MgB₂

Schematic of the crystal structure of 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 T_c and uncovering the basic physics. Although so far higher T_c has not been found, we now know that MgB₂ is akin to other LTS intermetallics, with high T_c 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.

Research Focus

Understanding Basic Properties
In collaboration with many groups around the world, we continue to focus on measuring basic properties, including critical temperature, critical fields, critical current density, resistivity, specific heat and anisotropy. The central question being addressed is whether small variations in sample chemistry, preparation or structure produce discernable differences in basic properties.

Controlling Fabrication

Understanding the phase diagram and potential for alloying
Magnesium diboride can be prepared from a stoichiometric mixture of pure elements. If this is done carefully, samples with high T_c, low resistivity and relatively low H_c2 result. A key question is whether H_c2 can be raised by controllably alloying MgB₂ with a third element, as is the case in many other type-II superconductors, to produce properties more favorable for high-field applications. Our early thin film work found evidence for oxygen alloying and nanoprecipitates of MgO when films are prepared by pulsed laser deposition from an MgB₂ target. Scattering by these defects increased resistivity and H_c2, although T_c was also decreased. We are currently investigating whether nanoparticles added to bulk samples could produce a similar effect, in collaboration with colleagues at Imperial College. A related question is how variations from stoichiometry, such as Mg deficiency, affect the superconducting properties.

Cross Section of MgB₂ wire.

Understanding how to make useful forms of MgB₂
We are working extensively on developing wires and thin films of MgB₂. Round wires are best suited for making stable, tightly packed cables for high field magnets, while tapes can be used in cables for electric power applications. Both of these conductors begin as powder-in-tube composites, and chief concerns are fabricating long, uniform lengths, chemical reactions with sheath materials, and connectivity of the powder core. Recently, epitaxial thin films have been made by a 2-step technique, in which a boron film is deposited on a sapphire substrate and then converted to MgB₂ by a reaction with Mg vapor. Present work addresses making epitaxial films by a single step process, developing methods to integrate MgB₂ with other materials, and exploring superconducting electronic devices.

Exploring Science Issues
A number of unique features of MgB₂ raise additional scientific challenges. Recent theoretical and experimental work suggests that MgB₂ may exhibit two-gap superconductivity. We are therefore curious whether there are novel effects due to the coupling between the gaps, and whether this produces new properties when quantized field lines move along grain boundaries or when MgB₂ is exposed to microwave radiation. The competition between thermal fluctuations and flux pinning is also being explored.

Recent Publications

Magneto-optical studies of the uniform critical state in bulk MgB₂
Magneto-optical results indicate a uniform Bean critical state behavior in polycrystalline MgB₂ with no electromagnetic granularity characteristics. From the measured magnetic flux profiles the temperature dependence of the critical current density has been extracted, which is in qualitative agreement with the result of global magnetization measurements.

Electronic anistropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-Axis oriented MgB₂ thin films
Using in-field resistivity measurements, the electronic anisotropy in c-axis textured MgB₂ thin films is estimated to be 1.9 ± 0.2. It is also observed that Hc2|| is strongly enhanced up to 39 T by alloying, more than twice that is seen in bulk samples. The H - T phase diagram indicates that flux pinning disappears at H* ~ 0.8 Hc2^, indicating a weak but noticeable effect of thermal fluctuations.

To download publications you need Adobe Reader, available for free by clicking on the icon at left.

For more information contact David Larbalestier at larbalestier@asc.magnet.fsu.edu or
(850) 645-7483, or Alex Gurevich at gurevich@asc.magnet.fsu.edu or (850) 645-7754.