A large volume pressure cell for high temperatures

Abstract

Studies of matter under very high pressure at synchrotron radiation sources are mostly done using pressure cells with single-crystal diamond anvils. In some cases the available volume (≤ 10^?3mm³)in such cells causes problems especially at high temperature and for crystal synthesis. To ensure sufficient homogeneity of pressure and temperature, the use of cells with large sample volumes (≥ 1 mm³) is necessary.

Existing devices for such measurements are compared with a novel setup which consists of a toroidal anvil arrangement and a lightweight (50 kg) press with 250 tonnes (2.5 MN) capacity. Preliminary tests of this instrument with synchrotron radiation are reported.

Presented at the IUCr Workshop on ‘Synchrotron Radiation Instrumentation for HighPressure Crystallography’. Daresbury Laboratory 20-21 July 1991     

13C-NMR Sequence Analysis. XVIII. Tacticity of Poly(D,L-β-amino Acids)

D, L-Cis-2-aminocyclobutane-1-carboxylic acid NCA, D, L-cis-and trans-2-aminocyclohexane-1-carboxylic acid N-carboxyanhydride (NCA) and D, L-cis-2-aminocyclohexane-1-carboxylic acid NCA were polymerized under various conditions. The ¹³C-NMR spectra of the resulting β-polyamides measured in trifluoro-acetic acid show splittings of all signals reflecting diads and triads. Poly-D, L-3-aminobutyric acid obtained by anionic polymerization of D, L-4-methyl acetidinone does not display tacticity effects in its ¹³C-NMR spectrum. Hence it is concluded that tacticity effects are observable only if both α- and β-carbons have a substituent. Furthermore, it was found that the reaction conditions do not have a strong influence on the stereospecificity of the NCA-polymerization. In all cases nearly random sequences of D and L-units were obtained.     

Geometrical limitations in the intramolecular addition of unsymmetrical allylsilanes to enones

The geometrical constraints of the fluoride-initiated and Lewis acid-catalyzed intramolecular addition of allylsilanes to enone systems were shown to be governed by kinetic-ring-size preferences. Molecules containing adjacent quaternary carbon atoms can be prepared using this method.     

Megawatt VUV xenon laser employing coaxial electron-beam excitation

3 nsec laser pulses, of bandwidth 1.3 nm, are obtained from a 10 J, 600 keV coaxial diode electron-beam pumping arrangement. Uniform pumping, with a well defined cylindrical geometry, facilitates experimental investigation of the laser parameters. Gas heating limits the laser repetition rate. While mirror damage at present limits the peak power to ∽ 1 MW, higher powers seem available. The addition of helium results in a drastic reduction of peak molecular xenon fluorescence.     

The effect of rate of stimulation on perception of spectral shape by cochlear implantees

The effect of rate of stimulation on spectral shape perception was measured in six users of the Nucleus CI24 cochlear implant. Three spectral shapes were created by using three profiles of current across seven electrode positions. Each current profile was replicated in three stimuli that interleaved stimulus pulses across the seven electrodes with cycle rates (rate per electrode) of 450, 900, and 1800 Hz. The stimulus space resulting from a multidimensional scaling experiment showed a clear dimension related to the rate of stimulation that was orthogonal to the dimension related to the spectral shapes. A second experiment was performed with the same subjects to investigate whether the perceptual dimension related to rate in Experiment 1 could be attributed to different perceptual flatness of the profiles at different rates. In Experiment 2, the rate of stimulation was fixed at 900 Hz and three profiles were created for each spectral shape that differed in flatness. This experiment did not, however, result in an independent perceptual dimension related to the flatness of the profile. In conclusion, rate of stimulation provided an independent perceptual dimension in the multiple-electrode stimuli, in spite of the rates being not discriminable or barely discriminable in single-electrode stimulation.     

Collapsing cristobalitelike structures in silica analogues at high pressure

The cristobalitelike forms of the ternary silica analogues BPO4 and BAsO4 were investigated at high pressure by x-ray diffraction and theoretical methods. The behavior of these compounds represents an extreme case in which the tilt angle of the constituent tetrahedra increases in a spectacular way at high pressure resulting in a major change in topology from a cristobalitelike framework towards a "collapsed cristobalite" structure. These compounds provide the first examples of the collapse of a framework structure to a close-packed form in a continuous manner without an intervening phase transition.     

New mechanism for dislocation blocking in strained layer epitaxial growth

Dislocation interactions play a critical role in plasticity and heteroepitaxial strain relaxation. We use real time transmission electron microscopy observations of the interaction between threading and misfit dislocations in SiGe heterostructures to investigate interactions quantitatively. In addition to the expected long-range blocking of threading segments, we observe a new short-range mechanism which is significantly more effective. Simulations show that this reactive blocking occurs when two dislocations with the same Burgers vector reconnect.     

A high temperature diffraction-resistance study of chalcopyrite, CuFeS2

The electrical, magnetic and structural properties of synthetic chalcopyrite, CuFeS₂, have been studied up to 873 K using DC resistance measurements performed in-situ during neutron powder diffraction experiments. Under ambient conditions the material adopts the accepted structural model for CuFeS₂ in the space group I4?2d, with the magnetic moment of the Fe³⁺ cations aligned along [001]. The electrical resistivity is around 0.3 Ω cm under ambient conditions, consistent with semiconductor character, and decreases slightly with increase in temperature until a more abrupt fall occurs in the region 750-800 K. This abrupt change in resistivity is accompanied by a structural transition to a cubic zinc blende structured phase (space group F4?3m) in which Cu⁺ and Fe³⁺ cations are disordered over the same tetrahedral crystallographic sites and by a simultaneous loss of long-range magnetic order. The implications of these results are discussed in the context of previous studies of the chalcopyrite system.     

Thermal degradation and flame retardance in copolymers of methyl methacrylate with diethyl(methacryloyloxymethyl)phosphonate

Methyl methacrylate (MMA) has been free radically copolymerized, both in bulk and in solution, with diethyl(methacryloyloxymethyl)phosphonate (DEMMP), to give polymers which are significantly flame retarded when compared with PMMA, as indicated by the results of limiting oxygen index (LOI) measurements, UL 94 tests, and the results of cone calorimetric experiments. The physical and mechanical properties of the copolymers are similar to those of PMMA, except that the bulk copolymers are slightly crosslinked, and are better than those of PMMA flame retarded to a similar extent by some phosphate and phosphonate additives. Examination of the some of the gaseous products of pyrolysis and combustion, and of chars produced on burning, show that flame retardation occurs in the copolymers by both a condensed-phase and a vapour-phase mechanism. The condensed-phase mechanism is shown to involve generation of phosphorus acid species followed by reaction of these with MMA units giving rise to methacrylic acid units. The methacrylic acid units subsequently form anhydride links, which probably impede depolymerization of the remaining MMA sequences, resulting in evolution of less MMA (the major fuel when MMA-based polymers burn). By undergoing decarboxylation, leading to interchain cyclisation and, eventually, to aromaticisation, the anhydride units are probably also the principal precursors to char.     

Nonaqueous catalytic water oxidation

The complex [Ru(Mebimpy)(bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2'-bipyridine] and its 4,4'-(PO(3)H(2)CH(2))(2)bpy derivative on oxide electrodes are water oxidation catalysts in propylene carbonate and 2,2,2-trifluoroethanol (TFE) to which water has been added as a limiting reagent. The rate of water oxidation is greatly enhanced relative to that with water as the solvent and occurs by a pathway that is first-order in H(2)O; an additional pathway that is first-order in acetate appears when TFE is used as the solvent.