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.

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Doping effect of diamond nanoparticles on the superconducting properties of MgB 2 bulk material has been studied. It is found that the superconducting transition temperature T c of MgB 2 is suppressed by the diamond-doping, however, the irreversibility field H irr and the critical current density J c are systematically enhanced. Microstructural analysis shows that the diamond-doped MgB 2 superconductor consists of tightly-packed MgB 2 nano-grains (~50-100 nm) with highly-dispersed and uniformly-distributed diamond nanoparticles (~10-20 nm) inside the grains. High density of dislocations and diamond nanoparticles may take the responsibility for the enhanced flux pinning in the diamond-doped MgB 2 . *Corresponding author: email: c.cheng@unsw.edu.au 2 I IntroductionSince the discovery of superconductivity at 39 K in MgB 2 [1], significant progress has been made in improving the performance of MgB 2 materials [2][3][4][5][6][7]. MgB 2 offers the possibility of wide engineering applications in the temperature range 20-30 K, where conventional superconductors, such as Nb 3 Sn and Nb-Ti alloy, cannot play any roles due to their low T c .However, the realization of large-scale applications for MgB 2 -based superconductivity technology essentially relies on the improvement of the pinning behaviour of MgB 2 in high fields. As it has poor grain connection and a lack of pinning centres, MgB 2 often exhibits a rapid decrease in critical current density, J c , in high magnetic fields. Fortunately, through the formation of nanoparticle structures in bulk MgB 2 [3][4][5] and thin films [7], the problem of the poor grain connection can be solved, and the flux pinning force can also be significantly enhanced due to an increase of pinning centres served by grain boundaries. In order to improve further the performance of MgB 2 , it is necessary to introduce more pinning centres, especially those consisting of nano-sized second-phase inclusions which often provide strong pinning forces.Nano-diamond, prepared by the detonation technique, has been widely used as an additive to improve the performance of various materials [8]. However, nano-diamond-doping effect on the superconducting properties MgB 2 has never been reported although carbon with other forms has been used as dopants in MgB 2 [9][10][11]. The high dispersibility of the nanodiamond powder makes it possible to form a high density of nano-inclusions in MgB 2 matrix.In this article, we have investigated the doping effect of nano-diamond on the superconducting properties of MgB 2 . Our results show that the nano-diamond-doped MgB 2 consists of tightly-packed MgB 2 nano-grains (~50-100 nm) with diamond nanoparticles (~10-20 nm) wrapped within the grains. This unique microstructure provides the samples with a good grain connection for the MgB 2 phase and a high density of flux-pinning centres served 3 by the diamond nanoparticles. Compared to the MgB 2 bulk materials doped with other nanoparticles [3][4][5][6][7], the irreversibility line has been significantly improved and ...

We prepared a series of Mg 1Ϫx Zr x B 2 bulk samples with different x values by the solid-state reaction at ambient pressure and investigated the effects of the Zr doping on phase compositions, microstructure, and flux pinning behavior. The results indicate that the a axis increases with the increase of the Zr doping and the c axis remains unchanged. ZrB 2 phases form and a small amount of Zr atoms may substitute Mg in Zr-doped samples. Critical temperature drops with increasing the Zr doping. It is found that critical current density J c can be significantly enhanced by the Zr doping and the best data are achieved in Mg 0.9 Zr 0.1 B 2 . For this sample, J c values are remarkably improved to 1.83ϫ10 6 A/cm 2 in the self field and 5.51ϫ10 5 A/cm 2 in 1 T at 20 K. Also, the irreversibility field is increased by the Zr doping. The high density, fine grain size of MgB 2 , and small ZrB 2 particles in Zr-doped samples may be responsible for the enhancement of J c . Also, the field dependence of J c at different temperatures was briefly discussed.