FR2583734A1 - PROCESS FOR THE PRODUCTION OF SILICON NITRIDE FROM POLYCARBOSILANE. - Google Patents

Abstract

PROCESS FOR THE PRODUCTION OF SILICON NITRIDE WHISKEYS, CONSISTING OF THE COOKING OF A POLYCARBOSILANE IN THE FORM OF POWDER OR FIBER FIBERS, AT 1300C OR ABOVE, IN AN AMMONIA GAS ATMOSPHERE OR IN A GASEOUS CURRENT OF GASEOUS NITROGEN AND GASEOUS AMMONIA, IN A COOKING OVEN.

Description

i

  PROCESS FOR THE PRODUCTION OF SILICON NITRIDE FROM POLYCARBOSILANE.

  The present invention relates to a process for the production of silicon nitride whiskers, and, more particularly, to a process for the production of silicon nitride whiskers, which exhibit good compatibility with the metal at the time of production. a

metal reinforced with fibers.

  Various types of process have been implemented to date to produce silicon nitride, like this

  is shown by the following chemical reaction schemes.

  The methods thus implemented comprise a method consisting in reducing silica having a high purity with coke, at an elevated temperature, in accordance with the chemical reaction scheme <1), a method consisting in reacting silicon tetrachloride with l ammonia in the vapor phase, in accordance with the reaction scheme (2), a process consisting in decomposing the silicon imide in the vapor phase (reaction scheme (3)), and a process consisting in directly nitriding the metallic silicon (reaction scheme (4) )):: 3SiO2 t 6C t 2N2 Si3N4 + 6CO ... (1) 3SiC14 r 4NH3- Si3N4 t 12HC1 ... (2) 3Si (NH) 2 Si3N4 t 2NH3 ... (3) 3Si t 2N2> Si3N4. (4) In the production of silicon nitride by conventional methods, it is difficult to separate the product perfectly from its starting materials. Consequently, silicon nitride having a high purity is not obtained. If such a less pure silicon nitride is used as a reinforcing material for fiber-reinforced metals (hereinafter designated by the abbreviation "MRF"), the silicon nitride is not D compatible with the metal, which does not allow to obtain

  an NRF with excellent characteristics.

  The present inventors have carried out studies in an attempt to discover a process for the production of an excellent fibrous silicon nitride, which has enabled them to discover that if a polycarbosilane is cooked in the form of powder or of fibers spun, at 1300 ° C. or above, in an ammonia atmosphere, silicon nitride whiskers are obtained, and that, if the whiskers thus obtained are used in the production of a metal reinforced by fibers (] RF), we

  obtains an NRF with excellent characteristics.

  They have thus achieved the present invention.

  The essence of what constitutes the present invention is to obtain silicon nitride whiskeys by baking a polycarbosilane in the form of a powder or spun fibers, at 1300 ° C. or above,

  in an atmosphere of gaseous ammonia.

  The polycarbqsilane is represented by the following formula: l 1iSi-CH2 + n R2 in which Ri and R2 each represent an alkyl group containing from 1 to 5 carbon atoms or a phenyl group, these residues Ri and R2 possibly being identical at the same time , and n is an integer from 1 to 5. The reaction making it possible to obtain the desired silicon nitride is represented by the following reaction scheme: at least 1300QC Ri tSi-CH2tnt NH3 - Si3N4 R2 S The present invention will be explained in more detail detail with reference to the accompanying drawing in which: - Figure i is an X-ray diffraction spectrum of the silicon nitride obtained in accordance with the present invention; - Figure 2 is a photomicrograph (magnification: 3000) of the silicon nitride of Figure 1; and - Figure 3 shows schematically an apparatus

  for the production of silicon nitride.

  We will now explain, with reference to the accompanying drawing, a common apparatus for the synthesis of nitride.

of silicon.

  Referring to FIG. 3, it can be seen that the reference number 1 designates a nitrogen gas bomb having a high purity, the reference number 2, a gaseous ammonia bomb having a high purity, the figures 3 and 4 each designate a flow meter, and 5 and 6 reference numbers each designate a shut-off valve. The reference numeral 7 designates a gas mixer, for mixing the ammonia and nitrogen gases, the tubes for supplying the mixer with ammonia and nitrogen gases being provided with check valves 8 and 9 respectively. The mixture of nitrogen and ammonia gases is dried in a drying column 10, to be sent to an oven

cooking 11.

  For its baking in the baking oven 11, the polycarbosilane is loaded into a nacelle placed in said oven, after which the pressure prevailing in the latter is reduced using a vacuum pump 12, then it is heated, while by passing a mixture of ammonia and nitrogen gas through it, which makes it possible to convert the polycarbosilane present in the furnace into the desired silicon nitride. Furthermore, the reference numeral 13 in the drawing designates a pressure gauge, and the reference numeral 14, a device

  to prevent backflow.

  The silicon nitride obtained in accordance with the present invention is in the form of whiskeys and if used in the production of an NRF, it will have good compatibility with the metal due to its low content of impurities, which provides an RXF with high resistance. Furthermore, the process for the production of silicon nitride in accordance with the present invention is simple in comparison with the conventional processes. More specifically, it consists

  only to treat a polycarbosilane, at 1300 ° C or above

  above, in an ammonia atmosphere.

  The KRF comprising silicon nitride whiskers of the present invention can be subjected to a conventional aluminum treatment technique. Thus, it can be extrusion molded, rolled, and forged to obtain round bars, sheets, deformed articles and pipes, all of which are excellent in terms of tensile modulus, tensile strength and volume. fibrous. Exe = pie Hundred grams <100 g) of particulate polycarbosilane (its average molecular weight is approximately 2000, and its melting point is 220 to 230 ° C) were loaded into an alumina basket, which is placed in a small tubular oven (heat generating element: Siliconit) and then undergo baking inside this oven, for one hour, at 1400 ° C, while 0.5 l / min is allowed to flow . of each of the ammonia and nitrogen gases of high purity, which makes it possible to obtain

a cooked body.

  The cooked body thus obtained was subjected to X-ray diffraction, which made it possible to find peaks respectively at 43.4, 38.9, 35.3 ', 34.8, 31.0, 22.9 and 20.6, for 26, as shown in Figure 1, and it was confirmed that it was a-SisN '. In addition, a photomicrograph with a 3000 magnification of the baked body indicated that the fibers were in the form of whiskerset had a diameter of 0.1 & 2.2 µm and a length of at least 0.2 mm, as this is depicted on the

Figure 2.

  Test 1 (MRF containing silicon nitride whiskeys) An MRF containing silicon nitride whiskers was obtained by a high pressure forging process, in accordance with JIS Natrix A6061. The NRF thus obtained was measured to determine its volume of fibers (vf) with, as a result, the value of 30%. Observation of the internal part of the MRF revealed that the NRF had no vacuum and that both the fibers and the matrix were bonded together enough to form a

consolidated composite.

  The MRF was also measured to determine its mechanical properties at room temperature (293K), with, as results, a tensile strength value of 80 kg / mmi (v 80.10 Pa) and a tensile modulus

of 15 tonnes / mm2 (_ 15.1010 Pa).

  On the other hand, the vf of the classic product was

  % and its tensile strength of 20 kg / crumb (20.107 Pa).

  ERai 2 A sheet-type MRF, containing silicon nitride whiskers, was obtained from the NRF ingot manufactured in Test 1, by rolling. The result of

  the measurement of the vf of the product thus obtained was 30%.

  Observation of the internal part of the product showed that the latter had a consolidated composite structure in which there was no void and the adhesion of the fibers to the matrix was satisfactory. The result of the measurement of the mechanical characteristics of the product & the ambient temperature <293K> was 80 kg / min (80.10 Pa) for the tensile strength, and 15 tonnes / mm2 (15.1010 Pa) for the

traction module.

  A bar-shaped MRF, containing the silicon nitride whiskeys, was produced from the NRF ingot

  manufactured & Test 1, by means of extrusion molding.

  The result of the measurement of vf of the product thus obtained was 30%. It was confirmed that the product was a good composite, which exhibited little fiber breakage and whose

  fibers were oriented in the direction of extrusion.

  The mechanical properties measured at room temperature (293K> were 100 kg / mm9 (100.107Pa) for the resistance to

  traction, and 18 tonnes / mu2 (18.10 Pa) for the traction module.

  Furthermore, the tensile strength of the product at elevated temperature was found to be 80 kg / mm2 (80.107 Pa) at 300 "C, which was a very high value by

  comparison with that of conventional aluminum alloys.

Claims (5)

  1. Process for the production of silicon nitride, characterized in that it consists in baking a polycarbosilane in the form of a powder or spun fibers, & 1300 ° C. or above,   in an atmosphere of gaseous ammonia.   2, A method for the production of silicon nitride according to claim 1, characterized in that the atmosphere of gaseous ammonia consists of a current   gaseous ammonia and nitrogen gas mixed.   3. A process for the production of silicon nitride according to claimlcu2, characterized in that the polycarbosilane has an alkyl group containing from 1 to 5 carbon atoms and a phenyl group in the recurring pattern.   4. A process for the production of silicon nitride according to claima1u2caractérisé in that the polycarbosilane has two alkyl groups each containing 1 to 5 carbon atoms in the recurring unit. 5. Method for the production of silicon nitride according to claimlou2caractérisé in that the polycarbosilane has two phenyl groups in the recurring pattern.