Flux pinning and superconducting properties of melt-textured NEG-123 superconductor with TiO_2 addition

摘要

We studied the effect of TiO_2 doping on flux pinning and superconducting properties of a melt-grown (Nd_(0.33)Eu_(0.33)Gd_(0.33)) Ba_2Cu_3O_y + 35 mol% Gd_2BaCuO_5 (70 nm in size) composite (NEG-123) processed in Ar-1% O_2 atmosphere. As indicated by similar, sharp superconducting transitions, the small quantities of TiO_2 used in our experiments did not deteriorate superconducting properties of the NEG material. Transmission electron microscopy (TEM) analysis found 20-50 nm Ti-based particles in the NEG-123 matrix. However, we have not observed the clouds of <10 nm sized particles in the NEG-123 matrix, as in the case of recently reported NEG-123 composites doped by Mo and Nb nanoparticles. Nevertheless, quite a good J_c-B performance in the 0.1 mol% Ti-doped sample, namely 550 kA/cm~2 at the self-field and at the secondary peak field (4.5 T) was achieved at 65 K, while 320 kA/cm~2 was obtained at zero-field at 77 K, and 50 kA/cm~2 at 90.2 K. The pinning effectiveness decreased with increasing Ti content above 0.2 mol%. The analysis of the pinning force showed that higher concentration of Ti (>0.2 mol%) increased the amount of normal pins (δl pinning), indicated by the Fp(h) peak shift from h = 0.42-0.36. The maximum pinning effect in a broad field range could be achieved by optimizing Ti content and adding sub-micron Gd-211 particles.