Influence of Free Carbon Elimination on Microstructure and Property of SiC Fibers

doi:10.15541/jim20150502

Abstract

Abstract:

To prepare SiC fibers with different free carbon contents, polycarbosilane (PCS) fibers cured with unsaturated hydrocarbons were pyrolyzed at 1000℃ under controlled hydrogen/nitrogen atmosphere and subsequently heat treated at 1500℃ under nitrogen atmosphere. The process of carbon removal during pyrolysis was investigated using chemical elemental analysis, FTIR, and AES analysis. The microstructure and properties were examined by SEM, TEM, XRD, density measurements, tensile tests and resistivity measurements. The results show that the carbon content in SiC fibers decreases with H₂ concentration increasing. The hydrogen atmosphere suppresses H₂ evolution and helps to remove excess carbon as CH₄ during pyrolysis. Although thin carbon-enriched films are present on the fiber surfaces, the distribution of silicon and carbon is uniform in the fiber cores. The microstructure and properties of the resulting SiC fibers are very dependent on their C/Si chemical compositions. The β-SiC grain size increases with a decrease in the carbon content because the excess carbon aggregates at the grain boundary and impedes the grain growth. Moreover, the removal of free carbon also results in fiber densification, decrease of porosity and improvement of fiber specific resistivity, tensile strength and tensile modulus. Therefore, the nearly stoichiometric SiC fiber has good comprehensive performance.

Key words: free carbon, SiC fiber, structure and property, hydrogen

CLC Number:

TQ343

CAO Shi-Yi, WANG Jun, WANG Hao, WANG Xiao-Zhou. Influence of Free Carbon Elimination on Microstructure and Property of SiC Fibers[J]. Journal of Inorganic Materials, 2016, 31(5): 529-534.

Figures/Tables 11

Table1 Relationship between element content and H2 concentration

H₂/vol%	C content/wt%	Si content/wt%	C/Si atomic ratio
0	37.32	60.47	1.44
25	33.97	63.49	1.25
50	31.90	65.27	1.14
75	30.46	67.59	1.05
100	28.24	68.56	0.96

Fig. 1 SEM images of SiC fibers (C/Si= 1.05)

Fig. 2 FTIR spectra of cured PCS fibers (a), SiC fibers heat-treated at 1000℃ in pure N2 (b) and 75% H2/25%N2 (c)

Fig. 3 AES depth profile recorded from the surface of SiC fiber pyrolyzed in pure N2 (a) and 75% H2/25%N2 (b)

Fig. 4 XRD patterns of SiC fibers with different C/Si atomic ratios

Fig. 5 HRTEM images of SiC fibers

Fig. 6 Relationship between density and C/Si atomic ratio of SiC fibers

Table 2 Relationship between porosity and C/Si atomic ratio of SiC fibers

C/Si atomic ratio	Mole composition	Calculated density, ρ_t/%	Pore fraction, P/%
1.44	SiC+0.44C	3.06	13.1
1.25	SiC+0.25C	3.12	10.6
1.14	SiC+0.14C	3.16	8.2
1.05	SiC+0.05C	3.19	7.5
0.96	0.96SiC+0.04Si	3.18	8.5

Fig. 7 Relationship between tensile strength, tensile modulus and C/Si atomic ratio of SiC fibers

Fig. 8 Relationship between elongation at break and C/Si atomic ratio of SiC fibers

Fig. 9 Relationship between resistivity and C/Si atomic ratio of SiC fibers

References 15

[1]	BUNSELL A R, PIANT A.A review of the development of three generations of small diameter silicon carbide fibres. Journal of Materials Science, 2006, 41(3): 823-839.
[2]	ZHAO D F, WANG H Z, LI X D.Development of polymer derived SiC fiber.Journal of Inorganic Materials, 2009, 24(6): 1097-1104.
[3]	SHIMOO T, KATASE Y, OKAMURA K, et al.Cabon elimination by heat-treatment in hydrogen and its effect on thermal stability of polycarbosilane-derived silicon carbide fibers.Journal of Materials Science, 2004, 39(20): 6243-6251.
[4]	ZHANG G J, WU Y B, LIU C J, et al.Preparation of nearly stoichiometric SiC fibers derived from PCS by the control of atmosphere.Journal of Xiamen University (Natural Science). 2006, 45(5): 683-687.
[5]	赵爽. PIP工艺制备SiC/SiC复合材料的结构、性能与辐照行为研究. 长沙: 国防科技大学博士学位论文, 2013.
[6]	SHA J J, HINOKI T, KOHYAM A.Microstructure and mechanical properties of Hi-Nicalon™ Type S fibers annealed and crept in various oxygen partial pressures.Materials Characterization, 2009, 60(8): 796-802.
[7]	HASEGAWA Y, OKAMURA K.Synthesis of continuous silicon carbide fibre: Part 3 Pyrolysis process of polycarbosilane and structure of the products.Journal of Materials Science, 1983, 18(12): 3633-3648.
[8]	TANG X Y, ZHANG L, TU H B, et al.Decarbonization mechanisms of polycarbosilane during pyrolysis in hydrogen for preparation of silicon carbide fibers.Journal of Materials Science, 2010, 45(21): 5749-5755.
[9]	TAZI HEMIDA A, PAILLER R, NASLAIN R.Continuous SiC-based model monofilaments with a low free carbon content part I: from the pyrolysis of a polycarbosilane precursor under an atmosphere of hydrogen.Journal of Materials Science, 1997, 32(9): 2359-2366.
[10]	SHIMOO T, OKAMURA K, ITO M, et al.High-temperature stability of low-oxygen silicon carbide fiber heat-treated under different atmosphere.Journal of Materials Science, 2000, 35(15): 3733-3739.
[11]	TAKEDA M, SAEKI A, SAKAMOTO J I, et al.Effect of hydrogen amosphere on pyrolysis of cured polycarbosilane fibers.Journal of the American Ceramic Society, 2000, 83(5): 1063-1069.
[12]	YAO R Q, WANG Y Y, FENG Z D.The effect of high temperature annealing on tensile strength and its mechanism of Hi-Nicalon SiC fibres under inert atmosphere.Fatigue & Fracture of Engineering Materials & Structures, 2008, 31(9): 777-787.
[13]	JING M, TAN T T, WANG C G, et al.Comparison on the micro-structure of Toray T800H and T800S carbon fiber.Materials Science and Technology, 2015, 23(2): 45-52.
[14]	TAN T T, WANG C G, JING M, et al.Study on relationship between microstructure and mechanical property of PAN-based carbon fiber.Journal of Functional Materials, 2012, 43(16): 2226-2230.
[15]	WANG D Y, SONG Y C, JIAN K.Effect of composition and structure on the specific resistivity of continuous silicon carbide fibers.Journal of Inorganic Materials, 2012, 27(2): 162-168.