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Magnesium Diboride

November 2, 2015

While labor cost typically dominates the more mundane things in our lives, like automobile repair, the cost of some advanced technology items are set by the expensive materials they contain. One example is the catalytic converter that's found in every internal combustion automobile. You can tell that automotive technology has advanced, since a decade ago I would not have needed to preface "automobile" with "internal combustion" in the last sentence.

Catalytic converters have been required since the mid-1970s as a means of reducing air pollution from unburned fuel and nitrogen oxides. As I wrote in a previous article (Faux Palladium, January 14, 2011), the principal active materials in these converters are platinum and palladium. While palladium is relatively unknown outside of technical circles, everyone knows that platinum is expensive.

The current price for platinum is about $950/Troy ounce. About 160 million metric tons of platinum were mined in 2014.[1] The current palladium price is about $600/Troy ounce. About 190 million metric tons of palladium were mined in 2014.[1] For price comparison, the current price of gold is about $1,100/Troy ounce.

A catalytic converter core (detail)Detail of the core of a catalytic converter.

The catalyst is on the surface of a ceramic body that allows sufficient exhaust gas flow while maintaining enough residence time for the catalysis to proceed.

(Wikimedia Commons image by Global-Kat Recycling, modified.)

Platinum and palladium aren't the only expensive materials in an automobile. Our motorized windows, doors, door locks, and the loudspeakers on our radios, all contain magnets made from rare-earth metals. In 2014, China mined 95 million metric tons of rare earth oxide equivalents, from which rare earth metals are refined, of the world's total production of 110 million metric tons.[2] In 2012, the last year for which data are available, the oxide of neodymium, the rare earth from which the best magnets are made, sold for $117/kilogram.[3]

Rare earths are also a component of some high temperature superconductors. The mechanism for superconductivity in these materials is apparently by electron-electron interaction, as distinct from the phonon-electron interaction of conventional (BCS theory) superconductors. The later achieved superconducting temperatures as high as 30 K before the discovery of the superconductivity of magnesium diboride (MgB2) with a superconducting temperature of 39 K.

Structure of magnesium diborideStructure of magnesium diboride, MgB2

The magnesium atoms sit between the holes of the boron hexagonal layers.

(Created with Inkscape.)

Since magnesium diboride is a brittle material, it's made into superconducting wire by many of the same processing techniques used for high temperature superconductors. However, it has the advantage that its alloying elements are common and inexpensive. Magnesium is found at a concentration of about 25,000 ppm in Earth's crust, and slightly less than a million metric tons of magnesium were produced annually worldwide in 2014.[4] While boron is less abundant, about 10 ppm, about 3.75 million metric tons of boron were produced annually worldwide in 2014.[5]

Interestingly, magnesium diboride was discovered to be a superconductor only in 2001, many years after its first synthesis and characterization in 1953.[6-7] The melting point of magnesium is 652°C, so MgB2 is synthesized quite easily by heating a mixture of boron (melting point, 2076 °C) up to that temperature and letting the exothermic reaction heat the material until the compound is completely formed. The product is MgB2 powder.

Strong magnetic fields will kill a material's superconductivity, but so-called type-II superconductors are somewhat resistant to this effect. That's good, since the primary application of superconductors is the generation of large magnetic fields. Magnesium diboride is fortunately a type-II superconductor; and, as is commonly true for superconductors, small alloy additions can allow superconductors to function at even higher fields.

Carbon, since it has an atomic size close to that of boron, will substitute for boron in MgB2. Just a 5% substitution of carbon for boron more than doubles the ability of MgB2 to withstand a magnetic field.[8] In theory, this allows the creation of one tesla (10,000 gauss) magnets that operate at 25 kelvin (K), a temperature at which liquid hydrogen (boiling point, 20 K) can be used as a refrigerant instead of liquid helium.

Magnesium diboride is interesting since it sits between the conventional (BCS) superconductors and the high temperature superconductors. Theorists at Lawrence Berkeley National Laboratory and the University of California, Berkeley, found that the odd features of MgB2 superconductivity arise from two separate populations of electrons residing in two different kinds of bonds among the atoms.[9-10] These two different types of bonds can be seen in the schematic of the MgB2 structure, as pictured above.

The hexagonal honeycomb layers of boron atoms that sandwich layers of magnesium atoms in MgB2 are bonded like a plane of graphite. However, boron has one electron less than carbon, and that's what causes the unusual electron distribution in the material. Since the sigma bonds in the boron plane are only partially filled, lattice vibrations - the phonons in the phonon-electron interaction - have a much large affect.[9-10] This insight into how a layered structure helps to achieve superconductivity at higher temperatures might lead to the discovery of other high temperature superconductors.[10]

References:

  1. Mineral Commodity Summary, Platinum-Group Metals, United States Geological Survey Web Site.
  2. Mineral Commodity Summary, Rare Earths, United States Geological Survey Web Site.
  3. 2012 Minerals Yearbook (February, 2015), United States Geological Survey Web Site.
  4. Mineral Commodity Summary, Magnesium Metal, United States Geological Survey Web Site.
  5. Mineral Commodity Summary, Boron, United States Geological Survey Web Site.
  6. Morton E. Jones and Richard E. Marsh, "The Preparation and Structure of Magnesium Boride, MgB2," J. Am. Chem. Soc., vol. 76, no. 5 (March, 1954), pp 1434-1436, DOI: 10.1021/ja01634a089.
  7. Jun Nagamatsu, Norimasa Nakagawa, Takahiro Muranaka, Yuji Zenitani. and Jun Akimitsu, "Superconductivity at 39 K in magnesium diboride," Nature, vol. 410, no. 6824 (March 1, 2001), pp. 63-64, doi:10.1038/35065039.
  8. Taking superconductors to new heights, DOE/Ames Laboratory Press Release, June 28, 2004.
  9. Hyoung Joon Choi, David Roundy, Hong Sun, Marvin L. Cohen, and Steven G. Louie, "The origin of the anomalous superconducting properties of MgB2," Nature, vol. 418, no. 6899 (August 15, 2002), pp. 758-760, doi:10.1038/nature00898.
  10. Paul Preuss, "A Most Unusual Superconductor and How It Works," Lawrence Berkeley Laboratory Press Release, August 14, 2002.

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