Structure and superconducting performance of YBa2Cu3O7-δ bulk added with carbon nanotubes prepared by thermal treatment method

YBa 2 Cu 3 O 7-δ (Y123) superconductors with different concentrations of single-walled carbon nanotubes (CNTs) ( x = 0.2–1.0 mass%) were synthesized using a thermal treatment method. The influence of CNT addition on the structural, microstructural, and superconducting properties was investigated using thermogravimetric analysis (TGA/DTG), X-ray diffraction (XRD), field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (FESEM–EDX), AC susceptibility (ACS), and the four-point probe (4PP) technique. Thermal stability was examined using TGA-DTG analysis. XRD analysis confirmed that the orthorhombic Y123 phase remained dominant in all samples. Minor secondary phases, including Y 2 BaCuO 5 (Y211) (0.2–12.3%), were detected in all CNT-added samples, while the Y 2 Ba 4 Cu 7 O 15−δ (Y247) phase (4.2%) was identified at x = 0.8 mass% CNT. FESEM observations revealed a reduction in the average grain size with CNT incorporation. AC susceptibility measurements showed a slight increase in the onset transition temperature ( T c-onset ), rising from 91.7 K for the undoped sample to 92.3 K for the sample containing 0.4 mass% CNT. The peak temperature ( T p ) of the imaginary component shifted towards higher temperatures for the samples with x = 0.4 and 0.8 mass% CNT. Electrical resistivity measurements confirmed superconducting behaviour in all samples, with the sharpest superconducting transition (Δ T c ) observed at x = 0.4 mass% CNT. The critical current density ( J c ), determined from the current–voltage characteristics, increased from 2.04 A cm −2 for the undoped Y123 sample to 8.1 A cm −2 for the sample containing 0.4 mass% CNT. These results indicate that an optimal CNT addition improves grain connectivity and flux-pinning behaviour, thereby enhancing the superconducting performance of Y123 ceramics.