Intermediate Pressure Range Research Articles

Optical emission features of laterally colliding laser-produced plasmas at different ambient pressures

Optical emission characteristics of seed plasma and stagnation region of laterally colliding laser produced aluminium plasmas are investigated under various background gas pressures of air and argon ambient. The intensity of ionic and neutral emissions from seed plasma region increases with the increase of ambient pressure. Neutral emission from the stagnation region increases whereas the intensity of ionic emission decreases with the increase of ambient pressure. Decrease of collision rate between the seed plasmas results in the decrease of ionic emission from the interaction region. For higher inter seed plume separation, maximum spectral emission from stagnation region is observed at an intermediate pressure range of 10−2 mbar. Emission intensity and the life time of both ionic and neutral emission are greater in heavier Ar ambient. In the case of seed plasmas, the electron density decreases whereas the electron temperature increases with the increase of ambient gas pressure and is greater in Ar ambient. The variations of plasma parameters are quite less in stagnation region.

Effective capillary pressure and permeability of a granular material during imbibition in a closed column

We present a method to determine the effective capillary pressure and permeability of a column packed with a granular material by means of imbibition experiments using a closed column and measuring both the liquid uptake and pressure differential that develops ahead of the liquid front. The method is based on finding an intermediate range of pressure differential values, where a Washburn approximation is no longer valid but a recently obtained effective description accurately reproduces the experimental results. To determine this intermediate range of pressures we calculate the goodness of fit corresponding to both the Washburn pressure-solution, valid at short times, and the implicit pressure-solution, which includes the non-linear dependence of the pressure on the liquid uptake and is compatible with finely divided solids. Using a closed column also allow us to compare the effective capillary pressure with the maximum advancing pressure measured using Bartell’s static method. We perform experiments using liquid polydimethylsiloxane, and glass beads or lactose powder. We show that an effective description of the liquid imbibition is possible and determine the corresponding capillary pressure and permeability that characterize the granular system. We observe that the heterogeneity of the system reduces the characterization range. However, it was possible to characterize lactose powder columns, where the heterogeneity leads to spontaneous bubbling and static measurements are not possible.

The role of pressure and defects in the wurtzite to rock salt transition in cadmium selenide.

Using molecular dynamics and path sampling techniques we investigated the effect of pressure and defects in the wurtzite to rock salt transition in cadmium selenide (CdSe). In the pressure range 2-10 GPa, rate constants of transition are in the order of 10-23 to 105 s-1 for the transformation of a relatively small wurtzite crystal consisting of 1024 atoms with periodic boundary conditions. The transition paths predominantly evolve through an intermediate 5-coordinated structure, as reported before, though its typical lifetime within the transition paths is particularly long in the intermediate pressure range (4-6 GPa). The defects were created by removing Cd-Se pairs from an otherwise perfect crystal. The removals were either selected fully randomized or grouped in clusters (cavity creation). We find that the rate of transition due to the defects increases by several orders of magnitude even for a single pair removal. This is caused by a change in the transition mechanism that no longer proceeds via the intermediate 5-coordinated structure, when defects are present. Further, the cavity creation yields a lower rate than the fully randomized removal.

Transition from ballistic to thermalized transport of metal-sputtered species in a DC magnetron

Tunable diode-laser induced fluorescence technique has been optimized to accurately measure the titanium (Ti)-sputtered atom velocity distribution functions (AVDFs) in a magnetron discharge operating in DC mode. The high spatial and spectral resolution achieved reveals some features of the transport of the metal-sputtered atoms and their thermalization. The two groups of thermalized and energetic atoms have been very well separated compared to previous works. Hence, the fitting of the energetic atom group shows dumping from modified Thompson to Gauss distribution when the product pressure-distance from the target increases. In parallel, sputtered metal transport from the target has been simulated using the Monte Carlo collision (MCC) approach. Direct comparison between numerical and experimental results led to an improved cross-section for Ti–Ar momentum transfer, based on the ab initio formulas of the interaction potential derived from noble gas interaction. The accuracy of the experimental data enabled the numerical parametric study of the angular distribution and cut-off energy for the initial distribution of sputtered atoms to reveal the precise characterization of the initial conditions. A very good overall agreement is obtained for measured and calculated AVDFs. Comparison between the measured and modeling results emphasized the major role played by the argon (Ar) ions, not only in the sputtering process, but in the neutral metal transport by the gas rarefaction near the target. The microscopic description provided by the MCC model clearly reveals different transport regimes: ballistic, diffusive and back-scattering, which provide new insight into the thermalization of sputtered species in the intermediate pressure range.

Double Superconducting Dome and Triple Enhancement of T_{c} in the Kagome Superconductor CsV_{3}Sb_{5} under High Pressure.

CsV_{3}Sb_{5} is a newly discovered Z_{2} topological kagome metal showing the coexistence of a charge-density-wave (CDW)-like order at T^{*}=94 K and superconductivity (SC) at T_{c}=2.5 K at ambient pressure. Here, we study the interplay between CDW and SC in CsV_{3}Sb_{5} via measurements of resistivity, dc and ac magnetic susceptibility under various pressures up to 6.6GPa. We find that the CDW transition decreases with pressure and experience a subtle modification at P_{c1}≈0.6-0.9 GPa before it vanishes completely at P_{c2}≈2 GPa. Correspondingly, T_{c}(P) displays an unusual M-shaped double dome with two maxima around P_{c1} and P_{c2}, respectively, leading to a tripled enhancement of T_{c} to about 8K at 2GPa. The obtained temperature-pressure phase diagram resembles those of unconventional superconductors, illustrating an intimated competition between CDW-like order and SC. The competition is found to be particularly strong for the intermediate pressure range P_{c1}≤P≤P_{c2} as evidenced by the broad superconducting transition and reduced superconducting volume fraction. The modification of CDW order around P_{c1} has been discussed based on the band structure calculations. This work not only demonstrates the potential to raise T_{c} of the V-based kagome superconductors, but also offers more insights into the rich physics related to the electron correlations in this novel family of topological kagome metals.

Extension of finite-strain equations of state to ultra-high pressure

Eulerian and Lagrangian finite-strain expansions are extended into the ultra-high pressure range by way of an isothermal closed-form EoS, a broken power-law density depending on four parameters determined by least-squares regression. The EoS is put to test with high-pressure data sets of copper up to 60 TPa and can be used to extrapolate data sets obtained in the GPa range to ultra-high densities approaching the Thomas-Fermi free-electron regime. In the low and intermediate pressure range up to a few hundred GPa, the EoS admits finite-strain ascending series expansions, which coincide with the third-, fourth- and fifth-order Birch-Murnaghan and Lagrangian EoSs, subject to the finite-strain expansion parameter used. The pressure evolution of the compression modulus of copper is obtained from the regressed EoS. A closed-form expression of the free energy over the full pressure range up to the Thomas-Fermi limit is derived and compared with finite-strain theory.

Structural transition, metallization, and superconductivity in quasi-two-dimensional layered PdS2 under compression

Based on first-principles simulations and calculations, we explore the evolution of crystal structure, electronic structure and transport properties of quasi 2D layered PdS2 under uniaxial stress and hydrostatic pressure. The coordination of the Pd ions plays crucial roles in the structural transition, electronic structure and transport properties of PdS2. An interesting ferroelastic phase transition with lattice reorientation is revealed under uniaxial compressive stress, which originates from the bond reconstructions of the unusual PdS4 square-planar coordination. By contrast, the layered structure transforms to 3D cubic pyrite-type structure under hydrostatic pressure. In contrast to the experimental proposed coexistence of layered PdS2-type structure with cubic pyrite-type structure at intermediate pressure range, we predict that the compression-induced intermediate phase showing the same structural symmetry with the ambient phase, except of sharply contracted interlayer-distances. The coordination environments of the Pd ions have changed from square-planar to distorted octahedra in the intermediate phase, which results in the bandwidth broaden and orbital-selective metallization. In addition, the superconductivity comes from the cubic pyrite-type structure protected topological nodal-line states. The strong correlations between structural transition, electronic structure and transport properties in PdS2 provide a platform to study the fundamental physics of the interplay between crystal structure and transport behavior, and the competition between diverse phases.

Quantum chemical and kinetic study of the CCl2 + HCl → CHCl3 insertion reaction

The CCl2 + HCl → CHCl3 (1) insertion reaction has been investigated by ab initio molecular orbital and kinetic calculations. The geometries of reactants, products and transition state were optimized with a large number of density functional theory (DFT) formulations combined with the 6-311++G(3df,3pd) basis set. The reaction barriers calculated with G4(MP2)//DFT and G4//DFT theories of 1.16 ± 0.27 and 0.61 ± 0.26 kcal mol−1 are consistent with the barrier height of ΔE0# = 1.54 ± 0.30 kcal mol−1 estimated from the room temperature experimental rate constant. Calculated enthalpies of reaction of −53.97 ± 0.09 (G4(MP2)//DFT), −55.27 ± 0.09 (G4//DFT) and −56.52 ± 3.90 kcal mol−1 (DFT) agree well with experimental values. The obtained rate constants over the 300–2000 K temperature range at the high and low pressure limit can be expressed as k1,∞(G4(MP2)//DFT) = (4.8 ± 1.8) × 10−14 (T/300 K)2.12±0.19 and k1,∞(G4//DFT) = (9.8 ± 3.4) × 10−14 (T/300 K)1.77±0.16 in cm3 molecule−1 s−1, and k1,0 = [HCl] 3.40 × 10−27 (T/300 K)−6.57 exp(−2218 K/T) cm3 molecule−1 s−1. Falloff curves for the intermediate pressure range obtained for the reactions (1, −1) with these rate constants and a derived central broadening factor of Fcent = 0.16 + 0.84 exp(−T/331 K) + exp(−7860 K/T) were compared with reported experimental rate data.

Volume and surface loss of O(3P) atoms in O2 RF discharge in quartz tube at intermediate pressures (10–100 Torr)

The O(3P) atom loss has been studied in O2 RF plasma in the quartz tube in the intermediate pressure range (10–100 Torr) when the transition from the surface to volume loss is observed. Space- and time-resolved actinometry on Ar and Kr atoms was used to study O(3P) atom loss. The research has shown that such a transition actually takes place. However, it was revealed that the gas temperature plays a significant role in this. Gas temperature was measured spectroscopically using the band emission. It was demonstrated that the gas temperature in the plasma volume was rather high (>1200 K) due to the high values of the specific input RF power which in turn led to a significant O(3P) loss rate decrease in the discharge volume. The atomic oxygen loss is limited by the O(3P) surface recombination as well as by the volume recombination in a thin layer near the wall. It leads to a low integral loss rate and provides a high oxygen dissociation degree. Analysis based on measured [O(3P)]/N, Tgas, O3 spatial profiles and surface loss model including recombination with chemisorbed and physisorbed atoms has revealed the importance of both surface and volume losses. High values of the specific input RF power also lead to increase of the gas temperature near the wall and the temperature of the internal tube surface. As a result, the O atom surface loss rate increases and the volume loss rate near the wall decreases, so overall the contributions of both O(3P) volume and surface recombination are comparable at pressures up to 100 Torr. The O(3P) loss kinetics at intermediate pressures appears to be a rather complex phenomenon and requires at least 1D or even 2D modeling for its correct description. Using global models under similar conditions may lead to dubious results in a detailed study of kinetic mechanisms and processes, but it can be useful for a simple analysis of experimental results.

Interplay between Single and Cooperative H2 Adsorption in the Saturation of Defect Sites at MgO Nanocubes

The configurations associated with reversible and irreversible adsorption of hydrogen on MgO escape consensus. Here, we report the dissociation of H2 on MgO nanocubes, which was examined by combining Fourier transform infrared spectroscopy and density functional theory (DFT)-based simulations. We found that the use of ultrahigh vacuum is essential for identifying the very first adsorption stages. Hydrogen pressure was varied from 10–8 mbar to near ambient, resulting in IR spectra of richer complexity than in current state of the art. Models with oxygen at regular corners (O3C) and Mg at inverse corners (MgIC) were identified to be the most reactive and to split H2 irreversibly already in the lowest pressure regime (PH2 < 10–3 mbar). The continuous increase in intensity of the corresponding IR bands (3712/1140 cm–1) in the intermediate range of pressures (10–3–1 mbar), along with the appearance of bands at 3605/1225 cm–1, was demonstrated to stem from cooperative adsorption mechanisms, which could be therefore considered as the main origin of irreversible hydrogen adsorption. At PH2 > 1 mbar, fully reversible adsorption was shown to occur at O4C (either on mono- or diatomic steps) and Mg3C sites. Another OH/MgH couple (3697/1030 cm–1) that became reinforced at high PH2 but remained stable upon pumping was correlated to O3C and MgIC in multiatomic steps. The difference in adsorption and desorption sequences confirmed the proposed cooperative adsorption of H2 molecules. Our study provides new insights into the mechanisms that can be beneficial for understanding the chemistry of H2 and other hydrogen-containing molecules, such as CH4, on oxide surfaces, but also for the advancement of hydrogen-storage technologies.

Ostwald's rule of stages and metastable transitions in the hydrogen-water system at high pressure.

Although the hydrogenous analogue of the D2-D2O system has been well explored in the regimes above 1 GPa, and below 0.2 GPa, there have been very few studies in the region between these pressures. The recent discovery in the range 0.5-0.7 GPa of a new phase, C0, that possesses a new clathrate structure with a new H2O network, along with the proposal of another structure stable at similar conditions, has prompted further studies of the hydrogen water system in this intermediate pressure region. Here, we report the results of neutron-diffraction experiments that observed transitions from metastable to stable structures in the D2-D2O system around 0.2-0.3 GPa between 130 K and 280 K. These metastable structures were observed in the stability region of the sII hydrogen hydrate clathrate and computational studies of their relative enthalpies suggest that transition sequence observed is in line with Ostwald's 'Rule of Stages'.

Slag Chemistry of High-Alumina Iron Silicate Slags at 1300 °C in WEEE Smelting

New fluxing chemistries are appearing in secondary copper smelters, where the use of raw materials from various waste electric and electronic equipment sources as significant copper feedstocks associate greatly increased fractions of alumina in the slag, compared with the conventional raw materials utilized in primary copper smelting processes. In extreme cases, alumina concentrations close to the saturation level may be reached, and the practical alumina loads to the slag may be above 10 wt% (Al2O3). This affects the physical properties of the molten slag and enhances stabilities of the solid primary phases, thus having an impact on its saturation boundaries and solubilities. We examined fundamental phase-equilibrium data of high-alumina slags by an equilibration-quenching method in equilibrium with metallic copper at 1300 °C, in the oxygen pressure range of P(O2) = 10−10–10−5 atm. We measured the chemical compositions of the molten slag and copper, and solid alumina-iron spinel directly by the Electron X-ray Microprobe technique and provide novel phase-equilibrium data for the poorly known intermediate oxygen pressure range from air to iron saturation.

Crystallisation of the amorphous ices in the intermediate pressure regime

The crystallisation behaviour of very high-density amorphous ice (VHDA) and unannealed high-density amorphous ice (uHDA) has been studied in situ by volumetry and ex situ by powder x-ray diffraction in the intermediate pressure range 0.7–1.8 GPa employing different heating rates (0.5, 5 and 30 K min−1). This study shows that at pressures >1 GPa the crystallisation behaviour of VHDA and uHDA is basically the same for all heating rates. That is, parallel crystallisation is almost entirely suppressed with mainly ice XII forming. This contrasts former results reporting parallel crystallisation to approximately levelled phase mixtures of ice IV and ice XII even at higher pressures for uHDA. We speculate this to be due to formation of microcracks upon decompression in earlier works, but not in the present one. Crystallisation temperatures Tx are up to 16 K higher than previously reported, raising the low-temperature border to no man’s land and opening a considerably larger window for future studies on non-crystalline water. The results indicate uHDA to contain heterogeneities on the nanoscale, but VHDA to be rather homogeneous with nano-crystallites being largely absent. Upon transforming uHDA to VHDA, the nano-scale heterogeneities disappear for >1 GPa whereas microcracks do not.

COMBUSTION AND PERFORMANCE STUDIES OF GLYCIDYL AZIDE POLYMER AND ITS MIXTURES AS HYBRID ROCKET FUEL

The combustion of glycidyl azide polymer (GAP) was investigated. The GAP fuel tests were conducted in a strand burner with nitrogen gas purge and in a lab-scale hybrid rocket with gas oxygen as oxidizer. In the strand burner, at intermediate pressure range 3−6 MPa, burning rates of both cured and uncured GAP were nearly the same and strongly depend on the chamber pressure. At pressure larger than 6 MPa, uncured GAP shows less dependence on pressure, and the GAP ratio of mixtures begins to have dominant role on burning rate. Transition pressure region was observed, which was in between the pressure dependence to composition dependence. The fluctuating burning rate was the main appearance in the transition pressure region. The Zel'dovich formula shows good prediction for burning rate at intermediate pressure range, but has much deviation at high pressure due to the surface instability. A smaller sample tube can restrict the burning surface from twisting, and the resulting burning rate shows good agreement with Zel'dovich formula prediction. Uncured 100% GAP can reach its maximum burning rate at certain pressure less than 10 MPa. Based on the assumption that condensed phase reaction is the only heat source which dominates the burning rate, the prediction of maximum burning rate of GAP mixture is also possible. In a lab-scale hybrid rocket test, GAP/PEG solid fuel demonstrated a very good burning rate under mild chamber pressure and oxygen flux. Instantaneous burning rate was successfully measured by ultrasonic techniques, and the results implied complex internal ballistic behavior.

Plasma interactions determine the composition in pulsed laser deposited thin films

Plasma chemistry and scattering strongly affect the congruent, elemental transfer during pulsed laser deposition of target metal species in an oxygen atmosphere. Studying the plasma properties of La0.6Sr0.4MnO3, we demonstrate for as grown La0.6Sr0.4MnO3-δ films that a congruent transfer of metallic species is achieved in two pressure windows: ∼10−3 mbar and ∼2 × 10−1 mbar. In the intermediate pressure range, La0.6Sr0.4MnO3-δ becomes cation deficient and simultaneously almost fully stoichiometric in oxygen. Important for thin film growth is the presence of negative atomic oxygen and under which conditions positive metal-oxygen ions are created in the plasma. This insight into the plasma chemistry shows why the pressure window to obtain films with a desired composition and crystalline structure is narrow and requires a careful adjustment of the process parameters.

Role of the 245 phase in alkaline iron selenide superconductors revealed by high-pressure studies

Here we show that a pressure of about 8 GPa suppresses both the vacancy order and the insulating phase, and a further increase of the pressure to about 18 GPa induces a second transition or crossover. No superconductivity has been found in compressed insulating 245 phase. The metallic phase in the intermediate pressure range has a distinct behavior in the transport property, which is also observed in the superconducting sample. We interpret this intermediate metal as an orbital selective Mott phase (OSMP). Our results suggest that the OSMP provides the physical pathway connecting the insulating and superconducting phases of these iron selenide materials.

Applicability of Child–Langmuir collision laws for describing a dc cathode sheath in N2O

It is established which of the Child–Langmuir collision law versions are most appropriate for describing the processes in the cathode sheath in the N2O. At low pressure (up to 0.3 Torr), the Child–Langmuir law version relating to the constant ion mobility holds. At N2O pressure values starting from 0.75 Torr and above, one has to employ the law version for which it is assumed that the ion mean free path within the cathode sheath is constant. In the intermediate pressure range (between 0.3 and 0.75 Torr), neither of the Child–Langmuir law versions gives a correct description of the cathode sheath of the glow discharge in the N2O.

Role of Dynamical Instability in theAb InitioPhase Diagram of Calcium

In the 32-119 GPa pressure range and at room temperature, a simple cubic phase was reported for calcium in many different experiments. Standard linear response theory, both within density functional perturbation theory and frozen phonon calculations, presents dynamical instabilities for the simple cubic structure in the whole pressure range. Many other possible candidate phases, as well as several possible stabilization mechanisms for the simple cubic phase, have been proposed as the result of ab initio predictions but the role of temperature on the relative stability of the different phases has not been systematically investigated. We revisit the stability of the three most important candidate phases of calcium for the intermediate pressure range and for various temperatures, taking explicitly into account thermal corrections relative to electronic as well as phononic entropy and anharmonic contributions. This corrects the discrepancies among previous theoretical results and experiments and presents a different picture of the temperature driven phase transition, which results from dynamical anharmonic stabilization of simple cubic and destabilization of the tetragonal phase.

On the Defect Structure and Transport Properties of Co3O4 Spinel Oxide

AbstractDefect structure and transport properties of Co3O4 cobalt oxide have been studied as a function of temperature and oxygen pressure, using marker and thermogravimetric techniques. It has been found that the oxidation of CoO to form Co3O4 follows parabolic kinetic, being thus diffusion controlled. Marker experiments have demonstrated that cation sublattice of Co3O4 oxide is predominantly disordered, but the defect structure is rather complex. At very low oxygen pressures, close to the dissociation pressure of the oxide, interstitial cations are the predominant point defects, while at high pressures cation vacancies predominate. This behavior is reflected in complex dependence of the parabolic rate constant of CoO oxidation on oxygen pressure. At low pressures, namely, parabolic rate constant of the reaction increases with oxygen pressure, reaching then virtually constant value in intermediate pressure range and increases again in highest pressure range. Theoretical analysis of kinetic results in terms of point defect thermodynamics confirmed the above conclusion, concerning complex defect structure of Co3±yO4 oxide. Finally, self-diffusion coefficient of cations in the discussed oxide has been calculated as a function of temperature and oxygen pressure from kinetic rate measurements, using Wagner's theory of metals oxidation.

Lithium Amide (LiNH2) Under Pressure

Static high pressure lithium amide (LiNH(2)) crystal structures are predicted using evolutionary structure search methodologies and intuitive approaches. In the process, we explore the relationship of the structure and properties of solid LiNH(2) to its molecular monomer and dimer, as well as its valence-isoelectronic crystalline phases of methane, water, and ammonia all under pressure. A NaNH(2) (Fddd) structure type is found to be competitive for the ground state of LiNH(2) above 6 GPa with the P = 1 atm I4[overline] phase. Three novel phases emerge at 11 (P4[overline]2(1)m), 13 (P4(2)/ncm), and 46 GPa (P2(1)2(1)2(1)), still containing molecular amide anions, which begin to form N-H···N hydrogen bonds. The P2(1)2(1)2(1) phase remains stable over a wide pressure range. This phase and another Pmc2(1) structure found at 280 GPa have infinite ···(H)N···H···N(H)···H polymeric zigzag chains comprising symmetric N···H···N hydrogen bonds with one NH bond kept out of the chain, an interesting general feature found in many of our high pressure (>280 GPa) LiNH(2) structures, with analogies in high pressure H(2)O-ices. All the predicted low enthalpy LiNH(2) phases are calculated to be enthalpically stable with respect to their elements but resist metallization with increasing pressure up to several TPa. The possibility of Li sublattice melting in the intermediate pressure range structures is raised.