FR2567873A1 - PROCESS FOR OBTAINING WHISKEYS OF TYPE B SILICON CARBIDE AND CONTINUOUS REACTION FURNACE FOR ITS IMPLEMENTATION - Google Patents

Abstract

THE OBJECT OF THE PRESENT INVENTION IS A PROCESS FOR OBTAINING TYPE B SILICON CARBIDE WHISKERS AND A CONTINUOUS REACTION OVEN FOR ITS IMPLEMENTATION. THE PROCESS IS CHARACTERIZED IN THAT IT CONSISTS OF MIXING CARBON POWDER AND SILICON BIOXIDE POWDER, PLACING THE RESULTING MIXTURE IN A CONTAINER WITH A REACTION FOLLOWING AN APPARENT DENSITY OF NOT GREATER THAN 0.23GCM, AND IN HEATING IT MIXED FOR REACTION PURPOSES IN AN INERT GAS ATMOSPHERE AT A TEMPERATURE BETWEEN 1500 AND 2000C. APPLICATION TO THE MANUFACTURE OF TYPE B SILICON CARBIDE WHISKERS.

Description

-1-

  PROCESS FOR OBTAINING WHISKEYS OF SILICON CARBIDE

  TYPE B AND CONTINUOUS REACTION FURNACE FOR ITS IMPLEMENTATION

  The present invention relates to a process for obtaining silicon carbide whiskers of type and

  a continuous reaction oven for obtaining these whiskers.

  More particularly, the invention relates to a process for the economical manufacture of high quality and high yielding type B silicon carbide whiskers as well as a particularly advantageous furnace for obtaining these whiskers. It is already known to produce whiskers of silicon carbide, which will be called simply whiskers, from solid raw materials by methods using carbon powder as a source of carbon and silicon dioxide as a source of silicon ( Japanese Patent Applications SHO 54 (1979) -17,720 and SHO 57 (1982) -111,300). These methods are all based on the principle of growth of whisker crystals by causing the transfer of the reaction gas from the raw material to a location other than the one where this raw material is disposed. Thus, it is necessary to strictly regulate the temperature and the partial pressure of gas. Devices designed to implement these processes are therefore so complicated that they make it difficult to mass produce whiskers. As a result, these processes have the disadvantage of producing whiskers with low yields. The inventors have carefully investigated a process capable of producing a reaction between carbon powder and silicon dioxide powder. They discovered that the reaction provoked in the appropriate zone of the raw material is carried out more slowly and that the whiskers are obtained more easily with a satisfactory reaction efficiency by the determination of the apparent density of the mixture of the raw materials put in place in a reaction tank. The present invention was carried out on the basis of

this discovery.

  The first object of the invention is to provide a process for obtaining whiskers of high purity very easily and with a high yield. A second object of the invention is to provide a vertical continuous heating electric furnace for the mixture of carbon powder and silicon dioxide powder which must be heat-treated at rest, said mixture being hereinafter simply called reaction mixture, to be treated at high temperature while being contained in moving jet trays. In particular, this oven advantageously serves to

  device for making whiskers.

  The first aspect of the invention resides in a process for obtaining whiskers which comprises mixing the carbon powder with silicon dioxide powder, filling a reaction vessel or container with the aid of the mixture. resulting in an apparent density of not more than 0.23 g / cm, and heating the mixture under a

  inert gas at a temperature between 1500 and 2000C.

  The second aspect of the present invention resides in a vertical continuous reaction furnace provided with electric heaters and having a core tube axially disposed in an oven casing, chillers and air ducts connected to the core tube in proximity to the two. opposite open ends of the furnace shell, means for bringing line reaction vessels to the upper part of the core tube and means for supporting and discharging the reaction vessels at the lower part of said core tube, so that the mixture of raw materials in treatment contained in the

  reaction undergoes heat treatment continuously.

  Other features and advantages of the invention

  will emerge from the following description of modes of

  embodiment of the device of the invention, description given to

  By way of example only and with reference to the appended drawings in which: FIG. 1 is a vertical section of a preferred embodiment of a continuous reaction oven according to the invention;

  FIG. 2 is a vertical section of another embodiment of FIG.

the oven according to the invention;

  3 is an enlarged partial sectional view of a

  part of the upper stage of an oven in the vicinity of the zone of introduction of the reaction vessels;

  FIG. 4 is an enlarged partial view in section of a part

  part of the lower floor of the kiln to the right of the extraction zone; Figure 5 is a top view showing the arrangement of a

  device for supporting and evacuating the reaction vessels

  along line A-A 'of Figures 1 and 2; and FIGS. 6A to 6D are perspective views showing reaction vessels for the apparatus according to the invention. The carbon powder to be used in the process of the invention is not specifically limited. As an example of a usable carbon powder, there may be mentioned pitch coke, petroleum coke, coal, anthracite, carbon black, activated carbon, and soot resulting from the incomplete combustion of organic compounds such as acetylene, available as a powder. The carbon powder must advantageously have an apparent density not

greater than 0.12 g / cm3.

  As an example of a silicon dioxide powder that may actually be used in the invention, mention may be made of silicon dioxide, white carbon, silicic acid, and silica flour, available in the form of a powder. In this case, the particle size of the powder is not specifically defined. The ratio of the mixture of the silicon dioxide powder and the carbon powder is generally set according to the apparent density of each of the powders to be mixed. Normally it is desirable that the carbon powder be incorporated into the mixture with a slight excess of the equivalent weight indicated by the formula SiO 2 + 3C + SiC + 2CO, because any unreacted carbon powder remaining in the the reaction device can be easily removed by oxidation at about 750 ° C, while any unaltered silicon dioxide powder remaining in the reaction device can not be removed unless treated with hydrofluoric acid. During mixing it is necessary that the

  two powders are mixed as uniformly as possible.

  If they are not mixed uniformly, it may occur in the resulting mixture of particulate SiC or

  remains of SiO2 in unaltered form.

  The mixed raw materials are subjected to a thermal reaction after filling in a reaction vessel made of graphite, carbon or silicon carbide. The bulk density of the raw material mixture disposed in the reaction vessel must not be greater than 0.23 g / cm. If the bulk density exceeds 0.23 g / cm3 the resulting silicon carbide appears as divided fine particles or short whiskers. Even below the limit of 0.23 g / cm3, the bulk density is not less than 0.03 g / cm3 and

  not more than 0,15 g / cm3 is particularly desirable.

  because the produced whiskers have a relative size

  large and uniform in a satisfactory manner. If the

  apparent capacity is less than 0.03 g / cm3, the yield is lower although the produced whiskers have the same

size and shape.

  The thermal reaction takes place at a temperature of between 1500 and 2000 C. If this temperature is below 1500 ° C., the time required for the reaction is excessively long. If it exceeds 20000C, the resulting silicon carbide is partly in the form of fine particles and the yield of the whiskers is not high enough. Since the reactivity of the carbon powder and the silicon dioxide powder is variable according to the types of raw materials, the temperature of the

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  The reaction must be appropriately selected within the range mentioned, depending on the types of materials used.

first used.

  The term "inert gas" refers to a gas such as argon or carbon monoxide which is chemically inactive in the reaction system employed in the invention. This term

  does not concern nitrogen or any nitrogen-based gas.

  Nitrogen and nitrogen-based gases are undesirable because the reaction in the presence of either of these gases produces silicon nitride whiskers. The thermal reaction takes place after the air in the reaction vessel has been replaced by the inert gas mentioned above. The carbon powder that remains in the unaltered state 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 micrometer in diameter and 50 to 200 microns in length. There is substantially no formation of particulate silicon carbide. By X-ray diffraction analysis, it is found that the product consists essentially of SiC type. Whiskey manufacturing yield is almost 100%, based on the SiO2 component of the materials

first mixed.

  The advantage of the present invention lies in the fact that by a simple process, high quality whiskers are easily obtained with a yield of almost% and that the whiskers have a high purity because the reaction implemented does not require of

catalyst.

  The furnace used for heat treatment of the mixture of carbon powder and silicon dioxide powder is not limited to the vertical continuous reaction furnace according to the present invention. Furnaces of other modes of construction, including horizontal furnaces, can be used to carry out the heat treatment according to the invention. The continuous reaction furnace according to the present invention is a vertical furnace comprising electric heaters as illustrated in axial vertical section in FIGS. 1 and 2. This furnace comprises a core tube 2 arranged axially through a vertical furnace. 1, refrigerating devices, 3,3 'and a gas inlet 7 and a gas outlet 7' connected to the core tube 2 at the two open ends of the furnace, means 11 for bringing containers or reaction vessels 10 in line at the upper stage of the core tube and means 16 for supporting and discharging the reaction vessels 10a, 10b at the lower stage of the core tube 2, so that the mixture to be treated in the reaction vessels is heat-treated so

continue in these containers.

  The vertical kiln 1 is a thermally insulated refractory structure and designed according to the conventional technique to withstand the working temperature conditions involved. If the furnace is to withstand heat treatment at temperatures as high as about 2400 ° C, for example, it may be designed so that the core portion is made of carbon or graphite refractories, the outer coating being formed of refractory bricks. silica-alumina type and thermal insulating fibers, and

  the outer shell being made of steel materials.

  The electric heaters 4,4 'arranged in the vertical furnace 1 are preferably of the direct heating type from the point of view of thermal efficiency. Optionally, indirect heating type electric heaters may

be used if necessary.

  Specifically, electric heaters 4,4 'of the direct heating type are obtained by producing the core tube 2 itself with a resistance heating material and connecting 6,6' bus bars to the tube core 2 respectively by means of cooling sleeves such as for example sleeves water jacket, 5'de copper as shown in Figure 1. With this arrangement, the desired temperature rise is reached by generating the heat through the heart tube 2 itself. The resistance heating material is suitably selected to achieve the particular temperature of the heat treatment operated by the furnace. When the heating temperature is between 1000 and 2400 C, for example, silicon carbide, carbon, or graphite is adopted as the heat-resistant material. The core tube may consist of only one type of heat-resistant material or a mixture of two or more kinds of heat-resistant materials. It is possible to use a single type of heat resistant material in the low temperature areas of the core tube and another type of material in the high temperature zone. Other combinations of types of heat resistant materials may be used. The busbars 6, 6 'can be connected to the core tube 2 inside the oven if necessary. In contrast, the indirect heating type electric heaters 4,4 'are obtained by inserting a resistive heating element 8 made of a strong heating material as described above, inside the oven so as to surround the tube of core 2 and connecting the busbars 6, 6 'to the resistive heating element 8 via said cooling sleeves 5, 5' as illustrated in FIG. 2. In this arrangement the temperature of the core tube 2 is raised indirectly. by the heat generated by the resistive heating element 8. In this case, the core tube 2 may be made of any heat-resistant thermal conductive material selected to achieve the heating temperature. For example, it may consist of silicon nitride, silicon carbide,

carbon, or graphite.

  The core tube 2 serves to apply heat to the reaction vessels 10 as they move through it and, at the same time, itself forms a passage for moving the containers. It can be made of a suitable material. When the electric heaters 4, 4 'are of indirect heating type, the core tube can be divided into several longitudinally aligned sections in the oven. It is preferable to cover the exposed surface of the core tube with a coating of a material insensitive to oxidation or wear such as for example an oxide, a carbide, or a nitride, depending on the type of the atmosphere of -8-

Ciaul rage.

  The coolers 3,3 'completely surround the core tube 2 so as to prevent the surface of the core tube from being worn or oxidized and, at the same time, to cool the reaction vessels circulating inside the core tube. It can be spiral wound cooling ducts around the core tube or shirts

  threaded on the heart tube for example.

  The gas inlet 7 and the gas outlet 7 'are ducts arranged to regulate the atmosphere inside the core tube 2. These ducts are connected to a gas source, to a gas suction device , a recovery and circulation device (not shown), etc ... for various purposes such as forming or maintaining an inert atmosphere (using argon or carbon monoxide) inside of the furnace, the removal of the gas formed as a result of the application of heat, and the supply of a reactive gas used for example in a sintering reaction. At the opposite ends of the core tube 2 are connected graphite guide tubes 14, 14 'which, as illustrated in FIGS. 3 and 4, pinch the seals 9, 9' serving to seal the air from inside the core tube, and, at the same time, serving as a guide to the reaction vessels 10 at the inlet and the outlet of the core tube 2. The joints 9, 9 'are made of a flexible material such as raw rubber, silicone rubber, or a fluorine resin and are of a shape having a diameter smaller than the outer diameter of the reaction vessels 10, so that they closely follow the outer walls of the containers in transit inside. heart tube and ensure the preservation of a good seal at

  the air inside the heart tube.

  The feeding device 11 serves to bring the reaction vessels 10 into the core tube at the upper stage of the tube 2. It comprises a horizontal transfer device 13 for transferring the reaction containers 10 containing the mixture to be treated and arranged on a support plate 12, in the direction of the upper part of the core tube, a guide tube 14 for guiding the containers 10 in the right position, and a vertical transfer device for forcibly moving down the reaction containers 10. The means 16 for supporting and discharging the reaction containers 10 are used for discharging the containers out of the core tube via a guide tube disposed in the bottom of the oven and cooperating with the device feed 11 to the upper floor of the oven. It comprises a lifting device 17 for holding the reaction containers in place during the heat treatment, a feed device 18 for laterally evacuating the reaction vessels extracted from the furnace base by lowering the stem of the lifting device 17, and a waiting table 19 serving as a receptacle for the reaction vessels leaving the furnace. The lifting device 17 and the feeding device 18 independently comprise a fork-shaped arm 21 with two fingers and a support piece 20 able to move inside said fork, arranged at the end of the fork.

  their respective actuating rod.

  Figure 5 illustrates an arrangement in which the support member 20 is disposed on the elevating device 17 and the arm 21 on the delivery device 18. A reverse arrangement may be provided if necessary. The active parts of the horizontal transfer device 13, the vertical transfer device 15, the lifting device 17 and the feed device 18 above can be suitably selected from the various devices such as actuated cylinder and piston devices. hydraulically or pneumatically and electrically actuated screw and gear devices capable of moving the rods

  following movements back and forth safely.

  The basic configuration of the continuous reaction oven

  vertical according to the present invention has just been described.

  Naturally, one can add various devices commonly mounted on ovens or substitute some

components equivalent means.

  For example, the oven 1 may comprise a thermometer or a pressure gauge 22 for controlling the temperature or pressure inside the core tube 2, an inspection window (not shown) or a conducting bar flexible, or locking and control devices for oven components. The support plate 12 and the waiting table 19 are horizontal fixed plates with a smooth surface. Optionally it may be sloping fixed plates with a smooth surface or moving plates such as belt or roller conveyors. In addition, the horizontal transfer device 13 of the feed device 11 may be an automatic transfer device 23 capable of gripping the reaction vessels in the manner of a gripper between arms to support them from the support plate. to transfer them directly into the core tube as shown in Figure 2, instead of a simple device arranged to horizontally move back and forth the actuating rod. The fork-shaped arm 21 of the elevating device 17 or of the feed device 18 may be arranged to open or close in order to perform a gripping movement in the manner of a gripper instead of being

  arranged to perform a simple displacement.

  The reaction vessels 10 may be made of a suitable heat-resistant material selected according to the processing conditions (temperature and atmosphere) and the properties of heat resistance, thermal conduction and corrosion resistance. When

  the heating temperature reaches 15000C, for example, the-

  containers may be made of refractory materials to

  base of silicon nitride, alumina or silica-alumina.

  When they have to withstand flames with temperatures exceeding 2000 C, they may consist of refractory materials based on silicon carbide, carbon or graphite. They can be formed with the help of a single

  material or multiple layers of various materials.

  The reaction vessels 10 may be of the open type or of the closed and sealed type. Typical examples of the shape of reaction vessels are illustrated in FIGS. 6. More precisely, the reaction vessels 10 may be in the form of simple tubes (FIG. 6A), tubes pierced with holes for the passage of the gas formed and the reaction gas - 11 (FIG. 6B), tubes having a projecting central part forming an integral part of the tubes in order to ensure an efficient conduction of heat towards the core tube (FIG. 6C), and of closed tubes provided with a cover to allow airtight closure (Figure 6D). The reaction vessels 10 are provided on their upper and lower parts with depressions and corresponding projections arranged to allow the containers to be

  stacked steadily on top of each other.

  The outer diameter of the reaction vessels 10 may be appropriately selected. The thermal efficiency of the treatment is increased as well as the ease of separation of the dust adhering to the containers during their transit, inversely proportional to the clearance existing between the inner wall of the core tube 2 and the outer wall of the reaction vessels. As a result, the outer diameter of the reaction vessels 10 must be chosen so that the clearance is at least about 2 mm. When this game is less than 2 mm, although the electric heating energy of the core tube for obtaining uniformly shaped whiskers can be preserved, the possibility of depositing foreign matter in the interval between the core tube 2 and the reaction vessels 10 increases and can

  terminate the operation.

  Preferably the clearance is of the order of 2 to 15 mm. If the clearance exceeds 15 mm, the energy consumption exceeds normal. The inner diameter of the heart tube, although not precisely defined, is mostly included

between 50 and 500 mm.

  The oven described above can work in different ways. By way of example, a typical operation of the oven illustrated in FIGS. 1 and 5 will be described. The inert gas is introduced through the inlet pipe 7 'to empty the inside of the tube of the core 2 in which containers are stacked. reaction 10 empty and to create an inert atmosphere. Then, water is sent into the coolers (3,3 'and 5,5'), and electricity is sent into the conductive bars 4,4 'to raise the temperature

  from the heart tube to the prescribed level.

  Then, the reaction vessel 10a is removed at the base of the furnace by the cooperation of the vertical transfer device 15 and the lifting device 17. The actuating rod of the feed device 18 is advanced so that the reaction vessel 10Ob is received on the upper part of the fork-shaped arm 21 and the reaction vessel 10a is pushed to the side by the end of said arm 21. The reaction vessel 10a thus pushed by the arm 21 is moved laterally to the bottom. of the oven and transferred to the waiting table 19. Next, the support piece 20 fixed to the end of the actuating rod of the lifting device 17 is moved upwards in the gap between the arms of the arm fork 21 until it takes over the reaction vessel 10b located above. Then, the actuating rod of the feed device 18 is retracted to return the arm 21 to its original position. By repeating the above process, the reaction vessels 10 stacked one above the other at the top of the core tube 2 can be discharged at the base of the furnace one after the other. At the same time, at the upper part of the oven, after the reaction vessel 10a has been removed from the base of the oven, the supply means 11 are actuated to present one by one to the core tube 2 the reaction vessels 10 filled in advance of the mixture to be treated by means of a suitable measuring device (not shown). More particularly, the reaction vessels 10 on the support plate 12 are moved laterally towards the core tube 2 by the operation of the horizontal transfer device rod 13. The guide tube 14 aligns the reaction vessels 10 one by one with respect to the axis of the core tube 2. Then, the reaction vessels are lowered inside the core tube 2 by

  lowering the vertical transfer device 15.

  By controlling the rate at which the reaction vessels are withdrawn from the furnace base and circulating inside the core tube, the reaction vessels are held in the highest desired temperature zone for a predetermined period of time, so that the mixture to be treated contained in the reaction vessels can receive the prescribed heat treatment. During this time, the inert gas may be introduced through the inlet conduit 7, or the gas formed may be withdrawn through the outlet tube 7 'to regulate the atmosphere within the core tube. In this case, the flow of the gas and that of the mixture being treated is effected in opposite directions, the control of the atmosphere during the application of the heat is carried out very easily and the heat of the gas is used. to preheat the mixture to

  treat. Thus, the oven has a high efficiency.

  Even when the volatile component from the mixture being processed, because the temperature of the mixture rises, causes the formation of a deposit, since the ascending gas comes into countercurrent contact with the reaction vessels, there is has no possibility that this volatile component recombines with the current mixture of

  treatment inside the reaction temperature zone.

  Similarly, when the volatile component forms a deposit on the wall of the core tube, this deposit is scraped by the reaction vessels during their descent by friction on the walls of the core tube. As a result, this deposit can not interfere with the operation of the oven.

  During the heat treatment at a high temperature, the mixture to be treated is heated in a static state with respect to the reaction vessels because it is preserved inside the reaction vessels and is not exposed to external shocks. Even when the mixture to be treated consists of raw materials having low shape retention properties, it can undergo a

  heat treatment at high temperatures.

  This vertical furnace is suitable for various forms of heat treatment at various temperatures up to about 24000C. It works advantageously for obtaining silicon carbide whiskers and provides the following desirable effects. Thus, the present invention contributes to

  very important way to the economy.

  a) the mixture to be treated is subjected to heat treatment while being kept stable in reaction vessels which are automatically transferred by gravity. Thus, it receives the heat treatment statically with respect to the reaction vessels. No way to

  Conveying is necessary inside the oven.

  b) even when any deposit is formed in the core tube, it is scraped during the descent of the reaction vessels. Thus we do not encounter the disadvantages of the horizontal furnace. c) since the flow of gas and the displacement of the mixture to be treated are effected in opposite directions, the adjustment of the atmosphere inside the furnace can be carried out easily and the heat of the gas can be advantageously used to

  preheat the mixture to be treated.

  d) because of the above effects, the furnace constitutes an ideal device for obtaining silicon carbide whiskers which comprises a reaction in

gas phase.

EXAMPLE 1

  Any carbon powder and any silicon dioxide powder were mixed uniformly. The resulting mixture was poured over a height of mm into graphite tubular reaction vessels (FIG. 6B) and introduced into a graphite core tube 3 m in length and 150 mm internal diameter of a vertical furnace. The other reaction conditions as well as the

  The results are shown in Table 1 below.

  During the period of temperature rise until the inside of the oven reaches a constant state and during the cooling of the oven after stopping operation, the inside of the core tube has been 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 to

3 $ 678? 3

  750 C in an electric oven to eliminate by combustion the

unaltered carbon powder.

  The operation of the oven was continued for 30 days as described above. During this time, the inside of the heart tube has not been closed by any deposit and the graphite containers and the core tube have not

  revealed no wear due to oxidation.

COMPARATIVE EXAMPLE 1

  Mixtures of various carbon powders and various silicon dioxide powders were heat-treated by following the procedure of Example 1 except for different reaction conditions as shown in Table 1. The results are also shown in FIG. the

table 1.

  : uT Ja ::: ::: -TeInDT: -: - :: oOZZ: 80'0: 5: 00'1: oi TITS: ú0'0eu [: -q gTa-e; ed:: -a'ed DTS :: -; 9Dep aFnS: -d aidtuaxs:

:::: AT: :::::: :

  : -Te uoud :::: -uF @ 3xaR :: ú ::: 8 ::: :::: *: Tuou. 05105: + ú: Sl 80'0 5 00'1 a.ITS: ú0'0: k-P.pep aTns:; D helps x:

::::: To: At :::::

  k. OS-OZ. Z 0581. 0úC0 s 5 .. TS.z0 Og4o0 uqeqDZ-; çTxeed:

... ::: :::::.

  :: 001-o0: 01 z'o :: OO ODITS The O: auoq: 1-1 gJexed :: :: :: ':::' :: :: - aeD ap TTON WODu eIdLxS: :: -o: - : I - ::: r ': p u-ze ::: T eTd1s :: :: :: 00T-0u: lll: Z: 008T: ZZ'0 :: 00'O: @ @ o.rITS: Zz'0: gF4e oq 4: II 4atdz:

::::. :::: ::::

  OOZ-05 Z-H + 1: Z: 0081:> 1'0 t, Z'O: 3p auTx2ea: Zl'O -XeD aP aTON: ú-1 @ Tdu'xS:: * :: o -:: o: :: ú1: - p eoIs: - ::: 3 i :: 0OZ0S: llll: Z: 08T: 80'0: S: OO'IFS ú0'0: ap aFnS: ZI aTdaxs: ::::: : ::: - :: u qT :: :: OOZOu: 1I: Z: OSLI: Sú0'0'0 :: 0-tiTS: ú0'0: ,, ae, p aTns: 1-1 atcIdxs: :: MA :::: Tc:: - ..: _: _ __ _ _ _ __: __ :: _ :::: ed &: ::::: (q ::: :: :::: ::: (): m oCas: (o) aaIg3, :( D /) :(: 0/6) 3usq (uo / 6): ouesqns :: (ur,): slaa: sru: @posaid a: a6uelI :: a -u4ed: "_.:a'4u:dd:l:p * kk4.kkkkkk k * kkk klTaP UI The (T aMO :: (E): sap: sduaq: a mk - n dg: np: ( c *: suap 9I P suap sadizez $ g: PmaHSFt: UOF; E = OZ :: - - -: aXua ::; iodie-T): - -: - - -: - ^ -: - - - - :: :: S3p: @p (T) uor "ceq; r: _edd: ZOFS / D: aDTITs ap: o,

  :: m.n6uoi: uoDT4puo: ap sUolTPTPUoDq :: Tsue :: pxo-q p aTlpnod -

  ::::: :: :: :: co T, 4 EriEl j -17- (1) The highest temperature figures represent the highest values obtained by the instantaneous temperature measurement of the outer surface of the center of the heating element using a radiation thermometer. The residence time values represent the times required for samples passing through the zones at a temperature exceeding 1400 ° C. in Comparative Example 1-3 and zones with a temperature exceeding 1500 ° C. in all

the other examples.

  (2) The marks in the table represent the results obtained by burning samples of silicon carbide produced in order to remove the residual carbon, by appreciating the whiskers obtained by the observation SEM, and by classifying the percentages of whiskers in silicon carbide according to a six-degree scale, in which +++++ represents 100%, ++++ from 100 to 95%, +++ from 95 to 90%, ++ from 90 to 60%, + 60 to 0%, and - 0%. The yield of Type 8 SiC (granules and whiskers) based on the SiO 2 content of the raw material used was 100% in all examples except Comparative Examples 1-3 and 1-4. In Comparative Example 1-4, SiC from typed a

  also been found in large quantities.

  (3) The ranges represent those of the lengths of most whiskers. Each of the samples naturally had whiskers longer than the upper limit and some with a length of less than

  the lower limit of the range concerned.

-18-

EXAMPLE 2

  Any carbon powder and any silicon dioxide powder were mixed uniformly. The resulting mixture was poured into tightly plugged graphite tube reaction vessels of 40 mm ID, 48 mm ID and mm height. The containers were placed in the center of a horizontal electrical resistance reaction furnace and heated under a stream of argon at room temperature to a predetermined level. The other reaction conditions are shown in Table 2 below. After completion of the thermal reaction, the mixture was removed from the graphite vessels, placed in a ceramic crucible, and heated at 750 ° C in an electric oven to burn off the unaltered carbon powder. the

table 2.

COMPARATIVE EXAMPLE 2

  Various blends of carbon powders and silicon powders were subjected to a heat treatment by following the procedure of Example 2 except for the different reaction conditions shown in Table 2.

  The results are also shown in Table 2.

TABLE 2

   oudDiesit: Condition. :: eP oudre debiofde ee Lengtho carbon powder d ide eLngusur d. (C / Sio2: rents: Tem ra =: formmationf: wskers Remexques Ci2 'éa mt lwbiskers: ReMaus :( -p- --- du = - (ture of To: (3): C Ncm dsea 1 dansit o _ density port mixed: heated: whiskers (,): Noma of the Noma de lao e sdstece apparent = - Ud o to relate: mOlo) (g / cm ') (c) (1) (2) substatice, g / cm substance: (g / ao3) oo ee es Exenpe2-1 Acetate Soy: 0.03: Silios: 0 /, O104 5: 0.035 os 1600: 50-200 00 eene e: Example 2 Acetyl salt: 0.03 Silica: 0.04 0.035: 1600 s: 50-200 eene 0e 0 ee Exempt 2-2 Su2-acetone: 003 Si: 0.05: 0.08: 1800: ++ ± ++: 50-200:: Example 2-3: Carbon black: 0.12: Flour of: 0.22: 4 0.14: 1750: +: 50-200: rle. Oe ee C: ne: silica: Example 2-4Active carbon: 0.22: Flour of: 0.22 o 4 0, 22: 1750: + +: 30-150::. :: silica - O o: Exemnple cOm : Black of car-: 1800 30100 0.17: Silica '1.00: 4. 0.24: 1800 310: parative 2-1: bone: s: ::: Exeople no 2 l pari2.Carbon active S5ilice, 1 , 00 5 0.30 1800 + 10-50: parative Example 2: Cleavage: 0.03 Silica: 1.00: 0.08: 1400 +. altered i: parative 2-3: lene: :: ::: a! ira head: ::::: e:: e :::: carrying: Exerple ccm-: acety Soy5 008 2100 - SiC parti- : 0.03: Silica: 1005 0.08: 2: parative 2-4 plain notch: ::: To ::: -: cu! Aren ::: ::: e: r: Tr1t:

::::: e :.

  -2o0- (1) The residence time at the heating temperature was 6 hours in Comparative Example 2-3 and 1 hour in

all the other examples.

  (, i) The marks shown represent the results obtained by burning samples of silicon carbide produced to remove residual carbon, by evaluating the whiskers obtained by microscopic observation, and by classifying the percentages of silicon carbide whiskers with the same scale. than that used in Table 1. The yield of Sic type (granules and whiskers) on the basis of the SiO2 content of the raw material used was 100% in all the examples

  except Comparative Examples 2-3 and 2-4.

  (3) The ranges represent those of the lengths of most whiskers. Each of the samples naturally had whiskers longer than the upper limit and some with a length of less than

  that of the lower limit of the range concerned.

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Claims (13)

R E V E N D I C AT IONS - =: = .-. =: = -. =: =: =: =: = .- = _   Process for obtaining type silicon carbide whiskers, characterized in that it comprises mixing carbon powder and silicon dioxide powder, placing the resulting mixture in a reaction vessel at a bulk density of not more than 0.23 cm, and heating said mixture for reaction in an inert gas atmosphere at a temperature between 1500 and 2000 C.   2. Method according to claim 1, characterized in that the bulk density is between 0.03 and 0.15 g / cm3. 3. Process according to claim 1, characterized in that the apparent density of said carbon powder is not not more than 0.12 g / cm3.   4. Process according to claim 1, characterized in that the carbon powder consists of at least one element selected from the group formed by pitch coke, petroleum coke, coal, anthracite, black and white. carbon, activated carbon and incomplete combustion soot acetylene.   5. Process according to claim 1, characterized in that said silicon dioxide powder consists of at least one element chosen from the group formed by silicon dioxide, white carbon, silicic acid and silica. silica flour.   6. Process according to claim 1, characterized in that said inert gas consists of at least one element = oisi among the group formed by argon and carbon monoxide. 7. Vertical continuous reaction furnace comprising electric heaters, characterized in that it comprises a core tube arranged axially inside an oven casing, chillers and air ducts arranged at the two opposite ends of the furnace. said furnace casing, means for bringing reaction containers one by one to the upper portion of said core tube, and means for supporting and discharging said reaction vessels at the lower end of the core tube; so as to subject the raw material mixture contained in said reaction vessels to a heat treatment   continuous inside said core tube.   Vertical continuous reaction furnace according to claim 7, characterized in that said means for supporting and discharging said reaction vessels comprise a lifting device, a feed device, and a stand-up table, said lifting devices and with a two-finger fork-shaped arm and a support piece capable of moving inside said fork, arranged at the ends of the   actuating rods of said devices.   9. vertical continuous reaction furnace according to claim 7, characterized in that said feed device comprises a horizontal transfer device, a   vertical transfer device and a support plate.   Vertical continuous reaction furnace according to Claim 7, characterized in that the difference between the internal diameter of the core tube and the external diameter of the   reaction vessels is not less than 2 mm.   Vertical continuous reaction furnace according to claim 10, characterized in that said difference is between 2 and 15 mm.   12. Continuous vertical reaction furnace according to claim 7, characterized in that said means for supporting and discharging the reaction vessels comprise, at a distance from the surface of said support plate, a device for gripping, in the manner of a gripper, the reaction vessels   and a device for transferring them.   13. Continuous vertical reaction furnace according to claim 8, characterized in that said fork-shaped arm with two fingers comprises a device for gripping at   the manner of a clamp said reaction vessels.