HONEYCOMB STRUCTURE BODY

Abstract

Honeycomb structural body comprising: a fired honeycomb body in which a plurality of through holes with a partition wall between them are arranged parallel to each other in the longitudinal direction, wherein the fired honeycomb body contains ceria-zirconia composite oxide particles and inorganic fibers, and a b-axis thermal expansion coefficient measured in a direction along a b-axis, which is a direction perpendicular to a longitudinal direction of the honeycomb structural body, is higher than an a-axis thermal expansion coefficient measured in a direction along an a-axis, which is the longitudinal direction of the honeycomb structural body, wherein at least 60% of the inorganic fibers observed in a cross-sectional image of the honeycomb structural body cut in a cross-sectional direction perpendicular to the longitudinal direction have a ratio of the length of a long axis to the length of a short axis (long axis / short axis) of 1.00 to 1.30, wherein the length of the long axis is the length of a long axis of a cross section of an inorganic fiber shown in the cross-sectional image, wherein the length of the short axis is the length of a perpendicular bisector of the long axis.

Description

TECHNICAL FIELD

[0001] The present invention relates to a honeycomb structural body. STATE OF THE ART

[0002] Exhaust gases emitted from internal combustion engines of automobiles and the like contain harmful gases such as carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbons (HC). An exhaust catalyst that decomposes such harmful gases is also called a three-way catalyst. A typical three-way catalyst includes a catalyst layer formed by washing a slurry containing precious metal particles with catalytic activity onto a honeycomb monolithic substrate made of cordierite or the like.

[0003] Patent Literature 1 discloses an exhaust gas catalyst including a monolithic substrate containing ceria-zirconia composite oxide particles and θ-phase alumina particles, wherein noble metal particles are supported on the monolithic substrate. CITATION LIST - Patent Literature Patent Literature 1: JP 2015-85241 A Patent literature 2: EP 3 593 884 A1 Patent literature 3: US 2009 / 0 291 835 A1 SUMMARY OF THE INVENTION - Technical Problem

[0004] In the exhaust gas catalyst disclosed in Patent Literature 1, the monolithic substrate contains ceria-zirconia composite oxide particles and θ-phase alumina particles.

[0005] A monolithic substrate containing a ceria-zirconia composite oxide has a high thermal expansion coefficient and may be prone to cracking due to temperature changes. In particular, in the case of a failure mode called ring-off cracking, in which the monolithic substrate fractures in a b-axis direction (a direction perpendicular to a longitudinal direction of a honeycomb structural body) due to cracking, the monolithic substrate may fall out.

[0006] The present invention has been made to solve the above problem and aims to provide a honeycomb structural body containing a ceria-zirconia composite oxide and is less susceptible to ring-off cracking. - Solution to the problem

[0007] The present invention provides a honeycomb structural body comprising a fired honeycomb body in which a plurality of through holes are arranged parallel to each other in the longitudinal direction with a partition wall therebetween, wherein the fired honeycomb body contains ceria-zirconia composite oxide particles and inorganic fibers, and a b-axis thermal expansion coefficient measured in a direction along a b-axis, which is a direction perpendicular to a longitudinal direction of the honeycomb structural body, is higher than an a-axis thermal expansion coefficient measured in a direction along an a-axis, which is the longitudinal direction of the honeycomb structural body.

[0008] In the honeycomb structural body, the b-axis thermal expansion coefficient is higher than the a-axis thermal expansion coefficient. In this case, when a temperature difference occurs in the honeycomb structural body, a larger tensile stress is exerted on the b-axis direction of the honeycomb structural body, and cracks initially occur in a direction along the a-axis. Cracking in this direction is not ring-off cracking. Even if cracks occur in this direction, the honeycomb structural body is less likely to fall out. Therefore, such cracking is a minor failure mode compared to the case of ring-off cracking.

[0009] In other words, the honeycomb structural body of the present invention is considered to be a honeycomb structural body that is less susceptible to ring-off cracking.

[0010] In the honeycomb structural body of the present invention, the a-axis thermal expansion coefficient is preferably 7.5 × 10 -6 / K or more and less than 8.0 × 10 -6 / K.

[0011] In the honeycomb structural body of the present invention, the b-axis thermal expansion coefficient is preferably 8.0 × 10 -6 / K or more and 8.5 × 10 -6 / K or less.

[0012] In the honeycomb structural body of the present invention, a difference between the b-axis thermal expansion coefficient and the a-axis thermal expansion coefficient ((b-axis thermal expansion coefficient) - (a-axis thermal expansion coefficient)) is preferably 0.1 to 1.0 × 10 -6 / K.

[0013] When the a-axis thermal expansion coefficient, the b-axis thermal expansion coefficient, and the difference between the b-axis thermal expansion coefficient and the a-axis thermal expansion coefficient are adjusted within these ranges, a honeycomb structural body that is even less susceptible to ring-off cracking can be produced.

[0014] In the honeycomb structural body of the present invention, at least 60% of the inorganic fibers observed in a cross-sectional image of the honeycomb structural body cut in a cross-sectional direction perpendicular to a longitudinal direction have a ratio of the length of a long axis to the length of a short axis (long axis / short axis) of 1.00 to 1.30, where the length of the long axis is the length of a long axis of a cross section of an inorganic fiber shown in the cross-sectional image, and the length of the short axis is the length of a perpendicular bisector of the long axis.

[0015] Regarding the orientation of the inorganic fibers defined above, if the long axis / short axis ratio is 1.00, it means that the inorganic fibers are completely oriented parallel to the longitudinal direction (hereinafter also referred to as the "a-axis direction") of the honeycomb structural body. Inorganic fibers with a long axis / short axis ratio of 1.00 to 1.30 are considered to be oriented in the a-axis direction, although they are slightly inclined from the a-axis direction. The presence of such inorganic fibers in a proportion of 60% or more means that many inorganic fibers are oriented in the a-axis direction.

[0016] When many inorganic fibers are oriented in the a-axis direction as described above, the thermal expansion coefficient in the a-axis direction decreases. Thus, a honeycomb structural body that is less susceptible to ring-off cracking can be produced. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view schematically showing an exemplary honeycomb structural body of the present invention. DESCRIPTION OF EMBODIMENTS[Honeycomb structural body]

[0017] The honeycomb structural body of the present invention will be described.

[0018] Fig. 1 is a perspective view schematically showing an exemplary honeycomb structural body of the present invention.

[0019] As in Fig. 1, a honeycomb structural body 10 comprises a fired honeycomb body 11 in which a plurality of through holes 12 are formed in the longitudinal direction (a direction indicated by a double arrow L in Fig. 1) arranged parallel to each other with a partition wall 13 therebetween, and an outer peripheral wall 14 at the outermost periphery.

[0020] The fired honeycomb body 11 contains cerium oxide-zirconia composite oxide particles (hereinafter also referred to as “CZ particles”) and inorganic fibers.

[0021] If the honeycomb structural body 10 contains only one fired honeycomb body 11, as in Fig. 1, the fired honeycomb body 11 is the honeycomb structural body.

[0022] In the honeycomb structural body of the present invention, the fired honeycomb body contains CZ particles and inorganic fibers.

[0023] As described later, the fired honeycomb body is prepared by extruding a raw material paste containing CZ particles and inorganic fibers and firing the resulting extrudate.

[0024] Whether the honeycomb structural body of the present invention contains the CZ particles or not can be confirmed by X-ray diffraction (XRD).

[0025] The honeycomb structural body may further contain aluminum oxide particles and an inorganic binder.

[0026] In the honeycomb structural body of the present invention, the b-axis thermal expansion coefficient measured in a direction along the b-axis, which is a direction perpendicular to the longitudinal direction of the honeycomb structural body, is higher than the a-axis thermal expansion coefficient measured in a direction along the a-axis, which is the longitudinal direction of the honeycomb structural body.

[0027] In the honeycomb structural body, the b-axis thermal expansion coefficient is higher than the a-axis thermal expansion coefficient. In this case, when a temperature difference occurs in the honeycomb structural body, a larger tensile stress is exerted on the b-axis direction of the honeycomb structural body, and cracks initially occur in a direction along the a-axis. Cracking in this direction is not ring-off cracking. Even if cracks occur in this direction, the honeycomb structural body is less likely to fall out. Therefore, such cracking is a minor failure mode compared to the case of ring-off cracking.

[0028] In other words, the honeycomb structural body of the present invention is considered to be a honeycomb structural body that is less susceptible to ring-off cracking.

[0029] The a-axis thermal expansion coefficient and the b-axis thermal expansion coefficient can be measured using a thermal expansion meter.

[0030] The thermal expansion gauge can be DIL 402C, which is available from NETZSH, for example.

[0031] The measurement temperature range is 40 °C to 800 °C, the heating rate is 10 °C / min, and the gas flow rate is 100 ml / min in air. The reference material is sapphire.

[0032] The size of a test piece is 3.5 mm × 3.5 mm (3 cells × 3 cells) × 25 mm.

[0033] The a-axis thermal expansion coefficient and the b-axis thermal expansion coefficient are measured for each of the five test pieces, and the average is calculated as the measurement result.

[0034] The a-axis thermal expansion coefficient measured in this way is preferably 7.5 × 10 -6 / K or more and less than 8.0 × 10 -6 / K. The b-axis thermal expansion coefficient is preferably 8.0 × 10 -6 / K or more and 8.5 × 10 -6 / K or less.

[0035] Further, a difference between the b-axis thermal expansion coefficient and the a-axis thermal expansion coefficient ((b-axis thermal expansion coefficient) - (a-axis thermal expansion coefficient)) is preferably 0.1 to 1.0 × 10 -6 / K.

[0036] When the a-axis thermal expansion coefficient, the b-axis thermal expansion coefficient, and the difference between the b-axis thermal expansion coefficient and the a-axis thermal expansion coefficient are controlled within these ranges, a honeycomb structural body that is even less susceptible to ring-off cracking can be manufactured.

[0037] At least 60% of the inorganic fibers observed in a cross-sectional image (hereinafter also simply referred to as "cross-sectional image of the honeycomb structural body") of the honeycomb structural body cut in a cross-sectional direction perpendicular to the longitudinal direction have a ratio of the length of a long axis to the length of a short axis (long axis / short axis) of 1.00 to 1.30, where the length of the long axis is the length of a long axis of a cross section of an inorganic fiber shown in the cross-sectional image, where the length of the short axis is the length of a perpendicular bisector of the long axis.

[0038] In a cross-sectional image of the honeycomb structural body, a long axis is drawn in each of the circular cross sections and the substantially elliptical cross sections of the inorganic fibers, and a perpendicular bisector of the long axis is drawn as a short axis.

[0039] If the ratio of the long axis to the short axis (long axis / short axis) drawn as described above is 1.00, it means that the inorganic fibers, each with a true circular cross-section, are oriented completely parallel to the longitudinal direction (hereinafter also referred to as the "a-axis direction") of the honeycomb structural body. Inorganic fibers with a long axis / short axis ratio of 1.00 to 1.30 are considered to be oriented in the a-axis direction, although they are slightly inclined from the a-axis direction.

[0040] The long-axis / short-axis ratio is calculated for each of the 100 inorganic fibers shown in the cross-sectional image, and the proportion of inorganic fibers each having a long-axis / short-axis ratio of 1.00 to 1.30 is determined. At least 60% of the inorganic fibers shown in the cross-sectional image have a long-axis / short-axis ratio of 1.00 to 1.30. This means that many inorganic fibers in the honeycomb structural body are oriented in the a-axis direction. If a cross-sectional image contains fewer than 100 inorganic fibers, multiple cross-sectional images are randomly acquired until 100 inorganic fibers are acquired, and the values are calculated.

[0041] When many inorganic fibers are oriented in the a-axis direction, the thermal expansion coefficient in the a-axis direction decreases. Thus, a honeycomb structure can be produced that is less susceptible to ring-off cracking.

[0042] The average fiber length of the inorganic fibers is not limited, but is preferably 10 to 90 µm.

[0043] The average fiber diameter of the inorganic fibers is not limited, but is preferably 1 to 5 µm.

[0044] The aspect ratio of the inorganic fibers is preferably 5 to 300, more preferably 10 to 200, even more preferably 10 to 100.

[0045] The average fiber length of the inorganic fibers can be adjusted by appropriately changing the crushing conditions of the fibers.

[0046] The average fiber diameter of the inorganic fibers can be adjusted by appropriately changing the fiber diameter of an inorganic fiber precursor.

[0047] The content of inorganic fibers is preferably 5 to 20 wt% based on the weight of the fired honeycomb. When the content of inorganic fibers is 5 to 20 wt%, the effect of sufficiently increasing strength can be achieved while maintaining exhaust gas conversion performance.

[0048] The inorganic fibers can be formed of any material. Examples include alumina, silica, silicon carbide, silica-alumina, glass, potassium titanate, and aluminum borate. Two or more of these can be used in combination. Of these, alumina fibers are preferred.

[0049] The alumina fibers are inorganic fibers containing 70 wt% or more of alumina and may contain other elements such as Si in addition to Al.

[0050] In addition to the alumina fibers, the inorganic fibers may also include silica fibers, silicon carbide fibers, glass fibers, aluminum titanate fibers, or the like.

[0051] The alumina fibers may be either crystalline alumina fibers or amorphous alumina fibers, and may be mixed fibers of crystalline alumina fibers and amorphous alumina fibers.

[0052] The alumina fibers are analyzed by powder X-ray diffraction (XRD) to determine those with a diffraction peak in 2θ from 25° to 30° as crystalline alumina fibers and those without a diffraction peak in this range as amorphous alumina fibers.

[0053] The honeycomb structural body of the present invention may comprise a single fired honeycomb body or a plurality of fired honeycomb bodies. When the honeycomb structural body includes a plurality of fired honeycomb bodies, these plurality of fired honeycomb bodies are preferably held together with an adhesive layer.

[0054] In the honeycomb structural body of the present invention, the fired honeycomb body preferably contains 25 to 75 wt% of CZ particles.

[0055] When the fired honeycomb contains 25 to 75 wt% CZ particles, cerium can exhibit a higher oxygen storage capacity (OSC).

[0056] Examples of the shape of the honeycomb structural body according to the present invention include a round column shape, a prism, a cylindrical shape, a column shape having an end surface with a race track shape, and a prism with rounded corners (e.g., a triangular column shape with rounded corners).

[0057] In the honeycomb structural body of the present invention, all partition walls preferably have the same thickness. Specifically, the partition walls of the fired honeycomb body preferably have a thickness of less than 0.14 mm. The thickness is preferably 0.05 mm or more.

[0058] In the honeycomb structural body of the present invention, the shape of the through holes of the fired honeycomb body is not limited to a quadrangular columnar shape. For example, a triangular columnar shape or a hexagonal columnar shape may be used.

[0059] The shapes of the through-holes may differ from each other, but they are preferably the same. In other words, the through-holes each surrounded by the partition walls in a cross-section perpendicular to the longitudinal direction of the fired honeycomb body preferably have the same size.

[0060] In the honeycomb structural body of the present invention, the density of the through holes in a cross section perpendicular to the longitudinal direction of the fired honeycomb body is preferably 31 to 155 pieces / cm 2 .

[0061] In the honeycomb structural body of the present invention, the fired honeycomb body preferably has a porosity of 45 to 70%.

[0062] The fired honeycomb body with a porosity of 45 to 70% can achieve a balance between high mechanical strength and exhaust gas conversion performance.

[0063] If the fired honeycomb body has a porosity of less than 45%, the percentage of pores that can contribute to internal gas diffusion in the partition walls is low, which may lead to poor exhaust gas conversion performance. If the fired honeycomb body has a porosity of more than 70 vol%, the porosity is so high that the honeycomb structure has poor mechanical properties, and the honeycomb structure is easily cracked, broken, or the like during use.

[0064] The porosity of the fired honeycomb body can be measured by a weighing method described below. (1) The fired honeycomb body is cut into a size of 10 cells × 10 cells × 10 mm to obtain a test sample. The test sample is washed with deionized water and acetone under ultrasonic cleaning, and dried in an oven at 100°C. The test sample with a size of 10 cells × 10 cells × 10 mm is a sample cut so that the sample contains 10 longitudinally oriented through holes × 10 transversely oriented through holes, the outermost through holes, and the partition walls forming the through holes, with the longitudinal length being 10 mm. (2) Using a measuring microscope (Measuring Microscope MM-40, available from Nikon, 100x magnification), the cross-sectional dimension of the sample is measured, and the volume is determined from a geometric calculation (if the volume cannot be determined from a geometric calculation, the volume is measured by measuring the water-saturated weight and the weight in water). (3) The weight of the sample is calculated from the calculated volume and the true density of the sample measured with a pycnometer, assuming that the sample is a completely dense body. A measurement method using a pycnometer is as described in (4) below. (4) The fired honeycomb body is pulverized to 23.6 cm 3The powder is dried at 200 °C for 8 hours. The true density is then measured according to JIS R 1620:1995 using the Auto Pycnometer 1320 available from Micromeritics. The evacuation time is 40 minutes. (5) The actual weight of the sample is measured using an electric balance (HR202i, available from A & D). (6) The porosity of the fired honeycomb body is determined by the following formula. (Porosity of the fired honeycomb body) = 100 − (actual weight of the test sample / weight of the test sample assuming that the test sample is a completely dense body) × 100 [%]

[0065] Even if a noble metal is directly supported on the honeycomb structural body of the present invention, changes in the porosity of the fired honeycomb body due to the noble metal are small enough to be ignored.

[0066] In the honeycomb structural body of the present invention, the fired honeycomb body may further contain alumina particles and an inorganic binder.

[0067] The alumina particles are preferably θ-phase alumina particles.

[0068] When the alumina particles are θ-phase alumina particles, due to their high heat resistance, the honeycomb structural body supporting a noble metal can exhibit high exhaust gas conversion performance even after long-term use.

[0069] The inorganic binder is preferably boehmite. This is because a large portion of the boehmite is converted to γ-alumina after firing.

[0070] In the honeycomb structural body of the present invention, a noble metal is preferably supported on the fired honeycomb body.

[0071] Examples of the precious metal include platinum group metals such as platinum, palladium, and rhodium.

[0072] The amount of the precious metal carried in the entire fired honeycomb body is preferably 0.1 to 15 g / l, more preferably 0.5 to 10 g / l.

[0073] The term "amount of precious metal supported" as used herein refers to the weight of precious metal per apparent volume of the honeycomb structural body. The apparent volume of the honeycomb structural body includes the volume of the pores. If the honeycomb structural body contains an adhesive layer, the apparent volume includes the volume of the adhesive layer.

[0074] In the honeycomb structural body of the present invention, a peripheral coating layer may be formed on the outer periphery of the fired honeycomb body. [Method for producing a honeycomb structural body]

[0075] Next, a method for producing the honeycomb structural body of the present invention will be described.

[0076] The honeycomb structural body of the present invention can be manufactured by a method comprising: a molding step of molding a raw material paste containing, for example, CZ particles, alumina particles, inorganic fibers, and an inorganic binder into a honeycomb formed body in which a plurality of through holes are arranged longitudinally parallel to each other with a partition wall therebetween; a drying step of drying the honeycomb formed body obtained in the molding step; and a firing step of firing the honeycomb formed body dried in the drying step to produce a fired honeycomb body. (Forming step)

[0077] In the forming step, CZ particles and inorganic fibers are first mixed together to produce a raw material paste.

[0078] The raw material paste may further contain alumina particles, an inorganic binder, an organic binder, a pore former, a molding aid, a dispersion medium, or the like.

[0079] The CZ particles serve as a co-catalyst (oxygen storage) for the exhaust catalyst. CZ particles are preferably those that form a solid solution of cerium oxide and zirconium oxide.

[0080] The CZ particles may further contain a rare earth element in addition to cerium. Examples of the rare earth element include scandium (Sc), yttrium (Y), lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb), dysprosium (Dy), ytterbium (Yb), and ruthenium (Lu).

[0081] The amount of cerium oxide in the CZ particles is preferably 20 wt% or more, more preferably 40 wt% or more. At the same time, the amount of cerium oxide is preferably 90 wt% or less, more preferably 80 wt% or less. The amount of zirconia in the CZ particles is preferably 60 wt% or less, more preferably 50 wt% or less. Such CZ particles have a low heat capacity. Thus, the temperature of the honeycomb structural body rises easily, thereby achieving better warm-up performance.

[0082] The average particle size of the CZ particles is preferably 1 to 50 µm. The average particle size of the CZ particles is more preferably 1 to 30 µm. When the CZ particles have an average particle size of 1 to 50 µm, the resulting honeycomb structural body has a larger surface area and can thus have a higher oxygen storage capacity.

[0083] The alumina particles may be of any type, but θ-phase alumina particles (hereinafter sometimes referred to as “θ-alumina particles”) are preferred.

[0084] The use of θ-phase alumina particles as a divider between CZ particles can inhibit the thermal sintering of alumina particles during use, allowing for sustained catalytic function. Furthermore, the use of θ-phase alumina particles can increase heat resistance.

[0085] The alumina particles can have any average particle size. However, to improve gas conversion performance and heating performance, the average particle size is preferably 1 to 10 µm, more preferably 1 to 5 µm.

[0086] The average particle sizes of the CZ particles and the alumina particles can be determined using a laser diffraction particle size distribution meter (Mastersizer 2000, available from Malvern Panalytical).

[0087] Any material can be used to form the inorganic fibers. Examples include alumina, silica, silicon carbide, silica-alumina, glass, potassium titanate, and aluminum borate. Two or more of these can be used in combination. Of these, alumina fibers are preferred.

[0088] The inorganic binder is preferably boehmite.

[0089] Boehmite is alumina monohydrate with a composition of AlOOH and exhibits good dispersibility in media such as water. Therefore, boehmite is preferably used as an alumina binder.

[0090] The use of boehmite can reduce the moisture content of the raw material paste and improve formability.

[0091] Any organic binder can be used. Examples include methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, polyethylene glycol, phenolic resin, and epoxy resin. Two or more of these can be used in combination.

[0092] Any pore-forming agent can be used. Examples include acrylic resins, coke, and starch.

[0093] The pore former is used to introduce pores into a fired honeycomb body when a fired honeycomb body is manufactured.

[0094] Any molding aid can be used. Examples include ethylene glycol, dextrins, fatty acids, fatty acid soaps, and polyalcohols. Two or more of these can be used in combination.

[0095] Any dispersion medium can be used. Examples include water, organic solvents such as benzene, and alcohols such as methanol. Two or more of these can be used in combination.

[0096] When CZ particles, alumina particles, alumina fibers, and the alumina binder are used as materials of the raw material paste, the percentage of each of these materials, based on the total solids remaining in the raw material paste after the firing step, is preferably as follows: CZ particles: 25 to 75 wt%; alumina particles: 15 to 35 wt%; alumina fibers: 5 to 20 wt%; and alumina binder: 5 to 20 wt%.

[0097] When preparing the raw material paste, mixing / kneading is preferred. A device such as a mixer or attritor can be used for mixing, or a device such as a kneader can be used for kneading.

[0098] In the molding step, the raw material paste containing CZ particles and inorganic fibers is extruded into a honeycomb molded body in which a plurality of through holes with a partition wall therebetween are arranged parallel to each other in the longitudinal direction.

[0099] At this time, the manufacturing conditions and molding conditions of the raw material paste are preferably adjusted by one or more of the following methods (these methods may be used in combination) so that the b-axis thermal expansion coefficient of the honeycomb structural body is higher than its a-axis thermal expansion coefficient. (1) The fiber length of the inorganic fibers is adjusted. When the inorganic fibers have a longer average fiber length, the inorganic fibers can be easily oriented in the a-axis direction. As a result, the thermal expansion coefficient in the a-axis direction decreases.

[0100] For example, the inorganic fibers preferably have an average fiber length of 10 to 90 µm. (2) The flowability of the raw material paste is adjusted (viscosity is adjusted). If the raw material paste is soft and has good flowability, the inorganic fibers can be easily oriented in the a-axis direction. As a result, the thermal expansion coefficient in the a-axis direction decreases. For example, the raw material paste preferably has a shear stress of 500 Pa s or less at a shear rate of 500 (1 / s). (3) The forming pressure during extrusion is adjusted. When the forming pressure is high, the inorganic fibers are easily oriented in the a-axis direction. As a result, the thermal expansion coefficient in the a-axis direction decreases.

[0101] For example, the forming pressure is preferably 4 to 5 MPa. (4) A mesh is placed in an extruder, and the mesh size (unit) is adjusted. When the mesh size is large, the inorganic fibers can easily orient themselves along the a-axis. As a result, the thermal expansion coefficient decreases along the a-axis.

[0102] The shape of the honeycomb molded body is not limited, but it is preferably a round column. The round column preferably has a diameter of 150 mm or less.

[0103] Alternatively, the honeycomb molded body can be prismatic. The prismatic shape is preferably a quadrangular columnar shape. (drying step)

[0104] The honeycomb body is then dried to obtain a dried honeycomb body (drying step).

[0105] In the drying step, a dryer such as a microwave dryer, a hot air dryer, a dielectric dryer, a reduced pressure dryer, a vacuum dryer, or a freeze dryer is used to dry the honeycomb formed body to produce a dried honeycomb body. (Firing step)

[0106] In the firing step, the dried honeycomb obtained in the drying step is fired into a fired honeycomb. In this step, the dried honeycomb is degreased and fired. Therefore, this step can also be referred to as the "degreasing / firing step," but for convenience, it is referred to as the "firing step."

[0107] The temperature in the firing step is preferably 800°C to 1300°C, more preferably 900°C to 1200°C. The duration of the firing step is preferably 1 to 24 hours, more preferably 3 to 18 hours. The atmosphere of the firing step is not limited, but an atmosphere with an oxygen concentration of 1 to 20% is preferred.

[0108] The honeycomb structural body of the present invention can be manufactured by the above steps. (Other steps)

[0109] The method for producing a honeycomb structural body of the present invention may further comprise a supporting step of allowing a noble metal to be supported on the fired honeycomb body, if necessary.

[0110] Examples of the method for enabling a noble metal to be supported on the fired honeycomb body include a method in which the fired honeycomb body or the honeycomb structural body is immersed in a solution containing noble metal particles or a noble metal complex, and the fired honeycomb body or the honeycomb structural body is then pulled out and heated.

[0111] When the honeycomb structural body includes a peripheral coating layer, a noble metal may be supported on the fired honeycomb body before the peripheral coating layer is formed, or a noble metal may be supported on the fired honeycomb body or the honeycomb structural body after the peripheral coating layer is formed.

[0112] In the method for producing a honeycomb structural body of the present invention, the amount of the noble metal supported in the supporting step is preferably 0.1 to 15 g / L, more preferably 0.5 to 10 g / L.

[0113] When the method for producing a honeycomb structural body of the present invention includes forming a peripheral coating layer on the outer periphery of the fired honeycomb body, the peripheral coating layer can be formed by applying a peripheral coating layer paste to the outer periphery of the fired honeycomb body, excluding both end surfaces, and then solidifying the peripheral coating layer paste by drying. A paste having the same composition as the raw material paste can be used as the peripheral coating layer paste. EXAMPLES

[0114] Examples that further disclose the present invention are described below. The present invention is not limited to these examples. [Production of a honeycomb structural body](Example 1)

[0115] The following materials were mixed / kneaded into a raw material paste: CZ particles (average particle size: 2 µm) (26.5 wt%); θ-alumina particles (average particle size: 2 µm) (13.2 wt%); alumina fibers (average fiber diameter: 3 µm; average fiber length: 60 µm) (5.3 wt%); boehmite as an alumina binder (11.3 wt%); methylcellulose as an organic binder (7.8 wt%); an acrylic resin as a pore former (1.9 wt%); graphite also as a pore former (2.3 wt%); polyoxyethylene oleyl ether (surfactant) as a molding aid (4.3 wt%); and deionized water (27.4 wt%). [Forming step]

[0116] The raw material paste was extruded into a round columnar honeycomb body using an extruder.

[0117] A mesh was placed into the extruder.

[0118] The mesh size and forming pressure are as shown in Table 1. The mesh number was 42 and the forming pressure was 4.5 MPa. [Drying step]

[0119] The honeycomb molded body was dried using a microwave dryer with a power of 1.8 A and a microwave irradiation time of 110 seconds. [Firing step]

[0120] The resulting dried honeycomb body was degreased / fired at 1100 °C for 10 hours, thereby producing a fired honeycomb body according to Example 1. The fired honeycomb body had a round column shape with a diameter of 117 mm and a length of 80 mm, in which the density of through holes was 77.5 pieces / cm 2 (500 cpsi) and the thickness of the partition was 0.127 mm (5 mil). (Examples 2 and 3 and Comparative Example 1)

[0121] Honeycomb structural bodies were prepared as in Example 1, except that a raw material paste was prepared and extruded by changing the average fiber length of the alumina fibers in the raw material paste and the mesh number and the molding pressure in the molding step.

[0122] Table 1 shows the results. [Measurement of the thermal expansion coefficient]

[0123] The a-axis thermal expansion coefficient and the b-axis thermal expansion coefficient were measured using a thermal expansion meter.

[0124] The thermal expansion gauge was DIL 402C, available from NETZSH.

[0125] The measurement temperature range was 40 °C to 800 °C, the heating rate was 10 °C / min, and the gas flow rate was 100 ml / min in air. The reference material was sapphire.

[0126] The size of a test piece was 3.5 mm × 3.5 mm (3 cells × 3 cells) × 25 mm.

[0127] The a-axis thermal expansion coefficient and the b-axis thermal expansion coefficient were measured for each of the five test pieces, and the average was calculated as the measurement result. Table 1 shows the results. [Measurement of the degree of orientation]

[0128] Cross-sectional images of the honeycomb structural bodies of the examples and comparative examples were taken. The long axis / short axis ratio was calculated for each of the 100 inorganic fibers shown in each cross-sectional image, and the proportion of inorganic fibers with a long axis / short axis ratio of 1.00 to 1.30 was determined. This proportion was regarded as the "orientation degree." Table 1 shows the results. [Observation of crack formation state and crack size]

[0129] Hot gas was injected into the honeycomb structure to increase its temperature to 1000 °C, followed by cooling to room temperature in 15 seconds. This cycle was repeated 30 times to conduct a thermal shock test to induce cracks in the honeycomb structure.

[0130] After cracking the honeycomb structure, the crack direction and maximum crack size were measured. Table 1 shows the results.

[0131] The direction of cracking is indicated as “a-axis direction” for cracking along the a-axis and as “ring-off” for ring-off cracking (cracking along the b-axis direction). [Table 1] aluminum oxide fiber Forming conditions Physical properties of the honeycomb structure Average fiber length (µm) Mesh size Forming pressure (MPa) a-axis thermal expansion coefficient (× 10 -6 / K) b-axis thermal expansion coefficient (× 10 -6 / K) Difference (× 10 -6 / K) Degree of orientation (%) Crack formation direction Maximum crack size (mm) Example 1 60 #42 4,5 7,8 8,3 0,5 70 aA. 10 Example 2 70 #36 4,1 7, 9 8,2 0,3 60 aA. 15 Example 3 90 #50 4,7 7,6 8,4 0,8 80 aA. 15 Comparison example 1 40 #30 4,1 8,0 8,0 0 55 Ring-off 100 (aA.= a-axis direction)

[0132] As shown in Table 1, in each of the honeycomb structural bodies of Examples 1-3, the b-axis thermal expansion coefficient is higher than the a-axis thermal expansion coefficient, and cracks are oriented in the a-axis direction. Furthermore, the crack size is smaller.

[0133] This shows that it is possible to manufacture a honeycomb structural body that is less susceptible to ring-off cracking when a temperature difference occurs in the honeycomb structural body by adjusting the ratio between the a-axis thermal expansion coefficient and the b-axis thermal expansion coefficient. LIST OF REFERENCE SYMBOLS 10 honeycomb structural bodies 11 fired honeycomb body 12 through holes 13 Partition wall 14 outer peripheral wall

Claims

[1] Honeycomb structural body comprising: a fired honeycomb body in which a plurality of through holes with a partition wall between them are arranged parallel to each other in the longitudinal direction, wherein the fired honeycomb body contains ceria-zirconia composite oxide particles and inorganic fibers, and a b-axis thermal expansion coefficient measured in a direction along a b-axis, which is a direction perpendicular to a longitudinal direction of the honeycomb structural body, is higher than an a-axis thermal expansion coefficient measured in a direction along an a-axis, which is the longitudinal direction of the honeycomb structural body, wherein at least 60% of the inorganic fibers observed in a cross-sectional image of the honeycomb structural body cut in a cross-sectional direction perpendicular to the longitudinal direction have a ratio of the length of a long axis to the length of a short axis (long axis / short axis) of 1.00 to 1.30, wherein the length of the long axis is the length of a long axis of a cross section of an inorganic fiber shown in the cross-sectional image, wherein the length of the short axis is the length of a perpendicular bisector of the long axis. [2] The honeycomb structural body according to claim 1, wherein the a-axis thermal expansion coefficient is 7.5 × 10 -6 / K or more and less than 8.0 × 10 -6 / K. [3] The honeycomb structural body according to claim 1 or 2, wherein the b-axis thermal expansion coefficient is 8.0 × 10 -6 / K or more and 8.5 × 10 -6 / K or less. [4] The honeycomb structural body according to any one of claims 1 to 3, wherein a difference between the b-axis thermal expansion coefficient and the a-axis thermal expansion coefficient ((b-axis thermal expansion coefficient) - (a-axis thermal expansion coefficient)) is 0.1 to 1.0 × 10 -6 / K.

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