26
1 Introduction
very timely to re-examine this model as a physical basis from which to understand
mechanically-induced reactions in EMs.
1.4 Research Concept and Aims
The development of new EMs is an active area of fundamental research. Amongst
the main target characteristics of new EMs, ensuring low sensitivity to mechanical
perturbation is simultaneously a top priority and notoriously difficult. This difficulty
is largely due to the fact that no underlying mechanism for EM sensitivity is yet
known.
It is generally accepted that initiation of an EM requires localisation of energy
within the material. This localisation can be described by hot-spots. The origin of
these hot-spots remains widely debated, although a number of mechanisms (Sect. 1.2)
have been proposed. Some theories suggest hot-spots to be rooted in equilibrium
temperature increases, while others describe non-equilibrium, athermal processes.
Many models (Sect. 1.3) have been proposed in an attempt to identify a set of
physical parameters to understand and describe the impact sensitivity of EMs. The
‘band gap criterion’ remains the most popular amongst solid state models. While
many models are promising, they often lack a physical basis and are restricted to
subsets of energetic materials. A particularly promising physical model is rooted in
the so-called up-pumping of vibrational energy (Sect. 1.2.2). This model describes the
transfer of energy from an initial mechanical impulse, and the mechanism by which
it transitions into localised molecular energy. Importantly, the up-pumping model is
not isolated from previous hot-spot models, as the initial energy can originate via
any phenomenon, including adiabatic compression, fracture, plastic deformation,
contact stresses, amongst others. Hence, it offers a physical basis for converting hotspot generation mechanisms into a chemical reaction. Considerable experimental
work has validated the up-pumping phenomenon, and numerical modelling has been
used to rationalise many of its general features. However, only very limited work has
focused on applying these concepts to test sets of EMs for the prediction of impact
sensitivity. Those that have done so have been based on low resolution inelastic
neutron scattering spectra [119, 120], or very limited consideration of vibrational
structure from calculation [123, 124], or on Raman spectroscopy [121]. The models
employed in these studies have been limited to very simple (and often different)
energy-transfer models and typically require the inclusion of additional experimental
data. Before a model based on up-pumping based model can therefore be employed
to predict impact sensitivity, a fully ab initio model must be developed.
This work therefore conducted with the following aims:
• Investigate the vibrational properties of a range of energetic materials.
• Consider possible target vibrational modes for simple EMs.
• Consider the ‘band-gap criterion’ for a range of EMs.
1 Introduction
very timely to re-examine this model as a physical basis from which to understand
mechanically-induced reactions in EMs.
1.4 Research Concept and Aims
The development of new EMs is an active area of fundamental research. Amongst
the main target characteristics of new EMs, ensuring low sensitivity to mechanical
perturbation is simultaneously a top priority and notoriously difficult. This difficulty
is largely due to the fact that no underlying mechanism for EM sensitivity is yet
known.
It is generally accepted that initiation of an EM requires localisation of energy
within the material. This localisation can be described by hot-spots. The origin of
these hot-spots remains widely debated, although a number of mechanisms (Sect. 1.2)
have been proposed. Some theories suggest hot-spots to be rooted in equilibrium
temperature increases, while others describe non-equilibrium, athermal processes.
Many models (Sect. 1.3) have been proposed in an attempt to identify a set of
physical parameters to understand and describe the impact sensitivity of EMs. The
‘band gap criterion’ remains the most popular amongst solid state models. While
many models are promising, they often lack a physical basis and are restricted to
subsets of energetic materials. A particularly promising physical model is rooted in
the so-called up-pumping of vibrational energy (Sect. 1.2.2). This model describes the
transfer of energy from an initial mechanical impulse, and the mechanism by which
it transitions into localised molecular energy. Importantly, the up-pumping model is
not isolated from previous hot-spot models, as the initial energy can originate via
any phenomenon, including adiabatic compression, fracture, plastic deformation,
contact stresses, amongst others. Hence, it offers a physical basis for converting hotspot generation mechanisms into a chemical reaction. Considerable experimental
work has validated the up-pumping phenomenon, and numerical modelling has been
used to rationalise many of its general features. However, only very limited work has
focused on applying these concepts to test sets of EMs for the prediction of impact
sensitivity. Those that have done so have been based on low resolution inelastic
neutron scattering spectra [119, 120], or very limited consideration of vibrational
structure from calculation [123, 124], or on Raman spectroscopy [121]. The models
employed in these studies have been limited to very simple (and often different)
energy-transfer models and typically require the inclusion of additional experimental
data. Before a model based on up-pumping based model can therefore be employed
to predict impact sensitivity, a fully ab initio model must be developed.
This work therefore conducted with the following aims:
• Investigate the vibrational properties of a range of energetic materials.
• Consider possible target vibrational modes for simple EMs.
• Consider the ‘band-gap criterion’ for a range of EMs.