192 research outputs found
CVD growth of carbon nanostructures from zirconia: mechanisms and a method for enhancing yield.
By excluding metals from synthesis, growth of carbon nanostructures via unreduced oxide nanoparticle catalysts offers wide technological potential. We report new observations of the mechanisms underlying chemical vapor deposition (CVD) growth of fibrous carbon nanostructures from zirconia nanoparticles. Transmission electron microscope (TEM) observation reveals distinct differences in morphological features of carbon nanotubes and nanofibers (CNTs and CNFs) grown from zirconia nanoparticle catalysts versus typical oxide-supported metal nanoparticle catalysts. Nanofibers borne from zirconia lack an observable graphitic cage consistently found with nanotube-bearing metal nanoparticle catalysts. We observe two distinct growth modalities for zirconia: (1) turbostratic CNTs 2-3 times smaller in diameter than the nanoparticle localized at a nanoparticle corner, and (2) nonhollow CNFs with approximately the same diameter as the nanoparticle. Unlike metal nanoparticle catalysts, zirconia-based growth should proceed via surface-bound kinetics, and we propose a growth model where initiation occurs at nanoparticle corners. Utilizing these mechanistic insights, we further demonstrate that preannealing of zirconia nanoparticles with a solid-state amorphous carbon substrate enhances growth yield.This material is based upon work supported by the National
Science Foundation under Grant No. 1007793 and was also
supported by Airbus group, Boeing, Embraer, Lockheed Martin,
Saab AB, Hexcel, and TohoTenax through MIT’s Nano-
Engineered Composite aerospace STructures (NECST) Consortium.
This research was supported (in part) by the U.S. Army
Research Office under Contract W911NF-13-D-0001. This work
was performed in part at the Center for Nanoscale Systems
(CNS), a member of the National Nanotechnology Infrastructure
Network (NNIN), which is supported by the National
Science Foundation under NSF Award No. ECS-0335765. CNS
is part of Harvard University. This work was carried out in part
through the use of MIT Microsystems Technology Laboratories.
Stephan Hofmann acknowledges funding from EPSRC under
grant EP/H047565/1. Piran Kidambi acknowledges the
Lindemann Trust Fellowship.This is the final published version. It first appeared at http://pubs.acs.org/doi/abs/10.1021/ja509872y
First-principles calculation of positron lifetimes and affinities in perfect and imperfect transition-metal carbides and nitrides
Morphological effects on IR band profiles: Experimental spectroscopic analysis with application to observed spectra of oxygen-rich AGB stars
To trace the source of the unique 13, 19.5, and 28 m emission features
in the spectra of oxygen-rich circumstellar shells around AGB stars, we have
compared dust extinction spectra obtained by aerosol measurements. We have
measured the extinction spectra for 19 oxide powder samples of eight different
types, such as Ti-compounds (TiO, TiO, TiO, TiO,
AlTiO, CaTiO), -, -,
---AlO, and MgAlO in the infrared region
(10 - 50 m) paying special attention to the morphological (size, shape,
and agglomeration) effects and the differences in crystal structure. Anatase
(TiO) particles with rounded edges are the possible 13, 19.5 and 28 m
band carriers as the main contributor in the spectra of AGB stars, and
spherically shaped nano-sized spinel and AlTiO dust grains are possibly
associated with the anatase, enhancing the prominence of the 13 m feature
and providing additional features at 28 m. The extinction data sets
obtained by the aerosol and CsI pellet measurements have been made available
for public use at http://elbe.astro.uni-jena.deComment: 17 pages, 8 figures, Accepted 24 March 2009 for publication in A&
Age-related variations in evoked potentials to auditory stimuli in normal human subjects
Self-Diffusion Coefficient of fcc Mg: First-Principles Calculations and Semi-Empirical Predictions
The Precision Determination of Lattice Constants by the Powder and Rotating Crystal Methods and Applications
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