Magnetic Materials: Superconductors

Superconductors

Superconductors are materials which exhibit no electrical resistance below a certain temperature defined as the critical temperature (T_C). Prior to 1986, the highest T_C reported was 20 K for Nb₃Ge and Nb₃Sn.⁵ In 1986-87, a group lead by Johannes Bednorz and Karl Müller reported the ceramic oxides La_2-xBa_xCuO_4-x and YBa₂Cu₃O₇ (product 328626) superconduct above the boiling point of nitrogen (77 K).^6,7 Materials whose T_C is greater than the boiling point of nitrogen (a common, readily available, cryogenic coolant) are referred to as high-temperature superconductors (HTS). For their work Bednorz and Müller were awarded the Nobel Prize in Physics in 1987.⁸

Other more exotic compounds such as fullerides have also exhibited superconducting properties. Fullerides of the formula A_x@C₆₀ (A = K, Rb, Cs) are reported to have superconducting character.⁹ Although superconductive compounds have been known for nearly a century, the relatively mundane compound magnesium boride has only recently been demonstrated to exhibit superconductivities. Magnesium boride, MgB₂ (product 553913) is not only superconductive but its critical temperature is surprisingly high for a simple ceramic material (T_c = 39 K).¹⁰ Figure 1 shows an image of a MgB₂ wire segment with a tungsten boride core. The wire is formed by reaction of magnesium vapor with a boron filament. The grain structure seen in this image is visible under polarized light.¹¹ Table 1 for a comparison of some critical temperatures.

*Highest reported Tc for an element **Highest reported T_C to date

Superconductivity is governed not only by a critical temperature but also by a critical magnetic field (H_C) and a critical current density (J_C). The critical magnetic field refers to an applied magnetic field, such that, if an applied field becomes too large (greater than H_C) superconductivity will be lost. Critical temperature and critical field are inversely proportional such that just below T_C, the superconducting state can only be maintained in a very weak applied field, whereas, near 0 K, a larger applied field can be tolerated (Figure 2). Similarly, J_C is the maximum current that can be passed through a superconducting material before it reverts back to a non-superconducting state. This is a critical factor for power applications such as practical superconductor-based electronics would have a J_C greater than 106 amp·cm^-1.

Aside from traditional metals based superconductors and HTS cuprate-based ceramics, more recent work has focused upon molecular and fullerene based superconductors.