Internal Deformation, Evolution, and Fluid Flow in Basement-Involved Thrust Faults, Northwestern Wyoming

An integrated field , microstructure, fracture statistic , geochemistry , and laboratory permeability study of the East Fork and White Rock fault zones , of similar age and tectonic regime but different structural level and hydrogeologic history , provides detailed information about the internal deformation and fluid flow processes in fault zones . The primary conclusions of this research are: 1) Fault zones can be separated into subzones of protolith, damaged zone , and gouge /cataclasite , based on physical morphology and permeability structure . At deep structural levels, gouge/cataclasite zones are more evolved (thicker with increased grain size reduction) due to strain localization , higher pressure and temperature, and fluid/rock interaction ; 2) Deformation mechanisms evolved from primarily brittle fracturing and faulting in the damaged zone to extreme, fluid-enhanced chemical breakdown and cataclasis which localized strain in the fault core. Deformation in the deep-level-fault core may be a combination of frictional and quasiplastic mechanisms, and is largely controlled by extremely fine-grained clays, zeolites , and other phyllosilicates that may have acted as a thermally pressurized, fluid-saturated lubricant; 3) Permeability in fault zones was temporally heterogeneous and anisotropic (permeability of damaged zone\u3eprotolith\u3egouge /cataclasite, permeability along fault\u3e permeability across fault); 4) Volume loss was concentrated in the fault cores and was negligible at intermediate structural levels and high at deep structural levels in the semi-brittle to brittle regime ; 5) Fluid flow and solute transport were concentrated upwards and subparallel to the fault in the damaged zone ; 6) Faults at both the local and regional scale acted as fluid flow conduit/barrier systems depending upon the evolutionary stage and interval in the seismic cycle ; 7) Fluid/rock volume ratios , fluid flux , and fluid/rock volume ratios over time ranged from ⋍ 103 to 104, 10-6 ms-1 to 10-9 ms-1, and 0.05 L/m3 rock•yr to 0.50 L/m3 rock•yr, respectively, suggesting that enormous quantities of fluids passed through the fault zones; 8) Box counting fractal analyses of fault zone fractures showed that fracture spatial and density distribution is scale-invariant at the separate scales of outcrop , hand-sample , and thin section, but self-affine from outcrop to thin-section scale; 9) Linear fractal analysis depicts clustering and density distribution as a function of orientation, and may be a quick, robust method of estimating two-dimensional fracture permeability; and 10) Fractal analysis of fractures is not a comprehensive statistical method, but can be used as another supplemental statistical parameter

Anisotropic Shock Sensitivity of Cyclotrimethylene Trinitramine (RDX) from Compress-and-Shear Reactive Dynamics

We applied the compress-and-shear reactive dynamics (CS-RD) simulation model to study the anisotropic shock sensitivity of cyclotrimethylene trinitramine (RDX) crystals. We predict that, for mechanical shocks between 3 and 7 GPa, RDX is most sensitive to shocks perpendicular to the (100) and (210) planes, whereas it is insensitive for shocks perpendicular to the (120), (111), and (110) planes. These results are all consistent with available experimental information, further validating the CS-RD model for distinguishing between sensitive and insensitive shock directions. We find that, for sensitive directions, the shock impact triggers a slip system that leads to large shear stresses arising from steric hindrance, causing increased energy inputs that increase the temperature, leading to dramatically increased chemical reactions. Thus, our simulations demonstrate that the molecular origin of anisotropic shock sensitivity results from steric hindrance toward shearing of adjacent slip planes during shear deformation. Thus, strain energy density, temperature rise, and molecule decomposition are effective measures to distinguish anisotropic sensitivities. We should emphasize that CS-RD has been developed as a tool to distinguish rapidly (within a few picoseconds) between sensitive and insensitive shock directions of energetic materials. If the high stresses and rates used here continued much longer and for larger systems, it would ultimately result in detonation for all directions, but we have not demonstrated this

First Principles Study of the Ignition Mechanism for Hypergolic Bipropellants: N,N,N′,N′-Tetramethylethylenediamine (TMEDA) and N,N,N′,N′-Tetramethylmethylenediamine (TMMDA) with Nitric Acid

We report quantum mechanics calculations (B3LYP flavor of density functional theory) to determine the chemical reaction mechanism underlying the hypergolic reaction of pure HNO_3 with N,N,N′,N′-tetramethylethylenediamine (TMEDA) and N,N,N′,N′-tetramethylmethylenediamine (TMMDA). TMEDA and TMMDA are dimethyl amines linked by two CH_2 groups or one CH_2 group, respectively, but ignite very differently with HNO_3. We explain this dramatic difference in terms of the role that N lone-pair electrons play in activating adjacent chemical bonds. We identify two key atomistic level factors that affect the ignition delay: (1) The exothermicity for formation of the dinitrate salt from TMEDA or TMMDA. With only a single CH_2 group between basic amines, the diprotonation of TMMDA results in much stronger electrostatic repulsion, reducing the heat of dinitrate salt formation by 6.3 kcal/mol. (2) The reaction of NO_2 with TMEDA or TMMDA, which is the step that releases the heat and reactive species required to propagate the reaction. Two factors of TMEDA promote the kinetics by providing routes with low barriers to oxidize the C: (a) formation of a stable intermediate with a C–C double bond and (b) the lower bond energy for breaking the C–C single bond (by 18 kcal/mol comparing to alkane) between two amines. Both factors would decrease the ignition delay for TMEDA versus TMMDA. The same factors also explain the shorter ignition delay of 1,4-dimethylpiperazine (DMPipZ) versus 1,3,5-trimethylhexahydro-1,3,5-triazine (TMTZ). These results indicate that TMEDA and DMPipZ are excellent green replacements for hydrazines as the fuel in bipropellants

ReaxFF-lg: Correction of the ReaxFF Reactive Force Field for London Dispersion, with Applications to the Equations of State for Energetic Materials

The practical levels of density functional theory (DFT) for solids (LDA, PBE, PW91, B3LYP) are well-known not to account adequately for the London dispersion (van der Waals attraction) so important in molecular solids, leading to equilibrium volumes for molecular crystals ∼10-15% too high. The ReaxFF reactive force field is based on fitting such DFT calculations and suffers from the same problem. In the paper we extend ReaxFF by adding a London dispersion term with a form such that it has low gradients (lg) at valence distances leaving the already optimized valence interactions intact but behaves as 1/R^6 for large distances. We derive here these lg corrections to ReaxFF based on the experimental crystal structure data for graphite, polyethylene (PE), carbon dioxide, and nitrogen and for energetic materials: hexahydro-1,3,5-trinitro- 1,3,5-s-triazine (RDX), pentaerythritol tetranitrate (PETN), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and nitromethane (NM). After this dispersion correction the average error of predicted equilibrium volumes decreases from 18.5 to 4.2% for the above systems. We find that the calculated crystal structures and equation of state with ReaxFF-lg are in good agreement with experimental results. In particular, we examined the phase transition between α-RDX and γ-RDX, finding that ReaxFF-lg leads to excellent agreement for both the pressure and volume of this transition occurring at ∼4.8 GPa and ∼2.18 g/cm^3 density from ReaxFF-lg vs 3.9 GPa and ∼2.21 g/cm^3 from experiment. We expect ReaxFF-lg to improve the descriptions of the phase diagrams for other energetic materials

Anisotropic shock sensitivity for β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine energetic material under compressive-shear loading from ReaxFF-ℓg reactive dynamics simulations

We report here the predictions on anisotropy of shock sensitivity and of chemical process initiation in single crystal β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-HMX) using compressive shear reactive dynamics (CS-RD) model with ReaxFF-ℓg reactive force field. Analysis of resolved shear stress induced by uniaxial compression along three shock directions normal to (110), (011), and (010) planes leads to identify eight slip systems as candidates for shear deformation. For each of the eight slip systems, non-equilibrium reactive dynamics simulations were carried out to determine thermal, mechanical, and chemical responses to shear deformation. Shock direction normal to (010) plane exhibits large shear stress barriers arising from steric hindrance between molecules of adjacent layers leading to local dramatic energy and temperature increases under shear flow that in turn accelerate chemical bond breaking and initial product formation processes, promoting further molecular decomposition and eventually transition to detonation. This suggests that single crystal β-HMX is sensitive to shocks in direction normal to (010) plane. Shock directions normal to (110) and (011) planes reveal significantly less steric hindrance, leading to more modest energy and temperature increases followed by slower chemical reaction initiation. Thus, shock directions normal to (110) and (011) planes are less sensitive than shock direction normal to (010) plane, which agree with interpretations from currently available plate impact experiments on HMX. This validation of CS-RD and ReaxFF for characterizing sensitivity of single crystal energetic materials indicates that these methods can be applied to study sensitivity for more complex polymer bonded explosives and solid composite propellants having complex microstructures, corrugated interfaces, as well as defects

Oxygen Hydration Mechanism for the Oxygen Reduction Reaction at Pt and Pd Fuel Cell Catalysts

We report the reaction pathways and barriers for the oxygen reduction reaction (ORR) on platinum, both for gas phase and in solution, based on quantum mechanics calculations (PBE-DFT) on semi-infinite slabs. We find a new mechanism in solution: O_2 → 2O_(ad) (E_(act) = 0.00 eV), O_(ad) + H_2O_(ad) → 2OH_(ad) (E_(act) = 0.50 eV), OH_(ad) + H_(ad) → H_2O_(ad) (E_(act) = 0.24 eV), in which OH_(ad) is formed by the hydration of surface O_(ad). For the gas phase (hydrophilic phase of Nafion), we find that the favored step for activation of the O_2 is H_(ad) + O_(2ad) → HOO_(ad) (E_(act) = 0.30 eV) → HO_(ad) + O_(ad) (E_(act) = 0.12 eV) followed by O_(ad) + H_2O_(ad) → 2OH_(ad) (E_(act) = 0.23 eV), OH_(ad) + H_(ad) → H_2O_(ad) (E_(act) = 0.14 eV). This suggests that to improve the efficiency of ORR catalysts, we should focus on decreasing the barrier for Oad hydration while providing hydrophobic conditions for the OH and H_2O formation steps

Thermal Decomposition of Hydrazines from Reactive Dynamics Using the ReaxFF Reactive Force Field

We report reactive dynamics (RD) studies on: the decomposition of bulk hydrazine (N_2H_4); the decomposition of bulk monomethyl-hydrazine (CH_3N_2H_3), hereafter referred to simply as methyl-hydrazine; the decomposition of hydrazine in the presence of hydrogen peroxide (H_2O_2); and decomposition hydrazine on catalytic surfaces Pt[100] and Pt[111] under various conditions. These studies use the ReaxFF reactive force field to describe the multitude of chemical reactions in these systems for a variety of reaction conditions in order to show that this approach leads to realistic decomposition mechanisms and rates. In particular, we determined how the decomposition of hydrazine is affected by temperature, pressure, and heating rate. We analyzed chemical reaction mechanism of the decomposition of hydrazine at the studied conditions and found that at lower temperatures the initial product from hydrazine decomposition is NH_3, whereas at higher temperatures H_2 and N_2 are the dominant early products. Prominent intermediates observed during these decompositions include N_2H_3, N_2H_2, and NH_2, in agreement with quantum mechanical studies (7.3 ps at 3000 K). As the heating rate is decreased, the onset for hydrazine decomposition shifts to lower temperatures. Using a constant heating rate, we found that higher pressure (increased density) favors formation of NH_3 over N_2 and H_2. In studies of the catalytic decomposition of hydrazine on surfaces Pt[100] and Pt[111], we found that the presence of a Pt-catalyst reduces the initial decomposition temperature of hydrazine by about 50%. We found that the Pt[100]-surface is 20 times more active for hydrazine decomposition than the Pt[111]-surface, in qualitative agreement with experiments. These studies indicate how ReaxFF RD can be useful in understanding the chemical processes involved in bulk and catalytic decomposition and in oxidation of reactive species under various reaction conditions