Nanorods

Incorporating nonsuperconducting particles in the shape of nanorods should produce extended pinning centers resembling continuous columnar defects introduced by high-energy heavy ion irradiation. Nanorods of MgO were used for improving the vortex pinning in Bi2Sr2Ca2Cu3O10, [132] Bi2Sr2CaCu2O8 [128], and Tl2Ba2Ca2Cu3Oz [133] superconductors. The diameter of the nanorods was 5-50 nm and length was about 2 /m. They were orientated preferably along two directions in the crystal: parallel and perpendicular to the crystalline c-axis [128].

Doping increased the value of Jc about 10 times, at all temperatures (Fig. 6) [128, 132, 133]. The increase was more pronounced at higher fields and temperatures. The values of Jc depended approximately inversely on the diameter of the nanorods. The irreversibility line was shifted to higher fields and temperatures with the doping (Fig. 6). All this is consistent with the pinning by columnar defects. Further proof for that was doping of Bi2Sr2CaCu2O8 with MgO nanoparticles of spherical shape [128]. This resulted in an increase of Jc only at temperatures lower than 25 K, consistent with point-defect pinning.

Bi2Sr2CaCu2O8 was also doped with carbon nanotubes [134, 135]. The carbon nanotubes consisted of one or more cylindrical shells of graphite. Their outer and inner diameters were respectively 2-20 and 1-3 nm. Their length was several micrometers. Embedding the nanotubes into Bi2Sr2 CaCu2O8 gave increased vortex pinning at temperatures lower than 53 K. This increase of pinning was ascribed to the nanotubes, which are of size and shape comparable to the irradiation induced columnar defects.

Needle-shaped particles of CuO were also incorporated into Bi2Sr2CaCu2O8 [136]. The CuO particles were of diameter of about 50 nm and length 3 /m. Jc increased by more than an order of magnitude at 30 K and H > 0.2 T (Fig. 7). Such large improvement of Jc was attributed to the small size of the particles and even distribution within the crystal.

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