Showing papers on "Phase (matter) published in 1978"

Raman spectroscopic method for determining the crystallinity of polyethylene

Abstract: The Raman spectrum of partially crystalline polyethylene can be described as a superposition of three components, which originate from the orthorhombic crystalline phase, a meltlike amorphous phase, and a disordered phase of anisotropic nature, where chains are stretched but have lost their lateral order. The mass fractions involved in the three phases can be derived directly from the integral intensities of characteristic bands without an additional calibration procedure. A comparison of the results obtained for a variety of samples shows agreement with the crystallinities derived from the density, and the small-angle and wide-angle x-ray diagrams. Data indicate that the disordered anisotropic phase is located at a transition zone between crystalline and amorphous layers. Application of the method in a temperature-dependence experiment permits a detailed examination of partial melting.

Effect of stress-induced phase transformation on the properties of polycrystalline zirconia containing metastable tetragonal phase

Abstract: Polycrystalline zirconia containing a high content of metastable tetragonal phase shows high strength (∼ 700 MPa), high fracture toughness (Kc = 6 to 9 MN m−3/2) and small grain size (<0.3jμm). The strength and grain size remain nearly constant over a wide range of tetragonal phase content (100 to 30%). At a low concentration of tetragonal phase <30%, there is a rapid decrease in strength accompanied by a rapid increase in grain size. These results are explained by means of a stress-induced phase transformation in the metastable tetragonal phase.

The polymorphic phase behaviour of phosphatidylethanolamines of natural and synthetic origin

Abstract: 1. The polymorphic phase behaviour of aqueous dispersions of phosphatidylethanolamines isolated from human erythrocytes, hen egg yolk and Escherichia coli have been investigated employing 31P NMR techniques. All species exhibit well defined, reversible bilayer to hexagonal (H11) phase transitions as the temperature is increased. The temperatures at which these transition take place (10, 25--30 and 55--60 degrees C for erythrocyte, egg yolk and E. coli phosphatidylethanolamine, respectively) are sensitive to the fatty acid composition, occurring at a temperature up to 10 degrees C above the high temperature end of the hydrocarbon phase transition as detected by differential scanning calorimetry. In some cases the bilayer to hexagonal (H11) transitions may also be detected employing calorimetric techniques. 2. The addition of equimolar concentrations of cholesterol to these naturally occurring phosphatidylethanolamines does not dramatically affect the bilayer-hexagonal (H11) transition temperature, producing changes of up to 10 degrees C. 3. 18 : 1t/18 : 1t phosphatidylethanolamine undergoes the bilayer to hexagonal (H11) phase transition as the temperature is increased through the interval 50--55 degrees C. Alternatively, hydrated 12 : 0/12 : 0 phosphatidylethanolamine remains in the bilayer phase at temperatures up to 90 degrees C (50 degrees C above the hydrocarbon phase transition temperature). 4. The presence of 100 mM NaCl or 10 mM CaCl2 in aqueous dispersions of egg yolk phosphatidylethanolamine does not alter the temperature-dependent polymorphic phase behaviour significantly. However, at 40 degrees C, increasing the p2H above 8.0 results in progressive inhibition of the hexagonal (H11) phase and the appearance of a phase possibly of cubic structure at p2H 9.0. At p2H 10.0 the bilayer phase is preferred. 5. It is suggested that in biomembranes containing phosphatidylethanolamine as a majority species (such as that of E. coli) the fatty acid composition may primarily reflect the need to maintain bilayer structure. Alternatively, it is pointed out that in mammalian membranes such as that of the erythrocyte, phosphatidylethanolamine tends to destabilize bilayer structure. The resulting possibility that transitory non-bilayer lipid configurations may occur may be directly related to many important properties of biological membranes.

A thermodynamic analysis of the amorphous to crystalline calcium phosphate transformation.

Abstract: A thermodynamic analysis of the precipitation of amorphous calcium phosphate (ACP) and its transformation to crystalline apatite has been made. A nearly constant ion product, over a wide variety of conditions, was obtained for a tricalcium phosphate (TCP)-like phase suggesting that the molecular unit which governs the solubility of ACP may be similar in composition to TCP. The introduction of 10% acid phosphate into the formula for the TCP ion product improves the fit of experimental data and results in an invariant ion product. The stability of ACP in solution was found to be dependent upon its thermodynamic instability with respect to an octacalcium phosphate (OCP)-like phase. The dependence of the induction period for the amorphous to crystalline transformation upon the pH and the Ca/P ratio of the solution is best explained by the assumption that an OCP-like phase is initially nucleated on the surfaces of the ACP particles. The events that occur in the immediate post-transition period suggest the hydrolysis of this OCP-like material to an apatitic phase.

Brownian motion of highly charged poly(L-lysine). Effects of salt and polyion concentration

Abstract: The Brownian motion of a single sample of high-molecular-weight poly(L-lysine) [(Lys)n, n = 955] has been studied by dynamic light scattering over a wide range of NaBr concentrations and at three different polyion concentrations. A substantial decrease in scattered intensity is associated with the transition from the ordinary phase to the low-salt extraordinary phase. At the salt concentration where the transition takes place the relaxations are non-exponential and appear to exhibit at all angles a rapid relaxation (τ ≅ 10 μsec) that is presumed to be a manifestation of the kinetics of the transition process. The K2 dependence of the slow relaxation rates in the extraordinary phase has been confirmed within the experimental error. The extrapolated infinite-dilution values of the diffusion coefficients in the ordinary phase are observed to decline precipitously below 10−2M salt to astonishingly small values, indicating a dramatic rise in the friction factors of the isolated polyions. An extensive discussion of these findings in relation to the theory employed here and to existing data in the literature is also given.

Phase behavior of synthetic phosphatidylglycerols and binary mixtures with phosphatidylcholines in the presence and absence of calcium ions.

Abstract: Using differential thermal analysis, scanning calorimetry and light scattering, transition temperatures and enthalpy data for the gel to liquid crystalline phase transitions of five synthetic phosphatidylglycerol sodium salts (PG-Na+) were measured. The values obtained were almost identical with literature values for the corresponding phosphatidylcholines (PC). However, transition temperatures for the fully protonated forms of the saturated phosphatidylglycerols (PG-H+) were approximately 20 degrees C higher. For binary mixtures of PG-Na+ and PC in which the acyl chains of the two species were identical, the width of the thermal transition for the phase change was not appreciably greater than that observed with either of the two components alone. In contrast, mixing of PG-Na+ and PC with different chain lengths increases the transition width. In the presence of Ca2+, narrow transitions were also observed with mixtures of PG and PC when the chain length of the PG-Ca2+ was equal to or two carbons shorter than the PC but the transition width was clearly increased when the chain length of the PG-Ca2+ was two carbons longer than the PC. Mixing lipids with greater differences in chain length or mixing saturated lipids with unsaturated lipids in the presence of Ca2+ produced two minima in the thermograms, clearly indicative of phase separation. In sum, these results provide evidence for a high degree of miscibility of the phosphoglycerol and phosphocholine head groups, either in the presence or absence of Ca2+, such that the characteristic phase behavior of each mixture is determined primarily by differences in the hydrocarbon chain structure.

Interactions in the Co/Si thin‐film system. I. Kinetics

Abstract: Interactions in the Co/Si thin‐film system were investigated by MeV backscattering and x‐ray‐diffraction techniques. It was found that Si diffuses through the Co layer and accumulates at the sample surface at about 300 °C. Increasing the temperature causes the growth of the Co2Si phase, followed by the simultaneous growth of the CoSi phase. The growth rates for both phases have a square root of time dependence. The activation energies of growth are 1.5 and 1.9 eV for the Co2Si and CoSi phase, respectively. A model for the growth of multiple phases is suggested. The transformation between CoSi2 and CoSi is found to be a reversible reaction.

Chemical‐state effects in Auger electron spectroscopy

Abstract: We have used Auger spectroscopy as a probe of local chemical environment both in the gas and condensed phases using a systematically chosen series of molecules [H2O, CH3OH, (CH3)2O, CH4, C2H4, and C2H2]. For the series of gas phase molecules, H2O, CH3OH, (CH3)2O, and CH4, where oxygen and carbon are, respectively, in similar bonding arrangements, characteristic fingerprint spectra (methanelike for C and waterlike for O) are shown to result. Additional fine structure, which is dependent on the specific molecular environment, appears on the spectra. In contrast, dramatic differences are observed for the series CH4, C2H2, and C2H4 in which major differences in hybridization exist at the carbon site. H2O, CH3OH, and (CH3)2O were studied both in the gas phase (electron excited) and in the condensed phase (x‐ray excited). The O(KVV) (K level–valence–valence transition) and C(KVV) spectra are shown to be similar when comparing the gas–solid results only if the multilayer spectra are properly corrected for electr...

Martensitic Transformation as a Typical Phase Transformation in Solids

Abstract: Publisher Summary The chapter presents a discussion on martensitic transformation as a typical phase transformation in solids. Martensitic transformation is a polymorphic phase transition in solids consisting of the regular rearrangement of the crystal lattice, the constituent atoms of which do not interchange places. The transformation usually proceeds by means of the nucleation of new phase crystals inside the initial phase. These crystals form a complex heterophase system with a characteristic structure—that is, morphology, an internal substructure, and mutual arrangement of the individual phase components of the system. For different solids and under different conditions, both the kinetics and the morphology of the products may differ essentially. Nevertheless, some structure-morphological, thermodynamic, and kinetic features do exist that may be considered as typical of the martensitic transformations. The chapter presents a figure on the concrete data on the transformation in the Fe (30%) Ni alloy and other ferrous alloys. Regarding crystal nucleation kinetics, two types of martensitic transformations are distinguished: athermal and isothermal transformations. The chapter also presents the three general theoretical approaches of thermodynamics and kinetics of martensitic transformations. The concept of this study is based on the idea that martensitic transformation may be considered as a special type of phase transition (the so-called strain transition).

The microstructural location of the intergranular metal‐oxide phase in a zinc oxide varistor

Abstract: High‐resolution electron microscopy of a commerical ZnO‐based varistor reveals that the Bi‐rich intergranular phase is found at all three‐ and four‐grain junctions and that the majority of the ZnO grains are not surrounded by an oxide‐barrier film as previously believed.

Interfacial Tension and Phase Behavior of Surfactant Systems

Abstract: Perturbations of system parameters are shown to affect the surfactant partitioning between oil and water through changes in the relative standard chemical potentials of the surfactant in the 2 phases. For the systems investigated, low interfacial tensions are associated with near-unity values of the partition coefficient. The conditions necessary for optimum low tension and phase behavior at high surfactant concentrations are identical to those required for a tension minimum at low surfactant concentrations, where solibilization effects may not be visible. The effects of alcohol and other system parameters on surfactant chemical potential and hence on interfacial tension are noted. (18 refs.)

Hot-pressing and the ?-? phase transformation in silicon nitride

Abstract: The kinetics of densification and the kinetics of theα-β phase transformation have been measured during the hot-pressing of silicon nitride ceramics using magnesia as additive. Two mechanisms of densification have been identified. The first is a very rapid particle rearrangement, liquid-enhanced above 1550° C, which operates up to relative densities of about 0.65. The kinetics of the much slower decelerating second stage obey the Coble hot-pressing equation and the rate of densification is found to be proportional to the amount of additive. The controlling mechanism is believed to be diffusion in a boundary second phase, and values for the diffusion coefficient,D b, of the rate-controlling species in the boundary phase for temperatures above and below 1550° C are given. The kinetics of theα toβ transformation, the greater part of which occurs after densification is complete, are described by a first order reaction; the dependence of rate on the quantity of additive and on temperature is similar to that found for densification, and a similar controlling mechanism is believed to be responsible for the two processes.

Ordered oxygen overlayer associated with chemisorption state on Al(111)

Abstract: We present experimental evidence of a unique, ordered chemisorption phase in the initial interaction of oxygen with the Al(111) surface. At high oxygen exposure or high temperature, this phase is shown to transform irreversibly to a bulklike aluminum oxide. The measured temperature dependence, as well as the low-energy electron diffraction, suggests a threefold, centered bonding site. A comparison between calculated and experimental valence-band density of states for the oxygen-covered Al(111) surface is made for the estimated oxygen-atom-substrate-surface distance.

Raman spectroscopic studies of the OH stretching region of low density amorphous solid water and of polycrystalline ice Ih

Abstract: Extensive experimental studies of the OH stretching region of the Raman spectra of polycrystalline ice Ih and low density amorphous solid water (H2O (as)) are reported. Analysis of these spectra, using insights from theoretical calculations, leads to the following conclusions: (i) The overall span and distribution of features in the spectra result from strong intermolecular coupling of OH oscillators. This coupling is so strong that the solids cannot be considered to be composed of weakly perturbed water molecules, and the identification of features of their spectra with molecular vibrations is not useful. (ii) At the low end of the OH stretching region there is a totally symmetric mode—that is the amplitudes of motion on all oscillators are in phase, but these amplitudes vary erratically from site to site in the solid. The temperature dependence of the shift in frequency of this mode is likely all accounted for by thermal expansion, the temperature dependence of its FWHM is accurately described as due to scattering by optical phonons with effective frequency 200 cm−1. (iii) The earlier proposed interpretation of the Raman spectrum of H2O(as) by Venkatesh, Bates and Rice [J. Chem. Phys. 63, 1065 (1975)] is withdrawn. Their interpetation was based on the assumptions that they had observed the spectrum of high density H2O (as), which we now believe to be incorrect, and that the solid could be considered an assembly of weakly perturbed water molecules, which we now believe to be untenable.

Effect of cholesterol on the molecular motion in the hydrocarbon region of lecithin bilayers studied by nanosecond fluorescence techniques.

Abstract: Effects of cholesterol on the dynamic structure of the hydrocarbon region of dipalmitoyllecithin vesicles were examined. Decays of the emission anisotropy and the fluorescence intensity of 1,6-diphenyl-1,3,5-hexatriene embedded in lecithin-cholesterol vesicles were measured over a temperature range of 10--60 degrees C. The emission anisotropy decreased rapidly with time and then leveled off. The rotational motion of the probe was analyzed by a model of wobbling diffusion confined in a cone. Cholesterol (10--50 mol%) decreased the cone angle in the liquid-crystalline phase and increased it in the gel phase. In the presence of 33 mol% cholesterol, the wobbling diffusion constant increased in the gel phase and changed little in the liquid-crystalline phase. The viscosity in the cone decreased in the gel phase and remained almost unchanged in the liquid-crystalline phase in the presence of 33 mol% cholesterol. The total fluorescence intensity followed a singel exponential decay independently of the cholesterol content 0--50 mol%.

Growth morphology of the tetragonal phase in partially stabilized zirconia

Abstract: The zirconia-rich, metastable tetragonal phase in partially stabilized zirconia—magnesia, zirconia—calcia and zirconia—yttria is examined using electron microscopy and electron and X-ray diffraction. The tetragonal phase precipitate distribution is that normally associated with homogeneously nucleated coherent precipitation. An attempt is made to explain the growth morphology of the tetragonal phase in terms of the cubic—tetragonal lattice parameter mismatch. It is found that the tetragonal phase is retained at room temperature provided coherency with the cubic matrix is retained. Once coherency is lost, due to growth strains or mechanical influences, the precipitate reverts to the room temperature stable monoclinic form.