摘要
为探究影响共晶炸药晶体稳定性和爆轰能量的关键因素,基于第一性原理方法对六硝基六氮杂异伍兹烷/奥克托今(CL‑20/HMX)、六硝基六氮杂异伍兹烷/三硝基甲苯(CL‑20/TNT)、苯并三氧化呋咱/1,3,5‑三硝基苯(BTF/TNB)、3‑硝基‑1,2,4‑三唑‑5‑酮/5,6,7,8‑四氢四唑并三嗪 (NTO/TZTN)等16种共晶炸药的晶体结构、分子间相互作用、物理化学性质参数、晶体稳定性及爆轰能量进行了研究。结果表明,分子间氢键强度小于21 kJ·mo
图文摘要
The effect of hydrogen bonding in regulating crystal stability and detonation energy for 16 reported cocrystal explosive crystals.
寻求含能材料高能量和高安全性之间的最佳平衡是新型含能材料设计的关键问题,共晶技术是解决上述问题的潜在有效途
共晶炸药虽有平衡安全性和能量的潜力,但现有共晶的合成常以能量的损失为代
采用High Accuracy Atomistic Simulation for Energetic Materials (HASEM
本研究以苯并三呋喃/三硝基苯甲胺(BTF/MATNB)、苯并三呋喃/三硝基苯胺(BTF/TNA)、苯并三呋喃/三硝基甲苯(BTF/TNT)、苯并三呋喃/三硝基苯(BTF/TNB)、苯并三呋喃/三硝基氮杂环丁烷(BTF/TNAZ)、六硝基六氮杂异伍兹烷/苯并三呋喃(CL‑20/BTF)、 CL‑20/TNT、CL‑20/DNB、六硝基六氮杂异伍兹烷/丁内酯(CL‑20/Butyolactone)、六硝基六氮杂异伍兹烷/二甲基甲酰胺(CL‑20/DMF)、CL‑20/NMP、CL‑20/HMX、六硝基六氮杂异伍兹烷/二恶烷(CL‑20/DIOXANE)、NTO/TZTN,苦味酸/硝基萘(Picric acid/Nitronaphthalene)、三硝基甲苯/硝基萘(TNT/Nitronaphthalene)共16种共晶炸药为研究对象,以单晶X射线衍射技术获取的晶格信息和原子坐标作为输入,基于共轭梯度法优化晶体结构。当每个原子的残余力小于0.03 eV/Å,且晶体内应力分量小于0.1 GPa时,晶体结构达到平衡态。

图1 16种共晶炸药晶胞体积和晶格常数的计算与实验结
Fig.1 Comparison of the calculated and experimental values of the unit cell volumes and the lattice constants of 16 cocrystal explosives
假设共晶炸药由n种组分分子构成,第i种组分分子的计量百分比为Ki,%;该组分分子在晶体结构中的Hirshfeld面表面积为Si,
(1) |
采用键能表征氢键非键部分的相互作用强度,kJ·mo
(2) |
式中,为态密度,为氢原子与受体原子的哈密顿矩阵交叠量,为费米能级。
对于晶体结构,采用晶格能LE,eV表征各组分分子结合成为晶体的强度:
(3) |
对于团簇结构,分子结合的稳定程度采用相互作用能,eV表征,
(4) |
式中,表示第i种组分分子在真空下的总能量,eV;表示共晶晶体的总能量,eV;表示团簇在真空下的总能量,eV。值越大,表示晶体结构越稳定;的负值越大,表示团簇结构越稳定。
氮含量由单位体积内的氮元素质量表征:
(5) |
式中,ρcocrystal为共晶炸药的晶体密度,g·c
为避免不同实验测量条件引起数据差异的影响,本研究中所有晶体结构、分子间相互作用、结构稳定性、爆轰性能的数据均为HASEM软件的计算结果。

a. Hydrogen bond amount

b. Hydrogen bond strength
图2 16种共晶炸药晶格能与氢键的关系
Fig.2 Correlation between the lattice energy and hydrogen bond of 16 cocrystal explosives.
如
爆速(Vd)和爆压(pd)基于共晶炸药的晶体结构直接计算获得,因而包含了炸药的化学组成及分子的空间堆积效应。

a. Correlation between density and detonation velocity

b. Correlation between density and detonation pressure

c. Correlation between nitrogen density and detonation velocity

d. Correlation between nitrogen density and detonation pressure

e. Correlation between oxygen balance and detonation velocity

f. Correlation between oxygen balance and detonation pressure
图3 16种共晶炸药的晶体密度、氮含量、氧平衡与爆速、爆压的关系
Fig.3 Correlation between the crystal density, nitrogen density, and oxygen balance with the detonation performance of the 16 cocrystal explosives.
由于共晶炸药的组分分子往往从已合成的高氮分子中选取,使得共晶炸药的氮含量大多分布于0.5~0.92 g·c
由此可见,未来新型共晶设计的主要方向,将是加强分子间氢键的强度,而非仅增大氢原子计量比、增加氢键数量以提升晶体稳定性。该策略能兼顾较好的氧平衡、氮含量和晶体密度,从而实现共晶炸药晶体稳定性和爆轰能量的实质性提升。
CL‑20和HMX分子边缘的‑NO2基团之间有较强的排斥作用,使得CL‑20/HMX共晶的氢键强度较弱(如

图4 CL‑20/HMX共晶、CL‑20晶体及HMX晶体中氢键含量的对比
Fig.4 Comparison of the hydrogen bonding amount among CL‑20/HMX, CL‑20, and HMX crystal structures
在CL‑20/HMX共晶结构中,组成共晶的CL‑20分子包括γ、β和ζ三种构象,不同的硝基取向使得CL‑20分子与HMX分子形成了较密实堆积(packing coefficient,PC,77.7%),取得了较好的晶体密度(2.00 g·c
因此,若采用先进的实验技术调整CL‑20、HMX分子的构象类型和取向,有望进一步提高不同组分分子之间的氢键强度、氢键数量和晶体密度,实现CL‑20/HMX共晶晶体稳定性和爆轰能量的进一步提升。

a. Symmetry of TATB

b. Construction of CL‑20/TATB structure

c. Stable CL‑20/TATB structure

d. Intermolecular interactionenergies of the TATB/CL‑20 bimolecular clusters
图5 TATB/CL‑20双分子团簇的堆积模型及其结构稳定性评估
Fig.5 Stacking modes and stability evaluation of the designed TATB/CL‑20 bimolecular clusters
基于共轭梯度算法进行结构优化后,24种团簇结构对应的分子间相互作用能如
为评估CL‑20与TATB的相互作用强度,将该能量与19种具有广泛代表性的炸药小分子之间的相互作用能 (E1~E19)进行对比。炸药E1~E19涵盖了─NO2、─NH2、苯环等传统炸药常见基团和结构,其相互作用分布可覆盖典型炸药的结合强度范
由此可见,CL‑20、TATB分子之间可以形成氢键,并具有可观的相互作用能。若采用先进的实验技术调整CL‑20分子的构象和取向,有望与TATB分子形成三维氢键网络而成功合成CL‑20/TATB共晶晶体。
基于第一性原理方法计算了CL‑20/HMX、CL‑20/TNT、BTF/TNB、NTO/TZTN等16种共晶炸药的晶体结构、氢键数量和氢键强度等分子间相互作用,氮含量、晶体密度、氧平衡等物性参数,晶格能、分子间相互作用能等能量稳定性参数,以及爆压、爆速等爆轰参数。相对于传统单组分炸药,所研究的共晶炸药具有较好的氮含量和氧平衡分布,但晶体密度普遍较小,导致爆轰能量相对于单组分炸药并无明显优势。本文主要结论如下:
(1) 氢键是影响共晶炸药晶体稳定性的关键因素,氢键数量的增大和氢键强度的增加共同促进分子间的结合;分子间氢键强度大于21 kJ·mo
(2) 以CL‑20共晶炸药为例,共晶设计需提升分子间氢键的强度,而非仅增大氢原子计量比、增加氢键数量以提升晶体稳定性。该策略能兼顾较好的氧平衡、氮含量和晶体密度,从而实现共晶炸药晶体稳定性和爆轰能量的实质性提升。
(责编: 高 毅)
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