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The influence of lattice strain and Mg vacancies on the superconducting properties of MgB2 samples has been investigated. High quality samples with sharp superconducting transitions were synthesized. The variation in lattice strain and Mg vacancy concentrations were obtained by varying the synthesis conditions. It was found that high strain (~1%) and the presence of Mg vacancies (~ 5 %) resulted in lowering the Tc by only 2 K.Comment: 3 figures, HTML+GIF format to be published in AP

The critical current density (Jc) of hot isostatic pressed (HIPed) MgB2 wires, measured by d.c. transport and magnetization, is compared with that of similar wires annealed at ambient pressure. The HIPed wires have a higher Jc than the annealed wires, especially at high temperatures and magnetic fields, and higher irreversibility field (Hirr). The HIPed wires are promising for applications, with Jc>106 A/cm2 at 5 K and zero field and >104 A/cm2 at 1.5 T and 26.5 K, and Hirr ~ 17 T at 4 K. The improvement is attributed to a high density of structural defects, which are the likely source of vortex pinning. These defects, observed by transmission electron microscopy, include small angle twisting, tilting, and bending boundaries, resulting in the formation of sub-grains within MgB2 crystallites.Comment: 13 pages,3 figure

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This work studies the influence of microstructures and crystalline defects on the superconductivity of MgB 2 , with the objective to improve its flux pinning. A MgB 2 sample pellet that was hot isostatic pressed (HIPed) was found to have significantly increased critical current density (J c ) at high fields than its un-HIPed counterpart. The HIPed sample had a J c of 10000 A/cm 2 in 50000 Oe (5 T) at 5K. This was 20 times higher than that of the un-HIPed sample, and the same as the best J c reported by other research groups. Microstructures observed in scanning and transmission electron microscopy indicate that the HIP process eliminated porosity present in the MgB 2 pellet resulting in an improved intergrain connectivity. Such improvement in intergrain connectivity was believed to prevent the steep J c drop with magnetic field H that occurred in the un-HIPed MgB 2 pellet at H > 45000 Oe (4.5 T) and T = 5 K. The HIP process was also found to disperse the MgO that existed at the grain boundaries of the un-HIPed MgB 2 pellet and to generate more dislocations in the HIPed the pellets. These dispersed MgO particles and dislocations improved flux pinning also at H<45000 Oe. The HIPing process was also found to lower the resistivity at room temperature.74.70. Ad, 74.60.Ge, 74.62.Bf, 74.25.Fy 1

MgB 2 samples prepared by solid-state reaction were investigated using high-resolution transmission electron microscopy (HREM), X-ray energy-dispersive spectroscopy (EDX), electron energy-loss spectroscopy (EELS), and energy-filtered imaging. Large amounts of coherent precipitates with a size range from about 5 nm up to about 100 nm were found in the MgB 2 crystallite matrices. The precipitates are of different shapes including sphere, ellipsoid, and faceted polyhedron depending on the size of the precipitates. EDX and EELS analyses confirm that smaller precipitates contain magnesium, boron and oxygen while larger faceted precipitates contain mainly magnesium and oxygen, implying that the oxygen content increases with precipitate size. HREM and electron diffraction investigations found that the precipitates have the same crystal lattice structure as that of MgB 2 but with various composition modulations depending on the composition of the precipitates. The precipitates transform to the MgO phase after long exposure to residual oxygen in flowing Ar gas at high temperatures. The effect of the precipitates in different size ranges on flux pinning is discussed.

A theory is presented to explain the dependence of the superconducting transition temperature T c on the changes in the phonon frequency spectrum and electronic density of states which result from lattice disorder. Numerical calculations of T c are presented for films composed of crystalline granules, for films composed of amorphous granules, and for homogeneous amorphous metals. The calculations are in good agreement with experimental values of T c .Experiments 1 ' 2 on disordered films of a variety of metals and on disordered dilute Sn-Cu alloys 3 have shown that the superconducting transition temperature T c increases with increasing lattice disorder. None of the explanations proposed 4 ' 5 for the enhancement of T c in superconducting films has been applicable to all experimental situations discussed in this paper. Although no calculations are presented for the case of very thin films, 4 the theory proposed here is applicable to films composed of small metallic crystallites, 1 ' 6 films composed of amorphous metallic granules, 1 ' 2 ' 6 and homogeneous amorphous superconducting alloys. In particular, a two-parameter model for granular films of nearly-free-electron metals yields numerical values for the enhancement of T c which are in good agreement with the experimental values found by Buckel and Hilsch 1 and von Minnigerode. 2 The theory proposed here is based on the assumption that the average amplitude of ionic vibrations is larger in a disordered lattice than a perfect crystal. For disordered films which consist of small crystalline granules, 6 this increase in the average amplitude of ionic vibrations results from the many ions which are in lattice positions of reduced symmetry near the surface of the granules. Since these ions are held in place by weaker ionic forces than those found in a bulk crystal, they undergo localized ionic vibrations of larger amplitude and lower frequency than those found in a bulk crystal. As a result, the formation of granules (1) increases the average amplitude of ionic vibrations and decreases the average phonon frequency (^ph)> anc * (2) broadens the peaks in the phonon density of states D(w). For homogeneous amorphous metals, the increased average amplitude of ionic vibrations results from the weakened forces acting on all of the ions. In this case, the first effect is usually more important than the second. Both the effects found in homogeneous amorphous metals and those found in crystalline granular films occur in the case of disordered films which consist of amorphous granules.The increase in the average amplitude of the ionic vibrations increases the average electronphonon coupling constant. This increases both T c and the phonon contribution 7 X to the electron mass renormalization constant Z(0) = (1 + X)Z e . Here, Z e is the Coulomb mass renormalization constant. 8 Note that X is approximately given by 7where N(Q) is the electronic band density of states at the Fermi level, M is the ionic mass, (oOp^2) is the average squared phonon frequency, and (J 2 ) is the aver...