On the thermodynamics of plasticity during quasi-isentropic compression of metallic glass

Entropy production in quasi-isentropic compression (QIC) is critically important for understanding the properties of materials under extremeconditions. However, the origin and accurate quantification of entropy in this situation remain long-standing challenges. In this work, a framework is established for the quantification of entropy production and partition, and their relation to microstructural change in QIC. Cu50Zr50is taken as a model material, and its compression is simulated by molecular dynamics. On the basis of atomistic simulation-informed physicalproperties and free energy, the thermodynamic path is recovered, and the entropy production and its relation to microstructural change aresuccessfully quantified by the proposed framework. Contrary to intuition, entropy production during QIC of metallic glasses is relativelyinsensitive to the strain rate ˙γ when ˙γ ranges from 7.5 × 10^(8) to 2 × 10^(9)/s, which are values reachable in QIC experiments, with a magnitudeof the order of 10^(−2)kB/atom per GPa. However, when ˙γ is extremely high (>2 × 10^(9)/s), a notable increase in entropy production rate with˙γ is observed. The Taylor–Quinney factor is found to vary with strain but not with strain rate in the simulated regime. It is demonstrated thatentropy production is dominated by the configurational part, compared with the vibrational part. In the rate-insensitive regime, the increase inconfigurational entropy exhibits a linear relation to the Shannon-entropic quantification of microstructural change, and a stretched exponential relation to the Taylor–Quinney factor. The quantification of entropy is expected to provide thermodynamic insights into the fundamentalrelation between microstructure evolution and plastic dissipation.

聚偏氟乙烯/苯乙烯-马来酸酐共聚物复合膜中的偏析行为及膜性能

采用非溶剂相转化法(NIPS)制备聚偏氟乙烯/苯乙烯-马来酸酐共聚物(PVDF/SMA)复合膜。首先,探究苯乙烯-马来酸酐共聚物(SMA)的添加量对复合膜表面偏析行为的影响;其次,探究添加剂聚乙烯吡咯烷酮(PVP)和十二烷基硫酸钠(SDS)对复合膜中SMA偏析行为的影响。结果表明,随着SMA添加量的增加,复合膜中SMA的偏析程度增强,在SMA的质量分数为2%时,复合膜的纯水通量为176.28 L/(m^(2)·h),对牛血清白蛋白(BSA)截留率为89.11%,但是随着SMA添加量的进一步增加,膜表面的孔结构发展成大裂纹,当SMA质量分数为5%时,对BSA的截留性能降低至53.87%。添加PVP后,SMA的表面偏析行为没有变化,而添加PVP/SDS后复合膜中SMA的表面偏析程度减弱,但SDS的存在增强了PVP在膜表面的富集,复合膜的纯水通量提高至409.51 L/(m^(2)·h),同时抗污染性能提高。

Towards the Same Line of Liquid–Liquid Phase Transition of Dense Hydrogen from Various Theoretical Predictions

For a long time,there have been huge discrepancies between different models and experiments concerning the liquid-liquid phase transition(LLPT)in dense hydrogen.We present the results of extensive calculations of the LLPT in dense hydrogen using the most expensive first-principle path-integral molecular dynamics simulations available.The nonlocal density functional rVV10 and the hybrid functional PBEO are used to improve the description of the electronic structure of hydrogen.Of all the density functional theory calculations available,we report the most consistent results through quantum Monte Carlo simulations and coupled electron-ion Monte Carlo simulations of the LLPT in dense hydrogen.The critical point of the first-order LLPT is estimated to be above 2000 K according to the equation of state.Moreover,the metallization pressure obtained from the jump of dc electrical conductivity almost coincides with the plateau of equation of state.

Unified first-principles equations of state of deuterium-tritium mixtures in the global inertial confinement fusion region

Accurate knowledge of the equation of state(EOS)of deuterium–tritium(DT)mixtures is critically important for inertial confinement fusion(ICF).Although the study of EOS is an old topic,there is a longstanding lack of global accurate EOS data for DT within a unified theoretical framework.DT fuel goes through very wide ranges of density and temperature from a cold condensed state to a hot dense plasma where ions are in a moderately or even strongly coupled state and electrons are in a partially or strongly degenerate state.The biggest challenge faced when using first-principles methods for obtaining accurate EOS data for DT fuel is the treatment of electron–ion interactions and the extremely high computational cost at high temperatures.In the present work,we perform extensive state-of-the-art ab initio quantum Langevin molecular dynamics simulations to obtain EOS data for DT mixtures at densities from 0.1 g/cm3 to 2000 g/cm3 and temperatures from 500 K to 2000 eV,which are relevant to ICF processes.Comparisons with average-atom molecular dynamics and orbital-free molecular dynamics simulations show that the ionic strong-coupling effect is important for determining the whole-range EOS.This work can supply accurate EOS data forDTmixtures within a unified ab initio framework,as well as providing a benchmark for various semiclassical methods.

Lattice Thermal Conductivity of MgSiO_(3) Perovskite and Post-Perovskite under Lower Mantle Conditions Calculated by Deep Potential Molecular Dynamics

Lattice thermal conductivity(κlat)of MgSiO_(3) perovskite and post-perovskite is an important parameter for the thermal dynamics in the Earth.Here,we develop a deep potential of density functional theory quality under entire thermodynamic conditions in the lower mantle,and calculate theκlatby the Green-Kubo relation.Deep potential molecular dynamics captures full-order anharmonicity and considers ill-defined phonons in low-κlatmaterials ignored in the phonon gas model.Theκlatshows negative temperature dependence and positive linear pressure dependence.Interestingly,theκlatundergos an increase at the phase boundary from perovskite to post-perovskite.We demonstrate that,along the geotherm,theκlatincreases by 18.2% at the phase boundary.Our results would be helpful for evaluating Earth’s thermal dynamics and improving the Earth model.

Anomalous Thermal Transport across the Superionic Transition in Ice

Superionic ices with highly mobile protons within stable oxygen sub-lattices occupy an important proportion of the phase diagram of ice and widely exist in the interior of icy giants and throughout the Universe.Understanding the thermal transport in superionic ice is vital for the thermal evolution of icy planets.However,it is highly challenging due to the extreme thermodynamic conditions and dynamical nature of protons,beyond the capability of the traditional lattice dynamics and empirical potential molecular dynamics approaches.By utilizing the deep potential molecular dynamics approach,we investigate the thermal conductivity of ice-Ⅶ and superionic ice-Ⅶ’’ along the isobar of P = 30 GPa.A non-monotonic trend of thermal conductivity with elevated temperature is observed.Through heat flux decomposition and trajectory-based spectra analysis,we show that the thermally activated proton diffusion in ice-Ⅶ and superionic ice-Ⅶ′′contribute significantly to heat convection,while the broadening in vibrational energy peaks and significant softening of transverse acoustic branches lead to a reduction in heat conduction.The competition between proton diffusion and phonon scattering results in anomalous thermal transport across the superionic transition in ice.This work unravels the important role of proton diffusion in the thermal transport of high-pressure ice.Our approach provides new insights into modeling the thermal transport and atomistic dynamics in superionic materials.

Transport properties of warm and hot dense iron from orbital free and corrected Yukawa potential molecular dynamics

The equation of states,diffusions,and viscosities of strongly coupled Fe at 80 and 240 eV with densities from 1.6 to 40 g/cm^(3) are studied by orbital-free molecular dynamics,classical molecular dynamics with a corrected Yukawa potential and compared with the results from average atom model.A new local pseudopotential is generated for orbital free calculations.For low densities,the Yukawa model captures the correct ionic interaction behavior around the first peak of the radial distribution function(RDF),thus it gives correct RDFs and transport coefficients.For higher densities,the scaled transformation of the Yukawa potential or adding a short range repulsion part to the Yukawa potential can give correct RDFs and transport coefficients.The corrected potentials are further validated by the force matching method.

Two-temperature warm dense hydrogen as a test of quantum protons driven by orbital-free density functional theory electronic forces

We consider a steady-state(but transient)situation in which a warm dense aggregate is a two-temperature system with equilibrium electrons at temperature T_(e),ions at T_(i),and T_(e)≠T_(i).Such states are achievable by pump–probe experiments.For warm dense hydrogen in such a twotemperature situation,we investigate nuclear quantum effects(NQEs)on structure and thermodynamic properties,thereby delineating the limitations of ordinary ab initio molecular dynamics.We use path integral molecular dynamics(PIMD)simulations driven by orbital-free density functional theory(OFDFT)calculations with state-of-the-art noninteracting free-energy and exchange-correlation functionals for the explicit temperature dependence.We calibrate the OFDFT calculations against conventional(explicit orbitals)Kohn–Sham DFT.We find that when the ratio of the ionic thermal de Broglie wavelength to the mean interionic distance is larger than about 0.30,the ionic radial distribution function is meaningfully affected by the inclusion of NQEs.Moreover,NQEs induce a substantial increase in both the ionic and electronic pressures.This confirms the importance of NQEs for highly accurate equation-of-state data on highly driven hydrogen.For Te>20 kK,increasing Te in the warm dense hydrogen has slight effects on the ionic radial distribution function and equation of state in the range of densities considered.In addition,we confirm that compared with thermostatted ring-polymer molecular dynamics,the primitive PIMD algorithm overestimates electronic pressures,a consequence of the overly localized ionic description from the primitive scheme.

Ionic self-diffusion coefficient and shear viscosity of high-Z materials in the hot dense regime

High-Zmaterials exhibit a broad range of variation of the charge state in the hot dense regime,and so ionic structures becomecomplexwith increasing density and temperature owing to ionization.Taking high-Z uranium as example,we study its electronic and ionic structures in the hot dense regime by combining an average-atommodelwith the hypernetted chain approximation.The electronic structure is described by solving theDirac equation,taking account of relativistic effects,including broadening of the energy levels,and the effect of other ions via correlation functions.On the basis of the electronic distribution around a nucleus,the ion pair potential is constructed using the modified Gordon–Kim model in the frame of temperaturedependent density functional theory.Because of the presence of ion–ion strong coupling,the bridge function is included in the hypernetted chain approximation,whichis usedto calculate the correlation functions.To take account of the influenceon transportpropertiesof the strong correlation of electrons with highly charged ions,we perform both classical and Langevin molecular dynamics simulations to determine ion self-diffusion coefficients and the shear viscosity,using theGreen–Kubo relation and an ion–ion pair potential with good convergence.We show that the influence of electron–ion collisions on transport properties becomes more important as the free electron density increases owing to thermal ionization.

Efficient terahertz generation from van der Waalsα-In_(2)Se_(3)

Two-dimensional(2D)van der Waals materials have attracted tremendous attention due to their versatile physical properties and flexible manipulation approaches.Among the various types of van der Waals materials,α-In_(2)Se_(3)is remarkable for its intrinsic 2D ferroelectricity and high-performance opto-electronic properties.However,the study of theα-In_(2)Se_(3)system in terahertz(THz)radiation is scarce,although it is promising for electrically controlled THz field manipulation.We investigate theα-In_(2)Se_(3)in different thicknesses and report that the THz generation efficiency induced by femtosecond laser pulses can be largely improved by reducing the thickness from the bulk.Furthermore,we reveal the surge current in thin film coupled with THz emission exhibits a different Auger recombination mode,which is helpful in understanding the mechanism and provides insights into the design of 2D highly efficient THz devices.

Full-scale ab initio simulations of laser-driven atomistic dynamics

The coupling of excited states and ionic dynamics is the basic and challenging point for the materials response at extreme conditions.In the laboratory,the intense laser produces transient nature and complexity with highly nonequilibrium states,making it extremely difficult and interesting for both experimental measurements and theoretical methods.With the inclusion of laser-excited states,we extend an ab initio method into the direct simulations of whole laser-driven microscopic dynamics from solid to liquid.We construct the framework of combining the electron-temperature-dependent deep neural-network potential energy surface with a hybrid atomistic-continuum approach,controlling non-adiabatic energy exchange and atomistic dynamics,which enables consistent interpretation of experimental data.By large-scale ab initio simulations,we demonstrate that the nonthermal effects introduced by hot electrons play a dominant role in modulating the lattice dynamics,thermodynamic pathway,and structural transformation.We highlight that the present work provides a path to realistic computational studies of laser-driven processes,thus bridging the gap between experiments and simulations.

Electron localization enhanced photon absorption for the missing opacity in solar interior

The internal solar structure predicted by the standard solar model disagrees with the helioseismic observations even by utilizing the most updated physical inputs, such as the opacity and element abundances. By increasing the Rosseland mean, the decade-old open problem of the missing opacity can be resolved. Herein, we propose that the continuum electrons in the radiative processes lose phases and coherence as matter waves, giving rise to a phenomenon of transient spatial localization. It not only enhances the continuum opacity but also increases the line widths of the bound-bound transitions. We demonstrate our theoretical formulation by investigating the opacity of solar mixtures in the interior. The Rosseland mean demonstrates an increase of 10%-26% in the range of 0.3 R⊙-0.75 R⊙. The results are compared with the recent experimental data and the existing theoretical models. Our findings provide novel clues to the open problem of the missing opacity in the solar interior and new insight on the radiative opacity in the hot dense-plasma regime.