The use of MgB 2 in superconducting applications still awaits for the development of a MgB 2 -based material where both current-carrying performance and critical magnetic field are optimized simultaneously. We achieved this by doping MgB 2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in polycrystalline samples. The optimum nominal DWCNT content for increasing the critical current density, J c is in the range 2.5-10%at depending on field and temperature. Record values of the upper critical field, H c2 (4K) = 41.9 T (with extrapolated H c2 (0) ≈ 44.4 T) are reached in a bulk sample with 10%at DWCNT content. The measured H c2 vs T in all samples are successfully described using a theoretical model for a two-gap superconductor in the dirty limit first proposed by Gurevich et al.

This work describes in detail the simultaneous enhancement of the upper critical field ͑H c2 ͒ and the critical current density ͑J c ͒ of MgB 2 bulk samples doped with nano-SiC particles, as well as single-walled and double-walled ͑dw͒ carbon nanotubes ͑CNTs͒. The magnetization properties were examined in a superconducting quantum interference device magnetometer, and four-probe transport measurements were performed using a 50 T pulsed magnet to determine H c2 ͑T͒. We found that the J c enhancement is similar in all doped samples at 5 K but nano-SiC addition is more effective to improve the flux pinning in the high temperature range ͑T ജ 20 K͒; this improvement cannot solely be attributed to the C incorporation to the lattice but also to the presence of other types of defects ͑i.e., several kinds of nanoinclusions͒. CNTs produce a better C incorporation that is more effective to enhance H c2 ͓i.e., dwCNT-doped samples reached a record H c2 ͑0͒ϳ44 T value for bulk MgB 2 ͔. All the H c2 ͑T͒ curves obtained for different types of doping can be successfully described using a model for a two-gap superconductor in the dirty limit.

By applying a combination of characterisation tools, changes in structural and superconducting properties with nominal Mg non‐stoichiometry in MgxB2 are found. The non‐stoichiometry produces enhanced in‐field critical current densities (Jc's) and upper critical field / irreversibility field (Hc2/Hirr(T)) values. Upper critical fields of ∼ 21 T (4.2 K) were obtained in nominal Mg‐deficient samples compared to ∼ 17 T (4.2 K) for near‐stoichiometric samples.

a b s t r a c tIn this work the Ti 2 InN phase is investigated by X-ray diffraction, magnetic and resistivity measurements. X-ray powder patterns suggest that all peaks can be indexed with the hexagonal phase of Cr 2 AlC prototype. Electrical resistance as a function of temperature reveals superconductivity below 7.3 K. M(H) hysteresis loops show typical type-II superconductivity. Using R(H) versus T measurements we estimated

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The discovery of superconductivity at 39 K in MgB 2 has been catching the attention of scientists due to the possibility of applying the material in magnets and electronic devices operating with cryocoolers (temperatures around 20 K). In the present work, a methodology to optimize the critical current densities of this material is described. MgB 2 bulk samples were prepared and analyzed with the addition of other diborides with the same C32 hexagonal structure as MgB 2 (TaB 2 , ZrB 2 , VB 2 , and AlB 2 ) and with simultaneous addition of SiC. Microstructural characterization, performed using SEM+EDS and XRD, was extremely important to determine the distribution and compositional characterization of the superconducting phase. Magnetic superconducting characterization using SQUID was performed to determine the best material addition. As a result we could analyze the behavior of each addition on the current transport capacity, including an analysis of the magnetic flux pinning mechanisms.Index Terms-Addition of diborides and SiC, crystallography, MgB 2 superconductors, microstructure, superconducting properties.

We report on measurements and numerical simulations of the behavior of MgB2 superconductors when magnetic field components are applied along mutually perpendicular directions. By closely matching the geometry in simulations and measurements, full quantitative agreement is found. The critical state theory and a single phenomenological law, i.e. the field dependence of the critical current density Jc(B), are sufficient for a full quantitative description of the measurements. These were performed in thick strips of carbon nanotube doped MgB2 samples. Magnetization was measured in two orthogonal directions using a SQUID magnetometer. Magnetic relaxation effects induced by the application of an oscillatory perpendicular field were observed and simulated numerically. The measurements confirm the numerical predictions, that two relaxation regimes appear, depending on the amplitude of the applied magnetic field. The overall agreement constitutes a convincing validation of the critical state model and the numerical procedures used.

This paper describes the synthesis and modification of MgB 2 bulk samples to produce better transport superconducting materials. The methodology uses a mixture of MgB 2 with another diboride, TaB 2 , which has the same C32 hexagonal structure as MgB 2 . The simultaneous addition of TaB 2 and SiC, which contributes carbon doping, is also investigated. As an important result, the critical current densities (J c ) were enhanced at low magnetic fields (0-2 T) when TaB 2 alone was added. In addition, J c was enhanced in the entire range of the measured applied magnetic fields when TaB 2 was simultaneously added with SiC. Some explanations for this J c enhancement are presented.