Elements, superconducting

Fig. 1 Proton surface excess, x = Z — xN, as a function of mass number. Nuclide periodicity predicts maximal surface spin to occur in the regions as marked, in general agreement with the measured spin and elemental superconductivity of odd mass number nuclides...

It is used in arc-welding rods for stabilized grades of stainless steel. Thousands of pounds of niobium have been used in advanced air frame systems such as were used in the Gemini space program. The element has superconductive properties superconductive magnets have been... 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

Difficulties in separating and isolating the lanthanoids delayed their widespread use in technology. However, today they are studied intensely, because superconducting materials often contain lanthanoids (Fig. 1.64). All the actinoids are radioactive. None of the elements following plutonium occurs naturally on Earth in any significant amount. Because they can be made only in nuclear reactors or particle accelerators, they are available only in small quantities. 

Why Do We Need to Know This Material The elements in the last four groups of the periodic table illustrate the rich variety of the properties of the nonmetals and many of the principles of chemistry. These elements include some that are vital to life, such as the nitrogen of proteins, the oxygen of the air, and the phosphorus of our bones, and so a familiarity with their properties helps us to understand living systems. Many of these elements are also central to the materials that provide the backbone of emerging technologies such as the nanosciences, superconductivity, and computer displays. 

The generally accepted theory of electric superconductivity of metals is based upon an assumed interaction between the conduction electrons and phonons in the crystal.1-3 The resonating-valence-bond theory, which is a theoiy of the electronic structure of metals developed about 20 years ago,4-6 provides the basis for a detailed description of the electron-phonon interaction, in relation to the atomic numbers of elements and the composition of alloys, and leads, as described below, to the conclusion that there are two classes of superconductors, crest superconductors and trough superconductors. 

Fig. 3.—The curve represents the calculated values of the superconductivity critical temperature. The circles are the experimental values for the elements and for binary alloys between adjacent elements.

Several structural features, including electron transfer between atoms of different electronegativity, oxygen deficiency, and unsynchronized resonance of valence bonds, as well as tight binding of atoms and the presence of both hypoelectronic and hyperelectronic elements, cooperate to confer metallic properties and high-temperature superconductivity on compounds such as (Sr.Ba.Y.LahCuO,-,. 

The predictions made by Mendeleev provide an excellent example of how a scientific theory allows far-reaching predictions of as-yet-undiscovered phenomena. Today s chemists still use the periodic table as a predictive tool. For example, modem semiconductor materials such as gallium arsenide were developed in part by predicting that elements in the appropriate rows and columns of the periodic table should have the desired properties. At present, scientists seeking to develop new superconducting materials rely on the periodic table to identify elements that are most likely to confer superconductivity. 

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. 

Superconducting only in thin films or under high pressure in a crystal modification not normally stable. Critical temperatures for those elements from [32, Chapter 12]. 

The fabrication of logic elements using such devices allows in principle the construction of a large capacity, compact, high-speed computer [50], Major problems with the technology are that large fan-out ratios are difficult to achieve and that superconducting circuits have a very low inherent impedance and so are difficult to couple with conventional elements at room temperature. 

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