3.1 Introduction
67
upwards; the latter is substantially faster and hence vibrational energy continues to
scatter upwards. The general process of up-conversion occurs in two stages:[17].
1. Scattering of the phonon bath modes leads to excitation of intermediate librational
modes, known as doorway modes.
2. The ‘hot’ doorway mode subsequently scatters with additional phonon bath
modes.
This process ultimately allows up-conversion of energy from the initially excited
phonon region into the localised molecular modes, Fig. 3.1. Experiment has found
such energy conversion processes to occur on the order of picoseconds [18], and occur
more rapidly around defects [6, 20]. These processes therefore occur considerably
faster than the thermally-induced chemical decomposition in energetic materials
(EM) [14]. The rate of vibrational energy transfer is on the same time scale as the
primary events associated with an explosion [8], and has prompted interest in this
model to explain mechanically-induced initiation of EMs [21, 22].
The initial stage of a chemical explosion involves the rupture of a covalent bond.
Within the proposed vibrational energy transfer model [1, 23], it follows that the
vibrational energy must ultimately localise into a particular molecular vibration, the
target mode, Q T . When this target mode is sufficiently populated, distortion along
its eigenvector reduces the energy separation of the frontier orbitals, and athermal
bond rupture ensues [24–27].
The apparent structural simplicity of azide-based EMs, where only a single covalent bond (N-N bond) exists for initiation, offer a particularly interesting challenge for
understanding impact sensitivity. Azides have been known for over a century [28],
and their initiation mechanism has been the subject of research for nearly threequarters of a century [29]. Despite this apparent simplicity, the sensitivity of these
materials span orders of magnitude. For example, NaN 3 is completely insensitive
to impact, while Pb(N 3 ) 2 is a common primary explosive used in detonators [15].
Broadly, the azide materials can be classified into three structural types: ionic, polymeric and molecular [30]. The ionic materials have been reported to be less sensitive
than the polymeric or molecular materials [31]. The existence of covalent bonds in
the latter two material types has been used to rationalise a decrease in N-N bond
strengths and hence increase sensitivity, although the change in N-N dissociation
energies typically varies only slightly [30]. This rationale is therefore insufficient
to explain the range of sensitivities observed, both within and between structural
classifications, or between polymorphic forms [32].
The breadth of physical and chemical characteristics displayed by azide-based
EMs is vast, which in conjunction with the relative simplicity of azide-based chemistry [33], continues to keep the development of azide-based materials an active area
of research [33–36]. However, without a means to predict the relative sensitivity of
new materials a priori, the preparation of new azide materials remains very hazardous.
67
upwards; the latter is substantially faster and hence vibrational energy continues to
scatter upwards. The general process of up-conversion occurs in two stages:[17].
1. Scattering of the phonon bath modes leads to excitation of intermediate librational
modes, known as doorway modes.
2. The ‘hot’ doorway mode subsequently scatters with additional phonon bath
modes.
This process ultimately allows up-conversion of energy from the initially excited
phonon region into the localised molecular modes, Fig. 3.1. Experiment has found
such energy conversion processes to occur on the order of picoseconds [18], and occur
more rapidly around defects [6, 20]. These processes therefore occur considerably
faster than the thermally-induced chemical decomposition in energetic materials
(EM) [14]. The rate of vibrational energy transfer is on the same time scale as the
primary events associated with an explosion [8], and has prompted interest in this
model to explain mechanically-induced initiation of EMs [21, 22].
The initial stage of a chemical explosion involves the rupture of a covalent bond.
Within the proposed vibrational energy transfer model [1, 23], it follows that the
vibrational energy must ultimately localise into a particular molecular vibration, the
target mode, Q T . When this target mode is sufficiently populated, distortion along
its eigenvector reduces the energy separation of the frontier orbitals, and athermal
bond rupture ensues [24–27].
The apparent structural simplicity of azide-based EMs, where only a single covalent bond (N-N bond) exists for initiation, offer a particularly interesting challenge for
understanding impact sensitivity. Azides have been known for over a century [28],
and their initiation mechanism has been the subject of research for nearly threequarters of a century [29]. Despite this apparent simplicity, the sensitivity of these
materials span orders of magnitude. For example, NaN 3 is completely insensitive
to impact, while Pb(N 3 ) 2 is a common primary explosive used in detonators [15].
Broadly, the azide materials can be classified into three structural types: ionic, polymeric and molecular [30]. The ionic materials have been reported to be less sensitive
than the polymeric or molecular materials [31]. The existence of covalent bonds in
the latter two material types has been used to rationalise a decrease in N-N bond
strengths and hence increase sensitivity, although the change in N-N dissociation
energies typically varies only slightly [30]. This rationale is therefore insufficient
to explain the range of sensitivities observed, both within and between structural
classifications, or between polymorphic forms [32].
The breadth of physical and chemical characteristics displayed by azide-based
EMs is vast, which in conjunction with the relative simplicity of azide-based chemistry [33], continues to keep the development of azide-based materials an active area
of research [33–36]. However, without a means to predict the relative sensitivity of
new materials a priori, the preparation of new azide materials remains very hazardous.