JP2014193782A - Porous sintered body, production method of porous sintered body, honeycomb structure, and honeycomb filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a porous sintered body in which three conditions of a low thermal expansion, a high thermal conductivity, and a low young ratio that become necessary when being used as an exhaust gas treatment member of an internal combustion engine are satisfied, by which an extrusion mold is hardly abraded, and for which special silicon is not required.SOLUTION: A honeycomb structure 10 that a honeycomb filter 1 includes is formed by a porous sintered body in which silicon is used as a chief ingredient, the silicon is covalently bonded, and a porosity is 20-70%.

Description

  The present invention relates to a porous sintered body used for exhaust gas treatment, a manufacturing method thereof, a honeycomb structure using the porous sintered body, and a honeycomb filter.

  The porous sintered body is used for an exhaust gas processing member of an internal combustion engine. For gasoline engines, the cordierite porous sintered body is used for exhaust gas catalyst support, and for diesel engines, the cordierite porous sintered body is used for supporting exhaust gas treatment oxidation cordierite, aluminum titanate, A porous sintered body such as silicon carbide is used for DPF (Diesel Particulate Filter) for collecting and purifying particulate matter contained in exhaust gas.

  In such a porous sintered body used for an exhaust gas processing member of an internal combustion engine, in order to withstand high temperature and rapid temperature change, it is required to have a low thermal expansion coefficient and a high thermal conductivity, It is also required to have a low Young's modulus, which does not easily cause distortion.

  Table 1 shows the density, melting point, heat capacity, and thermal conductivity of dense sintered bodies (dense bodies) of silicon carbide (SiC), cordierite, and aluminum titanate that have already been put to practical use as such porous sintered bodies. , Thermal expansion coefficient, Young's modulus and Vickers hardness. The source of numerical values will be described later in the section of the embodiment. Moreover, when comparing the dense sintered body and the porous sintered body of the same raw material, the thermal conductivity and Young's modulus are lower in the porous sintered body containing pores, but the thermal expansion coefficient is It will be almost the same.

  Moreover, what uses silicon (Si: metal silicon, also called silicon) as an auxiliary material is proposed as a porous sintered compact used for an exhaust gas processing member. For example, Patent Document 1 describes a silicon carbide particle to which metal silicon (Si) is added, and Patent Documents 2 and 3 are made of a composite material of ceramic particles and crystalline silicon (Si). Things are listed.

JP 2002-154882 A (published on May 28, 2002) JP 2005-064128 (published July 14, 2005) Special table 2004-031100 gazette (released on April 15, 2004)

  However, all of the previously proposed porous sintered bodies have their merits and demerits, and the low thermal expansion coefficient, high thermal conductivity, and low required for porous sintered bodies used for exhaust gas treatment members of internal combustion engines. The three conditions of Young's modulus could not be satisfied. Moreover, even if these conditions could be satisfied, the extrusion mold was easily worn or special silicon was required.

  That is, although it is a dense sintered body, as shown in Table 1, silicon carbide (SiC) is advantageous for thermal shock because of its high heat resistance and high thermal conductivity. Although the coefficient of thermal expansion is not low, it can withstand use. However, the Young's modulus is high and distortion is likely to occur. In terms of manufacturing, there is also a problem that the extrusion mold is easily worn because the particles are hard.

  Cordierite has a very low coefficient of thermal expansion and a low Young's modulus. In terms of manufacturing, since the extrusion raw materials (talc, aluminum hydroxide, kaolin, etc.) are low in hardness, the extrusion mold is hardly worn. However, the thermal conductivity is low.

  Aluminum titanate, like cordierite, has a very low coefficient of thermal expansion, a very low Young's modulus, and a low hardness of the extrusion raw material. However, as with cordierite, the thermal conductivity is low. In addition, there is a problem that the microcracks are inherent in the material due to the difference in the coefficient of thermal expansion of the crystal, so that the strength is reduced and it is difficult to use.

  Further, the porous sintered body obtained by adding metal silicon to silicon carbide particles described in Patent Document 1 can be sintered at a relatively low temperature while using silicon carbide particles by including metal silicon, and is inexpensive. It is described that it can be manufactured. However, since silicon carbide having hard particles is used, the extrusion mold is likely to be worn.

  In the porous sintered body made of a composite material of ceramic particles and crystalline silicon described in Patent Document 2, the ceramic particles are bonded via silicon. The thermal conductivity of this porous sintered body varies greatly depending on the crystallinity of silicon, and in order to make the thermal conductivity excellent, the crystallinity of silicon needs to satisfy a predetermined condition, It is necessary to use special silicon.

  The present invention has been made in view of the above problems, and its purpose is to satisfy the three conditions of low thermal expansion coefficient, high thermal conductivity, and low Young's modulus that are required when used for an exhaust gas treatment member of an internal combustion engine. An object of the present invention is to provide a porous sintered body that is satisfactory and that does not easily wear an extrusion mold and does not require special silicon.

  Today, in the exhaust gas treatment of an internal combustion engine, the improvement of combustion control technology has progressed, and the use temperature does not exceed 900 ° C. for DPF applications. The inventors of the present application have made extensive studies focusing on this point. As a result, the above three conditions required when a porous sintered body mainly made of silicon, which has been considered to be inferior in heat resistance, is used for an exhaust gas treatment member of an internal combustion engine, are satisfied. The present inventors have found that the present invention is satisfactory and that it is difficult to cause wear of the extrusion mold and that no special silicon is required.

  The porous sintered body of the present invention is characterized in that silicon is a main component, silicon is covalently bonded, and the porosity is 20 to 70%.

  The porous sintered body of the present invention having such characteristics satisfies the three conditions of low thermal expansion coefficient, high thermal conductivity, and low Young's modulus, which are required when used for exhaust gas treatment members of internal combustion engines. Moreover, it is difficult to cause wear of the extrusion mold, and no special silicon is required as a raw material.

  Specifically, in the porous sintered body of the present invention, the thermal expansion coefficient is not as low as that of cordierite and aluminum titanate, but can be lower than that of silicon carbide that can withstand use.

  The thermal conductivity can be made higher than silicon carbide, which is considered to be much higher than cordierite and aluminum titanate. This is because silicon is the main component and silicon is covalently bonded. Here, not all silicon needs to be covalently bonded, and some silicon may be glass-bonded. However, as the proportion of glass bonds increases, the thermal conductivity decreases.

  Young's modulus is less than half that of silicon carbide, although not as low as cordierite or aluminum titanate.

  In addition, it has been confirmed that the amount of wear of the extrusion mold during the molding process can be reduced to about 1/3 compared to the processing of the extruded material using silicon carbide as a raw material. Further, it is not necessary to use silicon having a special crystal structure.

  Furthermore, it can function stably as DPF by having a porosity of 20 to 70%. When the porosity is less than 20%, the ability to collect particulate matter cannot be guaranteed, and conversely, when the porosity exceeds 70%, the strength that can be used is obtained. It is because it cannot be done.

  The porous sintered body of the present invention further has a metal powder of AL, Fe, Ni, Ti, B, P, or Ca, or AL, Fe, Ni, Ti, B, P as a sintering aid. Or it is good also as a structure containing the powder of the compound of Ca and silicon.

  According to the above configuration, the strength of the porous sintered body can be increased.

  The method for producing a porous sintered body according to the present invention includes a mixing step of mixing a sintering aid and a binder substance into main raw material silicon, a molding step of molding the mixture obtained in the mixing step, and the molding step And a degreasing step for degreasing the molded product obtained in (1) above, and a sintering step for firing and sintering the degreased molded product obtained in the degreasing step in an oxygen-free atmosphere.

  By the above method, the porous sintered body of the present invention can be obtained. Covalent bonding can be achieved by setting the sintering step under an oxygen-free atmosphere.

  In the method for producing a porous sintered body according to the present invention, in the sintering step, the molded product is sintered in an inert gas atmosphere or in a vacuum at a temperature range of 1200 to 1400 ° C. Also good.

  By the above method, the porous sintered body of the present invention in which silicon is covalently bonded can be stably produced.

  In the method for producing a porous sintered body of the present invention, a pore former that disappears at least in the sintering step may be further mixed in the mixing step.

  By the said method, the porous sintered compact of this invention which has a porosity of 20 to 70% can be manufactured stably.

  In the method for producing a porous sintered body of the present invention, in the molding step, a honeycomb structure having a plurality of cells extending in one direction formed by being partitioned by cell walls is formed by extrusion molding. You can also

  By the above method, a honeycomb structure made of the porous sintered body of the present invention can be obtained.

  Further, the present invention comprises a honeycomb structure having a plurality of cells extending in one direction formed by the porous sintered body of the present invention and defined by cell walls, and the honeycomb structure, The honeycomb filter in which the end portions on one side in the extending direction of the plurality of cells in the honeycomb structure are alternately sealed by the sealing portions is also included in the category.

  According to the present invention, the three conditions of low thermal expansion coefficient, high thermal conductivity, and low Young's modulus, which are required when used for an exhaust gas treatment member of an internal combustion engine, are satisfied, and it is difficult to wear the extrusion mold. In addition, there is an effect that a porous sintered body that does not require special silicon, a manufacturing method thereof, a honeycomb structure using the porous sintered body, and a honeycomb filter can be provided.

1 is a perspective view showing an overview of a honeycomb filter according to an embodiment of the present invention. It is a schematic cross section of a plane parallel to the axial direction of the honeycomb filter.

[Embodiment 1]
Hereinafter, the present invention will be described with reference to embodiments. The porous sintered body according to the present embodiment can be used for, for example, an exhaust gas processing member of an internal combustion engine. It can be used for supporting an exhaust gas catalyst for a gasoline engine, for supporting an oxidation catalyst for exhaust gas treatment for a diesel engine, or for a DPF for collecting and purifying particulate matter contained in exhaust gas.

  The porous sintered body according to the present embodiment has silicon (Si) as a main component, and silicon is covalently bonded. The porous sintered body having such a configuration satisfies the three conditions of low thermal expansion coefficient, high thermal conductivity, and low Young's modulus, which are required when used for an exhaust gas processing member of an internal combustion engine, and is an extrusion process. It is difficult to wear the metal mold, and there is no need to use special silicon as a raw material.

  Specifically, in the porous sintered body of the present embodiment, the coefficient of thermal expansion is lower than that of silicon carbide, though not as low as that of cordierite and aluminum titanate. Thermal conductivity is even higher than silicon carbide, which is considered to be much higher than cordierite and aluminum titanate. This is because silicon is the main component and silicon is covalently bonded. Here, not all silicon needs to be covalently bonded, and some silicon may be glass-bonded. However, as the proportion of glass bonds increases, the thermal conductivity decreases. Young's modulus is less than half that of silicon carbide, although not as low as cordierite or aluminum titanate.

  Moreover, the hardness of the silicon used as the extrusion material when performing the molding process is less than half that of silicon carbide. For this reason, abrasion of the extrusion mold used for the molding process hardly occurs. Further, it is not necessary to use silicon having a special crystal structure as silicon as the main component.

  The porosity of the porous sintered body is 20 to 70%. By having a porosity of 20 to 70%, it can function stably as a DPF. When the porosity is less than 20%, the ability to collect particulate matter cannot be guaranteed, and conversely, when the porosity exceeds 70%, the porosity can be used to obtain a strength that can be used. This is because they cannot.

  As silicon which is a main constituent element of the porous sintered body, it is preferable to use silicon having a particle diameter of 20 to 60 μm. The reason why the added amount is in such a range is that when used for DPF, if the particle diameter is less than 20 μm, the pressure loss increases, and conversely, if it exceeds 60 μm, soot cannot be collected. . In applications other than DPF, if the particle size is less than 20 μm, the powder filling rate increases and the amount of pore former added increases, making it difficult to manufacture, and conversely, if it exceeds 60 μm, the catalyst is supported. This is because the specific surface area becomes small.

  As the binder material, for example, a combination of methylcellulose and water can be used. The amount of methylcellulose added is 10 to 20 wt% with respect to the total weight of silicon and pore former. The reason why the added amount is in such a range is that when the added amount is less than 10 wt%, extrusion molding cannot be performed, and conversely, if it exceeds 20 wt%, degreasing in the next step becomes difficult.

  In order to stably obtain a porous sintered body having a porosity of 20 to 70%, it is preferable to add a pore former. As the pore former, for example, starch can be used. The addition amount of the pore former is 0 to 30 wt% with respect to the total weight of silicon and the pore former. The reason why the addition amount is in such a range is that when the addition amount exceeds 30 wt%, degreasing becomes difficult, and it becomes difficult to maintain the shape of the molded product after degreasing. The pore diameter of the pore former is preferably 20 to 60 μm, which is the same as the particle diameter of silicon.

  In order to adjust the strength of the porous sintered body, silicon having a particle size larger than that of silicon as a main raw material may be included.

  Further, as a sintering aid, a metal powder of AL, Fe, Ni, Ti, B, P, or Ca, or a compound of AL, Fe, Ni, Ti, B, P, or Ca and silicon. Powder or the like may be added.

A mixture obtained by mixing (adjusting) the above materials is extruded using an extrusion die, and formed into a desired shape such as a honeycomb structure. Thereafter, the molded product is dried, and methylcellulose and starch are removed in a degreasing process. Degreasing, a heat treatment with the addition of some _{O 2} to _{N 2} during or _{N 2.} When the degreasing temperature is 400 ° C. or lower, silicon is not oxidized and is easy. However, when adding some O ₂ in N _2, while the amount of O ₂ added to adjust to oxidize only a binder material, it can be degreased at 400 ° C. or higher.

  Next, the degreased molded product is fired and sintered in an oxygen-free atmosphere (sintering step). Specifically, for example, sintering is performed in an argon stream or in a vacuum. The sintering temperature is a temperature lower than the melting point of silicon and is preferably 1200 to 1400 ° C. The sintering temperature is set to such a range when the temperature is below 1200 ° C., the mechanical strength of the sintered porous body after sintering becomes insufficient. On the other hand, when the temperature is higher than 1400 ° C., silicon melts. This is because the shape of the molded product cannot be maintained. Although silicon oxidizes to the inside at 1000 ° C. or higher, here, since it is baked in an oxygen-free atmosphere, surface oxidation occurs, but oxidation to the inside of silicon does not occur.

[Embodiment 2]
Hereinafter, the present invention will be described with reference to embodiments. FIG. 1 is a perspective view showing an overview of a honeycomb filter according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of a plane parallel to the axial direction of the honeycomb filter.

  As shown in FIGS. 1 and 2, the honeycomb filter 1 has a columnar honeycomb structure 10 made of a porous sintered body according to the present invention. A plurality of cells 11 extending in one direction are formed inside the cylindrical main body of the honeycomb structure 10. Each cell 11 has a cross-sectional shape that is substantially square in a direction perpendicular to the axial direction (cell extending direction) and is defined by a porous cell wall 12. The porous cell wall 12 serves as a particulate matter (hereinafter, PM) collecting member.

  Each cell 11 provided in the honeycomb structure 10 is sealed (sealed) by sealing a filler at one end in the axial direction (stretching direction), and a sealing portion 21 is formed. ing. The sealing part 21 is provided with a plurality of cells 11 alternately at both ends in the axial direction of the honeycomb structure 10.

  The outer peripheral portion of the honeycomb structure 10 is covered with an outer peripheral coating layer 15. The outer peripheral covering layer 15 is made of a ceramic layer, and is formed by sintering an outer peripheral covering material applied to the outer peripheral portion of the honeycomb structure 10. The outer peripheral coating layer 15 is not necessarily required. In addition, in FIG. 2, description of the outer periphery coating layer 15 is abbreviate | omitted.

  As shown in FIG. 2, the honeycomb filter 1 having such a configuration is arranged such that the axial direction, which is also the extending direction of the cells 11, is parallel to the flow of exhaust gas. The exhaust gas flows in from a cell (inflow side cell) 11 that is located upstream of the flow and whose cell end is not sealed. The exhaust gas that has flowed into the cell 11 passes through the micropores of the porous cell wall 12 and is located on the downstream side of the flow to the adjacent cell (outflow side cell) 11 where the cell end is not sealed. And move out of it.

  As the exhaust gas passes through the fine holes in the cell wall 12, PM contained in the exhaust gas is collected in the cell wall 12. The collected PM is removed from the cell wall 12 by regenerating and heating the honeycomb filter 1, thereby regenerating the honeycomb filter 1.

  In FIG. 1, the honeycomb structure 10 is exemplified as a columnar shape in which the cross-sectional shape of the surface perpendicular to the axial direction of the honeycomb filter 1 is circular, but the cross-sectional shape is not particularly limited. For example, it may be oval, square, rectangular, or polygonal. Such a honeycomb structure 10 can be molded in advance into a desired shape by using an extruder. Further, the optimum value of the cross-sectional size of the plane perpendicular to the axial direction of the honeycomb filter 1 is determined by the engine displacement.

  On the other hand, the cross-sectional shape of the cell 11 is preferably substantially square. However, it is not necessarily limited to this, and other shapes may be used. The thickness of the cell wall 12 is not particularly limited, and may be 0.2 to 0.4 mm, for example. Further, the number of cells in the unit area is not particularly limited, and may be, for example, 200 to 300 cpsi. Although the thickness of the outer peripheral coating layer 15 is not particularly limited, it is generally set to 0.3 mm to 1.0 mm.

  As the material of each part in the honeycomb filter 1, existing conventional materials can be used except for the honeycomb structure 10 using the porous sintered body according to the present invention.

  For example, as a filler for sealing the cell end, ceramic clay such as aluminum oxide (alumina), aluminum titanate, silicon carbide, silicon nitride, cordierite, mullite, apatite, or cement material should be used. Can do. These may be used alone or in combination. Among these, aluminum oxide is particularly preferable from the viewpoint of versatility as a cement material.

  The outer peripheral coating layer 15 is made of a ceramic layer, and a material obtained by blending inorganic particles such as inorganic balloon, colloidal silica, bentonite, an inorganic binder, or the like with the material of the honeycomb structure 10 described above is used.

  The honeycomb filter 1 is a so-called integral type in which the honeycomb structure 10 is formed of one porous ceramic fired body. However, a divided type in which a honeycomb segment body, which is a plurality of porous ceramic fired bodies formed in a prismatic shape, is bonded to each other through a joint portion may be used.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  The present invention will be described in more detail with reference to examples.

  To 140 parts by weight of # 350 under silicon powder, 10 parts by weight of starch as a pore-forming material, 15 parts by weight of methylcellulose as a binder material, and water were added and mixed to prepare an extruded goat (mixture). Extruded goblet was extruded using an extrusion die to form a honeycomb structure of 35 mm × 35 mm × 100 mm as a molded product.

The honeycomb structure thus formed was dried and then degreased by heating to 300 ° C. in a stream of N ₂ + 5% O ₂ to remove methylcellulose and starch. Next, the degreased molded product (degreasing molded product) was placed in a sintering furnace and sintered at a temperature of 1350 to 1400 ° C. in an argon atmosphere.

  The porosity (DPF porosity), the density (DPF density), the thermal conductivity, the thermal expansion coefficient, and the Young's modulus of the honeycomb structure of the example manufactured as described above were measured. Table 2 shows the measurement results. The DPF porosity and DPF density were measured using an Archimedes density meter. The thermal expansion coefficient was measured using a differential thermal expansion coefficient measuring apparatus. The thermal conductivity was measured using a thermophysical property measuring apparatus by a hot disk method. Young's modulus was measured using a pulse measurement method.

  Table 2 also shows the density, melting point, heat capacity, thermal conductivity, thermal expansion coefficient, Young's modulus, and Vickers hardness of dense sintered bodies of silicon (Si), silicon carbide (SiC), cordierite, and aluminum titanate. (Reference: Kazuo Ohno, Waseda University Doctoral Dissertation, Ceramics Engineering Handbook, Japan Ceramic Society, 1989, TOTO Ceramics Property List, Acera Ceramics Property List, Mitsubishi Materials Electronics Chemicals Corporation Silicon Catalog, Shinyo Silicon Property Data)

As can be seen from Table 2, in the porous sintered body (porous body) of the example using silicon (Si), the thermal expansion coefficient is 3 × 10 ⁻⁶ / ° C., and cordierite or aluminum titanate. Although not so low, it was lower than silicon carbide.

  The thermal conductivity was 70 W / m · K. Silicon carbide, which is said to be much higher than cordierite and aluminum titanate, is a compact body of about 81 W / m · K. If the porosity is also 50%, it is half that of 40 W / m · K. It is assumed that the degree. The thermal conductivity is 70 W / m · K, which is sufficiently higher than this.

  The Young's modulus was 85 GPa. Although it is not as low as cordierite and aluminum titanate, dense silicon carbide is 411 GPa, and when the porosity is also 50%, it is assumed that it is about 200 GPa, which is half that. It can be said that the Young's modulus is sufficiently lower than 85 GPa.

  Moreover, when comparing the hardness of each raw material, the hardness of silicon (Vickers hardness) is lower than that of silicon carbide, and wear of the extrusion mold can be suppressed. In addition, the hardness of the talc and aluminum hydroxide used as the raw material for cordierite is 10 GPa or less.

  Regarding the wear of the extrusion mold, a 35 × 35 × 100 mm honeycomb structure was molded using the extrusion mold, and the wear amount of the mold calculated from the change in the cell partition wall thickness was measured. It was 1/3 or less when silicon carbide was molded. From this, it was confirmed that the extrusion mold was hardly worn.

  The present invention can be suitably used as an exhaust gas purification filter for collecting particulate matter contained in exhaust gas of a diesel engine.

DESCRIPTION OF SYMBOLS 1 Honeycomb filter 10 Honeycomb structure 11 Cell 12 Cell wall 15 Perimeter coating layer 21 Filler

Claims (8)

  A porous sintered body comprising silicon as a main component, silicon being covalently bonded, and a porosity of 20 to 70%.   As a sintering aid, metal powder of AL, Fe, Ni, Ti, B, P, or Ca, or powder of AL, Fe, Ni, Ti, B, P, or a compound of Ca and silicon is used. The porous sintered body according to claim 1, comprising: A mixing step of mixing a sintering aid and a binder material into the main raw material silicon,
A molding step of molding the mixture obtained in the mixing step;
A degreasing step of degreasing the molded product obtained in the molding step;
And a sintering step in which the degreased molded product obtained in the degreasing step is fired and sintered in an oxygen-free atmosphere.   In the said sintering process, sintering of a molded object is implemented in the temperature range of 1200-1400 degreeC in inert gas atmosphere or a vacuum, The manufacture of the porous sintered compact of Claim 3 characterized by the above-mentioned. Method.   The method for producing a porous sintered body according to claim 3, wherein in the mixing step, a pore former that disappears at least in the sintering step is further mixed.   The porous structure according to claim 4 or 5, wherein in the forming step, the porous structure according to claim 4 or 5 is formed by extrusion forming into a honeycomb structure having a plurality of cells extending in one direction formed by being partitioned by cell walls. Of manufacturing a sintered material.   A honeycomb structure comprising a plurality of cells extending in one direction formed by being partitioned by cell walls, comprising the porous sintered body according to claim 1.   A honeycomb filter comprising the honeycomb structure according to claim 7, wherein one end in the extending direction of the plurality of cells in the honeycomb structure is alternately sealed with a sealing portion between adjacent cells.

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