Dependence of particle morphology and size on the mechanical sensitivity and thermal stability of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

doi:10.1016/j.jhazmat.2008.02.009

Journal of Hazardous Materials

Volume 159, Issues 2–3, 30 November 2008, Pages 222-229

https://doi.org/10.1016/j.jhazmat.2008.02.009 Get rights and content

Abstract

Three kinds of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) samples with spherical (β-phase), needle (γ-phase) and polyhedral (β-phase) shapes were fabricated by wet milling, solvent/non-solvent and riddling methods, respectively. By changing the technical conditions, HMX powders with different particle sizes were obtained for each kind of sample. All as-prepared samples were characterized by laser granularity measurement, scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Taking advantage of mechanical sensitivity tests, slow cook-off tests and differential scanning calorimetry (DSC) analysis, the mechanical sensitivity and thermal stability of HMX samples were found to depend on particle size and morphology. Results indicated that particle size played a significant role in the safety of HMX, and that morphology regulated the experimental results, i.e., for each kind of HMX samples, the mechanical sensitivity and thermal stability of HMX changed if the particle size differed. However, the trends of these changes exhibit much variance if the microstructure of the HMX particles is altered. Consequently, the difference in safety for these kinds of samples has to do with their specific morphology.

Introduction

The wide application of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) has prompted vigorous efforts to understand and improve its safety. Among the factors affecting the safety of explosives (such as physical and chemical structures, charge diameter and density, etc.), microstructure and size of explosive particles play significant roles [1], [2], [3], [4].

It is not strange to study the effects of particle size on mechanical sensitivity of explosives, but the reported results are controversial. Liu found that the friction sensitivity of explosives decreased almost linearly as the particle size was reduced from 154 to 10 μm [5]. However, Yang investigated the friction sensitivity of RDX particles sized 8.95, 12.78, 54.89 and 640 μm, and the results showed no self-consistent trend [6]. One report has even shown an inverse relationship between friction sensitivity and particle size of HMX [7]. Fortunately, the reports about impact sensitivity tests were more consistent, i.e., the impact sensitivity of explosive always fell as the particles size decreased. Zhang compared the impact sensitivity of HMX with different particle sizes and found that the explosive probability decreased significantly if the particles size is less than 2 μm, which is attributed to the slighter crystal deficiency inside the smaller explosive particles [8], [9]. Simpson et al. also found that explosives with smaller particle size are more difficult to ignite, but that combustion following ignition has an increased likelihood of resulting in detonation [10], [11].

The effect of morphology on the safety and stability of explosive particles is also important. However, few researchers addressed this factor, which may be the cause of the discrepancy noted above. For example, if we study the relationship between mechanical sensitivity and particle size, we should select HMX particles with similar microstructure. Otherwise, the results would fluctuate greatly because the potential factor, i.e., the effect of morphology is neglected. Herein, we used three methods to fabricate three kinds of HMX samples with differing morphologies. For each kind of HMX samples, the effect of particle size on their safety is studied. Furthermore, as a highlight of this study, the influence of microstructure of HMX particles on its safety is discussed in detail.

Section snippets

Materials and fabrication

Raw HMX powders (d₅₀ = 85.9 μm, d₉₀ = 258.2 μm) were purchased from the Yinguang chemical plant of China. Using wet riddling, wet milling and solvent/non-solvent methods, three kinds of HMX samples were made from raw HMX. By controlling technical parameters, HMX particles with different average particle sizes (d₅₀) were obtained within each of the three kinds of samples (shown in Table 1).

Samples test

Sample morphologies were examined using a LEO field-emission scanning electron microscope (SEM). Particle size

Sample characterization

Fig. 1 provides SEM images of raw HMX powders and part of as-prepared samples. The differences in morphology among the four kinds of samples shown above are obvious. The raw HMX (Fig. 1(a)) has prismatic type microstructure with smooth particle surfaces. SEM image of Fig. 1(b) shows that HMX particles prepared after several hours of wet milling yield the nearly spherical crystal morphology. In Fig. 1(c), groups of needle-shaped particles representing loose agglomerates are visible, whereas the

Discussion

In combining the experimental results and theories together, we suppose that the unique microstructure of each kind of HMX particles relates to the “hot spot” growth mechanism [14]. In friction sensitivity tests, the friction among HMX particles is the main factor forming “hot spots”, which then cause detonation. For spherical β-HMX samples, the contact area upon particles surfaces and bulk density are apparently higher than those of needle particles when they undergo the same frictional

Conclusion

In Section 1, we not only summarized many reports about the effect of particle size on the mechanical sensitivity of explosives, but also showed the discrepancies among them. Therein, we speculated that the different microstructures of explosive particles caused the inconsistency in previous studies’ experimental results. In order to confirm our supposition, i.e., to investigate the influence of morphology on mechanical sensitivities and thermal stabilities of explosive, three kinds of HMX

Acknowledgements

This work was performed by several research groups in our center. The authors would like to thank Chengfang Che of National Special Superfine Powders Engineering Research Center in Nanjing University of Science & Technology for her enthusiastic support in granularity measurements and DSC analysis, and Dr. Haridwar Singh of High Energetic Materials Research Lab (HEMRL) of India who provided many helpful suggestions for our experiments.

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Dependence of particle morphology and size on the mechanical sensitivity and thermal stability of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

Abstract

Introduction

Section snippets

Materials and fabrication

Samples test

Sample characterization

Discussion

Conclusion

Acknowledgements

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Aqueous solubility and alkaline hydrolysis of the novel high explosive hexanitrohexaazaisowurtzitane (CL-20)

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