Revisiting the phase diagram of hard ellipsoids

In this work, the well-known Frenkel-Mulder phase diagram of hard ellipsoids of revolution [D. Frenkel and B. M. Mulder, Mol. Phys. 55, 1171 (1985)] is revisited by means of replica exchange Monte Carlo simulations. The method provides good sampling of dense systems and so, solid phases can be accessed without the need of imposing a given structure. At high densities, we found plastic solids and fcc-like crystals for semi-spherical ellipsoids (prolates and oblates), and SM2 structures [P. Pfleiderer and T. Schilling, Phys. Rev. E 75, 020402 (2007)] for x : 1-prolates and 1 : x-oblates with x ≥ 3. The revised fluid-crystal and isotropic-nematic transitions reasonably agree with those presented in the Frenkel-Mulder diagram. An interesting result is that, for small system sizes (100 particles), we obtained 2:1- and 1.5:1-prolate equations of state without transitions, while some order is developed at large densities. Furthermore, the symmetric oblate cases are also reluctant to form ordered phases.     

On the phase behavior of hard aspherical particles

We use numerical simulations to understand how random deviations from the ideal spherical shape affect the ability of hard particles to form fcc crystalline structures. Using a system of hard spheres as a reference, we determine the fluid-solid coexistence pressures of both shape-polydisperse and monodisperse systems of aspherical hard particles. We find that when particles are sufficiently isotropic, the coexistence pressure can be predicted from a linear relation involving the product of two simple geometric parameters characterizing the asphericity of the particles. Finally, our results allow us to gain direct insight into the crystallizability limits of these systems by rationalizing empirical data obtained for analogous monodisperse systems.     

Structure and phase transitions of the lanthanide metals

The structures and phase transitions of the lanthanide metals can be related to f orbital contributions to the bonding. With increasing availability of the f orbitals the structure sequence hexagonal closest packed, double hexagonal closest packed, δ-samarium, cubic closest packed, and body-centered cubic is observed. Increases in temperature and/or pressure result in an increased availability of the f orbitals resulting in predictable phase transitions.     

Spectrochemical analysis of hard metals by a rotating disc technique: effect of impregnation and complexing agents

A spectrographic method has been developed for the simultaneous determination of the constituents W, Co, Ti, Ta and Nb and contaminants Fe, Ni, Or, Mn, Mg and Ca in hard metals. The samples were dissolved in hydrofluoric acid and nitric acid and the solutions stabilized with citric acid. Graphite discs were waterproofed in order to ensure control of the introduction of solution into the source. For the five types of hard metal samples studied no interelement effect was observed. Titanium has also been determined by means of a double-beam optical spectrometer.     

High-pressure structures and phase transformations in elemental metals

At ambient conditions the great majority of the metallic elements have simple crystal structures, such as face-centred or body-centred cubic, or hexagonal close-packed. However, when subjected to very high pressures, many of the same elements undergo phase transitions to low-symmetry and surprisingly complex structures, an increasing number of which are being found to be incommensurate. The present critical review describes the high-pressure behaviour of each of the group 1 to 16 metallic elements in detail, summarising previous work and giving the best present understanding of the structures and transitions at ambient temperature. The principal results and emerging systematics are then summarised and discussed.     

Liquid-crystal phase diagrams of binary mixtures of hard spherocylinders

We have built the liquid crystal phase diagram of several binary mixtures of freely rotating hard spherocylinders employing a second-order virial density functional theory with Parsons scaling, suitably generalized to deal with mixtures and smectic phases. The components have the same diameter and aspect ratio of moderate value, typical of many mesogens. Attention has been paid to smectic-smectic demixing and the types of arrangement that rods can adopt in layered phases. Results are shown to depend on the aspect ratio of the individual components and on the ratio of their lengths. Smectic phases are seen not to easily mix together at sufficiently high pressures. Layered phases where the longer rods are the majority component have a smectic-A structure. In the opposite case, a smectic-A(2) phase is obtained where the shorter particles populate the layers and the longer ones prefer to stay parallel to the latter in the interlayer region.     

Thermal phase transition behaviour of liquid crystals in submicron pores formed with stretched polymer fibrils

This paper discusses thermal phase transitions in a nematic liquid crystal material, filling a highly strained porous polyolefin film. Ruptured and aligned polymer fibrils, whose minimum diameter reaches several tens of nm, formed polymer networks in the porous film. From observations of temperature-controlled composite films using a polarizing microscope, it was noted that the isotropic-to-nematic phase transition temperatures of submicron liquid crystal domains, located in or near molecularly aligned streak-like polymer areas, were significantly higher than those located at some distance from these areas. It was assumed that the highly aligned polymer chains in the fine fibrils promote the nematic phase of the liquid crystal. The stretched porous polymer, exhibiting spatial ordering on a submicron scale, is thus suitable for the control of thermal phase transitions in a liquid crystal.     

Direct phase identification with SIMS by evaluation of atomic and cluster ion intensities: A Study of calcium sulfide formation in hard metals during sintering

A SIMS method for the direct identification of micro and nanophases based on the quantitative evaluation of atomic and cluster ion intensities is described and applied to the study of the formation of CaS phases in hard metals.     

Physicochemical analysis in metals science

The survey concerns the history of physicochemical analysis as a field of science and highlights its role in metals science.     

Chemical hysteresis in phase transitions in the terbium oxide-oxygen system

A thermogravimetric study of hysteresis in the TbO_xO₂ system has provided insight into phase transitions occurring among the fluorite-related rare earth oxides. A series of isobaric scanning loops at 380 Torr have been made. The scans were between TbO_1.5(?) and TbO_1.714(ι) at higher temperatures and between TbO_1.714(ι) and TbO_1.818(δ) at lower temperatures. Corresponding isobaric studies were made utilizing high temperature X-ray powder diffraction to augment the TGA experiments.It was confirmed that sections of the lower temperature loop were dependent on the rate of temperature change while the higher temperature loop was entirely reproducible. Interior scanning loops made within the lower hysteresis loop showed univariant behavior typical of a single phase when reversed in the δ′δ pseudophase region, otherwise it exhibited bivariant behavior. The upper hysteresis loop showed bivariant behavior throughout the interior of the loop. Some thermodynamic aspects and the microdomain concept as applied to hysteresis are also considered.     

Fractionation analysis of oxyanion-forming metals and metalloids in leachates of cement-based materials using ion exchange solid phase extraction

A simple and versatile solid phase extraction (SPE) method has been developed to determine the anionic species of As, Cr, Mo, Sb, Se and V in leachates of cement mortar and concrete materials in the pH range 3-13. The anionic fractions of these elements were extracted using a strong anion exchanger (SAX) and their concentrations were determined as the difference in element concentration between the sample and the SAX effluent. Inductively coupled plasma mass spectrometry (ICP-MS) was used off-line to analyse solutions before and after passing through the SAX. The extraction method has been developed by optimizing sorbent type, sorbent conditioning and sample percolation rate. Breakthrough volumes and effect of matrix constituents were also studied. It was found that a polymer-based SAX conditioned with a buffer close to the sample pH or in some cases deionised water gave the best retention of the analytes. Optimal conditions were also determined for the quantitative elution of analytes retained on the SAX. Extraction of the cement mortar and concrete leachates showed that most of the elements had similar distribution of anions in both leachate types, and that the distribution was strongly pH dependent. Cr, Mo and V exist in anionic forms in strongly basic leachates (pH > 12), and significant fractions of anionic Se were also detected in these solutions. Cr, Mo, Se and V were not determined as anions by the present method in the leachates of pH < 12. Anionic As and Sb were found in small fractions in most of the leachates.     

Chemical potential of a hard sphere fluid adsorbed in model disordered polydisperse matrices

We consider a model for adsorption of a simple fluid in disordered polydisperse adsorbents. The fluid consists of hard sphere particles. On the other hand, the adsorbents of this study are modeled as a collection of hard spheres with their diameter obeying a certain distribution function. Our focus is in the evaluation of the chemical potential of the fluid immersed in such a polydisperse material. It permits us to obtain porosity and pore size distribution for the adsorbent, as well as a set of adsorption isotherms. The latter have been calculated theoretically and by grand canonical Monte Carlo simulations. We observe that the width of assumed polydispersity distribution affects all the properties of the system. Nevertheless, the effect of matrix packing is dominant in determining adsorption for this class of models. We are convinced that the matrix structures generated via more sophisticated algorithms would exhibit stronger effects of polydispersity on the entire set of properties of adsorbed simple fluids.     

Chemical speciation of trace metals in seawater: a review.

The recent development of the chemical speciation of trace metals in seawater is described. The speciation studies reveal that metal ion complexation is one of the most important processes in seawater; especially, most bioactive trace metals, such as Fe(III) and Cu, exist as complexes with ligands in dissolved organic matter. The organic ligands in seawater are characterized with metal ions selected by the HSAB concept. A strong organic ligand, which originates from marine microorganisms, is classified as a hard base including carboxylates. The free organic ligand concentrations in seawater are buffered by complexation with excess amounts of Ca and Mg in seawater. The chemical equilibrium model suggested that the concentrations of bioactive free metal ions are at an optimal level to activities of marine microorganisms. For chemical speciation, it is important to have a better understanding of the ecological roles of trace metals in seawater.