PAN原丝在预氧化和碳化过程中微观结构的演变
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摘要
聚丙烯腈(PAN)基碳纤维优异的力学性能是由其结构决定的,而碳纤维最终的结构形态是在制备过程中逐渐形成的,与其制造工艺、原丝和预氧丝的结构密切相关。因此,弄清碳纤维的结构特征、掌握原丝和预氧丝的结构对碳纤维结构的影响规律,以及碳纤维结构与性能之间的关系,对提高PAN基碳纤维的性能具有重要意义。本文采用傅里叶变换红外光谱(FTIR)、元素分析仪、X射线衍射仪(XRD)、场发射扫描电镜(FESEM)、透射电镜(TEM)以及高分辨透射电镜(HRTEM)等测试技术,分别系统研究了湿纺原丝和干喷湿纺原丝在预氧化和碳化过程中纤维晶体结构、化学结构、微观结构和力学性能的演变规律,分析了碳纤维在石墨化过程中的结构转变,并对碳纤维结构与性能的相关性问题作了探讨。
湿纺PAN原丝的结晶度在预氧化过程中逐渐降低,在碳化过程中随着碳化温度的升高逐渐增加。由于预氧化反应首先在非晶区进行,然后扩展到晶区,这导致晶粒尺寸(L_c)在235℃温度以前先增大,在高于235℃温度逐渐减小。在碳化过程中(002)晶面的晶面间距d_(002)随温度的升高在700℃前先增大,在700℃后减少,而晶粒尺寸L_c随碳化温度的升高有先减小后增大的变化趋势。
湿纺原丝中C、H、N、O四种元素含量在预氧化前期变化幅度较小,在预氧化中后期,氧化反应比较剧烈,C、N、H含量逐渐减少,O含量剧烈增加。在碳化过程中预氧化纤维中O和H元素在700℃以前减少幅度较大,在700℃以后缓慢减少;而N元素在700℃以前减少幅度相对较少,在700℃以后显著减少,最终形成了含碳量为94.81%的碳纤维。体密度在预氧化过程中逐渐增加,而线密度在预氧化过程中变化比较复杂,有先减小再增大后减小的趋势,这是由于线密度的变化受多重因素的影响。纤维体密度在275～500℃和700～1000℃增加幅度较大,对应着同一阶段线密度缓慢降低;在500～600℃和1000～1200℃阶段纤维体密度增加幅度较少,对应着同一阶段线密度剧烈降低。
采用FESEM技术对湿纺原丝和干喷湿纺原丝及其碳纤维的表面形貌和断面形貌进行研究,结果表明,湿纺原丝表面存在贯穿纤维并平行于纤维轴方向的沟槽,经过预氧化和碳化处理后沟槽有合并的趋势。干喷湿纺原丝及碳纤维表面光滑无沟槽。湿纺原丝在碳纤维制备过程中纤维的拉伸断裂形貌经历了由韧性断裂到脆性断裂的转变,原丝中的原纤随预氧化和碳化反应的进行细度降低,原纤之间结合更加紧密。采用TEM对湿纺原丝、预氧化纤维和碳纤维的微观组织进行分析,发现原丝的结构不均匀,存在皮芯结构,芯部组织随着预氧化反应的进行变得粗大。经过低温碳化和高温碳化后,纤维皮层和芯部组织的致密性和结构稳定性得到改善。
选区电子衍射分析碳化过程中二维乱层石墨结构的形成和演变表明:在碳化过程中,预氧化纤维中非晶衍射环的弥散性逐渐降低,并出现(002)晶面的取向衍射弧和(100)晶面、(110)晶面的多晶衍射环,随着碳化温度的升高,(002)晶面的取向衍射弧强度增强并且弧长变短,(100)和(110)晶面的多晶衍射环强度增强并且衍射环周围的弥散性光晕逐渐减少,以上结果表明在碳化过程中石墨层沿纤维轴排列逐渐致密和整齐,取向性明显提高,石墨微晶的晶区逐渐完善,结晶度增加。
HRTEM研究湿纺PAN原丝在碳纤维制备过程中微观结构的变化表明:PAN原丝的HRTEM结构由无序区和相对有序区组成,有序区和无序区之间不存在严格的分界面而是连续过渡的。275℃处理的预氧化纤维的HRTEM结构以非晶组织为主。在碳化过程中,随着热解过程的进行纤维的结构继续发生变化。碳网层面的数目和大小增加,碳网平面间的堆积密度及其沿纤维轴的取向逐渐增加,具体表现为:单个石墨层沿纤维轴方向上的平均长度由1-2 nm增加到4 nm,石墨微晶尺寸L_c和L_α增加,石墨层间非晶组织减少,纤维的结晶度逐渐提高,相邻石墨层连贯性逐渐增加,石墨层排列更加致密有序使d_(002)由0.36 nm减小到0.35 nm,逐渐形成了二维乱层石墨结构的碳纤维,这一高度取向的二维过渡态物相使碳纤维具有良好的拉伸性能。
通过对比研究实验室自制碳纤维和日本东丽T700碳纤维的XRD、元素组成、断面形貌和HRTEM微观结构,发现T700碳纤维具有较好性能的原因在于:T700碳纤维具有较好的晶体结构完整性,T700碳纤维在石墨化程度、微晶内石墨层的堆砌层数、石墨层的有序排列、择优取向性等方面均优于自制碳纤维;T700碳纤维的含碳量高于自制碳纤维,要制得高强度碳纤维,高温碳化的温度需根据不同的工艺条件来加以选择;T700碳纤维中含有较少的内部孔洞、孔隙、石墨层排列紊乱等结构缺陷,致密性和结构均匀性好于实验室自制碳纤维,致密化和均质化是提高碳纤维拉伸强度的主要技术途径。
对日本东丽T300碳纤维进行2350℃石墨化处理后,其微观结构由二维乱层石墨结构向三维有序的石墨结构转变,纤维中残留的N、H、O等非碳原子进一步被脱除,碳的质量分数由95.04%增加到99.27%,纤维中只含有少量N、H、O元素。碳六元环的平面环数增加,石墨微晶不仅长大(表现为L_c和L_α增大),结晶度提高到51%,而且石墨微晶沿纤维轴的择优取向提高,使层间距d_(002)逐渐减小为0.3359 nm,石墨化程度提高,结构逐步向理想石墨靠近。结构的变化直接导致碳纤维在石墨化过程中模量增加,拉伸强度和断裂伸长率减小,碳纤维完全转化为脆性材料。石墨纤维中存在比较多的结构连接弱点,这也使T300碳纤维在石墨化处理后拉伸强度有了大幅度的下降。
干喷湿纺原丝的体密度在碳纤维制备过程中逐渐增大,而线密度的变化受几种因素的综合影响,变化规律比较复杂,呈现增大、减小、增大、减小、增大最后减小的变化趋势。在预氧化前期(温度低于220℃),纤维取向度降低,衍射峰强度降低,晶粒尺寸减小;在220～230℃范围内,环化反应在非晶区(无序区)进行,准晶结构生长,微晶尺寸增大,结晶度增加;当预氧化温度继续升高,环化反应扩展到晶区(有序区),有序区向无序区转变,晶粒尺寸逐渐减小,结晶度降低。在碳化过程中,伴随着非碳原子的驱除,碳原子通过环化、缩聚等反应形成碳网层面,碳原子之间的结合力提高,L_c增大,结晶度提高。
HRTEM研究干喷湿纺原丝在预氧化和碳化过程中微观结构变化表明,干喷湿纺原丝HRTEM结构为准晶结构,其中分布着无序区和有序区,有序区和无序区之间也是连续过渡的.在预氧化过程中,随着PAN线形分子链向平面梯形结构转变,有序区逐渐向无序区转变。在碳化过程中,碳网平面形成,并随着碳化温度的升高,碳网层面沿纤维轴方向择优取向。
与湿纺制得的原丝和碳纤维相比,干喷湿纺由于能制得较高密度、高结晶度、结构均匀、表面光滑无沟槽的原丝和碳纤维,从而使干喷湿纺制得的碳纤维拉伸强度得到大幅度提高。与湿纺的方法相比,干喷湿纺是一种制备高性能碳纤维的优异的纺丝方法。
The excellent properties of PAN-based carbon fibers are determined by their structure. The structure and morphology of carbon fibers gradually form during the preparation process, and are correlated with the technique, the structure of PAN precursor fibers and preoxidized fibers. Therefore, in order to obtain high quality carbon fibers, it is important to fully understand the structure of carbon fibers, master the influence of the structure of PAN precursor fibers and preoxidized fibers on carbon fibers, and deeply study the correlation between structure and properties. In this paper several technologies, such as Fourier transform infrared spectroscopy (FTIR), elemental analyzer, wide angle X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were used to systemically investigate the evolution of crystal structure, chemical structure, microstructure and properties of wet-spinning and dry-jet wet-spinning PAN precursor fibers during proxidation and carbonization process, analysis the structural change of carbon fibers during graphitization, and discuss the correlation between the structure and properties.
The crystallinity of wet-spinning PAN precursor fibers gradually reduces during preoxidation process, while it gradually increases during carbonization process. Because preoxidation reactions firstly take place in amorphous regions and then transits to crystal regions, crystallite size (L_c) firstly increases below 235℃, and gradually decreases above 235℃. During carbonization, d_(002) firstly increases below 700℃, and gradually decreases above 700℃. L_c firstly decreases, then increases again with the carbonization temperature rising.
At the initial stage of preoxidation, there is little variation in C, H, N, O content. At the later stage, due to the acute oxidation reaction, the content of O increases obviously along with the gradual decrease of C, H and N. During carbonization, O and H contents of preoxidized fibers obviously decreases below 700℃, and gradually decreases above 700℃. While N content gradually decreases below 700℃, and obviously decreases above 700℃, as a result, carbon fibers with 94.81% of C content is formed. The bulk density gradually increases during preoxidation, while the variation of linear density is complex. Influenced by many factors, linear density firstly decreases, then increases, and at last decreases again. The bulk density obviously increases between 275 and 700℃, and between 700 and 1000℃, corresponding to the gradual decrease of linear density. The bulk density gradual increases between 500 and 600℃, and between 1000 and 1200℃, corresponding to the gradual decrease of linear density.
FESEM study on surface morphology and fracture morphology of wet-spinning and dry-jet wet-spinning fibers shows that the grooves exist on the surface of wet-spinning precursor fibers. During preoxidation and carbonization process, the grooves combine with each other. Dry-jet wet-spinning precursor fibers have a smooth surface. The tensile fracture morphologies of wet-spinning PAN precursor fibers transform from tough fracture to brittle fracture throughout the whole preparation process. The diameter of fibrils of PAN precursor fibers gradually decrease, along with the closer bonding between fibrils. Deeply investigation was carried out on the structure of PAN precursor fibers, preoxidation fibers and carbon fibers by TEM. The results show that the structure of PAN precursor fibers is inhomogeneous and has skin-core structure. With the process of preoxidation reaction, the core texture grows thick. After low temperature and high temperature carbonization, the compactness and stability in skin and core region are improved.
Selected-area electron diffraction study on the formation and evolution of two-dimensional turbostratic graphite shows that during carbonization, the dispersion of amorphous diffractive ring gradually decreases, and orientation diffraction arc of (002) crystal planes and polycrystalline diffraction ring of (100) and (110) crystal planes appear. With the rising of carbonization temperature, the intensity of orientation diffraction arc of (002) crystal planes strengthens along with the arc length shortening, and more over the intensities of polycrystalline diffraction ring of (100) and (110) crystal planes strengthen, and halo disturbance of diffraction ring gradually decreases. Above results indicate that during carbonization, graphite lamella closes up further, the orientation of graphite lamella along the axis gradually increases, and the crystallinity of graphite crystallite increases.
HRTEM study on the microstructural evolution of PAN precursor fibers during the formation of carbon fibers shows that the HRTEM microstructure of PAN precursor fibers is quasicrystal structure, which is composed of amorphous region and relative ordered region. There have no apparent interfaces between amorphous region and relative ordered region. Amorphous structure is observed in preoxidized fibers. When preoxidized fibers are carbonized, with the process of pyrogenation, changes of the structure take place. The amount and size of aromatic plane continues to increase, carbon planes widen and improve further, and the orientation of graphite lamella along the axis gradually increases. Above phenomena represent in detail that the length of single graphite lamella gradually increases, the size of graphite crystallite (L_c and L_a) increases, amorphous texture between graphite lamella decreases, as a result the crystallinity of graphite crystallite increases. The continuity of adjacent graphite lamella gradually increases, graphite lamella close up further and the interplanar spacing decreases, and two-dimensional turbostratic graphite structure is formed gradually, which guarantees high tensile strength for carbon fibers.
XRD, elemental composition, fracture morphology, HRTEM microstructure were investigated by comparing self-produced carbon fibers with Toray T700 carbon fibers. The results indicate that Toray T700 carbon fibers have better completeness of crystal structure, better graphitization degree, more graphitization stack layers and higher orientation. The C content of Toray T700 carbon fibers is higher than that of self-produced carbon fibers, so in order to obtain high performance carbon fibers, the temperature of high temperature carbonization needs to be selected according to various technologies. T700 carbon fibers have lesser structural defects, such as holes, small openings and disorder of graphitization layers. As a result, structural compactness and homogenization are the main technology to improve tensile strength of carbon fibers.
After being graphitized at 2350℃, the microstructure of Toray T300 carbon fibers transforms from two-dimensional turbostratic graphite structure to three-dimensional order graphite structure. The N, H and O content of fibers are further released, and the wt. % of C content increases from 95.04% to 99.27%. The number of the C six-membered ring increases, and graphite crystallites grow up, representing the increase of L_c, L_a, and crystallinity. The orientation degree of graphite crystallites along fiber axis improves, which makes the interplanar spacing shorten to 0.3359 nm, and the graphitization degree increase. Above-mentioned changes of structure result in the increase of the young's modulus, the decrease of tensile strength and elongation at break. After graphitization, carbon fibers convert into brittle materials completely. More structural weaknesses exist in graphite fibers, which make the tensile strength of T300 carbon fibers decrease after graphitization.
The bulk density of dry-jet wet-spinning PAN precursor fibers gradually increases in the formation of carbon fibers, while the variation of linear density is complex. Influenced by many factors, linear density firstly increases, decreases, then increases, decreases, and at last increases, decreases again. Because of the decrease of orientation degree of fibers at the initial stage of preoxidation (below 220℃), the diffraction peak intensity weakens, and moreover, the crystallite size decreases. Because preoxidation reactions firstly take place in amorphous regions at the temperature range of 220-230℃, quasicrystal structure grows up, crystallite size increases, and the crystallinity increases. With the rising of preoxidation temperature, cyclization reaction spreads to crystal regions, as a result, ordered regions transit to amorphous regions, crystallite size and the crystallinity both decrease. During carbonization process, along with the release of non-carbon elements, C atoms form C basal planes through cyclization and condensation polymerization reaction, the combined force between C atoms strengthens. Consequently L_c and crystallinity increase.
HRTEM study on the microstructural evolution of dry-jet wet spinning PAN precursor fibers during preoxidation and carbonization indicates that The HRTEM microstructure of dry-jet wet-spinning is quasicrystal structure, which is composed of amorphous region and relative ordered region. There have no apparent interfaces between amorphous region and relative ordered region. During preoxidation process, along with the transformation from PAN linear molecular chains to planar ladder structure, ordered regions gradually transit to amorphous regions. During carbonization process, carbon basal planes form, and with the carbonization temperature increasing, carbon basal planes orient along the fiber axis.
Compared with wet-spinning PAN precursor fibers and the resultant carbon fibers, dry-jet wet spinning PAN precursor fibers and carbon fibers with high density, high crystallinity, uniform structure and smooth surface guarantee high tensile strength for carbon fibers. Compared with wet-spinning, dry-jet wet spinning is a relatively better spinning technology for producing carbon fibers with high performance.
湿纺PAN原丝的结晶度在预氧化过程中逐渐降低,在碳化过程中随着碳化温度的升高逐渐增加。由于预氧化反应首先在非晶区进行,然后扩展到晶区,这导致晶粒尺寸(L_c)在235℃温度以前先增大,在高于235℃温度逐渐减小。在碳化过程中(002)晶面的晶面间距d_(002)随温度的升高在700℃前先增大,在700℃后减少,而晶粒尺寸L_c随碳化温度的升高有先减小后增大的变化趋势。
湿纺原丝中C、H、N、O四种元素含量在预氧化前期变化幅度较小,在预氧化中后期,氧化反应比较剧烈,C、N、H含量逐渐减少,O含量剧烈增加。在碳化过程中预氧化纤维中O和H元素在700℃以前减少幅度较大,在700℃以后缓慢减少;而N元素在700℃以前减少幅度相对较少,在700℃以后显著减少,最终形成了含碳量为94.81%的碳纤维。体密度在预氧化过程中逐渐增加,而线密度在预氧化过程中变化比较复杂,有先减小再增大后减小的趋势,这是由于线密度的变化受多重因素的影响。纤维体密度在275～500℃和700～1000℃增加幅度较大,对应着同一阶段线密度缓慢降低;在500～600℃和1000～1200℃阶段纤维体密度增加幅度较少,对应着同一阶段线密度剧烈降低。
采用FESEM技术对湿纺原丝和干喷湿纺原丝及其碳纤维的表面形貌和断面形貌进行研究,结果表明,湿纺原丝表面存在贯穿纤维并平行于纤维轴方向的沟槽,经过预氧化和碳化处理后沟槽有合并的趋势。干喷湿纺原丝及碳纤维表面光滑无沟槽。湿纺原丝在碳纤维制备过程中纤维的拉伸断裂形貌经历了由韧性断裂到脆性断裂的转变,原丝中的原纤随预氧化和碳化反应的进行细度降低,原纤之间结合更加紧密。采用TEM对湿纺原丝、预氧化纤维和碳纤维的微观组织进行分析,发现原丝的结构不均匀,存在皮芯结构,芯部组织随着预氧化反应的进行变得粗大。经过低温碳化和高温碳化后,纤维皮层和芯部组织的致密性和结构稳定性得到改善。
选区电子衍射分析碳化过程中二维乱层石墨结构的形成和演变表明:在碳化过程中,预氧化纤维中非晶衍射环的弥散性逐渐降低,并出现(002)晶面的取向衍射弧和(100)晶面、(110)晶面的多晶衍射环,随着碳化温度的升高,(002)晶面的取向衍射弧强度增强并且弧长变短,(100)和(110)晶面的多晶衍射环强度增强并且衍射环周围的弥散性光晕逐渐减少,以上结果表明在碳化过程中石墨层沿纤维轴排列逐渐致密和整齐,取向性明显提高,石墨微晶的晶区逐渐完善,结晶度增加。
HRTEM研究湿纺PAN原丝在碳纤维制备过程中微观结构的变化表明:PAN原丝的HRTEM结构由无序区和相对有序区组成,有序区和无序区之间不存在严格的分界面而是连续过渡的。275℃处理的预氧化纤维的HRTEM结构以非晶组织为主。在碳化过程中,随着热解过程的进行纤维的结构继续发生变化。碳网层面的数目和大小增加,碳网平面间的堆积密度及其沿纤维轴的取向逐渐增加,具体表现为:单个石墨层沿纤维轴方向上的平均长度由1-2 nm增加到4 nm,石墨微晶尺寸L_c和L_α增加,石墨层间非晶组织减少,纤维的结晶度逐渐提高,相邻石墨层连贯性逐渐增加,石墨层排列更加致密有序使d_(002)由0.36 nm减小到0.35 nm,逐渐形成了二维乱层石墨结构的碳纤维,这一高度取向的二维过渡态物相使碳纤维具有良好的拉伸性能。
通过对比研究实验室自制碳纤维和日本东丽T700碳纤维的XRD、元素组成、断面形貌和HRTEM微观结构,发现T700碳纤维具有较好性能的原因在于:T700碳纤维具有较好的晶体结构完整性,T700碳纤维在石墨化程度、微晶内石墨层的堆砌层数、石墨层的有序排列、择优取向性等方面均优于自制碳纤维;T700碳纤维的含碳量高于自制碳纤维,要制得高强度碳纤维,高温碳化的温度需根据不同的工艺条件来加以选择;T700碳纤维中含有较少的内部孔洞、孔隙、石墨层排列紊乱等结构缺陷,致密性和结构均匀性好于实验室自制碳纤维,致密化和均质化是提高碳纤维拉伸强度的主要技术途径。
对日本东丽T300碳纤维进行2350℃石墨化处理后,其微观结构由二维乱层石墨结构向三维有序的石墨结构转变,纤维中残留的N、H、O等非碳原子进一步被脱除,碳的质量分数由95.04%增加到99.27%,纤维中只含有少量N、H、O元素。碳六元环的平面环数增加,石墨微晶不仅长大(表现为L_c和L_α增大),结晶度提高到51%,而且石墨微晶沿纤维轴的择优取向提高,使层间距d_(002)逐渐减小为0.3359 nm,石墨化程度提高,结构逐步向理想石墨靠近。结构的变化直接导致碳纤维在石墨化过程中模量增加,拉伸强度和断裂伸长率减小,碳纤维完全转化为脆性材料。石墨纤维中存在比较多的结构连接弱点,这也使T300碳纤维在石墨化处理后拉伸强度有了大幅度的下降。
干喷湿纺原丝的体密度在碳纤维制备过程中逐渐增大,而线密度的变化受几种因素的综合影响,变化规律比较复杂,呈现增大、减小、增大、减小、增大最后减小的变化趋势。在预氧化前期(温度低于220℃),纤维取向度降低,衍射峰强度降低,晶粒尺寸减小;在220～230℃范围内,环化反应在非晶区(无序区)进行,准晶结构生长,微晶尺寸增大,结晶度增加;当预氧化温度继续升高,环化反应扩展到晶区(有序区),有序区向无序区转变,晶粒尺寸逐渐减小,结晶度降低。在碳化过程中,伴随着非碳原子的驱除,碳原子通过环化、缩聚等反应形成碳网层面,碳原子之间的结合力提高,L_c增大,结晶度提高。
HRTEM研究干喷湿纺原丝在预氧化和碳化过程中微观结构变化表明,干喷湿纺原丝HRTEM结构为准晶结构,其中分布着无序区和有序区,有序区和无序区之间也是连续过渡的.在预氧化过程中,随着PAN线形分子链向平面梯形结构转变,有序区逐渐向无序区转变。在碳化过程中,碳网平面形成,并随着碳化温度的升高,碳网层面沿纤维轴方向择优取向。
与湿纺制得的原丝和碳纤维相比,干喷湿纺由于能制得较高密度、高结晶度、结构均匀、表面光滑无沟槽的原丝和碳纤维,从而使干喷湿纺制得的碳纤维拉伸强度得到大幅度提高。与湿纺的方法相比,干喷湿纺是一种制备高性能碳纤维的优异的纺丝方法。
The excellent properties of PAN-based carbon fibers are determined by their structure. The structure and morphology of carbon fibers gradually form during the preparation process, and are correlated with the technique, the structure of PAN precursor fibers and preoxidized fibers. Therefore, in order to obtain high quality carbon fibers, it is important to fully understand the structure of carbon fibers, master the influence of the structure of PAN precursor fibers and preoxidized fibers on carbon fibers, and deeply study the correlation between structure and properties. In this paper several technologies, such as Fourier transform infrared spectroscopy (FTIR), elemental analyzer, wide angle X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were used to systemically investigate the evolution of crystal structure, chemical structure, microstructure and properties of wet-spinning and dry-jet wet-spinning PAN precursor fibers during proxidation and carbonization process, analysis the structural change of carbon fibers during graphitization, and discuss the correlation between the structure and properties.
The crystallinity of wet-spinning PAN precursor fibers gradually reduces during preoxidation process, while it gradually increases during carbonization process. Because preoxidation reactions firstly take place in amorphous regions and then transits to crystal regions, crystallite size (L_c) firstly increases below 235℃, and gradually decreases above 235℃. During carbonization, d_(002) firstly increases below 700℃, and gradually decreases above 700℃. L_c firstly decreases, then increases again with the carbonization temperature rising.
At the initial stage of preoxidation, there is little variation in C, H, N, O content. At the later stage, due to the acute oxidation reaction, the content of O increases obviously along with the gradual decrease of C, H and N. During carbonization, O and H contents of preoxidized fibers obviously decreases below 700℃, and gradually decreases above 700℃. While N content gradually decreases below 700℃, and obviously decreases above 700℃, as a result, carbon fibers with 94.81% of C content is formed. The bulk density gradually increases during preoxidation, while the variation of linear density is complex. Influenced by many factors, linear density firstly decreases, then increases, and at last decreases again. The bulk density obviously increases between 275 and 700℃, and between 700 and 1000℃, corresponding to the gradual decrease of linear density. The bulk density gradual increases between 500 and 600℃, and between 1000 and 1200℃, corresponding to the gradual decrease of linear density.
FESEM study on surface morphology and fracture morphology of wet-spinning and dry-jet wet-spinning fibers shows that the grooves exist on the surface of wet-spinning precursor fibers. During preoxidation and carbonization process, the grooves combine with each other. Dry-jet wet-spinning precursor fibers have a smooth surface. The tensile fracture morphologies of wet-spinning PAN precursor fibers transform from tough fracture to brittle fracture throughout the whole preparation process. The diameter of fibrils of PAN precursor fibers gradually decrease, along with the closer bonding between fibrils. Deeply investigation was carried out on the structure of PAN precursor fibers, preoxidation fibers and carbon fibers by TEM. The results show that the structure of PAN precursor fibers is inhomogeneous and has skin-core structure. With the process of preoxidation reaction, the core texture grows thick. After low temperature and high temperature carbonization, the compactness and stability in skin and core region are improved.
Selected-area electron diffraction study on the formation and evolution of two-dimensional turbostratic graphite shows that during carbonization, the dispersion of amorphous diffractive ring gradually decreases, and orientation diffraction arc of (002) crystal planes and polycrystalline diffraction ring of (100) and (110) crystal planes appear. With the rising of carbonization temperature, the intensity of orientation diffraction arc of (002) crystal planes strengthens along with the arc length shortening, and more over the intensities of polycrystalline diffraction ring of (100) and (110) crystal planes strengthen, and halo disturbance of diffraction ring gradually decreases. Above results indicate that during carbonization, graphite lamella closes up further, the orientation of graphite lamella along the axis gradually increases, and the crystallinity of graphite crystallite increases.
HRTEM study on the microstructural evolution of PAN precursor fibers during the formation of carbon fibers shows that the HRTEM microstructure of PAN precursor fibers is quasicrystal structure, which is composed of amorphous region and relative ordered region. There have no apparent interfaces between amorphous region and relative ordered region. Amorphous structure is observed in preoxidized fibers. When preoxidized fibers are carbonized, with the process of pyrogenation, changes of the structure take place. The amount and size of aromatic plane continues to increase, carbon planes widen and improve further, and the orientation of graphite lamella along the axis gradually increases. Above phenomena represent in detail that the length of single graphite lamella gradually increases, the size of graphite crystallite (L_c and L_a) increases, amorphous texture between graphite lamella decreases, as a result the crystallinity of graphite crystallite increases. The continuity of adjacent graphite lamella gradually increases, graphite lamella close up further and the interplanar spacing decreases, and two-dimensional turbostratic graphite structure is formed gradually, which guarantees high tensile strength for carbon fibers.
XRD, elemental composition, fracture morphology, HRTEM microstructure were investigated by comparing self-produced carbon fibers with Toray T700 carbon fibers. The results indicate that Toray T700 carbon fibers have better completeness of crystal structure, better graphitization degree, more graphitization stack layers and higher orientation. The C content of Toray T700 carbon fibers is higher than that of self-produced carbon fibers, so in order to obtain high performance carbon fibers, the temperature of high temperature carbonization needs to be selected according to various technologies. T700 carbon fibers have lesser structural defects, such as holes, small openings and disorder of graphitization layers. As a result, structural compactness and homogenization are the main technology to improve tensile strength of carbon fibers.
After being graphitized at 2350℃, the microstructure of Toray T300 carbon fibers transforms from two-dimensional turbostratic graphite structure to three-dimensional order graphite structure. The N, H and O content of fibers are further released, and the wt. % of C content increases from 95.04% to 99.27%. The number of the C six-membered ring increases, and graphite crystallites grow up, representing the increase of L_c, L_a, and crystallinity. The orientation degree of graphite crystallites along fiber axis improves, which makes the interplanar spacing shorten to 0.3359 nm, and the graphitization degree increase. Above-mentioned changes of structure result in the increase of the young's modulus, the decrease of tensile strength and elongation at break. After graphitization, carbon fibers convert into brittle materials completely. More structural weaknesses exist in graphite fibers, which make the tensile strength of T300 carbon fibers decrease after graphitization.
The bulk density of dry-jet wet-spinning PAN precursor fibers gradually increases in the formation of carbon fibers, while the variation of linear density is complex. Influenced by many factors, linear density firstly increases, decreases, then increases, decreases, and at last increases, decreases again. Because of the decrease of orientation degree of fibers at the initial stage of preoxidation (below 220℃), the diffraction peak intensity weakens, and moreover, the crystallite size decreases. Because preoxidation reactions firstly take place in amorphous regions at the temperature range of 220-230℃, quasicrystal structure grows up, crystallite size increases, and the crystallinity increases. With the rising of preoxidation temperature, cyclization reaction spreads to crystal regions, as a result, ordered regions transit to amorphous regions, crystallite size and the crystallinity both decrease. During carbonization process, along with the release of non-carbon elements, C atoms form C basal planes through cyclization and condensation polymerization reaction, the combined force between C atoms strengthens. Consequently L_c and crystallinity increase.
HRTEM study on the microstructural evolution of dry-jet wet spinning PAN precursor fibers during preoxidation and carbonization indicates that The HRTEM microstructure of dry-jet wet-spinning is quasicrystal structure, which is composed of amorphous region and relative ordered region. There have no apparent interfaces between amorphous region and relative ordered region. During preoxidation process, along with the transformation from PAN linear molecular chains to planar ladder structure, ordered regions gradually transit to amorphous regions. During carbonization process, carbon basal planes form, and with the carbonization temperature increasing, carbon basal planes orient along the fiber axis.
Compared with wet-spinning PAN precursor fibers and the resultant carbon fibers, dry-jet wet spinning PAN precursor fibers and carbon fibers with high density, high crystallinity, uniform structure and smooth surface guarantee high tensile strength for carbon fibers. Compared with wet-spinning, dry-jet wet spinning is a relatively better spinning technology for producing carbon fibers with high performance.
引文
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