Neighboring Tubes Research Articles

Introducing thermally stable inter-tube defects to assist off-axial phonon transport in carbon nanotube films

Through integrated molecular dynamics (MD) simulations and experimental studies, we demonstrated the feasibility of an ion-irradiation-and-annealing based phonon engineering technique to enhance thermal conductivity of carbon nanotube (CNT) films. Upon ion irradiation of CNT films, both inter-tube defects and intra-tube defects are introduced. Our MD simulations show that inter-tube defects created between neighboring tubes are much more stable than intra-tube defects created on tube graphitic planes. Upon thermal annealing, intra-tube defects are preferentially removed but inter-tube defects stay. Consequently, axial phonon transport increases due to reduced phonon scattering and off-axial phonon transport is sustained due to the high stability of inter-tube defects, leading to a conductivity enhancement upon annealing. The modeling predictions agree with experimental observations that thermal conductivities of CNT films were enhanced after 2 MeV hydrogen ion irradiations and conductivities were further enhanced upon post irradiation annealing.

Formation of a Flare-Productive Active Region: Observation and Numerical Simulation of NOAA AR 11158

We present a comparison of the Solar Dynamics Observatory (SDO) analysis of NOAA Active Region (AR) 11158 and numerical simulations of flux-tube emergence, aiming to investigate the formation process of this flare-productive AR. First, we use SDO/Helioseismic and Magnetic Imager (HMI) magnetograms to investigate the photospheric evolution and Atmospheric Imaging Assembly (AIA) data to analyze the relevant coronal structures. Key features of this quadrupolar region are a long sheared polarity inversion line (PIL) in the central δ-sunspots and a coronal arcade above the PIL. We find that these features are responsible for the production of intense flares, including an X2.2-class event. Based on the observations, we then propose two possible models for the creation of AR 11158 and conduct flux-emergence simulations of the two cases to reproduce this AR. Case 1 is the emergence of a single flux tube, which is split into two in the convection zone and emerges at two locations, while Case 2 is the emergence of two isolated but neighboring tubes. We find that, in Case 1, a sheared PIL and a coronal arcade are created in the middle of the region, which agrees with the AR 11158 observation. However, Case 2 never builds a clear PIL, which deviates from the observation. Therefore, we conclude that the flare-productive AR 11158 is, between the two cases, more likely to be created from a single split emerging flux than from two independent flux bundles.

Phonon transport assisted by inter-tube carbon displacements in carbon nanotube mats

Thermal transport in carbon nanotube (CNT) mats, consisting of randomly networked multi-walled carbon nanotubes (MWNTs), is not as efficient as in an individual CNT because of the constrained tube-to-tube phonon transport. Through experiments and modeling, we discover that phonon transport in CNT mats is significantly improved by ion irradiation, which introduces inter-tube displacements, acting as stable point contacts between neighboring tubes. Inter-tube displacement-mediated phonon transport enhances conductivity, while inter-tube phonon-defect scattering reduces conductivity. At low ion irradiation fluence, inter-tube thermal transport enhancement leads to thermal conductivity increase by factor > 5, while at high ion irradiation fluence point defects within tubes cause a decrease in thermal conductivity. Molecular dynamics simulations support the experimentally obtained results and the proposed mechanisms. Further conductivity enhancement in irradiated mats was obtained by post-irradiation heat treatment that removes majority of the defects within the tubes without affecting thermally stable inter-tube displacements.

Pair tunneling, phase separation, and dimensional crossover in imbalanced fermionic superfluids in a coupled array of tubes

We study imbalanced fermionic superfluids in an array of one-dimensional tubes at the incipient dimensional crossover regime, wherein particles can tunnel between neighboring tubes. In addition to single-particle tunneling (ST), we consider pair tunneling (PT) that incorporates the interaction effect during the tunneling process. We find that with an increase of PT strength, a system of low global polarization evolves from a structure with a central Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state to one with a central BCS-like fully-paired state. For the case of high global polarization, the central region exhibits pairing zeros embedded in a fully paired order. In both cases, PT enhances the pairing gap, suppresses the FFLO order, and leads to spatial separation of fully paired and fully polarized regions, the same as in higher dimensions. Thus, we show that PT beyond second-order ST processes is of relevance to the development of signatures characteristic of the incipience of the dimensional crossover.

Photoconductive, free-standing crystallized TiO2 nanotube membranes

We report controllable fabrication and photoconductive properties of free-standing crystallized TiO2 nanotube (TNT) membranes. The TNT membranes are synthesized by a two-step anodic procedure, in which dependence of the TNT morphology and detachment on the applied voltage is experimentally present. Due to the large surface area-to-volume ratio and high quality crystal structure, the membrane exhibits a sensitive spectral response to the ultra violet (UV) light. The spectral response at 320nm is ∼30A/W under 5.0V bias, corresponding to external quantum efficiency of 10,695%. The photoresponse origins from internal gain induced by desorption of oxygen molecules from the nanotube surfaces and reduction of the barrier at neighboring tubes under UV illumination. This work extracts intrinsic photoconductive properties of TNT membranes and is beneficial to the future design of optoelectronic devices.

Torsional Alfvén waves in partially ionized solar plasma: effects of neutral helium and stratification

Ion-neutral collisions may lead to the damping of Alfven waves in chromospheric and prominence plasmas. Neutral helium atoms enhance the damping in certain temperature interval, where the ratio of neutral helium and neutral hydrogen atoms is increased. Therefore, the height-dependence of ionization degrees of hydrogen and helium may influence the damping rate of Alfven waves. We aim to study the effect of neutral helium in the damping of Alfven waves in stratified partially ionized plasma of the solar chromosphere. We consider a magnetic flux tube, which is expanded up to 1000 km height and then becomes vertical due to merging with neighboring tubes, and study the dynamics of linear torsional Alfven waves in the presence of neutral hydrogen and neutral helium atoms. We start with three-fluid description of plasma and consequently derive single-fluid magnetohydrodynamic (MHD) equations for torsional Alfven waves. Thin flux tube approximation allows to obtain the dispersion relation of the waves in the lower part of tubes, while the spatial dependence of steady-state Alfven waves is governed by Bessel type equation in the upper part of tubes. Consecutive derivation of single-fluid MHD equations results in a new Cowling diffusion coefficient in the presence of neutral helium which is different from previously used one. We found that shorter-period (< 5 s) torsional Alfven waves damp quickly in the chromospheric network due to ion-neutral collision. On the other hand, longer-period (> 5 s) waves do not reach the transition region as they become evanescent at lower heights in the network cores. Propagation of torsional Alfven waves through the chromosphere into the solar corona should be considered with caution: low-frequency waves are evanescent due to the stratification, while high-frequency waves are damped due to ion neutral collisions.

Hydrodynamics of Jets From Guillotine Steam Generator Tube Rupture: Modeling, Analytical Results, Computational Fluid Dynamics Calculation, and Comparison With Experimental Data

In this work we study the hydrodynamics of characteristic gas jets resulting from guillotine breaks of steam generator tube rupture sequences (SGTR) in pressurized nuclear power reactors. As an initial step towards describing an “in-bundle” gas jet, a hydrodynamic model of free gas jets emerging from a guillotine break under prototypical SGTR conditions has been developed. First we have studied the jet characteristic for an isolated tube; the analytical model estimates variables such as trajectories, centerline velocities, velocity distribution, and Reynolds stresses. We have performed model comparisons with experimental data for different experimental conditions with different mass flow rates, and we have found good agreement of the model with the experimental results. Additionally, an “ad hoc” expression has been derived for the centerline jet velocity, which has been experimentally confirmed. Consistently with the experimental data and the computational fluid dynamics (CFD) calculations the analytical model predicts no outflow near the jet center. As a complementary issue, we have performed CFD calculations for a guillotine tube rupture when the tube is surrounded by several rows of neighboring tubes, in this case the jet trajectories are affected by the Coanda effect near the tubes.

Influence of Intertube Additional Atoms on Sliding Behaviors of Double-Walled Carbon Nanotube

The effects of intertube additional atoms on the sliding behaviors of double-walled carbon nanotubes (DWCNTs) are investigated using molecular dynamics (MD) simulation method. The interaction between carbon atoms is modeled using the second-generation reactive empirical bond-order potential coupled with the Lennard—Jones potential. The simulations indicate that intertube additional atoms of DWCNT can significantly enhance the load transfer between neighboring tubes of DWCNT. The improvement in load transfer is guaranteed by the addition of intertube atoms which are covalently bonded to the inner and outer tubes of DWCNT. The results also show that the sliding behaviors of DWCNT are strongly dependent of additional atom numbers. The results presented here demonstrate that the superior mechanical properties of DWCNT can be realized by controlling intertube coupling. The general conclusions derived from this work may be of importance in devising high-performance CNT composites.

Origin of conductivity crossover in entangled multiwalled carbon nanotube networks filled by iron

A realistic transport model showing the interplay of the hopping transport between the outer shells of iron-filled entangled multiwalled carbon nanotubes (MWNTs), and the diffusive transport through the inner part of the tubes, as a function of the filling percentage, is developed. This model is based on low-temperature electrical resistivity and magneto-resistance (MR) measurements. The conductivity at low temperatures showed a crossover from Efros-Shklovski variable range hopping (VRH) to Mott VRH in three dimensions (3D) between the neighboring tubes as the iron weight percentage is increased from 11$%$ to 19$%$ in the MWNTs. The MR in the hopping regime is strongly dependent on temperature as well as magnetic field and shows both positive and negative signs, which are discussed in terms of wave-function shrinkage and quantum-interference effects, respectively. A further increase of the iron percentage from 19$%$ to 31$%$ gives a conductivity crossover from Mott VRH to 3D weak localization (WL). This change is ascribed to the formation of long iron nanowires at the core of the nanotubes, which yields a long dephasing length (e.g., 30 nm) at the lowest measured temperature. Although the overall transport in this network is described by a 3D WL model, the weak temperature dependence of inelastic scattering length expressed as ${L}_{\ensuremath{\varphi}}$ \ensuremath{\sim} ${T}^{\ensuremath{-}0.3}$ suggests the possibility for the presence of one-dimensional channels in the network due to the formation of long Fe nanowires inside the tubes, which might introduce an alignment in the random structure.

Nanotube bundles and tube-tube orientation: A van der Waals density functional study

We study the binding energy, intertube distance and electronic structure of bundles consisting of single walled carbon nanotubes of the same chirality. We model various nanotube structures (chiralities) and orientations with van der Waals density functional theory. The orientation of the tubes in the bundle strongly influences the properties of the bundles if the chirality of the tubes shares symmetry with the trigonal bundle structure, meaning chiralities which have a $60^\circ$-rotational symmetry (C$_6$-axis), e.g. ($12,0$) bundles. The bundle structure breaks the symmetry depending on the arrangement of the neighboring tubes. Pseudogaps open in the electronic density of states and intertube distances ($\pm5$-10%) vary in dependence of the relative orientation of the tubes in the bundle. Bundles of C$_6$-axis armchair tubes have metallic configurations. A $15^\circ$ rotation off the high symmetry configuration (AA-stacked) of a ($6,6$) bundle shows metallic behavior and has higher binding energy than the high symmetry configuration. We find binding energies between $19 meV/atom$ and $35 meV/atom$, depending on the chirality of the tubes. The intertube distances are between $3.2 {\AA}$ and $3.4 {\AA}$ but independent of orientation for non C$_6$-axis tubes.

Performance analysis of a two-stage thermal energy storage system using concrete and steam accumulator

Abstract This paper presents a thermal analysis of a two-stage thermal energy storage system, in which concrete is used in the high-temperature heat storage stage and steam accumulator is used in the low-temperature stage. To analyze the thermal characteristics of the whole system, a numerical simulation model for the two-stage thermal storage system was developed, based on the conservation principle of mass and energy. The results show that the performance of the high-temperature concrete storage unit depends greatly on the concrete conductivity and the distance between the neighboring steel tubes. Also, the steam discharging speed of steam accumulator directly affects the heat behavior inside the concrete block. The founded simultaneous decline in the energy quality level for the steam accumulator and the concrete system may benefit cascade utilization of thermal energy. Results of the investigation will assist in the optimum design of the thermal energy storage system used for solar thermal power plant.

Hydrogen-induced self-assembly of helical carbon nanostructures from ethanol over SiO2 catalysts

Helical carbon nanostructures from ethanol over silica are successfully prepared through the addition of hydrogen during chemical vapor deposition. The helical structure formation appears to occur through hydrogen etching reactions enabling graphitic caps between neighboring tubes to merge, triggering the self-assembly of a helical structure. The experimental data is supported by thermodynamic calculations which correlate the hydrogen radical concentration with the observed efficiency of helical structure formation. Moreover, the calculations show an increase in water vapor content as one increases hydrogen addition. This can account for the improved crystallinity of the samples with higher hydrogen doses.

Experimental study on frosting suppression for a finned-tube evaporator using ultrasonic vibration

An experimental study was conducted to investigate the possibility and the effect of frost release from a finned-tube evaporator by using ultrasonic vibrations in natural convection. A compressor was adopted as cold source, and a finned-tube evaporator was placed horizontal on the frames inside a chamber. The aluminum fins surface was covered with hydrophilic coatings whose contact angle of the water drop of the surface is 37.7°, and the fins had wing-cases to enlarge the heat-exchange area. The thickness of a fin was approximately 0.1 mm, and the gap distance between any two fins of the evaporator was about 1.55 mm in average. An ultrasonic transducer with resonance 28.2 kHz and rating power 60 W was adopted as a vibrator to excite the evaporator. A microscopic image system was used to observe and record the whole frosting process with/without ultrasonic vibrations, a method of frost thickness measurement on the double sides of a fin was present and realized through digital image processing technology. From microscopic view, the deposited frost nearly block the fins gaps when the compressor worked for 32 min without ultrasonic vibration, while almost 2/3 of the gaps between two neighbor fins were unblocked when the compressor worked for 92 min with ultrasonic vibration, which shows that the frost deposited on the fins surfaces could be suppressed with high frequency ultrasonic vibrations. From macroscopic view, the frost growing on the fins are being vibrated to fall off and accumulated under the evaporator to form frost piles. Although the fins above the braze tubes were deposited with frost, the fins between any two neighboring tubes were unblocked. Experiments show that the basic ice layer on the fins cannot be removed with ultrasonic vibrations, but frost crystals and frost branches on the ice layer can be fractured and removed effectively, the frost growing on the evaporator can be suppressed by using ultrasonic vibrations. The main mechanism of ultrasonic frost suppression maybe mainly attributed to the high frequency ultrasonic mechanical vibrations that can break up frost crystals and frost layers, then frost will fall off with the gravity, not to ultrasonic cavitation effect or heat effect.

A106 高速実用炉蒸気発生器の細管亀裂開口からの高圧蒸気ジェット噴流測定(OS8 高速増殖炉の蒸気発生器(Na-水反応))

One of the design basis accidents in sodium-cooled fast reactor is sodium-water reaction at steam generator (SG). In case of a defect occurred on a heat transfer tube, the high-pressure water/vapor will spout into the low-pressure sodium surrounding outside the tube. As sodium is ordinarily quite reactive with water, this will initiate sodium-water reactions accompanied by high chemical heat generation. The liquid droplet in the reaction steam outflow would impinge on neighboring tubes to cause erosion, while the chemical reaction will cause corrosion, eventually may lead to secondary tube failure. Focusing on the erosion part, this study is to evaluate the liquid droplet impingement erosion (LDIE) rate on neighboring tubes caused by SG heat transfer tube rupture. In this paper, as a basic study, the pressure and temperature distribution of high-pressure two-phase free jet into the air is measured.

Femtosecond luminescence decay due to exciton energy transfer in single-walled carbon nanotube bundles

We investigated luminescence decay kinetics in single-walled carbon nanotubes in large bundles excited by femtosecond pulses. The time constant of luminescence decay becomes longer with decrease in photon energy: 40 fs at 1.2 eV and 380 fs at 0.6 eV. This behavior is explained by exciton energy transfer from an excited to neighboring tubes.

Bright Luminescence and Exciton Energy Transfer in Polymer-Wrapped Single-Walled Carbon Nanotube Bundles

Selective solubilization of semiconducting single-walled carbon nanotubes (SWNTs) by polymer wrapping leads to bright luminescence even in bundles because of suppression of deexcitation processes via metallic SWNTs. We investigate photoluminescence (PL) properties of polymer-wrapped SWNTs in a paper form by means of PL excitation spectroscopy and time-resolved PL spectroscopy. PL excitation spectra indicate bright luminescence and the existence of exciton energy transfer from wide to narrow gap tubes. Time-resolved measurements show that the luminescence decay becomes slower with decreasing gap energy. The observed decay behavior is analyzed by a simple rate equation model describing both intratube exciton decay and exciton energy transfer to neighboring tubes. It is found that the intratube exciton decay rate and the effective transfer rate per tube at the center-to-center distance of ∼1.9 nm are 1.1 × 1011 s−1 and 2.7 × 1011 s−1, respectively. These findings are relevant for understanding of energy-tran...

Phonon dephasing and population decay dynamics of the G-band of semiconducting single-wall carbon nanotubes

The dephasing and population decay dynamics of optical phonons are studied for semiconducting single-wall carbon nanotubes (SWCNTs) using broadband time-resolved coherent anti-Stokes Raman scattering and time-resolved incoherent anti-Stokes Raman scattering. By simply adjusting the spectral bandwidth of a continuum pulse, we are able to directly measure both the total dephasing time, ${T}_{2}$, and the population decay time, ${T}_{1}$, of the G-band sequentially in the same sample, which allows for one to exclude artifacts due to comparison of dynamics values measured with different sample conditions and different measurement schemes. The values of ${T}_{1}$ and ${T}_{2}/2$ are presented for two different SWCNT samples: bundles in a film on glass and a dispersion in water. While the measured ${T}_{1}$ values are similar for the two samples, the pure dephasing times, ${T}_{2}^{\ensuremath{\ast}}/2$, determined from the ${T}_{2}/2$ and ${T}_{1}$ measurements are faster in bundled SWCNTs than in isolated dispersion. This suggests that neighboring tubes in the film perturbs the vibrational mode more strongly than surrounding surfactants and that the pure dephasing dynamics is more sensitive to the perturbation.

Effect of the Geometry of the Anodized Titania Nanotube Array on the Performance of Dye-Sensitized Solar Cells

Highly ordered TiO2 nanotube arrays are superior photoanodes for dye-sensitized solar cells (DSSCs) due to reduced intertube connections, vectorial electron transport, suppressed electron recombination, and enhanced light scattering. Performance of the cells is greatly affected by tube geometry, such as wall thickness, length, inner diameter and intertube spacing. In this paper, effect of geometry on the photovoltaic characteristics of DSSCs is reviewed. The nanotube wall has to be thick enough for a space charge layer to form for faster electron transportation and reduced recombination. When the tube wall is too thin to support the space charge layer, electron transport in the nanotubes will be hindered and reduced to that similar in a typical nanoparticle photoanode, and recombination will easily take place. Length of the nanotubes also plays a role: longer tube length is desired because of more dye loading, however, tube length longer than the electron diffusion length results in low collecting efficiency, which in turn, results in low short-circuit current density and thus low overall conversion efficiency. The tube inner diameter (pore size) affects the conversion efficiency through effective surface area, i.e., larger pore size gives rise to smaller surface area for dye adsorption, which results in low short-circuit current density under the same light soaking. Another issue that may seriously affect the conversion efficiency is whether each of the tube stands alone (free from connecting to the neighboring tubes) to facilitate infiltration of dye and fully use the outer surface area.

Investigation of Effect of Cut Fins on Carbon Dioxide Gas Cooler Performance

In fin-and-tube heat exchangers operating at large temperature differences (such as carbon dioxide gas coolers), the heat transfer due to fin conduction between neighboring tubes degrades the heat exchanger performance. In this paper, a validated segment-by-segment heat exchanger model capable of accounting for tube-to-tube conduction in two dimensions, as well as simulating continuous and discontinuous cut fins has been employed to study the effect of cut fins on heat exchanger performance. The effect of two separate configurations of cut fins on heat exchanger performance was studied for 36 test cases, and a heat load gain of up to 12% was shown with cut fins. Additionally, for a target heat load, fin material savings of up to 40% were shown. Further, gains of up to 20% in the inlet quality of an evaporator were shown, considering a 7.2°C evaporating temperature and isenthalpic expansion. Finally, by placing constraints on the length of cuts and maximizing heat load, guidelines for location of cut fins were established.

Mechanical Peeling of Free‐Standing Single‐Walled Carbon‐Nanotube Bundles

An in situ electron microscopy study is presented of adhesion interactions between single-walled carbon nanotubes (SWNTs) by mechanically peeling thin free-standing SWNT bundles using in situ nanomanipulation techniques inside a high-resolution scanning electron microscope. The in situ measurements clearly reveal the process of delaminating one SWNT bundle from its originally bound SWNT bundle in a controlled-displacement manner and capture the deformation curvature of the delaminated SWNT bundle during the peeling process. A theoretical model based on nonlinear elastica theory is employed to interpret the measured deformation curvatures of the SWNTs and to quantitatively evaluate the peeling force and the adhesion strength between bundled SWNTs. The estimated adhesion energy per unit length for each pair of neighboring tubes in the peeling interface based on our peeling experiments agrees reasonably well with the theoretical value. This in situ peeling technique provides a potential new method for separating bundled SWNTs without compromising their material properties. The combined peeling experiments and modeling presented in this paper will be very useful to the study of the adhesion interactions between SWNTs and their nonlinear mechanical behaviors in the large-displacement regime.