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The compressibility of cubic white and orthorhombic, rhombohedral, and simple cubic black phosphorus

Description: The effect of pressure on the crystal structure of white phosphorus has been studied up to 22.4 GPa. The ?alpha phase was found to transform into the alpha' phase at 0.87 +- 0.04 GPa with a volume change of 0.1 +- 0.3 cc/mol. A fit of a second order Birch- Murnaghan equation to the data gave Vo = 16.94 ? 0.08 cc/mol and Ko = 6.7 +- 0.5 GPa for the alpha phase and Vo = 16.4 +- 0.1 cc/mol and Ko = 9.1 +- 0.3 GPa for the alpha' phase. The alpha' phase was found to transform to the A17 phase of black phosphorus at 2.68 +- 0.34 GPa and then with increasing pressure to the A7 and then simple cubic phase of black phosphorus. A fit of a second order Birch-Murnaghan equation to our data combined with previous measurements gave Vo = 11.43 +- 0.05 cc/mol and Ko = 34.7 +- 0.5 GPa for the A17 phase, Vo = 9.62 +- 0.01 cc/mol and Ko = 65.0 +- 0.6 GPa for the A7 phase and , Vo = 9.23 +- 0.01 cc/mol and Ko = 72.5 +- 0.3 GPa for the simple cubic phase.
Date: March 10, 2010
Creator: Clark, Simon M & Zaug, Joseph
Partner: UNT Libraries Government Documents Department

Optical Switching in VO2 films by below-gap excitation

Description: We study the photo-induced insulator-metal transition in VO2, correlating threshold and dynamic evolution with excitation wavelength. In high-quality single crystal samples, we find that switching can only be induced with photon energies above the 670-meV gap. This contrasts with the case of polycrystalline films, where formation of the metallic state can also be triggered with photon energies as low as 180 meV, well below the bandgap. Perfection of this process may be conducive to novel schemes for optical switches, limiters and detectors, operating at room temperature in the mid-IR.
Date: March 14, 2008
Creator: Dipartimento di Fisica, Universita?di Brescia, Italy
Partner: UNT Libraries Government Documents Department

Molecular Simulation of Phase Equilibria for Complex Fluids

Description: The general area of this project was the development and application of novel molecular simulation methods for prediction of thermodynamic and structural properties of complex polymeric, surfactant and ionic fluids. Over this project period, we have made considerable progress in developing novel algorithms to meet the computational challenges presented by the strong or long-range interactions in these systems and have generated data for well-defined mod-els that can be used to test theories and compare to experimental data. Overall, 42 archival papers and many invited and contributed presentations and lectures have been based on work supported by this project. 6 PhD, 1 M.S. and 2 postdoctoral students have been associated with this work, as listed in the body of the report.
Date: September 9, 2009
Creator: Panagiotopoulos, Athanassios Z.
Partner: UNT Libraries Government Documents Department

Exploring pulse shaping for Z using graded-density impactors on gas guns (final report for LDRD project 79879).

Description: While isentropic compression experiment (ICE) techniques have proved useful in deducing the high-pressure compressibility of a wide range of materials, they have encountered difficulties where large-volume phase transitions exist. The present study sought to apply graded-density impactor methods for producing isentropic loading to planar impact experiments to selected such problems. Cerium was chosen due to its 20% compression between 0.7 and 1.0 GPa. A model was constructed based on limited earlier dynamic data, and applied to the design of a suite of experiments. A capability for handling this material was installed. Two experiments were executed using shock/reload techniques with available samples, loading initially to near the gamma-alpha transition, then reloading. As well, two graded-density impactor experiments were conducted with alumina. A method for interpreting ICE data was developed and validated; this uses a wavelet construction for the ramp wave and includes corrections for the ''diffraction'' of wavelets by releases or reloads reflected from the sample/window interface. Alternate methods for constructing graded-density impactors are discussed.
Date: October 1, 2005
Creator: Furnish, Michael David; Reinhart, William Dodd; Anderson, William W. (Los Alamos National Laboratory, Los Alamos, NM); Vogler, Tracy John; Hixson, Rob (Los Alamos National Laboratory, Los Alamos, NM) & Kipp, Marlin E.
Partner: UNT Libraries Government Documents Department

LDRD final report on using chaos for ultrasensitive coherent signal detection.

Description: A quantum optical approach is proposed and analyzed as a solution to the problem of detecting weak coherent radiation in the presence of a strong incoherent background. The approach is based on the extreme sensitivity of laser dynamical nonlinearities to the coherence of external perturbation. This sensitivity leads to dynamical phase transitions that may be employed for detecting the presence of external coherent radiation. Of particular interest are the transitions between stable and chaotic states of laser operation. Using a baseline scheme consisting of a detector laser operating with a Fabry-Perot cavity, we demonstrated significant qualitative and quantitative differences in the response of the detector laser to the intensity and coherence of the external signal. Bifurcation analysis revealed that considerable modification to the extension of chaotic regions is possible by tailoring active medium and optical resonator configurations. Our calculations showed that with semiconductor lasers, destabilization can occur with a coherent external signal intensity that is over six orders of magnitude smaller than the detector laser's intracavity intensity. Discrimination between coherent and incoherent external signal also looks promising because of the over four orders of magnitude difference in intensity required for inducing chaos-like behavior. These results suggest that the proposed approach may be useful in laser sensor applications, such as satellite Laser Threat Warning Receivers (LTWR).
Date: November 1, 2006
Creator: Chow, Weng Wah; Wieczorek, Sebastian Maciej & Torrington, Geoffrey Kenneth
Partner: UNT Libraries Government Documents Department

Thermographic analyses of the growth of Cd1-xZnxTe single crystals

Description: Bulk Cd1-xZnxTe (0<x<0.1) single crystals for gamma-ray detectors are grown mainly from near-stoichiometric melts. We discuss the influence of the thermal pre-history of the melts (superheating, thermo-cycling, and cooling rate) on various physical properties based on our thermographic analyses, electrical conductivity and viscosity measurements. Increasing the Zn content causes non-monotonic dependencies in the quality of the crystals structure.
Date: August 1, 2010
Creator: Kopach, O. V.; Bolotnikov, A.; Shcherbak, Larysa P.; Fochuk, Petro M. & James, Ralph B.
Partner: UNT Libraries Government Documents Department

Theoretical Treatment of the Thermophysical Properties of Fluids Containing Chain-like Molecules

Description: This research program was designed to enhance our understanding of the behavior of fluids and fluid mixtures containing chain-like molecules. The original objective was to explain and predict the experimentally observed thermophysical properties, including phase equilibria and dynamics, of systems containing long flexible molecules ranging in length from alkanes to polymers. Over the years the objectives were expanded to include the treatment of molecules that were not chain-like. Molecular dynamics and Monte Carlo computer simulations were used to investigate how variations in molecular size, shape and architecture influence the types of phase equilibria, thermodynamic properties, structure and surface interactions that are observed experimentally. The molecular insights and theories resulting from this program could eventually serve as the foundation upon which to build correlations of the properties of fluids that are both directly and indirectly related to the Nation’s energy resources including: petroleum, natural gas, and polymer solutions, melts, blends, and materials.
Date: November 14, 2008
Creator: Hall, Carol K.
Partner: UNT Libraries Government Documents Department

LDRD final report : raman spectroscopic measurements to monitor the HMX beta-delta phase transition.

Description: The HMX {beta}-{delta} solid-solid phase transition, which occurs as HMX is heated near 170 C, is linked to increased reactivity and sensitivity to initiation. Thermally damaged energetic materials (EMs) containing HMX therefore may present a safety concern. Information about the phase transition is vital to predictive safety models for HMX and HMX-containing EMs. We report work on monitoring the phase transition with real-time Raman spectroscopy aimed towards obtaining a better understanding of physical properties of HMX through the phase transition. HMX samples were confined in a cell of minimal free volume in a displacement-controlled or load-controlled arrangement. The cell was heated and then cooled at controlled rates while real-time Raman spectroscopic measurements were performed. Raman spectroscopy provides a clear distinction between the phases of HMX because the vibrational transitions of the molecule change with conformational changes associated with the phase transition. Temperature of phase transition versus load data are presented for both the heating and cooling cycles in the load-controlled apparatus, and general trends are discussed. A weak dependence of the temperature of phase transition on load was discovered during the heating cycle, with higher loads causing the phase transition to occur at a higher temperature. This was especially true in the temperature of completion of phase transition data as opposed to the temperature of onset of phase transition data. A stronger dependence on load was observed in the cooling cycle, with higher loads causing the reverse phase transitions to occur at a higher cooling temperature. Also, higher loads tended to cause the phase transition to occur over a longer period of time in the heating cycle and over a shorter period of time in the cooling cycle. All three of the pure HMX phases ({alpha}, {beta} and {delta}) were detected on cooling of the heated samples, either in pure ...
Date: November 1, 2000
Creator: Renlund, Anita Mariana; Tappan, Alexander Smith & Miller, Jill C.
Partner: UNT Libraries Government Documents Department

Microstructures and properties of materials under repeated laser irradiation

Description: This research program has explored the stability of alloys under pulsed laser irradiation. Two primary directions were investigated: (i) phase transitions during a single laser pulse, and (ii) phase stability under repeated laser irradiation. The first theme was primarily concerned with both the crystalline to amorphous phase transition and the transition of liquids and glasses to crystalline matter. The second project examined the phase evolution during laser pulsing in situations where plastic deformation was prevalent (high-energy laser pulses). Both computer simulation and experimental programs were undertaken. Our work using computer simulations had several notable successes. For the work connected with multiple pulsing, we used molecular dynamics (MD) to simulate the behavior of alloys under severe plastic deformation. We found that during high strain-rate deformation atomic mixing of chemical species is random, independent of the detailed thermochemical properties of the system. This result contrasts with recent reports. In this work, we also developed two new methods of analyzing atomic mixing, one is based on relative mean square displacements (RMSD) of atoms and the other, Burgers vector analysis (BVA), on nearest neighbor displacements. The RMSD analysis is valuable in that it specifies the length scales over which deformation processes take place, and we applied it to understand deformation in nanocrystalline, amorphous and large-grained systems. The BVA analysis, on the other hand, reveals if the damage is homogeneous. Finally we showed that at elevated temperatures, the phase stability is not determined from a simple competition between shearing events and vacancy diffusion, which has long been assumed, but rather atomic mixing in the shearing events is temperature dependent. This work is significant in that it elucidates the fundamental mechanisms that underlie high strain rate deformation, and it provides computational tools for other researchers to perform related work. Our work on MD simulation also examined ...
Date: February 12, 2007
Creator: Averback, Robert & Bellon, Pascal
Partner: UNT Libraries Government Documents Department