Improved Critical Current Density of MgB₂–Carbon Nanotubes Composite

In the present study, we report a systematic study of doping/admixing of carbon nanotubes (CNTs) in different concentrations in MgB₂. The composite material corresponding to MgB₂−x at.% CNTs (35 at.% ≥ x ≥ 0 at.%) have been prepared by solid-state reaction at ambient pressure. All the samples in the present investigation have been subjected to structural/microstructural characterization employing XRD, Scanning electron microscopic (SEM), and Transmission electron microscopic (TEM) techniques. The magnetization measurements were performed by Physical property measurement system (PPMS) and electrical transport measurements have been done by the four-probe technique. The microstructural investigations reveal the formation of MgB₂–carbon nanotube composites. A CNT connecting the MgB₂ grains may enhance critical current density due to its size (∼ 5–20 nm diameter) compatible with coherence length of MgB₂ (∼5–6 nm) and ballistic transport current carrying capability along the tube axis. The transport critical current density (J _ct) of MgB₂ samples with varying CNTs concentration have been found to vary significantly e.g., J _ct of the MgB₂ sample with 10 at.% CNT addition is ∼2.3 × 10³ A/cm² and its value for MgB₂ sample without CNT addition is ∼7.2 × 10² A/cm² at 20 K. In order to study the flux pinning effect of CNTs doping/admixing in MgB₂, the evaluation of intragrain critical current density (J _c) has been carried out through magnetic measurements on the fine powdered version of the as synthesized samples. The optimum result on J _c is obtained for 10 at.% CNTs admixed MgB₂ sample at 5 K, the J _c reaches ∼5.2 × 10⁶ A/cm² in self field, ∼1.6 × 10⁶ A/cm² at 1 T, ∼2.9 × 10⁵ A/cm² at 2.6 T, and ∼3.9 × 10⁴ A/cm² at 4 T. The high value of intragrain J _c in 10 at.% CNTs admixed MgB₂ superconductor has been attributed to the incorporation of CNTs into the crystal matrix of MgB₂, which are capable of providing effective flux pinning centres. A feasible correlation between microstructural features and superconducting properties has been put forward.