Essential factors for the critical current density in superconducting MgB₂: connectivity and flux pinning by grain boundaries

Abstract

Electrical connectivity and flux pinning strength of grain boundaries are essential factors in determining the critical current density in superconducting polycrystalline MgB2. The effect of these factors is quantitatively investigated for a series of MgB2 bulk samples prepared by various methods. The electrical connectivity is evaluated using the percolation model and the obtained electrical connectivity is used for the estimation of the residual resistivity of superconducting MgB2 grains. The elementary pinning force of grain boundaries is evaluated using the theoretical result of Yetter et al based on the electron scattering mechanism with the residual resistivity. It is found that the critical current density is approximately proportional to the product of the electrical connectivity, the elementary pinning force of grain boundaries and the reciprocal grain size. This confirms that the critical current density is dominantly determined by the electrical connectivity and the flux pinning strength of grain boundaries. The flux pinning property in MgB2 under the condition of full electrical connectivity is compared with that in Nb3Sn. The obtained result shows that the flux pinning ability of pure MgB2 is comparable to or even higher than that of Nb3Sn, indicating a much higher potential in carbon-doped MgB2. This proves that the MgB2 is a promising superconductor for practical applications.