Lu Zhipeng, Xue Xianggui, Zhang Chaoyang
Institute of Chemical Materials, China Academy of Engineering Physics (CAEP), P. O. Box 919-327, Mianyang, Sichuan 621900, China.
Phys Chem Chem Phys. 2017 Nov 29;19(46):31054-31062. doi: 10.1039/c7cp06363f.
Dihydroxylammonium 5,5'-bistetrazole-1,1'-diolate (TKX-50) is a new and attractive energetic material that outperforms numerous common explosives because of its excellent properties and performance, and is thus a promising candidate to replace some of them. Nevertheless, knowledge of its physico-chemical properties, in particular, the underlying mechanism for it undergoing external stimuli for complete decay still remains poor. In the present study, we ascertain a preferred slip system of (010)/[101] and a shear-induced phase transition of TKX-50 with the aid of theoretical calculations. In other words, a new phase of TKX-50, γ-TKX-50, is observed to be formed by shearing TKX-50 along a slip system of (010)/[101] or (010)/[101[combining macron]] with a space group of P2/a, elevated energy of 9.4 kcal mol and a unit cell expanded 4%, relative to the original TKX-50. Moreover, γ-TKX-50 can most readily be formed by shearing TKX-50 along (010)/[101] with a lowest energy barrier of 18.6 kcal mol, which is much below that for TKX-50 decay. The predicted elastic constants of γ-TKX-50 verify its mechanical stability with decreased mechanical anisotropy relative to the original TKX-50. In addition, we find that, after phase transition, the hydrogen bonding is weakened, while the electrostatic repulsion of HH increases, which disfavors the proton transfer from NHOH to CON to facilitate the thermal decay of TKX-50. This suggests that the shear-induced transition from TKX-50 to γ-TKX-50 can enhance thermal stability by elevating the energy barrier for proton transfer, potentially contributing to the low mechanical sensitivity of TKX-50. Hopefully, this study would enrich the insight into the underlying mechanism of TKX-50 against external thermal-mechanical stimuli. Moreover, in combination with the newly found heat-induced phase, the shear-induced phase observed in the present study and the original one, there are at least three phases for TKX-50.
5,5'-双四唑-1,1'-二醇二羟铵盐（TKX-50）是一种新型且具有吸引力的含能材料，因其优异的性能而优于众多常见炸药，因此是替代其中一些炸药的有前途的候选材料。然而，对其物理化学性质的了解，特别是其在外部刺激下完全分解的潜在机制仍然不足。在本研究中，我们借助理论计算确定了（010）/[101]的优先滑移系和TKX-50的剪切诱导相变。换句话说，观察到通过沿（010）/[101]或（010）/[101[上加横线]]的滑移系剪切TKX-50形成了一种新的TKX-50相，即γ-TKX-50，其空间群为P2/a，能量比原始TKX-50升高了9.4千卡/摩尔，晶胞膨胀了4%。此外，γ-TKX-50最容易通过沿（010）/[101]剪切TKX-50形成，其最低能垒为18.6千卡/摩尔，远低于TKX-50分解的能垒。预测的γ-TKX-50弹性常数验证了其力学稳定性，且相对于原始TKX-50，力学各向异性降低。此外，我们发现相变后氢键减弱，而HH间的静电排斥增加，这不利于质子从NHOH转移到CON以促进TKX-50的热分解。这表明从TKX-50到γ-TKX-50的剪切诱导转变可以通过提高质子转移的能垒来增强热稳定性，这可能是TKX-50机械感度较低的原因。希望本研究能丰富对TKX-50抵抗外部热机械刺激的潜在机制的认识。此外，结合新发现的热致相、本研究中观察到的剪切致相以及原始相，TKX-50至少有三个相。
