bluuebunny/crossref_metadata_2025_split huggingface dataset:
Complex Permittivity Studies of Polyaniline
Authors: Buckley Leonard J. , Dudeck Kenneth E. 
Published: March 2011

ABSTRACTA study of the complex dielectric properties at microwave and millimeterwave frequencies on several polyanilines was performed. The polymers investigated were based on emeraldine salts. The chemically synthesized emeraldine base was protonated with acids of various pH levels. Aqueous HCI (pH range 0.12 to 1.07) and Tosylic (pH range 0.22 to 1.13) were used. Complex permittivities of the emeraldine base and salts were determined at X-band and R-band using waveguide measurements. Characterization techniques were developed to reduce the measurement error. This included the electrical determination of the sample holder length and sample position. The real component of the dielectric constant was found to vary from 4 to 107 at Xband and from 3.8 to 926 at R-band. The dielectric loss spanned 3 orders of magnitude at X-band and 4 orders of magnitude at Rband. The frequency dependence was relatively level within each band with a slight decrease in each permittivity component with increasing frequency. The DC conductivity ranged from 10−4 S/cm to 10−1 S/cm for the emeraldine salts. The time dependence of the complex permittivity and DC conductivity was examined.
Observed Trends in The Dielectric Properties of Minerals at Elevated Temperatures
Authors: Holderfield Sonya P. , Salsman J. B. 
Published: March 2011

ABSTRACTResearchers at the U.S. Bureau of Mines' Tuscaloosa Research Center have measured the dielectric properties of over 60 minerals. Thirty-eight of these minerals have been identified as containing elements such as Co, Pt, and Ni, which are considered to be critical and strategic to the Nation. The dielectric properties were measured over the temperature range of 25° to325° C and microwave frequencies of 300 MHz to 3 GHz.The data were analyzed to determine if trends could be detected in the dielectric properties of minerals having the same crystalline structure and/or mineralogical classification. Such trends would make possible realistic assumptions about how these classes of minerals would be affected by microwave energy without specific knowledge of their dielectric properties.
Dynamic Model for Calculating Heating Patterns During Microwave Sintering
Authors: Tucker James , Smith Ray , Iskander Magdy F. , Andrade Octavio M. 
Published: March 2011

ABSTRACTAnalysis of dynamic development of heating patterns during microwave sintering provides vital information on the evolution of the heating process and the contributions from the various components in a complex sintering arrangement (picket fence) to the heat-transfer mechanism. Measured heating patterns often provide overall effects, and it is difficult to isolate and control the various contributions. To this end, results from numerical simulation may be significant.In this paper we describe a thermal model that calculates the temperature distribution in ceramic samples and insulation under realistic sintering conditions. The calculation process involves a two-step procedure. The first step is to calculate the microwave power deposition in the sample and surrounding insulation. 3D FDTD calculations described in a companion paper are used for this purpose [1] The other step involves calculation of the temperature distribution using a 3D finite-difference heat-transfer program developed in our department.Results illustrating the effect of thickness of insulation and the placement of SiC rods in picket-fence arrangement are presented. Also, the need to measure additional parameters such as thermal conductivity and density of green samples as a function of temperature during sintering is discussed.
High Temperature Dielectric Measurements on Ceramics
Authors: Aral M , Binner J G P , Carr G E , Cross T E 
Published: March 2011

ABSTRACTThe use of microwave frequency radiation as an alternative energy source for processing materials is currently receiving considerable worldwide attention. The ability of a material to extract energy from the microwave field depends upon its dielectric properties, however these change as the material heats. There is, therefore, a need for obtaining measurements of the microwave dielectric properties at elevated temperatures. This paper outlines some of the current work on high temperature dielectric measurements which is in progress at the University of Nottingham as part of the Materials and Microwave Processing Group initiative and some results on engineering ceramics are presented.
Multikilowatt Variable Frequency Microwave Furnace
Authors: Bible D. W. , Lauf R. J. , Everleigh C. A. 
Published: March 2011

ABSTRACTWe describe a new type of microwave processing furnace in which the frequency can be varied continuously from 4 to 8 GHz and the power level varied from zero up to 2.5 kW. The extraordinary bandwidth of this furnace is achieved by using a traveling wave tube (TWT) amplifier originally developed for electronic warfare applications. The TWT is a linear beam device characterized by a traveling electromagnetic wave that continuously extracts energy longitudinally along the path of an electron beam. The TWT, unlike other microwave tubes such as the magnetron, klystron, gyrotron, and others, does not depend upon resonant RF fields and is therefore capable of wide bandwidth operation.
Structural Studies of Submonolayers of Carbon Atoms on Graphite
Authors: Ge M. , Sattler K. , Xhie J. , Venkateswaran N. 
Published: March 2011

ABSTRACTSubmonolayer coverages of carbon adsorbed on highly-oriented pyrolytic graphite were examined by scanning tunneling microscopy under ultra-high vacuum condition. Linear carbon wires were found on atomically flat graphite surfaces. The wires had different thicknesses, from single atomic width to about lnm. The long wires extended to over several hundred nanometers. Two directions, graphite β-β direction and 30° rotated, were preferred for the long wire orientation. Parallel wire alignment, with several nanometers of inter-wire spacings were observed. Carbon particles, from 0.7 to 2 nm in diameter were found to be attached to the carbon wires. Particles from different wires formed parallel linear chains about perpendicular to the wire direction.
One- and Two-Dimensional Carbon Clusters: Isomers, Structures and Isomer Abundances.
Authors: Helden Gert von , Hsu Ming-Teh , Kemper Paul R. , Bowers Michael T. 
Published: March 2011

ABSTRACTThe new technique of ion structural chromatography is applied to carbon clusters. The results indicate that C5+ and C6+ are purely linear but C7+, C8+, C9+ and C1O+ have both linear and monocyclic ring structures. From C11+ to C20+ only monocyclic ring structures are observed. At C21+, a new family of planar ring structures appears. The first 3 dimensional structure occurs at C29+ and the first fullerene at C30+. Isomer structure is verified by the comparison of experimental mobilities with those derived from theory for the various structures. For C20+ only the monocyclic ring is observed experimentally but electronic structure calculations suggest more compact structures might be lower in energy. The results are discussed in terms of possible growth mechanisms for C60.
Determination of the Permeability of Carbon Aerogels by Gas Flow Measurements
Authors: Kong F-M. , Hulsey S.S. , Alviso C.T. , Pekala R.W. 
Published: March 2011

ABSTRACTCarbon aerogels are synthesized via the polycondensation of resorcinol and formaldehyde, followed by supercritical drying and pyrolysis at 1050 °C in nitrogen. Because of their interconnected porosity, ultrafine cell structure and high surface area, carbon aerogels have many potential applications, such as in supercapacitors, battery electrodes, catalyst supports, and gas filters. The performance of carbon aerogels in the latter two applications depends on the permeability or gas flow conductance in these materials. By measuring the pressure differential across a thin specimen and the nitrogen gas flow rate in the viscous regime, we calculated the permeability of carbon aerogels from equations based upon Darcy's law. Our measurements show that carbon aerogels have apparent permeabilities on the order of 10−12 to 10−10 cm2 for densities ranging from 0.44 to 0.05 g/cm3. Like their mechanical properties, the permeability of carbon aerogels follows a power law relationship with density and average pore size. Such findings help us to estimate the average pore sizes of carbon aerogels once their densities are known. This paper reveals the relationships among permeability, pore size and density in carbon aerogels.
C60 Transformations at High Pressures
Authors: Yoo C. S. , Nellis W. J. , Sattler M. L. , Musket R. G. , Hinsey N. , Brocious W. 
Published: March 2011

ABSTRACTC60 molecules have been studied at both shock and static high pressures. Under shock compressions C60 fullerenes are stable into the 13-17 GPa pressure range. The onset of a fast (∼0.5 μs) reconstructive transformation to graphite occurs near 17 GPa. The graphite recovered from 27 GPa and about 900 K is relatively well ordered with La = 100 Å. Above 50 GPa a continuous transformationto an amorphous state is observed in recovered specimens. A transparent, metastable carbon phase was recovered from thin films of C60, shocked to 69 GPa and 2200 K and then rapidly quenched to 1000 K. The selected area diffraction patterns indicate thatthe metastable carbon contains an amorphous diamond and n-diamond. Under hydrostatic compressions C60 molecules transform reversibly to a semi-transparent phase in the pressure range of 15-25 GPa with a large pressure hysteresis. The high pressure phaseconsists of interconnected strongly interacting C60 agglomerates, or networksof fullerenes, whose stability continuously increases with increase of pressure. Above 27 GPa the transition becomes irreversible, and the material recovered from high pressureis metastable and diamond-like at ambient conditions. These pressure-induced transitions are explained in terms of nr-electron rehybridization between C60 molecules, which occurs at substantially decreased intermolecular distances.
Rotations, Vibrations and Structure in Solid C60: Investigations by Neutron Scattering
Authors: Kamitakahara W. A. , Copley J. R. D. , Cappelletti R. L. , Rush J. J. , Neumann D. A. , Fischer J. E. , Mccauley J. P. , Smit A. B. 
Published: March 2011

ABSTRACTIn a series of experiments on solid C60, the low-energy rotational dynamics of the molecules, the higher-energy vibrational spectra, and aspects of disorder in the static structure, have been studied. The nearly spherical shape of the C60 molecule, and the occurrence of an orientational phase transition at Tc=256 K, make the C60 solid an excellent system for investigations of orientational dynamics. Coherent quasi-elastic neutron scattering above Tc is described quantitatively by a rotational diffusion model. Below Tc, inelastic scattering froma soft librational mode is observed between 2 and 3 meV. Models of the strong, temperaturedependent diffuse scattering due to orientational disorder are discussed. Finally, measurements of the density of states for the intramolecular ( E ≥ 33 meV) vibrational modes are also presented.
Raman Scattering and Superconductivity of Alkali-Fullerene Materials
Authors: Lannin J. S. , Mitch M. G. , Bacsa W. , Chase S. J. 
Published: March 2011

ABSTRACTRaman scattering measurements in alkali—fullerene alloys in ultrathin and thin films provide evidence for variations in electron—phonon coupling. For x — 3similar behavior of Rb3 C60 films of different thickness support substantial electron—phonon induced damping of specific Hg(i) modes derived from intramolecular modes of C60. In 400A thick films a reduction of induced scattering from Raman inactive C60 modes substantiates the importance ofHg(2), but not Hg(3) modes for phonon—mediated superconductivity. In contrast to RbxC60 and KxC60 ultrathin film solid solutions, similar Raman spectra for NaxC60 indicate substantially reduced coupling consistent with the absence of superconductivity in this system.
A Model Calculation for the Vibrational Modes in C60
Authors: Jishi R.A. , Mirje R.M. , Dresselhaus M.S. 
Published: March 2011

ABSTRACTA force constant model for the vibrational modes in C60, based on bond-stretching and anglebending interactions, is presented. The results of this model are compared with the experimental data available from Raman, infrared, and high resolution electron energy loss spectroscopies, as well as neutron inelastic scattering measurements. Excellent agreement is obtained between the calculated and experimental mode frequencies. The pressure dependence of the Ramanand infrared-active mode frequencies is calculated within a simple model and is compared to available experimental data.
Dodecahedron- and Bowl-Shaped Structures C20
Authors: Slanina Zdenék , Adamowicz Ludwik 
Published: March 2011

ABSTRACTPurely carbonaceous aggregates C20have been studied by the AM1 quantumchemical method. In addition to one dodecahedron-shaped structure possessingC1symmetry another three-dimensional species is revealed, viz. a bowl-shaped structureofC5vsymmetry (and also one two-dimensional and two one-dimensional species). Temperature dependence of the relative stabilities of both three-dimensional structures is evaluated, showing that in the relevant temperature region the fullerenic species is prevailing. However, in a very high temperature region a relative-stability interchange has been predicted.
Study of Linear and Nonlinear Microscopic Polarizabilities of C60
Authors: Westin Erik , RosÉn Arne 
Published: March 2011

ABSTRACTThe low energy part of the linear optical response spectrum and lowest order hyperpolarizability γ(3) of C60 are calculated by a sum over state approach using single particle wavefunctions and. energy levels determined from local density calculations. Lorentz local field factors, as well as a RPA correction are introducedto facilitate comparison with the dielectric function ε(ω) determined fromfilms of C60. γ( 2 ) for a centrosymmetric molecule such asC60 is zero and the lowest non-zero contribution to the polarizability is γ( 3 ). Reasonable agreement is found with linear optical response experiments if a RPA screening is used. However, SHG and THG experiments on C60 in solid or solution form, yields values closer to the unscreened results.
Photoemission Study of Single Crystal C60
Authors: Wu J. , Shen Z.-X. , Dessau D. S. , Cao R. , Marshall D. S. , Pianetfa P. , Lindau I. , Yang X. , Terry J. , King D. M. , Wells B. O. 
Published: March 2011

ABSTRACTWe report angle-resolved photoemission data from single crystals of C60 cleaved in UHV. Unlike the other forms of pure carbon, the valence band spectrum of C60 consists of many sharp features that can be essentially accounted for by the quantum chemical calculations describing individual molecules. This suggests that the electronic structure of solid C60 is mainly determined by the bonding interactions within the individual molecules. We also observe remarkable intensity modulations of the photoemission features as a function of photon energy, suggesting strong final state effects. Finally, we address the issue of the band width of the HOMO state of C60. We assert that the width of the photoemission peak of C60 does not reflectthe intrinsic band width.
An Nmr Study of the Occupation of C60 Interstitial Sites by Oxygen Molecules
Authors: Assink Roger A. , Loy Douglas A. , Schirber James E. , Morosin Bruno 
Published: March 2011

ABSTRACTThe 13C NMR of FCC C60 under magic angle spinning (MAS) conditionsyields linewidths on the order of 1 Hz at fields of 4.7 T. The spectrum consists of a primary resonance at 143.7 ppm and a minor peak shifted 0.7 ppm downfield. The intensity of the minor resonance relative to the primary resonance was found to vary from 0.6 to 5.5 % depending on the sample history. The downfield shift obeys Curie's law and isattributed to the Fermi contact coupling interaction between paramagnetic oxygen molecules and all 60 carbon atoms of rapidly rotating adjacent C60 molecules. Exposure of the sample to 1 kbar oxygen for 1 3/4 hours resulted in a spectrum of 7 evenly spaced resonances corresponding to 0 to 6 of the adjacent octahedral interstitial sites being filled with oxygen molecules. At ambient pressure, the oxygen diffused out of the lattice on time scales ranging from hours to days.
Theory for New Carbon-Based Materials
Authors: Brenner D.W. , Mowrey R.C. , Mintmire J. W. , Harrison J.A. , Robertson D.H. , Lyons M. , Dunlap B.I. , White C.T. 
Published: March 2011

ABSTRACTWe review results of our local-density-functional-based cluster calculations and molecular dynamics simulations of fullerenes and related structures. These include predictions of cohesive energies, electronic structures, and photoelectron spectra for a number of pure and chemically substituted fullerenes, studies of the resilience of C60 under severe compression and during surface collisions, simulations of the trapping of Hein the interior of C60, predictions of the strain energy, electronic and elastic properties of graphitic tubules, and simulations of the folding and curling of graphitic ribbons.
Novel Hybrid Composites Based on Carbon Foams
Authors: Hager Joseph W. , Lake Max L. 
Published: March 2011

ABSTRACTThe extraordinary mechanical properties of commercial carbon fiber are due to the unique graphitic morphology of the spun filaments. Contemporary advanced structural composites exploit these properties by creating a disconnected network of graphitic filaments held together by an appropriate matrix. Carbon foam derived from a blown mesophase pitch precursor can be considered to be an interconnected network of graphitic ligaments. As such interconnected networks, they represent a potential alternative reinforcing phase for structural composite materials. Based on this notion, consideration is given to novel forms of graphitic carbon and the processing routes to create hybrid composites, such as net shape fabrication and fiber placement processes.
Polarons and Triplet Excitons in C60: An Optically Detected Magnetic Resonance (Odmr) and Light-Induced Esr (Lesr) Study
Authors: Lane P. A. , Swanson L. S. , Ni Q.-X. , Shinar J. , Engel J , Barton T. J. , Wheelock J. , Jones L. 
Published: March 2011

ABSTRACTThe photoluminescence (PL), X-band ODMR. and LESR of C60 films and C60 isolated in a toluene/polystyrene glass matrix (C60:T/PS) is described. The delocalized triplet LESR and ODMR of C60:T/PS are similar to previously reported LESR. In films, however, the ODMR indicates that the delocalized triplet is larger and distorted by neighboring molecules. In addition, another, localized triplet, and a narrow PL-enhancing line at g = 2.0017 ± 0.0005 are observed. These features are similar to those observed in several π-conjugated polymers. The former is attributed to a localized triplet of size similar to a 5- or 6-membered ring, tentatively on a face adjacent to another C60 molecule; the latter is believed to result from intermolecular polaron recombination.
Early Nucleation of Diamond in a Combustion Flame
Authors: McClure Michael T. , von Windheim Jesko A. , Glass Jeffrey T. , Prater John T. 
Published: March 2011

ABSTRACTThe early stages of a gas phase pretreatment for diamond nucleation in an oxy-acetylene flame were investigated. The pretreatment involved a low oxygen-to-acetylene ratio (Rf = 0.93) performed at 15 mm from the torch on a scratched Si substrate. The nature of the carbon species deposited was analyzed using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy. The pretreatment process produced diamond crystals after 30 seconds and a complete film in the center of the deposition area after 180 seconds. Furthermore, the pretreatment process kept the initial Siob2 layer less than the thickness generated by the control conditions, (Rf = 0.97, d = 10mm). The success of the pretreatment was attributed to changes in the flame structure and chemistry from the control conditions.
In-Vacuo Surface Analytical Study of Diamond Nucleation on Copper Vs. Silicon
Authors: Wolter S. D. , Stoner B. R. , Ma G.-H. M. , Glass J. T. 
Published: March 2011

ABSTRACTA study was performed on polycrystalline copper versus that of Si(100) utilizing a negative substrate bias to enhance diamond nucleation. The biasing pretreatment and subsequent growth of the diamond were performed via microwave plasma chemical vapor deposition and the initial stages of nucleation were characterized by in-vacuo surface analysis. The biasing pretreatment step proved to have a tremendous influence on the nucleation density pertaining to Si(100) substrates, however, the nucleation density on polycrystalline copper was only increased slightly. A highly graphitic surface coverage of roughly 10A was evident on the copper substrates prior to the detection of diamond and was quite stable in thickness in as early as 15 minutes of biasing. The Si(100) substrates, however, were characterized by the formation of a carbide with some form of nondiamond carbon present on the surface throughout the biasing pretreatment.
Defect Structure of Diamond Film at the Interface with Amorphous Carbon
Authors: Chang Li 
Published: March 2011

ABSTRACTDiamond films prepared by microwave plasma chemical vapor deposition, using a gas mixture of methane and hydrogen with ethanol, were formed on silicon substrates. Highresolution transmission electron microscopy was employed to characterize the microstructure at interface regions. It was found that the diamond crystals were grown on an amorphous carbon layer. Twins and stacking faults were observed at the regions interfaced with the amorphous carbon layer, suggesting that the defects may already exist in the nucleation stage and at the very first stage of growth. Also, some diamond nuclei embedded in the amorphous layer were observed.
Theoretical Investigation of Fluorinated and Hydrocenated Diamond <100> Filks
Authors: Pederson Mark R. , Pickett Warren E. 
Published: March 2011

ABSTRACTTo investigate some of the fundamental differences between halogen and hydrogen assisted diamond film growth we have performed several calculations related to the <100> diamond surface. The models used in these investigations include ten-layer periodic slabs of free standing fluorinated diamond films as well as isolated clusters [C21F6H20]. For purposes of comparison, we have also performed calculations on models of the hydrogenated <100> surface. The calculations are performed within the density-functional framework using LCAO and LAPW computational methods. We have considered two geometries of a monofluoride surface. The first surface, best described as an ideal l×l surface with a monolayer of ionically bonded fluorines, exhibits a metallic density of states in contrast to a 2×l reconstructed surface with chemically bonded fluorines that is found to be insulating. We compare theoretical carbon core level shifts with experimental values and discuss growth models based on these surface calculations.