Small Pressure Coefficient Research Articles

Modelling transient unloading-triggered dynamic responses in rock mass under a non-hydrostatic geo-stress

Transient excavation unloading of in situ stress affects the stability of surrounding rock in the deep tunnel engineering. This paper focuses on the dynamic responses triggered by the transient unloading of a non-hydrostatic geo-stress field in deep-buried engineering. With resort to the integral transformation, the frequency-domain responses of stress, displacement and velocity components induced by the transient unloading are obtained under a non-hydrostatic in situ stress. Based on the numerical inversion of the Laplace transformation, the corresponding time-domain theoretical results are determined. Agreement of the current solutions with the existing results and the numerical simulations verifies the proposed scheme in this paper. The elastic analytical results indicate that the influence of unloading path on the stress redistribution is characterized by the unloading rate. The higher the unloading rate is, the larger the stress magnitude is. The smaller the unloading time is, the more remarkable vibration is. The elasto-plastic dynamic numerical responses are investigated on the basis of a self-developed code, elasto-plastic cellular automaton (EPCA), which is a module of CASRock. The numerical results demonstrate that the transient excavation unloading results in the stress redistribution and concentration of surrounding rock mass, inducing damage for the case of exceeding the capacity of rock mass. For the small lateral pressure coefficient (less than 0.25), the tensile-shear failure is the major damage mechanism of surrounding rock, while the shear failure is the major damage mechanism for the large lateral pressure coefficient. The analytical and numerical results can provide theoretical basis for the support and reinforcement of underground tunnel.

Layered Hydride LiH4 with a Pressure-Insensitive Superconductivity.

For hydride superconductors, each significant advance is built upon the discovery of novel H-based structural units, which in turn push the understanding of the superconducting mechanism to new heights. Based on first-principles calculations, we propose a metastable LiH4 with a wavy H layer composed of the edge-sharing pea-like H18 rings at high pressures. Unexpectedly, it exhibits pressure-insensitive superconductivity manifested by an extremely small pressure coefficient (dTc/dP) of 0.04 K/GPa. This feature is attributed to the slightly weakened electron-phonon coupling with pressure, caused by the reduced charge transfer from Li atoms to wavy H layers, significantly suppressing the substantial increase in the contribution of phonons to Tc. Its superconductivity originates from the strong coupling between the H 1s electrons and the high-frequency phonons associated with the H layer. Our study extends the list of H-based structural units and enhances the in-depth understanding of pressure-related superconductivity.

Research of quality indicators of gears with a complex non-evolvent profile of the teeth flanks

The article substantiates the relevance of the study of gears with a complex non-involute profile of the side surfaces of the teeth, which in some applications have advantages over involute gears and are devoid of some of their drawbacks associated with quality indicators.A technique for obtaining mating surfaces of the teeth of non-invasive gears as envelopes of the specified surfaces of the teeth of tools is described. A scheme for forming pairs of non-involute gears, from which a gearing can be composed, is proposed. It is shown that to obtain the mating surfaces of the teeth of two non-involute gears, two tool rails can be used with the profiles of the side surfaces of the teeth opposite to each other. As a nonlinear profile of the tooth lateral surface of the tool rail, some part of one of the simulated flat kinematic curves is considered. A description of the program is given, which allows you to calculate the geometric characteristics of the shaped profiles of the gear pair wheels, visualize the shaping process, and also determine the quality indicators of the gearing. Thus, the prerequisites were created for choosing from the resulting geometric modeling of the curve field of such tooth profiles of the tools, which would provide the most rational combination of the tooth profiles of the gears processed by them and the required quality parameters of the gear teeth. The results of the study of the pressure ratio between the teeth of a gear and the overlap ratio of gears when choosing the shape of the tooth profiles are presented. A series of numerical experiments for gearing, formed by pairs of tool rails with different profiles of the side surfaces of the teeth - straight, convex and concave, as well as convex-concave were performed. It is shown that non-involute gearing can have large reduced radii of curvature (and consequently smaller pressure coefficients) at the points of tangency of the profiles compared to involute gearing with a slight increase or decrease in the gearing overlap ratio. The most preferable is the variant of the rails with convex and concave tooth profiles, which provides the best values of both quality indicators of the engagement.

Finite-time blowup of smooth solutions for the relativistic generalized Chaplygin Euler equations

In this paper, the Cauchy problem of the 3+1-dimensional relativistic Euler equations for generalized Chaplygin gas with non-vacuum initial data is considered. It is shown that for large background energy-mass density and small pressure coefficient, the smooth solutions of the relativistic Euler equations for generalized Chaplygin gas with the generalized subluminal condition will blow up on finite time when the initial radial component of the generalized momentum is sufficiently large. Moreover, our blowup condition is independent of the signs of the generalized mass.

ANALYSIS OF QUALITATY INDICATORS OF NON-EVOLVENT GEARING

The article substantiates the relevance of the study of gears with a complex non-involute profile of the side surfaces of the teeth, which in some applications have advantages over involute gears and are devoid of some of their drawbacks associated with quality indicators. A technique for obtaining mating surfaces of the teeth of non-invasive gears as envelopes of the specified surfaces of the teeth of tools is described. A scheme for forming pairs of non-involute gears, from which a gearing can be composed, is proposed. It is shown that to obtain the mating surfaces of the teeth of two non-involute gears, two tool rails can be used with the profiles of the side surfaces of the teeth opposite to each other. As a nonlinear profile of the tooth lateral surface of the tool rail, some part of one of the simulated flat kinematic curves is considered. A description of the program developed in accordance with the described method is given, which allows you to calculate the geometric characteristics of the shaped profiles of the gear pair wheels, visualize the shaping process, and also determine the quality indicators of the gearing. Thus, the prerequisites were created for choosing from the resulting geometric modeling of the curve field of such tooth profiles of the tools, which would provide the most rational combination of the tooth profiles of the gears processed by them and the required quality parameters of the gear teeth. The results of the study of the pressure ratio between the teeth of a gear and the overlap ratio of gears when choosing the shape of the tooth profiles are presented. A series of numerical experiments for gearing, formed by pairs of tool rails with different profiles of the side surfaces of the teeth - straight, convex and concave, as well as convex-concave were performed. It is shown that non-involute gearing can have large reduced radii of curvature (and consequently smaller pressure coefficients) at the points of tangency of the profiles compared to involute gearing with a slight increase or decrease in the gearing overlap ratio. The most preferable is the variant of the rails with convex and concave tooth profiles, which provides the best values of both quality indicators of the engagement.

Pressure-dependent Raman spectra of Ba5(PO4)3Cl alforsite

The pressure-dependent Raman spectra of synthetic alforsite, Ba5(PO4)3Cl, were investigated up to 34.9 GPa using a DAC at room temperature. During compression to greater than 20 GPa, new Raman active peaks of Ba5(PO4)3Cl were observed. The Raman frequencies of all observed bands for Ba5(PO4)3Cl alforsite increase continuously with increasing pressure. The quantitative analysis of PO4 internal vibrational pressure dependences for different Raman bands in alforsite shows that the ν3 anti-symmetric stretching modes have larger pressure coefficients (from 4.24 to 5.46 cm−1/GPa) whereas, the ν4 anti-symmetric bending vibrations have smaller pressure coefficients (from 1.16 to 2.04 cm−1/GPa). The external modes show larger pressure coefficients (from 4.71 to 5.54 cm−1/GPa). The PO4 internal modes in Ba5(PO4)3Cl alforsite give isothermal mode Gruneisen parameters varying from 0.147 to 0.488, which yields an average PO4 internal mode Gruneisen parameter of 0.314. On the other hand, the external modes give isothermal mode Gruneisen parameters from 1.583 to 2.030. The external modes mainly contribute to the bulk Gruneisen parameter since the bulk thermochemical Gruneisen parameter was determined as 1.44.

Zinc-blende and wurtzite GaAs quantum dots in nanowires studied using hydrostatic pressure

We report both zinc-blende (ZB) and wurtzite (WZ) crystal phase self-assembled GaAs quantum dots (QDs) embedded in single GaAs/AlGaAs core-shell nanowires. Optical transitions and single-photon characteristics of both kinds of QDs have been investigated by measuring photoluminescence (PL) and time-resolved PL spectra upon application of hydrostatic pressure. We find that the ZB QDs are of direct band-gap transition with short recombination lifetime (\ensuremath{\sim}1 ns) and higher pressure coefficient (75--100 meV/GPa). On the contrary, the WZ QDs undergo a direct-to-pseudodirect band-gap transition as a result of quantum confinement effect, with remarkably longer exciton lifetime (4.5--74.5 ns) and smaller pressure coefficient (28--53 meV/GPa). These fundamentally physical properties are further examined by performing state-of-the-art atomistic pseudopotential calculations.

Equation of state and electronic properties of EuVO4: A high-pressure experimental and computational study

Structural, elastic and electronic properties of zircon-type and scheelite-type EuVO4 are investigated experimentally, by in-situ X-ray diffraction using synchrotron radiation, and theoretically within the framework of the density functional theory (DFT) and using the PBE prescription of the exchange-correlation energy. This study was motivated by the fact that the previous knowledge of the equation of state (EOS) was inconclusive due to a large scatter of the experimental and theoretical data, and by the lack of information on the dependence of the electronic structure with pressure.Under the applied experimental conditions, the zircon-type structure transforms to a scheelite-type one at 7.4(2) GPa, whereas the calculations yield a lower zircon–scheelite-coexistence pressure of 4.8 GPa. The experimental part of the study shows that the bulk modulus of the zircon-type phase is 119(3) GPa, perfectly supported by the DFT-calculated value, 119.1 GPa. The bulk modulus for the scheelite-type polymorph is higher, with an experimental value of 135(7) GPa and a theoretical one of 137.4 GPa. Compared to those reported in previous experimental and DFT or semiempirical works, the present values for the zircon-type phase are comparable or slightly lower, whereas those for the scheelite-type phase are markedly lower. Discrepancies between the present results and earlier reported ones are attributed to differences in details of the experimental method such as the pressure transmitting medium and the pressure calibration method.The calculated band structure confirms that zircon-type EuVO4 is a direct-gap semiconductor, with a bandgap energy at zero pressure of 2.88 eV. Under compression, the bandgap of the zircon phase increases with a coefficient of 10.3 meV/GPa up to the transition pressure, at which point the present calculations show a small drop of the bandgap energy. Above the transition pressure, the bandgap energy of the scheelite phase becomes almost constant, with a small pressure coefficient of just 1.5 meV/GPa.

Dilatometric Studies on Single Crystalline Barlowite – A Structurally Perfect Spin-1/2 Kagome System

We present results of high-resolution thermal expansion measurements on single crystalline barlowite – a structurally perfect spin-1/2 kagome system. The data reveal strongly pronounced and anisotropic second-order phase transition anomalies at the Néel transition at TN = 16K. From these data, together with literature results on the specific heat, the uniaxial-pressure dependences of TN are derived. We find a rather large positive pressure coefficient for uniaxial pressure along the hexagonal c axis of ∂TN/∂pc = (2.3 ± 0.2) K/GPa and smaller negative in-plane pressure coefficient of ∂TN/∂pin-plane = -(0.6 ± 0.03) K/GPa. These effects result in a small positive pressure coefficient under hydrostatic-pressure conditions of ∂TN/∂phydr = (1.1 ± 0.2) K/GPa. Bond-lengths considerations indicate that inter-layer Cu-O bonds, being larger than those typically found in stable Cu-O complexes, are responsible for this behavior.

Pressure-dependent Raman spectra of β-Ca3(PO4)2 whitlockite

The pressure dependence of Raman spectra for whitlockite β-Ca3(PO4)2 was investigated up to 18.0 GPa using a diamond-anvil cell at room temperature. The Raman frequencies of all observed bands for β-Ca3(PO4)2 continuously increase with increasing pressure. The quantitative analysis of pressure dependence of Raman bands for the sample shows that the ν 3 asymmetric and ν 1 symmetric stretching vibrations are with the larger pressure coefficients (from 3.44 to 4.59 cm−1 GPa−1) and that the ν 4 bending and ν 2 deforming vibrations are with the smaller pressure coefficients (from 1.46 to 3.12 cm−1 GPa−1). Combined with previous result, the isothermal mode Gruneisen parameters of β-Ca3(PO4)2 were calculated. The splitting of the PO4 symmetric stretching ν 1 vibrations changes during compression and disappears around 15.4 GPa, which may be attributed to the evolution of PO4 tetrahedra under high pressure.

Effects of internal strain and external pressure on electronic structures and optical transitions of self-assembled InxGa1−xAs/GaAs quantum dots: An experimental and theoretical study

The optical emissive transitions from the ground and excited states of the self-assembled InxGa1−xAs/GaAs quantum dots (QDs) at room temperature were experimentally measured as a function of the external hydrostatic pressure by means of the confocal micro-photoluminescence technique. The ground state transition is very weak under zero external pressure and the photoluminescence is dominant by the excited state transition. However, the intensity of the ground state transition monotonically increases with increasing the external pressure and eventually become the dominant transition. Their pressure coefficients (PCs) were determined to be 6.8 and 7.1 meV/kbar, respectively, which were astonishingly smaller than those of GaAs bulk and the InGaAs/GaAs reference quantum well. The emission peak from the higher order excited states had a much smaller PC (∼0.5 meV/kbar). The influence of the built-in strain and external hydrostatic pressure on the electronic structures and optical transitions of various InxGa1−xAs/GaAs QDs was theoretically investigated by using the eight-band k·p method. Good agreement between the theoretical and experimental results was achieved, firmly revealing that the internal built-in strain in the dot system is mainly responsible for the experimental findings.

Dimensional analysis of two-phase flow including a rate-dependent capillary pressure–saturation relationship

The macroscopic modelling of two-phase flow processes in subsurface hydrosystems or industrial applications on the Darcy scale usually requires a constitutive relationship between capillary pressure and saturation, the P c( S w) relationship. Traditionally, it is assumed that a unique relation between P c and S w exists independently of the flow conditions as long as hysteretic effects can be neglected. Recently, this assumption has been questioned and alternative formulations have been suggested. For example, the extended P c( S w) relationship by Hassanizadeh and Gray [Hassanizadeh SM, Gray WG. Mechanics and thermodynamics of multiphase flow in porous media including interphase boundaries. Adv Water Resources 1990;13(4):169–86] proposes that the difference between the phase pressures to the equilibrium capillary pressure is a linear function of the rate of change of saturation, thereby introducing a constant of proportionality, the coefficient τ. It is desirable to identify cases where the extended relationship needs to be considered. Consequently, a dimensional analysis is performed on the basis of the two-phase balance equations. In addition to the well-known capillary and gravitational number, the dimensional analysis yields a new dimensionless number. The dynamic number Dy quantifies the ratio of dynamic capillary to viscous forces. Relating the dynamic to the capillary as well as the gravitational number gives the new numbers DyC and DyG, respectively. For given sets of fluid and porous medium parameters, the dimensionless numbers Dy and DyC are interpreted as functions of the characteristic length and flow velocity. The simulation of an imbibition process provides insight into the interpretation of the characteristic length scale. The most promising choice for this length scale seems to be the front width. We conclude that consideration of the extended P c( S w) relationship may be important for porous media with high permeability, small entry pressure and high coefficient τ when systems with a small characteristic length (e.g. steep front) and small characteristic time scale are under investigation.

Pressure dependence of the optical properties of wurtzite and rock‐salt Zn1–xCoxO thin films

AbstractIn this paper we investigate the electronic structure of Zn1–x Cox O by means of optical absorption measurements under pressure. Thin films of Zn1–x Cox O with different Co content (from 1 to 30%) were prepared by pulsed laser deposition on mica substrates. Absorption spectra exhibit three main features that are clearly correlated to the Co content in the films: (i) absorption peaks in the infrared associated to crystal‐field‐split internal transitions in the Co 3d shell, with very small pressure coefficients due to their atomic character; (ii) a broad absorption band below the fundamental edge associated to charge transfer transitions, that exhibit relatively large pressure coefficients, indicating that the Co 3d final states must be strongly hybridized to the conduction band; and (iii) a blue‐shifted fundamental absorption edge associated to band to band transitions with a pressure coefficient close to that of pure ZnO. In the up‐stroke the transition pressure from wurtzite to rock‐salt phase decrease almost linearly as the Co increases, from 9.5 GPa in pure ZnO to about 6.5 GPa for x = 30%. In the down‐stroke pressure we observe a similar behavior, yielding a metastable rock‐salt phase at room pressure, after a pressure cycle up to 15 GPa. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High Interband Transitions in β-FeSi2 under Pressure

The effect of pressure on high interband transitions up to 3 eV in β-FeSi2 is studied by measuring reflectivity spectra up to 16 GPa. The evaluated linear pressure coefficients for the interband transitions are all positive and about 8 meV/GPa for the transitions below 1.6 eV and about 16 meV/GPa for those beyond 1.6 eV. The previously determined pressure coefficient of the direct absorption edge (0.875 eV) is 15.9 meV/GPa. This characteristic indicates that the density of states near the energy gap consists of two different bands, with large- and small-pressure coefficients.

Hydrostatic pressure coefficients of the photoluminescence of InAs/GaAs quantum dots

There is a significant diminution in the hydrostatic pressure coefficients (PCs) of the photoluminescence spectrum for InAs/GaAs quantum dots in comparison with that of bulk binary. We study this phenomenon with the nonlinear elasticity theory. The variation of the lattice and elastic constants plays an important role in the change of PCs, which causes the obvious decrease of the built-in strains of InAs/GaAs quantum dots under the hydrostatic pressure. The decrease will induce the novel change of the energy gap and the electronic confined energy. Finally, the change is shown in the measured small PCs of photoluminescence from quantum dots. Also the calculation reveals that the PCs are sensitive to the sizes of quantum dots because of the change of the electronic confined energy. The smaller the quantum dot is, the stronger the influence of this change on the PCs becomes. This effect gives rise to the increase of PCs when dot size is reduced.

Hydrostatic pressure coefficients of the photoluminescence of InAs/GaAs quantum dots

There is a significant diminution in the hydrostatic pressure coefficients (PCs) of the photoluminescence (PL) for InAs/GaAs quantum dots (QDs) in comparison wi th that of bulk binary. We study this phenomenon with the nonlinear elasticity t heory. The variation of the lattice and elastic constants plays an important rol e in the change of PCs, which causes the obvious decrease of the built-in strain s in InAs/GaAs QD. Therefore, the energy gap and the electronic confined energy change with pressure subsequently. It is the main reason for the measured small PCs of PL from QDs. Also the calculation reveals that the PCs are sensitive to t he sizes of QD. The smaller the quantum dot is, the more greatly the change of t he electronic confined energy affects the PCs. This effect gives rise to the inc rease of PCs when dot size is reduced.

Effects of hydrostatic pressure on optical properties of InN and In‐rich group III‐nitride alloys

AbstractWe have used the diamond anvil cell technique to measure the hydrostatic pressure dependence of the optical absorption edge and photoluminescence peak energy of MBE grown InN, In‐rich In1–xGaxN (0 < x < 0.5) and In1–xAlxN (x = 0.25) alloys. Together with previous reports, our results show that the pressure coefficient of the absorption edge varies monotonically from about 3.0 ± 0.1 meV/kbar for InN to 4.2 meV/kbar in GaN and to 4.9 meV/kbar in AlN. The photoluminescence measurements yield significantly smaller pressure coefficients than the coefficients of the bandgap, which is attributed to emission associated with recombination via highly localized defect states. Samples which give small photoluminescence pressure coefficient usually have large Stokes shift. Based on the results of the absorption and photoluminescence measurements we are able to determine the absolute deformation potentials of the conduction and valence band edges of these alloys. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reflection and absorption spectra of β-FeSi 2 under pressure

High-pressure optical reflection and absorption study of β-FeSi 2 were carried out up to 5.0 and 4.8 GPa, respectively at room temperature. By the absorption experiment, the pressure coefficient of the direct band gap was determined to be 15.9 meV/GPa. On the other hand, the pressure dependence of dielectric functions were obtained by using Kramers–Kronig relations for the reflection spectra and the pressure coefficients of three different higher energy gap were tentatively evaluated to be 8.73, 8.63 and 16.7 meV/GPa, respectively. These pressure coefficients are quite smaller than those of II–VI, III–V and group IV common semiconductors. We propose that the small pressure coefficient is caused by not only the larger bulk modulus compared with common semiconductors but also the shift of the valence band maximum of β-FeSi 2 to higher energy with increasing pressure.

Photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots under pressure

The photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots was investigated at 15 K under hydrostatic pressure up to 9 GPa. Photoemission from both the ground and the first excited states in large InAs dots was observed. The pressure coefficients of the two emissions were 69 and 72 meV/GPa, respectively. A nonlinear elasticity theory was used to interpret the significantly small pressure coefficients of the large dots. The sequential quenching of the ground and the excited state emissions with increasing pressure suggests that the excited state emissions originate from the optical transitions between the first excited electron states and the first excited hole states.

Pressure Dependence of Optical Transitions in In-rich Group III-Nitride Alloys

ABSTRACTThe hydrostatic pressure dependence of the optical transitions in InN, In-rich In1-xGaxN (0 < x < 0.5) and In1-xAlxN (x = 0.25) alloys is studied using diamond anvil cells. The absorption edges and the photoluminescence peaks shift to higher energy with pressure. The pressure coefficient of InN is determined to be 3.0±0.1 meV/kbar. Together with previous experimental results, our data suggest that the pressure coefficients of group-III nitride alloys have only a weak dependence on the alloy composition. Photoluminescence gives much smaller pressure coefficients, which is attributed to emission involving highly localized states. This indicates that photoluminescence might not be an accurate method to study the pressure dependence of the fundamental bandgaps of group III-nitrides.