Honeycomb structure

Abstract

To provide a honeycomb structure capable of inhibiting a temporal increase in resistance while suppressing deterioration in its thermal shock resistance due to thermal expansion differences in the honeycomb structure.SOLUTION: A honeycomb structure 1 according to the present invention comprises: a honeycomb structure part 2 that includes an outer peripheral wall 20, and partition walls 21 disposed inside the outer peripheral wall 20 to partition to form a plurality of cells 21a that form a flow path extending from one end face to the other end face, where the outer peripheral wall 20 and the partition walls 21 are made of ceramics containing silicon carbide and silicon; and a pair of electrode layers 3 that are provided on the outer surface of the outer peripheral wall 20 so as to face each other across the central axis of the honeycomb structure part 2, and are used for the inflow and outflow of current relative to the honeycomb structure part 2. In the surface of the honeycomb structure part 2 orthogonal to an extending direction of the cells 21a, when the axial center of the honeycomb structure part 2 is defined as a position with a radius of 0% and the outer edge of the honeycomb structure part 2 is defined as a position with a radius of 100%, the SiO2 crystalline mass ratio (Ro) in an outer edge region 2or with a radius from 99% to 100% inclusive is greater than the SiO2 crystalline mass ratio (Rc) in a central region 2cr with a radius of 1% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a honeycomb structure. [Background technology]

[0002] Generally, an electrically heated carrier (EHC) is known in which electrodes are placed on a honeycomb structure and the honeycomb structure itself is heated by passing electricity through it, thereby raising the temperature of the catalyst supported on the honeycomb structure to an activation temperature before starting the engine, thereby purifying the exhaust gas emitted immediately after the internal combustion engine starts.

[0003] The resistance of EHC increases due to oxidation. If the resistance increase due to oxidation is significant, there is a risk that the catalyst will not reach its activation temperature within the specified time. For example, the increase in resistance may cause current concentration, resulting in localized heat generation, which may cause cracks in the honeycomb structure due to temperature differences within the honeycomb structure, or damage to the metal electrodes that introduce current into the honeycomb structure.

[0004] The following Patent Document 1 proposes that an increase in resistance of an EHC be suppressed by subjecting a honeycomb structure to an oxidation treatment and allowing the honeycomb structure to contain 1.0 mass % or more of cristobalite (SiO2 crystals). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-145495 Summary of the Invention [Problem to be solved by the invention]

[0006] As described in the above Patent Document 1, an increase in the resistance of the honeycomb structure over time can be suppressed by increasing the proportion of SiO2 crystals. However, since SiO2 crystals have a larger thermal expansion coefficient than silicon carbide and silicon-containing ceramics that form the outer peripheral wall and partition walls, if the proportion of SiO2 crystals contained in the honeycomb structure as a whole is increased, there is a concern that the thermal shock resistance of the honeycomb structure may be deteriorated.

[0007] The present invention has been made to solve the above-mentioned problems, and one of its objects is to provide a honeycomb structure that can suppress an increase in resistance over time while suppressing a deterioration in thermal shock resistance due to differences in thermal expansion of the honeycomb structure. [Means for solving the problem]

[0008] Item 1. In one embodiment, the present invention relates to a honeycomb structure comprising: an outer peripheral wall; and partition walls disposed inside the outer peripheral wall to define a plurality of cells that form flow paths extending from one end face to the other end face, the outer peripheral wall and the partition walls being made of a ceramic containing silicon carbide and silicon; and a pair of electrode layers provided on an outer surface of the outer peripheral wall to face each other across a central axis of the honeycomb structure, the electrode layers being used for inputting and outputting current to and from the honeycomb structure, wherein, on a surface of the honeycomb structure part perpendicular to the extending direction of the cells, when the axial center of the honeycomb structure part is defined as a position of 0% radius and the outer edge of the honeycomb structure part is defined as a position of 100% radius, the SiO2 crystal mass fraction (Ro) in an outer edge region of a radius of 99% or more and 100% or less is larger than the SiO2 crystal mass fraction (Rc) in a central region of a radius of 1% or less.

[0009] Item 2. The present invention may relate to the honeycomb structure according to Item 1, wherein the ratio (Ro / Rc) of the SiO2 crystal mass fraction (Ro) of the outer edge region to the SiO2 crystal mass fraction (Rc) of the central region is 1.1 or more and 2.4 or less.

[0010] Item 3. The present invention may relate to the honeycomb structure according to Item 2, wherein the ratio (Ro / Rc) is 1.4 or more and 2.3 or less. [Effects of the Invention]

[0011] As described above, the SiO2 crystals can suppress an increase in the resistance of the honeycomb structure over time. On the other hand, when a high-temperature fluid is passed through the honeycomb structure, the temperature of the outer peripheral region is less likely to rise than that of the central region, and even if the outer peripheral region is configured to thermally expand more easily than the central region, the thermal shock resistance of the honeycomb structure is less likely to deteriorate. According to one embodiment of the honeycomb structure of the present invention, the SiO2 crystal mass fraction (Ro) of the outer peripheral region at 99% or more and 100% or less of the radius is greater than the SiO2 crystal mass fraction (Rc) of the central region at 1% or less of the radius, so that an increase in resistance over time can be suppressed while suppressing a deterioration in the thermal shock resistance of the honeycomb structure due to the difference in thermal expansion. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a perspective view showing a honeycomb structure according to an embodiment of the present invention; [Figure 2] FIG. 2 is an explanatory view showing a state in which electrodes are attached to the honeycomb structure of FIG. [Figure 3] FIG. 2 is a front view showing a surface of the honeycomb structure part perpendicular to the extending direction of the cells in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to each embodiment, and the components can be modified and embodied without departing from the spirit of the present invention. Furthermore, various inventions can be formed by appropriately combining multiple components disclosed in each embodiment. For example, some components may be omitted from all the components shown in the embodiments. Furthermore, components of different embodiments may be appropriately combined.

[0014] Fig. 1 is a perspective view showing a honeycomb structure 1 according to an embodiment of the present invention, and Fig. 2 is an explanatory view showing a state in which an electrode 4 is attached to the honeycomb structure 1 of Fig. 1. The honeycomb structure 1 according to the embodiment of the present invention has a honeycomb structure portion 2 and a pair of electrode layers 3.

[0015] As shown in FIG. 1, the honeycomb structure 2 has an outer peripheral wall 20 and partition walls 21 disposed inside the outer peripheral wall 20 to define a plurality of cells 21a that form flow paths extending from one end face to the other end face. The honeycomb structure 2 may be a columnar member. A columnar shape can be understood as a three-dimensional shape having a thickness in the extension direction of the cells 21a (axial direction of the honeycomb structure 2). The ratio (aspect ratio) of the axial length of the honeycomb structure 2 to the diameter or width of the end face of the honeycomb structure 2 is arbitrary. The columnar shape may include a shape (flat shape) in which the axial length of the honeycomb structure 2 is shorter than the diameter or width of the end face.

[0016] The outer shape of the honeycomb structure part 2 is not particularly limited as long as it is columnar, and can be other shapes such as a columnar shape with circular end faces (cylindrical shape), a columnar shape with oval end faces, a columnar shape with polygonal end faces (quadragonal, pentagonal, hexagonal, heptagonal, octagonal, etc.), etc. The size of the honeycomb structure part 2 is set to 2000 to 65000 mm2 in order to increase heat resistance (suppress cracks in the circumferential direction of the outer peripheral wall 20). 2 It is preferable that the thickness is 5000 to 25000 mm 2 It is more preferable that the end faces of the pillar-shaped honeycomb structures are polygonal. A plurality of pillar-shaped honeycomb structures each having a polygonal end face can be joined together and used.

[0017] Although there are no limitations on the shape of the cells 21a in a plane (cross section or end face) perpendicular to the extending direction of the cells 21a, a square, a hexagon, an octagon, or a combination thereof is preferred. Among these, a square and a hexagon are preferred. By using such a cell shape, the pressure loss when exhaust gas flows through the honeycomb structure portion 2 is reduced, and the purification performance of the catalyst is improved.

[0018] The thickness of the partition walls 21 that define the cells 21a is preferably 0.1 to 0.8 mm, and more preferably 0.1 to 0.6 mm. When the thickness of the partition walls 21 is 0.1 mm or more, it is possible to prevent a decrease in the strength of the honeycomb structure section 2. When the thickness of the partition walls 21 is 0.8 mm or less, it is possible to prevent an increase in pressure loss when exhaust gas flows through the honeycomb structure section 2 when the honeycomb structure section 2 is used as a catalyst carrier and a catalyst is carried thereon. In the present invention, the thickness of the partition walls 21 is defined as the length of a portion that passes through the partition walls 21, of a line segment that connects the centers of gravity of adjacent cells 21a in a plane perpendicular to the extension direction of the cells 21a.

[0019] The honeycomb structure portion 2 has a cell density of 4 to 150 cells / cm in a plane perpendicular to the extending direction of the cells 21a. 2 Preferably, the number of cells is 7 to 100. 2 By setting the cell density in this range, it is possible to increase the purification performance of the catalyst while minimizing the pressure loss when exhaust gas flows through it. 2 If the cell density is 150 cells / cm or more, a sufficient catalyst carrying area is ensured. 2 If the cell density is less than this value, when the honeycomb structure part 2 is used as a catalyst carrier to support a catalyst, excessive pressure loss during the flow of exhaust gas is suppressed. The cell density is a value obtained by dividing the number of cells by the area of ​​one end face part of the honeycomb structure part 2 excluding the outer wall 20 part.

[0020] Providing the honeycomb structure 2 with the peripheral wall 20 is useful from the viewpoints of ensuring the structural strength of the honeycomb structure 2 and suppressing leakage of the fluid flowing through the cells 21a from the peripheral wall 20. Specifically, the thickness of the peripheral wall 20 is preferably 0.05 mm or more, more preferably 0.10 mm or more, and even more preferably 0.15 mm or more. However, if the peripheral wall 20 is made too thick, the strength becomes too high, which disrupts the strength balance with the partition walls 21 and reduces thermal shock resistance. Therefore, the thickness of the peripheral wall 20 is preferably 1.0 mm or less, more preferably 0.7 mm or less, and even more preferably 0.5 mm or less. Here, the thickness of the peripheral wall 20 is defined as the thickness in the direction normal to the tangent of the peripheral wall 20 at the measurement point when the portion of the peripheral wall 20 whose thickness is to be measured is observed on a plane perpendicular to the extension direction of the cells 21a.

[0021] The honeycomb structure part 2 is made of a conductive material and is heated by passing an electric current through it. The honeycomb structure part 2 may be made of a conductive ceramic. There are no particular restrictions on the volume resistivity of the honeycomb structure part 2 as long as it can generate heat by Joule heat when a current is passed through it, but it is preferably 0.1 Ωcm or more and 200 Ωcm or less, and more preferably 1 Ωcm or more and 200 Ωcm or less. In the present invention, the volume resistivity of the honeycomb structure part 2 is a value measured at 25°C by a four-terminal method.

[0022] In the honeycomb structure 1 of this embodiment, the honeycomb structure portion 2 is made of ceramics in which the outer peripheral wall 20 and the partition walls 21 contain silicon carbide and silicon (hereinafter, sometimes referred to as a silicon carbide-silicon composite material). The ceramics contain, for example, 90 mass % or more, or, for example, 95 mass % or more of silicon carbide and silicon in total. With this configuration, the volume resistivity of the honeycomb structure portion 2 at 25°C can be set to a predetermined range (for example, 0.1 Ωcm to 200 Ωcm, or, for example, 1 Ωcm to 200 Ωcm, or further, for example, 10 Ωcm to 100 Ωcm). As a result, even when electricity is applied to the honeycomb structure portion 2 from a high-voltage power supply of, for example, 200 V or more, excessive current flow can be suppressed, and the desired heat generation can be achieved by the appropriate current flow. The ratio of the "mass of metallic silicon as a binder" contained in the honeycomb structure part 2 to the sum of the "mass of silicon carbide particles as aggregate" contained in the honeycomb structure part 2 and the "mass of metallic silicon as a binder" contained in the honeycomb structure part 2 is preferably 10 to 40 mass%, and more preferably 15 to 35 mass%. The ceramic may contain substances other than the silicon carbide-silicon composite material. Examples of such substances include aluminum and strontium.

[0023] The silicon carbide-silicon composite typically contains silicon carbide particles as an aggregate and silicon as a binder that bonds the silicon carbide particles. In the silicon carbide-silicon composite, for example, a plurality of silicon carbide particles are bonded by silicon so as to form pores between the silicon carbide particles. That is, the outer peripheral wall 20 and the partition walls 21 containing the silicon carbide-silicon composite may be, for example, porous. When porous, the porosity of the outer peripheral wall 20 and the partition walls 21 is preferably 35 to 60%, more preferably 35 to 45%. The porosity is a value measured using a mercury porosimeter. Furthermore, the outer peripheral wall 20 and the partition walls 21 may be dense. When dense, the porosity of the outer peripheral wall 20 and the partition walls 21 may be 10% or less, or may be 5% or less.

[0024] An oxide film having a thickness of 0.1 μm to 5.0 μm may be formed on the silicon surface. By forming such an oxide film on the silicon surface, it is possible to realize a honeycomb structure 1 having an excellent balance between oxidation resistance and thermal shock resistance in a high-temperature environment. More specifically, it is possible to suppress an increase in the resistance of the honeycomb structure 1 in a high-temperature oxidizing atmosphere and to reduce the thermal expansion coefficient (thermal expansion coefficient) of the honeycomb structure 1. 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, it is possible to further suppress an increase in resistance in a high-temperature oxidizing atmosphere and further reduce the thermal expansion coefficient. The thickness of the oxide film can be determined, for example, from an image taken with a scanning electron microscope (SEM).

[0025] The oxide film can be formed, for example, by subjecting the honeycomb structure 1 or the honeycomb structure part 2 to an oxidation treatment (note that, as will be described later, a predetermined amount or more of SiO2 crystals (cristobalite) can also be formed by the oxidation treatment). The oxidation treatment temperature in the oxidation treatment is, for example, 1300°C or less, for example, 1200°C or less, for example, 1150°C or less, for example, 1100°C or less, for example, 1050°C or less, for example, 1000°C or less, or for example, 950°C or less. On the other hand, the oxidation treatment temperature is, for example, 750°C or more, for example, 800°C or more. When the oxidation treatment temperature is within this range, an oxide film of the predetermined thickness can be formed. As a result, an increase in the resistance of the honeycomb structure 1 in a high-temperature oxidizing atmosphere can be suppressed, and the thermal expansion coefficient of the honeycomb structure 1 can be reduced. Preferably, by setting the oxidation treatment temperature to 1150°C or less, these effects become more pronounced. The oxidation treatment time can be changed depending on the oxidation treatment temperature. For example, when the oxidation treatment temperature is 1200°C or higher, the oxidation treatment 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 oxidation treatment temperature is 1150°C or lower, the oxidation treatment 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 oxidation treatment time is too long, the thermal expansion coefficient may increase, which may cause problems such as reduced thermal shock resistance. If the oxidation treatment time is too short, the oxide film (and / or SiO2 crystals described below) may not be sufficiently formed. The oxidation treatment may be performed in an air atmosphere or a water vapor atmosphere (for example, by supplying a gas with a water vapor content adjusted to 10% by volume to 30% by volume on a nitrogen basis during the oxidation treatment). Under the same oxidation treatment conditions, performing the oxidation treatment in a water vapor atmosphere can form an oxide film with a more preferable thickness (and / or a more preferable amount of SiO2 crystals, as described below), and can further enhance the effects of suppressing the increase in resistance and reducing the thermal expansion coefficient.The oxidation treatment conditions may be those for the first oxidation treatment described below, and after this first oxidation treatment, a second oxidation treatment may be performed to make the SiO2 crystal mass fraction (Ro) of the outer peripheral region 2or larger than the SiO2 crystal mass fraction (Rc) of the central region 2cr (and at least a part of the intermediate region 2ir).

[0026] The oxide film is essentially composed of silicon oxide. As described above, the oxide film may be formed on the surface of silicon serving as a binder. Therefore, the oxide film may be formed on the surface of silicon carbide particles in addition to the silicon surface, or may be formed on other parts within the structure of the outer wall 20 and the partition wall 21. Since the oxidation of silicon in a high-temperature environment is essentially the main cause of the increase in resistance, by forming a predetermined oxide film on the silicon surface of the honeycomb structure 1 or the honeycomb structure portion 2 in advance, the effect of suppressing the increase in resistance and the effect of reducing the thermal expansion coefficient can be efficiently obtained.

[0027] The average pore diameter of the outer wall 20 and the partition walls 21 of the honeycomb structure portion 2 is preferably 2 to 15 μm, more preferably 4 to 8 μm. The average pore diameter is a value measured by a mercury porosimeter.

[0028] The electrode layers 3 are used for inputting and outputting current to and from the honeycomb structure section 2, and are provided on the outer surface of the outer peripheral wall 20 so as to face each other across the central axis of the honeycomb structure section 2. In FIG. 1, only one of the pair of electrode layers 3 is shown. In a plane perpendicular to the extension direction of the cells 21a, 0.5 times the central angle of each electrode layer 3 may be 15 to 89°. Note that the central angle of the electrode layer 3 may be the angle formed by two line segments connecting both ends of the electrode layer 3 to the center of the honeycomb structure section 2 in a cross section of the honeycomb structure section 2 perpendicular to the extension direction of the cells 21a. When the honeycomb structure section 2 is cylindrical, this angle may be the interior angle of a sector formed by the two line segments and the electrode layer 3.

[0029] The thickness of the electrode layer 3 is preferably 0.01 to 5 mm, more preferably 0.01 to 3 mm. By setting the thickness within this range, uniform heat generation is possible. If the thickness of the electrode layer 3 is thinner than 0.01 mm, the electrical resistance may be high, resulting in inconsistent heat generation. If the thickness is thicker than 5 mm, the honeycomb structure 1 may be damaged during canning. Furthermore, if the electrode layer 3 is too thin, the effect of adjusting the outer diameter when varying the outer diameter of the honeycomb structure 1 in the extension direction of the cells 21a due to variations in the thickness of the electrode layer 3 is reduced, as described below. If the electrode layer 3 is too thick, the holding force of the portions without the electrode layer 3 is reduced, resulting in the honeycomb structure 1 being easily detached during a vibration test. Furthermore, if the electrode layer 3 is too thin, the resistance of the electrode layer 3 is insufficient, which facilitates current flow through the honeycomb structure 2, destabilizing the heat generation distribution and leading to reduced purification performance and damage to the substrate. On the other hand, if the electrode layer 3 is too thick, current flows easily through the electrode layer 3, which facilitates heat generation, destabilizing the heat generation distribution and leading to reduced purification performance and damage to the substrate.

[0030] The pair of electrode layers 3 in this embodiment each includes a separator 30 and a first partial electrode layer 31 and a second partial electrode layer 32 separated by the separator 30. The separator 30 may be a slit provided between the first partial electrode layer 31 and the second partial electrode layer 32. The slit may be filled with a material having a higher volume resistivity than the first partial electrode layer 31 and the second partial electrode layer 32. The first partial electrode layer 31 and the second partial electrode layer 32 are strip-shaped with a predetermined width in the circumferential direction of the honeycomb structure section 2, and the separator 30 is linear and narrower than the first partial electrode layer 31 and the second partial electrode layer 32. However, the arrangement of the separator 30 and the first partial electrode layer 31 and the second partial electrode layer 32 is not limited to this form as long as they can be connected to the electrode 4.

[0031] As shown in FIG. 2, an electrode 4 may be fixed on the electrode layer 3. The electrode 4 may be made of metal. An external power source such as a battery may be connected to the electrode 4 via a power cable. By applying a voltage to the honeycomb structure 1 through the electrode 4, the honeycomb structure 1 can generate heat. This allows the catalyst supported on the honeycomb structure 1 to be heated to an activation temperature before the engine is started.

[0032] The electrode 4 may have a connection portion 41 arranged on the outer peripheral surface of the honeycomb structure 1 (on the electrode layer 3) and a lead portion 42 led out from the connection portion 41. Although not shown in detail, the connection portion 41 may be configured in a comb shape having a plurality of teeth, some of which may be connected to the first partial electrode layer 31 (see FIG. 1) and the other teeth may be connected to the second partial electrode layer 32 (see FIG. 1). The connection portion 41 may be formed in an arc shape that follows the outer peripheral surface of the honeycomb structure 1. The lead portion 42 may be erected from one end of the connection portion 41, and a power cable may be connected to this lead portion 42.

[0033] From the viewpoint of making it easier for electricity to flow through the electrode layers 3 , the volume resistivity of the electrode layers 3 is preferably 1 / 200 or more and 1 / 10 or less of the volume resistivity of the honeycomb structure portion 2 .

[0034] The material of the electrode layer 3 can be a conductive ceramic, a metal, or a composite material (cermet) of a metal and a conductive ceramic. Examples of metals include a single metal such as Cr, Fe, Co, Ni, Si, or Ti, or an alloy containing at least one metal selected from the group consisting of these metals. Examples of 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).

[0035] In a manufacturing method of the honeycomb structure 1 having the electrode layers 3, first, an electrode layer forming raw material containing ceramic raw materials is applied to the side surface of the dried honeycomb body, and then dried to form a pair of unfired electrode layers extending in a band shape in the extension direction of the cells 21a on the outer surface of the peripheral wall 20, sandwiching the central axis of the dried honeycomb body, thereby producing a dried honeycomb body with unfired electrode layers. Next, the dried honeycomb body with the unfired electrode layers is fired to produce a fired honeycomb body having a pair of electrode layers 3. In this way, the honeycomb structure 1 having the electrode layers 3 is obtained.

[0036] By supporting a catalyst on the honeycomb structure portion 2, the electrically heated carrier can be used as a catalyst body. Examples of the catalyst include precious metal catalysts and other catalysts. Examples of precious metal catalysts include three-way catalysts and oxidation catalysts in which a precious metal such as platinum (Pt), palladium (Pd), or rhodium (Rh) is supported on the pore surface of alumina and a promoter such as ceria or zirconia is included, and NOx storage reduction catalysts (LNT catalysts) containing alkaline earth metals and platinum as nitrogen oxide (NOx) storage components are included. Examples of catalysts that do not use precious metals include NOx selective reduction catalysts (SCR catalysts) containing copper-substituted or iron-substituted zeolites. Two or more catalysts selected from these catalysts may also be used. The catalyst support method is not particularly limited, and can be performed in accordance with conventional methods for supporting a catalyst on the honeycomb structure 1.

[0037] Next, Fig. 3 is a front view showing the surface of the honeycomb structure section 2 perpendicular to the extending direction of the cells 21a in Fig. 1. In Fig. 3, the illustration of the outer peripheral wall 20, the partition walls 21 and the cells 21a is omitted.

[0038] 3, in the surface of the honeycomb structure 2 perpendicular to the extending direction of the cells 21a, when the axial center of the honeycomb structure 2 is set at the position of radius 0% and the outer edge of the honeycomb structure 2 is set at the position of radius 100%, the region of radius 99% to 100% is called the outer edge region 2or, and the region of radius 1% or less is called the central region 2cr. The surface of the honeycomb structure 2 perpendicular to the extending direction of the cells 21a may be the end faces appearing on both ends of the honeycomb structure 2, or may be the cross section of the honeycomb structure 2. The outer edge of the honeycomb structure 2 may be the outer surface of the outer wall 20.

[0039] In the honeycomb structure 1 of the present embodiment, the SiO2 crystal mass fraction (Ro) (wt%) of the outer edge region 2or is set to be larger than the SiO2 crystal mass fraction (Rc) (wt%) of the central region 2cr. The SiO2 crystal content can be measured by, for example, X-ray diffraction.

[0040] As described above, in the honeycomb structure 1 of this embodiment, the honeycomb structure portion 2 is formed of ceramics containing silicon carbide and silicon, and the peripheral wall 20 and partition walls 21 are made of SiO2 crystals (cristobalite). Typically, SiO2 crystals (cristobalite) can be formed in an oxide film on the silicon surface formed by oxidizing the honeycomb structure 1. As described in the above-mentioned Patent Document 1, increasing the proportion of SiO2 crystals can suppress an increase in the resistance of the honeycomb structure 1 over time. However, since SiO2 crystals have a larger thermal expansion coefficient than the ceramics containing silicon carbide and silicon that constitute the peripheral wall 20 and partition walls 21, increasing the overall proportion of SiO2 crystals in the honeycomb structure 1 raises concerns that the thermal shock resistance of the honeycomb structure 1 may deteriorate. On the other hand, when a high-temperature fluid is passed through the honeycomb structure 1, the temperature of the outer peripheral region 2or is less likely to increase than that of the central region 2cr. Therefore, even if the outer peripheral region 2or is configured to be more thermally expandable than the central region 2cr, the thermal shock resistance of the honeycomb structure 1 is less likely to deteriorate. In the honeycomb structure 1 of this embodiment, by making the SiO2 crystal mass fraction (Ro) of the outer edge region 2or larger than the SiO2 crystal mass fraction (Rc) of the central region 2cr, it is possible to suppress the deterioration of thermal shock resistance due to the difference in thermal expansion of the honeycomb structure 1 while suppressing the increase in resistance over time.

[0041] The SiO2 crystal mass fraction of the intermediate region 2ir between the peripheral region 2or and the central region 2cr may decrease continuously or stepwise from the SiO2 crystal mass fraction (Ro) of the peripheral region 2or to the SiO2 crystal mass fraction (Rc) of the central region 2cr. The SiO2 crystal mass fraction of at least a part of the intermediate region 2ir may be the same as the SiO2 crystal mass fraction (Ro) of the peripheral region 2or or the SiO2 crystal mass fraction (Rc) of the central region 2cr.

[0042] Although not limited thereto, when the honeycomb structure 1 is subjected to oxidation treatment, the SiO2 crystal mass fraction (Ro) of the outer peripheral region 2or can be made larger than the SiO2 crystal mass fraction (Rc) of the central region 2cr (and at least a part of the intermediate region 2ir) by filling the cells 21a of the central region 2cr (and at least a part of the intermediate region 2ir) with the same material as the outer peripheral wall 20 and the partition wall 21. The material filled in the cells 21a of the central region 2cr (and at least a part of the intermediate region 2ir) may be removed after the oxidation treatment.

[0043] The oxidation treatment of the honeycomb structure 1 may include a first oxidation treatment in which the entire honeycomb structure 1 is oxidized, and a second oxidation treatment in which, after the first oxidation treatment, the cells 21a in the central region 2cr (and at least a part of the intermediate region 2ir) are filled with the same material as the outer peripheral walls 20 and the partition walls 21, and then the honeycomb structure 1 is additionally oxidized. By performing the second oxidation treatment, the SiO2 crystal mass fraction (Ro) in the outer peripheral region 2or can be made larger than the SiO2 crystal mass fraction (Rc) in the central region 2cr (and at least a part of the intermediate region 2ir). From this point of view, the second oxidation treatment may also be called an additional oxidation treatment of the outer peripheral region 2or.

[0044] The oxidation treatment temperature in the second oxidation treatment is, for example, 1300°C or lower, for example, 1200°C or lower, for example, 1150°C or lower, for example, 1100°C or lower, for example, 1050°C or lower, for example, 1000°C or lower, or for example, 950°C or lower. On the other hand, the oxidation treatment temperature in the second oxidation treatment is, for example, 750°C or higher, for example, 800°C or higher. If the oxidation treatment temperature is within this range, SiO2 crystals can be added to the outer peripheral region 2or, etc. The oxidation treatment time can vary depending on the oxidation treatment temperature. For example, when the oxidation treatment temperature is 1200°C or higher, the oxidation treatment time is preferably 1 hour to 100 hours, more preferably 20 hours to 80 hours, and even more preferably 30 hours to 70 hours. If the oxidation treatment time is too long, the thermal expansion coefficient may increase, which may cause problems such as reduced thermal shock resistance. If the oxidation treatment time is too short, SiO2 crystals may not be sufficiently added to the outer peripheral region 2or, etc. The second oxidation treatment may be carried out in an air atmosphere or in a water vapor atmosphere (for example, by supplying a gas containing 10% to 30% by volume of water vapor on a nitrogen basis during the oxidation treatment). Under the same oxidation treatment conditions, more SiO2 crystals can be added by carrying out the oxidation treatment in a water vapor atmosphere.

[0045] It is preferable that the ratio (Ro / Rc) of the SiO2 crystal mass fraction (Ro) of the outer peripheral region 2or to the SiO2 crystal mass fraction (Rc) of the central region 2cr is 1.1 or more and 2.4 or less. When the ratio (Ro / Rc) is in this range, it is possible to more reliably suppress the deterioration of the thermal shock resistance of the honeycomb structure 1 due to the difference in thermal expansion while suppressing the increase in resistance over time. It is also sometimes preferable that the ratio (Ro / Rc) is 1.4 or more and 2.3 or less. When the ratio (Ro / Rc) is in this range, it is possible to more reliably suppress the deterioration of the thermal shock resistance of the honeycomb structure 1 due to the difference in thermal expansion while suppressing the increase in resistance over time.

[0046] In addition, since the outer peripheral wall 20 and the partition walls 21 contain SiO2 crystals, it is possible to suppress an increase in resistance over time of the honeycomb structure 1 compared to when no SiO2 crystals are contained. Although not limited thereto, the SiO2 crystal mass fraction (Ro) of the outer peripheral region 2or can be 1 wt% or more and 20 wt% or less.

[0047] Although the preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Example]

[0048] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The evaluation items in the examples are as follows.

[0049] (1) Mass ratio of SiO2 crystals The SiO2 crystal mass fraction was measured as follows. Images of the interior of the outer wall 20 or partition wall 21 of the honeycomb structure 1 obtained in the examples and comparative examples were taken using a scanning electron microscope (SEM). The locations where silicon element (Si) and oxygen element (O) were confirmed on the silicon surface by SEM-EDX were identified as oxide films. Next, an X-ray diffraction pattern of the oxide film was obtained using an X-ray diffractometer (D8 ADVANCE, manufactured by Bruker AXS) (main measurement conditions: CuKα characteristic X-ray, tube voltage 10 kV, tube current 20 mA, diffraction angle 2θ = 5° to 100°). Next, the obtained X-ray diffraction data was analyzed by the Rietveld method using analysis software TOPAS (manufactured by Bruker AXS), and the diffraction peak intensity of the SiO2 crystal was quantified.

[0050] (2) Resistance increase rate Test samples were cut out from the honeycomb structures 1 obtained in the examples and comparative examples. The volume resistivity R of the cut test samples was measured. The test samples were subjected to a durability test at 900°C for 100 hours in an atmosphere of air and 10% water vapor, and the volume resistivity R of the test samples after the test was 100 The initial volume resistivity R and the volume resistivity R of the test sample after the test were measured. 100 Ratio of 100 / R was calculated, and the ratio R 100 / R ratio R 250 The ratio of / R was used as the resistance increase rate (%) for Comparative Example 2 and the Example.

[0051] (3) Thermal shock resistance A heating and cooling test of the honeycomb structure 1 was carried out using a propane gas burner tester equipped with a metal case for housing the honeycomb structure 1 and a propane gas burner capable of supplying heated gas into the metal case. The heated gas was combustion gas generated by burning propane gas with a gas burner (propane gas burner). The heating and cooling test was carried out to evaluate the thermal shock resistance by checking whether cracks occurred in the honeycomb structure 1. Specifically, the honeycomb structure 1 was first housed (canned) in the metal case of the propane gas burner tester. Then, gas (combustion gas) heated by the propane gas burner was supplied into the metal case so that the gas passed through the honeycomb structure 1. The temperature conditions (inlet gas temperature conditions) of the gas flowing into the metal case were set as follows: First, the temperature was raised to 200°C in 180 seconds and held at that temperature, then raised to the designated temperature in 10 seconds and held at the designated temperature for 290 seconds, and then cooled to 100°C in 600 seconds and held at 100°C for 10 minutes. This series of heating, cooling, and holding operations was repeated three times. The designated temperatures were 800°C and 900°C, and the thermal shock resistance of the honeycomb structure 1 was evaluated based on the following evaluation criteria. Evaluation: No cracks occurred at the specified temperature of 900°C. Evaluation △: Cracks occurred at the specified temperature of 900°C. Rating ×: Cracks occurred at the specified temperature of 800°C.

[0052] <Comparative 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 as a binder, a water-absorbent resin as a pore-forming material, and water were added to the ceramic raw material to prepare a molding raw material. The molding raw material was kneaded using a vacuum kneader to prepare cylindrical clay. The binder content was 8 parts by mass per 100 parts by mass of the silicon carbide powder and metallic silicon powder. The pore-forming material content was 3 parts by mass per 100 parts by mass of the silicon carbide powder and metallic silicon powder. The water content was 31 parts by mass per 100 parts by mass of the silicon carbide powder and metallic silicon powder. 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 by high-frequency dielectric heating, then dried at 120°C for 2 hours using a hot air dryer, and processed to the specified outer dimensions to obtain a dried honeycomb body. The obtained dried honeycomb body 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 had a circular end face with an outer diameter (diameter) of 40 mm. The obtained honeycomb fired body was subjected to oxidation treatment in an air atmosphere at 1000° C. for 20 hours, to obtain a honeycomb structure 1.

[0053] <Comparative Example 2> The temperature and time of the oxidation treatment were changed from those of Comparative Example 1. That is, the above-mentioned honeycomb fired body was subjected to oxidation treatment in an air atmosphere at 1250° C. for 40 hours, and a honeycomb structure 1 was obtained.

[0054] <Examples 1 to 3> The cells 21a in the circular region from the axial center to a radius of 25 mm of the honeycomb structure 1 obtained in Comparative Example 1 were filled with the same material as the outer peripheral wall 20 and the partition walls 21, and the honeycomb fired body was then subjected to an additional oxidation treatment. The oxidation treatment temperature was 1250°C or 1300°C, and the oxidation treatment time was 30 to 80 hours.

[0055] The SiO2 crystal mass fraction (Ro) (wt%) of the outer peripheral region 2or and the SiO2 crystal mass fraction (Rc) (wt%) of the central region 2cr, as well as the resistance increase rate, were investigated for the honeycomb structures 1 of Comparative Examples 1 and 2 and Examples 1 to 3, and the thermal shock resistance was also evaluated. The results are shown in Table 1 below.

[0056] [Table 1]

[0057] By comparing Comparative Examples 1 and 2, it can be seen that increasing the overall proportion of SiO2 crystals contained in honeycomb structure 1 can improve the resistance increase rate of honeycomb structure 1, but the thermal shock resistance of honeycomb structure 1 deteriorates.

[0058] In the honeycomb structures 1 of Examples 1 to 3, the SiO2 crystal mass fraction (Ro) (wt%) of the outer peripheral region 2or is set to be larger than the SiO2 crystal mass fraction (Rc) (wt%) of the central region 2cr. Compared with Comparative Examples 1 and 2, the honeycomb structures 1 of Examples 1 to 3 are able to suppress an increase in resistance over time while suppressing deterioration in thermal shock resistance due to differences in thermal expansion of the honeycomb structure 1. These results confirm the advantage of having the SiO2 crystal mass fraction (Ro) (wt%) larger than the SiO2 crystal mass fraction (Rc) (wt%) of the central region 2cr.

[0059] It can be seen that when the ratio (Ro / Rc) of the SiO2 crystal mass fraction (Ro) of the outer peripheral region 2or to the SiO2 crystal mass fraction (Rc) of the central region 2cr is 1.1 or more and 2.4 or less as in Examples 1 to 3, it is possible to more reliably suppress the increase in resistance over time while suppressing the deterioration of the thermal shock resistance due to the thermal expansion difference of the honeycomb structure 1. Furthermore, it can be seen that when the ratio (Ro / Rc) is 1.4 or more and 2.3 or less as in Examples 1 and 2, it is possible to more reliably suppress the deterioration of the thermal shock resistance due to the thermal expansion difference of the honeycomb structure 1 while suppressing the increase in resistance over time compared to Comparative Examples 1 and 2. [Explanation of symbols]

[0060] 1: Honeycomb structure 2: Honeycomb structure 20:Outer wall 21: Bulkhead 21a: Cell 2cr: central area 2or: outer edge area 3: Electrode layer

Claims

1. a honeycomb structure portion having an outer peripheral wall and partition walls disposed inside the outer peripheral wall to define a plurality of cells forming flow paths extending from one end face to the other end face, the outer peripheral wall and the partition walls being made of ceramics containing silicon carbide and silicon; a pair of electrode layers provided on the outer surface of the outer peripheral wall so as to face each other across the central axis of the honeycomb structure section, and used for inputting and outputting current to and from the honeycomb structure section; Equipped with On the surface of the honeycomb structure part perpendicular to the extending direction of the cells, when the axial center of the honeycomb structure part is set as the position of radius 0% and the outer edge of the honeycomb structure part is set as the position of radius 100%, SiO in the outer edge region of the radius 99% or more and 100% or less 2 The crystalline mass fraction (Ro) is SiO in the central region of 1% or less of the radius. 2 greater than the crystalline mass fraction (Rc); Honeycomb structure.

2. SiO of the central region 2 SiO of the outer edge region relative to the crystal mass fraction (Rc) 2 The ratio (Ro / Rc) of the crystalline mass fraction (Ro) is 1.1 or more and 2.4 or less; The honeycomb structure according to claim 1 .

3. The ratio (Ro / Rc) is 1.4 or more and 2.3 or less. The honeycomb structure according to claim 2 .

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