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The boron-assisted catalytic graphitization of carbonaceous materials is an attractive methodology for enhancing their electrical properties by modulating the chemical structures of pristine carbons. In this study, polyacrylonitrile-based carbon fibers (CFs) with highly developed microstructures were prepared by boron-assisted catalytic graphitization. Hydrothermally immersed CFs in boric acid were heat-treated up to 2700 °C, and their chemical structures were traced to investigate the boron-assisted catalytic graphitization mechanism. Boron from the gasified boron-related functional groups of CFs diffused into the CFs to form boron carbide, B 4 C. The boron-substituted CFs exhibited a highly developed crystalline structure that could not be achieved by heat treatment alone, indicating that boron accelerated the graphitic carbon structure. As a result, boron-assisted catalytic graphitization at 2700 °C simultaneously enhanced the electrical and thermal conductivities of CFs, with values of 3677.8 S/cm and 365.9 W/mK, respectively, which were 2.8 and 2.4 times higher than those of heat-treated CFs at 2700 °C. In addition, CFs were used to prepare CF papers using a wet-laid process, and their heat generation and thermal management capabilities were evaluated. Considering affordable CFs compared to nanomaterials, we believe that our study provides a feasible approach for fabricating heating elements and heat sinks. [Display omitted] [ABSTRACT FROM AUTHOR]

Carbon fibers (CFs) have been considered as a next-generation material in high-performance applications such as aerospace, defense, sporting goods, wind energy, automobiles, electronics, and civil engineering industries due to their superior mechanical properties including high tensile strength and modulus and characteristics such as low density, good corrosion and low impact resistance, low coefficient of thermal expansion, and high electric conductivity. In this paper, we introduced the brief overview of the most widely produced and used polyacrylonitrile(PAN)-based CFs. In particular, the studies on developing high performance CFs by theoretical and experimental approaches were presented. Theoretical tensile strength of CFs can be ideally up to 100 GPa with the perfectly developed graphitic structure and its orientation along fiber direction. However, commercially available CFs reveal only tensile strength of 7 GPa because of the existing defects, misordered carbon crystals, and turbostratic structure. Those originate from manufacturing processes such as spinning, stabilization, and carbonization, which generate significant morphological and structural defects in both exterior and interior of CFs. The review covered the research on parameters related with the tensile properties and optimizing processes to increase the tensile strength of PAN-based CFs. [Display omitted] [ABSTRACT FROM AUTHOR]

Through comparing carbons in the graphite allotrope family, namely isotropic graphite (IG), exfoliated-graphite-based flexible graphite (FG), graphite-flake thick film (not sintered), carbon fibers (PAN, pitch) and highly-oriented pyrolytic graphite (HOPG), all differing in the microstructure, this work addresses for the first time the factors (including the structure-property relationship) that govern the electric permittivity (kHz frequency) of carbons. The permittivity is more sensitive to the microstructural heterogeneities than the resistivity, which only slightly influences the permittivity. Smaller crystallite dimensions promote the permittivity. PAN-based carbon fibers exhibit higher permittivity than pitch-based carbon fibers, IG, FG or HOPG. The crystallographic preferred orientation is not influential to the permittivity, though it influences the resistivity. Interfaces (e.g., the grain boundaries in IG and the interface between the carbon layer ribbons in a fiber) promote the permittivity, being sites for the carrier-atom interaction that enables the polarization. A large number of sites is more important than a large site size for promoting the permittivity. The fiber diameter is not influential to both permittivity and resistivity. The thick film exhibits the highest relative permittivity (5 × 105), due to its low electrical connectivity. IG (grain size 12 μm) gives the lowest relative permittivity (5 × 102), due to its coarse microstructure. [Display omitted] [ABSTRACT FROM AUTHOR]

Carbon fiber (CF) is a versatile material renowned for its excellent mechanical, thermal, and electrical properties. Polyacrylonitrile (PAN)-based CFs dominate the market in part due to their high tensile strength, rendering them suitable for structural applications in a wide variety of applications ranging from sporting goods to aerospace. Over five decades of commercial development of PAN-based CF has resulted in a range of varieties with different tensile moduli and tensile strengths. The microstructures, nanostructures, and crystal structures of PAN-based CF all play pivotal roles in the macroscale properties of this material. In particular, the crystal structure and crystallite orientation in CF is closely related to the mechanical properties. The crystal structure of PAN-based CF generally consists of turbostratic carbon, a disordered form of graphite, the characteristics of which can be effectively characterized in a bulk format through wide-angle x-ray diffraction (WAXD). In this work, we employed a three-part approach to the analysis of WAXD patterns collected from four intermediate modulus PAN-based CFs. The approach incorporates a Scherrer analysis, a Debye analysis, and an orientational analysis to provide precise estimates of crystallite sizes, crystallite distributions, and crystallite orientations with the fiber axis. The results presented here suggest that intermediate modulus PAN-based CF mostly consists of small turbostratic crystallites (<4 nm), with larger crystallites having increased orientation with the fiber axis. The results here imply the presence of curvature and/or wrinkling of the turbostratic layers within the CF structure. [Display omitted] [ABSTRACT FROM AUTHOR]

Capacitance-based nondestructive flaw evaluation (NDE), electric permittivity, piezoelectricity (capacitance-based stress/strain self-sensing) and piezoresistivity (resistance-based stress/strain self-sensing) of carbon-carbon composite (C/C, density 1.5 g/cm3, PAN-based carbon fiber) are unprecedentedly reported. The in-plane capacitance measured using coplanar electrodes decreases monotonically with increasing cumulative artificially inflicted damage (1–62 blind/through holes, diameter 1.00 mm), indicating NDE feasibility. The in-plane relative permittivity is 1.1 × 104 (2 kHz); the in-plane DC resistivity is 2.4 × 10−3 Ω.cm. These values are similar to those previously reported for PAN-based carbon fiber. The permittivity increases reversibly by up to 16% with increasing elastic tensile strain (up to 0.0081%). However, the contribution of the stress-dependent permittivity to the piezoelectric effect is negligible. The observed in-plane direct piezoelectric effect is primarily due to the reversible monotonic decrease of the electric field output with the increasing tensile strain. The piezoelectric coupling coefficient d 33 is negative, -(8.1 ± 0.2) × 10−9 pC/N, in contrast to the positive value of +(1.7 ± 0.3) × 10−8 pC/N for PAN-based carbon fiber. The in-plane piezoresistivity gage factor is negative, −7804 ± 429, which is much higher in magnitude than −1830 ± 47 previously reported for PAN-based carbon fiber. The differences in d 33 and gage factor between C/C and PAN-based carbon fiber are attributed mainly to the carbon matrix in the C/C. Image 1 [ABSTRACT FROM AUTHOR]

The electric permittivity is a fundamental material property concerning the dielectric behavior. Colossal permittivity (relative permittivity exceeding 10,000) is attractive for capacitors, sensors, actuators and mechanical energy harvesters. It was discovered in this work in continuous polyacrylonitrile (PAN) based carbon fiber (Teijin's Tenax HTS45) along the fiber axis. This is the first report of colossal permittivity in carbon materials. The relative permittivity (12230 ± 990, 2 kHz) is much higher than that of mesophase-pitch-based carbon fiber (Cytec's Thornel P-25, 4384 ± 257, 2 kHz). The two fibers exhibit similar resistivity (1.52 × 10−3 and 1.24 × 10−3 Ω.cm for the PAN-based and pitch-based fibers, respectively). For both fibers, the permittivity is high, due to the physical continuity and mobile charge carriers. The higher permittivity of the PAN-based fiber compared to the pitch-based fiber is attributed to the higher degree of graphitization (as indicated by the lower interplanar spacing) of the PAN-based fiber. However, the smaller crystallite sizes and larger azimuthal spread of the carbon layers around the fiber axis cause the PAN-based fiber to be not higher in conductivity than the pitch-based fiber, in spite of the higher degree of graphitization. The smaller crystallite sizes and larger azimuthal spread are detrimental more to the conductivity than the polarization. Image 103662 [ABSTRACT FROM AUTHOR]

Polyacrylonitrile (PAN) fibers containing up to 40 wt% cellulose nanocrystals (CNC) have been successfully stabilized and carbonized to make carbon fibers. Under the batch process, PAN/CNC based carbon fibers exhibit a tensile strength in the range of 1.8–2.3 GPa, and tensile modulus in the range of 220–265 GPa, which are comparable to the control PAN-based carbon fibers. Based on the transmission electron microscopy study, individually distributed CNC regions surrounded by PAN matrix were observed in the stabilized and carbonized fibers. Carbonized fibers show two distinct stabilized and carbonized regions. This study shows that biphasic carbon fibers can be made from the PAN/CNC system. Image 1 [ABSTRACT FROM AUTHOR]

The stress analysis and fracture toughness of high tensile strength polyacrylonitrile (PAN)-based (T1000GB, IMS60, and T300), high modulus PAN-based (M60JB), high modulus pitch-based (K13D), and high ductility pitch-based (XN-05) single carbon fibers were investigated using notched specimens and a focused ion beam. The maximum stress of T1000GB, IMS60, T300, M60JB, K13D, and XN-05 single fibers was calculated as 30.5, 29.4, 25.6, 44.3, 68.6, and 15.5 GPa, respectively. The critical stress intensity factor of T1000GB, IMS60, T300, M60JB, K13D, and XN-05 single carbon fibers was calculated as 1.91, 1.82, 1.67, 3.09, 4.84, and 1.02 MPa m 1/2 , respectively. The critical energy release rate of T1000GB, IMS60, T300, M60JB, K13D, and XN-05 single carbon fibers was calculated as 478, 502, 763, 3064, 13266, and 237 J/m 2 , respectively. These results indicate that a linear relationship exists between the fracture toughness (critical stress intensity factor and critical energy release rate) and tensile modulus and maximum stress. The results indicate that the maximum stress is an effective parameter for evaluating the fracture toughness of PAN-based and pitch-based single carbon fibers. [ABSTRACT FROM AUTHOR]

We present a feasible method, named as radius profiling, to study the structure–mechanical heterogeneity of polyacrylonitrile (PAN)-based carbon fibers (CFs). CF monofilaments were peeled layer by layer by plasma etching, and their electrical resistances and mechanical properties were measured by a multimeter and a single-filament tensile tester, respectively. The method allows the structure and mechanical properties of CF monofilament to be explored with high spatial resolution. The measurement results show that the modulus distribution along the fiber radius direction is highly dependent on the local crystallinity, and the maximums of both modulus and crystallinity appear at approximately 1.1–1.6 μm beneath the surface. The observation is used to explain the high modulus of PAN-based CFs. [ABSTRACT FROM AUTHOR]

The continuous changes in orientation angle (OA) across longitudinal sections of high strength polyacrylonitrile-based carbon fibers with circular cross sections were investigated by transmission electron microscopy. A contrast ring was observed in some cross-sections, displaying two contrast zones in the longitudinal section. Two types of OA distribution features were found in the absence or presence of contrast zones. Contrast zones were confirmed as the symbol of the skin-core structure and the boundaries between the skin and core. The high preferred orientation of the basal planes was first identified in the contrast zone, which presented an OA inflexion point (OIP) in the OA plot. The OIP was also detected in the contrast zone for the self-prepared carbon fibers. The OIP and contrast ring shifted towards the core as the preoxidation time increased. For a preoxidation time of 8 h, both the OIP and contrast ring disappeared. A mechanism, based on the stress-induced alignment of the basal planes, was proposed to account for the formation of the OIP. [ABSTRACT FROM AUTHOR]

Thermally sensitive tetramethylammonium oxalate (TMAO) is incorporated as an in situ porogen into freestanding nonwoven nanofibers of polyacrylonitrile (PAN) produced by electrospinning. Supercapacitor electrode materials are prepared by a series of thermal treatments on these fibers, including stabilization followed by a single step carbonization and CO 2 activation. The specific surface areas of the resultant carbon nanofibers (CNFs) are controlled by varying the amounts of TMAO in the precursor fibers. The electrochemical properties of the carbon nanofibers are characterized by cyclic voltammetry and galvanostatic charge–discharge tests. The highest surface area (2663 m 2 g −1 ) and best electrochemical performance are obtained from PAN containing 0.1 wt% of TMAO (T10), which gives a maximum specific capacitance of 140 F g −1 at a scan rate of 10 mV s −1 in comparison to CNFs from PAN alone, which yielded only 90 F g −1 . At a discharge current density of 1 A g −1 , the obtained energy and power densities are 68 Wh kg −1 and 1.7 W kg −1 , respectively, with capacitance retention of 80% after 1000 cycles. [ABSTRACT FROM AUTHOR]

Polyacrylonitrile (PAN) and PAN/carbon nanotube (CNT) based carbon fibers at various CNT content have been processed and their structural development was investigated using high resolution transmission electron microscope (HR-TEM). In CNT containing carbon fibers, the CNTs act as templating agents for the graphitic carbon structure development in their vicinity at the carbonization temperature of 1450 °C, which is far below the graphitization temperature of PAN based carbon fiber (>2200 °C). The addition of 1 wt% CNT in the gel spun precursor fiber results in carbon fibers with a 68% higher thermal conductivity when compared to the control gel spun PAN based carbon fiber, and a 103% and 146% increase over commercially available IM7 and T300 carbon fibers, respectively. The electrical conductivity of the gel spun PAN/CNT based carbon fibers also showed improvement over the investigated commercially available carbon fibers. Increases in thermal and electrical conductivities are attributed to the formation of the highly ordered graphitic structure observed in the HR-TEM images. Direct observation of the graphitic structure, along with improved transport properties in the PAN/CNT based carbon fiber suggest new applications for these materials. [ABSTRACT FROM AUTHOR]

Understanding the atomistic mechanisms of tensile failure in carbon fibers is important for fiber manufacturing and applications. Here we design structural faults with atomistic details, pertaining to polyacrylonitrile (PAN) derived fibers, and probe them using large-scale molecular dynamics simulations to uncover trends and gain insight into the effect of local structure on the strength of the basic structural units (BSUs) and the role of interfaces between regions with different degrees of graphitization. Besides capturing the expected strength degrading with increasing misalignment, the designed basic structural units reveal atomistic details of local structural failure upon tensile loading. Fracture initiation is nearly always associated with the interface of the misoriented crystallite and its environment. [ABSTRACT FROM AUTHOR]

Abstract: A range of polyacrylonitrile (PAN) and pitch based carbon fibre types (high, standard and intermediate modulus fibres) have been characterised using both physical and chemical techniques, the results highlighting the heterogeneity of the fibre. Nano-indentation showed variation in stiffness between different fibres of the same type as well as variation along a 20μm length of a single fibre. Tensile tests showed variance of approximately 25% in tenacity for three different carbon fibre types but less variability in modulus with values from 8% to 19%. Raman spectroscopy showed variation in the graphitic content both between fibres of different origin as well as variation, with 0.5μm spatial resolution, along the length of a single fibre. Inverse gas chromatography surface energy measurements of larger samples of fibres were carried out using the novel approach of incremental surface coverage by varying the probe molecule concentration and revealed different levels of energetic heterogeneity for PAN based fibres collected at different stages of carbon fibre production. The heterogeneity of the unoxidised fibres (collected after carbonisation) was restricted to about 15% of the fibre surface whereas the surface oxidised fibre sample (collected after the electrolytic oxidation bath) was heterogeneous over more than 30% and the sized fibres were shown to be quite homogeneous. [Copyright &y& Elsevier]

Polyacrylonitrile-based carbon fiber bundles, with two sides fixed, were heated in a graphitization furnace with temperature profiles similar to those used in the preparation of carbon/carbon composites. The microstructure of the microvoids formed was characterized by small-angle X-ray scattering and high resolution transmission electron microscopy (HR-TEM). Results showed that the average radius of gyration of the microvoids decreases, and their length, width, cross-sectional area and volume fraction decrease followed by a slight increase with increasing temperature, with the minima found at around 2300°C. The HR-TEM images showed that the microvoids were formed by stacking defects of the carbon layers. The ordering of the amorphous carbon layers accounted for the initial decrease of size and volume fraction of the voids with increasing temperature, while the growth and ordering of the layers at high temperatures gave rise to increases in their size and volume fraction. [New Carbon Materials 2013, 29(1): 41–46] [ABSTRACT FROM AUTHOR]