The effect of a detonation nanodiamond coating on the thermal decomposition properties of RDX explosives

A well-dispersed and uniformly shaped detonation nanodiamond (DND) was produced and coated over micron scale RDX in various amounts to form four kinds of DND coating composites (NDRs). In order to confirm the optimal coating amount and its effect on the thermal properties, the thermal decomposition and kinetics were studied by DSC, TG and DPTA techniques. The critical temperature of thermal explosion (T_b) and the self accelerating decomposition temperature (T_SADT) both exhibit an interesting volcano-shaped changing trend and rank in an increasing order of NDR4 < NDR1 < RDX < NDR3 < NDR2. This indicates that the DND coating amount, ranging from 1/7 to 1/5, provides NDRs with better thermal safety than RDX. The thermolytic kinetic parameters (E_a and A) and activation thermodynamic parameters (ΔS^≠, ΔH^≠ and ΔG^≠) are sorted in the following order: NDR1 < NDR4 < NDR2 < NDR3. The gas emission and reaction rate constant of the initial thermal decomposition have the same order. The results show that the DND coating could improve the reactivity of the NDRs and the effect is proportional to the coating amount. However, excessive coating that is more than 1/3 conversely hinders decomposition and gas diffusion, like a layer of protective shell. The isoconversional activation energy (E_a) varies with the conversion extent (α) at the initial stage of α = 0.1-0.5, which indicates that the thermal decomposition of the NDRs is a multi-step process including the secondary reaction or catalytic reaction. However, the E_a values are almost independent of α when α = 0.6-0.9, with the mean values in an increasing order of NDR1 < NDR4 < NDR2 < NDR3.