallic silicon powder is 100 parts by mass.

[0034] Any suitable material can be used as the pore-forming material, as long as it disappears during firing and forms pores. Examples of pore-forming materials include graphite, starch, foamed resin, superabsorbent resin, and silica gel. The content of the pore-forming material can also be appropriately set according to the desired configuration of the resulting honeycomb structure. The content of the pore-forming material is preferably 0.5 to 10 parts by mass, when the total mass of silicon carbide powder and metallic silicon powder is 100 parts by mass. The average particle size of the pore-forming material is preferably 10 μm to 30 μm. If the average particle size of the pore-forming material is too small, pores may not be formed sufficiently. If the average particle size of the pore-forming material is too large, the molding material may clog the die during molding. The average particle size of the pore-forming material can be measured, for example, by laser diffraction.

[0035] The water content can also be appropriately set according to the desired configuration of the resulting honeycomb structure. The water content is preferably 20 to 60 parts by mass, when the total mass of silicon carbide powder and metallic silicon powder is 100 parts by mass.

[0036] Next, the molding materials are kneaded to form the clay. Any suitable device or mechanism can be used for kneading. Specific examples include a kneader and a vacuum clay mixer.

[0037] Next, the clay is extruded to form a honeycomb structure. During extrusion molding, a die can be used that has a configuration corresponding to the desired overall shape, cell shape, partition wall thickness, cell density, etc., of the honeycomb structure. For example, a wear-resistant cemented carbide can be used as the material for the die. The partition wall thickness, cell density, outer wall thickness, etc., of the honeycomb structure (i.e., the configuration of the die) can be appropriately set to correspond to the desired configuration of the resulting honeycomb structure, taking into account shrinkage during drying and firing, which will be described later.

[0038] Next, the honeycomb molded body is dried to obtain a dried honeycomb body. Any suitable drying method can be used. Specific examples include electromagnetic heating methods such as microwave heating drying and dielectric heating drying (e.g., high-frequency dielectric heating drying); and external heating methods such as hot air drying and superheated steam drying. In one embodiment, a two-stage drying process may be performed. The two-stage drying process includes drying a certain amount of moisture using an electromagnetic heating method, and then drying the remaining moisture using an external heating method. Such a two-stage drying process allows the entire molded body to be dried quickly and uniformly without cracking. More specifically, the two-stage drying process includes removing 30% to 99% by mass of moisture from the honeycomb molded body before drying using an electromagnetic heating method, and then reducing the moisture content of the dried honeycomb body to 3% by mass or less using an external heating method. Dielectric heating is preferred as the electromagnetic heating method, and hot air drying is preferred as the external heating method.

[0039] Next, the honeycomb dried body is calcined to obtain a honeycomb calcined body. In one embodiment, pre-calcination may be performed before calcination. Pre-calcination allows for effective removal of binders and the like. Pre-calcination may be performed, for example, in an air atmosphere at 400°C to 500°C for 0.5 to 20 hours. Calcination may be performed, for example, in an inert atmosphere such as nitrogen or argon at 1400°C to 1500°C for 1 to 20 hours. Pre-calcination and calcination may be performed using any suitable means. Pre-calcination and calcination may be performed, for example, using an electric furnace or a gas furnace.

[0040] Finally, the honeycomb calcined body is heat-treated to form an oxide film on the silicon surface and / or cristobalite within the structure, thereby obtaining a honeycomb structure. The heat-treatment conditions are as described in Section A-1 above.

[0041] B. Electrically heated carrier The illustrated example of an electrically heated carrier 200 comprises a honeycomb structure 100 and a pair of electrode layers 120, 120 arranged on the outer circumference of the honeycomb structures 100, 100 (typically, facing each other across the central axis of the honeycomb structures). Metal terminals (not shown) are connected to each of the electrode layers 120, 120. One metal terminal is connected to the positive terminal of a power source (e.g., a battery), and the other metal terminal is connected to the negative terminal (e.g., a battery).

[0042] The electrode layer extends over, for example, 80% or more of the length between the two end faces of the honeycomb structure 100, preferably 90% or more of the length, and more preferably over the entire length. This configuration has the advantage that the current spreads easily in the axial direction of the electrode layer.

[0043] The electrode layer thickness is preferably 0.01 mm to 5 mm, and more preferably 0.01 mm to 3 mm. This range enhances uniform heating. The electrode layer thickness is defined as the thickness of the outer surface of the electrode layer in the direction normal to the tangent at the measurement point, when the measurement point is observed in a cross-section perpendicular to the cell's stretching direction.

[0044] By making the volume resistivity of the electrode layer lower than that of the honeycomb structure, electricity flows more preferentially through the electrode layer, and when current is applied, the electricity spreads more easily in the direction of the cell's flow path and circumferentially. The volume resistivity of the electrode layer is preferably 1 / 200 or more, 1 / 10 or less, and more preferably 1 / 100 or more and 1 / 20 or less, of the volume resistivity of the honeycomb structure. The volume resistivity of the electrode layer is the value measured at 25°C using the four-terminal method.

[0045] As the material of the electrode layer, conductive ceramics, metal, or a composite material (cermet) of metal and conductive ceramics can be used. As the metal, for example, single metals such as Cr, Fe, Co, Ni, Si, or Ti, or alloys containing at least one metal selected from the group consisting of these metals can be mentioned. Examples of the conductive ceramics include, but are not limited to, silicon carbide (SiC), and metal compounds such as metal silicides such as tantalum silicide (TaSi2) and chromium silicide (CrSi2). Specific examples of the composite material (cermet) of metal and conductive ceramics include a composite material of metal silicon and silicon carbide, a composite material of metal silicides such as tantalum silicide and chromium silicide, metal silicon, and silicon carbide, and further, from the viewpoint of reducing thermal expansion, one or more insulating ceramics such as alumina, mullite, zirconia, cordierite, silicon nitride, and aluminum nitride are added to one or more of the above-mentioned metals. A composite material can be mentioned.

[0046] The metal terminals may be a pair of metal terminals arranged such that one metal terminal faces the other metal terminal across the central axis of the honeycomb structure. When a voltage is applied through the electrode layer, the metal terminals can be energized to generate heat in the honeycomb structure by Joule heat. Therefore, the electric heating type carrier can also be preferably used as a heater. The voltage to be applied can be appropriately set according to the purpose and the like. The voltage to be applied is preferably 12V to 900V, and more preferably 48V to 600V.

[0047] As the material of the metal terminals, there is no particular limitation as long as it is metal, and single metals and alloys can also be adopted. From the viewpoints of corrosion resistance, electrical resistivity, and linear expansion coefficient, for example, it is preferable to use an alloy containing at least one selected from the group consisting of Cr, Fe, Co, Ni, and Ti, and stainless steel and Fe-Ni alloy are more preferable.

[0048] In the electric heating type carrier 200, typically, the catalyst can be supported on the partition wall 30 of the catalyst of the honeycomb structure 100. By supporting the catalyst on the partition wall, when exhaust gas is passed through the cell 20, CO, NO in the exhaust gas xThis makes it possible to convert hydrocarbons and other substances into harmless materials through catalytic reactions. The catalyst may preferably contain noble metals (e.g., platinum, rhodium, palladium, ruthenium, indium, silver, gold), aluminum, nickel, zirconium, titanium, cerium, cobalt, manganese, zinc, copper, tin, iron, niobium, magnesium, lanthanum, samarium, bismuth, barium, and combinations thereof. These elements may be contained as elemental metals, metal oxides, or other metal compounds. The amount of catalyst supported may be, for example, 0.1 g / L to 400 g / L.

[0049] In the electrically heated carrier 200, applying a voltage to the honeycomb structure 100 causes current to flow, generating heat in the honeycomb structure due to Joule heating. This allows the catalyst supported on the honeycomb structure (essentially the partition wall) to be heated to its activation temperature before or during engine startup. As a result, exhaust gas can be adequately treated (typically purified) even during engine startup. According to the embodiment of the present invention, as described in Section A above, the increase in resistance of the honeycomb structure under a high-temperature oxidizing atmosphere can be suppressed, and the thermal expansion coefficient of the honeycomb structure can be reduced. As a result, the electrically heated carrier maintains stable exhaust gas treatment (typically purification) performance over a long period of time, and can suppress destruction, cracking, and other damage to the honeycomb structure even with repeated engine starts and stops.

[0050] The electrically heated carrier is typically housed in any suitable cylindrical member to constitute an exhaust gas treatment device. The exhaust gas treatment device is typically installed in the exhaust gas flow path for the exhaust gas from an automobile engine. [Examples]

[0051] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The evaluation items in the examples are as follows.

[0052] (1) Thickness of the oxide film Images were acquired of the interior of the partition walls or outer walls of the honeycomb structures obtained in the examples and comparative examples using a scanning electron microscope (SEM). Using SEM-EDX, areas on the Si surface where silicon (Si) and oxygen (O) elements were observed were identified as oxide films, and their thickness was determined.

[0053] (2) Determination of cristobalite content The amount of cristobalite was measured as follows. An X-ray diffraction pattern of the oxide film was obtained using an X-ray diffractometer (Bruker AXS, D8 ADVANCE). (Main measurement conditions: characteristic X-rays of CuKα, tube voltage 10kV, tube current 20mA, diffraction angle 2θ = 5°~100°). Next, the X-ray diffraction data obtained by the Rietveld method was analyzed using the analysis software TOPAS (Bruker AXS) to quantify the diffraction line peak intensity of cristobalite.

[0054] (3) Resistance increase rate Test samples were cut from the honeycomb structures obtained in the examples and comparative examples. The volume resistivity R0 of the cut test samples was measured. These test samples were subjected to a 250-hour durability test at 950°C in a water vapor atmosphere, and the volume resistivity R0 of the test samples after the test was measured. 250 The ratio R was measured. 250 / R0 was defined as the resistance increase rate (dimensionless).

[0055] (4) Thermal expansion coefficient A test sample measuring 3 mm (vertical) x 3 mm (horizontal) x 20 mm (length) was cut from the honeycomb structure obtained in the examples and comparative examples. The average linear thermal expansion coefficient (thermal expansion rate) of this test sample was measured in the longitudinal direction of the test sample at temperatures from 40°C to 800°C, in accordance with JIS R1618:2002.

[0056] (5) Strength / Young's modulus ratio Test samples measuring 3 mm (vertical) x 4 mm (horizontal) x 70 mm (length) were cut from the honeycomb structures obtained in the examples and comparative examples. The Young's modulus E (GPa) at room temperature was measured for these test samples in accordance with JIS R1602. Furthermore, the four-point bending strength σ (MPa) at room temperature was measured for the samples for which Young's modulus was measured, in accordance with JIS R1601. The ratio σ / E was defined as the strength / Young's modulus ratio. The obtained strength / Young's modulus ratios σ / E were ranked according to the following criteria. AA: Ratio σ / E is between 0.65 and 0.85. A: Ratio σ / E is 0.45 or greater and less than 0.65

[0057] (6) Thermal shock resistance A heating and cooling test of a honeycomb structure was conducted using a propane gas burner test machine equipped with a metal case for housing the honeycomb structure and a propane gas burner capable of supplying heating gas into the metal case. The heating gas was combustion gas generated by burning propane gas in a gas burner (propane gas burner). The thermal shock resistance was evaluated by checking whether or not cracks occurred in the honeycomb structure during the heating and cooling test. Specifically, first, the obtained honeycomb structure was canned into the metal case of the propane gas burner test machine. Then, gas heated by the propane gas burner (combustion gas) was supplied into the metal case and passed through the honeycomb structure. The temperature conditions of the heating gas flowing into the metal case (inlet gas temperature conditions) were as follows: First, the temperature was raised to a specified temperature in 5 minutes, held at the specified temperature for 10 minutes, then cooled to 100°C in 5 minutes, and held at 100°C for 10 minutes. This series of operations—heating, cooling, and holding—is referred to as the "heating and cooling operation." Afterward, cracks were checked in the honeycomb structure. Then, the "heating and cooling operation" was repeated while increasing the specified temperature by 25°C increments from 825°C. The thermal shock resistance of the honeycomb structure was evaluated based on the following evaluation criteria. Rating AA: No cracks occurred at the specified temperature of 1000°C. Evaluation A: No cracks occurred at the specified temperature of 950°C to 975°C, but cracks occurred at 1000°C. Evaluation B: No cracks occurred at the specified temperature of 900°C to 925°C, but cracks occurred at 950°C.

[0058] <Example 1> A ceramic raw material was prepared by mixing silicon carbide powder and metallic silicon powder in a mass ratio of 75:25. Hydroxypropyl methylcellulose was added as a binder and a water-absorbent resin as a pore-forming agent, along with water, to the ceramic raw material to create a molding material. The molding material was kneaded in a vacuum clay mixer to produce cylindrical clay blocks. The binder content was 8 parts by mass when the total amount of silicon carbide powder and metallic silicon powder was 100 parts by mass. The pore-forming agent content was 3 parts by mass when the total amount of silicon carbide powder and metallic silicon powder was 100 parts by mass. The water content was 31 parts by mass when the total amount of silicon carbide powder and metallic silicon powder was 100 parts by mass. The average particle size of the silicon carbide powder was 20 μm. The resulting clay was extruded to form a hexagonal cell structure. The resulting honeycomb molded body was dried using high-frequency dielectric heating, then dried in a hot air dryer at 120°C for 2 hours, processed to the specified external dimensions, and a dried honeycomb body was obtained. The obtained honeycomb dry material was fired in an Ar atmosphere at 1450°C for 0.5 hours to produce a cylindrical honeycomb fired body. The obtained honeycomb fired body was subjected to a heat treatment in a water vapor atmosphere at 950°C for 50 hours to obtain a honeycomb structure. The resulting honeycomb structure had circular end faces with an outer diameter of 80 mm, a height (length in the flow direction of the cell) of 80 mm, and an outer wall thickness of 0.5 mm. The cell density was 93 cells / cm³. 2 The septum thickness was 150 μm, the porosity of the septum was 40%, and the average pore diameter of the septum was 8 μm. In the obtained honeycomb structure, an oxide film with a thickness of 0.39 μm was formed on the silicon surface of the silicon carbide-silicon composite material. In addition, the cristobalite content in the obtained honeycomb structure was 2.2 mass%. The obtained honeycomb structure was subjected to the evaluations described in (3) to (6) above. The results are shown in Table 1.

[0059] <Examples 2-12 and Comparative Example 1> For Examples 2-8, 11-12, and Comparative Example 1, honeycomb structures were obtained in the same manner as in Example 1, except that the honeycomb-fired bodies were heat-treated under the conditions shown in Table 1. For Examples 9-10, honeycomb structures were obtained in the same manner as in Example 1, except that the honeycomb-fired bodies were heat-treated under the conditions shown in Table 1, and the amount of porosity material was controlled to increase the porosity of the honeycomb structure compared to Example 1. The thickness of the oxide film and the cristobalite content in the obtained honeycomb structures are shown in Table 1. The obtained honeycomb structures were subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0060] <Comparative Example 2> A honeycomb structure was obtained in the same manner as in Example 1, except that the honeycomb calcined body was not subjected to heat treatment. The thickness of the oxide film and the cristobalite content in the obtained honeycomb structure are shown in Table 1. The obtained honeycomb structure was subjected to the same evaluation as in Example 1. The results are shown in Table 1.

[0061] [Table 1]

[0062] As is clear from Table 1, by forming an oxide film of a predetermined thickness on the silicon surface of the silicon carbide-silicon composite material constituting the honeycomb structure, and / or by setting the cristobalite content in the honeycomb structure to a predetermined value or higher, the resistance increase rate of the honeycomb structure after the heating and cooling durability test can be suppressed to within an acceptable range, and the thermal expansion coefficient of the honeycomb structure can be reduced. Furthermore, as is clear from comparing Examples 1-5 with Examples 6-8, it can be seen that the above characteristics tend to improve further by lowering the heating temperature and increasing the heating time. More specifically, in Examples 1-5, the resistance increase rate of the honeycomb structure after the heating and cooling durability test can be suppressed to within an acceptable range, while the thermal expansion coefficient of the honeycomb structure can be made very small. On the other hand, in Examples 6-8, the resistance increase rate is suppressed well, but the thermal expansion coefficient is not sufficiently small, and depending on the heating conditions, the thermal shock resistance may be insufficient. In addition, as is clear from Examples 11 and 12, by performing heat treatment at high temperatures for a long period of time, the cristobalite content in the honeycomb structure can be significantly increased, resulting in a significant suppression of the resistance increase rate and a good strength / Young's modulus ratio. Furthermore, as is clear from comparing Example 4 (low porosity) and Example 10 (high porosity), and Example 8 (low porosity) and Example 9 (high porosity), even when heat treatment is performed under the same conditions, the strength / Young's modulus ratio can be improved by increasing the porosity. [Industrial applicability]

[0063] The honeycomb structure according to an embodiment of the present invention and the electrically heated carrier using the same can be suitably used for the treatment (purification) of automobile exhaust gases. [Explanation of Symbols]

[0064] 10a 1st end surface 10b 2nd end face 20 cells 30 Bulkhead 40 Peripheral wall 100 Honeycomb Structure 120 Electrode layer 200 Electrically heated NT

Claims

1. It has a partition wall that defines a cell extending from a first end face to a second end face and forming a fluid flow path, and an outer peripheral wall, The partition wall and the outer periphery wall are made of ceramics containing silicon carbide and silicon. An oxide film with a thickness of 0.1 μm to 0.2 μm is formed on the silicon surface. The coefficient of thermal expansion is 4.20 ppm / K to 4.35 ppm / K. Honeycomb structure.

2. It has a partition wall that defines a cell extending from a first end face to a second end face and forming a fluid flow path, and an outer peripheral wall, The partition wall and the outer periphery wall are made of ceramics containing silicon carbide and silicon. It contains 1.5% to 3.5% by mass of cristobalite. The coefficient of thermal expansion is 4.20 ppm / K to 4.35 ppm / K. Honeycomb structure.

3. A honeycomb structure according to claim 1 or 2, A pair of electrode layers are arranged on the outer surface of the outer peripheral wall, with the central axis of the honeycomb structure in between, A pair of metal terminals connected on the pair of electrode layers, An electrically heated carrier having the following characteristics.

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