Session Z24: Materials at Extreme Conditions

The Importance of Temperature for Vented Chamber Calorimetry

Many accurate, calorimetric-based methodologies exist to measure the energy released from reactive materials. However, there is a need to extract energy data from non-ideal, application specific environments. High-velocity impact testing of reactive materials is one such case. A common methodology used to derive energy release in high-velocity impact testing is Vented Chamber Calorimetry (VCC). VCC relies on the measurement of both quasi-static pressure and temperature change in a vented chamber to implicitly determine energy release during impact, fragmentation, and combustion of reactive material projectiles. Due to the extreme conditions present in these experiments, VCC is often done without temperature measurements. Temperature can be mathematically estimated based on assumptions of ideal gas, however, it will be shown that this greatly reduces the accuracy of the measurement. A series of experiments were performed on a High-Velocity Impact-Ignition Testing System (HITS) with consolidated aluminum (Al) and molybdenum trioxide (MoO3) projectiles. The projectiles were launched at 1300 m/s into a vented chamber with varying volumes. Quasi-static pressure measurements were collected alongside pyrometry and thermography to determine the importance of temperature as a data point in energy derivations. Results show temperature is vital to the accurate derivation of energy. Measurements taken without temperature data assume an almost negligible temperature change in the chamber. Data from pyrometry and thermography suggest far greater average temperatures than theoretically predicted. This incongruity manifests in a greater than 100% difference for calculated energy release. Results indicate that in a highly dynamic system, pressure does not correlate directly with temperature to quantify energy released.   

Experimentally Simulating Ballistic Impact Events in High and Low Altitude Environments

Intermetallic, aluminum and zirconium, and thermite, molybdenum trioxide and aluminum, projectiles were launched using a high velocity impact ignition testing system (HITS) to study their reactivity in argon and air environments. The projectiles were consolidated from powder media and launched through a steel target plate before impacting an anvil. The experiments were completed in two test chambers. The first was a calorimetric chamber that was semi-sealed and provided quasi-static pressure data. Analysis of the pressure data revealed energy released from the system and an understanding of the environmental influence on performance. Further analysis of the pressure data included comparison to the theoretical heat of reaction and allowed quantification of the completeness of the metal reaction events. The second used a visual chamber to collect high speed videos of the penetration and impact events. The qualitative penetration and impact events are consistent with pressure data for illustrating fragmentation and reaction. The results of the test series demonstrated the thermite benefited from an on-board oxidizer for better material performance in an inert environment while the intermetallic benefited from two metal oxidization reactions in an air environment.   

Strengthening of Aluminum Powder Pellets under High-Velocity Impact

Aluminum powder (3-4.5 micron-particle diameter) was consolidated into pellets and used as projectiles in high-velocity impact experiments with a modified .410 caliber powder gun. Typically, the forces induced on the pellet after gun-powder ignition will fragment the pellet such that structural integrity is compromised and the pellet fractures before entering the explosion chamber. A metallurgical treatment was developed to alter the material properties of the Al pellets, such that they retain their structural form throughout flight in the explosion chamber. A treatment of annealing and quenching was applied to the pellet in such a way as to reorder the Al particle microstructure, which was observed by SEM imaging. Untreated consolidated pellets were used as a control sample and compared with treated pellets that were shot via a powder gun at 1300m/s into an explosion chamber where pressure and high-speed video data were collected. For untreated projectiles, pressure generation was very inconsistent, as the pellets normally failed before reaching the explosion chamber. The treated pellets maintained their structural integrity throughout flight in the chamber, observed by video, and resulted in consistently high-pressure generation. This study provides a qualitative and quantitative understanding of how the preparation technique for consolidated powder projectiles can influence their mechanical properties and resulting reactivity upon impact. Results provide a fundamental perspective for the mechanisms that improve the mechanical integrity of consolidated powder projectiles.   

Quantitative Characterization of Metallic Composite Particle Combustion using X-ray Phase Contrast Imaging and Machine Learning

Reactive metallic powders have a rich set of applications as energetic materials, including as additives in propellants, explosives, tools for bioagent defeat, and pyrotechnics. Microexplosions are often observed during the combustion of Al-Zr bimetallic powders in air, leading to a prolonged burn. Rapid bubble growth caused by the accumulation of nitrogen from air is hypothesized to be the underlying mechanism. To better understand this mechanism, x-ray phase contrast imaging was recently adopted to observe in situ bubble nucleation and growth during combustion of Al-Zr powders. We report here a high-throughput framework using machine learning to identify and track particles from x-ray phase contrast imaging videos. We use convolutional neural networks to identify particles, a Kalman filter to track trajectories, and additional tools to characterize burning particles. This framework enables us to automatically extract, organize, and analyze quantitative data from large amounts of video data.  Using it, we are able to better determine the relationship between bubble growth and microexplosions. The framework can also be applied to combustion analysis of other energetic materials, and we believe it will be a valuable tool for quantitative characterization of combustion.    

Pressure-dependent nonlinear optical response from TaAs

The topological Weyl semimetal TaAs is a polar metal that has recently been shown to exhibit a giant optical second harmonic response at infrared frequencies. In this talk, I will present hydrostatic pressure-dependent second harmonic generation rotational anisotropy measurements from TaAs single crystals using a diamond anvil cell based setup. Across a pressure-tuned tetragonal-to-hexagonal structural phase transition, we observe pronounced changes in both the magnitude and symmetry of the second harmonic generation signal, which are dictated primarily by the presence or absence of a polar axis. Potential application of this technique to other materials will be discussed.   

Interpretable Performance Models for Energetic Materials using Parsimonious Neural Networks

Predictive models for the performance of explosives and propellants are important for their design, optimization, and safety. Thermochemical codes can predict certain properties from fundamental properties such as density and formation energies that can be obtained from first principles. The Kamlet-Jacobs equations provide a computationally inexpensive alternative, but still require these fundamental properties as inputs and are limited in their ability to generalize to different types of explosives. Such easy to evaluate models are desirable for the efficient screening of large numbers of candidate materials, beyond what is possible with computationally intensive methods. Therefore, we use parsimonious neural networks (PNNs) to learn interpretable models for the detonation velocity and pressure for explosives using data collected from open literature. PNNs use evolutionary optimization to create models that balance accuracy and complexity. For both detonation velocity and pressure, we establish a family of interpretable models that are pareto optimal in accuracy and simplicity space. The Kamlet-Jacobs models lie close to but not at the pareto front. We extract expressions from these models and draw conclusions based on the functional forms of the terms discovered.   

Application of Periodic Domains in Multiscale Simulations of Large MaterialDeformation

Many multiscale simulations use periodic computational domains to consider lower length scale physics and to calculate closure quantities, such as the stress, to drive the macroscopic continuum level calculations. For materials undergoing large or extreme deformations, the computation domains often become highly distorted and need to be reinitialized. Reinitialization causes the loss of history information and accuracy of the simulation.

To avoid this difficulty, we introduce a method (Wang and Zhang, CMAME, 374, 2021; Barclay and Zhang, JCP, 435, 2021) by decomposing the velocity gradient matrix into an upper triangular part and an antisymmetric part representing a pure rotation. We show that the upper triangular part can always be accommodated by a cuboid domain, and the antisymmetric part can be canceled by performing the simulation in a rotating frame of reference. In this way, the domain reinitialization is avoided. Numerical examples are presented to show advantages of this method.   

Picosecond Laser Ablative Mass Spectrometry of Metallic Targets

Laser ablative mass spectrometry was used to study the molecular fragments produced due to picosecond laser ablation of metallic targets under vacuum conditions (~10^-5 Torr). Mass spectrometer used is a residual gas analyzer (RGA300) with 3 or 8eV ion energy, operating in first stability zone. Fundamental wavelength from Nd:YAG laser (1064nm) delivering 30ps pulses with 10Hz repetition rate and energy varied between 5-45mJ was observed to affect the ablation of target, resulting in enhanced plasma plume expansion along with the decomposition mechanism of target, which is measured in terms of partial pressures by RGA. The experimental setup was also used to study the plasma plume evolution. To understand the hydrodynamics of the ablated mass, one dimensional radiation hydrodynamic(1D-RHD) studies using modified MULTI-fs code was performed. Multiphoton ionization was incorporated into the code. The simulated plasma properties such as temperature and electron density were experimentally corroborated using LIBS data obtained from the same experimental setup. This work aims at establishing a relation between the plasma plume properties and mass spectrum which eventually can be used for the study of reaction pathways.   

Compression of helium-nitrogen mixtures: evidence for loading-path dependent phase separation and structure transition via Raman methods

Nitrogen is one of the most fundamental and abundant elements in the universe, investigating its phase behaviors under high pressure is of great significance in many areas. Vos et al. studies the helium-nitrogen mixtures under isobaric cooling, which aims to further understand the behaviors of nitrogen. In our study, Raman spectra and visual observation are used to investigate the phase transition of nitrogen-helium mixtures with helium concentrations of 95%, 70%, 50%, 40% and 20% along the 295 K isothermal loading pressure 1, unloading pressure and loading pressure 2. Several two-phase separation and phase transition have been observed and futher investigated in different He:N₂ mixtures and the high-quality (N₂)₁₁He single crystal surrounded by helium-rich fluid (F₂) are obtained above 8.8 GPa. The structure, as determined by single-crystal x-ray diffraction at 11.6 GPa, is hexagonal with unit cell parameters a=8.1246 Å, c=9.6145 Å. Furthermore, pressure-induced nitrogen molecule ordering at 32.6 GPa and a structural phase transition at 110 GPa are observed in (N₂)₁₁He. Finally, at 187 GPa, a pressure-induced transition to an amorphous state is identified by the simultaneous disappearance of Raman active modes, as well as a change from transparency to opacity.   

The D'yakov-Kontorovich Instability of Shock Fronts in Condensed Media

The studies of shock compression of condensed media and shock-front stability started simultaneously in the 1940s within nuclear weapons projects. Notwithstanding the impressive progress made in both fields since then, the fundamental shock-front instability theoretically discovered by D'yakov (1954) and Kontorovich (1957) (DK) in the USSR still challenges our understanding of shock compression in condensed media. DK predicted non-decaying oscillations of an isolated planar shock front, accompanied by spontaneous acoustic emission. This subtle effect is only possible under strict constraints on the equation of state and shock strength resulting in a specific shape of the Hugoniot curve. It took 20 years since the DK discovery to find realistic shock-compression conditions for its manifestation (in copper, Bushman, 1976) and many more years till its first numerical demonstration (in van der Waals fluid, Bates & Montgomery, 2000). The discussion about the nature of this elusive phenomenon continues in the literature to this day, still bringing unexpected findings. We present a theoretical stability analysis for an expanding accretion shock applicable to an arbitrary equation of state and shock strength. An explicit dispersion relationship for the perturbation growth rate is given.   

Raman spectra of hydrocarbons under extreme conditions of pressure and temperature: a first-principles study

Hydrocarbons are of great importance in carbon-bearing fluids in deep Earth and in ice giant planets at extreme pressure (P)-temperature (T) conditions. Raman spectroscopy is a powerful tool to study the chemical speciation of hydrocarbons; however, it is challenging to interpret Raman data at extreme conditions. We performed ab initio molecular dynamics simulations coupled with the modern theory of polarization to calculate Raman spectra of methane, ethane, and propane up to 48 GPa and 2000 K. Our method includes anharmonic temperature effects. We studied the pressure and temperature effects on the Raman bands,  and identified the characteristic Raman modes for the C-C and C-C-C bonds. To the best of our knowledge, it is the first time to calculate the Raman spectra of hydrocarbons at extreme P-T conditions. Our result may help to interpret in-situ Raman data of hydrocarbons at extreme P-T conditions,  with important implications for studying the hydrocarbon reactions in the deep carbon cycle inside Earth and the compositions of ice giant planets.   

Filament Induced Acoustic Spectroscopy:Sensing Molecular Aerosols

Filamentation is a dynamic, intense propagation regime of the ultrashort pulses due to the periodic and dynamic competition between Kerr Self focusing and plasma defocusing. Filaments so formed confine and propagate within them, fs light pulses over large distances, with peak intensities > 10¹³ Wcm^-2, within a mm spot size. This intense plasma channel radiates energy into its surrounding atmosphere in the form of optical emissions, RF emissions, THz emissions as well as intense acoustic shockwaves. The periodic spatially varying plasma radiates acoustic pulses proportional to the plasma density and unique to the medium in which they are generated, hence can be exploited for molecular sensing applications. In this report, we present our work on the generation and characterization of acoustic emissions from filaments in Tryptophan aerosols. The acoustic pulses generated in ambient air had acoustic emissions as a broad peak centered at 120 kHz, whereas the filament generated in tryptophan aerosol had multiple emission frequencies at 75kHz, 90kHz, 120kHz and 140kHz. The spectral intensity and peak over pressures were dependent on molecule concentration. We propose this as a proof of concept, “Filament Induced Acoustic Spectroscopy” technique for standoff sensing of molecular aerosols.   

New phases of hydrates at High Pressure

High pressures will induce new phases compounds, stoichiometry and physical phenomenon at extreme conditions. Hydrates, one of the star materials at high pressure, will show completely different properties from atmosphere with important application prospects in proton transport, nuclear quantum effect, etc. and they are also related to the evolution and behavior of gas giants, ice planets and our mother earth. This report mainly focus on two new compounds of hydrates which are newly predicted at high pressure: 1) [H4O]²⁺ ions form in the HF-H2O system, enhancing the basic understanding of the ionization of water under high pressure and acid-base theory; 2) A new phase of H2O·MgO·MgSiO3 which could exist as water preserver in the first 50-100 million years of Earth’s history and related the origin of sea on the earth.