Physical properties, composite superconductor

Soon after the discovery of the quaternary borocarbide superconductors in 1994 a remarkable progress in the investigation of their physical properties could be asserted (Muller and Narozhnyi, 2001b). One reason for this rapid progress is the favorable synthesis properties of this class of materials, which resulted in high-quality polycrystalline samples over a wide range of compositions, as well as thin... 

A phase is a structurally homogeneous portion of matter. Regardless of the number of chemical constituents of a gas, there is only one vapor phase. This is true also for the liquid form of a pure substance, although a mixture of several liquid substances may exist as one or several phases, depending on the interactions among the substances. On the other hand a pure solid may exist in several phases at different temperatures and pressures because of differences in crystal structure (Reference 1). At the phase transition temperature, the chemical composition of the solid remains the same, but a change in the physical properties often will take place. Such changes are found in ferroelectric crystals (example BaTiOj) which develop a spontaneous polarization below in superconductors (example Pb) which lose all electrical resistance below the transition point, and in many other classes of solids. 

This should come as no surprise, since the physical behavior of materials is non-linear and unpredictable, especially when materials are formulated or in combination. Two examples will suffice high temperature ceramic superconductors and insulators above their critical temperatures or at non-ideal stoichiometries composite structures may show several times the strength or impact resistance than would be expected from their component materials. Materials discovery will always require a good deal of trial and error, factors that may be mitigated by techniques that permit the simultaneous synthesis of large numbers of materials, followed by rapid or parallel screening for desired properties. 

Gross stoichiometry variations in these modular systems are often acconunodated by the formation of other series members or the incorporation of planar faults, often equivalent to isolated lamellae of other series members, into the structure. However, many individual phases also show composition variation which is sometimes accommodated by random populations of point defects and in other phases by ordering and the generation of new structures. In both cases composition flexibility is important in influencing the physical and chemical properties of these materials. In particular this effect has been well studied in the cuprate superconductors, where changes in composition have repercussions for the superconducting transition temperature, r, of the phase. 

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