Advances in High Temperature Superconductors and their applications

Book excerpt: Prof. Heike Kamerlingh Onnes discovered superconductivity while measuring resistivity of mercury. Surprisingly the resistivity of mercury ceased at 4.2 K and this phenomenon was known as superconductivity. He realized the importance of this discovery in producing large magnetic fieldspl. delateIt was realized that superconductivity is in a new thermodynamic state with peculiar electric and magnetic properties. This paved the way to discover more superconductors. Simple elements such as Tin, Indium or lead showed the highest critical temperature (Tc) 7.2 K. They were called as Type 1 superconductors. Niobium-nitride was found to superconduct at 16 K at 1941 and Vanadium-silicon showed superconductive properties at 17.5 K at 1953. Nb alloys and binary or more complex compounds such as Nb3Sn (Tc – 18 K), Nb-Ti (Tc -9 K), Ga, V with Tc,23 K became type II superconductors. Thereafter, there was not much improvement in the development of superconductor although wonderful applications were expected from superconductors. After three decades, Fullerenes, like ceramic superconductors, are discovered. A decade ago MgB2 was discovered with Tc = 39 K. These superconductors were routinely produced into formof wires for producing larger magnetic fields. In all these cases cooling was effectively done by liquid Helium. A comprehensive microscopic theory of superconductivity in metals was proposed in 1957 by John Bardeen, Leon Cooper and Robert Schrieffer (the so-called “BCS” theory) for which they received the Nobel Prize in Physics. In a major breakthrough, George Bednorz and Karl Mueller discovered a brittle ceramic superconductivity in the family of cuprates at 30 K in 1986 and a new era began. Inspired by the work of Bednorz and Mueller on high temperature superconductivity (HTS), Paul Chu and his associates at the University of Houston discovered in 1987, 123 compounds. That is, YBCO (Yttrium1- Barium2-Copper3- Oxygen7) and iso-structural RBCO (Rare-earth1-Barium2-Copper3-Oxygen7) have a Tc of 93 K. Prior to 1987, all superconducting materials had lower critical temperatures (Tc’s) and therefore functioned only at temperatures near the boiling point of liquid helium (4.2 K) or liquid hydrogen (20.28 K), with the highest being Nb3Ge at 23 K. They were known as low temperature superconductors. YBCO was the first material to become superconducting above 77 K, (boiling point of liquid nitrogen) and subsequently a series of high temperature superconducting materials were discovered. These superconducting materials are widely known as High temperature superconductors as these Tc’s exceeded the limit prescribed by BCS theory. HTSCs are potentially valuable as liquid nitrogen is cheaper than liquid helium. YBCO possesses superior superconducting and physical properties. YBCO receiver coils in NMR-spectrometers have improved the resolution NMR spectrometers by a factor of 3 compared to that achievable with conventional coils. Paul Chu’s group holds the current Tc-record of 164 K in the mercury barium based cuprate superconductor under pressure. Their work led to a rapid succession of new high temperature superconducting materials, ushering in a new era in material science, chemistry and technology. Added to this the structure of Bi2Sr2Ca2Cu2O10(BiSCCO) high temperature superconductive compound having T= 110 K was reported. In 1993, mercuric-cuprates, perovskite ceramic superconductors with the transition temperatures Tc =138 K was also reported.