On the nucleation mechanism of the β-δ phase transition in the energetic nitramineoctahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocineL. Smilowitz, B. F. Henson, M. Greenfield, A. Sas, B. W. Asay, and P. M. Dickson Citation: The Journal of Chemical Physics 121, 5550 (2004); doi: 10.1063/1.1782491 View online: http://dx.doi.org/10.1063/1.1782491 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/121/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Initial chemical events in shocked octahydro-1,3,5,7-tetranitro-1,3,5,7- tetrazocine: A new initiationdecomposition mechanism J. Chem. Phys. 136, 044516 (2012); 10.1063/1.3679384 Interfacial and volumetric kinetics of the β → δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine based on the virtual melting mechanism J. Chem. Phys. 124, 026101 (2006); 10.1063/1.2140698 The β–δ phase transition in the energetic nitramine-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine: Kinetics J. Chem. Phys. 117, 3789 (2002); 10.1063/1.1495399 The β–δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine:Thermodynamics J. Chem. Phys. 117, 3780 (2002); 10.1063/1.1495398 Equation of state, phase transition, decomposition of β-HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) athigh pressures J. Chem. Phys. 111, 10229 (1999); 10.1063/1.480341

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NOTES

On the nucleation mechanism of the b-d phase transition in theenergetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

L. Smilowitz, B. F. Henson, M. Greenfield, A. Sas, B. W. Asay, and P. M. DicksonLos Alamos National Laboratory, Los Alamos, New Mexico 87545

~Received 28 April 2004; accepted 22 June 2004!

@DOI: 10.1063/1.1782491#

We recently published1,2 two papers on theb-d solidstate phase transition in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine~HMX !. In these publications, we modeled thephase transition kinetics as a nucleation and growth phenom-enon with growth kinetics controlled by the thermodynamicsof the melt phase of HMX and nucleation kinetics fit empiri-cally. In this paper, we present data and discuss additionalconstraints on the nucleation mechanism in this system. Inour previous work, we discussed a mechanism that wasgreatly affected by the presence of a binder in the HMXbased formulation PBX 9501.3 PBX 9501 undergoes thephase transition in a more coherent way and at lower tem-peratures than its component HMX crystals. Since then, wehave further evidence for this and have separated out thechemical components of the binder to confirm that it is thenitroplasticizer4 which controls the nucleation rate in PBX9501. While the exact role in the mechanism has not beenworked out, HMX has been shown to be soluble in thenitroproponal/nitroformal mixture4 and we hypothesize thatthis solubility moves the lowest energy nucleation site to thecrystal surface, at the HMX binder interface. The HMX dis-solving in the nitroplasticizer serves as the nucleation site forthe phase transition to begin. In what follows we present dataon four formulations of HMX that support this hypothesis.

The materials used in this study are HMX crystals with aparticle size distribution centered at;120mm, PBX 9501, aformulation consisting of 95% HMX and a binder made fromestane and nitroplasticizer,3 and LX 14 consisting of 95.5%HMX in 4.5% estane.5 LX 14 is a formulation very similar toPBX 9501 but without the presence of nitroplasticizer, allow-ing us to separate the effects of the estane and the nitroplas-ticizer on the phase transition kinetics.

We first attempted to image defects within an HMXcrystal with a scanning confocal microscope and then imagethe onset of the phase transition in the crystal upon heating,using second harmonic generation~SHG! microscopy.6 Thecrystal is heated at 0.5 °C/min to a temperature of 165 °C andthen at 0.1 °C/min to 172 °C and held at this temperature foran hour to observe the transition. SHG images are taken at

intervals during this time to observe the progress of the tran-sition with integration times on the order of 10 s. Figure 1shows an overlay of the confocal image of a crystal with anSHG image taken early on in the transition process. Whilemany structural defects are observable in the optical image,the regions of convertedd-HMX imaged by SHG do notreflect any obvious, previously observable, feature of thecrystal. Furthermore, the separately converting regionswithin the crystal indicate separate and unrelated nucleationevents. Such images illustrate that the phase transition nucle-ation cannot be uniquely correlated with optically observabledefects in the crystal.

In the original papers, we noted that nucleation of thephase transition occurred at generally higher temperature andover a broader range of times and temperatures for isolatedcrystals of HMX than for the plastic bonded formulationPBX 9501. In Fig. 2 the effects of nitroplasticizer on thekinetics of the phase transition are demonstrated by compar-ing four different formulations based on HMX; HMX crys-tals pressed into a pellet without binder, HMX crystals wetwith nitroplasticizer, PBX 9501~HMX plus estane and nitro-plasticizer!, and LX 14 ~HMX plus estane!. These samplesallow us to separate the effects of the two individual compo-nents of the PBX 9501 binder—estane and nitroplasticizer.We observe that the HMX and LX 14 have qualitativelysimilar behaviors, while the HMX wet with nitroplasticizerbehaves qualitatively like PBX 9501. This is consistent withthe nitroplasticizer providing lower energy nucleation sitesby acting as a solvent for HMX at elevated temperature. Thetemperature profile used on all of the samples is a 5 °C/minramp to an isotherm at 163 °C. The sample is held at 163 °Cuntil the SHG signal plateaus, indicating that growth of thephase transition has been completed. Then a second ramp to172 °C is performed. For the PBX 9501 and HMX wet withnitroplasticizer, there is no further increase in SHG uponstepping up the temperature. This indicates that the fullsample has been transformed at the lower temperature. Forthe LX 14 and HMX samples, however, a further increase inSHG is observed at the higher temperature indicating that not

JOURNAL OF CHEMICAL PHYSICS VOLUME 121, NUMBER 11 15 SEPTEMBER 2004

LETTERS TO THE EDITORThe Letters to the Editor section is divided into three categories entitled Notes, Comments, and Errata. Letters to the Editor arelimited to one and three-fourths journal pages as described in the Announcement in the 1 July 2004 issue.

55500021-9606/2004/121(11)/5550/3/$22.00 © 2004 American Institute of Physics

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all of the HMX crystals in the sample had undergone thephase transition at the lower temperature. Furthermore, thepristine HMX sample had not reached a plateau in SHG sig-nal level by the end of the experiment, indicating that it wasstill undergoing the phase transition. The LX 14 behavesqualitatively like the HMX, but shows a phase transition oc-curring at slightly lower temperature than the HMX. An ex-planation of this slight lowering of temperature in the HMXestane formulation consistent with the hypothesis of bindersolubility would simply involve a very reduced, though non-zero, solubility of HMX in the estane as well as the nitro-plasticizer.

We hypothesize that the range of phase transition tem-

peratures observed for a distribution of HMX crystals un-coupled by binder reflects a range of nucleation energiescharacterizing different sites. We further hypothesize that dif-ferent solvent inclusions remaining within the crystals fromthe production of HMX would provide sites with differingnucleation energies that would correlate with the solubility ofHMX in the particular solvent included. In the PBX 9501formulation, the nitroplasticizer acts as a solvent for HMXcrystals at elevated temperature and acts as the lowest energynucleation site. Because all the HMX crystals in the sampleare in contact with the same rate limiting defect, PBX 9501undergoes the phase transition with a much more uniformrate than observed in the same distribution of HMX crystals.Conversely, formulations of these crystals with binders thatdo not readily dissolve HMX even at elevated temperaturesand therefore which do not couple nucleation will exhibit abroad range of nucleation energies~temperatures! as ob-served in the samples of Fig. 2.

These results impact the interpretation of much past andpresent work on this system. These effects are fully consis-tent with the SHG microscopy described in our previouspaper2 where we showed that for an isothermal bed of HMXcrystals some fraction of crystals in the bed underwent thetransition while some fraction did not. It also accounts forthe steps seen in the integrated SHG signal from a bed ofHMX crystals as opposed to a smooth sigmoid observed inPBX 95012 and for the structure observed recently in differ-ential scanning calorimetry data from samples of HMXcrystals.7 This variability in nucleation energy with samplepreparation~i.e., purification steps for pure HMX, the pres-ence of different binders in HMX formulations!, also ex-plains the observed ranges of phase transition kinetics re-

FIG. 1. An overlay of the confocal white light image of a single crystal ofHMX and the initial stages of thed phase conversion imaged via SHG~shown in green!.

FIG. 2. Comparison ofb-d phase tran-sition kinetics for samples of pressedHMX ~solid circles!, LX 14 ~opensquares!, PBX 9501~solid diamonds!,and HMX wet with nitroplasticizer~open circles!.

5551J. Chem. Phys., Vol. 121, No. 11, 15 September 2004 On the nucleation of a b-d phase transition

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cently reported,8 and the 40 °C range of transitiontemperatures reported in the past. All of these results areconsistent with the nucleation and growth model previouslypresented when the role of defects on nucleation energy aretaken into account. It should also be mentioned that the re-versibility of the transition has only been observed in ourlabs in the presence of nitroplasticizer. We have not beenable to drive HMX in the delta phase back to beta phaseHMX. It does, however, spontaneously transform to alphaphase upon storage at room temperature over periods of daysto months, depending on conditions such as the presence ofdecomposition products which we are not controlling.

Finally, this mechanism may fail sufficiently close to theequilibrium temperature where a distribution in energy fordifferent nucleation sites might be observable even in PBX9501. Also, we have observed that conducting very longheating experiments at temperatures close to the equilibriumtransition temperature in open cells can drive off nitroplasti-

sizer and change the state of the material enough to affect amore HMX like behavior.

1B. Henson, L. Smilowitz, B. Asay, and P. Dickson, J. Chem. Phys.117,3780 ~2002!.

2L. Smilowitz, B. Henson, B. Asay, and P. Dickson, J. Chem. Phys.117,3789 ~2002!.

3PBX9501 is composed of a bimodal distribution of HMX crystals~;120and 30mm! bonded with Estane and nitroplasticizer which is the eutecticmixture of bis~2,2-dinitropropyl!acetal and bis~2,2-dinitropropyl!formal. Itis 94.9% HMX, 2.5% estane, 2.5% nitroplasticizer, and 0.1% antioxidant,Irgonox.

4The nitroplasticizer is the eutectic mixture of bis~2,2-dinitropropyl!acetaland bis~2,2-dinitropropyl!formal.

5B. M. Dobratz and P. C. Crawford, Report No. UCRL--52997-Chg.2;1985 ~unpublished!.

6L. Smilowitz, Q. X. Jia, X. Yang, D. Q. Li, D. McBranch, S. J. Buelow,and J. M. Robinson, J. Appl. Phys.81, 2051~1997!.

7R. Weese, J. Maienschein, and C. Perrino, Thermochim. Acta401, 1~2003!.

8G. Scholtes, Impact, Friction, and Shear Workshop, Santa Fe, NewMexico, 2003.

5552 J. Chem. Phys., Vol. 121, No. 11, 15 September 2004 Smilowitz et al.

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