Improved inter-tube coupling in CNT bundles through carbon ion irradiation

Carbon ion irradiation of carbon nanotube (CNT) bundles to enhance mechanical performance is investigated using classical molecular dynamics. Strategies to achieve inter-tube cross-linking for improved shear response without a drastic reduction in tensile strength due to induced defects are considered. Deposition energies of 50–300 eV/ion, fluences of 4 × 10¹³ to 2 × 10¹⁴ cm⁻², and dosages of 2–60 MGy on 7-tube bundles are studied. Within 100–200 eV/ion, the level of cross-linking is directly proportional to dosage and therefore controllable. Lower energy irradiation produces smaller-sized defects so ∼100 eV/ion is the preferred energy. More than 10 different types of cross-link and a variety of defects are created. The defect level becomes excessive if either the energy or the fluence is set too high. Extension to larger bundles however is significantly more challenging. In 19-tube bundles, ∼500 eV/ion is required to form cross-links with the centre CNT, and at this energy careful control of fluence is required to avoid excessive damage. Thus ion irradiation for improving mechanical properties is best suited to small bundles. However, a scenario whereby small bundles are irradiated prior to twisting into ropes is suggested as a possible future method for producing macro-scale cross-linked CNT fibres.     

Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films

Diamond-like carbon (DLC) coatings were successfully deposited on carbon nanotube (CNT) films with CNT densities of 1 × 10⁹/cm², 3 × 10⁹/cm², and 7 × 10⁹/cm² by a radio frequency plasma-enhanced chemical vapor deposition (CVD). The new composite films consisting of CNT/DLC were synthesized to improve the mechanical properties of DLC coatings especially for toughness. To compare those of the CNT/DLC composite films, the deposition of a DLC coating on a silicon oxide substrate was also carried out. A dynamic ultra micro hardness tester and a ball-on-disk type friction tester were used to investigate the mechanical properties of the CNT/DLC composite films. A scanning electron microscopic (SEM) image of the indentation region of the CNT/DLC composite film showed a triangle shape of the indenter, however, chippings of the DLC coating were observed in the indentation region. This result suggests the improvement of the toughness of the CNT/DLC composite films. The elastic modulus and dynamic hardness of the CNT/DLC composite films decreased linearly with the increase of their CNT density. Friction coefficients of all the CNT/DLC composite films were close to that of the DLC coating.     

Rapid oxidative activation of carbon nanotube yarn and sheet by a radio frequency,atmospheric pressure,helium and oxygen plasma

Carbon nanotube yarn and sheet were activated using radio frequency, atmospheric pressure, helium and oxygen plasmas. The nanotubes were exposed to the plasma afterglow, which contained 8.0 × 10¹⁶ cm⁻³ ground state O atoms, 8.0 × 10¹⁶ cm⁻³ metastable O₂ (¹Δ_g), and 1.0 × 10¹⁶ cm⁻³ ozone. X-ray photoelectron spectroscopy and infrared spectroscopy revealed that 30 s of plasma treatment converted 25.2% of the carbon atoms on the CNT surface to oxidized species, producing 17.0% alcohols, 5.9% carbonyls, and 2.3% carboxylic acids. The electrical resistivity increased linearly with the extent of oxidation of the CNT from 4 to 9 × 10⁻⁶ Ω m. On the other hand, the tensile strength of the yarn was decreased by only 27% following plasma oxidation.     

Thermal fatigue and hypothermal atomic oxygen exposure behavior of carbon nanotube wire

In low earth orbit (LEO), components of space systems are exposed to damaging hypothermal atomic oxygen and thermal fatigue. Carbon nanotube (CNT) wires are candidate materials for different applications in space systems. Thirty-yarn CNT wire’s behavior was evaluated when exposed to hypothermal atomic oxygen and thermal fatigue. CNT wire specimens were exposed to a nominal fluence of hypothermal atomic oxygen of 2 × 10²⁰ atoms/cm². The erosion rate due to hypothermal collision between atomic oxygen and CNT wires was calculated to be 2.64 × 10⁻²⁵ cm³/atom, which is comparable to highly ordered pyrolytic graphite. The tensile strength of CNT wire was not affected by this exposure, and a minor reduction of electrical conductivity (2.5%) was found. Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy analysis showed erosion of surface layer with depleted carbon and increased oxygen. Thermal fatigue excursion of 5000 cycles from 70 to −50 °C at a rate of 55 °C/min showed no loss in tensile strength; however a large decrease in conductivity (18%) was seen. SEM analysis showed that the thermal fatigue created buckling of yarn and fracture of individual CNTs bundles. These reduced the effective area and electrical conductivity of CNT wire.     

Overcoming the quality–quantity tradeoff in dispersion and printing of carbon nanotubes by a repetitive dispersion–extraction process

Dispersion-printing processes are essential for the fabrication of various devices using carbon nanotubes (CNTs). Insufficient dispersion results in CNT aggregates, while excessive dispersion results in the shortening of individual CNTs. To overcome this tradeoff, we propose here a repetitive dispersion–extraction process for CNTs. Long-duration ultrasonication (for 100 min) produced an aqueous dispersion of CNTs with sodium dodecylbenzene sulfonate with a high yield of 64%, but with short CNT lengths (a few μm), and poor conductivity in the printed films (∼450 S cm⁻¹). Short-duration ultrasonication (for 3 min) yielded a CNT dispersion with a very small yield of 2.4%, but with long CNTs (up to 20–40 μm), and improved conductivity in the printed films (2200 S cm⁻¹). The remaining sediment was used for the next cycle after the addition of the surfactant solution. 90% of the CNT aggregates were converted into conductive CNT films within 13 cycles (i.e., within 39 min), demonstrating the improved conductivity and reduced energy/time requirements for ultrasonication. CNT lines with conductivities of 1400–2300 S cm⁻¹ without doping and sub-100 μm width, and uniform CNT films with 80% optical transmittance and 50 Ω/sq sheet resistance with nitric acid doping were obtained on polyethylene terephthalate films.     

Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors

A carbon material consisting of hollow carbon spheres anchored on the surface of carbon nanotubes (CNT–HCS) has been synthesized by an easy chemical vapor deposition process using a CNT–MnO₂ hybrid as template. An electrode made of this material exhibits a maximum specific capacitance of 201.5 F g⁻¹ at 0.5 A g⁻¹ and excellent rate performance (69% retention ratio at 20 A g⁻¹). It has impressive cycling stability with 90% initial capacitance retained after 5000 cycles at 5 A g⁻¹ in 6 mol L⁻¹ KOH. Symmetric supercapacitors based on CNT–HCS achieve a maximum energy density of 11.3 W h kg⁻¹ and power density of 11.8 kW kg⁻¹ operated within a wide potential range of 0–1.6 V in 1.0 mol L⁻¹ Na₂SO₄ solution.     

Surface modifications of activated carbon by gamma irradiation

Four commercial activated carbons with different chemical and textural characteristics were modified by gamma irradiation under five different conditions: irradiated in absence of water, in presence of ultrapure water, in ultrapure water at pH = 1.0 and 1000 mg L⁻¹ Cl⁻, in ultrapure water at pH = 7.5 and 1000 mg L⁻¹ Br⁻, and in ultrapure water at pH = 12.5 and 1000 mg L⁻¹ NO₃⁻. Changes in surface chemistry were studied by X-ray photoelectron spectroscopy; pH of point of zero charge, total acidic groups and total basic groups, which were determined by assessment with HCl and NaOH; and textural changes were determined by obtaining the corresponding adsorption isotherms of N₂ and CO₂. Outcomes show that the activated carbon surface chemistry can be modified by gamma irradiation and that the changes depend on the irradiation conditions. Modifications in the sp² hybridization of the surface carbons suggest that the irradiated carbons undergo graphitization. Measurements of structural parameters indicate that the irradiation treatment does not modify the textural properties of the carbons. Finally, studies of pristine and irradiated activated carbons using diffuse reflectance spectroscopy with the Kubelka–Munk function revealed a reduction in band gap energy in the irradiated carbons associated with an increase in sp² hybridization of the carbon atoms.     

Preparation-microstructure-property relationships in double-walled carbon nanotubes/alumina composites

Double-walled carbon nanotube/alumina composite powders with low carbon contents (2–3 wt.%) are prepared using three different methods and densified by spark plasma sintering. The mechanical properties and electrical conductivity are investigated and correlated with the microstructure of the dense materials. Samples prepared by in situ synthesis of carbon nanotubes (CNTs) in impregnated submicronic alumina are highly homogeneous and present the higher electrical conductivity (2.2–3.5 S cm⁻¹) but carbon films at grain boundaries induce a poor cohesion of the materials. Composites prepared by mixing using moderate sonication of as-prepared double-walled CNTs and lyophilisation, with little damage to the CNTs, have a fracture strength higher (+30%) and a fracture toughness similar (5.6 vs 5.4   MPa m^1/2) to alumina with a similar submicronic grain size. This is correlated with crack-bridging by CNTs on a large scale, despite a lack of homogeneity of the CNT distribution.     

The effective interfacial shear strength of carbon nanotube fibers in an epoxy matrix characterized by a microdroplet test

The tensile properties of continuous carbon nanotube (CNT) fibers spun from a CNT carpet consisting of mainly double- and triple-walled tubes, and their interfacial properties in an epoxy matrix, are investigated by single fiber tensile tests and microdroplet tests, respectively. The average CNT fiber strength, modulus and strain to failure are 1.2 ± 0.3 GPa, 43.3 ± 7.4 GPa and 2.7 ± 0.5%, respectively. A detailed study of strength distribution of CNT fiber has been carried out. Statistical analysis shows that the CNT fiber strength is less scattered than those of MWCNTs as well as commercial carbon and glass fibers without surface treatment. The effective CNT fiber/epoxy interfacial shear strength is 14.4 MPa. Unlike traditional fiber-reinforced composites, the interfacial shear sliding occurs along the interface between regions with and without resin infiltration in the CNT fiber. Guidelines for microdroplet experiments are established through probability analysis of variables basic to specimen design.     

A high volume and low damage route to hydroxyl functionalization of carbon nanotubes using hard X-ray lithography

The efficient functionalization of large quantities of carbon nanotubes entangled in a non-woven fashion into bucky-papers has been demonstrated through exposure to hard X-rays generated by a synchrotron source. The X-ray beam solely functionalized the carbon nanotube outer walls and an optimum X-ray exposure energy between 1048 J cm⁻² and 2096 J cm⁻² has been found to achieve maximum hydroxyl group density. Sol–gel reaction between a commercial fluoro-silane and the hydroxyl-modified carbon nanotubes was successfully performed resulting in an even distribution of fluoride atoms on the carbon nanotube surface, opening the way for the mass production of functionalized carbon nanotubes.     

High performance field emission of carbon nanotube film emitters with a triangular shape

We report novel two-dimensional (2D) shaped carbon nanotube (CNT) field emitters using triangular-shaped CNT films and their field emission properties. Using the 2D shaped CNT field emitters, we achieved remarkable field emission performance with a high emission current of 22 mA (equivalent to an emission current density >10⁵ A/cm²) and long-term emission stability at 1 mA for 20 h. We also discuss the field emission behavior of the 2D shaped CNT field emitter in detail.     

A high-performance three-dimensional micro supercapacitor based on ripple-like ruthenium oxide–carbon nanotube composite films

A micro-supercapacitor with a three-dimensional configuration has been fabricated using an inductively coupled plasma etching technique. A ruthenium oxide–carbon nanotube (CNT) composite with a ripple-like morphology is successfully synthesized using a cathodic deposition technique while using silica-based three-dimensional microstructures as a template. The desired network of carbon nanotubes in the composite facilitates electrolyte penetration and proton exchange/diffusion. A single three dimensional microelectrode is studied using cyclic voltammetry, and a specific capacitance of 272 mF·cm⁻² is observed at 5 mV s⁻¹ in a neutral Na₂SO₄ solution. The accelerated cycle life is tested at 80 mV s⁻¹, and a satisfactory cyclability is observed. When placed on a chip, the symmetric cell exhibits good supercapacitor properties, the specific capacitance up to 37.23 mF cm⁻² and specific power density up to 19.04 mW cm⁻² were obtained at 50 mA cm⁻².     

Surface properties of vertically aligned carbon nanotube arrays

This study focuses on the structural changes of vertically aligned carbon nanotube (CNT) arrays while measuring their adhesive properties and wetting behaviour. CNT forests grown by chemical vapor deposition with a height of ~ 100 µm, an outer CNT diameter of ~ 10 nm and a density of the order of ~ 10¹⁰ CNTs/cm² show an average adhesion of 4 N/cm² when pressed against a glass surface. The applied forces lead to the collapse of the regular CNT arrays which limits their reusability as functional dry adhesives. Goniometric water contact angle (CA) measurements on CNT forests show a systematic decrease from an initial value of ~ 126° to a final CA similar to highly orientated graphite. Environmental scanning electron microscopy shows that this loss of hydrophobicity is due to an evaporation induced compaction of CNTs together with the loss of their vertical alignment. We observe the formation of cellular patterns for controlled drying.     

Failure mechanisms of carbon nanotube fibers under different strain rates

Strong and uniform carbon nanotube (CNT) fibers with tensile strength around 1.2 GPa were prepared from vertically aligned CNT arrays, and their mechanical properties were studied using a wide range of tensile strain rates. The cyclic load/unload process, polarized Raman measurements, and fiber fracture surfaces were also used to study the failure mechanism of the CNT fibers. It is found that the fibers exhibit a strain-rate strengthening effect, and have different failure mechanisms at high and low strain rates. The key factors that limit the mechanical properties of the CNT fibers were then investigated based on a failure mechanism analysis: inter-tube slippage happens at low strain-rates, and “cascade-like” breaking dominates at high strain-rates. The maximum strength of the fibers appears at high strain rates, and is mainly determined by the CNT alignment.     

Improved radiation damage tolerance of titanium nitride ceramics by introduction of vacancy defects

TiN and TiN_0.7 were irradiated using a 100 keV Ar-ion beam at 600 °C to target doses of 3 × 10¹⁷ ions cm⁻². SRIM estimation, GIXRD and fluorescence analysis have been performed to evaluate the effect of pre-existing vacancy defect on the radiation tolerance. The lattice parameter of TiN increased after irradiation due to interstitial atoms and vacancies in as-irradiated TiN. In contrary, the lattice parameter decreased for as-irradiated TiN_0.7, which indicates that the nitrogen atom vacancies in TiN_0.7 acted as sinks for displacement atoms generated by irradiation to limit interstitial atoms existing. The intensity of peaks in fluorescence spectrum of as-irradiated TiN was higher than that of as-irradiated TiN_0.7. That attributed to the presence of color centers formed by Frenkel defects in as-irradiated TiN. All of the results indicate that introducing vacancy defect in materials would offer capability to realize self-heal of irradiation damage.     

Fabrication and properties of lightweight SiOC modified carbon-bonded carbon fiber composites

SiOC modified carbon-bonded carbon fiber composites (CBCFs) with densities of 0.38, 0.61, 0.94 g cm⁻³ were prepared by precursor infiltration and pyrolysis method using dimethoxydimethylsilane and methyltrimethoxysilane as precursors. The densification behavior was investigated by analyzing the microstructure of CBCF-SiOC (CS) composites with different densities. The mechanical properties and oxidation resistance of the CS composites were studied. Results indicate that the CS composites with the density of 0.94 g cm⁻³ exhibit better mechanical and anti-oxidation properties.     

Effective production of fluorescent nanodiamonds containing negatively-charged nitrogen-vacancy centers by ion irradiation

Fluorescence from negatively-charged nitrogen-vacancy centers (NV⁻s) in diamonds has unique optical properties with none of the undesirable effects such as photo-bleaching and photo-blinking. In addition, the spin-dependent fluorescence intensity of NV⁻s allows us to perform optically detected magnetic resonance (ODMR) investigation for evaluating the presence of NV⁻s and for the electronic local environment. In this work, we irradiated H⁺, He⁺, Li⁺ and N⁺ ions to nanodiamonds with a median size of 26 nm at various irradiation energies and doses for improving the NV⁻ concentration. ODMR observations of the nanodiamonds showed that ion irradiation increased the number of nanodiamonds containing NV⁻s up to 200 ppm, whereas without ion irradiation, only few NV⁻s were found. The number of nanodiamonds containing NV⁻s at various ion irradiation doses was not monotonous, but had maxima at certain irradiation doses. These results suggest a competition in two opponent roles of vacancies, effective for pairing with nitrogen atoms and as defects for developing damage in crystalline. We also found that sharp and strong ODMR signals were obtained from nanodiamonds irradiated at the optimal condition for the highest yield of NV⁻s. We concluded that He⁺ ion irradiations with 60 or 80 keV at a dose of 1 × 10¹³ ions cm^–2 are the conditions required for the most efficient production of a high quantity of nanodiamonds containing NV⁻s.     

The effect of Ne+ ion implantation on polyimide film

The fluence of Ne⁺ ion irradiation on the surface modification of polyimide (Kapton HN type) film was investigated. The irradiation of ion implantation onto a polyimide film was performed, and the surface chemical structure was analyzed in detail by X-ray photoelectron spectroscopy (XPS). An acceleration voltage of 100 keV was used in the ion implantation with different doses from 5 × 10¹⁴ to 5 × 10¹⁷ ion cm^?2 and a beam current density of 10 μA cm^?2. The elemental ratios of carbon, oxygen and nitrogen were calculated from 1s peaks of the corresponding elements. The results showed that the content of carbon in the surface layer increased after ion irradiation, while the ratios of oxygen decreased after irradiation, especially in the case of the polyimide film treated at ion fluence. The O1s spectra after ion irradiation are related to the rearrangement of those recoil atoms and the ion incorporated into the film and the formation of new types of bond, such as C–O and O–O.     

Studies of carbon ion self-implantation into hydrogenated amorphous carbon films

The properties of polymer-like amorphous hydrogenated carbon thin films with low defect density have been studied. These films were implanted with carbon ions with a dose range of 10¹²–10¹⁶ cm⁻². The purpose of the study is to investigate the effects of ion beam damage on this type of film. Optical absorption measurements observe a narrowing of the optical band gap, suggesting the introduction of a large number of defect states subsequent to the implantation resulting in the broadening of the band tails, only after a threshold ion dose of 10¹⁵ cm⁻². Nuclear reaction analysis suggests also a reduction in the hydrogen content of the film which coincides with film thinning.     

Dense carbon monoliths for supercapacitors with outstanding volumetric capacitances

A commercially available dense carbon monolith (CM) and four carbon monoliths obtained from it have been studied as electrochemical capacitor electrodes in a two-electrode cell. CM has: (i) very high density (1.17 g cm⁻³), (ii) high electrical conductivity (9.3 S cm⁻¹), (iii) well-compacted and interconnected carbon spheres, (iv) homogeneous microporous structure and (v) apparent BET surface area of 957 m²g⁻¹. It presents interesting electrochemical behaviors (e.g., excellent gravimetric capacitance and outstanding volumetric capacitance). The textural characteristics of CM (porosity and surface chemistry) have been modified by means of different treatments. The electrochemical performances of the starting and treated monoliths have been analyzed as a function of their porous textures and surface chemistry, both on gravimetric and volumetric basis. The monoliths present high specific and volumetric capacitances (292 F g⁻¹ and 342 F cm⁻³), high energy densities (38 Wh kg⁻¹ and 44 Wh L⁻¹), and high power densities (176 W kg⁻¹ and 183 W L⁻¹). The specific and volumetric capacitances, especially the volumetric capacitance, are the highest ever reported for carbon monoliths. The high values are achieved due to a suitable combination of density, electrical conductivity, porosity and oxygen surface content.