JPH01145377A - Silicon carbide honeycomb structure and production thereof - Google Patents

Abstract

PURPOSE:To obtain the title honeycomb structure freed from developing melt damage or thermal shock fracture when heated for the purpose of its reuse, by forming partition walls made of porous material having three-dimensional network structure, and also forming the mean size of the open pores in said network structure so as to become gradually larger from the partition walls located at the center of the structure towards those walls located on the periphery. CONSTITUTION:The objective silicon carbide honeycomb structure in which numerous penetrating holes 1a are arranged side by side in the axis direction through thin partition walls 1b. The constitution of this honeycomb structure is as follows: the partition walls 1b are made of a porous material having three-dimensional network structure composed mainly of lamellar crystals with an average aspect ratio of 2-50, and the average size of the open pores in said network structure is formed so as to become stepwise or continuously larger from the partition walls located at the center of the structure towards those walls located on the periphery (in the direction expressed by the arrow).

Description

Detailed Description of the Invention [Objective of the Invention] (Industrial Field of Application) The present invention relates to a silicon carbide honeycomb structure and a method for manufacturing the same, and more specifically relates to a silicon carbide honeycomb structure and a method for manufacturing the same. The present invention relates to a silicon carbide honeycomb structure that can efficiently carry out reactions, mass transfer, etc., and that can prevent melting damage or thermal shock destruction of the honeycomb structure due to heating during reuse, and a method for manufacturing the same.

(Prior art) For example, one end surface of countless through holes connected in a honeycomb shape through thin partition walls tb as shown in FIGS. A ceramic honeycomb structure consisting of a porous partition wall in which the other end surface of the through hole adjacent to the sealed through hole is filled with a sealing material 3 and sealed is used in automobile diesel engines and other applications. It is known as an exhaust gas purification device that adsorbs and purifies particulate carbon contained in exhaust gas from various combustion devices.

Conventionally, honeycomb structures mainly composed of cordierite or silicon carbide have often been used, but in the case of honeycomb structures mainly composed of cordierite, the partition walls are formed during extrusion molding. Ceramic particles are easily oriented in the extrusion direction, making it difficult for fluid to pass through the partition wall, resulting in a large pressure loss.Also, since ceramic particles are plate-shaped and have a relatively smooth surface, the contact area of the granules is small. However, there is a problem in that the heat transfer described above cannot be carried out efficiently.

On the other hand, with silicon carbide as the main component, the ratio of pores in the partition walls is relatively small at 30 to 40%, so the ventilation resistance is large and the effective contact area with gas or liquid particles is reduced. There is a problem that many of them are not suitable for uses such as catalyst carriers and filters because of their small amount.

The present inventor first developed a honeycomb structure that solves these problems by creating a three-dimensional network structure in which plate crystals intertwine in a complex manner in multiple directions, and the proportion of pores is relatively high. A honeycomb structure having silicon carbide porous partition walls has been proposed in Japanese Patent Application No. 59-143235.

(Problems to be Solved by the Invention) This honeycomb structure has a larger effective specific surface area than conventional ones, and can easily take fluid into the porous structure from the flow in the axial direction of the honeycomb. Since the fluid flow generated on the surface becomes turbulent, uniformity due to diffusion, stirring, etc. within the flow is promoted, and heat transfer occurring on the partition wall surface,
Chemical reaction.

This has the effect of effectively carrying out mass transfer, etc. Furthermore, since it is mainly composed of silicon carbide, it has a higher melting point than those whose main component is cordierite, and has the property of being able to withstand high temperatures when heated for reuse.

However, in such cases, i.e., when fine particulate carbon adsorbed and collected on the partition wall is heated with a burner or heater for the purpose of reusing the honeycomb structure, the combustion heat of the collected carbon itself often accumulates in the central part. Although the main component is silicon carbide, there is a problem in that the thin partition walls forming the through-holes in the portions may be melted or destroyed by thermal shock, making subsequent use impossible.

The present invention provides a novel silicon carbide honeycomb structure that does not reduce the effects of the silicon carbide honeycomb structure described above and is free from melting damage or thermal shock destruction even when heated for the purpose of reuse. The purpose is to provide a method for producing the same.

[Structure of the Invention] (Means for Solving the Problems) The silicon carbide honeycomb structure of the present invention has a silicon carbide honeycomb structure in which a large number of through holes are adjacent to each other in the axial direction with thin partition walls in between. , the partition wall is made of a porous body having a three-dimensional network structure mainly composed of plate crystals with an average aspect ratio in the range of 2 to 50, and the average pore diameter of the open pores of the network structure is , the honeycomb structure is characterized in that it is formed to increase in size stepwise or continuously as it goes from the central partition wall to the outer peripheral partition wall.

The reason why the plate-like crystals intertwine in a complicated state to form a three-dimensional network structure in the partition wall is that sintering is allowed to proceed under predetermined conditions, which will be described later.

The reason why the average aspect ratio is set to 2 to 50 is because when it is less than 2, the pores formed by the silicon carbide crystals become smaller compared to the volume occupied by the crystals, and it is possible to have a high porosity and a large pore diameter. This is because it becomes difficult. On the other hand, 5
If it exceeds 0, the strength of the joint between the plate crystals will be low, and the strength of the porous body will be extremely low.
This is because, as a result, it becomes difficult to maintain the shape of the honeycomb structure. A more preferable aspect ratio is in the range of 3-30.

Note that the 7 spectral ratio (R) of the silicon carbide plate crystals referred to here is the maximum length (X) of each plate crystal observed in an arbitrary cross section of the sintered body and the average thickness in the minor axis direction ( Y)
In other words, it is the value expressed as RX/Y.

In addition, the average short axis thickness of the plate crystals is 1 to 500 μm.
is preferable, and 3 to 300Q is more preferable, because if it is smaller than Ig, the pores formed by the plate-like crystals become smaller and the flow rate becomes smaller. If it is large, the number of joints in the plate crystals will be small and the joint strength will be low, and as a result,
This is because it becomes difficult to retain the shape.

Preferably, the plate crystals account for at least 20 parts by weight based on 100 parts by weight of the porous body.
When the amount is less than 0 parts by weight, the number of pores formed by the crystal becomes smaller than the capacity occupied by the crystal, and the heat transfer and
The effective area for chemical reactions or mass transfer to take place is reduced. In addition, since the bonding area of the plate crystals decreases,
This is because the mechanical strength of the porous body is significantly reduced. Among these, it is most preferable that the amount is at least 40 parts by weight.

The silicon carbide honeycomb structure of the present invention has partition walls made of a porous material having the above-described three-dimensional network structure, and further, the average pore diameter of the open pores of the network structure is It is characterized in that it is formed to become larger stepwise or continuously as it approaches the outer peripheral partition wall.

The reason is 1. As mentioned above, if the average pore diameter of the partition walls made of porous material constituting a honeycomb structure is uniform throughout the structure, combustion heat during reuse tends to accumulate in the center. On the other hand, in the case of the present invention, in which the average pore diameter increases from the center to the outer periphery, the combustion heat does not remain in the center but is smoothly dissipated to the outer periphery, so that heating during reuse is possible. This is because it is possible to prevent melting damage or thermal shock destruction of the partition walls caused by.

The average pore diameter of the pores in the network structure is preferably within the range of 1 to 50 μm. This is because if it is less than 1-, the resistance to fluid passage becomes small, while if it exceeds 50-, the strength of the porous body to which it faces is reduced. Preferably it is in the range of 2 to 30 μm. In addition, the value of the average pore diameter is a value obtained by mercury intrusion method.

Therefore, the average pore diameter of the porous body constituting the partition walls of the honeycomb structure of the present invention is within the above range.
As shown by the arrows in the figures and the arrows in the wfJz figures, the honeycomb structure has its minimum size at the center and gradually or continuously increases in size toward the outer periphery.

Further, the open porosity of the network structure is preferably 20 to 95% by volume. This is because if it is smaller than 20% by volume, some of the pores become independent pores and the effective surface area becomes smaller. , and if it is larger than 95% by volume,
This is because although the effective surface area increases, the shape retention of the honeycomb structure cannot be maintained. Among these, 30 to 90% by volume is more preferable.

Furthermore, it is preferable that the specific surface area of the silicon carbide partition wall is at least 0.05 m''/g, and further preferably 0.05 m''/g.
．． Most preferably, it is 2rn'/g, where the specific surface area is the value determined by the BET method using nitrogen absorption.

Next, a method for manufacturing a silicon carbide honeycomb structure of the present invention will be explained.

The method for producing a silicon carbide honeycomb structure of the present invention includes a first step of using silicon carbide powder as a starting material and adding a crystal growth aid if necessary to obtain a mixture; adding a molding binder to the mixture and forming it into a honeycomb shape. A second step of obtaining a molded body;
a third step of inserting the molded body into a heat-resistant container and firing it within a temperature range of 2000 to 2500°C while blocking the intrusion of outside air; In the method for producing a silicon honeycomb structure, when obtaining the formed body in the second step, at least one selected from aluminum, boron, calcium, chromium, iron, lanthanum, lithium, yttrium, silicon, nitrogen, oxygen, and carbon is used. A type of element or a compound thereof (
(hereinafter simply referred to as "transition layer formation success agent" in some cases) is present in the molded body so as to create a concentration gradient, and the average pore diameter of the open pores of the network structure varies from the central partition wall to the outer peripheral partition wall of the honeycomb structure. It is characterized by being formed so that it becomes larger stepwise or continuously as it approaches.

First, the reason why it is preferable to use silicon carbide powder as the starting material in the first step is that β-type silicon carbide crystals are low-temperature stable crystals that are synthesized at relatively low temperatures. This is because it easily undergoes a phase transition to a high temperature stable α type crystal such as 4H, 6H or 15R type to form a plate-shaped crystal, and also has excellent crystal growth properties. Especially 60
By using a starting material containing at least % by weight of β-type silicon carbide, the porous body targeted by the present invention can be suitably produced. Among these, it is advantageous to use a starting material containing 70% by weight or more of β-type silicon carbide.

Examples of crystal growth aids include aluminum, boron, iron, carbon, and the like.

Next, in the second step, a molding binder such as methyl cellulose, polyvinyl alcohol, or water glass is added to the mixture obtained in the first step, and the mixture is formed into a honeycomb shape by extrusion molding, sheet molding, press molding, etc. Get a body. At least one element selected from aluminum, boron, calcium, chromium, iron, lanthanum, lithium, yttrium, silicon, nitrogen, oxygen, and carbon or a compound thereof is present so as to create a concentration gradient within the molded body. . This method is carried out by directly applying a solution containing the compound to the molded body, or by removing the molding binder from the molded body to make it porous, and then impregnating it in the same manner.

Among the substances mentioned above, concentration gradients occur in aluminum, boron, calcium, chromium, iron, lanthanum,
Lithium and yttrium have the function of accelerating the growth rate of crystal grains in silicon carbide, and where these substances exist, a large number of plate-like crystal nuclei are generated, and plate-like crystals are formed in each part. As a result of this development, the size of the plate-like crystals that are formed is restricted, and areas where a large amount of these substances are present can form a three-dimensional network structure with a very fine structure.

On the other hand, silicon, nitrogen, oxygen, and carbon act to slow down the growth rate of silicon carbide crystal grains, contrary to the above substances, and where these substances exist, plate crystal nucleation occurs. As a result, each plate crystal grows relatively large, and the areas where many of these substances exist form a large three-dimensional network structure. This is because it can be done.

Therefore, in order to obtain a silicon carbide honeycomb structure formed such that the average pore diameter of the open pores of the network structure increases stepwise or continuously from the central partition wall to the outer peripheral partition wall of the honeycomb structure. Among the above transition layer forming aids, aluminum, boron, calcium, chromium, iron, lanthanum, lithium, and yttrium are contained near the center of the honeycomb formed body and sintered by the method described below, or silicon, nitrogen , a method in which oxygen and carbon are contained near the outer periphery of the honeycomb formed body and sintered by the method described below, and a method in which both methods are used in combination.

Note that if a large amount of the transition layer forming aid remains in the sintered body, the inherent properties of silicon carbide will be lost, so it is desirable that the amount remaining in the sintered body be as small as possible. It is preferably 10 parts by weight or less, and more preferably 5 parts by weight or less.

Next, as a third step, the obtained molded body is sealed in a heat-resistant container and heated to a
Calcinate within a temperature range of 0°C.

The reason why baking is performed while sealing in a heat-resistant container and blocking outside air from entering is that it allows adjacent silicon carbide crystals to fuse together and promotes the growth of plate-shaped crystals. This is because they intertwine in a complex manner to form a three-dimensional network structure.

In addition, the growth of plate crystals can be promoted by:
This is thought to be because movement of silicon carbide between silicon carbide particles by evaporation-recondensation and/or surface diffusion can be promoted.

In contrast, when conventional pressureless sintering, atmospheric pressure sintering, or sintering under reduced pressure was tried, not only was it difficult to grow plate-shaped crystals, but the joints of silicon carbide particles became a bottleneck. The shape became constricted, and the strength of the sintered body decreased.

As the heat-resistant container, it is preferable to use a heat-resistant container made of at least one material selected from graphite, silicon carbide, tungsten carbide, molybdenum, and molybdenum carbide.

In addition, setting the firing temperature to 2000 to 2500°C is 2
If it is lower than 000℃, the growth of particles is insufficient and it is difficult to make the partition wall into a porous body with high strength.If it is higher than 2500℃, the sublimation of silicon carbide becomes active. This is because the developed plate-like crystals conversely become thinner, and as a result, it becomes difficult to obtain a porous body with high strength. More preferably 2100-230
It is within the range of 0°C.

[Example] Burial 1" The silicon carbide fine powder used as the starting material was one in which 80% by weight consisted of β-type crystals. This starting material contains 0.01% of B as an impurity. C is 0.5, An is 0.0
1. N is 0.2, Fe is 0.08 atomic weight part, and traces of other elements are incorporated, and the total amount of these impurities is 0.8
It was 1 part by atomic weight. Further, the average particle size of this starting material was 0.3) us, and the specific surface area was 18.7 m''/g.

To this starting material were added 10 parts by weight of methylcellulose as a molding binder and 20 parts by weight of water. This was kneaded and extrusion molded to a diameter of 130mm and a length of 120m.
5. A silicon carbide honeycomb molded body was obtained in which the thickness of the partition walls of the through holes was 0.3+ue and the number of through holes was 200 per square inch.

This molded body was heated to 500°C at a heating rate of 1°C/min in an oxidizing atmosphere to remove the organic binder by oxidation.Then, 40% phenol resin was applied to a portion 20 mm from the outer periphery of the molded body. It was impregnated with alcohol solution and then dried. As a result, free carbon is contained at 8% in a portion 20 μl from the outer periphery, and gradually decreases toward the inside, and in a portion 20 μl from the center, free carbon is 0.3%.
% was included.

Thereafter, this molded body was placed in a graphite crucible with a porosity of 20%, and fired L7 in an Ar gas atmosphere at 1 atm.

For firing, the temperature was raised to 2150°C at a rate of 2°C/min and held at the maximum temperature for 4 hours.

2-5 1-4 Same as Example 1, but in addition to adding phenol resin, alumina sol (0,
05-particles) When an aqueous solution was added to make the An content 0.2% by weight (Example 2), BN fine powder (particle size 0, 2. 20mm diameter, and the B content is 0.1% by weight.
The case of binding (Example 3), the same as Example 1 but without the addition of phenol resin (Comparative Example 1)
, when 0.4% by weight of B was added to the whole (Comparative Example 2),
Same as Example 1, but when the firing temperature was 2300°C and the holding time was 10 hours (Example 4), and when the firing temperature was 2050°C and the holding time was 2 hours. (Example 5), when the firing temperature was 1800°C (Comparative Example 3), and when the firing temperature was 2550°C (Comparative Example 4), the results of the partition structure, performance, etc. of the honeycomb structure are shown below. In the table, a indicates a partition wall located near the center of the honeycomb structure, b indicates a partition wall located on the outer periphery thereof, and C indicates a partition wall located further on the outer periphery thereof.

As is clear from the table, the honeycomb structure of the present invention has an average pore diameter that gradually increases from the partition walls in the center to the partition walls in the outer periphery. When used as a particulate trap filter in a diesel engine with
In each example, the thickness of the particles continuously changed from the center toward the outer periphery.

Therefore, the honeycomb structure according to the present invention has an excess of 02
When ignited at 800℃, for example, in Example 1, the temperature at the outer periphery was 1000℃, and the temperature at the center was 1020℃. The difference was small, there was no melting loss, and there were no problems with thermal shock resistance.

[Effects of the Invention] The silicon carbide honeycomb structure of the present invention has a three-dimensional network structure in which plate crystals are intricately intertwined, so that heat transfer, chemical reaction, and mass transfer occurring on the partition wall surface are suppressed. In addition, the average pore diameter increases from the central partition wall to the outer peripheral partition wall of the structure, so even when heated for the purpose of reuse, combustion heat will not be transferred to the central part. There is no accumulation and it is possible to prevent melting and thermal shock destruction of partition walls.

[Brief explanation of the drawing]

FIG. 1 is a plan view of the honeycomb structure of the present invention, and FIG. 2 is a schematic vertical cross-sectional view thereof.

Claims (7)

[Claims] (1) In a silicon carbide honeycomb structure in which a large number of through holes are adjacent to each other in the axial direction across thin partition walls, the partition walls are mainly composed of plate crystals having an average aspect ratio of 2 to 50. The honeycomb structure is made of a porous body having a three-dimensional network structure, and the average pore diameter of the open pores of the network structure increases stepwise or continuously from the central partition wall to the outer peripheral partition wall of the honeycomb structure. A silicon carbide honeycomb structure characterized by being formed as follows. (2) The average thickness of the plate crystals in the minor axis direction is 1 to 500.
The silicon carbide honeycomb structure according to claim 1, wherein the silicon carbide honeycomb structure has a diameter of .mu.m. (3) The silicon carbide honeycomb structure according to claim 1 or 2, wherein the plate crystals are contained in an amount of at least 20 parts by weight based on 100 parts by weight of the porous body. (4) The average pore diameter of the open pores of the three-dimensional network structure is 1
The silicon carbide honeycomb structure according to any one of claims 1 to 3, which has a thickness of 50 μm. (5) The silicon carbide honeycomb structure according to any one of claims 1 to 4, wherein the three-dimensional network structure has an open porosity of 20 to 95% by volume. (6) The silicon carbide honeycomb structure according to any one of claims 1 to 5, wherein the silicon carbide porous body has a specific surface area of at least 0.05 m^2/g. (7) A first step of using silicon carbide powder as a starting material and adding a crystal growth aid if necessary to obtain a mixture; a second step of adding a molding binder to the mixture to obtain a honeycomb-shaped body; A third step of inserting the molded body into a heat-resistant container and firing it within a temperature range of 2000 to 2500°C while blocking the intrusion of outside air; In the method for producing a raw honeycomb structure, when obtaining the molded body in the second step, at least one selected from aluminum, boron, calcium, chromium, iron, lanthanum, lithium, yttrium, silicon, nitrogen, oxygen, and carbon. elements or their compounds are present so as to cause a concentration gradient in the molded body, and the average pore diameter of the open pores of the network structure is gradually or continuously as it goes from the central partition wall to the outer peripheral partition wall of the honeycomb structure. 1. A method for manufacturing a silicon carbide honeycomb structure, which is characterized by forming the silicon carbide honeycomb structure so that it becomes larger.