Comparative study of hot and cold press and their effect on superconducting properties of coronene added bulk MgB2

Highlights: • Effects of hot and cold pressing on C₂₄H₁₂ addition into MgB₂ has been investigated. • Cold pressing strongly increases lattice strain, confirming more C substitution. • Hot pressing method minimizes the dependence of T_con the C₂₄H₁₂ addition. • Hot pressing ensures smaller grain size and stronger flux pinning force. • Electron–phonon interaction properties changes depending on the pressing method. -- Abstract: In this study, the superconducting properties of 4 wt% coronene (C₂₄H₁₂) added polycrystalline bulk MgB₂ samples prepared by hot pressing and cold pressing, were reported in detail. The results were compared to determine one of which pressing methods is more useful to substitute of carbon released from C₂₄H₁₂ into MgB₂ structure and to increase the superconducting properties of MgB₂. The lattice strain, crystallite size, residual resistivity, superconducting transition temperature (T_c), critical current density (J_c), flux pinning force density (F_p) and vertical/lateral levitation force (F_z, F_x) were analysed for the hot and cold pressed samples. The a-lattice parameter and T_c value reduced sharply for 4 wt% C₂₄H₁₂ added cold pressed sample, resulting in a notable decrease of increment ratio of J_c and F_p values, while the depressing of T_c is lower for 4 wt% C₂₄H₁₂ added hot pressed sample. The cold pressing strongly increased the lattice strain, confirming more C substitution, while the hot pressing ensured smaller crystallite size. The J_c and F_p values were respectively found as 4.6 × 10³ A/cm², 1.3 × 10⁴ A/cm², 5.8 × 10³ A/cm² and 1.8 × 10⁸ N/m³, 5.1 × 10⁸ N/m³, 2.3 × 10⁸ N/m³ for the pure sample and, 4 wt% C₂₄H₁₂ added hot and cold pressed samples at 20 K, 4 T. The maximum repulsive and attractive levitation force values at 20 K in ZFC and FC regimes were obtained for the 4 wt% C₂₄H₁₂ added hot pressed sample. Finally, the hot pressing provides an effective way to substitute C into MgB₂ lattice without a remarkable decrease of T_c value, ensuring more homogenous and compacted structure, more electron–phonon interaction and strong flux pinning force.