New packaging solutions for high pressure treatments of food

Abstract

High pressure is a promising technology for developing new processes in food treatment. In most cases, pre-packaged foods are used in high pressure treatments (HP). Consequently, the behavior of the package under treatment conditions is an important factor. The following work was devoted to the evaluation of different packages under high pressure conditions in the presence of different substances used as simulants. Three main characterizations were carried out after HP treatment: mechanical resistance, barrier properties and integrity.

The experimental results using multilayer plastic films (PA/PE) have led to the selection of several solutions which may be used as packages for high pressure treatments.     

High-pressure phases of plutonium monoselenide studied by X-ray diffraction

Abstract

Plutonium monoselenide was studied under high pressure up to 47 GPa, at room temperature, using a diamond anvil cell in an energy dispersive X-ray diffraction facility. At ambient pressure, PuSe has the NaC1-type (B1) structure. The compound has been found to undergo a second-order crystallographic phase transition at around 20 GPa. This phase can be described as a distorted B1 structure, with a rhombohedral symmetry. PuSe transforms to a new phase at around 35 GPa, which can be indexed in the cubic CsCl-type (B2). The volume collapse at this phase transition is 11%. When releasing pressure, we observed a strong hysteresis to the inverse transformation down to 5 GPa. From the pressure-volume relationship, the bulk modulus has been determined to B ₀ = 98 GPa and its pressure derivative as B ₀ = 2.6. These results are compared to those obtained with other actinide monmictides and monochalcogenides.     

High pressure raman studies of GaAs/A1As superlattices: folded-acoustic phonons and resonant second-order scattering

Abstract

Materials containing light elements and characterized by a dense structure present specific physico-chemical properties.

The carbonitrides C_xN_y, and in particular C₃N₄, have attracted the attention of the scientific community during these last ten years.

The main chemical problem in the synthesis of such materials is to introduce nitrogen into the lattice, due to the strong stability of N₂.

In order to prevent nitrogen loss, the use of high pressures and the selection of nitriding media can be developed.

The present contribution describes the main results obtained through two routes:

(i) the thermal decomposition of precursors containing carbon and nitrogen under high pressure conditions (2-6GPa) (without or with nitrogen solvent)

(ii) the high pressure condensation of “organic” precursors containing carbon and nitrogen.     

Effect of Pressure on Phonon Modes in Wurtzite Zinc Oxide

The pressure dependence of first- and second-order Raman frequencies of wurtzite ZnO has been measured up to the wurtzite-rocksalt phase transition pressure of 8.3 GPa. A small increase of the LO-TO splitting with increasing pressure is observed. This effect is related to the combined pressure dependences of the electronic dielectric constant in the phonon region and Born's transverse dynamic effective charge. Our results indicate a rather weak dependence of the dynamic charge on pressure, a behavior which is similar to that found for GaN, AlN, and SiC and different from that of other polar tetrahedral semiconductors.     

New trends in baro-enzymology

Abstract

The recent interest on high pressure biotechnology (mainly in food industry) requires fundamental studies on the pressure behavior of biochemical constituants¹ In this laboratory, we use pressure as a thermodynamic parameter, such as temperature, to determine the energetic quantities of enzyme reactions². Two essential requirements for the study of the mechanism of enzyme action are that, first, a simple rate constant has to be measured (determination of a composite rate constant, such as kcat, can lead to ambiguous results) and, second, a maximum number of physico-chemical ways must be used to perturb the system under study. To assess this simple usually very rapid rate constant, cryoenzymology was used. It was then possible to probe, at constant pressure, the thermodynamics of the interconversion of two successive intermediates, thereby obtaining the classical ΔG? ΔS? and ΔH? parameters. Ify however, we can also vary another intensive parameter, namely the pressure, it is possible to determine the activation volume (ΔV?) of the reaction. In addition to pressure and temperature, a third variable has to be considered also : the nature of the cryosolvent which allows experiments to be extended to subzero temperatures in the first place (with the medium being kept fluid) but which can also act as a perturbing agent thereby inducing controlled and reversible changes in equilibrium and me processes. The interdependence of the two variables, namely temperature and pressure, which is predicted by the general equation for the standard variation of free energy ΔG = f(T, P), is presented. This approach will be illustrated using different model reactions where data are analysed according to the classical transition state theory.     

The influence of high pressure on the solidification of supercooled Se melt

Abstract

Under pressures between 1.75 and 8 GPa a supercooling of Se melt and an average grain size of the samples quenched under pressure with constant cooling rate 100 K/s were measured. In framework of the classic theory of nucleation and grain growth the numerical evaluation of surface tension and activation energy of crystal growth was performed. The comparison of properties of supercooled Se melt to that of Pb and In reveals anomalies on the pressure dependences of Se melt properties.

This circumstance is discussed in connection with the semiconductor-metal transition discovered earlier in Se melt.     

A practical second-order electromagnetic model in the quasi-specular regime based on the curvature of a ‘good-conducting’ scattering surface

Abstract

This letter presents an approximate second-order electromagnetic model where polarization coefficients are surface dependent up to the curvature order in the quasi-specular regime. The scattering surface is considered ‘good-conducting’ as opposed to the case for our previous derivation where perfect conductivity was assumed. The model reproduces dynamically, depending on the properties of the scattering surface, the tangent-plane (Kirchhoff) or the first-order small-perturbation (Bragg) limits. The convergence is assumed to be ensured by the surface curvature alone. This second-order model is shown to be consistent with the small-slope approximation of Voronovich (SSA-1+SSA-2) for perfectly conducting surfaces. Our model differs from SSA-1 + SSA-2 in its dielectric expression, to correct for a full convergence toward the tangent-plane limit under the ‘good-conducting’ approximation. This new second-order formulation is simple because it involves a single integral over the scattering surface and therefore it is suitable for a vast array of analytical and numerical applications in quasi-specular applications.     

Pressure reduction of parasitic parallel conduction in InGaAs/InP heterostructures containing LT-InP layers

Abstract

We have measured classical and quantum (Shubnikov-de Haas effect) magnetotrans-port properties of InGaAs/InP heterostructures in which phosphorus antisite defects incorporated in low temperature MBE-grown InP (LT-InP) layers are the main source of electrons. The heterostructures show strong parasitic parallel conduction, which is reduced under hydrostatic pressure. The comparison of the experimental results with the calculations of the potential profile and the charge distribution in the heterostructures enables to identify all the conduction channels in the structures and unambiguously proves that the parasitic parallel conduction is due to spontaneous formation of the quantum well in the LT-InP buffer.     

New scientific opportunities for high pressure research by energy-dispersive XMCD

ABSTRACT

XMCD under pressure is used to study the magnetic properties of the transition metal (TM) systems for over 15 years. We present the technique and how it has been developed. The energy dispersive XAS spectrometer is particularly suited for these studies. The effect of pressure on TM magnetism is discussed. Recent studies performed at different edges illustrate the information that can be obtained through XMCD. Finally, some results obtained on TMs are presented, either at the L_II,III edges of 5d metals or at the K edge of 3d metals, which correspond to the energy ranges that can be probed when using diamond anvil cells for high pressure. Different cases are treated: pure 3d metals, alloys, magnetic insulator and inorganic compounds.     

Effect of uniaxial stress on the dielectric properties of BaTiO3+0.1wt.%Eu2O3 ceramics

ABSTRACT

BaTiO₃+0.1wt.%Eu₂O₃ ceramics were prepared by a solid-state reaction method. The dielectric behavior of these ceramics as a function of uniaxial pressure has been systematically studied. The external stress showed obvious effects on these properties. An increase of the Curie point (T_c) and decrease of the Curie–Weiss temperature (T₀) was observed with increasing pressure, resulting in an increase in the first-order nature of the phase transformation (T_c–T₀ increases). Broadening and flattening of the permittivity versus temperature curves near their maximum was found. The pressure behavior of thermal hysteresis and the ??/?T vs. T plot suggests that the phase transition changes to second-order type with increasing pressure. Furthermore, the Curie–Weiss constant obtained from a modified Curie–Weiss law strongly decreases with increasing pressure, suggesting that the mechanism of phase transition is going to order–disorder type.     

High pressure since P. W. Bridgman - A personal view

Abstract

In this paper an attempt is made to review high pressure research over the past three or four decades from a personal viewpoint. Focus is first on two technical developments; the introduction of shock wave techniques in solids, and the extension of the static pressure range by a factor of 20–30.

The major emphasis is on the introduction of high pressure into the mainstream of modern science through the concept that pressure is a powerful, indeed an essential tool in understanding molecular and electronic properties of matter. These properties are classified under the headings (q) structure (x-rays) (b) molecular dynamics (c) pressure tuning spectroscopy (d) chemical and electronic rate phenomena (e) chemical synthesis.     

Properties of low-Dimensional metals at high pressure

Abstract

Low dimensional metals, in which the conduction electrons are confined to stacks or within layers, provide an important area of research into interactions in the electron gas and its coupling to the lattice. The metallic state is commonly unstable at low temperatures, and can be replaced by a wide variety of ground states, including the charge density wave (CDW) state, spin density wave (SDW) state, superconductivity etc. High pressure has always been an important experimental tool in this field, as many of the materials studied have high compressibilities, and it is often possible to switch from one ground state to another under pressure. We consider here the properties under pressure of a series of organo-metallic charge transfer salts in which the metallic behaviour is due to intermolecular delocalisation of the π electron systems of the ligand groups around the transition metal atom.     

Alternative ensemble averages in molecular dynamics simulation of hard spheres

ABSTRACT

We consider application to the hard sphere (HS) model of the mapped-averaging framework for generating alternative ensemble averages for thermodynamic properties. Specifically, we develop and examine new formulas for the pressure, the singlet and pair densities, and the cavity-correlation function inside the HS core; the pressure formula in particular is constructed such that it gives an ensemble average that exactly corrects the second-order virial equation of state. The force plays a central role in mapped-averaging expressions, and we write them in a way that accounts for the impulsive, event-driven nature of the HS dynamics. Comparison between results obtained conventionally versus mapped averaging finds that the latter has some advantage at low density, while both perform equally well (in terms of uncertainties for a given amount of sampling) at higher densities.     

Effect of nitrogen pressure on the combustion synthesis of titanium nitride

Abstract

Titanium nitride, TiN, has attractive physical and chemical properties such as hardness, chemical stability and electrical conductivity. It is a typical material with a wide range of stoichiometry. It can be synthesised by high pressure combustion synthesis. The composition and microstructures can vary with the experimental conditions especially with thermal treatment and nitrogen pressure.     

Pressure Effects on Anisotropic Low-Dimensional Superconductors Close to an Electronic Topological Transition

Hydrostatic pressure or anisotropic stress can modify the electronic properties of a metal, thus inducing a change in the topology of its Fermi surface. In the case of low-dimensional anisotropic superconductors (such as high- T c cuprates, some organic salts, or heavy Fermion compounds), such changes result in a nonmonotonic dependence of several properties on the critical parameter z , measuring the distance of the chemical potential from the electronic topological transition (ETT). This has to be contrasted with the monotonic, nearly step-like z -dependence of the same quantities in the 3D case. Such a non-monotonic behavior is in agreement with the trend observed for T c as a function of pressure and other material specific quantities in several high- T c cuprate compounds. On the other hand, higher pressures than those reported in the literature (～ 10 kbar) should be investigated, in order to find evidence for ETT effects in the BEDT-TTF-based salts.     

Pressure dependence of electronic transitions of Fe,Co and Ni ions in layered dihalides

Abstract

The near infra-red absorption peaks due to transition metal ions in four halides of Fe, Co and Ni have been studied as a function of pressure. The behaviors of these ions' absorption peaks under pressure are found to be quite different. While the energy of the absorption peak in Col₂ increases with pressure similar to the behavior of transition metal ions in cubic crystal fields, the absorption peaks in the Fe halides are found to be almost independent of pressure. In Nil₂ two absorption peaks exhibit level-crossing at about 2GPa. The results have been interpreted in terms of a theory proposed by da Silva and Falicov [Phys. Rev. B 45,11511 (1992)] in which pressure changes the trigonal component of the crystal field at the transition metal ions.     

Thorium under strong compression—a test case for the evaluation of EOS data by different forms and procedures

Abstract

X-ray diffraction on Thorium under pressures to 300 GPa at ambient temperature provides new EOS data for both the (low pressure) fcc phase as well as for the (high pressure) bct phase. A detailed evaluation of these data and a comparison with previous results shows systematic differences in the fitted parameters resulting from the use of different EOS forms and from the correlations in the parameters used in all these fitting procedures. The absolute uncertainties in these parameters are elucidated and special attention is given to the compatibility of different EOS forms for phases which are related by second order transitions.     

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     

Computer simulation to the porosity

Abstract

Computer simulations to the porosity of starting materials for the synthesis of polycrystalline diamond (PCD) bodies are described. The logarithmic normal distributed spheres were deposited in three dimensions by means of random coordinates. Conditions for distances between the particles have to be satisfied. Thus one can realize a defined overlapping, contact or distance. Results were obtained for the porosity dependence as a function of standard deviation of the size distribution and particle overlapping. The comparison of calculated and measured porosity under normal pressure showed good agreement. The filling of pores yielded the estimation of pore size distribution and a clear reduction of the pore volume.