Ti 변성 폴리카르보실란으로부터 C-SiC-TiC 계 복합섬유의 합성

강 필 현 · 양 현 수

충남대학교 대학원 공업화학과

Synthesis of C-SiC-TiC Composite fiber Ti modified Polycarbosilane

Phil H. Kang, Hyun S. Yang

Dept. of Industrial Chemistry, Graduate School, Chungnam National Univ., Taejon

305-764, Korea

1. INTRODUCTION

In inert or reducing atmospheres, carbon materials have excellent properties at high temperatures. For example, the mechanical strength of several different grades of graphite was highest at a temperature of about 2500℃. Hence, carbon materials have been used widely in applications requiring high temperature stability, such as in the aerospace industry. However, there is a problem to be solved; that is, carbon materials are oxidized easily in air. there are two technical methods of oxidation protection for carbon materials. One is to coat the surface of carbon materials by refractory materials. The other is to compose carbon materials  with ceramics, such as SiC. In this study, we note that polytitanocarbosilane(PTC) as a SiC/TiC precursor has attracted special interest, because PTC is pyrolyzed to be SiC/TiC in the same way as  carbon fibers are made, and we can expect to get a homogeneous C/SiC/TiC composite fiber. A C/SiC/TiC composite fiber was obtained by heating a C/PTC precursor, which was prepared from the carbon fiber impregnated with a mixed solution polycarbosilane and titanium tetra butoxide. The reduction of the precursor fiber was investigated. The reduction heat-treatment was done from 1200 to 1500℃. The reaction composite fiber aws examined by X-ray diffraction, SEM observation and chemical analysis.

2. EXPERIMENTAL

1. Experimental Procedure

Anhydrous HPLC p-xylene and sodium metal were put into a four-necked flask, and heated to the boiling point of p-xylene in a stream of nitrogen gas, and dimethyldichlorosilane was added dropwise over 1 hour. After the addition, the mixture was heated under reflux for 15 hours to form a precipitate. The precipitate

was filtered, and washed with methanol and then with deionized water, and dryed at vacuum drying oven for 24 hours to afford polydimethylsilane as a white fine powder. The polydimethylsilane was put into a stainless steel autoclave, and heat-polymerized in an nitrogen atmosphere at 470℃ and about 100 atmospheres for 14 hours to obtain crude polycarbosilane. The crude polycarbosilane was allowed to cool at room temperature and n-hexane was added. The crude polycarbosilane was thus taken out as a solution in n-hexane. The n-hexane was evaporated by using rotaryevaporator, and the residue was concentrated at 280℃ and 1 mmHg for 1 hour. The resulting solid was treated with acetone to remove fraction of low molecular weight and obtained yellowish solid polycarbosilane.

Polycarbosilane and titanium tetra butoxide were weighed, and anhydrous HPLC p-xylene was added to the mixture of form a homogeneous mixed solution. The solution was subjected to refluxing reaction at 130℃ for 10 hours with stirring in an atmosphere of nitrogen gas. After the refluxing reaction, p-xylene solvent was distilled off, and obtained polytitanocarbosilane at 60℃ and 1 mmHg for 1 hour.

The obtained PTC was dissolved in xylene. Carbon fiber aws immersed in PTC solution by using dipping coater. The composite fiber was placed on a graphite setter and heat treated at 100℃/hr and held for 1 hour at 1200℃, 1300℃, 1400℃, and 1500℃ in nitrogen to make C/SiC/TiC composite fiber.

2. Analysis

To investigate structure of C/PTC composite fiber and  its conversion into carbon and titanium modified silicon carbide fiber, the FT-IR, XPS and XRD analysis was measured.

Infrared spectra were measured with a Perkin Elmer 1330 spectrometer by the KBr pellet method in the 4000∼400 cm-1 range.

Gel permeation chromatography(GPC) was taken with a Hitachi HCL-635A with A-803 Shodex packing and tetrahydrofurane eluent at a flow rate at room temperature of 1 □/min.

Thermogravimetric analysis(TGA) was performed using a Dupont 9900 thermogravimetric analyzer(high temperature type) connected to a system 7/4 thermal analysis controller by raising the temperature at 10℃/min up to 1300℃ in a nitrogen gas flow of 50 cm3/min and also X-ray diffraction patterns were recorded with a Phillips diffractometer using CuK□ radiation with a Ni filter from 20 to 80o.

Also X-ray photoelectron spectroscopy(XPS) analysis was carried out with a V. G. SCIENTIFIC ESCALAB MKII spectrometer using AlK□1,2 X-ray radiation at 1486.6 eV with operating at 10-7 Pa.

3. RESULTS AND DISCUSSION

The C/PTC composite fiber was pyrolyzed under nitrogen at various temperatures up to 1500℃ and was chracterized by FT-IR, XPS and X-ray diffraction analysis.

Fig. 1 represented the FT-IR spectra of conversion of the C/PTC into carbon titanium modified silicon carbide fiber. The absorption intensity at about 1080cm-1 in titanium modified silicon carbide fiber ontained at 1300℃ begins to decrease abruptly at about 1400℃ and thereafter. This suggests that oxides with Si-O bonds react with free carbon and evolved CO gas, that is, the amorphous titanium modified silicon carbide fiber crystallized to □-SiC and TiC, with evolution of SiO and CO gas.

Fig. 2 represented the X-ray diffraction patterns of C/PTC composite fiber heat treated up to 1500℃ for 1 hour. At 1000℃, the fiber is poorly crystalline SiC phases. The broad peak around a diffraction line (111) of □-SiC at 2□=35.8 degrees indicates that the crystallization of the SiC matrix is just starting. At 1300℃, the peaks due to crystalline TiC are sharpening as well as those due to the SiC phase that can be identified as the cubic polytype SiC3C. New sharp peaks present are due to crystalline TiC phase. At 1400℃ for 1 hour, the only crystalline phases are TiC and SiC3C. The titanium modified silicon carbide fiber obtained in this experimental was heated to 1300℃ over 1 hour in an atmosphere of nitrogen, and then fired at 1500℃ for 1 hour to afford a black solid. As shown in Fig. 2, the X-ray powder diffraction pattern of this ceramic fiber shows a diffraction line (111) of □-SiC at 2□=35.8°, a diffraction line (220) of □-SiC at 2□=60.1°, a diffraction line (311) of □-SiC at 2□=72.1°, a diffraction line of (200) of TiC at 2□=42.4°, a diffraction line (111) 0f TiC at 2□=36.4°, a diffraction line (220) of TiC at 2□=61.4°and a diffraction line (113) of TiC at 2□=73.5°. From the fact that the diffraction lines of TiC are shifted to a higher angle side than those of conventional TiC and its lattice constant differed from conventional TiC, it is presumed that the resulting titanium modified silicon carbide fiber was a composite carbide composed mainly of □-SiC, TiC, a solid solution of □-SiC and TiC and TiC1-x (wherein 0<x<1).

REFERENCES

1. S. Yajima, Y. Hasegawa, J. Hayashi and M. Iimura: J. Mater. Sci., 13, 2569         (1978).

2. Y. Hasegawa, J. Hayashi and S. Yajima: ibid., 15, 720 (1980).

3. Y. Hasegawa: ibid., 18, 3633 (1983).

4. Y. Hasegawa and K. Okamura: ibid., 21, 321 (1986).