Superconductivity discovered

The high temperature superconductivity discovered in the Bi-Sr-Ca-Cu-O system was found to be related to a homologous series of compounds with an idealized formulation Bi2Sr2Ca iCu 02n+4 (n = 1 to 3 or more). The n = I, 2, 3 phases have Tc values of 10-20, 90 and 11 K, respectively. The presence of superstructure modulations which are, in general, incommensurate with the basic structure was first discovered by HRTEM and ED (Shaw et al 1988, Gai et al 1988). The periodicity of the modulation is found to be about 4.7 multiplied by the a-lattice parameter. The compounds can be prepared from solid state reactions of the component oxides in stoichiometric proportions and heating between 800-900 °C in air. 

Oxide superconductors have been known since the 1960s. Compounds such as niobium oxide [12034-57-0] NbO, TiO, SrTi02, and AWO, where A is an alkah or alkaline earth cation, were found to be superconducting at 6 K or below. The highest T observed in oxides before 1986 was 13 Kin the perovskite compound BaPb Bi O, x = 0.27. Then in 1986 possible superconductivity at 35 K in the La—Ba—Cu—O compound was discovered (21). The compound composition was later determined to be La 85 A the Y—Ba—Cu—O system was pubUshed in 1987 and reported a transition... 

H. Kammerling Onnes (Nobel Prize for Physics, 1913) discovered superconductivity in Leiden in 1911 when he cooled mercury to the temperature of liquid helium. Many other materials, mostly metals and alloys, were subsequently found to display superconductivity at very low temperatures. 

Nonstoichiometry is relatively common among mixed metal oxides, in which more than one metal is present. In 1986 it was discovered that certain compounds of this type showed the phenomenon of superconductivity on cooling to about 100 K, their electrical resistance drops to zero (Figure 20.9). A typical formula here is YBa2Cu30 where x varies from 6.5 to 7.2, depending on the method of preparation of the solid. 

The gap in superconductivity between the fifth and sixth groups of the periodic table, discovered by Matthias,24 is seen to correspond to the transition from crest to trough superconductivity. It does not require for its explanation the assumption20- 25 that there are mechanisms of superconductivity other than the electron-phonon interaction. 

The transition between normal conductivity and superconductivity occurs at different temperatures for different materials. In general, Tc is near 4.2 K, the boiling point of liquid helium. For this reason, any device that makes use of superconductivity must be immersed in a bath of liquid helium. Little wonder that superconductivity was not discovered until early in this century and remained a laboratory curiosity until the mid-1980s. 

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. 

Superconductivity has also been discovered in rather exotic materials, including the following Buckminsterfullerene (Cgo) doped with ICI Carbon nanotubes (superconductivity in just one direction) Nickel borocarbides, which contain Ni2 B2 layers alternating with R C sheets, where R is a rare earth element such as Er and organic superconductors that contain planar organic cations and oxoanions. Chemists and physicists continue to study these and other families of superconductors. 

Superfluidity is just one of the surprising new properties discovered through low-temperature research. Another example is superconductivity, described in our Chemistry and Technology Box in Chapter JT. The 2003 Nobel Prize in physics went to three theoreticians who developed theories explaining these phenomena. 

In 1908, Kamerling-Onnes got the liquefaction of helium (discovered by Janssen e Lockyer during the solar eclipse of 18 August 1868). Kamerlingh-Onnes obtained in Leiden 60 cc of liquid helium extracted from several tons of monazite sable imported from India. Kamerlingh-Onnes himself discovered the X-transition and the superfluidity in 4He and in 1911 the superconductivity of Hg, a particularly pure substance at that time. In the race towards lower and lower temperatures, Kamerling-Onnes, pumping on liquid 4He, obtained 0.7K in 1926. 

The phenomenon of superconductivity was discovered at the beginning of the twentieth century by the Dutch physicist H. Kamerlingh Onnes, during the first attempts to liquefy helium (which at atmospheric pressure boils at 4.2 K). After refining the technique of helium liquefaction, in 1911, Onnes attempted to measure the electrical resistance of metals at these extraordinary low temperatures, and realized that at 4 K the resistance of mercury, as well as that of other metals indicated in Figure 1, became too low to be measured. This change in electrical property became the indication of the new superconductive physical state. The temperature below which materials become superconducting is defined as the critical temperature, Tc. 

Hence, it seems safe to conclude that the next century will see considerable research on materials and alchemy using quantum theory. If the physical models improve and computations continue to get more accessible, many experimentalists will have access to these tools. In science, all decisions are ultimately made by experiment, and most new discoveries are made by observing physical systems. Perhaps it is not too outrageous to suggest that, if theory continues to improve as it has, theorists may discover new states of matter and properties such as superconductivity and magnetism using their computers and analytic modeling. 

Since Onnes s discovery, chemists have discovered other metals that superconduct at extremely low temperatures. A niohum metal alloy was found to exhihit superconductivity at 23 K, for example. But scientists wanted to find materials that exhihit superconductivity at much higher temperatures, because refrigerants like liquid helium are very expensive. 

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