ntaining unit coated with a graphite fabric impregnated with a polyamide-imide of the following structure: ##STR16## and units of: ##STR17## wherein one carbonyl group is meta to and one carbonyl group is para to each amide group and wherein Z is a trivalent benzene ring or lower-alkyl-substituted trivalent benzene ring, R1 and R2 are different and are divalent aromatic hydrocarbon radicals of fro