Superconducting ceramics research

Until 1986 many experiments were carried out using metals and alloys. However, in 1986 Georg Bednorz and Alex Muller, two researchers of the IBM laboratory in Ruschlikon near Zurich, published an article in the Zeitschrift fur Physik in which they announced that they had made a superconducting ceramic material. It turned out to be a compound made of barium, copper, lanthanum and oxygen, which became superconducting at 35 K. They were awarded the Nobel prize for this discovery. After this, superconductors developed rapidly, at least as far as critical temperature is concerned. 

This extraordinary discovery of superconductivity in a ceramic material led to an explosion of research on other ceramic systems. The most notable... 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Early work on superconductors concentrated on metals or metal mixtures (alloys). Niobium alloys are particularly good superconductors, and in 1973 a niobium alloy, Nb3Ge, was found to have Tc — 23 K, the highest known value for a metal superconductor. In 1986, a ceramic oxide with formula La2- Ba CuOq was found to show superconductivity at 30 K. Through intense research efforts on ceramic oxides, YBa2 C U3 Oj-, with Tc — 93 K, was discovered in 1987. 

The record for the highest superconducting temperature in the year 2004 is 138 K, held by a nonstoichiometric ceramic oxide, HgQ g Tig 2 Ba2 Ca2 C1I3 Og 33. This is still far below room temperature, but research continues. 

Challenges to Established Theories, It is interesting to note that some theoreticians struggle with describing how superconductivity occurs at high temperatures in the newer, ceramic superconductors. This is understandable because the classic theory of superconductivity is tied to metals. Most ceranuc superconductors discovered to date incorporate distinctive layers of copper and oxygen atoms, One question posed by some researchers, Is the mechanism of high-temperature superconductivity the same in hole superconductors as it is in electron superconductors ... 

Research chemists found that they could modify the conducting properties of solids by doping them, a process commonly used to control the properties of semiconductors (see Section 3.13). In 1986, a record-high Ts of 35 K was observed, surprisingly not for a metal, but for a ceramic material (Section 14.24), a lanthanum-copper oxide doped with barium. Then early in 1987, a new record T, of 93 K was set with yttrium-barium-copper and a series of related oxides. In 1988, two more oxide series of bismuth-strontium-calcium-copper and thallium-barium-calcium-copper exhibited transition temperatures of 110 and 125 K, respectively. These temperatures can be reached by cooling the materials with liquid nitrogen, which costs only about 0.20 per liter. Suddenly, superconducting devices became economically viable. 

In 1986, IBM researchers K. Alex Muller and Georg Bednorz paved the path to superconductivity at slightly higher temperatures using a ceramic alloy as a medium. Shortly thereafter, a team led by University of Houston physicist Paul Chu created a ceramic capable of superconductivity at temperatures high enough to encourage true commercialization. 

The discovery in 1986 of high-temperature superconductivity in ceramic cuprates of perovskite structure started a period of very intensive research of transition metal oxides. Soon afterwards, in 1993, the colossal magnetoresistance effect was discovered in manganite perovskites, again leading to an increasing research activity in the field of magnetic oxides. It is... 

Fig. 4.58 (a) Micrograph of lead section see text for details (b) Bi-2212 current leads the longer lengths are approx. 3x25x300 mm. (Courtesy of the IRC in Superconductivity and Department of Materials Science, University of Cambridge, UK, ABB Corporate Research, Switzerland and Advanced Ceramics Ltd., UK.)... 

Matthias, it now turns out, was an incredible prophet. The very recent discovery of a family of ceramic superconductors similar in crystal structure to the compound with which Roy had been experimenting has galvanized the interests of researchers throughout the world. For not only do these ceramics superconduct in a strange new way, but they do so at temperatures higher than ever before dreamed possible. Some scientists even predict that yet-to-be-discovered ceramics will superconduct at temperatures at or above room temperature, a development that would represent a breakthrough of such enormous proportions that it would drastically change the very way we use electricity. 

The other bit of information researchers picked up was that when the crystal was in its nonsuperconducting shape, it had six oxygen atoms per molecule in its chains in its superconducting form, it had seven. Moreover, the researchers found that if they annealed their ceramic—the step that followed heating it in the oven—in an inert atmosphere, it lost oxygen atoms when they annealed it under oxygen, the supply was replenished. The arrangement of... 

IBM researchers Alex Muller and Georg Bednorz make a ceramic compound of lanthanum, barium, copper, and oxygen that superconducts at 35° K. 

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