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High-Pressure and High-Temperature Synthesis and In Situ High-Pressure Synchrotron X-ray Diffraction
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High-Pressure, High-Temperature Studies of Phase Transitions in SrOsO[subscript 3] - Discovery of a
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High Pressure-Temperature Single-Crystal Elasticity of Grossular: Implications for the Low-Velocity
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Phase Transitions of Fe-, Al- and Ca-Bearing Orthopyroxenes at High Pressure and High Temperature: I
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Single-Crystal Elasticity of Phase E at High Pressure and Temperature: Implications for the Low-Velo
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Diamond and methane formation from the chemical decomposition of polyethylene at high pressures and temperatures
Abstract Polyethylene (C 2 H 4 ) n was compressed to pressures between 10 and 30 GPa in a diamond anvil cell (DAC) and laser heated above 2500 K for approximately one second. This resulted in the chemical decomposition of the polymer into carbon and hydrocarbon reaction products. After quenching to ambient temperature, the decomposition products were measured in the DAC at pressures ranging from ambient to 29 GPa using a combination of x-ray diffraction (XRD) and small angle x-ray scattering (SAXS). XRD identified cubic diamond and methane as the predominant product species with their pressure–volume relationships exhibiting strong correlations to the diamond and methane equations of state. Length scales associated with the diamond products, obtained from SAXS measurements, indicate the formation of nanodiamonds with a radius of gyration between 12 and 35 nm consistent with 32–90 nm diameter spherical particles. These results are in good agreement with the predicted product composition under thermodynamic and chemical equilibrium.
Experimental measurements of the viscosity and melt structure of alkali basalts at high pressure and temperature
Abstract Volcanic eruptions are shallow phenomena that represent the final stage of density- and viscosity- driven processes of melt migration from source rocks at upper mantle depths. In this experimental study, we investigated the effect of pressure (0.7–7.0 GPa) and temperature (1335–2000 °C) on the viscosity and the atomic melt structure of a synthetic anhydrous primitive alkaline basalt, an analogue of the pre-eruptive magma that likely feeds the Campi Flegrei Volcanic District at present day. Obtained viscosities (0.5–3.0 Pa s), mobility (0.1–0.4 g cm 3 Pa −1 s −1 ) and ascent velocity (1.5–6.0 m yr −1 ) are presented to support geochemical and geophysical observations of Campi Flegrei as a critical volcanic district currently undergoing gradual magma recharge at depth.
Experimental and theoretical confirmation of an orthorhombic phase transition in niobium at high pressure and temperature
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The yielding behavior of SiC under high pressure and temperature conditions
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High-pressure and temperature neural network reactive force field for energetic materials
Reactive force fields for molecular dynamics have enabled a wide range of studies in numerous material classes. These force fields are computationally inexpensive compared with electronic structure calculations and allow for simulations of millions of atoms. However, the accuracy of traditional force fields is limited by their functional forms, preventing continual refinement and improvement. Therefore, we develop a neural network-based reactive interatomic potential for the prediction of the mechanical, thermal, and chemical responses of energetic materials at extreme conditions. The training set is expanded in an automatic iterative approach and consists of various CHNO materials and their reactions under ambient and shock-loading conditions. Further, this new potential shows improved accuracy over the current state-of-the-art force fields for a wide range of properties such as detonation performance, decomposition product formation, and vibrational spectra under ambient and shock-loading conditions.
Phonon thermodynamics and elastic behavior of GaAs at high temperatures and pressures
The phonons of wurtzite and zinc blende GaAs were calculated at simultaneously elevated temperature and pressure, and elastic constants were calculated as functions of pressure. Pressure caused instabilities of shorter-wavelength transverse acoustic modes in both wurtzite and zinc blende GaAs, causing them to fall to zero at 18 and 20GPa, respectively. Here, the Born stability criteria, which depend on elastic constants and only long wavelength phonons, therefore overestimated the pressure needed to induce instability at 0 K. At elevated temperatures, explicit anharmonicity pushes the onset of instability to higher pressures in both wurtzite and zincblende GaAs. Phonon linewidth and densities of states data showed that the quasiharmonic approximation failed to account for temperature-induced phonon frequency shifts, and the quasiharmonic approximation became less reliable at elevated pressure. In general, the number of three-phonon processes increased with pressure, thereby increasing the temperature-driven broadening of phonon spectral lineshapes.
TiO2-Coated Optical Fibers for Distributed pH Monitoring at High Pressures and Temperatures
SPIE Defense and Commercial Sensing, Orlando, FL, April 30-May 4, 2023
Sound velocity of neon at high pressures and temperatures by Brillouin scattering
Abstract not provided
Quarterly Management Document – FY20, 1st Quarter, Physics-based Creep Simulations of Thick Section Welds in High Temperature and Pressure Applications
During the first quarter of FY20, efforts focused on calibrating a computational model using experimental data. The experimental creep data obtain from specimens containing weld metal exhibit considerable inherent variability making calibration of the computational difficult and may not result in an accurate model.
Quarterly Management Document – FY20, 2nd Quarter, Physics-based Creep Simulations of Thick Section Welds in High Temperature and Pressure Applications
During the 2nd quarter of FY20, calibration of the creep model was made using experimental creep data on base metal at temperatures between 700-800 C. The model simulated primary and secondary creep behavior at these temperatures but tended to over estimate creep strain in the tertiary regime at 700 C while underestimating it at higher temperatures. Additional issues were also discovered and will be looked at in greater detail in the 3rd quarter of FY20.