High Magnetic Field Strength tolerated By Nanocrystalline Superconductor - New Technology

Scientists at the University of Durham have developed a superconductor which can withstand a magnetic field of almost 100 Tesla and shows promise for more efficient magnets for use in higher-resolution MRIs, better nuclear fusion reactors and particle accelerators.

Strong magnetic fields are harmful to superconductors as they either try to push magnetic fields out from the inside or they limit them on so-called fluxons, a type of tube in which the magnetic field lines are enclosed. With an increase in magnetic field the fluxons multiply and take up more and more space in the superconductor. Once the external field reaches a critical size, the superconductor then consists entirely of fluxons on the inside and it is no longer superconducting. The tolerance towards external magnetic fields could be increased if the fluxons created were finer.

The circumference of a fluxon is dependent on the free path length of a superconducting electron. The free path length is the distance over which an electron is able to move rectilineally before it is scattered. If these electrons are scattered more often, they reduce the size of the fluxons. To achieve this, PbMo₆S₈ was ground into a superfine powder, with grains measuring approx. 20 nanometres. The powder was pressed and heated to create a new piece of superconducting material. The result was a nanocrystalline superconductor whose many grain boundaries and dislocations greatly reduce the free path length of the electrons.

The results showed that the nanocrystalline PbMo₆S₈ tolerates a field of almost 100 Tesla. Comparitively, Nb₃Sn only withstands almost 28 Tesla. At present the material is very brittle and unsuitable for being drawn into necessary wires.

Posted 24^th July 2003