Insulating materials for super-conductors, their characteristics at cryogenic temperature

Abstract

Liquid nitrogen (LN/sub 2/) and cryogenic nitrogen (CGN/sub 2/) are found suitable not only as coolants but also as insulating materials for high critical temperature superconductors (HTSC). Experimental studies have been carried out to evaluate the dielectric strength of LN/sub 2/, under varying field conditions. Experiments were also performed with CGN/sub 2/ and solid insulting materials immersed in LN/sub 2/ using spherical electrodes. The breakdown strength of insulating materials has been found to increase significantly at cryogenic temperatures in comparison to atmospheric temperatures. The increase in breakdown strength of insulating materials like cotton tape, empire tape, Leatheroid paper, varnished paper, Presspahn, Bakelite sheet and mica is comparatively large with respect to materials like P.V.C., minimax and perspex. No apparent physical change was noticed in case of cotton tape, empire tape, Leatheroid paper, varnished paper, Presspahn, Bakelite sheet and mica, when immersed in LN/sub 2/ while minimax P.V.C. and perspex sheet got hardened and colour of rest pieces of these materials got faded till they remained immersed in LN/sub 2/.