Partial Melting Temperature Research Articles

Dopants in high-T c superconductors from rapidly quenched Bi1.6Pb0.4Ca2Sr2Cu3Ow glasses

High-Tc superconductors with a variety of metal dopants (M=V, Nb, Mo, Te, Ta and W, ordered by atomic number) were prepared by the heat treatment of rapidly quenched glass precursors. The optimum heat-treatment temperature, Topt, was observed in all the glass-derived superconductors with different dopants; in all cases Tc (zero) higher than 105 K was observed on heat treatment at Topt for each dopant. Topt was related to the temperature at which a partial melting of the specimen occurred, as determined by differential thermal analyses. The dopants lower the partial melting temperature and thus lower Topt for the formation of the 110 K phase.

Effect of silver coating on the superconducting and mechanical properties of solution-spun Y 1Ba 2Cu 3O x filament

The effect of Ag coating on solution-spun Y 1Ba 2Cu 3O x filaments is examined. The Ag coating lowers the optimum sintering and partial melting temperatures and improves mechanical properties without degrading superconducting properties. The Ag coating results in enhancement of flux pinning in the sintered filament and improvement of weak links in the partially melted filament at 77 K.

Crystallization process and superconducting properties of Bi-Pb-Sr-Ca-Cu-O thick films crystallized from an amorphous state

Abstract We have studied the crystallization process in the Bi-Pb-Sr-Ca-Cu-O system from an amorphous state, and we have characterized the superconducting properties of thick film samples prepared on MgO substrates by annealing. A rapidly quenched sample showed amorphous nature. After this sample was annealed at 500°C, Bi2Sr2CuO x phase was found. At 650°C, the Bi2Sr2CaCu2O y phase appeared. The sample at 800°C consisted of the Bi2Sr2CaCu2O y phase, while the Bi2Sr2Ca2Cu3O z phase occurred at 850°C. Thick film samples were prepared by annealing at 855°C, the partial melting temperature, for 1-400 h. A short-time annealed sample showed semiconducting behavior in the normal conducting state, but longer annealing showed the more remarkable metal-superconducting transition. This was considered to be due to oxygen deficiency and weak coupling at grain boundaries. The highest Tc(0) is 91 K in a sample annealed for 400 h., although it consisted of high-Tc and low-Tc mixed phases.

The influence of sintering conditions on the critical current density and the microstructure of Ag-sheathed Bi2Sr2CaCu2Ox superconducting tapes

Ag-sheathed Bi2Sr2CaCu2Ox tape was fabricated by a combination of partial melting, subsequent cooling, annealing and quenching. The partial-melting temperature TH affects the growth of the 2212 grains. The promotion of the growth at TH>or approximately=885 degrees C enhances the JC value at 77 K and 0 T, until the appearance of microcracks at TH>or approximately=895 degrees C. The annealing temperature TL also effects JC and yields a maximum between 840 degrees C<or approximately=TL<or approximately=860 degrees C. Needle-shaped precipitates appeared at TL<or approximately=860 degrees C and porous microstructures at TL<or approximately=840 degrees C cause the weak links at the grain boundaries and reduce JC. The optimum cooling period tS from TH to TL is around 10 h, which corresponds to a cooling rate of 3.8 degrees C h-1. The prolongation of the period tK during annealing at TL enhances JC by the improvement of the weak links and reaches the highest value of JC=6.5*103 A cm-2 at 77 K and 0 T for tK=150 h.

Synthesis, Microstructure, Critical Current Density, Magnetization and Relaxation of Hot-Pressed YBa2Cu3O6+δ Through Partial Melting Post-Oxygenation

ABSTRACTHot-pressed YBa2Cu3O6+δ superconductor with plate-like grains annealed at the partial melting temperature ( ∼ 1,000 C) followed by slow cooling in oxygen atmosphere has resulted in enhanced magnetization hysteresis loops which show weak dependence on magnetic field. The X-ray diffraction pattern exhibits the significant features associated with the reflection (003) and (006), and a higher compaction density in hot-pressed samples compared with cold-pressed samples was observed from the SEM micrographs. Morphology study and microstructural analysis by SEM and TEM, respectively, indicate that grains were well aligned during hot-pressing mostly with their c-axes parallel to the pressing axis, and many intersecting twins were actually formed during post-oxygenation. Local oxygen-deficient regions likely existing in these hot-pressed and oxygenated samples are believed to act as effective flux pinning centers at high temperatures (∼ 50 K and above). The measurement of magnetic relaxation was employed to interpret the observed pinning behavior. The critical magnetization current density (Jc) calculated using the Bean model and the average grain dimension is in the order of 104 Amp/cm2 at 75 K up to 1 T. Because of a shorter period of oxygenation and a weak field dependence of Jc values, the process of hot-pressing combined with partial melting post-oxygenation is technologically practical in forming various shapes as desired from oxide superconductors.

Metal Dopants in Bi‐Pb‐Ca‐Sr‐Cu‐O High‐Tc Superconductor Thick Films Prepared by Melt Solidification

Metal‐doped superconductor thick films Bi1.6Pb0.4M0.1 Ca2‐Sr2Cu3Ox (M = V, Nb, Mo, In, Sn, Sb, and W) were prepared using a melt–solidification procedure. The optimum heat‐treatment temperature Topt was observed in all the films with different dopants; Topt was related to the temperature at which a partial melting of the films occurred as determined by differential thermal analysis. The dopants in the melt‐solidified films lower the partial melting temperature and thus lower Topt for the formation of the 110 K phase.

Phase diagram studies of the BiO 1.5[sbnd]PbO [sbnd]SrO [sbnd]CaO [sbnd]CuO system and the formation process of the “2223 (high- Tc)” phase

The phase relation in the BiO 1.5 PbO SrO CaO CuO system and the formation process of the “2223” phase for a nominal composition Bi 0.9 Pb 0.2 SrCaCu 1.6 O z were studied. At 825°C a small amount of liquid phase was formed that triggered the 2223 phase formation. Solid phases formed below the partial melting temperature were identified, and the melting points of various combinations of these were measured. The lowest liquid formation temperature was obtained at 825°C from a combination of the “2201” phase and Ca 2 PbO 4 . The liquid hardly wetted 2223 but did the “2201” and the “2212” phases very well. Reaction with the liquid has been concluded to convert 2212 phase into 2223.

Optimum annealing temperature dependence on initial lead content in dc magnetron sputtered Bi(Pb)-Sr-Ca-Cu-O thin films

Influence of the initial lead content in Bi(Pb)-Sr-Ca-Cu-O thin films on the annealing conditions, which make the films fully superconducting above 100 K is reported. We have found that the optimum annealing temperature was very close to the temperature of the film partial melting and decreased with the increase of the lead content in the film. The increase of the lead content also dramatically accelerated formation of the 110-K phase, resulting in short (1 hour) annealing times.

Formation Mechanism of Bi-Based High-TcSuperconductors in the Melt-Quenching Method

Abstract Bi-based high-Tc superconductors were prepared by using the melt-quenching method. The values of critical temperature Tc and critical current density Jc for the samples of Bi0.8 Pb0.2SrCa-CuxOy (x=1.5, 1.8 and 2.0) obtained by annealing at 840°C for 250h decreased with increasing Cu content, and the glass-ceramics with x=1.5 exhibited superconductivity with a Tc (zero) of 106K and a Jc (77K, zero magnetic field) of 250A/cm2. The formation of the high-Tc phase in the superconducting (Bi, Pb)2Sr2Ca2Cu3Oy ceramics was largely enhanced by the addition of a very small amount of Sb or Mo element. It was found that the lowering of the partial melting temperature at around 870°C caused by the coexistence of Pb and Sb or Mo elements was very important for the formation of the high-Tc phase.

Improvement in critical current density of Bi2Sr2Ca1Cu2Ox tapes synthesized by doctor-blade casting and melt growth

Bi2Sr2Ca1Cu2Ox superconducting tape with an excellent Jc at 4.2 K has been synthesized by a modified doctor-blade process. A green tape prepared by doctor-blade casting is laid on a silver foil and heat treated at various temperatures below and above its partial melting temperature. The Jc of the tape increases dramatically when heat treated in a narrow temperature range just above the melting temperature. A highly oriented layer structure with c-axis alignment is obtained by the growth of aligned Bi2Sr2Ca1Cu2Ox grains in the partially melted state. This oriented microstructure produces an exceptional increase in Jc . Typical Jc ’s of the tape at 4.2 K are 50 000 A/cm2 at 0 T, 22 000 A/cm2 at 10 T, and 17 000 A/cm2 at 23 T. Note the small magnetic field dependence in Jc above 10 T.

Ｂｉ−Ｓｒ−Ｃａ−Ｃｕ−Ｏ系超伝導体の高Ｔｃ相生成に及ぼすＢａ添加の効果

The effect of partial substitution of Ba for Sr or Ca in the BiSrCaCu2Oy on the formation of the high-Tc phase was studied. A series of samples BiSr1-xBax CaCu2 Oy and BiSrCa1-x Bax Cu2 Oy, (x=0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5, 1.0) was prepared by the conventional ceramic method. The differential thermoanalysis and microstructural observation revealed that the substitution of Ba for Sr lowered the melting point and partial melting temperature and widened the temperature range of the partial melting with increasing Ba substitution(x≤0.1) for Sr. Tc measurements and X-ray diffraction analyses showed that the fraction of the high-Tc phase was at maximum when the BiSr1-x Bax CaCu2 Oy, (x=0.1) was sintered in the partial melting region. The partial substitution of Ba for Ca also lowered the melting point and partial melting temperature, but showed little effect on the formation of the high-Tc phase.

Ｂｉ系超伝導ガラスセラミックスにおける高温相の生成

We prepared superconducting Bi1.6Pb0.4-xTexSr2Ca2Cu3Oy ( x=0.1, 0.2, 0.3 and 0.4 ) glass-ceramics by the melt-quenching method and examined the effect of Te addition on the formation of high-Tc phase. It was found that the formation of the high-Tc phase was largely enhanced by the addition of a small amount (x=0.1 and 0.2 ) of Te element. In the Bi1.6Te0.4Sr2Ca2Cu3Oy glass-ceramics, the main superconducting phase was the low-Tc phase. It was concluded that the lowering of the partial melting temperature at around 870°C caused by the coexistence of Pb and Te elements was very important for the formation of the high-Tc phase. The critical temperature of the sample ( x=0.2 ) annealed at 850°C for 100h was 103K.

Effect of oxygen partial pressure on superconductivity in Bi 2Sr 1.5Ca 1.9Cu 3.4O x and synthesis of single crystals of undoped Bi 2Sr 2Ca 2Cu 3O x ( Tc≈108 K)

Samples of Bi 2Sr 1.50Ca 1.87Cu 3.37O x were annealed at oxygen pressures between 0.0065 and 0.21 atm O 2 at temperatures above and below the partial melting temperature T m. Maximum fractions of the 110 K-superconductor Bi 2Sr 2Ca ( n−1) Cu n O (4+2 n) with n=3 (ca. 50% from AC susceptibility) were found just above T m, while n=1 was the stable phase above ≈ T m+15°C. Crystal growth of n=3 was possible at 0.035 atm O 2 and T m+≈10°C. Quenching experiments at 0.05 atm provide evidence of the disproportionat ion equilibrium n=2→( n=1)+( n=3) in a narrow temperature range at 0.05 atm O 2. Diamagnetic shielding below 40–60 K exhibited in AC susceptibility measurements on sintered pellets only is due to intergrain contacts.

Crust and upper-mantle velocity structure beneath the northern and central Indian Ocean from the phase and group velocity of Rayleigh and Love waves

Fundamental-mode Rayleigh and Love waves generated by nine earthquakes, which occurred in the central Indian Ocean (with epicentres extending to 40° S) and recorded at seven WWSSN stations of central Asia, have been used to determine the phase and group velocity along various paths across the northern and central Indian Ocean. The dispersion characteristics of Rayleigh and Love waves show a thick crust of 23 km thickness. This anomalous crustal thickening, having quasi continental-oceanic structure, may be explained by assuming the gradual transformation of top mantle material into material having either crustal like velocity or slightly lower than Moho velocity (P-wave velocity 7.72 km s −1, S-wave velocity 4.45 km s −1). The velocity structure down to a depth of 200 km has been obtained across the northern and central Indian Ocean. A low-velocity zone of 90 km with P-wave velocity of 7.85 km s −1 and S-wave velocity of 4.37 km s −1 is estimated to be centred at a depth of 78 km from the water surface. This low shear velocity in the mantle may be caused by the partial melting and elevated temperature there. The observed dispersion data for Rayleigh and Love waves also exhibit a strong anisotropy along different paths in the crust and upper mantle. The observed dispersion data for Rayleigh and Love waves that cross the Ninety-east Ridge are characterized by lower phase and group velocities, as compared to waves having paths in a more western direction than this ridge axis. However, the phase velocities for Rayleigh and Love waves are characterized by lower values across the Ninety-east Ridge, as compared to the normal oceanic structure.

The Significance of Terrestrial Electrical Conductivity Variations

The degree to which different materials of the earth can conduct electricity varies enormously, from the insulating qualities of cold, quartz-rich crys­ talline rock to the metallic behavior of the liquid core. A well-recognized physical property, the specific resistance or resistivity expresses quanti­ tatively the resistance to current flow, and the fact that it can be sensed through measurements on the earth's surface makes it an important geophysical parameter. The history of the investigation of resistivity is similar to that of other geophysical quantities, such as seismic velocity. When it was recognized that the earth as a whole was in some degree a conductor, the initial aim was to determine a global average value of resistivity; this was followed by the search for a function describing the radial variation. While the latter effort is by no means completed, a great deal of recent work has been concerned with departures from the average behavior and that aspect forms the basis of this paper. Lateral variations in electrical resistivity may provide information on the composition and water content of the crust and on the temperature and degree of partial melting in zones of the mantle. The success of any investigation depends on the adequacy of electromagnetic measurements at the earth's surface, the ability to mathematically infer a distribution of resistivities on the basis of induction theory, and, by no means least, the knowledge, through laboratory mea­ surements, of electrical properties of earth materials under different phys­ ical conditions. Recent progress in each of these aspects is considered. First, a word as to units. Resistivity is conveniently expressed in ohm­ meters (Qm), the resistance of a meter cube of the material. As an indication