自由碳的脱除对SiC纤维微观结构和性能的影响

doi:10.15541/jim20150502

摘要/Abstract

摘要：

采用加氢烧成法脱碳, 制备了不同自由碳含量的连续SiC纤维。通过元素分析、红外、X射线衍射和拉伸试验等手段对纤维的脱碳过程、元素组成、微观结构和性能进行了分析。结果表明: 加氢烧成通过抑制脱H₂反应、促进脱CH₄反应而实现有效脱碳, 且氢气浓度越高, 纤维中的碳含量越低。纤维芯部元素分布均匀, 表明该方法可以实现均匀脱碳, 但表面出现很薄的富碳层, 这是纤维经氢气处理后表面吸附氧形成的富氧层在高温烧成时分解所致。自由碳的脱除引起纤维晶粒长大, 密度增加, 孔隙率降低, 电阻率升高, 拉伸强度与拉伸模量提高。近化学计量SiC纤维具有优异的综合性能。

关键词: 自由碳, SiC纤维, 结构与性能, 氢气

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

中图分类号:

TQ343

曹适意, 王军, 王浩, 王小宙. 自由碳的脱除对SiC纤维微观结构和性能的影响[J]. 无机材料学报, 2016, 31(5): 529-534.

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.

图/表 11

表1 纤维元素含量与氢气浓度的关系

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

图1 SiC纤维的SEM形貌照片

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

图2 不熔化纤维(a)与预烧纤维(b,c)红外图谱

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)

图3. 纯N2(a)和75% H2/25%N2(b)气氛下烧成SiC纤维的表层俄歇深度分析

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

图4 不同C/Si原子比纤维的XRD图谱

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

图5 SiC纤维的HRTEM照片

Fig. 5 HRTEM images of SiC fibers

图6 纤维密度与C/Si原子比的关系

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

表2 纤维孔隙含量与C/Si原子比的关系

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

图7 纤维拉伸强度、拉伸模量与C/Si原子比的关系

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

图8 纤维断裂伸长率与C/Si原子比的关系

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

图9 纤维电阻率与C/Si原子比的关系

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

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