Influence of sintering time and quenching on the formation of highT c phase

Influence of sintering time and quenching in Bi_{2 −x} Pb _x Ca₂Sr₂Cu₃O _y (x=0.0, 0.1, 0.2, 0.25, 0.3 and 0.4) samples have been studied by resistance and XRD measurements. In samples sintered at 850°C for 4 days,T _c(0) increases with Pb concentration.T _c(0) increased from 81 K forx=0.0 to 109 K inx=0.30 sample and then decreased. Increasing the sintering time to 10 days decreased theT _c Quenching further decreased theT _c(0). From X-ray diffraction patterns, the intensity peaks of low and highT _c phases have been measured. The addition of Pb promotes highT _c-phase. Sintering time, slow cooling and rapid quenching studies show that there is an optimum sintering time and cooling rate to produce a highT _c-phase.     

Enhancement of superconductivity in Pb-Bi-Sn and Pb-Bi-In alloy filaments produced by glass-coated melt spinning

The enhancement ofT _c in Pb-Bi-Sn and Pb-Bi-In system alloy filaments produced by glass-coated melt spinning was investigated as a means of producing a new type of superconducting filament with highT _c. Long filaments of Pb-Bi-Sn alloy withT _c higher than 10 K and Pb-Bi-In alloy withT _c higher than 9 K were obtained from the molten state at a temperature of 1500 K with a winding speed of 2.63 m sec^–1. For example, a Pb₄₅Bi₃₅Sn₂₀ filament withT _c of 10.1 K was 15 m in diameter and polycrystalline with a grain size of 100 nm. The structure of the filament was a mixture of , tin and bismuth phases and a metastable phase of mixed structure of bismuth and supersaturated solid solution of tin in -phase was detected. TheT _c of the filament decreased on heat treatment. A metastable phase of mixed structure of bismuth and -phase was also detected for a Pb₄₅Bi₄₅In₁₀ filament withT _c of 9.3 K. As the metastable phase for the Pb-Bi-In filament was more unstable than that for the Pb-Bi-Sn filament, theT _c of the filament was drastically decreased by heat treatment. The metastable phase was considered to play an important role in the enhancement ofT _c for Pb-Bi-Sn and Pb-Bi-In alloy filaments.     

Effect of stress on the superconducting transition temperature of SrTiO3

The superconducting transition temperatures of several specimens of reduced SrTiO₃ and of Nb-doped SrTiO₃ have been investigated as functions of hydrostatic and uniaxial compressive stresses up to 1.8 kbars. Decreases inT _c as large as 0.12 K have been observed in specimens under hydrostatic pressure. Because of the lowT _c and small compressibility of SrTiO₃, (lnT _c)/P and (lnT _c)/(lnV) are orders of magnitude greater than the corresponding effects in elemental superconductors. The effect of uniaxial stress onT _c varied with the direction of stress. Compression along a [111] direction caused large decreases inT _c, while both small increases and small decreases inT _c have been observed for [100] compression. It is believed that the present results reveal the presence of a sensitive volume dependence in one or more of the parameters important to superconductivity in SrTiO₃, and that no significant electron-transfer effects occurred in the range of stresses of this experiment.     

Superconducting thin films of Bi-Pb-Sr-Ca-Cu-O synthesized by the spray-inductively coupled plasma technique

Superconducting thin films of Bi-Pb-Sr-Ca-Cu-O were synthesized on polycrystalline MgO substrates by injecting atomized aqueous solutions of appropriate nitrates into an inductively coupled r.f. plasma (ICP) under atmospheric pressure (the spray-ICP technique). 1–2 m thick films composed ofc-axis oriented phases (c=2.4 and 3.0 nm) could be synthesized in runs of 15–20 min. The phase compositions of the films were varied fromc=2.4 nm phase toc=3.0 nm phase through their mixed phases by increasing the distance of the substrates from the ICP. In some cases the mixed phases contained a small amount ofc=3.7 nm phase. Annealing in air (850 °C for 10 min and then 800 °C for 5 h) convertedc=2.4 nm phase toc=3.0 nm phase. The best as-deposited film showedT _c (resistance = 0) at 55 K, and after annealing its resistance began to drop around 110 K and reached zero at 75 K.     

Superconductivity and microstructures of fluorinated and chlorinated YBa2Cu3Oz

Superconductivity and microstructures of fluorinated and chlorinated YBa₂Cu₃O _z were investigated with BaF₂ and BaCl₂ as the fluorination and chlorination agents. The incomplete decomposition of BaF₂ and BaCl₂ led to inhomogeneous phase and elemental distributions as shown by energy dispersive analysis of X-rays (EDAX) and X-ray diffraction (XRD) analysis. A high (y 2) chlorine concentration stabilized a tetragonal perovskite structure in the YBa₂Cu₃Cl _y O _z compositions while an orthorhombic structure was observed in samples with a lower chlorine concentration. The maximum superconductivity transition temperature,T _c, was 95 K in these fluorinated and chlorinated samples. Thus, our data are contradictory to the claims of high transition temperatures (155 K) recently reported for some of these compositions. The microstructural and phase composition data reported here could provide useful information in the search for possibly highT _c phases in these fluorinated and chlorinated compositions.     

Role of barium addition on the properties of bismuth-based superconductors

The effect of the addition of barium on the zero resistance temperature,T _c, of Bi_1.6Pb_0.4Sr_1.6Ca₂Ba _x Cu₄O _y ,x= 0.4, 0.5, 0.6, 0.8 and 1.6 was studied. The added barium had the effect of raisingT _c to a higher temperature region, although too much barium gave rise to semiconducting resistance temperature behaviour. X-ray diffraction analysis showed that as the barium concentration,x, increased from 0.4 to 0.8, a decrease in the lowT _c phase and peaks due to CuO and BaBiO₃ appeared, whereas an increase in the peaks due to the highT _c phase and BaCuO₂ were seen. Critical current densities were also measured in zero field at 77 K.     

Effect of Sintering Duration on the Thermal Conductivity of (Bi, Pb)-2223 Superconducting Pellets Between 10 and 150 K

The influence of sintering duration on the electrical resistivities and thermal conductivities of (Bi_0.8Pb_0.2)₂Sr₂Ca₂Cu₃O_9.8+ pellets with 0.11 < < 0.54 is reported between 10 and 150 K. The results indicate a gradual transformation of the 2212 phase to the 2223 phase and this transformation starts within 5 h of sintering in air at 840°C. The thermal conductivity of the pellets sintered for shorter durations display two maxima at T _c0 and around 110 K, respectively. The shape and magnitude of these maxima depend on the relative amount of the 2212 and 2223 phases present in the pellets. While the magnitude of the total thermal conductivity over the measured temperature range is strongly influenced by the duration of sintering, the phonon-dominant nature of heat transport is retained. The relative contribution of the electronic part (_E) to the total thermal conductivity () remains small and does not change appreciably with sintering time.     

Interplay of low and highT c phases in Bi-Sr-Ca-Cu-O superconductor

In the Bi-Sr-Ca-Cu-O system stringent conditions of heat-treatment lead to the formation of a mixture of both the low and highT _c phases and obtaining a single-phase material becomes extremely difficult. This study reports preparation of samples with single superconducting transitions at ∼ 75 K and ∼ 108 K; the compositions of which correspond ton=2,3 in the series Bi₂Sr₂Ca _n−1Cu _n O_{4 + 2n} . X-ray diffraction studies show that the lowerT _c material is a relatively pure phase while the higherT _c phase only co-exists with the lowerT _c phase. The most obvious effect of doping the system with lead is to make the reaction take place faster and thereby increase the volume fraction of the 110K phase.     

Preparation and degradation behaviour of Bi-cuprate superconductors

The role of preparation conditions and the effect of addition of Pb in Bi-Sr-Ca-Cu-O (BSCCO) superconductor, on theT _c,J _c and grain orientation have been studied. Calcination at a temperature higher than the melting point of Bi₂O₃ after the prereaction at 800°C leads to formation ofc-axis oriented nearly single phase material. The presence of lead yields a nearly highT _c phase (2223) exhibiting a maximum transition temperatureT _c=110 K and ΔT _c=2 K. The addition of lead and grain orientation together result in an increase inJ _c by nearly two orders of magnitude. The degradation behaviour was studied in terms of changes inT _c,J _c and structure on exposure to atmosphere up to 250 days. Samples with 0.6 Pb were found to be superconducting withT _c=97 K even after 250 days. The highT _c phase was stable against degradation. Our studies indicate that the degradation of leaded BSCCO is a surface phenomenon rather than bulk phenomenon.     

The preparation of high transition temperature superconducting Pb-Bi-Ge alloy filaments using the method of glass-coated melt spinning

The melt spinning of Pb-Bi-Ge alloys with Pyrex glass was investigated as a means of producing a superconducting long filament with highT _c. Continuous filaments with maximumT _c of more than 10 K of Pb_100-x-y Bi _x Ge _y (15x37 and 725) were obtained from the molten state at 1500 K with a winding speed of 0.95 m sec^–1. The Pb₄₉Bi₃₃Ge₁₈ filament, which was 34×10^–6 m diameter and a ductile material with a tensile strength of 20 MPa and elongation of 2.7%, exhibited superconductivity at the highestT _c of 14.3 K. These filaments were found to be polycrystalline with a grain size of more than 5000×10^–10 m and had a mixed structure of germanium (diamond) (h c p) and bismuth phases. The germanium element distributed homogeneously in the filament.     

Growth of highT c Bi-Sr-Ca-Cu-O thick films

Thick films of Bi-Sr-Ca-Cu-O were deposited on (100) MgO substrates by screen-printing technique with the starting composition 1112. To attain the superconducting state, the films were subjected to two-step heat-treatment. R-T and XRD have been studied for films annealed at different durations of the second step. InitiallyT _c (R=0) increased from 77 to 103 K as the annealing duration was increased after whichT _c decreased. Kinetics of the growth of highT _c phase is discussed in the light of our results.     

Isotactic polypropylene/polyisobutylene blends: morphology-structure-mechanical properties relationships

Morphological observations by optical and scanning electron microscopy, wide (WAXS) and small (SAXS) angle X-ray scattering, differential scanning calorimetry (DSC) and mechanical tests have been performed on sheet specimens of isotactic polypropylene (iPP)/polyisobutylene (PIB) blends obtained under different crystallization conditions. Two kinds of morphologies have been observed, particularly at high crystallization temperatureT _c, on thin sections of the same sheets: a spherulitic one in the centre and a row-like structure on the edges. The size of the spherulites, as well as the thickness of the row-like regions, decreases with diminishingT _c, and seemsto be independent of the amount of rubber. The adhesion among the spherulites and between the spherulites and the row-like regions seems to become poorer with higherT _c. The rubber particles seem to be evenly dispersed into the iPP matrix for samples quenched at low temperatures, whereas for samples isothermally crystallized (at highT _c) their concentration seems to be slightly higher at the border of the spherulites than in the centre. The overall crystallinity measured by DSC and by WAXS is an increasing function ofT _c and decreases with increasing amount of PIB. The index of iPP phase, quite low indeed (max 3%), drops with loweringT _c and with enhancing PIB percentage. The long spacingL for a given quenching temperatureT _q is independent of PIB content, whereas for isothermally crystallized samples at low undercooling varies differently according toT _c. The lamellar thicknessL _c is always a decreasing function of rubber content. Stress-strain analysis shows a more and more brittle behaviour both with increasingT _c (beyondT _c=122° C all the specimens are very brittle irrespective of PIB amount) and PIB amount in accordance with the morphological observations. Some tentative hypotheses have been made to explain the observed behaviour.     

Superconducting beryllium films

Resistance-vs.-temperature measurements were made on a series of beryllium films condensed on liquid-helium cooled surfaces. Two vapor sources designed to reduce contamination were used and films were prepared on both crystalline quartz and glass substrates. The samples were superconducting with a transition temperature ofT _c =9.6±0.1 K as indicated by a sharp falloff of resistance on cooling. This qualitatively confirms earlier reports of the superconductivity of quenched beryllium films. The transition curves were, however, appreciably sharper and the transition temperature about a degree higher than previously reported. Good agreement found from sample to sample indicates that the residual impurity concentration was small enough to be unimportant and that the observed transition temperature is characteristic of pure beryllium. The phase of beryllium responsible for the highT _c value disappeared on annealing in the range 40–60 K. No indication was found of a reportedT _c 6-K phase. Beryllium films thicker than about 750 Å broke up during deposition, indicating the presence of large stresses.The work at Karlsruhe was supported by the Deutsche Forschungsgemeinschaft, Germany.     

Density of states and superconductivity in ScTc2 and ScRe2

The low-temperature specific heats of ScTc₂ and ScRe₂ are reported. The newly discovered record highT _c for a Laves phase (10.9 K) in ScTc₂ is explained qualitatively by its factor of two largerN(0) than in ScRe₂ (T _c=0.93 K). Comparison of the data for ScTc₂ with that for HfV₂ (T _c=8.4 K), which has three times theN(0) as for ScTc₂, suggests that the electron-phonon coupling strength is also an important factor in the highT _c of ScTc₂.     

Structural and morphological evolution from a freeze-dried precursor to the La1.85Sr0.15 CuO4 superconductor during thermal processing

The crystalline phases and morphology of solids with a composition of La_1.85Sr_0.15CuO₄ were studied in terms of thermal processing conditions of freeze-dried acetates. Samples were characterized by X-ray diffraction, scanning electron microscopy and iodometric titration techniques. Results indicate that a full development of the superconducting phase only occurs after pressing sintering and annealing under oxygen of the powders treated at T _f=1253 K for 8 h. The structural evolution observed in this case involves a solid-state reaction between La_1.67Sr_0.33Cu₂O_5–, La(OH)₃ and CuO. The existence of transient liquid phase at temperatures around 600 K generates local composition fluctations (lanthanum deficiency) where the non-superconducting ternary compound La_1.67Sr_0.33Cu₂O_5– develops. The selection of high heating rates (50 K min^–1) below 773 K minimizes the deleterious effect of transient melting and, at the same time, ensures a high reproducibility in the morphology of the final material obtained.     

Effect of HfN Microadditions on the Microstructure and Superconducting Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + x Ceramics

The effect of ultrafine hafnium nitride additions (0.05–0.2 wt %) on the microstructure, phase composition, density, and superconducting properties (T _c and j _c) of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 + x} ceramics is studied. The elemental composition of individual phases is determined, and the magnetic-field distribution in the ceramics is examined. The introduction of hafnium nitride increases the density of the ceramics and improves their microstructural stability at sintering temperatures between 840 and 850°. The low hafnium solubility in the superconductor offers the possibility of attaining higher magnetizations and 77-K critical current densities.     

Superconductivity of 2212 phase in Bi-based high-Tc superconductors

In the Bi-based high-T _c superconductors, three superconducting transition points were observed above the liquid-N₂ temperature range. Allotropes of the 2212 phase were found. These allotropes were metastable and can interchange with the 2212 phase, and theirT _c's vary from 85 to 100 K.     

Preparation, Structure, and Properties of (Bi,Pb)2Sr2Ca2Cu3O10 + δ Ceramics with Si3N4 Additions

The microstructure, phase composition, texture, and superconducting properties (T _c, T _c, j _c(T), and R(T) at H= 0 and 5 mT) of (Bi,Pb)₂Sr₂Ca₂Cu₃O_{10 +} ceramics (sintering at 840°C for 36 h) with ultrafine Si₃N₄ additions (0.05–0.2 wt %) are studied. The introduction of 0.05–0.1 wt % Si₃N₄ is shown to reduce the width of the superconducting transition by 2–3 K and to raise the critical current density at temperatures below 95 K.     

Formation of the high-Tc phase in lead-doped Bi-Sr-Ca-Cu-O superconductors

The characteristics of Bi-Sr-Ca-Cu-O superconductors have been investigated as a function of the amount of Pb addition. The volume fraction of the high-T _c phase increases with increasing Pb addition up to x=0.3 in (Bi_1–xPb_x)₂Sr₂Ca₂Cu₃O_y and above that it decreases. The grain size is increased and density is decreased with increasing Pb addition, due to two-dimensional grain growth which results from formation of the high-T _c phase. The specimen with x=0.3 has a transition temperature of 104 K and high magnetic susceptibility due to the fact that most of the volume fraction is of the high-T _c phase. Pb addition cause formation of Ca₂PbO₄ as a secondary phase and it produces a partially melted liquid phase below the sintering temperature, which acts as a flux and promotes formation of the high-T _c phase.     

Specific heat capacity of molybdenum chalcogenides. III. Measurements of the heat capacity of Sn1 − x Ga x Mo5S6. An investigation of the singularities at low temperatures

The heat capacity of molybdenum chalcogenides with different Sn/Ga content was measured at temperatures of T 60 K. The occurrence of two phases of the chalcogenides studied could clearly be seen for higher Ga concentrations. In the miscibility gap there coexists a superconducting phase (T _c= 12.5 K) SnGa_0.25Mo₅S₆ with the ferromagnetic GaMo₅S₆ phase (T _curie = 19 K). As for the pure ternary compound GaMo₅S₆, a singularity of the heat capacity could also be observed in the range of the structural change (T = 45 K). All the singularities are discussed.