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The effect of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) addition on the phase formation and the superconducting properties of GdBa₂Cu₃O${}_{7-\delta }$ phase has been studied. Therefore, composite superconductor samples of type (CNTs)${}_x$ GdBa₂Cu₃O${}_{7-\delta }$, 0.0 $\le x\le$ 0.1 wt.% have been synthesized by a standard solid-state reaction technique. The samples have been characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results of XRD show an enhancement in the phase formation up to 0.06 wt.% and 0.08 wt.% for SWCNTs and MWCNTs, respectively. SEM and TEM reveal that CNTs form an electrical network resulting in well-connected superconducting grains. The electrical properties of the prepared samples have been examined by electric resistivity and I–V measurements, and their results reinforce the XRD, SEM and TEM. Consequently, both $T_c$ and $J_c$ improve as the addition percentage increases up to 0.06 wt.% and 0.08 wt.% for SWCNTs and MWCNTs, respectively.

Gd-123 superconducting phase was prepared by solid-state reaction technique. Single-walled carbon nanotubes (SWCNTs) were added in Gd-123 superconducting matrix with different concentrations during the final sintering process to obtain (SWCNTs)${}_x$ GdBa₂Cu₃O${}_{7-\delta }$ (x = 0.0–0.1 wt.%) nanoparticles–superconductor composite. The influence of SWCNTs addition on the phase formation, structural, morphological, superconducting and dielectric properties of Gd-123 phase was investigated. It was found that SWCNTs addition enhance the phase formation and does not change the crystal structure of the host Gd-123 superconducting phase. The superconducting properties of Gd-123 samples were improved after the addition of SWCNTs up to x = 0.06 wt.% due to the enhancement in intergrain connectivity by healing up of micro-cracks and reduction of defects, while these properties were retarded with further increase in x. The dielectric response of (SWCNTs)${}_x$ Gd-123 superconducting phase with x = 0.0, 0.01, 0.04, 0.05, 0.06 and 0.1 wt.% was measured from 100 KHz to 5 MHz at 77 K. The results reveal that for both real (${𝜀}^{\prime }$) and imaginary (${𝜀}^{″}$) parts of dielectric constant, the frequency of dispersion increased by increasing SWCNTs amount up to 0.06 wt.%, then this frequency shifted to lower values for x$>$ 0.06 wt.%. The results were discussed according to the presence and interference of both interfacial and dipolar polarizations.