Transcript No Slide Title

Polymorphism and Phase
Transitions in Energetic
Materials
Thomas B. Brill
Department of Chemistry
University of Delaware
Newark, DE 19716
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
Primary Explosive: Mild impetus leads to a short, strong
shock wave
Reactants
DH
Products
time
Secondary Explosive: Strong impetus leads to a long duration
shock wave
Reactants
DH
Products
time
Nitramines
NO2
N
NO2
N
O2 NN
O2 NN
NNO2
NNO2
N
NO2
RDX
HMX
O
N
R
R
N
N
O
N
R
O
N
R
N
Furazans
R
N
H
N
Furoxans
Tetrazoles
R
N
N
N
N
Nitrate esters: RONO2
Aliphatic Nitro Derivatives: RNO2
Organic Azides: RN3
R
Tetrazines
Peroxides: ROOR
Inorganics: ClO4- , NO3- , CNO- , N3-
Structure-Property Correlations can
be Found in Energetic Materials
• Decomposition
• Combustion
• Detonation
Decomposition Characteristics of
Nitramines
1640
TNTO
Asymmetric NO
2
stretch (cm
-1
)
1620
CL-20
1600
DNTO
DNFP
Less NO2
1580
RDX
1560
BCMN
o-HMX
DATH
1540
DNCP
BCEN
1520
1500
1480
1.33
AZTC
HMX
More NO2
DPT
DMNA
1.34
1.35
1.36
1.37
1.38
1.39
1.4
1.41
1.42
N-NO2 bond distance (Angstroms)
T. B. Brill and Y. Oyumi, J. Phys. Chem. 90, 2697 (1986).
Burning Rates of 5-Substituted
Tetrazoles
100
N=NR
NNO2
NO2
Mass burning rate (g cm
2
/s)
Tz
Cl
CN
Br
H
NHTz
10
OH
NH2
1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Hammett sigma
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.
Impact Sensitivity as a Function of the
Energy Transfer Rate into the Phonon
Mode Structure
8
Pb styphnate
7
gamma-HMX
1/h 50 (m
-1
)
6
5
4
RDX
beta-HMX
3
2
picric acid
styphnic acid
1
TATB
0
0
5
10
15
20
25
30
Energy Transfer Rate at 425 cm -1
K. L. McNesby and C. S. Coffey, J. Phys. Chem. B, 101, 3097
(1997).
Why are Solid-Solid Phase
Transitions and Polymorphism
Important in Energetic
Materials?
Density Considerations
I.
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.
Detonation Velocity and Density of
Selected Explosives
Explosive Density,
g/cm3
Det. Velocity,
m/sec
HMX
1.90
9160
RDX
1.80
8754
PETN
1.78
8695
DATB
1.79
7520
Tetryl
1.70
7560
TNT
1.65
6950
Defects and Crystal Properties
II.
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.
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.
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.
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.
Raman Spectra of ClO4- Fundamentals of NH4ClO4
orthorhombic
cubic
T. B. Brill and F. Goetz, J. Chem. Phys. 65, 1217 (1976)
The E Bending Mode of ClO4- in NH4ClO4
Phase transition takes
place when the ClO4ion begins free tumbling
in the crystal lattice.
Cubic Phase
Orthorhombic Phase
T. B. Brill and F. Goetz, J. Chem. Phys. 65, 1217 (1976)
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.
Ammonium Nitrate Phase Transition Scheme
-200oC
-18oC
32oC
84oC
125oC
VI-------V-------IV--------III----------II---------I--Tetragonal
Orthohombic
tripyramidal
Orthorhombic Tetragonal
Cubic
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.
HMX: A Highly Valued Energetic Material
NO2
N
O2NN
NNO2
N
NO2
HMX
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 hemihydrate (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?
Sensitivity to impact: d > g > a > b
Large volume expansion (7%) occurs during the
b-d phase transition
The Molecular Conformation Change in the
b-d Phase Transition of HMX
HMX Phase Transition Scheme
T. B. Brill and R. J. Karpowicz, J. Phys Chem. 86, 4260 (1982)
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)
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)
Arrhenius Plot for the b-d Solid Phase Transition of HMX
T. B. Brill and R. J. Karpowicz, J. Phys Chem. 86, 4260 (1982)
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).
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 submillisecond 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).
b-d-HMX Phase Transition Kinetics
Conclusion: The b-d-HMX phase transition occurs
during combustion of HMX crystals.
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).
Temperature Dependence of 14N NQR Coupling
Constants is Related to the xyz Torsional Motions
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.
Pressure
affects the
b-d phase
transition
Raman
spectra of
the effect of
pressure on
HMX at
187oC
b-HMX
d-HMX
b-HMX
Pressure Dependence of the b-d-HMX Phase Transition
R. J. Karpowicz and T. B. Brill, AIAA J. 20, 1586 (1982)
Total Ion Current CH4-CI MS of HMX
3m HMX
175m HMX
Thermally cycled
175m 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.
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)
Other Examples of
Polymorphism and Phase
Transitions in Energetic Solids
Polymorphism in TNDBN
NO2
N
O2NC
CNO2
N
NO2
TNBDN
Y. Oyumi, T. B. Brill and A. L. Rheingold, J. Phys. Chem. 90,2526 (1986)
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.
Thermally-Induced Solid-Solid Phase Transitions of TNDBN
Y. Oyumi, T. B. Brill and A. L. Rheingold, J. Phys. Chem. 90, 2526 (1986)
CL-20 (HNIW): The Most Highly Valued
Explosive
O2NN
O2NN
NNO2
NNO2
O2NN
NNO2
C L-20
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
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 > g > a > b
So far, phase transformations in CL-20 have not
been a problem as they could be in HMX
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.
Plastic Crystal Formation in Explosives
The high temperature phase of many explosives is
plastic (translationally ordered but rotationally
disordered).
NO2
N
NO
N
O2NC
ONN
NO2
N
CNO2
O
NNO
N
NO2
TRDX
O2NN
N
TNBDN
N
NO2
N
NO2
NNO2
N
N
NO2
DNFP
NH4ClO4
NH4NO3
OHMX
Can be determined and studied by solid-state NMR,
IR, DTA, etc
Enthalpy Differences in Cyclic vs.
Acyclic Compounds
 SDH for phase transitions plus melting for seven
cyclic energetic compounds is 35±4 cal/gm
 SDH for phase transitions plus melting of ten acyclic
energetic compounds is 66±18 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)
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.