T NP was then set to 1500 K, each NP was monitored briefly in argon to establish the baseline rate of mass change (near zero in all cases), and then at time = zero, O 2 was added to the trap atmosphere at a constant pressure (P O2 = 4.3 x 10 -5 Torr for graphite, Si, and Hf; 2.25 x 10 -5 Torr for carbon black), and the NP mass was monitored at 1500 K for up to 10 hours to follow the evolution of the kinetics.