H. Cui

Fast and highly anisotropic thermal transport through vertically aligned carbon nanotube arrays

This letter reports on fast and highly anisotropic thermal transport through millimeter-tall, vertically aligned carbon nanotube arrays (VANTAs) synthesized by chemical vapor deposition on Si substrates. Thermal diffusivity measurements were performed for both longitudinal and transverse to the nanotube alignment direction, with longitudinal values as large as 2.1±0.2cm2∕s and anisotropy ratios as large as 72. Longitudinal thermal conductivities of 15.3±1.8W∕(mK) for porous 8±1vol% VANTAs in air and 5.5±0.7W∕(mK) for epoxy-infiltrated VANTAs already exceed those of phase-changing thermal interface materials used in microelectronics. Data suggest that further improvements are possible through optimization of density and defects in the arrays.

Growth behavior of carbon nanotubes on multilayered metal catalyst film in chemical vapor deposition

Abstract The temperature and time dependences of carbon nanotube (CNT) growth by chemical vapor deposition are studied using a multilayered Al/Fe/Mo catalyst on silicon substrates. Within the 600–1100 °C temperature range of these studies, narrower temperature ranges were determined for the growth of distinct types of aligned multi-walled CNTs and single-walled CNTs by using high-resolution transmission electron microscopy and Raman spectroscopy. At 900 °C, in contrast to earlier work, double-walled CNTs are found more abundant than single-walled CNTs. Defects also are found to accumulate faster than the ordered graphitic structure if the growth of CNTs is extended to long durations.

In situ control of the catalyst efficiency in chemical vapor deposition of vertically aligned carbon nanotubes on predeposited metal catalyst films

Premature termination of growth, presumably because of catalyst deactivation, is an undesirable side effect of chemical vapor deposition of vertically aligned carbon nanotubes on predeposited metal catalyst films. The addition of ferrocene, an effective precursor for in situ Fe formation, was found to enhance carbon nanotubegrowth rates and extend growth to 3.25 mm thick carbon nanotube films. Ferrocene was introduced into the gas stream by thermal evaporation concurrently with acetylene using a specially constructed source. The key factor facilitating the growth of thick carbon nanotube films was the independent and precise control of the ferrocene amount in the feedstock. The carbon nanotube films were characterized by scanning and transmission electron microscopy, and Raman spectroscopy. The temperature dependence of the carbon nanotubegrowth with ferrocene exhibits a steep drop at high substrate temperatures and a loss of vertical alignment at 900u200a° C . The negative temperature coefficient of the growth rate suggests that the reaction mechanism of vertically aligned carbon nanotubegrowth is governed by a heterogeneous intermediate step.

Integrated tungsten nanofiber field emission cathodes selectively grown by nanoscale electron beam-induced deposition

We report on the fabrication and operation of integrated field emission cathodes containing single tungsten (W) nanofibers selectively grown by nanoscale electron beam induced deposition (EBID). A nonorganometallic precursor, WF6, was used to deposit metallic W fibers. Vacuum electrical testing reveals electrons were successfully extracted from the W nanofiber tip and were collected by a phosphor anode. Direct current versus voltage (I–V) curves exhibited Fowler–Nordheim behavior, indicating the occurrence of cold field emission. Electrical testing of these devices indicated that EBID direct-write is a promising technique for direct production or repair of field emission cathodes.

Operation of individual integrally gated carbon nanotube field emitter cells

In this work, we examine the operation of individual field emitter cells contained in a field emitter array composed of integrally gated multiwalled carbon nanotube (MWNT)-based field emission cathodes. These devices were found to behave in a manner consistent with a multiple emission site model of Fowler–Nordheim field emission. These results show considerable variation in the operational characteristics of cells contained within the same array and indicate that data obtained from arrays of cells are not necessarily indicative of individual cell performance.