Exhaust gas filter and method for making the same

Abstract: An exhaust gas filter for internal combustion engine and of honeycomb structure comprising a ceramic fiber and inorganic binder, the ceramic fiber consists mainly of Al₂O₃ and SiO₂, the inorganic binder including Al₂O₃ and SiO₂ to bind the said ceramic fiber, and the inorganic binder is of single glass phase or of mixed phase of a glass phase and a crystal phase keeping the same crystal structure in the temperature range of 20 to 1200°C, whereby, in the inorganic binder, any crystal such as quartz which produces abnormal expansion due to crystal phase transition is excluded, and occurrance of crack is prevented..

Description: FIELD OF THE INVENTION This invention relates to an exhaust gas filter of an internal combustion engine, especially Diesel engine, to clean the exhaust gas by trapping and removing the particulates such as soot exhausted from the engine, to prevent air pollution. BACKGROUND OF THE INVENTION In recent years, air pollution by NO x or floating small particulates is developing in large cities. As for the small particulates floating in the air 20 to 30% of them are said to be produced by the black smoke exhausted by Diesel engines of automobiles. As the mostly expected counter-measure against the black smoke, there is provided an exhaust gas cleaner which, placed in the exhausting system, comprises a filter to trap the small particulates in the exhaust gas and a refreshing apparatus to burn the trapped particulates. Among the conventional filters, a ceramic monolithic type is known as is disclosed in USP 4364761. This conventional ceramic monolithic filter comprises many long cells arranged side by side to form a honeycomb structure, one cell having an inlet at one end thereof and being plugged at the other end, and an adjacent cell being plugged at the same end as the inlet of the said one cell but having an outlet at the other end. When the exhaust gas enters the inlet and passes along the porous wall between the cells, the particulate in the gas is trapped. If the amount of trapped particulates increases, the porous walls are choked, of course, by the particulates resulting in an increase of back pressure of the engine. Accordingly, it is necessary, when the trapped particulates exceed a certain amount, to remove the particulates to suppress load increase of the engine due to increased back pressure. The particulates consist of solid carbon and soluble organic fraction (SOF) which is soluble in an organic solvent such as dichloromethane, both being combustible and burned if heated over 600°C, although the temperature varies somewhat depending upon the kind or loading condition of the engine. Thus the exhaust gas cleaner is provided with a filter regenerating apparatus such as an electric heater or gas burner. For the exhaust gas filter, it is required not to be choked and not to be molten when heated for regenerating and is usually a porous material of large porosity and high melting temperature. For an exhaust gas filter to meet the above described requirement, USP 4652286 discloses a filter which is made by sintering a mixture of silica-alumina fiber and silica-alumina clay in 1200°C to make a sheet which is assembled in a honeycomb structure. This filter, however, after repetition of particulate trappings and heating-regenerated producing cracks in the wall, which is not satisfactory. Japanese Patent JP-A-63134020 relates to a corrugated honeycomb filter for the treatment of diesel exhaust gases, and consists of heat-resistant inorganic fibres, ceramic material powder and Cerium (IV) Oxide. Sheets of the above are made into a honeycomb structure using an adhesive and a plug material, and this is then sintered at a temperature between 1200°C and 1350°C to obtain an exhaust gas filter. SUMMARY OF THE INVENTION It is an object of the present invention to provide a gas filter especially for an exhaust gas cleaner of an internal combustion engine with cracking suppressed and with high durability. According to the present invention there is provided an exhaust gas filter comprising a ceramic fiber including Al 2 0 3 and Si0 2 , an inorganic binder including Al 2 0 3 and Si0 2 to bind the ceramic fiber, characterised in that the organic binder further includes potassium in an amount of 0.8 to 4.2 weight % in terms of K 2 O, and that the inorganic binder is of a homogeneous phase of glass or of a heterogeneous phase of a glass phase and a crystal phase, the crystal phase of which has a crystal structure which has no crystal transformation in the temperature range of 20 to 1200°C. Such a filter has, in its inorganic binder, no crystal phase as quartz which causes abnormal volume expansion or shrinkage as a result of crystal phase transition, and consequently crack occurrence in the porous wall is suppressed and the durability is improved. Further, according to an embodiment of the present invention, the ceramic fiber including alumina and silica is applied with a crystal nucleus of Cr 2 O 3 etc. With this crystal-nucleus the mullite crystal which generates at the time of sintering is suppressed, and the mechanical strength of the porous wall and the durability of the filter are further improved. BRIEF DESCRIPTION OF THE DRAWINGS Fig.1 is a sectional view of an exhaust gas filter embodying the present invention. Fig.2 is a perspective view of the same exhaust gas filter. Fig.3 is a sectional view showing the skeleton of the system in which the exhaust gas filter is included. DETAILED DESCRIPTION OF THE INVENTION Using the conventional exhaust gas filter with sintered sheets of the mixture of silica-alumina fiber and silica-alumina clay, the present inventors investigated the cause of the cracks after repeatedly trapping and heating particulates, and found, by thermo-mechanical analysis, an abnormal volume-expansion near 573°C. This is caused by the volume expansion produced by the transformation of quartz, which is a content of the clay in the filter, from low temperature crystal phase to high temperature crystal phase. As described above, the particulates deposited in the filter are burned out at 600°C. If the exhaust gas filter, when heated for regenerating had its particulates burned out only partially, the part of the filter with particulates burned out was heated above 600°C, and the part of the filter with particulates not burned was heated below 550°C. Accordingly it was found that, in a wall between a burning path and a not-burning path, the surface of the burning path expands abnormally due to crystal transformation of quartz, and cracks of the wall result. In some kinds of clays, crystal phases other than quartz such as cristobalite or tridymite were observed. When these crystals are involved, abnormal expansion due to crystal transformation to high temperature crystal phase was noticed in 150-230°C temperature range. This was the reason that cracks occur when exhaust gas near to 500°C flows through an exhaust gas filter at room temperature, Based on the above investigation of cracking, the present inventors tried many experiments on the materials and processes to obtain an exhaust gas filter with excellent durability. The constituent material for exhaust gas filters is required to have high heatresisting property for burning the particulates. As a typical ceramic fiber of high heat-resisting property, alumina fiber, aluminosilicate fiber, aluminoborosilicate fiber and mullite are known. Among them, alumina fiber and mullite are not appropriate for the exhaust gas filter, since the former is weak against thermal shock or steep temperature gradients due to its big thermal expansion coefficient and the latter is mechanically weak as it is a polycrystal. Thus the inventors selected aluminosilicate fiber and aluminoborosilicate fiber as the material. Aluminosilicate fiber with Al 2 O 3 and SiO 2 of 50/50 weight% and nominal usable temperature 1260°C is marketed. Applying differential thermal analysis to this aluminosilicate fiber, a high heat-generating peak is found in the vicinity of 980°C. This heat-generating peak corresponds to the crystalization peak of mullite crystal (3Al 2 O 3 ·2SiO 2 ). Therefore, when aluminosilicate fiber, which is in amorphous state at a normal temperature, is heated, mullite crystal nuclei begin to precipitate at the temperature below 950°C, and, at the vicinity of 980°C, the crystal growth reaches the climax. As the weight ratio of Al 2 O 3 and SiO 2 in a mullite crystal is 72:28, aluminosilicate fiber with Al 2 O 3 :SiO 2 =50:50, at the temperature over 980°C, is of a state such that needle-like crystals of mullite are dispersed in a SiO 2 -rich aluminosilicate glass. Thus aluminosilicate fiber after sintering is under such state. Since the mullite crystals, having no crystal transformation, are stable from room temperature up to 1200°C, the highest temperature expected for the filter, the filter according to the present invention does not have abnormal expansion due to crystal transformation and crack occurrance, if any mullite crystal exists. The temperature 1260°C mentioned above as the temperature aluminosilicate fiber can withstand is the one determined based on shrinking of board-type products used as heat-insulating material, and does not limit the sintering temperature for the usage of the present invention. This fiber, in spite of eutectic temperature as high as about 1760°C, has its fibrous structure destroyed at high temperature as a result of viscosity decrease due to its nature as glass. However, it keeps its fibrous structure up to 1600°C sintering temperature according to an observations using an electron microscope, and the sintering at a temperature as high as 1600°C is possible. Although the existence of mullite crystal does not cause cracking as described above, sintering at high temperature and for many hours brings abnormal growth of crystal particles, and a mechanically strong exhaust gas filter is not obtained. Accordingly, the growth of mullite crystal particles is to be suppressed. To suppress the growth of needle crystals of mullite, it is enough to have the numbers of crystal nuclei increased and the crystal size minimized. As the crystal nucleus Cr 2 O 3 , TiO 2 , ZrO 2 or such metal oxide which is relatively easily precipitated as crystal from glassy state is preferrable. Aluminosilicate fiber with Cr 2 O 3 additive and aluminosilicate fiber without any crystal nucleus were applied to differential thermal analysis, and the area of heating peak by the crystallization of the former was confirmed to be far smaller than that of the latter. Also, in electronmicroscopic photographs of crystal particles after sintering, needle-like crystals of 0.5µm average length were observed for the aluminosilicate fiber without any additive, but very small crystals only of average length less than 0.1µm were found for the aluminosilicate fiber with Cr 2 O 3 . herefore, to supress the crystal growth of mullite, addition of crystal-nucleus preferrably Cr 2 O 3 is preferrable. As for the content of Cr 2 O 3 , 1.5-3.5 weight% is suitable; less than 1.5% is not enough for nucleus suppression, while more than 3.5 weight% results in the sheet being hard to work. The length of the ceramic fiber is preferrably within the range of 0.1 to 10mm. With the length less than 0.1mm, the twisting of the fibers is not enough and the strength of ceramic sheet before sintering is insufficient resulting in hard working, and, with the length more than 10mm, the fibers twist each other too much forming large flocks, and unevenness of sheet thickness is greater. In the following the inorganic binder according to the present invention is explained. When the aluminosilicate fiber for example is adopted as a material for an exhaust gas filter, sufficient strength is not obtained for itself, so that addition of inorganic binder is necessary. Characteristics necessary for the inorganic binder are high heat resistance, low thermal expansion, and reactivity with ceramic fiber, and, accordingly, for that purpose glass or ceramics comprising complex oxide of Al 2 O 3 and SiO 2 is adequate. Further, to obtain high adhesive strength without losing the form of the ceramic fiber, use of the inorganic binder with eutectic or softening temperature lower than that of ceramic fiber is preferrable. For that purpose, those containing a small amount of oxide of alkaline metal (Li, Na, K etc.) or alkaline-earth metal (Ca) is preferrable. Thus, as the raw material for the inorganic binder, glass of alkaline-metal oxide (alkaline-earth metal oxide)-SiO 2 -Al 2 O 3 , feldspar such as potash feldspar or plagioclase, or ceramic material consisting mainly of clay mineral such as sericite or illite, or the mixture of those is preferrable. When ceramic material powder of a feldspar or clay mineral, or the mixture of these is adopted as the inorganic binder, sintering at a temperature equal to or higher than the temperature at which a glass phase is formed from the above crystal content will produce a filter material with high mechanical strength. Even if the sintering temperature is equal to or higher than the temperature at which a glass phase is formed, however, the quartz or its modified crystal, cristobalite and tridymite still remains in the inorganic binder after sintering, provided the sintering temperature is relatively low. These crystals have their crystal transformation temperature between the normal temperature and 1200°C, the latter being the highest temperature the filter is used, and the existence of these crystal phases causes cracking. In consequence, according to the present invention, sintering is carried out at a temperature equal to or higher than that at which the crystal phase of quartz melts into a glass phase. In so doing, the inorganic binder becomes a single phase of glass or a mixed phase of a glass phase and another crystal phase such as of mullite which does not transform below 1200°C, preventing cracking. Further, the glass phase in the inorganic binder becomes silica-rich, resulting in low thermal expansion, improvement of mechanical strength and heatresistance, and thermal-shock-resistance of the exhaust gas filter is also obtained. Among material powders containing an alkaline metal or alkaline-earth metal, especially preferrable is clay such as sericite consisting mainly of SiO 2 , Al 2 O 3 and K 2 O and feldspar due to the high durability of the gas filter and easy treatment in manufacturing with an appropriate sintering temperature. If sericite clay, with sericite as the main constituent and quartz, kaolinite or pyrophyllite as impurities, is sintered at a temperature equal to or higher than 1350°C, the quartz crystal melts into the glass phase, resulting in complete separation to glass and mullite phase, and small thermal expansion coefficient and high mechanical strength with low sintering temperature thanks to the existence of the glass phase. For instance, Murakami clay ( a natural material marketed by Kyoritsu Ceramic Materials Co., Ltd.) can have its thermal expansion coefficient reduced to 3.6×10 -6 from the conventional 4.5 × 10 -6 by completely glassifying it. Further, with the glass phase becoming silica-rich, the glass transition temperature and yielding point also rises. For example the yielding temperature rises to 1000°C from 950°C for Murakami clay, improving the heat resistance property of the filter. The temperature at which a quartz crystal melts into the glass phase depends on the conposition of the ceramic material powder and especially the content of alkaline metal. For the case of sericite clay, the content of potassium is a big factor for determining the sintering temperature. The Murakami clay for example, with potassium of about 5 weight% calculated in terms of K 2 O, is completely separated to mullite phase and glass phase when heated up to or over 1350°C. Generally, the larger the alkaline metal content is, the lower the glassifying temperature is, and the higher the sintering temperature is. For the filter regenerated by heating-burning high heatshock-resisting property, i.e. low thermal expansion coefficient, is required, so that potassium content is preferred to be within 1.8 to 3.2 weight% range in terms of K 2 O. However. if the regenerating is done by any method other than heating, potassium content is not necessarily limited. The ratio of combining the said ceramic fiber and inorganic binder depends on the porosity necessary for the filter, and is preferred to be 25 to 75 weight% of ceramic fiber and 25 to 75 weight% of inorganic binder. If the inorganic binder content is less than 25%, the mechanical strength is insufficient, and if larger than 75% the porosity is not enough. Embodiment 1 One thousand grams of aluminosilicate fiber (Ibiwool bulk by Ibiden Co.,Ltd.) with an average fiber diameter 3µm chopped to 0.1 to 10mm length was dispersed in 600kg water to form a suspension. The suspension was mixed with 1000g of sericite (Murakami clay) which contains about 5 weight% potassium in terms of K 2 O, and stirred, and further mixed and stirred, with 30g of emulsion of vinyl-acetate and acrylate copolymer. The suspension was then mixed with 100g of aluminum sulfate and further with aqueous solution of sodium hydroxide such that the slurry thus made had a pH of about 5.5. After adding 200g of 15% aqueous solution of cationic polyacrylamid to the slurry to form a flock suspension, the suspension was diluted with water to 1500ℓ and made to a ceramic paper by a usual papermaking machine. On the other hand, a paste-like plug material was prepared by mixing 2000g of shattered aluminosilicate fiber, 2000g of sericite clay, and polyvinyl alcohl. A part of the ceramic paper was made corrugated on a corrugate machine and glued to another part of the paper with adhesive deposited on the peak of corrugation, the adhesive being prepared by kneading 2000g of alminosilicate fiber, 2000g of sericite cray, and potato starch. While glueing, an end of the glued corrugated paper is filled with plug material, which, after sintering, becomes the plug 2 in Fig.1 and Fig.2. The free peaks of the above glued corrugated sheet were further glued and fixed to another plain sheet, and the body thus formed was rolled and the other end was filled by plug material to become the plug 3. When the rolled up corrugated paper is taken off the rolling machine the centre of the roll occupied by the core becomes empty. The empty center part was filled with plug material, which is to become the core 4 after sintering. Thus the assembled body is sintered for 2 hours under 1350°C temperature. As a result, organic contents disapears, and aluminosilicate fibers are consolidated by the glassified sericite clay, and an exhaust gas filter of honeycomb structure of fibrous ceramic having inlets 1a and outlets 1b for the exhaust gas on one side and on the other side respectively was obtained. The exhaust gas filter 5 was placed in a case 13 of an exhaust gas cleaning apparatus as shown in Fig.3 with a heat-insulating body 6 consisting of thermally expanding ceramic fiber. For trapping particulates, a valve 10 is fixed to the side of filter 5, whereby the exhaust gas from the engine 8 is, after passing exhaust gas pipe 9, sent to the filter 5. In the filter 5, the particulates in the exhaust gas are trapped, and the exhaust gas without particulates is sent, via an exhaust gas pipe 14, to the outer world or turbocharger. When the trapped particulates exceed a certain amount, the valve 10 is changed to the bypass 12 side, and the regenerating of the filter 5 is carried out. With an electric heater 7 applied with electric power, the exhaust gas filter 5 is heated and the particulates near the entrance of the filter 5 are burned. Then the heater 7 is switched off, and air is sent by an air pump 11, to transfer the flame to the exit of the filter to promote the burning of the particulates. When the burning of particulates is over, the valve 10 is changed to the filter 5, and particulate trapping begins again. An engine 8 was applied with an exhaust gas cleaner with the exhaust gas filter of the present embodiment and was operated for 2 hours, and the increase of back pressure while trapping particulates was found to be 60mmHg, the same value as the conventional one. When, after particulate trapping, heating-regenerating by heater 7 is being done, the particulates burn off at most parts of the exhaust gas filter, but do not at other parts due to heat loss to the outer atmosphere, the temperature difference between the parts being nearly 300°C. While the conventional filter could withstand only about 200°C temperature difference, the exhaust gas filter according to the embodiment can withstand as high as 500°C temperature difference, due to the mechanical strength increased to two times and the thermal expansion coefficient reduced by more than 20%. Repetition testing of the trapping and heating of particulates proved that the filter is not destroyed even after 200 cycles of trapping and heating. Further, black smoke was not observed after 1000 cycles, showing that the filter is carrying out the duty as a filter up to 1000 cycles. Instead of the aluminosilicate fiber, aluminoborosilicate fiber can have the similar function. Embodiment 2 An exhaust gas filter was prepared, with sintering temperature of 1500°C for the sintering to make a honeycomb structure, instead of 1350°C at the Embodiment 1. Repetition testing of heating-regenerating of this filter similar to that in Embodiment 1 was carried out without destroying of the filter after more than 1000 cycles. Thus, it was found that increasing the sintering temperature brought an increase of mechanical strength of the filter and the increase of number of times of repeated regenerating. Embodiment 3 An exhaust gas filter was prepared, with aluminosilicate fiber (SC bulk 1400B, Nippon Steel Chemical Co., Ltd.) containing 2.5 weight% Cr 2 O 3 instead of aluminosilicate fiber in Embodiment 1. To verify the crystal-growth suppression effect differential thermal analysis was applied to the aluminosilicate fiber with Cr 2 O 3 and the one according to Embodiment 1 using differential thermal analyser DTA8110D of Rigaku Corp. with Cr 2 O 3 without an additive Peak temperature of crystallization 980°C 980°C Peak area of DTA 10µV·s/mg 40µV·s/mg As a result, as shown in Table 1, it was confirmed that the area of heating peak by crystallization of aluminosilicate fiber with Cr 2 O 3 is far smaller than that of fiber without additive. Also, an electromicroscopic photograph of crystal particles after sintering was inspected, and needle-like crystals with more than 0.5µm average length for the fiber without additive was observed, but only a minute crystal of equal to or less than 0.1µm average was observed forthe fiber with Cr 2 O 3 . The exhaust gas filter prepared as explained above was applied on a exhaust gas cleaning apparatus of Fig. 3. After 2 hours of the engine operation and particulate trapping, the increase of back pressure was 60mmHg, the same as for Embodiment 1. Also, repetition of particulate trapping and heating-regenerating 1000 times did not bring black smoke. The mechanical strength increased to more than 1.2 times that of the filter of Embodiment 1, and increase of duration resulted. Embodiment 4 To investigate the effect K 2 O content in a inorganic has on the thermal characteristics, physical properties were compared on the five samples of sericite clay with K 2 O content 1, 2, 3, 4 and 5 weight% (tolerance of ±0.2% is possible, as natural products) from Shokozan mine, Showa Mining Co., Ltd. whose constituents given by analysis are as shown in Table 2. Constituents of Samples Sample No.1 No.2 No.3 No.4 No.5 Designated K 2 O content (%) 0.8-1.2 1.8-2.2 2.8-3.2 3.8-4.2 4.8-5.2 K 2 O (%) 1.2 2.0 2.9 3.9 5.1 SiO 2 (%) 88.7 80.6 70.2 60.5 65.0 Al 2 O 3 (%) 7.1 13.8 22.3 29.8 24.8 Ig Loss (%) 1.6 2.8 3.8 4.7 3.8 These samples were sintered in an electric furnace for 2 hours at temperatures 1200, 1300, 1400, 1500, and 1600°C, and applied to themomechanical analysis (by analyser TMA8140, Rigaku Corp.), the results being shown in Table 3, wherein the "sintering-possible temperature" means the temperature, after sintering under which temperature only mullite and glass phase remain and quartz and cristobalite disappear. Sample No.1 No.2 No.3 No.4 No.5 Sintering-possible temperature (°C) ≧1600 ≧1500 ≧1400 ≧1350 ≧1300 Thermal expansion coefficient(10 -6 /°C) 0.8 1.9 2.8 3.7 3.9 Glass transition temperature (°C) 1000 890 880 850 870 Yielding temperature (°C) 1300 1150 1100 1030 1010 Five kinds of exhaust gas filter were made with 5 kinds of clay of Table 1 as the material powder for inorganic binder, when the quantity of organic binder was adjusted to the same porousity as obtained for different sintering temperatures peculiar to clays. These 5 kinds of filter were tested, applying on the said exhaust gas filter cleaning apparatus and being heated repeatedly under the same conditions, to obtain the number of repetitions until cracking or melting of material occurs; the results are given in Table 4. Clay sample No.1 No.2 No.3 No.4 No.5 K 2 O content (%) 0.8-1.2 1.8-2.2 2.8-3.2 3.8-4.2 4.8-5.2 Sintering temperature (°C) 1600 1500 1400 1350 1300 Cycle of duration >1000 >1000 >1000 200 10 As is observed in Table 4, the lower the K 2 O content is, the larger the cycles of duration against heating-regenerating are, especially, with K 2 O content less than 3.2 weight%, duration of more than 1000 cycles is possible. Among the samples, however, the filter from No.1 clay had its aluminosilicate fiber partially deformed due to the high sintering temperature. Accordingly, the most favorable K 2 O content is 1.8 to 3.2 weight%, and the most favorable sintering-temperature is 1400 to 1500°C. Embodiment 5 Natural sericite clay contains small amounts of clays such as kaolinite, pyrophyllite and quartz as impurities. On samples obtained by adding, to 50 weight part of sericite clay with 3.9 weight% K 2 O (Showa Mining Co.), 50 weight part of kaolinite (Tsuchiya Kaolin Ind. 5M kaolin), 50 weight part of pyrophyllite (Showa Mining Co.), or silica sand (Tsuchiya, natural silica sand No.3) to make the K 2 O contents about 2.0 weight%, the sintering-possible temperatures were examined. The result is shown in Table 5. Additive Kaolinite Pyrophyllite Silica(quartz) sintering-possible temperature (°C) 1500 1500 not usable As is observed, the clays with kaolinite or pyrophyllite can be used under likewise condition as sericite with 2% K 2 O, but the clay with the addition of quartz had much crystallization of cristobalite, and could not melt into glass phase, had too large an expansion coefficient, and was not suitable for application. Then, of the three exhaust gas filters obtained by manufacturing methods similar to those of Embodiment 1, but with three clays of the above described present embodiment, similar tests of repeated heating were applied. The result is shown in the Table 6. As is indicated, sericite clay with addition of kaolinite or pyrophyllite to reduce the K 2 O content to 1.8-3.2% can work against more than 1000 times of heating-regenerating. Additive Kaolinite Pyrophyllite Silica Sintering temperature (°C) 1500 1500 1500 Duration cycle >1000 >1000 1

Claims: An exhaust gas filter comprising a ceramic fiber including Al 2 0 3 and Si0 2 , an inorganic binder including Al 2 0 3 and Si0 2 to bind the ceramic fiber, characterised in that the organic binder further includes potassium in an amount of 0.8 to 4.2 weight % in terms of K 2 O, and that the inorganic binder is of a homogeneous phase of glass or of a heterogeneous phase of a glass phase and a crystal phase, the crystal phase of which has a crystal structure which has no crystal transformation in the temperature range of 20 to 1200°C. The exhaust gas filter of Claim 1, wherein the ceramic fiber consists of aluminosilicate or of aluminoborosilicate. The exhaust gas filter of Claim 1, wherein the ceramic fiber further includes a crystal nucleus. The exhaust gas filter of Claim 3, wherein the crystal nucleus is Cr 2 0 3 . The exhaust gas filter of Claim 1, wherein the potassium is an amount of 1.8 to 3.2 weight % in terms of K 2 O. The exhaust gas filter of any of Claims 1 to 5, wherein the inorganic binder is of a heterogeneous phase of a glass phase and a mullite phase. A method of manufacturing an exhaust gas filter comprising the steps of preparing a mixture of raw materials comprising a ceramic fiber including Al 2 0 3 and Si0 2 and an inorganic binder material powder including Al 2 0 3 and Si0 2 ; manufacturing a honeycomb structure with a porous wall, an inlet of the exhaust gas leading to the porous wall, and an outlet of the exhaust gas after passing the porous wall from the mixture; and sintering the honeycomb structure; characterised in that the inorganic binder material powder further includes K 2 O of 0.8 to 4.2 weight %, and that the sintering is carried out at the temperature range in which the inorganic binder material powder changes to a homogeneous glass phase or to a heterogeneous phase with a glass phase and a crystal phase of which the crystal structure has no crystal transformation in the temperature range of 20 to 1200°C. The method of manufacturing an exhaust gas filter of Claim 7, wherein the ceramic fiber further contains Cr 2 0 3 of 1.5 to 3.5 weight%. The method of manufacturing an exhaust gas filter of Claim 7, wherein the inorganic binder material powder comprises a sericite. The method of manufacturing an exhaust gas filter of Claim 7, wherein the K 2 0 is in an amount of 1.8 to 3.2 weight%. The method of manufacturing an exhaust gas filter of claim 7, wherein the inorganic binder comprises sericite clay including K 2 O in an amount of 1.8 to 3.2 weight %; the method further comprising the step of manufacturing a honeycomb structure comprising steps of making a planar sheet and corrugated sheet with the mixture, gluing the planar sheet to the corrugated sheet so that cavities are formed, pouring paste for plug-forming into the cavities, the paste comprising aluminosilicate fiber, sericite clay, and inorganic binder, and combining and winding both the sheets into a cylindrical form; and the step of sintering is carried out at the temperature range of 1400-1500°C.

Inventors: KENICHI NAKAMURA, KUNIO KIMURA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: B01D39
IPC codes: C04B35, F01N3, B01D39, C04B38, B01D46