Assessment of the Potential of the La_2-xSr_xCuO₄ Superconductor for Power Applications

Since the discovery of superconductor materials in 1900s, the demand for superconductors with high critical temperature (T_c) and high critical current density (J_c) has grown increasingly urgent. Currently, YBaCuO (YBa₂Cu₃O_7−x) is the most widely used superconductor, boasting a T_c over 90 K and a J_c exceeding 107 A/cm². However, challenges such as controlling oxygen vacancy content, grain angular orientation, and grain boundary issues persist. To address these challenges, attention has once again turned to the original cuprate superconductor, La_2−xSr_xCuO₄ (LSCO). Although its T_c only reaches 40 K, advances in refrigeration technology have mitigated this limitation, and insights gained from LSCO research can be applied to improve the superconductive properties of other cuprate superconductors.

This thesis explores the fabrication and post-treatment techniques for LSCO ceramics. An advanced technique, Spark Plasma Sintering (SPS), was employed to investigate the structural and superconductive properties of LSCO ceramics compared to those fabricated using conventional sintering methods. While the SPS technique suppressed superconductivity, it achieved higher density compared to conventionally sintered ceramics.

To restore and enhance the performance of SPS LSCO ceramics, annealing was applied. High-temperature annealing restored the superconducting T_c of SPS LSCO ceramics to the standard 38 K of La_1.84Sr_0.16CuO₄, and the critical current density reached over 60000 A/cm² when the annealing temperature reached 1300 °C. This value approaches the J_c along the c-axis of LSCO crystal, indicating the potential applicability of this annealing treatment. However, surface decomposition of LSCO into complex oxides occurred, impacting the structural properties of SPS LSCO ceramics.

Given the angular dependence of cuprate superconductors, a two-step forging method was applied to study the texturing effects on SPS LSCO ceramics. Postforging, the superconductive performance showed some fluctuations compared to
the one-step SPS specimens, but subsequent annealing restored and improved performance, with a maximum Jc of about 35,000 A/cm².

In the context of fusion energy, superconductors are crucial in magnetic coils used to control high-temperature plasma. However, neutron production in fusion reactors can damage superconductors. This thesis also investigates the effects of
fast neutron irradiation on LSCO ceramics. Neutron irradiation introduced additional pinning centers within the lattice, and the impact was found to be grain-size-dependent. The highest J_c observed in this study was over 120,000 A/cm² in irradiated SPS LSCO ceramics, marking the highest J_c reported for such materials.

In summary, this research applied Spark Plasma Sintering to the fabrication of LSCO superconducting ceramics, demonstrating significant improvements in current properties through various post-treatments. Given the structural similarities
among cuprate superconductors, these findings could contribute to improving other complex cuprate superconductors, paving the way for their practical applications in daily life.