Helium permeation in composite materials for cryogenic application

Glass fibre composites are attractive alternatives to metals for cryogenic applications because of their high specific strength and stiffness and their low electrical and thermal conductivity. A disadvantage is the ease of permeability of He through the material at room temperature, and after damage accumulation at 4.2 K. For this study, a special experimental leak detector was built based on a controlled helium flow through a diaphragm and mass spectroscopic measurement. Temperature cycles between room temperature (RT) and 77 K have no effect on permeation rates at 300 K. Tensile tests producing damage at room temperature showed different effects on permeation flow of He, caused by cracking. At 4.2 K, the limit for using this material is given by matrix cracking. The glass fibre volume fraction is preponderant in controlling the coefficient of He permeation. It has been found that composite materials such as glass fibre reinforced epoxy can be used effectively for cryogenic applications using a vacuum as thermal insulation. (C) 1998 Elsevier Science Ltd. All rights reserved.