利用二维小角X射线散射技术(SAXS),结合纤维孔结构解析理论及分形原理得到了炭纤维形成过程中纺丝、预氧化、低温和高温炭化等四个阶段样品的微孔结构信息.结果表明:原丝中孔隙沿纤维轴向择优取向,呈长梭状,其长轴与短轴的平均尺寸分别为24.3 nm和19.2 nm,长径比为1.27.遗留到预氧化阶段的原丝孔洞使得预氧化纤维出现高达1.85的长径比极大值,这可能与原丝线性结构向预氧丝耐热梯形结构转化有关.炭化阶段微孔尺寸迅速减小,长轴、短轴分别达到3.56 nm和2.85 nm左右,长径比也减小至1.24.分形状态研究表明:表面分形维数D_S值介于2.42～2.88之间,且随工艺的进行逐渐增大,低温炭化阶段变化幅度较大,说明各级产品在微观结构上越来越复杂,亦证明低温炭化是促进炭纤维微观结构转变的重要的工艺段.
Microvoids,formed in spinning,oxidation and carbonization during the preparation of PAN-based carbon fibers,were investigated by two-dimensional small angle X-ray scattering.Results indicated that microvoids in PAN fibers had a preferred elongation and orientation along the fiber axis with an average length of 24.3 nm,diameter of 19.2 nm and aspect ratio of around 1.27.The size of the microvoids remained almost unchanged during oxidation,while their aspect ratio increased to 1.85,possibly due to a crystalline transformation from a linear structure in PAN fibers to a heat-resistant ladder structure in oxidized fibers.However,the size of the microvoids decreased dramatically to about 3.56 nm in the long axis and 2.85 nm in the short axis during carbonization.The surface fractal dimension (Ds) increased in these processes from 2.42 to 2.88 and the most severe change occurred in low-temperature carbonization,indicating that surface roughness increased during processing,and carbonization was the most important process for such a transformation.
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