JPH0813706B2 - Silicon carbide / metal silicon composite and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon carbide / metal silicon composite formed from silicon-accumulated biomass and a method for producing the same.

[0002]

2. Description of the Related Art Generally used widely
In the case of a catalyst, the catalytically active component is a fine particle (ideally a molecular level
It is used by being dispersed and supported on a substance having a large surface area.
Have been. Materials with large surface area used here
Is the catalyst carrier. When the catalyst is put to practical use, for example,
When a gaseous or liquid reactant is brought into contact with the catalyst, the catalyst surface
The reaction proceeds by itself. Gas or liquid with catalyst in the reactor
In order to make intimate contact, many shapes of catalyst beds are used as catalyst beds.
A monolith-shaped object with a thick through hole, or a shape
Supporting catalytically active components on a foam with irregular through holes
The one used is used. Such monolithic substances and
The foam is the catalyst structure support. This support is catalytically active
Efficiently contact the components with the reactants, as well as the reaction temperature
Is chemically and physically stable, and has mechanical strength
Must be equipped. Generally, catalysts are highly active at low temperatures
Is used as a means to obtain. But up
Recently, exhaust gas purification catalyst for gasoline vehicles, diesel vehicle exhaust gas purification
The need for catalysts that operate at high temperatures such as catalysts is increasing.
You. The maximum operating temperature of the catalysts currently in practical use is
850, a three-way catalyst used to purify exhaust gas from Sorin cars
℃. This catalyst activates platinum, rhodium and palladium.
Supported on crystalline alumina, which is made of cordierite.
Noris-type catalyst structure
You. This catalyst sinters alumina at temperatures above 1000 ° C
Occurs, and the surface area decreases accordingly. At the same time
Aggregation of platinum, which is a catalytically active component, occurs, resulting in catalytic activity.
Drops sharply. In addition, the catalyst structure support
Dierite has a softening point of 1300 ° C, but
It is known that it cannot withstand long-term use at 0 ℃
There is. In recent years, there have been many uses of diesel fuel that are economically advantageous
However, compared to gasoline fuel, the C / N ratio is large,
Withstands temperatures above 850 ° C due to high combustion temperatures
Catalyst development is needed. Diesel engine combustion temperature
The temperature is high because the diesel fuel has a high C / H. So
As a result, N in the air₂And O₂Reacts with thermal NOx
Is generated. To reduce the amount of thermal NOx produced
It is conceivable to use an exhaust gas purifying catalyst. So
In order to reduce the amount of air,₂But
Incomplete combustion occurs due to the decrease of CO and C in the exhaust gas.
The amount of mHn (hydrocarbon) increases. These reductions
NOx generated using a reactive gas₂At the same time
CO and CmHn are CO₂And H₂Oxidizes to O and purifies exhaust gas
Become Such a catalyst is called a three-way catalyst, and its reaction
Is expressed by the following equation. CO + O₂ → CO₂  CmHn + O₂ → CO₂+ H₂O NOx + CO → CO₂+ N₂  NOx + CmHn → CO₂+ H₂O + N₂ In addition to heat resistance, the catalyst is required to have excellent thermal shock resistance.
You. That is, the temperature rises sharply when the engine starts.
However, the temperature drops sharply when the engine stops.

When a three-way catalyst is used to purify exhaust gas from a diesel engine, the amount of air is adjusted,
Generates appropriate CO and CmHn in the exhaust gas and reduces and removes NOx, but unburned fine carbon particles (particulates) for incomplete combustion of diesel fuel with a high C / H ratio
Is generated. For diesel engines, CO, CmHn,
When NOx is reduced, particulate matter is generated, so that particulate matter adheres to the catalyst surface, resulting in reduction in catalyst activity and new pollution due to release of particulate matter to the atmosphere. . As a currently considered measure, a method of capturing particulates contained in exhaust gas with a filter, periodically removing the filter, removing the particulates in the filter by combustion, and attaching the filter again and using it again is promising. It is considered. This filter also has a high exhaust gas temperature, and even when the particulates are removed by combustion, the temperature is high in the places where the particulates exist, so that this filter is also required to have heat resistance and thermal shock resistance.

As catalysts used at high temperatures, combustion catalysts can be mentioned in addition to the above-mentioned automobile exhaust gas purification catalysts. The combustion catalyst, on the catalyst is contacted with CH ₄ or H ₂ flammable gas molecules and O _2, such as, oxidation reactions, i.e., a catalyst to produce a combustion reaction. By using a combustion catalyst, a flame-retardant gas can be completely combusted or even at a low temperature, so that it is used for deodorization by completely combusting a malodorous component. In recent years, high temperature (1
O ₂ and N _{2 in the} air when burnt at 500 ° C. or higher)
It is considered that NOx (thermal NOx) generated by the reaction of the is a cause of air pollution and acid rain, and various measures have been taken. To reduce this thermal NOx, burn 1
This can be achieved by carrying out the reaction at a temperature of 500 ° C. or lower (at 1500 ° C. or lower, the equilibrium constant of the reaction of N ₂ + O ₂ → NOx is small and the amount of NOx generated is small). To recover the heat generated by combustion and use it efficiently, 1
It is necessary to burn at a temperature close to this at 500 ° C or lower. However, a catalyst that can be used at such a high temperature has not been developed. As described above, the highest temperature among the practical catalysts used is 850 ° C., which is a gasoline vehicle exhaust gas purification catalyst.

The automobile exhaust gas purification catalyst described here,
Since the particulate trapping filter and the combustion catalyst are exposed to a gas containing oxygen at high temperature, heat resistance,
Oxidation resistance is required. Furthermore, since it is used for combustion, the temperature changes rapidly due to ignition and extinguishing, so thermal shock resistance is also required. Cordierite is used as a catalyst structure support. The softening point of this cordierite is 1360 ° C, but it is difficult to use it at a temperature of about 1000 ° C for a long time.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a material having excellent heat resistance, thermal shock resistance and oxidation resistance, and a method for producing the same.

[0007]

Means for Solving the Problems The present inventors have made intensive studies to solve the above problems, and as a result, completed the present invention. That is, according to the present invention, there is provided a silicon carbide / metal silicon composite having a structure in which silicon carbide formed by heat-treating silicon-accumulated biomass under an argon or nitrogen atmosphere is integrally bonded via metal silicon. Provided. Also,
According to the present invention, a paste-like mixture composed of silicon carbide, metallic silicon, and an organic solvent formed by heat-treating silicon-accumulated biomass under an argon or nitrogen atmosphere is molded into a predetermined shape, and the organic solvent is removed from the mixture. To form a solid molded body, and then heat the molded body at a temperature not lower than the melting point of the metallic silicon and not higher than the melting point of the silicon carbide to melt the metallic silicon, and then cooling. A method for producing a silicon carbide / metal silicon composite is provided. Furthermore, a heat-resistant catalyst structure and a filter using the composite as a material are provided.

The silicon carbide used as a raw material in the present invention is formed by heat-treating silicon-accumulated biomass in an argon or nitrogen atmosphere. In addition, the silicon-accumulated biomass referred to in the present specification means a plant containing a silica component (silicon-accumulated plant) or its leaves, parts such as stems, specifically, rice husks or straws such as wheat, or straw, Bamboo leaves, corn and takusa leaves or stems are examples.

In order to produce silicon carbide from this silicon-accumulated biomass, first, as the silicon-accumulated biomass, for example, a rice hull is used as it is or, if necessary, 50 to 100 μm.
And crushed to a particle size of 100 to 1200 ° C. in an argon or nitrogen gas atmosphere to obtain a rice husk thermal decomposition product (a mixture of silica (SiO ₂ ) and carbon (C)). In this case, carbon is generated by the thermal decomposition reaction of organic substances such as cellulose and lignin in the hull. In this step, since the purpose is to prepare a rice husk thermal decomposition product, the heat treatment atmosphere may be either argon or nitrogen. The thermal decomposition product obtained is a mixture of amorphous silica and amorphous carbon. The production of rice husk thermal decomposition product is
O ₂ is 25 to 55% by weight, C is 40 to 75% by weight, and impurities (K ₂ O, Na ₂ O, CaO, Al ₂ O ₃ etc.) are 0.
10 to 10% by weight. Then, similarly in an argon atmosphere, 1300 to 2000 ° C., preferably 1300 to 160
By heat treatment in the temperature range of 0 ° C, silicon carbide (S
A mixture of iC) and carbon (C) is obtained. As the reaction atmosphere, nitrogen can be used in addition to argon. The rice husk thermal decomposition product generally contains C in a larger amount than the stoichiometric amount of the reaction reaction (SiO ₂ + 3C → SiC + 2CO) in which SiC is produced from SiO ₂ and C, and even when heated in a nitrogen atmosphere. It does not form silicon nitride. The heat treatment time varies depending on the type of silicon accumulated biomass used and the heating temperature,
It is in the range of 0.1 to 12 hours. The generated SiC is β-
It is SiC, and C is amorphous carbon. In order to remove C in the product, the produced mixture of SiC and C is 400 to 800 ° C. in air, preferably 500 to 70.
By treating at a temperature of 0 ° C., C is burned off and Si
Get C. As the processing temperature is higher, a part of SiC is oxidized to be SiO ₂ . In order to remove this SiO ₂ , it is treated with a hydrogen fluoride (HF) solution. However, when the treatment temperature in air is low, the amount of SiO ₂ produced is very small, and this HF treatment can be omitted.

In order to produce the silicon carbide / metal silicon composite of the present invention (hereinafter also simply referred to as a composite), first, silicon carbide formed from silicon-accumulated biomass and metal silicon are ground and mixed by using a grinder. To do. The dimensions of silicon carbide and metallic silicon used as the raw materials for pulverization and mixing are not particularly limited, and may be powdery or lumpy, but in order to efficiently perform pulverization and mixing, it is preferable to perform fine pulverization in advance. A vibration mill, a planetary ball mill, or the like is used as the crusher. The use ratio of silicon carbide formed from silicon-accumulated biomass and metallic silicon is silicon carbide: 5 to 95% by weight, preferably 33 to 50% by weight, metallic silicon: 5 to 95% by weight, preferably 50 to 67% by weight. Is. By this pulverization and mixing, the average particle size is 0.1 to 30 μm, preferably 0.1 to 30 μm.
A powder mixture of 10 μm of silicon carbide and metallic silicon is obtained.

In the present invention, the powder mixture obtained as described above is then dispersed in an organic solvent to give a paste-like substance having an appropriate viscosity. As the organic solvent, it is preferable to use an easily volatile organic solvent having a boiling point of 100 ° C. or less, such as ethanol, methanol, or acetone. The paste-like material thus formed is molded into a predetermined shape, and then the organic solvent is removed from the paste-like material to obtain a solid molded body. The shape of the molded body is arbitrary, and may be pellet, spherical, tubular, plate-shaped, honeycomb-shaped or the like. The organic solvent can be removed from the pasty material by a conventional means such as heating or vacuum.

Next, the molded body composed of silicon carbide and metallic silicon formed from the silicon-accumulated biomass obtained as described above is used as a molded body in the atmosphere of oxygen-free argon to melt the metallic silicon (1414 ° C.). At the temperature above the melting point of silicon carbide, preferably at a temperature of 1414 to 1500 ° C. for 0.1 to 5 hours, preferably 0.1 to 2 hours, and then cooled to obtain a silicon carbide / metal silicon composite. Get the body. In this step, a porous composite can be produced by treating the temperature as close to the melting point of metallic silicon as possible and for a relatively short time, for example, for less than 0.5 hours. In particular, when a powder mixture in which fine metal silicon particles adhere to the surface of silicon carbide agglomerated particles is used as a raw material,
A composite having continuous pores with a large pore size can be produced. By the method of the present invention, a SiC-Si composite having the shape of a pellet, a honeycomb, or a porous body having continuous pores can be manufactured. Since the produced SiC-Si composite has a structure in which the solidified material of the metallic silicon melt covers (surrounds) the surface of the SiC particles and the SiC particles are integrally bonded via the metallic silicon, silicon carbide is It is shielded from oxygen-containing atmospheres and protected from oxidation. Metallic silicon has excellent oxidation resistance, and when a metallic silicon lump having a cubic shape with one side of 5 mm is heat-treated in air at 1200 ° C. and 1300 ° C. for 1 hour, the weight change is 0.08% by weight and It is 1% by weight, and the change in weight is the same even if the treatment is carried out for 30 hours. On the other hand, when all the metallic silicon is oxidized, a weight increase of 114% by weight occurs. Therefore, it can be seen from this that the oxidation reaction occurs only in the thin surface layer of metallic silicon. In the case of the composite body of the present invention, at a temperature equal to or higher than the melting point of metallic silicon, metallic silicon melts, SiC comes into contact with air, and oxidation of SiC proceeds. Therefore, SiC-Si
The maximum temperature at which the oxide of the composite can be prevented is not higher than the melting point of metallic silicon.

[0013]

According to the present invention, a stable SiC-Si composite can be easily produced in an oxidizing atmosphere up to 1414 ° C. Since the SiC-Si composite of the present invention is excellent in heat resistance and oxidation resistance and is also excellent in thermal shock resistance, it is a high temperature combustion catalyst structure support, a diesel engine exhaust gas purification catalyst structure support, and a diesel engine. It can be used as a filter for collecting particulates discharged from the engine.

[0014]

Next, the present invention will be described in more detail with reference to examples. Example 1 20 g of rice husk was filled in a quartz tube having an inner diameter of 60 mm, and treated with argon at 300 ml / min and 500 ° C. for 1 hour to obtain a rice husk thermal decomposition product. SiO ₂ in the thermal decomposition product
Is 38.5% by weight, C is 57.7% by weight, K ₂ O, Al
Impurities such as ₂ O ₃ and CaO were 3.8% by weight.
10 g of this thermal decomposition product was filled in a reaction tube made of mullite, and this was installed in an electric furnace of a siliconite heating element. Argon is flown in the reaction tube at 100 ml / min, 1500 ° C., and 3
After time treatment, a mixture of SiC and C was obtained. This mixture was treated at 700 ° C. for 1 hour under 100 ml / min of air to burn out C. The product was then treated with HF solution to remove SiO ₂ in the product. The surface area of the rice husk SiC powder obtained by the above operation is 60 m ² / g
It contained 8.7% by weight of oxygen. The shape of the SiC powder is 0.01 to 1 μm and the diameter is 0.01.
The curled SiC whiskers having a size of μm and a length of 2 to 3 μm were aggregated to form secondary particles having a particle size of 50 to 100 μm. Further, commercially available β-SiC particles were used. The commercially available SiC powder had a surface area of 7 m ² / g and had an average particle size of 0.3 μm and a maximum particle size of 4 μm. The metal silicon powder used had an average particle size of 10 μm and a purity of 98% by weight. Next, the SiC powder and the metal silicon powder were precisely weighed so that the SiC / Si weight ratio was 1.0, 0.67, and 0.5, and pulverized and mixed for 15 minutes with a planetary ball mill. After mixing, an appropriate amount of methanol was added to prepare a SiC-Si mixed powder paste. This paste was put into a glass tube with an inner diameter of 5 mm and extruded, and a SiC-Si with a diameter of 5 mm and a height of 5 mm was formed.
Mixed powder pellets were prepared. Next, this pellet was treated in an argon stream at 1500 ° C. for 1 hour to obtain SiC-Si.
Composite pellets were prepared. In addition, using a glass tube having an inner diameter of 30 mm, a paste having a SiC / Si weight ratio of 0.5,
A SiC-Si mixed powder pellet having a diameter of 30 mm and a height of 20 mm was prepared. Before the methanol in the pellets is volatilized, 21 through holes are opened in the height direction of the pellets with a metal rod having a diameter of 2 mm, and heat treatment is performed in the same manner as in the case of producing SiC-Si composite pellets. −
I was prepared. Further, in the case of manufacturing SiC-Si composite pellets, the paste is poured into a mold made of a glass tube having a diameter of 30 mm in which 10 glass rods having a diameter of 3 mm are vertically installed to prepare SiC-Si mixed powder pellets. Heat treatment was performed in the same manner as in 1. to produce a SiC-Si composite honeycomb-II. The SiC-Si composite pellets, Honeycomb-I, and Honeycomb-II produced by the above methods were treated at 1200 ° C and 1300 ° C for 1 to 30 hours in an air stream, and the change in weight of the sample was traced. The results are shown in Tables 1 and 2.

[0015]

[Table 1]

Table 1 shows the weight changes of SiC-Si composite pellets, honeycombs and rice husk SiC powder for comparison, which were treated in an air stream at 1200 ° C. Rice husk S
Weight increase due to oxidation of iC is 39% by weight after 1 hour treatment
Is. Therefore, it can be seen from Table 1 that in the rice husk SiC-Si composite, the oxidation reaction of SiC and Si is significantly suppressed. The smaller the SiC / Si ratio, the greater the effect of suppressing the oxidation reaction, and the honeycomb-shaped SiC-Si composite is also suppressed in the oxidation reaction. In the SiC-Si composite manufactured from the commercially available SiC powder, the weight increase due to the oxidation reaction is larger than that in the rice husk SiC-Si composite.
The weight increase shows a constant value and the oxidation reaction is suppressed in the treatment for 15 hours or more. Table 2 shows 13
The change in weight of the pellet and the honeycomb after treated at 00 ° C is shown. Compared to the case of 1200 ° C shown in Table 1, SiC-
Although the oxidation of the Si composite is in progress, a SiC / Si composite pellet having a SiC / Si weight ratio of 0.67 or less, a SiC-Si composite honeycomb having a weight ratio of 0.5, and a commercially available SiC having a weight ratio of 0.5. In the -Si composite pellet, the oxidation reaction is suppressed.

[0017]

[Table 2]

Example 2 Aggregated particles having a particle size of 76 μm or more were sieved and separated from the chaff prepared in Example 1. The metallic silicon powder was pulverized with a planetary ball mill for 15 minutes to make the particle size uniform to 1 μm or less. Next, 75% by weight of SiC powder and 25% by weight of metallic silicon powder were precisely weighed and mixed for 24 hours at 30 revolutions / min in a ball mill filled with nylon balls, and then the SiC agglomerated particles having a large particle size were mixed. A mixed powder having fine metal silicon particles attached thereto was manufactured. Methanol was added to this mixed powder to form a paste. This paste has a diameter of 30
It was put into a mm mold and made into pellets having a diameter of 30 mm and a height of 20 mm. This was processed at 1420 degreeC in argon for 3 hours, and the SiC-Si composite pellet was manufactured. SiC-
Si composite pellets were treated with emery abrasive paper Nos. 200, 400 and 600 and then with an alumina abrasive having a particle size of 1 μm to prepare pellet cross-section samples. From an optical microscope observation of the cross section of the pellet, continuous pores were observed, and it was confirmed that a composite body in which the metal silicon solidified material surrounds the inside and the periphery of the SiC particles was formed. The diameter of the continuous pores is about 15 μm on average, and it can be used as a filter. This SiC-Si composite pellet is 1200 and 1300 degreeC.
Table 3 shows the change in weight of the pellets when treated under the air atmosphere.

[0019]

[Table 3]

It is clear that the oxidation reaction can be suppressed at 1200 and 1300 ° C. in a SiC-Si composite prepared from particles having metallic silicon particles attached to the outer surface of SiC particles.

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Claims (5)

[Claims] 1. A silicon carbide / metal silicon composite having a structure in which silicon carbide formed by heat-treating silicon-accumulated biomass under an argon or nitrogen atmosphere is integrally bonded via metal silicon. 2. The composite according to claim 1, wherein the content of silicon carbide formed from silicon-accumulated biomass is 5 to 95% by weight. 3. A heat-resistant catalyst structure support comprising the silicon carbide / metal silicon composite according to claim 1. 4. A heat resistant filter comprising the silicon carbide / metal silicon composite according to claim 1. 5. A paste-like mixture comprising silicon carbide, metallic silicon and an organic solvent, which is formed by heating silicon-accumulated biomass under an atmosphere of argon or nitrogen, is molded into a predetermined shape, and the organic solvent is removed from the mixture. Characterized in that a solid molded body is formed by heating the molded body at a temperature not lower than the melting point of the metallic silicon and lower than the melting point of the silicon carbide to melt the metallic silicon, and then cooled. Method for producing silicon carbide / metal silicon.