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Description
This work describes the basic properties of MgB$_2$ ($2001$) and iron pnictide superconductors ($2008$) discovered in the third millennia. Magnetic and transport $J_c(B)$ and $R(T)$ measurements of superconducting wires made of these two materials have been done at low temperatures and external magnetic fields up to $12$ T [1-3]. While MgB$_2$ wires show promising behaviour for the windings generating low or medium DC external fields and also for AC currents and AC fields, iron pnictide conductors are more suitable for high DC magnetic fields. But, high strain tolerances against the Lorentz forces of iron pnictide wires are needed at high magnetic fields, which requires sufficient mechanical reinforcement of soft Sr-$122$/Ag composite e.g. by stainless steel [4-5]. Magnetization AC losses of these superconductors were measured at temperature between $20$ K and $40$ K, AC field of $70$ mT and frequencies $72$ Hz and $144$ Hz. It was found that AC losses are not affected only by superconducting filaments but also by used metallic sheath materials. In the case of Sr-$122$/Ag wire with pure Ag sheath, high contribution of eddy-current losses was observed [6]. Eddy current losses in MgB$_2$ wires were effectively superseded by low purity Cu and Al sheaths. The obtained limits in engineering current density, stress tolerances and AC losses of these third millennia superconductors are presented, compared and discussed in relation to possible practical applications.
References
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