FI84167C - FOERFARANDE FOER FRAMSTAELLNING AV EN KISELKARBIDPRODUKT UR KISELDIOXID OCH EN ORGANISK KOLKAELLA. - Google Patents

Description

1 84167

Process for the preparation of a silicon carbide product from silica and an organic carbon source - Preparation for the production of silicon carbide from an organic carbon source 5

The invention relates to a process for the preparation of a silicon carbide product by bringing together and heating silica and an organic carbon source under reducing conditions.

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Silicon carbide is mainly used in the form of powders for sintering, coated particles such as blended fibers, and pure fibers. Silicon carbide is prepared from silicon and carbon [1], silica and carbon [2, 3], silicon, silica and carbon [4], silica (silica) and an organic carbon source [5, 6] or directly from an organosilicon compound [6, 7].

Lee et al. [5] investigated the production of silicon carbide powder from 20 husks of rice. Rice husks normally contain 15-20% ash, which consists mainly (about 95%) of silica. The final material is mainly cellulose, which acts as a carbon source for the silicon carbide reaction. The treatment was thermal and the starting material was untreated rice husks as well as rice husks dissolved in 10% FeCl 3 solution for 1 hour and further in 9% NIUOH solution for 1 hour, after which the husks were washed. On thermal pretreatment, the product was evaporated for about 2 h at 900 ° C in air, followed by treatment at 1350 ° C in neat carbon monoxide. Thanks to carbon monoxide, silicon carbide has not yet formed at this stage.

The thermally pretreated mixture was reacted to silicon carbide at different temperatures and different pressures of carbon monoxide gas. Temperatures ranged from 1,350 to 1,470 ° C in various experiments. Silicon carbide formation was monitored at various reaction times up to 30 minutes. Finally, the excess carbon was removed by burning for 3 hours at 700 ° C, if necessary.

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The silicon carbide formed was cubic in crystalline form with a particle size of the order of about 0.1 micrometers and about 10% of the product in the form of silicon carbide wiskers. The preparation of silicon carbide products of a certain shape, such as fibers, was not shown.

Wei et al. [6] used colloidal silica in their experiments, which was heated together with organic carbon sources. Initially, the colloidal silica was mixed with a carbon source which was cane sugar in aqueous solution, petroleum pitch in toluene, or phenolic resin dissolved in acetone. Upon mixing, the silica particles were completely covered with a carbonaceous material. In this case, the dispersion formed by colloidal silica bound the matrix solution and formed a gel upon evaporation of the solvent. The dried gel was calcined at 500 ° C under argon to give a fine mixture of silica and carbon. Depending on the batch size and starting material, the calcined gels were allowed to react for 4-16 hours at 1600 ° C. Excess carbon was then removed by oxidation in air at 600 ° C and residual silica was removed by treatment with 20% hydrofluoric acid. The resulting powders consisted mainly of round beta-silicon carbide particles of 10-20 nanometers in size. Fiber products were not made by this method.

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As mentioned above, the most important silicon carbide products are fibers with high strength values and extremely good heat resistance and good chemical resistance. Silicon carbide coated fibers are made by reacting dichloro-30 rimethylsilane vapor on the surface of a tungsten wire.

A mixture fiber of carbon and silicon carbide has also been prepared by immersing the carbon fiber in a molten silicon bath. Blended fibers of silicon carbide and carbon have also been made as a continuous filament by chemically depositing hydrogen-35 and silane gases on a carbon filament substrate in the same manner as in the manufacture of boron fibers. Silicon carbide coated boron fibers are also known.

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Continuous pure silicon carbide fiber with high tensile strength has also been produced in the art [7]. In this case, the starting material is dimethyldichlorosilane, which is dechlorinated with lithium to give dodecamethylcyclohexacilane, which is then converted into an organosilicon polymer. The organosilicon polymer solution is melt-spun into fibers with a diameter of about 10-20 micrometers. To break the organic bonds of the polymer, the organosilicon fiber is heat treated in vacuo and then converted to a continuous silicon carbide fiber having a diameter of less than or equal to 10 micrometers. The length and diameter of the silicon carbide fiber can be easily controlled by the spinning process. Thermogravimetry has shown that organosilicon fiber carbonizes between 300-800 ° C. Above this temperature, the formation of silicon carbide begins, with the fiber treated at about 1,300 ° C giving the best strength values.

Kajiwara [9] has described the formation of beta-silicon carbide fibers from the SiOa-C-NaF system and the dependence of the formed fibers on the particle size of the carbon used in the system.

The reaction temperature was about 1,400 ° C and the reaction time was 1-2 hours. The length of the resulting beta-silicon carbide fibers was less than 3 mm, i.e. a typical Wiskers size.

Silicon carbide products have been used in applications where, in addition to good mechanical properties, good heat resistance, oxidation resistance, corrosion stability and thermal shock resistance are required. Silicon carbide can be made into pure products by sintering, but an interesting area of application is found in composites in which 30 silicon carbide fibers are used to reinforce plastic, ceramic and metal matrices.

As indicated above, β-sic filament fibers can only be prepared by synthesizing from the monomer to the second polymer fiber from which the silicon carbide filament is made by heating, or by depositing silicon carbide on the finished metal, carbon, or boron fiber to form a blended fiber product.

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The object of the present invention is to provide a new method for producing silicon carbide products, with which products of a certain shape, such as filament fiber or film, can be easily produced. It is also an object of the invention to prepare said products from the cheapest possible starting materials. The invention also seeks to utilize available synthesis and processing techniques to minimize the manufacturing costs of silicon carbide products.

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The above objects have been achieved in the present invention by a new method for producing a silicon carbide product, which is mainly characterized by what is stated in the characterizing part of claim 1. Thus, it has been found that the incorporation of base-soluble silica into a conventional viscose-type process gives a product containing both silica and a carbon-rich source which can be easily converted to the desired shape prior to heat treatment to silicon carbide.

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According to the new method, the silica base solution and the alkali cellulose xanthate base solution are first mixed together into a multicomponent solution. The multicomponent solution is then shaped into the desired product, if necessary. In this case, of course, only those forms into which this type of multicomponent solution can be modified are possible. The molded or unmoulded multicomponent solution is then treated with an acidic regeneration solution to provide a substantially solid silica-cellulose intermediate. If necessary, the intermediate can be washed and dried and catalyst or carbon can be added before heating to form the final silicon carbide product.

The silica used in the product of the invention is substantially amorphous silica. It can also be an aqueous glass solution containing an excess of NaOH, which is able to homogeneously mix with a basic solution of viscose, i.e. alkali cellulose xanthate s 84167. The silica can be, for example, a phlogopite extracted silica body or doped silica.

At the beginning of the process of the present invention, a silica base solution and an alkali cellulose saxate base solution are mixed together to form a multicomponent solution. In this mixing step, a basic silica solution obtained by dissolving about 0.5 to 25% by weight of said substantially soluble silica in an aqueous NaOH solution having a concentration of about 10 to 60% by weight is used. The alkali cellulose xanthate solution is preferably a viscose prepared from soluble cellulose by mercerization, maturation, embroidery, post-maturation and filtration. The NaOH content of the viscose is preferably between about 2 and 10% by weight and the Na cellulose xanthate content is between about 5 and 20% by weight, calculated as alpha-cellulose.

For viscose, preferably with some NaOH added to improve miscibility, the preferred viscosity is between 3 and 25 Pas.

According to a preferred embodiment, the preparation of the product is started by mixing together the silica base solution and the viscose in such a proportion that the multicomponent solution contains about 20-80% by weight, preferably about 36-45% by weight, of NaOH base, Na-cellulose xanthate to alpha-cellulose 25 based on about 5-70% by weight, preferably about 5-44% by weight and silica between about 0.1-70% by weight, preferably about

5-40% by weight, all proportions calculated on the dry matter.

Next, a viscous multicomponent solution of silica base solution and viscose is converted to the desired shape. This step is not necessary if the product is in the form of spontaneously precipitated particles. According to one embodiment, the shaping takes place by transferring the multicomponent solution 35 between the parallel and spaced plates, whereby a liquid preform of the film or plate is formed. The preform is then solidified by immersing the plates with their multicomponent solutions in an acidic regeneration solution. According to another preferred embodiment, 6 84167, said multicomponent solution is pressed through a die into one or more profiles, e.g. through a slit nozzle into a film or most preferably through a spinning nozzle into a filament fiber.

5 In this case, the product solidifies automatically so that the regeneration solution is immediately on the outlet side of the nozzle.

A solid silica-cellulose intermediate is formed when the desired multi-component solution is treated with an acidic regeneration solution containing any suitable acid such as HCl or H 2 SO 4. The regeneration solution preferably contains about 5-15% by weight of H 2 SO 4, about 20-40% by weight of Na 2 SO 4 and about 0.0-1.5% by weight of ZnSO 4 and its temperature is regeneration. - i.e. in the solidification step preferably about 40-60 ° C.

The silica-cellulose intermediate, which is preferably a filament fiber or film, is washed or dried, if desired, before being heat treated to form the silicon carbide product.

The silica-cellulose intermediate, which is preferably solidified, is preferably heated to a final temperature of about 900 to 1600 ° C and preferably to a temperature of about 1,200 to 1,500 ° C. The heating, which is carried out under reducing conditions, takes place by raising the temperature slowly or gradually, whereby the moisture is first removed from the product, after which the regenerated cellulose, which acts as a carbon source, begins to carbonize. Above the carbonization temperature, a reaction between silica and carbon begins, which after various steps results in a silicon carbide product having a substantially the same shape as the silica-cellulose intermediate. Slow heating takes place, for example, at a rate of approx. 5 ° C per minute. The heat treatment is preferably performed by maintaining the silica-cellulose intermediate at said final temperature for about 100-300 minutes, preferably about 130-200 minutes.

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To promote the silicon carbide formation reaction, a soluble iron salt may be added to the silica base solution and / or viscose, which promotes carbide formation, if necessary.

At some point in the process, activated carbon, which is preferably in the form of a suspension of a liquid such as ethanol, may also be added to the mixture to be treated.

Often the silicon carbide product contains unreacted carbon which can be easily burned off by heating the product under oxidizing conditions. The oxidation preferably takes place between about 500 and 1000 ° C, more preferably between about 700 and 900 ° C.

According to a preferred embodiment 15 of the process according to the invention, the process for producing a viscose fiber is utilized by initially mixing the basic silica solution with the viscose at the stage when the viscose is dissolved or preferably when it has been post-ripened and filtered. In this case, care must be taken to ensure that the multicomponent solution 20 contains enough sodium so that the silicic acid does not regenerate the cellulose too early from its sodium xanthate form.

In the following, the multicomponent solution of silica, base and viscose-25 further goes through a conventional viscose process, so that the molding and regeneration steps comprise pressing through a spinneret or other die, respectively, directly into an acid regeneration bath solution containing silica and cellulose-containing fibers. Thereafter, the product can be washed and dried in the same manner as in the conventional viscose process.

The process according to the invention is thus a final, heating step, with the exception of substantially similar to the viscose process, and can use equipment from any viscose process. Thus, it is much cheaper and more practical than prior art methods. The products obtained by the conventional viscose process are excellently suited for the heating step in which the silica-cellulose intermediate is heated and maintained at a high temperature to provide the silicon carbide product.

The following examples are intended to illustrate the present invention.

Example 1 60 g of silica-containing silica was dissolved in 380 grams of a 10% alkaline solution (17% NaOH) for 2 hours at 95 ° C. The insoluble impurity was separated to give a solution with a SiO 2 content of 12.3%. The SiO 2 -containing solution was mixed with 300 g of 400 g of viscose containing 5.7% by weight of NaOH and 9.0% by weight of alpha-cellulose. The viscose-15 ratio of viscose was 7.0 Pas.

Air and gas bubbles were removed from the silica-viscose solution, which had been reduced to 1.4 Pas, by pressure. The solution was fed through holes 20 (70 micrometers) of the spinneret into a spinning bath containing 10%

HaSO 4, 20% Na 2 SO 4 and 0.5% ZnSO 4, the fiber bundle was raised to galettes where it was stretched by 70%. The spinning speed was about 20 m / s. The spun fiber was washed, cleaned and dried. The ash content of the dried silica-viscose filament fiber was 32%.

The filament was transferred to a thermogravimetric test furnace equipped with an argon atmosphere. The weight loss of the sample was measured with a temperature program with a heating rate of 30 5 ° C / min up to a holding temperature of 1400 ° C. The sample was kept at this temperature for about 2 hours, as shown in Figure 1, curve (A). The experiment produced silicon carbide filament fiber in such a way that the carbon released during the pyrolytic decomposition of organic compounds and the silica in the fiber react with each other to form silicon carbide:

SiO 2 (s) + 3C (s) = SiC (s) + 2CO (g) (1), 84167 This synthesis is likely to be divided into partial reactions:

SiO 2 (s) + C = SiO (g) + CO (g) (2)

SiO (g) + 2C = SiC (s) + CO (g) (3) 5 The thermogram of this example is shown in Figure 1 (A). After synthesis, residual carbon was removed by annealing a sample of silicon carbide filament at 700 ° C for 3 hours in an air atmosphere. During this time, the mass of the sample decreased by 7.4%.

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The synthesis product was identified as beta-silicon carbide by X-ray diffraction. Figure 2 (A) shows the reflections given by the crystal planes (111).

Figure 3 shows a silica-viscosity filament according to this example before heat treatment, Figure 4 shows a silicon carbide fiber prepared according to this example.

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Example 2

The silica-viscose filament fiber was prepared in the same manner as in Example 1. The silica-viscose filament fiber was pretreated by mixing the fibers into a suspension of activated carbon and ethanol before thermogravimetric analysis.

The pretreated sample was transferred to a thermogravimeter test room with an argon atmosphere. The heating rate was 5 ° C / minute 30 and the final holding temperature was 1,400 ° C. The heating program is shown in Figure 1 (B). It was assumed that the course of the reaction was the same as in Example 1.

After synthesis, residual carbon was removed by annealing 35 samples for 3 hours at 700 ° C in an air atmosphere. This reduced the weight of the sample by 61.8%. Such a large weight reduction was due to activated carbon mixed with the fibers.

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The synthesis product was identified as beta-silicon carbide by X-ray diffraction. Figure 2 (B) shows the reflections given by (111) crystal walls.

5 The following compares the results given by the examples and draws conclusions from them.

Results obtained 10 Figure 1 shows the determined thermograms. Three clear weight drops are observed in both samples: 1. A change in weight of about 7% in the temperature range of 100-200 ° C due to the removal of moisture.

2. Changes of 49% (untreated starting filament) and 36% by weight occur between 200-800 ° C, mainly due to the pyrolytic decomposition of organic compounds.

20 3. At temperatures above 1 250 ° C a change in weight due to the formation of silicon carbide is observed.

The weight changes observed in step 2 per fiber unit 25 are the same magnitude due to the difference in weight of activated carbon in the sample.

The change in weight observed in step 3 is about twice as large per unit fiber as the pretreated sample compared to the untreated sample.

After removal of residual carbon after synthesis by annealing the samples for 3 hours at 700 ° C in an air atmosphere, the observed weight changes were -7.4% (untreated starting material-35) and -61.8%. Because the transfer conditions in the solid-phase reaction are cumbersome, the low residual carbon content of the untreated sample suggests sub-chiometric reaction conditions with respect to carbon.

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Figure 2 compares the reflections given by the crystal planes of both products (111). The wider intensity of the untreated sample indicates a smaller crystal size or amorphous structure, but distortion of the crystal lattice caused by impurities such as oxygen can also cause the phenomenon.

It will be apparent from the foregoing that the process of the present invention has been able to produce a silicon carbide product, e.g., fiber, directly from silica and an organic carbon source.

12 841 67 • Bibliography

Literature: Diploma thesis number: Title: 1. 72 Konijenburg 4. 73 Kennedy et al.

2. Ref. Ilkka Kojo Ackeson

Manufacture of ceramic powders

M.Sc., TKK / V, 1986, p. 68 3. 71 Wei et al.

5. 74 Lee et al.

6. 75 Wei et al.

7. Yajima et al., J. Am. Ceramic Soc., 59 (1976), 325 8. Yajima et al., Chem. Lett., 1975, 931-34 9. Kajiwara, M., J. Mater, Sci., 1988 (7), 900

Claims (13)

13 841 67 A process for preparing a silicon carbide product by combining and heating silica and an organic carbon source under reducing conditions, characterized in that 5 a) the silica base solution and the liquid viscose are mixed together into a multicomponent solution, b) the multicomponent solution is to obtain a substantially solid silica-cellulose product which, if desired, is further molded as desired, and d) heating the substantially solid silica-cellulose product to form a silicon carbide product. 15 Process according to Claim 1, characterized in that step a) is carried out in such a way that the resulting mixture contains about 20 to 80% by weight, preferably about 36 to 45% by weight, of sodium hydroxide, sodium cellulose xanthate calculated as alpha-cellulose n 5-70% by weight, preferably about 5-44% by weight, and silica about 0.1-70% by weight, preferably about 5-40% by weight, all on a dry weight basis. Process according to Claim 1 or 2, characterized in that in step a) a base solution of silica obtained by dissolving about 0.5 to 25% by weight of soluble silica in an aqueous NaOH solution having a concentration of n. 10-25% by weight. Process according to Claim 1, 2 or 3, characterized in that viscose with a NaOH content of between about 2 and 10% by weight and a Na cellulose xanthate content, calculated as alpha-cellulose, of between about 5 and 5% is used. 20% by weight, and that its viscosity is preferably between 35 and about 3-25 Pas. Process according to Claim 4, characterized in that the viscose used in step a) is viscose taken, post-matured and filtered from a conventional viscose process, to which sodium hydroxide is preferably added. Process according to one of the preceding claims, characterized in that a soluble iron salt is added to the solution or solutions before step b) in order to promote the carbide formation of step d). Method according to one of the preceding claims, characterized in that in step b) the multicomponent solution is brought into the desired shape by passing it between parallel, separate plates or pressing it through a die into one or more profiles, such as through a slit nozzle into a film or preferably through a spinneret into filament fiber resulting in the formation of a shaped carbide product in step d). Process according to one of the preceding claims, characterized in that in step c) the shaped multicomponent solution is treated with an acidic regeneration solution containing about 5 to 15% by weight of acid, e.g. HCl or HaSO 4. 20-40% by weight of Na 2 SO 4 and about 0.0-1.5% by weight of ZnSO 4 and preferably having a temperature of about 40-60 ° C. 25 Process according to one of the preceding claims, characterized in that the heating of step d) is carried out under reducing conditions by slowly or stepwise heating to a final temperature of 900 to 1600 ° C, preferably to a final temperature of about 1200 to 1500 ° C. Process according to claim 9, characterized in that the heating of step d) is carried out by keeping the silica-cellulose intermediate after said heating at said final temperature for about 100-300 min, preferably about 120-200 min. is 84167 Process according to one of the preceding claims, characterized in that at some point in the process activated carbon is added to the substance to be treated, preferably in suspension in a liquid such as ethanol. 5 Process according to one of the preceding claims, characterized in that a viscose process is utilized in that the silica base solution according to step a) is mixed with viscose in a step in which the viscose is dissolved and preferably when the viscose is post-matured and filtered and passed through. then as a substantially homogeneous solution with viscose, the process such that steps b) and c) comprise pressing through a spinneret into an acidic spinning bath solution, after which the filament is preferably washed and dried before being heated according to step d) to form a silicon carbide substantially filament product. Process according to one of the preceding claims, characterized in that the finished silicon carbide product is kept under oxidizing conditions at an elevated temperature, preferably between about 500 and 1000 ° C, most preferably between about 700 and 900 ° C, in order to burn off the excess carbon. 84167 BC