complete phonon dispersion curves, the various pathways to vibrational energy
up-pumping were explored, namely via overtone and combination pathways. In
particular, the relative rates of up-pumping into the N
À
3 bending mode were
investigated. Remarkable agreement is found between these up-pumping rates and
the relative ordering of the impact sensitivity of these azides.
The calculated vibrational structures of organic molecular EMs were first compared
with experimental inelastic neutron scattering spectra and found to provide accurate
representation of the low-temperature vibrational structure of these complex crystals.
The decomposition pathways for organic EMs are not known, and hence, no target
frequency could be unambiguously identified. Instead, the up-pumping model was
developed for these materials by investigating the total rate of energy conversion into the
internal vibrational manifold. A number of qualitative trends were identified, which may
provide a mechanism for the rapid classification of EMs from limited vibrational
information. The overtone pathways were found to offer a good agreement with
experimental impact sensitivities of these compounds. However, the increased complexity of the vibrational structure of the organic EMs as compared to the azides required
a more thorough treatment of the up-pumping mechanism to correctly reflect experimental sensitivities. The effects of temperature on up-pumping were also explored.
The sensitivity of organic EMs is known to differ across polymorphic forms.
Most notable are the HMX polymorphs. The calculated vibrational structure of two
HMX polymorphs was confirmed by inelastic neutron scattering spectroscopy. The
up-pumping model developed for molecular organic EMs was therefore extended to
a comparison of these two HMX polymorphs. The polymorphic forms of FOX-7
were also investigated under the premise of the up-pumping model. Upon heating,
FOX-7 undergoes two polymorphic transformations, which increases the layering
of the materials. It therefore offered an opportunity to explore the widely held
hypothesis that layered materials are less sensitive than non-layered materials. The
metastable c-form was successfully recovered, and its experimental impact sensitivity investigated by the BAM drop-hammer method. However, upon impact, the
c-polymorph appeared to convert to the a-form and initiate at the same input
energy. Hence, a considerable deficiency of experimental methods is identified
when studying polymorphic materials. FOX-7 was therefore explored within the
framework of the up-pumping model. The inelastic neutron scattering spectrum was
collected for c-FOX-7, which confirmed the calculated vibrational structure. It was
shown that within the up-pumping model, the layered c-polymorph is predicted to
be less sensitive than the a-form and results from a decrease in the maximum
phonon-bath frequency. Hence, a new mechanism is proposed to describe the
insensitivity of layered compounds.
The work presented in this thesis explores the applications of vibrational
up-pumping to rationalize and predict the relative impact sensitivities of a range of
EMs. Despite the approximations employed in construction of the model, it leads to
excellent correlation with experimental results in all cases. This work therefore
opens the door to a new fully ab initio approach to designing new EMs based solely
on knowledge of the solid-state structure.
viii
Abstract
up-pumping were explored, namely via overtone and combination pathways. In
particular, the relative rates of up-pumping into the N
À
3 bending mode were
investigated. Remarkable agreement is found between these up-pumping rates and
the relative ordering of the impact sensitivity of these azides.
The calculated vibrational structures of organic molecular EMs were first compared
with experimental inelastic neutron scattering spectra and found to provide accurate
representation of the low-temperature vibrational structure of these complex crystals.
The decomposition pathways for organic EMs are not known, and hence, no target
frequency could be unambiguously identified. Instead, the up-pumping model was
developed for these materials by investigating the total rate of energy conversion into the
internal vibrational manifold. A number of qualitative trends were identified, which may
provide a mechanism for the rapid classification of EMs from limited vibrational
information. The overtone pathways were found to offer a good agreement with
experimental impact sensitivities of these compounds. However, the increased complexity of the vibrational structure of the organic EMs as compared to the azides required
a more thorough treatment of the up-pumping mechanism to correctly reflect experimental sensitivities. The effects of temperature on up-pumping were also explored.
The sensitivity of organic EMs is known to differ across polymorphic forms.
Most notable are the HMX polymorphs. The calculated vibrational structure of two
HMX polymorphs was confirmed by inelastic neutron scattering spectroscopy. The
up-pumping model developed for molecular organic EMs was therefore extended to
a comparison of these two HMX polymorphs. The polymorphic forms of FOX-7
were also investigated under the premise of the up-pumping model. Upon heating,
FOX-7 undergoes two polymorphic transformations, which increases the layering
of the materials. It therefore offered an opportunity to explore the widely held
hypothesis that layered materials are less sensitive than non-layered materials. The
metastable c-form was successfully recovered, and its experimental impact sensitivity investigated by the BAM drop-hammer method. However, upon impact, the
c-polymorph appeared to convert to the a-form and initiate at the same input
energy. Hence, a considerable deficiency of experimental methods is identified
when studying polymorphic materials. FOX-7 was therefore explored within the
framework of the up-pumping model. The inelastic neutron scattering spectrum was
collected for c-FOX-7, which confirmed the calculated vibrational structure. It was
shown that within the up-pumping model, the layered c-polymorph is predicted to
be less sensitive than the a-form and results from a decrease in the maximum
phonon-bath frequency. Hence, a new mechanism is proposed to describe the
insensitivity of layered compounds.
The work presented in this thesis explores the applications of vibrational
up-pumping to rationalize and predict the relative impact sensitivities of a range of
EMs. Despite the approximations employed in construction of the model, it leads to
excellent correlation with experimental results in all cases. This work therefore
opens the door to a new fully ab initio approach to designing new EMs based solely
on knowledge of the solid-state structure.
viii
Abstract