MgB2

In January 2001, a totally new superconductor was discovered, with a surprising critical temperature of 39 K - MgB2. This discovery stimulated a global flurry of work seeking higher Tc and uncovering the basic physics.

Although so far higher Tc has not been found, we now know that MgB2 is akin to other LTS intermetallics, with high Tc coming from the exceptionally high vibrational energies in the graphite-like boron planes. Thus, MgB2 appears to obey conventional models of superconductivity, and this more simple view (as compared to HTS) opens up a wide range of practical opportunities. In addition, since magnesium and boron are both cheap and abundant, practical long-length multifilament conductors might one day be cheaper than niobium-based LTS and sterling silver-clad HTS. MgB2 conductors might occupy a low to mid-field niche, operating in liquid helium or liquid hydrogen. The density of MgB2 is comparable to aluminum, perhaps leading to new lightweight applications as well.

Our goal in the Applied Superconductivity Center is to understand and explore the potential for MgB2. With help from collaborators at Princeton and Ames, we found early on that grain boundaries are not obstacles to current flow, unlike the situation in HTS. This means that random polycrystalline forms, such as round wires made by powder-in-tube (PIT) techniques, can carry substantial critical current densities. This does not mean that powder-based conductors are without obstacles to current flow, however. For example, MgO and amorphous regions are revealed when dense MgB2 samples are probed with a transmission electron microscope. Avoiding these unwanted phases remains a central issue for developing wires, tapes and other polycrystalline forms.

The layered crystal structure of MgB2 produces anisotropic properties when fields and electric currents are applied in various directions with respect to the boron planes. This anisotropy is revealed in textured samples, in which all grains share a common crystallographic orientation, and in single crystals. Thin films in particular are examples of textured samples, because grains line up against a flat substrate. By using textured thin films and, more recently, epitaxial thin films, a phase diagram of the fields and temperatures that limit superconductivity and current transport has been mapped out. While upper critical fields (Hc2, at which superconductivity is destroyed) determine possibilities for applications, practical limits are set by irreversibility fields (H*, above which current flow is no longer lossless), coolant liquids and refrigeration capacity.

Last modified on 28 September 2014