Influence of diffusion-annealing temperature on physical and mechanical properties of Cu-diffused bulk MgB2 superconductor

This study reports not only the effect of Cu diffusion on physical and mechanical properties of bulk MgB2 superconductors with the aid of Vickers microhardness (H-v) measurements but also the diffusion coefficient and the activation energy of copper (Cu) in the MgB2 system using the resistivity measurements for the first time. Cu diffusion is examined over the different annealing temperature such as 650, 700, 750, 800 and 850 A degrees C via the successive removal of thin layers and resistivity measurement of the sample. Further, Vickers microhardness, elastic modulus, yield strength, fracture toughness and brittleness index values of the samples studied are evaluated from microhardness measurements. It is found that all the results obtained depend strongly on the diffusion annealing temperature and applied load. The microhardness values increase with ascending the annealing temperature up to 850 A degrees C owing to the increment in the strength of the bonds between grains but decreasing with the enhancement in the applied load due to Indentation Size Effect behaviour of the bulk samples. Moreover, the diffusion coefficient is observed to enhance from 2.84 x 10(-8) to 3.22 x 10(-7) cm(2) s(-1) with the increase of the diffusion-annealing temperature, confirming that the Cu diffusion is more dominant at higher temperatures compared to lower ones. Besides, temperature dependence of the Cu diffusion coefficient is described by the Arrhenius relation D = 2.66 x 10(-3) exp(-1.09 +/- A 0.05 eV/k(B)T) and the related activation energy of the Cu ions in the MgB2 system is obtained to be about 1.09 eV. Based on the relatively low value of activation energy, the migration of the Cu ions primarily proceeds through defects such as pore surfaces and grain boundaries in the polycrystalline structure, resulting in the improvement of the physical and mechanical properties of the bulk MgB2 samples.