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Experimental issues in in-situ synchrotron x-ray diffraction at high pressure and temperature by using a laser-heated diamond-anvil cell

Description: An integrated technique of diamond-anvil cell, laser-heating and synchrotron x-ray diffraction technologies is capable of structural investigation of condensed matter in an extended region of high pressures and temperatures above 100 GPa and 3000 K. The feasibility of this technique to obtain reliable data, however, strongly depends on several experimental issues, including optical and x-ray setups, thermal gradients, pressure homogeneity, preferred orientation, and chemical reaction. In this paper, we discuss about these experimental issues together with future perspectives of this technique for obtaining accurate data.
Date: December 1, 1997
Creator: Yoo, C.S.
Partner: UNT Libraries Government Documents Department

Shock Initiation of Nitromethane

Description: The shock initiation processes of nitromethane have been examined by using a fast time-resolved emission spectroscopy at a two-stage gas gun. a broad, but strong emission has been observed in a spectral range between 350 and 700 nm from shocked nitromethane above 9 GPa. The temporal profile suggests that shocked nitromethane detonates through three characteristic periods, namely an induction period, a hock initiation period, and a thermal explosion period. This paper discusses temporal and chemical characteristics of these periods and present the temperature of the shock-detonating nitromethane at pressures between 9 and 15 GPa.
Date: December 31, 1993
Creator: Yoo, C. S. & Holmes, N. C.
Partner: UNT Libraries Government Documents Department

Vibrational Spectra of Dense Molecular Fluids in a Laser-Heated DAC: Implications to Shock Compressed Fluids

Description: Recent technical advances have made it possible to obtain very useful spectroscopic information about simple molecules at temperatures and pressures exceeding 2000K and 10 GPa inside a diamond-anvil cell, which is well above any melting point for such systems. This is accomplished by obtaining vibrational spectra via Coherent Anti-Stokes Raman Spectroscopy in conjunction with CW laser heating using a tungsten toroid as a laser target. By the simultaneous use of these techniques, vibrational spectra with relatively high signal to noise can be obtained despite the enormous thermal background generated by the incandescence of extremely hot laser heated material. Temperatures can be measured not only by fitting the Planck radiation to a graybody, but by the spectroscopic evidence of a Boltzmann distribution of molecules in their vibrationally excited quantum levels. Additionally, this technique allows for obtaining data at pressures and temperatures outside the region between the shock hugoniot and isentrope, complementing shock wave experiments.
Date: July 16, 2003
Creator: Baer, B J & Yoo, C-S
Partner: UNT Libraries Government Documents Department

Boron nitrides synthesized directly from the elements at high pressures and temperatures

Description: We use angle-resolved synchrotron x-ray diffraction, laser sample heating, and diamond-anvil cells to follow in-situ chemical reactions directly between elemental boron and nitrogen. The structures of the solid reaction products vary with pressure. Below 10 GPa, hexagonal BN is the product; cubic or wurzite BN form at higher pressures. Under nitrogen-rich conditions, another hexagonal allotrope occurs which seems to be a new highly transparent, low density h`-BN. No direct reactions occur at ambient temperature even at pressures as high as 50 GPa, implying that a large activation barrier limits the kinetics of these exothermic processes. Laser heating overcomes the large kinetic activation barrier and initiates spontaneous, self-sustaining exothermic reactions even at moderate pressures.
Date: November 1, 1996
Creator: Nicol, M.; Yoo, C.S.; Akella, J. & Cynn, H.
Partner: UNT Libraries Government Documents Department

Elementary reactions of nitrogen and oxygen with boron and carbon at high pressures and temperatures

Description: The direct elementary reactions among the first and second row elements often yield novel super hard, high energy density, and wide band-gap optical materials. The reactions of oxygen and nitrogen with boron and carbon have been investigated at high pressures and temperatures by using an integrated technique of diamond-anvil cell, laser-heating, x-ray diffraction, Raman spectroscopy. A wide range of products has been synthesized and characterized in-situ at high pressures, including {alpha}-CO{sub 2}, B{sub 2}0{sub 3}-I,B{sub 2}0{sub 3}-II, c-BN, h-BN, h{sup `}-B, amorphous carbon nitrides. The elementary reactions occur exothermically and result in highly polycrystallized products with an exception in carbon-nitrogen reactions. The implication of the elementary reactions to energetic materials applications is discussed.
Date: August 1, 1997
Creator: Yoo, C.S.; Cynn, H. & Nicol, M.F.
Partner: UNT Libraries Government Documents Department

Polymeric Carbon Dioxide

Description: Synthesis of polymeric carbon dioxide has long been of interest to many chemists and materials scientists. Very recently we discovered the polymeric phase of carbon dioxide (called CO{sub 2}-V) at high pressures and temperatures. Our optical and x-ray results indicate that CO{sub 2}-V is optically non-linear, generating the second harmonic of Nd: YLF laser at 527 nm and is also likely superhard similar to cubic-boron nitride or diamond. CO{sub 2}-V is made of CO{sub 4} tetrahedra, analogous to SiO{sub 2} polymorphs, and is quenchable at ambient temperature at pressures above 1 GPa. In this paper, we describe the pressure-induced polymerization of carbon dioxide together with the stability, structure, and mechanical and optical properties of polymeric CO{sub 2}-V. We also present some implications of polymeric CO{sub 2} for high-pressure chemistry and new materials synthesis.
Date: November 2, 1999
Creator: Yoo, C-S.
Partner: UNT Libraries Government Documents Department

Equation of state of unreacted high explosives at high pressures

Description: Isotherms of unreacted high explosives (HMX, RDX, and PETN) have been determined to quasi-hydrostatic high pressures below 45 GPa, by using a diamond-anvil cell angle-resolved synchrotron x-ray diffraction method. The equation-of-state parameters (bulk modulus B<sub>o</sub>, and its derivatives B<sup>'</sup> ) are presented for the 3rd-order Birch-Murnaghan formula based on the measured isotherms. The results are also used to retrieve unreacted Hugoniots in these high explosives and to develop the equations of state and kinetic models for composite high explolsivcs such as XTX-8003 and LX-04. The evidence of shear-induced chemistry of HMX in non-hydrostatic conditions is also presented.
Date: August 14, 1998
Creator: Yoo, C-S
Partner: UNT Libraries Government Documents Department

Time-resolved temperatures of shocked and detonating energetic materials

Description: Chemical processes occurring in shock-compressed and detonating high explosives have been studied using fast time-resolved emission spectroscopy and a two-stage gas-gun. The spectral characteristics of emission from shock-compressed nitromethane, tetranitromethane and single crystals of pentaerythritol tetranitrate are typically very broad and structureless, likely representing thermal emission. Assuming the thermal emission from a gray-body, the emission intensity can be correlated to the temperature changes in shock-compressed and detonating high explosives. The authors report Chapman-Jouguet temperatures of 3,800 K for nitromethane, 2,950 K for tetranitromethane, and 4,100 K for pentaerythritol tetranitrate. In this paper the authors also compare the data with the chemical equilibrium models.
Date: August 14, 1995
Creator: Yoo, C.S.; Holmes, N.C. & Souers, P.C.
Partner: UNT Libraries Government Documents Department

Detonation in shocked homogeneous high explosives

Description: We have studied shock-induced changes in homogeneous high explosives including nitromethane, tetranitromethane, and single crystals of pentaerythritol tetranitrate (PETN) by using fast time-resolved emission and Raman spectroscopy at a two-stage light-gas gun. The results reveal three distinct steps during which the homogeneous explosives chemically evolve to final detonation products. These are (1) the initiation of shock compressed high explosives after an induction period, (2) thermal explosion of shock-compressed and/or reacting materials, and (3) a decay to a steady-state representing a transition to the detonation of uncompressed high explosives. Based on a gray-body approximation, we have obtained the CJ temperatures: 3800 K for nitromethane, 2950 K for tetranitromethane, and 4100 K for PETN. We compare the data with various thermochemical equilibrium calculations. In this paper we will also show a preliminary result of single-shot time-resolved Raman spectroscopy applied to shock-compressed nitromethane.
Date: November 1, 1995
Creator: Yoo, C.S.; Holmes, N.C. & Souers, P.C.
Partner: UNT Libraries Government Documents Department

Shock temperature measurements of iron to 350 GPa

Description: Shock temperatures of iron have been measured to 350 GPa, using high quality well-characterized iron films deposited on diamond substrates. The authors have found an inflection in the (P,T)- iron Hugoniot between 250 GPa and 300 GPa, indicating iron melting at 6,350 K and 6,720 K, respectively. An extrapolation yields the iron melting temperature of 6,870 ({+-}500) K at 330 GPa, the pressure of the Earth inner core-outer core boundary. In this paper, they present this new iron melting data together with a brief description of the experimental method to measure the shock temperatures of iron.
Date: July 1, 1993
Creator: Yoo, C. S.; Holmes, N. C.; Ross, M. & See, E.
Partner: UNT Libraries Government Documents Department

Structures of High Density Molecular Fluids

Description: The goal of this proposal is to develop an in-situ probe for high density molecular fluids. We will, therefore, use Coherent Anti-Stokes Raman Spectroscopy (CARS) applied to laser heated samples in a diamond-anvil cell (DAC) to investigate molecular fluids at simultaneous conditions of high temperatures (T &gt; 2000K) and high pressures (P &gt; 10 GPa.) Temperatures sufficient to populate vibrational levels above the ground state will allow the vibrational potential to be mapped by CARS. A system capable of heating and probing these samples will be constructed. Furthermore, the techniques that enable a sample to be sufficiently heated and probed while held at static high pressure in a diamond-anvil-cell will be developed. This will be an in-situ investigation of simple molecules under conditions relevant to the study of detonation chemistry and the Jovain planet interiors using state of the art non-linear spectroscopy, diamond-anvil-cells, and laser heating technology.
Date: February 1, 2002
Creator: Baer, B; Cynn, H; Iota, V & Yoo, C-S
Partner: UNT Libraries Government Documents Department

Novel high energy density materials: Synthesis by megabar hot pressing. LDRD final report

Description: The goal of this work was to demonstrate proof-of-principle existence of a new class of high energy density materials (HEDMs). These proposed novel solids are derived from first and second row elements arranged in a uniform, three-dimensional network. Thus, every bond in these systems is energetic, in contrast to conventional energetic materials that store energy only within individual molecules. Recent predictions have suggested that a number of possible compounds including a polymeric form of nitrogen can be synthesized at high pressures and recovered metastably at ambient conditions. Specifically, polymeric nitrogen is predicted to have an energy density about three times that of a typical explosive. Such extended solid HEDMs offer dramatic new opportunities as explosives, monopropellants, or as environmentally clean fuels. The authors utilized the laser heated diamond anvil cell as the synthesis route for establishing proof-of-principle existence. They conducted high pressure studies of pure molecular nitrogen samples and completely revised the previously published equation-of-state. They also pursued studies of carbon monoxide, a compound that is isoelectronic with nitrogen and exhibits very similar high pressure phase transformations. Carbon monoxide polymerizes under pressure into a solid that can be recovered and may be energetic.
Date: April 1, 1996
Creator: Lorenzana, H.E.; Yoo, C.S.; Lipp, M.; Barbee, T. III; McMahan, A.K. & Mailhiot, C.
Partner: UNT Libraries Government Documents Department

Progress towards the synthesis of polymeric nitrogen

Description: Current conventional energetic compounds rely on strong covalent bonds within individual molecules for energy storage. A new class of energetic compounds has been recently proposed that entirely replaces weak van der Waals interactions with strong covalent bonds arranged in a continuous, uniform network, thus tremendously enhancing the energy per volume. In particular, recent theoretical calculations have suggested that a phosphorus-like or polymeric form of nitrogen may exist metastably at atmospheric pressure as a hard, insulating solid with an enhanced energy per unit volume. It is predicted that the polymeric phase of nitrogen should be stable at high pressure. And therefore the megabar diamond anvil cell might provide the ideal vehicle for carrying out proof-of-existence experiments. currently, the authors are bringing to bear technologies for achieving multimegabar pressures and temperatures of several thousand K. These conditions are necessary to rearrange the bonds of strongly covalent systems into highly energetic configurations. There is no doubt that the transformations will show strong hysteresis making the initial synthesis difficult, but for these very same reasons, these new compounds potentially will be metastable at ambient conditions in their energetic state. They discuss their results and progress to date, indicating that they are well on their way to understanding the high pressure equation-of-state of sold N{sub 2}.
Date: June 1, 1994
Creator: Lorenzana, H. E.; McMahan, A. K.; Yoo, C. S. & Barbee, T. W. III
Partner: UNT Libraries Government Documents Department

Ultrafast Materials Probing with the LLNL Thomson X-Ray Source

Description: The use of short laser pulses to generate very high brightness, ultra short (fs to ps) x-ray pulses is a topic of great interest. In principle, fantosecond-scale pump-probe experiments can be used to temporally resolve structural dynamics of materials on the time scale of atomic motion. The development of sub-ps x-ray pulses will make possible a wide range of materials and plasma physics studies with unprecedented time resolution. The Thomson scattering project at LLNL will provide such a novel x-ray source of high power using short laser pulses and a high brightness, relativistic electron bunch. The system is based on a 5mm-mrad normalized emittance photoinjector, 100 MeV electron RF linac, and a 300 mJ, 35 fs solid-state laser system. The Thomson source will produce ultra fast pulses with x-ray energies (60 kev) capable of probing into high-Z metals.
Date: September 3, 2002
Creator: Springer, P; Anderson, S; Brown, W; Barty, C; Cauble, R; Crane, J et al.
Partner: UNT Libraries Government Documents Department