Showing papers on "Transition temperature published in 1981"

The physical and structural properties of superconducting A15-type Nb-Sn alloys

Abstract: Bulk samples covering the entire homogeneity range of the Nb-Sn A15-phase were prepared by a new method: levitation melting under high argon pressure. The variations of the lattice parameter, superconducting transition temperature, resistivity and critical field slope were measured as a function of composition. A low-temperature X-ray diffraction study was undertaken in order to fix the compositional limit of the tetragonal phase. The theoretical expectations for the critical field slopes at the transition temperature, Tc, based on actually-observed alloy parameters, were found to be in good agreeement with measured values.

Hexagonal phases in phospholipids with saturated chains: phosphatidylethanolamines and phosphatidic acids.

Abstract: The structure of phospholipids with saturated chains was investigated by differential scanning calorimetry and by X-ray diffraction. It is shown that both phosphatidylethanolamines and phosphatidic acids can exhibit a hexagonal phase at high temperature. The temperature of the transition to the hexagonal phase is dependent on chain length and sodium salt concentration. Increasing the chain length or the sodium salt concentration results in a decrease in the transition temperature. In addition to this transition at high temperature, a calorimetric transition at low temperature, a calorimetric transition at low temperature is detected in some phospholipids.

The Re-entrant Nematic, Enhanced Smectic A Phases and Molecular Composition

Abstract: Phase diagram and X-ray studies of mixtures of 8OCB (cyanooctyloxybiphenyl) and 4O8 (butyloxybenzilidene octylaniline)and 8OCB and 6OCB (cyanohexyloxybiphenyl) demonstrate: 1. 4O8 molecules inhibit the pairing of the 8OCB molecules. One 4O8 molecule is attributed with the breakup of one 8OCB pair. 2. The maximum pressure at which the smectic A phase is stable increases as the number of 8OCB pairs decreases. 3. The temperature at which the smectic A phase is stable is maximum when the mixture is most polar [no 8OCB pairs at all]. For these mixtures, there is no nematic phase. 4. There is no smectic A phase when all of the 8OCB molecules are paired. 5. When the nematic-smectic A transition temperature of the mixture approaches the nematic-isotropic transition temperature of the pure compound, the nematic phase is completely suppressed. Assuming that the smectic A order parameter is coupled to the nematic order parameter and the coupling depends on temperature and composition, a Landau description i...

Kinetics of low temperature structural relaxation in two (Fe‐Ni)‐based metallic glasses

Abstract: Structural relaxation processes below the glass transition temperature have been investigated calorimetrically via isothermal annealing and the time shift method for (Fe.5Ni.5)75P16B6Al3 and Fe40Ni40P14B6 glasses. The relaxation spectra are described by broad distributions in both activation energies and frequency factors. The low temperature anneals stabilize a portion of the relaxation spectrum and the annealed sample upon heating shows an endothermic reaction above the annealing temperature. The effects of successive annealing are additive provided that the anneals are followed by a lower temperature one. The results may account for many aging behaviors, such as the wide variation in activation energies, ’’reversibility’’ in Curie temperature and the cross‐over phenomenon.

High-resolution studies of cubic perovskites by elastic neutron diffraction. II. SrTiO3, KMnF3, RbCaF3 and CsPbCl3

Abstract: For pt.I see ibid., vol.12, p.5393 (1979). A programme of elastic neutron-diffraction experiments has been undertaken to derive (and compare) atomic distribution functions, with high spatial resolution, for the ABX3 antiferroelectric perovskites SrTiO3, KMnF3, RbCaF3 and CsPbCl3 in their cubic phase. The background and motivation of the work, and some first results for CsPbCl3, have been published in paper I. Data collected 5-10K above the cubic to tetragonal transition temperature, Tc, have been used in least-squares refinements of parameters describing anharmonic distributions for the metal and X ions. For KMnF3 and RbCaF3 the same parameters have been refined from data collected approximately 100K above Tc-at room temperature. The metal ions are found to have relatively small anharmonicity, which decreases on cooling. By contrast the X ions have an anharmonicity which becomes anomalously enhanced near Tc. This effect increases strongly through the series SrTiO3 to KMnF3 to RbCaF3 to CsPbCl3: for the O-ion distribution in SrTiO3 no anharmonic parameter (i.e. above second order) is significantly non-zero, even at Tc+5K, whilst the Cl-ion distribution in CsPbCl3 at Tc+5K has some significant sixth-order parameters. The form and temperature dependence of the X-ion anharmonicity are examined in relation to current attempts to understand some structural phase transitions in terms of 'precursor' order (clusters), already present in the high-temperature phase.

Sub‐sub Tg structural relaxation in glassy polymers

Abstract: Structural relaxation in a temperature range from 32 to 62° K glass transition has been investigated, for the first time, for a nearly monodisperse polystyrene using a differential scanning calorimeter and a thermal mechanical analyzer. Low temperature anneals in the vicinity of 320 K stabilize the glassy structure and lead to volume contraction. Upon heating, the annealed samples show an excess endothermic peak and exhibit a gradual expansion associated with structural recovery above the annealing temperature. Significantly, the samples recover the initial heat content and volume without reheating through the glass transition temperature. This sub‐Tg structural relaxation behavior is in many respects distinct from that commonly observed near the glass transition. Kinetics of relaxation processes are also discussed.

Translational mobility of glycophorin in bilayer membranes of dimyristoylphosphatidylcholine

Abstract: The translational diffusion of the integral membrane sialoglycoprotein from erythrocyte membranes, glycophorin, incorporated into bilayer membranes of dimyristoyl-phosphatidylcholine at a protein/lipid molar ratio of 1:4500 was examined by using the fluorescence redistribution after photobleaching technique. A plot of the diffusion coefficient vs. temperature shows a sharp decrease in the rate of diffusion at about 15 degrees C. This sharp diffusion transition is at a temperature some 9 degrees C lower than the calorimetrically measured lipid gel-liquid crystalline phase transition temperature of the system. The difference between the diffusion transition temperature and the lipid phase transition temperature is attributed toi a localized fluidizing effect of the protein upon the gel phase lipid. The value of the diffusion coefficient above 15 degrees C was found to be (1-2) x 10(-8) cm(2)s(-1), and below 15 degrees C it was lower than about 5 x 10(-11)cm(2)s(-1). The fluorescence recovery in the bleached area as a consequence of diffusional redistribution appeared to be due to a single diffusing species at temperatures above 15 degrees C and due to more than one diffusing species below this temperature.

Magnetic field dependence of the nematic-isotropic transition temperature

Abstract: The nematic-isotropic transition temperature was measured by means of an optical technique as a function of applied magnetic field for the material octyl-cyanobiphenyl. The results represent the first unambiguous demonstration of the magnetic field term in the thermodynamic equation for a liquid-crystal phase transition.

Two-dimensional phase transition in granular NbN films

Abstract: The resistance of several granular NbN films has been measured as a function of temperature as they approach the superconducting state. The lowest-resistance data deviate from earlier reported power-law behavior above a phase-ordering transition T/sub c/j, but are consistent with the exponential expression describing the resistance above a two-dimensional phase-transition temperature, T/sub 2D/, of the type proposed by Kosterlitz and Thouless. At T/sub 2D/ a finite supercurrent appears.

Piezoelectricity in γ‐form Nylon 11

Abstract: X‐ray studies of unpoled and poled γ‐form Nylon 11 films indicate that the polarization mechanism is not due to a field‐induced crystal transformation from the γ to the α form, suggesting the existence of a polar γ form. The temperature dependence of d31 shows no decrease in polarization at the α‐γ transition temperature (∼95 °C), indicating that the hydrogen bonds in the poled film are not able to randomize. These results suggest the possibility that our understanding of the γ‐form structure of Nylon 11 is incomplete.

Phosphorus interdiffusivity inα-Fe binary and alloy systems

Abstract: The isothermal segregation kinetics of phosphorus to Fe and Fe-based alloy surfaces have been monitored from 783 to 923 K using Auger electron spectroscopy. The P segregation kinetics are consistent with a model which assumes bulk diffusion of P to be the rate controlling mechanism in the segregation process. The activation energy and preexponential for P diffusion in Fe calculated from the diffusion data are 314 ± 30 kJ · mol−1 and 8 × 105 cm2s in the temperature range studied. Similar results are found for the carbon free Fe-based alloys studied. Phosphorus segregation kinetics for a carbon containing alloy are more rapid, with evidence of a dependence on carbon concentration. The activation energy and pre-exponential calculated in this study for carbon-free alloys are considerably higher than published values measured at higher temperatures by conventional techniques. It is pointed out that an increase in activation energy with decreasing temperature is consistent with observations in several studies of Fe and other ferromagnetic alloys of an increase in activation energy for diffusion below the ferromagnetic transition temperature.

Processing of melt processable liquid crystal polymer by control of thermal history

Abstract: A process for the extrusion of melt processable liquid crystal polymer which provides improved processability characteristics is provided. The process comprises the steps of subjecting the liquid crystal polymer to an elevated temperature between the DSC transition temperature and the degradation temperature of the polymer; cooling the liquid crystal polymer to a processing temperature which is between the DSC transition temperature and the elevated temperature and at which the liquid crystal polymer is less subject to degradation than at the elevated temperature; and extruding the liquid crystal polymer into the form of a shaped article having mechanical properties which are improved over those of articles formed from liquid crystal polymer which has not been subjected to the elevated temperature. Preferably, the liquid crystal polymer exhibits at the elevated temperature a reduction by at least a factor of 5 in the melt viscosity of the liquid crystal polymer, as compared to the melt viscosity at the DSC transition temperature, without significant degradation of the liquid crystal polymer. The reduction in melt viscosity is substantially retained by the liquid crystal polymer at the processing temperature, and the liquid crystal polymer is extruded into the form of a shaped article while the reduction in melt viscosity is thus retained. By employing the process of the present invention, melt processable liquid crystal polymers can be conveniently processed at temperatures near the DSC transition temperature rather than at the higher temperatures commonly encountered in the prior art at which degradation is more likely to occur.

Neutron study of the incommensurate phase in (N(CD3)4)2ZnCl4 crystal

Abstract: Using the neutron scattering technique, the wavevector of the modulation has been carefully measured as a function of temperature in the incommensurate phase of d-TMATC-Zn. The modulation curve is qualitatively compared with a constant- mu Devil's staircase of the one-dimensional model. A great difference appears between the behaviour of h and d compounds, which is attributed to the isotope effect. Integrated intensities of first-order satellites follow a linear law over a 10 degrees C temperature range below the transition temperature Ti=24.7 degrees C. Odd-order satellites have been observed up to the seventh order.

Superconductivity in liquid metallic hydrogen

Abstract: By solving the Eliashberg equations for the gap function, the super-conducting transition temperature of a proposed low-temperature liquid state of metallic hydrogen is found to be comparable to that obtained, with similar approximations, for the solid. This indicates that metallic hydrogen may be a superconducting liquid in the density range 1.6> or =r/sub s/> or =1.3, where the melting point is expected to be fairly low.

Disorder in cobalt single crystals

Abstract: Disorder in both allotropic phases of pure single-crystalline cobalt was studied by elastic neutron scattering in order to separate bulk from surface effects and to remove diffuse inelastic contributions. The intensity variation along 10l, measured at different temperatures, was analysed quantitatively in terms of Jagodzinski's disorder theory. The values found for the degree of disorder in the h.c.p. phase were lower than those reported before for powder samples and remain nearly unaffected when approaching the transition temperature. The f.c.c. phase is always (below and above the transition) well ordered. However, the temperature behaviour of the f.c.c. and the h.c.p. precursor regimes in the h.c.p. and the f.c.c. modifications, respectively, is different. The connection between disorder and the martensitic h.c.p.-f.c.c. transformation is discussed.

Test of a year-annealed glass for the cohen-grest percolation transition

Abstract: The first order transition recently postulated by Cohen and Grest to lie somewhere between the normal glass transition temperature T g and the so-called “ideal glass” transition temperature T o . will be difficult to observe if it exists because of the long relaxation times needed for any structural changes in this temperature regime. We have recently identified a system in which the possibility for such a transition occurring would seem unsually high. The glass Ag 7 I 4 AsO 4 has a structural relaxation function such that it equilibrates at room temperature, 40 K below T g , in a matter of weeks. Samples of this glass which have lain in an equilibrium state 40 K below T g for a year or more have been characterized thermally to examine the possibility that a first order transition has occurred. The conclusion is that despite the favorable circumtances of the case, and the large thermal spike observed on reheating the annealed glass, nothing unaccounted for by normal annealing processes has occurred.

Critical behavior of the plasmon resonance at the metal-insulator transition in V O 2

Abstract: A large red shift in the energy of the plasmon resonance in metallic V${\mathrm{O}}_{2}$ has been observed by decreasing the temperature near the metal-to-insulator phase transition (${T}_{c}=68$\ifmmode^\circ\else\textdegree\fi{}C). Below the transition temperature the plasmon resonance disappears. The plasmon resonance has been studied from thermoreflectance spectra of an oriented single crystal. Polarized spectra varying the crystal temperature have been recorded. This experiment gives direct evidence that the metal-insulator transition in V${\mathrm{O}}_{2}$ is a Mott-type transition from delocalized conduction states to localized states on the V-V dimers. 1977

Reentrant phase transition of granular superconductors

Abstract: It is shown that the effects of the charging energy on the mean-field transition temperature of a granular superconductor depend in a profound manner on the choice of the interval for the phase phi of the order parameter. For -infinity

Coupling and isolation: Critical field and transition temperature of superconducting granular aluminum

Abstract: Measurements have been made of the critical magnetic field of superconducting granular aluminum (dapprox.30 A) down to temperatures well below T/sub c/(t/sup 2/<0.1). The results show that it is possible to have the coupling between grains weak enough and the temperature low enough so that the critical field is that of the isolated grain while at the same time the large fluctuations which exist in isolated grains of this size are suppressed. The results for T/sub c/, together with those from other sources, indicate that the maximum T/sub c/ is a function of grain size only.