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:

ExplosivesPropellantsGas generatorsPyrotechnics

Primary Explosive: Mild impetus leads to a short, strong shock wave

Secondary Explosive: Strong impetus leads to a long duration shock wave

Reactants

Products

Products

Reactants

H

H

time

timetime

O2NN NNO2

NO2N

RDX

O2NN

NNO2

NNO2

NO2N

HMX

Nitramines

N

O

NR

R

Furazans

N

O

NR

R

O

Furoxans

N

NNH

N

R

Tetrazoles

N

N N

N

R

R

Tetrazines

Nitrate esters: RONO2

Aliphatic Nitro Derivatives: RNO2

Organic Azides: RN3

Peroxides: ROOR

Inorganics: ClO4-, NO3

-, CNO-, N3-

Structure-Property Correlations can be Found in Energetic Materials

• Decomposition

• Combustion

• Detonation

TNTO

CL-20

DNTO

DNFP

RDX

AZTC

HMX

BCMNo-HMX

DPT

DATH

DNCPBCEN

DMNA

1480

1500

1520

1540

1560

1580

1600

1620

1640

1.33 1.34 1.35 1.36 1.37 1.38 1.39 1.4 1.41 1.42

N-NO2 bond distance (Angstroms)

Asy

mm

etri

c N

O2

stre

tch

(cm

-1)

Less NO2

More NO2

Decomposition Characteristics of Nitramines

T. B. Brill and Y. Oyumi, J. Phys. Chem. 90, 2697 (1986).

N=NR

NO2

CN

TzNNO2

Cl

Br

HNHTz

OH

NH2

1

10

100

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

Hammett sigma

Mas

s b

urn

ing

rat

e (g

cm

2 /s)

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.

Pb styphnate

gamma-HMX

RDX

beta-HMX

styphnic acidpicric acid

TATB0

1

2

3

4

5

6

7

8

0 5 10 15 20 25 30

Energy Transfer Rate at 425 cm-1

1/h

50 (

m-1

)

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).

Why are Solid-Solid Phase Transitions and Polymorphism

Important in Energetic Materials?

I. Density directly affects the detonation velocity.

Di = Do + M(i –o)

Most applications of energetic materials involve volume-limited situations. Therefore, the highest density polymorph is desired.

Density Considerations

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

Detonation Velocity and Density of Selected Explosives

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.

Defects and Crystal Properties

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.

The Rate of the Phase Transformation

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

T. B. Brill and F. Goetz, J. Chem. Phys. 65, 1217 (1976)

orthorhombic

cubic

T. B. Brill and F. Goetz, J. Chem. Phys. 65, 1217 (1976)

The E Bending Mode of ClO4- in NH4ClO4

Cubic Phase

Orthorhombic Phase

Phase transition takes place when the ClO4

- ion begins free tumbling

in the crystal lattice.

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.

VI-------V-------IV--------III----------II---------I---

-200oC -18oC 32oC 84oC 125oC

Tetragonal Orthohombic Orthorhombic Tetragonal Cubic

tripyramidal

Ammonium Nitrate Phase Transition Scheme

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

High density for an organic compound: 1.90 g/cm3

High detonation velocity: 9200 m/s

Exists in three polymorphs () and one hemi-hydrate ().

The -HMX phase transition occurs at 165-180oC, but reversion can require days.

Could this phase transition cause a deflagration to detonation transition?

O2NN

NNO2

NNO2

NO2N

HMX

Sensitivity to impact: > > >

Large volume expansion (7%) occurs during the phase transition

The Molecular Conformation Change in the Phase Transition of HMX

T. B. Brill and R. J. Karpowicz, J. Phys Chem. 86, 4260 (1982)

HMX Phase Transition Scheme

IR Spectra Showing the Progress of the 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 Solid Phase Transition of HMX

T. B. Brill and R. J. Karpowicz, J. Phys Chem. 86, 4260 (1982)

Arrhenius Plot for the 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 -HMX Phase Transition

-HMX is centrosymmetric whereas -HMX is noncentrosymmetric. -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).

-HMX Phase Transition Kinetics

Conclusion: The -HMX phase transition occurs during combustion of HMX crystals.

14N Nuclear Quadrupole Resonance Study of Mechanism of the -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

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.

Molecular Motion in -HMX

Pressure affects the phase transition

Raman spectra of

the effect of pressure on

HMX at 187oC

X

X

-HMX

Pressure Dependence of the -HMX Phase Transition

R. J. Karpowicz and T. B. Brill, AIAA J. 20, 1586 (1982)

3 HMX

175 HMX

Thermally cycled 175 HMX

Total Ion Current CH4-CI MS of HMX

R. J. Karpowicz and T. B. Brill, AIAA J. 20, 1586 (1982)

Solvent is trapped in the large crystals of HMX. It is released when the phase transition occurs.

Small crystals of HMX do not trap solvent

Once HMX is cycled through the phase transition and back, the trapped solvent is gone.

Other Examples of Polymorphism and Phase

Transitions in Energetic Solids

Polymorphism in TNDBN

O2NC

NNO2

CNO2

NO2N

TNBDN

Y. Oyumi, T. B. Brill and A. L. Rheingold, J. Phys. Chem. 90,2526 (1986)

Temperature Dependence of the IR Spectrum of TNDBN

Few changes in the N-NO2 regions. More differences in the C-H modes.

Transitions also measured by DTA

Y. Oyumi, T. B. Brill and A. L. Rheingold, J. Phys. Chem. 90, 2526 (1986)

Thermally-Induced Solid-Solid Phase Transitions of TNDBN

O2NN NNO2

NNO2

O2NN NNO2

O2NN

CL-20

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

Polymorphs of CL-20

2.044 gm/cm3

1.918 gm/cm3

1.992 gm/cm3 (a hydrate)

1.989 gm/cm3

Stability trend: > > >

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).

Can be determined and studied by solid-state NMR, IR, DTA, etc

O2NC

NNO2

CNO2

NO2N

TNBDN

ONN NNO

NON

TRDX

NNO2

NO2N

N

O

N

DNFP

O2NN NNO2

NNO2

NNO2

OHMX

NH4ClO4 NH4NO3

Enthalpy Differences in Cyclic vs. Acyclic Compounds

H for phase transitions plus melting for seven cyclic energetic compounds is 35±4 cal/gm

H 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

Polymorphism and phase transformations in energetic compounds occasionally have a major impact on the outcome. The best known examples are the -HMX and the IV/III-AN phase transformations.

For most energetic materials the problem of polymorphism arises in the desirability of formulating the most dense form.