Ceramic oxide superconducting materials

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. 

The amount and positions (atomic locations) of oxygen atoms in the superconductors are highly critical and determine the properties of the superconductor. The oxygen vacancies (or deficiency) can be ordered in these materials. Neutron-diffraction experiments were required to determine the population parameters and the atomic positions of oxygen in these structures. The superconducting transition temperature in these "ceramic" oxides is a critical balance between the oxygen content and a proper mix of Cu2+ and Cus+ ions generated in the anneal or post-heat treatment. 

The new superconducting ceramics offer a way out, if they can be made in some workable form, probably a thin film, and so long as the circuits can be operated at around 80° K or higher. The oxides could then be used on the circuit chips themselves to connect transistors and other devices and to make connections between chips. There is also the possibility that the superconducting properties in the new materials can be manipulated by, say, moving oxygen in and out as needed, or that the ceramics will turn out for some reason to be more suitable than conventional superconducting materials for use in transistors. 

There are a vast number of different sorts of superconducting compound. Examples of these are summarized in Table 7.2. They include organic polymers and intercalation compounds (such as RbjCgjj), as well as ceramic oxides and sulfides. In all these compounds the critical temperature is still below the boiling point of liquid nitrogen. Hence, these materials would be very expensive to use in everyday life (as a litre of liquid helium costs 20 times as much as liquid nitrogen). 

Reactions of nanoscale materials are classified with respect to the surrounding media solid, liquid, and gas phases. In the solid phase, nanoscale crystals are usually connected with each other to form a powder particle (micron scale) or a pellet (milli scale) see Figure 14.1. Two or more materials (powder or pellet) are mixed and fired to form a new material. The nanosized structure is favored, due to the mixing efficiency and high reaction rate. Alloys (metals), ceramics (oxides), cement (oxides), catalysts (metals and oxide), cosmetics (oxides), plastics (polymers), and many functional materials are produced through solid reaction of nanoscale materials. One recent topic of interest is the production of superconductive mixed oxides, where control of the layered stracture during preparation is a key step. 

The observed disappearance of electrical resistance when mercury and a few other alloys were cooled at liquid He temperature, led to die term superconductors. Only in 1986 did an oxide exhibit superconductivity and at higher transition temperatures, T. The following year the ceramic oxide YB2Cu307. (or YBCO) was found to become superconductor at a temperature >77 K (the boiling point of nitrogen). The ease of its preparation and its study makes it an interesting material. 

Electrical and Electronic Applications. Silver neodecanoate [62804-19-7] has been used in the preparation of a capacitor-end termination composition (110), lead and stannous neodecanoate have been used in circuit-board fabrication (111), and stannous neodecanoate has been used to form patterned semiconductive tin oxide films (112). The silver salt has also been used in the preparation of ceramic superconductors (113). Neodecanoate salts of barium, copper, yttrium, and europium have been used to prepare superconducting films and patterned thin-fHm superconductors. To prepare these materials, the metal salts are deposited on a substrate, then decomposed by heat to give the thin film (114—116) or by a focused beam (electron, ion, or laser) to give the patterned thin film (117,118). The resulting films exhibit superconductivity above Hquid nitrogen temperatures. 

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