6.1 General Conclusions
181
The first two models listed in Table 6.1 can be assessed purely from spectroscopic
data. This may prove useful for rapid screening of newly synthesised materials.
Unlike with the azide-based materials, consideration of the combination pathways
performed very poorly at predicting the impact sensitivity ordering for these materials. Instead, the sensitivity of these EMs appears to correlate best with the structure
of the doorway region, since:
(1) overtone up-pumping of the doorway region performed well on its own;
(2) The two-layered approach led to significant improvements over the pure
combination-based model.
While the model based on the overtone up-pumping of the doorway frequencies (the model chosen by Bernstein [2] and Coffey [1]) performed well, the most
successful model was that based on the temperature-dependent construction of the
two-layered model. The difference between the performance of the temperatureindependent and temperature-dependent two-layer models demonstrates the importance of considering more closely the rate rather than purely the number of uppumping pathways. This new two-layered model is constructed from a mixture of
the two independent models that have previously been proposed in the literature,
and therefore represents the first unified approach to predicting the relative impact
sensitivities of EMs.
The largest barrier to the successful application of this model is the simulation
of the full phonon dispersion curves, particularly for large organic EMs. It was
promising to find that (due to low vibrational dispersion) the same trends were
observed when only the zone-centre vibrational frequencies were used for the input.
Hence it was possible to add TATP (a highly sensitive material) to the model.
Applying both the overtone up-pumping model and the temperature-dependent twolayer model, TATP was successfully identified as being the most sensitive compound
in the test set.
In the final chapter of this thesis, the up-pumping model was tested on two HMX
polymorphs: δ- and β-HMX. The δ-form is well known to be more sensitive to
impact than the β-form. The calculated vibrational structure of δ-HMX was verified by comparison to inelastic neutron scattering spectroscopy. Consideration of
the overtone up-pumping (third model, Table 6.1) and the temperature-dependent,
two-layer model both suggested that the δ-form should be considerably more sensitive than the β-polymorph. This clearly demonstrated that the up-pumping models
are sensitive not only to different molecules, but also to the crystal structure. The
series of temperature-related FOX-7 polymorphs (α-, β- and γ-FOX-7) were therefore explored. As the structure becomes increasingly layered (α < β < γ) this
series offered an opportunity to explore why layered materials appear less sensitive
than non-layered materials. BAM fall hammer testing of γ-FOX-7 suggested that it
exhibited the same impact sensitivity as α-FOX-7. However, X-ray powder diffraction measurements showed that upon impact the γ-form transformed to the α-form,
and hence the impact sensitivity of the former could not be directly measured. The
full phonon-dispersion curves for the three polymorphs of FOX-7 showed that the
maximum frequency of the phonon bath ( max ) decreased with increased layering.
181
The first two models listed in Table 6.1 can be assessed purely from spectroscopic
data. This may prove useful for rapid screening of newly synthesised materials.
Unlike with the azide-based materials, consideration of the combination pathways
performed very poorly at predicting the impact sensitivity ordering for these materials. Instead, the sensitivity of these EMs appears to correlate best with the structure
of the doorway region, since:
(1) overtone up-pumping of the doorway region performed well on its own;
(2) The two-layered approach led to significant improvements over the pure
combination-based model.
While the model based on the overtone up-pumping of the doorway frequencies (the model chosen by Bernstein [2] and Coffey [1]) performed well, the most
successful model was that based on the temperature-dependent construction of the
two-layered model. The difference between the performance of the temperatureindependent and temperature-dependent two-layer models demonstrates the importance of considering more closely the rate rather than purely the number of uppumping pathways. This new two-layered model is constructed from a mixture of
the two independent models that have previously been proposed in the literature,
and therefore represents the first unified approach to predicting the relative impact
sensitivities of EMs.
The largest barrier to the successful application of this model is the simulation
of the full phonon dispersion curves, particularly for large organic EMs. It was
promising to find that (due to low vibrational dispersion) the same trends were
observed when only the zone-centre vibrational frequencies were used for the input.
Hence it was possible to add TATP (a highly sensitive material) to the model.
Applying both the overtone up-pumping model and the temperature-dependent twolayer model, TATP was successfully identified as being the most sensitive compound
in the test set.
In the final chapter of this thesis, the up-pumping model was tested on two HMX
polymorphs: δ- and β-HMX. The δ-form is well known to be more sensitive to
impact than the β-form. The calculated vibrational structure of δ-HMX was verified by comparison to inelastic neutron scattering spectroscopy. Consideration of
the overtone up-pumping (third model, Table 6.1) and the temperature-dependent,
two-layer model both suggested that the δ-form should be considerably more sensitive than the β-polymorph. This clearly demonstrated that the up-pumping models
are sensitive not only to different molecules, but also to the crystal structure. The
series of temperature-related FOX-7 polymorphs (α-, β- and γ-FOX-7) were therefore explored. As the structure becomes increasingly layered (α < β < γ) this
series offered an opportunity to explore why layered materials appear less sensitive
than non-layered materials. BAM fall hammer testing of γ-FOX-7 suggested that it
exhibited the same impact sensitivity as α-FOX-7. However, X-ray powder diffraction measurements showed that upon impact the γ-form transformed to the α-form,
and hence the impact sensitivity of the former could not be directly measured. The
full phonon-dispersion curves for the three polymorphs of FOX-7 showed that the
maximum frequency of the phonon bath ( max ) decreased with increased layering.