Vertically Aligned Carbon Nanotubes Research Articles

Flexible Ultra-Wideband Terahertz Absorber Based on Vertically Aligned Carbon Nanotubes

Ultra-wideband absorbers have been extensively used in wireless communications, energy harvesting, and stealth applications. Herein, with the combination of experimental and theoretical analyses, we develop a flexible ultra-wideband terahertz absorber based on vertically aligned carbon nanotubes (VACNTs). Measured results show that the proposed absorber is able to work efficiently within the entire THz region (e.g., 0.1-3.0 THz), with an average power absorptance of >98% at normal incidence. The absorption performance remains at a similar level over a wide range of incident angle up to 60°. More importantly, our devices can function normally, even after being bent up to 90° or after 300 bending cycles. The total thickness of the device is about 360 μm, which is only 1/8 of the wavelength for the lowest evaluated frequency of 0.1 THz. The new insight into the VACNT materials paves the way for applications such as radar cross-section reduction, electromagnetic interference shielding, and flexible sensing because of the simplicity, flexibility, ultra-wideband operation, and large-scale fabrication of the device.

Scalable electric-field-assisted fabrication of vertically aligned carbon nanotube membranes with flow enhancement

We report a solution-based approach for fabricating vertically aligned carbon-nanotube (VACNT) membranes, which improves on the unaligned nature of CNT mixed-matrix membranes. This approach addresses important challenges regarding scalability and incorporation of different types and sizes of nanotube as pores in a membrane. In a new, solvent-deposition approach, CNTs are dispersed and then aligned and concentrated via electro-deposition from an organic solvent, using a composite AC + DC electric field to form aligned CNT arrays. This enables VACNT number densities that are two orders-of-magnitude higher than possible from direct suspension of CNTs in the liquid oligomer. The solvent is then replaced by a UV-curable oligomer, which is selectively cured to form macroscopic membranes of highly controlled thickness. After plasma etching to open pores, the VACNT membranes are shown to be permeable, and to have pore sizes consistent with flow through internal channels of the CNTs. In a first for field-aligned, solution-fabricated VACNT membranes, the pores show substantial, 100–300×, gas-flow enhancement compared to theory, similar to membranes fabricated via less scalable methods (i.e., by CVD growth of already aligned CNT forests). The solution-based, electric-field-assisted approach aligns nanotubes of different sizes and types grown by any means, and is scalable to the efficient fabrication of large-area VACNT membranes for a variety of important applications.

Elastic properties of vertically aligned carbon nanotubes: A molecular dynamics study

In the present study, the mechanical properties of vertically aligned carbon nanotube (VACNT) arrays in the most general case of anisotropic elasticity is studied. To quantify the elastic stiffness coefficients, continuum mechanics is employed, in which the constitutive relation is formulized in terms of the interatomic potential obtained via molecular dynamics (MDs) simulations. Detailed investigations into elastic stiffness coefficients with different geometrical parameters are performed and, consequently, their sensitivity on length and radius parameters is hereby studied. With such study it would be possible to tune the mechanical properties of VACNTs.

Height and morphology dependent heat dissipation of vertically aligned carbon nanotubes

The continuous miniaturization of electronic devices substantially increases their power density, and consequently, requires effective cooling of these components. Vertically aligned carbon nanotubes (VA-CNTs) constitute one of the most promising materials for use as a high-end heat dissipation element due to their high thermal conductivity and large surface area. However, the lack of a clear understanding of the heat transfer mechanisms of VA-CNTs has so far impeded their large-scale use as cooling elements. Our infrared micro-thermography analysis revealed that the heat dissipation of VA-CNTs is determined mainly by their height, such that the heat dissipation behavior of tall samples was dominated by convection from the carbon nanotube (CNT) sidewalls. The mechanism of heat transfer in short VA-CNTs, in contrast, was determined by their morphology. Short VA-CNTs with highly organized CNT formations or with low thermal conductance exhibited convective heat dissipation similar to that of tall VA-CNTs, while other short VA-CNTs exhibited heat transfer dominated by conduction along the CNTs. This study provides important guidelines regarding the parameters that can be changed to optimize the performances of VA-CNTs in thermal applications. These applications include cooling elements in electronic devices, where convection is required, or thermal interface materials, where conduction is required.

Highly Stretchable, Directionally Oriented Carbon Nanotube/PDMS Conductive Films with Enhanced Sensitivity as Wearable Strain Sensors.

Recent interest in the fields of human motion monitoring, electronic skin, and human-machine interface technology demands strain sensors with high stretchability/compressibility (ε > 50%), high sensitivity (or gauge factor (GF > 100)), and long-lasting electromechanical compliance. However, current metal- and semiconductor-based strain sensors have very low (ε < 5%) stretchability or low sensitivity (GF < 2), typically sacrificing the stretchability for high sensitivity. Composite elastomer sensors are a solution where the challenge is to improve the sensitivity to GF > 100. We propose a simple, low-cost fabrication of mechanically compliant, physically robust metallic carbon nanotube (CNT)-polydimethylsiloxane (PDMS) strain sensors. The process allows the alignment of CNTs within the PDMS elastomer, permitting directional sensing. Aligning CNTs horizontally (HA-CNTs) on the substrate before embedding in the PDMS reduces the number of CNT junctions and introduces scale-like features on the CNT film perpendicular to the tensile strain direction, resulting in improved sensitivity compared to vertically-aligned CNT-(VA-CNT)-PDMS strain sensors under tension. The CNT alignment and the scale-like features modulate the electron conduction pathway, affecting the electrical sensitivity. Resulting GF values are 594 at 15% and 65 at 50% strains for HA-CNT-PDMS and 326 at 25% and 52 at 50% strains for VA-CNT-PDMS sensors. Under compression, VA-CNT-PDMS sensors show more sensitivity to small-scale deformation than HA-CNT-PDMS sensors due to the CNT orientation and the continuous morphology of the film, demonstrating that the sensing ability can be improved by aligning the CNTs in certain directions. Furthermore, mechanical robustness and electromechanical durability are tested for over 6000 cycles up to 50% tensile and compressive strains, with good frequency responses with negligible hysteresis. Finally, both types of sensors are shown to detect small-scale human motions, successfully distinguishing various human motions with reaction and recovery times of as low as 130 ms and 0.5 s, respectively.

Nanoscale transport characteristics and catalyst utilization of vertically aligned carbon nanotube catalyst layers for fuel cell applications: Comprehensive stochastic modeling of composite morphological structures

Nanoscale transport characteristics and catalyst utilization of vertically aligned carbon nanotube (VACNT) catalyst layers (CLs) are evaluated using a fully statistical modeling approach based on the inherent random nature of the catalyst layer structures for fuel cell applications. Composite morphological structures of the catalyst layers are stochastically modeled with a 95% confidence level, and transport phenomena inside the catalyst layers are simulated using the D3Q19 lattice Boltzmann method (LBM). The effective diffusion coefficient of VACNT catalyst layers is predicted to be higher than that of the conventional catalyst layer, despite a relatively small pore diameter and a low-Knudsen diffusion coefficient. Consequently, the VACNT catalyst layers exhibit improved catalyst utilization compared to the conventional catalyst layers. These statistical results obtained from a series of numerical experiments confirm that the PEFC catalyst layers containing the VACNT catalyst supports can provide more efficient reactant transport, resulting in enhanced catalyst utilization for electrochemical reactions.

Fast microfocus x-ray tube based on carbon nanotube array

A full vacuum-sealed macrofocus x-ray tube with a vertically-aligned ring-shaped carbon nanotube (CNT) emitter grown by microwave plasma enhanced chemical vapor deposition is presented in this paper. The external grid allowed the CNT-based x-ray tube to exhibit transient switching on and off. The total emission current was 200 μA, which corresponds to a maximum emission current density of 10.1 A/cm2 from the ring-shaped CNT emitter when the grid voltage was 2.4 kV. The optimized focus electrode controlled the beam convergence on the target to produce a very small x-ray focal spot size less than 5 μm. Consequently, this microfocus x-ray tube could produce x-ray images with very high spatial resolution. X-ray fluoroscopy images of a multilayer printed circuit board (PCB) and field programmable gate array show distinct gold PCB traces with approximately 20 μm width.

Wetting of AgCuTi alloys on quartz fiber reinforced composite modified by vertically aligned carbon nanotubes

Due to the poor wettability of the AgCuTi alloy on the quartz fiber reinforced composite (QFSC), a reliable joining of the QFSC to itself or to other metals can be hardly achieved. In this study, vertically aligned carbon nanotube (VA-CNT) composed of multi-wall carbon nanotube (MWCNT) was synthesized on the QFSC surface by a plasma enhanced chemical vapor deposition method. As a result, the final contact angle decreased from 96.5° (without VA-CNT modification) down to 30.6° (with VA-CNT modification) at 870 °C with 10min holding duration. Besides, the reaction layers at the AgCuTi/QFSC interface transformed into continuous ones and the infiltration effects became more obvious after the VA-CNT modification. Finally, a possible mechanism how VA-CNT enhanced the wettability of the AgCuTi alloy on the QFSC surface was proposed. Defective sites on the MWCNT surface with a high chemical reactivity and the nanoscale capillary structure of the VA-CNT turned out to be the essential factors to promote the wetting process. This novel surface modification approach can offer new insights on addressing the wetting issues in composites preparation, brazing and soldering, etc..

Wafer-scale vertically aligned carbon nanotubes for broadband terahertz wave absorption

Materials with high and broadband absorption characteristics in the terahertz (THz) range are desirable for many applications. In this paper, we propose, fabricate and experimentally demonstrated a wafer-scale vertically aligned carbon nanotube (VACNT) array for broadband THz wave absorption. The effects of VACNT parameters on the absorption performance are investigated within the THz and infrared spectra using the Maxwell-Garnett theory, revealing that the absorption in the THz range can be greatly enhanced by suitable selections of the length, volume fraction and vertical alignment factor of CNTs. A VACNT array with an average CNT length of ∼600 μm is fabricated on a 4-inch silicon substrate. Experimental results measured by a THz time-domain spectroscopic system show an average power absorptance of ∼98% from 0.3 to 2.5 THz, and agree well with the numerical modelling. This device can be used as a cost-effective near-perfect absorber across the THz and infrared regions for thermal emission and imaging, electromagnetic interference shielding, stealth and energy harvesting applications.

High performance reverse osmosis membrane with carbon nanotube support layer

Vertically aligned carbon nanotube (VACNT) wall membrane with its ultrafast water permeability shows a great promise as a novel support layer for membranes because of its ultrahigh porosity and hydrophobicity. In this work, we utilize the wall membrane as a support layer in forming a reverse osmosis (RO) membrane. For the purpose, a polyamide (PA) selective layer is grown by interfacial polymerization directly onto the top surface of the VACNT support layer. This RO membrane delivers a flux of 128.6 liter m−2h−1 (LMH) and 98.3% salt rejection at 15.5 bar, opening an avenue for a leap over the water permeability at the rejection level that has been pegged at 5 LMH bar−1over the past ten years. Formation of a thin PA layer by a modified preparation method and ultrahigh porosity of VACNT support membrane are the main factors for the high performance RO membrane.

Flexible free-standing vertically aligned carbon nanotube on activated reduced graphene oxide paper as a high performance lithium ion battery anode and supercapacitor

Here, controlled growth of vertically aligned carbon nanotubes (VACNTs) on free-standing porous activated reduced graphene oxide (a-rGO) paper was fabricated using plasma-enhanced chemical vapor deposition method. The electrochemical performance of prepared film was investigated to provide effective electrode for 3D flexible high-performance lithium-ion batteries (LIBs) and supercapacitors. The results revealed that the prepared electrode exhibited a high specific capacitance of 347 F/g at 0.5 A/g in 1 M KOH electrolyte, 60% more than non-activated rGO-paper (218 F/g). The VACNTs on a-rGO have increased the accessible surface area and acted as efficient electrical conducting paths, which improved the power density. The free-standing flexible supercapacitor fabricated using such a film exhibited a sufficient electrochemical behaviour with high power density of 407 kW kg−1 at 5 Wh.kg−1 at a current density of 0.5 A/g. Since VACNTs with low sp2 hybridization defect lead to cyclic stability, suitable for high-performance LIB anodes. This 3D flexible anode electrode demonstrated a high initial discharge capacity of 1401 mAhg−1 with a large reversible charge capacity of 958 mAhg−1 at 150 mAg−1. The charge and discharge capacity have reached a stable value of 459 mAhg−1 after 100 cycles with a coulombic efficiency of ∼100% which is much higher than most carbon structures.

Multizone Rapid Thermal Processing to Overcome Challenges in Carbon Nanotube Manufacturing by Chemical Vapor Deposition

For the scalable production of commercial products based on vertically aligned carbon nanotubes (VACNTs), referred to as CNT forests, key manufacturing challenges must be overcome. In this work, we describe some of the main challenges currently facing CNT forest manufacturing, along with how we address these challenges with our custom-built rapid thermal processing chemical vapor deposition (CVD) reactor. First, the complexity of the multistep processes and reaction pathways involved in CNT growth by CVD limits the control on CNT population growth dynamics. Importantly, gas-phase decomposition of hydrocarbons, formation of catalyst nanoparticles, and catalytic growth of CNTs are typically coupled. Here, we demonstrated a decoupled recipe with independent control of each step. Second, significant run-to-run variations plague CNT growth by CVD. To improve growth consistency, we designed various measures to remove oxygen-containing molecules from the reactor, including air baking between runs, dynamic pumping down cycles, and low-pressure baking before growth. Third, real-time measurements during growth are needed for process monitoring. We implement in situ height kinetics via videography. The combination of approaches presented here has the potential to transform lab-scale CNT synthesis to robust manufacturing processes.

In-situ Functionalization of Metal Electrodes for Advanced Asymmetric Supercapacitors.

Nanostructured metal-based compound electrodes with excellent electrochemical activity and electrical conductivity are promising for high-performance energy storage applications. In this paper, we report an asymmetric supercapacitor based on Ti and Cu coated vertical-aligned carbon nanotube electrodes on carbon cloth. The active material is achieved by in-situ functionalization using a high-temperature annealing process. Scanning and transmission electron microscopy and Raman spectroscopy confirm the detailed nanostructures and composition of the electrodes. The TiC@VCC and CuxS@VCC electrodes show a high specific capacity of 200.89 F g−1 and 228.37 F g−1, respectively, and good capacitive characteristics at different scan speeds. The excellent performance can be attributed to a large surface area to volume ratio and high electrical conductivity of the electrodes. Furthermore, an asymmetric supercapacitor is assembled with TiC@VCC as anode and CuxS@VCC as cathode. The full device can operate within the 0–1.4 V range, and shows a maximum energy density of 9.12 Wh kg−1 at a power density of 46.88 W kg−1. These findings suggest that the metal-based asymmetric electrodes have a great potential for supercapacitor applications.

Water Vapor Condensation from Atmospheric Air by Super-Hydrophobic VACNTs Growth on Stainless Steel Pipes

AbstractEnergy-efficient condensation of steam contained in atmospheric air has emerged as a solution to the water scarcity. Academic and industrial research works that seeks to develop water collection devices with high efficiency has great relevance for the scientific community. In this work, we aim to show that modified carbon nanotubes forest can remove the condensed drops easier than a hydrophobic and a super-hydrophilic surface. In addition, this result was reached at high super saturation level which is an innovative aspect of this work. The Vertically Aligned Carbon Nanotubes (VACNTs) were grown on steel pipes. We used a CO2 laser and an O2 plasma to perform the post treatments that changed the CNTs to super-hydrophobic and super-hydrophilic, respectively. In addition, the CO2 laser treatment added a second level of roughness in the surface by etching the nanotubes walls. A polyethylene coating attached the carbon nanotubes to the substrate. We experimentally demonstrated a 24% higher vapor condensation rate at high supersaturations levels.

Vertically Aligned Carbon Nanotubes Capacitive Sensors

Capacitive sensors are key components in a wide range of sensing applications, such as gas detectors and safety systems. While the mainstream silicon sensors demonstrated good performances, vertically aligned carbon nanotubes (VA-CNTs) outperform them and, therefore, are an attractive candidate material for capacitive applications owing to their extremely high surface area. Specifically, their top (crust) layer is characterized by a porous and dense morphology that further enhances the capacitive area and, consequently, induces an electrostatic fringe field and an enhancement of the capacitance. In this paper, we study how the porosity of VA-CNTs determines their electrostatic behavior. We observed that the porous surface of the crust layer generates a significant enhancement of the capacitance comparing to parallel plate capacitors. The rough surface of the crust layer results in amplification of the VA-CNT effective capacitor area, which further increases with the electrostatic gap. As-grown VA-CNTs with lower porosity demonstrate higher capacitance; however, densified samples and samples reinforced with conductive nano-particles did not show an enhancement of the capacitance, due to a significant change in their CNT formation. Thus, this paper emphasizes the influence of the morphological structure of VA-CNTs on their capacitance and enriches the material library that can be used for capacitive sensing.

Comparative study of electron field emission from randomly-oriented and vertically-aligned carbon nanotubes synthesized on stainless steel substrates

Randomly-oriented carbon nanotubes (CNTs) and vertically-aligned CNTs have been synthesized by a thermal chemical vapor deposition (CVD) process and a plasma enhanced CVD process, respectively, on stainless steel substrates without any external catalyst. Surface topography studies reveal that polishing and chemical etching result in favorable catalytic conditions for nucleation and growth of CNTs. Scanning electron microscopy and transmission electron microscopy observations reveal the growth of CNTs with catalyst particle at the tips. In comparison to randomly-oriented CNTs, vertically-aligned CNTs demonstrate better field emission properties with lower turn-on electric field of ∼2.0 V/μm, lower threshold electric field of ∼3.2 V/μm, and a 2.5-fold increase in the field enhancement factor. The vertical alignment of the emitters benefits the emission process by reducing the screening effect and streamlining the path of ejected electrons directly onto the anode. Vertically-aligned CNTs on conducting substrates are promising emitters in cold cathode vacuum electronics because of their direct contact with the substrate and efficient performance at low operating voltages.

Data Analytics Enables Significant Improvement of Robustness in Chemical Vapor Deposition of Carbon Nanotubes Based on Vacuum Baking

The root causes underlying run-to-run variations in the synthesis of vertically aligned carbon nanotubes (VACNTs) by chemical vapor deposition can be attributed to the sensitivity of the process to...

Growth of vertically-aligned carbon nanotubes on graphite for electric double-layer capacitors

Recently, electric double-layer capacitors (EDLCs) have attracted great attention as an energy storage device with high input-output power density. In this study, we synthesized carbon nanotubes (CNTs)/graphite with high surface area as a novel electrode component for EDLC. CNTs were grown on a defective graphite surface by alcohol catalytic chemical vapor deposition for the first time. We found that oxidative plasma treatment time for graphite surface attributes to selective growth of laterally-aligned (LA) or vertically-aligned (VA) CNTs. Electrochemical analysis reveals VACNTs/graphite possesses the highest electrostatic capacity of 1.5 mF cm−2 among pristine graphite, LACNTs/graphite, and VACNTs/graphite. We suggest that increase of specific surface area with CNTs results in the increase of electrostatic capacity for the EDLC electrode. The results indicate that VACNTs/graphite electrodes are expected to improve the performance of EDLCs.

Applying Aluminum⁻Vertically-Aligned Carbon Nanotube Forests Composites for Heat Dissipation.

Vertically-aligned carbon nanotube forests (VACNTs) with excellent axial heat dissipation properties were formed on aluminum foil to dissipate heat. In addition, the heat dissipation efficiency of aluminum–VACNTs composites in this work was compared with that of commercially available mainstream thermal sheets under the same natural cooling conditions. Chemical vapor deposition (CVD) was employed as a synthesis method using a three-segment high-temperature furnace. Subsequently, the temperature changes in a heating body with the aluminum–VACNTs composites was measured over time subject to natural cooling. In addition, the performance was compared with copper and pyrolytic graphite sheets. The experimental results revealed that the heat dissipation efficiency of the flexible aluminum–VACNTs composites was higher than that of clean aluminum foil, a copper sheet, and a pyrolytic graphite sheet by up to 56%, 40%, and 20%, respectively. Moreover, this work also verified the height of the carbon nanotube (CNT) did not influence the heat dissipation efficiency, indicating that the time cost of synthesis could be reduced.

One-pot preparation of iron/alumina catalyst for the efficient growth of vertically-aligned carbon nanotube forests

The catalytic growth of vertically-aligned carbon nanotubes (VA-CNTs) forest usually requires thin catalyst films deposited by multi-step and costly physical vapor deposition techniques. Here, we demonstrate that an efficient catalyst and its supporting layer for VACNT growth can be prepared by using a simple solution of Fe(NO3)3 and Al(NO3)3 deposited on silica in a single step. This process being much simpler and cheaper than existing preparation methods, it can easily be transferred to industry for the low-cost, thin and large-area coating of catalyst for VA-CNT growth. Our study shows that aluminum hydroxides preferentially react with the SiO2 surface while iron hydroxides tend to form oxide or hydroxide nanoparticles, thus allowing preparation of an aluminum-based buffer layer with iron-based nanoparticles at its surface. Optimization of the Fe/Al ratio and salt concentrations yielded catalysts with performances similar to standard Fe/Al2O3 catalysts prepared by physical vapor deposition.