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Polymer-derived carbon ceramics are highly desirable for lightweight, high strength, extreme environment material architectures, but their mechanical performance as a function of structure and processing is not currently understood and cannot be predicted. In this study, the mechanical behavior for bulk-scale pyrolytic carbons (PyCs) made via polymer pyrolysis of phenol-formaldehyde precursors is established as a function of heat treatment temperature and the resulting average nano- and meso-scale order and disorder via X-ray diffraction, Raman spectroscopy, and Fourier transform infrared spectroscopy. The PyCs exhibit crystallite evolution on both the atomic- and meso-scale for pyrolysis temperatures of 600 °C to 1000 °C, whereas only atomic-scale crystallite evolution is observed for pyrolysis temperatures of 1000 °C to 1400 °C. The measured Vickers hardness of the PyCs is observed to scale non-monotonically as a function of the pyrolysis temperature reaching a peak at -4 GPa for samples prepared at 1000 °C. New modeling results, based on the elastic constants of disordered graphite, indicate that this counter-intuitive Vickers hardness scaling, which is a decades-old open question, originates from the PyC inter-layer shear elastic constant and the crystallite aspect ratio evolution with processing temperature. PyCs studied here are shown to be the lightest super-hard materials, having Vickers hardness-to-density ratios that are comparable to super-hard carbides, oxides, nitrides, and phosphides. [ABSTRACT FROM AUTHOR]

Abstract: Intrinsic and scale-dependent properties of carbon nanotubes (CNTs) have led aligned CNT architectures to emerge as promising candidates for next-generation multifunctional applications. Enhanced operating regimes motivate the study of CNT-based aligned nanofiber carbon matrix nanocomposites (CNT A-CMNCs). However, in order to tailor the material properties of CNT A-CMNCs, porosity control of the carbon matrix is required. Such control is usually achieved via multiple liquid precursor infusions and pyrolyzations. Here we report a model that allows the quantitative prediction of the CNT A-CMNC density and matrix porosity as a function of number of processing steps. The experimental results indicate that the matrix porosity of A-CMNCs comprised of ∼1% aligned CNTs decreased from ∼61% to ∼55% after a second polymer infusion and pyrolyzation. The model predicts that diminishing returns for porosity reduction will occur after 4 processing steps (matrix porosity of ∼51%), and that >10 processing steps are required for matrix porosity <50%. Using this model, prediction of the processing necessary for the fabrication of liquid precursor derived A-CMNC architectures, with possible application to other nanowire/nanofiber systems, is enabled for a variety of high value applications. [Copyright &y& Elsevier]

Polymer-derived pyrolytic carbons (PyCs) are often desirable for low density high-temperature structural applications when they can be reinforced with high stiffness and strength fibers to address brittleness. Nanofibers, such as aligned carbon nanotubes (A-CNTs), can be an ideal reinforcement owing to their high mass-specific properties, and while modeling suggests that properties of A-CNTs at high volume fractions (i.e., 10–30 vol%) could yield significant enhancements, it is unknown how CNT confinement influences processing, as such materials have never been synthesized. Here, we report process development demonstrating the first successful fabrication of fully infused, void-free A-CNT carbon matrix nanocomposites (A/C-NCs) via polymer infiltration pyrolysis (PIP) that establishes a platform to study nanofiber-reinforced polymer-derived ceramics. The size of the average A/C-NC graphitic crystallites increases as CNT vol% increases up to 30 vol%, and the inter-layer separation of the PyC graphitic crystallites decreases, evidencing a nanoscale confinement effect observed in concert with increased mechanical performance. Vickers microhardness testing in the axial CNT direction of A/C-NCs shows agreement with prior data for low vol% CNTs in PyC and mechanical modeling, where specific hardness increases from ∼ 3.3 GPa/(g/cm3) for PyC to ∼ 6.7 GPa/(g/cm3) for 30 vol% A/C-NCs, demonstrating A/C-NCs as an advantaged superhard lightweight material. [Display omitted] • Fabrication of pyrolytic carbon composites with 1-30 vol% aligned carbon nanotubes. • Vacuum- and capillary-assisted infiltration of phenolic resin enable full infusion. • Four polymer infiltration pyrolysis cycles decrease porosity and increases density. • Graphitic crystallites in matrix are longer and denser with nanotube reinforcement. • Specific microhardness increases up to 30 vol% carbon nanotubes, rivaling diamond. [ABSTRACT FROM AUTHOR]

Abstract The exceptional static and dynamic physical properties of poly(urethane-urea) (PUU) elastomers make them prime candidates for impulsive loading structural applications, such as blast protection coatings. Since the theoretical physical properties of carbon nanotubes (CNTs) are among the best for any currently known material, a number of previous studies explored the use of CNTs as nanoscale fillers to enhance the properties of PUU nanocomposites. However, due to the challenges inherent in dispersing CNTs in a PUU matrix and the resulting random orientation of the CNTs, these previous works observed marginal improvements in physical properties, and were unable to establish clear structure-property relations. Here, we report the synthesis of aligned-CNT (A-CNT) reinforced PUU polymer nanocomposites (A-PNCs) by infusing A-CNT forests with a stoveable PUU, and establish process-structure-property relations that quantify the contribution of CNT confinement on the PUU mechanical response. This stoveable process was achieved using blocked isocyanate which prevented polymerization until the blocks were removed with heat. PUUs of two distinct compositions were explored: one with 40 wt% hard-segment content (PUU211) and the other with 66 wt% hard-segment content (PUU541). Thermogravimetric analysis indicates that A-CNTs enhance the thermal stability of the hard-segment phase in PUU A-PNCs at 340 °C by up to 45% over the baseline PUUs. Atomic force microscopy reveals that the elongated nanophase hard-segment formations along the CNT axis observed only in the nanocomposites were of similar characteristic size to the average inter-A-CNT spacing (∼70 nm), indicating a strong influence of A-CNTs on the size and orientation of hard-segment nanophases, as corroborated via small angle X-ray scattering. Nanoindentation testing reveals that PUU A-PNCs possess significant elastic anisotropy, and exhibit enhanced longitudinal effective indentation moduli of ∼460 MPa (>3 × that of the PUU211 baseline) and ∼1350 MPa (∼1.5 × that of the PUU541 baseline) for PUU211 and PUU541 nanocomposites, respectively. This difference in magnitude of CNT reinforcement efficacy indicates that CNT confinement leads to significant hard-segment re-organization in the PUU211 A-PNCs, whereas the interconnected network of hard-segments in the PUU541 is affected by CNT templating to a lesser extent. Dynamic nanoindentation testing results are consistent with these interpretations, where longitudinally-loaded PUU211 A-PNCs are found to exhibit a >3 × enhancement in storage modulus at 1 Hz of ∼730 MPa, whereas the longitudinally-loaded PUU541 A-PNCs exhibit a slightly enhanced storage modulus enhancement at 1 Hz of 2190 MPa (∼1.5 × that of the PUU541 baseline). Reinforcement of PUUs with A-CNTs is a promising way to tune the physical properties of the PNCs; higher A-CNT packing densities, where the inter-CNT spacing could approach the nanophase characteristic diameter, could further enhance the PUU performance in ballistic protection applications. [ABSTRACT FROM AUTHOR]

• Measurements of thermal diffusivity and conductivity of single-walled carbon nanotube (SWCNT) forests with biaxial mechanical densification. • Anomalous increase of thermal diffusivity with increase in volume fraction beyond threshold. • New thermal conduction path for densified forest of single-walled carbon nanotubes. Needs of a material for thermal management in reduced-sized electronic devices drive single-walled carbon nanotubes (SWCNTs) to be one of the most promising candidates due to their excellent thermal properties. Many numerical and experimental studies have reported on understanding thermal properties of the SWCNT for thermal device applications. In the present study, thermal diffusivity and conductivity of SWCNT forests in the axial direction with various volume fractions of SWCNTs were measured by laser flash analysis technique and the same properties of an individual SWCNT were derived. The volume fraction was controlled up to 25 vol% by biaxial mechanical densification which maintains SWCNT alignment. While the thermal diffusivity of SWCNT forests was almost constant, it increased when the volume fraction was higher than 17 vol%, suggesting that some SWCNTs, which did not serve as a thermal path in the case of lower volume fraction, were connected with the other SWCNTs. Through the increase of inferred thermal conductivity equivalent to an individual SWCNT after 17 vol%, we also realized that the enhancement of volume fraction of SWCNT forest diminished thermal boundary resistance in good agreement with the tendency inferred from the reported numerical data. [ABSTRACT FROM AUTHOR]

X-ray micro-computed tomography (μCT) is used to quantify morphology in AS4/8552 (autoclave) and IM7/M56 (Out-of-Autoclave, OoA) aerospace-grade advanced unidirectional-ply carbon fiber prepreg composites, revealing several previously unreported features. The micron-scale (1 μm voxel size) three-dimensional datasets combined with automated, objective algorithms, revealed the following previously unreported features of AS4/8552 and IM7/M56 laminates, respectively: all ply interfaces analyzed have misplaced microfibers at densities of 1–2 per mm2 of interface area that can contribute to the mean thickness of the interlaminar regions of 8.6 μm and 14.4 μm; all ply interfaces have elongated (aspect ratio > 10 and presumed to extend indefinitely) periodic resin pockets along the microfiber direction of the plies bounding the interlaminar region that we term tow-aligned resin pockets (TARPs), with typical thicknesses that are 2–3X greater than the average interlaminar thickness; overall void fractions are low at ~0.002 vol% and ~0.001 vol%, comprised primarily of newly-quantified "sub-microvoids" with an average volume of 26–31 μm3 that are equally pervasive in both materials, numbering ~300 per mm3. The new interlaminar region and void tools were also utilized to analyze laminates with aligned carbon nanotubes (A-CNTs), termed "nanostitches", incorporated between plies to reinforce the interlaminar regions. The addition of A-CNTs increased the interlaminar thickness by 2.2 μm and 8.0 μm for the AS4/8552 and IM7/M56 systems, respectively, but did not affect the quantity or distribution of voids or TARPs. These newly-identified features are relevant to the mechanical performance of such composites, as they may have positive or negative effects on damage initiation and progression. [ABSTRACT FROM AUTHOR]