Graduation Year

2009

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Physics

Major Professor

Ivan I. Oleynik, Ph.D.

Co-Major Professor

Venkat R. Bhethanabotla, Ph.D.

Committee Member

Dale E. Johnson, Ph.D.

Committee Member

Lilia M. Woods, Ph.D.

Keywords

equation of state, first principles, shear stress, explosives, compression

Abstract

First-principles calculations employing density functional theory (DFT) were

performed on the energetic materials PETN, HMX, RDX, nitromethane, and a recently

discovered material, nitrate ester 1 (NEST-1). The aims of the study were to accurately

predict the isothermal equation of state for each material, improve the description of these

molecular crystals in DFT by introducing a correction for dispersion interactions, and

perform uniaxial compressions to investigate physical properties that might contribute to

anisotropic sensitivity.

For each system, hydrostatic-compression simulations were performed. Important

properties calculated from the simulations such as the equilibrium structure, isothermal

equation of state, and bulk moduli were compared with available experimental data to

assess the agreement of the calculation method. The largest contribution to the error was

believed to be caused by a poor description of van der Waals (vdW) interactions within

the DFT formalism.

An empirical van der Waals correction to DFT was added to VASP to increase

agreement with experiment. The average agreement of the calculated unit-cell volumes

for six energetic crystals improved from approximately 9% to 2%, and the isothermal

EOS showed improvement for PETN, HMX, RDX, and nitromethane. A comparison was

made between DFT results with and without the vdW correction to identify possible

advantages and limitations.



Uniaxial compressions perpendicular to seven low-index crystallographic planes

were performed on PETN, HMX, RDX, nitromethane, and NEST-1. The principal

stresses, shear stresses, and band gaps for each direction were compared with available

experimental information on shock-induced sensitivity to determine possible correlations

between physical properties and sensitivity. The results for PETN, the only system for

which the anisotropic sensitivity has been thoroughly investigated by experiment,

indicated a possible correlation between maximum shear stress and sensitivity. The

uniaxial compressions that corresponded to the greatest maximum shear stresses in HMX,

RDX, solid nitromethane, and NEST-1 were identified and predicted as directions with

possibly greater sensitivity. Experimental data is anticipated for comparison with the

predictions.

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