Effect of cobalt on the high T c phase of the Bi1.6Pb0.4Sr2Ca2Cu3Oy system

A.c. susceptibility, X-ray diffraction (XRD), SEM and porosity studies have been performed on Bi_1.6Pb_0.4Sr₂Ca₂Cu_3–x Co _x O _y (x=0, 0.05, 0.1) superconductors. XRD and SEM results have shown that (i) the compound with x=0 contains mixed phases of 2223 and 2212, (ii) the increase in concentration of cobalt (from x=0 to 0.1) helps the compound to attain a structure of strong 2201 and a small amount of Ca₂CuO₃ impurity phase. The onset temperatures of the diamagnetic signal of these superconductor samples with x=0, 0.05, and 0.1 as observed from a.c. susceptibility measurements are 106, 60 and 50 K, respectively. The highest onset temperature, 50 K, observed in the sample with x=0.1 rather than the usually reported value of 20 K associated with 2201 phase in bismuth oxide compounds, may be due to the presence of Ca₂CuO₃ impurity phases. SEM and porosity results show that the cobalt helps to increase the grain and pore sizes.     

Appropriate thermal procedure for the preparation of high-T c phase of Bi1.6Pb0.4Sr2Ca2Cu3Oy superconductor

The high-T _c phase of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_y has been synthesized using an appropriate thermal procedure. D.c. electrical resistivity and X-ray diffraction studies have been done. The measurements show that a slow cooling process is necessary for a better control of the thermal process. The variation in T _c as a function of annealing time is also reported. Indexed X-ray diffraction patterns indicate the formation of high- as well as low-T _c phases.     

Zero resistance at 117 K in Bi1.6Pb0.4Sr2Ca2Cu2+xOy compounds

In the Pb-doped Bi-Sr-Ca-Cu-O system, optimization of the composition and heat treatment conditions at which a greater amount of the high-T _c phase forms has been reported in our early paper [1], where the temperature of zero resistance was 107K. Recently, we have achieved zero resistance at 117 K and observed an a.c. susceptibility step at around 150 K by changing the Cu composition in the Bi^1.6Pb^0.4Sr²Ca²Cu^2+x O ^y system (x=0, 0.4, 0.8, 1.2, and 1.6).     

HighT c of liquid-quenched Bi1.6Pb0.4Sr2Ga2Cu3Ox

AlthoughT _c cannot be found for a liquid-quenched Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x glassy sample, a highT _c is found after annealing for 24 h at 1100 K. The maximum offset temperature of the superconducting transition is 113.3 K at 2.2 × 10^–2mAmm^–2. The maximumT _{c off} is larger than that (the maximumT _{c off} is 103.4 K at 2.0 × 10^–2 mAmm^–2) of sintered specimens before liquid quenching.     

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.     

Characterization of Bi1.6Pb0.4Sr2Ca2Cu3O y ceramic superconductor prepared via coprecipitation method at different sintering time

The Bi(Pb)-2223 superconductor has been prepared via coprecipitation (COP) method from solutions of metal acetates and 2-propanol solution of oxalic acid at low temperature (0–2 °C). The metal oxalates powder was subjected to precalcination of 12 h at 730 °C, followed by 24 h calcination at 845 °C. The pelletized powder was sintered for 24, 48 and 100 h at 850 °C. The dominance of high-T _C phase was observed for all samples as evidenced in the single step transition of (R–T) curves. The T _{C(R = 0)} for samples sintered at 24, 48 and 100 h were 102, 102 and 104 K, respectively. XRD data showed the tetragonal structure for all samples followed by the enhancement of the 2223 phase as sintering time increased. Ac magnetic susceptibility measurements showed the improvement of the grain connectivity as sintering time increased. SEM micrographs showed large flaky grains of ∼7 μm in size and randomly distributed, which belong to 2223 phase. The degree of grains alignment increased as the sintering time increased.     

The stability of high-T c phase in Bi1.6Pb0.4Sr2Ca2Cu3Oz compounds prepared by hot isostatic pressing

By using a sequence of hot isostatic pressing (HIP) and intermediate cold pressing, it has been possible to produce high-density Bi-based superconductors containing almost 100% high-T_c 2223 phase, without any post-fabrication heat treatment. The X-ray data showed no decomposition of high-T_c phase in hot isostatically pressed Bi-based superconductors. By using this technique, Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_z (where Z≈10) superconductors could be densified to 6.06 g cm^?3 at an HIP temperature of 850 °C, and at an HIP temperature of 870 °C the density was close to 6.15 g cm^?3. These densities could be compared with about 3 to 4 g cm^?3 obtained for conventionally sintered compounds. X-ray diffraction data at various stages of fabrication, microstructure and electrical resistivity data are presented.     

Formation of high-T c Bi1.6Pb0.4Sr2Ca2(Cu0.9Ti0.1)3O x

Influences of sintering conditions onT _c are studied for Bi_1.6Pb_0.4Sr₂Ca₂(Cu_0.9Ti_0.1)₃O _x . HighT _c values are obtained at 1123±5 K sintering temperature and over 50 h sintering time. In order to explain theT _c changes, morphological discussion is attempted.     

Thermoelectric Power of Deoxygenated Bi1.6Pb0.4Sr2Ca2Cu3O10 + δ Sintered Superconducting Pellets

The thermal variation of the electrical resistivity and thermoelectric power of Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_{10 + δ} pellets subjected to various degrees of deoxygenation is reported. The temperature dependence of the electrical resistivities of deoxygenated samples displays gradual transformation from metallic-like to semiconductor-like features in the normal state. All the samples however, show superconducting transition, but increasing deoxygenation depresses T _C0 from 102 to 45 K. Gross features of the temperature variation of thermoelectric power observed in properly oxygenated (Bi, Pb)-2223 cuprates are retained in all the deoxygenated samples. Our results on electrical resistivity and thermoelectric power in the normal state have been found to be consistent with a two-band model.     

Melt-textured Bi1.6Pb0.4Sr2Ca2Cu3O11 bulk superconductors fabricated in a simple tube furnace

Using the simplified version of the melt-textured growth (MTG) technology with a simple tube furnace, we have fabricated superconductors satisfying the nominal composition Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₁. We would note that the material used in the fabrication was prepared by mixing a precursor Bi_1.6Pb_0.4Sr₂Cu₃O _x and CaO powder. This two step technique was found to be superior to the single step solid state reaction method after many trials. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) morphologies show that the melt-textured bulk samples are made up of stacks of highly textured single crystal-like layers. X-ray analysis as well as d.c. magnetization measurements were carried out and theJ _c value was found to be 1.3×10³ A cm^–2 at 77 K using the Bean model. At this stage, thoughJ _c is not so high as that of the best samples obtained from other complicated methods involving special (hot) pressing and sintering techniques, we do not need to apply any mechanical treatment at or after the heating procedure.     

Dopants in high-T c superconductors from rapidly quenched Bi1.6Pb0.4Ca2Sr2Cu3O w glasses

High-T _c 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, T _opt, was observed in all the glass-derived superconductors with different dopants; in all cases T _c (zero) higher than 105 K was observed on heat treatment at T _opt for each dopant. T _opt 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 T _opt for the formation of the 110 K phase.     

Evolution of 2223 phase in the Bi2Pb0.6Sr2Ca2Cu3.1Oy system

The role of cationic ratios (Sr/Ca and Bi/Pb) in the evolution of high-T _c phase (2223) in samples prepared under different sintering conditions, starting with a composition of Bi₂Pb_0.6Sr₂Ca₂Cu_3.1O_y, has been investigated by employing energy-dispersive X-ray analysis, scanning electron microscopy and X-ray diffraction. As manifested by the observed decrease in Sr/Ca ratio, an increase in sintering temperature from 822 to 852 °C increases the disorder in Sr-O and Ca layers. The observed increase in the volume fraction of 2223 phase and the contraction inc-axis parameter have been explained on the basis of the observed decrease in Sr/Ca ratio. It thus appears that the disorder caused by the intersubstitution of Ca and Sr in SrO and Ca layers and partial replacement of Bi by Pb in the structure promote the evolution and growth of 2223 phase.     

Influence of Ar ion etching on Tc of liquid-quenched Bi1.6Pb0.4Sr2Ca2Cu3Ox superconductor by sheet plasma source

The influence of argon-ion etching is investigated for liquid-quenched high-T_c Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O_x. The argon-ion irradiation has no effect on the T_c value for a dose of 4.05 × 10¹⁷ (ions mm^–2). However, doses in excess of this level greatly decreases the T_c value. Therefore, a critical irradiation does value (D_c) to maintain a T_c value above 100 K is defined and determined. The D_c is about 4.94 × 10¹⁷ (ions mm^–2) for argon-ion irradiation at the low acceleration energy of 1 keV.     

Effects of sintering temperature on the superconducting and microstructural properties of Bi1.7Pb0.4Sr1.6Ca2.4Cu3.6OY/Ag2O composites

The effects of sintering temperature on the superconducting and microstructure properties of Bi_1.7Pb_0.4Sr_1.6Ca_2.4Cu_3.6O _Y (BPSCCO)/Ag₂O (0–50 wt%) superconductors were investigated. Based on the differential thermal analysis data, it was found that the addition of Ag₂O to the BPSCCO system lowered the partial melting temperature (peritectic point), thereby promoting extra liquid formation in this system and affecting the stability of 2223 high-T _c phase of these composites. For example, the T _c (zero) of the BPSCCO/Ag₂O (10 wt%) composite which was sintered at 843 °C in air was depressed by as much as 52 K. However, the addition of Ag₂O (10–50 wt%) in the BPSCCO samples resulted in no significant effect on T _c when samples were treated under a lower sintering temperature (827 °C) in air. The correlations of superconducting properties with microstructures of these materials are discussed.     

A comprehensive study on mechanical properties of Bi1.8Pb0.4Sr2MnxCa2.2Cu3.0Oy superconductors

This study manifests the crucial change in the mechanical performances of Bi_1.8Pb_0.4Sr₂Mn_xCa_2.2Cu_3.0O_y superconductor samples (x = 0, 0.03, 0.06, 0.15, 0.3 and 0.6) prepared by conventional solid-state reaction method by use of Vickers microhardness (H_v) measurements carried out at different applied loads, (0.245 N ≤ F ≤ 2.940 N). Load dependent microhardness, load independent microhardness, Young’s (elastic) modulus and yield strength values being account for the potential technological and industrial applications are evaluated from the hardness curves and compared with each other. It is found that the H_v, elastic modulus and yield strength obtained decrease (increase) with the enhancement of the applied load for the undoped (doped) samples. Surprisingly, the results of the H_v values illustrate that the samples doped with x = 0.03, 0.06, 0.15, 0.3 and 0.6 exhibit reverse indentation size effect (RISE) feature whereas the pure sample obeys indentation size effect (ISE) behavior. Furthermore, the experimental results are examined with the aid of the available methods such as Meyer’s law, proportional sample resistance model (PSR), elastic/plastic deformation (EPD), Hays–Kendall (HK) approach and indentation-induced cracking (IIC) model. The results inferred show that the hardness values calculated by PSR and EPD models are far from the values of the plateau region, meaning that these models are not adequate approaches to determine the real microhardness value of the Mn doped Bi-2223 materials. On the other hand, the HK approach is completely successful for the explanation of the ISE nature for the pure sample while the IIC model is obtained to be the best model to describe the hardness values of the doped materials exhibiting the RISE behavior. Additionally, the bulk porosity analysis for the samples reveals that the porosity increases monotonously with the increment in the Mn inclusions inserted in the Bi-2223 system, presenting the degradation of the grain connectivity.