Abstract
Dense samples (relative density > 93%) of bulk MgB2 with GeO2 additions were obtained by Spark Plasma Sintering. The critical current density Jc of the added samples is improved at high magnetic fields when compared to the pristine sample. The optimum composition is for MgB2(GeO2)0.005. For this sample, a Jc(20 K) = 102 A/cm2 is obtained at 5.1 T versus 3.9 T for the pristine sample. Ge substitution in the crystal lattice of MgB2 can be considered negligible, and Tc,onset and Tc,midpoint from magnetization measurements scatter within 0.2 and 0.6 K, respectively. TEM investigations show some specific details at nano scale: the tendency to form secondary phases (25–100 nm) with sphere-like or irregular shapes is observed and discussed. Samples are composites and the residual strain of MgB2 is constant for pristine and GeO2-added samples. Therefore, pinning enhancement leading to improvement of Jc for the GeO2 added samples is purely a ‘microstructure’ effect due to the presence of secondary phases. The point pinning is determined to be the predominant mechanism. Addition of a higher amount of GeO2 is shifting the pinning mechanism toward a grain boundary pinning.
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