when the fibers reach a density of at least 1 35 g cm 3 the precursor fibers usually have a density of about 1 18 g cm 3 the cyclized intermediate fibers may be converted to carbon or graphite fibers by methods known in the art e g heating the intermediate fibers in an inert gas at 800 u00b0 to 1500 u00b0 c or higher for a short period of time carbon fibers will have a density of at least 1 70 g cm 3 in the carbonization step the atmosphere must be non oxidizing nitrogen or argon are preferred media the cyclization of the fiber is usually carried out at constant fiber length or slight draw by application of tension as is known in the art thermal cyclization of acrylic fiber becomes runaway unless slow heating rates are employed the present invention allows faster cyclization to be achieved by use of selected ammonium salts which reduce the heat surges differential scanning calorimetry a thermal analyzer du pont 1090 thermal analyzer is used to provide the dsc scans in each case a measured weight of fiber was inserted in the sample holder sealed in air and heated under a flow of nitrogen at the rate indicated in the figures the rate of heat evolution is recorded at the various temperatures the sample weights were normalized to permit comparison heat flux index refer to fig5 a base line is drawn for the dsc scan and the height of the highest peak of heat flux above the base line is measured the sides of the highest peak are extended to the.