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CL-20晶体来自中国工程物理研究院化学材料研究所。制备工艺为:首先以苄胺和乙二醛为原料(摩尔比2.2∶1),在介质乙腈/水(体积比11∶1)中以有机酸为催化剂在0 ℃下缩合16~18 h,制取六苄基六氮杂异伍兹烷(HBIW)。然后以Pd/C作为催化剂,乙酸酐作为溶剂,溴苯作为氢解助剂,将HBIW在氢气中氢解18 h。再用热氯仿对产物进行萃取、蒸馏得到粗品,用乙腈进行重结晶得到较纯的四乙酰基二苄基六氮杂异伍兹烷(TADBIW)。最后对TADBIW二次脱苄并分离产物,再将脱苄产物硝化,制得CL-20。
实验采用的测量样品为直径13 mm、厚度1.5~2.0 mm的圆形药片。制备流程为:取适量CL-20晶体在玛瑙研钵中轻轻研磨,通过200目筛过滤得到颗粒直径小于74 μm的样品粉末。再取70 mg样品粉末与400 mg聚四氟乙烯(PTFE)混合均匀,经压片机压制成药片。PTFE是一种熔点高(327 ℃)且对太赫兹波段吸收弱的合成材料,在样品制备中加入PTFE既能有效防止CL-20过度吸收,还可大幅提升样品的压制成型性能。
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实验采用的太赫兹时域光谱系统(THz-TDS)主要由飞秒激光器(MaiTai,Spectra-Physics)和太赫兹时域光谱测试仪(Z-3,Zomega)构成,如图1所示。系统的工作原理为:首先,由飞秒激光器产生中心频率为800 nm,脉宽为80 fs,震荡频率为80 MHz的激光脉冲,并经过偏振分束棱镜(CBS)分离为100 mW和20 mW的泵浦光与探测光。泵浦光经反射和透射镜组聚焦入射至光导天线间隙(200 μm),产生光生载流子,根据麦克斯韦方程[16],光生载流子在外加电场的调制下辐射出太赫兹波。探测光在经过延迟平移台后与携带有样品信息的太赫兹波共线入射至碲化锌<110>晶体(ZnTe)(厚度2 mm),由光电探测器实现相干探测。为避免水分引起的太赫兹波强吸收对实验产生干扰,将系统置于密闭空间并持续充入干燥空气。测量环境条件保持相对湿度<1%,温度23 ℃。
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将THz-TDS系统测量的参考信号和样品信号分别记为
$ {E_{{\text{ref}}}}\left( t \right) $ 和$ {E_{{\text{sam}}}}\left( t \right) $ ${{E}}_{\mathrm{s}\mathrm{a}\mathrm{m}}\left({t}\right)$ 。其中,参考信号为太赫兹波经过干燥空气的信号,样品信号为太赫兹波透过待测样品的信号。对上述时域信号分别进行快速傅里叶变换,得到频域信号$ {E_{{\text{ref}}}}\left( \omega \right) $ 和$ {E_{{\text{sam}}}}\left( \omega \right) $ 。通过公式(1)和(2)计算获得振幅比$ \rho \left( \omega \right) $ 和相位差$ \varphi \left( \omega \right) $ :$$ \rho \left( \omega \right) = \frac{{\mathop A\nolimits_{{\text{sam}}} }}{{\mathop A\nolimits_{{\text{ref}}} }} $$ (1) $$ \varphi \left( \omega \right) = \mathop \varphi \nolimits_{{\text{sam}}} - \mathop \varphi \nolimits_{{\text{ref}}} $$ (2) 折射率
${n}(\omega)$ 和吸收系数${α}(\omega)$ 计算公式如下所示:$$ n\left( \omega \right) = \varphi \left( \omega \right)\frac{c}{{\omega d}} + 1 $$ (3) $$ \alpha \left( \omega \right) = \frac{2}{d}\ln \left( {\frac{{4n\left( \omega \right)}}{{\rho \left( \omega \right){{\left[ {n\left( \omega \right) + 1} \right]}^2}}}} \right) $$ (4) 式中:c为空气中光速;ω为角频率(
$ \omega = 2\pi f $ );d为样品厚度。计算获得的吸收数据通过Origin2018软件中的Savitzky-Golay方法进行平滑处理,窗口点数设置为33,多项式阶数为5。基于DFT对ε和γ相CL-20的低频振动特性开展模拟计算。DFT计算利用Materials Studio软件包的DMol3模块来完成,使用的晶胞参数来自于剑桥晶体数据库(Cambridge Crystallographic Data Centre, CCDC)。计算过程中首先根据能量最小化原则对CL-20进行几何结构优化,在优化结果偏差较小的基础上开展振动频率和强度计算,计算结果最终通过洛伦兹曲线拟合绘制成吸收谱。
Terahertz spectral properties of temperature induced phase transition of CL-20
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摘要: 2,4,6,8,10,12-六硝基-2,4,6,8,10,12-六氮杂异伍兹烷(HNIW, CL-20)是一种具有笼形结构的高能化合物,在军事及民用领域具有广阔的应用前景。为了深入理解CL-20晶体相变物理机制,采用太赫兹时域光谱技术(THz-TDS)对23.0~179.8 ℃升温范围内的吸收光谱进行了研究。热作用下太赫兹光谱的显著变化表明136.8 ℃时CL-20开始发生不可逆相变,结合固态密度泛函理论(DFT)计算鉴别该转变为ε→γ相变。并且,对CL-20低频振动特性的分析表明,分子笼型骨架的振动模态在相变中发生了显著改变,分子间广泛的范德华相互作用是其重要来源。此外,骨架外硝基基团的旋转振动演变与分子的氢键作用密切相关。该研究为进一步理解CL-20在高温加载下发生相变乃至爆轰/爆燃过程中的复杂物理机制提供了参考,对新型CL-20基优质炸药的设计和合成具有重要意义。Abstract: 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexazazoisowoodane (HNIW, CL-20) is a high-energy com- pound with a cage structure, which has broad application prospect in military and civil fields. In order to further understand the physical mechanism of the phase transition of CL-20 crystal, the absorption spectra in the temperature range of 23.0-179.8 ℃ are studied by using the terahertz time-domain spectroscopy (THz-TDS). The significant change of terahertz spectrum under thermal action indicated that the irreversible phase transition of CL-20 begins at 136.8 ℃. Combined with the results of solid-state density functional theory (DFT), the transition is identified as ε→γ-CL-20. Moreover, the analysis of the low-frequency vibration characteristics of CL-20 shows that the vibration modes of the molecular cage skeleton change significantly during the phase transition. The extensive van der Waals interactions between molecules are the source of this change. In addition, the evolution of the rotational vibration of the nitro group outside the framework is closely related to the molecular hydrogen bonding. This study provides a reference for further understanding the complex physical mechanism of CL-20 phase transition and detonation/deflagration under temperature loading. It is of great significance for the design and synthesis of high-quality explosives based on CL-20.
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