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    Comparative study of hot and cold press and their effect on superconducting properties of coronene added bulk MgB2
    (Elsevier, 2019) Erdem, O.
    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% C(24)H(12)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 x 10(3) A/cm(2), 1.3 x 10(4) A/cm(2), 5.8 x 10(3) A/cm(2) and 1.8 x 10(8) N/m(3), 5.1 x 10(8) N/m(3), 2.3 x 10(8) N/m(3) 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.
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    Investigation of Hot Press and Cold Press Effects on Structural, Electrical and Magnetic Properties of Pyrene Added Bulk MgB2
    (Springer, 2024) Erdem, O.
    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 degrees C, after hot pressing at 200 degrees 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 Tc 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 x 10(3) A/cm(2), 1.62 x 10(4) A/cm(2), 6.90 x 10(3) A/cm(2) and the Hirr values while J(c) = 100 A/cm(2) 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 Delta rho value, and thus increased flux pinning ability.