PROCESS OF PRODUCING A CERAMIC HONEYCOMB STRUCTURAL BODY

Abstract: This invention provides a method of firing a ceramic honeycomb structure which comprises varying the oxygen concentration in a firing atmosphere in a temperature range where a molding aid and/or a pore forming material is difficultly burnt or in a temperature range where they are burnt, to inhibit abrupt heat buildup of the aid etc., increasing the oxygen concentration in the atmosphere in a temperature range where the pore forming material and/or the molding aid is difficultly burnt to promote their combustion so as to reduce the temperature difference between the internal and external parts of the honeycomb structure, and making uniform the internal and external characteristics without causing any crack inside and at the end parts of the honeycomb structure and without causing internal melt loss. In this manner the method of the invention enables satisfactory firing. Moreover, since it is not necessary to reduce the firing rate, the firing schedule can be shortened and production efficiency can be improved. Accordingly, the present invention is particularly effective for a ceramic honeycomb structure.

Description: The present invention relates to a process of producing a ceramic honeycomb-structural body, and more particularly to a method for firing a honeycomb-structural body of ceramic material. It is the common practice in the preparation of a honeycomb-structural body that a green body of ceramic mixture prepared by mixing ceramic material and a molding aids and/or a pore-forming agent is molded firstly to a desired shape in a forming mold and then extruded to obtain a desired honeycomb-structural body. Thereafter the thus-prepared body is fired in a continuous-feed furnace or in a periodic kiln under a predetermined temperature to attain the final ceramic honeycomb-structural body. In the firing of such a honeycomb-structural body, several materials are employed to be mixed into the ceramic material, such an organic binder and a surface-active agent as methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, starch paste, wheat flour, glycerol, such molding aids as wax and the like, and such a pore-forming material as graphite, starch, sawdust and the like, which all exhibit a differential specificity, as reviewed below. More particularly, these molding auxiliaries and pore-forming material may generally suffer from the inconvenience that they can hardly be heated from the outside of the honeycomb-structural body during the firing operation, while once they are fired, they may be burned abruptly to generate heat, and additionally each of such molding aids has a different ignition temperature. For this reason, there may possibly be generated a substantial difference in the temperatures between the interior and the exterior of such a honeycomb-structural body. With such a differential temperature, it is possible that there may be generated cracks in the inside or end surface of the honeycomb-structural body or melting in the interior thereof. Also, owing to a substantial difference in the degree of firing of such molding auxiliaries and/or pore-forming agent existing in the interior and the exterior of the honeycomb structure, the properties of such honeycomb-structural body may possibly become non-uniform between the interior and the exterior of the structure (for instance, if a pore-forming agent in a certain area has burned abruptly, it cause a generate irregularly larger pores in the specific area where there is such an abrupt burning). In this respect, therefore, it is generally the practice in the firing operation of such a ceramic green body to employ a smaller rate of temperature increase rate for the purpose of suppressing an abrupt firing in the interior of a ceramic green body in an attempt to prevent the generation of cracks and interior melting of the green body and the resultant non-uniformity between the interior and the exterior in the properties of the fired products. However, with a lower temperature increase rate in the firing operation, the problem arises that the schedule of operation may naturally be extended, and the efficiency of production may be reduced accordingly. US-A-4055614 describes a method of firing a green ceramic body in which the body is covered with a metal foil, capable of being oxidized at high temperature. The foil limits the concentration of oxygen at the interior of the body, to prevent abrupt firing of the interior. In view of the foregoing, an object of the present invention is to obviate the above-mentioned such problems by providing an efficient firing process for manufacturing of ceramic honeycomb body. The present invention is set out in claim 1. By the present invention, it is arranged that the concentration of oxygen in the firing atmosphere is increased in the range of temperatures where a molding aid or a pore-forming agent may hardly be burned so that such agents may be forced to be burned on one hand, and the concentration of oxygen in the firing atmosphere is decreased in the range of temperatures where such agents may be fired abruptly so that they may be suppressed from being fired on the other hand, and consequently, there can be attained such advantageous effects that a differential temperature existing between the interior and the exterior of a ceramic honeycomb structure being fired may well be held smaller so that the generation of cracks in the inside and the end surfaces of the structure as well as melting in the interior thereof may efficiently be prevented from occurring, and that the fired product may turn to be uniform in quality and properties throughout. Moreover, as it is not required to reduce the rate of temperature increase during the firing operation, the efficiency in production can then be made better accordingly. Fig. 1 of the drawing is a cross-sectional view showing generally the construction of a periodic kiln used for the practice of a firing process according to the present invention. In the drawings, the following items are shown:-10: a sample; 11: a periodic kiln; 12: a carriage; 13: a receiving plate; 14: a furnace burner; 16: a direct fire shield plate; 17: a thermocouple; 18: an oxygen sensor Explanation of one mode of performing the invention:- In preparation of the firing of a ceramic honeycomb-structural body to be manufactured according to the present invention, firstly, ceramic material of a desired grain size is mixed homogeneously and this is then admixed with a molding aids and/or a pore-forming agent to obtain a plasticized batch of moldable ceramic mixture, the thus-prepared batch of plasticized mixture is subjected to the extrusion step to be molded to a desired shape, and thereafter is dried to obtain a body ready to be fired in a furnace. Next, in the following step of firing, it is arranged according to the present invention that the concentration of oxygen in the firing atmosphere is adjusted accordingly to a differential temperature between the interior and the exterior of the molded body which is being measured during the firing operation, so that the degree of burning of molding aids and pore-forming agent may be controlled, and so that the prepared molded body may be fired optimally to obtain a desired ceramic honeycomb-structural body. For this, there are provided at least two thermocouples in appropriate positions of the interior and the exterior of a specific molded article to be fired for the purpose of measuring a differential temperature therebetween. This differential temperature to be measured between the interior and the exterior of the molded material by such thermocouples may be either a positive value or a negative one. A positive value of differential temperature represents that a temperature in the interior is lower than that in the exterior of the article out of the range of temperatures in which a molding aids and a pore-forming agent may be burned, while a negative value represents that an interior temperature is higher than an exterior temperature of the article owing to heat generated from such admixed agents in the temperature range that they are being burned, respectively. More specifically, the measuring of a differential temperature is adapted to identify a range where the sense of differential temperature may change, namely, either from the positive to the negative or vice versa , to enable appropriate control of the concentration of oxygen in the firing atmosphere. In practice, the control of such a concentration of oxygen in the atmosphere may be considered by way of the following means. 1) For increasing the instant concentration of oxygen in the furnace: 1 ○ increasing the air-fuel ratio in a furnace burner by the introduction of an increased diffusion rate of air. 2 ○ adding the instant percentage of oxygen gas in the combustion gas to be fed to the burner. 2) For decreasing the instant concentration of oxygen in the furnace: 1 ○ decreasing the air-fuel ratio in a furnace burner by decreasing the quantity of combustion gas fed to the burner. 2 ○ adding nitrogen gas to the combustion gas to be fed to the burner. Additionally, for the accurate control of the concentration of oxygen, an oxygen sensor is provided in an appropriate position in the atmosphere of the furnace. Also, in consideration of the fact that as a pore-forming agent is admixed thoroughly in the molded ceramic body, it has generally a limited chance to contact with oxygen in the firing atmosphere so that it is rather hard for it to be fired during the firing operation, and that once it is fired, it can hardly be extinguished, it is then preferred to provide the firing atmosphere with an excessive concentration of oxygen. In practice, it is preferable to employ appropriate molding aids selected from the group consisting of an organic binder, a surface-active agent, waxes, such as, for example, methyl cellulose, carboxymethyl cellulose, poly(vinyl alcohol), starch paste, wheat flour, glycerol, and an appropriate pore-forming agent selected from the group consisting of graphite, starch, sawdust. Example 1 Ceramic materials such as kaolin and alumina are prepared to be mixed together in an appropriate mixing ratio to obtain mullite composition, to which glycerol or a surface-active agent is admixed as a molding aid, which is to be plasticized, molded and then dried in the following steps for attaining a desired molded article. Thus-prepared molded article 10 was put onto a top plate 13 positioned on a traveling carriage 12, which is movable through the interior of a tunnel furnace 11, in the manner as shown in Fig. 1, and then subjected to the firing operation under the conditions shown in Table 1 In the firing operation, while a plurality of burners 14 mounted in the opposed side walls of the furnace were provided as heating means, it is preferred that solid shield plates 16 made of mullite-contained material are arranged in position between outer posts of the top plate 13 so that a plurality of prepared shaped articles (ceramic honeycomb-structural bodies 10) is not subjected directly to firing from the burners. Also, there were provided two thermocouples 17 at appropriate positions in the inside and the outer surface of one of the prepared shaped articles 10 disposed within the tunnel furnace 11, and further, an oxygen sensor 18 was placed in the side wall of the furnace in such a manner that its sensing head extends there through into the interior space of the furnace 11. Preferably, it may be arranged that there are provided a plurality of openings 19 in receiving plates 15 which oppose each of the honeycomb-structural body 10 disposed thereupon so that the atmosphere of the furnace may be assisted to pass efficiently through the interior of the honeycomb-structural body 10, and also provided an exhaust passageway 21 extending through the carriage 12 and the furnace bottom to an exhaust blower for directing a mass of atmosphere to the outside so that no substantial difference in temperatures may exist between the interior and the exterior of the honeycomb-structural body. With this arrangement, the furnace atmosphere was heated to the maximum temperature of 1400°C and held at this temperature for two and half hours with the prepared shaped article disposed in position, and then cooled at the cooling rate of 150°C/hr. The results obtained from this firing operation are shown in Table 1. Sample No. Product by the invention Reference product 1 2 3 Composition of fired product Mullite Mullite Mullite Starting material Kaolin & Alumina Kaolin & Alumina Kaolin & Alumina Molding aids Glycerol Surfactant Glycerol Pore-forming agent None None None Temperature rising rate (°C/hr) 100∼ 500°C 65 65 50 100∼1200°C 100 100 100 O₂ Concentration (%) 200∼ 300°C 10 10 19 500∼ 800°C 15 15 15 800∼1000°C 12 12 12 Range where differential temperature between product interior and exterior is negative (°C) When molding aids fired 200∼400 150∼350 200∼300 When pore-forming agent fired None None None Absolute differential temperature under above condition (Max) (°C) When molding aids fired 20 30 100 When pore-forming agent fired None None None Crack generating rate (%) 0 0 55 Interior melt generating rate (%) 0 0 0 Difference in bores at the interior and exterior of the product (µ) 0.5 1.0 0.5 Evaluation ○ ○ X As is evaluated from Table 1, in the case that the oxygen concentration in the furnace atmosphere is not controlled as in the conventional manner, it is found that the differential temperature between the interior and the exterior of the molded shaped article turned to be a negative value in the range of temperatures from 200 to 300°C owing to heat generated from the fired molding aids, and that its absolute differential temperature was as large as 100°C maximum. In contrast, however, with the Samples Nos. 1 and 2 processed with the oxygen concentration of 10% in that range of temperatures according to the present invention, it is noted that the negative range of temperatures were from 200 to 400°C and from 150 to 350°C respectively, and also its absolute differential temperature was as small as 20°C and 30°C maximum, respectively. From a visual check of thus-fired products processed under such conditions for the crack generation rate (%), the interior melt generation rate (%) and the difference in bores formed in the interior and the exterior of the products (µ), it is observed that the products according to the present invention turned out with no generation of cracks, in contrast to the case of the comparative product which had a crack generating rate of 55%. Example 2 Ceramic materials such as talc, kaolin and alumina are prepared to be mixed together with an appropriate mixing ratio to obtain cordierite composition, to which mixture starch paste or methyl cellulose is admixed as a molding aid, and further with sawdust or graphite as a pore-forming agent, which is to be plasticized, molded and then dried in the following steps for attaining a desired molded shaped article. Thus-prepared shaped article was mounted onto the receiving plate in the interior of a tunnel furnace 11 in the same manner as in the Example 1, and then subjected to the firing operation under the conditions shown in Table 2. In the firing operation, the furnace atmosphere was raised to the maximum temperature of 1350°C and held at this temperature for six hours with the prepared shaped article disposed in position, and then cooled off. The results obtained from this firing operation is shown in Table 2. Sample No. Product by the invention Reference product 1 2 3 Composition of fired product Cordierite Cordierite Cordierite Starting material Talc, Kaolin & Alumina Talc, Kaolin & Alumina Talc, Kaolin & Alumina Molding aids Starch paste Methyl cellulose Starch paste Pore-forming agent Sawdust Graphite Sawdust Temperature rising rate (°C/hr) 100∼ 500°C 80 90 60 100∼1200°C 100 115 60 O₂ Concentration (%) 200∼ 300°C 8 6 18 500∼ 800°C 21 21 15 800∼1000°C 10 9 12 Range where differential temperature between product interior and exterior is negative (°C) When molding aids fired 200∼400 200∼400 200∼300 When pore-forming agent fired 500∼1000 500∼1000 600∼900 Absolute differential temperature under above condition (Max) (°C) When molding aids fired 25 15 120 When pore-forming agent fired 30 20 80 Crack generating rate (%) 0 0 75 Interior melt generating rate (%) 0 0 30 Difference in bores at the interior and exterior of the product (µ) 2 0.5 15 Evaluation ○ o ○ X As is evaluated from Table 2, in the case of comparative products for which the concentration of oxygen in the furnace atmosphere is not controlled as in the conventional manner, it is found that the differential temperature between the interior and the exterior of the molded shaped article was a negative value in the range of temperatures from 200 to 300°C and from 600 to 900°C, and that its absolute differential temperature was 120°C and 80°C maximum, respectively. Also, it is observed that the differential temperature was a positive value in the range of temperature from 500 to 800°C, and that its absolute differential temperature was 80°C. In contrast, however, with the Samples Nos. 1 and 2 processed with the oxygen concentration of 8 and 10%; and 6 and 9% in those ranges of temperatures tending to a negative value of differential temperature according to the present invention, it is noted that their absolute differential temperatures turned out to be 25°C and 30°C; and 15°C and 20°C maximum, respectively. Also, with the oxygen concentration of 21% in the furnace atmosphere in the range of temperatures where the differential temperature was a positive value, its absolute differential temperature turned to be 50°C. From the visual check-up of thus-fired products processed under such conditions for the cracks generation rate (%), the interior melt generation rate (%) and the difference in bores formed in the interior and the exterior of the products (µ), it was observed that the products according to the present invention have a small order of dispersion of bores as of 2 and 0.5 µ, respectively, in contrast to the case of a product compared with the difference in bores was as large as 15 µ. While the invention has been described by way of embodiments with a certain degree of particularity, it is to be understood that the invention is not restricted to the details of such embodiments, but various changes and modifications may be made in the invention by those skilled in the art. As is apparent from the detailed description presented hereinbefore, according to the firing process for the manufacture of a ceramic honeycomb-structural body of the present invention, the abrupt heat generation of a molding aids and/or a pore-forming agent admixed to a ceramic material may be suppressed by decreasing the oxygen concentration in the firing atmosphere in the range of firing temperatures for such an admixed agent on one hand, and in reverse the firing of such an admixed agent may be facilitated by increasing the concentration of oxygen in the firing atmosphere in the range of temperatures where such an admixed agent can hardly be fired on the other hand, thereby to afford a substantial decrease in a differential temperature existing between the interior and the exterior of the burned honeycomb-structural body, which may consequently effect the prevention of cracks in the inside or in the end surface of such a product or interior melts thereof from being generated, and which may contribute to the attainment of uniformity of the specific properties in the interior and the exterior of such a product, whereby an optimal firing may be afforded, accordingly. Moreover, since it is not required to make the temperature rising rate smaller during the firing operation, the schedule of firing operation can be made shorter, and the efficiency in production can be improved accordingly.

Claims: A process of producing a ceramic honeycomb-structural body comprising mixing a ceramic material with at least one additive selected from molding aids and pore-forming agents to prepare a green ceramic material, extruding the green material to form a honeycomb-structural body and firing the body, wherein said additive is removed by firing with control of oxygen concentration in the firing atmosphere outside the body in dependence on the removal rate of said additive by decreasing said oxygen concentration in the temperature range of decomposition of said additive relative to said oxygen concentration at a temperature range at which said additive is not or is hardly burned from the body. A process as defined in claim 1, wherein the oxygen concentration in the firing atmosphere outside the body is higher in the range of firing temperatures which is lower than the temperature of decomposition of said additive than at the temperature of decomposition of said additive. A process as defined in claim 1 or claim 2 wherein said oxygen concentration outside the body is controlled during firing in dependence on a temperature difference between the interior and exterior of the body. A process as defined in claim 3 including providing temperature sensors at the interior and exterior of the body and detecting said temperature difference with said sensors.

Inventors: MIYAHARA, KAZUHIRO NIPPON GAISHI M.K. APT. 201
Applicants: NGK INSULATORS, LTD.
Application year: 1989
Main classification: C04B38
IPC codes: F27B9, C04B35, C04B38