Spherical Particles

Pinning centers formed by spherical particles incorporated into superconducting matrix have pinning properties similar to discontinuous defects introduced by irradiation. Chemically stable compounds are used, such as MgO [122, 123], ZrO2 [124], and SiC [125]. They were incorporated into Bi2Sr2Ca2Cu3O10, Bi2Sr2CaCu2O8, and YBa2Cu3O7 superconductors, respectively. SiC nanoparticles were also used for doping of silver-sheathed Bi2Sr2Ca2Cu3O10 tapes [126, 127]. Spherical particles of diameter about 20 nm were thoroughly mixed with precursor powder for production of HTSs. The superconductor was formed in thermal treatment, in which a liquid phase occurred before the formation of the superconductor. Because the liquid adhered to the surface of the nanoparticles, they were incorporated into the superconducting crystal. Increase of Jc with this doping was usually less than 10 times at 5 K, whereas there was no significant increase at high temperatures [128]. The doping levels were generally kept below 5% by weight.

Doping of YBa2Cu3O7 with SnO2 and CeO2 particles resulted in a moderate increase of Jc [129-131]. However, using nanosize particles did not have any advantage over the micrometer-size particles [129]. This was attributed to poor adherence of the molten phase of the superconductor to these particles.

Figure 6. (a) Field dependence of critical current density for Tl2 Ba2Ca2Cu3Oz high-temperature superconductor doped with MgO nanorods. Measurements at 50,60, and 90 K are represented by squares, triangles, and circles, respectively. Open and solid symbols represent Jc before and after introduction of MgO nanorods. (b) Temperature dependence of the critical current density for the same samples. The field was 0.5 and 0.8 T, represented by circles and triangles, respectively. (c) Irreversibility line for the same samples. Open and solid symbols represent the data before and after doping with MgO nanorods, respectively. Reprinted with permission from [133], P. Yang and C. M. Lieber, Appl. Phys. Lett. 70, 3158 (1977). © 1977, American Institute of Physics.

Figure 6. (a) Field dependence of critical current density for Tl2 Ba2Ca2Cu3Oz high-temperature superconductor doped with MgO nanorods. Measurements at 50,60, and 90 K are represented by squares, triangles, and circles, respectively. Open and solid symbols represent Jc before and after introduction of MgO nanorods. (b) Temperature dependence of the critical current density for the same samples. The field was 0.5 and 0.8 T, represented by circles and triangles, respectively. (c) Irreversibility line for the same samples. Open and solid symbols represent the data before and after doping with MgO nanorods, respectively. Reprinted with permission from [133], P. Yang and C. M. Lieber, Appl. Phys. Lett. 70, 3158 (1977). © 1977, American Institute of Physics.

Figure 7. Field dependence of critical current density for Bi2Sr2 CaCu2O8 crystals: pure (open symbols) and doped with nanosize CuO rods (solid symbols). Measurements were performed at 30 K. Reprinted with permission from [136], X. L. Wang et al., J. Appl. Phys. 81, 533 (1997). © 1997, American Institute of Physics.

Figure 7. Field dependence of critical current density for Bi2Sr2 CaCu2O8 crystals: pure (open symbols) and doped with nanosize CuO rods (solid symbols). Measurements were performed at 30 K. Reprinted with permission from [136], X. L. Wang et al., J. Appl. Phys. 81, 533 (1997). © 1997, American Institute of Physics.

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