SYNTHESIS AND PROPERTIES OF HG-BASED HIGH-T-C SUPERCONDUCTORS

The formation of Hg-based HTSC materials was investigated through the development of non-conventional techniques. The former was based on a single-step synthesis at temperatures significantly lower than usually reported. The basic idea was to exploit the low vapour pressure of Hg O at low temperatures (similar to 600 degrees C) and in oxygen atmosph ere to favour the solid-state reaction between HgO and precursors. Rec ently, a novel alternative approach essentially based on the oxygen an d mercury vapours reactivity for the synthesis of the superconducting phases has been successfully developed. Basically, solid precursors (p owders, 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 pre ssure 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 respe ctive 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 an d makes safer the preparation of these materials, especially in view o f applications.