Superconducting materials shaping

Direct transport measurements are generally employed on thin films, tapes and coated conductors. They quickly reach their natural limits as the currents go up and/or the cross-section of the conductor increases. They are fast, i.e., can be started immediately after the magnetic field is set, and trace the I-V characteristics, which is of course most valuable if flux creep or thermally assisted flux flow (Kes et al. 1989) prevails, because the electric field is directly determined. On the other hand, must be defined by a criterion e.g., 1 [xV/cm. It will be affected by the presence of stabilising materials, the silver tube or conducting substrates, and thus generally refers to the overall Jc, wfrich can be converted to the critical current density of the superconductor if the shape and the volume fraction of the superconducting material are known. 

Five groups of materials based on Ti may be distinguished [1.51-53] commercially pure (i. e., commercially available) Ti (cp-Ti), low-alloy Ti materials, Ti-base alloys, intermetallic Ti-Al materials, and highly alloyed functional materials TiNi shape memory alloys, Nb-Ti superconducting materials (Sect. 4.2.1), and Ti-Fe-Mn materials for hydrogen storage. 

To prepare high-frequency (hf) structures from niobium, massive turned bars are usually used [1]. The drawback to this process are great losses of the superconducting material and, consequently, high cost of the article. Moreover, it is difficult to produce intricately shaped structures by this method. 

In earlier works [2,3] it was shown that electrodeposition combined with galvanoplastics can be used to make monophase high-purity NbsSn coatings on intricately shaped articles with low surface roughness, which meet on the whole the requirements placed upon the hf structures. The main demands for the superconducting material for a hf cavity have been discussed in detail [4j. 

Other uses are as superconductive materials of TiNb, the shape memory alloy of Ti-Ni, the hydrogen occlusion alloy of Ti—Fe, and in computer equipment as nonmagnetic substance, artificial bones, dental roots, cardiac valves and cardiac pacemakers as nontoxic and biocompatible materials [3,5]. 

Yttrium—barium—copper oxide, YBa2Cu202 is a newly developed high T material which has been found to be fully superconductive at temperatures above 90 K, a temperature that can be maintained during practical operation. The foremost challenge is to be able to fabricate these materials into a flexible form to prepare wines, fibers, and bulk shapes. Ultrapure powders of yttrium—barium—copper oxide that are sinterable into single-phase superconducting... 

In some cases the stability of HTSC materials in contact with electrolytes is quite satisfactory, so they can be used for electrochemical measurements. Such measurements are made for various reasons. A number of workers have used cyclic voltam-mograms to characterize the state of HTSC materials. A constant shape of these curves over a certain length of time was evidence for conservation of the superconducting state during this time interval. 

Technology as we have witnessed over the years has been outpacing any other development. Some of the developments in technology such as co-extrusion in copper, sintered material objects of required shapes, use of thin-film techniques to produce a tape for electrical connections and the fabrication of tapes and wires of superconductor Nb3Sn lend hope for future technological developments in superconducting rare earth materials such as borocarbides and YBaaCujOy. 

There s also the possibility that the silver mixture will allow the superconducting proximity effect to occur. (Superconductivity can occur between two superconductors that are physically separated so long as the barrier is a conducting metal.) That tunneling effect may be the key to developing materials that can be machined, drawn into wires, or cast into various production shapes. Preliminary experiments have shown that the new silver composite is quite strong and can be machined into several useful forms. 

A crucial concern in the application of superconducting ceramics is to devise ways to fabricate the new materials in desired shapes such as wires. This will be quite a challenge because these superconductors are ceramics and have the brittleness and fragility typical of ceramic materials. 

---