High-field superconductors

M MogSeg and related Chevrel phases High field 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 varying 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 discovered in 1911 for supercooled liquid mercury, it was not until 1957 that a theory was developed for this phenomenon. 

Figure 1. Relationship between critical field and temperature of the best high field superconductors.

From Montgomery s model it follows that high-field superconductor coils of similar geometrical shape (a and jS constant) wound with identical wire type with equal distances... 

Other ternary molybdenum sulfides known as Chevrel phases are recognized as high field superconductors (59), but recently these compounds were found to be active as hydrotreating catalysts. Ooi and co-workers (60) reported that supported cobalt Chevrel phase sulfide catalysts present higher HDS activity than commercial C0M0S/AI2O3 catalysts. 

Some properties of high field superconductors with magnetic impurities" (with G. Sarma), Solid State Commun.. 4, 449 (1966). 

Current availability of individual lanthanides (plus Y and La) in a state of high purity and relatively low cost has stimulated research into potential new applications. These are mainly in the field of solid state chemistry and include solid oxide fuel cells, new phosphors and perhaps most significantly high temperature superconductors... 

Some of the above discussed precursor phenomena are also observed prior to diffusion driven phase transformations. A typical example are the conventional EM tweed images obtained in the tetragonal parent phase in high Tc superconductors and other ceramics. In a recent survey by Putnis St e of such observations it was concluded that in these cases the tweed contrast resulted from underlying microstructures fomied by symmetry changes driven by cation ordering. These symmetry changes yield a fine patchwork of twin related domains which coarsen when the transfomiation proceeds. However, in view of the diffusion driven character of the latter examples, these cases should be clearly separated from those in the field of the martensites. 

Metals and semiconductors are electronic conductors in which an electric current is carried by delocalized electrons. A metallic conductor is an electronic conductor in which the electrical conductivity decreases as the temperature is raised. A semiconductor is an electronic conductor in which the electrical conductivity increases as the temperature is raised. In most cases, a metallic conductor has a much higher electrical conductivity than a semiconductor, but it is the temperature dependence of the conductivity that distinguishes the two types of conductors. An insulator does not conduct electricity. A superconductor is a solid that has zero resistance to an electric current. Some metals become superconductors at very low temperatures, at about 20 K or less, and some compounds also show superconductivity (see Box 5.2). High-temperature superconductors have enormous technological potential because they offer the prospect of more efficient power transmission and the generation of high magnetic fields for use in transport systems (Fig. 3.42). 

Flexible superconducting tapes provide promise of uses for superconductors in motors, generators, and even electric transmission lines. Meanwhile, superconducting magnets cooled to the temperature of liquid helium already are in use. High-field nuclear magnetic resonance (NMR) spectrometers have become standard instruments in chemical research laboratories, and the same type of machine (called an MRI spectrometer) is used for medical diagnosis in hospitals worldwide. 

The recent discovery of high Tc superconductors means that applications of electrochemistry in this field are still in the initial phases... 

Lyubchenko LS, Stepanov SV, Lyubchenko ML, Sherle AI, Epstein VP, Dadali AA, Malinsky J (1992) Low-field microwave absorption in high-Fc superconductors based on olygo- and polyphthalocyanines. Phys Lett A162 69-78... 

Figure 2 The Meissner Effect, or the levitation of a strong magnet by the internal diamagnetic field of a high Te superconductor. 

In terms of the electronic age, which includes the invention of the radio, television, calculator, and computer, it has been claimed that the discovery of high Tc superconductors has resulted in a "third electronics revolution" preceded by the transistor (1947), and the vacuum tube (1904). It now appears that we have shifted from "silicon valley" to a "copper-oxide valley" with the new discoveries in the field of high Tc superconductivity. 

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