CORDIERITE HONEYCOMB STRUCTURAL BODY AND METHOD OF PRODUCING THE SAME

Abstract: Cordierite honeycomb structural bodies having coefficient of thermal expansion of not more than l.l0×l0⁻⁶, excellent thermal shock resistance and very low leakage, suitable for use in heat exchangers are provided by using very fine particles of kaoline and talc in the manufacturing process and/or by restricting total pore volume of pore diameter 5µm or over to not more than 0.04 cc/g in the product. Talc of average particle diameter not more than 7 µm, preferably 5 µm, and kaoline of average particle diameter of not more than 2 µm and not more than l/3 of the talc particle diameter is used. Preferably alumina and/or aluminium hydroxide used has an average particle diameter of not more than 2 µm, and preferably silica if used is highly pure amorphous silica of average particle diameter of not more than 8 µm.

Description: The present invention relates to a cordierite honeycomb structural body, particularly to a highly airtight cordierite honeycomb structural body for rotary accumulative type or heat transfer type heat exchanger, which is excellent in thermal shock resistance, airtight property and heat resistance, and a method of producing the same. Lately, the desire for materials of excellent heat resistance and thermal shock resistance has increased along with improvement of industrial technology. The thermal shock resistance of ceramics is influenced not only by the coefficient of thermal expansion, heat transfer coefficient, strength, elastic modulus, Poisson's ratio and so on in a material, but also by the size, shape and heating and cooling conditions in an article. It is known that among these factors influencing the thermal shock resistance, particularly, the contribution of the coefficient of thermal expansion is large; above all when heat flow rate is large, the thermal shock resistance is largely influenced only by the coefficient of thermal expansion. Therefore, there is a strong demand for the development of low thermal expansion materials of excellent thermal shock resistance. Heretofore, cordierite ceramics has been known as a relatively low thermal expansion material. However, the densification of cordierite ceramics by sintering is difficult in general. Especially, for producing low thermal expansion cordierite ceramics of the coefficient of thermal expansion of 2.0×l0⁻⁶/°C or less in a temperature range of from room temperature to 800°C, batch composition of raw materials have to be used wherein an amount of impurities such as alkali, (calcium, potassium, sodium) and the like which function as fluxes in firing the batch composition is limited to a very minor amount, so that cordierite honeycomb structural bodies obtained by firing the batch composition have very few amount of glass phases and hence are very porous and not dense. Accordingly, when such a cordierite ceramics is, for example, formed into a honeycomb structure to apply to a rotary accumulative type heat exchanger, fluid leakage is generated due to high porosity of open pores between the heating fluid and the heat recovery fluid because of pores, particularly communicated pores, on the surface of the partition wall defining the penetration holes of the honeycomb structural body, with the result that the heat exchanging efficiency and all efficiency of heat exchanger-using system decrease, which is a serious defect. Thus, there has been a demand for a low thermal expansion and highly airtight cordierite honeycomb structural body having an improved thermal shock resistance. Heretofore, low thermal expansion of cordierite ceramics has been publicly known. For instance, US-A-3,885,977 specification discloses an oriented cordierite ceramics of coefficient of thermal expansion of less than ll×l0⁻⁷(l/°C) in at least one direction in a temperature range of 25-l,000°C, and describes a planar orientation given to platelets during processing of stacked clay, which can be platelet clay as a cause of such orientation. However, the cordierite ceramics uses coarse talc of particle diameter of l0-20 µm and clay of wide particle diameter of 0.l-l0 µm, and also there is no disclosure about porosity. Further, US-A-3,950,175 specification discloses that a porous cordierite ceramics having open pores of a pore diameter of more than l0 µm can be obtained in an amount of at least 20% by substituting silica or silica alumina source material such as pyrophyllite, kyanite, quartz or fused silica for a partial or whole amount of talc or clay in raw materials. However, control of pore size to lower levels is not described at all. Furthermore, US-A-4,280,845 discloses a positive correlation between the average particle diameter of talc particles and the average pore diameter of pores in cordierite ceramics. However, in order to restrict the coefficient of thermal expansion to not more than l.0×l0⁻⁶/°C, the talc particles should be made coarse as the average particle diameter of l0-50 µm. Therefore, this disclosure does not lead to the combination of low thermal expansion and high airtightness in a cordierite ceramic. Finally, US-A-4,489,774 discloses a rotary accumulative type heat exchanger consisting of highly airtight cordierite ceramics, wherein a filling material adhesively seals open pores on the surface of the partition wall defining the penetration holes of the honeycomb structural body consisting essentially of cordierite ceramics of a porosity of 20-45%. This method described requires further steps such as applying a slip as a filling material on the fired cordierite honeycomb and refiring it, so that there are drawbacks of risk of plugging of holes of the cells and of high cost. The present invention aims to obviate or reduce these shortcomings and provide a highly airtight cordierite honeycomb structural body attaining compatibility between the airtight property and the low thermal expansion property, which is difficult in the conventional cordierite ceramics. The invention in one aspect provides a cordierite honeycomb structural body as set out in claim 1. The invention also provides a method of producing a cordierite honeycomb structural body as set out in claim 9. The invention further provides a cordierite honeycomb structural body as set out in claim 4. The average particle diameters specified here for talc and kaolin are average diameters calculated by weight where blends are used, of all talc or kaolin used, including calcined talc or kaolin. Reference is now made to the accompanying drawings, in which:- Fig. l is a graph showing a relation between the amount of pores of a pores diameter of not less than 5 µm and the leakage amount of test pieces under air pressure of l.4 kg/cm². Fig. 2 is a graph showing particle size distribution curves of fine talc raw materials used in the following Examples, in particular talcs (A) to (F) of Table l; Fig. 3 is a graph showing particle size distribution curves of fine kaoline raw materials used in the following Examples, in particular kaolines (A) to (E) of Table l; Figs. 4A and 4b are schematic perspective views showing an embodiment of rotary accumulating type heat exchanger of the present invention; and Fig. 5 is a graph showing relations between the amount of pores of a pore diameter of not less than 2 µm and the leakage amount of test pieces under air pressure of l.4 kg/cm². The inventors have found out that in a cordierite honeycomb structural body, the total pore volume of a pore diameter of not less than 5 µm is restricted to not more than 0.04 cm³/g by using very fine talc of an average particle diameter of not more than 7 µm and very fine kaolin of an average particle diameter of not more than 2 µm and not exceeding one-third (l/3) of the average particle diameter of talc, whereby a substantially airtight cordierite honeycomb structural body of the coefficient of thermal expansion of not more than l.0×l0⁻⁶/°C in a temperature range of 40-800°C is produced. Restriction of the average particle diameter of talc particles to not more than 7 µm is effective for making the pore diameter small. Further, use of kaolin particles of an average particle diameter of not more than 2 µm is effective for controlling the porosity. Furthermore, use of a combination of talc particles of an average particle diameter of not more than 7 µm and kaolin particles of an average diameter of not exceeding one-third (l/3) of the average particle diameter of talc accelerates the orientation of the cordierite crystals at the partition walls of the cordierite honeycomb structural body, which contributes to low thermal expansion. According to the invention, the honeycomb structural body has a chemical composition in weight basis of 42-56%, preferably 47-53% of SiO₂, 30-45%, preferably 32-38% of Al₂O₅ and l2-l6%, preferably l2.5-l5% of MgO. The honeycomb structure may additionally contain unavoidably present components such as TiO₂, CaO, KNaO, Fe₂O₃ in total amount of not more than 2.5%, and the content of P₂O₅ is less than 0.l%. The reason for restricting these contents is due to the fact that the crystalline phases consist essentially of cordierite crystal phase and the high thermal expansion glass phases resulting from impurities are removed. The inventors have also found that in firing of the dried extruded article of honeycomb structure, the heating temperature is preferably elevated at an average temperature increase rate of 20-60°C/hr in a temperature range of l,l00-l,350°C immediate before the firing, and thereafter it is preferably fired at a temperature of l,350-l,440°C. If the average temperature increase rate is less than 20°C/hr, the coefficient of thermal expansion may be large, and if it exceeds 60°C/hr, the pore diameter may become large in firing, as a result of which the airtight property is deteriorated. Usually, it is preferably within a range of 30-50°C/hr. In addition, the crystal phases can consist essentially of cordierite crystal phase by firing the dried extruded article at a temperature of l,350-l,440°C. The reason for restricting the preferred cell density per unit area to not less than 62 cells/cm² (400 cells/in²) and restricting the partition wall thickness to not more than 203 µm (8 mil) in the honeycomb structural body is due to the fact that the honeycomb structural body can act as a heat exchanger of high heat exchanging efficiency and low pressure loss. The cell shape is not intended to be a limitations but can usually be a triangle, square, rectangle, hexagon, or others, and provided with a fin, and so on. The reason for restricting the total pore volume of a pore diameter of not less than 5 µm to not more than 0.04 cm³/g, preferably not more than 0.026 cm³/g is due to the fact that the fluid leakage through thin walls of the honeycomb structure as shown in Figs. 4a and 4b results mainly from pores of a pore diameter of not less than 5 µm. In order to satisfy this condition, the porosity is preferably restricted to not more than 30%. Referring to Figs. 4a and 4b, a rotary cordierite heat regenerator l of heat accumulator type comprises a plurality of matrix segments 2 of honeycomb structural body, each of which matrix segments 2 mainly consists of cordierite. The present invention includes use of calcined talc and calcined kaolin which are effective for preventing the formation of cracks in the honeycomb structural bodies caused by shrinkage, etc. during the drying and firing steps, when using the fine raw materials kaolin and talc. Use of a higher temperature for calcining talc and kaolin incurs increase of porosity and the coefficient of thermal expansion of the resultant honeycomb structural bodies, so that the calcining temperature should preferably be made as low as possible, if calcined talc or kaolin is used. The unexpected effects of the present invention cannot be attained, unless the same limits for the fine particles of calcined talc and kaolin are used as for the raw talc and kaolin. The other raw materials for cordierite, namely, an alumina source material such as alumina, aluminum hydroxide, etc., and a silica source material such as amorphous silica, quartz sand, etc., can be the same as those used conventionally. However, the amount of impurities such as alkali in the chemical composition of the raw materials should be properly adjusted, and particle size of raw materials should be properly adjusted by excluding coarse particles, depending on thickness of the partition walls of the honeycomb structural bodies to be produced. Thus, according to the invention, there may be produced a cordierite honeycomb structural body comprising crystal phases consisting essentially of cordierite, total pore volume of a pore diameter of not less than 5 µm of not more than 0.04 cm³/g and coefficient of thermal expansion of not more than l.0×l0⁻⁶/°C in a temperature range of 40-800°C, by preparing the mixture of very fine talc of an average particle diameter of not more than 7 µm, very fine kaolin of an average particle diameter of not more than 2 µm and not exceeding one-third (l/3) of the average particle diameter of talc and other raw materials so as to produce the cordierite of a chemical composition on a weight basis of 42-56%, preferably 47-53% of SiO₂, 30-45%, preferably 32-38% of Al₂O₃, l2-l6%, preferably l2.5-l5% of MgO and less than 0.1% P₂O₅, adding a plasticizer and a binder to the mixture to obtain a plasticized deformable batch, extruding the plasticized batch to form a honeycomb structural body, drying the extruded honeycomb structural body, elevating the temperature of the dried honeycomb structural body at an average temperature increase rate of 20-60°C/hr in a temperature range of l,l00-l,350°C immediately before the firing , and firing the honeycomb structural body at a temperature of l,350-l,440°C. The firing process is usually carried out for about 0.5-l2 hours. The resulting honeycomb structural body has crystalline phases consisting essentially of not less than 90% by weight of cordierite. Other crystals consist essentially of mullite and spinel inclusive of sapphirine, but the amounts thereof are not more than 2.5% by weight, respectively. When the total pore volume of a pore diameter of not less than 5 µm is not exceeding 0.04 cm³/g, particularly not exceeding 0.026 cm³/g, the honeycomb structural body substantially exhibits an airtight property of a leakage amount of not more than l00 g/sec.m², particularly not more than 50 g/sec.m² under pressure of l.4 kg/cm², and consequently is suitable to be used for a heat exchanger. The inventors have also found out that, if in a cordierite honeycomb structure, the total pore volume of a pore diameter of not less than 2 µm is restricted to not more than 0.05 cm³/g or the total pore volume of a pore diameter of not less than 2 µm in the porosity of less than 25% to not more than 0.08 cm³/g by using very fine talc of an average particle diameter of not more than 5 µm, very fine kaolin of an average particle diameter of not more than 2 µm and not exceeding one-third (l/3) of the average particle diameter of talc, and very fine alumina and/or aluminum hydroxide of an average particle diameter of not more than 2 µm, or highly pure amorphous silica of an average particle diameter of not more than 8 µm therewith, a substantially airtight cordierite honeycomb structural body of the coefficient of thermal expansion of not more than l.0×l0⁻⁶/°C in a temperature range of 40-800°C is produced. Restriction of the average particle diameter of talc particles to not more than 5 µm is effective for making the pore diameter small. Further, use of kaolin particles of an average particle diameter of not more than 2 µm is effective for controlling the porosity. Furthermore, use of a combination of talc particles of an average particle diameter of not more than 5 µm and kaolin particles of an average diameter of not exceeding one-third (l/3) of the average particle diameter of talc accelerates the orientation of the cordierite crystals at the partition walls of the cordierite honeycomb structural body, which contributes to low thermal expansion. Use of alumina and/or aluminum hydroxide particles of an average particle diameter of not more than 2 µm attains fine pore diameter and low porosity, which has not been obtained only with the use of fine talc and kaolin particles. Further, among various aluminas, alumina of low sodium content is used to obtain a stable fine pore diameter. Restriction of the average particle diameter of highly pure amorphous silica particles to not more than 8 µm is effective for controlling the pore diameter and the porosity. In this case, it is important that the silica is amorphous. For instance, crystalline silicas such as quartz and so on unfavorably result in the considerable increase in porosity. In addition to fine alumina and/or aluminum hydroxide particles, use of amorphous silica gives considerably low thermal expansion. Control of the particle sizes of the raw materials such as alumina and/or aluminum hydroxide or amorphous silica renders it unnecessary to especially limit the average temperature raising rate, so that the rapid rate of not less than 50°C/hr capable of attaining low thermal expansion becomes applicable, which is seen to be preferred to improve the thermal shock resistance. In addition, the crystalline phases can consist essentially of cordierite crystalline phase by firing the honeycomb structural body at a temperature of l,350-l,440°C. The reason for as is preferred restricting the total pore volume of a pore diameter of not less than 2 µm to not more than 0.05 cm³/g, preferably not more than 0.03 cm³/g or the total pore volume of a pore diameter of not less than 2 µm in the porosity of less than 25% to not more than 0.08 cm³/g, preferably not more than 0.05 cm³/g is due to the fact that the fluid leakage through thin walls of the honeycomb structure as shown in Figs. 4a and 4b results mainly from pores of a pore diameter of not less than 2 µm. In order to satisfy this condition, the porosity is preferably restricted to not more than 30%, preferably not more than 25%. The present invention can include use of calcined talc and calcined kaoline even when using fine alumina and/or aluminum hydroxide. Thus, according to the invention, there is produced a cordierite honeycomb structural body comprising crystalline phases consisting essentially of cordierite, total pore volume of a pore diameter of not less than 2 µm of not more than 0.05 cm³/g or, in the porosity of less than 25%, of not more than 0.08 cm³/g, and coefficient of thermal expansion of not more than l.0×l0⁻⁶/°C in a temperature range of 40-800°C, by preparing the mixture of very fine talc of an average particle diameter of not more than 5 µm, very fine kaolin of an average particle diameter of not more than 2 µm and not exceeding one-third (l/3) of the average particle diameter of talc, very fine alumina and/or aluminum hydroxide of an average particle diameter of not more than 2 µm and other raw materials so as to produce the cordierite of a chemical composition in weight basis of 42-56%, preferably 47-53% of SiO₂, 30-45%, preferably 32-38% of Al₂O₃, l2-l6%, preferably l2.5-l5% of MgO, and less than 0.1% 8₂O₅, adding a plasticizer and a binder to the mixture to obtain a plasticized deformable batch, extruding the plasticized batch to form a honeycomb structural body, drying the extruded honeycomb structural body, and firing the honeycomb structural body at a temperature of l,350-l,440°C. Further, according to the invention there is produced a cordierite honeycomb structural body comprising crystalline phases consisting essentially of cordierite, total pore volume of a pore diameter of not less than 2 µm of not more than 0.05 cm³/g or, in the porosity of less than 25%, of not more than 0.08 cm³/g, and coefficient of thermal expansion of not more than 0.6×l0⁻⁶/°C in a temperature range of 40-800°C, by preparing the mixture of very fine talc of an average particle diameter of not more than 5 µm, very fine kaolin of an average particle diameter of not more than 2 µm and not exceeding one-third (l/3) of the average particle diameter of talc, very fine alumina and/or aluminum hydroxide of an average particle diameter of not more than 2 µm, highly pure amorphous silica of an average particle diameter of not more than 8 µm and other raw materials so as to produce the cordierite of a chemical composition in weight basis of 42-56%, preferably 47-53% of SiO₂, 30-45%, preferably 32-38% of Al₂O₃, l2-l6%, preferably l2.5-l5% of MgO and less than 0.1% 1'₂O₅, adding a plasticizer and a binder to the mixture to obtain a plasticized deformable batch, extruding the plasticized batch to form a honeycomb structural body, drying the extruded honeycomb structural body, and firing the honeycomb structural body at a temperature of l,350-l,440°C. When the total pore volume of a pore diameter of not less than 2 µm is not exceeding 0.05 cm³/g, particularly not exceeding 0.03 cm³/g, the honeycomb structural body substantially exhibits an airtight property of a leakage amount of not more than 30 g/sec.m², particularly not more than 20 g/sec.m² under pressure of l.4 kg/cm², and consequently is excellent for use for a heat exchanger. In the honeycomb structural body according to the invention, the total pore volume of a pore diameter of not less than 5 µm is not more than 0.04 cm³/g and the porosity is preferably not more than 30%, so that the leakage amount is small and the heat exchanging efficiency is high. Further, the coefficient of thermal expansion is small as not more than l.0×l0⁻⁶/°C in a temperature of 40-800°C, so that the thermal shock resistance is high. Further, in the preferred honeycomb stuctural body according to the invention, the total pore volume of a pore diameter of not less than 2 µm is not more than 0.05 cm³/g or the total volume pores of a pore diameter of not less than 2 µm in the porosity of less than 25% is not more than 0.08 cm³/g, so that the leakage amount is smaller and the heat exchanging efficiency is higher. In addition, the coefficient of thermal expanion is small as not more than l.0×l0⁻⁶/°C, preferably very small as not more than 0.6×l0⁻⁶/°C in a temperature of 40-800°C, so that the thermal shock resistance is very higher. Furthermore, it is possible to make the walls thin as the partition wall thickness of not more than 203 µm (8 mil), so that the pressure loss is small and a high density of cells is attained. Therefore, a ceramic heat exchanger of high heat exchanging efficiency can be obtained. Furthermore, the pore diameter is small and the porosity is also small, so that the strength of the honeycomb structural body is higher than that of the conventional honeycomb structural body and consequently thin walls can be attained. Hereinafter, the present invention will be explained in more detail with reference to Examples. Example l Raw materials of chemical compositions and particle sizes as shown in the following Table l are mixed based on the batch composition and the particle sizes of talc and kaolin as shown in Table 2 to prepare batches of Nos. l-30 of Table 2. To l00 parts by weight of each batch is added 4.5 parts by weight of methylcellulose and a suitable amount of water and the resulting mixture is kneaded to produce an extrudable blend. The used raw materials are preliminarily passed through a sieve of 63 µm. Thereafter, each blend of the respective batch is formed by a publicly known extrusion means into a cylindrical honeycomb structural body of a diameter of 93 mm and a height of l00 mm having a rectangular cell structure (short side/long side = l/l.73), a cell number of l67 per cm² and a partition wall thickness of l20 µm. Each honeycomb structural body is dried and fired at a firing condition as shown in Table 2 to obtain sintered honeycomb structural bodies. The sintered bodies are evaluated on coefficient of thermal expansion (CTE) in a temperature range of 40-800C, porosity, total pore volume of pore diameter of not less than 5 µm, leakage amount, amount of cordierite crystals, and thermal shock resistance as properties of the sintered bodies. The results of the evaluation are also shown in Table 2. All the sintered bodies have chemical compositions of P₂O₅ content of less than 0.l%. Example 2 The batch of No. 4 of Table 2 are extruded from dies of different cell structures in the same manner as in Example l, and the green honeycomb structural bodies are fired to produce cylindrical honeycomb structural bodies of Nos. 3l-40 of a diameter of 93 mm and a height of l00 mm and cell structures as shown in Table 3. Thus produced honeycomb structural bodies are evaluated on pressure loss, leakage amount and coefficient of thermal expansion (CTE). The results of the evaluation are also shown in Table 3. Referring now to the graph of Fig. l, this shows the relationship between total pore volume (cm³/g) of a pore diameter of not less than 5 µm and the leakage of test pieces (g/sec.m²) measured in the manner described as a note to Table 2, both for samples taken from Table 2 and for other samples on which pore size measurements and leakage measurements were made in the same way. Example 3 Raw materials of chemical compositions and particle sizes as shown in the following Table 4 are mixed based on the batch composition and the particle sizes of talc and kaolin as shown in Table 5 to prepare batches of Nos. l0l-l40 of Table 5. To l00 parts by weight of the mixed batch of the raw materials are added 4.5 parts by weight of methylcellulose and a suitable amount of water and the resulting mixture is kneaded to produce an extrudable blend. The raw materials are all preliminarily treated to pass a sieve of 63 µm. Then, each blend of the respective batch is formed by a publicly known extrusion means into a cylindrical honeycomb structural body of a diameter of 93 mm and a height of l00 mm having a partition wall thickness of l20 µm, a cell number of l67 per cm² and a rectangular cell structure (short side/long side = l/l.73). The honeycomb structural bodies of respective batches are dried and fired at firing conditions shown in Table 5 to obtain sintered honeycomb structural bodies. The sintered bodies are evaluated on coefficient of thermal expansion (CTE) in 40-800°C, total pore volume of a pore diameter of not less than 2 µm, leakage amount, amount of cordierite crystals, and thermal shock resistant as properties of the sintered bodies. The results of the evaluation are also shown in Table 5. All the sintered bodies have chemical compositions of P₂O₅ content of less than 0.l%. As seen from Table 5, test Nos. l02-l05, l08-ll5, ll7, ll8 and l20 wherein talc of an average particle diameter of not more than 5 µm and kaolin of an average particle diameter of not more than 2 µm and not exceeding one-third (l/3) of the average particle diameter of talc, are mixed with alumina and/or aluminum hydroxide of an average particle diameter of not more than 2 µm, and test Nos. l22, l23, l25, l27 and l29-l40, wherein highly pure amorphous silica of an average particle diameter of not more than 8 µm is further added thereto, satisfy the total pore volume of a pore diameter of not less than 2 µm preferred according to the invention. Further, relations between the leakage amount and the total pore volume of a pore diameter of not less than 2 µm in test pieces are determined at both porosities of less than 25% and not less under air pressure of l.4 kg/cm² respectively to obtain results shown in Fig. 5. In Fig. 5, white circles represent data on the porosity of less than 25% and black circles represent data on the porosity of not less than 25%. As seen from Fig. 5, in order to achieve the leakage amount of 30 g/sec.m² preferred in the invention, it is required that when the porosity is less than 25%, the total pore volume of a pore diameter of not less than 2 µm is not more than 0.08 cm³/g and when it is not less than 25%, such a total volume is not more than 0.05 cm³/g. Examples of test Nos. l0l and l24 use talc of a particle diameter of not less than 5 µm, test No. l06 uses alumina of a particle diameter of not less than 2 µm, test Nos. ll6, ll9 and l26 use aluminum hydroxide of a particle diameter of not less than 2 µm, test No. l07 makes a ratio of average particle diameter of kaolin to that of talc not less than one-third (l/3), test No. l2l makes an amount added of silica not less than 8.0% and test No. l28 uses crystalline silica as a silica source material, so that the preferred total pore volume and/or the thermal expansion property are not satisfied. Example 4 Batch of test No. l36 in Table 5 is formed into honeycomb structures by extrusion from dies of different cell structures in the same way as in Example 3, and the extruded articles are dried and fired to produce cylindrical honeycomb structural bodies Nos. l4l-l50 of a diameter of 93 mm and a height of l00 mm having cell structure as shown in Table 5. Respective honeycomb structural bodies are evaluated on pressure loss, leakage amount and coefficient of thermal expanion (CTE). the results of evaluation are shown in the following table 6 and the shape of the resulting honeycomb structural body is represented in Fig. 4. As described in Tables 2 and 3, according to the invention, it is possible to obtain a honeycomb structural body of total pore volume of a pore diameter of not less than 5 µm of not more than 0.04 cm³/g and coefficient of thermal expantion (CTE) of not more than l.0×l0⁻⁶/°C, which has an improved thermal shock resistance, a small leakage amount and an improved heat exchanging efficiency. According to the invention, it is also possible to obtain a honeycomb structure of total pore volume of a pore diameter of not less than 2 µm of not more than 0.05 cm³/g or, in the porosity of less than 25%, of not more than 0.08 cm³/g and coefficient of thermal expansion of not more than l.0×l0⁻⁶/°C or 0.6×l0⁻⁶/°C, which has an improved thermal shock resistance, a very small leakage amount and an improved heat exchanging efficiency. The cordierite honeycomb structural bodies according to the invention are very useful in a broader field.

Claims: A cordierite honeycomb structural body comprising a crystalline phase consisting essentially of cordierite, total pore volume of a pore diameter of not less than 5 µm of not more than 0.04 cm³/g, coefficient of thermal expansion of not more than 1.0x10 -6 /°C in the temperature range of 40-800°C, and a chemical composition on a weight basis of 42-56% of SiO₂, 30-45% of Al₂O₃ and 12-16% of MgO as main components and less than 0.1% of P₂O₅. A cordierite honeycomb structural body as defined in claim 1, wherein the total volume of a pore diameter of not less than 5 µm is not more than 0.026 cm³/g. A cordierite honeycomb structural body as defined in claim 1 or claim 2 wherein the cell density per unit area is not less than 62 cells/cm² (400 cells/in²) and the partition wall thickness is not more than 203 µm (8 mil). A cordierite honeycomb structural body, comprising a crystalline phase consisting essentially of cordierite, total pore volume of a pore diameter of not less than 2 µm of not more than 0.05 cm³/g or, in the case where the porosity is less than 25%, of not more than 0.08 cm³/g, coefficient of thermal expansion of not more than 1.0x10 -6 /°C in the temperature range of 40-800°C, and a chemical composition on a weight basis of 42-56% of SiO₂, 30-45% of Al₂O₃ and 12-16% of MgO as main components and less than 0.1% of P₂O₅. A cordierite honeycomb structural body as defined in claim 4, wherein the total volume of a pore diameter of not less than 2 µm is not more than 0.03 cm³/g or, in the case where the porosity is less than 25%, of not more than 0.05cm³/g. A cordierite honeycomb structural body as defined in claim 4 or claim 5, wherein the cell density 2 per unit area is not less than 62 cells/cm² (400 cells/in²) and the partition wall thickness is not more than 203 µm (8 mil). A cordierite honeycomb structural body as defined in any one of claims 4 to 6 wherein the coefficient of thermal expansion is not more than 0.6x10 -6 /°C in the temperature range of 40-800°C. A heat exchanger including at least one cordierite honeycomb structural body as defined in any one of claims 1 to 7 as heat exchanging element. A method of producing a cordierite honeycomb structural body as defined in any one of claims 1 to 7, comprising preparing a mixture including very fine talc of an average particle diameter of not more than 7 µm and very fine kaolin of an average particle diameter of not more than 2 µm not exceeding one-third (1/3) of the average particle diameter of talc and other raw material(s) suitable to produce a cordierite of a chemical composition on a weight basis of 42-56% of SiO₂, 30-45% of Al₂O₃, 12-16% of Mgo and less than 0.1% of P₂O₅, adding a plasticizer and a binder to the mixture so as to obtain a plasticized deformable batch, extruding the plasticized batch to form a honeycomb structural body, drying the extruded honeycomb structural body, and firing the dried honeycomb structural body at a temperature of 1,350-1,440°C. A method as defined in claim 9 wherein the dried honeycomb structural body is fired at an average temperature increase rate of 20-60°C/hr in the temperature range of 1,100-1,350°C immediately before the firing at the temperature of 1,350°C-1,440°C. A method as defined in claim 9 or claim 10, wherein the very fine talc used has an average particle diameter of not more than 5 µm. The method as defined in any one of claims 9 to 11 wherein the kaolin used has an average particle diameter of not more than 1 µm. A method as defined in claim 9 wherein the very fine talc used has an average particle diameter of not more than 5 µm, and the mixture also includes very fine alumina and/or aluminum hydroxide of an average particle diameter of not more than 2 µm. A method as defined in claim 13 wherein the very fine talc used has an average particle diameter of not more than 5 µm, and the mixture includes very fine alumina and/or aluminium hydroxide of an average particle diameter of not more than 2 µm and highly pure amorphous silica of an average particle diameter of not more than 8 µm. The method as defined in claim 14 wherein the amount added of the highly pure amorphous silica is not more than 8%. A method as defined in claim 13 or claim 14 wherein the content of Na₂O in the alumina used in the raw materials for producing the cordierite is not more than 0.12%. A method as defined in claim 13 or claim 14 wherein the kaolin used has an average particle diameter of not more than 1 µm.

Inventors: ASAMI, SEIICHI, HAMAGUCHI, KUNIKAZU, HAMANAKA, TOSHIYUKI, WATANABE, KEIICHIRO
Applicants: NGK INSULATORS, LTD.
Application year: 1986
Main classification: B01J35
IPC codes: B01J35, C04B35, C04B38, B01J21