The influence of pitch-binder coke content on the properties and irradiation behavior of molded graphite

A series of needle-coke graphites were prepared with various fractions of pitch-binder coke in the range 5.5–14.2 wt-%. The specimens were irradiated to 9.70 × 10²¹n/cm² (E > 0.18 MeV) at 1225°C. Apparent bulk density and apparent crystallite size L_c were dependent on the binder-coke content. Crystallite orientation and thermal expansivities were not affected. Dimensional and volumetric changes were sensitive to binder-coke content; specimens deficient in binder-coke showed volumetric turnaround at lower fluences and higher expansion rates at high fluences. Specimens of 5.5 wt-% binder coke expanded 20 vol-%, whereas those of 11.7 wt-% binder coke expanded only 4 vol-%. Thermal expansivity increases closely followed the volume changes and were large in specimens deficient in binder coke. Unbonded coke particles expanded at three times the rate of coke particles bonded with pitch. The needle-coke particles were restrained by the binder coke during expansion at high fluences. Large cracks formed between filler particles during expansion and were responsible for the large volume expansions.