Sugali Pavan Kumar Naik

Layered oxychalcogenide Sr₂ZnCu₂(S_1-x Se _x )₂O₂ compounds were synthesized via a solid-state reaction. Density functional theory calculations indicate different valence band maximum, Sr₂ZnCu₂Se₂O₂ was composed of Cu 3d–Se 4p, while Sr₂ZnCu₂S₂O₂ was dominated by the Zn 3d–O 2p hybrid orbital. Band gap was decreased as the Se-containing is increasing, from 2.5 eV to 2.2 eV. In contrast, electrical conductivity of the Se-substituted samples considerably increased from 10⁻³ S cm⁻¹ at x = 0 to 10⁻¹ S cm⁻¹ at x = 0.25, and was almost the same at x = 1. Based on this feature, Sr₂ZnCu₂(S_1-x Se _x )₂O₂ can be applied as a transparent conductor material.

The fabrication of RE Ba ₂ Cu ₃ O _{7- δ} ( RE BCO, RE = rare earth elements) bulks in single grain form and their flux pinning depend strongly on the initial conditions of the RE ₂ BaCuO ₅ ( RE 211) phase used as precursor. We recently demonstrated a novel method of employing high-energy ultrasonication to create fine RE 211 particles ( RE 211 _Ultra ) with irregular shapes along with sharp edges and damaged surfaces. The developed method is convenient, free of contaminants, and low cost as compared to other methods currently utilized. Integrating these RE 211 _Ultra in the top-seeded melt growth (MG) method, we improved the initial peritectic growth reactivity and thus, the superconducting properties of YBCO and GdBCO bulk single grains. Melt growth experiments were carried out on different RE BCO ( RE = Y and Gd) systems with the addition of 30 mol% of RE 211 _Ultra . The RE 211 _Ultra particles boosted the initial peritectic reaction i.e., growth rate during the MG process due to their sharp edges, and damaged surfaces in their nanometer size. The left-back fine-sized RE 211 particles increased the interfacial defects and hence, the flux pinning. In the present work, we summarize the effect of ultrasonication of RE 211 particles, their impact on peritectic reaction and improvement of superconducting properties of various REBCO bulks. Employing RE 211 _Ultra powders for infiltration growth (IG) processing, we enabled a good control of the RE 211-phase content and particle size in the final bulk YBCO single grains. The field-dependence properties of MG and IG-processed of YBCO superconductors are significantly improved without any additional chemical addition/doping.

Recently, an improvement in the flux pinning performance of REBa₂Cu₃O_7−δ (REBCO) bulk was achieved employing high energy ultrasonic irradiation (up to 300 W and 60 min) prior to the infiltration-growth process. Here, we demonstrate that a higher ultrasonic power and shorter duration treatment (450 W for 30 min) of the RE ₂BaCuO₅ (RE211, RE= Y, Gd) powder produces individual, nanometer-sized and surface damaged RE211 particles (RE211_Ultra). We study the growth of YBCO and GdBCO systems via the top-seeded melt-growth method, with the addition of 30 mol% of RE211_Ultra, which were pre-treated by high-energy ultrasonication and compared with the conventional method. Isothermal growth experiments clarified that the addition of RE211_Ultra particles enables faster and more effective crystal growth via an improved peritectic reaction due to their size in the nanometer range and presence of sharp edges, which are crucial for growing large, single grain bulks of REBCO. Microstructural investigations by scanning electron microscopy indicated the presence of two differently sized RE211 inclusions within the bulk GdBCO and YBCO superconductors. All samples showed an onset of superconductivity at ∼92 K or above. Utilizing the effective growth temperature window and the addition of RE211_Ultra, single grain bulks of GdBCO and YBCO were fabricated by the slow-cooling method. The field dependent critical current density (J _c) of the bulk samples using RE211_Ultra were found to exhibit superior performance over the standard GdBCO/YBCO samples due to significant changes of sample growth and of their microstructures. Here, we discuss various factors affecting the addition of RE211_Ultra on the development of the microstructure, growth of bulk, single grain materials and further, the superconducting properties of different REBCO superconductors.

Flux-Pinning Docking Interfaces (FPDI) in satellite systems were developed using bulk superconductors and permanent magnets in previous works. However, such FPDIs have limited magnetic field strength, consist of heavy-weight material, and can only be used with a single purpose, i.e., as chasing or docking satellite. Replacing the magnetic material in the FPDI by a trapped field (TF)-magnet would enable the interface to operate for both purposes, i.e., generating a (stronger) magnetic field and trapping it. We show the requirements for such a system and discuss the possible gains when using a TF-FPDI in satellites. To reduce the system weight, the use of superconducting foams as superconducting material is discussed in detail. Furthermore, the use of superconducting foams, the size of which can be easily upscaled, may also comprise the function of the damping material, so even more weight could be saved for the payload.

In this study, high-density magnesium diboride (MgB₂) bulk superconductors were synthesized by spark plasma sintering (SPS) under pressure to improve the field dependence of the critical current density (J_c-B) in MgB₂ bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and J_c-B using two methods, ex situ (sintering MgB₂ synthesized powder) and in situ (reaction sintering of Mg and B powder), respectively. As a result, we found that higher density with suppressed particle growth and suppression of the formation of coarse particles of MgB₄ and MgO were found to be effective in improving the J_c-B characteristics. In the ex situ method, the degradation of MgB₂ due to pyrolysis was more severe at temperatures higher than 850 °C. The sample that underwent SPS treatment for a short time at 850 °C showed higher density and less impurity phase in the bulk, which improved the J_c-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and J_c-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (f_P) analysis revealed the possibility of refined MgO inclusions and MgB₄ phase as new pinning centers, which greatly contributed to the J_c-B properties. The contributions of the sintering conditions on f_P for both synthesis methods were analyzed.

Various intrinsic defect structures of polycrystalline CaKFe ₄ As ₄ (CaK1144) superconductor were investigated probing scanning transmission electron microscopy (STEM) and correlated to their high J _c up to T _c and large applied magnetic fields.

New CrAs-based layered mixed-anion compounds Sr₂ScCrAsO₃ (SrScO-21113) and Ba₃Sc₂Cr₂As₂O₅ (BaScO-32225) were synthesized, and their electronic structures and physical properties were investigated. The structures of these compounds comprise stacking of the anti-fluorite CrAs layer and perovskite-like SrScO or BaScO layers. The lattice constants of these compounds are relatively longer than those of the related compounds, such as BaCr₂As₂, owing to the insertion of a large perovskite blocking layer of SrScO/BaScO. While there are variations in the crystal structure of this system, such as 21113 and 32225, their chemical stability calculated by the first-principles calculations indicated that SrScO-21113 is energetically favorable compared to SrScO-32225. The formation energies of BaScO-32225 and BaScO-21113 are close to each other; in the experiment, while there was an indication of BaScO-21113 formation, only BaScO-32225 was formed as a single phase because of the low chemical stability of BaScO-21113. The partial density of states indicates that the majority of states are obtained from the 3d⁴-electrons of the Cr element hybridized modestly with p electrons at the Fermi energy. The magnetic properties of these compounds were paramagnetic, and they were different from related compounds, such as BaCr₂As₂, probably because of their long a-axis lengths. The temperature dependences of the electrical resistivities of both samples were in good agreement with the electronic band structure calculations. The variety of structures in the series of compounds with a CrAs layer results in different physical properties, and further development of new compounds may bring novel functionalities, such as superconductivity.