Supercritical Fluid Phase Separations Induced by Chemical Reactions
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chemically reactive systems to examine a possible supercritical fluid phase separation and its effect on post-detonation product mixtures. In this regard we have earlier used the statistical mechanical chemical equilibrium (CHEQ) code1 to investigate the occurrence of supercritical fluid phase separations in binary and ternary mixtures of N 2 , CO 2 , and H 2 0 molecules and in detonation products of a condensed explosive, PBX-9404. 2 The most significant result of our analysis was that the N2-H 20 system may exhibit a supercritical fluid phase separation in the same (T, P) range as we encounter in explosives experiments. This prediction has been validated by diamond-anvil-cell experiments up to P less than 3 GPa and T less than 1000 K.2 The most significant practical result of such a prediction for explosives is that, by incorporating the N2-fluid phase separation in modeling the detonation of high explosives, we obtain better agreement with the experimental detonation data. Most modern high explosives are "composites" with two main components; explosive molecules and organic binders to hold, mold, and pack the explosive powders. Recently, binders containing fluorine atoms are drawing interest for the safety reason. Typical ones are Kel-F with polymeric composition (C8H 2Cl 3F1 I)n and viton A with composition (C5H 3.5F6.5)n . LX-17 uses the KelF binder to mold high explosive, TATB (= C 6 H6N6 0 6 ), while LX-04 uses the viton A binder to mold HMX (= CAH 8N8 OA). The former is insensitive and the latter more sensitive. Results presented below show that the fluorine chemistry can critically influence the supercritical 127 Mat. Res. Soc. Symp. Proc. Vol. 499 © 1998 Materials Research Society
fluid phase separation between the N2-rich and the N2 -poor phases in detonation product mixtures and that it in turn affects the detonation properties of LX- 17 and LX-04. FORMULATION
Theory LX-17 is a composite explosive with C2 .29H2.18N 2.1502. 15F 0.2CI0. 054 per 100 g, while 100 g of LX-04 is represented by C 1.5 5H 2 .58N 2 .30 0 2 .30 F0 .52. The amount of Cl in LX-17 is small and will be neglected in the present work. We assume that these explosives produce gaseous species (N2 , C0 2, CO, H20, H2 , NH 3, 02, NO, etc.) and condensed carbon in a graphite, diamond, or liquid phase. The Gibbs free energy of this heterogeneous system can be obtained as a sum of the Gibbs free energies for the gaseous mixture phase and the condensed carbon phases. The Gibbs free energy for carbon has been discussed in detail and will not be repeated here.3 We sketch below only the pertinent portions of the fluid-phase Gibbs free energy. We use exp-6 potentials to represent interactions between a (i, j) pair of detonation product species in a fluid mixture; 0 ij =
{6 exp[taij(1-r/rij )]-a(rij/r)6}.(1 E// a/j- 6 i rr).
For polar molecules such as H 20 and HF, the well depth Eis made temperature dependent by introducing an additional parameter X, (2) E= C0 (l + /T), which describes the increasingly attractive hydrogen-bonding character of t
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