[en] We report the results of an investigation of the influence of magnetic fields on hysteretic ac losses in bulk MgB₂ superconductor by using Hall probe ac susceptibility. The external magnetic field in this study had an ac part with frequency 10 Hz and magnitude in the range 240-1200 A m^-1 and no dc part. We have measured the imaginary part of the magnetic susceptibility and analysed data by using Bean's critical state model of cylindrical geometry for four different temperature values 39.55, 39.51, 39.47 and 39.41 K. The result of this analysis indicates that Bean's model is adequate to explain the experimental findings. Calculated hysteretic ac loss versus applied magnetic field curve is given by using the critical state model. We have also measured the magnetization versus applied magnetic field and determined the first critical magnetic field of a MgB₂ sample as 500 A m^-1 at 35 K

[en] In this work, we report on the effect of Y3+ doping on structural, mechanical and electrical properties of Bi-2202 phase. Samples of Bi2Sr1.9Ca0.1−xYxCu2O7+δ with x=0, 0.025, 0.05, 0.075 and 0.10 are elaborated in air by conventional solid state reaction and characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM) combined with EDS spectroscopy, density, Vickers microhardness and resistivity measurements. A good correlation between the variations of the bulk density and the Vickers microhardness with doping is obtained. The SEM photograph shows that the samples are composed of grains with a flat shape that characterizes the Bi-based cuprates. Quantitative EDS analysis confirms the reduction of Ca content and the increase of Y content when x is increased. The variation of resistivity with temperature shows that only samples with x=0, 0.025 and 0.05 present an onset transition to the superconducting state. The higher onset transition temperature is obtained for x=0.025 and is about 93.62K. The transition is wide and is realized in two steps confirming then the presence of the low Tc Bi-2201 phase in the samples. For x=0.075 and 0.10, a transition to a semiconducting state is seen at low temperatures. Some physical parameters are extracted from these curves and discussed. (Author)

[en] Highlights: ► Effect of MgB₂ inclusions on the microstructural, electrical, mechanical and superconducting properties of Bi-2223 phase. ► dc resistivity, transport critical current density, SEM, XRD and bulk density measurements for identification of the samples. ► An improvement in the superconducting and structural properties up to concentration level of x = 0.05. ► From the content of x = 0.1 onwards, divergence from the optimization of the hole concentration. ► Degradation of the bond strength between the atoms in the Bi-2223 system with the excess MgB₂ individuals. -- Abstract: This study reports the effect of MgB₂ inclusions on the microstructural, electrical, mechanical and superconducting properties of Bi_1.8Pb_0.4Sr₂(MgB₂)_xCa_2.2Cu_3.0O_y ceramics with x = 0, 0.01, 0.03, 0.05, 0.1, 0.3, 0.5 and 1.0 by use of bulk density, dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transport critical current density (J_c) and Vickers microhardness (H_v) measurements. The samples studied in this work are prepared by conventional solid-state reaction method at the annealing temperature of 840 °C for 48 h. For the potential technological and industrial applications, the important characteristics such as the normal state resistivity, density, porosity, critical (onset and offset) transition temperature, variation of critical temperature, self-field critical current density, crystallinity, phase purity, lattice parameter, texturing, surface morphology, Vickers microhardness and elastic modulus values are obtained for the pure and MgB₂-doped samples and compared with each other. It is found that all the properties given above are sensitively dependent upon the MgB₂ concentration in Bi-2223 matrix. The critical transition temperature and critical current density values of the samples are observed to improve significantly for the optimum doping level of x = 0.05. The maximum T_c^onset of 121.3 K, T_c^offset of 114.1 K and J_c of 463 A/cm² for the sample doped with x = 0.05 whereas the minimum values are found to be 118.3 K, 101.3 K and 235 A/cm² for the material doped with x = 1.0 as against 118.6 K, 109.4 K and 306 A/cm², respectively, for the undoped sample. This may be attributed to the fact that the Bi-2223 system changes from the optimally doped to an underdoped position with the excess of the MgB₂ nanoparticles. Furthermore, XRD measurements illustrate that both pure and MgB₂ doped superconductors (new system) contain only Bi-2223 and Bi-2212 phases (no different phase of MgB₂ or any other cation) and exhibit the polycrystalline superconducting phases with the variable intensity of diffraction lines, confirming that the MgB₂ inclusions are incorporated into the Bi-2223 crystal structure in the form of nanoparticles. Besides, the smallest (largest) lattice parameter a (c) is noticed to belong to the sample doped with x = 0.05 while the largest (smallest) cell parameter a (c) is observed for the maximum doping level of MgB₂ (x = 1.0). As for SEM images, similar to the XRD investigations, the former sample has the best crystallinity, texturing, grain connectivity, lowest porosity and largest grain size while the worst surface morphology is observed for the latter sample. Additionally, the bulk porosity analyses for the samples demonstrate that the bulk porosity decreases monotonously with the MgB₂ content in the Bi-2223 system up to x = 0.05 after which the porosity starts to increase regularly towards to maximum value, leading to the degradation of the grain connectivity. At the same time, the Vickers microhardness values indicate that the samples doped with x = 0.01, 0.03 and 0.05 (towards optimally doped state) exhibit reverse indentation size effect (RISE) behavior; on the other hand, the others (towards underdoped state) present indentation size effect (ISE) feature. To sum up, the MgB₂ addition in the Bi-2223 superconducting system promotes the velocity of the Bi-2223 phase formation up to the concentration level of x = 0.05 beyond which the microstructural, electrical, mechanical and superconducting properties of the Bi-2223 materials studied are found to degrade systematically and in fact reach to their local minimum points for the doping level of x = 1.0

[en] Superconducting Bi₂Sr₂Ca_1−xY_xCu₂O₈ ceramics samples have been prepared by Sol-Gel methods using citrate process. The influences of the conditions of preparation of oxide powder compound on structural and superconducting properties have been investigated by X ray diffraction (XRD), Scanning Electron Microscopy (SEM) equipped with EDS. The critical transition temperatures Tc have been determined by resistivity versus temperature measurements. Cell parameters samples were calculated from XRD patterns. The polyacryl amide gel makes the citrate process easier, more rapid and affords the possibility of synthesis of high quality oxide powders.

[en] Superconducting BiPbSrCaCuO thin films were prepared on MgO(001) and Ag/MgO substrates using an electron beam (e-beam) evaporation technique. The effects of annealing temperature and Ag diffusion on the crystalline structure and some superconducting properties, respectively, were investigated by X-ray diffraction, atomic force microscopy, and by measurements of the critical temperature and the critical current density. It was shown that an annealing of both types of films at 845 or 860 C resulted in the formation of mixed Bi-2223 and Bi-2212 phases with a high degree of preferential orientation with the c-axis perpendicular to the substrates. The slight increase of the critical temperature from 103 K to 105 K, the enhancement of the critical current density from 2 x 10³ to 6 x 10⁴ A/cm², and the improved surface smoothness are due to a possible silver doping from the substrate. (Abstract Copyright [2002], Wiley Periodicals, Inc.)

[en] Highlights: • Iso-valence substitution of Ca²⁺ for Sr²⁺ in Bi₂Sr_1.6-xCaxLa_0.4CuO_6+δ leads to the appearance of the high temperature transition. • It improves the bulk onset critical transition temperature, the transition width and the residual resistivity. • The best results for the activation energy of vortices and the irreversibility field are seen for x = 0.4 of Ca content. - Abstract: In this work, the effect of Ca²⁺ iso-valence substitution for Sr²⁺ on properties of Bi₂Sr_1.6La_0.4CuO_6+δ superconductors is presented. Samples series with nominal composition of Bi₂Sr_1.6-xCa_xLa_0.4CuO_6+δ (x= 0, 0.2, 0.4, 0.6 and 0.8) are prepared by a solid-state reaction method. When Ca content is increased, the X-ray diffraction technique shows that the cell parameters a and c decrease, while b one is almost constant. The scanning electron microscopy analysis reveals that the substitution has no significant effect on the porosity and the grain size of the samples. The physical properties of the samples are studied by the analysis of the magneto-resistivity curves measured under magnetic fields in the range 0-1 T. As Ca is added, the results show that the high temperature transition appears and is pushed up to 94.87 K for x=0.8. The substitution also improves the bulk onset critical transition temperature, the transition width, the residual resistivity, the activation energy of vortices and the irreversibility field. The best results are seen for x=0.4 of Ca content.

[en] We have investigated the effect of annealing temperature on the microstructure, magnetic and mechanical properties of MgB₂ superconducting samples employing X-ray diffraction (XRD), scanning electron microscopy (SEM), ac susceptibility and Vickers microhardness measurements. XRD patterns and SEM micrographs are used to obtain information about lattice parameters and grain size, respectively. These measurements indicate that MgB₂ grain size, lattice parameters, and critical temperature are increased, and grain connectivity is improved, with increasing the sintering temperature up to 850 deg. C. It is also observed that the Vickers microhardness of the samples is dependent of the sintering temperature and applied load. In addition, we calculate the load dependent mechanical properties of MgB₂ samples such as the Young's modulus, yield strength, and fracture toughness. The possible reasons for the observed improvements in microstructure, superconducting and mechanical properties due to annealing temperature are discussed.

[en] The effect of the cooling rates on oxygen content of silver sheathed Bi-2223 bulk superconducting samples was investigated performing dc electrical resistivity, transport critical current density, ac susceptibility and x-ray diffraction (XRD) measurements. The bulk samples with Ag sheathing were annealed under identical conditions and cooled with rates of 25 deg. C / h, 50 deg. C / h, 75 deg. C / h, and 100 deg. C / h. We estimated the transition temperatures from both dc resistivity and ac susceptibility as a function of temperature measurements. It is observed that T_c and J_c depend on cooling rates of the samples. T_c and J_c decrease with increasing cooling rates. The imaginary part of ac susceptibility measurements is used to calculate inter-granular critical current density using the Bean Model. J_c(T_p) is also seen to decreases with increasing cooling rates. The peak temperature, T_p, in the imaginary part of the ac susceptibility is shifted to a lower temperature with increasing cooling rates as well as increasing ac magnetic fields. The force pinning density decreased with increasing the cooling rates. XRD patterns are given to determine lattice parameter c and obtain information about Bi-2223 and Bi-2212 phases.

[en] In this study, in order to investigate the effect of Au doping on mechanical properties of YBCO, Au-doped (from x = 1 wt% to x = 20 wt%) and pure YBa₂Cu₃O_7-x samples were prepared by standard solid-state reaction method. The indentation load versus diagonal length of the samples under various indentation loads in the range of 0.245–2.940 N is measured. The micro-indentation measurements showed that the load-dependent micro-hardness value for Au-doped samples is lower in comparison with that of the pure sample. The values of the hardness are found to be load and Au content-dependent. For each sample, the reverse indentation size effect (RISE) is observed in which micro-hardness value increased with increasing the load. In addition, we extract the load-independent (true) micro-hardness using Meyer’s law, Kick’s law, proportional specimen resistance, modified proportional specimen resistance, the Hays-Kendall, and the indentation-induced cracking (IIC) models and compare the true hardness with the apparent hardness. Among the investigated models, IIC model is determined to be the most successful one describing the mechanical properties of our samples.

[en] The aim of the present study is to investigate the effect of diffusion-doped silver on some physical properties of Bi_1.8Pb_0.4Ca_2.2Sr₂Cu₃O_x superconducting samples and to calculate the diffusion coefficient and the activation energy of silver. The present work consists of three parts: (a) optimization of annealing temperature, (b) the effect of Ag diffusion-doped on microstructure and superconducting properties of Bi(Pb)CaSrCuO, and (c) calculation of diffusion coefficient of silver. First, to investigate the optimum annealing temperature we prepared Bi_1.8Pb_0.4Ca_2.2Sr₂Cu₃O_x ceramic superconductors annealed at 830, 835, 840, 845, and 850 deg. C using the solid state reaction method. The investigations consist of XRD, SEM, dc resistivity, and transport critical current density measurements. The highest T_c and J_c values were observed for the sample annealed at 840 deg. C for 48 h (B840). Large grain size, denser surface, and high volume fraction of the high-T_c phase were obtained for the sample B840. The diffusion doping of Bi_1.8Pb_0.4Ca_2.2Sr₂Cu₃O_x by silver increased the critical current density from 123 to 696 A/cm² and the transition critical temperature by about 3 K compared with the undoped sample. Ag-doping increased the amount of high-T_c phase and improved the surface morphology. It also caused an increase of the lattice parameter c by an amount of 0.15%. The temperature dependence of the silver diffusion coefficient in the range 600-800 deg. C is described by D=2.9x10^-4exp(-1.05 eV/k_BT). Possible reasons for the observed improvement in the structural and superconducting properties of the samples due to silver diffusion are discussed.