Cryogenic Materials Series – serie

This was the third meeting in the series of special topical conferences on Non-Metallic materials at low temperatures. The first meeting was in Munich in 1978, the second in Geneva (1980) and so Heidelberg 1984 seemed an obvious time to review some of the hopes and objectives of the earlier meetings. It is also appropriate to consider the changing needs of the cryogenic community and how best the theory and practice of Non-metallic materials can be applied to suit this dynamic young science. The aims and objectives of the International Cryogenic Materials Board in sponsoring this meeting remain the same. Namely, to provide a forum where practicing Engineers can meet with materials suppliers and researchers in an attempt to ensure that a real understanding exists between the two sides of the Cryogenic Materials Community. In this atmosphere, real problems can be addressed together with full discussions of tried and tested practical solutions. It is in this way that knowledge and confidence may grow hand in hand with the logical growth of the industry.

This, the second special topical conference on the properties of Non-Metallic Materials at Low Temperatures, was sponsored by the International Cryogenic Materials Conference Board. The potential for plastics materials in the field of cryogenics is vast and as yet only partly explored. In addition, many other materials, which qualify for the title non-metallic but are not 'plastics', have numerous possible outlets in low temperature technology. This conference aimed at providing a forum, whereby specialists from Industry, the Universities and from Government sponsored Institutions could assemble to discuss the extent of our current knowledge. As it transpired, the meeting was also to high­ light the considerable gaps that still exist in our fundamental understanding of the low temperature behaviour of these materials. On this theme, during the course of the conference, a reference was made to an almost forgotten quotation by Lord Kelvin, who said: "When you cannot measure what you are speaking about, when you cannot express in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the stage of a science, whatever the matter be." This simple statement sums up the aims, objectives and hopefully the achievements of this conference. To discuss and disseminate the current knowledge on non-metallic materials in order that realistic predictions of in-service performance may be made.

The need for alternate energy sources has led to the develop­ ment of prototype fusion and MHD reactors. Both possible energy systems in current designs usually require the use of magnetic fields for plasma confinement and concentration. For the creation and maintenance of large 5 to 15 tesla magnetic fields, supercon­ ducting magnets appear more economical. But the high magnetic fields create large forces, and the complexities of the conceptual reactors create severe space restrictions. The combination of re­ quirements, plus the desire to keep construction costs at a mini­ mum, has created a need for stronger structural alloys for service at liquid helium temperature (4 K). The complexity of the required structures requires that these alloys be weldable. Furthermore, since the plasma is influenced by magnetic fields and since magnet­ ic forces from the use of ferromagnetic materials in many configur­ ations may be additive, the best structural alloy for most applica­ tions should be nonmagnetic. These requirements have led to consideration of higher strength austenitic steels. Strength increases at low temperatures are achieved by the addition of nitrogen. The stability of the austenitic structure is retained by adding manganese instead of nickel, which is more expensive. Research to develop these higher strength austenitic steels is in process, primarily in Japan and the United States.

Cryogenics is an emerging technology filled with promises. Many cryogenic systems demand the use of nonmetallics and composites for adequate or increased performance. Thermal and electrical insulations, potting for superconducting magnets' mechanical sta­ bility, and composite structures appear to be some of the most significant applications. Research on nonmetallics at cryogenic temperatures has not progressed to the degree of research on metals. Nor can room temperature research be extrapolated to low tempera­ tures; most polymers undergo a phase transformation to the glassy state below room temperature. Research by producers, for the most part, has not been prac­ tical, because, except for LNG applications, the market for large material sales is not imminent. There are, however, many government­ stimulated developmental programs. Research on nonmetallics thus is dictated by development project needs, which require studies orien­ ted toward prototype hardware and specific objectives. As a result, research continuity suffers. Under these conditions, periodic topical conferences on this subject are needed. Industry and uni­ versity studies must be encouraged. Designers and project research material specialists need to exchange experiences and data. Low­ temperature-oriented research groups, such as the National Bureau of Standards and the Institute for Technical Physics - Karlsruhe, must contribute by assisting with fundamentals, interpreting proj­ ect data, and contributing to project programs through their materials research.

Compound superconductors with the Al5 structure carry the promise of the second generation in practical superconductivity. They will provide higher operating magnetic fields at higher tem­ peratures than the preceding alloy superconductors. To fulfill this promise, their brittle nature must be accommodated in a fila­ mentary structure. Achieving this has been no simple task and imaginative research and clever production techniques have led to many usable conductor configurations. In addition, several new and exciting possibilities are being proposed; for example, in situ processing promises easier production with improved strain toler­ ance. It is timely, therefore, to take measure of what we have achieved and to assess our understanding so that we may choose, with some confidence, paths for future research and potential ap­ plications. To meet these needs, the International Cryogenic Materials Conference Board has sponsored this special topic conference on Filamentary Al5 Superconductors bringing together superconductiv­ ity researchers, superconducting wire producers, and high field magnet users to discuss current research problems. That the in­ formation exchange was intensive and successful is evidenced by the excellent papers in this volume. In order to capture some of the synergistic wisdom generated in discussions at the conference several people were asked to assemble and interpret the comments and concerns of the fusion, high energy physics, solid state phys­ ics, and metallurgy groups.