2. 嘉兴学院生物与化学工程学院, 浙江 嘉兴 314001
2. College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China
太安(PETN, 季戊四醇四硝酸酯,
感度即安全性,是高能物质最重要性能之一。先前的微观理论判据主要是通过量子化学计算求得的, 用于高能分子较多[15]。对于成千上万原子的超晶胞或复合材料, 近年来已有关于HMX/AP[16, 17]、HMX基PBX[18, 19]、RDX完美和缺陷晶体及RDX基PBX[20, 21]感度判别的工作发表。我们所建议的引发键最大键长(
力学性能也是高(含)能材料最重要的性能之一。过去的十年, 本课题组已开辟了高能复合材料(PBXs、发射药和推进剂)弹性力学性能的预测研究[22-30], 主要都是基于静态力学分析。而收集MD平衡后的原子运动全轨迹加以分析的波动法[31], 虽然耗机时较多, 但对力学性能的预测相对要准确可靠, 故本研究试以波动法求得PETN晶体的力学性能。
在物质结构-性能微观理论研究中, 经典MD模拟较量子力学(QM)计算有一个优点, 就是能求得庞大复杂体系结构、性能随温度递变的统计平均规律。因为研究易爆燃物质的感度和力学性能及其温度效应极为重要, 故而本实验以PETN晶体为研究对象, 选择构建其(4×3×4)超晶胞, 并选择其较为钝感的(100)面[3, 13, 32]加以切割分面, 将所得这两种模型, 在5种不同温度(195, 245, 295, 345,395 K)和NPT系综下, 进行MD模拟研究, 重点探讨结构、能量和力学性能及其温度效应, 寻求和核准微观感度理论判据。
2 模型搭建和模拟细节 2.1 力场选择和模型搭建本研究之所以选择COMPASS力场[33], 是因为该力场能较好地适用于凝聚态, 包括硝酸酯及其相关物[34-36]。以PETN的X射线衍射晶体数据[37-38]为依据, 构建PETN(4×3×4)超晶胞并置于具周期性边界条件的周期箱中; 每个周期箱中含96个PETN分子, 共2784个原子, 即为PETN晶体模型。沿PETN(4×3×4)超晶胞(100)晶面方向进行“切割”, 并使Z轴平行晶轴
将搭建好的PETN和PETN(100)两种晶体模型分别在COMPASS[33]力场、NPT系综下进行MD周期性模拟研究。模拟过程中温度和压力的控制采用Anderson [39]和Parrinello[40]方法, 范德华(vdW)和静电作用(Coulomb)分别用Atom-Based[41]和Ewald[42]方法, 截断半径取9.5×10
表 1给出PETN晶体295 K下NPT-MD模拟所得晶胞参数和密度。为方便比较, 表中还给出了实验值[38]和Sorescu等的理论计算结果[1, 10, 43]。由表 1可见, 本工作模拟所得晶胞参数与前人的实验和计算值吻合较好, 晶胞参数相差很小或相等; 与实验密度相对误差也只有0.6%。由此进一步表明COMPASS力场对PETN晶体有较好的适用性, 也表明本研究的MD模拟确已达到平衡, 所得平衡结构可信。
MD模拟能提供各种体系在广义平衡结构下的键长统计分布, 包括在不同温度下的键长分布, 这是MD很有意义的优势所在。PETN的引发键一般认为是分子中的O—
由图 4可见, PETN和PETN(100)的引发键键长分布均呈近似对称的高斯型分布, 最可几键长和平均键长较为接近。PETN晶体引发键键长在1.350~1.480Å范围的占97%, PETN(100)在1.345~1.485Å范围的占98%。由表 2可见, 它们在295 K下的
定义硝酸酯类体系中引发键(O—
表 3和表 4给出PETN和PETN(100)晶体在不同温度下的
内聚能密度(CED)是单位体积1 mol凝聚态克服分子间作用力变为气态时所需能量。在MD模拟中CED是vdW力与静电力之和, 即分子的非键力。基于MD模拟轨迹所得不同温度下PETN晶体和PETN(100)的内聚能密度及其分量。见表 5。
由表 5可见, 随温度升高, PETN和PETN(100)的CED、vdW力和静电力均单调递减, 表明它们由晶态变为气态时所需能量变小, 这与温度升高感度增大的实验事实相一致。由此表明, 在一定条件下, PETN晶体的CED也可用于热感度相对大小的理论判据; 且在每个温度下PETN和PETN(100)的CED及其分量数值都惊人地一致, 表明CED的统计平均值不受MD模型的影响。
3.5 力学性能比较弹性模量是评价材料刚性的指标, 是材料抵抗弹性形变能力的度量[44]。而弹性性能和塑性性能是相关联的[45], 通常表示阻止塑性形变能力的硬度和拉伸强度与剪切模量呈正比; 断裂强度与本体模量呈正比; 本体模量与剪切模量的比值(
根据广义虎克定律, 反映应力应变关系的弹性系数应有36个; 因
通过PETN和PETN(100)晶体两种模型在5个温度下的NPT-MD模拟, 得出如下主要结论:
(1) 模拟所得PETN晶胞参数和密度与实验值接近, 表明COMPASS力场适用、所得平衡结构可信。
(2) 模拟求得PETN晶体的引发键(O—
(3) PETN晶体的引发键连双原子作用能(
(4) 由波动法求得PETN晶体的力学性能。弹性系数和弹性模量随温度升高而减小, 表明各向同性递增, 刚性降低。
(5) 切割分面研究PETN晶体, 能获得较确定的结构、相互作用能和力学性能及其温度效应。
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