We report temperature and magnetic-field dependence of flux pinning force in bulk $\mathrm{Mg}{\mathrm{B}}_2$ with variable grain size. The samples are prepared by advanced methods, allowing minimizing effects of porosity, impurities, and inclusions of secondary phases. The effects of grain connectivity, flux-creep phenomena, and grain size on critical current density and flux pinning curves are analyzed. We have compared the field dependence of the pinning force for a range of samples with the predictions of theoretical models accounting for the effect of the grain size. There is qualitative agreement between grain-boundary pinning mechanism proposed by Hampshire and Jones [J. Phys. C 21, 419 (1987)] and the experimentally observed grain-size dependence of pinning force in bulk $\mathrm{Mg}{\mathrm{B}}_2$.

In this paper we analyse numerically and experimentally the quench behavior of Cu-stabilized second-generation high temperature superconducting (2G HTS) wires at self-field and in adiabatic conditions. The electric field profiles along the superconductor after applying an energy pulse to the conductor have been obtained together with the parameters characterizing the quench: minimum quench energy (MQE) and the normal zone propagation velocity (NZPV). The analysis has been performed at different temperatures, T, between 72 K and 77 K, and at different applied currents, I/I_c(T) <; 1, I_c being the critical current. Our analysis shows that the numerical results obtained by finite element method (FEM) are closer to the experimental MQE values when the contribution of the thermal mass of the heater is taken into account.

Superconducting Ag or Cu sheathed MgB2 wires have been fabricated by conventional powder-in-tube techniques. The in-situ reaction of Mg and B powders with atomic proportions of 1:2 and 1.33:2 inside Ag or Cu tubes at low temperatures have been searched. An active alloying of Mg with Ag or Cu sheaths at temperatures below 660 C has been observed. For Ag sheaths, there is a strong diffusion of Mg, which does not allow the formation of large amounts of MgB2 at the core. For Cu sheaths, the diffusion of Mg is stopped by the formation of a MgCu2 layer around the superconducting core and an appropriated excess of Mg leads to superconducting cores with Tc=39 K and good Jc values, thus being possible the development of Cu/MgB2 composite wires.