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Application of solid-phase transition kinetics to the properties of HMX

1983; American Institute of Aeronautics and Astronautics; Volume: 21; Issue: 2 Linguagem: Inglês

10.2514/3.8072

1533-385X

Richard J. Karpowicz, Larisa Gelfand, T. B. Brill,

Chemical Thermodynamics and Molecular Structure

Introduction T chemical HMX (octahydro-l,3,5,7-tetranitro1,3,5,7-tetrazocine) is an important nitramine monopropellant. Recently we have determined the pressuretemperature phase diagram for the conversion of the 0 polymorph to the 6 polymorph.' This phase transformation involves both a major disruption of the crystal lattice from the monoclinic to the hexagonal crystal system' and a ring conformation change of 0 (chair) to 6 (chair-chair) shown in Fig. 1. The volume expansion associated with the first-order 0—6 phase transition (the density is 1.90 g/cm for (3 and 1.78 g/cm for 6) will lead to strain and microfracturing in the crystallites. Hence, the mechanical properties and combustion characteristics of HMX could be affected by this transition. The phase transformation temperature depends on the applied pressure'' and the particle size of HMX.' At very high pressures where the temperature of the phase transition is high enough to produce considerable decomposition, the enthalpy change A// for the conversion is similar to the activation energy of decomposition. Hence, at high pressure, the 0-*6 transformation could be a source of fracturing and heat release in the thermal wave below a burning surface. Such events concern deflagration-to-detonation transitions and combustion modeling of HMX, particularly with larger particle sizes. Intimately coupled to the thermochemical analysis of the 0 — 6 transition is the kinetics of the process. While a number of thermochemical studies are now available,''' no kinetic data on this phase transition have been obtained. This void is undoubtedly due to the difficulty of measuring the rates of solid-solid transitions. In fact, the entire domain of firstorder solid-solid phase transitions in organic materials at elevated temperatures is virgin territory. We have determined the Arrhenius parameters by using Fourier transform/infrared (FT-IR) spectroscopy. Full reports describing the methodology of phase transition kinetics studies by FT-IR and the kinetic stabilities, applications, and significance of other HMX polymorphs appear elsewhere.' This Note deals with the relationship of the 0 —6-HMX solid-phase transition kinetics to propellants.