HMX不同晶型热膨胀特性及相变的ReaxFF分子动力学模拟

Thermal expansion behaviors and phase transitions of HMX polymorphs via ReaxFF molecular dynamics simulations

HMX(octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine)晶体不同晶型对温度的响应对于深入认识含能材料在外加载荷下的感度、稳定性、相变等具有重要意义.采用ReaxFF-lg势函数和等温等压分子动力学(NPT-MD)方法,研究了β-,δ-和α-HMX三种固态晶型在T=303—503K内的晶体结构和分子结构.结果表明,ReaxFF-lg势函数能够合理地描述HMX晶体的热膨胀行为,计算得到的晶体结构和热膨胀系数与实验数据比较符合.三种晶型的线膨胀系数表明,β-HMX晶体具有明显的热膨胀各向异性;δ-HMX晶体c轴的热膨胀特性与a,b轴略有不同;α-HMX晶体三个方向的热膨胀特性基本相同.三种晶型的体膨胀系数为δ-HMX>α-HMX>β-HMX,表明δ-HMX晶体对温度最敏感,这可能是δ-HMX晶体具有更高感度的原因.在T=443K时,β-HMX晶体的晶胞参数突变,且部分分子构型由"椅式"转变为"船式-椅式".采用两相热力学模型(2PT)得到的亥姆赫兹自由能表明,β-HMX晶体在T=423—443K时发生相变.β-,δ-和α-HMX晶体分别在T=303—423K,T=443—503K和T=363—423K内稳定存在.

The response to temperature of HMX polymorph is important for understanding the sensitivity, stability, and phase transitions of energetic material. Using ReaxFF-lg with isothermal-isobaric molecular dynamics （NPT-MD） methods, the crystal and molecular structures of β-, δ-, and α-HMX crystals in a temperature range of 303-503 K and at atmospheric pressure are investigated. The calculated crystal structures and thermal expansion coefficients are in general agreements with experimental results, indicating that ReaxFF-lg potential can correctly describe the thermal expansion of HMX polymorph. The linear thermal expansion coefficients indicate that the thermal expansion of β-HMX is anisotropic, the thermal expansion along e axis is slightly different from those for a and b axes for 6-HMX, and the thermal expansion along a, b, and c axes are almost the same for α-HMX. The volume expansion coefficients for the three phases decrease in the following sequence： δ-HMX〉 α-HMX〉 β-HMX, showing that 6-HMX is the most sensitive to temperature in the three crystals, which may be the reason for higher sensitivity of this phase. Sharp changes in lattice parameter and molecular conformation transformation from ＂chair＂ to ＂boat-chair＂ occur for β-HMX when temperature reaches 443 K. Helmholtz free energy derived from the two-phase thermodynamics （2PT） model suggests a phase transition for β-HMX at T = 423- 443 K. The β-, δ-, and α-HMX crystals are stable in the temperature ranges of 303-423 K, 443-503 K, and 363-423 K, respectively.