Synthesis and properties of Hg-based high-T _c superconductors

Summary

The formation of Hg-based HTSC materials was investigated through the development of non-conventional techniques. The former was based on a singlestep synthesis at temperatures significantly lower than usually reported. The basic idea was to exploit the low vapuur pressure of HgO at low temperatures (≈600°C) and in oxygen atmosphere to favour the solid-state reaction between HgO and precursors. Recently, a novel alternative approach essentially based on the oxygen and mercury vapours reactivity for the synthesis of the superconducting phases has been successfully developed. Basically, solid precursors (powders, pellet or thick-film forms) are processed in a two-temperature zone reactor which allowed the independent control of precursors and HgO source temperatures, the partial pressure of HgO and the total pressure during the reaction. In the latter approach, a very low mercury vapour partial pressure (intrinsically below 1–2 atm) was demonstrated to be effective for synthesizing the superconducting phase. The respective merits of both these alternative techniques will be discussed in terms of structural, morphological and magnetic measurements. The low reaction pressure common to both the techniques simplifies largely and makes safer the preparation of these materials, especially in view of applications.