Invited review “sol-gel” preparation of high temperature superconducting oxides

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Abstract

This review article focuses on the sol-gel preparation of high temperature superconducting oxides wherein different classes of gel technologies were utilized. These involve: 1) the sol-gel route based upon hydrolysis-condensation of metal-alkoxides, 2) the gelation route based upon concentration of aqueous solutions involving metal-chelates, often called as “chelate gel” or “amorphous chelate” route, and 3) the organic polymeric gel route. This paper reviews the current status of these sol-gel processes, and illustrates the underlying chemistry involved in each sol-gel technology. It is demonstrated that the chemical homogeneity of the gel is often disturbed by the differences in the chemistries of the cations. Prior to gelation the starting precursor solution containing various forms of metal-complexes must be chemically modified to overcome this problem. Illustration of a variety of strategies for success in obtaining a homogeneous multicomponent gel with no precipitation is focal point of this review article.

strictly speaking, sol-gel must include processing, in which a sol (often defined as a suspension of small but discrete particles remaining dispersed in a liquid phase) undergoes a transition to a gel characterized by an infinite three-dimensional network structure spreading uniformly throughout the liquid medium. Thus a gel has to be distinguished either from a viscous liquid of one continuous phase or from a gelatinous precipitate in which a liquid phase is not involved in the solid network. It is generally accepted that a true gel can be prepared in two major chemical ways: (1) a molecular route based upon hydrolysis and polycondensation of metal alkoxides (alkoxide gels) and (2) a network formation route based upon destabilization of dispersed colloidal particles in a liquid medium (particulate gels). According to the rigorous definition of gel, some solution processes, started from metal-organics (other than metal alkoxide), would not be classified into a true sol-gel process. In this paper, however, the term “sol-gel” is broadly used to describe the preparation of inorganic materials by a variety of solution routes. A chemical process which can produce a solid matter or a highly viscous liquid matter with no precipitation is treated as the so-called “sol-gel” process, even when it is likely that the system contains no infinite rigid network.