The β–δ phase transition in the energetic nitramine-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine: Kinetics
- 22 August 2002
- journal article
- research article
- Published by AIP Publishing in The Journal of Chemical Physics
- Vol. 117 (8), 3789-3798
- https://doi.org/10.1063/1.1495399
Abstract
In this paper we present second harmonic generation(SHG)experiments designed to confirm the mechanism and quantify the transformation kinetics of the β–δ solid state phase transition in the organic nitramine molecule octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The β phase adopts a centrosymmetric crystallographic configuration (P2 1 /c) while the δ phase adopts a noncentrosymmetric one (P6 1 (P6 5 )). As expected, this results in a very poor generation of SHG intensity from the β phase, while the δ phase is very efficient, rivaling KH 2 PO 4 in absolute efficiency. SHG thus provides a very high sensitivity zero background probe of the δ phase. We discuss the use of this signal as a quantitative measure of the δ phase mole fraction in ensembles of free HMX crystals and crystals embedded in a visco–elastic polymer matrix. We report imaging experiments where the spatial characteristics of the transformation are shown to be consistent with nucleation from a low density of initial sites, followed by rapid growth. We also report experiments where the total integrated SHG is measured and used to infer the transition progress as a function of time in a series of isothermal experiments on both β–δ conversion and δ–β reversion. Additionally, reversibility experiments are reported which are used to verify both the volumetric mechanism of SHG generation in this system and the independence of these results to the internal stress state of the polycrystalline samples. We compare the measured SHG intensity as a function of time for a range of temperatures with predictions of the two state kinetic model presented in the accompanying paper [B. F. Henson et al., J. Chem. Phys. 117, 3780 (2002)]. We perform a set of parameter optimization calculations based on agreement with the predictions of the model. Optimization does not significantly change the kinetic parameters that are thermodynamically constrained by the model, but there is a distribution of parameters necessary to reproduce the nucleation kinetics observed. In particular, a striking difference in nucleation kinetics is observed between samples of free crystals and crystals embedded in a visco–elastic polymer matrix.Keywords
This publication has 10 references indexed in Scilit:
- The β–δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine: ThermodynamicsThe Journal of Chemical Physics, 2002
- Dynamic Measurement of the HMX-Phase Transition by Second Harmonic GenerationPhysical Review Letters, 1999
- Imaging nanometer-thick patterned self-assembled monolayers via second-harmonic generation microscopyJournal of Applied Physics, 1997
- Theory of second harmonic generation in randomly oriented speciesChemical Physics, 1995
- Molecular Theory of Harmonic GenerationAdvances in Chemical Physics, 1993
- Theory of second-harmonic generation in strongly scattering mediaPhysical Review B, 1991
- Solid phase transition kinetics. The role of intermolecular forces in the condensed-phase decomposition of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocineThe Journal of Physical Chemistry, 1982
- The crystal structure of the δ-form of 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (δ-HMX)Acta Crystallographica Section B: Structural Science, Crystal Engineering and Materials, 1974
- A Powder Technique for the Evaluation of Nonlinear Optical MaterialsJournal of Applied Physics, 1968
- The crystal structure of α-HMX and a refinement of the structure of β-HMXActa Crystallographica, 1963