Showing papers on "Phase (matter) published in 1976"

Critical Behaviour of Conductivity and Dielectric Constant near the Metal-Non-Metal Transition Threshold

Abstract: A system consisting of randomly distributed metallic and dielectric regions is considered. The metal-non-metal transition takes place when the volume fraction of the metallic phase approaches the percolation threshold. It is shown that the static dielectric constant diverges near the threshold. Critical indexes are introduced which describe the behaviour of the conductivity and the dielectric constant near the threshold as functions of the volume fraction and frequency. The case of non-zero dc conductivity of dielectric regions is considered also. It is shown that all indexes describing the critical behaviour of complex conductivity can be expressed by two indexes which are known from computer and model experiments. The results of computer calculations of Webman et al. are analysed.

First phase nucleation in silicon–transition‐metal planar interfaces

Abstract: What is the first compound that nucleates in planar solid silicon–transition‐metal binary couple reactions whose members form bulk equilibrium compounds? We propose, for couples annealed at low temperatures, the following simple rule: The first compound nucleated in planar binary reaction couples is the most stable congruently melting compound adjacent to the lowest‐temperature eutectic on the bulk equilibrium phase diagram. The predictions of this rule are compared with experimental results.

Multiphase Microemulsion Systems

Abstract: Economical microemulsion flooding inevitably involves microemulsion phases immiscible with oil, water or both; and oil recovery is largely affected by displacement efficiency during the immiscible regime. Therefore, it is pertinent to study interfacial tension in relation to multiphase microemulsion behavior. Three basic types of multiphase systems are identified and used to label phase transitions that occur when changes are made in salinity, temperature, oil composition, surfactant structure, cosolvent, and dissolved solids in the aqueous phase. Directional effects of these changes on phase behavior, interfacial tension, and solubility number are tabulated for the alkyl aryl sulfonates studied, and conjectured to hold for all anionic surfactant systems. This approach provides a convenient method for preliminary screening of surfactants for tertiary oil recovery. Interfacial tensions were found to correlate with solubility number in the various microemulsion phases. (18 refs.)

Organic Linear Conductors as Systems for the Study of Electron-Phonon Interactions in the Organic Solid State

Abstract: A model calculation shows that each phonon band which couples to the electron density in an organic linear-chain semiconductor effectively develops an infrared activity along the chain axis. The origin of the effect lies in phase oscillations of additional charge-density-wave distortions which inevitably arise in the presence of electron-phonon interactions. This suggests that organic linear conductors may constitute unique systems for the study of electron-phonon interactions in the organic solid state.

Elastic and Optical Properties of Some 4′-n-Alkyl-4-Cyanobiphenyls

Abstract: The bend elastic constants (k 33) and order parameters (S) derived from optical measurements in the nematic phase of pentlyl, hexyl, heptyl and octyl cyanobiphenyls (5CB, 6CB, 7CB and 8CB respectively) are reported. The odd-even effect is clearly seen in the K 33 values of all four compounds and in the S values of the first three. 5CB, 6CB and 7CB show only the nematic mesophase but their elastic constants indicate the existence of smectic-like short range order. For example, even the lowest homologue, 5CB, does not obey the law k 33 ∞ S 2 given by the mean field throry. 7CB, has about the same order parameter as 5CB but a much higher elastic constant. 8CB, which shows a smectic A phase as well, exhibits a lower k 33 than 7CB near the nematic-isotropic transition point but a pronounced pretransitional increase near the A-N transition.

Interplay of the monohydride phase and a newly discovered dihydride phase in chemisorption of H on Si(100)2 × 1

Abstract: We have established that chemisorption of atomic hydrogen on Si(100) forms a previously unreported dihydride phase, Si(100)1 \ifmmode\times\else\texttimes\fi{} 1::2H, as well as the monohydride phase Si(100)2 \ifmmode\times\else\texttimes\fi{} 1:H. The interplay of these phases as exposure and temperature are varied casts new light on surface structure and bonding, and on the kinetics of thermal desorption. These studies suggest that the reconstruction inherent in the clean Si(100)2\ifmmode\times\else\texttimes\fi{} 1 surface is achieved by the pairing of adjacent surface rows (pairing model) rather than by surface vacancies (vacancy model).

Neutron scattering study of nitrogen adsorbed on basal-plane-oriented graphite*

Abstract: Thermal-neutron scattering has been used to investigate the structure of nitrogen films adsorbed on Grafoil, a basal-plane-oriented graphite. Diffraction scans were made at coverages between 1/3 of a monolayer and 7/4 monolayers over a temperature range from 10 to 90 K. The observed line shapes were analyzed and found to be generally consistent with the predictions of two-dimensional diffraction theory. From the data at least three distinct surface phases can be identified: (i) a low-temperature epitaxial phase with a triangular lattice structure, (ii) a more compressed low-temperature phase with the same structure but not in registry with the substrate, and (iii) one or more high-temperature disordered phases. Changes in the slope of the adsorption isotherm are found to correlate well with the observed transitions from one phase to another.

Kinetics and Mechanism of Emulsion Polymerization

Abstract: The course of emulsion polymerization may be considered as involving three intervals: Interval I, where particle formation takes place. The end of this interval is not dependent upon the degree of conversion, but on the total amount of polymer formed. With usual recipes, it ends at about 1-5% conversion. Interval II lasts from the end of Interval I until monomer disappears as a separate phase. In this interval, the particle number is usually found to be constant, the particle volume increases proportional to conversion, the monomer concentration in the particles is approximately constant, and therefore the termination is also constant within the particles. Interval III starts when the monomer disappears as a separate phase. The transition from Intervals II to III is determined by the degree of conversion and differs for different monomers. In cases where the solubility of monomer in the water phase is low, the monomer present in the aqueous phase may usually be neglected compared to the monomer p...

Fluids in the evolution of granitic magmas: Consequences of finite CO2 solubility

Abstract: The small but finite solubility of CO 2 in granitic magmas under crustal conditions, together with the common occurrence of CO 2 in likely magma source materials, suggests that granitic magmas will often be accompanied by a CO 2 -H 2 O fluid phase during their ascent in the crust. Polybaric and isobaric calculations have been made for model systems with varying total volatile content, initial CO 2 /H 2 O ratios, crystallization rates, and closed-system or open-system conditions. The calculations demonstrate that the presence of CO 2 in an evolving magma system can result in greatly differing values of H 2 O activity (and hence H 2 O content, phase equilibria, and physical properties of the magma). Specifically, if the mass ratio CO 2 /H 2 O is ≥0.4 and the initial mass ratio of total volatiles to silicate magma is ≥0.05, then, if little or no loss of the fluid phase occurs during magma evolution, the activity of H 2 O will remain nearly constant. This is in strong contrast to all other possible cases in which the activity of H 2 O increases rapidly with decreasing pressure and (or) anhydrous phase crystallization, invariably reaching a value of unity. It is also demonstrated that if CO 2 is present in a fluid phase in the magma source region, then there will be a fluid present throughout the evolutionary history of the magma. The presence of fluid bubbles in the magma should considerably alter many properties of the magma system such as heat transfer, mass transfer, and viscosity.

Transferability of Phase Shifts in Extended X-Ray Absorption Fine Structure

Abstract: Phase shifts in extended x-ray-absorption fine-structure (EXAFS) measurements have been empirically determined for atom pairs. For photoelectron energies g 100 eV it is shown that these phase shifts, because they are essentially independent of chemical environment, can be used with EXAFS spectra to determine interatomic distances typically to accuracies of 0.02 \AA{}.

Magnetocrystalline Anisotropy of Low Temperature Phase of Magnetite

Abstract: The magnetocrystalline anisotropy of low temperature phase of magnetite (Fe 3 O 4 ) was measured for a monoclinic single phase specimen by using computerized fully-automatic torque magnetometer. The anisotropy is expressed by \begin{aligned} E_{\text{a}}=K_{a}\alpha^{2}_{a}+K_{b}\alpha^{2}_{b}+K_{aa}\alpha^{4}_{a}+K_{bb}\alpha^{4}_{b}+K_{ab}\alpha^{2}_{a}\alpha^{2}_{b}-K_{u}\alpha^{2}_{111}, \end{aligned} where α a , α b and α 111 are direction cosines of the magnetization with respect to the monoclinic a -, b - and cubic [111] axes, respectively, the last of which coincides with the longest cube diagonal. The values of the anisotropy constants at 4.2 K are: K a =25.5, K b =3.7, K u =2.1, K a a =1.8, K b b =2.4 and K a b =7.0 in 10 5 erg/cm 3 . It was found that K a , K b and K u exhibit the temperature dependence of an activation type with the activation energy of about 0.02 eV. It was also found that the constants K a a , K b b and K a b are well expressed in terms of cubic K 1 . The mechanism of the an...

The effect of chain length of bonded organic phases in reversed phase high performance liquid chromatography

Abstract: Bonded hydrocarbon phases have been prepared by the reaction of organotrichlorosilanes with silica (Lichrosorb Si-100, 10 μm, Merck) using conditions for maximum coverage. Alkyl phases, CH3 (CH2)n-With n=7, 10, 12, 14, 17 and 20 and arylakyl phases, Ph (CH2)n—with n=0,2,4,6 were investigated. In reverse phase chromatography using water/methanol phases a linear relationship was between the capacity ratios k′ and the amount of coverage, independent of chain length. The selectivity, however, depends on the chain length of the bonded phase and molecular structure of the solute, whereas for halogenated benzene derivatives the selectivity is constant, and changing the mobile phase composition shows only a minor effect. The selectivity of 4,4′-dibromodiphenyl in relation to the benzene derivatives is strongly influenced by the chain length and solvent composition. The utilization of chemically bonded organic long chain phases with maximum coverage makes it possible to reduce the water content in the water/methanol phase increasing the efficiency and loading capacity. The extent of maximum coverage was slightly dependent on the chain length and showed only a 10% decrease from n-octyl-to heneicosyl phase. Treatment of the bonded organic phase with TMCS was not effective with long chain materials, but further reaction occurred with bonded phenyl phase, which could be shown by IR-analysis.