Superconductivity elements exhibiting

In the course of the broad research effort on preparing radically new superconducting materials, other inorganic systems were found to exhibit this phenomenon a sampling of these are presented below. The unusual feature of these materials is the fact that few contain "the best" superconducting elements in their chemical formulations. 

Table 7.1 indicates some elements of the Periodic Table which have been shown to have a superconducting transition under normal conditions of temperature and pressure. Other elements exhibit superconductivity under exceptional conditions, e.g. under pressure (Si, Y), or when prepared as thin films (Li, Cr) or irradiated by a-particles (Pd). for the elements is generally below 10 K (maximum Nb, = 9.25 K). Small increases in these critical temperatures can be achieved by using high pressure to force the atoms closer together. 

TABLE 3. Elements Exhibiting Superconductivity Under or After Application of High Pressure... 

Those readers not familiar with superconductivity in organic materials may find the Tc values rather low. However, they are comparable to values for inorgaiuc metallic elements. Here is a list of some selected examples FcCNb) = 9.25 K, rc(Pb) = 7.20 K, rc(a-Hg) = 4.15 K, rc(Sn) = 3.72 K, Tc(Al) = 1.17 K, Tc(ri) = 0.40 K, etc. It is interesting to note that copper does not exhibit a superconducting transition. The highest known Tc values of any material correspond to the copper-oxide series with Tc 138 K as the absolute record for the thallium-doped mercury-cuprate compound. 

Non-metals, such as silicon, can also become superconducting when pressure is applied. At 120-130 kbar pressure, silicon exhibits a Tc of 6.7 to 7.1 K. Sulfur has also recently been converted into the superconducting state at 200 kbar with a transition temperature of 5.7 K. In 1989, hydrogen was obtained in the condensed state and under 2.5 megabars pressure, it becomes opaque. This observation indicates that the element is possibly transforming into a metal. Several... 

For more than 20 years, little progress was made in the understanding of superconductors and only more substances exhibiting the effect were found. More than 20 metallic elements can be made superconducting under suitable conditions (Figure 10.2), as can thousands of alloys. It was not until 1933 that Meissner observed a new effect. 

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. 

Properties Lustrous metal. D 7.87, mp 1312C, bp 3000C reacts slowly with water. Soluble in dilute acid insoluble in water. Exhibits a high degree of magnetism, especially at low temperatures salts are colorless has highest neutron absorption cross-section of any known element has superconductive properties, burns in air to form the oxide. Combustible. 

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

Superconductivity at temperatures above 90 K has been observed also in perovskites where Y has been replaced by all the rare earth elements except for Ce, Pr, and Pm. These isostructural derivatives of 8820030, although they do not possess higher transition temperatures, may exhibit other important alternative properties. These superconducting characteristics will certainly extend the use of perovskites to many fields, including integrated electronics and electromagnetic applications. 

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