Method of producing cordierite honeycomb ceramics

Abstract: Cordierite honeycomb ceramics comprises a cordierite phase as a main ingredient. A value of [Fe 2 O 3 wt%/(MgO wt%+Fe 2 O 3 wt%)]x100 is 2-10, where a ferric component is calculated as Fe 2 O 3 . A thermal expansion coefficient of the cordierite honeycomb ceramics is less than 0.5x10 -6 /°C within a temperature range of 40-800°C in a direction parallel to a flow passage of a honeycomb body. In the cordierite honeycomb ceramics including a ferric component, a ferric component is added from a talc as raw materials. The cordierite honeycomb ceramics having a low thermal expansion coefficient is obtained by mixing raw materials for cordierite generation to obtain a batch, extruding the batch into a honeycomb formed body, drying the honeycomb formed body, and firing the dried honeycomb formed body at a temperature range of 1350-1450°C. After that, the sintered honeycomb body is immersed in an acid solution if necessary.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of producing a cordierite honeycomb ceramics having a low thermal expansion coefficient and an excellent heat resistivity, which is especially used as a catalyst carrier for an exhaust gas from automobiles and a filter for trapping soots included in an exhaust gas from a diesel engine. Related Art Statement Cordierite honeycomb ceramics have excellent heat resistivity, and thus they are used for a catalyst carrier for an exhaust gas from automobiles and a filter for trapping soots included in an exhaust gas from a diesel engine. In these fields, a high thermal shock resistivity is also required in addition to an excellent heat resistivity. If a thermal shock resistivity is low, various problems, such as a crack generation and so on, occur due to rapid heating, rapid cooling and temperature difference in a body of cordierite honeycomb ceramics. Therefore, the life of cordierite honeycomb ceramics is shortened. In order to make a thermal shock resistivity high, it is necessary to make the thermal expansion coefficient of cordierite honeycomb ceramics low. Heretofore, various experiments are performed for making the thermal expansion coefficient of cordierite honeycomb ceramics low. Japanese Patent Laid-open Publication No. 64-3067 (JP-A-64-3067) discloses a method of making a thermal expansion coefficient of cordierite honeycomb ceramics low by controlling the particle size of raw materials for cordierite generation such as talc, kaolin and silica so as to improve a thermal-shock resistivity. However, in this method, it is necessary to limit porosity to 30-42% so as to maintain an excellent thermal shock resistivity, and thus a filter made of cordierite honeycomb ceramics according to this method is not preferably used for the filter for trapping soots included in an exhaust gas from a diesel engine. Moreover, Japanese Patent Laid-open Publication No. 49-129704(JP-A-49-129704) discloses a method such that a body of cordierite ceramics is brought into contact with an acid solution so as to improve thermal shock resistivity of the body of cordierite ceramics. In the body of cordierite ceramics treated by an acid solution, thermal shock resistivity is temporarily improved, since the thermal expansion coefficient is lowered. However, if the body is maintained for a long time in a high temperature condition such as the filter for trapping soots included in an exhaust gas from a diesel engine, the thermal expansion coefficient becomes high again, and thus an effect of improving a thermal shock resistivity disappears. Generally, in order to make the thermal expansion coefficient low on producing cordierite honeycomb ceramics, amounts of impurity components in raw materials are lowered. For example, if an amount of CaO component including in raw materials for cordierite generation is increased, the thermal expansion coefficient of cordierite honeycomb ceramics becomes high. Moreover, as for a ferric component and the like, it is generally understood that it is better that a ferric component and the like is not included in cordierite honeycomb ceramics. This is because a ferric component is liable to be dissolved in a solid state in chlorite, which exists in talc as an impurity component. That is to say, a chlorite crystal having a ferric solid soltion becomes a glass phase by firing, and becomes a cause of increasing thermal expansion coefficient of cordierite honeycomb ceramics. Japanese Patent Laid-open Publication No. 61-178038 (JP-A-61-178038) discloses a catalyst carrier in which Fe 2 O 3 component is mixed in cordierite honeycomb ceramics and a coloring of the catalyst carrier is performed while maintaining a thermal expansion coefficient under 1.6x10 -6 /°C. However, in this case, Fe 2 O 3 component functions only to color the catalyst carrier, and thermal shock resistivity is affected since the thermal expansion coefficient is high. US-A-3958058 proposes a method of producing a low expansion honeycomb body by leaching a cordierite body with strong mineral acid to selectively remove Al 2 O 3 and MgO with a weight loss of 7 to 32%. Fe 2 O 3 is mentioned as an impurity. Summary of the Invention It is an object of the invention to eliminate the drawbacks mentioned above and to provide a method of producing cordierite honeycomb ceramics which can maintain an excellent thermal shock resistivity and a small-variation of thermal expansion coefficient, if it is used under a high temperature for a long time, by improving a batch component for cordierite honeycomb ceramics. According to the invention, there is provided a method of producing a cordierite honeycomb ceramics body as set out in claim 1. The thermal expansion coefficient of the body produced can be less than 0.5x10 -6 /°C within a temperature range of 40-800°C in a direction parallel to a flow passage of the honeycomb body. By the method, it is possible to obtain cordierite honeycomb ceramics which can maintain an excellent thermal shock resistivity and a small variation of a thermal expansion coefficient, if it is used under a high temperature for a long time, by improving a batch component for cordierite honeycomb ceramics. Description of the Preferred Embodiment During many experiments, the inventors formed cordierite honeycomb ceramics including an excessive amount of ferric component as compared with a normal cordierite honeycomb ceramics, and treated the thus obtained cordierite honeycomb ceramics by an acid solution. As a result, we found cordierite honeycomb ceramics which can maintain an excellent thermal shock resistivity and a small variation of a thermal expansion coefficient, if it is used under a high temperature for a long time. In the cordierite honeycomb ceramics body produced the amount of iron satisfies the condition that a value of [Fe 2 O 3 wt%/(MgO wt%+Fe 2 O 3 wt%)] ×100 (hereinafter, referred as M value) is 2-10, where the iron component is calculated as Fe 2 O 3 , and the thermal expansion coefficient satisfies the condition that it is less than 0.5x10 -6 /°C within a temperature range of 40-800°C in a direction parallel to a flow passage of a honeycomb body. If a thermal expansion coefficient is not less than 0.5x10 -6 /°C, cracks are generated due to a thermal shock, and thus a life of cordierite honeycomb ceramics is shortened. In the present invention, since an excess amount of iron is included in a cordierite phase, it is possible to maintain a low thermal expansion coefficient, if the cordierite honeycomb ceramics body is immersed in an acid solution and then used under a high temperature. Moreover, in the cordierite honeycomb ceramics body, it is preferred to set the M value to 3-7. Further, it is preferred that a porosity is more than 45%, and a thermal expansion coefficient is less than 0.3x1 -6 /°C. In this case, if a porosity is not more than 45%, the cordierite honeycomb ceramics body is not effectively used for a filter for trapping soots included in an exhaust gas from a diesel engine, since a pressure loss is increased. The method according to the invention may be performed by mixing raw materials for cordierite generation such as talc, alumina, silica with forming agents and surface active agents to obtain a batch, extruding the batch into a honeycomb shape to obtain a formed honeycomb body, drying the formed honeycomb body, and firing the honeycomb body after drying in a temperature range of 1350-1450°C. In this case, the M value in the talc is set to 2-10. Moreover, if the firing temperature is not more than 1350°C, a cordierite phase is not generated, and if the firing temperature is not less than 1450°C, a cordierite phase is melted. As mentioned above, since an excess amount of ferric component is included in a cordierite phase from raw materials for cordierite generation, it is possible to maintain a low thermal expansion coefficient, if the cordierite honeycomb ceramics body is immersed in an acid solution and then used under a high temperature. Further, it is preferred to set the M value to 3-7. Generally, the thermal expansion coefficient of the cordierite honeycomb ceramics body after firing can be decreased by immersing it in an acid solution. In the production of a cordierite honeycomb ceramics body having a cordierite phase in a main ingredient and the M value of 2-10 produced according to the present invention, the weight reduction rate in acid immersion is 0.5-2.0%. If this weight reduction rate is not more than 0.5%, an effect of low thermal expansion coefficient is small, and if the weight reduction rate is not less than 2.0%, strength of the cordierite honeycomb ceramics body is extremely lowered, since a large amount of cordierite crystal is dissolved together with a impurity phase to be dissolved. As for an acid solution, use is made of hydrochloric acid, sulfuric acid, fluoric acid, nitric acid, and so on, but it is preferred to use a nitric acid since it is inexpensive. As for the M value of the cordierite honeycomb ceramics body, if the M value is not more than 2, a low thermal expansion coefficient obtained by the acid solution treatment is returned to a large value by the use under a high temperature, and thus an effect of the acid solution treatment such as the improvement of the thermal shock resistivity is not maintained. Moreover, if the M value is not less than 10, a softening point of the cordierite honeycomb ceramics body is decreased, and thus a variation of a dimension and a breakage by melting occur. In the method of producing cordierite honeycomb ceramics according to the invention, in order to add a ferric component in the cordierite phase, use is made of talc including an excess amount of iron component, i.e. a talc having about 0.3 wt% of Fe 2 O 3 . Therefore, an increase of a thermal expansion coefficient due to iron component as an impurity can be eliminated, and thus a heat resistivity after an acid solution treatment can be improved. As for talc, it is preferred to use talc including no chlorite crystal. This is because, if a large amount of chlorite is included in talc, the iron component is dissolved in a chlorite crystal as an impurity, and a thermal expansion coefficient is liable to be increased due to the ferric component as an impurity. Moreover, if an iron component is added by itself, it is preferred-to use FeO component so as to improve a heat resistivity after the acid solution treatment. This is because FeO component is liable to be replaced with Mg component in a cordierite crystal as compared with Fe 2 O 3 component, and thus Fe component is liable to be dissolved in a cordierite crystal. However, in this case, a thermal expansion coefficient before the acid solution treatment is increased, and thus a thermal shock resistivity is not improved largely. From the results of the various experiments mentioned above, it is assumed that an iron component dissolved in a cordierite crystal is effective to obtain an excellent heat resistivity after the acid solution treatment. Usually, almost all the iron component included in cordierite honeycomb ceramics is dissolved in chlorite crystal which is included in talc as an impurity. In the present invention, as for talc raw materials, use is made of talc in which the iron component is dissolved by replacing Mg component in talc crystal not in chlorite crystal. Therefore, a thermal expansion coefficient after firing is not affected by the iron component as compared with the case in which an iron component exists in the cordierite honeycomb ceramics as an impurity. It is known that the reason for decreasing a thermal expansion coefficient of the cordierite honeycomb ceramics by the acid treatment is that microcracks generated between cordierite crystals are opened and a thermal expansion does not occur so much due to the opened microcracks. Moreover, if the cordierite honeycomb ceramics body after the acid solution treatment is aged under a high temperature, the microcracks are closed by an amorphous substance generated on a surface of the cordierite crystal, and the effect of reducing a thermal expansion disappears. In this case, it is assumed that the amorphous substance is made of a glass phase generated by a reaction between an impurity component in a cordierite fired body and a magnesia. The inventors find that an iron component as an impurity is decreased by dissolving an iron component in a cordierite crystal and that the iron component dissolved in a cordierite fired body is served as controlling the reaction between the impurity and the magnesia. Moreover, the inventors find that a phase generated by the dissolved iron component and the iron component as the impurity is included in the amorphous phase and that a deposition temperature of the phase included in the amorphous phase is increased. Therefore, the microcracks are not closed if used in a high temperature, and thus a low thermal expansion coefficient is not lost and is maintained. Hereinafter, actual embodiments will be explained. METHOD OF PRODUCING Producing of cordierite honeycomb ceramics Cordierite honeycomb ceramics are produced according to the invention as follows. At first, raw materials for cordierite generation such as talc, kaolin, alumina, silica and the like are mixed with forming agents such as methyl cellulose, surface active agent and the like and solvents such as water, alcohol and the like to obtain a batch having a chemical composition of SiO 2 :42-56 wt%, Al 2 O 3 :30-45 wt% and MgO:12-16 wt% whose center composition is a theoretical chemical composition of a cordierite. The thus obtained batch is extruded into a honeycomb formed body. In this case, 0-50 wt% of graphite may be added as pore-forming agent if necessary. Then, the honeycomb formed body is fired under a temperature range of 1350-1450°C at which a ratio of cordierite generation proceeds sufficiently. In this case, since an included iron component is liable to be free at near 1200°C, it is preferable to set a temperature ascending rate less than 100°C/h till 1000-1300°C so as to dissolve an iron component sufficiently in a cordierite crystal. Producing of ceramic filter In the present invention, by adding an iron component into a cordierite crystal from talc raw materials, it is possible to obtain a porous ceramic filter having an excellent thermal shock resistivity which can maintain low thermal expansion properties after an acid solution treatment if used under a high temperature. Raw materials for cordierite generation shown in Table 1 were prepared. Then, from the raw materials for cordierite generation, raw materials for cordierite honeycomb ceramics having various amounts of iron component were produced by using talc raw materials including various amounts of iron component. As for specimens according to comparative examples, raw materials to which an iron component is added by itself were also prepared. Then, these raw materials were mixed with forming agents and solvents to obtain batches, and the batches were extruded into honeycomb formed bodies having rib thickness: 0.45 mm, diameter: 43.8mm and length:152mm. Then, the thus obtained honeycomb formed bodied were fired. After that, both ends of the honeycomb fired bodies were sealed in a checker pattern such that if one end of each flow passage was opened, the other end of this flow passage was closed to obtain ceramic filters. Batch No. Specimen No. Talc (wt%) particle size µm (Fe 2 O 3 wt%) Kaolin (wt%) particle size µm Alumina (wt%) particle size µm Silica (wt%) particle size µm Thermal expansion coefficient ×10 -6 /40-800°C before acid solution treatment after acid solution treatment 1 2 38.2% 60.0µm (0.03 wt%) 23.8% 8.7 µm 28.0% 6.0 µm 10.0% 100.5 µm 1.03 0.19 2 6 38.8% 30.0 µm (0.03 wt%) 41.7% 8.5 µm 19.5% 5.5 µm - 0.52 0.11 3 12 40.7% 22.3 µm (0.03 wt%) 16.0% 3.3 µm 30.8% 1.8 µm 12.5% 19.8 µm 0.17 -0.21 Acid solution treatment The thus obtained ceramic filter was immersed in a nitric acid solution of 1.5 normality and 93°C, whose solution amount was 10 times as large as a volume of the ceramic filter, so as to perform an acid solution treatment within a range of weight reduction rate of 1±0.2%. PERFORMANCE ESTIMATION Experiment condition With respect to the thus obtained ceramic filter, a heat treatment was performed under the atmosphere at 1200°C during 100 hours to estimate a heat stability of a lowering effect of a thermal expansion coefficient due to the acid solution treatment. Estimation method As for the estimation for a thermal shock resistivity, a thermal expansion coefficient in a range of 40-800°C of respective ceramic filters was measured by a thermal expansion coefficient measuring apparatus. Moreover, in order to estimate a heat stability of a thermal expansion coefficient, a deterioration rate of the effect of the acid solution treatment (hereinafter, referred as deterioration rate) was measured. This deterioration rate was obtained form a heat recovery rate after a heat treatment of 1200°C for 100 hours. If this deterioration rate is low, the effect of the acid solution treatment is maintained in a high temperature, and thus it shows a good heat stability. Moreover, with respect to the ceramic filters, a cordierite composition and amounts of Fe 2 O 3 and CaO were measured by a fluorescent X-ray analysis. Further, in order to make clear an effect of the iron component addition according to the invention, not only a total amount of iron component (Fe 2 O 3 amount) but also a ratio of a iron amount with respect to a sum of a ferric amount and a magnesia amount (M value) were measured. In the case that an iron component is dissolved in a cordierite crystal, since an iron component is liable to be replaced with a magnesia component, the effect of an iron component addition with respect to the heat stability of the thermal expansion coefficient after the acid solution treatment can be estimated by the M value. Moreover, respective ceramic filters was cut out to a specimen having a diameter of 25,4 mm (1 inch) and a length of 25,4 mm (1 inch), and a transformation rate under a high temperature of the cut out specimen was measured from a transformation rate after a heat treatment at 1450°C for 10 minutes. As for a porosity, it was measured by a mercury-porosimeter. Estimation results Tables 2-6 show the results of the examples according to the invention and the results of the comparative examples. In the Tables 2-6, properties of respective specimens are also shown. As shown in the Tables 2-6, the batches having various thermal expansion coefficients can be obtained by varying raw materials to be used, firing conditions and so on, and the batches show various corrosion resistivities with respect to an acid solution due to the conditions of a crystal phase, a porosity, and so on, thereby they show various decreasing rate on a thermal expansion coefficient after the acid solution treatment. Therefore, it is necessary to estimate a heat stability of the acid solution treatment batch by batch. Estimation of known method In the Table 2, the specimen Nos. 1 and 2 show known examples. Usually, an iron component is seen as an impurity which increases a thermal expansion coefficient, and thus it is estimated that the batch including no iron component is preferred for improving a thermal shock resistivity. However, as shown in Table 2, in the known method, if decrease of thermal expansion coefficient is performed by the acid solution treatment, a thermal expansion coefficient is recovered to a level before the acid solution treatment even under a temperature of 1100°C. Therefore, in the batches according to the known examples, an effect of the acid solution treatment is not maintained. Example 1 It is possible to improve heat stability of the acid solution treatment by adding an iron component in the talc raw materials of the specimen No. 2 mentioned above. In this case, the iron component can be stably dissolved in a talc crystal, and thus it is possible to dissolve an iron component easily in a cordierite crystal by using the talc raw materials mentioned above as a ferric source. In the Table 3, various properties of the systems based on the batch No. 1 (specimen No. 2) in the Table 1 are shown as the example 1. In the specimen No. 2 according to the known example, a deterioration rate is 88%, and it is confirmed that thermal shock resistivity is decreased to a level before the acid solution treatment. From the results of decrease of the deterioration rate and the thermal shock resistivity in respective batches, it is confirmed that, if an increase of a thermal shock resistivity is controlled under 60% with respect to a deterioration rate according to the known example, an improving effect of the acid solution treatment with respect to a thermal shock resistivity can be maintained certainly. Therefore, in the Table 3, the deterioration rate of a thermal shock resistivity (hereinafter, referred as deterioration rate) is indicated by a percentage with respect to that of the specimen No. 2 (100%), and it is assumed that the specimens, in which the deterioration rate is under 60%, can maintain the effect of the acid solution treatment. As shown in the specimen Nos. 3-5 in the Table 3, the deterioration rate can be improved, if the M value is made larger i.e. an iron component is added from the talc raw materials. Example 2 In the Table 4, various properties of the systems based on the batch No. 2 (specimen No. 6) in the Table 1 are shown as the example 2. The batch systems mentioned above show a low thermal expansion coefficient before the acid solution treatment as compared with the example 1 and an excellent corrosion resistivity. Moreover, in the batch systems mentioned above, a thermal expansion rate after the acid solution treatment is not decreased so much i.e. about 0.4×10 -6 /°C. Also in the batch systems mentioned above, it is possible to improve the deterioration rate by making the M value larger as is the same as the example 1. Further, as shown in the specimen Nos. 7 and 11, the deterioration rate can be improved largely by making the M value larger, but a transformation rate under a high temperature becomes larger corresponding to the increase of a thermal expansion coefficient. Moreover, in these cases, if the M value not less than 10, a part of the body starts to be melted at an upper-limit temperature in a normal use. Therefore, it is necessary to limit the M value to less than 10. Moreover, from the results shown in the example 1 and the example 2, the M value of the specimens, in which the deterioration rate is under 60%, is more than 2, and they show an excellent heat stability. Example 3 In the Table 5, various properties of the systems based on the batch No. 3 (specimen No. 12) in the Table 1 are shown as the example 3. The batch systems mentioned above show an extremely low thermal expansion coefficient, and the batches after the acid solution treatment show a heat contractibility. In the batch systems mentioned above, it is possible to improve the deterioration rate by making the M value larger as is the same as the examples 1 and 2. Moreover, the specimens having the M value more than 2 have an excellent heat stability. As mentioned above, if the M value of the batch systems is set to a range of 2-10, all the batch systems can maintain the effect of the acid solution treatment as compared with the known examples even at a high temperature over 1200°C, and also they show an excellent heat stability. Comparative example In the Table 6, various properties of the systems, in which a ferric component is added by itself in the batch No. 3 (specimen No. 12) in the Table 1, are shown as the comparative example. As shown in the Table 6, it is possible to form the cordierite ceramic filter in the method of the invention which includes an iron component by adding an iron component by itself. However, if the specimen Nos. 16, 20 and 21 are compared with the specimen Nos. 13, 9 and 10 having the same M value as those of the specimen Nos. 16, 20 and 21, in which the iron component is added from the talc raw materials, the deterioration rates of the specimen Nos. 16, 20 and 21 are higher than those of the specimen Nos. 13, 9 and 10. Moreover, also in these cases, if the M value is not less than 10, shapes of them are not maintained in a high temperature, and they are not used as they are. As mentioned above, in the case that an iron component is added by itself, it is possible to use if the M value is in a range of 2-10, and the effect of the acid solution treatment can be maintained in a high temperature. However, the effect of the acid solution treatment is not so large as compared with the specimens in which an iron component is added from the talc raw materials. According to the invention, it is possible to obtain a batch, which can maintain the improving effect of the thermal shock resistivity after the acid solution treatment in a normal use, by setting the M value in a range of 2-10. In this case, it is preferred to use the talc raw materials as a ferric source. It should be noted that, in the embodiments mentioned above, the present invention is applied to the production of a filter only, but the present invention can be applied to the production of other products such as a catalyst carrier, a heat exchange device and so on.

Claims: A method of producing a cordierite honeycomb ceramics body comprising the steps of (i) mixing raw materials for cordierite generation, including talc, with forming agents and solvents to obtain a batch, wherein said talc has an iron content such that the value of [Fe 2 O 3 wt%/(MgO wt%+Fe 2 O 3 wt%)]x100 is 2-10, the iron content being calculated as Fe 2 O 3 , (ii) extruding said batch into a honeycomb shaped body, (iii) drying said honeycomb shaped body, (iv) firing the dried honeycomb shaped body at a temperature range of 1350-1450°C, and (v) immersing the fired cordierite honeycomb ceramics body having a cordierite phase as a main ingredient, wherein the value of [Fe 2 O 3 wt%/(MgO wt%+Fe 2 O 3 wt%)]x100 is 2-10, the iron content being calculated as Fe 2 O 3 , in an acid solution, the weight reduction of said fired honeycomb body in the acid solution treatment step being in the range of 0.5-2.0%. The method of producing cordierite honeycomb ceramics according to claim 1, wherein said talc has an iron content such that the value of [Fe 2 O 3 wt%/ (MgO wt%+Fe 2 O 3 wt%)]x100 is 3-7, where the iron content is calculated as Fe 2 O 3 . The method of producing cordierite honeycomb ceramics according to claim 1 or 2, wherein said acid solution is a nitric acid solution.

Inventors: KOTANI WATARU, KUMAZAWA KAZUHIKO, ONO YOSHIRO, KOTANI, WATARU, KUMAZAWA, KAZUHIKO, ONO, YOSHIRO, KUMAZAWA, KAZUHIKO, NGK TAKEDA-KITA-SHATAKU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1995
Main classification: C04B35
IPC codes: B01J35, B01D39, B01J32, B01D53, B01J21, F02B3, C04B35, C04B38, C04B41