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Chemical Transformation of Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) Induced by Low Energy Electron BeamIrradiation

Tao Xu,* Ying Xiong, Fachun Zhong, Lin Wang, Xiaofei Hao, Hui Wang

Institute of Chemical Materials, Chinese Academy of Engineering Physics, 621900, Mianyang, P. R. Chinae-mail: xuzhtao@126.com

Received: May 13, 2010; revised version: June 21, 2010

DOI: 10.1002/prep.201000059

Abstract

The effects of 8.0 � 10�17 J (500 eV) and 3.2 � 10�19 J (2 eV)electrons on chemical structure of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) were studied in situ, under ultra-highvacuum conditions using a combination of X-ray photoelectronspectroscopy (XPS) and quadrupole mass spectrometry. XPSdata indicated that electrons impact by 8.0 � 10�17 J for 30 scaused a decrease in nitro group concentration, and a little shiftin the binding energy of the nitrogen 1s peak. Such a phenomen-on was found at very low kinetic energy (3.2� 10�19 J) with timeevolution. Quadrupole mass spectrometry detected gas desorp-tion after electron irradiation included H2O and H2 mostly. Mi-croscopy-IR spectroscopic investigations also proved that the in-tensity of nitro groups of HMX after irradiation decreased com-pared with those of the pristine HMX. We attributed the struc-ture changes obtained by XPS and IR spectroscopy result in achemical transformation, which was associated with low-energydissociative electron attachment (DEA) of surface contaminantsfollowed by deoxidization reactions to form the product mole-cules.

Keywords: HMX (Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazo-cine), Electron Irradiation, Chemical Transformation

1 Introduction

HMX is one of the most popular explosives with promis-ing energy, and used as a main constitute in polymerbonded explosives (PBX) [1]. In both cold war “partners”– USA and USSR since 1950th there were some paperspublished on interaction of HMX, RDX and other ener-getic materials with X-ray radiation, electron beams, UV/Vis spectroscopy etc. [2–6]. Past studies of the effects ofelectron irradiation on explosives have utilized muchhigher energy up to 9.6� 10�10 J (60 MeV) to create arapid and uniform energy deposition, and the exothermic

reactions in explosives were found which offered a newdynamic technique for studying the thermal initiationprocess. The high dose electron energy had an instant andmostly thermal effect on the explosives, and the mecha-nism was considered to be equal to the thermal decompo-sition ignoring the interaction of low energy electronsand explosive molecules. However, interactions betweenHMX and low energy electron beams are unavoidableduring the nuclear weapon stockpile and chemical orphysical analysis including electron microscopy, electrondiffraction methods, electron probe microanalysis(EPMA) and so on. Although electron beams are gener-ally known to be destructive to organic and polymericfilms, there have been few studies of the effects of lowenergy electron beam irradiation on explosives. The elec-tron dose required for significant damage and the natureof the resulting product are especially important. Theseare critical issues with respect to the sensitivity and safetyduring the explosive processing, stockpile and usage.

Several studies of electron irradiation-induced damageand the dependence on beam energy and irradiated dosehave been published [7–12]. They can be divided intophysical and chemical effects. The physical “knock-ondamage” in which direct electron collision with atomscaused a change in the crystal structure or the removal ofan atom, has a threshold at beam energy [13, 14].Chemical damage caused by electron-beam irradiation isvaried with the different materials and the irradiationconditions. Recently, low-energy electron-induced struc-tural and chemical degradation in the film has also beenobserved with a number of techniques for n-alkane thio-late and other functionalized thiolate-based SAMs. Fromthese works it has become clear that there is significantdamage introduced into C�H, Au�S, and S�C bonding

Propellants Explos. Pyrotech. 2011, 36, 499 – 504 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 499

sites, particularly leading to C�H bond rupture and thecreation of unsaturated C�C bonds [16–18].

In order to understand the effects of low energy elec-tron irradiation on HMX, especially for chemical struc-ture changes, the elemental composition of the HMX sur-face was studied by XPS using the monochromated X-raysource combined with the electron gun. Quadrupole massspectroscopy has been used to directly detect species re-moved by electron impact. The surface structure of explo-sives before and after electron irradiation was further de-termined by Microscopy-IR spectroscopic analyses. Theresults indicated the chemical structure of HMX has beentransformed due to the nitro groups decreased whereasthe atomic ratio of nitrogen remained the same. A trans-formation mechanism was proposed as three reactionsteps associated with low-energy dissociative electron at-tachment (DEA) of surface contaminants followed by de-oxidization reactions to form the product molecules.HMX undertaking the chemical transformation at suchlow energy electron irradiation has never been reportedbefore, which gives not only an adequate interpretationof interaction mechanism but also helps to understandthe sensitivity and safety issue during the explosive proc-essing, stockpile and usage.

2 Experimental

2.1 Preparation of HMX Samples

Commercial-grade HMX (with a purity >99%) was pro-vided by the Yinguang Chemical Plant, China. RawHMX powders were pressed into small charges (F=5 �3 mm), whose central point was selected to be irradiatedand characterized.

2.2 Electron-Beam Irradiation and XPS Characterizationof HMX Samples

The elemental composition and chemical states of HMXwere obtained by a VG250 X-ray photoelectron spectros-copy (VG Scientific, UK) equipped with a twin anode X-ray source, monochromated X-ray source, electron gun,and a hemispherical analyzer. The electron flood gun fab-ricated in X-ray photoelectron spectroscopy (XPS) hasbeen used as the irradiation source. The electron gun andthe X-ray source were located in the same vacuum cham-ber, which ensured the electron beam and X-ray beamwas focused on the same region of the sample in order toget the in situ irradiation and observations, as illustratedin Figure 1. The electron kinetic energy at 8.0� 10�17 Jand 3.2� 10�19 J was used in our work, and the irradiationtime was varied according to different electron energy.XPS spectra were recorded using Al-Ka source, operatedat a power of 150 W and at constant pass energy of100 eV for survey scan and 20 eV for narrow scan. TheC�H 284.6 eV was used to calibrate the sample bindingenergies, and determined the respective elements accord-ing to the reference book. After electron-beam irradia-

tion and XPS measurement, the surface color of HMXwas changed from white to yellow, and the pressure inthe vacuum chamber of XPS instrument increased from2�10�7 Pa to 4 �10�6 Pa, which might suggest some kindsof substance out gassed from the HMX.

2.3 Mass Spectrometry

For detecting neutral desorption, a quadrupole-based re-sidual gas analyzer (RGA) was installed in the analysischamber of the ESCALAB such that it was in line-of-sight of the HMX sample, which could be biased at differ-ent voltages with respect to ground.

2.4 Microscopy-IR Spectroscopic Characterization

The chemical nature of the components formed with thesurface region of the electron-beam irradiated HMXcharge was investigated by means of Microscopy-IR spec-troscopy. The IR spectra were recorded on a Nicolet 800FTIR spectrometer with a resolution of 4 cm�1, in therange of 4000 cm�1�400 cm�1. The magnification is about32� 15 mm.

2.5 Theoretical Calculations of HMX Molecule

DFT method as implemented in Gaussian 98 with basisset B3P86/6-311g (d, p) was used for HMX molecularstructure optimization and single point calculations. Vi-bration frequencies were calculated at the same level totake account of the zero point energy and to identify thestationary structures.

3 Results

3.1 In-situ XPS Characterization

Elemental analysis of the HMX surface (before electronirradiation) by XPS shows only carbon, nitrogen, andoxygen to be present; XPS does not detect hydrogen.

Figure 1. Sketch of electron irradiation in a XPS system. Theelectron beam and X-ray beam are focused on the same regionof the sample in order to get in-situ observation.

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Figure 2 shows the XPS N1s narrow scan spectra of HMXbefore and after electron irradiation with a kinetic energyof 8.0� 10�17 J. The N1s peak consists of two components,attributable to N*�N and N*�O, which corresponds tothe binding energy at 401 eV and 407 eV respectively.After irradiated for 30 s, the N1s peak was totallychanged to a wide, single peak as shown in Figure 2. TheN*�O peak at 407 eV decreased sharply and almost dis-appeared, and the N*�N peak was tended to widen andshift to higher binding energy at 402 eV. The results indi-cated the chemical structure of the HMX surface may bedramatically influenced by the electron irradiation,whereas the nitro groups� excitation by the electron beamleads to the chemical transformation.

In order to investigate the interaction mechanism be-tween the explosive and electron beams, the lower kineticenergy of 3.2 �10�19 J was used; the chemical transforma-tion of HMX was also found. The XPS N1s spectra withtime evolution were shown in Figure 3. In HMX molecu-lar structure, elemental nitrogen has two chemical stateswith equal molar ratio. Therefore, the intensity of N*�Nand N*�O peak is almost same theoretically. After elec-tron irradiation for 10 min, 20 min, 40 min and 60 min,the intensity of the N�O peak decreased slowly and thefull width of half maximum (FWHM) of the N�N peakincreased. For further study of the irradiation effect, theXPS quantitative analysis has been done and the elemen-tal concentration of HMX was obtained. From the valuesin Table 1, we found the nitro group concentration de-creased with time evolution, and changed from 48 at-%in the pristine HMX to 25 at-% after irradiated for100 min. The results showed that even very low electronenergy can cause chemical transformation of HMX withtime evolution. But the elemental concentration of nitro-gen was almost the same during the low electron energyirradiation, as shown in Table 2, which indicated the nitrogroups might be transformed to another structure includ-

ing nitrogen atoms instead of disassociated from HMXmolecular structure.

Figure 2. XPS of the N1s region of HMX before and after 8.0 �10�17 J electron irradiation.

Figure 3. Time evolution of XPS N1s spectra of HMX with3.2 �10�19 J electron irradiation.

Table 1. XPS quantitative results of nitro-group percentage as afunction of irradiation time with 3.2 � 10�19 J kinetic energy*.

Irradiation time/min NO2/at-% Irradiation time/min NO2/at-%

0 48.0 60 38.510 46.1 70 37.420 43.6 80 36.630 41.4 90 32.640 39.8 100 25.250 39.1

* Experimental uncertainties of composition were estimated tobe below 7%.

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Chemical Transformation of HMX by Low Energy Electron Beam Irradiation

Figure 4 shows neutral gases desorption from the HMXsample, with the data obtained using the quadrupole-based residual gas analyzer in the ESCALAB analysischamber. Before electron irradiation, the signals at m/z=28, 18, 16 and 2 are attributed to H2O and CO2 absorbedin the background chamber. After irradiation for 15 minat 3.2 �10�19 J, there are mostly neutral H2 and water de-tected which corresponded with the m/z at 18 and 2 re-spectively. H2 was not only produced by water electrolysisas the intensity of H2 peak increased higher than water.The molecular hydrogen must certainly result from thecombination of neutral or ionic hydrogen atoms originat-ed from C�H bond cleavage, which has been proved byother scientists in ESD experiments on the monolayersby low-energy electrons [15–18]. Detailed mechanism willbe discussed later. While no nitrogen-containing speciesare detected, extensive electron bombardment results indecrease of nitro groups from the surface on the HMXcharge. This result must further indicate that the nitrogroups are transformed to another structure instead ofdissociated from the HMX molecule.

3.2 Microscopy-IR Spectroscopic Analysis

Figure 5 shows the IR spectra of HMX before and afterelectron irradiation. The bands at 1590 cm�1 and1354 cm�1, observed on two spectra, are assigned to asym-metrical and symmetrical N�O stretching vibration. Thebands at 1318 cm�1 and 975 cm�1 are attributed to the C�N skeletal stretching modes in the HMX ring. Otherweak peaks are not found because the intensity of micros-copy-IR spectra is much lower than FT-IR spectra. Theelectron irradiation on the HMX charge results in a re-duction of the intensity of the n(N�O). The intensity ofN�O and C�N bands changed relatively with respect tothe corresponding values in the spectra of the pristineHMX, indicated the nitro groups decreased while the C�N percentage increasing after electron irradiation.

4 Discussion

Generally, the interaction mechanism between electronsand molecules is quite different from the interaction be-tween photons and molecules. Electrons will pass theenergy partially to the electrons of one molecule after in-teracted with each other, and diffuse as inelastic ones.Therefore, the relativistic energy of electrons is muchlower than the incident energy. Many reports [7–10] onthe high dose electron-beam irradiation indicate that irra-diation might cause heating and melting effects on mostsolid materials. However, HMX undertakes the chemicaltransformation even at such low electron energy, this isnot reported until now.

To understand such chemical transformation of HMX,the structure of b-HMX was geometry-optimized byB3P86/6-311G (d, p) method in our work. The results sug-gested that b-HMX has Ci symmetry which involves twokinds of bonds, i.e. A and E forms (as shown in Figure 6).

Table 2. Surface atomic percentage from XPS survey spectra asa function of irradiation time with 3.2 �10�19 J kinetic energy.

Irradiation time/min C1s/at-% N1s/at-% O1s/at-%

0 30.8 34.5 34.510 30.8 34.2 34.920 33.6 33.5 32.930 33.7 32.5 33.740 33.1 33.6 33.150 34.4 33.5 32.060 32.8 33.6 33.570 34.2 33.5 32.280 33.6 35.1 31.290 35.5 34.1 30.2

100 34.8 34.8 30.2

Figure 4. Mass spectrometry of neutral desorption from HMXsurface by 3.2 � 10�19 J electron irradiation.

Figure 5. Microscopy-FTIR spectra of HMX before and afterelectron irradiation.

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The dissociation energies of A and E form N�NO2 bondsare 192 kJ·mol�1 and 178 kJ·mol�1 respectively. Since theelectron energy used in the study is much lower than thebond dissociation energy of N�NO2 bonds, it is notenough to remove the nitro groups from the molecularstructure.

As we all know, the surface of most solid materials is acomprehensive and active region which may absorb manygases and contaminants from the atmosphere, mostly N2,H2O and hydrocarbon. Following the experimental resultsand the seminal works of Sanche [14–18], it is now under-stood that resonant dissociative electron attachment(DEA) is a significant channel for the initial depositionof energy into condensed phase systems and the genera-tion of molecular fragments, and that in many cases thesurface processes can be directly compared to their gas-phase analogues. DEA can be described as a two-stepprocess initiated by the capture of the incident electronleading to electronic excitation to a repulsive anionic po-tential energy surface; this is then followed by the frag-mentation into anionic and neutral species [15]. Row-ntree et al. have studied desorption of H� from con-densed amorphous hydrocarbon films, and reported astrong resonant process at Ei =10 eV for this dominantanion. It is proved that the most prominent processes inalkyl matrix are progressive conformational and disorder-ing and cleavage of C�H and C�C bonds by low-energyelectron bombardment [16–19].

In the process of irradiation, electrons will react withthe materials near the surface first and H2 produced ac-cordingly, described as Reaction step 1.

Reaction step 1: DEA process of contaminants ab-sorbed on the HMX surface.

R0�CHx þ e� ! ðR0�CHxÞ*� ! R0�CHx�1 þH� ð1aÞ

H� þ R0�CHx ! R0�CHx�1 þH2 þ e� ð1bÞ

Reaction step 2: DEA process of H2 molecule

H2 þ e� ! ðH2Þ*� ! HþH� ð2aÞ

H2 produced by contaminants may excited by the elec-trons again and then become some active particles withcertain electronic energy.

Reaction step 3: deoxidation reacted between the nitrogroups of HMX and hydrogen electrons

The active hydrogen particles will act as the deoxidiz-ing addition and intend to attack the electropositiveatoms. The nitro groups react with hydrogen electronsand will be deoxidized, meanwhile H2O molecules areproduced. It is correlated with the experimental results ofXPS that the concentration of the nitro group decreasedand the vacuum pressure dropped after irradiation.

Meanwhile, the effect of X-ray irradiation during XPSmeasurement on HMX was considered under the sameexperimental conditions and the results indicated therewas not any obvious difference by comparing the bindingenergy and the relative intensity before and after X-rayirradiation. Note that the major effect of X-rays is medi-ated by photons and secondary electrons, which is not assame as the primary electrons excitation.

It is a generally accepted idea that nitro groups repre-sent the primary cause of initiation reactivity of polynitroexplosives [20]. If the nitro groups transform to anotherstructure, the reaction center might change or lose the re-activity in some cases. Consequently, the properties suchas detonation, sensitivity, safety of HMX explosive mightbe changed after electron irradiation.

Figure 6. Optimized structures of b-HMX, bond length in10�10 m.

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Chemical Transformation of HMX by Low Energy Electron Beam Irradiation

5 Conclusion

The effects of electron irradiation on HMX were studiedin a XPS. It is found that the surface chemical structureof HMX is changed even under low electron energy irra-diation with time evolution. The nitro groups are trans-formed to another structure including nitrogen atoms in-stead of disassociated from the HMX molecular structurebecause the concentration of the nitro group decreaseswhereas the atomic ratio of nitrogen remains almost thesame. Microscopy-IR spectra also prove that the intensityof nitro groups of HMX after irradiation decrease com-pared with those of the pristine. The transformationmechanism was associated with low-energy dissociativeelectron attachment (DEA) of surface contaminants fol-lowed by deoxidization reactions to form the productmolecules. The results indicate that care must be takenwhen using electron beams as source energy. Furtherstudies of these effects in a wider range would be valua-ble to investigate the chemical reactions taking place inmore detail.

Acknowledgments

This work was financially supported by the Special Foundationof CAEP (2011 A0302013). We would thank Dr. Zhong Wei andDr. Chaoyang Zhang for their guidance and instructions.

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