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3 Vibrational Up-Pumping: Predicting Impact Sensitivity of Some …
3.2 Aims
Simple inorganic azide energetic materials display a huge variation in impact sensitivity behaviour, and development of new azide-based materials is of high interest
to the energetics community. A physical basis for their initiation has not yet been
elucidated, although some qualitative trends, including bond lengths, cation ionization potentials and symmetry breaking of azide vibrational modes with energetic
behaviour have been noted [37, 38]. More physical models have also been proposed,
noting that crack propagation rates may be linked to sensitivity [15], based on formation of hot-spots at crack tips or deformation pile-ups. However, no unified mechanism has yet been proposed to explain the sensitivity relationships observed for
these compounds. The sensitivity of energetic materials is a complex phenomenon,
and the underlying mechanism can differ. For example, initiation may occur due
to hot-spot formation in a bulk composition, or at extended defects within single
crystallites (Chap. 1.2). Which mechanism dominates (and thus the type and magnitude of hot-spot that forms) depends on the nature of the prepared sample and is
largely irreproducible. Regardless of the mechanism by which the energy is generated, its localisation can be sought in terms of the vibrational up-pumping mechanism
(Chap. 1.2.2). At the most fundamental level is the intrinsic sensitivity of a material.
This describes the propensity of the ideal material (i.e. defect-free) to react under
mechanical perturbation and will form the basis for the work presented here.
This chapter aims to build a model for ideal crystalline materials, based on a
vibrational up-pumping approach, in order to rationalise and predict the relative
sensitivity ordering of a test set of azide-based EMs. To that end, the work presented
here sought to:
1. Identify a vibrational mode (target mode, Q T ) that is responsible for the initial
decomposition of the explosophoric azido anion.
2. Investigate the pathways to vibrational up-pumping.
3. Correlate the relative rate of vibrational up-pumping to impact sensitivity.
3.3 Test Set of Energetic Azides
The model presented here is constructed from a selected series of crystalline energetic azides, Table 3.1 and Fig. 3.2, selected to cover a range of reported experimental sensitivities and cover the three main structural types. Of the ionic species
selected, two are based on molecular cations: triaminoguananidinium azide (TAGZ)
and ammonium azide (NH 4 N 3 ). The experimental measurement of EM sensitivity is
highly unreliable, with many conflicting reports in the literature [39]. In many cases,
conflicting reports are due to crystal size, purity, defect concentration, as well as both
environmental and experimental conditions [40–42].
Literature discrepancies are particularly prevalent across the azide materials, most
notably for the ordering of the more sensitive materials. Due to these large discrepancies, exact values are not quoted in developing the model in this chapter, but instead
3 Vibrational Up-Pumping: Predicting Impact Sensitivity of Some …
3.2 Aims
Simple inorganic azide energetic materials display a huge variation in impact sensitivity behaviour, and development of new azide-based materials is of high interest
to the energetics community. A physical basis for their initiation has not yet been
elucidated, although some qualitative trends, including bond lengths, cation ionization potentials and symmetry breaking of azide vibrational modes with energetic
behaviour have been noted [37, 38]. More physical models have also been proposed,
noting that crack propagation rates may be linked to sensitivity [15], based on formation of hot-spots at crack tips or deformation pile-ups. However, no unified mechanism has yet been proposed to explain the sensitivity relationships observed for
these compounds. The sensitivity of energetic materials is a complex phenomenon,
and the underlying mechanism can differ. For example, initiation may occur due
to hot-spot formation in a bulk composition, or at extended defects within single
crystallites (Chap. 1.2). Which mechanism dominates (and thus the type and magnitude of hot-spot that forms) depends on the nature of the prepared sample and is
largely irreproducible. Regardless of the mechanism by which the energy is generated, its localisation can be sought in terms of the vibrational up-pumping mechanism
(Chap. 1.2.2). At the most fundamental level is the intrinsic sensitivity of a material.
This describes the propensity of the ideal material (i.e. defect-free) to react under
mechanical perturbation and will form the basis for the work presented here.
This chapter aims to build a model for ideal crystalline materials, based on a
vibrational up-pumping approach, in order to rationalise and predict the relative
sensitivity ordering of a test set of azide-based EMs. To that end, the work presented
here sought to:
1. Identify a vibrational mode (target mode, Q T ) that is responsible for the initial
decomposition of the explosophoric azido anion.
2. Investigate the pathways to vibrational up-pumping.
3. Correlate the relative rate of vibrational up-pumping to impact sensitivity.
3.3 Test Set of Energetic Azides
The model presented here is constructed from a selected series of crystalline energetic azides, Table 3.1 and Fig. 3.2, selected to cover a range of reported experimental sensitivities and cover the three main structural types. Of the ionic species
selected, two are based on molecular cations: triaminoguananidinium azide (TAGZ)
and ammonium azide (NH 4 N 3 ). The experimental measurement of EM sensitivity is
highly unreliable, with many conflicting reports in the literature [39]. In many cases,
conflicting reports are due to crystal size, purity, defect concentration, as well as both
environmental and experimental conditions [40–42].
Literature discrepancies are particularly prevalent across the azide materials, most
notably for the ordering of the more sensitive materials. Due to these large discrepancies, exact values are not quoted in developing the model in this chapter, but instead