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Polymorphism and Phase Transitions in Energetic Materials

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.

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Polymorphism and Phase Transitions in Energetic Materials

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  1. Polymorphism and Phase Transitions in Energetic Materials Thomas B. Brill Department of Chemistry University of Delaware Newark, DE 19716

  2. 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

  3. time time time 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

  4. N i t r a m i n e s N O 2 N O 2 N N O N N N N O 2 2 O N N N N O 2 2 N N O 2 R D X H M X O N N R N R N O R O N R N N R N H F u r a z a n s T e t r a z o l e s F u r o x a n s R N i t r a t e e s t e r s : R O N O 2 N N A l i p h a t i c N i t r o D e r i v a t i v e s : R N O 2 N N O r g a n i c A z i d e s : R N 3 R P e r o x i d e s : R O O R T e t r a z i n e s - - - - I n o r g a n i c s : C l O , N O , C N O , N 4 3 3

  5. Structure-Property Correlations can be Found in Energetic Materials • Decomposition • Combustion • Detonation

  6. Decomposition Characteristics of Nitramines T. B. Brill and Y. Oyumi, J. Phys. Chem.90, 2697 (1986).

  7. 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.

  8. 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).

  9. Why are Solid-Solid Phase Transitions and Polymorphism Important in Energetic Materials?

  10. 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.

  11. Detonation Velocity and Density of Selected Explosives

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. Raman Spectra of ClO4- Fundamentals of NH4ClO4 orthorhombic cubic T. B. Brill and F. Goetz, J. Chem. Phys. 65, 1217 (1976)

  17. 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)

  18. 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.

  19. -200oC -18oC 32oC 84oC 125oC VI-------V-------IV--------III----------II---------I--- Tetragonal Orthohombic Orthorhombic Tetragonal Cubic tripyramidal Ammonium Nitrate Phase Transition Scheme

  20. 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.

  21. 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?

  22. Sensitivity to impact: d > g > a > b Large volume expansion (7%) occurs during the b-d phase transition

  23. The Molecular Conformation Change in the b-d Phase Transition of HMX

  24. HMX Phase Transition Scheme T. B. Brill and R. J. Karpowicz, J. Phys Chem. 86, 4260 (1982)

  25. 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)

  26. 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)

  27. Arrhenius Plot for the b-d Solid Phase Transition of HMX T. B. Brill and R. J. Karpowicz, J. Phys Chem. 86, 4260 (1982)

  28. 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).

  29. 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).

  30. b-d-HMX Phase Transition Kinetics Conclusion: The b-d-HMX phase transition occurs during combustion of HMX crystals.

  31. 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).

  32. Temperature Dependence of 14N NQR Coupling Constants is Related to the xyz Torsional Motions

  33. 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.

  34. b-HMX Pressure affects the b-d phase transition Raman spectra of the effect of pressure on HMX at 187oC d-HMX b-HMX

  35. Pressure Dependence of the b-d-HMX Phase Transition R. J. Karpowicz and T. B. Brill, AIAA J. 20, 1586 (1982)

  36. 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)

  37. Other Examples of Polymorphism and Phase Transitions in Energetic Solids

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

  39. 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.

  40. Thermally-Induced Solid-Solid Phase Transitions of TNDBN Y. Oyumi, T. B. Brill and A. L. Rheingold, J. Phys. Chem. 90, 2526 (1986)

  41. 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

  42. 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

  43. 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.

  44. 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

  45. 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)

  46. 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.

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