Graduation Year

2007

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Physics

Major Professor

Ivan I. Oleynik, Ph.D.

Keywords

Materials modeling, Density functional theory, Equation of state, PETN, HMX, RDX, Nitromethane

Abstract

First-principles density functional theory calculations were performed on a number of important energetic molecular crystals, pentaerythritol tetranitrate (PETN), cyclotetramethylene tetranitramine (HMX), cyclotrimethylene trinitramine (RDX), and nitromethane. Simulations of hydrostatic and uniaxial compressions, as well as predictions of ground-state structures at ambient conditions, were performed using the DFT codes CASTEP and VASP. The first calculations done with CASTEP using GGA-PW yielded reasonable agreement with experiment for the calculated isothermal EOS for PETN-I from hydrostatic compression data, yet the EOS for beta-HMX shows substantial deviation from experiment. Interesting anisotropic behavior of the shear-stress maxima were exhibited by both crystals upon uniaxial compression.

It was predicted that the <100> direction, the least sensitive direction of PETN, has significantly different values for shear stress maxima tau-yx and tau-zx, in contrast to the more sensitive directions, <110> and <001>. In addition, non-monotonic dependence of one of the shear stresses as a function of strain was observed upon compression of PETN in the <100> direction. VASP calculations were later performed, and the results yielded good qualitative agreement with available experimental data for the calculated isothermal EOS and equilibrium structures for PETN-I, beta-HMX, alpha-RDX, and nitromethane. Using VASP, uniaxial compression simulations were performed in the <100>, <010>, <001>, <110>, <101>, <011>, and <111> directions for all crystals up to the compression ratio V/V0 =0.70.

The VASP calculations of PETN reproduced the CASTEP results of significantly different values of tau-yx and tau-zx for the insensitive <100> compression, and relatively high and equal values of tau-yx and tau-zx for the sensitive <110> and <001> compressions. A correlation between this behavior of shear stress upon uniaxial compression and sensitivity was suggested, and predictions of anisotropic sensitivity of HMX, RDX, and nitromethane were made. Further analysis of the VASP results for PETN do not indicate a correlation between sensitivity and shear stress maxima as a function of longitudinal stress, where longitudinal stress is an appropriate experimental independent variable for comparison. The validity of a correlation between shear stress maxima and sensitivity requires further investigation. Further characterization of the anisotropic constitutive relationships in PETN was performed.

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