A barrier to increasing the critical current density of bulk untextured polycrystalline superconductors in high magnetic fields

The critical current density J(c) in optimised untextured polycrystalline superconducting wires is still less than 1% of the depairing current density (J(d)) in high magnetic fields. This paper considers whether there is a barrier to increasing J(c). of order ln(kappa)/2 kappa(2) lower than J(d) (kappa: Ginzburg-Landau parameter). Consideration of the wave-like properties of the superelectrons (diffraction effects) and zero pinning of fluxons that flow along normal weak-link grain boundaries leads to:J(c)(B,T) = alpha(T)(1 - B/B-c2(T)) exp(-B/beta(T)), where for thin grain boundaries: alpha(T) = 0.46(HcP1/2/lambda(GL)S(x))(ln(kappa)/kappa(2)) = 0.82 P-1/2 B(c2)(3/2)ln(kappa)/phi(0) kappa(4) mu(0)S*, beta(T) = phi(0)/2 root 3 pi d(n)(v2), where H-c is the thermodynamic critical field strength, S* is the Fermi surface enhancement factor equal to one for simple metals. P is the purity parameter equal to one for clean materials. B-c2 is the upper critical field and d(n)* is the effective thickness of the grain boundary, alpha(T) is essentially the depairing current density multiplied by a diffraction term. beta(T) characterises the suppression of the order parameter by the magnetic field at grain boundaries. The values of J(c) predicted are similar to state-of-the-art wire values. (C) 1998 Elsevier Science B.V.