magnesium diboride

In January 2001, Akimitsu’s research group in Japan revealed that MgB2 turns out to be superconducting at 39 Kelvin, making it one of the highest known transition temperatures (Tc) of any superconductor. Hundreds of papers have been produced in the first rush to examine this material, but researchers using different techniques have reported many different, often unusual, and sometimes conflicting properties. Magnesium diboride is an intermetallic compound that loses its resistance at 32 K in nanocrystalline form while 39 K in the wire-form. Relatively high temperature superconductors are celebrated nowadays for they no longer require, at least minimize, the use of cryogenics to maintain their superconducting state. The compound also has a pronounced difference in the resistivity of about 10 μΩ cm at low temperature and of about 0.5 μΩ cm at above Tc.

The structure of MgB2 plays a very important role in its conducting property. It is a simple hexagonal AlB2-type of structure which is typical of borides.

The structure is consisting of layers of boron that is separated by closed-packed layers. Located at the center of the hexagons that are formed by boron atom are the magnesium atoms. These magnesium atoms donate electron density to the boron layers.

Preparation of MgB2 powder (Canfield, et al., 2008). Stoichiometric amounts of powdered B and Mg at were reacted in 950◦C for approximately an hour to form MgB2 powder. The vapor pressure of Mg is approximately 200 Torr at 950◦C. Given this, it is assumed that MgB2 forms via a process of diffusion
of Mg vapor into the boron grains.

Preparation of MgB2 wires (Canfield, et al., 2008). One hundred micrometer diameter of boron fiber10 and M were sealed in a tantalum tube to produce MgB2 wire. The said tube was sealed in quartz and was then placed for approximately an hour into a 950◦C box furnace. The reaction container was removed from the furnace and was allowed to quench to room temperature.

Preparation of Nanocrystalline MgB2 by Ball Milling Method (Lorenz, et al., 2006). Nanocrystalline MgB2 is synthesized by high-energy ball milling of Mg and B. Milling is conducted in Ar atmosphere using jar and balls made from tungsten carbide (WC). 20 h of milling time was done and the Mg and B were partially reacted through cold alloying to form MgB2. The reaction was completed by hot uniaxial pressing at 973 K/640 MPa for 10 min. This treatment resulted in a complete conversion of Mg and B to MgB2 with only minor traces of impurity phases left.

References:

Brown, T. L., et al (1994). Chemistry: The Central Science. Englewood Cliffs, New Jersey: A Simon & Schuster Company
Canfield P. C., and S L Bud’ko. (2002). Magnesium diboride: one year on. Physics World. pp 29-34.
Canfield, P and Crabtree G. (2003). Magnesium boride: better late than never. Physics today. pp. 34-40
Canfield P. C., et al., (2008). Superconductivity in Dense MgB2 Wires. Iowa: Ames Laboratory, U.S. Department of Energy and Department of Physics and Astronomy Iowa State University. Pp 1-4.
Lee, J.D. (1992). Conscise Inorganic Chemistry (4th ed.).Singapore: Fong and Sons Printers Pte Ltd.
Lorenz, B. Et al., (2006). Superconducting properties of nanocrystallineMgB2. Supercond. Sci. Technol. 19 (2006) 912–915
Physicsweb, Nature and LANL. (2003, May 1). Magnesium diboride Thoery confirmed. Retrieved September 13, 2008, from http://www.superconductors.org/news.htm.