Abstract Carbon fibers with high axial stiffness and strength have been used to reinforce polymer-matrix materials in advanced composites. However, these fibers exhibit large anisotropies in the radial and axial directions. In addition, limited data has been reported on the elastic properties of fibers in other directions than longitudinal and transversal directions, and on the relationship between elastic properties and fiber structures. In this study, we prepared five epoxy composites each containing a different type of carbon fiber and used a nanoindentation method in combination with finite element analysis to investigate five elastic constants of the carbon fibers. The results demonstrate that the crystallite size and the orientation of its graphite-crystal inclusions affect the elastic constants c11, c12, c13 and c33, while the carbon fibers with well-aligned large-sized crystallite possess a large mechanical anisotropy. In contrast, the shear modulus (c44) is less affected by the alignment of the crystallite. Instead, the fibers with larger interlayer spacing of (002) basal planes and large crystallite size exhibit a smaller c44.

A study of 1304 data points collated over 266 papers statistically evaluates the relationships between carbon nanotube (CNT) material characteristics, including: electrical, mechanical, and thermal properties; ampacity; density; purity; microstructure alignment; molecular dimensions and graphitic perfection; and doping. Compared to conductive polymers and graphitic intercalation compounds, which have exceeded the electrical conductivity of copper, CNT materials are currently one-sixth of copper's conductivity, mechanically on-par with synthetic or carbon fibers, and exceed all the other materials in terms of a multifunctional metric. Doped, aligned few-wall CNTs (FWCNTs) are the most superior CNT category; from this, the acid-spun fiber subset are the most conductive, and the subset of fibers directly spun from floating catalyst chemical vapor deposition are strongest on a weight basis. The thermal conductivity of multiwall CNT material rivals that of FWCNT materials. Ampacity follows a diameter-dependent power-law from nanometer to millimeter scales. Undoped, aligned FWCNT material reaches the intrinsic conductivity of CNT bundles and single-crystal graphite, illustrating an intrinsic limit requiring doping for copper-level conductivities. Comparing an assembly of CNTs (forming mesoscopic bundles, then macroscopic material) to an assembly of graphene (forming single-crystal graphite crystallites, then carbon fiber), the ≈1 µm room-temperature, phonon-limited mean-free-path shared between graphene, metallic CNTs, and activated semiconducting CNTs is highlighted, deemphasizing all metallic helicities for CNT power transmission applications. [ 25 , 26 , 27 , 168 , 174 , 175 , 176 ] Figure 3 a,b) From data surveyed across the experimental literature: a) electrical conductivities and b) tensile strengths of CNTs, other carbon‐based conductors, and benchmark materials. Filled‐in shapes denote doped materials, as well as the right‐most box plot in each subcategory. Unfilled shapes and the left‐most box plots in each subcategory represent undoped materials. Green lines indicate benchmarks. Key: ) unaligned MWCNT materials; ) aligned MWCNT materials; ) unaligned FWCNT materials; ) aligned FWCNT materials; ) conductive polymers; ) graphitic intercalation compounds; ) carbon fiber and graphite. Unaligned MWCNTs and FWCNTs have lower strengths than their aligned counterparts (measured parallel to alignment direction) and are comparable to paper (3 to 100 MPa). Both aligned MWCNT and aligned FWCNT materials have larger strengths, with maximum clustered values reaching 4500 MPa. This is better than Kevlar (2800 MPa) and Dyneema (3900 MPa). It is also substantially greater than traditional conductive and high‐strength alloys (Copper 210 MPa, aluminum 241 MPa, steel 690 MPa, titanium 827 MPa) and approaches the best synthetic fibers, carbon fiber (2300 to 7100 MPa) and Zylon (5800 MPa). b) Depiction of the multifunctional metric (conductivity multiplied by tensile strength), partitioned by CNT categories. c) Dependence of conductivity and strength on fiber diameter. Key: ) unaligned MWCNT material; ) aligned MWCNT materials; ) unaligned FWCNT materials; ) aligned FWCNT materials; ) conductive polymers; )graphitic intercalation compounds; ) carbon fiber and graphite. M, F, B indicate individual MWCNTs, FWCNTs, and CNT bundles respectively. “ x ” indicated annotated benchmarks. Only in (b) do filled in shapes indicate doped materials. Ellipses help identify trends and are adjusted to cover 90% of the points. Key: ) unaligned MWCNT material; ) aligned MWCNT materials; ) unaligned FWCNT materials; ) aligned FWCNT materials; ) carbon fiber, diamond, and graphite. M, F, B indicate individual MWCNTs, FWCNTs, and CNT bundles respectively. Only in (a) do filled in shapes indicate doped materials, as does the right‐most box plots in each subcategory. Ellipses help identify trends and are adjusted to cover 90% of the points. This also includes unaligned films of predominantly metallic SWCNTs; [ 172 , 329 ] though the CNTs individually are metallic, this does not mean the bulk material will have the standard metal‐like increase in resistance with increasing temperature. Alternatively, more ordered materials, such as CNT cables with aligned microstructure, [ 15 ] less disordered carbon fibers, [ 320 ] and conductive polymers, [ 380 , 381 ] ) conductive polymers; ) graphitic intercalation compounds; ) carbon fiber and graphite. The ellipses help identify trends and are adjusted to cover 90% of the points. Table 24 Log conductivity versus log R 300K / R 10K Details of popular transport models are discussed elsewhere, [ 68 , 376 ] although to better understand the meaning of Figure 11d , we briefly address the origins of the semiconductor‐like resistance‐temperature response as intrinsic CNT elements are brought together into an assembly. Considering that single‐crystal graphite and graphitized carbon fiber can have a fully metal‐like resistance response with temperature (Figure 11a , b ), this should still be possible for individual MWCNTs provided the defect density is lowered. 4.2. Leading Properties of the Ultimate CNT Cable Yarns made from spinning MWCNT forests (AS‐CNTs) have a competitive degree of microstructure alignment, long CNT length, and purity compared to aligned FWCNT material, although in terms of strength they are somewhat lower than aligned FWCNT material and, in terms of electrical conductivity, they are significantly lower. For these reasons, we see CNT cables being fundamentally superior to single‐crystal graphite and graphitized carbon fiber as a high‐mobility host for a doping chemical species. Although recent experimental evidence deemphasizes the importance of electronic species control, for the theoretical reasons above, we still expect that a sufficiently doped semiconducting SWCNTs will be superior to metallic SWCNTs, doped or not, in a CNT cable with aligned microstructure. 4.4.