Hongwei Qiu

RDX-based nanocomposite microparticles for significantly reduced shock sensitivity

Cyclotrimethylenetrinitramine (RDX)-based nanocomposite microparticles were produced by a simple, yet novel spray drying method. The microparticles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and high performance liquid chromatography (HPLC), which shows that they consist of small RDX crystals (∼0.1-1 μm) uniformly and discretely dispersed in a binder. The microparticles were subsequently pressed to produce dense energetic materials which exhibited a markedly lower shock sensitivity. The low sensitivity was attributed to small crystal size as well as small void size (∼250 nm). The method developed in this work may be suitable for the preparation of a wide range of insensitive explosive compositions.

Nanoscale 2CL-20·HMX high explosive cocrystal synthesized by bead milling

Energetic nanoscale 2CL-20·HMX, a cocrystal of CL-20 and HMX in a 2 : 1 molar ratio, was prepared by a novel method of bead milling an aqueous suspension of e-CL-20 and β-HMX. The conversion of the coformers to the cocrystal form was monitored by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis of specimens sampled at various milling times. Complete conversion to the cocrystal form was achieved by 60 minutes of milling. Rounded 2CL-20·HMX cocrystal particles with a mean size below 200 nm were produced. A mechanism for the conversion of the coformers to the cocrystalline form is postulated based on the experimental results. As an inherently safe manufacturing method, the aqueous bead milling process has great potential in advancing cocrystal research and applications in the field of energetic materials.

Inkjet-printed graphene for flexible micro-supercapacitors

Here we report our multi-institutional effort in exploring inkjet printing, as a scalable manufacturing pathway of fabricating graphene electrodes for flexible micro-supercapacitors. This effort is founded on our recent discovery that graphene oxide nanosheets can be easily inkjet-printed and thermally reduced to produce and pattern graphene electrodes on flexible substrates with a lateral spatial resolution of ∼50 µm. The highest specific energy and specific power were measured to be 6.74 Wh/kg and 2.19 kW/kg, respectively. The electrochemical performance of the graphene electrodes compared favorably to that of other graphene-based electrodes fabricated by traditional powder consolidation methods. This paper also outlines our current activities aimed at increasing the capacitance of the printed graphene electrodes and integrating and packaging with other supercapacitor materials.

Dependence of Raman Spectral Intensity on Crystal Size in Organic Nano Energetics.

Raman spectra for various nitramine energetic compounds were investigated as a function of crystal size at the nanoscale regime. In the case of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20), there was a linear relationship between intensity of Raman spectra and crystal size. Notably, the Raman modes between 120 cm−1 and 220 cm−1 were especially affected, and at the smallest crystal size, were completely eliminated. The Raman spectral intensity of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), like that of CL-20s, depended linearly on crystal size. The Raman spectral intensity of 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), however, was not observably changed by crystal size. A non-nitramine explosive compound, 2,4,6-triamino-1,3,5- trinitrobenzene (TATB), was also investigated. Its spectral intensity was also found to correlate linearly with crystal size, although substantially less so than that of HMX and CL-20. To explain the observed trends, it is hypothesized that disordered molecular arrangement, originating from the crystal surface, may be responsible. In particular, it appears that the thickness of the disordered surface layer is dependent on molecular characteristics, including size and conformational flexibility. Furthermore, as the mean crystal size decreases, the volume fraction of disordered molecules within a specimen increases, consequently, weakening the Raman intensity. These results could have practical benefit for allowing the facile monitoring of crystal size during manufacturing. Finally, these findings could lead to deep insights into the general structure of the surface of crystals.