Stephen A. Sheffield

The phase diagram of ammonium nitrate

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.

Initiation of EDC-37 measured with embedded electromagnetic particle velocity gauges

Planar shock wave initiation of the explosive EDC-37 was studied using multiple embedded magnetic particle velocity gauges. EDC-37 consists of (by weight) 91% HMX, 1% nitrocellulose, and 8% K10, a liquid eutectic mixture of di-nitro-ethyl-benzene and tri-nitro-ethyl-benzene. Its nominal density is 1.841±0.002 g/cm3. The Hugoniot for EDC-37 was measured as US=2.4+2.4uP, similar to other explosives of this density containing at least 90% HMX. The run distance to detonation, X*, vs. shock pressure P, was measured as log(X*)=2.0−1.5 log(P). This curve is shifted to a few GPa higher pressure compared to that of other plastic bonded explosives containing 90–95% HMX. This shift is most likely due to the low void content of EDC-37. Like other high density HMX based explosives, EDC-37 initiates with wave profiles having a little increase in particle velocity at the shock front (heterogeneous initiation), and also a second wave which starts small at the impact surface, grows as it travels, and reaches a large ampli...

Equation of state and high pressure properties of a fluorinated terpolymer: THV 500

We present the results of an investigation of the static compressive and dynamic (shock) responses of a fluorinated terpolymer of tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride (Dyneon THV 500), in an effort to further understand its behavior under static and dynamic high pressures, and elucidate its equation of state properties. Fluorinated polymers, and, in particular, their copolymers, have garnered increasing attention by the static high pressure and shock wave communities, due to their widespread use in engineering applications, and formulation into energetic materials as binders. Shock wave compression experiments performed at two laboratories showed good consistency, and provide the first Hugoniot data for this fluorinated terpolymer. The Hugoniot of THV 500 is in general agreement with that of the related fluoropolymers polytetrafluoroethylene and poly(chlorotrifluoroethylene-co-vinylidene fluoride), reported previously. The static compressive data, combined with measurement of the am...

Reduction of detonating liquid nitromethane’s chemical reaction-zone length by chemical sensitization

We examine the effect of the addition of small amounts of the organic base diethylenetriamine [NH2(CH2CH2)NH(CH2CH2)NH2] on the chemical reaction-zone length (CRZ) of detonating liquid nitromethane (CH3NO2). This is done by making accurate measurements of the detonating materials’ diameter-effect curves (i.e., detonation speed versus lateral charge size) as a function of the amount of chemical sensitizer added. Detonation speed experiments were performed with additions of the organic base in amounts between 0.00 and 0.25wt%. Reductions in the CRZ of as much as 25% were produced by base addition. Most of the reduction in length is produced by very small amounts of base addition—i.e., ca. 0.05wt% of base or less (i.e., 1 molecule of the base per 3300 nitromethane molecules or less). Measured detonation speeds are given for five compositions of nitromethane and base as a function of charge internal diameter. Absolute CRZs are estimated using a value of liquid nitromethane’s CRZ obtained by other means. Earli...

PHASE TRANSITION AND DECOMPOSITION OF 90% HYDROGEN PEROXIDE AT HIGH PRESSURES

Physical and chemical changes of 90 wt% hydrogen peroxide have been investigated to pressures of 12 GPa by using a diamond-anvil cell, synchrotron x-ray diffraction, and Raman spectroscopy. Hydrogen peroxide freezes at 1.5 GPa and ambient temperature into a tetragonal structure (P41212, Z=4, denoted as H2O2-I. This is the same transition that has previously been reported in this material at 253 K. The unit cell parameters at 6.3 GPa are a=3.759 A, c=7.397 A, and V=15.74 cm3/mol, representing 21% compression from that at ambient pressure. H2O2-I has been found to transform into a high pressure phase H2O2-II at 7.5 GPa, and it decomposes into water and oxygen at the onset of melting, which may be incongruent. In contrast to water, hydrogen peroxide exhibits a relatively simple polymorphism and a positive initial slope of the melting curve at high pressures.

Laser-driven MiniFlyer induced gold spall

A laser-launched miniature flyer system (MiniFlyer) is being used to study the dynamic properties of materials. A 3-mm diameter and 0.05-mm thick flyer plate is accelerated by a laser-pulse-induced plasma contained between a clear window substrate and the flyer plate. The substrate is coated with carbon, aluminum oxide, and aluminum to enhance the plasma formation process. The flyer impacts a gold target plate of 0.10 or 0.26 mm thickness, producing a shock. The shock pulse interacts with the free surface and reflects as a rarefaction wave, producing tension in the foil. Dynamic measurements of the free surface particle velocity were made using VISAR (Velocity Interferometer System for Any Reflector). Cross-sections of the gold targets exhibit spall planes at the expected locations.

Shock-induced chemistry of phenylacetylene

Gas gun-driven shock compression experiments of phenylacetylene using embedded electromagnetic gauging were used to obtain in situ particle velocity wave profiles at multiple Lagrangian positions at several shock input conditions. At shock conditions above 6 GPa, the input shock wave evolved over time and distance into a complex multiple wave structure due to shock-driven chemical reactions. The 3-wave structure was marked by a fast risetime 2nd wave, slower risetime 3rd wave, and unusual wave dynamics in the 1st wave. From the measured shock and particle velocities, the 1st wave, and intermediate and final product states associated with the chemical reactions were determined. A thermodynamically complete unreacted equation of state was calibrated to estimate the temperature rise along the shock locus. Use of this EOS with the measured 2nd and 3rd wave risetimes yielded highly statesensitive global reaction rates as a function of the shock locus.

Mass-spectroscopic observations of glycine subjected to strong shock loading.

We have made time-of-flight mass-spectroscopic observations of 85/15 wt % water/glycine solutions and of crystalline alpha-glycine subjected to strong shock loading. The shockwaves were produced by placing the materials in contact with detonating solid explosives. In the solution observations, we have done experiments with glycine molecules composed of ordinary isotopes and with molecules labeled with (13)C, (15)N, and D atoms. The primary reason for conducting this research was to examine whether glycine molecules can survive exposure to strong shock loading, e.g., as might occur in the entry of a meteor into the earths atmosphere. Our results show that glycine molecules can withstand the rigors of shock environments that generate pressure and temperature up to 180 kbar and 3200 K. Glycine in a 85 H(2)O/15 glycine wt % solution (i.e., one molecule of glycine to ca. 24 H(2)O molecules) exists primarily in its zwitterionic form. In both the solution and crystal experiments, we observed zwitterionic dimers, trimers, and, possibly, tetramers, after the materials were shocked. This implies that the solvating water molecules in the solution experiments must reside on the exterior of groups of solvated glycine molecules. We report quantum-chemical calculations, using density functional theory, that predict that two glycine zwitterions are bound together by ca. 15.72 kcal when immersed in an Onsager model of water. Our observations allow us to place lower-bound estimates on the lifetime of glycine zwitterions under our conditions. We have examined our data to determine whether dipeptide formation has occurred and found no evidence that it has. Compressible fluid-mechanical calculations were performed to estimate the pressures, temperatures, and the time scales present in the experiments.

Shock equation of state of a multi-phase epoxy-based composite (Al–MnO2-epoxy)

There are several studies in the literature regarding the equation of state of alumina-epoxy composites. Although these single component systems interact in a complex manner with shock waves, the addition of a second metal or ceramic particulate can result in even more complex interactions. This paper presents the shock equation of state results on a multi-phase composite Al–MnO2-epoxy. Equation of state experiments were conducted using three different loading techniques—single stage light gas gun, two stage light gas gun, and explosive loading—with multiple diagnostic techniques. The Us−up relationship is shown to be linear, with deviations from linearity at low, and possibly high, pressures due to the behavior of the epoxy binder. The experimental equation of state data is compared to volume averaged and mesoscale mixture models.

EQUATION OF STATE OF AMMONIUM NITRATE

Ammonium nitrate (AN) is a widely used fertilizer and mining explosive. AN is commonly used in ammonium nitrate‐fuel oil (ANFO), which is a mixture of explosive‐grade AN prills and fuel oil in a 94:6 ratio by weight. ANFO is a non‐ideal explosive with measured detonation velocities around 4 km/s. The equation of state properties and known initiation behavior of neat AN are limited. We present the results of a series of gas gun‐driven plate impact experiments on pressed neat ammonium nitrate at 1.72 g/cm3. No evidence of initiation was observed under shock loading to 22 GPa. High pressure x‐ray diffraction experiments in diamond anvil cells provided insight into the high pressure phase behavior over the same pressure range (to 25 GPa), as well as a static isotherm at ambient temperature. From the isotherm and thermodynamic properties at ambient conditions, a preliminary unreacted equation of state (EOS) has been developed based on the Murnaghan isotherm and Helmholtz formalism [1], which compares favorably...