Neutron irradiation of MgB₂bulk superconductors

Abstract

Sintered samples of MgB 2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10 B(n,α) 7 Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.

Strong competition between the δl and δTc flux pinning mechanisms in MgB2 doped with carbon containing compounds

“…The research toward improvement of j c properties in MgB 2 has been mostly aimed at enhancing the H irr , the flux pinning ability, and H c2 by irradiation or chemical doping. [2][3][4][5][6][7][8] Chemical doping has been proved to be a simple and effective way to introduce pinning centers into MgB 2 superconductor or to improve the upper critical field. It is believed that carbon containing dopants, such as nano-SiC, 5 carbon nanotubes, 6 nanocarbon, 7 hydrocarbons, 8 silicone oil, 9,10 malic acid, 11 etc., are especially effective in enhancing the H c2 , and hence the j c properties of MgB 2 in high magnetic fields.…”

Influence of the upper critical-field anisotropy on the transport properties of polycrystalline MgB2

“…In MgB 2 thermal effects should play only a minor role due to its comparatively low Ginzburg number. Nevertheless, the critical currents become too small for power applications at fields well below H c2 , even at 4.2 K. Fortunately, the upper critical field of MgB 2 can be rather easily enhanced by certain preparation conditions [1,2,3], doping [4,5,6,7,8] or irradiation [9,10,11], and exceeds 30 T (at 0 K). Even for such "high-H c2 " materials, the application range is limited to around 10 T in polycrystalline samples.…”

Current Percolation and Anisotropy in PolycrystallineMgB2

“…Especially thermal fluctuations lead to an artificial increase of the percolation threshold at high temperatures, but also material inhomogeneities enhance p c . The latter explains the increase of p c after irradiation (Table I) due to the large inhomogeneity of the defect structure following neutron irradiation [34], which leads to a significant broadening of the transition even at zero field (△T c ). The wire was irradiated in a cadmium cover [35], which reduces the inhomogeneity of the resulting defect structure and the enhancement of p c .…”

“…The samples were neutron irradiated in a fission reactor, in order to change the reversible properties. Details of the irradiation experiments can be found in [34,35]. The upper critical field of a similar bulk sample (from the same pellet) was determined by the onset of the resistive transition (Fig.…”