5.5 Conclusions
175
5.5 Conclusions
Polymorphism is very prevalent amongst energetic materials, and can lead to drastic
changes in a material’s sensitivity to impact. Most notable are the δ- and βpolymorphs of HMX. The former has been reported to be as sensitive to impact as a
primary explosive material, while the latter exhibits much lower sensitivity to impact.
Application of the up-pumping model was able to reproduce these experimental
findings and assessed δ-HMX as being a highly sensitive material. This therefore
demonstrates that the up-pumping model is sensitive to polymorphic modifications.
It was therefore applied to a second polymorphic energetic material, FOX-7.
Under ambient conditions, FOX-7 exists in the α-form, which adopts a
herringbone-type structure. When heated, these layers flatten, and are nearly flat
in the γ-form. This form was recovered to ambient conditions and its impact sensitivity measured using a BAM fall hammer. This suggested that the layered γ-form had
the same impact sensitivity as the α-form, despite the general principle that layered
materials are insensitive. X-ray powder diffraction, however, revealed that the γform undergoes transformation to the α-form on impact, and hence it is not known
which polymorphic phase was in fact tested. The mechanism for this impact-induced
transformation is not yet known, and may be the result of pressure, temperature or
their combination. The up-pumping model was therefore applied to the FOX-7 polymorphs. Both the overtone-based model and the temperature-dependent two-layered
model suggested that the layered γ-form should be notably less sensitive than the
α-form. It is suggested that the reduction in sensitivity is the result of a decrease in
max (an observation noted in both the INS spectra and simulated phonon density
of states plots) that results from the increased layering. This reduction in max is
observed across all layered materials studied thus far. Hence, a new structurally-based
mechanism for the decreased sensitivity of layered materials has been proposed.
Due to the γ → α transition, BAM hammer testing appears incapable of directly
measuring the impact sensitivity of the γ-form. Current experimental approaches are
inadequate for the investigation of polymorphic materials. Furthermore, this transition demonstrates the importance of considering structural transformations during
the initiation process of EMs, and the ability of the up-pumping model to assist in
the interpretation of experimental results.
5.6 Suggestions for Further Work
It is clear from this chapter that the up-pumping models can be applied to polymorphic
series. However, the sample size used here is limited. It is therefore of great interest
to extend this work to a broader set of polymorphic materials. To do this, it will be
necessary to conduct experimental investigations on the sensitivity of polymorphic
materials, many of which have yet to be thoroughly analysed.
175
5.5 Conclusions
Polymorphism is very prevalent amongst energetic materials, and can lead to drastic
changes in a material’s sensitivity to impact. Most notable are the δ- and βpolymorphs of HMX. The former has been reported to be as sensitive to impact as a
primary explosive material, while the latter exhibits much lower sensitivity to impact.
Application of the up-pumping model was able to reproduce these experimental
findings and assessed δ-HMX as being a highly sensitive material. This therefore
demonstrates that the up-pumping model is sensitive to polymorphic modifications.
It was therefore applied to a second polymorphic energetic material, FOX-7.
Under ambient conditions, FOX-7 exists in the α-form, which adopts a
herringbone-type structure. When heated, these layers flatten, and are nearly flat
in the γ-form. This form was recovered to ambient conditions and its impact sensitivity measured using a BAM fall hammer. This suggested that the layered γ-form had
the same impact sensitivity as the α-form, despite the general principle that layered
materials are insensitive. X-ray powder diffraction, however, revealed that the γform undergoes transformation to the α-form on impact, and hence it is not known
which polymorphic phase was in fact tested. The mechanism for this impact-induced
transformation is not yet known, and may be the result of pressure, temperature or
their combination. The up-pumping model was therefore applied to the FOX-7 polymorphs. Both the overtone-based model and the temperature-dependent two-layered
model suggested that the layered γ-form should be notably less sensitive than the
α-form. It is suggested that the reduction in sensitivity is the result of a decrease in
max (an observation noted in both the INS spectra and simulated phonon density
of states plots) that results from the increased layering. This reduction in max is
observed across all layered materials studied thus far. Hence, a new structurally-based
mechanism for the decreased sensitivity of layered materials has been proposed.
Due to the γ → α transition, BAM hammer testing appears incapable of directly
measuring the impact sensitivity of the γ-form. Current experimental approaches are
inadequate for the investigation of polymorphic materials. Furthermore, this transition demonstrates the importance of considering structural transformations during
the initiation process of EMs, and the ability of the up-pumping model to assist in
the interpretation of experimental results.
5.6 Suggestions for Further Work
It is clear from this chapter that the up-pumping models can be applied to polymorphic
series. However, the sample size used here is limited. It is therefore of great interest
to extend this work to a broader set of polymorphic materials. To do this, it will be
necessary to conduct experimental investigations on the sensitivity of polymorphic
materials, many of which have yet to be thoroughly analysed.