Phase Transition Boundary Research Articles

Mid-temperature thermoelectric performance of zone-melted Sb2(Te,Se)3 alloys near phase transition boundary

(Bi,Sb)2(Te,Se)3 alloys are widely used commercial thermoelectric (TE) materials for solid-state refrigeration around room temperature. The composition-induced structural phase transition could be realized by varying the compositions in these alloys, which may largely alter the electronic structure and phonon dispersion. Among them, the Se-alloyed Sb2Te3 accompanied with structural transition is seldom reported. Herein, the interrelations of Se-alloying induced changes in structural phase transition, band structure and TE properties of p-type zone-melted Sb2Te3-xSex (x = 1.5–2.4) alloys near phase transition boundary are systematically investigated. The results demonstrate that Sb2Te3-xSex shows a structural transition from a rhombohedral phase to mixed structure at x = 2.0. The carrier concentration and bandgap at room temperature of Sb2Te3-xSex (x = 1.5–2.4) constantly decrease with increasing Se contents x. The zT peak of the Sb2TeSe2 matrix is improved and shifted to higher temperature by optimizing carrier concentration via Ag doping. A maximum zT of ∼0.4 is obtained at 680 K in Sb1.97Ag0.03TeSe2 alloy, about 100% enhancement compared with the undoped sample.

Force-induced desorption of 3-star polymers in two dimensions

We investigate the phase diagram of a self-avoiding walk model of a 3-star polymer in two dimensions, adsorbing at a surface and being desorbed by the action of a force. We show rigorously that there are four phases: a free phase, a ballistic phase, an adsorbed phase and a mixed phase where part of the 3-star is adsorbed and part is ballistic. We use both rigorous arguments and Monte Carlo methods to map out the phase diagram, and investigate the location and nature of the phase transition boundaries. In two dimensions, only two of the arms can be fully adsorbed in the surface and this alters the phase diagram when compared to 3-stars in three dimensions.

Absolute Primary Pressure Scale to 120 GPa: Toward a Pressure Benchmark for Earth's Lower Mantle

AbstractSelf‐consistent absolute primary pressure scale available to whole Earth's lower mantle pressure regime was established by the simultaneous measurement of densities and acoustic wave velocities of NaCl‐B2 phase up to ~120 GPa without any use of prior pressure standards. While our pressure scale shows excellent agreement with the theoretical predictions, some of the previous secondary pressure scales constructed based on the prior pressure standard by the shock wave data significantly deviate from our primary scale reaching around 10 GPa at most within lower mantle pressure regime, which corresponds to ~170‐km difference in depth. Our newly established pressure scale would thus be a new benchmark to deal with the controversy on the depth of the boundaries of Earth's layered structure, such as magnesium‐silicate postperovskite phase transition boundary at the bottom of the Earth's mantle which might represent the D″ seismic discontinuity.

Model of movement of the free boundary of hydride formation in a cylindrical shell of zirconium alloy

One of the most important requirements for the materials (zirconium alloys) used in the reactor active zone is low hydrogen absorptivity, since hydrogen-induced embrittlement may cause zirconium cladding damage. Depending on the hydrogen content and operation temperature, hydrogen may be present in zirconium alloys as a solid solution or as hydrides. Hydrides have the greatest embrittlement effect on alloys as they can initiate and enlarge cracks. The problem is to model the dynamics of the moving boundary of phase transition and to estimate the concentration distribution in the hydride and the solution. This paper presents a mathematical model of zirconium alloy hydrogenation taking into account the phase transition (hydride formation) and the iterative computational algorithm for solving the nonlinear boundary-value problem with free phase boundary based on implicit difference schemes. The study was carried out under state order to the Karelian Research Centre of the Russian Academy of Sciences (Institute of Applied Mathematical Research KarRC RAS).

ИНЖЕКЦИЯ ЖИДКОГО ДИОКСИДА УГЛЕРОДА В ПЛАСТ, НАСЫЩЕННЫЙ МЕТАНОМ И ЕГО ГАЗОГИДРАТОМ

Актуальность исследования связана с разработкой теоретических основ технологий добычи газа из газогидратных месторождений методом замещения. Рассматривается метод инжекции диоксида углерода, позволяющий значительно снизить энергетические затраты на разработку природных газогидратных залежей. Целью исследования является выяснение особенностей протекания процесса замещения метана диоксидом углерода в газогидрате при инжекции жидкого диоксида углерода в газогидратный пласт. Объект: пористый пласт конечной протяженности, насыщенный метаном и его газогидратом, исходные давление и температура которого соответствуют условиям стабильного существования газогидрата метана. Методы. На основе уравнений механики сплошной среды построена математическая модель тепломассопереноса в природном пласте, сопровождающаяся замещением метана на диоксид углерода в газогидрате. Принято, что в рассматриваемом случае в пласте возникают две характерные зоны, разделенные подвижной границей фазовых переходов. В первой (ближней) зоне поры насыщены жидкой двуокисью углерода и ее газогидратом, а во второй (дальней) зоне содержатся метан и его газогидрат. Результаты. Получены численные решения для полей давления и температуры при инжекции жидкого диоксида углерода в газогидратный пласт конечной протяженности. Построены зависимости температуры на границе замещения от давления инжекции и проницаемости пласта. Установлено, что при достаточно низких значениях давления инжекции и проницаемости величина температуры на границе замещения может подниматься выше равновесного значения температуры диссоциации газогидрата метана на газ и воду. Это соответствует возникновению второй подвижной межфазной границы, на которой происходит разложение газогидрата метана. Найдена зависимость предельной температуры закачиваемой жидкой двуокиси углерода, выше которой необходимо учитывать образование смеси метана и воды, от значений давления на правой и левой границах пласта и его проницаемости. Установлено, что режим с разложением газогидрата реализуется при высоких значениях давления на правой границе пласта и низких значениях проницаемости и давления, под которым закачивается двуокись углерода. Получены зависимости скорости границы замещения, а также времени полной замены газогидрата метана на газогидрат диоксида углерода во всем пласте от давления на правой и левой границах пласта, а также от его проницаемости.

Decomposition of Methane Hydrate with Heat Exposure

The paper presents a mathematical model of dissociation of methane hydrate during thermal exposure to a porous layer. The problem of the impact of a heat source (hot fluid) on a porous reservoir of finite length, initially saturated with methane hydrate, is considered. The temperature distributions in the system are constructed at different points in time. The influence of the temperature of the heat source on the nature of the movement of the phase transition boundary is investigated.

High-alumina pyroxenite xenoliths from Quaternary basalts of NW Spitsbergen - evidence of continental crust delamination

The origin of spinel-garnet pyroxenite from xenoliths in Quaternary basaltoids of Spitsbergen Island (Svalbard Archipelago) is discussed. The rocks have a high concentration of Al2O3 and MgO and low Cr. The primary magmatic association Spl-Opx-Cpx and the high Al content in pyroxenes provide evidence for the formation of these rocks as cumulates of hydrous basaltic melts in the lower crust at a pressure of ~1.2 GPa. Transformation of the texture from the magmatic hypidiomorphic to the metamorphic granoblastic and growth of garnet at the expense of spinel and high-alumina pyroxenes indicate transport of rocks to a depth below the spinel/garnet phase transition boundary in the Cr2O3-free CaO-MgO-Al2O3-SiO2 system. The parameters of the Grt-Opx equilibrium range within 1060-1120 °C and 2.2-2.6 GPa. Thus, magmatic pyroxenite was transported to a great depth into the mantle, which provides evidence for delamination in the region of the lower continental crust containing ultramafic cumulates.

Effect of L-Lysine on the Phase Transition Temperature in a Three-Component Water/Sodium Bis(2-ethylhexyl)sulfosuccinate/Isopropyl Myristate System

Novel prospective transport systems are prepared that consist of lyotropic liquid crystals (LLCs), water/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/isopropyl myristate. The temperature boundaries of phase transitions at different water contents and in the presence of guest molecule L-lysine amino acid and the types of mesophases formed are determined via differential scanning calorimetry (DSC), polarization optical microscopy (POM), and measurements of the wetting angle. It is shown that upon a reduction in the H2O content and the introduction of L-lysine, the thermal stability of a system and the temperature of a phase transition increase, while the mass loss upon heating is diminished. The concentration dependences of thermodynamic properties allow us to control the phase transition temperatures that determine the processes of the controlled release of bioactive molecules from the studied LLC-system.

High-Alumina Pyroxenite Xenoliths from Quaternary Basalt of Spitsbergen Island: Evidence for Continental Crust Delamination

The origin of spinel–garnet pyroxenite from xenoliths in Quaternary basaltoids of Spitsbergen Island (Svalbard Archipelago) is discussed. The rocks have a high concentration of Al2O3 and MgO and low Cr. The primary magmatic association Spl–Opx–Cpx and the high Al content in pyroxenes provide evidence for the formation of these rocks as cumulates of hydrous basaltic melts in the lower crust at a pressure of ~1.2 GPa. Transformation of the texture from the magmatic hypidiomorphic to the metamorphic granoblastic and growth of garnet at the expense of spinel and high-alumina pyroxenes indicate transport of rocks to a depth below the spinel/garnet phase transition boundary in the Cr2O3-free CaO–MgO–Al2O3–SiO2 system. The parameters of the Grt–Opx equilibrium range within 1060–1120°C and 2.2–2.6 GPa. Thus, magmatic pyroxenite was transported to a great depth into the mantle, which provides evidence for delamination in the region of the lower continental crust containing ultramafic cumulates.

Автомодельные процессы теплопереноса в прозрачном для излучения твердом теле с поглощающим включением при наличии фазовых превращений в системе

The paper considers the problem of determining temperature field parameters in a radiation-trans-parent isotropic solid body containing an absorptive inclusion, when the system features phase transitions. We identify sufficient conditions, meeting which ensures the possibility of self-similar heat transfer process taking place in the system under con-sideration. We qualitatively investigated physical properties of the self-similar process under study and determined its specifics. We provide a theoretical validation of implementing a thermostating mode of the moving phase transition boundary in the heat transfer process investigated

Generalized effective-potential Landau theory for the two-dimensional extended Bose-Hubbard model

We analytically study the quantum phase diagrams of ultracold dipolar Bose gases in an optical square lattice at zero temperature by using the generalized effective-potential Landau theory (GEPLT). For a weak nearest-neighbor repulsion, our analytical results are better than the third-order strong-coupling expansion theory calculation. In contrast to a previous quantum Monte Carlo (QMC) simulation, we analytically calculate phase transition boundaries up to the third-order hopping, which are in excellent agreement with QMC simulations for second-order phase transition.

Inhomogeneous magnetization of a thin film of a ferromagnetic semiconductor in an electric field

A theoretical model describing the spontaneous magnetization of a ferromagnetic semiconductor (InMn)As film in the presence of an external electric field directed across the film is considered. It is assumed that the ions of a manganese impurity with spin 5/2 are acceptors, have a uniform spatial distribution inside the semiconductor, and do not change their position under the action of an external field. The motion of holes with spin ½ changes their spatial distribution under the action of the field. The exchange interaction between manganese ions and holes allows the formation of magnetization that is non-uniform across the film thickness.In particular, the existence of a piecewise continuous solution describing the presence of a phase transition boundary for magnetization inside a ferromagnetic semiconductor film is shown.

Hole distribution in a film of a ferromagnetic semiconductor in the presence of an external electric field

The results of numerical calculations for the mathematical model proposed for describing the magnetization in a thin film of a ferromagnetic semiconductor at temperatures below the Curie temperature in the presence of an external electric field are presented. The theoretical prediction of the existence of a piecewise continuous solution, which describes the presence of the phase transition boundary for magnetization inside the film, is confirmed. The location of this phase transition boundary depends on the external electric field and temperature.

Strain engineering of electro-optic constants in ferroelectric materials

Electro-optic effects allow control of the ow of light using electric fields, and are of utmost importance for today’s information and communication technologies, such as TV displays and fiber optics. The search for large electro-optic constants in films is essential to the miniaturization and increased efficiency of electro-optic devices. In this work, we demonstrate that strain-engineering in PbTiO3 films allows to selectively choose which electro-optic constant to improve. Unclamped electro-optic constants larger than 100 pm V−1 are predicted, either by driving the softening of an optical phonon mode at a phase transition boundary under tensile strain, or by generating the equivalent of a negative pressure via compressive strain to obtain large piezoelectric constants. In particular, a r13 electro-optic coefficient twice as large as the one of the commonly used LiNbO3 electro-optic material is found here when growing PbTiO3 on the technologicallyimportant Si substrate.

Phase transition boundary between fcc and hcp structures in Fe-Si alloy and its implications for terrestrial planetary cores

Phase transition boundary between fcc and hcp structures in Fe-Si alloy and its implications for terrestrial planetary cores

On the theory of hydrate formation decomposition under thermal influence

A mathematical model of the process of decomposition of gas hydrate during heat exposure is proposed and developed. Based on the proposed technological scheme and the corresponding theoretical model, the problem of the action of a heat source on a porous layer of finite length, initially saturated with methane hydrate, is considered. The task describes the heating and simultaneous extraction of gas into a combined well. According to the adopted scheme, a coolant in the form of hot water is supplied to the annular channel, and the internal well communicates with the formation and gas is produced there, which was formed during the hydrate decomposition as a result of thermal exposure. The influence of the temperature of the heat source on the evolution of thermal fields around the well, on the nature of the motion of the phase transition boundary, is studied, and the law of its motion is obtained. The heat consumption for heating the formation and the evolution of gas output over the considered time interval at various values of the heating temperature and pressure drop are analyzed. The dynamics of the gas mass flow rate and the energy efficiency of methane production at various values of the temperature difference between the reservoir and the fluid injected into the heat pipe are revealed. A quasistationary solution is obtained that corresponds to the case when a pressure is maintained in the well equal to the equilibrium value for the initial temperature of the gas hydrate formation. The dependence of the energy efficiency of the proposed method of gas production on the porosity of the formation is analyzed. It was established that with a twofold increase in the hydrate content of the formation, this value grows by about ten percent. The obtained solutions make it possible to determine the most favorable heat exposure regimes. Moreover, this solution is in good agreement with the numerical results obtained by a more general theoretical model.

Solution of the Problem of the Associated Petroleum Gas Injection into a Porous Medium Saturated with Methane and Ice

A system of nonlinear differential equations is proposed for describing the process of the gas hydrate formation during the injection of methane into a porous reservoir of finite length. This reservoir initially saturated with methane and ice. The case of phase transformations “ice + gas → gas hydrate” occurring in an extended region is considered. It has been established that over time the extended region of hydrate formation degenerates into a frontal surface. It is shown that at low pressures of the injected gas it is possible to stop the phase transition boundary.

Application of the Fokker-Planck Kolmogorov equation to determining the mean value of nucleons generated by the direct ejection process

The statistical description of the process of direct nucleon ejection is the subject of this paper. This description is based on the generalized Fokker-Planck-Kolmogorov equation. The basic proposal is this: deterministic equations and their solutions have the mean values of the stochastic model of the ablation problem. The problem of deformation of the phase transition front is considered. The study is carried out by using the introduced stability position for the dispersion of solutions for mean values. The result of the study is the conclusion that the influence of the Markov diffusion coefficient leads to distortion of the original shape of the boundary phase transition front. The effect of the initial aspiration to resist changing the shape of the phase transition front was found.

Effects of Fe and Al incorporations on the bridgmanite–postperovskite coexistence domain

The postperovskite phase transition of Fe and Al-bearing MgSiO3 bridgmanite, the most aboundant mineral in the Earth's lower mantle, is believed to be a key to understanding seismological observations in the D″ layer, e.g., the discontinuous changes in seismic wave velocities. Experimentally reported phase transition boundaries of Fe and Al-bearing bridgmanite are currently largely controversial and generally suggest wide two-phase coexistence domains. Theoretical simulations ignoring temperature effects cannot evaluate correctly two-phase coexistence domains under high-temperature. We show high-pressure and high-temperature phase transition boundaries for various compositions with geophysically relevant impurities of Fe2+SiO3, Fe3+Fe3+O3, Fe3+Al3+O3, and Al3+Al3+O3 derived from the ab initio finite-temperature free energies calculated combining the internally consistent LSDA+U method and a lattice dynamics approach. We found that at ∼2500K, incorporations accompanied by Fe3+ expand the two-phase coexistence domains distinctly, implying that D″ seismic discontinuities likely arise from the phase transition of Fe2+-bearing bridgmanite.

Segregation-Induced Nanofaceting Transition at an Asymmetric Tilt Grain Boundary in Copper

We show that chemistry can be used to trigger a nanofaceting transition. In particular, the segregation of Ag to an asymmetric tilt grain boundary in Cu is investigated. Aberration-corrected electron microscopy reveals that annealing the bicrystal results in the formation of nanometer-sized facets composed of preferentially Ag-segregated symmetric Σ5{210} segments and Ag-depleted {230}/{100} asymmetric segments. Our observations oppose an anticipated trend to form coarse facets. Atomistic simulations confirm the nanofacet formation observed in the experiment and demonstrate a concurrent grain boundary phase transition induced by the anisotropic segregation of Ag.