Room temperature superconductors determination

Gloom for Oxide Superconductors Dismayed at the progress through the years, even with the most promising room-temperature metallic, binary oxides, many scientists abandoned the search for new high temperature oxide superconductors. Also, it should be mentioned that a deep-rooted prejudice had developed which claimed that the BCS theory had imposed a maximum transition temperature limit of 25 K for all superconducting materials, and that this temperature had already been achieved in certain alloys of niobium. Some scientists, however, were steadfast in their determination to break this barrier, optimistic in their outlook, and they continued their search for this unusual phenomenon in other metallic oxide systems. 

A determined search for superconductivity in metallic oxides was initiated in mid-summer of 1983 at the IBM, Zurich Research Laboratories in Riischliken, Switzerland. This research effort was an extension of previous work (145) on oxides, namely, Sr1.xCaxTiOs, which exhibited some unusual structural and ferro-electric transitions (see Section 2.2a). During the summer of 1983, the superconductivity research was focussed on copper-oxide compounds. Muller had projected the need for mixed Cu2+/Cu3+ valence states, Jahn-Teller interactions (associated with Cu2+ ions), and the presence of room temperature metallic conductivity to generate good superconductor candidates. These researchers then became aware of the publication by Michel, Er-Rakho, and Raveau (146) entitled ... 

The structures that have been determined so far include those of the pure, electrically insulating Qo (ref. 5) and the heavily doped KfcCfto (ref. 6). At room temperature, solid forms a face-centred cubic (f.c.c.) lattice with 10.0 A intercluster separ-ation KeCftQ has a body-centred cubic structure, with K. atoms in distorted tetrahedral sites. No superconductivity was observed in that material. Cs6Cso has the same structure. We have found that the superconductor K3Q0 has a f.c.c. structure derived from that of C o by incorporating K ions into all of the octahedral and tetrahedral interstices of the host lattice. 

In order to realize the potential of these ceramics they must be fabricated, and with such a low decomposition temperature sintering to high strength is proving a problem. Hardness determinations will prove useful in characterizing the mechanical properties of these and other ceramic superconductors, but as yet little has been reported. Table 6.21 contains hardness values obtained for the YBa2Cu307 superconductor at room temperature and at liquid Nj temperatures (77 K). As expected, the Vickers hardness rises to 3.1 GPa at 77 K and some success with sinter additives is achieved because the standard material hardness is raised from 2.2 to 2.5 GPa after sintering in their presence. Indentation hardness techniques have been used to establish the Kic value of 1.1 MPam. ... 

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