Polymorphism and Phase Transitions in Energetic Materials

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Polymorphism and Phase Transitions in Energetic
Materials
Thomas B. Brill Department of Chemistry University
of Delaware Newark, DE 19716
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Energetic Materials
Compounds that release heat and/or gaseous
products at a high rate upon stimulus by heat,
impact, shock, spark, etc.
Applications
Explosives Propellants Gas generators Pyrotechnics
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Primary Explosive Mild impetus leads to a
short, strong shock wave
Reactants
DH
Products
Secondary Explosive Strong impetus leads to a
long duration shock wave
Reactants
DH
Products
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• Structure-Property Correlations can be Found in
  Energetic Materials
• Decomposition
• Combustion
• Detonation

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Decomposition Characteristics of Nitramines
T. B. Brill and Y. Oyumi, J. Phys. Chem. 90, 2697
(1986).
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Burning Rates of 5-Substituted Tetrazoles
V. P. Sinditskii, A. E. Fogelzang, A. I.
Egorshev, V. V. Serushkin, and V. Y. Kolesov,
Solid Propellant Chemistry, Combustion and Motor
Interior Ballistics, Prog. Astronaut. Aeronaut.
Vol. 185, edited by V. Yang, T. B. Brill, and W.
Z. Ren, (AIAA, Reston, VA) 2000, p. 99.
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Impact Sensitivity as a Function of the Energy
Transfer Rate into the Phonon Mode Structure
K. L. McNesby and C. S. Coffey, J. Phys. Chem. B,
101, 3097 (1997).
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Why are Solid-Solid Phase Transitions and
Polymorphism Important in Energetic Materials?
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Density Considerations

• Density directly affects the detonation velocity.

Di Do M(ri ro)
Most applications of energetic materials involve
volume-limited situations. Therefore, the
highest density polymorph is desired.
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Detonation Velocity and Density of Selected
Explosives
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Defects and Crystal Properties

• Defect density can increase during a phase
  transition. The material may become more
  sensitive because the decomposition reactions
  begin at defects. These sites become hot
  spots.
• Examples of defects that lead to hot spots are
  shear bands and dislocations, fractures, and
  voids.

III. The shock sensitivity of explosive crystals
can depend on the crystal orientation. Stress
can be relieved if a glide plane exists.
Polymorphs can differ in this respect.
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The Rate of the Phase Transformation
IV. If the rate of the transition is fast
enough, then the phase transition might occur in
the crystal during combustion and lead to
fracture and increased surface area. The result
may be a transition from combustion to
detonation.
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Morphology
V. Crystal morphology is important when making a
formulation. Needles and leaves are difficult to
process at high solid loadings. Prisms and
spheres are preferred.
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Ammonium Perchlorate NH4ClO4

• Most common oxidizer used in solid rocket
  propellants.
• Is usually mixed with Al, a rubber-like binder
  and catalysts. AP makes up about 80 of the
  formulation.
• Monoclinic cubic phase transition
  occurs at 254oC.
• Phase transition occurs on the crystal surface
  during combustion.

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Raman Spectra of ClO4- Fundamentals of NH4ClO4
orthorhombic
cubic
T. B. Brill and F. Goetz, J. Chem. Phys. 65, 1217
(1976)
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The E Bending Mode of ClO4- in NH4ClO4
Cubic Phase
Phase transition takes place when the ClO4- ion
begins free tumbling in the crystal lattice.
Orthorhombic Phase
T. B. Brill and F. Goetz, J. Chem. Phys. 65, 1217
(1976)
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Ammonium Nitrate NH4NO3

• AN is a widely used oxidizer and fertilizer with
  a jaded history.
• When mixed with fuel oil, it becomes a powerful
  explosive widely used industrially.
• Between -20oC and 125oC AN exhibits 5 polymorphs
  at 1 atm.
• The IV/III transformation occurs at 32oC and
  involves a 3.7 volume expansion.
• Several cycles through IV/III reduces AN prills
  to caky dust. Breaking up caked AN has resulted
  occasionally in detonation.

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Ammonium Nitrate Phase Transition Scheme
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Phase Stabilization of AN (PSAN) or How to avoid
the IV/III Transformation at 32oC
rNH4/rNO3- 0.76 in AN vs. 0.73 needed for the
NiAs structure of AN(III). Replacement of NH4
(1.48 pm) by K (1.33 pm) contracts the cell
dimensions and stabilizes AN(III). The reduced
cell dimensions hinder the onset of rotational
libration of NO3-, which is responsible for the
III/II transformation. Hence AN(III) is stable
to a higher temperature.
The Result AN(III) can be stabilized over a
wide temperature range.
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HMX A Highly Valued Energetic Material
High density for an organic compound 1.90
g/cm3 High detonation velocity 9200 m/s Exists
in three polymorphs (a,b,d) and one hemi- hydrate
(g). The b-d-HMX phase transition occurs at
165-180oC, but reversion can require days. Could
this phase transition cause a deflagration to
detonation transition?
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Sensitivity to impact d gt g gt a gt b Large volume
expansion (7) occurs during the b-d phase
transition
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The Molecular Conformation Change in the b-d
Phase Transition of HMX
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HMX Phase Transition Scheme
T. B. Brill and R. J. Karpowicz, J. Phys Chem.
86, 4260 (1982)
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IR Spectra Showing the Progress of the b-d Solid
Phase Transition of HMX at 185oC
T. B. Brill and R. J. Karpowicz, J. Phys Chem.
86, 4260 (1982)
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First Order Rate Plot for b-d Solid Phase
Transition of HMX
T. B. Brill and R. J. Karpowicz, J. Phys Chem.
86, 4260 (1982)
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Arrhenius Plot for the b-d Solid Phase Transition
of HMX
T. B. Brill and R. J. Karpowicz, J. Phys Chem.
86, 4260 (1982)
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Extrapolation of HMX Phase Transition Kinetics
into the Combustion Regime
R. J. Karpowicz, L. S. Gelfand and T. B. Brill,
AIAA. J. 21, 310 (1983).
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Fast Kinetic Measurement of the b-d-HMX Phase
Transition
b-HMX is centrosymmetric whereas d-HMX is
noncentrosymmetric. d-HMX emits a strong second
harmonic signal (SHG) that can be used to measure
the rate of conversion on the sub-millisecond
time scale.
B. F. Henson, B. W. Asay, R. K. Sander, S. F.
Son, J. M. Robinson and P. M. Dickson, Phys. Rev.
Lett., 82, 1213 (1999).
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b-d-HMX Phase Transition Kinetics
Conclusion The b-d-HMX phase transition occurs
during combustion of HMX crystals.
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14N Nuclear Quadrupole Resonance Study of
Mechanism of the b-d-HMX Phase Transition
A. G. Landers, T. B. Brill and R. A. Marino, J.
Phys. Chem. 85, 2618 (1981).
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Temperature Dependence of 14N NQR Coupling
Constants is Related to the xyz Torsional Motions
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Molecular Motion in b-HMX
Torsion about z dominates
Torsion about x,y dominates
Torsion about x,y inertial axes breaks the HMX
molecule free from the strongest intermolecular
interactions of the crystal lattice.
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b-HMX
Pressure affects the b-d phase transition
Raman spectra of the effect of pressure on HMX at
187oC
d-HMX
b-HMX
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Pressure Dependence of the b-d-HMX Phase
Transition
R. J. Karpowicz and T. B. Brill, AIAA J. 20, 1586
(1982)
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Total Ion Current CH4-CI MS of HMX
3m HMX
Small crystals of HMX do not trap solvent
Solvent is trapped in the large crystals of HMX.
It is released when the phase transition occurs.
175m HMX
Thermally cycled 175m HMX
Once HMX is cycled through the phase transition
and back, the trapped solvent is gone.
R. J. Karpowicz and T. B. Brill, AIAA J. 20, 1586
(1982)
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Other Examples of Polymorphism and Phase
Transitions in Energetic Solids
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Polymorphism in TNDBN
Y. Oyumi, T. B. Brill and A. L. Rheingold, J.
Phys. Chem. 90,2526 (1986)
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Temperature Dependence of the IR Spectrum of TNDBN
Transitions also measured by DTA
Few changes in the N-NO2 regions. More
differences in the C-H modes.
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Thermally-Induced Solid-Solid Phase Transitions
of TNDBN
Y. Oyumi, T. B. Brill and A. L. Rheingold, J.
Phys. Chem. 90, 2526 (1986)
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CL-20 (HNIW) The Most Highly Valued Explosive
Extremely high density for an organic compound
2.04 g/cm3 Extremely high detonation velocity
9800 m/s Drawbacks are high cost and high shock
sensitivity
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Polymorphs of CL-20
e 2.044 gm/cm3 g 1.918 gm/cm3 a
1.992 gm/cm3 (a hydrate) b 1.989 gm/cm3
Stability trend e gt g gt a gt b
So far, phase transformations in CL-20 have not
been a problem as they could be in HMX
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Relations Between Molecular Structure and Phase
Transformations/Polymorphism
Plastic crystal formation in the high-temperature
phase is seen for many but not all
compounds. Enthalpy change differences can be
found that depend on the molecular shape.
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Plastic Crystal Formation in Explosives
The high temperature phase of many explosives is
plastic (translationally ordered but rotationally
disordered).
Can be determined and studied by solid-state NMR,
IR, DTA, etc
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Enthalpy Differences in Cyclic vs. Acyclic
Compounds

• SDH for phase transitions plus melting for seven
  cyclic energetic compounds is 354 cal/gm
• SDH for phase transitions plus melting of ten
  acyclic energetic compounds is 6618 cal/gm

Conclusion is that on average the crystal lattice
of rod-like molecules is harder to disrupt than
globular molecules.
Y. Oyumi and T. B. Brill, Thermochim. Acta, 116,
125 (1987)
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Some Concluding Remarks
For most energetic materials the problem of
polymorphism arises in the desirability of
formulating the most dense form.
Polymorphism and phase transformations in
energetic compounds occasionally have a major
impact on the outcome. The best known examples
are the b-d-HMX and the IV/III-AN phase
transformations.