Session R33 - High Pressure: Primarily Experimental I.
MIXED session, Thursday morning, March 23
213B, MCC

[R33.001] High Pressure Physics

This abstract not available.

[R33.002] Thermal Transport at Static High-Pressures

Static properties and dynamic processes at high pressures and high temperatures are critically dependent on thermal properties of materials. Measurements of thermal properties at high pressures have only slowly developed through the years. Here we present a novel method, utilizing gem anvil cells, to measure specific heat, thermal conductivity and thermal diffusivity. The method involves launching a thermal wave in a sample. Subsequently, a localized sensor measures the temporal behavior of the temperature at a fixed point downstream in the material. A pulsed laser is used to deliver the heating pulse, while time-resolved fluorescence from a ruby sphere is used to measure the temperature. The thermal properties are inferred from the temporal behavior of the temperature and the heat conduction equations with appropriate boundary conditions. The thermal properties of table salt (NaCl) are obtained using this method. Impact to current and future high-pressure research, including materials science and geophysics will be addressed.

[R33.003] Thermal conductivity of TmSeTe

The rare earth compound TmSeTe is a mixed valent semiconductor at ambient pressure. On increasing pressure the substance reveals a phase transition into an excitonic insulator phase(B. Bucher et al., PRL 66, (1991) 2717). The transition into the coherent excitonic groundstate might be interpreted as a Bose condensation. Different methodes have been applied on TmSeTe in order to search for features which are typical for a Bose condensate.

The thermal conductivity of TmSeTe has been measured for pressures up to 20 kbar and temperatures down to 2 K. The thermal conductivity is enhanced within the excitonic phase. Furthermore the elastic properties of TmSeTe revealed an extremely small compressibility which must be attributed to the formation of the coherent ground state.

[R33.004] Hugoniot of Polytetrafluorethylene (Teflon) at Initial Temperature of 250 C*

General interest continues in the behavior of materials when they are subjected to dynamic loading while being at high initial temperature. Certain materials are used as insulating materials for in-situ gauges in shock wave experiments. Teflon is one of the highly preferred materials because of its electrical insulating properties for such experiments. To improve accuracy of the experimental data using gauges insulated by Teflon requires the measurement of its Hugoniot at high initial temperatures. Gas gun experiments with Teflon initially at 250 C were conducted at stresses between 50 and 150 kbar. *This work was performed under the auspices of the United States Department of Energy by the Lawrence Livermore National Laboratory under Contract N0. W-7405-ENG-48.

[R33.005] Nonhydrostatic Stress Effects in Diamond Coated Rhenium and Diamond Coated Beryllium Gaskets at Ultra High Pressures

The nonhydrostatic component in a diamond anvil cell was studied in the case of a diamond coated rhenium gasket (J. Phys.: Condens. Matter 11, 6515 (1999)) and a diamond coated beryllium gasket by energy dispersive x-ray diffraction technique. The studies were carried out at the Advanced Photon Source (APS) at the Argonne National Laboratory using a focused 5 micron x 5 micron x-ray beam. In the case of diamond coated rhenium gasket at 200 GPa, axial diffraction geometry was employed and two-theta diffraction angle was increased from 5 to 30 degrees. Large uniaxial stress effect was measured and the measured pressure using rhenium equation of state increased with increase in diffraction angle. In the case of diamond coated beryllium gasket, low absorption in beryllium allowed us to go through the gasket and utilize the radial diffraction geometry to study crystallographic planes of a copper sample parallel to the face of the diamond anvil. The utility of diamond-coated gaskets in ultra high-pressure research using diamond anvil cells will be discussed.

[R33.006] High pressure studies to 100 GPa with perforated diamond anvils

Diamonds is being routinely used as anvils for generating static pressures to 300 GPa. Thickness of a typical 0.3C anvils pair is \sim5 mm resulting in very small transmission for soft X- or \gamma-rays (E < 12 keV). We report successful attempts* to use partial and fully drilled conical holes in diamond anvils. Fully drilled anvils, designated as "diamond backing plate" (DBP),in conjunction with miniature anvils made of \sim0.01 C diamonds (0.2 mm culets, \sim0.8 mm height)directly mounted on the DBP 1.7 mm culet, sustained pressures to 100 GPa using Re gaskets. Damages to the DBP caused by the shattered miniature anvil were minimal. The realtive large 1.3 mm diameter base of the DBP and the reduced thickness of the anvils allow for improved mechanical strength of the anvil, considerable increased S/N in ^57Fe Mössbauer apectra, better resolution in angle dispersive XRD and EXAFS by using soft X-rays. Work partially supported by ISF grant 88/97. *DBP and miniature anvils made by DANVIL'S (Ramot Ltd.)

[R33.007] Solid Bodies Behaviour Under Submegabar and Megabar Pressure, Obtained by DAC

The use of metal deformable containers - gaskets between diamond anvils in apparatus (DAC) helps to carry out studies under quasi- static conditions at high pressure. The deformable container- metal gasket together with the basic functions of pressure generation and formation of cell for object study provides the anvils peripheral area thus reducing a probability of failure at high pressures.

Several theoretical models were used to study the metal gasket behaviour during the generation of pressure and the support of the anvils peripherial area. The study of the metal gasket behaviour under loading was based upon the elastic- plastic deformation study of a gasket made from stainless steels in the pressure range studied.

The operation of the gasket in the DAC is analized and some practical relevant conclusions for users of the DACs in the submegabar pressure range are given. Two distinct regimes are found: first regime where the gasket is at the maximum thickness and the DAC is intrinsically unstable, and the other one where the gasket in thin and the cell is stable in operation, so that there are good reasons to use pre- indented gaskets, of thickness rather less than usually recomended. Force- pressure plots are shown to give valuable information in routine use, and their interpretation is discussed. Our calculations show that in the range of submegabar pressure in the working cell, i.e. into the pressure range, which is of interest to studies of biological objects, there is the maximum in distribution of contact loads along cell radius. As a result the decrease of cell diameter with the growth of contact loads is observed. We suppose that the making allowance for this effect is necessary condition at the development of specific experimental methods.

The obtained experimental and theoretical results makes it possible to choose the optimum of gasket parameters and properties for the deformable containers, thus providing the submegabar pressure generetion functions and the support of the diamond anvils peripheral area.

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[R33.008] Pulsed Field High Pressure Measurements of Reduced Dimensional Systems : Cryogenics and Diamond Anvil Cells

Exploration of the reduced dimensional systems such as high temperature superconductors and molecular conductors often requires magnetic fields that can only be generated using pulsed magnets. High pressure work in pulsed fields has been limited due to the eddy current heating of the typically metallic pressure vessel which leads to dramatic heating of the sample . This rapid sample heating occurs within the 10-20 msec pulse duration of the magnet, severely compromising the integrity of the data. We have successfully performed high pressure millikelvin work in pulsed fields to 60 T by addressing the issues of eddy current heating, vibrations induced in the cryostat tails, and open loop voltages. Methods to perform high pressure electrical transport to 2 GPa and optical measurements to 5 GPa at millikelvin temperatures using small 8 to 10 mm diameter plastic diamond anvil cells will be presented along with preliminary data.

[R33.009] HIGH PRESSURE STUDIES ON THE STRUCTURAL INFLUENCE ON THE ELECTRONIC PROPERTIES OF SEXITHIOPHENE SINGLE CRYSTALS

Sexithiophene shows electronic properties strongly dependent on the order in the films. The focus of this study are structural properties of sexithiophene single crystals. We perform Raman spectroscopy at 10 K and under pressure up to 80 kbar to probe structural changes, phase transitions and interchain interactions. Upon changing pressure we detect changes in frequency, position and intensity of the Raman modes. The observed stiffening with increasing pressure, is indicative for increasing force constants. Finally we discuss the effect of both the variations in the intermolecular interactions (Davydov splitting) and in the conformational properties of the oligomer, induced by the applied pressure.

[R33.010] Pressure-induced solid-solid phase transitions of HMX*

The chemical activation energy barrier of metastable anisotropic solid phase decomposition reactions depends, in part, on crystal symmetry. Further, a detailed understanding of solid-solid phase transitions is critical for accurate modeling of energetic material decomposition. Indeed, delta phase HMX is more sensitive to shock initiated chemical reactions than the beta phase polymorph. Thus, to learn more about pressure-induced beta to delta transition dynamics we have employed a second harmonic generation optical diagnostic. In addition we have monitored the corresponding pressure and temperature throughout the transition. In some instances we include real-time digital micro-photographs that temporally correspond to each optical pressure measurement. Our goal is to quantify the isothermal kinetic phase diagram of HMX and the corresponding stability fields. X-ray analysis before and after the pressure-induced transition provides an added degree of certainty to inferences made from our optical measurements. These experiments provide a unique data set for the understanding of ignition conditions that lead to the stable or metastable decomposition reactions of delta phase HMX.

[R33.011] ELASTIC PRECURSOR DECAY IN SINGLE CRYSTAL HMX

The decay of the elastic precursor shock was measured for two orientations of single crystal cyclotetramethylene tetranitramine (HMX) in plane impact experiments. Elastic shock amplitude was measured for distances of propagation from 1 to 4.5 mm. It was measured for impact stresses of 1.4 and 2.4 GPa. The response was similar but not identical for the two orientations. Stress relaxing behavior is observed.

[R33.012] Physical Principals Plastification of Solid under Pressure and its Applications in the Treatment Processes

Hydrostatic pressure is the powerful method of action on the mechanisms of plastic deformation and fracture of solids. This effect is widely used in the technologies aimed to obtaining specimens with the desired physical and mechanical properties. The processes of hydrostatic treatment of materials is the most promising in this respect.

This report presents an effort to formulate physical causes of plastification of solid under deformation by high hydrostatic pressure, which is one of fundamentals for development of the processes of hydrostatic material processing.

The obtained theoretical and experimental results give evidence that the tendency to homogenization of the defect structure is the most general relationship under deformation of solids by pressure. The homogenization of the defect structure is realized by different methods depending on the nature of solids and the degree of compression in the experiment.

Now we propose the new efficient technologies of hydrostatic treatment of materials and formulate the main principles of regulation by the treatment regimes on the basis of physical models.

In particular, we research different technological processes: - the cryoforming of stainless steels on the basis of Fe-Cr-Ni solid solution under pressure; - the warm hydropressing of maraging steels and high-strenght steels containing the ageing austenite; - the treatment technologies of metastable steels on the basis of Fe-Mn and Fe-Mn-Cr solid solution with the application of both warm hydropressing and the back-pressure effect.

[R33.013] Pressure Dependent Mechanical Properties of Polymer-Particulate Composites

The mechanical response of a composite containing 3% polymer, 3% plastizer and 94% filler is being studied in uniaxial compression as a function of superimposed hydrostatic pressures between atmospheric and 138 MPa. The modulus defined as the initial slope increases exponentially and the yield strength increases linearly with pressure. At atmospheric pressure yield is followed by strain softening while at 34 MPa and above yield is followed by plastic flow and strain hardening. The strain hardening coefficient also increases with pressure. These results indicate that crack growth processes which account for the strain softening at atmospheric pressure are inhibited at the higher pressures and further that the dominant failure processes at the higher pressures involve plastic flow. The pressure dependence of the modulus and the yield strength as well as the temperature and strain rate dependence of these parameters at atmospheric pressure indicate the influence of the polymer component of the composite on these properties.

Part R of program listing