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Melting of Copper and Nickel at high pressure: the role of d-electrons

Description: Melting curves of Cu and Ni were measured to 97 GPa (3800 K) and 60 GPa (2970 K), respectively, in the laser-heated diamond cell. The measured melting temperatures of Cu are in good agreement with recent theoretical calculations. The melting slope (dT/dP) of Cu, which has a filled d-electron band, is about 2.5 times steeper than for Ni, which with one less electron, has a partially unfilled d-electron band. The relatively low melting slope obtained for Ni, measured using identical experimental methods as for Cu, is consistent with our previous measurements for other transition metals with partially filled d-bands, which are in serious disagreement with theoretical estimates. The present results confirm the key role d-shell electrons play in determining the high pressure melting curves.
Date: April 11, 2005
Creator: Japel, S; Boehler, R & Ross, M
Partner: UNT Libraries Government Documents Department

A Steinberg-Guinan model for High-Pressure Carbon, Diamond Phase

Description: Since the carbon, diamond phase has such a high yield strength, dynamic simulations must account for strength even for strong shock waves ({approx} 3 Mbar). We have determined an initial parametrization of two strength models: Steinberg-Guinan (SG) and a modified or improved SG, that captures the high pressure dependence of the calculated shear modulus up to 10 Mbar. The models are based upon available experimental data and on calculated elastic moduli using robust density functional theory. Additionally, we have evaluated these models using hydrodynamic simulations of planar shocks experiments.
Date: July 27, 2007
Creator: Orlikowski, D; Correa, A; Schwegler, E & Klepeis, J
Partner: UNT Libraries Government Documents Department

Final Report. IUT No. B560420 with UC Berkeley. Organic Chemistry at High Pressures &Temperatures

Description: We have successfully completed the research outlined in our proposal: Organic Chemistry at High Pressures and Temperatures. We have experimentally determined a phase diagram which documents the phases and reaction regimes of cyanuric acid , H{sub 3}C{sub 3}N{sub 3}O{sub 3} (1,3,5-triazine-2,4,6-trione), from 300 - 750 K and 0 - 8.1 GPa. We utilized a comparatively new technique to study thin samples of cyanuric acid in the diamond anvil cell in order to collect ambient temperature, high pressure FTIR and Raman data as well as the high-pressure, high-temperature data used in the phase diagram. These experiments made use of the CMLS High-pressure lab's diamond anvil facilities as well as the FTIR and Raman systems.
Date: March 20, 2007
Creator: Montgomery, W; Crowhurst, J C; Zaug, J M & Jeanloz, R
Partner: UNT Libraries Government Documents Department

Asymptotic Diffusion-Limit Accuracy of Sn Angular Differencing Schemes

Description: In a previous paper, Morel and Montry used a Galerkin-based diffusion analysis to define a particular weighted diamond angular discretization for S{sub n}n calculations in curvilinear geometries. The weighting factors were chosen to ensure that the Galerkin diffusion approximation was preserved, which eliminated the discrete-ordinates flux dip. It was also shown that the step and diamond angular differencing schemes, which both suffer from the flux dip, do not preserve the diffusion approximation in the Galerkin sense. In this paper we re-derive the Morel and Montry weighted diamond scheme using a formal asymptotic diffusion-limit analysis. The asymptotic analysis yields more information than the Galerkin analysis and demonstrates that the step and diamond schemes do in fact formally preserve the diffusion limit to leading order, while the Morel and Montry weighted diamond scheme preserves it to first order, which is required for full consistency in this limit. Nonetheless, the fact that the step and diamond differencing schemes preserve the diffusion limit to leading order suggests that the flux dip should disappear as the diffusion limit is approached for these schemes. Computational results are presented that confirm this conjecture. We further conjecture that preserving the Galerkin diffusion approximation is equivalent to preserving the asymptotic diffusion limit to first order.
Date: November 5, 2009
Creator: Bailey, T S; Morel, J E & Chang, J H
Partner: UNT Libraries Government Documents Department

Defect-related internal dissipation in mechanical resonators and the study of coupled mechanical systems.

Description: Understanding internal dissipation in resonant mechanical systems at the micro- and nanoscale is of great technological and fundamental interest. Resonant mechanical systems are central to many sensor technologies, and microscale resonators form the basis of a variety of scanning probe microscopies. Furthermore, coupled resonant mechanical systems are of great utility for the study of complex dynamics in systems ranging from biology to electronics to photonics. In this work, we report the detailed experimental study of internal dissipation in micro- and nanomechanical oscillators fabricated from amorphous and crystalline diamond materials, atomistic modeling of dissipation in amorphous, defect-free, and defect-containing crystalline silicon, and experimental work on the properties of one-dimensional and two-dimensional coupled mechanical oscillator arrays. We have identified that internal dissipation in most micro- and nanoscale oscillators is limited by defect relaxation processes, with large differences in the nature of the defects as the local order of the material ranges from amorphous to crystalline. Atomistic simulations also showed a dominant role of defect relaxation processes in controlling internal dissipation. Our studies of one-dimensional and two-dimensional coupled oscillator arrays revealed that it is possible to create mechanical systems that should be ideal for the study of non-linear dynamics and localization.
Date: January 1, 2007
Creator: Friedmann, Thomas Aquinas; Czaplewski, David A.; Sullivan, John Patrick; Modine, Normand Arthur; Wendt, Joel Robert; Aslam, Dean (Michigan State University, Lansing, MI) et al.
Partner: UNT Libraries Government Documents Department

Electrical Conductivity of the Lower-Mantle Ferropericlase

Description: Electrical conductivity of the lower-mantle ferropericlase-(Mg{sub 0.75},Fe{sub 0.25})O has been studied using designer diamond anvils to pressures over one megabar and temperatures up to 500 K. The electrical conductivity of (Mg{sub 0.75},Fe{sub 0.25})O gradually rises by an order of magnitude up to 50 GPa but decreases by a factor of approximately three between 50 to 70 GPa. This decrease in the electrical conductivity is attributed to the electronic high-spin to low-spin transition of iron in ferropericlase. That is, the electronic spin transition of iron results in a decrease in the mobility and/or density of the charge transfer carriers in the low-spin ferropericlase. The activation energy of the low-spin ferropericlase is 0.27 eV at 101 GPa, similar to that of the high-spin ferropericlase at relatively low temperatures. Our results indicate that low-spin ferropericlase exhibits lower electrical conductivity than high-spin ferropericlase, which needs to be considered in future geomagnetic models for the lower mantle. The extrapolated electrical conductivity of the low-spin ferropericlase, together with that of silicate perovskite, at the lower mantle pressure-temperature conditions is consistent with the model electrical conductivity profile of the lower mantle.
Date: April 19, 2007
Creator: Lin, J F; Weir, S T; Jackson, D D; Evans, W J; Vohra, Y K; Qiu, W et al.
Partner: UNT Libraries Government Documents Department

Tuning magnetic frustration on the diamond lattice of the A-site magnetic spinels CoA12-xGax04: lattice expansion versus site disorder

Description: The spinels CoB{sub 2}O{sub 4} with magnetic Co{sup 2+} ions on the diamond lattice A site can be frustrated because of competing near-neighbor (J{sub 1}) and next-near neighbor (J{sub 2}) interactions. Here we describe attempts to tune the relative strengths of these interactions by substitution on the non-magnetic B-site. The system we employ is CoAl{sub 2-x}Ga{sub x}O{sub 4}, where Al is systematically replaced by the larger Ga, ostensibly on the B site. As expected, Ga substitution expands the lattice, resulting in Co atoms on the A-site being pushed further from one other and thereby weakening magnetic interactions. In addition, Ga distributes between the B and the A site in a concentration dependent manner displacing an increasing amount of Co from the A site with increasing x. This increased inversion, which is confirmed by neutron diffraction studies carried out at room temperature, affects magnetic ordering very significantly, and changes the nature of the ground state. Modeling of the magnetic coupling illustrates the complexity that arises from the cation site disorder.
Date: January 1, 2008
Creator: Proffen, Thomas E; Melot, Brent C; Page, Katharine; Seshadri, Ramzy; Stoudenmire, E M; Balents, Leon et al.
Partner: UNT Libraries Government Documents Department

Preliminary numerical modeling results - cone penetrometer (CPT) tip used as an electrode

Description: Figure 1 shows the resistivity models considered in this study; log10 of the resistivity is shown. The graph on the upper left hand side shows a hypothetical resisitivity well log measured along a well in the upper layered model; 10% Gaussian noise has been added to the well log data. The lower model is identical to the upper one except for one square area located within the second deepest layer. Figure 2 shows the electrode configurations considered. The ''reference'' case (upper frame) considers point electrodes located along the surface and along a vertical borehole. The ''CPT electrode'' case (middle frame) assumes that the CPT tip serves as an electrode that is electrically connected to the push rod; the surface electrodes are used in conjuction with the moving CPT electrode. The ''isolated CPT electrode'' case assumes that the electrode at the CPT tip is electrically isolated from the pushrod. Note that the separate CPT push rods in the middle and lower frames are shown separated to clarify the figure; in reality, there is only one pushrod that is changing length as the probe advances. Figure 3 shows three pole-pole measurement schemes were considered; in all cases, the ''get lost'' electrodes were the leftmost and rightmost surface electrodes. The top frame shows the reference scheme where all surface and borehole electrodes can be used. The middle frame shows two possible configurations available when a CPT mounted electrode is used. Note that only one of the four poles can be located along the borehole at any given time; electrode combinations such as the one depicted in blue (upper frame) are not possible in this case. The bottom frame shows a sample configuration where only the surface electrodes are used. Figure 4 shows the results obtained for the various measurement schemes. The white lines ...
Date: December 19, 2006
Creator: Ramirez, A L
Partner: UNT Libraries Government Documents Department

The structure of a-C: What NEXAFS and EXAFS see

Description: Mechanically hard ha-C and soft sa-C amorphous carbon films of 2.9 and 2.2 g cm-3 approximate densities were prepared by filtered cathodic arc deposition and analyzed by near-edge x-ray absorption spectroscopy NEXAFS and extended x-ray absorption spectroscopy EXAFS to determine their structure. The analysis observed an insignificant level of pi bond conjugation in both kind of films. EXAFS distinguished two types of atomic environments in them: one semiordered with well defined bond lengths, and the other with so strong bond disorder that its contribution to EXAFS was undetectable. The proportion of atoms in the semiordered atomic environments was of less than 40percent in both films. Their bond lengths were similar to those of diamond in the ha-C films and to graphite in the sa-C. NEXAFS spectra analysis was based on the linear relation between sigma* energy and bond length. It served to quantify the proportion of sp3 bonded atoms in a-C, to deduce the average bond length of the atoms undetected by EXAFS, and to determine the level of bond conjugation in the films. The sp3 concentration estimated with the proposed method was of 44percent in the ha-C films and 10percent in the sa-C films. These values were consistent with the EXAFS results, but disagreed with those obtained based on the traditional pi*/sigma* intensity ratio method which overestimated sp3 concentrations. Annealing of the ha-C films up to its almost complete graphitization caused a gradual reduction in bond length of the semiordered environments with no differentiation between two phases, diamondlike and graphitelike, at any temperature. This resultsupport models that explain sp3 bond promotion in a-C as caused by the high compressive stress attained by a strongly disordered sp2 dense structure during film deposition.
Date: August 1, 2006
Creator: Hussain, Zahid; Diaz, J.; Monteiro, O.R. & Hussain, Z.
Partner: UNT Libraries Government Documents Department

DIAMOND SECONDARY EMITTER

Description: We present the design and experimental progress on the diamond secondary emitter as an electron source for high average power injectors. The design criteria for average currents up to 1 A and charge up to 20 nC are established. Secondary Electron Yield (SEY) exceeding 200 in transmission mode and 50 in emission mode have been measured. Preliminary results on the design and fabrication of the self contained capsule with primary electron source and secondary electron emitter will also be presented.
Date: October 9, 2005
Creator: BEN-ZVI, I.; RAO, T.; BURRILL, A.; CHANG, X.; GRIMES, J.; RANK, J. et al.
Partner: UNT Libraries Government Documents Department

Recovery of CVD Diamond Detectors using Laser Double Pulses

Description: A 5 x 0.25 mm Chemical Vapor Deposited (CVD) diamond detector, with a voltage bias of + 250V, was excited by a 400 nm laser (3.1 eV photons) in order to study the saturation of the wafer and its associated electronics. In a first experiment, the laser beam energy was increased from a few tens of a pJ to about 100 {micro}J, and the signal from the diamond was recorded until full saturation of the detection system was achieved. Clear saturation of the detection system was observed at about 40 V, which corresponds with the expected saturation at 10% of the applied bias (250V). The results indicate that the interaction mechanism of the 3.1 eV photons in the diamond (E{sub bandgap} = 5.45 eV) is not a multi-photon process but is linked to the impurities and defects of the crystal. In a second experiment, the detector was irradiated by a saturating first laser pulse and then by a delayed laser pulse of equal or smaller amplitude with delays of 5, 10, and 20 ns. The results suggest that the diamond and associated electronics recover within 10 to 20 ns after a strong saturating pulse.
Date: September 27, 2005
Creator: Dauffy, L S; Lerche, R A; Schmid, G J; Koch, J A & Silbenagel, C
Partner: UNT Libraries Government Documents Department

Static High Pressure Structural studies on Dy to 119 GPa

Description: Structural phase transitions in the rare-earth metal Dysprosium have been studied in a Diamond Anvil Cell (DAC) to 119 GPa by x-ray diffraction. Four transformations following the sequence hcp {yields} Sm-type {yields} dhcp {yields} hR24 (hexagonal) {yields} bcm (monoclinic) are observed at 6, 15, 43, and 73 GPa respectively. The hexagonal to monoclinic transformation is accompanied by a 6% reduction in volume, which is attributed to delocalization of the 4f electrons, similar to that seen in Ce, Pr, and Gd.
Date: November 12, 2003
Creator: Patterson, J R; Saw, C K & Akella, J
Partner: UNT Libraries Government Documents Department

ELECTRON AMPLIFICATION IN DIAMOND.

Description: We report on recent progress toward development of secondary emission ''amplifiers'' for photocathodes. Secondary emission gain of over 300 has been achieved in transmission mode and emission mode for a variety of diamond samples. Techniques of sample preparation, including hydrogenation to achieve negative electron affinity (NEA), have been adapted to this application.
Date: July 10, 2006
Creator: SMEDLEY, J.; BEN-ZVI, I.; BURRILL, A.; CHANG, X.; GRIMES, J.; RAO, T. et al.
Partner: UNT Libraries Government Documents Department

First Observation of an Electron Beam Emitted from a Diamond Amplified cathode

Description: We observed, for the first time, the emission of an electron beam from a hydrogenated diamond in the emission mode on a phosphor screen. Our experimental device is based on the following concept: the primary electrons of a few keV energy generate a large number of secondary electron-hole pairs in a hydrogenated diamond, and then the secondary electrons are transmitted to the opposite face of the diamond and emitted from its negative-electron-affinity (NEA) surface. Under our present conditions, the maximum emission gain of the primary electron is about 40, and the bunch charge is 50pC/0.5mm{sup 2}. Our achievement led to new understanding of the hydrogenated surface of the diamond. We propose an electron-trapping mechanism near the hydrogenated surface. The probability of electron trapping in our tests is about 70%. The hydrogenated diamond was demonstrated to be extremely robust. After exposure to air for days, the sample exhibited no observable degradation in emission.
Date: May 4, 2009
Creator: Chang,X.; Ben-Zvi, I.; Burrill, A.; Kewisch, J.; Rao, t.; Smedley, J. et al.
Partner: UNT Libraries Government Documents Department

DIFFERENTIAL THERMAL ANALYSIS OF IRRADIATED DIAMOND AND SILICON CARBIDE

Description: It was demonstrated by differential thermal analysis (DTA) that: 1. Catastrophic amounts of energy can be stored in diamond. 2. Even at low irradiations, the release takes place over several hundred degrees, indicating a spectrum of activation energies. 3. At higher irradiations, the stored energy release is considerably less than the increased energy content and seems not to have been completely released even at the highest temperatures reached. 4. There is some indication of an increased heat capacity below the temperature of stored energy release. It was shown by DTA that large amounts of energy can be stored in silicon carbide on irradiation. The release was found to be spread out over a greater range of temperatures than in diamond and indicated a larger and higher group of activation energies. Catastrophic release was not achieved. The amount of stored energy released over the range of temperatures used was 140 cal/g in a sample irradiated in a water-cooled testhole at HEW for an exposure of 265 Mwd/aT. (auth)
Date: December 1, 1954
Creator: Primak, W.
Partner: UNT Libraries Government Documents Department

The utility of diamond sensors for space flight

Description: Los Alamos National Laboratory has developed diamond sensors with interdigitated electrodes that operate in a photoconducting mode. The specific application for this work was for the Department of Energy`s instruments flown on the Global Positioning System satellites. Sensors have been fabricated and tested for their response to low-energy x-rays. These sensors can be operated to extremely high volumetric radiation doses. We find that the sensors are extremely useful for situations where the surface radiation dose is not excessive, but that this limit is exceeded for the GPS orbit. It is possible that further studies and special detector arrangements or auxiliary heating of the sensor may push this limit to higher values.
Date: March 1996
Creator: Higbie, P. R.; Han, S. S. & Wagner, R. S.
Partner: UNT Libraries Government Documents Department

Simplified models of growth, defect formation, and thermal conductivity in diamond chemical vapor deposition

Description: A simplified surface reaction mechanism is presented for the CVD of diamond thin films. The mechanism also accounts for formation of point defects in the diamond lattice, an alternate, undesirable reaction pathway. Both methyl radicals and atomic C are considered as growth precursors. While not rigorous in all details, the mechanism is useful in describing the CVD diamond process over a wide range of reaction conditions. It should find utility in reactor modeling studies, for example in optimizing diamond growth rate while minimizing defect formation. This report also presents a simple model relating the diamond point-defect density to the thermal conductivity of the material.
Date: April 1, 1996
Creator: Coltrin, M.E. & Dandy, D.S.
Partner: UNT Libraries Government Documents Department

Numerical analysis of the ultraprecision machining of copper

Description: Modeling of the ultraprecision machining process can aid in the understanding of the relative importance of various process parameters and ultimately lead to improved methods of generating ultraprecision surfaces such as those required for metal optics and single crystal microelectronics substrates. Any modeling method should be verified by direct comparison to experimental data. Until recently it has been difficult to accurately measure the cutting edge, or sharpness, of a diamond tool; and therefore, most models have assumed an infinitely sharp cutting tip. With the relatively new technology of the Atomic Force Microscope (AFM), the cutting edge of single crystal diamond tools can be quantitatively described. Ultraprecision machining experiments using an AFM characterized cutting tool and orthogonal geometry have been performed. These experiments have resulted in measured cutting and thrust forces for different depths of cut in copper (Te-Cu: 99.4-99.5% Cu, 0.5-0.6% Te, 4-5 micron grain size, 225 MPa yield strength) with a well characterized diamond tool. By using this actual tool tip geometry the authors have been able to develop a model that can predict cutting and thrust forces for depths of cut on the order of the sharpness of the tool. Forces predicted by this numerical model are compared to the experimentally measured forces.
Date: March 1, 1995
Creator: Stevens, R.; Anderson, C.; Rhorer, R. & Lucca, D.
Partner: UNT Libraries Government Documents Department

Diamond films grown from fullerene precursors

Description: Fullerene precursors have been shown to result in the growth of diamond films from argon microwave plasmas. In contradistinction to most diamond films grown using conventional methane-hydrogen mixtures, the fullerene-generated films are nanocrystalline and smooth on the nanometer scale. They have recently been shown to have friction coefficients approaching the values of natural diamond. It is clearly important to understand the development of surface morphology during film growth from fullerene precursors and to elucidate the factors leading to surface roughness when hydrogen is present in the chemical vapor deposition (CVD) gas mixtures. To achieve these goals, we are measuring surface reflectivity of diamond films growing on silicon substrates over a wide range of plasma processing conditions. A model for the interpretation of the laser interferometric data has been developed, which allows one to determine film growth rate, rms surface roughness, and bulk losses due to scattering and absorption. The rms roughness values determined by reflectivity are in good agreement with atomic force microscope (AFM) measurements. A number of techniques, including high-resolution transmission electron microscopy (HRTEM) and near-edge x-ray absorption find structure (NEXAFS) measurements, have been used to characterize the films. A mechanism for diamond-film growth involving the C{sub 2} molecule as a growth species will be presented. The mechanism is based on (1) the observation that the optical emission spectra of the fullerene- containing plasmas are dominated by the Swan bands of C{sub 2} and (2) the ability of C{sub 2} to insert directly into C-H and C-C bonds with low activation barriers, as shown by recent theoretical calculations of reactions of C{sub 2} with carbon clusters.
Date: July 1, 1995
Creator: Gruen, D.M.; Zuiker, C.D. & Krauss, A.R.
Partner: UNT Libraries Government Documents Department

Diamond radiation detectors I. Detector properties for IIa diamond

Description: The detector properties and carrier dynamics of type IIa diamonds are reasonably well understood. The trends in the electron and hole mobilities have been characterized as a function of temperature, impurity content, electric field and carrier density. The carrier lifetimes are coupled through the nitrogen impurity. This leaves us with typical samples with collection distances of 20 to 50 micrometers. The detailed dynamics of the carriers can be modeled using a rate equation analysis. Much progress has been made in understanding the detector properties of diamond, but continued progress has been limited by the geologic processes used to make the material, for example sample size and no synthesis control. CVD diamond promises to eliminate these restrictions.
Date: May 16, 1997
Creator: Kania, D. R.
Partner: UNT Libraries Government Documents Department