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Search Results: 1 - 10 of 31043 matches for " Z. Lockman "
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Growth Mechanism of Cubic-Silicon Carbide Nanowires
K. Y. Cheong,Z. Lockman
Journal of Nanomaterials , 2009, DOI: 10.1155/2009/572865
Abstract: Cubic-SiC nanowires were synthesized using activated carbon powder and Si substrate in vacuum at 1200–1350°C for 1–4 hours. The nanowires were grown according to the following proposed mechanisms: (1) diffusion of C/CO into Si substrate, (2) weakening of Si bond and atomic kick-out, (3) formation of Si-C in vapor phase, (4) formation of saturated SiC layer, (5) formation of pyramid-like SiC nanostructure, and (6) formation of SiC nanowires.
Protective Agent-Free Synthesis of Colloidal Cobalt Nanoparticles
M.D.L. Balela,Z. Lockman,A. Azizan,E. Matsubara
Journal of Physical Science , 2008,
Abstract: Herein, we report a facile low-temperature (80°C) synthesis route for colloidal cobalt (Co) nanoparticles by Co2+ reduction using hydrazine (N2H4) in a basic solution of ethylene glycol (EG). The colloidal particles were spherical and monodispersed, with mean diameter ranging from 2–7 nm increasing with the Co chloride concentration. However, precipitation of the nanoparticles resulted in weakly agglomerated spherical Co structures of submicron size. X-ray diffraction (XRD) of the precipitated powder revealed hexagonal close-packed-Co (hcp-Co). But since the major peaks of hcp and face-centered cubic-Co (fcc-Co) are overlapping, the presence of fcc-Co cannot be ruled out. Besides precursor concentration, it was found that pH, reaction temperature, and molar ratio of N2H4 to Co2+ also influence the reduction rate. An alkaline medium, elevated temperature (80°C) and high molar ratio of N2H4 to Co2+ promoted the formation of colloidal Co nanoparticles. Nonetheless, when the molar ratio of N2H4 to Co2+ was greater than 12, the mean particle size remained almost constant.
Improved Current Densities in MgB2 By Liquid-Assisted Sintering
S. K. Chen,Z. Lockman,M. Wei,B. A. Glowacki,J. L. MacManus-Driscoll
Physics , 2005, DOI: 10.1063/1.1947374
Abstract: Polycrystalline MgB2 samples with GaN additions were prepared by reaction of Mg, B, and GaN powders. The presence of Ga leads to a low melting eutectic phase which allowed liquid phase sintering and produces plate-like grains. For low-level GaN additions (5% at. % or less), the critical transition temperature, Tc, remained unchanged and in 1T magnetic field, the critical current density, Jc was enhanced by a factor of 2 and 10, for temperatures of \~5K and 20K, respectively. The values obtained are approaching those of hot isostatically pressed samples.
The Milky Way and its gas: Cold fountains and accretion
Lockman F.J.
EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20121908003
Abstract: The Milky Way is acquiring gas from infalling high-velocity clouds. The material enters a disk-halo interface that in many places is populated with HI clouds that have been ejected from the disk through processes linked to star formation. The Smith Cloud is an extraordinary example of a high-velocity cloud that is bringing > 106 M☉ of relatively low metallicity gas into the Milky Way. It may be part of a larger stream, components of which are now passing through the disk.
HI and Galactic Structure
Felix J. Lockman
Physics , 2002,
Abstract: HI observations in the 21cm line have been a principal tool for investigating the large-scale structure of the Milky Way. This review considers what was learned in the first decade after the discovery of the 21cm line, and how that knowledge has been expanded and refined in subsequent years. Topics include spiral structure, the Galactic nucleus, the thickness of the HI layer, and affairs in the outskirts of the Galaxy. New advances in instrumentation and computing, and a broad attack on problems using information from all wavelengths, are likely to keep HI studies of the Milky Way interesting for years to come.
High Velocity Cloud Complex H: A Satellite of the Milky Way in a Retrograde Orbit?
Felix J. Lockman
Physics , 2003, DOI: 10.1086/376961
Abstract: Observations with the Green Bank Telescope of 21cm HI emission from the high-velocity cloud Complex H suggest that it is interacting with the Milky Way. A model in which the cloud is a satellite of the Galaxy in an inclined, retrograde circular orbit reproduces both the cloud's average velocity and its velocity gradient with latitude. The model places Complex H at approximately 33 kpc from the Galactic Center on a retrograde orbit inclined about 45 degrees to the Galactic plane. At this location it has an HI mass > 6 10^6 Msun and dimensions of at least 10 by 5 kpc. Some of the diffuse HI associated with the cloud has apparently been decelerated by interaction with Galactic gas. Complex H has similarities to the dwarf irregular galaxy Leo A and to some compact high-velocity clouds, and has an internal structure nearly identical to parts of the Magellanic Stream, with a pressure P/k about 100 cm^{-3} K.
Discovery of a Population of HI Clouds in the Galactic Halo
Felix J. Lockman
Physics , 2002, DOI: 10.1086/345495
Abstract: A population of discrete HI clouds in the halo of the inner Galaxy has been discovered in 21cm observations made with the Green Bank Telescope. The halo clouds are seen up to 1.5 kpc from the Galactic plane at many longitudes. Their velocities follow Galactic rotation. A group of clouds chosen for detailed study are found to have, in the median, a peak NHI of 2x10^19, a diameter of a few tens of pc, a density of a few tenths cm^-3, and a mass in HI of 50 solar masses. Some halo clouds have narrow lines implying that their temperature must be <1000 K; some have a core-halo spectral structure. As much as half of the mass of the neutral halo may reside in these discrete clouds. They may have been formed in a Galactic fountain and are now returning to the disk.
Accretion Onto the Milky Way: The Smith Cloud
Felix J. Lockman
Physics , 2015,
Abstract: Active gas accretion onto the Milky Way is observed in an object called the Smith Cloud, which contains several million solar masses of neutral and warm ionized gas and is currently losing material to the Milky Way, adding angular momentum to the disk. It is several kpc in size and its tip lies two kpc below the Galactic plane. It appears to have no stellar counterpart, but could contain a stellar population like that of the dwarf galaxy Leo P. There are suggestions that its existence and survival require that it be embedded in a dark matter halo of a few 10^8 solar masses.
Nearby Galaxies: Templates for Galaxies Across Cosmic Time
F. J. Lockman,J. Ott
Physics , 2009,
Abstract: Studies of nearby galaxies including the Milky Way have provided fundamental information on the evolution of structure in the Universe, the existence and nature of dark matter, the origin and evolution of galaxies, and the global features of star formation. Yet despite decades of work, many of the most basic aspects of galaxies and their environments remain a mystery. In this paper we describe some outstanding problems in this area and the ways in which large radio facilities will contribute to further progress.
G28.17+0.05, A Giant Atomic/Molecular Cloud
Anthony Minter,Felix J. Lockman
Physics , 2001,
Abstract: HI 21 cm observations with the NRAO 140 Foot telescope have revealed a giant HI cloud (G28.17+0.05) in the Galactic plane that has unusual properties. The cloud is 150 pc in diameter, is at a distance of 5 kpc, and contains as much as 100,000 Solar Masses of atomic hydrogen. The cloud consists of a cold core, T~10 K, and a hotter outer envelope, T < 200 K. There is no observable difference in the HI line widths, ~7 km/s, between the core and the envelope. Anomalously-excited 1720 MHz OH emission, with a similar line width, is associated with the core of the cloud. The cloud core also exhibits 12CO and 13CO self-absorption which indicates that most of the cloud mass is in molecules. The total mass of the cloud is greater than 200,000 Solar Masses. The cloud has only a few sites of current star formation. If similar clouds are associated with other observed sites of anomalously-excited 1720 MHz OH emission, there may be as many as 100 more of these objects in the inner galaxy.
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