INORGANIC FIBER AGGREGATE, METHOD FOR PRODUCING INORGANIC FIBER AGGREGATE, HONEYCOMB STRUCTURE AND METHOD FOR PRODUCING HONEYCOMB STRUCTURE |
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| 申请号 | EP06714211.7 | 申请日 | 2006-02-21 | 公开(公告)号 | EP1726698B1 | 公开(公告)日 | 2013-07-10 |
| 申请人 | Ibiden Co., Ltd.; | 发明人 | OHNO, Kazushige c/o IBIDEN CO., LTD.; SHIMIZU, Masao c/o IBIDEN CO., LTD.; MATSUO, Chiemi c/o IBIDEN CO., LTD.; | ||||
| 摘要 | |||||||
| 权利要求 | |||||||
| 说明书全文 | The present invention relates to an inorganic fiber aggregated body, a method for manufacturing an inorganic fiber aggregated body, a honeycomb structured body using the inorganic fiber aggregated body, and a method for manufacturing a honeycomb structured body. Particulates, such as soot, contained in exhaust gases discharged from internal combustion engines of vehicles, such as buses and trucks, and construction machines, have raised serious problems as contaminants harmful to the environment and the human body. Conventionally, various kinds of filters, used for capturing particulates in exhaust gases and purifying the exhaust gases, have been proposed, and filters having a honeycomb structure have also been known. For example, a honeycomb structured body, which is manufactured by corrugating inorganic sheets obtained through a sheet-forming -process of inorganic fibers made of alumina, silica, mullite or the like, has been proposed (for example, see Patent Document 1). Moreover, a honeycomb structured body in which a catalyst supporting layer made of an inorganic material has been formed on the entire surface of a porous sintered body made of metal fibers has also been known (for example, see Patent Document 2) .
The honeycomb structured body using inorganic fibers, as disclosed in Patent Document 1, is easily subjected to erosion, and the resulting problem is that the inorganic fibers tend to scatter during the use. Moreover, the honeycomb structured body, as disclosed in Patent Document 2, is easily eroded, failing to satisfy the reliability as a honeycomb structured body. The present inventors have intensively studied so as to solve the above-mentioned problems, and found that, upon firmly fixing inorganic fibers to each other through an inorganic matter, the inorganic matter is fixed to a portion of the inorganic fibers so that the strength of the entire inorganic fiber aggregated body can be improved to consequently prevent loose inorganic fibers and erosion; thus, the inorganic fiber aggregated body of the present invention has been completed. Moreover, the present inventors have also found that the inorganic fiber aggregated body is appropriately applicable to a honeycomb structured body. The inorganic fiber aggregated body of the present invention comprises inorganic fibers and an inorganic matter, wherein the inorganic matter sticks firmly to a portion at a surface of the inorganic fibers, and the inorganic fibers are firmly fixed to each other through the inorganic matter. In the inorganic fiber aggregated body, the portion where the inorganic fibers are firmly fixed to each other corresponds to an intersection of the inorganic fibers, and the inorganic matter exists locally on the intersection of the inorganic fibers. Also, in the inorganic fiber aggregated body, the inorganic matter desirably contains silica, and the inorganic fibers desirably comprise at least one kind selected from the group consisting of silicon carbide, alumina, basalt, silica, silica alumina, titania and zirconia. The method for manufacturing an inorganic fiber aggregated body of the present invention comprises: mixing inorganic fibers A with inorganic fibers B and/or inorganic particles C that melt at a temperature at which the inorganic fibers A are neither melted nor sublimated; and heating the mixture at a temperature below a heat resistant temperature of the inorganic fibers A and above a softening temperature of the inorganic fibers B and/or the inorganic particles C. In the method for manufacturing an inorganic fiber aggregated body, the inorganic fibers B and/or the inorganic particles C desirably contain silica, and the inorganic fibers A desirably comprise at least one kind selected from the group consisting of silicon carbide, alumina, basalt, silica, silica alumina, titania and zirconia. Moreover, a compounding ratio of the inorganic fibers A to the inorganic fibers B and/or the inorganic particles C is desirably from 2 : 8 to 8 : 2. The method for manufacturing an inorganic fiber aggregated body desirably comprises a sheet-forming process or a fiber laminating process so that a sheet-shaped inorganic fiber aggregated body is manufactured, and more desirably comprises a process for subjecting said sheet-shaped inorganic fiber aggregated body to an acid treatment. The honeycomb structured body of the present invention is in accordance with claim 10. In the honeycomb structured body, desirably, each of the through holes is sealedat oneof ends of the honeycomb structured body, and the honeycomb structured body has a configuration so as to function as a filter. The honeycomb structured body desirably has a plate member mainly made of metal laminated on each end of a laminate of the inorganic fiber aggregated bodies. The honeycomb structured body desirably comprises a catalyst supported on at least a portion of the inorganic fibers. The method for manufacturing a honeycomb structured body of the present invention comprises: carrying out a process for preparing a sheet-shaped mixture by mixing inorganic fibers A with inorganic fibers B and/or inorganic particles C that melt at a temperature at which the inorganic fibers A are neither melted nor sublimated, and then forming the mixture into a sheet shape; carrying out a through hole formation process of forming through holes in the sheet-shaped mixture at the same time with preparation of the sheet-shaped mixture or after preparation of the sheet-shaped mixture; carrying out a process for heating the sheet-shaped mixture with the through holes formed therein at a temperature below a heat resistant temperature of the inorganic fibers A and above a softening temperature of the inorganic fibers B and/or the inorganic particles C so that a sheet-shaped inorganic fiber aggregated body is manufactured; and laminating the inorganic fiber aggregated bodies such that the through holes are superposed on one another. In the method for manufacturing a honeycomb structured body, the inorganic fibers B and/or the inorganic particles C desirably contain silica, and the inorganic fibers A desirably comprise at least one kind selected from the group consisting of silicon carbide, alumina, basalt, silica, silica alumina, titania and zirconia. In the method for manufacturing a honeycomb structured body, the compounding ratio of the inorganic fibers A to the inorganic fibers B and/or the inorganic particles C is desirably from 2 : 8 to 8 : 2. The method for manufacturing a honeycomb structured body desirably includes a process for subjecting the sheet-shaped inorganic fiber aggregated body to an acid treatment. Furthermore, the method for manufacturing a honeycomb structured body also desirably comprises laminating the sheet-shaped inorganic fiber aggregated bodies; and laminating a plate member mainly made of metal on each end of a laminate of the inorganic fiber aggregated bodies. According to the inorganic fiber aggregated body of the present invention, an inorganic matter sticks firmly to a portion at the surface of an inorganic fiber, and the inorganic fibers are firmly fixed to each other through the inorganic matter so that it is possible to exhibit an excellent strength. Moreover, in the above-mentioned inorganic fiber aggregated body, since the inorganic fibers are firmly fixed to each other through the inorganic matter, it becomes possible to prevent loose inorganic fibers, and also to make the inorganic fiber aggregated body hardly subjected to erosion. According to the method for manufacturing an inorganic fiber aggregated body of the present invention, by using the above-mentioned processes, the inorganic fiber aggregated body can be appropriately manufactured. In particular, since the portion where the inorganic fibers are firmly fixed to each other corresponds to an intersection of the inorganic fibers, the inorganic fiber aggregated body can be appropriately manufactured with a structure that the inorganic matter is locally present on the intersection of the inorganic fibers. The honeycomb structured body of the present invention, which is formed by using the inorganic fiber aggregated bodies of the present invention, is allowed to exert a sufficient strength when used as a filter for purifying exhaust gases containing particulates and the like, and further prevents loose inorganic fibers due to exhaust gases flowing into the honeycomb structured body, as well as preventing the honeycomb structured body from being hardly subjected to erosion, thereby exhibiting excellent reliability. Moreover, in the method for manufacturing the honeycomb structured body of the present invention, it becomes possible to appropriately manufacture the honeycomb structured body of the present invention by using the above-mentioned processes. First, the following description will explain the inorganic fiber aggregated body of the present invention. The inorganic fiber aggregated body of the present invention comprises inorganic fibers and an inorganic matter, wherein the inorganic matter sticks firmly to a portion at a surface of the inorganic fibers, and the inorganic fibers are firmly fixed to each other through the inorganic matter. The inorganic fiber aggregated body comprises an inorganic fiber and an inorganic matter. With respect to the material for the inorganic fiber, examples thereof include oxide ceramics such as silica-alumina, mullite, alumina, silica, titania and zirconia, nitride ceramics such as silicon nitride and boron nitride, carbide ceramics such as silicon carbide, and basalt. Each of these materials may be used alone, or two or more of these may be used in combination. Among those materials the desirable example is at least one kind selected from the group consisting of silicon carbide, alumina, basalt, silica, silica alumina, titania and zirconia. This is because the inorganic fiber aggregated body comprising these materials is excellent in heat resistance. The lower limit value of the fiber length of the inorganic fibers is desirably 0.1 mm, and the upper limit value thereof is desirably 100 mm. The fiber length of less than 0.1 mm makes it difficult to firmly fix the inorganic fibers to each other through an inorganic matter, sometimes failing to provide a sufficient strength; in contrast, the fiber length exceeding 100 mm makes it difficult to manufacture a homogenous inorganic fiber aggregated body, resulting in a failure to provide an inorganic fiber aggregated body having sufficient strength. The lower limit value of the fiber length is more desirably 0.5 mm, and the upper limit value thereof is more desirably 50 mm. The lower limit value of the fiber diameter of the inorganic fibers is desirably 0.3 µm, and the upper limit value thereof is desirably 30 µm. The fiber diameter of less than 0.3 µm tends to make each inorganic fiber easily bent, causing the resulting inorganic fiber aggregated body to be easily eroded. In contrast, the fiber diameter exceeding 30 µm makes it difficult to firmly fix the inorganic fibers to each other through an inorganic matter, resulting in a failure to provide a sufficient strength. The lower limit value of the fiber diameter is more desirably 0.5 µm, and the upper limit value thereof is more desirably 15 µm. Here, inorganic matters containing silica are desirably used as the inorganic matter. In each of the inorganic fiber aggregated bodies, the inorganic matter sticks firmly to a portion of the inorganic fiber, and the inorganic fibers are firmly fixed to each other through the inorganic matter. Here, the portion where the inorganic fibers are fixed to each other corresponds to an intersection of the inorganic fibers, and the above-mentioned inorganic matter exists locally on the intersection of the inorganic fibers. Referring to the drawings, this structure will be explained. In the inorganic fiber aggregated body 10 shown in In this manner, since the inorganic matter 12 sticks firmly to the intersection of the inorganic fibers 11 so that the strength of the inorganic fiber aggregated body 10 is improved, and it becomes possible to prevent loose inorganic fibers. Moreover, in the inorganic fiber aggregated body 10, the inorganic matter 12 is locally present on the intersection of the inorganic fibers 11. Consequently, the inorganic fiber 11 is covered by the inorganic matter 12 at the intersection with another inorganic fiber 11, but most of the other parts of the inorganic fibers 11 do not have the inorganic matter firmly fixed thereto. Here, the intersection of the inorganic fibers refers to an area covering a distance within approximately ten times the fiber diameter of the inorganic fiber from the portion where the inorganic fibers are made closest to each other. The tensile strength of the inorganic fiber aggregated body is desirably 0.3 MPa or more, more desirably 0.4 MPa or more. The tensile strength of less than 0.3 MPa tends to fail to provide sufficient reliability when the inorganic fiber aggregated bodies are used for a honeycomb structured body, as will be described later. Here, the tensile strength can be measured through the following process: forming the inorganic fiber aggregated body into a sheet shape; securing both ends of the inorganic fiber aggregated body by jigs; and carrying out measurement using an Instron-type universal tester. If the above-mentioned inorganic fiber aggregated body is used for a honeycomb structured body, which will be described later, with respect to the apparent density, the lower limit value thereof is desirably 0. 02 g/cm3, more desirably 0.05 g/cm3, and the upper limit value thereof is desirably 1.00 g/cm3. The apparent density of less than 0.02 g/cm3 tends to cause an insufficient strength. In contrast, the apparent density exceeding 1.00 g/cm3 tends to make the pressure loss of the honeycomb structured body too high. The lower limit value is more desirably 0.10 g/cm3, and the upper limit value is more desirably 0.50 g/cm3. Moreover, with respect to the porosity of the inorganic fiber aggregated body, the lower limit thereof is desirably 60% by volume, and the upper limit thereof is desirably 98% by volume. The porosity of less than 60% by volume tends to make the pressure loss of the honeycomb structured body too high, when used for the honeycomb structured body. In contrast, the porosity exceeding 98% by volume tends to fail to provide a sufficient strength, when used for the honeycomb structured body. The lower limit thereof is more desirably 70% by volume. Here, the apparent density and porosity can be measured through known methods, such as a weighing method, Archimedes method and a measuring method using a scanning electron microscope (SEM). In the inorganic fiber aggregated body, the intersection of the inorganic fibers to which the inorganic matter firmly sticks desirably occupies 20% or more of the entire intersections of the inorganic fibers. The rate of less than 20% tends to cause an insufficient strength in the inorganic fiber aggregated body. Here, the rate of the intersections of the inorganic fibers to which the inorganic matter firmly sticks is obtained by the following processes: observing a plurality of portions on the inorganic fiber aggregated body by a microscope; counting within each of the observation views the number of intersections of the inorganic fibers and the number of the intersections of the inorganic fibers to which the inorganic matter firmly sticks so as to obtain the rate; and calculating the average value of the obtained rates. The above-mentioned inorganic fiber aggregated body can be used for a honeycomb structured body, which will be described later, and in this case, it can also be applied to such a honeycomb structured body on which a catalyst is supported. Therefore, a catalyst may be supported on the inorganic fiber aggregated body. Referring to the drawings, the following description will discuss the inorganic fiber aggregated body of the present invention on which a catalyst is supported. The inorganic fiber aggregated body 20 shown in Moreover, as shown in These inorganic fiber aggregated bodies 20 and 30 are also included in one mode of the inorganic fiber aggregated body of the present invention. As described above, upon supporting a catalyst on the inorganic fiber aggregated body of the present invention, the catalyst may be supported on the entire surface of the inorganic fiber, or only on the portion at a surface of the inorganic fiber which is exposed after the inorganic matter has been firmly fixed thereto. Moreover, although not shown in the drawings, in the mode of the inorganic fiber aggregated body as shown in Furthermore, a catalyst may be supported only on a portion of the inorganic fiber and/or the inorganic matter. Examples of the catalyst include a catalyst comprising a noble metal such as platinum, palladium, rhodium and the like, although not limited thereto. In addition to the noble metals, an element such as an alkali metal (Group 1 in Element Periodic Table), analkali earthmetal (Group 2 in Element Periodic Table), a rare-earth element (Group 3 in Element Periodic Table) and a transition metal element, may be supported. A method to support a catalyst on the inorganic fiber aggregated body will be described below. The inorganic fiber aggregated body comprising the structure according to the present invention can be desirably used for, for example, a honeycomb structured body. Also, the inorganic fiber aggregated body comprising the structure according to the present invention can be manufactured according to the method for manufacturing an inorganic fiber aggregated body mentioned below. The following description will explain the method for manufacturing an inorganic fiber aggregated body of the present invention. The method for manufacturing an inorganic fiber aggregated body of the present invention comprises: mixing inorganic fibers A with inorganic fibers B and/or inorganic particles C that melt at a temperature at which the inorganic fibers A are neither melted nor sublimated; and heating the mixture at a temperature below a heat resistant temperature of the inorganic fibers A and above a softening temperature of the inorganic fibers B and/or the inorganic particles C. The following is a step by step explanation of the above-mentioned process for manufacturing an inorganic fiber aggregated body. Firstly, inorganic fibers A are mixed with inorganic fibers B and/or inorganic particles C that melt at a temperature at which the inorganic fibers A are neither melted nor sublimated, to obtain a mixture. The mixture obtained here includes those in which the inorganic fibers A and the inorganic fibers B and/or the inorganic particles C are dispersed in water. Examples of the inorganic fibers A may be the same with those inorganic fibers (inorganic fibers comprising silicon carbide, alumina or the like) described in the above-mentioned inorganic fiber aggregated body. The inorganic fibers A desirably comprise at least one kind selected from the group consisting of silicon carbide, alumina, basalt, silica, silica-alumina, titania and zirconia. The reason for this is because an inorganic fiber aggregated body with excellent heat resistance can be manufactured. With respect to the inorganic fibers B and/or the inorganic particles C, materials thereof are not particularly limited as long as they melt at a temperature where the inorganic fibers A are neither melted nor sublimated, and specific examples of the inorganic fibers B include inorganic glass fibers comprising silicate glass, silicate alkali glass, borosilicate glass or the like, and specific examples of the inorganic particles C include inorganic glass particles comprising silicate glass, silicate alkali glass, borosilicate glass or the like. The lower limit value of the fiber length of the inorganic fibers B is desirably 0.1 mm and the upper limit value thereof is desirably 100 mm. When the fiber length is less than 0.1 mm, it is difficult to fix the inorganic fibers A with each other using an inorganic matter, and thus a sufficient strength may not be obtained. On the other hand, the fiber length exceeding 100 mm makes it difficult to carry out a uniform dispersing process upon preparation of the mixture, thereby upon carrying out a heating process in the followingprocedures, because the inorganic fibers B and/or the inorganic particles C are not dispersed uniformly, a fixing portion at the intersection of the inorganic fibers A tend to become small. The lower limit value of the fiber diameter of the inorganic fibers B is desirably 0.3 µm, and the upper limit value thereof is desirably 30 µm. The fiber diameter of less than 0.3 µm tends to make each inorganic fiber easily bent, causing the resulting inorganic fiber aggregated body to be easily eroded and, in contrast, the fiber diameter exceeding 30 µm makes it difficult to firmly fix the inorganic fibers A to each other through an inorganic matter, resulting in a failure to provide a sufficient strength. The lower limit value of the particle diameter of the inorganic particles C is desirably 1 µm, and the upper limit thereof is desirably 100 µm. The particle diameter of less than 1 µm causes a need for a flocculant, and makes it difficult to carry out a uniform dispersing process. In contrast, upon preparing a mixture, the particle diameter exceeding 100 µm makes it difficult to carry out a uniform dispersing process, and tends to cause a smaller fixing portion at the intersection of the inorganic fibers A due to the fact that during a heating process, the inorganic fibers B and/or the inorganic particles C are not dispersed uniformly. Uponmixing the inorganic fibers A with the inorganic fibers B and/or the inorganic particles C, a compounding ratio (weight ratio) of the inorganic fibers A to the inorganic fibers B and/or the inorganic particles C is desirably from 2 : 8 to 8 : 2. When the compounding ratio of the inorganic fibers A is less than 2 : 8, the inorganic matter is more easily fixed in a manner so as to coat the surface of the inorganic fiber. As a result, the flexibility of the resulting inorganic fiber aggregate tends to become insufficient. In contrast, when the compounding ratio of the inorganic fibers A is more than 8 : 2, the number of the fixing portions between the inorganic fibers becomes smaller; thus, the strength of the resulting inorganic fiber aggregated body tends to become insufficient. Moreover, upon preparing the mixture, water and a dispersant may be added thereto, if necessary, so as to uniformly mix the inorganic fibers A with the inorganic fibers B and/or the inorganic particles C. Furthermore, an organic binder may be added thereto. By adding the organic binder, the inorganic fibers A, the inorganic fibers B and/or the inorganic particles C are made positively entangled with each other and the inorganic fibers B and/or the inorganic particles C are made to hardly come off the mutual inorganic fibers A before firing so that it becomes possible to more positively and firmly fix the inorganic fibers A to each other. On the mixture which has been thus prepared, a heating process is carried out at a temperature below the heat resistant temperature of the inorganic fibers A and above the softening temperature of the inorganic fibers B and/or the inorganic particles C. By carrying out this process, it is possible to manufacture an inorganic fiber aggregated body in which the inorganic fibers A are firmly fixed to each other through an inorganic matter that contains the inorganic fibers B and/or the inorganic particles C as materials or is composed of the same material as each of these, where most of the firmly fixed portions are at the intersections of the inorganic fibers A, and in which the inorganic matter that contains the inorganic fibers B and/or the inorganic particles C as materials or is composed of the same material as each of these, exists locally on the intersections. The heating temperature is appropriately selected by taking into consideration the combination among the inorganic fibers A in addition to the inorganic fibers B and/or the inorganic particles C. Here, the heat resistant temperature of the inorganic fibers A is exemplified as: alumina > 1300°C, silica > 1000°C, silicon carbide > 1600°C, silica-alumina > 1200°C. With respect to a specific heating temperature, although not generally determined since it depends on the heat resistant temperature and the softening temperature of the inorganic fibers and the inorganic matter, it is considered that the desirable temperature is 900 to 1050°C, if inorganic glass is used as the inorganic fibers B and/or the inorganic particles C. When the heating temperature is 900°C or less, although the inorganic matter sticks firmly to a portion at the surface of the inorganic fibers, the inorganic fibers may not be firmly fixed to each other; in contrast, the heating temperature exceeding 1050°C tends to cause cracks in the inorganic matter that firmly sticks to the inorganic fibers. In the method for manufacturing an inorganic fiber aggregated body in accordance with the present invention, after the mixture has been prepared, a sheet-forming process or a fiber laminating process, such as a blowing process, may be carried out, and the above-mentioned heating process may then be conducted so that a sheet-shaped inorganic fiber aggregated body is manufactured. When a sheet-shaped inorganic fiber aggregated body is manufactured through these processes, the resulting inorganic fiber aggregated body can be appropriately used as the honeycomb structured body of the present invention, which will be described later. In the sheet- forming process, the above-mentioned mixture is subjected to a sheet-forming process by using a mesh in which holes having a predetermined shape are formed with mutually predetermined intervals, and the resulting matter is dried at a temperature in the range of 100 to 200°C so that a sheet-processed sheet having a predetermined thickness is obtained. The thickness of each sheet-processed sheet is desirably 0.1 to 20 mm. Moreover, when, upon preparation of the mixture, water is added thereto to prepare the mixture, a drying process is desirably carried out prior to the heating process so that water is removed from the mixture. Moreover, in the method for manufacturing an inorganic fiber aggregated body of the present invention, an acid treatment maybe carried out on the sheet-shaped inorganic fiber aggregated body, manufactured by the above-mentioned method. By carrying out the acid treatment, the heat resistance of the inorganic fiber aggregated body can be improved. The acid treatment can be carried out by immersing the inorganic fiber aggregated body in a solution, such as hydrochloric acid and sulfuric acid. With respect to conditions of the acid treatment in the case where inorganic glass is used as the inorganic matter, the concentration of the treatment solution is desirably 1 to 10 mol/L, the treatment time is desirably 0.5 to 24 hours, and the treatment temperature is desirably 70 to 100°C. By carrying out the acid treatment under these conditions, components other than silica are eluted so that the heat resistance of the inorganic fiber aggregated body can be improved. In the method for manufacturing the inorganic fiber aggregated body of the present invention, an inorganic fiber aggregated body on which a catalyst is supported can also be manufactured. In this case, for example, a method in which a catalyst is preliminarily supported on the inorganic fibers A may be used. More specifically, for example, the inorganic fibers A are immersed in a slurry of an oxide on which a catalyst made of noble metal such as Pt has been supported, and then taken out thereof and heated so that the inorganic fibers A to which the catalyst has adhered is prepared; thus, an inorganic fiber aggregated body is manufactured by using the inorganic fibers A to which the catalyst has adhered so that the inorganic fiber aggregated body on which the catalyst is supported can be manufactured. Upon supporting the catalyst on the inorganic fibers A, after the inorganic fibers have been immersed in a slurry containing the catalyst, the inorganic fibers may be taken out, and then heated so that the catalyst can be directly adhered to the inorganic fibers. Moreover, another method may be used in which, after an inorganic fiber aggregated body has been manufactured through the above-mentioned process, this inorganic fiber aggregated body is immersed in a slurry of an oxide on which a catalyst made of noble metal such as Pt is supported or a slurry containing the catalyst, and then taken out and heated. The former method can manufacture an inorganic fiber aggregated body of a mode shown in Moreover, still another method may be used to support a catalyst thereon. More specifically, for example, after an inorganic fiber aggregated body has been manufactured, this inorganic fiber aggregated body may be immersed in a solution containing 10 g of CZ(nCeO2·mZrO2), 1 L (litter) of ethanol, 5 g of citric acid and an appropriate amount of a pH adjusting agent for about 5 minutes, and then subjected to a firing process at about 500°C so that the catalyst is supported thereon. In this case, the amount of catalyst adhered to the inorganic fiber aggregated body can be adjusted by repeating the above-mentioned immersing and firing processes. The following description will explain the honeycomb structured body of the present invention. The honeycomb structured body of the present invention is a pillar-shaped honeycomb structured body in which a plurality of through holes are placed in parallel with one other in a longitudinal direction with a wall portion therebetween, wherein the honeycomb structured body comprises the inorganic fiber aggregated bodies in accordance with the present invention, and the inorganic fiber aggregated bodies, each having through holes formed therein, are laminated in the longitudinal direction such that the through holes are superposed on one another. In the honeycomb structured body of the present invention, through holes are placed in parallel with each other in the longitudinal direction, and the through holes may be normal through holes with no sealed portions in both ends, or may be through holes with either one of the both ends being sealed (hereinafter, referred to as a bottomed hole). Upon forming an inorganic fiber aggregated body having the through holes, a sheet-forming process may be carried out by using a mesh in which holes corresponding to the through holes have been formed, or a machining process for forming holes at portions corresponding to the through holes may be carried out after an inorganic fiber aggregated body without holes corresponding to the through holes has been formed. If the through holes are prepared as normal through holes, the above-mentioned honeycomb structured body is not allowed to function as a filter; however, by allowing a catalyst to adhere to portions including the through holes, it is allowed to function as a converting device (catalyst supporting carrier) for toxic gases. In contrast, if a number of the through holes are prepared as the bottomed holes with either one end of which is sealed, the honeycomb structured body is allowed to function as a filter, and when a catalyst is adhered thereto, it can also function as a filter and a purifying and/or converting device for toxic gases. The following description will mainly discuss a honeycomb structuredbody that functions as a filter; however, as described above, the honeycomb structured body of the present invention may also have a function as the converting device for toxic gases. Since the honeycomb structured body of the present inventionuses the inorganic fiber aggregated body of the present invention as its constituent material, it has a sufficient strength, loose inorganic fibers are hardly caused, and the honeycomb structured body is also hardly subjected to erosion. Moreover, since its constituent materials are mainly inorganic fibers, it is possible to achieve a honeycomb structured body having a high porosity. For this reason, the pressure loss is reduced to a lower level, and the chance of particulates contacting to the catalyst adhered to the inorganic fibers increases so that particulates can be burned more easily. Furthermore, since its thermal capacity is small, the temperature of the honeycomb structured body is raised to an active temperature of the catalyst earlier by exhaust heat discharged from an internal combustion engine. In particular, in the case of a mode in which a filter is placed right below the engine so as to effectively utilize its exhaust heat, this effect is exerted more efficiently. If the filter is placed right below the engine, the space for the filter is extremely limited, with the result that the filter needs to be formed into a complicated shape; however, since the honeycomb structured body of the present invention is formed by laminating the inorganic fiber aggregated bodies in the longitudinal direction, it can easily be formed into an appropriate shape without losses of the materials. Upon carrying out a regenerating process, a great temperature difference is caused in the longitudinal direction of the filter due to burning of particulates; thus, a great thermal stress is imposed on the filter. However, the honeycomb structured body of the present invention has a structure in which the inorganic fiber aggregated bodies are laminated in the longitudinal direction so that, even if a great temperature difference occurs in the entire filter, the temperature difference occurring per each of the units is small and the resulting thermal stress is also small, making damages such as cracks hardly occur. In particular, the above-mentioned filter having a complicated shape becomes extremely fragile to a thermal stress because of its shape; however, the honeycomb structured body of the present invention makes damages such as cracks hardly occur as described above, even if it has a complicated shape. Moreover, in the honeycomb structured body of the present invention, since a catalyst can be adhered to the inorganic fiber aggregated bodies which are the constituent materials of the inorganic fiber aggregated bodies prior to the forming process thereof, the catalyst is adhered thereto in a more evenly dispersed state. Since the sheet-shaped inorganic fiber aggregated bodies are laminated in the longitudinal direction, the laminating process can be carried out, while the dispersion degree of the catalyst in the longitudinal direction and the kind of catalyst are freely combined in accordance with the intended purpose of the honeycomb structured body. Consequently, the honeycomb structured body of the present invention can improve its regenerating process and toxic gas purifying and/or converting function. Moreover, in the honeycomb structured body of the present invention, by alternately laminating inorganic fiber aggregated bodies having mutually different sizes of through holes, or by randomly laminating those aggregated bodies, irregularities can be easily formed on the surface of each wall portion of the honeycomb structured body. The formation of such irregularities on the surface of each wall portion presumably makes it possible to increase the filtering area, and consequently to reduce a pressure loss at the time of capturing particulates. Moreover, the irregularities allow the exhaust gas flow to form a turbulent flow, making it possible to reduce the temperature difference in the filter and consequently to prevent damages such as cracks due to thermal stress. With reference to the drawings, the honeycomb structured body of the present invention will be explained below. A honeycomb structured body 100 has a cylindrical shape in which a number of through holes 111 with either one of ends being sealed are placed in parallel with one another in the longitudinal direction with a wall portion 113 therebetween. In other words, as shown in The honeycomb structured body of the present invention is, as shown in Here, the expression "the inorganic fiber aggregated bodies are laminated in such a manner that the through holes are superposed on one another" refers to the configuration where through holes formed in the adjacent inorganic fiber aggregated bodies are allowed to communicate with each other. The respective inorganic fiber aggregated bodies may be bonded to each other by using an inorganic adhesive or the like, or may be simply laminated physically; and it is more desirable for the inorganic fiber aggregated bodies to be simply laminated physically. When the inorganic fiber aggregated bodies are simply laminated physically, it is possible to prevent the flow of exhaust gases from being blocked by a joining portion composed of the adhesive or the like, and consequently to prevent the pressure loss from becoming high. Here, in the case of the structure in which the respective inorganic fiber aggregated bodies are simply laminated physically, a laminated body is formed by laminating the inorganic fiber aggregated bodies in a casing (a can-type metal body) to be attached to an exhaust pipe, and a pressure is applied thereto. In the honeycomb structured body of the present invention, desirably, plate members (hereinafter, referred to also as metal plates), mainly made of metal, are laminated on each end of a laminate of the inorganic fiber aggregated bodies. The formation of the metal plates on the both ends of the inorganic fiber aggregated bodies makes the honeycomb structured body hardly subjected to erosion. Moreover, it also becomes possible to prevent a gap from the casing (metal container) and a gap between the mutual inorganic fiber aggregated bodies from being formed at high temperatures (in use) due to a difference in the thermal expansions from the casing (metal container), and consequently to prevent particulates in exhaust gases from leaking and causing a reduction in the capture efficiency of particulates. Moreover, since the strength in the end faces is increased, it becomes possible to prevent damages in the filter from occurring due to a pressure or the like of exhaust gases imposed on the end faces in use. In this case, for example, a metal plate may be used for only the portion at which one of the ends of a through hole is sealed. With respect to the material for the metal plate, not particularly limited, for example, chrome-based stainless steel and chrome-nickel-based stainless steel may be used. Here, through holes need to be formed at predetermined positions of the metal plate. In the honeycomb structured body of the present invention, an inorganic fiber aggregated body 110a on which a catalyst is supported is desirably used. When the catalyst capable of converting toxic gas components in exhaust gases such as CO, HC and NOx is supported on the honeycomb structured body of the present invention, it becomes possible to sufficiently convert the toxic gas components in exhaust gases through the catalyst reaction so that the reaction heat, generated in the catalyst reaction, can be utilized for burning and eliminating the particulates adhered to the wall portions 113. Moreover, by supporting a catalyst that helps to burn the particulates, the particulates can be burned and eliminated more easily. As a result, the honeycomb structured body of the present invention is allowed to improve its purifying function for exhaust gases and reduce energy required for burning the particulates. With respect to the inorganic fiber aggregated body on which a catalyst is supported, the description thereof has been given above. The catalyst may be supported on all the inorganic fiber aggregated bodies, or may be supported on only part of the inorganic fiber aggregated bodies. For example, if the porosity of each of the inorganic fiber aggregated bodies is altered depending on the material of each inorganic fiber aggregated body, the catalyst may be supported only on the inorganic fiber aggregated body that is made to have a high porosity. In this manner, in the honeycomb structured body of the present invention, the amount of the supported catalyst in the longitudinal direction, and the kind of the catalyst are freely altered in accordance with the intended purpose, and consequently, the honeycomb structured body of the present invention can improve its regenerating process and toxic gas purifying and/or converting function. Here, in the above-mentioned honeycomb structured body, the amount of the supported catalyst is desirably 0.01 to 1 g per 10 g of the inorganic fibers. Since the catalyst is supported thereon in this manner, the honeycomb structured body of the present invention is allowed to function as a filter for capturing particulates in exhaust gases, and also to function as a catalyst supporting carrier used for converting CO, HC and NOx contained in the exhaust gases. Here, the honeycomb structured body of the present invention in which the catalyst is supported is allowed to function as a gas purifying device in the same manner as the conventionally known DPFs (Diesel Particulate Filters) with catalyst. Therefore, the detailed explanation of functions of the honeycomb structured body serving as the catalyst supporting carrier is omitted. The desirable porosity of one whole honeycomb structured body of the present invention is the same with the desirable porosity of the above-mentioned inorganic fiber aggregated body of the present invention. With respect to the thickness of the wall portion, although not particularly limited, the lower limit value is desirably set to 0.2 mm and the upper limit value is desirably set to 10.0 mm. The thickness of the wall portion of 0.2 mm or less tends to reduce the strength, while the thickness of the wall portion exceeding 10.0 mm tends to make the pressure loss become high. More desirably, the lower limit value is set to 0.3 mm and the upper limit value is set to 6.0 mm. With respect to the density of the through hole on the cross section perpendicular to the longitudinal direction of the honeycomb structured body of the present invention, although not particularly limited, the lower limit value thereof is desirably set to 0.16 hole/cm2 (1.0 hole/in2), and the upper limit value thereof is desirably set to 62 holes/cm2 (400 holes/in2). The density of less than 0.16 hole/cm2 tends to reduce the filtering area, while the density exceeding 62 holes/cm2 tends to make the pressure loss become too high. The lower limit value of the density of the through hole is more desirably set to 0.62 hole/cm2 (4.0 holes/in2), and the upper limit value thereof is more desirably set to 31 holes/cm2 (200 holes/in2). With respect to the size of the through hole on the cross section perpendicular to the longitudinal direction of the honeycomb structured body of the present invention, although not particularly limited, the lower limit value thereof is desirably set to 0. 8 mm × 0.8 mm, and the upper limit value thereof is desirably set to 16 mm × 16 mm. Moreover, when sheet-shaped inorganic fiber aggregated bodies having different through hole sizes are used and laminated one after another, irregularities are formed on the inner surface of each through hole so that the filtering area becomes greater and thus it is considered that the pressure loss upon capturing particles can be reduced. Moreover, since the irregularities make the flowof exhaust gases to a turbulent flow, it is considered that the temperature difference in the filter can be made smaller to effectively prevent damages due to a thermal stress. The shape of the through hole on the plan view is not particularly limited to a quadrangular shape, and any desired shape, such as a triangle, a hexagon, an octagon, a dodecagon, a round shape and an elliptical shape, may be used. Although the shape of the honeycomb structured body shown in If the filter is installed right below the engine, the filter space is extremely limited, and a complex filter shape is sometimes required. However, in the honeycomb structured body of the present invention, even a complex shape, such as a filter 30 with a concave portion on one side as shown in The honeycomb structured body of the present invention having such a structure can be formed by using a method for manufacturing a honeycomb structured body of the present invention, which will be described later. The following description will explain the method for manufacturing a honeycomb structured body of the present invention. The method for manufacturing a honeycomb structured body of the present invention comprises: carrying out a process for preparing a sheet-shaped mixture by mixing inorganic fibers A with inorganic fibers B and/or inorganic particles C that melt at a temperature at which the inorganic fibers A are neither melted nor sublimated, and then forming the mixture into a sheet shape; carrying out a through hole formation process of forming through holes in the sheet-shaped mixture at the same time with preparation of the sheet-shaped mixture or after preparation of the sheet-shaped mixture; carrying out a process for heating the sheet-shaped mixture with the through holes formed therein at a temperature below a heat resistant temperature of the inorganic fibers A and above a softening temperature of the inorganic fibers B and/or the inorganic particles C so that a sheet-shaped inorganic fiber aggregated body is manufactured; and laminating the inorganic fiber aggregated bodies such that the through holes are superposed on one another. Referring to
In this process, a mixture is prepared by mixing the inorganic fibers A with the inorganic fibers B and/or the inorganic particles C by using the same method as the method explained in the method for manufacturing an inorganic fiber aggregated body of the present invention, and the resulting mixture is formed into a sheet shape through a sheet-forming process and a fiber laminating process explained in the method for manufacturing an inorganic fiber aggregated body of the present invention. (2) Next, a through hole forming process for forming through holes in the sheet-shaped mixture is carried out. More specifically, through holes having desired shapes can be formed at predetermined positions by using, for example, a stamping process. Moreover, in the sheet-forming process in the above-mentioned (1), by using a mesh in which holes having predetermined shapes are formed in a checkered pattern or a mesh inwhichholes are formed at portions corresponding to the through holes, a sheet-shaped mixture in which through holes are formed can be obtained. (3) Next, the heating process at a temperature below the heat resistant temperature of the inorganic fibers A and above the softening temperature of the inorganic fibers B and/or the inorganic particles C is carried out on the sheet-shaped mixture having through holes formed therein so that a sheet-shaped inorganic fiber aggregated body is manufactured. Here, with respect to the heating process, the same method as that of the heating process used in the method for manufacturing the inorganic fiber aggregated body of the present invention may be adopted. Moreover, after the sheet-shaped inorganic fiber aggregated body has been manufactured through the above-mentioned heating process, an acid treatment or quenching treatment may be carried out on the sheet-shaped inorganic fibers, if necessary. (4) Thereafter, the resultingsheet-shapedinorganic fiber aggregated bodies are laminated so that the through holes are superposed on one another so that a honeycomb structured body can be manufactured. More specifically, as shown in In this process, for example, in place of the sheet-shaped inorganic fiber aggregated bodies for both end portions, metal plates having through holes at predetermined positions may be laminated. Thus, a laminating process inwhich the sheet-shaped inorganic fiber aggregated bodies are laminated, with plate members mainly made of metal being laminated on the both ends thereof, is carried out, so that a honeycomb structured body in which the plate members mainly made of metal are laminated on each end of a laminate of the inorganic fiber aggregated bodies can be manufactured. If the honeycomb structured body of the present invention has a structure in which sheet-shaped inorganic fiber aggregated bodies are simply laminated physically in this manner, even if a certain degree of temperature distribution occurs in a honeycomb structured body to, after this honeycomb structured body has been placed in an exhaust passage, the temperature distribution per one sheet-shaped inorganic fiber aggregated body is small so that cracks and the like seldom occur. Furthermore, if sheet-shaped inorganic fiber aggregated bodies, which have different dimensions in the through holes, are manufactured and laminated, the bottomed holes are allowed to form irregularities; thus, bottomed holes having a larger surface area can be formed. With respect to the shape of the through holes, not particularly limited to a square shape, any desired shape, such as a triangle, a hexagon, an octagon, a dodecagon, a round shape and an elliptical shape, may be used. Although the use of the honeycomb structured body of the present invention is not particularly limited, it is desirably applied to, for example, an exhaust gas purifying device for a vehicle. As shown in In the exhaust gas purifying device 200 having the above-mentioned configuration, exhaust gases, discharged from an internal combustion system such as an engine, are introduced into the casing 123 through the introducing pipe 124, and allowed to pass through a wall portion (partition wall) from the through hole of the honeycomb structured body 20 so that, after particulates therein have been captured by this wall portion (partition wall) to purify the exhaust gases, the resulting exhaust gases are discharged outside through the exhaust pipe 125. In addition, when large amount of particles accumulate on the surface of the wall portion (partition wall) of the honeycomb structured body 20, and as a result the pressure loss becomes high, a regenerating process on the honeycomb structured body 20 is carried out by a prescribed means such as post-injection. The following description will discuss the present invention in detail by means of examples, but the present invention is not limited by these examples. First, alumina fibers (50 parts by weight), glass fibers (average fiber diameter: 9 µm, average fiber length: 3 mm) (50 parts by weight) and an organic binder (polyvinyl alcohol based fibers) (10 parts by weight) were dispersed in a sufficient amount of water, and this was sufficiently stirred to prepare a sheet-forming slurry. The slurry obtained in the process (1) was formed into a sheet by using a mesh having a diameter of 143.8 mm, and the resulting matter was dried at 135°C so that a sheet-shaped mixture having a thickness of 1 mm was obtained. Next, a stamping process was carried out on the resulting mixture so that through holes of 4.5 mm × 4.5 mm were formed at 2 mm intervals over almost the entire face of the sheet-shaped mixture. A heating process at 950°C was carried out on the sheet-shaped mixture obtained in the process (2) for one hour so that a sheet-shaped inorganic fiber aggregated body was obtained. The sheet-shaped inorganic fiber aggregated body obtained in the process (3) was immersed in 4 mol/L of a HCl solution at 90°C for one hour so as to carry out an acid treatment thereon, and then a quenching treatment was carried out at 1050°C for 5 hours. The porosity and the like of the sheet-shaped inorganic fiber aggregated body obtained in the processes (1) to (4) are as shown in Table 2. After a Ni-Cr alloy metal plate had been machined into a disc shape of 143.8 mm in diameter × 1 mm in thickness, a laser machining process was further carried out on the resulting plate so that a metal lamination member in which holes of 4.5 mm × 4.5 mm were formed with a checkered pattern was manufactured. Two plates of the metal lamination member were manufactured by this process in such a manner that the locations of holes formed therein were different from each other, thereby, after laminating the metal lamination members in the below-mentioned laminating process, a resulting honeycomb structured body has a configuration in which the sealed parts in the inlet side end face and outlet side end face are different. First, in a separated process, a casing (a can-type metal container) to which a pressing metal member was attached to one side thereof was placed, with the side bearing the pressing metal member attached thereto facing down. After one of the metal lamination members obtained in the process (5) had been laminated, 83 sheets of the sheet-shaped inorganic fiber aggregated bodies obtained in the process (4) were laminated thereon, and lastly, one of the metal lamination members was laminated thereon, and a pressing process was further carried out on the resulting product. Then, another pressing metal member was also placed on the other side and secured thereon so that a honeycomb structured body having a laminated body with a length of 75 mm was manufactured. In this process, the sheet-shaped inorganic fiber aggregated bodies were laminated one after another in such a manner that all the through holes formed in each sheet superpose with those corresponding holes formed in the adjacent sheet, and furthermore; the metal lamination members were laminated in such a manner that the sealed parts in the inlet side end face and outlet side end face of the honeycomb structured body were different (i.e. only one end of the overlapped through holes were sealed). The same processes as those in Example 1 were carried out, except that the shapes of inorganic fibers A (alumina fibers) and inorganic fibers B (glass fibers) were changed to those shown in Table 1, so that a honeycomb structured body was obtained. The same processes as those in Example 1 were carried out, except that the inorganic particles C (glass particles) were used in place of the inorganic fibers B (glass fibers), so that a honeycomb structured body was obtained. The same processes as those in Example 1 were carried out, except that the inorganic fibers shown in Table 1 were used as the inorganic fibers A in place of the alumina fibers, so that a honeycomb structured body was obtained. The same processes as those in Example 1 were carried out, except that the compounding amount of the inorganic fibers A (alumina fibers) and the inorganic fibers B (glass fibers) were set to the amount shown in Table 1, so that a honeycomb structured body was obtained. Alumina fibers (90 parts by weight) was dispersed in a sufficient amount of water, and 10 parts by weight of silica sol and 2.7 parts by weight of acrylic latex as organic binder were added thereto. Further, aluminum sulfate as a coagulant and polyacrylamide as a flocculant, both in a small amount, were added, and sufficiently stirred to prepare a sheet-forming slurry. The slurry obtained in the process (1) was formed into a sheet by using a mesh having a diameter of 143.8 mm, and the resulting product was dried at 150°C, and then a stamping process was carried out thereon as in the process (2) in Example 1 to obtain a sheet-shaped inorganic fiber aggregated body having a thickness of 1 mm in which through holes of 4.5 mm × 4.5 mm were formed over the entire face at 2 mm intervals. The porosity and the like of the sheet-shaped inorganic fiber aggregated body are shown in Table 2. A casing (a can-type metal container) to one side of which a pressing metal member was attached was placed with the side bearing the metal member attached thereto facing down. In the same manner as in Example 1, after laminating one of the metal laminationmembers, 83 sheetsof thesheet-shapedinorganic fiber aggregated bodies were laminated thereon, and lastly, one of the metal lamination members was laminated thereon. A pressing process was further carried out on the resulting product, and then another pressing metal member was also placed on the other side and secured thereon so that a honeycomb structured body comprising a laminated body with a length of 75 mm was obtained. The metal lamination members were laminated in such a manner that the sealed parts in the inlet side end face and outlet side end face of the honeycomb structured body were different (i.e. only one end of the overlapped through holes were sealed). A honeycomb structured body was obtained in the same manner as in Comparative Example 1, except that a Sic fiber having the size shown in Table 1 was used in place of alumina fibers in process (1) of Comparative Example 1 and polyorganosilane was used in place of silica sol in process (2) of Comparative Example 1. (1) Alumina fibers (90 parts by weight) was added to 2.7 parts by weight of acrylic latex as organic binder and a sufficient amount of water to disperse the alumina fibers, and a sheet-forming process and a stamping process were carried out on the resulting alumina fiber-dispersion solution in the same manner as in the process (2) of Comparative Example 1 to obtain an alumina-fiber sheet. (2) Next, the alumina-fiber sheet, obtained in the above-mentionedprocess (1), was immersed in an alcohol solution of iron nitrate (concentration: 0.5mol/L), and the alumina fiber sheet was then taken out, and dried to prepare a sheet-shaped inorganic fiber aggregated body. (3) Next, by using the same method as the process (4) of Comparative Example 1, the metal lamination members and the sheet-shaped inorganic fiber aggregated bodies were laminated on each other to obtain a honeycomb structured body. Theporosityof the sheet-shaped inorganic fiber aggregated body was measured by using a weighing method. In other words, a sample of the sheet-shaped inorganic fiber aggregated body (10 mm × 10 mm × 1 mm) was cut out, and two times of an ultrasonic washing process (for 5 minutes) was carried out on the sample by using ion exchange water. Next, an ultrasonic washing process (for 5 minutes) was carried out for one time on the resulting product by using acetone, and dried at 100°C for 5 hours, and the weight a (g) was measured by using an electronic scale. Next, the volume of only the base member (wall) portion of the inorganic fiber aggregated body was calculated through measurements on the wall thickness of longitudinal one row and lateral one row in the center, the cell width and the height by using an optical microscope to find the volume b (cm3) so that the bulk density c (g/cm3) of only the base member portion of the inorganic fiber aggregated body was found based upon a/b. Successively, the inorganic fiber aggregated body was pulverized into powder (volume: 23.6 cc), and dried at 200°C for 8 hours, and the degree of vacuum was measured for an exhausting time of 40 minutes by using an AutoPycnometer 1320 (made by MicroMeritics, Inc.) in compliance with JIS-R-1620 (1995); thus, the degree of vacuum d (g/cm3) was found, and by subs ti tuting the measured value to an expression of (1- c/d) × 100 (%), the porosity (%) was calculated. Table 2 shows the results. The pore diameter of the honeycomb structured body was measured by using a mercury porosimeter. In other words, a sample of the sheet-shaped inorganic fiber aggregated body (10 mm × 10 mm × 1 mm) was cut out, and two times of an ultrasonic washing process (for 5 minutes) was carried out on the sample by using ion exchange water. Next, an ultrasonic washing process (for 5 minutes) for one time was carried out on the resulting product by using acetone, and dried at 100°C for 5 hours. Next, the pore diameter of the result was measured by using an automatic porosimeter (AutoPore III9405, made by Shimadzu Corp.) in a range of 0.2 to 500 µm, in compliance with JIS-R-1655. In this case, the measurements were carried out for every 0.1 psia in the range of 100 to 500 µm, as well as for every 0.25 psia in the range of 0.2 to 100 µm, and the average pore diameter was obtained from the results of the measurements. Table 2 shows the results. In the same method as the method according to the respective examples and comparative examples, a sheet-shaped inorganic fiber aggregated body without through holes formed therein and having a size of 34 mm × 34 mm × 1 mm was manufactured separately, and with respect to this inorganic fiber aggregated body, both of the ends thereof were secured by jigs, and the tensile strength was measured by using an Instron-type universal tester (5582, made by Instron Corp.). Table 2 shows the results. Each of the same inorganic fiber aggregated bodies as the inorganic fiber aggregated bodies that had been measured by the tensile strength measurements in the above-mentioned evaluation (3) was observed by an SEM (magnification: ×150 to ×3000). With respect to the inorganic matter or the like used for firmly fixing the inorganic fibers to each other, if most of the inorganic matter existed locally on the intersection of the inorganic fibers, this state was evaluated as "existing locally", and if the inorganic matter was expanded over the entire inorganic fibers, this state was evaluated as "coated". Table 2 shows the results. Moreover, SEM observation photographs of the inorganic fiber aggregated bodies according to Examples 1, 10 and 11 as well as Comparative Examples 1 to 3 are shown in An air flow at 70°C was continuously applied to each of the honeycomb structured bodies according to examples and comparative examples at a flow rate 3 m/sec for one hour, and the weights of the honeycomb structured body before and after applying the air flow were measured, and those having a weight reduction of 1% or more were evaluated as "erosion present " while those having a weight reduction of less than 1% were evaluated as "erosion absent". Table 2 shows the results. In compliance with JIS A 1405, the sound absorbency in a range of 100 to 2000 Hz was measured on measuring samples manufactured through the following method. The same processes as those of the respective examples and comparative examples were carried out except that, in Examples 1 to 15 and Comparative Examples 1 to 3, the diameter was changed to 100 mm so that lamination members (sheet-shaped inorganic fiber aggregated bodies and metal lamination members) were manufactured, and these lamination members were laminated so that one metal lamination member was placed on each of the two ends, with 35 sheets of the sheet-shaped inorganic fiber aggregated bodies being placed between the metal lamination members, and this laminated body was next secured into a cylinder (can-type) jig (metal tube: 100 mm in inner diameter, 102 mm in outer diameter and 35 mm in height) so that a measuring sample was manufactured. With respect to the results of measurements, Table 2 shows the sound absorbency at 400 Hz. Here, Table 2 also shows firing conditions in each of the examples and the comparative examples. As clearly indicated by the results shown in Table 2, the inorganic fiber aggregated bodies according to the respective examples had a tensile strength of 0.3 Mpa or more, which was a high level, so that the honeycomb structured bodies using these inorganic fiber aggregated bodies were not subjected to erosion. In contrast, the inorganic fiber aggregated bodies according to the respective comparative examples had a tensile strength of 0.25 Mpa or less, which was a low level, with the result that the honeycomb structured bodies using these inorganic fiber aggregated bodies were subjected to erosion. Moreover, with respect to the inorganic fiber aggregated bodies according to the respective examples and comparative examples, when the results of SEM observation were compared, portions at which the inorganic fibers were firmly fixed to each other by the inorganic matter corresponded to the intersection of the inorganic fibers, with the inorganic matter existing locally on the intersection of the inorganic fibers, in the case of the inorganic fiber aggregated bodies according to Examples; in contrast, silica sol, polyorganosilane and iron nitrate, used for firmly fixing the inorganic fibers to each other, were expanded in a manner so as to coat the entire inorganic fibers, in the case of the inorganic fiber aggregated bodies according to the respective comparative examples (see As clearly indicated by these results, the inorganic fiber aggregated bodies of the present invention have a high tensile strength, and the honeycomb structured bodies, manufactured by using these inorganic fiber aggregated bodies, have a high tensile strength, and are free from the occurrence of erosion so that they are appropriately used as filters. Moreover, when the honeycomb structured bodies (inorganic fiber aggregated bodies) according to the respective examples and comparative examples were compared with each other with respect to the sound absorbency, it was found that, as shown in Table 2, the honeycomb structured bodies according to the respective examples had a superior absorbing rate in comparison with the honeycomb structured bodies according to the respective comparative examples. Here, although Table 2 only shows the results of measurements at 400 Hz, the same tendency was observed in the overall frequency bands (100 to 2000 Hz) that had been measured. Although the reason for these results obtained on the sound absorbency is not clearly verified, the following reason is considered. In other words, in the inorganic fiber aggregated body, it is considered that the sound absorbing function is exerted by the fact that sound energy, directed to the inorganic fiber aggregated body, is transmitted to the inside of the inorganic fiber aggregated body to make the fibers and air rattle so that the energy is converted into thermal energy. Here, upon comparison with two cases, that is, the case like the inorganic fiber aggregated bodies of Examples, in which the inorganic matter used for firmly fixing the inorganic fiber aggregated bodies to each other exists locally, and the case like the inorganic fiber aggregated bodies of the respective comparative examples, in which the entire inorganic fiber aggregated bodies are coated with the inorganic matter, the inorganic fibers tend to be more easily warped and more easily rattled if the inorganic matter exists locally; therefore, it is considered that the amount of energy to be converted from sound energy to thermal energy increases so that the inorganic fiber aggregated bodies of the respective examples exert a superior sound absorbing characteristic. As described above, the honeycomb structured body of the present invention is superior in strength and resistance to erosion as a filter, and is also capable of absorbing noise in an internal combustion engine; therefore, it is appropriately applicable to an exhaust gas purifying device. In addition, when the honeycomb structured body of the present invention is used for the exhaust gas purifying device, the honeycomb structured body exerts a strong bonding strength between the inorganic fiber aggregated bodies (high tensile strength), and is hardly subjected to adverse effects caused by rattling, pressure of exhaust gases and the like (scattered inorganic fibers due to cracks and the like and deterioration in the fibers); therefore, the sound-absorbing effect is hardly deteriorated.
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