Superconductor properties

In the previous reviewing periods (CHEC(1984), CHEC-II(1996)) theoretical methods were applied in order to understand certain chemical and physical properties of 1,3-dithiole compounds, especially 1,3-dithiolylium ions, 1,3-dithiol-2-one and l,3-dithiole-2-thione. Since 1995, theoretical methods have been applied, mainly to a particular class of compounds, TTFs, which as electron donors easily form charge-transfer (CT) complexes. These complexes show a wide variety of electronic behaviors leading to semiconductor, metal-like, or superconductor properties. 

Of great interest was the discovery of hi temperature superconductor properties in Cgo doped with combinations of alkali and other metals. [244, 245] These can be viewed as insertion compounds with the metal center occupying octahedral and tetrahedral interstitial ates in the fee lattice of Qo- Charge transfer occurs from the metal center to a vacant conduction band formed from interaction between the Cw and metal valence orbitals in the dose packed lattice. Since the fullerenes have stable hdlow cage structures, there is a possibility that dusters might be synthesized and stabilized within them. These hypothetical materials might be expected to have novel properties, induding superconductivity. 

To determine the hf characteristics of the cavity material it is convenient to express the superconductor properties (surface resistance, critical fields, etc.) using three basic and easily measured values the critical temperature Tc, the residual resistivity po and the Sommerfeld constant y. 

Thus far the importance of carbon cluster chemistry has been in the discovery of new knowl edge Many scientists feel that the earliest industrial applications of the fullerenes will be based on their novel electrical properties Buckminsterfullerene is an insulator but has a high electron affinity and is a superconductor in its reduced form Nanotubes have aroused a great deal of interest for their electrical properties and as potential sources of carbon fibers of great strength... 

CERAMCS-ELECTRONIC PROPERTIES AND MATERIALSTHUCTURE] (Vol5) -in superconductors [CERAMICS AS ELECTRICAL MATERIALS] (Vol5)... 

In the area of superconductivity, tetravalent thorium is used to replace trivalent lanthanides in n-ty e doped superconductors, R2 Th Cu0 g, where R = Pr, Nd, or Sm, producing a higher T superconductor. Thorium also forms alloys with a wide variety of metals. In particular, thorium is used in magnesium alloys to extend the temperature range over which stmctural properties are exhibited that are useful for the aircraft industry. More detailed discussions on thorium alloys are available (8,19). 

Dimensions. Most coUoids have aU three dimensions within the size range - 100 nm to 5 nm. If only two dimensions (fibriUar geometry) or one dimension (laminar geometry) exist in this range, unique properties of the high surface area portion of the material may stiU be observed and even dominate the overaU character of a system (21). The non-Newtonian rheological behavior of fibriUar and laminar clay suspensions, the reactivity of catalysts, and the critical magnetic properties of multifilamentary superconductors are examples of the numerous systems that are ultimately controUed by such coUoidal materials. 

Electrical Properties at Low Temperatures The eleciiical resistivity of most pure metalhc elements at ambient and moderately low temperatures is approximately proportional to the absolute temperature. At very low temperatures, however, the resistivity (with the exception of superconductors) approaches a residual value almost independent of temperature. Alloys, on the other hand, have resistivities much higher than those of their constituent elements and resistance-temperature coefficients that are quite low. The electrical resistivity of alloys as a consequence is largely independent of temperature and may often be of the same magnitude as the room temperature value. 

There are presently four famihes of high-temperature superconductors under investigation for practical magnet appheations. Table 11-25 shows that all HTS are copper oxide ceramics even though the oxygen content may vary. However, this variation generally has little effect on the phvsical properties of importance to superconductivity. 

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

XPS has been used in almost every area in which the properties of surfaces are important. The most prominent areas can be deduced from conferences on surface analysis, especially from ECASIA, which is held every two years. These areas are adhesion, biomaterials, catalysis, ceramics and glasses, corrosion, environmental problems, magnetic materials, metals, micro- and optoelectronics, nanomaterials, polymers and composite materials, superconductors, thin films and coatings, and tribology and wear. The contributions to these conferences are also representative of actual surface-analytical problems and studies [2.33 a,b]. A few examples from the areas mentioned above are given below more comprehensive discussions of the applications of XPS are given elsewhere [1.1,1.3-1.9, 2.34—2.39]. 

M. S. Dresselhaus, G. Dresselhaus, and R. Saito. In Physical Properties of High Temperature Superconductors IV, edited by D. M. Ginsberg, World Scientific Publishing Co., Singapore, 1994. Vol. IV, Chapter 7. 

Many metal sulfides have important physical properties.They range from insulators, through semiconductors to metallic conductors of electricity, and some are even superconductors. 

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