GB2162504A - Process and continuous reaction furnace for production of beta -type silicon carbide whiskers - Google Patents

Abstract

A process for the production of beta -type silicon carbide whiskers comprises mixing carbon powder with silicon dioxide powder, packing the resultant mixture in a reaction container in a bulk density of not more than 0.23 g/cm<3>, and heating the mixture for reaction in an atmosphere of inert gas at a temperature in the range of 1,500 DEG to 2,000 DEG C. A continuous reaction furnace suitable for the process has a core tube 2 axially disposed through a furnace shell 1, coolers 3,3' and air tubes disposed fast around the core tube and paired near opposite open ends of the furnace shell, means 11 for feeding reaction containers 10 in a row disposed at the uppermost part of the core tube, and means 16 for supporting and carrying out the reaction containers 10a, 10b disposed at the lowermost part of the core tube, and, therefore, enables the mixture of raw materials under treatment held in the reaction containers to be continuously subjected to a thermal treatment therein. <IMAGE>

Description

SPECIFICATION Process and continuous reaction furnace for production of fl-type silicon carbide whiskers FIELD OF THE INVENTION This invention relates to a process for the production of P-type silicon carbide whiskers and to a continuous reaction furnace for the production of the whiskers. More particularly, this invention relates to a process for economic production of fl-type silicon carbide whiskers of high quality in a high yield and to an furnace advantageous for the production of the whiskers.

DESCRIPTION OF THE PRIOR ART As means of producing silicon carbide whiskers (hereinafter referred to simply as "whiskers") from solid raw materials, methods using carbon powder as a carbon source and silicon dioxide as a silicon source have been known (as disclosed by Japanese Patent Publication SHO 54 (1979)-17,720 and Japanese Patent Application Laid-open SHO 57 (1982)-111,300, for example). These methods are invariably based on the principle that crystals for whiskers are grown by causing the reaction gas produced from solid raw material to be transferred to a site other than the site at which the raw materials are disposed. Thus, they are required to have the temperature and the partial pressure of gas rigidly regulated. The apparatuses adopted for working these methods, therefore, are so complicated as to render mass production of whiskers difficult.Further, these methods have the disadvantage of producing whiskers in poor yields.

SUMMARY OF THE INVENTION The inventors made a diligent study in search for a process capable of producing reaction between carbon powder and silicon dioxide powder. They have consequently found that the reaction caused within the region accommodating the raw materials proceeds more smoothly and that whiskers are produced easily with satisfactory reactional efficiency by selecting the bulk density in which the mixture of raw materials is packed in a reaction container. This invention has been perfected based on this discovery.

The first object of this invention is to provide a process for producing whiskers of high purity very easily in a high yield. The second object of this invention is to provide a vertical continuous heating electric furnace capable of enabling the mixture of powdered carbon with silicon dioxide powder which should be given thermally treatment at rest (hereinafter referred to simply as "reactant mixture") to be treated at a high temperature as held in reaction containers in motion.

Specifically, this is an furnace which serves advantegeously as an apparatus for the production of whiskers.

The first aspect of this invention resides in a process for the production of whiskers, which comprises mixing carbon powder with silicon dioxide powder, packing the resultant mixture in a reaction container in a bulk density of not more than 0.23 g/cm3, and heating the mixture under an atmosphere of inert gas at a temperature in the range of 1,500 to 2,000"C.

The second aspect of the present invention resides in a vertical continuous reaction furnace providing with electric heaters and having a core tube axially disposed through an furnace shell, coolers and air tubes disposed fast around the core tube as paired near opposite open ends of the furnace shell, means for feeding reaction containers in a roa disposed at the uppermost part of the core tube, and means for supporting and carrying out the reaction containers disposed at the lowermost part of said core tube, whereby the mixture of raw materials under treatment as held in the reaction containers will be enabled to undergo thermal treatment continuously.

BRIEF DESCRIPTION OF THE DRAWINGS The other functions and characteristic features of the present invention will become apparent from the further disclosure of the invention to be given herein below with reference to the accompanying drawings.

Figure 1 is a conceptual diagram illustrating a typical continuous reaction furnace as a preferred embodiment of this invention in a longitudinal cross section.

Figure 2 is a conceptual diagram illustrating another embodiment of this invention in longitudinal cross section.

Figure 3 is an enlarged cross section of paper of the upper stage of an furnace near the site of a packing material.

Figure 4 is an enlarged cross section of part of the lower stage of the furnace near the site of the packing material.

Figure 5 is a plan view illustrating the positional relation in a mechanism for supporting and carrying of reaction containers as seen from the line A-A' of the diagram of Fig. 1 and Fig. 2.

Figures 6A through 6D are perspective views illustrating reaction containers of the apparatus of this invention.

DETAILED DESCRIPTION OF THE INVENTION The carbon powder to be used in the process of the present invention is not specifically limited. Examples of the carbon powder effectively usable herein include pitch coke, petroleum coke, charcoal, anthracite, carbon black, activated carbon, and incomplete combustion soots of such organic compounds as acetylene, available in a powdered form. The carbon powder, to be used advantageously, is desired to have a bulk density of not more than 0.12 g/cm3.

Examples of the silicon dioxide powder effectively usable herein include silicon dioxide, white carbon, silicic acid, and silica flour, available in a powdered form. In this case, the grain size of the powder is not specifically defined. The mixing ratio of the silicon dioxide powder to the carbon powder is generally fixed depending on the bulk density of each of the powders being mixed.Normally it is desirable for the carbon powder to be incorporated in the mixture in a slight excess of the equivalent weight indicated by the formula, SiO2 + 3CoSiC + 2CO. This is because any unreacted carbon powder surviving in the reaction system can be easily removed by oxidation at about 750"C, whereas any unaltered silicon dioxide powder remaining in the reaction sytem cannot be removed unless it is treated with hydrofluoric acid. During the mixture, the two powders are desired to be mixed as uniformly as possible. If they are not mixed uniformly, the resultant mixture may suffer occurrence of particulate SiC or survival of SiO2 in its unaltered form.

The mixed raw materials are subjected to thermal reaction as packed in a reaction container made of graphite, carbon or silicon carbide. The bulk density in which the mixture of raw materials is packed in the reaction container is required to be not more than 0.23 g/cm3. If the bulk density exceeds 0.23 g/cm3, the resultant silicon carbide occurs in the form of finely divided particles or whiskers of small length. Even below the limit of 0.23 g/cm3, the bulk density which is not less than 0.03 g/cm3 and not more than 0.1 5 g/cm3 proves particularly desirable because the produced whiskers have a relatively long and satisfactorily uniform size. If the bulk density is less than 0.03 g/cm3, the productivity is inferior, although the produced whiskers have the same shape and size.

The thermal reaction is caused to proceed at a temperature in the range of 1,500 to 2,000"C. If this temperature is less than 1 ,500'C, the time required for the reaction is excessively long. If it exceeds 2,000"C, the resultant silicon carbide partly occurs in the form of fine particles and the yield of whiskers is not sufficiently high. Since the reactivity of the carbon powder and the silicon dioxide powder is variable with the kinds of the two raw materials, the temperature of the reaction is required to be selected suitably within the range mentioned, depending on the kinds of raw materials being used.

The term "inert gas" as used herein means a gas such as argon or carbon monoxide which is chemically inactive in the reaction system involved in this invention. Neither nitrogen gas nor any nitrogen compound gas is embraced by this term. Nitrogen gas and nitrogen compound gas are undesirable because the reaction performed in the presence of either of the gases byproduces silicon nitride whiskers. The thermal reaction is carried out after the air in the reaction container has been displaced with the aforementioned inert gas. The carbon powder which remains in an unaltered form after completion of the thermal reaction is removed by combustion at a temperature of about 750"C.

The silicon carbide produced by the reaction is in the form of whiskers 0.1 to 1 ym in diameter and 50 to 200 ym in length. Substantially no formation of particulate slicon carbide is recognized. By the analysis with X-ray diffraction, the product is found to be wholly fl-type SiC.

The yield of whiskers is almost 100%, based on the SiO2 component in the mixed raw materials.

The effect of the present invention resides in the fact that by a simple procedure, whiskers of high quality are easily obtained in a yield of almost 100% and the whiskers possess high purity because the reaction involved required no use of catalyst.

The furnace used for thermally treating the mixture of the carbon powder with the silicon dioxide powder is not limited to the vertical continuous reaction furnace which embodies one aspect of the present invention. Furnaces of various other constructions including horizontal furnaces are avaiable for the thermal treatment contemplated by this invention.

The continuous reaction furnace of the present invention is a vertical furnace incorporating electric heaters as illustrated in longitudinal cross section by the conceptual diagrams of Fig. 1 and Fig. 2. It has a core tube 2 axially disposed through a vertical furnace proper 1, coolers 3, 3' and a gas inlet 7 and a gas outlet 7' disposed fast around the core tube 2 as paired near the opposite open ends of the furnace proper, means 11 for feeding reaction containers 10 in a row disposed at the uppermost stage of the core tube, and means 1 6 for supporting and carrying out reaction containers 1 Oa, lOb disposed at the lowermost stage of the core tube 2, so that the mixture under treatment as packed in the reaction containers will be thermally treated therein continuously.

The vertical furnace proper 1 is in a refractory, thermally insulated construction suitably designed by the conventional technique to meet the working temperature conditions involved.

Where the furnace is expected to offer a thermal treatment at elevated temperatures as high as about 2,400"C, for example, it may be designed in a construction such that the core part will be formed of refractories of carbon or graphite, the outer layer formed of silica-alumina type refractory bricks and heat insulating fibers, and the outermost shell made of external materials of steel.

Electric heaters 4, 4' disposed in the vertical furnace proper 1 are desired to be of a directly heating type from the standpoint of thermal efficiency. Optionally, electric heaters of an indirectly heating type may be adopted when necessary.

To be specific, the electric heaters 4, 4' of the directly heating type are obtained by forming the core tube 2 itself with a resistance heating material and connecting bus bars 6, 6' to the core tube 2 respectively through the medium of cooling holders such as, for example, jacketed water-cooled holders 5, 5' of copper as illustrated in Fig. 1. In this arrangement, desired temperature elevation is attained by causing the core tube 2 itself to generate heat. The resistance heating material may be suitably selected to suit the particular temperature of the thermal treatment offered by the furnace. Where the heating temperature ranges from 1,000 to 2,400"C, for example, such heat-resistant material as silicon carbide, carbon, or graphite is adopted.The core tube can be formed solely of one kind of heat-resistant material or of a mixture of two or more kinds of heat-resistant material. It is permissible to use one kind of heatresistant material in the low-temperature zone of the core tube and another kind in the hightemperature zone. Various other combinations of the kinds of heat-resistant material are conceivable. The bus bars 6, 6' may be connected to the core tube 2 inside the furnace proper when necessary.

In contrast, the electric heaters 4, 4' of the indirectly heating type are obtained by burying a resistance heating member 8 made of such resistance heating material as described above inside the furnace proper in such a manner as to encircle the core tube 2 and connecting the bus bars 6, 6' to the resistance heating element 8 through the medium of the aforementioned cooling holders 5, 5' as illustrated in Fig. 2. In this arrangement, the temperature of the core tube 2 is indirectly elevated by causing the resistance heating element 8 to generate heat. In this case, the core tube 2 can be formed of any heat-resistant, thermally conductive material selected to suit the heating temperature. For example, it may be made of silicon nitride, silicon carbide, carbon, or graphite.

The core tube 2 serves to apply heat to the reaction containers 10 being transferred therethrough and, at the same time, constitutes itself as a passage for the transfer of the reaction containers. It can be formed of a suitably material. When the electric heaters 4, 4' are of the indirectly heating type, the core tube may be divided into a plurality of portions arranged in a row in the longitudinal direction of the furnace. It is proper for the exposed surface of the core tube to be covered with a coating layer of a material proof against oxidation or wear such as, for example, an oxide, a carbide, or a nitride, depending on the kind of the heating atmosphere.

The coolers 3, 3' are disposed around the entire circumference of the core tube 2 so as to prevent the surface of the core tube from wear by oxidation and, at the same time, cool the reaction containers in transit inside the core tube. They may be cooling tubes spirally wound around the core tube or jackets slipped over the core tube, for example.

The gas inlet 7 and the gas outlet 7' are pipes laid and adapted to adjust the atmosphere with in the core tube 2. They are connected to a gas source, a gas suction device, a gas recovery and circulation device (not shown), etc., to meet the various purposes such as the formation and retention of an inert atmosphere (using argon or carbon monoxide) within the furnace, the removal of the gas formed in consequence of heat application, and the supply of a reactive gas to be used such as in reaction sintering.

To the opposite ends of the core tube 2 are connected guide tubes 14, 14' of graphite which, as illustrated in the enlarged diagrams of Fig. 3 and Fig. 4, nip the packing materials 9, 9' serving to retain the airtightness of the interior of the core tube and, at the same time, guide properly the reaction containers 10 into and out of the core tube 2. The packing materials 9, 9' are made of a flexbile material such as raw rubber, silicon rubber, or fiuorine resin in a shape having a diameter smaller than the external diameter of the reaction containers 10, so that they will intimately conform with the outer walls of the reaction containers in transit inside the core tube and warrant safe retention of ample airtightness of the interior of the core tube.

The feeding device 11 serves to feed reaction containers 10 into the core tube through the uppermost stage of the core tube 2. It comprises a horizontal transfer device 1 3 for transferring the reaction containers 10 containing the mixture under treatment and mounted on a retainer base 1 2 toward the upper part of the core tube, a guide tube 14 for guiding the reaction containers 10 to the stated position, and a vertical transfer device 1 5 for transferring forcibly the reaction containers 10 downwardly.

The means 1 6 for supporting and carrying out the reaction containers 10 serves to discharge the reaction containers 10 out of the core tube via a guide tube disposed at the bottom of the furnace in cooperation with the feeding device 11 at the upper stage of the furnace. It comprises an elevating device 17 for holding the reaction containers in place during the course of the thermal treatment, a feeding device 1 8 for expelling sidewise the reaction containers discharged out of the base of the furnace by the descent of the rod of the elevating device 1 7, and a retaining bed 1 9 serving as a receptacle for the reaction containers departing from the furnace.The elevating device 17 and the feeding device 18 severally have an arm part 21 of the shape of three sides of a square and a supporting base 20 capable of operating within the gap in the arm part disposed, as opposed to each other, at the leading ends of the rods thereof.

Fig. 5 illustrates an arrangement where in the supporting base 20 is disposed on the elevating device 1 7 side and the arm part 21 of the shape of three sides of a square on the feeding device 1 8 side. This positional relation may be reversed when necessary. The operating parts of the horizontal transfer device 13, the vertical transfer device 15, the elevating device 17, and the feeding device 1 8 mentioned above may be suitably selected from among various mechanisms such as hydraulically or pneumatically actuated piston and cylinder mechanisms and electrically actuated worm and wormgear mechanism which are invariably capable of imparting a reciprocating motion to the rods.

The basic configuration of the vertical continuous reaction furnace of the present invention has been described. Naturally, it may incorporate other devices commonly accepted for furnace operation or may have some of its components substituted with equivalent means.

For example, the furnace proper 1 may be provided with a thermometer or pressure gauge 22 for the control of temperature or pressure inside the core tube 2, with an inspection window (not shown) or a flexible bus bar, or with interlocking and controlled devices for the components of the furnace. The retaining base 1 2 and the retaining bed 1 9 are horizontal stationary beds of flat smooth surfaces. Otherwise, they may be inclined stationary beds of flat smooth surfaces or moving beds such as belt conveyors or roller conveyors.Further, the horizontal transfer device 1 3 in the feeding device 11 may be a robot transfer device 23 capable of nipping reaction containers between arms, separating them from the surface of the retaining base, and transferring them directly onto the core tube as illustrated in Fig. 2 instead of a simple mechanism adapted to reciprocate the rod horizontally. The arm part 21 of the shape of three sides of a square provided in the elevating device 1 7 or the feeding device 1 8 may be adapted to open and close to produce a nipping motion instead of being designed in a motionless type.

The reaction containers 10 may be formed of a suitable heat-resistant material to be selected in due consideration of the conditions of treatment (temperature and atmosphere) and the properties of resisting heat, conducting heat, and withstanding corrosion. When the heating temperature is up to about 1,500 C, for example, they may be formed of refractories of silicon nitride, alumina, or silica-alumina. When they are expected to be resistant to flame of temperatures exceeding 2,000"C, they may be formed of refractories of silicon carbide, carbon, or graphite. They may be formed solely of one material or they may be formed of a plurality of layers of varying material.

The reaction containers 10 may be of an open type or a tightly closed type. Typical examples of the shape of the reaction containers are illustrated in Fig. 6. To be specific, the reaction containuers 10 may be in the form of simple tubes (Fig. 6A), tubes pierced with holes for passing the formed gas and the reaction gas (Fi'g. 6B), tubes having an internal protuberance integrally formed therein to ensure thorough conduction of heat to the core (Fig 6C, and blind tubes provided with a lid to permit airtight closure (Fig. 6D). The individual reaction containers 10 are provided on the upper and lower sides thereof with depressions and matched projections adapted to enable the reaction containers to be stably superposed one on top of another.

The external diameter of the reaction containers 10 can be suitably selected. The thermal efficiency of the treatment is heightened and the ease of separation of adhering dirt from the containers during their descent is increased in portion as the clearance between the inner wall of the core tube 2 and the outer wall of the reaction containers decreases. In due respect of this fact, the external diameter of the reaction containers 10.should be selected so that the clearance will be at least about 2 mm. When this clearance is less than 2 mm, while the electric power in heating the core tube for the production of whiskers of uniform shape can be saved, the possibility of foreign matter deposition in the gap between the core tube 2 and the reaction containers 10 will increase even to the extent of bringing about an interruption of the operation.

Preferably the clearance is in the range of 2 to 1 5 mm. If the clearance exceeds 1 5 mm, the power consumption is more than normally accepted.

The internal diameter of the core tube, though not specifically defined, is often in the range of 50 to 500 mm.

The furnace described above can be operated in numerous ways. A typical operation of the furnace constructed as illustrated in Fig. 1 and Fig. 5 will be described by way of example.

The inert gas is introduced through the gas inlet pipe 7' to displace the interior of the core tube 2 having empty reaction containers 10 piled up therein and create an inert atmosphere therein. Then, water is circulated through the coolers (3, 3' and 5, and 5'), and electricity is fed to the bus bars 4, 5' to elevate the temperature of the core tube to a prescribed level.

Then the reaction container 1 Oa is withdrawn to a position flush with the retaining bed at the base of the furnace by the cooperation of the vertical transfer device 1 5 and the elevating device 1 7. The rod of the feeding device 18 is advanced so that the reaction container 1 Ob will be received on the upper side of the arm part 21 of the shape of three sides of a square and pushed out by the front side of the arm part 21.Thus, the reaction container 1 Oa so held fast in the arm part 21 is moved sidewise out of the bottom of the furnace and transferred onto the retaining bed 1 9. Subsequently, the supporting base 20 connected to the leading end of the rod of the elevating device 17 is elevated through the gap in the arm part 21 of the shape of three sides of a square until it takes hold of the reaction container 1 Ob lying above. The rod of the feeding device 1 8 is subsequently retracted to return the arm part 21 of the shape of three sides of a square from the bottom of the furnace to its original position. By repeating the procedure described above, the reaction containers 10 piled up one on the top of another inside the core tube 2 can be transferred out of the base of the oven one after another.

In the meantime, on the upper side of the furnace, after the reaction container 1 Oa has been withdrawn from the bottom of the furnace, the feeding means 11 is actuated to feed the reaction containers 10 packed in advance with the mixture under treatment by a suitable metering device (not shown) one by one to the core tube 2. To be more specific, the reaction containers 10 on the retaining base 1 2 are moved sidewise onto the core tube 2 by advancing the rod of the horizontal transfer device 1 3. The guide tube 14 align the reaction containers 10 one by one relative to the axis of the core tube 2. Then, the reaction containers are fed downwardly into the core tube 2 by lowering the vertical transfer device 1 5.

By adjusting the speed at which the reaction containers are withdrawn from the bottom of the furnace and they are fed downwardly into the core tube, the reaction containers are retained in the desired highest temperature zone for a prescribed length of time, so that the mixture under treatment packed in the reaction containers may be given the prescribed thermal treatment.

In this while, the inert gas may be introduced through the gas inlet tube 7 or the formed gas may be withdrawn through the gas outlet tube 7' to adjust the atmospher within the core tube.

In this case, the flow of the gas and that of the mixture under treatment occur in opposite directions, the control of the atmosphere under application of heat is attained very easily and the heat of the gas is utilized for the purpose of preheating the mixture under treatment. Thus, the furnace enjoys high energy efficiency.

Even when the volatile component rising from the mixture under treatment while the temperature of the mixture is being elevated is suffered to form a deposit, since the ascending gas comes into counterflow contact with the reaction containers, there is no possibility of the volatile component mingling again with the mixture under treatment within the reaction temperature zone. Even when the volatile component forms a deposit on the wall of the core tube, the deposit is scraped off when the reaction containers while in descent rub against the walls of the core tube. The deposit, therefore, has no possibility of causing any operational hindrance.

During the thermal treatment at the elevated temperature, the mixture under treatment is heated statically relative to the reaction containers because it is held safely within the reaction containers without being exposed to external sh'ock. Even when the mixture under treatment happens to be formed of raw materials of poor shape-retaining property, it is given proper thermal treatment at the elevated temperatures.

The vertical furnace under discussion is suitable for various forms of thermal treatment at various temperatures up to about 2,400'C. It functions advantageously for the production of silicon carbide whiskers and mainfests the following desirable effects. Thus, this invention contributes immensely to economy.

(a) The mixture under treatment is subjected to the thermal treatment as it is held fast in the reaction containers which are automatically transferred by virtue of gravitation. Thus, it receives the thermal treatment statically relative to the reaction containers. No means of conveyance is required to be disposed within the furnace.

(b) Even when any deposit is formed on the core tube, it is scraped off by the descent of the reaction containers. Thus, the operational hindrance as encountered by the horizontal furnace is not suffered.

(c) Since the gas flow and the flow of the mixture under treatment occur in opposite directions, the adjustment of the atmosphere within the oven can be attained easily and the heat of the gas can be utilized advantageously for preheating the mixture under treatment.

(d) Owing to the effects enumerated above, the furnace constitutes an ideal apparatus for the production of silicon carbide whiskers which entails a gaseous-phase reaction.

Example 1: A varying carbon powder and a varying silicon dioxide powder were uniformly mixed. The resultant mixture was packed in a height of 100 mm in tubular reaction containers of graphite (Fig. 6B) and fed to a core tube of graphite 3 m in length and 1 50 mm in internal diameter in a vertical furnace. The other reaction conditions and the results are shown in Table 1.

During the period of temperature elevation until the interior of the furnace reached a constant state and during the cooling of the furnace after stop of the operation, the interior of the core tube was kept filled with argon gas. After completion of the thermal reaction, the reaction product was removed from the graphite containers, placed in a ceramic crucible, and heated at 750"C in an electric furnace to eliminate the unaltered carbon powder by combustion.

The operation of the furnace in the manner described above was continued for 30 days. In this while, the interior of the core tube was not clogged with any deposite and the graphite containers and the core tube were found to have undergone substantially no wear due to oxidation.

Comparative example 1: Mixtures of various carbon powders and various silicon doxide powders were subjected to thermal treatment by following the procedure of Example 1, except using different reaction conditions as indicated in Table 1. The results are shown in Table 1.

Table 1

Carbon powder Silicon dioxide C/SiO2 Bulk Reaction Condition Lengh powder density conditions (1) of wisker of (mol Bulk Bulk of Highest Retention formation wiskers Remarks ratio) Name of density Name of density mixture temperature time (2) (3) substance (g/cm ) substance (g/cm ) (g/cm ) ( C) (hr) ( m) Example 1-1 Acetylene soot 0.03 Silica 0.04 5 0.035 1750 2 +++++ 30#200 " 1-2 Acetylene soot 0.03 Silica 1.00 5 0.08 1850 2 +++++ 50#200 " 1-3 Carbon black 0.12 Silica 0.22 4 0.14 1800 2 ++++ 50#200 flour " 1-4 Activated carbon 0.22 Silica 0.22 4 0.22 1800 2 ++++ 30#150 flour Comparative Carbon black 0.17 Silica 1.00 4 0.24 1850 2 ++ 30#100 example 1-1 " 1-2 Activated carbon 0.20 Silica 1.00 5 0.30 1850 2 + 20#50 " 1-3 Acetylene soot 0.03 Silica 1.00 5 0.08 1450 3 + 50#150 Unaltered portion large " 1-4 Acetylene soot 0.03 Silica 1.00 5 0.08 2200 2 - - Fine particulate SiC (1) The numerals of highest temperature represent the highest values obtained by the actual temperature measurement of the outer surface of the center of the heating element by the use of a radiation thermometer. Those of retention time represent the durations required for samples to travel through zones at temperatures exceeding 1,400 C in Comparative example (Run Nos. 1-3) and zones at temperatures exceeding 1,500 C in all the other examples.

(2) The marks given herein represent the results obtained by firing produced silicon carbide samples to remove residual carbon, rating the produced whiskers by the SEM observation, and classifying the percentages of whiskers in silicon carbide on the six-point scale, wherein +++++ denotes 100%, ++++ 100 to 95%, +++ 95 to 90%, ++ 90 to 60%, + 60 to 0%, and - 0%. The yield of ss-type SiC (granules and whiskers) based on SiO2 content of raw material used was 100% in all the examples except Comparative examples 1-3 and 1-4. In Comparative example 1-4, &alpha;-type SiC was also contained to a great extent.

(3) The ranges represent those of lengths possessed by most whiskers. Each of the samples naturally included whiskers longer than the upper limit and one shorter than the lower limit of the relevant range.

Example 2: A varying carbon powder and a varying silicon dioxide were mixed uniformly. The resultant mixture was packed in stoppered tubular reaction containers of graphite 40 mm in internal diameter, 48 mm in external diameter, and 95 mm in height. The containers were placed at the center of a horizontal electric resistance reaction furnace and heated under a flow argon from room temperature to a prescribed level. The other reaction conditions were as shown in Table 2.

After completion of the thermal reaction, the mixture was removed from the graphite containers, placed in a ceramic crucible, and heated at 750"C in an electric furnace to remove the unaltered carbon powder by combustion. The results are shown in Table 2.

Comparative example 2: Various mixtures of carbon powders with silicon dioxide powders were subjected to the thermal treatment by following the procedure of Example 2, except using different reaction conditions indicated in Table 2. The results are shown in Table 2.

Table 2

Carbon powder Silicon dioxide Bulk Heating Condition Length of C/SiO2 powder density temperature of whisker whiskers Remarks (mol of ( C) formation (3) ratio) Bulk Bulk mixture (1) (2) ( m) Name of density Name of density (g/cm ) substance (g/cm ) substance (g/cm ) Example 2-1 Acetylene soot 0.03 Silica 0.04 5 0.035 1600 +++++ 50#200 " 2-2 Acetylene soot 0.03 Silica 1.00 5 0.08 1800 +++++ 50#200 " 2-3 Carbon black 0.12 Silica 0.22 4 0.14 1750 ++++ 50#200 fluor " 2-4 Activated carbon 0.22 Silica 0.22 4 0.22 1750 +++ 30#150 fluor Comparative example 2-1 Carbon black 0.17 Silica 1.00 4 0.24 1800 ++ 30#100 " 2-2 Activated carbon 0.20 Silica 1.00 5 0.30 1800 + 10#50 " 2-3 Acetylene soot 0.03 Silica 1.00 5 0.08 1400 + 30#100 Unaltered portion, large " 2-4 Acetylene soot 0.03 Silica 1.00 5 0.08 2100 - - Solely particulate SiC (1) The retention time at the heating temperature was 6 hours in Comparative example 2-3 and 1 hour in all the other examples.

(2) The marks given herein represent the results obtained by firing produced silicon carbide samples to remove residual carbon, rating the produced whiskers by microscopic observation, and classifying the percentages of whiskers in silicon carbide on the same scale as used in Table 1. The yield of ss-type SiC (granules and whiskers) based on SiO2 content of raw material used was 100% in all the examples except Comparative examples 2-3 and 2-4.

(3) The ranges represent those of lengths possessed by most whiskers. Each of the samples naturally included whiskers longer than the upper limit and one shorter than the lower limit of the relevant range.

Claims (11)

1. A process for the production of fl-type silicon carbide whiskers, characterised by the steps of mixing carbonaceous powder with silicon dioxide powder, packing the resultant mixture in a reaction container at a bulk density of not more than 0.23 g/cm3, and heating said mixture to cause it to react in an inert atmosphere at a temperature in the range of 1,500 to 2,000"C.

2. A process according to claim 1, wherein said bulk density is in the range of 0.03 to 0.1 5 g/cm3.

3. A process according to claim 1, wherein the bulk density of said carbonaceous powder is not more than 0.1 2 g/cm3.

4. A process according to claim 1, wherein said carbonaceous powder is at least one member selected from the group consisting of pitch coke, petroleum coke, charcoal, anthracite, carbon black, activated carbon and soot formed by the incomplete combustion of acetylene.

5. A process according to claim 1, wherein said silicon dioxide power is at least one member selected from the group consisting of silicon dioxide, white carbon, silicic acid, and silica flour.

6. A process according to claim 1, wherein said atmosphere comprises at least one of argon and carbon monoxide.

7. A vertical continuous reaction furnace having electric heaters, characterised by having a core tube axially disposed through a furnace shell, coolers and air tubes disposed closely around said core tube and paired near opposite open ends of said furnace shell, means for feeding into the top of the core tube a row of reaction containers and an assembly at the bottom of the core tube for removing reaction containers emerging therefrom, thereby enabling a mixture of raw materials held in said reaction containers to be continuously subjected to a thermal treatment.

8. A vertical continuous reaction furnace according to claim 7, wherein said means for removing assembly containers comprises a container supporting device, a container extracting device, said supporting device having a support base and said extracting device having an arm portion of the shape of three sides of a square capable of operatiing within the resulting gap in the arm portions, the support base and the arm portion being mounted at the ends of actuating rods.

9. A vertical continuous reaction furnace according to claim 7, wherein said extracting device comprises a horizontally-acting transfer device, a vertically acting transfer device, and a retainer base.

1 0. A vertical continuous reaction furnace according to claim 7, wherein the difference between the internal diameter of said core tube and the external diameter of said reaction containers is not less than 2 mm.

11. A vertical continuous reaction furnace according to claim 10, wherein said difference is in the range of 2 to 1 5 mm.

1 2. A vertical continuous reaction furnace according to claim 7, wherein said assembly for extracting said reaction containers is provided, separated from the surface of said support base, with a device for grasping said reaction containers and a device for transferring them.

1 3. A vertical continuous reaction furnace according to claim 8, wherein said arm part of the shape of three sides of a square has a device for grasping said reaction containers.