Aligned Carbon Nanotube Bundles Research Articles

Nanoscale Heat Transport of Vertically Aligned Carbon Nanotube Bundles for Thermal Management Applications.

Electronic devices continue to shrink in size while increasing in performance, making excess heat dissipation challenging. Traditional thermal interface materials (TIMs) such as thermal grease and pads face limitations in thermal conductivity and stability, particularly as devices scale down. Carbon nanotubes (CNTs) have emerged as promising candidates for TIMs because of their exceptional thermal conductivity and mechanical properties. However, the thermal conductivity of CNT films decreases when integrated into devices due to defects and bundling effects. This study employs a novel cross-sectional approach combining high-vacuum scanning thermal microscopy (SThM) with beam-exit cross-sectional polishing (BEXP) to investigate the nanoscale morphology and thermal properties of vertically aligned CNT bundles at low and room temperatures. Using appropriate thermal transport models, we extracted effective thermal conductivities of the vertically aligned nanotubes and obtained 4 W m-1 K-1 at 200 K and 37 W m-1 K-1 at 300 K. Additionally, non-negligible lateral thermal conductance between CNT bundles suggests more complex heat transfer mechanisms in these structures. These findings provide unique insights into nanoscale thermal transport in CNT bundles, which is crucial for optimizing novel thermal management strategies.

Molecular Dynamics study of buckling behavior of wavy vertically aligned carbon nanotube bundles

Experiments have yielded various reports on the deformation behavior of Vertically Aligned Carbon Nanotube bundles under compression. However, a decisive conclusion is still to be explored as various factors influence bundles' deformation behavior. In this study, we employed Molecular Dynamics simulations to investigate the influence of waviness, temperature, and bundle size on the buckling behavior of the bundle under axial compression. It was established that the waviness induced along the structure of the bundles can significantly reduce the collective load-carrying capacity of the bundle, regardless of its size. Furthermore, It was observed that there was a directly proportional relationship between the load-carrying capacity of the bundle and the waviness ratio of the nanotubes, as the increased waviness along the structure diminished the total buckling stress of the VACNT bundle. In addition, the effect of atmospheric temperature on the buckling behavior of the carbon nanotube bundles was investigated. The results suggest that the elevation of the surrounding temperatures leads to an increase in kinetic energy within the bundle. Thus, the pattern and rate at which buckling propagates along the VACNT structure are accelerated. Specifically, higher temperatures were shown to induce less predictable buckling patterns, which emphasizes the significant influence of ambient conditions on the mechanical behavior of VACNT bundles.

Processing of Composite Electrodes of Carbon Nanotube Fabrics and Inorganic Matrices via Rapid Joule Heating.

Composites of nanocarbon network structures are interesting materials, combining mechanical properties and electrical conductivity superior to those of granular systems. Hence, they are envisaged to have applications as electrodes for energy storage and transfer. Here, we show a new processing route using Joule heating for a nanostructured network composite of carbon nanotube (CNT) fabrics and an inorganic phase (namely, MoS2), and then study the resulting structure and properties. To this end, first, a unidirectional fabric of conductive CNT bundles is electrochemically coated with MoS2. Afterward, the conformally coated inorganic phase is crystallized via heat generated by direct current passing through the CNT ensemble. The Joule heating process is rapid (maximum heating rate up to 31.7 °C/s), enables accurate temperature control, and takes only a few minutes. The resulting composite material combines a high electrical conductivity of up to 1.72 (±0.25) × 105 S/m, tensile modulus as high as 8.82 ± 5.5 GPa/SG, and an axial tensile strength up to 200 ± 58 MPa/SG. Both electrical and mechanical properties are orders of magnitude above those of wet-processed nanocomposites of similar composition. The extraordinary longitudinal properties stem from the network of interconnected and highly aligned CNT bundles. Conductivity and modulus follow approximately a rule of mixtures, similar to a continuous fiber composite, whereas strength scales almost quadratically with the mass fraction of the inorganic phase due to the inorganic constraining realignment of CNTs upon stretching. This processing route is applicable to a wide range of nanocarbon-based composites with inorganic phases, leading to composites with specific strength above steel and electrical conductivity beyond the threshold for electronic limitations in battery electrodes.

(Digital Presentation) Thermoelectric and Electronic Transport Studies of Ultrahigh-Conductivity Aligned Carbon Nanotube Assemblies

Macroscopic assemblies of aligned carbon nanotubes (CNTs) have been doubling in conductivity every three years and have now surpassed 10 MS/m [1]. They are promising for replacing copper- or aluminum-based electrical cables in applications where flexibility or weight savings are critical considerations. Understanding of transport processes in these ordered CNT assemblies is critical towards further conductivity improvement; yet, fiber-level transport is still poorly understood. Here, we studied thermoelectric and electrical properties of aligned CNT fibers and bundles produced by solution spinning. We first measured thermoelectric properties while tuning the Fermi energy and demonstrated a giant thermoelectric power factor [2]. We then performed temperature- and magnetic field-dependent conductivity measurements. In contrast to the majority of transport studies of CNT networks [3,4], our aligned CNT fibers exhibited a metallic behavior in a wide temperature range (30-300 K), i.e., conductivity monotonically increasing with decreasing temperature, which we attribute to the excellent sample morphology. At temperatures below 30 K, we observed a gradual decrease of conductivity with decreasing temperature, together with negative magnetoresistance, consistent with the weak localization theory for disordered metals. We determined the dimensionality and coherence lengths of carriers via analysis of the weak localization behavior. In addition to macroscopic CNT fibers with diameters of ~10 μm, we also conducted conductivity measurements on individual crystalline CNT bundles (with diameters ~ 50 nm and lengths ~ 30 μm) that constitute the fibers.

Highly Sensitive Flexible Tactile Sensors in Wide Sensing Range Enabled by Hierarchical Topography of Biaxially Strained and Capillary-Densified Carbon Nanotube Bundles.

Flexible tactile sensors with high sensitivity have received considerable attention for their use in wearable electronics, human-machine interfaces, and health-monitoring devices. Although various micro/nanostructured materials are introduced for high-performance tactile sensors, simultaneously obtaining high sensitivity and a wide sensing range remains challenging. Here, a resistive tactile sensor is presented based on the hierarchical topography of carbon nanotubes (CNTs) prepared by a low-cost and straightforward manufacturing process. The 3D hierarchical structure of the CNTs over large areas is formed by transferring vertically aligned CNT bundles to a prestrained elastomer substrate and subsequently densifying them through capillary forming, providing a monotonic increase in the contact area as applied pressure. The deformable and hierarchical structure of CNTs allows the sensor to exhibit a wide sensing range (0-100kPa), high sensitivity (141.72kPa-1 ), and low detection limit (10Pa). Additionally, the capillary-formed CNT structure results in increased durability of the sensor over repeated pressures. Based on these advantages, meaningful applications of tactile sensors, such as object recognition gloves and multidirectional force perceptions, are successfully realized. Given the scalable fabrication method, 3D hierarchically structured CNTs provide an essential step toward next-generation wearable devices.

Photocurrent in carbon nanotube bundle: Graded Seebeck coefficient phenomenon

Polarized photovoltage of a suspended aligned carbon nanotube (CNT) bundle under uniform optical irradiation is discovered without additional structural modification or bias voltage. Such a phenomenon is very surprising considering the metallic behavior of the overall bundle and zero temperature difference between ends. The photovoltage characteristic time is found similar to the thermal response time under step Joule heating and implies a relation to the thermal behavior of the CNT bundle. A similar thermoelectric voltage is also observed during step Joule heating. Localized laser heating and scanning along the axial direction of the bundle uncovers a linear spatial variation of the local Seebeck coefficient. The Seebeck coefficient linearly decreases from root to tip of the CNT bundle with a rate of a few - µV∙K−1·mm−1. Deep investigation in both the microscopic and macroscopic structures of the CNT bundle reveals that the local alignment of CNT assemblies rather than the minor defects in individual CNTs brings about this linear distribution of Seebeck coefficient in space. The finding presents a new way for direct photon-to-electric energy conversion via Seebeck coefficient grading in CNT structures.

A surface charge governed nanofluidic diode based on a single polydimethylsiloxane (PDMS) nanochannel

HypothesisNanofluidic diodes have attracted intense attention recently. Commonly used materials to design these devices are membrane-based short nanopores and aligned Carbon nanotube bundles. It is highly desirable and very challenging to develop a nanofluidic diode based on a single PDMS nanochannel which is easier to be introduced into an integrated electronic system on a chip. Layer-by-layer (LBL) deposition of charged polyelectrolytes can change the size and surface properties of PDMS nanochannels that provides new possibilities to develop high-performance nanofluidic based on PDMS nanochannels. ExperimentsA novel design of nanofluidic diode is presented by controlling the surface charges and sizes of single PDMS nanochannels by surface modification using polyelectrolytes. Polybrene (PB) and Dextran sulfate (DS) are used to reduce the PDMS nanochannel size to meet the requirement of ion gating by LBL method and generate opposite surface charges at the ends of nanochannels. The parameters of such a nanofluidic diode are investigated systematically. FindingsThis nanofluidic diode developed in this work has high effective current rectification performance. The rectification ratio can be as high as 218 which is the best ever reported in PB/DS modified nanochannels. This rectification ratio reduces with high voltage frequency and ionic concentration whereas increases in shorter nanochannels.

Mechanical analysis of aligned carbon nanotube bundles under electric field

The alignment of carbon nanotubes (CNTs) has a superior role in the development of state-of-the-art electronic devices and advanced nanocomposites for diverse applications. In this regard, the implementation of an external electric field is one of the most promising methods in aligning CNTs. Estimation of the alignment time and mechanical loads on the CNTs’ body is necessary for controlling the alignment, helping protect the CNTs’ structure from the likely defects and generation of atomic disorders. Contrary to the most analytical investigations assessing the mechanical features of the alignment process through a 2D manner, we encounter the 3D phase in real-life applications. As such, herein, through a 3D simulation approach, the mechanical behaviors of CNT bundles are investigated and experimentally validated. We investigated the effect of bundle diameter and field amplitude on the alignment time and mechanical loads. This research reveals the hydrodynamic pressure drag force is the dominant player in exerting notable stresses to the CNTs’ body. Furthermore, Raman spectroscopy of post-processed CNT bundles confirmed that the utilization of strong fields can dissociate the thinner bundles from the main one and destruct the structure of CNTs and weaken their unique mechanical strength.

Behavior of the mechanical system composed of highly deformable structural elements

Carbon nanotubes (CNTs) are attractive for many applications because they possess a unique combination of mechanical and physical properties. Horizontally aligned CNT bundles under lateral compression behave as an elastic body with highly deformable elements, since their cross sections can collapse. Mechanical properties of such systems is poorly investigated in spite of the fact that they are promising for vibration and shock protection. Here we use a chain model with a reduced number of degrees of freedom in order to study the behaviour of a CNT bundle under uniaxial and biaxial lateral compression. Stress-strain curves are obtained and evolution of the CNT bundle structure is analyzed.

Mechanically durable and self-healing super-hydrophobic coating with hierarchically structured KH570 modified SiO2-decorated aligned carbon nanotube bundles

We prepared stably and hierarchically structured particles (ACNTB-SiO2-KH570) composed of silane coupling agent 3-methacryloxypropyltrimethoxysilane (KH570) modified nano-SiO2 and aligned carbon nanotube bundles (ACNTB), and developed a mechanically durable super-hydrophobic composite coating from epoxy (EP) adhesive precursor and ACNTB-SiO2-KH570 by dripping method, without using any fluoro-containing compounds. The results show that ACNTB-SiO2-KH570 particle film possesses excellent super-hydrophobicity because of the complicated nanotexture of ACNTB-SiO2-KH570. The composite coating with a very small content (4.3%) of ACNTB-SiO2-KH570 can show excellent super-hydrophobicity owing to the formation of rough surface with micro/nanoscale hierarchical structure. The water contact angle (CA) and sliding angle (SA) of the composite coating can reach 158.8 ± 2.5° and 2.5 ± 0.5°, respectively. Moreover, the composite coating not only displays good antifouling and self-cleaning performance, but also retains super-hydrophobicity after sand falling test, water jet impact and sandpaper abrasion cycles. The excellent mechanical durability of the composite coating is attributed to the synergistic effects of high strength and toughness ACNTB-SiO2-KH570 and the good interface interaction of the different components in coating. The composite coating can show good adhesions to various substrate and exhibit super-hydrophobicity. Furthermore, the damaged composite coating can recover its super-hydrophobicity after being heated to 300 °C, owing to the combination of the decomposition of partial EP polymers and the migration of SiO2 to the coating surface.

Reactive Polymer-Functionalized Aligned Multiwalled Carbon Nanotube Bundles-Induced Porous Poly(ethylene terephthalate) Fibers

Porous poly(ethylene terephthalate) (PET) fibers were fabricated by the melt-spinning method. Here, reactive polymer-functionalized aligned multiwalled carbon nanotube bundles (RP@ACNTB) were intro...

Ultrasensitive Strain Sensor Based on Separation of Overlapped Carbon Nanotubes

Although there have been remarkable improvements in stretchable strain sensors, the development of strain sensors with scalable fabrication techniques and which both high sensitivity and stretchability simultaneously is still challenging. In this work, a stretchable strain sensor based on overlapped carbon nanotube (CNT) bundles coupled with a silicone elastomer is presented. The strain sensor with overlapped CNTs is prepared by synthesizing line-patterned vertically aligned CNT bundles and rolling and transferring them to the silicone elastomer. With the sliding and disconnection of the overlapped CNTs, the strain sensor performs excellently with a broad sensing range (≥145% strain), ultrahigh sensitivity (gauge factor of 42 300 at a strain of 125-145%), high repeatability, and durability. The performance of the sensor is also tunable by controlling the overlapped area of CNT bundles. Detailed mechanisms of the sensor and its applications in human motion detection are also further investigated. With the novel structure and mechanism, the sensor can detect a wide range of strains with high sensitivity, demonstrating the potential for numerous applications including wearable healthcare devices.

Dynamic operation of liquid crystal cell with inherently nanogroove-featured aligned carbon nanotube sheets

The use of a highly aligned carbon nanotube (CNT) sheet as a multifunctional constituent for liquid crystal (LC) displays and electro-optic LC applications is assessed. The CNT sheet can perform a dual function: one is an alignment layer for LCs, replacing the commonly used rubbed polyimide film, and the other is a transparent conductive layer, taking the place of indium tin oxide. The hydrophobic treatment improved the adhesion quality between aligned CNT bundles and the glass substrate, which helps to preserve the inherently aligned nanogroove morphology of transparent CNT sheets. The test LC display cells, comprising 4-cyano-4′-pentylbiphenyl molecules sandwiched between CNT-sheet-on-glass substrates, demonstrate the operation characteristics comparable to that of the conventional cell under temperature variation and ac electric field of 1 kHz. The results offer a possibility of deploying multifunctional CNT-sheet alignment layers in LC-based devices, especially in the future flexible display applications.

Phase transformation as the mechanism of mechanical deformation of vertically aligned carbon nanotube arrays: Insights from mesoscopic modeling

Vertically aligned carbon nanotube (VACNT) arrays or “forests” behave mechanically as foams when compressed, exhibiting a characteristic nonlinear stress – strain response. However, the fiber structure of VACNT forests is unlike that of cellular foams, and the microscopic mechanisms of the deformation are quite different. While numerous studies have addressed the mechanical response of VACNT forests undergoing uniaxial compression, the underlying deformation mechanisms are not yet fully established. In this paper, we report the results of large-scale mesoscopic simulations of the uniaxial compression of a VACNT forest composed of 2-μm-long carbon nanotubes (CNTs) as well as three structurally distinct forests composed of 0.6-μm-long CNTs. The simulations reveal that the compressive deformation proceeds as a phase transformation from an original low-density phase composed of vertically aligned CNT bundles to a densified phase with horizontal alignment of CNTs. The two phases are separated by a well-defined interfacial layer, which advances during the compressive deformation through localized bending and folding of nanotubes. For the 2-μm-tall forest, the folding involves correlated displacements of multiple CNT bundles, hinting on the origin of the collective buckling behavior observed in experiments. The characteristic three-stage stress-strain dependence (an initial "elastic" peak followed by an extended plateau region and a sharp rise of stress in the densification regime), commonly observed in experimental probing of the mechanical properties of VACNT forests, is reproduced in all of the simulations, suggesting that the heterogeneous propagation of densification front may be the general mechanism of the mechanical deformation of VACNT forests.

The microbiome and development – an introduction

The use of a highly aligned carbon nanotube (CNT) sheet as a multifunctional constituent for liquid crystal (LC) displays and electro-optic LC applications is assessed. The CNT sheet can perform a dual function: one is an alignment layer for LCs, replacing the commonly used rubbed polyimide film, and the other is a transparent conductive layer, taking the place of indium tin oxide. The hydrophobic treatment improved the adhesion quality between aligned CNT bundles and the glass substrate, which helps to preserve the inherently aligned nanogroove morphology of transparent CNT sheets. The test LC display cells, comprising 4-cyano-4′-pentylbiphenyl molecules sandwiched between CNT-sheet-on-glass substrates, demonstrate the operation characteristics comparable to that of the conventional cell under temperature variation and ac electric field of 1 kHz. The results offer a possibility of deploying multifunctional CNT-sheet alignment layers in LC-based devices, especially in the future flexible display applications.

Production of CNT yarns for use as filaments in incandescent bulb: effect of carbon source and state of catalyst on production of CNT

Presence of highly aligned carbon nanotubes (CNTs) bundles is a prerequisite to producing good quality CNTs that will retain their original good electrical and mechanical properties when spun into yarn. In this study, effects of carbon source and state of catalyst on the CNT yield and quality during the production of CNTs were investigated. CNTs were produced in a Swirled Continuous CVD reactor using acetylene and methane as carbon sources and ferrocene as a catalyst to investigate the effect of carbon source and the production was carried out with the state of the catalyst altered to investigate effect of state of catalyst on the yield and quality of the CNTs. The quality and purity of the produced CNTs were checked with Raman and Scanning electron microscopy (SEM) provides information on the morphology of the produced CNTs. SEM images of the produced CNTs using acetylene gas reveal the presence of more nanoballs than those produced using methane under the similar conditions. However, amount of CNTs produced using methane is very low compared to the amount obtained when acetylene was used. The CNT yield from methane was enhanced when ferrocene (catalyst) was dissolved in cyclopentane. Results from SEM and Raman Spectroscopy indicates CNT bundles with longer lengths and reduced number of nanoballs when methane was used together with ferrocene dissolved in cyclopentane as compared to using ferrocene in a solid state.

Adsorptive Separation of CO2 from Multicomponent Mixtures of Flue Gas in Carbon Nanotube Arrays: A Grand Canonical Monte Carlo Study

Grand canonical Monte Carlo (GCMC) simulations have been performed to investigate the adsorption and separation behavior of ternary and quaternary gaseous mixtures of CO2, along with H2S, SO2, and N2, in bundles of aligned double-walled carbon nanotubes with a diameter of 3 nm and an intertube distance of 0.5 nm. All of the simulations are performed at 303 K and at pressures varying between 0 and 3 bar. The GCMC results are then compared to the ideal adsorbed solution theory (IAST) predictions. For the ternary mixture H2S–CO2–N2, the results show that CO2 has the highest adsorption among the three components. The IAST predictions agree reasonably well with the GCMC data for the ternary mixture, except for H2S. For the quaternary mixture H2S–SO2–CO2–N2, it is observed that initially CO2 has the highest adsorption up until around 2 bar, whereafter there is a crossover by SO2 to have the highest adsorption. IAST fails to predict the adsorption behavior of the quaternary mixture involving SO2.

Magnetic properties of aligned iron containing nitrogen-doped multi-walled carbon nanotubes

In this study, aligned nitrogen-doped multi-walled carbon nanotubes (A-CNTs) have been synthesized by Aerosol Assisted Chemical Vapour Deposition. Three solutions were nebulized (toluene, acetonitrile, 1:1 toluene/acetonitrile) and the as synthesized carbon nanostructures formed were collected from the three different reactions. After synthesis, the A-CNTs were purified by acid treatment and retained for further analysis. The effect of in-situ nitrogen doping on the internal structure of the aligned carbon nanotube bundles was studied by scanning electron microscopy. Transmission electron microscopy studies confirmed the formation of bamboo like tubes upon nitrogen doping. Thermal gravimetric analysis indicated that nitrogen doping does not significantly degrade the thermal stability of the A-CNT samples. The structural properties of the A-CNT after incorporation of nitrogen and the subsequent formation of bamboo-like tubes were studied by Raman spectroscopy. The presence of ferromagnetic impurities was identified by Mössbauer spectroscopy as α-Fe, γ-Fe, Fe2O3 and Fe3C, and these were further confirmed by SQUID hysteresis and temperature dependence magnetization data.

Tailoring the structure of aligned carbon nanotube bundle by reactive polymer for strengthening its surface interaction with thermosets and the excellent properties of the hybrid thermosets

Aligned carbon nanotube bundles (ACNTB) with multi-level hierarchical structures were tailored by reactive polymer vinyl-terminated polyphenylene ether (PPE) for the excellent integrated property of bismaleimide-triazine (BT) resin. The PPE-tailored ACNTB (ACNTB@PPE) has increased strength for the penetration of PPE into porous ACNTB strengthening the interaction between each CNT. The strong interaction at the interface of ACNTB@PPE and BT matrix can be created owing to the reaction of the vinyl group in PPE on the surface of ACNTB and maleimide group in BT. BT with 2% ACNTB@PPE composite shows the optimal flexural strength, fracture toughness and tensile strength, which are 88%, 115% and 77% higher than those of BT, respectively. The introduction of ACNTB@PPE slightly enhances the thermal property of BT. ACNTB@PPE can significantly improve the flame retardancy of BT composites. As compared to individual ACNTB, ACNTB@PPE effectively improves the integrated property of BT composites mainly due to the chemical interaction at the interface of ACNTB@PPE and BT matrix and the increased interaction between each CNT.

Tuning vertical alignment and field emission properties of multi-walled carbon nanotube bundles

We report the growth of vertically aligned carbon nanotube bundles on Si substrate by thermal chemical vapor deposition technique. Vertical alignment was achieved without any carrier gas or lithography-assisted deposition. Growth has been carried out at 850 °C for different quantities of solution of xylene and ferrocene ranging from 2.25 to 3.00 ml in steps of 0.25 ml at a fixed concentration of 0.02 gm (ferrocene) per ml. To understand the growth mechanism, deposition was carried out for different concentrations of the solution by changing only the ferrocene quantity, ranging from 0.01 to 0.03 gm/ml. A tunable vertical alignment of multi-walled carbon nanotubes (CNTs) has been achieved by this process and examined by scanning and transmission electron microscopic techniques. Micro-crystalline structural analysis has been done using Raman spectroscopy. A systematic variation in field emission (FE) current density has been observed. The highest FE current density is seen for the film grown with 0.02 gm/ml concentration, which is attributed to the better alignment of CNTs, less structural disorder and less entanglement of CNTs on the surface. The alignment of CNTs has been qualitatively understood on the basis of self-assembled catalytic particles.