Superconducting materials borides

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. 

Solid borides have high melting points, exceptionally high hardness, excellent wear resistance, and good immunity to chemical attack, which make them industrially important with uses as refractory materials and in rocket cones and turbine blades. Some metal borides have been found to exhibit superconductivity. 

Following the discovery of superconductivity in Hg in 1911, physicists, chemists, material scientists, metallurgists, electrical engineers, and others have found superconductivity in thousands of materials with values from a few millikelvin to 164 K [current record T, obtained in HgBa2Ca2Cu309 (Flg-1223) under high pressure see 17.3,10.2.5], These materials include elements, alloys, carbides, nitrides, borides, sulfides, organics, and oxides. 

In the fifteen years since publication of the first edition of Comprehensive Coordination Chemistry (CCC, 1987), group 5 chemistry has been part of the intensive development of ceramic, optical, and magnetic materials based upon metal borides, nitrides, phosphides, oxides, and sulfides. A major impetus came from the discovery of the high-temperature superconducting oxides. In addition, the search for new routes to these materials via sol-gel or chemical vapor deposition techniques has spurred growth in metal amido, oxo, alkoxo, thio, and carboxylato chemistry. 

This book summarizes the research on many different types of conducting materials in the field of CVD. In Chapter 2, Schulz and Marks report on superconducting films and Krauter and Rees, Jr. report on borides, silicides and nitrides in Chapter 8. In the present chapter we will focus on two classes of conducting materials, nitrides and transparent conducting oxides. It is possible that some aspects can also be found in other chapters of this book, e.g., CVD of Sn02. Nevertheless, they will be discussed in detail to give a consistent description on the state of the art. 

In this book, we briefly examine the different types of reactions and methods employed in the synthesis of inorganic solid materials. Besides the traditional ceramic procedures, we discuss precursor methods, combustion method, topochemical reactions, intercalation reactions, ion-exchange reactions, alkali-flux method, sol-gel method, mechanochemical synthesis, microwave synthesis, electrochemical methods, pyrosol process, arc and skull methods and high-pressure methods. Hydrothermal and solvothermal syntheses are discussed separately and also in sections dealing with specific materials. Superconducting cuprates and intergrowth structures are discussed in separate sections. Synthesis of nanomaterials is dealt with in some detail. Synthetic methods for metal borides, carbides, nitrides, fluorides, sili-cides, phosphides and chalcogenides are also outlined. 

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