Observed Pressure Dependence Research Articles

Kinetics of the Gas-Phase Reactions of syn- and anti-CH3CHOO Criegee Intermediate Conformers with SO2 as a Function of Temperature and Pressure.

Kinetics of reactions between SO2 and CH3CHOO Criegee intermediate conformers have been measured at temperatures between 242 and 353 K and pressures between 10 and 600 Torr using laser flash photolysis of CH3CHI2/O2/N2/SO2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy. The kinetics of syn-CH3CHOO + SO2 are pressure-dependent and exhibit a negative temperature dependence, with the observed pressure dependence reconciling apparent discrepancies between previous measurements performed at ∼298 K. Results indicate a rate coefficient of (4.80 ± 0.46) × 10-11 cm3 s-1 for the reaction of syn-CH3CHOO with SO2 at 298 K and 760 Torr. In contrast to the behavior of the syn-conformer, the kinetics of anti-CH3CHOO + SO2 display no significant dependence on temperature or pressure over the ranges investigated, with a mean rate coefficient of (1.18 ± 0.21) × 10-10 cm3 s-1 over all conditions studied in this work. Results indicate that the reaction of syn-CH3CHOO with SO2 competes with unimolecular decomposition and reaction with water vapor in areas with high SO2 concentration and low humidity, particularly at lower temperatures.

Barometric pressure correction to gamma-ray observations and its energy dependence

Cosmic rays (CRs) have been studied extensively in the last century to understand the processes in the universe as well as in the solar system. In today's satellite era, although many observations are made from space, CR observations from the ground are still viewed as a significant tool. These observations, however, mainly detect the secondary cosmic rays (SCRs) produced via nuclear spallation processes during the interactions of the primary CR with the atmospheric nuclei. Neutron, muon, and gamma are the major components of SCRs detected on the ground. It is well known that atmospheric pressure plays a vital role in the SCR flux observed on the ground. Barometric pressure correction is standard practice for neutron monitor (NM) data. For gamma-rays, however, being massless, their pressure dependence is not intuitive. Nevertheless, the pressure affects the particles such as e±, μ±, which produce gamma rays in the cascade. Subsequently, the indirect pressure dependence of the gamma-ray flux can be anticipated.We examine this aspect in detail by studying the gamma-ray counts detected by the NaI (Tl) detector. The present study confirms that there is no correlation between the atmospheric pressure and the total counts covering the entire energy range (150 keV–10 MeV) recorded by the NaI detector. However, the scenario differs when the fluxes of different energies are investigated separately. The gamma rays of energy below ∼3 MeV are primarily due to the radioactivity originating from the ground, whereas gamma rays above 3 MeV are mainly produced in the CR cascade. It is observed that the counts of energy above 3 MeV are well anti-correlated with the atmospheric pressure. The barometric coefficient obtained here matches well with that reported by the previous studies which used anti-coincidence methods. This may indicate that the role of directly detected muons and electrons by the NaI (Tl) in the observed pressure dependence is non-significant. It is demonstrated that applying the barometric correction formula to NaI (Tl) data successfully removes the pressure dependence in the flux above 3 MeV. Therefore, we suggest that the particle flux data above 3 MeV measured by NaI (Tl) detector needs to be corrected for the local atmospheric pressure variations.

Squeeze-Film Effect on Atomically Thin Resonators in the High-Pressure Limit.

The resonance frequency of membranes depends on the gas pressure due to the squeeze-film effect, induced by the compression of a thin gas film that is trapped underneath the resonator by the high-frequency motion. This effect is particularly large in low-mass graphene membranes, which makes them promising candidates for pressure-sensing applications. Here, we study the squeeze-film effect in single-layer graphene resonators and find that their resonance frequency is lower than expected from models assuming ideal compression. To understand this deviation, we perform Boltzmann and continuum finite-element simulations and propose an improved model that includes the effects of gas leakage and can account for the observed pressure dependence of the resonance frequency. Thus, this work provides further understanding of the squeeze-film effect and provides further directions into optimizing the design of squeeze-film pressure sensors from 2D materials.

MgPd2Sb : A Mg-based Heusler-type superconductor

We report the synthesis and physical properties of a full Heusler compound, MgPd$_2$Sb, which we found to show superconductivity below $T_c$ = 2.2 K. MgPd$_2$Sb was obtained by a two-step solid-state reaction method and its purity and cubic crystal structure (Fm-3m, a=6.4523(1) \r{A}) were confirmed by powder x-ray diffraction. Normal and superconducting states were studied by electrical resistivity, magnetic susceptibility, and heat capacity measurements. The results show that MgPd$_2$Sb is a type-II, weak coupling superconductor ($\lambda_{e-p}$ = 0.53). The observed pressure dependence of $T_c$ ($\Delta T_c / p \approx $ -0.23 K/GPa) is one of the strongest reported for a superconducting Heusler compound. The electronic structure, phonons, and electron-phonon coupling in MgPd$_2$Sb were theoretically investigated. The obtained results are in agreement with the experiment, confirming the electron-phonon coupling mechanism of superconductivity. We compare the superconducting parameters tothose of all reported Heusler-type superconductors.

Elasticity of single-crystal NaCl under high-pressure: simultaneous measurement of x-ray inelastic scattering and diffraction

ABSTRACT Elastic stiffness constants, C ijs, of single crystal of NaCl B1 phase have been determined at pressure up to 20 GPa using inelastic x-ray scattering to measure elastic wave velocities and x-ray diffraction to measure densities in the same experimental setup. The Voigt-Reuss-Hill average velocities calculated from the present Cijs are in good agreement with previous ultrasonic measurements of polycrystalline samples. Absolute pressures based on the present elastic constants and densities are proposed. In order to fit our results an equation of state with at least three parameters (an addition to a reference volume), such as the fourth-order form of Birch–Murnaghan equation or the higher order form of Vinet equation, is required, as fewer parameters do not reproduce the observed pressure dependence of the volume and bulk modulus.

First principle calculations of pressure dependent yielding in solute strengthened aluminium alloys

The pressure dependence of the yield stress in solute strengthened aluminium alloys is investigated by first principle calculations. The solute elements studied are magnesium, silicon and copper. A fixed boundary cluster model is employed to calculate the interaction energies between the edge dislocation and the solutes, while simultaneously controlling the hydrostatic pressure in the system. The results show a systematic increase in yield stress with increasing hydrostatic pressure for all solute elements. The calculated pressure dependence is in qualitative agreement with experiments, but underestimated quantitatively. It is suggested that the experimentally observed pressure dependence is caused by both the static and the transient dilatancy of dislocations. In contrast to magnesium and copper atoms, silicon atoms are found to interact non-elastically with dislocations within the core field, indicating that the favourable position for the silicon atoms is in the distorted sites in the matrix.

Combustion of aqueous HAN/methanol propellants at high pressures

“Green” monopropellants based on hydroxylammonium nitrate (HAN) are of interest for replacing highly toxic hydrazine in space propulsion systems and for other applications. However, their combustion mechanisms at high pressures are not well understood. In the present paper, decomposition of HAN at pressures up to 15 MPa was studied with differential scanning calorimetry, while combustion of an aqueous HAN/methanol solution (70.1 wt% HAN and 14.9 wt% methanol) was studied in a strand burner at pressures up to 30 MPa. The experiments in both setups were conducted in a nitrogen environment. It has been shown that the decomposition temperature of HAN significantly decreases with increasing pressure from atmospheric to 2 MPa and remains virtually constant with further increasing pressure to 15 MPa. The decrease with pressure is explained by suppressing evaporation of HNO3 and H2O formed during the decomposition process. The strand burner experiments have revealed an increase in the linear burning rate by over 50% at 12−14 MPa, explained by reaching the critical pressure of the liquid. Other observed pressure dependencies of the burning rate include a decrease at 9–12 MPa, a plateau at 14–19 MPa, a decrease at 20 MPa, and an increase at 22–30 MPa.

Magnetism driven by strong electronic correlations in the heavily carrier-doped iron oxypnictide LaFeAsO0.49H0.51

The magnetism of the second antiferromagnetic phase (AF2) arising in the iron-based LaFeAsO$_{1-x}$H$_{x}$ superconductor for $x\gtrsim0.4$ was investigated by muon spin rotation measurements under hydrostatic pressure up to 2.6 GPa. The N\'eel temperature ($T_{\rm N}$) obtained for a sample with $x=0.51$ exhibits considerably greater sensitivity to pressure than that in the pristine antiferromagnetic phase (AF1, $x\lesssim0.06$). Moreover, while the AF1 phase is always accompanied by the structural transition (from tetragonal to orthorhombic) at a temperature ($T_{\rm s}$) which is slightly higher than $T_{\rm N}$, the AF2 phase prevails at higher pressures above $\sim$1.5 GPa where the structural transition is suppressed ($T_{\rm s}=0$). These features indicate that the microscopic origin of the AF2 phase is distinct from that of AF1, suggesting that electronic correlation plays important role in the former phase. We argue that the orbital-selective Mott transition is a plausible scenario to account for the observed pressure dependence of $T_{\rm N}$ and $T_{\rm s}$ in the AF2 phase.

Comparative studies of electron-phonon coupling parameters in the luminescence lines for Zn2SiO4: Mn2+ and Zn2SiO4: En3+ crystals

The effects of temperature on the luminescence line 6A1─4T1 of Zn2SiO4: Mn2+ and luminescence lines 5D0 ─7FJ (J = 0, 1, 2, 3, 4) of Zn2SiO4: Eu3+ phosphors are analyzed in terms of both the static effect (characterized by the static parameter A) due to lattice thermal expansion and the dynamic effect (characterized by electron-phonon coupling parameter α′) caused by electron-phonon interaction. The static effect is estimated by dint of the pressure dependences of these line positions. It is found that the static and dynamic effects lead to the thermal blue shift and thermal red shift, respectively, and the observed zero thermal shifts of these line positions are due to the offset of the two effects. By help of an approximate processing, the electron-phonon coupling parameters α′ and static parameters A for these spectral lines are obtained although their thermal shifts are zero. Both the parameters A and α′ for the luminescence lines of 3dn ion Mn2+ in Zn2SiO4 are larger than those of 4fn ion Eu3+ in Zn2SiO4, in especial the parameter α′ in Zn2SiO4: Mn2+ is one order of magnitude greater than those in Zn2SiO4: Eu3+. The reasonableness of these estimated parameters is analyzed. It appears that the processing can extend the area of acquiring electron-phonon coupling parameters to the systems of spectral line with zero thermal shift (but its observed pressure dependence is not zero).

Saturation and pressure effects on the resistivity of titanium and two Ti-Al alloys

The electrical resistivities of α phase titanium and two Ti-Al alloys have been measured as functions of temperature, T, between 4 and 700 K and, in the range 175–700 K, as a function of pressure up to 1.2 GPa. All materials showed resistivity saturation effects at the highest temperatures. A “parallel resistivity” saturation model could be fitted to all data with excellent results if Mott-Fermi smearing, expected for a transition metal, was included by adding a term in T3 to the phonon-induced resistivity. However, in the standard saturation model the fitted resistivity parameters were not always realistic. A modified saturation model which partially retained Matthiessen's rule could be fitted equally well and gave numerically acceptable results for both residual, electron-phonon and saturation resistivities. This new model also fitted the T dependence of the pressure coefficients with a single set of coefficients, each valid for all three materials. Although simple free-electron and Debye models could apparently explain the observed pressure dependence of the impurity and electron-phonon resistivities, a model taking band structure changes with pressure into account showed that the electron-phonon interaction factor of titanium is practically independent of pressure while the plasma frequency has a strong pressure dependence. This model gave reasonable numerical results for the pressure dependence of both the residual, electron-phonon and saturation resistivities and also agreed with experimental data for the superconducting critical temperature Tc under pressure for α titanium.

Temperature and pressure dependence of local structural changes around Gd3+ centers in RAl3(BO3)4 crystals: Modeling zero-field splitting parameters

The semi-empirical superposition model (SPM) analysis is applied to predict the zero-field splitting parameters (ZFSPs) of Gd3+ ions doped into RM3(BO3)4 (RAB) as well as the structural changes around these paramagnetic centers located at possible cation sites in these crystals. Theoretical investigations are aimed at explanation of the temperature dependence of the local structural parameters of Gd3+:RAB (R = Y, Eu, Tm). The results of spin Hamiltonian (SH) analysis of electron magnetic resonance (EMR; EPR – electron paramagnetic resonance) spectra serve for fine-tuning the theoretically predicted zero-field splitting (ZFS) parameters (ZFSPs) obtained using SPM. This approach enables determination of the local structure changes around Gd3+ centers in RAB crystals and explains the observed temperature dependence of the ZFSPs. For Gd3+:EuAB, due to availability of the data, also the observed pressure dependence of the ZFSPs is considered. For this purpose, the distortions parameters, i.e.m the changes in the ligand’s distances ΔRi and the angular distortions Δθi, have been treated as variable parameters in SPM analysis. This procedure yields good matching of the calculated ZFSPs and the experimental ones, and enables determination of the corresponding local distortions. The present results may be useful in future studies aimed at technological applications of the Huntite-type borates with the formula RM3(BO3)4. The model parameters determined here may be utilized for ZFSP calculations for Gd3+ ions at octahedral sites in single-molecule magnets and single-chain magnets.

Pressure-induced itinerant electron metamagnetism in UCo0.995Os0.005Al ferromagnet

The effect of external hydrostatic pressure on magnetic properties is studied for the UCo0.995Os0.005Al single crystal. At ambient pressure, the ground state is ferromagnetic. Even lowest applied pressure 0.11GPa is sufficient to suppress ferromagnetism. A sharp metamagnetic transition is observed only in magnetic fields along the c axis of the crystal, similar to previously studied itinerant electron metamagnet UCoAl. Temperature dependence of the susceptibility for various pressures shows a broad maximum at Tmax ~ 20K. The experimental data are analyzed with the theory of itinerant electron metamagnetism, which considers anisotropic thermal fluctuations of the uranium magnetic moment. The observed pressure dependence of the susceptibility at Tmax and the temperature for the disappearance of the first-order metamagnetic transition are explained with the theory.

Modeling of Pressure Dependence of Interfacial Tension Behaviors of Supercritical CO2 + Crude Oil Systems Using a Basic Parachor Expression

Parachor based expressions (basic and mechanistic) are often used to model the experimentally observed pressure dependence of interfacial tension (IFT) behaviors of complex supercritical carbon dioxide (sc-CO2) and crude oil mixtures at elevated temperatures. However, such modeling requires various input data (e.g. compositions and densities of the equilibrium liquid and vapor phases, and molecular weights and diffusion coefficients for various components present in the system). In the absence of measured data, often phase behavior packages are used for obtaining these input data for performing calculations. Very few researchers have used experimentally measured input data for performing parachor based modeling of the experimental IFT behaviors of sc-CO2 and crude oil systems that are of particular interest to CO2 injection in porous media based enhanced oil recovery (EOR) operations.This study presents the results of parachor based modeling performed to predict pressure dependence of IFT behaviors of a complex sc-CO2 and crude oil system for which experimentally measured data is available in public domain. Though parachor model based on calculated IFT behaviors shows significant deviation from the measured behaviors in high IFT region, difference between the calculated and the experimental behaviors appears to vanish in low IFT region. These observations suggest that basic parachor expression based calculated IFT behaviors in low IFT region follow the experimental IFT behaviors more closely.An analysis of published studies (basic and mechanistic parachor expressions based on modeling of pressure dependence of IFT behaviors of both standard and complex sc-CO2 and crude oil systems) and the results of this study reinforce the need of better description of gas-oil interactions for robust modeling of pressure dependence of IFT behavior of these complex systems.

The Chemical Shift Baseline for High-Pressure NMR Spectra of Proteins.

High-pressure (HP) NMR spectroscopy is an important method for detecting rare functional states of proteins by analyzing the pressure response of chemical shifts. However, for the analysis of the shifts it is mandatory to understand the origin of the observed pressure dependence. Here we present experimental HP NMR data on the (15) N-enriched peptide bond model, N-methylacetamide (NMA), in water, combined with quantum-chemical computations of the magnetic parameters using a pressure-sensitive solvation model. Theoretical analysis of NMA and the experimentally used internal reference standard 4,4-dimethyl-4-silapentane-1-sulfonic (DSS) reveal that a substantial part of observed shifts can be attributed to purely solvent-induced electronic polarization of the backbone. DSS is only marginally responsive to pressure changes and is therefore a reliable sensor for variations in the local magnetic field caused by pressure-induced changes of the magnetic susceptibility of the solvent.

Modeling of Pressure Dependence of Interfacial Tension Behaviors of a Complex Supercritical CO2 + Live Crude Oil System Using a Basic Parachor Expression

Parachor based basic expressions are often used to model the experimentally observed pressure dependence of interfacial tension (IFT) behaviors of complex supercritical CO2 + crude oil (dead or live) mixtures at elevated temperatures. However, such modeling requires compositions and densities of the equilibrium liquid and vapor phases and molecular weights of various components present in the system. In the absence of measured data, often phase behavior packages are used for obtaining these input data for performing calculations. Very few researchers have used experimentally measured input data for performing parachor based modeling of the experimental IFT behaviors of complex supercritical CO2 + live crude oil systems. This study presents the results of a basic parachor model based modeling of pressure dependence of IFT behaviors of a complex supercritical CO2 + live crude oil system for which experimentally measured input data is available in public domain. Though the calculated IFT behaviors showed significant deviation from the experimentally measured behaviors at a given pressure, calculated IFT versus pressure trends appeared to follow the experimental observed trends closely. Also, despite a large variation in the compositions of feed supercritical CO2 + live crude oil mixtures, both trends (calculated and experimental) converged rapidly to attain a near zero IFT condition at more or less same pressure.

Charge Density and Bonding Properties of LaFeAsO under Pressure

Some interesting aspects of the electronic structure of iron oxypnictides depend sensitively on pressure. We investigate this sensitive pressure dependence in the parent compound LaFeAsO by full-potential electronic structure calculations based on density functional theory. We evaluate the sensitivity of the electronic structure to the Fe–As bond length as being due to the rather high degree of covalency associated with the bond, by visualizing the charge density distributions in detail. A strong pressure dependence in the charge density distributions is observed in the two bands, that form cylindrical electron sheets of the Fermi surface at ambient pressure. The bonding states associated with the Fe and As atoms rapidly grow up to 4 GPa and gradually start to decrease above 4 GPa. These pressure dependences of the covalency for the Fe–As bond are similar to the observed pressure dependences in superconductive transition temperatures. It is suggested that this result play an important role in exploring the origin of the superconductivity of LaFeAsO.

Direct Simulation Monte Carlo Analysis of Gas Transport in Microporous Structure Based on X-ray Computed Tomography

We present the analysis of gas transport in micro porous layers of polymer electrolyte fuel cells based on the three-dimensional structure data obtained from X-ray nano computed tomography (CT). The characteristic pore size is comparable to the mean free path of gas molecules. The polygonal surface representation of the porous structure is constructed from the volumetric CT image using the marching tetrahedrons algorithm. The diffusion fluxes of gas molecules through the porous layer are evaluated by the direct simulation Monte Carlo method, which can reproduce the transition between the Knudsen and molecular diffusion regimes. Our numerical simulation well reproduces the experimentally observed pressure dependence of the diffusion resistance. The effect of porous media morphology on gas transport is examined by an analysis of the trajectories of transmitted molecules through the porous layer.

Contribution of isotopologue self‐shielding to sulfur mass‐independent fractionation during sulfur dioxide photolysis

Signatures of sulfur mass‐independent fractionation (S‐MIF) are observed for sulfur minerals in Archean rocks, and for modern stratospheric sulfate aerosols (SSA) deposited in polar ice. Ultraviolet light photolysis of SO2 is thought to be the most likely source for these S‐MIF signatures, although several hypotheses have been proposed for the underlying mechanism(s) of S‐MIF production. Laboratory SO2 photolysis experiments are carried out with a flow‐through photochemical reactor with a broadband (Xe arc lamp) light source at 0.1 to 5 mbar SO2 in 0.25 to 1 bar N2 bath gas, in order to test the effect of SO2 pressure on the production of S‐MIF. Elemental sulfur products yield high δ34S values up to 140 ‰, with δ33S/δ34S of 0.59 ± 0.04 and Δ36S/Δ33S ratios of −4.6 ± 1.3 with respect to initial SO2. The magnitude of the isotope effect strongly depends on SO2 partial pressure, with larger fractionations at higher SO2 pressures, but saturates at an SO2 column density of 1018 molecules cm−2. The observed pressure dependence and δ33S/δ34S and Δ36S/Δ33S ratios are consistent with model calculations based on synthesized SO2 isotopologue cross sections, suggesting a significant contribution of isotopologue self‐shielding to S‐MIF for high SO2 pressure (>0.1 mbar) experiments. Results of dual‐cell experiments further support this conclusion. The measured isotopic patterns, in particular the Δ36S/Δ33S relationships, closely match those measured for modern SSA from explosive volcanic eruptions. These isotope systematics could be used to trace the chemistry of SSA after large Plinian volcanic eruptions.

Spin-ordering mediated orbital hybridization in CoO at high pressures

We demonstrate a strong dependence of high-resolution 1$s$3$p$ (or ${K}_{\ensuremath{\beta}}$) resonant x-ray emission spectroscopy (RXES) on local spin correlations at high pressure. We show that the pre-edge region in ${K}_{\ensuremath{\beta}}$ RXES of CoO can be separated into a local quadrupolar contribution and nonlocal dipolar intensity, which arises from the mixing of cobalt 4$p$ states with neighboring cobalt 3$d$ orbitals. For pressures of 0--12 GPa, we observe a twofold increase in nonlocal contributions with respect to local contributions. For pressures greater than 12 GPa, the ratio remains nearly constant. The observed pressure dependence and strong intensity changes cannot be explained by a conventional picture of bond-length shortening and is ascribed to increased intersite 4$p$-3$d$ mixing due to changes in magnetic ordering. These results are supported by theoretical calculations.

Pressure effect on element partitioning between minerals and silicate melt: Melting experiments on basalt up to 20 GPa

High pressure melting experiments of alkali basalt were performed from 3 to 20GPa, and the partitioning of major and trace elements between silicate melt and coexisting minerals was investigated using EPMA and LA-ICP-MS. Garnet existed at around the liquidus at all pressures, while clinopyroxene (cpx) was observed between 3 and 12GPa. K-hollandite was identified at 20GPa. The calculated partition coefficients (D∗-values) for garnet/melt and cpx/melt showed a similar pressure dependence: DNa∗ and DK∗ increase with pressure, while D∗-values of rare earth elements decreased. Apparent differences were found only in the behavior of DSr∗, which increased with pressure for garnet/melt but remained almost constant for cpx/melt. Partition coefficient profiles on PC-IR diagrams display a remarkable increase in peak position with increasing pressure for monovalent ions. However, the peak positions of divalent ions remained nearly constant and a slight decrease was found for the partition curve of trivalent ions. Under high pressure, both the crystal structure and ions are compressed. The observed pressure dependences in the peak positions of partition coefficient curves can be qualitatively explained by the difference in compressibility of monovalent and trivalent cations: the former being more compressible than the latter.