Articles you may be interested inTemperature-and field-dependent critical currents in [(Bi,Pb)2Sr2Ca2Cu3Ox]0.07(La0.7Sr0.3MnO3)0.03 thick films grown on LaAlO3 substrates

Static and dynamic bulk moduli (Ks and Kd) are measured as continuous functions of pressure from zero to 2‐3 kilobars for two sandstones, a tuff, limestone, granite, and oil shale. Results for the sandstones and granite are in good agreement with previously reported data with Ks/Kd varying from about 0.5 at atmospheric pressure to close to unity at pressures 2 kilobars and above. For rocks behaving elastically under static loading, the Ks/Kd ratio is inversely related to the microcrack density. For the limestone, time dependent deformation associated with pore collapse results in Ks/Kd ratios approaching 0.1 at high pressure. Upon unloading, while initially high ( ∼ 1.0) at high pressures, Ks/Kd becomes lower than values obtained during loading at low pressures ( < 1 kilobar) due to opening of microcracks generated during pore collapse. For the oil shale, in which few microcracks exist, Ks/Kd remains relatively constant with pressure at a value of about 0.7.

A commonly used model for a transversely isotropic cracked rock is that by Hudson (1980, 1981). This model is based on a simplified analysis of a thin circular crack, with displacement and stress conditions specified on the boundary. These papers have a second‐order correction in addition to the first‐order term in porosity/crack density. In this paper we compare the results of Hudson with those of Anderson et al. (1974) and Cheng (1978) using the long‐wavelength static approximation and the ellipsoidal crack model first proposed by Eshelby (1957). We show that the Hudson model and those based on the complete Eshelby theory agree for small‐aspect‐ratio cracks and small crack densities, as expected, provided the “weak material” version of Hudson's (1981) model is used. For larger crack densities but small aspect ratios, Hudson's first‐order term agrees with the Eshelby solution. The expansion in the second‐order term in crack density is an asymptotic series and not a uniformly converging series. Thus there is no general statement one can make about the accuracy of the second‐order expansion that is valid for a variety of situations. A new expansion based on the Padé approximation is proposed which is identical to Hudson's expansion up to second‐order in density. This expansion avoids some of the problems associated with Hudson's second‐order expansion such as increasing moduli with crack density at relatively small crack densities.

The microstructure of the Ti-doped MgB 2 which shows a significantly improved critical current density, J c ͓Appl. Phys. Lett. 79, 1154 ͑2001͔͒, is investigated. It is found that Ti does not occupy the atomic site in the MgB 2 crystal structure, but forms a thin TiB 2 layer ͑with a thickness about one unit cell of TiB 2 ͒ in the grain boundaries of MgB 2 . Besides, MgB 2 grains are greatly refined by Ti doping, forming a strongly coupled nanoparticle structure. It is argued that the unique microstructure of the MgB 2 nanoparticles with TiB 2 nanograin boundaries may take responsibility for the enhancement of J c in the Ti-doped MgB 2 bulk superconductor.

We present the superconducting properties and phase compositions of Mg 1Ϫx Zr x B 2 bulk samples fabricated by a solid-state reaction at ambient pressure. It is found that a small amount of Zr atoms may be introduced into the lattice of MgB 2 , while the majority of them forms ZrB 2 phase. The Mg 0.9 Zr 0.1 B 2 sample shows the highest J C of 2.1ϫ10 6 A/cm 2 in 0.56 T at 5 K and 1.83ϫ10 6 A/cm 2 in self-field at 20 K, higher irreversibility field and larger upper critical field in MgB 2 bulk samples. The combination of good grain connection, the reduction of grain size and small ZrB 2 particles in the sample may be responsible for the significant enhancement of J C in Zr-doped samples. This technique has a great potential to prepare high performance MgB 2 bulk samples and wires on an industrial scale.