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Sodium-Based Catalysis Of Carbon Nanotubes For Interlaminar Reinforcement Of Unidirectional Hierarchical Laminates
Despite the fact that numerous viable CNT techniques have been established over the last few decades, the metal-free synthesis strategy has not been substantially examined. The current work addresses the utilization of activated carbon (AC) as a supporting substrate combined with NaCl as a green catalyst for the synthesis of carbon nanotubes (CNTs) via a catalyzed chemical vapor deposition (cCVD) method. The effect of different AC–NaCl ratios (1:0, 1:1, 1:2, and 1:3) on CNT growth was investigated. The nano-particle yield was estimated and samples were characterized by BET, Electron microscopy (SEM and TEM), XRD, FTIR, and TGA analyses. The asymmetrical porous structure and high surface area of the AC clearly offer excellent uniform NaCl dispersion properties on the surface, resulting in a high catalyst–transition metal-free yield of CNTs forest growth. The results showed higher mass yield in the order AC–NaCl 1:2 > AC–NaCl 1:1 > AC–NaCl 1:3 ratio. AC and NaCl are excellent choices as substrate and catalyst combinations for the synthesis of metal-free MWCNTs as they are cost-effective and environmentally friendly.
Hybrid hierarchical carbon-nanotube (CNT)-based composites, such as radially-aligned CNT arrays grown onto microfiber filaments, have significant potential to expand the performance and functionality of fiber reinforced composites. Here, a novel method for high-yield growth of aligned CNTs on aerospace-grade carbon fibers (CFs) is demonstrated at the composite level for the first time. Fuzzy carbon fiber reinforced plastics (fuzzy CFRP) unidirectional composites with >60% microfiber volume fraction are fabricated via vacuum-assisted resin infusion of CNT-grafted tows using an unmodified aerospace-grade epoxy. Preservation of microfiber tensile modulus and strength are demonstrated by longitudinal composite tensile testing, consistent with single-fiber tensile tests. Fiber-matrix interface strength is also unchanged by the CNT growth as revealed through continuously-monitored fiber fragmentation tests. Taken together, the results provide needed new composite-level understanding of hierarchical structural composite laminates and motivate future work on structural CF composite laminates with integrated multifunctionality and improved interlaminar and intralaminar performance.
We present two strategies for growing aligned carbon nanotubes (CNTs) on carbon fibers with the goal of enabling production of nanoengineered carbon fiber advanced composites while preserving fiber tensile strength. The first strategy involves applying a protective conformal alumina barrier coating over the carbon fiber upon which metal catalyst nanoparticles suitable for CNT growth can be deposited without risk of damaging the carbon fiber surface. The second strategy leverages a class of novel non-metallic CNT growth catalysts based on active oxides such as zirconia (zirconium oxide) that, unlike metal catalysts, are chemically inert on the carbon fiber surface yet have been recently been shown to catalyze CNT growth. Single-fiber tensile tests for various stages of processing for barrier-coated fibers are presented and possible origins of tensile strength losses are discussed. Analysis of non-metallic CNT growth catalysts and considerations for their use on carbon fibers are presented. Results indicate paths forward for synthesizing the desired morphology of long, aligned (high yield) CNTs on carbon fibers while preserving properties of the carbon fibers - a critical step needed in the creation of carbon "fuzzy fiber" reinforced plastic advanced laminated composite materials.
Hybrid composite architectures employing traditional advanced composites and carbon nanotubes offer significant potential mechanical and multifunctional performance benefits. The architecture investigated here is composed of aligned fibers with carbon nanotubes grown radially on their surface. A novel process for rapidly growing dense, long, high-quality, aligned carbon-nanotube forests is employed. Two fundamental issues related to realizing hybrid composite architectures are investigated experimentally: wetting of the carbon nanotubes; by thermoset polymers and retention of mechanical (stiffness and strength) properties of the fibers after the carbon-nanotube growth process. Wetting of carbon-nanotube forests by two commercial polymers (including a highly viscous epoxy) is demonstrated at rates conducive to creating a fully dispersed carbon-nanotube/matrix region around the fibers in a typical composite. Single-fiber tension tests indicate no mechanical degradation for alumina fibers undergoing the carbon-nanotube growth process. Results indicate that hybrid carbon-nanotube/composite architectures are feasible, and future work focuses on mechanical and multifunctional property characterization of other hybrid architectures and scaling to a continuous carbon-nanotube growth process.
Hierarchical carbon fibers (CFs) sheathed with radial arrays of carbon nanotubes (CNTs) are promising candidates for improving intra- and interlaminar properties of advanced fiber-reinforced composites (e.g., graphite/epoxy) as well as high-surface-area electrodes for battery and supercapacitor architectures. While CVD growth of CNTs on CFs has been previously shown to improve apparent shear strength between fibers and polymer matrices (up to 60%), this has to date only been achieved at the expense of significant reductions in tensile strength (~30-50%) and stiffness (~10-20%) of the underlying fiber. Here we demonstrate two approaches for growing aligned and unaligned CNTs on CFs that enable preservation of fiber strength and stiffness. We observe that CVD-induced reduction of fiber strength and stiffness is primarily attributable to mechanochemical reorganization of the underlying fiber when heated untensioned above ~550°C in both hydrocarbon-containing and inert atmospheres. We show that tensioning fibers to ≥12% of tensile strength during CVD enables aligned CNT growth while simultaneously preserving fiber strength and stiffness even >700°C. We also show that CNT growth employing CO2/acetylene at 480°C without tensioning-below the identified critical strength-loss temperature-also preserves fiber strength. These results highlight previously unidentified mechanisms underlying synthesis of hierarchical CFs and demonstrate scalable, facile methods for doing so.
The feasibility of reinforcing conventional carbon fiber composites by grafting carbon nanotubes (CNTs) onto the fiber surface has been investigated. Carbon nanotubes were grown on carbon fibers using the chemical vapor deposition (CVD) method. Iron was selected as the catalyst and predeposited using the incipient wetness technique before the growth reaction. The morphology of the products was characterized using scanning electron microscopy (SEM), which showed evidence of a uniform coating of CNTs on the fiber surface. Contact angle measurements on individual fibers, before and after the CNT growth, demonstrated a change in wettability that can be linked to a change of the polarity of the modified surface. Model composites based on CNT-grafted carbon fibers/epoxy were fabricated in order to examine apparent interfacial shear strength (IFSS). A dramatic improvement in IFSS over carbon fiber/epoxy composites was observed in the single fiber pull-out tests, but no significant change was shown in the push-out tests. The different IFSS results were provisionally attributed to a change of failure mechanism between the two types of tests, supported by fractographic analysis.
This paper reviews published research into polymer composite laminates reinforced in the through-thickness direction with z-pins. Research into the manufacture, microstructure, delamination resistance, damage tolerance, joint strength and mechanical properties of z-pinned composites is described. Benefits of reinforcing composites with z-pins are assessed, including improvements to the delamination toughness, impact damage resistance, post-impact damage tolerance and through-thickness properties. Improvements to the failure strength of bonded and bearing joints due to z-pinning are also examined. The paper also reviews research into the adverse effects of z-pins on the in-plane mechanical properties, which includes reduced elastic modulus, strength and fatigue performance. Mechanisms responsible for the reduction to the in-plane properties are discussed, and techniques to minimise the adverse effect of z-pins are described. The benefits and drawbacks of z-pinning on the interlaminar toughness, damage tolerance and in-plane mechanical properties are compared against other common types of through-thickness reinforcement for composites, such as 3D weaving and stitching. Gaps in our understanding and unresolved research problems with z-pinned composites are identified to provide a road map for future research into these materials.
Three-dimensional reinforcement of woven advanced polymer–matrix composites using aligned carbon nanotubes (CNTs) is explored experimentally and theoretically. Radially-aligned CNTs grown in situ on the surface of fibers in a woven cloth provide significant three-dimensional reinforcement, as measured by Mode I interlaminar fracture testing and tension-bearing experiments. Aligned CNTs bridge the ply interfaces giving enhancement in both initiation and steady-state toughness, improving the already tough system by 76% in steady state (more than 1.5 kJ/m2 increase). CNT pull-out on the crack faces is the observed toughening mechanism, and an analytical model is correlated to the experimental fracture data. In the plane of the laminate, aligned CNTs enhance the tension-bearing response with increases of: 19% in bearing stiffness, 9% in critical strength, and 5% in ultimate strength accompanied by a clear change in failure mode from shear-out failure (matrix dominated) without CNTs to tensile fracture (fiber dominated) with CNTs.
We present an in-depth study of CNT growth on commercially-available woven alumina fibers, and achieve uniform growth of dense aligned CNTs on commercially-available cloths up to 5 × 10 cm in area. By systematically varying the catalyst concentration, catalyst pre-treatment time, and sample position within the tube furnace, we isolate key factors governing CNT morphology on fiber surfaces and classify these morphologies as related to the processing conditions. Synthesis employs a low-cost salt-based catalyst solution and atmospheric pressure thermal CVD, which are highly attractive approaches for commercial-scale processing. The catalyst solution concentration determines the uniformity and density of catalyst on the fibers, H2 exposure mediates formation of catalyst clusters, and thermal decomposition of the reactant mixture activates the catalyst particles to achieve uniform aligned growth. Under conditions for aligned CNT growth, uniform radially-aligned coatings are achieved with shorter CNT length, and these split into “mohawks” as the CNT length increases. Radially-aligned growth for 5 min adds a typical CNT mass fraction of 3.8% to the initial sample mass, and a uniform morphology exists throughout the weave. Composites prepared by standard layup techniques using these CNT “fuzzy” alumina fibers are attractive as integral armor layers having enhanced ballistic and impact performance, and serve as a model system for later implementation of this technology using carbon fibers.