Height Of Carbon Nanotubes Research Articles

Carbon nanotubes at the Graphene/hBN interface abnormally enhance its fracture toughness

Different from Van der Waals heterojunction, defects often appear at the interface of the planar heterostructures due to the low-dimensional structure characteristics. These defects lead to complex mechanical problems, and seriously affect application of planar heterostructures in flexible electronics devices. The most commonly used method to improve the toughness is to reduce/eliminate the interfacial structural defects. Using a defective Graphene/hBN interface, we show surprisingly that Carbon Nanotubes (CNTs) are able to enhance the toughness at the interface. It is found that interlayer stress transfer, bonding strain energies and energy release rate of the defective Graphene/hBN interface with CNTs are higher than those of free-defect Graphene/hBN interface, which is in strong contrast to the general view that interfacial defects reduce toughness. By combining the analysis of atomic mechanics and fracture mechanics theory, we find that the abnormal enhancement in interfacial toughness comes from the stress field localization arising from sp2 + sp3 hybrid state and out-of-plane deformation at the interface. In addition, the stress field at the distal end of the simulated models gradually decays with the CNTs height, resulting in the stress fields to be controlled at the interface. This abnormal mechanism provides a new dimension for enhancing toughness in two-dimensional heterostructures.

Water-Assisted Catalytic VACNT Growth Optimization for Speed and Height

The super-growth approach for carbon nanotubes synthesis is frequently used to boost the growth rate, catalyst lifespan, and height of vertically aligned carbon nanotubes. The elimination of amorphous carbon from catalyst particles, commonly made of iron, by injecting water vapor into a chemical vapor deposition process can enhance the purity, alignment, and height of carbon nanotubes and prevent the partial oxidation of the metallic catalyst. We present the development of a modified growth-optimized water-assisted super-growth vertically aligned carbon nanotube process by optimizing the catalyst layer structure and water vapor concentration for a carbon nanotube growth process for 4” diameter Si wafers. A significant finding is that under optimized water-assisted growth conditions over 4 mm, highly uniform tall, vertically aligned carbon nanotube structures can be grown with a minimum top crust layer of about ~5–10 μm thickness. This was achieved with a catalyst film comprising a >400 mm thermal SiO2 layer on top of a 4” diameter Si wafer that was overcoated with an e-beam batch process run that first deposited a 20 nm SiO2 layer, a 10 nm Al2O3 layer, and a 1.1 nm Fe layer, in a 4-h growth process step.

Cu/oriented-carbon nanotubes composite with ultralow thermal contact resistance

In this paper, a simple material preparation method is proposed to meet the requirements of efficient heat exchange interface materials for spacecraft on-orbit modular assembly. First, carbon nanotubes (CNTs) were uniformly dispersed on the surface of spherical copper (Cu) powder, and then the bulk was obtained by hot-pressing sintering. Finally, the Cu on the surface was removed by electrolysis, so that the CNTs can protrude from the surface to a certain height. After contacting with the matching materials, the exposed CNTs can fill the inevitable pores at the interface, increase the heat transfer channel, and effectively reduce the interface thermal contact resistance (TCR). By adjusting the type of CNT, contact pressure, the height of CNT protruding from the surface of Cu powder, the minimum TCR of the interface between the obtained Cu/oriented carbon nanotube (Cu/OCNT) composite and pure Cu was 25.4 mm2 K W−1 under 2.26 MPa, which is superior to most existing commercial thermal interface materials. Moreover, the heat exchange interface designed based on the prepared materials can be repeatedly contacted and separated, which is expected to provide a solution for the heat exchange of on-orbit modular assembly spacecraft.

Thermomechanical Reliability Investigation of Carbon Nanotube Off-Chip Interconnects for Electronic Packages

This article investigates the mechanical reliability benefits of carbon nanotube (CNT) off-chip interconnects for future electronic packages. Finite-element analysis (FEA) of a silicon chip attached to an organic substrate showed that CNT off-chip interconnects with an isotropic modulus of 1 MPa reduced the principal stresses in the silicon chip ~71%–93% compared with lead-free solder balls without underfill. In addition to reducing chip stresses, chip warpage was essentially eliminated by utilizing CNT interconnects compared with solder balls. Parameters of importance for the implementation of CNTs as interconnects such as CNT modulus and height were investigated with finite-element analysis. It was found that chip stresses increase with increasing CNT modulus, but for the modulus range reported in the literature and examined here (0.1–500 MPa), chip stresses were much less than packages utilizing solder balls. Due to the structure and morphology of CNT forests, the different material model types of fully isotropic versus transversely isotropic that can be used to model CNT interconnects were investigated. While the models yielded similar mechanical behavior in the finite-element analyses, the isotropic CNT model was mechanically conservative and simpler to implement and, therefore, sufficient for future thermomechanical reliability studies. To further demonstrate the thermomechanical reliability benefits, silicon chips with CNT off-chip interconnects were fabricated and assembled to organic substrates and subjected to cyclic thermal shock testing. During testing, the daisy chain circuit containing CNT interconnects survived 1660 cycles, demonstrating that CNT interconnects mechanically decouple the chip from the substrate and, therefore, reduce chip stresses caused by coefficient of thermal expansion (CTE) mismatch.

Explosive boiling of argon on a copper surface coated with graphene/CNT/Cu nanowire; a molecular dynamics study

Recent advances in nanoscience have improved surface properties as an important factor in heat transfer, leading to increased heat transfer. In this study, molecular dynamics is employed to study the explosive boiling of liquid argon on various Cu-based substrates. The smooth Cu surface is considered as the reference surface, and modified Cu-based surfaces coated with graphene sheet, carbon nanotubes (CNTs), and Cu-Nanowire-embedded-CNTs are investigated to find how the nanostructured coatings affect the heat transfer characteristics of the base surface. Different numbers of CNTs and CNT-Nanowires with various heights (10 Å, 20 Å, and 30 Å) are placed on the substrate. Liquid to vapor phase transition, density distribution of argon, temperature variations, and liquid argon cluster dynamics are studied. Heat transfer on the copper surface coated with graphene platelet is improved compared with the reference surface. Moreover, the results show enhanced heat transfer performance of the copper plate in presence of CNTs and CNT-Nanowires. According to the simulation results, increasing the number and height of CNTs and CNT-Nanowires on the surface improves the heat transfer performance. It is also found that the time taken for liquid argon to equilibrate with the temperature of the solid surface decreases in presence of CNTs and CNT-Nanowires.

Effects of total pressure on carbon nanotube synthesis, independent of feed pressure

While the effects of partial pressure of carbon feed on the growth of carbon nanotubes (CNTs) are well documented, the effects of total pressure in the growth chamber are not well understood. To shed some light, we investigated the growth of CNTs at different total pressures and a constant partial pressure using catalytic chemical vapour deposition method. It was observed that the growth height and diameter of CNTs decreased as the total pressure increased, probably due to the dilution of carbon feed, the increase in the boundary layer thickness and the decrease in the gas diffusion coefficient. Furthermore, the growth quality of the CNTs was found to be highest at intermediate total pressures. This can provide clues as to where the best temperature concentration is for high-quality CNTs.

The influence of the temperature and Ti and TiN sublayer material on carbon nanotubes growth

The influence of the temperature and Ti and TiN sublayer material on the height, diameter, density, and uniformity of carbon nanotubes (CNTs) growth is studied. It was found that on the TiN sublayer, CNTs form an array with a more uniform diameter distribution of CNTs than on the Ti sublayer (minimum dispersion 5.6 nm versus 8.1 nm). It is shown that for the Ti sublayer, with an increase in the growth temperature, an almost linear increase in the CNT height occurs. It was found that an increase in the CNT height is accompanied by a decrease in their diameter for both sublayers. For a given thickness of Ni, arrays with a density of 3–18 μm−2 were obtained on the Ti sublayer, and on a TiN sublayer in the range of 8–21 μm−2. The greatest uniformity of the surface distribution of CNTs is showed at 660 °C. CNT arrays on the Ti sublayer are characterized by a lower concentration of bundles than on the Ti sublayer.

Investigation of Early Stage of Carbon Nanotube Growth on Plasma-Pretreated Inconel Plates and Comparison with Other Superalloys as Substrates.

We investigate the early stage of carbon nanotube (CNTs) growth on Inconel 600 to address the effect of pretreatments such as annealing and plasma pretreatment on growth behavior. In addition, we compare the growth results to other Ni-based superalloys including Invar 42 and Hastelloy C276. The growth substrates were prepared using mechanical polish, thermal annealing and plasma pretreatment. The air annealing was performed at 725 °C for 10 min and plasma pretreatment was subsequently undergone with 10.5 W at 500 °C for 30 min. The annealed and plasma-pretreated substrates exhibited different surface morphologies on the surface and enhanced growth behavior of CNT was observed from the region of particulate surface. The optimized growth temperature, which produces the highest CNT height, was determined at 525 °C for Ni and Inconel 600 and 625 °C for Invar 42 and Hastelloy C276 substrates. The difference of optimal growth temperature is expected to the existence of high temperature elements such as Mn or Mo in the alloys. X-ray diffraction spectroscopy revealed that the formation of roughened oxide layers caused by the pretreatments would promote the nucleation and growth of the CNTs.

Investigation on growth and electronic characteristics of vertical carbon nanotubes

Carbon nanotubes (CNTs) as interconnects in integrated circuits (ICs) which are vertically aligned in growth with high tube density and long tube length are required. In this paper, we present a method to improve the height of CNTs. High-resolution transmission electron microscopic (TEM) images confirm CNTs top growth mode. By cutting and modifying the top of CNTs, the influences of different radii of apertures on interconnect resistance were studied. According to the analysis, we proposed a novel growth mechanism to improve growth height of CNTs interconnection structure and the top contact resistances of pre-cutting and post-cutting CNTs interconnection structure were forecasted. The result demonstrates that the electronic performances of the post-cutting CNTs interconnection structure with platinum (Pt)-protected layer are better than the ones of pre-cutting CNTs interconnection structure. The resistivity of the post-cutting CNTs interconnection structure with Pt-protected layer is [Formula: see text], which is much less than that of post-cutting CNTs interconnect structure [Formula: see text]. In what follows, the resistance of the contacted area of pre-cutting CNTs interconnection structure is 31 [Formula: see text], which is much less than that of post-cutting CNTs interconnection structure 439 [Formula: see text]. This constitutes a significant step to realize longer CNTs interconnects with complementary metal oxide semiconductor (CMOS) contact modules.

Morphology control of aligned carbon nanotube pins formed via patterned capillary densification

The exceptional intrinsic properties of aligned nanofibers, such as carbon nanotubes (CNTs), and their ability to be easily densified by capillary forces motivates their use as shape-engineerable materials. While a variety of self-assembled CNT structures, such as cell networks, micropillars, and pins have previously been fabricated via the capillary-mediated densification of patterned CNT arrays, predicting the critical pattern size (scr) that separates cell versus pin formation and the corresponding process-morphology scaling relations within the micrometer range are outstanding. Here, facile and scalable mechanical patterning and capillary densification techniques are used to establish scr by elucidating how the effective elastic modulus of aligned CNT arrays during densification governs the resulting pin geometries. Experiments and modeling show that this effective modulus scales with CNT height and is about an order of magnitude smaller for pins as compared to cell networks formed from bulk-scale (i.e. non-patterned) CNT arrays. Patterning therefore results in pins with a lower packing density (commensurate with double the wall thickness) and a larger characteristic length scale than bulk cell networks (i.e. scr ∼ 5 × cell width). CNT arrays with the initial randomly-oriented carbon ‘crust’ removed via oxygen plasma etching yield a higher degree of structural uniformity and better agreement with the proposed elasto-capillary model, which enables the use of capillary densification to predictively design hierarchical and shape-tunable materials for advanced thermal, electronic, and biomedical devices.

Patterned growth of carbon nanotube forests using Cu and Cu/Ag thin film reservoirs as growth inhibitors

Lithography and lift-off of the catalyst is the established way to pattern forests of carbon nanotubes (CNTs). We demonstrate an alternative technique where we pattern a thin film reservoir residing below the Al2O3 underlayer to partially or fully deactivate the iron catalyst on the surface, thus leading to short CNTs or no CNTs. High-resolution imaging suggests that mass transfer from the Cu or Cu/Ag alloy reservoir to the surface promotes interalloying with the catalyst, which is the mechanism for deactivation. The lower melting temperature of the Cu/Ag alloy is more effective than Cu in inhibiting CNT growth since it becomes partially liquid at the processing temperature, to faster alloy with the iron catalyst.We show how the lithographic patterning at the level of the reservoir translates into lithographic patterning of the CNT forest. This technique, besides avoiding contamination of the catalytic surface, is an important building block towards fabricating patterned forests with areas of different heights where the reservoir material will modulate the CNT height. Additionally, this research opens the door for testing additional materials as reservoirs and to analyze their effects on CNT growth to achieve fine modulation of the third dimension (height) in patterned CNT forests.

Correlation between density and hydrogen content in vertically aligned carbon nanotube forests by ion beam analysis

Interactions of vertically aligned multiwall carbon nanotubes (CNTs) with high energy He+ beams were studied using elastic recoil detection analysis and ion beam channeling. The relationship between the elastic recoil of hydrogen, the depth of He–H interactions, and the number of carbon atoms per volume (denoted as effective density) was calculated. Ion channeling was observed in CNT forests shorter than 40 μm. It was found that the effective density and hydrogen content were inversely correlated with the CNT height. In compliance with channeling and density calculations, the authors propose that this effect is due to the weakening of Van-der-Waals forces in taller CNT forests. The methodology suggested in this work may be extended to assessing densities of thin, highly porous materials.

Influence of synthesis parameters on CCVD growth of vertically aligned carbon nanotubes over aluminum substrate

In the past two decades, important results have been achieved in the field of carbon nanotube (CNT) research, which revealed that carbon nanotubes have extremely good electrical and mechanical properties The range of applications widens more, if CNTs form a forest-like, vertically aligned structure (VACNT) Although, VACNT-conductive substrate structure could be very advantageous for various applications, to produce proper system without barrier films i.e. with good electrical contact is still a challenge. The aim of the current work is to develop a cheap and easy method for growing carbon nanotubes forests on conductive substrate with the CCVD (Catalytic Chemical Vapor Deposition) technique at 640 °C. The applied catalyst contained Fe and Co and was deposited via dip coating onto an aluminum substrate. In order to control the height of CNT forest several parameters were varied during the both catalyst layer fabrication (e.g. ink concentration, ink composition, dipping speed) and the CCVD synthesis (e.g. gas feeds, reaction time). As-prepared CNT forests were investigated with various methods such as scanning electron microscopy, Raman spectroscopy, and cyclic voltammetry. With such an easy process it was possible to tune both the height and the quality of carbon nanotube forests.

Carbon Nanotubes: Free Form Growth of Carbon Nanotube Microarchitectures on Stainless Steel Controlled via Laser‐Stimulated Catalyst Formation (Adv. Mater. Interfaces 16/2017)

Norbert Hampp and co-workers describe site-selective carbon nanotube (CNT) growth on stainless steel, achieved by laser-stimulated self-organization of highly granular transition metal oxide precursors that transform into CNT catalysts upon thermal reduction, thus enabling the fabrication of free-form CNT microarchitectures with standard chemical vapor deposition equipment, in article number 1700508. This process facilitates control over CNT location and height as well as the alignment of CNT bundles.

Temperature-dependent selective growth of carbon nanotubes in Si/SiO2 structures for field emitter array applications

Temperature-dependent selective growth of Carbon Nanotubes (CNTs) in Si/SiO2 structures using ferrocene/xylene volatile catalyst source and its application in Field Emitter Array (FEA) is demonstrated in this work. CNTs are grown directly on Si/SiO2 substrates by volatile catalyst source (Ferrocene/Xylene) Chemical Vapor Deposition (CVD) technique and the effect of growth temperatures (760–880°C) on CNT height and crystallinity has been studied. Selective growth of CNTs on Si substrates is achieved at 790°C growth temperature. Using the obtained selective growth condition, CNT FEAs are fabricated by growing CNT bundles selectively on the Si surface of the pre-fabricated SiO2 pits on a Si wafer. Field emission current density above 100mA/cm2 is obtained from inter-pit separation distances of 4–10μm. These results show the potential of ferrocene/xylene catalyst source in achieving selective growth of CNTs in Si/SiO2 structures for FEA application.

Mechanical Behavior of Carbon Nanotube Forests Grown With Plasma Enhanced Chemical Vapor Deposition: Pristine and Conformally Coated

Plasma-enhanced chemical vapor deposition (PECVD) is a well-known method for the synthesis of carbon nanotube (CNT) forests with the electric field in the plasma sheath being responsible for the vertical orientation of CNTs. Here, we investigate the deformation mechanism and mechanical properties of pristine and conformally coated PECVD CNT forests under compressive loading. Our in situ indentation experiments reveal that local buckles form along the height of pristine CNTs progressing downward from the starting point at the tips. For CNT forests coated from their roots to top with alumina using atomic layer deposition (ALD), the deformation mechanism depends strongly on the coating thickness. The buckling behavior does not change significantly when the coating is 5-nm thick. However, with a 10-nm-thick coating, the nanotubes fracture—that is, at both the CNT core and alumina coating. Ex situ indentation experiments with a flat punch reveal 8- and 22-fold increase in stiffness with the 5- and 10-nm coating, respectively. Comparing the behavior of the PECVD forests with CNTs grown with thermal chemical vapor deposition (CVD) shows that the mechanical behavior of PECVD CNTs depends on their characteristic morphology caused by the growth parameters including plasma. Our findings could serve as guidelines for tailoring the properties of CNT structures for various applications in which CNT compliance or deformation plays a critical role.

Geometric scaling of artificial hair sensors for flow measurement under different conditions

Artificial hair sensors (AHSs) have been developed for prediction of the local flow speed and aerodynamic force around an airfoil and subsequent application in vibration control of the airfoil. Usually, a specific sensor design is only sensitive to the flow speeds within its operating flow measurement region. This paper aims at expanding this flow measurement concept of using AHSs to different flow speed conditions by properly sizing the parameters of the sensors, including the dimensions of the artificial hair, capillary, and carbon nanotubes (CNTs) that make up the sensor design, based on a baseline sensor design and its working flow condition. In doing so, the glass fiber hair is modeled as a cantilever beam with an elastic foundation, subject to the distributed aerodynamic drag over the length of the hair. Hair length and diameter, capillary depth, and CNT height are scaled by keeping the maximum compressive strain of the CNTs constant for different sensors under different speed conditions. Numerical studies will demonstrate the feasibility of the geometric scaling methodology by designing AHSs for aircraft with different dimensions and flight conditions, starting from the same baseline sensor. Finally, the operating bandwidth of the scaled sensors are explored.

Controllable deformation of salt water-filled carbon nanotubes using an electric field with application to molecular sieving

Precisely controlling the deformation of carbon nanotubes (CNTs) has practical application in the development of nanoscale functional devices, although it is a challenging task. Here, we propose a novel method to guide the deformation of CNTs through filling them with salt water and applying an electric field. With the electric field along the axial direction, the height of CNTs is enlarged by the axial electric force due to the internal ions and polar water molecules. Under an electric field with two mutually orthogonal components, the transverse electric force could further induce the bending deformation of CNTs. Based on the classical rod and beam theories, two mechanical models are constructed to verify and quantitatively describe the relationships between the tension and bending deformations of CNTs and the electric field intensity. Moreover, by means of the electric field-driven tension behavior of CNTs, we design a stretchable molecular sieve to control the flow rate of mixed gas and collect a single high-purity gas. The present work opens up new avenues in the design and fabrication of nanoscale controlling units.

Heat transfer augmentation in rectangular micro channel covered with vertically aligned carbon nanotubes

An experimental heat transfer investigation was carried out to examine the influence of carbon nanotubes (CNTs) layer deposits on the convective heat transfer performance inside rectangular microchannels. Successful synthesis of vertically aligned CNTs was achieved using a catalytic vapor deposition (CVD) process on a silicon sample substrate. By varying the synthesis time, on average 6μm and 20μm thick layers of CNTs were made with surface roughness of (Sa=1.062μm, Sq=1.333μm) and (Sa=0.717μm, Sq=0.954μm) respectively. The external surface area of the samples increased 7 times compared to the bare silicon chip. The heat transfer performance of each sample was measured inside two rectangular microchannels with cross-section of 125μm×9mm and 200μm×9mm. For the 125μm channel height, the 6μm and 20μm thick layer of CNTs resulted in 12% and 26% increase in pressure drop respectively. The pressure drop obtained from the 200μm channel height show a similar trend with an increase of 6% and 16.4% for 6μm and 20μm CNTs layer thickness respectively. An average heat transfer enhancement of 19% and 74% is obtained inside the 125μm height microchannel with 6μm and 20μm CNTs layer thickness respectively. Whereas, the average heat transfer enhancement of 22% and 62% are obtained inside the 200μm channel with respective CNTs layer thicknesses of 6μm and 20μm. Enhancements are attributed to an increase in surface area and effective thermal conductivity inside the thermal boundary layer. However, the frictional heating (viscous dissipation) of a particular nanostructured sample increases with a decrease in channel height. This difference in channel size results in stronger competition between heat transfer enhancement potential that can be achieved by the deposited surface and the decrease in Nusselt number due to viscous dissipation.

Study of Biomolecules Imaging Using Molecular Dynamics Simulations

The process of imaging a biomolecule by atomic force microscope (AFM) is modeled using molecular dynamics (MD) simulations. Since the large normal force exerted by the tip on the biosample in contact and tapping modes may damage the sample structure and produce irreversible deformation, the noncontact mode of AFM (NC-AFM) is employed as the operating mode. The biosample is scanned using a carbon nanotube (CNT) as the AFM probe. CNTs because of their small diameter, high aspect ratio and high mechanical resistance attract many attentions for imaging purposes. The tip–sample interaction is simulated by the MD method. The protein, which has been considered as the biomolecule, is ubiquitin and a graphene sheet is used as the substrate. The effects of CNT's geometric parameters such as the CNT height, the diameter, the tilt angle, the flexibility and the number of layers on the image quality have been evaluated.