High transition temperature superconductors

In practice the sensor is a plate 4-mm square mounted within a heater coil, and its characteristic is periodically restored to the standard state by heating to 450 °C for a few seconds. 

The following extract from a paper [31] published in 1968 makes interesting reading in the light of subsequent developments. 

Nevertheless, it is to be expected that, even if superconductors with transition temperatures well above 20 K are never found, superconductivity will still find its applications in technology. 

Since then there has been the totally unexpected breakthrough with the superconducting ceramics operating comfortably at liquid nitrogen temperature (77 K), and stimulating intensive R D efforts in the US, Japan and Europe. 

Probably the most obvious application for superconductors is in the transmission of electrical power and it seems that that this is close to being a reality. There are many other potential applications in the power electrical engineering field, for example power generators, motors, transformers, leads and fault current-limiters. 

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Lee, P.J. Engineering Superconductivity, Wiley-IEEE Press, New York, NY. 2001. Little, W.A. Experimental Constraints on Theories of High-Transition Temperature Superconductors, 5 Science, 1390 (December 9, 1988). 

Sun, J.Z. et al. Elimination of Current Dissipation in High Transition Temperature Superconductors, Science, 307 (January 19, 1990). 

High transition temperature superconductors behave as superconductors at unusually high temperatures... 

Lattice defects in the film can affect the electrical conductivity and electromigration in metallic films, and carrier mobility and lifetime in semiconductor materials. Generally high defect concentrations result in poor electromigration properties. Lattice defects have been shown to be important to the properties of the high transition temperature superconductor films. 

Niobium germanide, NbgGe, is a superconductor with a high transition temperature (T, = 20K). It is prepared by CVD by the coreduction of the chlorides as follows ... 

In early 1987, the composition and structure of the La-Ba-Cu -O superconductor was still unknown to the general public in the United States. By March of that year certain facts became known from Japanese publications. But at this point in time, a newer, higher Tc (> 90 K) material was announced. This new copper oxide superconductor was quite easy to prepare and, in addition to interested physicists, these new materials could be synthesized by ceramists, chemists, metallurgists, material scientists, or anyone with a knowledge of a chemical approach to solid-state materials. Even high school students developed simple methods for the synthesis of these compounds. The "high" transition temperature and the possible use of liquid nitrogen made research in superconductivity accessible to most scientists and laboratories. The media also capitalized on this worthy news report and published it in newspapers and also presented it on television as a news item. 

Soft metallic elements such as Al, In, Pb, Hg, Sn, Zn, Tl, Ga, Cd, V and Nb are type I superconductors. Alloys and chemical compounds such as Nb3Sn, V3Ga, and lZa3In, and some transition elements, are type II superconductors. Type II substances generally have a higher Tc than do type I superconductors. The recently discovered transition metal oxide superconductors have generated intense interest because they are type II superconductors with very high transition temperatures. Table 13.1 summarizes Tc for selected superconductors. 

The following summary of the essentials of the phenomenon and theory of superconductivity serves to introduce terminology and permits the high transition temperatures (Tc) superconductors (HTSs) to be seen in a historical... 

Chemical substitution has been used as a probe of the origin of the high transition temperature in YBa2Cu30y. The properties of this material are best understood when the material is considered to be a semiconductor doped to metallicity. When oxygen doped, the material becomes a 1 dimensional excitonic superconductor. The intrinsic semiconducting energy gap then provides the mechanism for the extremely... 

This heterogeneous Bauverband is found in Pm3n and built up by compounds of the 0-W-type (A1 S-type) (e.g., W3O or CraSi). A great number of compounds with this structure type are superconductors, e.g., NbaSn, partly with high transition temperature, and are used commercially therefore. 

The ternary metal nitride halides MNX (M=Ti, Zr, Hf X=C1, Br, I) contain two types of layer-structured polymorphs having different types of two-dimensional metal nitride networks [22]. Both are band insulators, and changed into superconductors with moderately high transition temperatures Ts up to 25.5 K upon electron-doping by means of intercalation through the interlayer space. There are two-layered polymorphs in MNX a-form with the FeOCl structure and p-form with the SmSI structure. p-ZrNCl has been synthesized by the reaction of Zr metal or ZrH powders were subjected to reaction with NH Cl at about 650 °C. 

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