Sunday, April 15, 2012
By definition, a superconductor exhibits no resistance to electrical conductivity, and will oppose an external magnetic field, a phenomenon referred to as the Meissner effect (Figure 1) . Many pure transition metals (e.g., Ti, Zr, Hf, Mo, W, Ru, Os, Ir, Zn, Cd, Hg) and main group metals (e.g., Al, Ga, In, Sn, Pb) exhibir superconductivity, many only when esposed to high pressure conditions. These materials are referred to as Type I or soft superconductors.
Binary and ternary alloys and oxides of these elements, as well as pure V, Nb, Gd, and Tc are referred to as Type II or high-field superconductors. In contrast to Type I, these materials exhibit conductive characteristics varyng from normal metallic to superconductive, depending on the magnitude of the external magnetic field. It is noteworthy to point out that metals with the highest electrical conductivity (e.g., Cu, Au) do not naturally possess superconductivity. Although this behavior was first discover in 1911 for supercooled liquid mercury, it was not until 1957 that a theory was developed for this phenomenon.
In order to exhibit superconductive behavior, early Type I and II materials needed to be cooled below the critical temperature (Tc) ranging from 0.015 K (for W) to 23 K (for Nb3Ge). An intriguing goal of current research is to increase the Tc to room temperature ("high-temperature superconductors", HTS), which would trivialize resistence-free aplications such as power grid lines and widespread levitated trains. In 1986, Muller and Bednorz at IBN made and important discovery toward this goal -the firsr high-temperature superconductor, La2-xSrxCuO4 (LSCO), with a critical temperature of 35 K. A year later, the first material with a critical point above the boling of nitrogen (77K) was discovered, known as YBa2Cu3O7-d (YBCO), with a critical point of 92 K. In more recent years, the highest-temperature cuprate based superconductors have been synthesized with a general formula: MvNwCaxCuyOz (where M=Y, Bi, Tl, or Hg; N=Ba or Sr; v=1 or 2; w=2 or 4; x=0,1, or 2; y= 1,2, or 3; z= 3, 4, 6, 7, 9, 10, or 15). To date, the highest temperature superconductived materials are thallium (e.g., TlBa2Ca2Cu3O9, Tc= 133K), mercury (e.g., Hg0.8Tl0.2Ba2Ca2Cu3O8.33, Tc= 138K), or lead-doped (e.g., (Hg0.75Pb0.15Tl0.1)Ba2Ca2Cu3O8+, Tc= 142 K).
Fahlman, Bradley D. Materials Chemistry. Reprinted 2008. Springer. Pages 38-39.