Investigation of Hot Press and Cold Press Effects on Structural, Electrical and Magnetic Properties of Pyrene Added Bulk MgB2

In this paper, the effects of hot pressing and cold pressing on the superconducting properties of pyrene (C₁₆H₁₀) added bulk MgB₂ were compared. For this aim, the polycrystalline disk-shaped MgB₂ samples with addition of 4 wt % C₁₆H₁₀ powder were fabricated by a pellet/closed tube method at 850 °C, after hot pressing at 200 °C and cold pressing. The XRD and SEM results showed that although a-lattice parameters reduce for both C₁₆H₁₀ added samples, the average crystallite size increases after cold pressing because of the accumulation and inhomogeneous distribution of C in the MgB₂ structure while it reduces for the C₁₆H₁₀ added hot pressed sample. The resistivity measurements showed that the increment of electron scattering is higher for the C₁₆H₁₀ added cold pressed sample than that of the C₁₆H₁₀ added hot pressed one which shows relatively better grain connectivity, and the T_c values of the samples are not appreciably affected by the pressing conditions. From the magnetisation measurements the in-field J_c values at 10 K and 5.5 T were obtained as 8.41 × 10³ A/cm², 1.62 × 10⁴ A/cm², 6.90 × 10³ A/cm² and the H_irr values while J_c = 100 A/cm² at 20 K were obtained as 5.99 T, 6.55 T and 6.19 T for the pure sample and the C₁₆H₁₀ added hot and cold pressed samples, respectively. The higher J_c and H_irr values obtained for the C₁₆H₁₀ added hot pressed sample is because of the homogenous C substitution into MgB₂ grains, as compatible with its lower Δρ value, and thus increased flux pinning ability.