Vertically Aligned Carbon Nanotube Arrays Research Articles

Thermal boundary resistance and thermal rectification in VACNT arrays integrated with SnZn alloys

Thermal rectification (TR) phenomena in carbon nanotubes (CNTs) have been previously foreseen through theoretical predictions; however, its experimental realization in bulk CNT arrays remains relatively unexplored. Herein, we have synthesized vertically aligned carbon nanotube (VACNT) arrays of ∼ 4.5 mm in length on a 4-inch silicon wafer by a combined bubble-assisted chemical vapor deposition method. By integrating these VACNT arrays with SnZn alloys, we have successfully developed devices capable of functioning across a wide temperature range spanning from 200 °C to 400 °C. Notably, the thermal boundary resistances (TBR) between the VACNTs and SnZn alloy exhibit pronounced dependence on the alloy's composition, exhibiting variations ranging from 0.12 cm2KW−1 to 1.12 cm2KW−1. Of particular significance, the TBRs of Sn50Zn50/VACNT compounds in the forward direction were found to fall within the range of 0.63–0.98 cm2KW−1 in the temperature range from120 °C to 180 °C, while in the backward direction, they exhibited values in the range of 0.13–0.34 cm2KW−1. These contrasting TBR values highlight a marked thermal rectification performance with a substantial TR coefficient between 0.49 and 0.66. Our comprehensive investigation sheds valuable insights into the TR effects within VACNTs and metal/alloy interfaces, presenting a promising avenue for the development of future thermal logic circuits and thermal transistors.

Optimizing the growth of vertically aligned carbon nanotubes by literature mining and high-throughput experiments

Vertically aligned carbon nanotube (VACNT) arrays with good mechanical properties and high thermal conductivity can be used as effective thermal interface materials in thermal management. In order to take advantage of the high thermal conductivity along the axis of nanotubes, the quality and height of the arrays need to be optimized. However, the immense synthesis parameter space for VACNT arrays and the interdependence of structural features make it challenging to improve both their height and quality. We have developed a literature mining approach combined with machine learning and high-throughput design to efficiently optimize the height and quality of the arrays. To reveal the underlying relationship between VACNT structures and their key growth parameters, we used random forest regression (RFR) and SHapley Additive exPlanation (SHAP) methods to model a set of published sample data (864 samples). High-throughput experiments were designed to change 4 key parameters: growth temperature, growth time, catalyst composition, and concentration of the carbon source. It was found that a screened Fe/Gd/Al2O3 catalyst was able to grow VACNT arrays with millimeter-scale height and improved quality. Our results demonstrate that this approach can effectively deal with multi-parameter processes such as nanotube growth and improve control over their structures.

Phase and orientation effects in X-ray attenuation of carbonaceous epoxy composites

Composites made of carbon nanotubes (CNTs) reinforced in an epoxy matrix have already been successfully used as electromagnetic interference shielding materials. Different weight fractions of randomly distributed and vertically aligned multiwall carbon nanotubes (MWCNTs), graphite, and carbon fiber reinforced in the epoxy matrix were used to make these composites. The comparisons of percentage attenuation of different composite materials were studied under both diagnostic or hard (narrow beam geometry) and soft X-rays (monochromatic X-rays beam). It was found that the concentration of CNTs and graphite in epoxy composites affects attenuation. The results also demonstrate that the percentage attenuation for CNTs (as made) epoxy composite is always higher than graphite epoxy and treated CNTs epoxy composite at any concentration at the same thickness and tube voltage. This study also confirmed that the metal catalyst particles present in CNTs contribute to a considerable attenuation of X-rays along with CNTs and also demonstrated that the increase in metal content inside CNTs increases the attenuation. For hard X-rays, the orientation of CNTs in composites affects attenuation, and this effect gradually weakens at higher tube voltages. Vertically aligned carbon nanotubes (VACNT) array epoxy, and the thin composites are not suitable to attenuate diagnostic X-rays coming from high tube potential. For thin samples, VACNT array epoxy composites always perform better on attenuation of soft X-rays than randomly oriented CNTs in composites with any other composition lower than 15 wt %. This study also showed that there is a difference in the attenuation of soft X-rays amongst CNTs, graphite, and carbon fiber epoxy composites, which confirms that the attenuation depends on the geometry of the carbon structures. There is a clear effect of the orientation of CNTs in VACNTs epoxy composite on soft X-ray attenuation, and it shows that the radial direction of CNTs attenuates more X-ray radiation than the tube axis direction.

Microporous vertically aligned CNT nanocomposites with tunable properties for use in flexible heat sinks

Effective thermal management of electronic systems depends on the heat transfer efficiency or the heat dissipation capability, and the thermal conductivity of heat sink components, which has a critical impact on the performance of the devices. The rapidly growing field of microelectronics creates an enormous need for next-generation flexible, lightweight heat sinks. In this work, flexible, microporous nanocomposites are fabricated utilizing a unique yet easily tunable processing method, targeting heat-sink applications. The highly porous and low-density nanohybrid structures were fabricated in a unique processing technique using conformally pyrolytic carbon (PyC) coated vertically aligned carbon nanotube (VACNT) arrays and polydimethylsiloxane (PDMS) infiltration. Simply by varying the concentration of the PDMS in the VACNT structure, the microporosity can be tuned from 50% to 93%, and at the same time, the density of the structure varies from 0.11 g/cm 3 to 0.51 g/cm 3 . The through-thickness thermal conductivity of the VACNT – PDMS nanocomposites did not vary substantially with increasing PDMS concentration, and the highest performance samples exhibited 14.1 W/mK thermal conductivity. The highly flexible nanocomposite structure also showed excellent mechanical resiliency and exhibited complete recovery from 80% compressive strain. The final heat-sink structure with fins was fabricated by a controlled laser etching technique. Analysis of the flexible VACNT array heat sink showed a significant ∼27% reduction in thermal resistance with an air velocity of 1.5 m/s, and about ∼40% improvement in the output performance of a thermoelectric generator (TEG) on which it was mounted. The high thermal conductivity of VACNTs and the large surface area provided by the microporous structure, as well as the laser-etched fins all together contributed to better thermal management performance.

Mechanical characterization of reinforced vertically-aligned carbon nanotube array synthesized by shock-induced partial phase transition: insight from molecular dynamics simulations

Despite intriguing mechanical properties of carbon nanotubes (CNTs), vertically-aligned carbon nanotube (VACNT) array does not possess a high strength against compression along the CNT axis and also the loadings perpendicular to the CNT axis. Here in this study, shock compression is introduced as a means for partial phase transition (PPT) in the VACNT array to reinforce the structure against the mentioned loadings. Molecular dynamics simulations are exploited to investigate the synthesis of a novel nanostructure from a VACNT array with 10 nm long (5, 5) CNTs. Employing Hugoniostat method, shockwave pressures of 6.6 GPa and 55 GPa are extracted from Hugoniot curves as the instability limit and the PPT point, respectively. Coordination analysis reveals the nucleation of carbon atoms in sp3 hybridization while preserving the dominant nature of CNT due to the high percent of sp2 hybridization. Recovery of the shocked samples yields the final structure to be tested for mechanical characteristics. Tensile and compression tests on the samples reveal that for the shockwave pressures below the PPT point, an increase of the shock strength leads to higher compliance in the VACNT array. However, beyond the PPT point the novel nanostructure shows an extraordinary strong behavior against loading along all directions.

A Flexible and Ultra‐Wideband Terahertz Wave Absorber Based on Pyramid‐Shaped Carbon Nanotube Array via Femtosecond‐Laser Microprocessing and Two‐Step Transfer Technique

AbstractHigh and uniform absorption capabilities of terahertz (THz) waves in an ultra‐broadband range is desirable for many THz functional devices. Nowadays, it is still challenging to fabricate flexible THz absorbers with a uniformly high absorptance across the entire THz band merely based on traditional bulk materials. Engineered metamaterials absorbers utilize impedance matching to reduce the surface reflection at a single frequency, and can achieve near‐unity power absorption within a relatively narrow bandwidth. In this work, a fabrication strategy combining a femtosecond‐laser microprocessing process and a two‐step‐transfer technique is demonstrated for the realization of vertically‐aligned carbon nanotube (VACNT) arrays with pyramid‐shaped unit cells for THz wave absorptions. To transfer the structured VACNT array from the silicon to the flexible PDMS/Cu/PET substrate, the temperature and pressure dependences of the transfer process are systematically investigated. The fabricated THz absorber demonstrates an average power absorptance over 98.9% from 0.1 to 2.5 THz, and can function well in bended states and after 300 times bending cycles. The proposed fabrication strategy is expected to be used for the patterning of VACNTs and other nanomaterials, and advance the development of novel THz devices for various applications.

DEVELOPMENT OF A BROADBAND ELECTROMAGNETIC RADIATION ABSORBER BASED ON MULTIWALL CARBON NANOTUBES 
 AND ITS APPLICATION IN BOLOMETRIC RECEIVERS

This work is devoted to the development of a technique for obtaining an array of multi-walled vertically aligned carbon nanotubes (VACNT) with a thickness of up to 120 μm on Si/Al2O3/Fe substrates and to the study of their absorbing properties in the THz spectral region, as well as to the assessment of their prospects as a broadband THz radiation absorber based on calculations of the spectral dependence of absorption coefficient for traditional and inverted-type bolometric devices. It is shown that the absorption of the VACNT array transferred onto the Revalpha polymer substrate reaches 70–80% in the wavelength range of 40–200 µm. Calculations show that traditional bolometers with an absorber based on VACNT have the best sensitivity at wavelengths less than 100 μm, and inverted bolometers also having a VACNT layer have the best sensitivity at wavelengths exceeding 50 μm, which makes them complementary to each other.

Electro-optic hybrid aligned nematic device utilizing carbon nanotube arrays and two-dimensional hexagonal boron nitride nanosheet as alignment substrates.

Hybrid-aligned nematic (HAN) liquid crystal (LC) devices have both fundamental and technological importance for their applications in LC adaptive lenses, low voltage LC displays, smart windows, and many more. We report the fabrication and characterization of a nanostructure-based HAN device employing vertically aligned carbon nanotube (VA-CNT) arrays as the homeotropic alignment agent on one side and two-dimensional (2D) hexagonal boron nitride (h-BN) as the planar alignment agent on the other side of the LC cell. The LC achieves the HAN configuration in the cell, i.e., homeotropic alignment at the VA-CNT side due to the π-π stacking interaction between the LC and CNTs, and planar alignment at the h-BN side due to the π-π stacking interaction between the LC and h-BN. When an applied electric field is ramped up across this VA-CNT/h-BN HAN cell, the LC (positive anisotropic) obtains a homeotropic state, requiring no threshold voltage to start the reorientation process; this effect is similar to that of a traditional polyimide (PI)-based HAN device. This VA-CNT/h-BN HAN cell successfully demonstrates the optical, electro-optical operations and the electric field-induced dynamic response. This study reveals that two inorganic nanostructured surfaces, VA-CNT arrays and 2D h-BN, can efficiently replace the organic PI alignment agents when needed and retain the HAN device's necessary electro-optical performances. These results substantially expand the fundamental understanding and the scope of utilizing various nanostructured surfaces for LC alignment mechanisms.

Low-temperature grown vertically aligned carbon nanotube array for an optimal infrared bolometer

Vertically aligned carbon nanotube (VACNT) arrays have been explored as an absorber of thermal-type photodetectors. A long and dense VACNT array absorbs a wide spectral range of incident light with high absorption rate, but has a high thermal mass that results in a low response speed. To achieve a small thermal mass, a shorter and less dense VACNT array is needed. In addition, the high temperature needed to grow the VACNTs is detrimental to the functional sensing materials of the photodetector. The height, density, and growth temperature of VACNTs need to be optimized to achieve a working absorber that has high absorption rate and a high response speed. In this work, a low-temperature plasma enhanced chemical vapor deposition process is used to prepare various VACNT arrays with different heights and densities by controlling the CNT growth parameters. The absorption coefficients of the resulting samples are measured with Fourier transform infrared spectroscopy. An effective medium theory (EMT) is adopted to establish a working model of the VACNTs. Using experimentally extracted CNT density and height as fitting parameters, the EMT model is fitted to obtain theoretical absorption coefficients, which are found to be comparable to the experimentally measured absorption coefficients. Our experimental and theoretical investigations pave the way for future studies to integrate CNTs with infrared photodetectors.

Alumina Thin-Film Deposition on Rough Topographies Comprising Vertically Aligned Carbon Nanotubes: Implications for Membranes, Sensors, and Electrodes

In this article, the limits of thin-film deposition on very rough topographies are demonstrated by depositing alumina on vertically aligned carbon nanotubes (VACNTs). Vapor deposition techniques are the enabling platforms of the thin-film industry, offering high material versatility and good coverage ability on relatively flat surfaces, leading to frequent use in a large array of applications, especially nanoscale electronic devices such as sensors and electrodes. However, when surface topography exhibits high roughness, even depositions that are not limited to line-of-sight show only partial coverage, significantly hindering performances. Our manufacturing process of VACNT/Al2O3 nanocomposites has three vaporous steps: CNT growth by chemical vapor deposition (CVD), functionalization via controlled thermal oxidation, and atomic layer deposition (ALD) of alumina. The same limited accessibility hinders each of these three steps. Morphological analyses show different CNT heights throughout the sample, with shorter CNTs in the middle, having less access to gases. As height differences between the center and peripheries escalate, sample centers may collapse under the tension. The limited accessibility of the center is manifested also in inhomogeneous oxygen contents, between sample centers and peripheries. Finally, a sharp transition in deposition quality occurs during the deposition process of Al2O3, from homogeneous to inconsistent, which is also linked to the accessibility differences between. Adjusting process parameters, we have successfully coated 1.8 mm-tall VACNT arrays with a homogeneous thin (few nm) Al2O3 layer and were able to increase the depth at which, thick (few dozens of nm) Al2O3 coating is uniform from 20 to 350 μm. However, when VACNTs were functionalized, the penetration depth was found to correlate negatively with center oxygen content. These results, indicating diffusion as a rate-setting step in complex topography coatings, can significantly improve deposition quality and enhance the performance of thin-film applications such as membranes, sensors, and electrodes for energy harvesting and storage.

V2O5/vertically-aligned carbon nanotubes as negative electrode for asymmetric supercapacitor in neutral aqueous electrolyte

Development of aqueous asymmetric supercapacitors (ASCs) is often limited by low specific capacitance of negative electrodes. Herein, a composite electrode with tiny vanadium pentoxide (V2O5) nanoparticles homogeneously decorated in vertically-aligned carbon nanotube arrays (VACNTs) is prepared by supercritical CO2 impregnation and subsequent annealing, and used as binder-free negative electrode for aqueous ASCs. Owing to its unique three-dimensional (3D) hierarchical nanostructure, the V2O5/VACNTs (VN) electrodes exhibit an ideal specific capacitance of 284 F g−1 in the potential range of −1.1 to 0 V vs SCE at 2 A g−1 and outstanding cycling stability in the Na2SO4 aqueous solution. An aqueous ASC device possessing wide potential range of 1.7 V was constructed with pure VACNTs and VN-350 as the positive and negative electrodes, respectively. The ASC delivers a high energy density of 32.3 Wh kg−1 at a power density of 118 W kg−1 and satisfactory cycling life with capacitance retention of 76% after 5000 cycles.

Vertically Aligned Carbon Nanotube Membranes: Water Purification and Beyond.

Vertically aligned carbon nanotube (VACNT) membranes have attracted significant attention for water purification owing to their ultra-high water permeability and antibacterial properties. In this paper, we critically review the recent progresses in the synthesis of VACNT arrays and fabrication of VACNT membrane methods, with particular emphasis on improving water permeability and anti-biofouling properties. Furthermore, potential applications of VACNT membranes other than water purification (e.g., conductive membranes, electrodes in proton exchange membrane fuel cells, and solar electricity–water generators) have been introduced. Finally, future outlooks are provided to overcome the limitations of commercialization and desalination currently faced by VACNT membranes. This review will be useful to researchers in the broader scientific community as it discusses current and new trends regarding the development of VACNT membranes as well as their potential applications.

Homeotropic liquid crystal device employing vertically aligned carbon nanotube arrays as the alignment agent.

Vertically aligned carbon nanotube (VA-CNT) arrays were grown on several chromium (Cr)-coated glass substrates using a plasma-enhanced chemical vapor deposition system. The CNTs were 2μm long and had a site density of 2×10^{9}cm^{-2} on the substrates. Two VA-CNT slides on Cr glass substrates were put together to design a homeotropic electro-optic liquid crystal (LC) device. A negative dielectric anisotropic LC was used in the device. The π-π stacking interaction between the LC and the VA-CNTs allows the LC material to align homeotropically in the cell. When an external electric field was applied using the transparent conducting Cr layers, the LC achieves a planar orientation above a threshold field. These results successfully demonstrate the optical, electro-optical operations, and the field-induced dynamic response of a homeotropic LC device employing the VA-CNT arrays as the homeotropic-alignment agent. This study significantly advances the range and understanding of nanostructured surfaces that provide vertical alignment of LCs.

Vertically aligned carbon nanotubes-coated aluminium foil as flexible supercapacitor electrode for high power applications

Vertically Aligned Carbon Nanotubes (VACNTs)-coated flexible aluminium (Al) foil is studied as an electrode for supercapacitor applications. VACNTs are grown on Al foil inside thermal Chemical Vapor Deposition (CVD) reactor. 20 nm thick layer of Fe is used as a catalyst while Ar, H2 and C2H2 are used as precursor gases. The effect of growth temperature on the structure of CNTs is studied by varying the temperature of CVD reactor from 550 °C to 625 °C. Better alignment of VACNTs arrays on Al foil is recorded at 600 °C growth temperature in comparison to other processing temperatures. Cyclic voltammetry results shows that VACNTs-coated Al foil has a specific capacitance of ~ 3.01 F/g at a scan rate of 50 mV/s. The direct growth of VACNT array results in better contact with Al foil and thus low ESR values observed in impedance spectroscopy analysis. This leads to a fast charge–discharge cycle as well as a very high value of power density (187.79 kW/kg) suitable for high power applications. Moreover, wettability study shows that the fabricated VACNT electrode has a contact angle of more than 152° which signifies that it is a superhydrophobic surface and hence shows lower specific capacitance in comparison to reported values for VACNT array. Therefore, it is necessary to develop suitable post-processing strategies to make VACNTs hydrophilic to realize their full potential in supercapacitor applications.

Investigation of the relationship between adhesion force and mechanical behavior of vertically aligned carbon nanotube arrays

The mechanical behavior of vertically aligned carbon nanotube (VACNT) arrays can largely impact their adhesion performance. In this paper, we fabricated various VACNT arrays to investigate the relationship between adhesion force and their mechanical behavior. High-volume fraction (3.4%) CNT arrays did not exhibit the applicable adhesion effect due to their intrinsic elastic property. Adhesion measurements on several low-density (less than 0.5%) VACNT arrays demonstrated that the adhesion performance is strongly related to the plastic deformation of the carbon nanotubes at the contact surface. Due to the nature of the growth of CNT arrays, the top region of the as-grown CNT arrays is denser and stiffer than the bottom region of the arrays. Therefore, compared with as-grown CNT arrays, the flipped CNT arrays reached higher adhesion efficiency (the ratio of adhesion force to preload) with lower preload due to the higher compliance at the top surface of the arrays. With cyclic loading under micro mechanical tests, stiffening of the surface and declining of adhesion force were also observed. These results illustrated that the mechanical compliance at the region near the contact interface is the dominant factor for the adhesion performance of VACNT arrays.

Water vapor-induced structure modification of vertically-aligned carbon nanotube arrays and successive thin film coating for enhanced field emission properties

Vertically-aligned carbon nanotubes (VCNTs) are used as electron source in various field emission applications owing to its high aspect ratio, chemical inertness, mechanical strength and electrical conductivity. Here, we demonstrate that surface structure modification along with thin film coating enhances the field emission performance, such as turn-on voltage, emission site density, and stability. In the present study, VCNTs with different heights were grown on silicon wafers by thermal chemical vapor deposition followed by the structure modification of VCNTs using capillarity-driven water vapor condensation. We obtained various surface morphologies by varying the water vapor exposure time and heating temperature. In addition, the structure-modified VCNTs surfaces were coated with W and SiO2 thin films using electron-beam evaporation. It was observed that W-coated VCNTs with modified surface morphology results in the best field emission performance.

Density control of vertically aligned carbon nanotubes and its effect on field emission properties

Vertically aligned carbon nanotubes (VACNTs) were synthesized directly on stainless steel substrate and their surface densities were tuned by an order of magnitude via growing them at different temperatures. The surface density of the VACNTs was correlated with the surface evolution of the substrate, a process in which catalyst nano hills were generated at different growth temperatures in the presence of reducing gas environment. Enhanced field emission (FE) properties including low turn-on electric field (ETurn-on = 0.66 V μm-1), low threshold electric field (ETh = 1.07 V μm-1), high field enhancement factor (β = 4084), and high FE stability for 600 min were obtained from VACNT arrays synthesized at 650 °C. It is speculated that the inherent features of our sample such as the conductive substrate, the low contact resistance between the substrate and the VACNTs, the uniform length and the appropriate spacing between the VACNTs have contributed to the excellent field emission properties. Simulation on the electrostatic field of various VACNT arrays was performed and the result is in good agreement with the experimental data.

Improving field emission properties of vertically aligned carbon nanotube arrays through a structure modification

Vertically aligned carbon nanotube (VACNT) emitters were synthesized directly on stainless steel substrate using DC plasma-enhanced chemical vapor deposition. Remarkable field emission (FE) properties, such as low turn-on electric field (ETO = 1.40 V/μm) and low threshold electric field (ETH = 2.31 V/μm), were observed from VACNT arrays with long length and moderate density. The FE performance was significantly enhanced by a uniquely bundled structure of VACNTs formed through a simple water treatment process. The FE properties of VACNTs were further improved by coating the exterior of CNTs with a uniform layer of crystalline SnO2 nanoparticles; the ETO and ETH were reduced to 1.18 and 2.01 V/μm, respectively. The enhancement of FE properties by SnO2 coating can be attributed to the morphological change of VACNTs caused by the solution phase coating process. The coated samples also exhibited an improved FE stability which is attributed to the enhancement of the mechanical strength and chemical stability of the VACNTs after the SnO2 coating. The VACNT emitters with characteristic features such as a conductive substrate, low contact resistance between the VACNTs and the substrate, uniform coating, and bundled morphology can be ideal candidates for FE devices.

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.

Vertically aligned carbon nanotube arrays as a thermal interface material

Vertically aligned carbon nanotube (VACNT) arrays are considered a promising candidate for a thermal interface material (TIM) due to their good stability, high thermal conductivity, and desirable mechanical properties. In the last few decades, extensive studies have been performed on the preparation and properties of VACNT arrays for TIM applications. In this perspective, we introduce recent progress on improving the intrinsic thermal conductivity, decreasing the thermal contact resistance, and the efficient transfer of VACNT arrays, then discuss challenges that need to be overcome, and predict the future of VACNT-based TIMs.