Investigation of Lu effect on YBa2Cu3O7−δ superconducting compounds

This study reports the effect of Lu addition on the microstructural and superconducting properties of YBa₂Lu_xCu₃O_7?δ (Y123) superconducting samples with x = 0, 0.1, 0.3, 0.5 and 0.7 by means of X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), electron dispersive X-ray (EDX), electrical resistivity and transport critical current density (J_c) measurements. The samples prepared by the liquid ammonium nitrate and derivatives are exposed to various annealing time (20, 40 and 60 h) and temperature (950, 960 and 970 °C), and the best ambient for the sample fabrication is determined to be 970 °C for 20 h. Zero resistivity transition temperatures (T_c), critical current densities (J_c), variation of transition temperatures, hole-carrier concentration, grain size, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the bulk superconducting samples prepared at 970 °C for 20 h are compared with each other. T_c and J_c values of the samples are inferred from the dc resistivity and the critical current measurements, respectively. The results show that the T_c value of the pure sample is about 90.6 K while the sample doped with 0.1 wt% Lu has the maximum T_c value (92.5 K). However, beyond x = 0.1, the T_c value is observed to decrease toward to 83.5 K with increment in the Lu addition. Similarly, the J_c values measured are found to reduce from 142 to 76 A/cm² with the addition. Moreover, XRD measurements show that both pure and Lu-doped samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines and contain Y123 and Y211 phase, confirming the incorporation of Lu atoms into the crystalline structure of the samples studied. At the same time, comparing of the XRD patterns of samples, the intensity ratio of the characteristic (110) and (013) peaks on the sample doped with 0.1 wt% Lu is more than that on the other samples prepared. Additionally, SEM images display that the sample doped with 0.1 wt% Lu obtains the best crystallinity, grain connectivity and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.7). Further, EDX results demonstrate that the Lu atoms doped are successfully introduced into the microstructure of the Y123 samples studied and the maximum Cu element level is observed for the sample doped with 0.1 wt%, explaining that why this sample obtains the best superconducting properties compared to others. According to all the results obtained, it is concluded that the 0.1 wt% Lu addition into the Y123 system improves the microstructural and superconducting properties of the samples studied.     

Comparative study on indentation size effect, indentation cracks and superconducting properties of undoped and MgB2 doped Bi-2223 ceramics

This study examines the evaluation mechanism of MgB₂ doped Bi_1.8Pb_0.4Sr₂(MgB₂)_xCa_2.2Cu_3.0O_y (0 ≤ x ≤ 1.0) superconducting ceramics prepared by conventional solid-state reaction method via dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and Vickers micro hardness (H_v) measurements. Variation of room temperature resistivity, critical transition temperatures (onset and offset), phase purity, cell parameter, texturing, grain connectivity, surface morphology, crystallinity and H _v values of the materials are deduced and compared with each other for the determination of the optimum doping level in the Bi-2223 system. It is found that all the properties given above depend strongly on the MgB₂ concentration. From dc resistivity investigations, each sample studied exhibits the superconducting behavior below their variable offset critical temperature values. The maximum onset (T _c ^onset ) and offset (T _c ^offset ) temperatures are found to be about 121.3 and 114.1 K, respectively, for the sample doped with x = 0.05. The minimum T _c ^onset of 118.6 K and T _c ^offset of 109.4 K are observed for the sample doped with x = 1.0. Similarly, XRD and SEM examinations indicate that there is an improvement in the crystal structures and surface morphologies of the superconducting materials with the increment of the MgB₂ inclusions in the Bi-2223 system up to x = 0.05 beyond which the crystallinity, grain connectivity and surface morphology start to degrade regularly and in fact reach to the worst structure appearance for the doping level of x = 1.0. Furthermore, the H_v measurement results being analyzed by Meyer’s law, proportional sample resistance (PSR), modified PSR, elastic–plastic deformation model, Hays–Kendall (HK) approach, Indentation-induced cracking model (IIC) allow us to derive the mechanical properties of the superconducting samples for the potential technological and industrial applications. According to the results obtained, HK approach, among the mechanical analysis methods, is determined as the most successful model for the samples (doped with x = 0, 0.1, 0.3, 0.5 and 1.0) exhibiting indentation size effect behavior whereas the IIC model is noted to be superior to other models for the other samples (doped with x = 0.01, 0.03 and 0.005) presenting reverse indentation size effect feature.     

Role of diffusion-annealing time on the superconducting, microstructural and mechanical properties of Cu-diffused bulk MgB2 superconductor

In this study, the effect of various annealing time (0.5, 1, 1.5 and 2 h) on microstructural, mechanical and superconducting properties of the Cu-diffused bulk MgB₂ superconducting samples is investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), Vickers microhardness (H _v ) and dc resistivity measurements for the first time. The critical transition temperature, grain size, phase purity, lattice parameter, surface morphology, crystallinity and room temperature resistivity values of the bulk samples prepared are compared with each other. Electrical-resistivity measurements show that the sample (annealed at 850 °C for 1 h), exhibiting the highest room temperature resistivity, obtains the maximum zero resistivity transition temperature (T _c ). From the XRD results, all the samples contain MgB₂ as the main phase with a very small amount of Mg₂Cu phase. Moreover, SEM investigations conducted for the microstructural characterization illustrate that not only does the grain size of the samples studied enhance gradually, but the surface morphology and grain connectivity also improve with the increase in the diffusion-annealing time up to 1 h beyond which all the properties obtained start to degrade. Indeed, the worst surface morphology is observed for the Cu-diffused bulk MgB₂ superconductor exposed to 2 h annealing duration. At the same time, Vickers microhardness, elastic modulus, load independent hardness, yield strength, fracture toughness and brittleness index values are calculated separately for the pure and Cu-diffused samples. It is found that the microhardness values depend strongly on the diffusion-annealing time. Furthermore, the diffusion coefficient of the Cu ion in the bulk MgB₂ superconductor is obtained to change from 1.63 × 10^?7 to 2.58 × 10^?7 cm² s^?1. The maximum diffusion coefficient is observed for the sample prepared at 850 °C for 1 h whereas the minimum one is noted for the sample annealed at 850 °C for 2 h, confirming that the annealing-time of 1 h is the best ambient to improve the mechanical, microstructural and superconducting properties of the samples produced.     

Role of diffusion-annealing temperature on the microstructural and superconducting properties of Cu-doped MgB2 superconductors

This study deals with not only investigate the effect of the copper diffusion on the microstructural and superconducting properties of MgB₂ superconducting samples employing dc resistivity as a function of temperature, scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements but also calculate the diffusion coefficient and the activation energy of copper for the first time. Electrical-resistivity measurements indicate that both the room-temperature resistivity value and zero resistivity transition temperatures (T _c ) increase with increasing the diffusion-annealing temperature from 650 to 850?°C. SEM measurements show that not only the surface morphology and grain connectivity improve but also the grain size of the samples increases with the increase in the diffusion-annealing temperature up to 850?°C. As for the XRD results, all the samples contain the MgB₂ phase only and exhibit the polycrystalline superconducting phase with more intensity of diffraction lines, leading to the increasement in the lattice parameter a and c. Additionally, the diffusion coefficient is observed to increase from 6.81?×?10^?8 to 4.69?×?10^?7?cm²?s^?1 as the diffusion-annealing temperature increases, confirming that the Cu diffusion at lower temperatures is much less significant. Temperature dependence of the Cu diffusion coefficient is described with the aid of the Arrhenius relation D?=?3.75?×?10^?3 exp (?1.15?±?0.10?eV/k _B T) and the corresponding activation energy of copper in MgB₂ system is found to be about 1.15?eV. The possible reasons for the observed improvement in microstructural and superconducting properties of the samples due to Cu diffusion are also discussed.     

Effect of Nb addition on magnetic,structural and superconducting properties of (Bi,Pb)-2223 superconductors

In this work, the effects of Nb₂O₅ addition with different ratios on the structural and magnetic properties of Bi_1.7?xPb_0.3Nb_xSr₂Ca₂Cu₃O_y (x = 0.00–0.20) superconducting samples were investigated. (Bi, Pb)-2223 superconducting samples were prepared by conventional solid-state reaction method. The phase formation, phase fraction and lattice parameters were determined from X-ray powder diffraction (XRD) measurements, the microstructure, surface morphology analyses of the samples were carried out using scanning electron microscope (SEM). Additionally, ac susceptibility measurements were done in order to determine the critical current density (J_c) and hole concentration (p) of the samples. AC susceptibility measurements were done at various ac fields (ranging from 20 to 160 A/m) to understand the effect of Nb addition on magnetic properties of Bi_1.7?xPb_0.3Nb_xSr₂Ca₂Cu₃O_y superconductor. Critical onset (T _c ^on ) and loss peak temperatures (T_p) were estimated from the ac susceptibility curves. It was observed from ac susceptibility measurements that the critical onset temperatures decreased from about 108–98 K with increasing Nb addition (x = 0.00–0.20). The imaginary part of susceptibility was used to calculate the intergranular critical current density (J_c) by means of the Bean’s model. X-ray diffraction analysis revealed that the samples consisted of a mixture of Bi-2223 and Bi-2212 phases as the major constituents and non-superconducting phase Ca₂PbO₄ as the minor. It was also shown from XRD measurements that volume fraction of high-T_c phase decreases with increasing Nb addition up to x = 0.20. The sample with Nb addition of x = 0.20 showed the highest volume fraction of Bi-2223 phase (86 %). When Nb addition was increased, the surface morphology and grain connectivity are found to degrade, the grain sizes decrease and porosity of the samples were observed to increase from SEM images except the sample with x = 0.20 Nb addition.     

Physical Properties and Diffusion-Coefficient Calculation of Iron Diffused Bi-2223 System

This study includes two parts: (I)?investigation of the effect of different annealing time (10?h, 30?h, and 60?h) on physical, superconducting, and microstructural properties of Fe-diffused Bi-2223 superconductor ceramics prepared by the conventional solid-state reaction method with the aid of the X-ray diffraction (XRD), scanning electron microscopy (SEM), dc resistivity (???CT) and transport critical current density (J _c ) measurements, and (II) determination of the diffusion coefficient and the activation energy of iron in the Bi-2223 system. In the former part, the zero-resistivity transition temperature (T _c ), phase purity, volume fraction, hole-carrier concentration, lattice parameters, surface morphology, texturing, crystallinity, grain connectivity, grain size, and room temperature resistivity values of the bulk samples are found and compared with each other. The results obtained show that both the zero resistivity transition temperature (T _c ) and transport critical current density (J _c ) regularly enhance with the increment in the diffusion-annealing time. The maximum T _c of 107±0.2 K and J _c of 50.0?A?cm^?2 are observed for the sample annealed at 830?°C for 60?h. As for the XRD investigations, according to the refinement of cell parameters done by considering the structural modulation, the enhancement in the diffusion-annealing is confirmed by both a decrease of the cell parameter a and an increase of the lattice parameter c of the samples, meaning that the greatest Bi-2223 phase fraction belongs to the sample annealed at 830?°C for 60?h. Moreover, SEM images display that the sample has the best crystallinity, grain connectivity, and largest grain size. Based on the results, the superconducting and microstructural properties improve with the increase in the diffusion-annealing time. In the latter part, Fe diffusion in the Bi-2223 system is examined in a range of 500?C830?°C by the variation of the lattice parameters evaluated from the XRD patterns. The temperature dependence of the Fe diffusion coefficient is described by the Arrhenius relation D=4.27×10^?5exp(?1.27±0.10) eV/k_BT, and the related activation energy of the iron in the Bi-2223 system is found to be about 1.27?eV. The relatively low value of activation energy obtained illustrates that the migration of the Fe ions primarily proceeds through defects such as pore surfaces and grain boundaries in the polycrystalline structure, leading to the improvement of the microstructural and superconducting properties of the samples, supported by the results of part?I. All in all, the aim of the present study is not only to analyze the role of diffusion-annealing time on superconducting and microstructural properties of Fe-diffused Bi-2223 superconductors, but also to find the diffusion coefficient and activation energy of Fe in the Bi-2223 system.     

Role of annealing time and temperature on structural and superconducting properties of (Bi,Pb)-2223 thin films produced by sputtering

This study reports the effect of annealing time (15 min, 1.5 and 3 h) and temperature (850, 860 and 870 °C) on the structural and superconducting properties of thin films by means of scanning electron microscopy (SEM), X-Ray analysis (XRD), electron dispersive X-Ray (EDX), resistivity and transport critical current density (J_c) measurements. Zero resistivity transition temperatures (T_c) of the films produced are estimated from the dc resistivity measurements. In addition, the phase and lattice parameters are determined from XRD patterns when the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. The results indicate that T_c values of the films obtained are observed to be in a range of 23–102 K. The T_c of the film annealed at 870 °C for 3 h is found to be the smallest (23 K) while the film annealed at 860 °C for 3 h is noted to obtain the maximum T_c value (102 K). On the other hand, the maximum (minimum) J_c is found to be about 2068 A/cm² (20 A/cm²) for the film annealed at 860 °C for 3 h (870 for 3 h). Moreover, according to the refinement of cell parameters done by considering the structural modulation, the greatest Bi-2223 phase fraction is noticed to belong to the film annealed at 860 °C for 3 h. Furthermore, SEM measurements show that the best surface morphology, largest grain size and grain connectivity are observed for that film. Based on these results, T_c and J_c values of the samples studied are found to depend strongly on the microstructure. As for EDX results, the elements used for the preparation of samples are observed to distribute homogeneously. The aim of this study is not only to investigate the changes of structural and superconducting properties of the films produced in the varied time and temperature but also to determine the best ambient for the film fabrication and show the feasibility of obtaining Bi-2223 film with tailored structure.     

The Influence of Postannealing on Structural and Superconducting Properties of Bi-2212 Ceramics

The effect of postannealing on the structural and superconducting properties of Bi-2212 sintered ceramic samples prepared by the solid state reaction method has been investigated. Postannealing times were varied from 0 h to 192 h at 700 ^°C. Electrical resistivity studies showed that postannealed samples at 96 h have the lowest room temperature values while critical transition temperature does not change significantly. XRD data have shown that the Bi-2212 phase content does not change noticeably, independently of the postannealing length. In addition, J _c values, calculated from the hysteresis loops using the Bean’s model, increased with increasing the postannealing time until 48 h and decrease for longer ones.     

Effect of Annealing Time and Temperature on Microstructural and Superconducting Properties of (Bi,Pb)-2212 Thin Films Produced by Magnetron Reactive Sputtering

This study deals with the role of various annealing time (1?h, 2?h, 4?h, 6?h and 8?h) and temperature (840 and 850?°C) on the microstructural and superconducting properties of thin films with the aid of scanning electron microscopy (SEM), X-Ray analysis (XRD), electron dispersive X-Ray (EDX), resistivity and transport critical current density (J _c ) measurements. The T _c , J _c , variation of transition temperatures, hole-carrier concentration, grain size, phase purity, lattice parameter, surface morphology, element distribution, crystallinity and resistivity (at room temperature) values of the films prepared are compared with each other. Critical transition temperatures (T _c ) of the samples are deduced from the dc resistivity measurement while critical current density values are estimated from the critical current and total cross-sectional area values. It is found that maximum T _c of 79.7?K and J _c of 1520?A/cm² are observed for the film annealed at 840?°C for 6?h as against 54.9?K and 30?A/cm² (minimum values), respectively, for the film annealed at 840?°C for 4?h. Moreover, SEM images indicate that the former has the best crystallinity, grain connectivity and largest grain size. Based on these results, T _c and J _c values of the samples studied are found to depend strongly on the microstructure. Additionally, EDX results show that the elements used for the preparation of all the samples are observed to distribute homogeneously. As for the XRD results, all the samples exhibit the polycrystalline superconducting phase with the changing intensity of diffraction lines. According to the refinement of cell parameters done by considering the structural modulation, the largest lattice parameter a and c are obtained for the film annealed at 840?°C for 8?h. To sum up, the aim of the present study is not only to investigate the changes of microstructural and superconducting properties of the samples fabricated in the varied time and temperature but to determine the best ambient for the film fabrication and show the feasibility of obtaining Bi-2212 film with tailored structure, as well.     

Analysis of Indentation Size Effect on Mechanical Properties of Cu-Diffused Bulk MgB2 Superconductor Using Experimental and Different Theoretical Models

This study indicates the change of the electrical, microstructural, physical, mechanical and superconducting properties of Cu-diffused bulk MgB₂ superconductors by means of scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), microhardness and dc resistivity measurements. The samples are prepared at different annealing temperatures in the range from 650 to 850 ^°C. Electrical and superconducting properties of samples are estimated from the dc electrical resistivity measurements. Moreover, microhardness measurements are performed to investigate the mechanical properties. Further, phase composition, grain sizes and lattice parameters are determined from the XRD measurements. At the same time, the surface morphology and grain connectivity of the samples are examined by SEM investigations. The measurements conducted demonstrate that both the Cu diffusion into the MgB₂ system and the increment in the diffusion-annealing temperature increase the critical transition temperatures. Similarly, microstructure and grain size improve while the voids and porosity decrease with the increase of the diffusion-annealing temperature. In addition, the experimental results of the microhardness measurements are investigated using the Meyer’s law, PSR (proportional specimen resistance), modified PRS (MPSR), elastic-plastic deformation model (EPD) and Hays–Kendall (HK) approach. The obtained microhardness values of the samples decrease with the increase of the diffusion-annealing temperature up to 850 ^°C. The Hays–Kendall approach is found to be the most successful model describing the mechanical properties of the samples studied in this work.     

Critical Current Density and Intergranular Coupling Study of the Dysprosium Oxide Nanoparticle Added Bi1.6Pb0.4Sr2Ca2Cu3O y Superconductor

The dysprosium oxide nanoparticles’ addition effects on structural, DC electrical resistivity, critical current density, and AC magnetic susceptibility properties of polycrystalline Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O _y samples are investigated. X-ray diffraction (XRD) analysis showed that both (Bi,Pb)-2223 and Bi-2212 phases coexist in the samples having orthorhombic crystal structure. Bi-2223 phase concentration increases with increasing dysprosium nanoparticle concentration. DC electrical resistivity, critical current density (J _c), and AC susceptibility measurements reveal that adding dysprosium nanoparticles to bismuth–strontium–calcium–copper–oxide (BSCCO) improves superconducting properties of this system and enhances its critical current density due to the improvement of the grain connectivity with dysprosium nanoparticle addition.     

Investigation of microstructural, Vickers microhardness and superconducting properties of YBa2Cu3−xGdxO7−δ (0 ≤ x ≤ 0.150) superconducting ceramics via experimental and theoretical approaches

This study manifests the change of pinning mechanism, electrical, structural, physical, mechanical and superconducting properties of YBa₂Cu_3?xGd_xO_7?δ superconductors samples prepared by the conventional solid-state reaction method (x = 0, 0.025, 0.050, 0.100 and 0.150) by use of dc resistivity, X-ray analysis (XRD), scanning electron microscopy (SEM) and Vickers microhardness measurements. Zero resistivity transition temperatures (T _c ^offset ) of the samples are deduced from the dc resistivity measurements. Additionally, the lattice parameters are determined from XRD measurements when the microstructure, surface morphology and microhardness of the samples studied are examined by SEM and mechanical measurements, respectively. The results obtained demonstrate that T _c ^offset values of the samples decrease slowly with the increase in the Gd content. The maximum T _c ^offset (92.0 K) is obtained for the pure sample prepared at 940 °C for 20 h in air atmosphere while the minimum value of 83.3 K is found for the sample doped with 0.150 Gd content. Moreover, it is obtained that J _c values reduce from 132 to 34 A/cm² with the enhancement of the Gd level in the crystalline structure. Further, the peak intensities belonging to Y123 (major) phase are obtained to decrease whereas the peak intensities of the minor phases such as BaCuO₂ and Y211 are found to enhance systematically with the increment in the Gd content in the system, illustrating that partial substitution of Cu²⁺ ions by Gd³⁺ ions are carried out successfully. Moreover, SEM images display that the undoped sample obtains the best crystallinity and connectivity between superconducting grains and largest grain size whereas the worst surface morphology is observed for the maximum doped sample (x = 0.150). At the same time, Vickers microhardness, elastic modulus, load independent hardness, yield strength, fracture toughness and brittleness index values, playing important roles on the mechanical properties, are computed for all the samples. The experimental results of the microhardness measurements are examined using the Meyer’s law, PSR (proportional specimen resistance), modified PRS, Elastic–Plastic deformation model (EPD) and Hays–Kendall (HK) approach. The microhardness values obtained increase with the enhancement of the Gd content in the samples. Besides, it is noted that the Hays–Kendall approach is the most successful model explaining the mechanical properties of the samples studied in this work.     

Influence of preparation conditions on superconducting properties of Bi-2223 thin films

We report electrical transport properties of Bi₂Sr₂Ca₂Cu₃O_10+x (Bi-2223) superconducting thin films fabricated by pulsed-laser deposition on SrTiO₃ substrate. The aim of the study was to investigate the influence of preparation conditions such as deposition temperature (T _S), annealing time (t _A) and deposition rate (r). A critical temperature (T _c) as high as 110 K and critical current density (J _c) of 6·2 × 10⁶ A/cm² at 20 K were obtained for T _S = 760°C, t _A = 4 h and r = 1·5 Å/s. We also investigated the effect of Li doping on Bi-2223 thin films. Li intercalation results in high resistive onset transition temperature and the resistivity shows broadening in magnetic field that increases with field. The large broadening of resistivity curve in magnetic field suggests that this phenomenon is directly related to the intrinsic superconducting properties of the copper oxide superconductors. The sudden drop in J _c at relatively low magnetic field (H < 0·5 tesla) is due to the effect of Josephson weak-links at the grain boundaries.     

Role of Gd content in Cu(1) and Cu(2) sites on electrical, microstructural, physical, mechanical and superconducting properties of YBa2Cu3−xGdxO7−δ ceramics

This study deals with the effects of partial Gd³⁺ substitution for the Cu sites on the electrical, microstructural, physical, mechanical and superconducting properties of YBa₂Cu_3?xGd_xO_7?δ ceramic superconductors with x = 0, 0.025, 0.050, 0.100 and 0.150 with the aid of dc resistivity, transport critical current density (J _c ), X-Ray analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-Ray (EDX), Vickers microhardness (H _v ) and density measurements. The samples studied in this work are prepared by the standard solid-state reaction method. The resistivity (at room temperature), critical (onset and offset) temperature, variation of transition temperature, critical current density, hole-carrier concentration, grain size, phase purity, lattice parameter, texturing, surface morphology, element distribution, density, porosity, crystallinity, Vickers microhardness and elastic modulus (E) values of the samples are obtained and compared with each other. The obtained results show that the room temperature resistivity systematically increases with the increment of the Gd content as a result of the hole filling when the onset (T _c ^onset ) and offset (T _c ^offset ) critical temperatures determined from the resistivity curves are found to decrease from 95.2 to 93.6 K and 92.0 to 83.3 K, respectively, showing the presence of impurities and weak links between the superconducting grains. As for the critical current density measurements, the J _c values decrease from 132 to 34 A/cm² as the Gd doping increases. The XRD results give that although the Gd³⁺ ions substituted tend to occupy both the Cu(1) and Cu(2) sites, the ions are more favorable for the Cu(2) site as a consequence no change of the crystal structure. Besides, the peak intensities belonging to major phase (Y123) decrease monotonously with the increment of the Gd content in the system; however, new peaks belonging to the minor phases start to appear after the doping level of x = 0.0250 beyond which these peaks enhance monotonously, resulting in the decrement of the grain size. Further, the Lotgering indices calculated from the XRD patterns indicate that the texturing of the Y123 grains reduces systematically with the Gd content. According to the SEM investigations, the microstructures of the samples prepared degrade slightly with the content up to the doping level of x = 0.025 after which the morphology suddenly deteriorates due to the appearance of the different phases in the system. EDX measurements show that not only do the elements used for the preparation of the Y123 superconductors with and without Gd content distribute homogeneously but also the level of Cu element rapidly decreases with the increment of the Gd content compared to the other elements, illustrating that the Cu²⁺ ions may partly be substituted by Gd³⁺ ions. Moreover, the porosity analyses for the samples depict that the porosity increases with the Gd content, leading to the degradation of the grain connectivity. We also discuss on the mechanical properties of the samples to examine both the elastic modulus and the strength of connection between superconducting grains.     

Vickers hardness measurements and some physical properties of Pr<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Bi-2212 superconductors

This study deals with the effect of Pr₂O₃ addition on the structural, superconducting and mechanical properties of Bi-2212 superconductor by means of X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), electron dispersive X-ray (EDX), Vickers microhardness and resistivity measurements. The samples studied are prepared using the standard solid-state reaction method. Onset transition temperatures (T _c ^onset) of the samples are estimated from the dc resistivity measurements. Furthermore, the phase ratio and lattice parameters a and c are determined from XRD patterns when the microstructure, surface morphology and element composition analyses of the samples are investigated by SEM and EDX measurements, respectively. Additionally, vickers microhardness, elastic modulus, yield strength and fracture toughness values of the samples are deduced from microhardness measurements. It is found that T _c ^onset values of the samples increase from 87 to 97 K with the Pr₂O₃ addition. According to the refinement of cell parameters done by considering the structural modulation, the doping is confirmed by both an increase of the lattice parameter a and a decrease of the cell parameter c of the samples in comparison with that of the pure sample. As for SEM measurements, it is obtained that the surface morphology and grain connectivity degrade with the increase of the Pr₂O₃ addition. Moreover, EDX images show that the elements used for the preparation of samples distribute homogeneously and the Pr atoms enter into the crystal structure by replacing Sr atoms. To sum up, the Pr₂O₃ addition is found to suppress the mechanical, microstructural and superconducting properties of the Bi-2212 superconductor.     

Effect of Postannealing Process on Bi2Sr2.1Ca0.9Cu2O8+δ Textured Superconductors

Bi-2212 samples prepared by the classical solid-state method have been grown from the melt using the Laser Floating Zone (LFZ) method. They have shown good grain alignment and transport critical current densities (J _c ). After postannealing processes designed to produce the Bi-2212 phase controlled decomposition, J _c values have been increased in an important manner. Maximum values have been achieved when samples were thermally treated at 680 °C for 168 h with improvements around 80 %, compared with the original textured samples. The results clearly indicate that postannealing processes, when adequately controlled, produce the formation of effective pinning centers which are responsible for the increase in the measured J _c values.     

Influence of Boron-Containing Dopants on Superconducting Properties of (Bi, Pb)-2223 HTS

The influence of addition of lead borate Pb(BO₂)₂ and boron oxide B₂O₃ on the phase evolution and superconducting properties of (Bi, Pb)-2223 HTSs synthesized by the solid-state reaction method in alumina crucibles has been studied. X-ray diffraction, resistivity, critical current density, and AC susceptibility measurements were performed on the prepared compounds. Obtained results have shown that boron-containing dopants lead to the drastic enhancement of the (Bi, Pb)-2223 phase formation. Boron-doped samples reveal a significant increase in both the zero resistivity temperature and transport critical current density compared to the undoped specimen. On the other hand, a high content of boron-containing dopants causes the appearance of a very low-T _c 2201 phase and leads to a deterioration of coupling between superconducting grain boundaries. Obtained results could enable us to develop a cheap and energy efficient fabrication technology for nearly single (Bi, Pb)-2223 phase superconducting materials via heat treatment of boron-incorporated precursors in an alumina crucibles.     

Improvement of the Nature of Indentation Size Effect of Bi-2212 Superconducting Matrix by Doped Nd Inclusion and Theoretical Modeling of New Matrix

Neodmium (Nd) inclusions at different stoichiometric ratios (x=0.0, 0.001 %, 0.005 %, 0.01 %, 0.05 %, 0.1 %) are doped in the Bi-2212 superconducting samples and the samples obtained are subjected to the sintering process at 840 ^°C constant temperature for 72 hours. The effect of Nd doping on the structural and mechanical properties of prepared samples is investigated by the standard characterization measurements. XRD and SEM measurements are performed to obtain information about surface morphology, phase ratios, lattice parameters and particle size. Moreover, Vickers microhardness (H _V ) measurements are exerted to investigate the mechanical properties of the all samples in detail. It is found that all the properties given above retrogress with the increase of the Nd concentration in the Bi-2212 superconducting core. However, the ISE nature of the materials improves systematically. Additionally, the experimental results of microhardness measurements are analyzed using Meyer’s law, PSR, MPSR, EPD models and HK approach. The results show that Hays–Kendall approach is determined as the most successful model.     

Study on Incommensurate Modulation Structures in Ca-Doped Bi-2201 System

The Ca-doped Bi-2201 system with nominal composition Bi₂Sr_2?x Ca _x CuO_6+δ (x=0.1,0.3,0.5,0.7,0.9) was prepared by sol–gel method and investigated by X-ray diffraction, transmission electron microscopy and resistance measurements. It can be found that the Ca-doped Bi-2201 system was composed of Bi-2201 phase containing Ca (main phase) and a small quantity of Bi₁₆(Sr,Ca)₁₄O₃₈ (impurity). For the main phase, the b ^? component of the modulation wave vector decreases while the c ^? component increases with the increase of calcium content. The evolutional mechanism of the modulation wave vector, the correlation between superconductivity and modulation wave vector were also discussed.     

Influence of Diffusion-Annealing Temperature on?the?Physico-Mechanical Properties of Au-doped Bi-2223 Superconductors

In order to investigate the influence of Au doping and diffusion-annealing temperature on the mechanical and superconducting properties of Bi-2223, Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃O _y superconductors were prepared by standard solid-state reaction methods. Doping of Bi-2223 was carried out by means of gold diffusion during sintering from an evaporated gold film on pellets. The investigation consisted of scanning electron microscopy, dc resistivity and hardness measurements. Electrical-resistivity measurements indicated that the room-temperature resistivity value decreased with decreasing diffusion-annealing temperature from 830 to 500?°C and these samples (G830, G800, G750, G700, G600 and G500) show the resistive behavior above the onset critical transition temperature with the zero-resistivity transition temperatures of 104 K, 80 K, 98 K, 95 K, 102 K and 103 K, respectively. To investigate mechanical properties of the samples, we have measured the diagonal length as a function of test load in the range of 0.245?C2.940 N. Mechanical properties (microhardness, Young??s modulus, yield strength and fracture toughness) of the samples are found to be load and diffusion-annealing temperature dependent. In addition, we have calculated the load independent hardness, Young??s modulus, yield strength, and fracture toughness of the samples. The possible reasons for the observed changes in superconducting and mechanical properties due to Au diffusion and diffusion-annealing temperature were discussed.