Helically Coiled Carbon Nanotubes Research Articles

Enhanced supercapacitor performance with binder-free helically coiled carbon nanotube electrodes

Despite their low specific surface area (<500 m2 g−1) relative to that of activated carbon (1500–2000 m2 g−1), carbon nanotubes are attractive as effective electrode materials for supercapacitors. Electrodes comprised of arrays of helically coiled carbon nanotubes (HCNTs) and multi-wall carbon nanotubes (MWCNTs) were prepared and evaluated as novel electrode materials for electric double layer capacitors (EDLCs). While both types of electrodes exhibited a linear dependence on array height and a diffusion limited behavior below the 1 V s−1 scan rate, the electrodes comprised of HCNT arrays exhibited better performance. Freestanding HCNT and MWCNT buckypapers were also prepared and used as electrodes, and the former showed a higher energy density relative to the latter with no loss in its power density and capacity fade. Collectively, this study concludes that HCNTs are well suited as binder-free electrodes that can be augmented with electroactive polymers for improved EDLC performance.

Helically coiled carbon nanotube electrodes for flexible supercapacitors

Novel highly flexible electrodes consisting of helically coiled carbon nanotubes (HCNTs) have been developed for flexible supercapacitors (SCs). Hierarchically mesoporous HCNTs are synthesized on unidirectional carbon fiber (UCF) by catalytic chemical vapor deposition. Free-standing HCNTs grown on UCF (HCNTF) is further used as electrode cum current collector for fabricating flexible SCs. Highly bendable UCF serves both as the substrate for the synthesis of HCNTs and as current collector for the SC. No separate secondary current collector is used in this study. The microstructure and surface morphology of the HCNTF hybrid is characterized by scanning electron microscopy and transmission electron microscopy, the chemical structure is determined by Raman spectroscopy, the chemical bonding states are examined by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy and the surface area is measured by Brunauer–Emmett–Teller surface area measurement. The electrochemical properties of HCNTF SC are examined by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge measurements. The HCNTF hybrid exhibits open mesoporous nanostructure with an average pore size of 3.8 nm. The bendability of HCNTF SC is tested by carrying out the galvanostatic charge/discharge measurement by bending the SC at different angles and the study reveals that the SC is highly bendable. The HCNTF SC exhibits a maximum gravimetric capacitance of 125.7 F g−1 with an area specific capacitance of 145.3 mF cm−2 at a current density of 0.28 mA cm−2. The supercapacitive performance of the HCNTF SC is found to be superior to that of the SCs utilizing straight-CNTs based electrodes. The present study opens-up the successful development of novel carbonaceous electrode with helical nanostructure for application particularly in SCs.

TRANSPORT IN HELICALLY COILED CARBON NANOTUBES: SEMICLASSICAL APPROACH

Semiconducting single wall carbon nanotubes (SWCNTs) exhibit high electron mobility in low electric field. Tube diameter and temperature have been found to strongly affect transport properties of SWCNTs. We have investigated electron mobility of helically coiled carbon nanotubes (HCCNTs). Electron and phonon band structures of HCCNTs are used in calculation of electron-phonon matrix elements. Scattering rates are calculated using the first order perturbation theory while taking care of energy and momentum conservation law. In order to obtain electron drift velocities, steady state simulation of charge transport is performed using Monte Carlo method.

Theoretical study of energy band splitting induced by spin–orbit interaction in helically coiled carbon nanotubes

Abstract Recent theoretical and experimental works on straight carbon nanotubes (SCNTs) have revealed that the curvature could enhance the spin–orbit interaction (SOI). Motivated by this, we further explore the effective SOI with another different curvature in helically coiled carbon nanotubes (HCCNTs) using the tight-binding model and perturbation approach. Finally, we derive the effective SOI in HCCNTs, and find the electron–hole asymmetric spin splitting. Interestingly, the asymmetric splitting largely depends on coil pitch p and coil radius r, which are not known in SCNTs. These results should be valuable for spintronic applications of carbon nanotubes.

TRANSPORT IN HELICALLY COILED CARBON NANOTUBES: SEMICLASSICAL APPROACH

Semiconducting single wall carbon nanotubes (SWCNTs) exhibit high electron mobility in low electric field. Tube diameter and temperature have been found to strongly affect transport properties of SWCNTs. We have investigated electron mobility of helically coiled carbon nanotubes (HCCNTs). Electron and phonon band structures of HCCNTs are used in calculation of electron-phonon matrix elements. Scattering rates are calculated using the first order perturbation theory while taking care of energy and momentum conservation law. In order to obtain electron drift velocities, steady state simulation of charge transport is performed using Monte Carlo method.

THERMAL CONDUCTANCE OF HELICALLY COILED CARBON NANOTUBES

Thermal conductivity is one of the most interesting physical properties of carbon nanotubes. This quantity has been extensively explored experimentally and theoretically using different approaches like: molecular dynamics simulation, Boltzmann-Peierls phonon transport equation, modified wave-vector model etc. Results of these investigations are of great interest and show that carbon- based materials, graphene and nanotubes in particular, show high values of thermal conductivity. Thus, carbon nanotubes are a good candidate for the future applications as thermal interface materials. In this paper we present the results of thermal conductance s of a model of helically coiled carbon nanotubes (HCCNTs), obtained from phonon dispersion relations. Calculation of s of HCCNTs is based on the Landauer theory where phonon relaxation rate is obtained by simple Klemens-like model.

Crossover from ballistic to diffusive thermal conductance in helically coiled carbon nanotubes

Thermal conductance in helically coiled carbon nanotubes (HCCNTs) is calculated out of phonon dispersion relations: anharmonicity is included through Brenner interatomic potential and scattering rates of the three‐phonon Umklapp processes are calculated exactly, for all allowed transport channels. Highlighted are the universal features of the quantized thermal conductance and crossover from the ballistic to the diffusive transport regime is studied in detail. Correlation between the geometrical parameters of HCCNTs and thermal conductance is investigated. Finally, the specific heat is calculated and compared to that of the other carbon materials.

Phonon transport in helically coiled carbon nanotubes

We perform theoretical studies on the phonon thermal transport in helically coiled carbon nanotubes (HCCNTs). The Grüneisen parameter, as a function of the phonon wave vector and phonon branch, is numerically evaluated for each vibrational mode, so that the three-phonon Umklapp scattering rates can be calculated exactly by taking into account all allowed phonon relaxation channels. We considered wide temperature range and heat conductor lengths from nano- to macro-scale. We examine the crossover from ballistic to diffusive transport regime and impact of HCCNT geometrical parameters on their heat conduction. Thermal conductivity in HCCNTs is found to be slightly lower than that in single walled carbon nanotubes (SWCNTs). This is interpreted by the competition among three factors. Firstly, threefold reduction of the Grüneisen parameter for the acoustic branches. Secondly, lower phonon group velocities. Finally, availability of purely acoustic scattering channels. Nevertheless, HCCNTs are predicted to be more suitable (than SWCNTs) for thermal management applications due to their spring-like shape. HCCNTs are extremely elastic, natural NanoVelcro material.

THERMAL CONDUCTANCE OF HELICALLY COILED CARBON NANOTUBES

Thermal conductivity is one of the most interesting physical properties of carbon nanotubes. This quantity has been extensively explored experimentally and theoretically using different approaches like: molecular dynamics simulation, Boltzmann-Peierls phonon transport equation, modified wave-vector model etc. Results of these investigations are of great interest and show that carbon- based materials, graphene and nanotubes in particular, show high values of thermal conductivity. Thus, carbon nanotubes are a good candidate for the future applications as thermal interface materials. In this paper we present the results of thermal conductance s of a model of helically coiled carbon nanotubes (HCCNTs), obtained from phonon dispersion relations. Calculation of s of HCCNTs is based on the Landauer theory where phonon relaxation rate is obtained by simple Klemens-like model.

Structural model of semi-metallic carbon nanotubes

Electronic band structure and electronic density of states (DOS) of single-walled helically coiled carbon nanotubes (HCCNTs) are numerically calculated by symmetry implemented density functional tight binding method. It is found that unlike straight single-walled carbon nanotubes, single-walled HCCNTs can be semi-metallic, having high DOS at Fermi level. We show that this is a consequence of the local structure of HCCNTs, i.e., that by changing positions of pentagons and heptagons within a monomer, transitions between metallic, semi-metallic, quasi-metallic, and semiconducting structures can be made.

On the structural rules of helically coiled carbon nanotubes

► We investigate the geometries of helically coiled carbon nanotubes. ► Two schemes, namely horizontal and vertical shifting, are studied. ► A new type of helically coiled carbon nanotubes is proposed. We discuss various possibilities for the geometries of helically coiled carbon nanotubes (HCCNTs), especially those derived from toroidal carbon nanotubes (TCNTs) containing nonhexagonal defects. Two schemes for the derivation of HCCNTs from a particular TCNT are studied, namely horizontal and vertical shifting. For both cases there is a parameter characterizing the skewness of the corresponding HCCNTs. In particular, we found that when the horizontal shift parameter (HSP) is larger than a certain value, depending on the parent TCNT chosen, the resulting HCCNT possesses a peculiar geometry that has not been reported, in which the molecular shape cannot be described by a tube rolling on a cylinder. The pitch angle as a function of HSP can be nicely approximated by simple trigonometry functions of HCCNT parameters. In the case of vertical shifting, the dependence of pitch angle on the shift parameter is relatively simpler and more intuitive.

Systematics of Toroidal, Helically-Coiled Carbon Nanotubes, High-genus Fullernens, and Other Exotic Graphitic Materials

We systematically examine the tiling rules of graphene on various 3D surfaces including torus, helicoid, high-genus closed surface, doubly periodic quasi-minimal surface, and torus knot. Firstly, a thorough examination on the problem of finding possible isomers of highly symmetric toroidal carbon nanotubes (TCNT), which are isomorphic to torus, is presented. We then show that a particular class of TCNTs with n-fold rotational symmetry and well-defined latitude coordinates, uniquely characterized by a set of four indices, can be used as starting building blocks for constructing graphitic structures with nontrivial topology. Particularly, we discovered that complex graphitic structures are categorized into two major groups: (A) Tubu-lar CNTs as complex as torus knots can be obtained by altering the nonhexagon distribution on the molecular surface. In general, carbon nanotubes along any space curve such as HCCNT, can be approached with the geometric manipulation schemes (B) A large family of porous graphitic structures, either closed (0D) or extended (2D, 3D), can be classified by assembling suitable sets of neck-like structures. The neck structure is obtained by peeling the outer-rim of a TCNT off leaving the central hole unchanged. Depending on the occasion, necks with different geometric features are used and the resulting porous molecules can be high-genus fullerenes (0D), doubly periodic supergraphene (2D), or triply periodic quasi-minimal surfaces (3D).

HELICALLY COILED CARBON NANOTUBES

Helically coiled carbon nanotubes are frequently synthesized, with well elaborated techniques of growth. This paper is a study of stability and conductivity of these configurations within simple theoretical model followed by the numerical approach.

Synthesis of helically coiled carbon nanotubes by reducing ethyl ether with metallic zinc

Helically coiled carbon nanotubes (HCNTs) with diameter of 50–100 nm were obtained on a large scale by reducing ethyl ether with metallic zinc at 700 °C. As the temperature was raised to 750 °C, most of the products were solid carbon spheres besides HCNTs. X-ray diffraction pattern and the Raman spectroscopy indicate that the product is graphite phase. HCNTs with diameter of 50–100 nm in large quantity were found in the products.