Stabilization of the 108 K superconducting phase in the Bi0.7Pb0.3SrCaCu1.8Oy system

Samples having the nominal composition Bi_0.7Pb_0.3SrCaCu_1.8O _y , were prepared by a solid-state reaction and characterized by energy-dispersive x-ray microanalysis, resistivity measurements, and magnetically modulated microwave absorption (MAMMA). Longer annealing times resulted in more homogeneous samples having a single superconducting transition to zero resistance at 108 and 105 K at 0.1 and 1 mA, respectively. The MAMMA measurements clearly show differences in sample homogeneity with different annealing times.     

Composition and superconductivity in Bi-Sr-Ca-Cu-O and substituted systems

The composition of the superconducting phase Bi₄(Sr_3±x Ca_3x )₌₆Cu₄O₁₆, has been determined by means of a sysmetic investigation of the relationship between composition and superconductivity, and phase analysis. The superconducting phase belongs to tetragonal system witha=0.3825 nm,c=3.082 nm, space group (S.G.) 14/mmm. The effect of strontium and calcium contents in the Bi(Sr_1±x Ca_1x )₌₂Cu₂O_5.5, system and substitution in the Bi_1–x A _x SrCaCu₂O₅ (A=V, Gd, Y) and BiSrCa_1–x Pb _x Cu₂O_5.05 systems on superconductivity, have been investigated. In the Bi-Sr-Ca-Cu-O system, there are two superconducting phases corresponding to transition temperatures of 110 and 80 K, respectively. The difference between the two superconducting phases may be due to the stacking difference of atoms along thec direction in the structure and their oxygen content.     

Effect of annealing and quenching on superconductivity in Pb and Sb-doped Bi2Sr2Ca2Cu3O x

Samples of the series Bi_1·9−x Pb _x Sb_0·1Sr₂Ca₂Cu₃O _y withx=0, 0·1, 0·2, 0·3 and 0·4 were prepared by the solid-state route. The X-ray and d.c. electrical resistivity data on furnace-cooled and quenched samples are presented. Though the starting composition is 2223, the end products were multiphase with 4334 as the major phase. A superconducting transition withT _c=100K was observed in the pure 2223 sample after quenching. The furnace-cooled samples were metallic, while samples withx=0·1, 0·2 and 0·3 were superconducting after quenching. The amount of the 4334 phase decreases with increasing Pb content. Quenching seems to be favourable for the formation of the 4334 phase.     

The Effect of Barium Doping on the Selective Structure of Bi-2223 Phase

The partial substitution of Sr by Ba in the two nominal compositions of Bi_1.8Pb_0.4Sr_2−x Ba _x Ca_2.2Cu₃O _y [x=0.0, 0.1, 0.2, and 0.3 (A group)] and Bi_1.66Pb_0.34Sr_2−x Ba _x Ca₂Cu₃O _y [x=0.0, 0.1, 0.2, 0.3, 0.4, and 0.5 (B group)] have been investigated by resistivity, ac susceptibility measurements and by XRD and SEM analysis. In general, the nature of the temperature dependence of resistivity and susceptibility measurements indicate the presence of a superconducting transition between grains coupled by weak links. However, the XRD and SEM analyses show that the relative composition of initial elements used in Bi-(2223) is essential to the site that is selected by the Ba ions. In the A group, Ba doping up to x=0.1 will improve the phase formation of Bi-2223, and improve the superconductivity properties of the samples. In the B group, although Ba doping up to x=0.1 will enhance the phase formation of Bi-2223, it will decrease the coupling between the grain and the superconductivity properties of these systems. The presence of lower Tc phases will begin to appear for x>0.1, in both of these systems. The superconductivity properties and the phase formation of Bi-(2223) will decrease as the Ba concentration increases.     

Effect of oxygen concentration on superconductivity of Bi-Sr-Ca-K-Cu-O compounds

In order to study the relation between the oxygen content and superconductivity samples of Bi₂Sr₂Ca_1–x K _x Cu₂O _y (x=0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) were prepared and treated under different conditions. It was found that the superconducting properties are sensitive to the oxygen concentration and theT _c,zero of Bi₂Sr₂Ca_1–x K _x Cu₂O _y can be shifted over a range of about 22 K by the changing oxygen concentration; no structural difference induced by oxygen were observed. The change in superconducting properties may be the result of a change in Cu average valence state. In addition, the change in superconducting phase with K⁺ concentration is also studied.     

Synthesis and properties of Bi1.6Pb2-xSr2-xCa2Cu3Oy

Bi_1.6Pb_0.4Sr_2-xCa₂Cu₃O_y (x = 0, 0.2, 0.4) high-T_c materials were prepared from oxide-carbonate mixtures and presynthesized mixed oxides. The 2223 superconducting phase was found to be formed most rapidly in thex = 0.4 sample if the Pb-and Ca-containing starting reagents were separated in the initial stage of synthesis. The highest superconducting transition temperature, T_c (R = 0) = 104.3 K, was attained in the stoichiometric 2223 material. The materials were characterized by electrical resistivity, magnetic susceptibility, and Hall effect measurements. The Hall data were used to evaluate carrier concentration and mobility. The 77-K resistivity of the materials was measured as a function of magnetic field.     

Role of lead in the growth of the highT c phase in the Bi2−x Pb x Sr2Ca2Cu3O y systems

Bi_2−x Pb _x Sr₂Ca₂Cu₃O _y superconducting samples with 0<x<0·3 have been synthesized and characterized using X-ray powder diffraction.T _c and superconducting volume fraction have been measured using a.c. magnetic susceptibility, d.c. electrical resistivity as well as X-band microwave surface resistance in the normal state. The data indicate the growth of the highT _c (2223) phase with corresponding reduction of the lowT _c (2122) phase with increasingx, up to 0·25. Beyond this value ofx there is a slight deterioration of the superconducting behaviour.     

Mixed state thermoelectric power of vanadium substituted 2223 (Bi, Pb) superconducting polycrystalline cuprates

Measurement of the mixed state thermoelectric power of pure and vanadium substituted 2223 pellets of the type (Bi_0.6Pb_0.2–y V _y )₂Sr₂Ca₂Cu₃O _x are presented for temperatures between 77 and 120 K. Mixed state Seebeck voltages for both pure and vanadium substituted samples show almost similar features. Our data forT>0.8T _c could be explained, attributing the observed dissipation to the normal quasiparticle excitations. For temperatures below 0.8T _c , we argue that the dominant contribution to the mixed state Seebeck voltage may originate from the dissipation induced by the fluctuations of the order parameter across the internal weak links.     

Phase relations and stoichiometry of superconducting Pbx Mo6 S8−y

The composition of Pb_xMo₆S_8?y can vary between 0.85 < x < 1.05 and 0.6 < y < 1.2 at 1100 °C and is shifted to lower Pb content at higher temperatures. The phase decomposes peritectically at about 1530 °C. At the approximate composition line Pb_xMo₆S₇, the rhombohedral lattice constant and the transition temperature T_c for superconductivity show maxima while the rhombohedral angle has a minimum. Small single crystals (～0.2 mm) of Pb_xMo₆S_8?y and larger crystals (～3 mm) of Cu_xMo₆S_8?y, Sn_xMo₆Se_8?y can be obtained in sealed Mo-crucibles     

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.     

Enhancement of (Bi,Pb)-2223 HTS Formation by Boron-Doping

Influence of boron-doping on the superconducting properties of (Bi,Pb)-2223 HTS ceramics prepared in an alumina crucibles has been investigated. X-ray diffraction, resistivity, and AC susceptibility measurements were performed on the undoped and boron-doped compounds. Obtained results have shown that B₂O₃ addition in the Bi_1.7Pb_0.3Ca₂Sr₂Cu₃O _y precursor enhances the formation of high-T _c phase. The boron-doped samples with starting composition Bi_1.7Pb_0.3Ca₂Sr₂Cu₃B _x O _y (x=0.05 and 0.5) reveal significant improvement in the zero resistivity temperature compared to the undoped sample (from 72 K up to 100 K). Boron-doping level x=1.5 results in a substantial degradation of the (Bi,Pb)-2223 phase.     

Role of Pb substitution and a study of synthesizing procedure for Bi-based superconductors

Bulk superconductors of the (Bi_1–x Pb _x )₂Sr₂Ca₂Cu₃O _y system have been synthesized by changing the Bi/Pb ratio. The effect of Pb substitution onT _c has been studied by standard d.c. resistivity measurements. An appropriate thermal procedure and time for the preparation of the 110 K phase has also been studied at length. The experiments indicate that the best results are obtained forx=0.2 and that a slow cooling process is necessary for a better control of the thermal process. Indexed X-ray diffraction patterns indicate the lattice parameters of low- and high-T _c phases asa _L=0.54004 nm,b _L=0.5445 nm,c _L=3.084 nm anda _H=0.5483 nm,b _H=0.5339 nm,c _H=3.772 nm, respectively. The observed superconducting behaviour is stable on thermal cycling between 77 and 300 K.     

Superconductivity in nearly single-phase Bi-Pb-Sr-Ca-Cu-O samples with different nominal compositions

The preparation conditions, phase composition, and superconducting properties of Bi-Pb-Sr-Ca-Cu oxide materials from different nominal compositions have been investigated. Nearly single-phase samples from Bi₂Pb_0.4Sr₂Ca₃Cu₄O _y , as well as from the proposed compositions Bi_1.8Pb_0.4Sr₂Ca₃Cu₄O _y and Bi_1.8Pb_0.4Sr₂Ca_2.5Cu_3.5O _y were obtained by a solid-state reaction in air. Samples with the third nominal composition showed the best superconducting properties (T _on=111 K and zero resistance atT ₀=103 K). A possible mechanism for the 2223 phase formation in the three investigated compositions has been discussed.     

Superconducting transition temperatures in the Ru-1232 system, RuSr2(Gd1? x Ln x Ce1.8Sr0.2)Cu2O z (Ln = Sm, Dy, and Ho)

We have investigated effects of the lanthanide element Ln and the composition changes on the superconducting transition temperatureT _c in the Ru-1232 system, RuSr₂(Gd_1−x Ln _x Ce_1.8Sr_0.2)Cu₂O _z (Ln = Sm, Dy, and Ho). At first, in the case of the samples with Ln = Sm among almost the single 1232 phase samples, the values of the superconducting onset temperatureT _co are almost the same forx=0.00−0.15, and each of the lattice parametersa andc is almost constant. While, in each of the cases of the samples with Ln = Dy and Ho, the sample withx=0.05 shows the maximum values for both the superconducting onset temperatureT _co and the zero resistivity temperatureT _cz. Especially for the sample with Ln = Dy, the values ofT _co andT _cz are 18.5 and 6.5 K, respectively. These are higher than those of the mother sample of RuSr₂(GdCe_1.8Sr_0.2)Cu₂O _z . Moreover, from variations ofT _co, lattice parameters ofa andc in the RuSr₂(Gd_1−x Dy _x Ce_1.8Sr_0.2)Cu₂O _z system as a function of Dy contentx, the relationship between the superconducting transition temperature and the lattice parameters in the present system are investigated.     

Stabilization of the 108 K superconducting phase in the Bi0.7Pb0.3SrCaCu1.8O y system

Samples having the nominal composition Bi_0.7Pb_0.3SrCaCu_1.8O _y , were prepared by a solid-state reaction and characterized by energy-dispersive x-ray microanalysis, resistivity measurements, and magnetically modulated microwave absorption (MAMMA). Longer annealing times resulted in more homogeneous samples having a single superconducting transition to zero resistance at 108 and 105 K at 0.1 and 1 mA, respectively. The MAMMA measurements clearly show differences in sample homogeneity with different annealing times.     

The effect of heat treatment on superconductivity of Zn‐doped YBa2Cu3‐xZnxO7‐y

YBa₂Cu_3‐xZn_xO_7‐y compounds with x = 0, 0.05, 0.15, and 0.30 have been synthesized by standard solid state reaction method. The crystal structure, lattice parameters, and oxygen content are not changed by the substitution of Zn for Cu since both valence state and ionic radius are almost identical for Zn and Cu elements in YBa₂Cu_3‐xZn_xO_7‐y. However, the superconducting transition temperature T_c decreases with the increase of Zn content, reflecting the T_c‐suppression effect of Zn substitution. Heat treatment experiments indicate that the heat treatment at low temperature is beneficial to improve the superconductivity of the sample. But T_c decreases with the increase of annealing temperature when the treatment temperature is above 300°C, and finally the superconductivity disappears at approximately 920°C, 700°C and 550°C for the samples with x = 0.0, 0.05 and 0.15, respectively. Our experiments indicate that the superconductivity of the sample with higher Zn content is more sensitive to the oxygen content, and a small decrease in the oxygen content can lead to a considerable decrease of T_c.     

Transport and Magnetic Properties of Bi1.7Pb0.4Sr1.5Ca2.5Cu3.6Ox/(LiCl)y Superconductors

We investigated the superconducting critical temperature, the intra- and intergranular critical current density, and the thermopower properties of Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiCl) _y samples. All these properties have been compared with those of Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiF) _y specimens. It was found that the critical temperature determined from resistive and AC complex susceptibility measurements show a maximum and the transition width shows a minimum for the intermediate values of y. Powder X-ray diffraction studies and the AC complex susceptibility measurements reveal that in our samples the amount of Bi₂Sr₂Ca₂Cu₃O_{10 +} high-temperature superconducting phase is maximum for y 0.02. The amount of LiCl in Bi_1.7Pb_0.4Sr_1.5Ca_2.5Cu_3.6O _x /(LiCl) _y changes the superconducting properties of the grains as well as of the intergrain matrix. The splitting of the peak in the temperature dependence of the imaginary part of the complex susceptibility, corresponding to the dissipation inside the grains, was also observed.     

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.     

Monitoring phase formation of Pb-substituted Bi-Sr-Ca-Cu-O superconducting samples at different preparative stages using Raman spectroscopy and X-ray diffraction

Raman spectroscopy and X-ray diffraction were used to follow structure and phase formation at various stages in order to study the effect of Pb substitution in (Bi_1–x Pb _x Sr₃Ca₃Cu₄O _y samples. We found that major reactions involving Bi₂O₃ occurred at around 700°C and that reactions with Ca, Cu, and Pb started at a lower temperature. The amount of Ca₂PbO₄ formed increases as a function of lead concentration and annealing temperature (up to 800°C), but excess lead substitution (50%) destroys the superconductivity. The high-temperature superconducting phase (2223) is only observed in the 15% and 20% leaded samples. These two samples exhibit a higher amount of Ca₂PbO₄ during intermediate processing stages, which suggests that the presence of the Ca₂PbO₄ is important for the formation of the high-T _c phase. It was found that theT _c increases with lead concentration (up to 20% Pb).