Patent ID: 12187650

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be explained below referring to the attached drawings. It should be noted that the present invention is not restricted to the embodiments below, and that modifications and improvements may be made within the scope of the present invention.

[1] Silicon Carbide Ceramic Honeycomb Structure

The silicon carbide ceramic honeycomb structure of the present invention comprises large numbers of axially penetrating flow paths partitioned by porous silicon carbide cell walls, the cell walls having a porosity of 35-50% and a median pore diameter of 8-18 μm,when a straight line C is drawn in parallel with the cell wall surface such that it passes a center in the direction of the thickness T of the cell wall, and straight lines are drawn in parallel with the straight line C such that they are separate from the straight line C by ±T/5 and ±2T/5 in the thickness direction of the cell wall, to measure the lengths (pore widths) of pore portions crossing each straight line, and the number of pores crossing each straight line, in a cell wall cross section perpendicular to the axial direction,an average pore width W determined by averaging the lengths (pore widths) of all measured pore portions being 10-25 μm, andthe number N of pores per length determined by dividing the total number of the measured pores by the total length of the straight lines used for measurement being 20-40/mm.

With such a structure, the ceramic honeycomb structure can effectively capture nanosized PM with good pressure loss after capturing PM, while maintaining high heat shock resistance.

The average pore width W determined by averaging the pore widths measured in the cell wall cross section is 10-25 μm. When the average pore width W is less than 10 μm, it is difficult to maintain low pressure loss after capturing PM. On the other hand, when it is more than 25 μm, the ratio of capturing nanosized PM is low. The lower limit of the average pore width W is preferably 12 μm, and the upper limit is preferably 23 μm, and more preferably 19 μm.

The number N of pores per length measured in the cell wall cross section is 20-40/mm. When the number N of pores per length is less than 20/mm, it is difficult to maintain low pressure loss after capturing PM. On the other hand, when it is more than 40/mm, the ratio of capturing nanosized PM is low. The lower limit of the number N of pores per length is preferably 22/mm, and the upper limit is preferably 37/mm.

The average pore width W and the number N of pores per length are determined by photographing a cell wall cross section perpendicular to the axial direction of the ceramic honeycomb structure by a scanning electron microscope (SEM), and processing the resultant SEM photograph by an image analyzer (Image-Pro Plus ver. 7.0 available from Media Cybernetics) as follows. First, the SEM photograph is processed to a binary image.FIG.4shows an example of binary images. As shown inFIG.5, in the photographed cross section of the cell wall12, a straight line C is drawn in parallel with the cell wall surface such that it passes a center in the direction of the thickness T of the cell wall, and straight lines are drawn in parallel with the straight line C such that they are separate from the straight line C by ±T/5 and ±2T/5 in the thickness direction of the cell wall. The pore widths (the lengths of pore portions crossing each straight line) and the number of pores crossing each straight line are measured with respect to the predetermined length of each straight line. A sum of the measured widths of all pores is divided by the total number of measured pores to determine the average pore width W, and the total number of measured pores is divided by the total length of the straight lines used for measurement to determine the number N of pores per length.

The porosity is 35-50%. When the porosity is less than 35%, it is difficult to maintain low pressure loss after capturing PM. On the other hand, when it is more than 50%, the ratio of capturing nanosized PM is low. The lower limit of the porosity is preferably 38%, and more preferably 40%. On the other hand, the upper limit of the porosity is preferably 49%, more preferably 48%, and most preferably 46%. The porosity of cell walls is measured by mercury porosimetry described below.

The median pore diameter is 8-18 μm. When the median pore diameter is less than 8 μm, it is difficult to maintain low pressure loss after capturing PM. On the other hand, when it is more than 18 μm, the ratio of capturing nanosized PM is low. The median pore diameter is preferably 10-15 μm. The median pore diameter is a pore diameter at which a cumulative pore volume is 50% of the total pore volume, in a pore size distribution curve of cell walls measured by mercury porosimetry described below.

The pore volume in a pore diameter range of 20 μm or more is preferably 10-20% of the total pore volume. When the pore volume at 20 μm or more is less than 10% of the total pore volume, it may be difficult to maintain low pressure loss after capturing PM. On the other hand, when it is more than 20%, the ratio of capturing nanosized PM may be low. It is preferably 12-18%.

The pore volume in a pore diameter range of 9 μm or less is preferably 3-25% of the total pore volume. When the pore volume at a pore diameter of 9 μm or less is less than 3% of the total pore volume, it may be difficult to maintain low pressure loss after capturing PM. On the other hand, when it is more than 25%, the ratio of capturing nanosized PM may be low. Its lower limit is preferably 4%, and its upper limit is preferably 23%.

The measurement of the cumulative pore volume by mercury porosimetry uses, for example, Autopore III 9410 available from Micromeritics. In the measurement of the cumulative pore volume by mercury porosimetry, a test piece (10 mm×10 mm×10 mm) cut out of each ceramic honeycomb structure is set in a measurement cell, the cell is evacuated, and mercury is then introduced into the cell under pressure to measure the volume of mercury intruded into pores in the test piece. Because mercury is introduced into finer pores under higher pressure, the relation between a pore diameter and a cumulative pore volume (cumulative volume of pores in a range from the maximum pore diameter to a particular pore diameter) can be determined from the pressure and the volume of mercury intruded into pores. Because mercury is introduced into pores successively from larger pore diameters to smaller pore diameters, the pressure is converted to pore diameter, and a cumulative pore volume determined by cumulating pore volumes from larger pore diameters to smaller pore diameters, which corresponds to the volume of mercury, is plotted against the pore diameter. The starting pressure of intruding mercury is herein 0.5 psi (0.35×10−3kg/mm2, corresponding to a pore diameter of about 362 μm), and the cumulative pore volume when mercury pressure reaches 1800 psi (1.26 kg/mm2, corresponding to a pore diameter of about 0.1 μm) is defined as the total pore volume.

The silicon carbide ceramic honeycomb structure of the present invention may be used as a honeycomb segment211as shown inFIG.2, and large numbers of honeycomb segments211may be integrally bonded with binder layers29to form a composite silicon carbide ceramic honeycomb structure210as shown inFIG.3. After integrally bonding large numbers of honeycomb segments211by binder layers29, the resultant composite structure is machined to have an outer periphery having a circular, oval, triangular, rectangular or any other desired shape in a cross section perpendicular to its flow paths, and the machined outer periphery is covered with a coating material to form an outer peripheral wall21.

The flow paths of the silicon carbide ceramic honeycomb structure110,210of the present invention on the exhaust-gas-introducing side25aor the exhaust-gas-discharging side25bmay be alternately plugged in a checkerboard pattern by a known method, to provide a ceramic honeycomb filter100,200. In the case of a ceramic honeycomb filter200formed by integral bonding, plugs26a,26bmay be formed in the honeycomb segments211before or after integrally bonded. These plugs may be formed on the end surface on the exhaust-gas-introducing or exhaust-gas-discharging side of the flow paths, or in inner portions of the flow paths inside the inlet-side or outlet-side end surface25a,26b.

[2] Production Method of Silicon Carbide Ceramic Honeycomb Structure

An example of the methods of the present invention for producing a silicon carbide ceramic honeycomb structure will be explained.

Ceramic particles comprising aggregate and binders are mixed and blended with an organic binder to form a moldable material, which is extruded in a honeycomb shape to obtain a green body. The green body is dried and then sintered. The above aggregate is composed of silicon carbide particles, and the above ceramic particles have a median particle diameter D50 of 35-45 μm. In a curve showing the relation between particle diameter and cumulative particle volume, the particle diameter D10 at a cumulative particle volume corresponding to 10% of the total particle volume is 5-20 μm, the particle diameter D90 at a cumulative particle volume corresponding to 90% of the total particle volume is 50-65 μm, and the particle size distribution deviation SD is 0.20-0.40, wherein SD=log(D80)−log(D20), D20 is a particle diameter at a cumulative particle volume corresponding to 20% of the total particle volume, D80 is a particle diameter at a cumulative particle volume corresponding to 80% of the total particle volume, and D20<D80.

Such method can produce a silicon carbide ceramic honeycomb structure having cell walls with porosity of 35-50% and a median pore diameter of 8-18 μm, as well as an average pore width W of 10-25 μm and the number N of pores per length of 20-40/mm in a cell wall cross section perpendicular to the axial direction.

The particle diameters of the ceramic particles can be measured by, for example, a particle diameter distribution meter (Microtrack MT3000 available from Nikkiso Co., Ltd.).FIG.6shows an example of the relations between the measured particle diameter and the cumulative particle volume determined by cumulating the volume of particles having particle diameters up to the particular one. In the curve shown inFIG.6, D10 (μm) is a particle diameter at a cumulative particle volume corresponding to 10% of the total particle volume, a median particle diameter (D50) (μm) is a particle diameter at a cumulative particle volume corresponding to 50% of the total particle volume, and D90 (μm) is a particle diameter at a cumulative particle volume corresponding to 90% of the total particle volume. The particle size distribution deviation SD is expressed by SD=log(D80)−log(D20), wherein D20 is a particle diameter (μm) at a cumulative particle volume corresponding to 20% of the total particle volume, and D80 is similarly a particle diameter (μm) at a cumulative particle volume corresponding to 80% of the total particle volume. Incidentally, D20<D80.

The ceramic particles have a median particle diameter D50 of 35-45 μm. When the median particle diameter D50 is less than 35 μm, pores formed in the cell walls have small diameters, making it difficult to maintain low pressure loss when PM is captured. On the other hand, when it is more than 45 μm, pores formed in the cell walls have too large diameters, lowering the ratio of capturing nanosized PM. The lower limit of the median particle diameter D50 is preferably 37 μm, and the upper limit is preferably 43 μm.

The ceramic particles have D10 of 5-20 μm. When the D10 is less than 5 μm, the percentage of fine pores deteriorating pressure loss characteristics is undesirably high among pores formed in the cell walls. On the other hand, when it is more than 20 μm, nanosized PM may not be effectively captured. The lower limit of D10 is preferably 7 μm, and the upper limit is preferably 18 μm.

The ceramic particles have D90 of 50-65 μm. When the D90 is less than 50 μm, it is difficult to maintain low pressure loss when PM is captured. On the other hand, when it is more than 65, the ratio of capturing nanosized PM is low. The lower limit of D90 is preferably 52 μm, and the upper limit is 63 μm.

The ceramic particles have a particle size distribution deviation SD of 0.20-0.40, wherein SD=log(D80)−log(D20), D20 is a particle diameter at a cumulative particle volume corresponding to 20% of the total particle volume, D80 is a particle diameter at a cumulative particle volume corresponding to 80% of the total particle volume, and D20<D80. When the SD is less than 0.20, the percentage of fine pores is high among pores formed in the cell walls, making it difficult to maintain low pressure loss when PM is captured. On the other hand, when it is more than 0.40, the percentage of large pores lowering the ratio of capturing nanosized PM is undesirably high. The lower limit of SD is preferably 0.22, and the upper limit is preferably 0.38.

The binder is preferably at least one selected from the group consisting of alumina particles, aluminum hydroxide particles, magnesium oxide particles, and magnesium hydroxide particles.

The organic binder may be methylcellulose, ethylcellulose, ethymethylcellulose, carboxymethylcellulose, hydroxypropylmethylcellulose, hydroxyethylcellulose, hydroxymethylcellulose, hydroxyethylethylcellulose, etc. Among them, hydroxypropylmethylcellulose and/or methylcellulose are preferable. The organic binder is preferably 5-15% by mass per 100% by mass of the starting material (silicon carbide particles+binder).

The ceramic particles comprising the aggregate and the binder are mixed with the organic binder, and then blended with water to form a plasticized moldable material. The amount of water added, which is controlled to provide the moldable material with consistency suitable for molding, is preferably 20-50% by mass of the starting material.

The formed moldable material is extrusion-molded through a known honeycomb-structure-molding die by a known method to form a honeycomb-structured green body. This green body is dried, and then machined on its end surfaces, outer peripheral surface, etc., if necessary. It is then sintered in a temperature rang of 1200-1350° C. in an oxidizing atmosphere to produce a silicon carbide ceramic honeycomb structure.

Though not restrictive, the drying method may be, for example, hot-air drying, microwave-heating drying, high-frequency-heating drying, etc.

By sintering in a temperature range of 1200-1350° C., binder particles (at least one selected from the group consisting of alumina particles, aluminum hydroxide particles, magnesium oxide particles, and magnesium hydroxide particles) are sintered to form binder layers binding silicon carbide particles. Because of sintering at such a relatively low temperature, the sintering cost for forming the binder layers can be made lower than ever. The sintering temperature of lower than 1200° C. insufficient binds the silicon carbide particles, failing to obtain sufficient strength. On the other hand, when it is higher than 1350° C., the heat shock resistance is low. Because of sintering in an oxidizing atmosphere, cost increase can be avoided in the sintering process.

The present invention will be explained below in further detail referring to Examples, without intention of restricting the present invention to thereto.

Examples 1-5

Silicon carbide particles and other particles having the particle diameters shown in Table 1 in the amounts shown in Table 1 were mixed with hydroxypropylmethylcellulose as an organic binder. Each of the resultant starting material mixtures was blended with water to form a plasticized moldable material, which was extrusion-molded through a honeycomb-structure-molding die in a screw-molding machine, to form a honeycomb-structured green body having a quadrilateral peripheral shape of 34 mm in each side and a length of 304 mm. This green body was dried at 120° C. for 2 hours by a hot-air drying machine, and then sintered at the highest temperature of 1300° C. in an oxidizing atmosphere to obtain each silicon carbide ceramic honeycomb structure of Examples 1-5 having a cell wall thickness of 8 mil (0.20 mm) and a cell density of 300 cpsi (46.5 cells/cm2).

Comparative Examples 1 and 7

The silicon carbide ceramic honeycomb structures of Comparative Examples 1 and 7 were produced in the same manner as in Example 1, except that the types and amounts of silicon carbide particles and binder particles were changed as shown in Table 1, that a degreasing step at 550° C. for 3 hours was added after drying the green body by hot air, and that sintering was conducted at the highest temperature of 1450° C. for 2 hours in an argon atmosphere.

Comparative Examples 2-6

The silicon carbide ceramic honeycomb structures of Comparative Examples 2-6 were produced in the same manner as in Example 1, except that each honeycomb-structured green body was molded by the same method as in Example 1 with the types and amounts of silicon carbide particles and binder particles changed as shown in Table 1, and that sintering was conducted at the highest temperature of 1300° C. in Comparative Examples 2 and 4-6 and at the highest temperature of 1400° C. in Comparative Example 3, in an oxidizing atmosphere.

One of the silicon carbide ceramic honeycomb structures formed in each of Examples 1-5 and Comparative Examples 1-7 was measured with respect to an average pore width, the number of pores per length, porosity, a median pore diameter, and a coefficient of thermal expansion.

(a) Average Pore Width and Number of Pores Per Length

The average pore width and the number of pores per length are measured as follows. A cell wall cross section of each ceramic honeycomb structure perpendicular to the axial direction is photographed by a scanning electron microscope (SEM) at a magnification of 200 folds. The resultant SEM photograph is measured by an image analyzer (Image-Pro Plus ver. 7.0 available from Media Cybernetics). Specifically, the SEM photograph is processed to a black-and-white binary image shown inFIG.4by the image analyzer. As shown inFIG.5, in the photographed cross section of a cell wall12, a straight line C is drawn in parallel with the cell wall surface such that it passes a center in the direction of the thickness T of the cell wall, and straight lines are drawn in parallel with the straight line C such that they are separate from the straight line C by T/5 and ±2T/5 in the thickness direction of the cell wall. The pore widths (lengths of pore portions crossing each straight line) and the number of pores crossing each straight line were measured in the predetermined length of each straight line. A sum of the measured pore widths was divided by the total number of measured pores to determine the average pore width W, and the total number of measured pores was divided by the total length of the measured straight lines to determine the number N of pores per length.

(b) Measurement of Porosity and Median Pore Diameter

The porosity and the median pore diameter were measured by mercury porosimetry. A test piece (10 mm×10 mm×10 mm) cut out of each ceramic honeycomb filter was set in a measurement cell of Autopore III available from Micromeritics, and the cell was evacuated. Thereafter, mercury was introduced into the cell under pressure to determine the relation between the pressure and the volume of mercury pressed into pores in the test piece. The pressure was converted to a pore diameter and cumulated from the larger pore diameter side to the smaller pore diameter side to determine a cumulative pore volume (corresponding to the volume of mercury), which was plotted against the pore diameter, thereby obtaining a graph showing the relation between pore diameter and cumulative pore volume. The mercury-intruding pressure was 0.5 psi (0.35×10−3kg/mm2), and constants used for calculating the pore diameter from the pressure were a contact angle of 130° and a surface tension of 484 dyne/cm. A cumulative pore volume at which the pressure of mercury was 1800 psi (1.26 kg/mm2corresponding to a pore diameter of about 0.1 μm) was regarded as the total pore volume.

From the mercury porosimetry measurement results, the total pore volume, and the median pore diameter at which the cumulative pore volume was 50% of the total pore volume were determined. These results are shown in Table 2.

(c) Measurement of Coefficient of Thermal Expansion

A test piece having a cross section shape of 4.5 mm×4.5 mm and a length of 50 mm was cut out of the ceramic honeycomb structure with its longitudinal direction substantially in alignment with the flow path direction, and heated from room temperature to 800° C. at a temperature-elevating rate of 10° C./minute to measure longitudinal length increase under a constant load of 20 g by a compression-load- and differential-expansion-type thermomechanical analyzer TMA (ThermoPlus available from Rigaku Corp.), to determine an average coefficient of thermal expansion between 40° C. and 800° C. The results are shown in Table 2.

End portions of flow paths in each silicon carbide ceramic honeycomb structure of Examples 1-5 and Comparative Examples 1-7 were alternately sealed by a plugging material slurry comprising silicon carbide particles, and the plugging material slurry was dried to form plugs. The silicon carbide ceramic honeycomb structure with plugs was used as a honeycomb segment, with its outer peripheral surface coated with a binder comprising silicon carbide particles and colloidal silica. 6×6 honeycomb segments were integrally bonded, and the resultant outer peripheral surface portion was machined to a circular shape in the cross section perpendicular to the axial direction. The machined outer peripheral surface was coated with a wall material comprising amorphous silica and colloidal silica, and dried to form an outer peripheral wall. In each of Examples 1-5 and Comparative Examples 1-7, two composite silicon carbide ceramic honeycomb filters each having an outer diameter of 266.7 mm, a length of 304.8 mm, a cell wall thickness of 8 mil (0.20 mm) and a cell density of 300 cpsi (46.5 cells/cm2) were obtained.

One of the ceramic honeycomb filters in each of Examples 1-5 and Comparative Examples 1-7 was measured with respect to pressure loss at an early PM-capturing stage and a number-based PM-capturing ratio after starting the capturing of PM, by the following methods.

(d) Pressure Loss after Capturing PM

The pressure loss after capturing PM was measured on each ceramic honeycomb filter fixed to a pressure loss test stand, by supplying combustion soot having an average particle diameter of 0.11 μm at a rate of 1.3 g/h together with air at a flow rate of 10 Nm3/min, and measuring pressure difference between the inlet side and the outlet side (pressure loss) when the amount of soot accumulated reached 2 g per 1 L of the volume of the filter. The pressure loss after capturing PM was evaluated by the following standard.Poor The pressure loss was more than 2.8 kPa.Fair The pressure loss was more than 2.5 kPa and 2.8 kPa or less.Good The pressure loss was more than 2.3 kPa and 2.5 kPa or less.Excellent The pressure loss was 2.3 kPa or less.
(e) Number-Based PM-Capturing Ratio

The number-based PM-capturing ratio was measured on each ceramic honeycomb filter fixed to a pressure loss test stand, by supplying combustion soot having an average particle diameter of 0.11 μm at a rate of 1.3 g/h together with air at a flow rate of 10 Nm3/min, and measuring the number of combustion soot particles flowing into and from the honeycomb filter per 1 minute by a scanning mobility particle sizer SMPS (Model 3936 available from TIS). The number-based PM-capturing ratio was calculated by the formula of (Nin−Nout)/Nin, wherein Ninrepresents the number of combustion soot particles flowing into the honeycomb filter, and Noutrepresents the number of combustion soot particles flowing from the honeycomb filter, in 1 minute between 40 minutes and 41 minutes after the start of supplying. The number-based PM-capturing ratio was evaluated by the following standard.

ExcellentThe PM-capturing ratio was 98% or more.GoodThe PM-capturing ratio was 96% or more and less than 98%.FairThe PM-capturing ratio was 95% or more and less than 96%.PoorThe PM-capturing ratio was less than 95%.

TABLE 1AggregateSiC ParticlesMedian ParticleD10D90No.(Mass %)Diameter (μm)(μm)(μm)Example 190.040.425.862.3Example 287.541.125.763.4Example 390.039.425.560.5Example 490.041.129.559.1Example 587.341.129.553.1Com. Ex. 180.0523776Com. Ex. 290.051.18.6147.3Com. Ex. 390.051.18.6147.3Com. Ex. 490.051.734.179.7Com. Ex. 590.051.734.179.7Com. Ex. 690.051.18.6147.3Com. Ex. 780.039.425.260.7BinderBinder%Median ParticleD10D90No.Particlesby MassDiameter (μm)(μm)(μm)Example 1Alumina5.385.02.111.6Magnesium4.623.11.15.5HydroxideExample 2Alumina7.365.02.111.6Magnesium5.133.11.15.5HydroxideExample 3Alumina5.894.92.211.7Magnesium4.113.11.15.5HydroxideExample 4Alumina5.894.92.211.7Magnesium4.113.11.15.5HydroxideExample 5Aluminum8.7410.82.120.3HydroxideMagnesium3.993.11.15.5HydroxideCom. Ex. 1Metallic20.0140.242SiliconCom. Ex. 2Alumina6.365.02.211.7Magnesium3.643.11.15.5HydroxideCom. Ex. 3Alumina6.365.02.211.7Magnesium3.643.11.15.5HydroxideCom. Ex. 4Alumina6.365.02.211.7Magnesium3.643.11.15.5HydroxideCom. Ex. 5Aluminum7.2810.82.120.3HydroxideMagnesium2.725.90.613.9HydroxideCom. Ex. 6Alumina4.285.02.211.7Magnesium5.723.11.15.5HydroxideCom. Ex. 7Metallic20.06.40.217.8SiliconCeramic Particles(Aggregate + Binder)Median ParticleD10D90No.Diameter (μm)(μm)(μm)SDExample 1399.4630.32Example 2385.8620.39Example 33813.4590.29Example 44017.6580.23Example 5367.6550.33Com. Ex. 14814.0760.33Com. Ex. 2425.61421.05Com. Ex. 3425.61421.05Com. Ex. 45017.2780.28Com. Ex. 55020.3780.28Com. Ex. 6425.31421.05Com. Ex. 7365.2580.44

TABLE 2AverageMedian PorePore widthNumber N ofPorosityDiameterPore Volume (%)No.W (μm)Pores (/mm)(%)(μm)≥20 μm≤9 μmExample 114.826.545.410.713.622.8Example 218.229.041.910.813.320.0Example 311.630.244.810.611.423.0Example 410.628.440.511.012.019.4Example 517.423.439.013.618.24.2Com. Ex. 137.711.850.222.061.11.8Com. Ex. 217.617.544.913.517.012.2Com. Ex. 328.79.739.426.982.25.8Com. Ex. 419.420.934.517.436.530.6Com. Ex. 519.420.950.713.214.58.9Com. Ex. 629.813.642.121.864.91.0Com. Ex. 718.616.742.310.519.333.5PressurePM-CapturingCTENo.LossRatio(×10−7/° C.)Example 1ExcellentExcellent43Example 2GoodExcellent44Example 3GoodExcellent45Example 4GoodExcellent47Example 5GoodExcellent43Com. Ex. 1FairPoor46Com. Ex. 2PoorGood44Com. Ex. 3GoodPoor53Com. Ex. 4PoorPoor44Com. Ex. 5GoodPoor46Com. Ex. 6PoorPoor45Com. Ex. 7PoorPoor46Note:(1) CTE means “coefficient of thermal expansion.”

It is clear from Tables 1 and 2 that the ceramic honeycomb filters of Examples 1-5 each having porosity, a median pore diameter, and an average pore width and the number of pores in a cell wall cross section within the ranges of the present invention exhibited better PM-capturing ratios with smaller pressure loss after capturing PM than those of the ceramic honeycomb filters of Comparative Examples 1-7 outside the ranges of the present invention in these requirements, as well as a coefficient of thermal expansion and thus heat shock resistance on the same level.