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Thermal Decomposition of Energetic Materials: Temporal Behaviors of the Rates of Formation of the Gaseous Pyrolysis Products from Condensed-Phase Decomposition of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine
Behrens, Richard
, p. 6706 - 6718 (2007/10/02)
The temporal behaviors of the rates of formation of the gaseous products from the pyrolysis of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) are determined by simultaneous thermogravimetric modulated beam mass spectrometry (STMBMS).The gaseous products formed from the pyrolysis of HMX, and its deuterium- and 15NO2-labeled analogues, between 210 and 235 deg C are H2O, HCN, CO, CH2O, NO, N2O, CH3NHCHO, (CH3)2NNO, 1-nitroso-3,5,7-trinitro-1,3,5,7-tetrazocine (ONTNTA), and their isotopic analogues.In addition, a nonvolatile residue (NVR) is formed.The NVR is a polyamide that decomposes between 250 and 780 deg C producing NH3, HCN, HNCO, H2NCHO, CH3NHCHO, and products that include long-chain hydrocarbons.The temporal behaviors of the rates of gas formation of the pyrolysis products along with the macroscopic and microscopic structure of the NVR indicate that complex physical processes and chemical mechanisms within the condensed phase of HMX control the decomposition.The temporal behaviors of the rates of gas formation show induction, acceleratory, and decay stages that are characteristic of either condensed-phase or autocatalytic decomposition.The NVR is composed of broken ellipsoidal shells whose diameters range from 0.3 to 5 μm.The shells appear to be remnants of gas-filled bubbles that are formed within the HMX particles during the pyrolysis.The pressures within the gas bubbles may exceed 7 MPa.The major portion of the pyrolysis products observed in the experiments comes from gases that have been contained within the bubbles.Gas released earlier in the decomposition are contained in smaller bubbles and therefore are formed under higher pressure conditions.This results in the variation of the relative rates of formation of the gas products.For example, the ratios of the rates of formation of CH2O to N2O is between 0.7 and 0.8 during the induction stage, decreases to a minimum of 0.36 during the acceleratory stage, and increases to ca. 1 at the end of the decay stage.Variations in the relative rates of formation of the other products are also observed.The changes in the rates of release of products from the initial to final stages of the decomposition indicate that different species are formed when the pyrolysis products are contained at different pressures.The effects of these different physical processes on the relation between the observed pyrolysis products and the underlying chemical decomposition mechanisms are discussed.
