Roberta N. Mulford

Performance of an ab initio equation of state for magnesium oxide

A thermodynamically complete ab initio equation of state (EOS) for MgO was obtained using electron density functional theory and the quasiharmonic phonon approximation, and adjusted to match the ambient density. This EOS was demonstrated to be consistent with isotherm, thermal expansivity, heat capacity and melting curve measured in static experiments, and reproduced density and temperature measurements under shock wave loading of bulk and porous periclase. The Gruneisen parameter of periclase at a given density was shown to be weakly dependent on temperature. The B1–B2 phase change was calculated to occur near 320 GPa on the principal Hugoniot. The melting locus of periclase, relevant to the Earths lower mantle pressures, was predicted to be accessible by shock wave loading of porous periclase, which could also put pressure and temperature bounds on B1–B2 transitions.

Preshock desensitization of PBX explosives

Preshocking delays initiation of PBX‐9404 and PBX‐9501, relative to unshocked material. In PBX‐9404 preshock experiment, a first shock of 2.3 GPa was followed 0.65 μs later by a second shock of 5.6 GPa. Both PBX explosives show clear desensitization while the preshock persists. In PBX‐9404, initiation of detonation occurs nearly as anticipated for the material, after coalescence of the preshock and main shock into a single wave. Multiple embedded magnetic gauges were used to measure the shock histories. Our data indicates a slighly longer run to detonation than expected, even though a single wave is initiating the material. A slight stress reduction at coalescence, as required by the shock dynamics, may be responsible for the overrun. A reactive wave is clearly evident while the preshock persists. The long run to detonation indicates that this reactive wave is not driving the initiation. A set of four preshock experiments were performed on PBX−9502, which is unreactive at these pressures to investigate th...

Equation of state of ammonia

Ammonia and water are critical components of extraterrestrial bodies, determining the density and physical properties of the Outer Planets, their moons, and of extrasolar planets. Ammonia is unusual in having a high heat capacity relative to other molecular species. Equations of state (EOS) are presented for ammonia and for mixtures of ammonia and water. Their properties are discussed in terms of chemical compositions that evolve as pressure and temperature are varied. The NH4OH hydrate of ammonia is known to exist as a separate molecular species at pressures above about 5 GPa, and an effort was made to include reaction between NH3 and H2O in the mixture EOS. The EOS are suitable for calculating structures of icy planets and exoplanets, and of impacts. mass-radius relations which bound the possible interpretations of composition and structure for extraterrestrial bodies of unknown composition, such as exoplanets.

IMPROVED EOS FOR DESCRIBING HIGH‐TEMPERATURE OFF‐HUGONIOT STATES IN EPOXY

Modelling of off‐Hugoniot states in an expanding interface subjected to a shock reveals the importance of a chemically complete description of the materials. Hydrodynamic experiments typically rely on pre‐shot target characterization to predict how initial perturbations will affect the late‐time hydrodynamic mixing. However, it is the condition of these perturbations at the time of shock arrival that dominates their eventual late‐time evolution. In some cases these perturbations are heated prior to the arrival of the main shock. Correctly modelling how temperature and density gradients will develop in the pre‐heated material requires an understanding of the equation‐of‐state. In the experiment modelled, an epoxy/foam layered package was subjected to tin L‐shell radiation, producing an expanding assembly at a well‐defined temperature. This assembly was then subjected to a controlled shock, and the evolution of the epoxy‐foam interface imaged with x‐ray radiography. Modelling of the data with the hydrodynam...

Reactive flow models for the desensitization of high explosive

Mechanical desensitisation of explosives (e.g. by a preshock) has important technological consequences for safety and performance. Many widely-used reactive flow models are not capable of reproducing the desensitisation observed in experiments such as double-shock particle velocity profiles. Improved models are under development, where we intend to incorporate enough flexibility to reproduce experimentally observed shock desensitisation while retaining plausible and computationally practical physical models for the equations of state (unreacted, partially-reacted and products), equilibration processes and reaction rate.

The combination bands (ν1+ν3, ν2+ν3) and overtone band (3ν3) of neptunium hexafluoride

Abstract Fourier transform, vibration-rotational spectra of NpF 6 measured at moderate to high resolution (0.1-0.008 cm −1 ) reveal well-defined PQR structures for the two combination bands, ν 1 +ν 3 and ν 2 +ν 3 . The resolved ground-state Q branch and rotational manifolds for each band provide accurate vibrational frequencies and allow detailed rovibrational analysis. We also report the observation of the 3ν 3 band spectrum at 5.3 μm. The transition strength for 3ν 3 is determined as S =1.7×10 −2 km mol −1 . The anharmonic constant x 33 ≈−0.77 cm −1 was estimated.

Sensitivity of PBX-9502 after ratchet growth

Ratchet growth, or irreversible thermal expansion of the TATB-based plastic-bonded explosive PBX-9502, leads to increased sensitivity, as a result of increased porosity. The observed increase of between 3.1 and 3.5 volume percent should increase sensitivity according to the published Pop-plots for PBX-9502 [1]. Because of the variable size, shape, and location of the increased porosity, the observed sensitivity of the ratchet-grown sample is less than the sensitivity of a sample pressed to the same density. Modeling of the composite, using a quasi-harmonic EOS for unreacted components [2] and a robust porosity model for variations in density [3], allowed comparison of the initiation observed in experiment with behavior modeled as a function of density. An Arrhenius model was used to describe reaction, and the EOS for products was generated using the CHEETAH code [4]. A 1-D Lagrangian hydrocode was used to model in-material gauge records and the measured turnover to detonation, predicting greater sensitivity to density than observed for ratchet-grown material. This observation is consistent with gauge records indicating intermittent growth of the reactive wave, possibly due to inhomogeneities in density, as observed in SEM images of the material [5].Ratchet growth, or irreversible thermal expansion of the TATB-based plastic-bonded explosive PBX-9502, leads to increased sensitivity, as a result of increased porosity. The observed increase of between 3.1 and 3.5 volume percent should increase sensitivity according to the published Pop-plots for PBX-9502 [1]. Because of the variable size, shape, and location of the increased porosity, the observed sensitivity of the ratchet-grown sample is less than the sensitivity of a sample pressed to the same density. Modeling of the composite, using a quasi-harmonic EOS for unreacted components [2] and a robust porosity model for variations in density [3], allowed comparison of the initiation observed in experiment with behavior modeled as a function of density. An Arrhenius model was used to describe reaction, and the EOS for products was generated using the CHEETAH code [4]. A 1-D Lagrangian hydrocode was used to model in-material gauge records and the measured turnover to detonation, predicting greater sensitivi...

Sensitivity of the TATB-based explosive PBX-9502 after thermal expansion

The sensitivity of TATB-based explosive PBX-9502 is affected by non-reversible thermal expansion, or “ratchet growth.” PBX-9502 is a plastic-bonded explosive consisting of 95 wt % TATB (2,4,6-trinitro-1,3,5-benzenetriamine) and 5 wt % Kel-F 800 binder (chlorotrifluoroethylene/vinylidine 3:1 copolymer). The magnitude of the increase in size and the corresponding increase in sensitivity is reported here for a particular pressing of PBX-9502, after repeated thermal cycling. The physical morphology of the expanded material is examined using scanning electron microscopy, in an effort to determine the increase in intergranular holes, intragranular cracks and fissures in the TATB crystals, and the change in the distribution of the Kel-F, all of which are suspected to affect the sensitivity of the material. These images support the proposed mechanism for ratchet growth. Sensitivity, growth of the reactive wave behind the shock front, and Hugoniot data are obtained from in-material particle velocity gauge records ...

Modelling Temperatures of Reacting Nitromethane

An equation of state (EOS) for unreacted nitromethane, CH3NO2 provides the accurate temperatures required to support a temperature dependent reaction rate within a reactive flow model. A quasiharmonic form based on the Gruneisen equations of state was used, normalised to shock wave data but with a more rigorous treatment of thermal modes. We use reactive flow models that include temperature as well as mechanical state to investigate shock initiation in nitromethane. Temperatures predicted by the model for various off‐Hugoniot states in reactive configurations show reasonable agreement with data from experimental measurements reported by several different workers, using different diagnostic methods. A thermochemical model was used for the reaction products, and Arrhenius reaction parameters reported in the literature reproduced reported reactive behavior of nitromethane.

Mesoscale modelling of shock initiation in HMX-based explosives

Hydrocode calculations we used to simulate initiation in single- and double-shock experiments on several HMX-based explosives. Variations in the reactive behavior of theee materials reflects the differences between binders in the material, providing information regarding the sensitivity of the explosive to the mechanical properties of the constituents. Materials considered are EDC-37, with a soft binder, PBX-9601, with a relatively malleable binder, and PIBX-9404, with a stiff binder. Bulk reactive behavior of these materials is dominated by the HMX component and should be comparable, while the mechanical response varies. The reactive flow model is temperature-dependent, based on a modified Arrhenius rate. Some unreacted material is allowed to react at a rate given by the state of the hotspot rather than the bulk state of the unreacted explosive, according to a length scale reflecting the hotspot size, and a time scale for thermal equilibration. The Arrhenius rate for HMX is wsumed to be the same for all compositions. The initiation data for different HMX-bwd explosives axe modelled by choosing plausible parameters to describe the reactive and dissipative properties of the binder, and hence the behavior of the hotspots in each formulation.