Hydrothermal Coating of Patterned Carbon Nanotube Forest for Structured Lithium‐Ion Battery Electrodes

Controlling the arrangement and interface of nanoparticles is essential to achieve good transfer of charge, heat, or mechanical load. This is particularly challenging in systems requiring hybrid nanoparticle mixtures such as combinations of organic and inorganic materials. This work presents a process to coat vertically aligned carbon nanotube (CNT) forests with metal oxide nanoparticles using microwave‐assisted hydrothermal synthesis. Hydrothermal processes normally damage delicate CNT forests, which is addressed here by a combination of lithographic patterning, transfer printing, and reduction of the synthesis time. This process is applied for the fabrication of structured Li‐ion battery (LIB) electrodes where the aligned CNTs provide a straight electron transport path through the electrode and the hydrothermal coating process is used to coat the CNTs with conversion anode materials for LIBs. These nanoparticles are anchored on the surface of the CNTs and batteries fabricated following this process show a fourfold longer cyclability. Finally, this process is used to create thick electrodes (350 µm) with a gravimetric capacity of over 900 mAh g^?1.     

Synthesis and characterization of vertically aligned carbon nanotube forest for solid state fiber spinning

Continuous carbon nanotubes (CNT) fibers were directly spun from a vertically aligned CNT forest grown by a plasma-enhanced chemical vapor deposition (PECVD) process. The correlation of the CNT structure with Fe catalyst coarsening, reaction time, and the CNTs bundling phenomenon was investigated. We controlled the diameters and walls of the CNTs and minimized the amorphous carbon deposition on the CNTs for favorable bundling and spinning of the CNT fibers. The CNT fibers were fabricated with an as-grown vertically aligned CNT forest by a PECVD process using nanocatalyst an Al2O3 buffer layer, followed by a dry spinning process. Well-aligned CNT fibers were successfully manufactured using a dry spinning process and a surface tension-based densification process by ethanol. The mechanical properties were characterized for the CNT fibers spun from different lengths of a vertically aligned CNT forest. Highly oriented CNT fibers from the dry spinning process were characterized with high strength, high modulus, and high electrical as well as thermal conductivities for possible application as ultralight, highly strong structural materials. Examples of structural materials include space elevator cables, artificial muscle, and armor material, while multifunctional materials include E-textile, touch panels, biosensors, and super capacitors.     

Aligning Solution‐Derived Carbon Nanotube Film with Full Surface Coverage for High‐Performance Electronics Applications

The main challenge for application of solution‐derived carbon nanotubes (CNTs) in high performance field‐effect transistor (FET) is how to align CNTs into an array with high density and full surface coverage. A directional shrinking transfer method is developed to realize high density aligned array based on randomly orientated CNT network film. Through transferring a solution‐derived CNT network film onto a stretched retractable film followed by a shrinking process, alignment degree and density of CNT film increase with the shrinking multiple. The quadruply shrunk CNT films present well alignment, which is identified by the polarized Raman spectroscopy and electrical transport measurements. Based on the high quality and high density aligned CNT array, the fabricated FETs with channel length of 300 nm present ultrahigh performance including on‐state current I_on of 290 µA µm^?1 (V_ds = ?1.5 V and V_gs = ?2 V) and peak transconductance g_m of 150 µS µm^?1, which are, respectively, among the highest corresponding values in the reported CNT array FETs. High quality and high semiconducting purity CNT arrays with high density and full coverage obtained through this method promote the development of high performance CNT‐based electronics.     

Effects of vertically aligned carbon nanotubes on shear performance of laminated nanocomposite bonded joints

Abstract

The main objective is to improve the most commonly addressed weakness of the laminated composites (i.e. delamination due to poor interlaminar strength) using carbon nanotubes (CNTs) as reinforcement between the laminae and in the transverse direction. In this work, a chemical vapor deposition technique has been used to grow dense vertically aligned arrays of CNTs over the surface of chemically treated two-dimensionally woven cloth and fiber tows. The nanoforest-like fabrics can be used to fabricate three-dimensionally reinforced laminated nanocomposites. The presence of CNTs aligned normal to the layers and in-between the layers of laminated composites is expected to considerably enhance the properties of the laminates. To demonstrate the effectiveness of our approach, composite single lap-joint specimens were fabricated for interlaminar shear strength testing. It was observed that the single lap-joints with through-the-thickness CNT reinforcement can carry considerably higher shear stresses and strains. Close examination of the test specimens showed that the failure of samples with CNT nanoforests was completely cohesive, while the samples without CNT reinforcement failed adhesively. This concludes that the adhesion of adjacent carbon fabric layers can be considerably improved owing to the presence of vertically aligned arrays of CNT nanoforests.     

Controlled partial embedding of carbon nanotubes within flexible transparent layers

Applications of carbon nanotubes (CNTs) like field emission displays, super-capacitors, and cell growth scaffolds can benefit from controllable embedding of the CNTs in a material such that the CNTs are anchored and protrude a desired length. We demonstrate a simple method for anchoring densely packed, vertically aligned arrays of CNTs into silicone layers using spin-coating, CNT insertion, curing, and growth substrate removal. CNT arrays of 51 and 120?μm in height are anchored into silicone layers of thickness 26 and 36?μm, respectively. Scanning electron microscopy (SEM) and optical microscopy are used to characterize the sample morphology, a 5.5?m?s(-1) impinging water jet is used to apply shear stress, and a tensile test shows that the silicone layer detaches from the substrate before the CNTs are ripped from the layer. The CNTs are thus well anchored in the silicone layers. The spin-coating process gives control over layer thickness, and the method should have general applicability to various nanostructures and anchoring materials.     

Tuning of vertically-aligned carbon nanotube diameter and areal density through catalyst pre-treatment

By controlling the timing and duration of hydrogen exposure in a fixed thermal process, we tuned the diameters of carbon nanotubes (CNTs) within a vertically aligned film by a factor of 2, and tuned the areal densities by an order of magnitude. The CNT structure is correlated with the catalyst morphology, suggesting that while chemical reduction of the catalyst layer is required for growth, prolonged H2 exposure not only reduces the iron oxide and enables agglomeration of the Fe film, but also leads to catalyst coarsening. Control of this coarsening process allows tuning of CNT characteristics.     

Air-assisted growth of ultra-long carbon nanotube bundles

We report an air-assisted chemical vapor deposition (CVD) method for the synthesis of super-long carbon nanotube (CNT) bundles. By mixing a small amount of air in the vapor phase catalyst CVD process, the catalyst lifetime can be dramatically increased, and extremely long dense and aligned CNT bundles up to 1.5?cm can be achieved. Electron microscopy characterization shows that the injection of air does not damage the CNT structures. Further, we have estimated that individual ultra-long CNTs can carry moderate current densities ～10(5)?A?cm(-2), indicating their possible use in nanoelectronic devices.     

Synthesis of regular nano-pitched carbon nanotube array by using nanosphere lithography for interconnect applications

In this paper, we report a method based on nanosphere lithography technology for the synthesis of nano-pitched vertically aligned multi-walled carbon nanotube array. A monolayer of polystyrene nanospheres with diameter of 650 nm was coated on silicon oxide layer to create hexagonally arranged patterns. A metal layer, which acted as a catalyst for carbon nanotube growth, was deposited on the patterns by e-beam evaporation method. Nano-sized metallic patterns were formed by removing the polystyrene nanospheres. Uniform CNT arrays with pitch of 800 nm were successfully synthesized from the metallic patterns by plasma enhanced chemical vapor deposition. Using nanosphere lithography, the pitch of the single CNT array can be well-controlled. Therefore, the as-grown CNTs have potential applications in advanced interconnects technology and other nano applications.     

Growth control of carbon nanotubes by plasma enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition (PECVD), which enables growth of vertically aligned carbon nanotubes (CNTs) directly onto a solid substrate, is considered to be a suitable method for preparing CNTs for nanoelectronics applications such as electron sources for field emission displays (FEDs). For these purposes, establishment of an efficient CNT growth process has been required. We have examined growth characteristics of CNTs using a radio frequency PECVD (RF-PECVD) method with the intention to develop a high efficiency process for CNT growth at a low enough temperature suitable for nanoelectronics applications. Here we report an effect of pretreatment of the catalyst thin film that plays an important role in CNT growth using RF-PECVD. Results of this study show that uniform formation of fine catalyst nanoparticles on the substrate is important for the efficient CNT growth.     

Growth of aligned carbon nanotubes on ALD-Al2O3 coated silicon and quartz substrates

The smooth surface of the amorphous Al₂O₃ film on either silicon or quartz, coated by atomic layer deposition (ALD), was changed to rough surface by annealing in either air or hydrogen at high temperature (745°C) due to the formation of nanosized pinholes and micrometre pimples during the crystallisation of the amorphous Al₂O₃. The rough surface makes the growth of long carbon nanotubes (CNTs) by chemical vapour deposition impossible. Nevertheless, we were able to develop new catalyst recipes for successful growth of vertically aligned CNTs on ALD-Al₂O₃ coated silicon and quartz substrates. The lengths of the CNTs reached 90?µm on silicon substrates and 180?µm on quartz substrates. Furthermore, it is observed that the adhesion of CNTs on silicon substrates is much stronger than that on quartz substrates.     

Influence of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> buffer layer on catalyst morphology and spinnability of carbon nanotube arrays

Carbon nanotube (CNT) fibers spun from vertically aligned CNT arrays hold great promise in promoting CNT’s practical applications. Their production and properties strongly depend on the spinnability of the arrays. Herein, we study the influence of Al₂O₃ buffer layer on catalyst morphology and the spinnability of CNT arrays. Long and vertically aligned CNT arrays have been obtained from a wide range of Al₂O₃ buffer layer thickness, but the spinnable ones have only derived from a narrow range of the thickness. It is further found that the Al₂O₃ buffer layer can regulate the size and size distribution of the catalyst particles through balancing surface diffusion and inter-layer diffusion. Small, dense, and uniform-distributed nanoparticles are fingerprinted as the optimal catalyst for growing long and spinnable CNT arrays and can be obtained at a proper thickness of buffer layer. By using a tailored tri-layered Fe/Al₂O₃/SiO₂ catalyst, the obtained CNT arrays could reach a height of 500–800 µm and are highly spinnable.     

取向碳纳米管膜/氰基树脂复合材料的制备与性能强化机制

以浮动催化化学气相沉积法(FCCVD)碳纳米管(CNT)膜为原料,通过氰基树脂溶液浸渍法制备CNT预浸膜,然后采用热辅助牵伸和热压固化的方法制备高取向CNT膜复合材料。详细分析了热处理的温度和树脂溶液浓度对CNT预浸膜拉伸性能的影响,从而得到合适的热辅助牵伸工艺,并考察固化工艺对复合材料性能的影响。在此基础上,从浸润特性、CNT取向程度和层间剪切性能方面揭示CNT膜复合材料力学性能的强化机制。结果表明与传统CNT膜牵伸工艺相比,CNT预浸膜热牵伸工艺更有利于制备高取向CNT膜复合材料。热牵伸的温度和树脂溶液的浓度是制备高取向、低孔隙CNT预浸膜的关键因素。通过固化工艺的改变可有效调控氰基树脂的反应程度碳纳米管薄膜/氰基树脂复合材料的拉伸性能。经高温后固化处理后,CNT膜/氰基树脂复合材料的拉伸强度和模量分别高达2 748 MPa和302GPa。优异的树脂浸润特性、层间剪切强度以及高的CNT取向度使CNT膜复合材料中CNT更有利于协同承载,从而提高其力学性能。     

Aligned carbon nanotube coating on polyethylene surface formed by microwave radiation

Multifunctional carbon nanotube (CNT) architectures have been created on polyethylene (PE) surface by a microwave welding process. The continuous and aligned CNT films drawn from super-aligned CNT arrays can significantly absorb microwave energy and act as a network of nanosized thermal sources to locally melt the PE substrate beneath, leading to polymer wrapping around individual nanotubes. Uniform and highly conductive CNT/PE nanocomposite layer was formed without undermining the original alignment of the CNTs. CNT patterns have also been precisely fabricated on PE samples. The PE/CNT/PE bonds showed high interfacial strengths, which were affected by the duration of microwave radiation. With ultra-low content of CNTs introduced as antistatic agents, the dissipation of surface charges on PE substrate has been tremendously improved.     

Predictive Synthesis of Freeform Carbon Nanotube Microarchitectures by Strain‐Engineered Chemical Vapor Deposition

High‐throughput fabrication of microstructured surfaces with multi‐directional, re‐entrant, or otherwise curved features is becoming increasingly important for applications such as phase change heat transfer, adhesive gripping, and control of electromagnetic waves. Toward this goal, curved microstructures of aligned carbon nanotubes (CNTs) can be fabricated by engineered variation of the CNT growth rate within each microstructure, for example by patterning of the CNT growth catalyst partially upon a layer which retards the CNT growth rate. This study develops a finite‐element simulation framework for predictive synthesis of complex CNT microarchitectures by this strain‐engineered growth process. The simulation is informed by parametric measurements of the CNT growth kinetics, and the anisotropic mechanical properties of the CNTs, and predicts the shape of CNT microstructures with impressive fidelity. Moreover, the simulation calculates the internal stress distribution that results from extreme deformation of the CNT structures during growth, and shows that delamination of the interface between the differentially growing segments occurs at a critical shear stress. Guided by these insights, experiments are performed to study the time‐ and geometry‐depended stress development, and it is demonstrated that corrugating the interface between the segments of each microstructure mitigates the interface failure. This study presents a methodology for 3D microstructure design based on “pixels” that prescribe directionality to the resulting microstructure, and show that this framework enables the predictive synthesis of more complex architectures including twisted and truss‐like forms.     

Turning refuse plastic into multi-walled carbon nanotube forest

Abstract

A novel and effective method was devised for synthesizing a vertically aligned carbon nanotube (CNT) forest on a substrate using waste plastic obtained from commercially available water bottles. The advantages of the proposed method are the speed of processing and the use of waste as a raw material. A mechanism for the CNT growth was also proposed. The growth rate of the CNT forest was ～2.5 μm min^?1. Transmission electron microscopy images indicated that the outer diameters of the CNTs were 20–30 nm on average. The intensity ratio of the G and D Raman bands was 1.27 for the vertically aligned CNT forest. The Raman spectrum showed that the wall graphitization of the CNTs, synthesized via the proposed method was slightly higher than that of commercially available multi-walled carbon nanotubes (MWCNTs). We expect that the proposed method can be easily adapted to the disposal of other refuse materials and applied to MWCNT production industries.     

Force output, control of film structure, and microscale shape transfer by carbon nanotube growth under mechanical pressure

We demonstrate that a film of vertically aligned multiwall carbon nanotubes (CNTs) can exert mechanical energy as it grows, and in our experiments the average force output is approximately 0.16 nN per CNT, for CNTs having an outer diameter of 9 nm and five walls. The film thickness after a fixed growth time and the alignment of CNTs within the film decrease concomitantly with increasing pressure which is applied by placing a weight on the catalyst substrate prior to growth, and CNTs grown under applied pressure exhibit significant structural faults. The measured mechanical energy density of CNT growth is significantly less than the energies of primary steps in the CNT formation process yet, based on the film volume, is comparable to the energy density of muscle and based on the volume of CNTs is comparable to hydraulic actuators. We utilize this principle to fabricate three-dimensional structures of CNTs which conform to the shape of a microfabricated template. This technique is a catalytic analogue to micromolding of polymer and metal microstructures; it enables growth of nanostructures in arbitrarily shaped forms having sloped surfaces and nonorthogonal corners and does not require patterning of the catalyst before growth.     

Flow-dependent directional growth of carbon nanotube forests by chemical vapor deposition

We demonstrated that the structural formation of vertically aligned carbon nanotube (CNT) forests is primarily affected by the geometry-related gas flow, leading to the change of growth directions during the chemical vapor deposition (CVD) process. By varying the growing time, flow rate, and direction of the carrier gas, the structures and the formation mechanisms of the vertically aligned CNT forests were carefully investigated. The growth directions of CNTs are found to be highly dependent on the nonlinear local gas flows induced by microchannels. The angle of growth significantly changes with increasing gas flows perpendicular to the microchannel, while the parallel gas flow shows almost no effect. A computational fluid dynamics (CFD) model was employed to explain the flow-dependent growth of CNT forests, revealing that the variation of the local pressure induced by microchannels is an important parameter determining the directionality of the CNT growth. We expect that the present method and analyses would provide useful information to control the micro- and macrostructures of vertically aligned CNTs for various structural/electrical applications.     

Flexible High‐Conductivity Carbon‐Nanotube Interconnects Made by Rolling and Printing

Applications of carbon nanotubes (CNTs) in flexible and complementary metal‐oxide‐semiconductor (CMOS)‐based electronic and energy devices are impeded due to typically low CNT areal densities, growth temperatures that are incompatible with device substrates, and challenges in large‐area alignment and interconnection. A scalable method for continuous fabrication and transfer printing of dense horizontally aligned CNT (HA‐CNT) ribbon interconnects is presented. The process combines vertically aligned CNT (VA‐CNT) growth by thermal chemical vapor deposition, a novel mechanical rolling process to transform the VA‐CNTs to HA‐CNTs, and adhesion‐controlled transfer printing without needing a carrier film. The rolling force determines the HA‐CNT packing fraction and the HA‐CNTs are processed by conventional lithography. An electrical resistivity of 2 mΩ · cm is measured for ribbons having 800‐nm thickness, while the resistivity of copper is 100 times lower, a value that exceeds most CNT assemblies made to date, and significant improvements can be made in CNT structural quality. This rolling and printing process could be scaled to full wafer areas and more complex architectures such as continuous CNT sheets and multidirectional patterns could be achieved by straightforward design of the CNT growth process and/or multiple rolling and printing sequences.     

Wafer-Scale Growth and Transfer of Aligned Single-Walled Carbon Nanotubes

Experimental demonstration of wafer-scale growth of well-aligned, dense, single-walled carbon nanotubes on 4" ST-cut quartz wafers is presented. We developed a new carbon nanotube (CNT) wafer-scale growth process. This process allows quartz wafers to be heated to the CNT growth temperature of 865degC through the alpha-beta phase transformation temperature of quartz (573degC) without wafer fracture. We also demonstrate wafer-scale CNT transfer to transfer these aligned CNTs from quartz wafers to silicon wafers. The CNT transfer process preserves CNT density and alignment. Carbon nanotube FETs fabricated using these transferred CNTs exhibit high yield. Wafer-scale growth and wafer-scale transfer of aligned CNTs enable carbon nanotube very large-scale integration circuits and their large-scale integration with silicon CMOS.     

Effects of Conformal Nanoscale Coatings on Thermal Performance of Vertically Aligned Carbon Nanotubes

The high aspect ratio and the porous nature of spatially oriented forest‐like carbon nanotube (CNT) structures represent a unique opportunity to engineer a novel class of nanoscale assemblies. By combining CNTs and conformal coatings, a 3D lightweight scaffold with tailored behavior can be achieved. The effect of nanoscale coatings, aluminum oxide (Al₂O₃) and nonstoichiometric amorphous silicon carbide (a‐SiC), on the thermal transport efficiency of high aspect ratio vertically aligned CNTs, is reported herein. The thermal performance of the CNT‐based nanostructure strongly depends on the achieved porosity, the coating material and its infiltration within the nanotube network. An unprecedented enhancement in terms of effective thermal conductivity in a‐SiC coated CNTs has been obtained: 181% compared to the as‐grown CNTs and Al₂O₃ coated CNTs. Furthermore, the integration of coated high aspect ratio CNTs in an epoxy molding compound demonstrates that, next to the required thermal conductivity, the mechanical compliance for thermal interface applications can also be achieved through coating infiltration into foam‐like CNT forests.