Essential factors for the critical current density in superconductingMgB2: connectivity and flux pinning by grain boundaries

Abstract

Electrical connectivity and flux pinning strength of grain boundaries are essential factorsin determining the critical current density in superconducting polycrystallineMgB2. The effect of these factors is quantitatively investigated for a series ofMgB2 bulk samples prepared by various methods. The electrical connectivity isevaluated using the percolation model and the obtained electrical connectivityis used for the estimation of the residual resistivity of superconductingMgB2 grains. The elementary pinning force of grain boundaries is evaluated using thetheoretical result of Yetter et al based on the electron scattering mechanism with theresidual resistivity. It is found that the critical current density is approximatelyproportional to the product of the electrical connectivity, the elementary pinningforce of grain boundaries and the reciprocal grain size. This confirms that thecritical current density is dominantly determined by the electrical connectivityand the flux pinning strength of grain boundaries. The flux pinning property inMgB2 under the condition of full electrical connectivity is compared with that inNb3Sn. The obtained result shows that the flux pinning ability of pureMgB2 is comparable to or even higher than that ofNb3Sn, indicating a much higher potential in carbon-dopedMgB2. This provesthat the MgB2 is a promising superconductor for practical applications.