Effects of Bioderived Calcium Oxide Doping on the Microstructure and Phase Formation of SmBa₂Cu₃O₇−δ Superconductors

Abstract

A superconductor is a class of materials capable of conducting electrical current without resistance or energy loss when cooled below a specific critical temperature. However, high temperature superconductors such as SmBa₂Cu₃O₇_−δ (SmBCO) often suffer from weak link behavior at grain boundaries, which can negatively affect microstructural homogeneity and phase stability, limiting their practical performance. SmBCO is a well-known high temperature superconductor belonging to the REBa₂Cu₃O₇_−δ family, exhibiting zero electrical resistance at relatively higher temperatures compared to conventional superconductors. To overcome grain boundary limitations and improve its microstructure, this study investigates the incorporation of calcium oxide (CaO) extracted from natural cockle shells (Cerastoderma edule) into SmBCO using the solid-state reaction method, chosen for its simplicity, cost-effectiveness, and ability to yield materials with good phase purity and structural stability. SmBCO samples were synthesized with varying CaO weight percentages of 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 wt.% to systematically examine the effect of CaO on grain connectivity, porosity, and phase formation. Comprehensive characterizations were performed using thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) to evaluate thermal stability, chemical bonding, crystalline structure, surface morphology, and elemental composition. Results indicate that CaO doping significantly enhances grain connectivity and reduces porosity, leading to improved microstructural homogeneity. XRD confirmed the formation of the primary SmBCO phase in all samples, with minor secondary phases appearing at higher CaO contents, suggesting an optimal doping threshold. Overall, the controlled addition of cockle shell–derived CaO provides an environmentally sustainable approach to improving the microstructural quality, phase stability, and performance of high-temperature SmBCO superconductors for advanced technological applications.