Lower Transition Pressure Research Articles

Pressure effect on magnetism and valence in ferromagnetic superconductor Eu(Fe0.75Ru0.25)2As2

Eu(Fe0.75Ru0.25)2As2 is an intriguing system with unusual coexistence of superconductivity and ferromagnetism, providing a unique platform to study the nature of such coexistence. To establish a magnetic phase diagram, time-domain synchrotron Mössbauer experiments in 151Eu have been performed on a single crystalline Eu(Fe0.75Ru0.25)2As2 sample under hydrostatic pressures and at low temperatures. Upon compression the magnetic ordering temperature increases sharply from 20 K at ambient pressure, reaching ∼49 K at 10.1 GPa. With further compression, the magnetic order is suppressed and eventually collapses. Isomer shift values from Mössbauer measurements and x-ray absorption spectroscopy data at Eu L 3 edge show that pressure drives Eu ions to a homogeneous intermediate valence state with mean valence of ∼2.4 at 27.4 GPa, possibly responsible for the suppression of magnetism. Synchrotron powder x-ray diffraction experiment reveals a tetragonal to collapsed-tetragonal structural transition around 5 GPa, a lower transition pressure than in the parent compound. These results provide guidance to further work investigating the interplay of superconductivity and magnetism.

Fluid Dynamic Characteristics and Flow Distribution ‎Structure Optimization of Axial Piston Pump Considering ‎Cavitation Bubble Evolution

An axial piston pump can produce a serious cavitation phenomenon in the high- and low-pressure ‎transition process. Cavitation bubbles expand, compress, rebound and collapse when they enter the high-‎pressure oil drainage area. This affects the outlet flow ripple as well as the pressure pulsation of the ‎piston pump. However, the effect of the cavitation bubbles is ignored in the current outlet flow ripple ‎model of axial piston pumps. It affects the optimization design of the axial piston pump distribution area ‎structure parameters with the objective of reducing the pressure and flow rate. Therefore, a method of ‎optimizing the fluid dynamic characteristics and the flow distribution area structure parameters of an axial ‎piston pump considering the cavitation bubble evolution is proposed. A single-cavity dynamic model was ‎established to study the bubble evolution as the piston chamber pressure changes. According to the ‎cavitation cloud (group cavitation) characteristics of the axial piston pump, theoretical models of the ‎outlet flow ripple and the pressure pulsation of a piston pump were established considering the cavitation ‎bubble characteristics. The influence of cavitation characteristics on the outlet flow ripples and pressure ‎pulsation of the axial piston pump was analyzed and compared with that without cavitation. Comparison ‎with the experimental results, verified that the outlet flow ripple model becomes more accurate when ‎cavitation bubble characteristics are considered. Based on the multi-agent particle swarm optimization ‎‎(MAPSO) algorithm, an optimization model of the piston pump outlet flow ripple was established ‎considering the cavitation bubble characteristics. The optimized design parameters for the flow ‎distribution area of the axial piston pump were evaluated. The proposed method can provide theoretical ‎guidance for the design of a low flow ripple axial piston pump.‎

Experimental Investigation of Secondary Flows and Length Reduction for a Low-Pressure Compressor Transition Duct

Abstract The need to reduce fuel-burn and emissions is pushing turbofan engines toward geared architectures with higher bypass ratios and small ultrahigh-pressure ratio cores. However, this increases the radial offset between compressor spools leading to a more challenging design for compressor transition ducts. For the duct connecting the fan to the engine core, this is further complicated by poor-quality flow generated at the fan hub, which is characterized by low total pressure and large rotating secondary flow structures. This paper presents an experimental evaluation of a new rotor designed to produce these larger flow structures and examines their effect on the performance of an engine sector stators (ESS) and compressor transition duct. Aerodynamic data were collected via five-hole probes, for time-averaged pressures and velocities and phase-locked hot-wire anemometry to capture the rotating secondary flows. The data showed that larger structures promoted mixing through the ESS increasing momentum exchange between the core and boundary layer flows. Measurements within the duct showed a continued reduction in the hub boundary layer, suggesting the duct had moved further from separation. Consequently, an aggressive duct with 12.5% length reduction was designed and tested and measurements confirmed the duct remained fully attached. Total pressure loss was slightly increased over the ESS, but this was offset by reduced loss in the duct due to improved flow quality. Overall, this length reduction represents a significant cumulative effect in reduced fuel-burn and emissions over the life of an engine.

Hybrid stars in the light of the merging event GW170817

We study quark-hadron hybrid stars with sharp phase transitions assuming that phase conversions at the interface are slow. Hadronic matter is described by a set of equations of state (EoS) based on the chiral effective field theory and quark matter by a generic bag model. Due to slow conversions at the interface, there is an extended region of stable hybrid stars with central densities above the density of the maximum mass star. We explore systematically the role of the transition pressure and the energy-density jump Δϵ at the interface on some global properties of hybrid stars, such as the maximum mass, the last stable configuration, and tidal deformabilities. We find that for a given transition pressure, the radius of the last stable hybrid star decreases as Δϵ raises resulting in a larger extended branch of stable hybrid stars. Contrary to purely hadronic stars, the tidal deformability Λ can be either a decreasing or an increasing function of the stellar mass M and for large values of the transition pressure has a very weak dependence on M. Finally, we analyze the tidal deformabilities Λ1 and Λ2 for a binary system with the same chirp mass as GW170817. In the scenario where at least one of the stars in the binary is hybrid, we find that models with low enough transition pressure are inside the 90 % credible region of GW170817. However, these models have maximum masses below 2 M⊙, in disagreement with observations. We also find that the LIGO/Virgo constrain (at 90% level) and the 2 M⊙ requirement can be simultaneously fulfilled in a scenario where all hybrid configurations have masses larger than 1.6 M⊙ and the hadronic EoS is not too stiff, such as several of our hybrid models involving a hadronic EoS of intermediate stiffness. In such scenario hybrid stars may exist in Nature but both objects in GW170817 were hadronic stars.

Implications of an improved water equation of state for water-rich planets

ABSTRACT Water (H2O), in all forms, is an important constituent in planetary bodies, controlling habitability and influencing geological activity. Under conditions found in the interior of many planets, as the pressure increases, the H-bonds in water gradually weaken and are replaced by ionic bonds. Recent experimental measurements of the water equation of state (EOS) showed both a new phase of H-bonded water ice, ice-VIIt, and a relatively low transition pressure just above 30 GPa to ionic bonded ice-X, which has a bulk modulus 2.5 times larger. The higher bulk modulus of ice-X produces larger planets for a given mass, thereby either reducing the atmospheric contribution to the volume of many exoplanets or limiting their water content. We investigate the impact of the new EOS measurements on the planetary mass–radius relation and interior structure for water-rich planets. We find that the change in the planet mass–radius relation caused by the systematic differences between previous and new experimental EOS measurements is comparable to the observational uncertainties in some planet sizes – an issue that will become more important as observations continue to improve.

Shape Dependence of Pressure-Induced Phase Transition in CdS Semiconductor Nanocrystals.

Understanding structural stability and phase transformation of nanoparticles under high pressure is of great scientific interest, as it is one of the crucial factors for design, synthesis, and application of materials. Even though high-pressure research on nanomaterials has been widely conducted, their shape-dependent phase transition behavior still remains unclear. Examples of phase transitions of CdS nanoparticles are very limited, despite the fact that it is one of the most studied wide band gap semiconductors. Here we have employed in situ synchrotron wide-angle X-ray scattering and transmission electron microscopy (TEM) to investigate the high-pressure behaviors of CdS nanoparticles as a function of particle shapes. We observed that CdS nanoparticles transform from wurtzite to rocksalt phase at elevated pressure in comparison to their bulk counterpart. Phase transitions also vary with particle shape: rod-shaped particles show a partially reversible phase transition and the onset of the structural phase transition pressure decreases with decreasing surface-to-volume ratios, while spherical particles undergo irreversible phase transition with relatively low phase transition pressure. Additionally, TEM images of spherical particles exhibited sintering-induced morphology change after high-pressure compression. Calculations of the bulk modulus reveal that spheres are more compressible than rods in the wurtzite phase. These results indicate that the shape of the particle plays an important role in determining their high-pressure properties. Our study provides important insights into understanding the phase-structure-property relationship, guiding future design and synthesis of nanoparticles for promising applications.

Pressure-Induced Phase Engineering of Gold Nanostructures.

Although phase engineering of a noble metal, gold (Au), is of critical importance for both fundamental research and potential application, it still remains a big challenge in wet-chemical syntheses. In this work, we report the irreversible transformation from the hexagonal 4H to face-centered cubic ( fcc) phase in Au nanoribbons (NRBs) through high pressure treatment, which has not been discovered in metals. The relative percentage of 4H and fcc phases in the recovered Au NRBs depends directly on the peak pressure applied to the original 4H Au NRBs, enabling a phase engineering of Au nanostructures. Interestingly, compared to the pure 4H Au NRBs, the crystal-phase-heterostructured 4H/ fcc Au nanorods require less energy to complete the phase transition process with a lower transition pressure and in a narrower range. Finally, the atom-based transformation pathway during the 4H-to- fcc phase transition is revealed experimentally, which is supported by the first-principle calculations. This work not only demonstrates the stability of 4H Au nanostructure and the pressure-induced 4H-to- fcc transition mechanism but also provides a strategy for the phase engineering of noble metal nanostructures.

Optimization of Powerful Two-stage Screw Centrifugal Pump

The article presents a refining method for a two-stage screw centrifugal pump by the joint usage of mathematical optimization software IOSO, meshing complex NUMECA and CFD software ANSYS CFX. The pump main parameters: high-pressure stage rotor speed was 13300 rpm; low-pressure rotor speed was 3617 rpm by gearbox; inlet total pressure was 0.4 MPa; outlet mass flow was 132.6 kg/s at the nominal mode. This article describes the process of simplifying the calculation model for the optimization. The parameters of camber lines of the low-pressure impeller, transition duct, and high-pressure impeller blades for two sections (hub and shroud) were chosen as optimization parameters. The blades of low-pressure impeller, transition duct and high-pressure impeller have changed during optimization. The optimization goal was the increase of the pump efficiency with preservation or slight increase in the pressure head. The efficiency was increased by 3%.

Phase transition and equation of state of dense hydrous silica up to 63 GPa

AbstractAlthough it has previously been considered to be essentially anhydrous, Al‐free stishovite can contain up to ∼1.3 wt % of H2O, perhaps through the direct substitution ( ), according to recent studies. Yet the stability of such substitution and its impact on the properties of silica and rutile‐structured hydrous phases (such as δ‐AlOOH and phase H) are unknown at the conditions of the deeper mantle. We have synthesized hydrous and anhydrous Al‐free stishovite samples at 723 K and 9 GPa, and 1473 K and 10 GPa, respectively. Synchrotron X‐ray diffraction patterns show that the unit cell volume of hydrous stishovite is 1.3% greater than that of anhydrous stishovite at 1 bar, suggesting significant incorporation of OH in the crystal structure (3.2 ± 0.5 wt % H2O). At 300 K, we found a lower and broader transition pressure from rutile type to CaCl2 type (28–42 GPa) in hydrous dense silica. We also found that hydrous silica polymorphs are more compressible than their anhydrous counterparts. After the phase transition, the unit cell volume of hydrous silica becomes the same as that of anhydrous silica, showing that the proton incorporation through a direct substitution can be further stabilized at high pressure. The lower pressure transition and the pressure stabilization of the proton incorporation in silica would provide ways to transport and store water in the lower mantle in silica‐rich heterogeneities, such as subducted oceanic crust.

Water hydraulic axial piston motor cavitation characteristics based on CFD technology

PurposeThe purpose of this paper is to analyze the cavitation characteristics of a water hydraulic axial piston motor (WHAPM) to improve the water motor performance, to reduce the vibration and noise and to prolong the service life of the motor.Design/methodology/approachThe computational fluid dynamics (CFD) software PumpLinx is chosen to do cavitation analysis of the WHAPM. In this case, first, cavitation mechanism of the water piston motor is analyzed in depth. Then, considering the effects of bubble dynamics, the rate of phase transition, turbulence effects and non-condensable gas, the full cavitation model is selected, the dynamic CFD numerical model of internal flow field on the water hydraulic piston motor is established based on PumpLinx software and the fluid cavitation inside is numerically studied. Finally, the influence of the valve plate and pistons on motor cavitation is analyzed.FindingsResearch results show that there are two serious cavitation regions: one is the pressure transition region of the valve plate that is near the top dead center, and the other is the low-pressure region of the piston that is near the low-pressure transition area. Moreover, the more serious cavitation area is on the valve plate region.Originality/valueThe simulation results show that the proposed algorithm is able to detect the cavitation characteristics of the water piston motor. Besides, it is deduced that valve-plate structure optimization is more important than pistons to reduce cavitation influence.

Zinc-blende to rocksalt transition in SiC in a laser-heated diamond-anvil cell

We explore the stability of the ambient pressure zinc-blende polymorph (B3) structure of silicon carbide (SiC) at high pressures and temperatures where it transforms to the rocksalt (B1) structure. We find that the transition occurs \ensuremath{\sim}40 GPa lower than previously measured when heated to moderately high temperatures. A lower transition pressure is consistent with the transition pressures predicted in numerous ab initio computations. We find a large volume decrease across the transition of \ensuremath{\sim}17%, with the volume drop increasing at higher formation pressures, suggesting this transition is volume driven yielding a nearly pressure-independent Clapeyron slope. Such a dramatic density increase occurring at pressure is important to consider in applications where SiC is exposed to extreme conditions, such as in industrial applications or planetary interiors.

Pressure-induced phase transition in Bi2Se3 at 3 GPa: electronic topological transition or not?

In recent years, a low pressure transition around GPa exhibited by the -type 3D topological insulators is attributed to an electronic topological transition (ETT) for which there is no direct evidence either from theory or experiments. We address this phase transition and other transitions at higher pressure in bismuth selenide (Bi2Se3) using Raman spectroscopy at pressure up to 26.2 GPa. We see clear Raman signatures of an isostructural phase transition at GPa followed by structural transitions at ∼10 GPa and 16 GPa. First-principles calculations reveal anomalously sharp changes in the structural parameters like the internal angle of the rhombohedral unit cell with a minimum in the c/a ratio near GPa. While our calculations reveal the associated anomalies in vibrational frequencies and electronic bandgap, the calculated invariant and Dirac conical surface electronic structure remain unchanged, showing that there is no change in the electronic topology at the lowest pressure transition.

Abnormal acoustic wave velocities in basaltic and (Fe,Al)‐bearing silicate glasses at high pressures

AbstractWe have measured acoustic VP and VS velocities of (Fe,Al)‐bearing MgSiO3 silicate glasses and an Icelandic basalt glass up to 25 GPa. The velocity profiles of the (Fe,Al)‐bearing and basaltic silicate glasses display decreased VP and VS with minima at approximately 5 and 2 GPa, respectively, which could be explained by the mode softening in the aluminosilicate networks. Our results represent the first observation of such velocity softening extending into the chemically complex basaltic glass at a relatively low transition pressure, which is likely due to its degree of polymerization, while the Fe and Al substitutions reduce sound velocities in MgSiO3 glass. If the velocity softening in the basaltic and silicate glasses can be used as analogs for understanding melts in Earth's interior, these observations suggest that the melt fraction needed to account for the velocity reduction in the upper mantle low‐velocity zone may be smaller than previously thought.

Structural properties of Alumnum nitride compound

Structural properties of Alumnum nitride in wurtzite, zinc-blende and rock-salt phases have been investigated by Full Potential-Linearized Augmented Plane Waves method based on Density Functional Theory within Local Density Approximation and seven Generalized Gradient schemes. It is found that, Alumnum nitride in wurtzite phase is the stable ground state structure and makes a transition to rock-salt phase at a low transition pressure (11.54 GPa). According to present total energy calculations, zinc-blende phase of Alumnum nitride also makes a transition to rock-salt phase, at a low transition pressure (10.17 GPa). Generalized Gradient functionals of Perdew-Wang-Engel-Vosko and Perdew-Burke-Ernzerhof are found to be more successful than other approximations considered in this work for providing the closest values of the structural features, such as, lattice constants, bulk moduli, first order pressure derivatives of bulk moduli and cohesive energies of Alumnum nitride three phases to available experimental ones. Although Generalized Gradient approaches of Perdew-Wang-Engel-Vosko, Perdew-Burke-Ernzerhof, Becke-Perdew-Wang and Perdew-Burke-Ernzerhof (revised) are found to be accurate schemes for elastic constants of rock-salt AlN, only Perdew-Wang-Engel-Vosko and Perdew-Burke-Ernzerhof functionals are observed to be more successful than the other schemes for supplying accurately both \(C_{11}\) and \(C_{12}\) of zinc-blende Alumnum nitride structure. Perdew-Wang-Engel-Vosko functional is observed to be superior to Perdew-Burke-Ernzerhof for elastic constants of wurtzite Alumnum nitride structure. Elastic constants of wurtzite Alumnum nitride obtained by self Perdew-Wang-Engel-Vosko approach and Martin’s transformation calculations in which elastic constants of zinc-blende Alumnum nitride are calculated with Perdew-Wang-Engel-Vosko scheme, are very close to the experimental ones. Hence, functional of Perdew-Wang-Engel-Vosko is decided to be the most accurate approximation among Local Density Approximation and other Generalized Gradient schemes considered in this work for all structural properties of Alumnum nitride three phases.

High pressure transformation of graphene nanoplates: A Raman study

High pressure Raman study on graphene nanoplates, graphite and micro-graphite has been carried out in a diamond anvil cell. A phase transformation has been observed in graphene nanoplates at 15GPa in the experiments with or without pressure medium, which can be explained by the interlayer coupling with sp3 bonds formed in the material. For graphite and micro-graphite, the transition pressure is 19GPa. Different transition pressures for them are attributed to the thickness difference. The lower transition pressure in graphene nanoplates has been discussed in the framework of the special limited-number layer structure and nucleation process in phase transition.

Equation of state of paramagnetic CrN fromab initiomolecular dynamics

Equation of state for chromium nitride has been debated in the literature in connection with a proposed collapse of its bulk modulus following the pressure induced transition from the paramagnetic cubic phase to the antiferromagnetic orthorhombic phase [F. Rivadulla et al., Nat Mater 8, 974 (2009); B. Alling et al., Nat Mater 9, 283 (2010)]. Experimentally the measurements are complicated due to the low transition pressure, while theoretically the simulation of magnetic disorder represent a major challenge. Here a first-principles method is suggested for the calculation of thermodynamic properties of magnetic materials in their high temperature paramagnetic phase. It is based on ab-initio molecular dynamics and simultaneous redistributions of the disordered but finite local magnetic moments. We apply this disordered local moments molecular dynamics method to the case of CrN and simulate its equation of state. In particular the debated bulk modulus is calculated in the paramagnetic cubic phase and is shown to be very similar to that of the antiferromagnetic orthorhombic CrN phase for all considered temperatures.

First-principles investigations of ferroelectricity and piezoelectricity in BaTiO3/PbTiO3superlattices

The pressure dependence of ferroelectricity and piezoelectricity of a tetragonal BaTiO${}_{3}$/PbTiO${}_{3}$ (BPT) short-period superlattice is investigated using first-principles calculations. Our results suggest that, as the applied pressure increases, the BPT superlattice first becomes paraelectric at low pressures and then transfers to another ferroelectric phase at much higher pressures. Furthermore, a large enhancement of piezoelectricity close to the phase-transition regions is predicted, similar to that previously predicted in PbTiO${}_{3}$. Comparing the BPT superlattice with bulk BaTiO${}_{3}$ and PbTiO${}_{3}$, we find that the BPT superlattice behaves very similarly to bulk BaTiO${}_{3}$ under high pressures for the first transition, while it has much lower transition pressure and zero-pressure spontaneous polarization than those for PbTiO${}_{3}$, although BPT has an equal number of BaTiO${}_{3}$ and PbTiO${}_{3}$ layers. However, for the second transition, BPT has a transition pressure close to the average of those for BaTiO${}_{3}$ and PbTiO${}_{3}$, and all three materials have similar pressure-induced polarization. Furthermore, our calculations indicate that the colossal enhancement in piezoelectricity is strongly correlated to phase transition when large atomic displacements can be generated by small external strain, but polarization rotation is not a necessary condition.

Z-BN: a novel superhard boron nitride phase

A superhard boron nitride phase dubbed as Z-BN is proposed as a possible intermediate phase between h-BN and zinc blende BN (c-BN), and investigated using first-principles calculations within the framework of density functional theory. Although the structure of Z-BN is similar to that of bct-BN containing four-eight BN rings, it is more energetically favorable than bct-BN. Our study reveals that Z-BN, with a considerable structural stability and high density comparable to c-BN, is a transparent insulator with an indirect band gap of about 5.27 eV. Amazingly, its Vickers hardness is 55.88 GPa which is comparable to that of c-BN. This new BN phase may be produced in experiments through cold compressing AB stacking h-BN due to its low transition pressure point of 3.3 GPa.

High pressure x-ray diffraction study of the volume collapse in Ba24Si100clathrate

The high pressure stability of the silicon type-III clathrate Ba${}_{24}$Si${}_{100}$ has been studied by x-ray diffraction (XRD) up to a maximum pressure of 37.4 GPa. The high pressure behavior of this Si type-III clathrate appears to be analogous to the structural type-I parent Ba${}_{8}$Si${}_{46}$. An isostructural volume collapse is observed at $~$23 GPa, a value higher than for Ba${}_{8}$Si${}_{46}$ (13--15 GPa). The crystallinity of the structure is preserved up to the maximum attained pressure without amorphization, which appears to be in contradiction with the interpretation given in a Raman spectroscopy study [Shimizu et al., Phys. Rev. B 71, 094108 (2005)]. Nevertheless, the XRD analysis shows the appearance of a type-III disordered nanocaged-based crystalline structure after the volume collapse. Moreover, we find that the volume collapse transformation is (quasi)reversible after pressure release. Additionally, a low pressure transition first evidenced by Raman spectroscopy is also observed in our XRD study at 5 GPa: The variation of the isotropic thermal factors of Ba atoms shows a clear discontinuity at this pressure while the average positions of Ba atoms remain identical.

First principles study of structural, electronic and optical properties of polymorphic forms of Rb 2Te

First-principles density functional theory calculations have been performed for structural, electronic and optical properties of three polymorphic forms of rubidium telluride. Our calculations show that the sequence of pressure induced phase transitions for Rb 2Te is F m 3 ¯ m → Pnma → P6 3/mmc which is governed by the coordination numbers of the anions. From our calculated low transition pressure value for the F m 3 ¯ m phase to the Pnma phase transition of Rb 2Te, the experimentally observed meta-stability of F m 3 ¯ m phase at ambient conditions seems reasonable. The electronic band structure has been calculated for all the three phases and the change in the energy band gap is discussed for the transitioning phases. The energy band gaps obtained for the three phases of Rb 2Te decrease on going from the meta-stable phase to the high-pressure phases. Total and partial density of states for the polymorphs of Rb 2Te has been computed to elucidate the contribution of various atomic states on the electronic band structure. Furthermore, optical properties for all the polymorphic forms have been presented in form of the complex dielectric function.