Converting natural gas into hydrogen and solid carbon materials using methane pyrolysis presents a promising opportunity to produce sustainable fuels and materials. The production of hydrogen and bulk carbon nanotubes (CNTs) via methane pyrolysis has been demonstrated independently, but concurrent production from the same reactor has remained elusive. Here we present a multi-pass floating catalyst chemical vapour deposition (FCCVD) reactor that converts methane into hydrogen and CNT aerogel. Whereas previous FCCVD CNT production consumed hydrogen, the multi-pass reactor recycles the carrier gas to eliminate the need for a hydrogen input. This results in a net output of 85 vol% hydrogen alongside CNT aerogel and a 446-fold increase in molar process efficiency. Furthermore, the demonstrated use of biogas to produce CNT aerogel enables a potential net sequestration of CO2 from the atmosphere. The results of this study have been extrapolated to a pilot-scale reactor, using data gathered at a commercial facility, to consider the challenges and opportunities associated with scale-up.

The carbon nanotube community swims in the sea of superlatives. Researchers expect mechanical performance to achieve two extremes, an ultrastrong fibre taking us into space, and a superlubricant saving energy otherwise lost as heat. We examine CNT fibres in the light of traditional yarn science and present an interpretation of properties which combines aspects of these two extremes of performance.

Manufacturing carbon nanotube (CNT) fibres on a large scale with properties that are a significant fraction of those measured for individual tubes or smaller bundles is crucial to achieving greater impact in industrial applications. We describe three macrostructured morphologies produced on a laboratory scale: CNT ribbons, ropes and tows, wherein tows are the most suitable for industrial scale-up, comprising a bundled structure of thousands of individual micro-fibres. For each morphology, we present the tensile failure mechanisms and mechanical performance observed over six orders of magnitude of strain rate. The tests were conducted on a conventional Hounsfield tensile tester (10(-4) to 10(-1) s(-1) strain rate) and a modified split-Hopkinson tensile bar tester (30-150 s(-1) strain rate). To avoid the problem of clamp failure, we present a novel method of testing loops, rather than uniaxial samples, as-produced from our chemical vapour deposition reactor. For material benchmarking and validation, we also tested unsized carbon fibre (Hexcel AS4) and Dyneema tows (SK25). All materials were used 'dry', i.e. not as a composite, for raw material and structure comparison. We show here that CNT tows exhibit both a mechanical performance and fracture behaviour which combines the ductility of Dyneema with the tenacity of carbon fibre. (C) 2020 Elsevier Ltd. All rights reserved.

Industrial-scale use of carbon nanotube (CNT) materials and prototype development is limited by the availability of economic, high-throughput production methods. Recent investigations have demonstrated the feasibility of producing direct-spun macroscopic CNT materials via floating catalyst chemical vapour deposition. However, few quantitative results have been reported regarding process yield and correlations with product quality. Validation of results is therefore challenging as identification of the key fundamental process parameters is hindered. This first meta-analysis quantifies atomic input rates and correlates them with product outputs to map the current parameter space of 55 successful conditions leading to spinnable aerogels. All mapped processes fall within a bulk residence time of 5-240 s, operating temperature of 1100-1500 degrees C and an atomic S:Fe of 0.1-10. Low (high) S/Fe ratios favour single (multi)-wall CNTs in the direct-spun product. A high atomic carbon dilution, with only 3% of the input atoms being C, is a common feature across many systems. Furthermore, we connect the findings to known catalyst and product growth behaviour, as well as the thermodynamics of intermediates, to create an emerging picture of direct-spun CNT product formation. Elucidation of the most important factors influencing material synthesis, and the relationships between them, provides opportunities for gains in industrial-scale synthesis. (C) 2019 Elsevier Ltd. All rights reserved.

The process for the direct spinning of CNT fibers has been made to work well on a laboratory scale by the diligence of teams at Cambridge over 15 years. However, in finding the process “sweet spot” in multidimensional parameter space, it is clear that there are a number of potentially critical factors that determine the ability to spin continuous fiber. When spinning is achieved, there is then a fine-tuning scenario that determines the structure of the fibers, including nanotube type, and ultimately the property spectrum of what is a remarkable new yarn-like carbon fiber. This chapter takes a critical path through a mass of research results from many laboratories. It leans particularly heavily on data from the Cambridge groups active in this area, as these can be and have been interrogated retrospectively. The background thermodynamics are addressed but against the understanding that the formation of a solid from gaseous components requires a nucleation stage and thus potentially a delay within the timescale of the continuous process. The possibility of the material from the ceramic reactor tubes affecting the nucleation of the catalyst particles is also addressed, as it opens up the possibility of further process control by the insertion of heterogeneous nuclei. A significant surprise is the prediction that most of the injected iron, once available on the breakdown of the precursor molecule (ferrocene), and some of the carbon would be lost as “plating out” on the reactor walls in the temperature range 750–1000°C. The primary role of sulfur, a necessary addition to ensure continuous spinning, is seen as one that, as a reducer of surface energy, enables homogeneous nucleation of the iron particles at 750°C without an energy barrier. However, the question is addressed as to whether sulfur also influences the actual growth of nanotubes from the catalyst particles. Its presence is known to favor graphite edge growth over “c” axis growth (laying down of successive new graphitic layers), and this can have profound effects on controlling the distribution of graphite in gray cast irons. Whether sulfur is critical to the carbon nanotube growth, mechanism remains to be seen, but it is certainly an additional factor. Industrial pilot plant development, continually informed by this improving understanding, is progressing in the hands of Tortech Nano Fibers Ltd., with 2–3 orders of magnitude increase in throughput achieved to date from a scaled-up version of the research reactors.

It is now 14 years since the publication of the first paper describing the continuous production of carbon nanotube fibres by the direct spinning process. Since then much effort and numerous papers have focused on optimisation of lab-scale spinning process and product properties. To some extent structure-property type research has outstripped studies to understand and eventually scale up the production process itself. Progress is such that the axial mechanical strength and stiffness of carbon nanotube fibres are already competing with other high performance fibres such as aramid, polyethylene and carbon, while both the electrical and thermal conductivities are one to two orders of magnitude better than PAN based carbon fibre. However, to undercut other high performance fibres on price while at the same time providing a wider ranging property spectrum, requires scale-up, although comparatively few publications in the literature have focused on this. Here the various key parameters which will come to bear during large-scale plant design are addressed, especially those associated with hydrogen supply and management. A vision of a 150 tonne per year plant is provided (Fig 9), but the vision is just that, and must be seen as informed speculation. (C) 2019 Published by Elsevier Ltd.

Industrial-scale use of carbon nanotube (CNT) materials and prototype development is limited by availability of economic, high throughput production methods. Recent investigations have demonstrated the feasibility of producing direct-spun macroscopic CNT materials via floating catalyst chemical vapour deposition. However, few quantitative results have been reported regarding process yield and correlations with product quality. Validation of results is therefore challenging as identification of the key fundamental process parameters is hindered. This first meta-analysis quantifies atomic input rates and correlates them with product outputs to map the current parameter space of 55 successful conditions leading to spinnable aerogels. All mapped processes fall within a bulk residence time of 5--150~ s, operating temperature of 1100--1500~ C and an atomic S: Fe of 0.1--10. Low (high) S/Fe ratios favour single (multi)-wall CNTs in the direct-spun product. A high atomic carbon dilution, only 3\% of the input atoms being C, is a common feature across many systems. Furthermore, we connect the findings to known catalyst and product growth behaviour, as well as the thermodynamics of intermediates, to create an emerging picture of direct-spun CNT product formation. Elucidation of the most important factors influencing material synthesis, and the relationships between them, provides opportunities for gains in industrial-scale synthesis.

Floating catalyst chemical vapor deposition (FC-CVD) methods offer a highly scalable strategy for single-step synthesis and assembly of carbon nanotubes (CNTs) into macroscopic textiles. However, the non-uniform axial temperature profile of a typical reactor, and differing precursor breakdown temperatures, result in a broad distribution of catalyst particle sizes. Spun CNT fibres therefore contain nanotubes with varying diameters and wall numbers. Herein, we describe a general FC-CVD approach to obtain relatively large yields of predominantly single-wall CNT fibres, irrespective of the growth promoter (usually a sulfur compound). By increasing carrier gas (hydrogen) flow rate beyond a threshold whilst maintaining a constant C : H2 mole ratio, CNTs with narrower diameters, a high degree of graphitization (G : D ratio ∼100) and a large throughput are produced, provided S : Fe ratio is sufficiently low. Analysis of the intense Raman radial breathing modes and asymmetric G bands, and a shift in the main nanotube population from thermogravimetric data, show that with increasing flow rate, the fibres are enriched with small diameter, metallic CNTs. Transmission electron microscopy corraborates our primary observation from Raman spectroscopy that with high total flow rates, the fibres produced consist of predominantly small diameter SWCNTs.

The reduction of cell size to the nanometre scale is one of the main current focuses in polymeric cellular materials research. The recent achievement of cell sizes in the range of 10-200 nm naturally gives rise to the question of what are the main limiting factors in obtaining further improvements. This paper explores the theoretical limits of this reduction in scale size, and presents an application of atomistic modelling strategies for the study of nanofoams produced by the gas dissolution technique. The main conclusion is that, as the cell size scale becomes closer to molecular sizes, atomic-scale interactions should be considered, and conventional rules for foams usually applied to continuous media should be reassessed due to the increasingly discrete nature of the polymeric material. The gas-polymer equilibrium for a range of different polymers (HDPE, PP, PVDF, PEI, PS, PES, PC and PMMA) were calculated using molecular dynamics (MD), in order to generate structures in the nanometric range and to study their stability. The results show that the minimum stable cell size is governed by the behaviour of the internal pressure in the polymer within small gas-filled voids, which may explain the differences between cell sizes observed experimentally in different polymers.

The paper focuses on the importance of using specific values (values where the axial property is normalised by dividing by density) for properties of CNT yarn-like fibres to avoid the huge uncertainties introduced by the state of lateral compression of the yarn, a difficulty not only associated with the measurement of cross sectional area, but exacerbated by the influence of winding forces and indeed gripping forces while under test. Even with the disciplined use of specific parameters, there are also serious issues regarding axial mechanical tests and measurements of thermal conductivity. These are discussed as well.

Lightweight high-performance macroscopic fibers and yarns of oriented carbon nanotubes (CNTs) with high tensile strength and stiffness and electrical conductivities are in high demand for a wide range of applications. Among various production methodologies, the direct continuous spinning of CNT fibers from a chemical vapor deposition reactor has attracted interest because of its scalability potential. However, the presence of non-CNT impurities limits achieving high fiber tensile and electrical performances reproducibly. Here, we present for the first time a one-step protocol to purify direct-spun CNT fibers by mild sonication in acetone. Sonication reduced the impurity content by ~42%, which led to enhanced CNT fiber performance. In comparison to untreated fibers, the purified fibers showed a 50% and 100% increase in specific strength and stiffness, respectively. The CNT fiber electrical conductivity increased by 3-fold after purification. This improvement in fiber performance was observed despite a reduction in the nanotube bundle orientation after sonication. The better performance of the purified fibers is attributed to improved CNT bundle packing and densification. Our process is simple, quick, effective, and uses readily available acetone and hence, shows promise as a post-production purification method to direct CNT fiber spinning.

During tensile tests of carbon nanotube (CNT) macrostructures (ribbons, ropes and tows) under dynamic strain rates (1000 s(-1)), we recorded temporally sporadic, spatially localized visible light emissions ("flashes") of less than 1.5 mu s duration. The flashes occurred at the fracture sites and were, depending on the sample morphology, either distributed randomly over time (for tows) or occurred all at once over larger areas of several pixels (for ribbons). In situ thermal camera measurements, as well as ex situ analysis by electron microscopy reveal a hierarchical mechanism of overall heating over the whole sample length during straining, and localized heating around the fracture surfaces. Temperatures around the fracture tip were calculated to be of 1800 K in average. We propose that the flashes are caused by charge separation due to CNT bond fracture and gas discharge of the surrounding gases. Triboluminescence, known for larger sugar crystals, has not been observed for carbon nanotubes previously. It results from the yarn-like morphology, the ultra-high strength and thermal conductivity of our CNT fibers, which at high strain rates concentrate the strain at CNT level and lead to CNT fracture, rather than bundle sliding. (C) 2017 Elsevier B.V. All rights reserved.

Thermal conductivity of carbon nanotube (CNT) films and micro-fibres synthesised by floating catalyst chemical vapour deposition was measured by the parallel thermal conductance method. CNT films showed in-plane thermal conductivities of 110 W m(-1) k(-1). Online condensation into a micro-fibre morphology a two-dimensional reduction in the transverse plane, including some axial stretching during solvent evaporation resulted in room-temperature thermal conductivity values as high as 770 +/- 10 W m(-1) k(-1), which is the highest thermal conductivity reported for CNT bulk materials to date. In specific terms, this matches the maximum thermal conductivity of heat-treated carbon fibre, but with a higher onset temperature for Umklapp scattering processes (300 K rather than 150 K). We selected four sample types to investigate effects of alignment, purity, and single- or multi-wall character on their thermal conductivity. For both the electrical and thermal conductivity of as-spun material, i.e. without any post-synthesis treatment, we show that the density and quality of CNT bundle alignment are still the predominant factors affecting these properties, outweighing the influence of single- or multi-walled character of the nanotubes. This raises the promise that, with optimal alignment and junction points, even higher values of thermal conductivity are achievable for macroscopic CNT fibres. (C) 2016 Published by Elsevier Ltd.

Cellulose is one of the world's primary natural resources, and is the most used material in the world. More than 20 times the volume of steel is used on an annual basis, and since ancient times cellulose, in the form of timber, paper and clothing, has underpinned the development of society. Despite its common use throughout human history, only in recent years has the true potential of cellulose as a high-end functional and sustainable material been realized, especially in the form of nanofibrous materials [1,2]. Natural cellulose is found in every plant as a hierarchical material. Within the trunk of the tree and its branches and roots, there are fibre vessel elements, which in the living tissues transport nutrients and water. In processes such as papermaking, these vessel elements are extracted and we typically then call them ‘fibres’. In fact, such fibres possess a layered structure of fibrils, embedded in other plant polysaccharides, and binding materials such as hemicelluloses, lignin, pectins and waxes. These fibrils, or cellulose nanofibrils (CNFs), can be extracted from the cell walls of the fibres through mechanical and/or chemical/enzymatic approaches. They represent not only a truly sustainable resource but also a functional material.Like many other biopolymers, celluloses possess a semicrystalline structure. This semicrystalline structure is susceptible to some degree to hydrolysis using strong acids, the so-called ‘amorphous’ regions being more susceptible than the crystalline ones [3,4]. This hydrolysis process liberates so-called ‘cellulose nanocrystals’ (CNCs), a colloidal form of cellulose stabilized by negative surface charge due to the presence of sulfate half-ester groups (if sulfuric acid is used) [4,5]. Such colloidal CNCs are interesting from an industrial perspective because they can be dispersed in water. As cellulose is known to be recalcitrant to a …

A protocol has been developed for the production of epoxy-based composites containing high-volume fractions of aligned carbon nanotubes. The nanotubes were fabricated as continuous fibres or aligned mats directly from the CVD reactor, in which they were synthesized. The block composites with highly aligned and tightly packed nanotube assemblies were prepared via epoxy resin infiltration, and their volume fraction, distribution, and internal porosity being analysed prior to mechanical testing. The samples were tested in both axial tension and three-point bending. The results show that the strength and stiffness enhancements were close to pro rata with the volume fraction of the carbon nanotubes added. The failure modes were distinctly different from those characteristic of the conventional aligned carbon fibre composites. The fracture surface showed considerable evidence of pull-out of bundles of (~50) nanotubes, but the pull-out appeared to involve the resin matrix which drew out along with the bundles. Subsidiary cracks were bridged by nanotube bundles giving structures reminiscent of crazes in glassy polymers, what constitutes the distinct toughness mechanism and higher resistance to the transverse cracks propagation.

Carbon nanotube (CNT) fibres are characterized by extreme anisotropy in their structure and physical properties. These fibres have been shown to have high axial strength, but poor shear strength between carbon nanotubes; for this reason it is difficult to transfer stress uniformly acrossthe fibre cross section. Here, Finite Element Analysis (FEA) is used to predict the stress distribution and the stress-strain curves of CNT fibres. The resultsdemonstrate that, in accordance with St. Venant principle,very considerable length-to-diameter ratios (> 103) are required to obtain a uniform stress distribution within the fibres even in the presence of low applied strain.

The potential toxicity of carbon nanotubes (CNTs) has been compared to pathogenic fibres such as asbestos. It is important to test this hypothesis to ascertain safe methods for CNT production, handling and disposal. In this study aspects reported to contribute to CNT toxicity were assessed: length, aspect ratio, iron content and crystallinity; with responses compared to industrially produced MWCNTs and toxicologically relevant materials such as asbestos. The impacts of these particles on a range of macrophage models in vitro were assessed due to the key role of macrophages in particle clearance and particle/fibre-induced disease. Industrially produced and long MWCNTs were cytotoxic to cells, and were potent in inducing pro-inflammatory and pro-fibrotic immune responses. Short CNTs did not induce any cytotoxicity. Frustrated phagocytosis was most evident in response to long CNTs, as was respiratory burst and reduction in phagocytic ability. Short CNTs, metal content and crystallinity had less or no influence on these endpoints, suggesting that many responses were fibre-length dependent. This study demonstrates that CNTs are potentially pathogenic, as they were routinely found to induce detrimental responses in macrophages greater than those induced by asbestos at the same mass-based dose.