RTwin spacing and its correlation with critical current density in melt-textured YBCO with yttria nanoparticle addition High pressure synthesis and sintering of MgB/sub 2/

Twin boundaries have been proved to be effective flux pinning centers in YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO). Increases in twin density yield higher values of critical current density. Twin spacings in melt-textured grown (MTG) YBCO with different amounts of yttria nanoparticles were investigated with a new x-ray method. Twin spacings from the x-ray method were compared with those from transmission electron microscopy to show applicability. From investigation, MTG-YBCO with 11.9 weight percent yttria and oxygenation temperature at 680(degree)C showed a colony size of 3.65 microns. This particular sample had a small twin spacing of 33 nm with a corresponding high value of critical current density (J/sub c/) of 1.2 * 10/sup 4/ A/cm/sup 2/. High pressure (HP) synthesized MgB/sub 2/ (from Mg scobs and amorphous B with Ta addition in the form of powder of about 2-10 wt%) exhibits critical current density j/sub c/>=10/sup 5/ A/cm/sup 2/ up to 4 T field at 10 K, 2.5 T at 20 K and 0.5 T at 30 K; j/sub c/>=10/sup 4/ A/cm/sup 2/ up to 7.3 T at 10 K, 4.5 T at 20 K and 1.5 T at 30 K; j/sub c/ >=10/sup 3/ A/cm/sup 2/ up to 10 T at 10 K and 6 T at 20 K. In our high-pressure apparatuses we can produce samples about 60 mm in diameter and 20-30 mm in height that might be used for superconductive devices such as electromotors, generators, flying wheels, current limiters, noncontact bearings, etc. In synthesis and sintering process Ta plays the role of an absorbent of impurity gases (hydrogen, nitrogen, etc.) and forms Ta/sub 2/H, TaH, TaN/sub 0.1/, etc., thus promotes the reduction of MgH/sub 2/ in samples, as well as, the impurity nitrogen and oxygen in Mg-B grains (black MgB/sub 2/ single crystals distributed over the matrix). In the materials with better superconductive characteristics, we observed some amount of unreacted Mg and higher density of MgB/sub 2/ single crystal inclusions. We have got some evidence that the amount of oxygen in the matrix Mg-B-O and the microdeformation of MgB/sub 2/ lattice influence the critical current density of the material. The HP-synthesized samples have exhibited a microhardness of H/sub v/=12.54 +- 0.86 GPa at 0.496-N load and a fracture toughness of 4.24 MN * m/sup -3/2/ at 4, 96-N load. The HP-sintered MgB/sub 2/ has a microhardness of H/sub v/=17.1+-1.11 GPa at 4.96 N-load, H/sub v/=10.12 +- 0.2 GPa and fracture toughness of K/sub 1C/=7.6 +- 2.0 MN * m/sup -3/2/ at 147.2 N-load and the Young modulus of 213 +- 18 GPa. The hardness (at 60-mN load) of MgB/sub 2/ single crystals located in a sample matrix is 35.6 +- 0.9 GPa that is higher than the hardness of sapphire (31.1 +- 2.0 GPa) and their Young modulus is 385 +- 14 GPa (sapphire has 416 +- 22 GPa) and that gives us grounds to assign MgB/sub 2/ to superhard materials as well.