JP2008030038A - Exhaust gas filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas filter which has pores suitable for trapping fine particles contained in an exhaust gas and is capable of sufficiently generating heat by itself by applying a small voltage. <P>SOLUTION: A filter for cleaning an exhaust gas of an internal combustion engine is made of a porous ceramic material. The porous ceramic material is obtained as a burned composite material comprising silicon carbide and an additive present in a neck section, which is the bonding section between silicon carbide particles. The additive is at least one of (1) a substance forming a liquid phase in the neck section, (2) a substance forming a solid solution in the neck section and (3) a substance which has a specific resistance smaller than silicon carbide. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas filter for removing particulates contained in gas discharged from an internal combustion engine, and more particularly to an exhaust gas filter suitable for use in a self-heating filter.

  Gases emitted from internal combustion engines such as diesel vehicles contain particulates that cause air pollution, and it has been confirmed that there are also highly carcinogenic substances in them. . For this reason, in recent years, a processing technique in which fine particles are collected by disposing an exhaust gas filter on the exhaust path of the internal combustion engine, and the collected fine particles are ignited and burned has been widely studied.

  Conventionally, materials made of porous ceramics such as cordierite and silicon carbide have been used for the exhaust gas filters as described above. Among these ceramic materials, particularly with respect to silicon carbide, attention has been paid to the characteristic that the heat resistance is high and the specific resistance value is low. Therefore, silicon carbide is considered promising as a material for forming a filter having excellent durability and regeneration efficiency.

  For example, what is shown as Patent Document 1 proposes a filter whose main component is silicon carbide.

JP-A-61-423

  By the way, silicon carbide having a low specific resistance has a property that it can be heated by directly applying a voltage. For this reason, it is generally considered that a so-called self-heating type exhaust gas filter can be obtained if a filter is made of porous silicon carbide.

  As the self-heating type exhaust gas filter as described above, for example, a foam-like structure formed by impregnating porous silicon carbide with silicon metal and self-heating when a voltage of about 400 volts is applied. There are things.

  However, in the case of this type of filter, the specific resistance value inevitably increases, and the value of the voltage to be applied also increases accordingly. For this reason, it has been pointed out that it cannot be mounted on a general vehicle having only a battery of about 20 volts.

In addition, as a self-heating type exhaust gas filter different from this, the following types can be considered. That is, it is a filter using as a material a ceramic sintered body in which aluminum nitride (or alumina) and carbon are contained in silicon carbide as a main component. In the case of this filter, the specific resistance value is as low as about 10 ⁻¹ Ω · cm, and therefore there is an advantage that even a small battery for general vehicles can be sufficiently heated.

  However, since the filter gives priority to heat generation, the filter has a relatively high density and is dense with few pores. For this reason, it was not necessarily said that it was suitable for collection of fine particles.

  Another object of the present invention is to provide an exhaust gas filter that can sufficiently generate heat by applying a small voltage, and that can reliably and easily obtain an exhaust gas filter having pores suitable for collecting fine particles. It is to provide a gas filter.

  In order to solve the above problems, in the present invention, in a filter made of porous ceramics for purifying exhaust gas of an internal combustion engine, the porous ceramics is a bonded portion of silicon carbide and silicon carbide crystal particles. A fired composite material with an additive present in the neck part, wherein the additive has a lower resistivity than a substance that forms a liquid phase in the neck part, a substance that forms a solid solution in the neck part, and silicon carbide Proposed is an exhaust gas filter comprising at least one of the substances.

  In the present invention, (1) the substance that forms a liquid phase in the neck part is aluminum oxide or silicon oxide, and (2) the substance that forms a solid solution in the neck part is tungsten carbide or titanium nitride. Alternatively, titanium diboride and (3) the substance having a lower specific resistance than silicon carbide is preferably boron nitride or aluminum nitride.

  According to the exhaust gas filter of the present invention, the exhaust gas filter has pores suitable for collecting fine particles contained in the exhaust gas, and the additive is present in the neck portion which is a bonded portion of the silicon carbide crystal particles. The specific resistance value of the sintered body can be reduced to an order of 1 to 2 orders lower than the lower limit in the present situation, and an excellent effect is achieved in that sufficient self-heating can be achieved by applying a small voltage.

  The present invention relates to a filter made of porous ceramics, wherein the porous ceramics is a fired composite material of an additive present in a neck part which is a bonded part of silicon carbide and silicon carbide crystal particles, and the additive is And a material that forms a liquid phase in the neck portion, a material that forms a solid solution in the neck portion, and a material having a lower specific resistance value than silicon carbide.

The reason why the “additive” is added to silicon carbide in the present invention will be described. It is known that silicon carbide is sintered by solid-phase sintering, and in that case, silicon atoms and carbon atoms diffuse through crystal particle bonding portions (neck portions). The lower limit of the specific resistance value of the sintered body at present is about 1 Ω · cm. Therefore, as a method for realizing a lower specific resistance value, for example, the following method can be considered. That is,
(1) The presence of a substance that forms a liquid phase at the neck,
(2) The presence of a substance that forms a solid solution in the neck portion, and (3) the presence of a substance having a low specific resistance value in the neck portion. That is, the method (1) secures an area where crystal grains are in contact with each other by forming a liquid phase at the neck portion, thereby lowering the specific resistance value. In the method (2), a solid solution is formed in the neck portion to secure an area where the crystal grains are in contact with each other, thereby reducing the specific resistance value. The method (3) is to lower the specific resistance value of the entire sintered body by lowering the electrical resistance of the neck portion. Examples of the substance belonging to the category (1) include the aluminum oxide and silicon oxide described above. Examples of the substance belonging to the category (2) include tungsten carbide, titanium nitride, and titanium diboride. Examples of the substance belonging to the category (3) include boron nitride and aluminum nitride. And according to these methods, it is possible to realize a value 1 to 2 orders lower than the lower limit of the specific resistance value of the sintered body in the present situation.

  In the case of the present invention, the additive is preferably coated with an organic resin having water repellency. The reason is to prevent the additive from reacting with water when the mixture of the silicon carbide powder and the additive is kneaded. For example, when the additive is aluminum nitride, if coating is applied, generation of ammonia gas due to reaction with water can be avoided, and the material can be kneaded.

  Note that as the water-repellent organic resin, a resin containing a hydrophobic group and nitrogen in the molecule may be used. Specific examples of such resins include paraffinic amines, paraffinic amides, olefinic amides, and nylons. The organic resin is preferably blended in an amount of 5 to 30 parts by weight with respect to the additive.

Further, in the filter of the present invention, the specific resistance value of the porous ceramic as the composite material is 10 ⁻² Ω · cm to 1 Ω · cm, the density thereof is 1.7 g / cm ^{3 to} 2.3 g / cm ³ , and The average pore diameter is preferably in the range of 1 μm to 50 μm. This is because a calorific value cannot be obtained when the specific resistance value is less than 10 ⁻² Ω · cm. On the other hand, if the specific resistance value exceeds 1 Ω · cm, a large applied voltage is required to raise the temperature to a predetermined temperature, which is disadvantageous for self-heating.

On the other hand, if the density is less than 1.7 g / cm ³ , the filter has too many voids, which weakens the strength. Moreover, the collection efficiency of microparticles | fine-particles will fall by the porosity becoming high. On the other hand, if the density exceeds 2.3 g / cm ³ , the filter becomes too dense, exhaust gas cannot be circulated inside, and particulate collection becomes impossible.

  If the average pore diameter is less than 1 μm, clogging of the filter due to the accumulation of fine particles becomes significant. On the other hand, if the average pore diameter exceeds 50 μm, fine particles cannot be collected, and the collection efficiency is improved. It will decline.

  Therefore, in order to be an excellent exhaust gas filter, particularly a self-heating type exhaust gas filter, it is preferable that all the above physical property values are satisfied.

  Further, the filter of the present invention contains 5 to 30 parts by weight of at least one additive selected from carbides, nitrides, borides and oxides excluding silicon carbide with respect to 100 parts by weight of silicon carbide. It is desirable to mix. When the amount of the additive is less than 5 parts by weight, the specific resistance value cannot be sufficiently lowered. On the other hand, if the amount of the additive exceeds 30 parts by weight, the original physical properties of silicon carbide may be impaired.

  Next, a method for manufacturing the exhaust gas filter of the present invention will be described in the order of steps. Step a: In this step, the surface of at least one additive powder selected from carbides, nitrides, borides and oxides excluding silicon carbide is coated with a water-repellent organic resin. In the present invention, it is desirable to select tungsten carbide as the carbide used for the additive. Further, it is desirable to select boron nitride, titanium nitride, or aluminum nitride as the nitride. Similarly, it is desirable to select titanium diboride as the boride and aluminum oxide or silicon oxide as the oxide.

  Next, step b: In this step, silicon carbide and the additive are dry-mixed. An organic binder and water are added to the resulting mixture and kneaded better. In this case, it is possible to use α-type silicon carbide powder, β-type silicon carbide powder or a mixed powder thereof as necessary. In this step, 5 to 30 parts by weight of the additive is mixed with 100 parts by weight of silicon carbide.

  Step c: In this step, the kneaded product obtained in the step b is dried after being molded or held in a porous shape. The drying temperature at this time is preferably 50 ° C to 100 ° C.

  Step d: In this step, the dried product obtained in the step c is degreased and fired in an inert atmosphere. The degreasing temperature at this time is preferably 400 ° C. to 800 ° C., and the firing temperature is preferably 1800 ° C. to 2200 ° C. The degreasing step thermally decomposes most of the carbon contained in the dried body, that is, the water repellent resin and the organic binder. And the porous ceramic sintered compact with which the value of a specific resistance value, a density, and the average pore diameter satisfy | filled said predetermined conditions is obtained by passing through the above processes.

  Hereinafter, an embodiment in which the present invention is embodied in an exhaust gas purifying apparatus for a diesel engine will be described in detail with reference to FIGS.

  As shown in FIG. 4, the exhaust gas purification device 1 includes a casing 2 made of metal pipe. The passage 2a of the casing 2 is connected to an exhaust pipe Ea of a diesel engine E as an internal combustion engine. In the casing 2, an exhaust gas filter 3 having a honeycomb structure is disposed in order to remove particulates in the gas discharged from the diesel engine E.

  As shown in FIGS. 1 to 3, the exhaust gas filter 3 has a prismatic shape (33 mm × 33 mm × 150 mm). In the filter 3, communication holes 3 a having a substantially square cross section are regularly formed along the axial direction of the filter 3. The communication holes 3a are separated from each other by an inner wall 3b having a thickness of 0.3 mm. One end on either the exhaust gas inflow side or the outflow side of each communication hole 3a is sealed in a checkered pattern by a sealing piece 3c made of a porous sintered body. As a result, cells C1 and C2 that are open only on either the inflow side or the outflow side of the filter 3 are formed. A silica film (not shown) is formed on the inner walls 3b of the cells C1 and C2. And the oxidation catalyst which consists of a platinum group element, another metal element, its oxide, etc. is carry | supported by the silica film.

  As shown in FIGS. 1 to 3, a pair of platinum electrodes 5 are provided on the outer wall surfaces of both end portions 4 of the filter 3 as self-heating electrodes. As shown in FIG. 4, these electrodes 5 are each connected to a battery (12V-2.5 kW) 7 via wiring 6.

Here, the flow of exhaust gas when the filter 3 is disposed at a predetermined position and the diesel engine E is started will be described. As shown by the arrow A1 in FIG. 2, the exhaust gas is first introduced into the cell C1 that opens to the inflow side of the filter 3. The exhaust gas then passes through the inner wall 3b and is adjacent to the cell C2.
That is, it is introduced into C2 opening on the outflow side. At this time, the movement of the fine particles contained in the exhaust gas is blocked by the inner wall 3b. Therefore, only the fine particles are trapped on the inner wall 3b. Then, the purified exhaust gas passes through the cell C2 opened to the outflow side, and is finally discharged from the filter 3.

  In the case of this exhaust gas purification device 1, when it is detected that a predetermined amount of particulates has been collected in the filter 3, the regeneration processing of the filter 3 is automatically performed. In the regeneration process, energization from the battery 7 to the filter 3 is started based on a signal from a detection unit (not shown). Then, the temperature of the filter 3 rises due to the self-heating of the filter 3, and the fine particles are eventually ignited. Thereafter, the filter 3 is heated to about 800 ° C. to 1000 ° C. and maintained at that temperature until the fine particles are combusted and disappeared. As a result, the filter 3 is regenerated to the original state before collecting the fine particles.

  Next, the characteristic evaluation test of the exhaust gas filter 3 will be described. In this characteristic evaluation test, as shown in Table 1, sample (1-16) of the example and sample (17, 18) of the comparative example were respectively produced.

  For example, in sample 1, an aluminum nitride powder was selected as an additive, and the surface of the powder was coated with a water-repellent organic resin (paraffinic amine). Next, 100 parts by weight of β-type silicon carbide powder and 15 parts by weight of aluminum nitride were dry mixed. Further, an organic binder (methyl cellulose) and water were added to the mixture in predetermined amounts and kneaded. The kneaded product was extrusion molded to obtain a honeycomb-shaped formed body. Next, after the communication hole 3a of the molded body was sealed with a paste for forming a sealing piece 3c made of a porous sintered body, the both ends 4 were impregnated with a platinum paste for forming an electrode 5. Then, the molded object, the paste for sealing pieces 3c, and the paste for electrodes 5 were dried using the dryer. And after degreasing the dry body at 600 degreeC, it was further baked at 2200 degreeC in argon atmosphere. Then, the filter 6 of the sample 1 was finally obtained by brazing the wiring 6 to both electrodes 5 of the sintered body.

Samples 2 to 18 were produced according to the above procedure. At that time, only the raw material composition was changed as shown in Table 1. Table 2 shows the results of analyzing the composition (SiC, additive, residual C) of the sintered body portion in the obtained samples 1 to 18. Table 3 shows the results of examining the average pore diameter (μm), specific resistance (Ω · cm), and density (g / cm ³ ) of Samples 1 to 18.

  Furthermore, the exhaust gas purification apparatus 1 was comprised using each said samples 1-18, and the quality of collection efficiency and the regeneration state were investigated by purifying exhaust gas. The results are also shown in Table 3. The quality of the collection efficiency was judged by measuring the amount of fine particles in the exhaust gas flowing out from the filter 3 during non-regeneration. In addition, the quality of the regenerated state was judged by measuring the time required until the fine particles were completely burned and removed by energization after a predetermined amount of fine particles were collected.

  In addition, this invention is not limited only to the said Example, It can be changed into the following structures. For example, (a) the filter 10 is not limited to the honeycomb-like filter 3 as shown in the above-described embodiment, and may be a filter 10 having a foam-like structure as shown in FIG. The filter 10 is manufactured by the following procedure, for example.

  First, the surface of at least one additive powder selected from carbides, nitrides, borides and oxides excluding silicon carbide is coated with a water-repellent organic resin. Next, after α-type silicon carbide powder, β-type silicon carbide powder or a mixed powder thereof and the additive are dry-mixed, an organic binder and water are added to the mixture and kneaded. Furthermore, by impregnating the kneaded product obtained in the previous step with a foamed resin such as urethane foam, the kneaded product is temporarily held in a predetermined porous shape. And it dries in this state and evaporates the water | moisture content in a kneaded material to some extent. After degreasing the dried product obtained in the previous step, it is further fired in an inert atmosphere such as argon. Then, while sintering of silicon carbide or the like proceeds, the foamed resin burns and disappears, and as a result, the filter 10 having a foam-like structure is obtained. An advantage of the manufacturing method of the filter 10 having the foam-like structure is that even a large-sized filter that is difficult to manufacture can be obtained relatively easily by extrusion or the like.

  The means for holding the kneaded material in a predetermined shape is not limited to the above foamed resin. That is, any device having continuous pores and capable of burning at a predetermined temperature can be used as the holding means.

  (B) As a filter structure, it is of course possible to adopt, for example, a three-dimensional network structure, a noodle shape, a fiber shape, etc. in addition to the above-described honeycomb structure or foam-like structure.

  (C) As shown in FIG. 6, for example, by combining an exhaust gas filter 11 including eight filters 3 having a substantially square cross section and four filters 12 having a substantially isosceles cross section, 1 Two filters 11 may be configured. With such a configuration, a large filter 11 as a whole can be obtained using the filters 3 and 12 that are not so large.

  The combination of the filters 3 and 12 is not limited to the one shown in FIG. 6, and the number of filters 3 and 12 to be used can be changed as a matter of course. Further, a filter 10 having a foam structure may be used instead of the filters 3 and 12 having a honeycomb structure.

  (D) The filters 3, 10, and 12 of the present invention are not necessarily used as a self-heating type. In other words, it may of course be used as a heated type that is heated by an external heat generating means such as a burner or a heater.

  (E) The self-heating electrode 5 can be produced by a method such as metal metallization or sputtering. Of course, a refractory metal other than platinum may be selected as the metal used at that time.

It is a perspective view which shows an exhaust gas filter. It is a partially broken expanded sectional view in the AA line of FIG. It is an expanded sectional view in the BB line of FIG. It is a schematic sectional drawing which shows an exhaust-gas purification apparatus. It is a perspective view which shows the exhaust-gas filter of the another example 1 which has a foam-like structure. It is a perspective view which shows the exhaust-gas filter of the other example 2 comprised by a some filter.

Explanation of symbols

E ... Diesel engine as internal combustion engine,
3, 10, 12 ... filter,
4 ... both ends,
5 ... (for self-heating) electrodes.

Claims (4)

In a filter made of porous ceramics for purifying exhaust gas of an internal combustion engine,
The porous ceramic is a fired composite material of silicon carbide and an additive present in a neck portion that is a bonding portion between silicon carbide crystal particles,
The additive is at least one of (1) a substance that forms a liquid phase in the neck part, (2) a substance that forms a solid solution in the neck part, and (3) a substance having a lower specific resistance than silicon carbide. An exhaust gas filter characterized by being.   2. The exhaust gas filter according to claim 1, wherein the substance that forms a liquid phase in the neck portion is aluminum oxide or silicon oxide.   2. The exhaust gas filter according to claim 1, wherein the substance that forms the solid solution in the neck portion is tungsten carbide, titanium nitride, or titanium diboride.   2. The exhaust gas filter according to claim 1, wherein the substance having a lower specific resistance than (3) silicon carbide is boron nitride or aluminum nitride.

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