1.School of Mechatronical Engineering, Beijing Institute of Technology, Beijing 100081, China 2.Institute of Chemical Materials, CAEP, Mianyang 621900, China
Fund Project:Project supported by the Joint Fund of the National Natural Science Foundation of China and the China Academy of Engineering Physics (Grant Nos. U1530262, U1330202), the National Natural Science Foundation of China (Grant No. 21875230), and the Presidential Foundation of CAEP (Grant No. YZJJLX2016005).
Received Date:15 January 2019
Accepted Date:27 March 2019
Available Online:01 May 2019
Published Online:20 May 2019
Abstract:The thermodynamic properties of insensitive high explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) based polymer bonded explosives (PBXs) are investigated by using first principle calculation and molecular dynamics simulation. The results include the phonon dispersion relations, interface thermal conductances, and thermal conductivities of TATB based PBXs. Both TATB and PVDF structures are optimized, in which the optimized lattice constants accord with previous results. The phonon dispersion relation of TATB and PVDF are calculated based on lattice dynamics. All interatomic force constants are calculated by the finite displacement method (numerical derivatives from perturbed supercells). The calculated phonon dispersion relation of TATB and heat capacity are in general agreement with experimental and theoretical results. The imaginary frequencies are observed in both TATB and PVDF dispersion relation. The imaginary frequencies are mainly due to the smaller calculated supercell size and temperature effect. The phonon mode of TATB and PVDF are assigned at Γ point. Based on the calculated phonon dispersion, some information including heat capacity, phonon density of states and phonon mode assignment is derived. The TATB possesses 144 phonon modes including 3 acoustic-phonon modes and 141 optical phonon modes. The anylized phonon mode of TATB shows that -NO2 dominates the phonon DOS in low frequency zone, phenyl rings dominate in middle frequency zone and -NH2 dominates in high frequency zone. By analyzing the phonon density of states and capacity, both TATB and PVDF imply that low-frequency vibration dominates the thermal conductivity. The thermal conductivity is determined for TATB by using the equlibrium molecular dynamics method and an established TATB force field. The TATB model is built with 2880 atoms. The structure of TATB is optimized by using molecular mechanics, then this system is relaxed by using a Nose-Hoover thermostat and barostat with a damping factor of 50 fs cin time steps of 0.1 fs. The calcultated thermal conductivity at room temperature shows good agreement with experimental result. The interface thermal conductance of TATB-PVDF is calculated by using a diffusive mismatch model. The interface thermal transport still follows Fourier’s law of heat conduction, and ballistic thermal transport mechanism is not involved. By using the above results, the thermal conductivity of mixture TATB-PVDF system is analized with a simple series model. The particle size smaller than 100 nm significantly suppresses the mixture system thermal conductivity. Keywords:triaminotrinitrobenzene/ first principle calculation/ molecular dynamics/ thermodynamic properties
声子色散关系是固体的一个重要物理量, 可以反映出晶格振动模式与动量、能量的关系. 声子色散关系中, Γ点处的晶格振动与材料的红外和Raman特性有关. 基于冻声子法, 我们得到了TATB, PVDF的声子色散关系和声子振动模式结果(见图2(a)和图2(b)以及图3(a)和图(b)). TATB晶体的单胞有48个原子, 共有144个振动模式, 其中包含3个声学支和141个光学支. 图 2 (a), (b) TATB和PVDF声子色散曲线; (c) TATB晶格热容随温度的变化; (d), (e) TATB和PVDF第一布里渊区高对称点积分路径 Figure2. The phonon dispersion relation of TATB (a), and PVDF (b); (c) the heat capacity of TATB as a function of temperature; the Brillouin zone and high symmetry points of TATB (d), and PVDF (e).
图 3 (a), (b) TATB和PVDF的Γ点声子模分析; (c), (d) TATB和PVDF的加权声子态密度和声子态密度(小图) Figure3. Decomposition of the gamma-point eigenmodes of TATB (a), and PVDF (b); the weighted phonon density of states of TATB (c), and PVDF (d).
表1TATB部分Raman活性模计算数值与文献结果的比较 Table1.Comparison of the Raman modes of TATB crystal obtained in the present and previous calculations with experimental results.