Method for producing silicon-carbide-based composite

Abstract

A method for producing a silicon-carbide-based composite. In the production of a silicon-carbide-based composite comprising a carbon-fiber-reinforced/silicon carbide composite (a C/SiC composite) or a silicon-carbide-fiber-reinforced/silicon carbide composite (a SiC/SiC composite), a film boiling method is carried out, using an organosilicon polymer having a chlorine-free polysilane skeleton and/or a chlorine-free polycarbosilane skeleton. The organosilicon polymer is in a liquid form at room temperature. The molar ratio of Si and C in the matrix of the C/SiC composite or the SiC/SiC composite is Si:C=1:1.08 to 1:1.43.

Claims

1. A method for producing a silicon-carbide-based composite, wherein in the production of a silicon-carbide-based composite comprising a carbon-fiber-reinforced/silicon carbide composite (a C/SiC composite) or a silicon-carbide-fiber-reinforced/silicon carbide composite (a SiC/SiC composite), a film boiling method is carried out using an organosilicon polymer having a chlorine-free polysilane skeleton.

2. The method for producing a silicon-carbide-based composite according to claim 1, wherein the organosilicon polymer is in a liquid form at room temperature.

3. The method for producing a silicon-carbide-based composite according to claim 1, wherein a composition ratio (molar ratio) of Si and C in a matrix of the C/SiC composite or the SiC/SiC composite is Si:C=1:1.08 to 1:1.43.

4. The method for producing a silicon-carbide-based composite according to claim 2, wherein a composition ratio (molar ratio) of Si and C in a matrix of the C/SiC composite or the SiC/SiC composite is Si:C=1:1.08 to 1:1.43.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an explanatory figure showing a mechanism of thermal decomposition of PDMS and conversion into PCS;

(2) FIG. 2 is a characteristic figure showing the molecular weight distribution of LPS;

(3) FIG. 3 is a characteristic figure showing the infrared absorption spectrum of LPS;

(4) FIG. 4 is a characteristic figure showing the 29-Si-NMR analysis spectrum of LPS;

(5) FIG. 5 is a figure showing an SIMS analysis image of SiC;

(6) FIG. 6 is a schematic figure showing a film boiling curve;

(7) FIG. 7 is a process figure showing a processing flow for producing a C/SiC composite;

(8) FIG. 8 is an external view showing an example of a FB treatment device;

(9) FIG. 9 is a characteristic figure showing density transitions of a composite;

(10) FIG. 10 is a microphotograph of a SiC matrix formed by FB treatment;

(11) FIG. 11 is a picture showing an external view of a C/SiC disk formed by FB treatment, and a characteristic figure showing results of X-ray diffraction; and

(12) FIG. 12 is a microphotograph of a SiC matrix formed by FB treatment, and a characteristic figure showing EPMA analysis results.

DESCRIPTION OF EMBODIMENTS

(13) The method of the present invention for producing a silicon-carbide-based composite will now be explained.

(14) As described above, the method of the present invention for producing a silicon-carbide-based composite is a method for producing a C/SiC composite or a SiC/SiC composite, and involves carrying out a FB method using an organosilicon polymer (LPS) having a chlorine-free polysilane skeleton, and/or a chlorine-free polycarbosilane skeleton.

(15) LPS

(16) Here, a typical example of LPS that may be cited is polydimethyl silane (PDMS), which is a polymer in which around 50 units of —SiMe.sub.2-(Me: —CH.sub.3) are linked.

(17) When such a polymer is heated, as shown in the equation on the left side of FIG. 1, the chains are broken one after another (breakage of Si—Si bonds), and the material thermally decomposes into polysilanes of low molecular weight.

(18) Since, from the result of analysis, the average molecular weight is considered to be about 400, accordingly the material is around m=5 to 6 in the formula of FIG. 1. Because of this low molecular weight, the material is liquid at room temperature. Therefore these polymers are appropriately termed LPS (liquid polysilanes). Si—H bonds are formed at the terminals of this polymer, probably by hydrogen being extracted from methyl groups in other polysilanes. Polysilanes with the above or lower molecular weight have higher vapor pressure, and volatilize out of the reaction system as gas. LPS is a low molecular weight substance generated by this decomposition, and is immediately separated from the system by distillation.
The structural formula can be expressed as H(SiMe.sub.2).sub.5-6H (where Me represents a methyl group), but this is not precise because there is a distribution of molecular weights.

(19) (1) Molecular Weight Distribution

(20) FIG. 2 shows the molecular weight distribution of LPS. The average molecular weight is estimated to be about 400 in polystyrene equivalent. The molecular weight is in the range of 133-3437.

(21) (2) Infrared Absorption Spectrum

(22) FIG. 3 shows the infrared absorption spectrum of LPS. It will be understood that LPS is an organosilicon polymer having the structure shown in FIG. 3.

(23) (3) 29-Si-NMR

(24) The results of molecular structure analysis of LPS by 29-Si-NMR are shown in FIG. 4. From the results of infrared spectral analysis and 29-Si-NMR analysis, it is estimated that the molecule is composed of around 7 Si atoms according to 29-Si-NMR, and the ratio of structural units is speculated as around SiC.sub.4:SiC.sub.3H:SiC.sub.3Si=1:3.31:1.08. This is not inconsistent with other analysis results such as FT-IR.

(25) SIMS Analysis of SiC by the MTS-CVI Method

(26) Whether or not chlorine (Cl) remains in a SiC matrix that has been formed by employing MTS (methyltrichlorosilane) as raw material can be checked by SIMS analysis.

(27) The results of SIMS analysis are shown in FIG. 5. Cl was detected by SIMS analysis, as shown in FIG. 5. Accordingly it will be understood that some Cl remains within the SiC matrix when MTS is employed as the raw material. Accordingly, since Cl remains when MTS is used as the raw material, it is possible to distinguish the resulting product from a SiC matrix that is formed by employing LPS as the raw material.
Furthermore since, as described above, it is difficult to apply SMP-10 to the FB method, accordingly it is clear that LPS is appropriate as a raw material for a SiC matrix to be formed around carbon fibers or silicon carbide fibers.
It should be understood that CVD-742, CVD-4000, and CVD-2000 are treatment agents having polycarbosilane skeletons that are applicable to the FB method.

(28) Film Boiling (FB) Treatment

(29) Film boiling (FB) is a phenomenon that is treated as a heat transfer problem in the chemical engineering field.

(30) FIG. 6 is a schematic figure showing a film boiling curve that expresses change of the film boiling phenomenon due to heat transfer at the solid-liquid interface as a relationship between the thermal flux (the heat transfer amount per unit area from the solid phase to the liquid phase) and the degree of surface superheating (the temperature difference at the solid phase/liquid phase interface).

(31) When the solid phase that is in contact with the liquid phase is superheated, convection, and then nucleate boiling take place, and, as the amount of the degree of surface superheating increases, the thermal flux also increases. Normally, the boiling point of the liquid phase corresponds to the latter half of the nucleate boiling region. If the solid phase temperature is raised further, then a vapor film is formed, and the thermal flux rather decreases, so that it becomes difficult to keep both the solid phase temperature and the liquid phase temperature constant (transition boiling). Thereafter, the system transitions again to the film boiling region in which the thermal flux increases along with further increase of the degree of surface superheating.

(32) When the film boiling phenomenon is utilized in the C/C conversion process, the interface between the carbon fiber and the liquid phase (treatment solution: a hydrocarbon based solvent, such as cyclohexane) is covered uniformly by a boiling vapor film, and a carbon matrix is precipitated due to thermal decomposition of this high density vapor. Since a high density vapor film is continuously created in the liquid, accordingly the densification proceeds at high speed.

(33) From the 1990s, research of the FB method has been conducted by P. David et al. at the CEA Royal Institute in France.

(34) The following findings have been obtained as features of the FB method.

(35) (1) The densification proceeds quickly, and the processing time is around 1/50 of the time in the case of the CVI method.

(36) (2) The amount of raw material used can be reduced to around 1/10 of the amount in the case of the CVI method.

(37) (3) The bonding between the fiber and the matrix is strong.

(38) (4) The carbon matrix that is obtained has the same quality as a CVI carbon matrix.

(39) (5) The higher the temperature is and the higher the pressure is, the greater is the rate of matrix generation.

(40) (6) The matrix generation rate is higher for solvents with high carbon content and low dissociation energy.

(41) Decarbonization Processing

(42) First of all, in order to know how far the decarbonization process has progressed, it is necessary accurately to comprehend the remaining amount of the C component. In general, it may be said that it is possible to comprehend the amount of a component by EPMA analysis, but it is difficult to accurately comprehend the amount of the C component with this method.
Thus, with the present invention, the contents of carbon (C), hydrogen (H), oxygen (O), and silicon (Si) are analyzed by the method shown in Table 1.

(43) Since, with this method, the material to be analyzed needs to be powdered, accordingly the SiC matrix formed by the FB method cannot be employed as the test sample for analysis. Accordingly, LPS which is used as a raw material is converted to PCS having an ultra high molecular weight (polycarbosilanization) by a liquid phase-gas phase thermal decomposition method, and is further calcined and converted to a SiC ceramic, which is used as the analysis test sample.

(44) Since the decarbonization process is carried out via a process in which the PCS (an organic ceramic polymer) is converted into a SiC ceramic, accordingly the decarbonization behavior may be confirmed by altering the firing conditions in a conversion process to ceramics.

(45) PCS, which is obtained by keeping it at 475° C. for 20 hours, is mainly used. Calcination in a hydrogen atmosphere can be carried out, using a tubular oven, by superheating the material up to a predetermined temperature at 200° C./hour in 40 volume percent H.sub.2/Ar (250 ml/min), and holding it for one hour. The holding temperature may, for example, be 1000° C., 1500° C., or 1600° C.

(46) TABLE-US-00001 TABLE 1 Analytical means of decarbonated SiC matrix Analytical Subject Method of investigation Device name Elementary C, H, O, Si C: SC-144DR Analysis amount analysis (Manufacturer: Leco) O, H: EMGA-930 (Manufacturer: HORIBA) Si: Decomposition with molten salt decomposition method using sodium carbonate, followed by ICP emission spectroanalysis by using ULTIMA2 (Manufacturer: HORIBA) FT-IR Investigation of FT/IR-4100 (KBr tablet chemical structure (Manufacturer: Jasco) method) XRD Investigation of SiC MiniFlex (Manufacturer: Rigaku) crystallization

(47) Examples of the results of elemental analysis under decarbonization process conditions is shown in Table 2. Only the amount of Si in the molten salt decomposition method has been considered to be unreliable. Therefore, the amount of Si has been obtained by subtracting the total of the other analysis values from 100%, and thus obtained amount has been considered reliable.

(48) TABLE-US-00002 TABLE 2 C/Si molar ratio depending on calcination condition - measurement results Calcination condition C/Si Temper- Element ratio Molar Class ature ° C. Atmosphere Si C H O ratio PCS.sub.LPS 1000 N.sub.2 60.05 36.89 1.12 1.94 1.43 1000 40 Vol % 65.19 31.24 0.74 1.78 1.12 1500 H.sub.2/Ar 67.59 31.41 0.16 1.83 1.08

(49) From these results, it will be understood that, in the present invention, the SiC matrix obtained from LPS, when decarbonized, has a Si/C ratio of around Si:C=1:1.08 to 1.43.

EXAMPLES

(50) In the following the present invention will be explained in more detail with reference to examples thereof, but the present invention is not limited to those examples.

Example 1

(51) A disk-shaped C/SiC composite was manufactured according to the process flow shown in FIG. 7.

(52) In detail, first, a disk shaped preform was made by employing a felt manufactured by Japan Vilene. Co. At this time, this felt consisted of 90 mass % of carbon fiber and 10 mass % of acrylic fiber with a PAN type carbon fiber (HTA) base and Vf is around 10%. Next, molding was performed. This molding was performed by tightening up a bolt until the Vf became 30%, thereby obtaining a molded disk-shaped body with a central hole, having molded dimensions of φ100 (outer diameter), φ30 (hole diameter)×25t (thickness).

(53) Next, the FB treatment device shown in FIG. 8 was constructed by employing this molded body that had been obtained, and FB treatment 1 was carried out. At this time, a body to be processed was formed by sandwiching the molded body described above between a graphite plate and a calcium silicate plate, and this was dipped into an LPS solution and the graphite plate was heated by induction heating. This heating process was carried out by holding for 6 hours at 1200° C. twice. The molded body after this processing was observed with microfocus X-ray CT.

(54) From the X-ray CT image, it has been understood that the overall density is low, but is homogeneous.

(55) Next, FB treatment 2 was carried out. At this time, the graphite plate was removed from the body to be processed described above. The body to be processed was dipped into LPS, and heat application was made (preform direct heating). This heating process was carried out by holding for 4 hours at 1200° C. twice.

(56) Next, the cut body obtained as described above was subjected to HTT treatment for one hour at 1500° C. in an argon atmosphere by employing a small graphitization furnace. In this HTT treatment, the temperature was raised to 1200° C. at 600° C. per hour and from 1200° C. to 1500° C. in 2.5 hours, and then the body was maintained at 1500° C. for one hour. The HTT treated body was subjected to X-ray diffraction analysis (diffraction angle 20°˜50°). It has been found that the SiC conversion progressed considerably due to this HTT treatment at 1500° C. The changes of density due to this series of processes are shown in FIG. 9.

(57) From FIG. 9 it will be understood that, after the final FB treatment, the density of the composite obtained changed to around 1.7 g/cm.sup.3. At this time, at the diametrical center, the generation of a SiC matrix by the FB treatment was confirmed, as shown in FIG. 10.

(58) Furthermore, according to the X-ray diffraction figure shown in FIG. 11, it was possible to confirm a high peak that indicates SiC bonding, and, according to the EPMA analysis shown in FIG. 12, it has also been found that the amount of C was excessive with respect to the stoichiometric ratio of SiC.

(59) While the present invention has been explained with a few embodiments and Examples, the present invention is not limited to these embodiments and Examples, and various changes can be made within the gist of the present invention.

INDUSTRIAL APPLICABILITY

(60) The present invention can be applied to liquid fuel rocket nozzles in the case of C/SiC and to turbine vanes for jet engines and generators and so on in the case of SiC/SiC, and thereby it is possible to obtain a compact and high density product with good manufacturing efficiency.