Pseudopotential Method Research Articles

Minimum superheat imposed by equations of state in modelling the phase transition

The boiling phenomenon has been studied extensively by using the pseudopotential lattice Boltzmann method (LBM) based on an equation of state (EOS) due to the algorithm simplicity. However, all reported studies have shown that a large superheat is either used to simulate the boiling or observed if a fixed heat flux is used. In the current study, insights into the large superheat required to initiate the phase transition in numerical simulations based on an EOS are obtained by theoretical analyses of the heating process, which show that density decrease is coupled with temperature increase through the adopted EOS at an almost constant pressure. Significant temperature increase from the saturation temperature is required to make the coupled density decrease large enough before having phase transition. The theoretically predicted minimum superheat is validated by the LBM simulation based on the Peng-Robinson EOS. A solution to reduce the minimum superheat is provided and verified by using a piecewise linear EOS in both theoretical analysis and LBM simulations. The conclusions are also valid for other simulation methods based on different EOS.

Ab-initio study of electronic, mechanical and thermodynamic properties of β-Ti–15Nb–xSi alloys for biomaterials applications

In this paper, we used the first-principles method to investigate the structural, electronic, mechanical and thermodynamic parameters of the ternary [Formula: see text]-Ti–15Nb–xSi alloys with [Formula: see text][Formula: see text]wt.%. We have carried out theoretical computations inside the density functional theory (DFT) utilizing the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) model. The random distribution of Nb atoms in the alloy was described by using both virtual crystal approximation (VCA), special quasirandom structure (SQS) and the coherent potential approximation (CPA) techniques, in combination with first-principles plane-wave pseudopotential (PW-PP) and exact muffin-tin orbital (EMTO) methods. We determined the elastic constants as well as the bulk, shear, Young’s modulus and Poisson’s ratio. Our structural results are in good agreement with the available experimental and theoretical results for the pure structure of the titanium. In addition, we have estimated the band structure and the density of state (DOS) for the electronic computations. Our findings demonstrate that all of the compounds are metallic, stable and meet the requirements for stability. Young’s modulus of Ti–15Nb–0.6Si and Ti–15Nb–1.6Si is 86.5[Formula: see text]GPa and 15.11[Formula: see text]GPa, respectively, which are similar to Young’s moduli of human bone (10–30[Formula: see text]GPa). All calculated parameters of the alloys decreased with increasing of Si concentration except for Poisson’s ratio, anisotropy and B/G ratio. Furthermore, all of the materials investigated showed ductile nature, and Young’s modulus values are needed for further applications. Excitations from the quasi-harmonic Debye approximation’s vibrational part were applied to the 0[Formula: see text]K free energy calculated via ab-initio calculations. The influence of temperatures up to 800 K on phase stability was investigated. These findings can be utilized to help designers create alternative low-modulus alloys for biomedical applications.

Lattice Boltzmann modeling of the coupled imbibition-flowback behavior in a 3D shale pore structure under reservoir condition

Imbibition and flowback of fracturing fluid usually occur in the shale matrix after hydraulic fracturing, which significantly impacts shale gas production and environmental protection. The rocks of deep shale gas reservoirs are under high-temperature and high-temperature conditions. There are rich micro-nano pores with various pore structures in deep shale. In addition, the flowback behavior is significantly affected by the imbibition behavior because the flowback begins after the end of the imbibition. Therefore, an accurate pore-scale description of the coupled imbibition-flowback behavior is crucial to understand the flowback mechanism and its impacts. In this paper, a pseudo-potential lattice Boltzmann method is employed to simulate the coupled imbibition-flowback behavior in a digital shale core, where the digital core is reconstructed by Markov Chain-Monte Carlo method based on scanning microscope images of deep shale cores. The microcosmic mechanism of the imbibition and flowback is studied under deep shale conditions. The influence of some factors, such as pore structure, fluid viscosity, wettability, and flowback pressure difference, on the flowback behavior of fracturing fluid is investigated. It is found that the fracturing fluid advances almost uniformly throughout the pore space during the imbibition process. The fracturing fluid is easy to adsorb on the pore wall, and the shale gas is located in the middle of the pore space. The viscous fingering is clearly observed during the flowback process, where shale gas flows through large pores to form a flow channel, and the fracturing fluid stays in tiny pores. The flowback rate increases gradually with the flowback time and eventually tends to be almost constant. The wettability, flowback pressure difference, and pore structure significantly influence the flowback behavior, while the fracturing fluid viscosity has a smaller effect on the flowback process.

Improved boundary conditions for lattice Boltzmann modeling of pool boiling at low temperature

The pseudopotential lattice Boltzmann method (LBM) becomes popular in simulating the boiling heat transfer problems. By using the interaction force determined from an equation of state, the boiling phenomenon can occur spontaneously according to the thermodynamics, which avoids using empirical models for boiling. However, this pseudopotential interaction model also brings additional cause of instability, which becomes severe at high density ratios. Therefore, most of its previous boiling simulations are conducted at high saturated temperatures with low density ratios. Since the instability usually stems from the disturbance at the interfaces, we investigated the scheme of computing the solid pseudopotential for the solid–fluid interaction and proposed using the average surrounding fluid properties as the virtual solid temperature in addition to density. Droplet evaporation and film boiling problems at high saturated temperatures are simulated, and very good agreement is obtained when compared with the available numerical and analytical solutions, respectively. We then applied the pseudopotential LBM model to simulate droplet evaporation and pool boiling at a low reduced temperature of Tr=0.592 with a density ratio of 1000, as demonstrations of the improved numerical stability. Different boiling regimes are observed by varying the superheat imposed at the bottom wall.

Study of low-level pool boiling in vapor chamber of wickless micro-heat pipe based on Lattice Boltzmann pseudopotential method—Effects of superheat and wall wettability

The lattice Boltzmann method, a mesoscopic numerical method with a solid kinetic basis, is used to establish a low-level pool boiling model to exhibit the transport phenomena inside an evaporation chamber of a wickless micro heat pipe using DI water as the working fluid. The effects of wall superheat and wall wettability on bubbles’ nucleation, growth, and detachment in the evaporation chamber were compared and analysed. Moreover, the influence of the wall superheat on the thermal performance of the micro heat pipe was specifically examined. It is found that the low-level pool boiling occurs with a wall superheat that is higher than 0.357T c, indicating a high heat flux for heat removal. Interestingly, when the relatively hydrophobic chamber wall is superheated by 0.46T c, there are more nucleation sites, and the complete bubbles cycle containing nucleation, growth, and detachment is longer. In contrast, on a hydrophilic surface under the same and sufficient superheat condition, the number of bubble nucleation sites is less than that on the hydrophobic surface, and the complete cycle of bubbles is shorter. It is worth mentioning that the model has values for predictions of homogeneous nucleation of bubbles and thermal transport on hydrophobic or hydrophilic surfaces.

Band Structure Calculation and Optical Properties of Ag3AsS3 Crystals

In the study, band structure calculation in the points of high symmetry of the first Brillouin zone and alongside the lines that connect them has been derived by using CASTEP programs in which the pseudopotential method with the basis in the form of plane-waves is realized. The calculated value of the lattice parameters using GGA functional is well correlated with experimental data. According to the band diagram that was built for the Ag3AsS3 crystal using GGA method, band gap has an indirect type. The calculated value of the band gap is = 1.22 еV. The experimental value of the band gap obtained by Tauc’s method is = 2.01 еV, = 2.17 еV. Full and partial density of N(E) states for contributions of separate atoms has been calculated. As a result, the top of the valence band is formed by 3p-states of S atoms and the bottom of the conduction band is formed by 5s-states of Ag atoms and 3p-states of S atoms.

Electronic and magnetic properties of CoSb3, Cr-doped CoSb3, and related compound thin films

The electronic, lattice, and magnetic properties of CoSb3, Cr-doped CoSb3, and related compound thin films were calculated by using the total energy pseudopotential method. A vacuum region and slab as a thin film were periodically repeated in the supercell. We considered three slab structures. One is symmetric, another is asymmetric, and the third is unusual. They are free-standing in the supercell. The magnetic moment of doped Cr atom is large in all Cr-doped cases. Magnetic moments of several Co atoms are enhanced in the asymmetric and unusual cases. The calculation results in this study indicate that electronic states of most thin films are ferromagnetic and they are energetically more favorable than nonmagnetic cases.

High-Temperature Thermodynamics of Uranium from Ab Initio Modeling

We present high-temperature thermodynamic properties for uranium in its γ phase (γ-U) from first-principles, relativistic, and anharmonic theory. The results are compared to CALPHAD modeling. The ab initio electronic structure is obtained from density-functional theory (DFT) that includes spin–orbit coupling and an added self-consistent orbital-polarization (OP) mechanism for more accurate treatment of magnetism. The first-principles method is coupled to a lattice dynamics scheme that is used to model anharmonic lattice vibrations, namely, Self-Consistent Ab Initio Lattice Dynamics (SCAILD). The methodology can be summarized in the acronym DFT + OP + SCAILD. Upon thermal expansion, γ-U develops non-negligible magnetic moments that are included for the first time in thermodynamic theory. The all-electron DFT approach is shown to model γ-U better than the commonly used pseudopotential method. In addition to CALPHAD, DFT + OP + SCAILD thermodynamic properties are compared with other ab initio and semiempirical modeling and experiments. Our first-principles approach produces Gibbs free energy that is essentially identical to CALPHAD. The DFT + OP + SCAILD heat capacity is close to CALPHAD and most experimental data and is predicted to have a significant thermal dependence due to the electronic contribution.

Ab-initio and Monte Carlo studies of the structural, electronic and magnetic properties of V-doped ZnO compound

Realizing room-temperature ferromagnetic interactions in diluted magnetic semiconductors is a precondition for the future application of next-generation spin-based information technologies. However, finding effective strategies to alter the inherent magnetic coupling remains difficult. This paper aims to propose an approach for manipulating the ferromagnetism of (0.12%, 0.18%, 0.25%) V-doped ZnO (V-ZnO). The influence of V doping on the structural, electronic, and magnetic properties of ZnO is studied using first principles pseudo-potential plane wave method within the density functional theory (DFT). According to the first-principles calculations, the ferromagnetic ground state is maintained by a half-metallic electronic structure resulting from substantial hybridization between V-3d and O-2p electrons, and the calculations suggest that the ferromagnetism could be governed by the interactions between cluster spines in the VO4 tetrahedra. With the magnetic coupling strengths obtained by the green’s function method with the Wannier functions as a localized basis, the Monte Carlo simulation based on the Classical Ising model predicts the ferromagnetism of Zn1−xVxO (x = 0.12%, 0.18%, 0.25%) with Tc = 55 K, 165 K and 345 K. The observed results show that, as a diluted magnetic semiconductor, V-doped ZnO is an excellent candidate.

Ab-initio study of the optical properties of 2D Yn+1Cn (n = 1, 2, and 3) MXenes and bulk of YC

Calculations are accomplished in the framework of density functional theory and pseudopotential method with local density approximation (LDA) and QUANTUM-ESPRESSO/PWSCF computational code. Large negative values of ε1ω represent that materials exhibit Drude-like behavior while at higher energies, interband transitions occur. According to the results of the imaginary part of the dielectric functionε2ω, it can be stated that the absorption starts from small energies, which indicates that the bulk of YC and Yn+1Cn (n = 1, 2, and 3) monolayers have no energy bandgap and show a metallic nature. The sharp plasmonic energy peak for the YC structure in the x-direction can be seen at 18.61 eV; also the values of sharp plasmonic energy peaks for the Y2C, Y3C2, and Y4C3 in the x-direction are 13.40 eV, 11.98 eV, and 10.14 eV respectively. The results of the reflection spectrum show that the bulk of YC and 2D Yn+1Cn (n = 1, 2, and 3) MXenes have large values due to the presence of free electrons and plasma fluctuations, and confirm the metallic nature of the mentioned compounds. For the bulk of YC, the refractive index prominent peaks approach zero within the photon energy range of 13 eV up to 16 eV, while the corresponding range for the 2D Yn+1Cn MXenes is narrower and equal to 6 eV up to 7 eV. Also, the absorption spectrum in the bulk structure of YC and Yn+1Cn MXenes starts from zero photon energy which verifies the metallic behavior of the studied structures.

Theoretical analysis of tunneling current in 4H-SiC Schottky barrier diodes under reverse-biased conditions based on the complex band structure

The tunneling current in heavily doped 4H-SiC Schottky barrier diodes under reverse-biased conditions is calculated based on the complex band structure by the empirical pseudopotential method. When the experimental values for effective mass and barrier height are assumed, the calculation result by the approximation assuming a parabolic complex band significantly underestimates the experimental tunneling current. In contrast, the calculation using the non-parabolic complex band by the empirical pseudopotential method we propose in this study reproduces the experimental result with better accuracy. These results imply that it is important to consider the non-parabolicity of the complex bands when calculating the tunneling current.

Pool boiling on micro-structured surface with lattice Boltzmann method

In this paper, pool boiling on micro-pillar structured surfaces is studied by using a three-dimensional pseudo potential phase-change lattice Boltzmann method (LBM). The joint enhancing effects on bubble nucleation and boiling performance are discussed in detail regarding various surface wettability and pillar geometrical parameters. Results show that on neutral and hydrophobic surfaces, enlarging the spacing of micro pillars delays the nucleation but can reduce the temperature inside vapor film and improve heat conduction. Despite that increasing the pillar height can improve heat flux, it is adverse to nucleation due to the increased cooling effect on the roots of micro pillars. In contrast, on hydrophilic surfaces, the impact of pillar spacing on nucleation is not monotonous and relatively much complicated. The heat flux is enhanced with increasing pillar spacing because of the extended three-phase contact line. And, the nucleation positions differ significantly by varying pillar geometrical parameters on hydrophilic surfaces.

First principle calculation of structural, electronic, magnetic, and elastic properties of ferromagnetic Cu2MnZ (Z = Pb, P, As, Bi, S, Se, and Te) Heusler alloys

We predicted new series of Heusler alloys from X2YZ type full Heusler Cu2MnZ (Z = Pb, P, As, Bi, S, Se, and Te) alloys, using density functional theory (DFT)-Projected Augmented Wave (PAW) pseudopotential method. We carried out detailed electronic structure and elastic properties calculations to find the suitability of this new series of Heusler alloys for device applications. From the structural, electronic, and magnetic calculations, we have found that Cu2MnZ (Z = Pb, P, As, Bi, S, Se, and Te) alloys are having a regular Heusler structure with ferromagnetic (FM) ground state. We explained the metallic and ferromagnetic nature of the proposed alloys with the help of spin-polarized density of states (DOS) and orbital-resolved DOS calculations. Interestingly, the contribution to the total magnetic moment in Cu2MnZ arises entirely from the Mn atoms. Further, elastic property calculation reveals that Cu2MnZ exhibit ductile nature and is found to be less compressible.

First principles study of the effect of (Mg, C) doping and Zn vacancies on the carrier activity, lifetime, visible light effect, and oxidation–reduction reaction of ZnO(0 0 1) monolayers

At present, some progress has been made in the study of Mg doping on the photocatalytic performance of ZnO monolayers. However, few studies on the effects of C impurities, Mg doping, and Zn vacancy coexistence on the electronic structure and photocatalytic performance of ZnO monolayers have been conducted. The preparation of ZnO through metal organic chemical vapor deposition growth technology inevitably generates C impurities, which are difficult to remove. To solve such problems, we investigated the effects of Mg doping and the coexistence of C impurities and Zn vacancies on the electronic structure and the photocatalysis of ZnO monolayers with a generalized gradient approximation plane-wave ultrasoft pseudopotential method based on the framework of density-generalized functional theory. Results showed that all doped systems had negative formation energy under O-rich conditions, indicating that the systems easily formed. The cohesion energy of Zn33CiCZnMgO36(Ci1−/2+/3+/4+,CZn2+) monolayers was relatively large, and the systems were stable. The absorption spectra of the doped systems red-shifted in a wavelength range of 425–800 nm compared with the absorption spectrum of the undoped Zn36O36 monolayer system. All doped systems were able to react with water to produce H2 and O2 at pH of 0 or 7. The Zn34CiCOMgO35 (Ci1−,CO2−) monolayer exhibited the strongest activity, the best absorption spectrum red shift, the smallest work function, the longest carrier lifetime, and the best HER ability. The Zn34CiCOMgO35 (Ci1−,CO2−) monolayer was considered a strong candidate for photocatalytic hydrogen production. Overall, this study has a theoretical reference value for the design and preparation of novel photocatalytic functional materials.

Theoretical Base of Pseudo Chemical Potential (PCP) Method

Pseudo chemical potential (PCP) method is a novel one based on the thermodynamic formalization, and the purpose of this work is to clarify theoretical base of PCP method and to perfect PCP method into a novel method for molecular calculation. Therefore, it is very important to clarify validity of thermodynamic formalization that becomes principle and methodology of this method for achievement of its purpose. The thermodynamic formalization in molecular calculation is to achieve molecular calculation by using principle and methodology of thermodynamics. In order to apply the method of thermodynamic formalization in molecular calculation, it must be clarified that molecule (atom) electron system, computational object of PCP method, can be considered to be “thermodynamic system”. In this paper, we have clarified the temperature zero limit (TZL) state of a finite temperature thermal equilibrium system, as physical base of the pseudo chemical potential (PCP) method, having thermodynamic properties and therefore being the research object of thermodynamic formalism. Furthermore, we have proved an existence of energy minimization principle, as the theoretical base of PCP method, and from it derived the variation equation and defined the electron charge distribution equation, and on the basis of it analyzed theoretically the electronegativity equalization principle. Resultantly, herein we have demonstrated theoretical validity of PCP method, as a novel method for calculation of molecular energies and charge distributions.

Theoretical study of second and third order elastic constants of β-SiC

First principles calculations of the second and third order elastic constants of the cubic silicon carbide β-SiC have been accomplished using the density functional theory within the local density approximationand the pseudopotential plane-wave method with the Hartwigsen Goedecker Hutter scheme. The secondorder elastic constants Cij compared fairly well with those previously measured by one of us using theBrillouin light scattering technique (P. Djemia) [1]. The ab-initio calculations include parameterspressure dependence determinations in relation with the structural transformation from a zinc Blende toa rocksalt structure. The transition pressure at which arises elastic instabilities is found to be Pt = 113GPa. Also, we investigate the effects of anharmonicity by calculating the third order elastic constantsC_ijk . The experimental values are still unknown and comparisons with our calculations are suitable in thefuture.
 

Theoretical investigation of fundamental physical properties of full-Heusler Co2VZ (Z= Al, Bi, Ga, Ge) compounds

A subclass of ternary intermetallic known as Heusler compounds is being employed in the steel industry to increase material strength. This study examines the structural, electrical, optical, and magnetic properties of Co2VZ (Z=Al, Bi, Ge, Si) compounds using two different methods. First is full potential linearized augmented plane wave (FP-LAPW) method by WIEN2k and second is pseudo potential method by Atomic Tool Kit-Virtual Nano-Lab. The equivalent energy gaps in the minority-spin of Co2VZ (Z= Al, Bi, Ga, Ge) with WIEN2k code, which displays 100% spin polarization, are 0.703, 0.384, 0.186, and 0.67 eV near to the Fermi level. Furthermore, it is discovered that these compounds are absolutely half-metallic ferromagnetic (HMF). With the exception of Co2VBi, which exhibits metallic properties, the aforementioned compounds exhibit 0.525, 0.0, 0.553, and 0.786 eV band gaps in the ATK-VNL code and indicate 100% spin polarization. The magnetic moments of the compounds Co2VZ (Z= Al, Bi, Ga, Ge) are found to be 2.976, 4.003, 1.989, and 3.001 µB, respectively, in the WIEN2k code. The relative magnetic moments of the aforementioned compounds are also 1.991, 3.947, 1.999, and 2.997 µB, according to the ATK-VNL code.

First Principle Study on the Influence Mechanism of Impurity Gas O<sub>2</sub> on the Adsorption Properties of Alloy ZrCo

The adsorption behavior of gases on the surface of ZrCo alloy has an important influence on its hydrogen storage performance. The adsorption behavior of O₂ on ZrCo(110) surface was investigated using the first principles based on the pseudopotential plane wave method. The results of adsorption energy and charge analysis showed that the most stable geometry configuration was B3(Zr-Co Bridge site) and the adsorption energy was −8.124 eV. The density of states and differential charge density analysis showed that the adsorption behavior of O₂ on ZrCo (110) surface was strong chemical adsorption, and the oxygen-oxygen bond broke. The essence of bonding between O atom and ZrCo (110) surface atom was that the electron orbit of O atom overlapped with the electron orbit of the surface atom. It was that the 2s and 2p orbital electrons of O atom overlapped with the 4p and 4d orbital electrons of Zr atom and the 3d orbital electrons of Co atom on the surface. The research results have a positive role in revealing the poisoning mechanism of ZrCo alloy in impurity gases.

New insights into the process of intrinsic point defects in PuO2.

Intrinsic point defects are known to play a crucial role in determining the physical properties of solid-state materials. In this study, we systematically investigate the intrinsic point defects, including vacancies (VPu and VO), interstitials (Pui and Oi), and antisite atoms (PuO and OPu) in PuO2 using the first-principles plane wave pseudopotential method. Our calculations consider the whole charge state of these point defects, as well as the effect of oxygen partial pressure. This leads to a new perspective on the process of intrinsic point defects in PuO2. We find that the antisite atoms OPu and PuO are more likely to appear in O-rich and O-deficient environments, respectively. Interestingly, the most energetically favorable type of Schottky defect is {2VPu3-: 3VO2+} in an O-rich environment, while {4VO1+: VPu4-} is preferred in an O-deficient environment. These results differ from the commonly known {VPu4-: 2VO2+} type of Schottky defect. Moreover, under O-deficient conditions, we predict that the stable cation Frenkel defect is {VPu4+: Pui4+}, while the most stable anion Frenkel defect is {VO2+: Oi2-} under O-rich conditions. Lastly, we find that the only two types of antisite pairs that can appear are {OPu5-: PuO5+} and {OPu6-: PuO6+}, with the latter being the more stable configuration. These unconventional defect configurations provide a new viewpoint on the process of intrinsic point defects in PuO2 and lay theoretical foundations for future experiments.

Nonlinear ion acoustic wave structure in electron–ion plasma in presence of fully relativistic electron beam

Analytical description of large amplitude nonlinear ion acoustic (IA) waves in the presence of a cold relativistic electron beam is presented. The pseudopotential method is adopted to observe the effects of various parameters like beam density, ion temperature, etc. on the IA wave structures. Furthermore, the maximum amplitude of the IA wave is estimated, and its variation with fully relativistic electron beam parameters is investigated. The maximum allowed amplitude of the wave is found to decrease with the increase in the electron beam density as well as in the ion temperature. The Mach number has the effect to increase the maximum amplitude of the electric field.