Supercooling of In2Bi and InBi Melts

The effect of melt overheating T ⁺ on the critical supercooling T ^– of liquid In₂Bi and InBi is studied by cyclic thermal analysis. It is shown that, the T ^– for In₂Bi is 2.0 K, independent of the melt preheating temperature. In contrast, the T ^– for InBi varies jumpwise with T ⁺: T ^– 1.0–1.6 K at T ⁺ < 5 K, and T ^– 16 K at T ⁺ = 5–300 K, independent of the cooling rate (varied from 0.002 to 8.0 K/s). The solidification behaviors of In₂Bi and InBi are shown to correlate with the structures of their liquid and solid phases.     

Use of the DIPPR Database for the Development of QSPR Correlations: Solid Vapor Pressure and Heat of Sublimation of Organic Compounds

A group-contribution method has been developed to estimate the heat of sublimation (H _sub) at the triple point for organic solids. The correlation was developed using H _sub values from a training set of 218 compounds, and it was tested by comparing predicted and solid vapor pressures for 87 compounds (1103 data points). The predicted solid vapor pressures were obtained from the H _sub correlation using the Clausius–Clapeyron equation. The absolute average deviation in the logarithm of vapor pressure was 0.371. The new method compares favorably with Bondi's method for prediction of H _sub and represents an improvement over other available methods for predicting solid vapor pressures.     

Correlation between Superconducting Gap and Pseudogap in High-T c Cuprates

We studied the low-temperature energy gap 2 ₀ on Bi ₂ Sr ₂ CaCu ₂ o ₈₊ (Bi2212) and La _2–x Sr _x CuCO ₄ (La214) systematically over a wide range of doping level p using STS, break junction tunneling spectroscopy, Raman scattering and low-T electronic specific heat data. We have also studied the electronic specific heat of La214 in the normal state at T > T _c , and confirmed that pseudogap behavior appears at around T*, below which the in-plane resistivity and magnetic susceptibility tend to be slightly suppressed. Similar suppression appears in and of Bi2212 below the onset temperature of pseudogap T*. It is pointed out in the present study that 2 ₀ is closely related to T* in both Bi2212 and La214 systems; T* 2 ₀ /4.3k _B . It is also pointed out that 2 ₀ is in almost linear proportion to k _B T _max ( T*), where T _max is the temperature exhibiting a broad peak in –T curves and k _B T _max can be considered to give a measure of the effective antiferromagnetic exchange energy J _eff. The factors in 2 ₀ k _B T _max J _eff are 1 for La214 and 2 for Bi2212, respectively. We also report that in both Bi22l2 and La214 systems T _c roughly scales with p ₀ except in highly doped samples, where T _c 2 ₀ .     

The Distribution of the Energy Gap and Josephson IcRn Product in Bi2Sr2CaCu2O8+x by Tunneling Spectroscopy

We present direct measurements of the density of states by tunneling spectroscopy on slightly overdoped Bi₂Sr₂CaCu₂O_8+x (Bi2212) single crystals at low temperature using break-junction and point-contact techniques. We find that (i) the variation of the gap magnitude, , between 20 and 36 meV is likely to be intrinsic to Bi2212, and (ii) there is a correlation between the maximum value of the Josephson I _c R _n product and the gap magnitude: I _c R _n decreases with the increase of . The maximum I _c R _n value of 26 mV is observed at = 20.5 meV. For = 36.5 meV, the maximum measured value of I _c R _n is 7.3 mV. We conclude that (i) the distribution of the Josephson I _c R _n product as a function of gap magnitude cannot be explained by the presence of a single energy gap in Bi2212, and (ii) the coherence energy scale in Bi2212 has the maximum Josephson strength.     

Excitation spectrum and the density of states for a superconducting bilayer composite

The single-particle excitation spectrum and the density of states (DOS) for a superconducting bilayer composite are calculated by solving the Bogoliubov equations. The bilayer consists of two different superconducting films in contact. The superconducting order parameters of the films ₁ and ₂ are assumed constant. For various film thicknesses the dependence of the DOS on the ratio ₁/₂ is shown. This dependence is most prominent for states with excitation energy less than ₂.     

Methodic error of conversion of analog signals into frequency-modulated pulse signals

Conclusions For an equiprobable distribution of the position of the beginning of interval T_i on the time axis, _m manifests itself as an uncorrelated random error.If the pulse duration is negligible in comparison with the time interval between adjacent pulses, the ME of the error _m is equal to zero for AFC's with uniform or nonuniform pulse sequences.The RMSD of the error _m varies with T_i, assuming a number of extremum values [_m]_max. For the same values of T_i andf, [_mn]_max exceeds [_mu]_max by a factor of not more than 2. If the maximum allowable values of the RMSD of the methodic error _m are assigned on the basis of experimental conditions, the minimum allowable measurement time of the secondary instrument which receives the AFC signal can be found by means of (7) and (11).Translated from Izmeritel'naya Tekhnika, No. 12, pp. 53–55, December, 1972.     

The unzipping model of fatigue crack growth and its application to a two-phase steel

The unzipping analysis, based on the alternate shear deformation process of two intersecting shear planes at a crack tip, is extended to study fatigue crack growth in a two-phase martensitic-ferritic steel. The unzipping crack increment a _uz is directly related to K and J in the case of small scale yielding. It is preferrable to use a _uz is directly related to K and J in the case of small scale yielding. It is preferable to use a _uz as a physical parameter to correlate with the growth rates of micro-cracks and fatigue cracks in a multi-phase material. In the case of micro-cracks, K is often not applicable because of extensive plastic deformation; and in the case of multi-phase material, neither K nor J is applicable because of material inhomogeneity. The effective K, K _eff, is defined in terms of a _uz. The relations between the endurance limit of a two-phase steel and crack nucleus size, ferrite layer thickness, the constraint by the strong martensite on crack tip deformation in the ferrite domain, and K _th's of the martensite and ferrite are analyzed.

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Résumé Une analyse de rupture progressive et continue des liaisons, basée sur un processus de déformation de cisaillement alterné de deux plants de cisaillement s'intersectant à l'extrémité d'une fissure, a été étendue à l'analyse de la propagation des fissures de fatigue dans un acier martensito-ferritique à deux phases. L'accroissement de la fissure a est directement en relation avec K et J dans le cas de déformation plastique de faible étendue. II est préférable d'utiliser a comme paramètre physique en corrélation avec les vitesses de croissance de microfissures et des fissures de fatigue dans un matériau à phases multiples. Dans le cas de microfissures, K n'est souvent pas applicable en raison de la déformation plastique importante. Dans le cas de matériau multiphase ni K ni J ne sont applicables en raison de l'inhomogénéité du matériau. La valeur effective K _eff est définie en terme de a. Les relations entre la limite d'endurance d'un acier à deux phases et la taille du nodule de fissuration, l'épaisseur de la couche de ferrite, la contrainte qu'exerce une zone martensitique dure sur le domaine ferritique, sur la déformation à l'extrémité de la fissure en domaine ferritique, et les valeurs de K de la martensite et de la ferrite sont analysées.

     

Effects of silver on the phase formation of the Bi-Pb-Sr-Ca-Cu-O superconductors

The effects of Ag substitution on the formation of the superconducting phases in the BPSCCO system have been investigated by Raman spectroscopy and XRD. The silver was found either to accelerate the formation of the 2212 phase or to reduce the formation temperature of the 2212 phase. Moreover, addition of even a small amount of silver (BiPbAg=821) poisons or inhibits the formation of the 2223 phase. However, there is no apparent effect on theT _c of both the 2212 and 2223 phases. In addition, the presence of silver does not alter the mechanism for the formation of the PBSCCO superconducting phases.     

Oxygen doping and temperature dependence of the tunneling spectroscopy on Bi2Sr2CaCu2O8+δ

We present local probe tunneling spectroscopy of Bi₂Sr₂CaCu₂O₈₊ single crystals for different oxygen concentrations, from optimally doped (T_c=92.2 K) to highly overdoped (T_c=56.0 K) phases. With increasing oxygen overdoping, the superconducting gap (_p) is reduced and the dip structure beyond _p at negative sample bias¹ shifts toward the Fermi energy. Apart from the shift in energy of these features, the generic shape of the tunneling spectra remains unchanged. The gap roughly scales with T_c, and 2_p/k_BT_c stays large even in the highly overdoped phase. We also present preliminary results on the temperature dependence of the tunneling spectra. They are consistent with a gap that is largely independent on temperature up to the vicinity of T_c     

Interaction between a dislocation and various divalent impurity in KCl single crystals for the Fleischer's model taking account of the Friedel relation

The interaction between a dislocation and the impurity in KCl : Mg²⁺ (0.035 mol% in the melt), KCl : Ca²⁺ (0.035 and 0.065 mol% in the melt) and KCl : Ba²⁺ (0.050 and 0.065 mol% in the melt) was investigated from the strain-rate cycling test during the Blaha effect measurement. This was carried out at 77–254 K. As a result, it was found that the Fleischer's model taking account of the Friedel relation seems to be suitable for KCl : Ca²⁺ and KCl : Ba²⁺. However, it was not appropriate for KCl : Mg²⁺. Furthermore, the values of T _c, H(T _c) and G ₀ were obtained for the specimens. T _c is the critical temperature at which effective stress is zero. H(T _c) and G ₀ are the enthalpy and the Gibbs free energy of activation for the breakaway of the dislocation from the impurity, respectively. H(T _c) was almost the same for the specimens except KCl : Mg²⁺. G ₀ increased with increasing the divalent cation size. In addition, the tetragonality around the divalent ion-positive ion vacancy pair was estimated on the basis of G ₀ for the each specimen.     

Asymptotic expansions for constant-composition dew-bubble curves near the critical locus

Explicit functional representations are developed for constant-composition dew and bubble curves near critical according to the modified Leung-Griffiths theory. The pressure and temperature incrementsP=P–P _c andT= T–T _c, where c denotes critical, are linearly transformed to new variablesP andT. In the transformed space, the coexistence curves are no longer double-valued and can be expressed as a nonanalytic expansion, where the coefficients are functions of the critical properties and their derivatives. A similar asymptotic expansion is developed forT in terms of the density increment=– _c. In the approximation that the critical exponents=0 and=1/3, the critical point in temperature-density space is shown to be a point of maximum concave upward curvature, rather than an inflection point as previously conjectured.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.Formerly National Bureau of Standards     

Enthalpy relaxation behaviour of Al-Si-Cr quasicrystalline and amorphous alloys upon annealing

Annealing-induced enthalpy relaxation behaviour was examined calorimetrically in quasicrystalline Al₆₂Si₁₉Cr₁₉ and amorphous Al₆₀Si₂₅Cr₁₅ alloys. When both alloys annealed at temperatures belowT _x are reheated, an excess endothermic reaction (enthalpy relaxation) occurs reversibly above the annealing temperature,T _a. The peak temperature of C _p,endo rises in a continuous manner with the logarithm of annealing time (t _a). The magnitudes of C _p,endo and H _endo of the amorphous alloy increase with increasingT _a while no appreciable change in C _p,endo and H _endo of the quasicrystal withT _a is seen. The activation energy,Q _m, for the enthalpy relaxation increases from 1.8 to 2.7 eV with the peak temperature of C _p T _m, for the amorphous alloy, whereas it remains constant (1.3 eV) for the quasicrystal. The endothermic reaction with smallO _m for the quasicrystal is thought to be attributable to the disappearance of short-range ordering of chromium and silicon atoms with stronger attractive interaction, which developed during annealing, i.e. the reversion phenomenon, in the unrelaxed localized regions with high free-energy isolately embedded in the more stable icosahedral structure. The similarity of the enthalpy relaxation behaviour between the quasicrystalline and amorphous phases allows us to infer that short-range atomic configuration is very similar between the quasicrystalline and amorphous phases.     

Transport properties and determination of the basic parameters of superconductors with overlapping bands

The thermoconductivity and ultrasonic attenuation in non-single-band super-conductors are investigated. It is proved that the ratio ₂/₁ atT T _c is equal to that atT=0. It is also shown that ultrasonic measurements and data on the phonon thermoconductivity can be used very effectively for definition of the basic parameters of the superconductors with overlapping bands.     

Analysis of thermal fatigue behaviour of brittle structural materials

The effect of the level of maximum temperature (T _max), the temperature range (T) and the mode of convective heat transfer on the thermal fatigue resistance of brittle structural materials is analysed. Expressions are derived for the number of thermal cycles to failure in terms of the appropriate mechanical and thermal properties, crack growth parameter, T andT _max. For simultaneous changes inT _max and T commonly used in practice, the change in thermal fatigue life is governed by both the thermal stress intensity exponent (n) and the activation energy (Q) for subcritical crack growth, in contrast to the results of other studies. For constantT _max but variable T, thermal fatigue life is affected byn only, whereas, for constant T but variableT _max, the value ofQ alone governs changes in fatigue-life. Heat transfer by natural or forced convection will result in differences in thermal fatigue resistance. Recommendations are made for the design and analysis of thermal fatigue experiments. Figures-of-merit for the selection of materials with high thermal fatigue resistance are presented.     

Superconductivity of cold-deposited beryllium films

The results are presented from an investigation of the transition temperatureT _k and energy gap2(0) as a function of the thickness of superconducting cold-deposited beryllium films. The transition temperature increases exponentially as the film thickness is decreased. In the interval of very small thicknesses a drop inT _k is observed. The thickness dependence of2(0) is analogous to the dependence ofT _k . In the investigated interval of thicknesses the ratio 2(0)/kT _k =3.6±0.1, indicating that the cold-deposited modification of beryllium is a superconductor with weak electron-phonon coupling. Superconducting beryllium films could only be produced up to thicknessesd _cr 200 Å. At thicknesses close tod _cr a transition to the non-superconducting phase is observed. The growth of the transition temperature with decreasing thickness is attributable to the influence of dimensional quantization of the transverse motion of electrons in the film. The drop inT _k for very small thicknesses (30 Å) is regarded as due to the influence of fluctuations. The functionsT _k (d) obtained by various authors for thin films of other superconductors, such as Al, Sn, Bi, Ga, and Pb, are discussed.     

Transport properties of CdGa2Se4 thin films

The d.c. conductivity was determined for CdGa₂Se₄ thin films in the temperature range 300–625 K for as-deposited and heat-treated films. The conductivity at room temperature of films of thickness 326 nm prepared at a temperature of 573 K was found to be 10^–12 ( cm)^–1. The dependence of the electrical conductivity on the annealing temperature in a vacuum of 1 Pa for a thin film of thickness 140 nm, shows that the electrical conductivity increases with increasing annealing temperature. However, the activation energies E and E decrease with increasing annealing temperature. The data of these annealed films are in agreement with the Meyer–Neldel rule. The thermoelectric power measurements indicate p-type conduction in the as-deposited films as well as for the heat-treated films. The p-type conduction is due to the cadmium deficiency as indicated by EDX. The difference between the value of the activation energy calculated from the thermoelectric power E _S and that obtained from the conductivity E indicates the presence of long-range potential fluctuations.     

Enthalpy relaxation behaviour of (Fe,Co, Ni)75Si10B15 amorphous alloys upon low temperature annealing

The effect of composition on the anneal-induced enthalpy relaxation for (Fe, Co, Ni)₇₅Si₁₀B₁₅ amorphous alloys was examined calorimetrically. Upon heating the sample annealed at temperatures well belowT _g (T _g<T _g–150 K), an excess endothermic reaction (enthalpy relaxation) occurs above the annealing temperature. The endothermic specific heat, C _p, evolves in a continuous manner with annealing time. The change in the magnitude of C _p withT _a is divided into two stages: a low-temperature stage which peaks at aboutT _g–150 K and a high-temperature peak just belowT _g. The activation energy,E _m, increases with the peak temperature of C _p,T _m, from 2.29 to 3.15 eV for the low-temperature peak and from 4.40 to 4.96 eV for the high-temperature peak. The low-temperature endothermic reaction is attributed to local and medium range atomic rearrangements and the high-temperature reaction to the long-range cooperative atomic regroupings. The magnitude of the low-temperature C _p is most pronounced for (Fe-Co)₇₅Si₁₀B₁₅, followed by (Fe-Ni)₇₅Si₁₀B₁₅, (Co-Ni)₇₅Si₁₀B₁₅, Fe₇₅Si₁₀B₁₅, Co₇₅Si₁₀B₁₅ and then Ni₇₅Si₁₀B₁₅. The reason for such a significant compositional effect of the enthalpy relaxation was investigated, based on the concept of distribution in relaxation time, which had been previously derived from the percolation theory, and was interpreted as due to the difference in the degree of local structure and compositional disorder in the as-quenched state; i.e. the higher the degree of short-range disorder (the larger the fraction of short relaxation time) the larger is the magnitude of the enthalpy relaxation (H _{, endo}). There existed a strong correlation between (H _{, endo}) or C _p and anneal-induced embrittlement tendency; the larger the (H _{, endo}) and C _p the larger is the embrittlement tendency. Such a strong correlation was interpreted based on the assumption that the internal strain which generated in the transition from as-quenched disordered state to a relaxed equilibrium state causes an enhancement of embrittlement tendency.     

Singular Band Behavior of the Extended Emery Model for the Superconducting Cuprates

Mean field slave-boson approximation is performed on the extended Emery model for the CuO₂ conducting plane. The model is parameterized by Cu–O charge transfer energy _pd , copper–oxygen overlap t ₀, oxygen–oxygen overlap t', and Coulomb interaction U on the copper site taken as infinite. Special emphasis is placed on the role of t in the renormalization trends of the effective band parameters _pf and t, replacing _pd and t ₀, at small doping . It is shown that small, negative t expands the range of stability of the metallic phase, changing, in the second order of the perturbation theory, the nature of the metal–insulator transition point. In the nonperturbative limit, t modifies strongly the renormalization of _pf , making it saturate at the value of 4t. Finite doping suppresses the insulating state approximately symmetrically with respect to its sign. The regime _pf 4t fits very well the ARPES spectra of Y123, Bi2212, and LSCO and also explans, in the latter case, the evolution of the FS with doping accompanied by the spectral weight-transfer from the oxygen to the resonant band.     

Josephson transport in Tl-cuprate bicrystal weak links

Superconducting epitaxial Tl₂Ba₂CaCu₂O₈ (Tl-2212) and Tl₂Ba₂CuO₆, (Tl-2201) thin films are synthesised on single crystal and bicrystal LaAlO₃ substrates by an ex-situ process. Tl-2212 films were optically smooth, with T_cs from 105 to 108K and T_c <2K, J_c reaching 2·10⁶A/cm² at 77K. Tl-2201 films exhibited tetragonal or orthorhombic symmetry depending on the thallium content. The T_c of the as-synthesised Tl-2201 films was in the range 40–55K. By oxygen depletion, T_c was increased by 15 – 20 K. Properties of weak links in Tl-2212 and Tl-2201 thin films are reported.     

On mode I and mode II energy release rates of an interface crack

The opening (mode I) and sliding (mode II) components of the energy that is released during an incremental extension of an interface crack between two different elastic materials are evaluated by the Irwin's crack closure method. Each component of the energies (G _I and G _II ) is expressed in terms of the functions of the length of the incremental crack extension (a) and the real and imaginary part of the complex stress intensity factor defined by Malyshev and Salganik. It is found that values of G _I /a and G _II /a oscillate violently when a approaches zero and that, hence, in contrast with the case for homogeneous materials, each energy release rate should be defined as G _I /a and G _II /a for an actual crack growth step size.