Floating Catalyst Chemical Vapor Deposition Research Articles

Computationally Guided Design of Large-Diameter Carbon Nanotube Bundles for High-Strength Materials

Large-diameter carbon nanotubes (CNTs) synthesized by floating-catalyst chemical vapor deposition (FC-CVD) assemble into bundles and subsequently into aerogel networks from which yarns and sheets are mechanically drawn. The CNT bundles exhibit unique microstructures with collapsed CNT packing, not found in other types of CNT yarns. At the same time, the bundle structure is not homogeneous and the wide variability of CNT cross-sectional shapes reflects the bundling process. Transmission electron microscopy (TEM) images allow detailed quantification of CNT diameters, shapes, and number of walls. Molecular dynamics (MD) simulations of CNT assemblies are subsequently built to match the observed CNT shape characteristics and mass densities. Contrary to an established notion of an applied “buckling” pressure requirement for radial collapse, MD demonstrated that interacting large-diameter CNTs can collapse spontaneously, suggesting that collapse can occur during bundling. Computational explorations of this mechanism yield conditions for large double-walled CNTs (with a mean diameter around 7.8 nm) to assemble into phases with significantly increased packing efficiencies and Young’s moduli approaching 1 TPa. The results may play a guiding role in advancing FC-CVD aerogel synthesis and processing methods to yield yarn and sheets with larger densities and superior mechanical properties.

Large‐Diameter Carbon Nanotube Transparent Conductor Overcoming Performance–Yield Tradeoff

AbstractThe floating catalyst chemical vapor deposition (FCCVD) method for producing single‐walled carbon nanotubes (SWNTs) has demonstrated great potential in transparent conductive film (TCF) application. In FCCVD, reducing the concentration of carbon nanotubes (CNTs) is a well‐agreed method of improving the conductivity of SWNT TCF, achieved by producing thinner and longer CNT bundles. However, this method decreases the yield dramatically, which has persisted throughout the TCF development. Here, the production of large‐diameter double‐walled CNT (DWNT) TCFs via FCCVD is reported, which overcomes the tradeoff between performance and yield. These TCFs of DWNTs with an average diameter of ≈4 nm have a low sheet resistance of 35 Ω sq−1 at 90% transmittance. The conductivity here aligns with the best‐performing SWNT TCFs reported to date, showing a production yield greater than two orders of magnitude. The main factor contributing to the high performance and yield is considered to be the large tube diameter, which greatly improves the yield threshold of CNT bundling and leads to long tube length and unique junctions broadening. Moreover, the application of DWNT TCFs in perovskite solar cells exhibits a power conversion efficiency of 17.4%, which has not been reported yet in indium‐free CNT‐based solar cells.

A Meta-Analysis of Conductive and Strong Carbon Nanotube Materials.

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.

The mechanical and electrochemical properties of polyaniline-coated carbon nanotube mat

The measured capacitance, modulus and strength of carbon nanotube-polyaniline (CNT-PANI) composite electrodes render them promising candidates for structural energy storage devices. Here, CNT-PANI composite electrodes are manufactured with electrodeposition of PANI onto the bundle network of CNT mats produced via a floating catalyst chemical vapour deposition process. PANI comprises 0% to 30% by volume of the electrode. The composition, modulus, strength and capacitance of the electrodes is measured in the initial state, after the first charge, and after 1000 charge/discharge cycles. Electrode modulus and strength increase with increasing CNT volume fraction; in contrast, the capacitance increases with increasing PANI mass. Charging or cycling reduce the electrode modulus and strength due to a decrease in CNT bundle volume fraction caused by swelling; the electrode capacitance also decreases due to a reduction in PANI mass. A micromechanical model is able to predict the stress-strain response of pre-charged and cycled electrodes, based upon their measured composition after pre-charging and cycling. The electrodes possess up to 63% of their theoretical capacitance, and their tensile strengths are comparable to those of engineering alloys. Their capacitance and strength decrease by less than 15% after the application of 1000 charge/discharge cycles. These properties illustrate their potential as structural energy storage devices.

Heat treatment of carbon nanotube hybrid material for textile applications

The paper describes the synthesis of carbon nanotube hybrid material and its post-processing treatment such as heat treatment of carbon nanotube (CNT) sheet to improve its properties for in textile applications. The CNT sheet is synthesized using the floating catalyst chemical vapor deposition (FCCVD) method. The floating catalyst method is a continuous process and can produce industrial scale nanotubes in a single step. The lightweight of the CNT material and its flexibility makes it a suitable candidate for textile and wearable applications. The synthesis process and applications of the new hybrid material are discussed along with the customization of the material.

Identification of Collapsed Carbon Nanotubes in High-Strength Fibers Spun from Compositionally Polydisperse Aerogels

Carbon Nanotubes (CNTs) of sufficiently large diameter and a few layers self-collapse into flat ribbons at atmospheric pressure, forming bundles of stacked CNTs that maximize packing and thus CNT interaction. Their improved stress transfer by shear makes collapsed CNTs ideal building blocks in macroscopic fibers of CNTs with high-performance longitudinal properties, particularly high tensile properties as reinforcing fibres. This work introduces cross-sectional transmission electron microscopy of FIB-milled samples as a way to univocally identify collapsed CNTs and to determine the full population of different CNTs in macroscopic fibers produced by spinning from floating catalyst chemical vapour deposition. We show that close proximity in bundles is a major driver for collapse and that CNT stoutness (number of layers/diameter), which dominates the collapse onset, is controlled by the growth promotor. Despite differences in decomposition route, different C precursors lead to similar distributions of the ratio layers/diameter. The synthesis conditions in this study give a maximum fraction of collapsed CNTs of 70$\%$ when using selenium as promotor, corresponding to an average of $0.25 layer/nm$.

A Novel Oriented CNT fiber/PDMS Elastic Conductive Composite with Reversible Two-Stage Conductivity

In recent years, intensive researches have been stimulated to explore the promising prospect for carbon nanotubes (CNTs) in the fabrication of novel polymer sensor composites. In this study, the fabrication and properties of a flexible and stretchable composite elastomer fabricated from direct-spun carbon nanotube fiber (yarns) were presented, and the novel CNT fiber/polydimethylsiloxane (PDMS) elastic conductive composite shows the reversible two-stage conductivity owing to its unique structure of CNTs fabricated by the floating catalyst chemical vapor deposition (FCCVD). As the strain increased from 0% to 10% (Stage I), stretching the oriented CNT fiber/PDMS elastic conductive composite induces a constant decrease in the conductive pathways and contact areas between CNTs depending on the stretching distance. However, this composite elastomer will retain almost stable electrical resistance while being stretched by over 10% (Stage II). Furthermore, the composite shows very little variation in resistance under 187 stretching–releasing cycles up to a pre-strain level of 6%, indicating the outstanding stability and repeatability in performance as stretchable conductors. The microstructure, reversible two-stage conductive properties and mechanism were also discussed.

Structural modification of carbon nanotube film toward multifunctional composites via a wet-compression method

Carbon nanotube film (CNTF) assembled by floating catalyst chemical vapor deposition (FFCVD) is a promising material for smart textile, multifunctional composites and soft robotics due to their lightweight, flexibility and electromechanical properties. However, bulk CNTF exhibit unsatisfactory properties, due to the loose and porous packing structures. In this study, an innovative and effective strategy was proposed to densify CNTF by a compression in acetone (wet compression). Due to low surface tension, acetone is easy to penetrate the CNTF and act as a lubricant thus decrease the interactions between the neighboring carbon nanotubes (CNT) under load. Therefore, the CNTF was condensed in thickness from 11.3 to 8.4 µm (34.5% decrease) with a CNT volume fraction increment (~ 11% to ~ 14.7%) under wet-compression treatment. Consequently, the wet-compressed CNTF tensile strength, electrical conductivity and gauge factor enhanced by 97%, 34% and 43% along the direction of CNT alignment and by 213%, 42% and 101% towards the CNT transverse direction. As a potential application, modified CNTF/epoxy composites were fabricated as strong de-icing parts of an airplane. The tensile strength increased to 742 MPa (by 21%) and effectively melted ice by 250 times weight ratio within 1 min; performing as strengthened lightweight multifunctional materials. To summarize, our work provides a universal approach to enhance the overall mechanical properties of the CNTF for several applications.

Simultaneous improvements in conversion and properties of molecularly controlled CNT fibres

Fibres of ultralong and aligned carbon nanotubes (CNT) have axial properties above reference engineering materials, proving to be exceptional materials for application in structural composites, energy storage and other devices. For CNT fibres produced by direct spinning from floating catalyst chemical vapor deposition (FCCVD), a scaled-up method, the challenge is to simultaneously achieve high process conversion and high-performance properties. This work presents a parametric study of the CNT fibre spinning process by establishing the relation between synthesis conditions, molecular composition (i.e. CNT type), fibre mechanical and electrical properties, and conversion. It demonstrates tensile properties (strength 2.1 ± 0.13 N/tex, modulus 107 ± 7 N/tex) above some carbon fibres, combined with carbon conversion about 5%, significantly above literature on similar materials. The combined improvement in conversion and properties is obtained by conducing the reaction at high temperature (1300 °C), using toluene as a carbon source, and through adjustment of the promotor to carbon ratio (S/C) to favor formation of few-layer, collapsed CNTs that maximize packing at relatively high conversions. Lower S/C ratios produce low-defect single-wall CNT, but weaker fibres. An increase in electrical conductivity to 3 × 105 S/m is also observed, with the data suggesting a correlation with longitudinal modulus.

High-strength carbon nanotube fibers with near 100% purity acquired via isothermal vacuum annealing

The Fe and carbonaceous particles are the main impurities in a carbon nanotube fiber (CNTF) fabricated by the floating catalyst chemical vapor deposition (FCCVD) method, which have an adverse effect on various applications, leading to strong demand for purification process to obtain high-purity CNTFs. However, the commonly used purification methods combining oxidation and acid refluxing cannot remove the impurities thoroughly, and will introduce undesired damage onto CNT wall at the same time. Herein, we propose an efficient purification strategy involving an isothermal annealing process under high vacuum to achieve mass preparation of CNTFs with high purity and mechanical/electrical properties. The Fe and carbonaceous impurities can be almost completely removed after vacuum annealing at 1800 °C. The purified CNTFs exhibit enhanced oxidation resistance due to the removal of Fe catalyst particles. The diffusion and evaporation of Fe atoms are most likely responsible for the removal of Fe particles enclosed with graphitic shells. The purified CNTF shows a high specific tensile strength of 1.82 ± 0.04 N/tex due to the high alignment of the CNT bundles within fibers, which can retain 80% of the value for pristine CNTF. The purified CNTF exhibits similar trends of electrical conductivity as the specific tensile strength and has a high conductivity of 3.65 ± 0.11 × 105 S/m. The large-scale preparation of high-purity CNTFs with high-performance will promote the real application of CNTFs.

Carbon nanotube sheet as a microporous layer for proton exchange membrane fuel cells

A multi-walled carbon nanotube (CNT) sheet was employed and investigated as a microporous layer (MPL) of proton exchange membrane fuel cell (PEMFC). The CNT sheet was synthesized via floating catalyst chemical vapor deposition method. The CNT sheet MPLs with the thickness of 15, 30, and 100 μm were prepared and compared with a commercial carbon-black MPL. As a result, it was found that the PEMFC with the 15-μm-thick CNT sheet MPL showed high electrochemical performance, and it outtopped the conventional PEMFC. The 15-μm-thick CNT sheet MPL increased the peak power density by 50.9% in air-supplied cathode and 20.9% in pure oxygen-supplied cathode compared to the conventional PEMFC. Through electrochemical impedance analyses, it was found that the CNT sheet MPL effectively decreased a charge transfer resistance, which is attributed to the improved reaction kinetics and mass transport through the MPL.

Catalyst-Mediated Enhancement of Carbon Nanotube Textiles by Laser Irradiation: Nanoparticle Sweating and Bundle Alignment

The photonic post-processing of suspended carbon nanotube (CNT) ribbons made by floating catalyst chemical vapor deposition (FC-CVD) results in selective sorting of the carbon nanotubes present. Defective, thermally non-conductive or unconnected CNTs are burned away, in some cases leaving behind a highly crystalline (as indicated by the Raman G:D ratio), highly conductive network. However, the improvement in crystallinity does not always occur but is dependent on sample composition. Here, we report on fundamental features, which are observed for all samples. Pulse irradiation (not only by laser but also white light camera flashes, as well as thermal processes such as Joule heating) lead to (1) the sweating-out of catalyst nanoparticles resulting in molten catalyst beads of up to several hundreds of nanometres in diameter on the textile surface and (2) a significant improvement in CNT bundle alignment. The behavior of the catalyst beads is material dependent. Here, we show the underlying mechanisms of the photonic post-treatment by modelling the macro- and microstructural changes of the CNT network and show that it is mainly the amount of residual catalyst which determines how much energy these materials can withstand before their complete decomposition.

Continuous growth of carbon nanotube films: From controllable synthesis to real applications

Due to their unique structures and superior mechanical, electrical, and other physical properties, carbon nanotubes (CNTs) have shown great potential as building blocks for a variety of novel high-performance materials. CNT films, which can be synthesized via CVD methods as well as wet processing approaches, have been proven to be promising for using as composite reinforcements, electric conductors, thermal management materials, etc. Floating catalyst chemical vapor deposition (FCCVD) is a simple and environmentally friendly process among the preparation methods of CNT films. It has the advantages of low cost and mass production, and is very helpful for the large-scale preparation of high-performance CNT films. In this review, we first briefly discuss development of FCCVD technology. Then the CNT growth mechanism in the FCCVD process and the relationship between key process parameters and quality/structure property of CNT film are discussed in-depth. The unique properties and the state-of-the-art applications of the CNT films in composite and some other application fields are overviewed in detail. Finally, the opportunities and remaining challenges toward its real applications are prospected.

Kinetics of Carbon Nanotube Aerogel Synthesis using Floating Catalyst Chemical Vapor Deposition

Kinetics of formation of carbon nanotube aerogel (CNT-a) by floating catalyst chemical vapor deposition (FC–CVD) has been studied for the first time by thermocatalytic decomposition of ethanol usin...

Initial competing chemical pathways during floating catalyst chemical vapor deposition carbon nanotube growth

Ferrocene (Fc) is an effective precursor for the direct synthesis of high quality single-walled carbon nanotubes (SWCNTs) via floating catalyst chemical vapor deposition (FCCVD). However, the formation mechanism of the Fe floating catalyst and the SWNCT growth precursors, such as carbon chains, during Fc decomposition are not well understood. Here, we report first principles nonequilibrium quantum chemical molecular dynamics simulations that investigate the decomposition of Fc during FCCVD. We examine the influence of additional growth precursors including ethylene, methane, CO, and CO2 on the Fc decomposition mechanism and show that the dissociation of these species into C2Hx radicals and C atoms provides the key growth agents for the nucleation of carbon chains from Fc-derived species such as cyclopentadienyl rings. Without an additional growth precursor, Fc decomposes via the spontaneous cleavage of Fe–C and C–H bonds, thereby enabling Fe atoms to cluster and form the floating catalyst. On the basis of these simulations, we detail the two competing chemical pathways present during the initial stages of FCCVD: Fe catalyst nanoparticle growth and carbon chain growth. The latter is accelerated in the presence of the additional growth precursors, with the identity of the precursor determining the nature of the balance between these competing pathways.

Fabrication of large-area uniform carbon nanotube foams as near-critical-density targets for laser–plasma experiments

Abstract Carbon nanotube foams (CNFs) have been successfully used as near-critical-density targets in the laser-driven acceleration of high-energy ions and electrons. Here we report the recent advances in the fabrication technique of such targets. With the further developed floating catalyst chemical vapor deposition (FCCVD) method, large-area ( $>25\kern0.5em {\mathrm{cm}}^2$ ) and highly uniform CNFs are successfully deposited on nanometer-thin metal or plastic foils as double-layer targets. The density and thickness of the CNF can be controlled in the range of $1{-}13\kern0.5em \mathrm{mg}/{\mathrm{cm}}^3$ and $10{-}200\kern0.5em \mu \mathrm{m}$ , respectively, by varying the synthesis parameters. The dependence of the target properties on the synthesis parameters and the details of the target characterization methods are presented for the first time.

Artificial neural network, Pareto optimization, and Taguchi analysis for the synthesis of single-walled carbon nanotubes

Floating-catalyst chemical vapour deposition (FCCVD) is a promising method for large-scale synthesis of single-walled carbon nanotube (SWCNT). The current work focuses on the comprehensive optimization studies with the objective of maximizing the yield and minimizing the diameter of SWCNTs considering the complex dependency and interaction effects between the process parameters. Data-analytics driven process parameter selection criterion was utilized for heat map generation of the eight process parameters. Orthogonal design of experiments encompassing Taguchi L-18 array and subsequent analysis of variance reveals strong interaction effects between the process parameters and demonstrates conflicting nature between the two objectives, yield and diameter of SWCNTs. Pareto optimization was used to capture the conflicting behaviour of the dual objectives in true sense. Artificial neural network (ANN) coupled with Taguchi analysis brings out the interaction effects of process parameters that could predict the output with more than 90% accuracy. The rigorous analysis concludes that the furnace temperature mostly affects the diameter, whereas, methane flow-rate determines the SWCNT yield. The optimum condition for higher SWCNT yield with minimum diameter was obtained at a lower methane and hydrogen flow while keeping furnace temperature and argon flow at the highest level and thiophene at the lowest level.

The effect of iron-carbon ratio and on carbon nanotube synthesis using camphor and ferrocene as carbon sources in the gauze reactor

Maximizing the quantity and quality of the product is the main goal of optimizing the material manufacturing process. The use of ferrocene as a carbon source and a catalyst in carbon nanotube (CNT) synthesis results in a significant amount of impurities. The addition of camphor as an additional carbon source is a way to overcome the high Fe/C ratio in ferrocene. This study aims to add camphor carbon sources to ferrocene to obtain an optimal Fe/C ratio to reduce the product's impurities. The application of a more reliable surface response method has not been well explored in studies of optimization of CNT synthesis. In this study, the response surface method (RSM) helps exploration in the parametric study of CNT synthesis. Ferrocene (Fe(C5H5)2) has an iron-carbon (Fe/C) ratio of 46.5%. The method used is Floating Catalyst Chemical Vapor Deposition (FCCVD), using a gauze-type 316 stainless steel substrate. Variations in the ratio of Fe/C in CNT synthesis were 0%, 6.8%, and 46.5% to obtain CNT results with the best quality and quantity. The Fe/C ratio of 6.8% has the best quality and highest quantity. The results of SEM, EDS, and TEM characterization showed that CNT at the variation of the Fe/C ratio of 6.8% had a CNT outside diameter of 20–40 nm, and the percentage of carbon mass was 86.64%. The yield at this ratio is 1.608 gs from 4 gs of precursor (40.2%). The RSM analysis with the optimum conditions for CNT yield were 49 min reaction time and 16% Fe/C ratio.

Deep-injection floating-catalyst chemical vapor deposition to continuously synthesize carbon nanotubes with high aspect ratio and high crystallinity

The deep-injection floating-catalyst chemical vapor deposition (DI-FCCVD) technique is introduced to continuously synthesize carbon nanotubes (CNTs) with high aspect ratio (AR>17000) and high crystallinity (IG/ID > 60) at high production rate (>6 mg/min). In this technique all reactants are injected directly and rapidly into high-temperature reaction zone through thin alumina tube; this process leads to simultaneous thermal decomposition of well-mixed catalyst precursors (ferrocene and thiophene), and thus to formation of uniformly-sized catalyst particles. Carbon nanotube fiber (CNTF) fabricated from high-AR CNT has specific strength of 2.94 N/tex and specific modulus of 231 N/tex, which are comparable to those of the state-of-the-art carbon fiber. Both DI-FCCVD and wet spinning methods are easily scalable to mass production, so this study may enable widespread industrial application of CNTFs.

Electrochemical activation to enhance the volumetric performance of carbon nanotube electrodes

In recent years, the floating catalyst chemical vapor deposition (FCCVD) method has established itself as one of the most commercially viable methods to produce carbon nanotubes (CNTs). However, at the final stage of the FCCVD process, densification of the web of CNTs to form a sheet causes a significant loss in the active surface area of the CNTs. This loss of surface area reduces the density of active materials that can be infiltrated into the CNT sheet. Reducing the amount of active material in the sheet reduces the charge storage capacity and causes low volumetric performance for CNT sheet in energy storage applications. Here, we are reporting the use of an electrochemical activation technique to chemically functionalize the CNT surface and recover the lost surface area. Chemical functionalization separates the CNTs and thereby increases the density of active materials that can be integrated into the CNT sheet. In order to avoid deterioration of the CNT structure from excessive electrochemical activation, we have optimized the procedure by comparing the performance of four samples of CNT sheets prepared using different numbers of cycles of electrochemical activation (5, 10, 20, 40 cycles). The performance of the sheet was analyzed using specific capacitance measurement, electrochemical impedance spectroscopy (EIS) analysis, and UV–vis absorption spectroscopy. In addition, the Randle-Sevcik plot was used to select the CNTs with the largest activated surface area and electrochemical specific capacitance (150F/g). Further analysis such as x-ray photoelectron spectroscopy (XPS) was also carried out to unveil a 5% increase of the various oxygen-containing functional groups at the surface of the activated CNTs. Polyaniline (PANI) was then deposited on the surface of the electrochemically altered CNTs via an oxidation polymerization process to create a CNT-PANI composite material. A fully fabricated device with CNT-PANI electrodes and Polyvinylidene fluoride − 1-Ethyl-3-methylimidazolium tetrafluoroborate (PVDF-EMIMBF4) electrolyte showed a significant specific capacitance improvement over a device with Poly(vinyl alcohol) -Sulfuric acid (PVA-H2SO4) electrolyte. The PVDF-EMIMBF4 capacitor device had a volumetric energy density of 833.3 µWh/cm3 at a current density of 0.5 mA/cm2. The capacitor has good mechanical flexibility and high capacitance retention (75% after 5000 cycles), which show the advantages of the electrochemical activation method.