Long-range Diffusion Research Articles

Correlation of Heterogeneous Local Martensite Tetragonality and Carbon Distribution in High Carbon Steel.

A novel approach for the correlation of local martensite tetragonality determined by electron backscatter diffraction and carbon distribution by atom probe tomography (APT) is presented. The two methods are correlated by site-specific sample preparation for APT based on the local tetragonality. This approach is used to investigate the local carbon distribution in high carbon steel with varying local martensite tetragonality. Regions with low tetragonality show clear agglomeration of carbon based on statistical nearest neighbour (NN) analysis, while regions with high tetragonality show only small elongated agglomerations of carbon and no significant clustering using NN analysis. The APT average bulk carbon content shows no quantitative difference between regions with low and high tetragonality, indicating that no significant long-range diffusion of carbon has taken place.

Long-Range Li-Ion 3D Diffusion in the High-Temperature Hexagonal Phase of LiBH4 Studied by 7Li Pulsed Field Gradient NMR and MD Simulations

Lithium diffusion pathways have been determined in three dimensions using molecular dynamics simulations and 7Li pulsed-field gradient spin-echo (PGSE) NMR diffusometry. Lithium ions arrange in multiple hexagonally arranged and intertwined columns oriented at different angles on (001) surfaces. It has been found that lithium jumps are characterized by zig–zag patterns with the main diffusion tensor axis aligned along the [001] direction. Additional diffusion tensor dimensions appear in the (001) planes through multiple jumps along the [001] direction. The simulation results were confirmed by non-linear spin-echo attenuation in the 7Li PGSE NMR experiment and three-dimensional diffusion model developed for Li+-ion conductivity. The exact values of the main diffusion tensor components D⊥ = (D11 + D22)/2 and D∥ = D33 were derived from the uni-dimensional projection of diffusion tensor under macroscopic powder averaging condition. The single ionic jump in the [001] direction was related to D∥, while translation on the (001) plane required at least two jumps through the zig–zag path and related to D⊥. Lithium diffusion in the [001] direction is faster than that in the (001) planes. The activation energies for D⊥ and D∥ were determined between 105 and 130 °C and varied between 0.47 and 0.51 eV, which suggests that both diffusion coefficients are related to the same mechanism of the jump. Pre-exponential factors for D∥ and D⊥ were also determined and equal to 4.8 × 10–6 m2 s–1 for D∥ and 3.8 × 10–7 m2 s–1 for D⊥, respectively, which confirms that molecular translations between (001) surfaces are not performed in a single jump.

Controlling the Thermal Stability, Threshold Voltage, and Thermoelectric Properties of Cuprous Sulfide Thermoelectrics.

Cu-ion liquid-like copper sulfide materials have excellent thermoelectric properties, while their applications are limited by their high-temperature decomposition and electric field-driven Cu precipitation issues. In particular, high thermoelectric properties and electric field-driven degradation are difficult to reconcile because liquid-like Cu ions are dominant in low κ and high ZT, while they cause electric field-driven degradation. Here, we control the sintering current and duration time to remove the Cu1.8S phase, thereby inhibiting the thermal decomposition of the copper sulfide samples, and introduce the Fe element into the sample matrix to improve its resistance to electric field-driven degradation. We reveal that the kinetic process of Cu1.8S phase decomposition can be suppressed by increasing the relative density of the sample or covering a layer of dense coating/film on the surface of the sample. However, as long as the Cu1.8S phase is present in the sample, it cannot maintain thermal stability above 450 °C. Furthermore, we find that the Fe element forms a nanogrid spinodal decomposition structure in the sample matrix, which acts as a barrier wall to prevent the long-range diffusion of liquid-like Cu ions and inhibit the electric field-driven degradation. The freely movable liquid-like Cu ions in the grid maintain a strong scattering of phonons in a short range, so the sample possesses low κ and high ZT. Then, a strategy to unify the high thermal decomposition temperature, high threshold voltage, and high thermoelectric performance of copper sulfide thermoelectric materials is proposed: transforming the Cu1.8S phase and introducing a liquid-like Cu ion migration barrier.

Inverse Faraday effect in massive Dirac electrons

We study the inverse Faraday effect (IFE) in a Dirac Hamiltonian with random impurities using Keldysh formalism and diagrammatic perturbation theory. The mass term in the Dirac Hamiltonian is essential for IFE, where the spin magnetic moment induced by circularly polarized light is proportional to the frequency of the incident light within the THz regime. For massive Dirac electrons, the corrections due to short-range impurities on spin magnetic moment vertex exhibits mixing of the spin magnetic moment vertex and spin angular momentum vertex. The spin magnetic density response is divergently enhanced by the vertex corrections near the band edge, indicting a long-range diffusion of spin density profile in massive Dirac electrons.

Re-examination of martensitic stabilization in Cu-based shape memory alloys Part II. Key factors determining occurrence of martensitic stabilization in Cu-based alloys

Present proposed mechanisms for ageing-induced martensitic stabilization cannot explain why the amount of martensites losing reverse transformation ability (RTA) positively depended on Ms temperatures in directly-quenched CuZnAl alloys owing to dynamic ageing during the cooling process of quenching and subsequent slow heating. We proposed and confirmed that the number of vacancies at martensites boundaries (Nvmb) was the key factor controlling the occurrence of martensitic stabilization and its resulting RTA or loss in shape memory effect, not the number of vacancies inside the martensites and their long-range diffusion. All reported results can be rationalized by the positive dependence of original Nvmb on the Ms temperature.

Coordination polymer design for fast proton conduction: Hybrid atomistic approach based on kinetic Monte Carlo and molecular dynamics methods

Fast proton conductors are important materials for catalysis and energy conversion applications. The glassy coordination polymers are an important class of proton conductors due to their good mechanical moldability; however, their conductivity has been limited to ca. 10 mS cm−1 at 100 °C. The systematic design of coordination polymers with fast proton conduction requires an atomistic simulation method that can describe long-range proton diffusion within an affordable computational time. The methodologies of atomistic simulations are separately limited and cannot fairly describe the long-range proton conduction in non-crystalline materials. In this work, we develop a hybrid approach that combines the molecular dynamics based on a conventional force-field and the kinetic Monte Carlo method, which allows for the large-scale (thousands of atoms) and long time (few nanoseconds) simulation of the long-range ionic diffusion in non-crystalline materials. Based on the developed approach, we propose and confirm a design concept for a fast proton-conducting coordination polymer based on Zn2+ ions and phosphoric acid.

Reduction of nanoscopic dynamics in the zwitterionic membrane by salt

The effect of addition of lithium chloride on the lateral diffusivity and internal motion of lipid in unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles was investigated using elastic incoherent neutron scattering, quasielastic incoherent neutron scattering, and differential scanning calorimetry. A shift of the main phase transition to a higher temperature indicated a broader temperature range of the thermodynamic stability of the ordered phase resulting from the salt addition. A universal stiffening effect exerted by lithium chloride on the lipid membrane manifested itself in (1) lower mean-square displacement and (2) decreasing diffusivities, both lateral and localized, in both the ordered gel and the disordered fluid phase state. A strong reduction in the lateral lipid diffusivity was likely primarily due to the strong interaction of Li+ ions with the head group of the zwitterionic DMPC lipid and was consistent with the previous reports of the effect of a salt addition on the self-diffusivity of zwitterionic lipids in a membrane. Besides the whole lipid long-range diffusivity, quasielastic neutron scattering allowed measurements of the local internal motion of the lipid. Despite the weak interaction between the ions and the uncharged lipid tails, in both the ordered and the disordered phase states, the local diffusivity of the lipid tails decreased by a factor of 2.2 in the presence of the salt, which closely correlates with the decrease in the diffusivity (that is, an increase in viscosity) of water in the LiCl salt solution by a factor of 2.4. Regardless of the phase state of the membrane, the decrease in the local diffusivity of the lipid seems to mimic the decrease in the diffusivity (which is inverse of the increase of viscosity) of the solvents.

Threshold dynamics of a nonlocal dispersal HIV/AIDS epidemic model with spatial heterogeneity and antiretroviral therapy

Antiretroviral therapy and long-range diffusion of HIV-infected individuals in heterogeneous environments can greatly impact the transmission and distribution of HIV/AIDS in a population. In this paper, we present a nonlocal dispersal HIV/AIDS epidemic model incorporating spatial heterogeneity as well as antiretroviral therapy to study the spatial and temporal dynamics of HIV/AIDS in China. To overcome the difficulty about a lack of compactness of the semiflow and the challenge about the existence and uniqueness of the principal eigenvalue of an eigenvalue problem without obvious compactness, we apply the operator semigroup and dissipative system theories. We establish the well-posedness and the existence of the global attractor for the system. Then we derive the basic reproduction number R0 which is defined as the spectral radius of the next generation operator by the renewal equation. Afterwards, we obtain the global dynamics of the system in terms of R0. Our numerical simulations suggest that: (1) the dynamics of HIV/AIDS transmission largely depends on the nonlocal dispersal kernel function; (2) it is necessary to monitor and design/refine rules for the mobility of HIV-infected population between remote and metropolitan areas to control the HIV/AIDS transmission; (3) antiretroviral therapy benefits not only each treated individual but also the entire community, and increasing the therapy coverage rate is one of the most effective ways to prolong the longevity of HIV-infected population. Moreover, spatial dispersal is also an important factor that cannot be ignored for designing antiretroviral therapy strategies.

Sodium diffusion in scheelite-type Na2Zr(MoO4)3 and Na4Zr(MoO4)4

Сomplex sodium-containing oxides are of interest for the search and development of new materials for many practical applications. This paper presents the results of multiscale experimental and theoretical studies aimed to explore the mechanism of sodium-ion diffusion in the scheelite-related Na2Zr(MoO4)3 and Na4Zr(MoO4)4. These molybdates were synthesized by the precursor (formate) method. Ab initio modeling of sodium migration predicts a barrier of 1.0–1.2 eV for long-range sodium diffusion, which is confirmed by the impedance spectroscopy and the 23Na NMR experiments. Our results allow us to assume the existence in Na4Zr(MoO4)4 of an additional, much faster motional process (with energy barrier of 0.5–0.6 eV) associated with localized ion jumps.

Microscopic diffusion in cationic vesicles across different phases

Understanding the phase behavior and microscopic diffusion mechanism of cationic vesicles is crucial for controlled-release kinetics in gene/DNA transfection. Here, we report our findings on the structure, phase behavior, and microscopic dynamics of a vesicle based on a cationic double-chained surfactant, dihexadecyldimethylammonium bromide (DHDAB), which is a promising candidate for a nonviral gene/DNA carrier. Small-angle neutron-scattering measurements reveal that DHDAB aggregates are in the form of unilamellar vesicles. Calorimetric studies show a strong thermal hysteresis in the heating and cooling cycles following different pathways. Two first-order transitions are observed at 303 and 318 K in the heating cycle. Fourier transform infrared spectroscopy reveals that the first transition (at 303 K) is associated with a polymorphic (solid-solid) transition from coagel to an intermediate crystalline (IC) phase, and subsequently the second transition (at 318 K) is an order-disorder transition from IC to the fluid phase. Interestingly, it is found that in the cooling cycle, the fluid phase directly transforms back into the coagel phase without passing through the IC phase. This is also confirmed by an elastic fixed window scan (EFWS) using incoherent neutron scattering. The phase behavior of DHDAB is in strong contrast to that of its longer-chain counterpart, dioctadecyldimethylammonium bromide (DODAB), where the intermediate gel phase was observed during the cooling cycle. Moreover, no polymorphic transition was observed in the heating cycle of DODAB, in which the coagel phase directly converts to the fluid phase. These sharply contrasting phase behaviors for DHDAB and DODAB suggest the strong dependence of membrane phase behavior to the alkyl chain length. EFWS measurements reveal that the phase transitions observed in the DHDAB membrane are associated with strong changes in the DHDAB dynamics. The changes in conformational entropy of DHDAB molecules estimated from EFWS confirm that the coagel-fluid and IC-fluid phase transitions are accompanied by strong changes in the conformations of the alkyl chains. The microscopic dynamics of DHDAB molecules in each of these phases is investigated by employing quasielastic neutron-scattering (QENS). In the ordered phases (coagel and IC), only localized internal dynamics of the DHDAB is observed in the QENS spectra. However, in the fluid phase, the presence of a long-range lateral diffusion is detected in addition to the localized internal dynamics of the alkyl chain. The lateral motion is found to follow Fickian diffusion. The internal dynamics of DHDAB depends on the phase state of the bilayer. In the ordered phase, the internal dynamics is described by a fractional uniaxial rotational diffusion model. However, in the fluid phase, where alkyl tails are disordered with significant gauche defects, the internal dynamics is described using localized translation within confined spheres. This study highlights the rich phase behavior of the DHDAB membrane and the surfactant dynamics across these phases.

Horizontal connectivity in V1: Prediction of coherence in contour and motion integration

This study demonstrates the functional importance of the Surround context relayed laterally in V1 by the horizontal connectivity, in controlling the latency and the gain of the cortical response to the feedforward visual drive. We report here four main findings: 1) a centripetal apparent motion sequence results in a shortening of the spiking latency of V1 cells, when the orientation of the local inducer and the global motion axis are both co-aligned with the RF orientation preference; 2) this contextual effects grows with visual flow speed, peaking at 150–250°/s when it matches the propagation speed of horizontal connectivity (0.15–0.25 mm/ms); 3) For this speed range, the axial sensitivity of V1 cells is tilted by 90° to become co-aligned with the orientation preference axis; 4) the strength of modulation by the surround context correlates with the spatiotemporal coherence of the apparent motion flow. Our results suggest an internally-generated binding process, linking local (orientation /position) and global (motion/direction) features as early as V1. This long-range diffusion process constitutes a plausible substrate in V1 of the human psychophysical bias in speed estimation for collinear motion. Since it is demonstrated in the anesthetized cat, this novel form of contextual control of the cortical gain and phase is a built-in property in V1, whose expression does not require behavioral attention and top-down control from higher cortical areas. We propose that horizontal connectivity participates in the propagation of an internal “prediction” wave, shaped by visual experience, which links contour co-alignment and global axial motion at an apparent speed in the range of saccade-like eye movements.

Long-range transport and ultrafast interfacial charge transfer in perovskite/monolayer semiconductor heterostructure for enhanced light absorption and photocarrier lifetime.

Atomically thin two-dimensional transition metal dichalcogenides (TMDs) have shown great potential for optoelectronic applications, including photodetectors, phototransistors, and spintronic devices. However, the applications of TMD-based optoelectronic devices are severely restricted by their weak light absorption and short exciton lifetime due to their atomically thin nature and strong excitonic effect. To simultaneously enhance the light absorption and photocarrier lifetime of monolayer semiconductors, here, we report 3D/2D perovskite/TMD type II heterostructures by coupling solution processed highly smooth and ligand free CsPbBr3 film with MoS2 and WS2 monolayers. By time-resolved spectroscopy, we show interfacial hole transfer from MoS2 (WS2) to the perovskite layer occurs in an ultrafast time scale (100 and 350fs) and interfacial electron transfer from ultrathin CsPbBr3 to MoS2 (WS2) in ∼3 (9) ps, forming a long-lived charge separation with a lifetime of >20ns. With increasing CsPbBr3 thickness, the electron transfer rate from CsPbBr3 to TMD is slower, but the efficiency remains to be near-unity due to coupled long-range diffusion and ultrafast interfacial electron transfer. This study indicates that coupling solution processed lead halide perovskites with strong light absorption and long carrier diffusion length to monolayer semiconductors to form a type II heterostructure is a promising strategy to simultaneously enhance the light harvesting capability and photocarrier lifetime of monolayer semiconductors.

Effect of Relaxations on the Conductivity of La1/2+1/2x Li1/2-1/2x Ti1-x Al x O3 Fast Ion Conductors.

Perovskite-type solid-state electrolytes, Li3xLa2/3–xTiO3 (LLTO), are considered among the most promising candidates for the development of all-solid-state batteries based on lithium metal. Their high bulk ionic conductivity can be modulated by substituting part of the atoms hosted in the A- or B-site of the LLTO structure. In this work, we investigate the crystal structure and the long-range charge migration processes characterizing a family of perovskites with the general formula La1/2+1/2xLi1/2–1/2xTi1–xAlxO3 (0 ≤ x ≤ 0.6), in which the charge balance and the nominal A-site vacancies (nA = 0) are preserved. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) investigations reveal the presence of a very complex nanostructure constituted by a mixture of two different ordered nanoregions of tetragonal P4/mmm and rhombohedral R3̅c symmetries. Broadband electrical spectroscopy studies confirm the presence of different crystalline domains and demonstrate that the structural fluctuations of the BO6 octahedra require to be intra- and intercell coupled, to enable the long-range diffusion of the lithium cation, in a similar way to the segmental mode that takes place in polymer-ion conductors. These hypotheses are corroborated by density functional theory (DFT) calculations and molecular dynamic simulations.

Superior radiation tolerance via reversible disordering-ordering transition of coherent superlattices.

Materials capable of sustaining high radiation doses at a high temperature are required for next-generation fission and future fusion energy. To date, however, even the most promising structural materials cannot withstand the demanded radiation environment due to irreversible radiation-driven microstructure degradation. Here we report a counterintuitive strategy to achieve exceptionally high radiation tolerance at high temperatures by enabling reversible local disordering-ordering transition of the introduced superlattice nanoprecipitates in metallic materials. As particularly demonstrated in martensitic steel containing a high density of B2-ordered superlattices, no void swelling was detected even after ultrahigh-dose radiation damage at 400-600 °C. The reordering process of the low-misfit superlattices in highly supersaturated matrices occurs through the short-range reshuffling of radiation-induced point defects and excess solutes right after rapid, ballistic disordering. This dynamic process stabilizes the microstructure, continuously promotes in situ defect recombination and efficiently prevents the capillary-driven long-range diffusion process. The strategy can be readily applied into other materials and pave the pathway for developing materials with high radiation tolerance.

Relative population of 1/2 and interstitial loops in alpha-Fe under irradiation: Effects of C15 cluster stability and loop one-dimensional movement

The relative stability of 1/2<111> and <100> dislocation loops, originating from anisotropic elasticity of alpha-Fe, was extensively exploited to explain experimentally observed variation in the relative population of the two loop types under varying irradiation temperatures. In this study, using cluster dynamics simulations taking into account the coevolution of C15 Laves phase structure (cluster) and loop reaction caused by defect one-dimensional (1-d) movement, we reveal new mechanisms that may underpin the variations in loop population of different Burgers vectors. We identify that C15 clusters have twofold roles in mediating loop evolution: (1) acting as buffers of self-interstitial atoms, (2) initiating loop nuclei via their collapses. The latter plays a critical role in nucleation of <100> loops and subsequently affects loop evolution of both types. We show that the loop relative population undergoes further re-arrangement via a transfer reaction driven by loop long-range diffusion. Our findings essentially demonstrate that, apart from the loop stability, the relative population of two loop types is governed by at least two other factors: C15 cluster stability and loop 1-d movement.

Reversible motions and disordered structure of soft particles in amorphous solids

In amorphous solids, soft vibrational modes derived from normal mode analysis can be utilized to identify the soft particles that are prone to irreversible rearrangements. However, the normal mode analysis approach cannot explain why the spatial distributions of clustered soft particles do not change over time. We define a softness parameter based on the vibrational density of states calculated directly from molecular dynamics simulations with both the harmonic vibrations and anharmonic relaxations being properly captured at finite temperatures. This parameter spontaneously correlates with the real space atomic motions and the dynamics heterogeneity. Using the softness parameter, we show that the softest particles are confined within rigid cages. These particles keep rearranging reversibly without long-range diffusion. The moderately soft particles rearrange irreversibly, and the hard particles mainly participate in vibrations without rearrangement. We also show that the soft particles form locally disordered structures, while the hard particles present strong ordering. These findings confirm the defective nature of soft particles, and provide insights on the nature of softness as the ability to rearrange, but not necessarily irreversibly.

Computational Study of Crystallography, Defects, Ion Migration and Dopants in Almandine Garnet

Almandine garnet has received considerable amounts of interest due to its application in manufacturing and engineering processes. Defect processes, Fe-ion diffusion pathways, and promising dopants on the Al, Fe, and Si sites are examined using classical pair potential simulations in almandine garnet. The cation antisite (Al–Si) defect cluster is the most favourable defect, highlighting the cation disorder in this material. A three-dimensional long-range Fe-ion diffusion pathway with an activation energy of 0.44 eV suggests that the ionic conductivity in this material is high. The most favourable isovalent dopants on the Fe, Al, and Si sites were found to be the Mn, Ga, and Ge, respectively. Subvalent doping of Ga on the Si site is a favourable process to increase the Fe content in this material.

Step, dip, and bell-shape traveling waves in a (2 + 1)-chemotaxis model with traction and long-range diffusion

Step, dip, and bell-shape traveling waves in a (2 + 1)-chemotaxis model with traction and long-range diffusion

Anormalous frequency-dependent conductivity in DC regime and long correlation length among mobile ions of β-alumina

Electrical conductivity measurements of single crystals Na, Ag, Rb and Cd β-alumina have been performed in the frequency range 10−2~107 Hz. Na β-alumina and Ag β-alumina are well known as “super”ionic conductor. Rb β-alumina and Cd β-alumina are “ordinary” solid electrolytes. The magnitude of conductivity depends on the kind of mobile ion strongly. One diffusion step of an ion will result in a change of the local potentials by the ion itself and by its neighbours. The motion of the mobile ions has to be regarded as a highly correlated process under many-particle effects. The present systematic investigation between “super”ionic conductor and “ordinary” solid electrolyte provided an experimental evidence of the many-particle effects in the long-range diffusion. The application of fractal concept for a random walk was helpful us to see the physical means of ion dynamics in the superionic conductor.

Management of invasive cells in soft biological tissues through modulated nonlinear excitations: Long-range effects

In the present paper, we investigate the contribution of long-range diffusion and diffusive stresses on cellular waves generated during an invasion in soft biological tissues. To that end, we consider a set of three coupled nonlinear partial differential equations describing spatiotemporal evolutions of cell density, extracellular matrix (ECM), and spatial displacements of a material point taken in the ECM. In addition to long-range diffusion, diffusive stresses, and proliferation, the model also accounts for ECM production-degradation. The linear stability analysis reveals that in presence of temporal perturbations, there is no stability-instability transition in the system. In presence of spatiotemporal perturbations, instability dominates but the stability-instability transition is controlled by proliferation rate and long-range diffusion of cells. Analytical solutions are constructed as well as their existence conditions provided. Amongst the structures recovered, breathers are associated with the transport of a high number of cells. Long-range diffusive stresses, saturation, and ECM depletion rate significantly modify the wave structure as well as the number of cells carried. On the other hand, Proliferation, ECM production rate, long-range haptotaxis, diffusion, and traction affect both spatial rearrangement and the number of cells carried by the wave. Through our analysis, numerical solutions remain positive, do not diverge from their analytical counterpart, meaning that the solutions proposed are stable and may be used to probe the inherent biological realities of the model considered.