106
3 Vibrational Up-Pumping: Predicting Impact Sensitivity of Some …
model, via compressibility and the capability of molecules to undergo necessary
geometric perturbations. This work therefore makes strides towards an overarching
understanding of initiation in energetic materials.
Despite its promise, the present model is based on idealised crystalline structures,
which do not exist in reality. The model therefore only offers insight into the intrinsic
potential of a material to initiate under mechanical perturbation. Further work will
be required to introduce non-ideal features, such as defects and surfaces. Further,
while the model here can justify the relative sensitivity of energetic materials, it is
not yet able to determine whether a material will be energetic in the first place. This
is to say, the model describes the propensity of a material to react under mechanical
perturbation, but it does not determine what that reaction will be. That aside, the
present contribution offers a powerful platform from which novel materials can be
designed in silico and offers novel insight into the structure-property relationships
of common energetic materials.
3.7 Suggestions for Future Work
The material presented in this chapter clearly identifies the up-pumping model as
a powerful tool to understanding the impact sensitivity of the crystalline energetic
azides. This offers a starting point for numerous follow-up investigations.
• The decomposition pathway of the azido anion within the crystal structure would
offer important validation of this model. While the CI was identified in the αNaN 3 lattice, it would be interesting to investigate how this translates across the
azides. In particular, how the PES associated with the bending mode changes as
the covalent bond character of the metal-anion increases. It will also be important
to further investigate the role of lattice-based eigenvectors in the reactivity of these
materials. As was demonstrated for α-NaN 3 , the highest frequency lattice mode
does permit formation of a reactive N
·
3 species. However, this pathway is unlikely
to be responsible for impact-induced initiation, given that α-NaN 3 is well known
to be insensitive to this form of mechanical stimulation.
• A lattice mode in α-NaN 3 was found to be sufficient to induce metallisation at
large eigenvector displacement. However, α-NaN 3 is not known to be sensitive to
impact. Further insight into why this vibrational mode does not lead to impactinduced initiation is therefore required.
• Investigating the initial stages of lattice excitation by dynamics simulations would
offer considerable insight into the up-pumping model. In particular, understanding
how the initial energy is inserted into the crystalline material, and how this initial
excitation varies as a function of material structural type.
• The model employed in the present contribution is based on T = 0 K. Hence, it does
not offer a mechanism for understanding the temperature dependence of impact
sensitivity. The introduction of temperature by means of thermal populations into
3 Vibrational Up-Pumping: Predicting Impact Sensitivity of Some …
model, via compressibility and the capability of molecules to undergo necessary
geometric perturbations. This work therefore makes strides towards an overarching
understanding of initiation in energetic materials.
Despite its promise, the present model is based on idealised crystalline structures,
which do not exist in reality. The model therefore only offers insight into the intrinsic
potential of a material to initiate under mechanical perturbation. Further work will
be required to introduce non-ideal features, such as defects and surfaces. Further,
while the model here can justify the relative sensitivity of energetic materials, it is
not yet able to determine whether a material will be energetic in the first place. This
is to say, the model describes the propensity of a material to react under mechanical
perturbation, but it does not determine what that reaction will be. That aside, the
present contribution offers a powerful platform from which novel materials can be
designed in silico and offers novel insight into the structure-property relationships
of common energetic materials.
3.7 Suggestions for Future Work
The material presented in this chapter clearly identifies the up-pumping model as
a powerful tool to understanding the impact sensitivity of the crystalline energetic
azides. This offers a starting point for numerous follow-up investigations.
• The decomposition pathway of the azido anion within the crystal structure would
offer important validation of this model. While the CI was identified in the αNaN 3 lattice, it would be interesting to investigate how this translates across the
azides. In particular, how the PES associated with the bending mode changes as
the covalent bond character of the metal-anion increases. It will also be important
to further investigate the role of lattice-based eigenvectors in the reactivity of these
materials. As was demonstrated for α-NaN 3 , the highest frequency lattice mode
does permit formation of a reactive N
·
3 species. However, this pathway is unlikely
to be responsible for impact-induced initiation, given that α-NaN 3 is well known
to be insensitive to this form of mechanical stimulation.
• A lattice mode in α-NaN 3 was found to be sufficient to induce metallisation at
large eigenvector displacement. However, α-NaN 3 is not known to be sensitive to
impact. Further insight into why this vibrational mode does not lead to impactinduced initiation is therefore required.
• Investigating the initial stages of lattice excitation by dynamics simulations would
offer considerable insight into the up-pumping model. In particular, understanding
how the initial energy is inserted into the crystalline material, and how this initial
excitation varies as a function of material structural type.
• The model employed in the present contribution is based on T = 0 K. Hence, it does
not offer a mechanism for understanding the temperature dependence of impact
sensitivity. The introduction of temperature by means of thermal populations into