Vertically Aligned Carbon Nanotube Films Research Articles

Thermal dissipation media for high power electronic devices using a carbon nanotube-based composite

Challenges in the thermal dissipation of an electronic package arise from the continuous increase in power density of higher-power devices. Carbon nanotubes (CNTs) are known as the highest thermal conductivity material (2000 W mK −1). This excellent thermal property suggests an approach in applying the CNTs in thermal dispersion materials to solve the aforementioned problems. In this work, we present an effect of thermal dissipation of the CNTs in the high-brightness light emitting diode (HB-LED) and micro-processor. For the thermal dissipation of the HB-LED, a vertically aligned carbon nanotube (VA-CNT) film on a Cu substrate was applied. Meanwhile, for the thermal dissipation of a micro-processor, the composite of commercial thermal paste/CNTs was used instead of the VA-CNTs. The experimental and simulation results have confirmed the advantages of the VA-CNT film and thermal paste/CNT composite as excellent thermal dissipation media for HB-LEDs, μ-processors and other high power electronic devices.

Electrochemical Charge Storage Properties of Vertically Aligned Carbon Nanotube Films: The Activation-Enhanced Length Effect

Improvement in the safety, efficacy, selectivity, and/or power consumption of devices used for neural stimulation requires a low impedance neural electrode interface that can rapidly store high charge densities without damaging the electrode or neural tissue. In this study, vertically aligned carbon nanotube (VACNT) films were synthesized and investigated in vitro for their potential use as a neural stimulation electrode. Materials and electrochemical (EC) characterization was performed prior and subsequent to EC activation, accomplished by slow potential cycling across the water window, to determine if activation improved the desired properties. EC characterization was performed at a wide range of frequencies using various techniques including cyclic voltammetry, electrochemical impedance spectroscopy, and high-rate potential transient measurements. The results indicated that EC activation, a technique often used to activate and greatly improve the performance of iridium oxide electrodes, was also favorable for VACNT electrodes. The effect of activation was more pronounced as the average length of the VACNTs was increased, especially at lower frequencies. The activation-enhanced length effect is believed to be a result of the VACNT film characteristics that are determined during the deposition process. A phenomenological model was developed to illustrate this concept and has important implications for various charge-storage applications.

Synthesis of vertically aligned carbon nanotubes and diamond films on Cu substrates for use in high-power electronic devices

Currently, most of the vertically aligned carbon nanotubes (VA-CNTs) and diamond films are mainly synthesised on flat silicon (Si) substrate. However, to achieve thermal dissipation in high-power electronic devices (HPEDs), the VA-CNTs and diamond films need to be attached to thermal dissipation metal substrates (like Cu, Ag, Al, etc.). In this paper, the fabrication process of the VA-CNTs and diamond films on Cu substrate is reported in detail. The VA-CNTs were synthesised by the thermal chemical vapour deposition (CVD) method. The VA-CNTs on Cu substrates were fabricated by two different methods: directly growing the VA-CNTs using thin catalytic metal layers such as Fe/Al or Cr/Al as a catalyst; transferring the VA-CNTs film that was pre-grown on Si substrate to Cu substrate. The diamond films were also directly grown on the Cu substrate by microwave plasma chemical vapour deposition (MPCVD). The grown VA-CNTs and diamond films were tested as the thermal dissipation media on a 0.5W InGaN LED chip. The VA-CNTs and diamond films greatly increased input current of the LED by more than 500 mA and 350 mA without reaching saturation. This is higher compared with that of the device packaged using normal commercial silver thermal paste. Initial experiment results on the LED demonstrated that the VA-CNTs and diamond films greatly improve the light's output power and that they are optimal choices for the thermal dissipation of HPED.

A metallization and bonding approach for high performance carbon nanotube thermalinterface materials

A method has been developed to create vertically aligned carbon nanotube (VACNT)thermal interface materials that can be attached to a variety of metallized surfaces.VACNT films were grown on Si substrates using standard CVD processing followed bymetallization using Ti/Au. The coated CNTs were then bonded to metallized substrates at220 °C. By reducing the adhesion of the VACNTs to the growth substrate during synthesis,the CNTs can be completely transferred from the Si growth substrate and usedas a die attachment material for electronic components. Thermal resistancemeasurements using a photoacoustic technique showed thermal resistances as low as1.7 mm2 K W − 1 for bondedVACNT films 25–30 µm in length and 10 mm2 K W − 1 for CNTs up to 130 µm in length. Tensile testing demonstrated a die attachment strength of40 N cm − 2 at room temperature. Overall, these metallized and bonded VACNT films demonstrateproperties which are promising for next-generation thermal interface material applications.

Fe-N-C oxygen reduction catalysts supported on vertically aligned carbon nanotubes

Abstract Non-noble metal electrocatalyst (Fe-N-C) for the oxygen reduction reaction (ORR) was sputter deposited onto films of vertically aligned carbon nanotubes (VACNT) and tested by the rotating ring disk electrode (RRDE) technique. The VACNT support was first synthesized at atmospheric pressure on a SiO2/Si wafer substrate in a simple tube furnace by iron-catalyzed chemical vapor deposition of ethylene and ammonia at a temperature of 720 °C. VACNT films, with CNT bundle height of 1.5 μm and CNT diameter of 50 nm, were loaded into a sputtering machine to deposit Fe-N-C materials onto the VACNT as support. Iron and carbon were sputtered in a nitrogen atmosphere in order to deposit an amorphous mixture of Fe-N-C material on the VACNT films. The Fe-N-C material supported on the VACNT films was then annealed in Ar at 800 °C to make the ORR electrocatalyst. The catalyst-coated VACNT was scraped off the SiO2/Si substrate and made into catalyst ink for testing by the RRDE technique. The impact of catalyst loading on the RRDE performance (disk current density and %H2O2) is discussed.

Effect of sidewall modification in the determination of friction coefficient of vertically aligned carbon nanotube films using friction force microscopy

The significance of the sidewall surface of vertically aligned carbon nanotubes (VACNTs) and the effect of humidity in the determination of VACNT film friction coefficient have been investigated. VACNT films of 2 μm thick were sidewall-modified by means of CF 4 and O 2 plasma treatments, and verified for the functionalization of the sidewalls. They were then characterized for wettability properties, as well as friction coefficient using friction force microscopy at different humidity levels. It was found that humidity had insignificant effect on the friction coefficient, and sidewall friction formed a major component of the friction force experienced by the tip. Sidewall modifications resulted in friction coefficient changes of up to 50%.

Electrophoresis Coating of Titanium Dioxide on Aligned Carbon Nanotubes for Controlled Syntheses of Photoelectronic Nanomaterials

Owing to its excellent semiconducting and photoelectronic properties, TiO2 has recently attracted a great deal of interest for a large variety of applications. Examples include the use of TiO2 nanoparticles/films as photocatalysts for environment protection, photoelectron mediators for sensors, photosensitizers in light-emitting devices, and solar cells. With the recent development in nanoscience and nanotechnology, there is now a pressing need to integrate multicomponent nanoscale entities into multifunctional materials and devices. In this regard, TiO2 nanoparticles have been deposited onto various catalytic supports to improve their photocatalytic activities. In particular, activated carbon fibers and spheres were used as a class of chemically stable mesoporous catalytic supports to provide multiple active sites and to allow effective diffusion of reactants for the photocatalytic reactions. Because of their unique one-dimensional electronic structure, large surface area, good chemical and thermal stability, and excellent mechanical properties, carbon nanotubes (CNTs) may work better than other carbon forms to support TiO2 for a wide range of applications, especially in photocatalytic and optoelectronic systems. As a consequence, some recent attempts have been made to coat nonaligned multiwalled CNTs with TiO2 thin films, [10] whilst several synthetic routes were devised to prepare nonaligned TiO2 nanotubes, [11] nanowires, or nanomembranes. It will be a significant advantage if we can coat vertically aligned CNTs (VACNTs) with TiO2 as the coaxial structure should allow the nanotube framework to provide a good mechanical stability, high thermal/electrical conductivity, and large surface/interface area necessary for efficient optoelectronic and sensing devices. The alignment structure will also facilitate surface modification for adding novel surface/interfacial characteristics to the TiO2 and VACNT hybrids, and allow the constituent nanotube devices to be collectively addressed through a common substrate/electrode. We previously prepared various vertically aligned conducting polymer–CNT coaxial nanowires by electrochemically depositing a concentric layer of an appropriate conducting polymer onto the individual aligned CNTs for advanced biosensing applications. In this Communication, we report the use of VACNTs not only as the support for electrophoretic coating with TiO2 but also as the template for producing aligned TiO2 nanotubes and nanomembranes. The resultant aligned TiO2–VACNT coaxial nanowires, TiO2 nanotubes, and TiO2 nanomembranes were demonstrated to possess novel photocurrent responses and photoinduced electron-transfer properties. In a typical experiment, we prepared VACNTs by pyrolyzing iron(II) phthalocyanine (FePc) on a Si substrate according to our published procedures. For electrophoresis, a Si-supported VACNT film thus produced was used as the cathode and a graphite rod as the anode. Both electrodes were immersed in a sol–gel solution approximately 2 cm apart from each other. The sol–gel solution was prepared by dissolving titanium (IV) isopropoxide (10 mL) in 30 mL of ethanol containing glacial acetic acid (12 wt %), followed by the addition of 0.6 mL of HCl in deionized water (pH 2) under magnetic stirring for 1 h. A potential of 1 V was then applied between the two electrodes for several minutes to electrophoretically deposit the positively charged, protonated TiO2 clusters onto the VACNT cathode. After the electrophoretic deposition, the samples were dried at 80 °C for l h. The chemical stoichiometry, crystal structure, and thickness of the resultant TiO2 coating can be regulated by controlling the electrophoresis conditions (e.g. applied voltage, deposition time) and a post-electrophoresis heat treatment for crystallization and densification (typically, 500 °C in air for 20 min). Figure 1a shows a typical scanning electron microscopy image of ca. 7 lm long VACNTs. The corresponding SEM image under higher magnification in Figure 1b shows an average diameter of ca. 50 nm for the as-synthesized CNTs. Upon electrophoresis, TiO2 clusters were deposited on sidewalls of the VACNTs as nanoparticles (Fig. 1c). Further electrophoresis deposition up to ca. 30 min caused additional TiO2 clusters to fill up the spaces between the pre-deposited TiO2 nanoparticles, leading to the growth of a continuous TiO2 coating along C O M M U N IC A IO N

High friction of a vertically aligned carbon-nanotube film in microtribology

The frictional behavior of a vertically aligned carbon-nanotube (VACNT) film against gold tips was studied in air. In these experiments, the film was 6 μm thick, the tip radii 4.5–30 μm, the applied forces smaller than 5 μN, and speeds were 0.15–24 μm/s. In spite of the null adhesion force to the gold tip, extremely high friction coefficients of 1.0–2.2 were found for the VACNT film. No dependence of friction on speed or tip radius was observed.