Diffusion Calculations Research Articles

Unveiling the oxidation mechanism of CrTaTiMo refractory medium-entropy alloys: A synergy of density functional theory and ab initio molecular dynamics

The demand for advanced materials capable of withstanding extreme high-temperature conditions has led to the development of novel High-Entropy Alloys (HEAs). Recent works indicate that forming a CrTaO4 protection layer gives HEAs excellent long-term oxidation resistance. In this study, the high-temperature oxidation resistance properties of CrTaTiMo Refractory Medium-Entropy Alloys (RMEAs) were assessed by Density Functional Theory (DFT) and ab initio Molecular Dynamics (AIMD). Through the oxygen adsorption and diffusion calculations, we demonstrated the preferential oxygen adsorption sequence on the surface of the RMEA as Ti, Ta, Cr, and Mo. Furthermore, our analysis identified the sites featuring Ta as subsurface atoms were the weakest locations for oxygen atom diffusion. The dynamic oxidation mechanism of oxygen molecules on CrTaTiMo RMEA was investigated by AIMD simulations. The results confirmed that the adsorption and dissociation of O2 molecules on the alloy surface. Additionally, the diffusion of the O atom took place at temperatures greater than 873 K and confirmed the O-attracting feature of Ta atoms. Moreover, electronic structure calculations confirmed the bonding of oxygen atoms with those four metal elements. This study could serve as a valuable reference for the strategic development of the CrTaTiMo-based RMEAs or RHEAs for high-temperature, long-term oxidation resistance applications.

Co-Activity Maximization in Online Social Networks

Social media with online social networks has risen to be a prevalent force in information diffusion and public discourse. Despite its popularity and convenience, social media has been criticized for contributing to societal and ideological polarization as the result of trapping users in an echo chamber and filter bubbles. An emerging line of research focuses on ways to redesign content or link recommendation algorithms to mitigate the polarization phenomenon. However, existing works mainly concentrate on node-level balancing, while omitting the balancing effect that can be incurred by edge interaction in social networks. In this article, we take the first step to study the problem (CoAM) that assuming two campaigns are present in a network, how we should select seeds for each so as to maximize the interaction/activity between the followers of two campaigns (co-activity) after the diffusion process is finished. We begin our analysis by showing the hardness of CoAM under two diffusion models that are generalized from wildly used diffusion models and its objective function is neither submodular nor supermodular. This encourages us to design a submodular function that acts as a lower bound to the objective, by exploiting which we are able to devise a greedy algorithm with a provable approximation guarantee. To overcome the #P-hardness of diffusion calculation, we further extend the notion of random reverse-reachable (RR) set to devise a scalable instantiation of our approximation algorithm. We experimentally demonstrate the quality of our approximation algorithm on datasets collected from real-world social networks.

Improving ab initio diffusion calculations in materials through Gaussian process regression

Improving <i>ab initio</i> diffusion calculations in materials through Gaussian process regression

Micropore system properties significantly influence the Na-type zeolite adsorption of the carbon disulfide impurity in mimetic coke oven gas

The experiments of the adsorption removal of CS2 impurity from a mimetic coke oven gas were performed by using a series of Na-type zeolites as adsorbents. Combining with the diffusion & simulation calculations and several characterizations, it's found both the development degree and the diffusion ability of microporous systems significantly influence the adsorption performance of these Na-type zeolites on which the physical absorption was predominant for CS2 molecules. Firstly, compared with NaZSM-5 or NaSAPO-34, the NaY with FAU-type framework exhibited a much higher breakthrough adsorption capacity, primarily due to its much higher microporous surface area & volume and larger-size channels, which ensured a broader adsorption space, a high-efficiency molecule-diffusion and a high adsorption utilization of micropores. Secondly, alkali-treatment introduced some mesopores into NaZSM-5 crystals but also resulted in a declined integrity of microporous structure, causing an obvious decline of adsorption performance. Thirdly, the nano-crystal NaZSM-5 or NaY exhibited a much higher breakthrough adsorption capacity than that of the corresponding micron-crystal zeolite, primarily attributed to both the increased microporous surface area & volume and the shortened micropore diffusion-path length of the former. In addition, a breakthrough adsorption capacity of up to 0.251 g/g was obtained in the first adsorption use of a nano-NaY-0.909 sample at 298 K, and it's 0.229 g/g in even the sixteenth time of operation (by then, total adsorption service hours were 80.58 h). Our work may provide an important reference for developing high-performance adsorbents for removing gaseous CS2 impurity in coke oven gas.

The luminescent semiconductor Pb7I6(CN2)4.

The development of new compounds in the domain of metal dinitridocarbonates is most efficiently performed via solid-state metathesis or simply by addition reactions. Our discovery of Pb7I6(CN2)4 is the result of a solid-state reaction of PbCN2 with PbI2 at 420 °C. Its crystal structure was solved and refined from X-ray diffraction data based on a single crystal with the space group P63/mmc. The crystal structure is based on a network of lead tetrahedra, lead trigonal bipyramids and lead octahedra interconnected by [NCN]2- and iodide. Properties of the material were investigated by diffuse reflection measurement, photoluminescence measurements, and electronic band structure calculations demonstrating that this material is a semiconductor.

Generating multi-group cross-sections using continuous-energy Monte Carlo method for fast reactor analysis

The deterministic two-step method, comprising multigroup cross-section generation and core calculation, is widely applied in fast reactor design and analysis. Monte Carlo (MC) methods with continuous energy and fine geometry provide high-precision cross-sections essential for advanced fast reactor neutronics analysis. This paper presents an analysis of integrating MC-generated homogenized cross-sections with various core solvers, demonstrating their effectiveness and potential improvements in fast reactor simulations. For diffusion core calculations, the superhomogénéisation (SPH) technique reduces control rod worth overestimation from 13.5% to 0.35% in the MET-1000 benchmark, improving power distribution predictions. In transport core calculations, the flux-moment homogenization technique (MHT) addresses reactivity overestimation by incorporating cross-section anisotropy, reducing error by 698 pcm. For Method of Characteristics (MOC) core calculations, transport-corrected multigroup cross-sections yield high precision in pin-by-pin power distribution for a 100 MWe lead-based fast reactor benchmark. While MC methods require significant computational resources, such as 62 CPU-hours for the MET-1000 core and 85.5 CPU-hours for the 100 MWe lead-based fast reactor core, they offer flexibility in geometry modeling. This work highlights MC multigroup cross-section generation methods applicable to diffusion, MOC, and transport core calculations for fast reactor analysis, suggesting further exploration into their performance in various reactor parameters and computational efficiency.

PWR core calculation based on pin-cell homogenization in three-dimensional pin-by-pin geometry

For the pressurized water reactor two-step calculation, the traditional assembly homogenization and two-group neutron diffusion calculation have been widely used. When it comes to the core pin-by-pin simulation, many models and techniques are different and unsettled. In this paper, the homogenization methods based on the pin discontinuity factors and super homogenization factors are used to get the pin-cell homogenized parameters. The heterogeneous leakage model is applied to modify the infinite flux spectrum of the single assembly with reflective boundary condition and to determine the diffusion coefficients for the SP3 solver which is used in the core simulation. To reduce the environment effect of the single-assembly reflective boundary condition, the on-line method for the SPH factors updating is applied in this paper, and the functionalization of SPH factors based on the least-squares method will be pre-made alone with the table of the group constants. The fitting function will be used to update the thermal-group SPH factors with a whole-core pin-by-pin homogeneous solution on-line. The three-dimensional Watts Bar Nuclear Unit 1 (WBN1) problem was utilized to test the performance of pin-by-pin calculation. And numerical results have demonstrated that PWR pin-by-pin core calculation has more accurate results compared with the traditional assembly-homogenization scheme.

Unveiling the potential of a BCN-biphenylene monolayer as a high-performance anode material for alkali metal ion batteries: a first-principles study.

Inspired by a freshly synthesized two-dimensional biphenylene carbon network, which features a captivating combination of hexagonal, square, and octagonal rings, we explored a similar biphenylene network composed of boron, carbon, and nitrogen (bpn-BCN) using first-principles calculations. There are six possible phases of borocarbonitrides, which are isoelectronic to biphenylene carbon networks with a stoichiometric ratio of 1 : 1 : 1 for boron (B), carbon (C), and nitrogen (N) atoms. All possible isoelectronic structures of the BCN combination of biphenylene networks are found to be stable, according to first-principles calculations. However, the geometry has a relatively large number of robust C-C and B-N bonds and strong partially ionic-covalent B-C and C-N bonds inside these bpn-BCN monolayers are effectively more stable. Furthermore, we employed first-principles calculations to investigate the electrochemical properties of the most stable geometry of BCN biphenylene as a potential anode material for alkali metal (AM) ion batteries. A global search has been made to find the most favourable alkali metal ion adsorption sites. The biphenylene monolayer has octagonal, square, and hexagonal motifs with different adsorption strengths. Furthermore, the partially ionic bond of B-N (due to the electronegativity difference) also supports the alkali metal ions for adsorption. The electronic properties of the stable phase of bpn-BCN reveal its narrow bandgap semiconductor nature. The ion diffusion calculations show a low activation barrier for Li, Na, and K of 0.65 eV, 0.26 eV, and 0.23 eV, respectively, indicating a fast charge/discharge rate. Furthermore, the theoretical capacities of the BCN biphenylene monolayer for Li (1057.33 mA h g-1), Na (647.27 mA h g-1), and K (465.98 mA h g-1) are found to be greater than those of commercial graphite. The average open-circuit voltage for AM decreases with increasing metal ion concentrations. It falls within a reasonable range of 0.34-1.89 V. Our results show that the BCN biphenylene monolayer could be a promising anode material in alkali metal ion rechargeable batteries.

Investigation of Hydrogen Diffusion Profile of Different Metallic Materials for a Better Understanding of Hydrogen Embrittlement

In this study, hydrogen diffusion profiles of different metallic materials were investigated. To model hydrogen diffusion, 1D and 2D mass diffusion models were prepared in MATLAB. Iron, nickel and titanium were selected as a material of choice to represent body-centered cubic, face-centered cubic, and hexagonal closed paced crystal structures, respectively. In addition, hydrogen back diffusion profiles were also modeled after certain baking times. Current results reveal that hydrogen diffusion depth depends on the microstructure, energy barrier model, temperature, and charging time. In addition, baking can help for back diffusion of hydrogen and can be utilized as hydrogen embrittlement prevention method. Since hydrogen diffusion is very crucial step to understand and evaluate hydrogen embrittlement, current set of results constitutes an important guideline for hydrogen diffusion calculations and ideal baking time for hydrogen back diffusion for different materials. Furthermore, these results can be used to evaluate hydrogen content inside the material over expensive and hard to find experimental facilities such as, thermal desorption spectroscopy.

High-temperature effects for transition state calculations in solids.

Transition state calculation is a critical technique to understand and predict versatile dynamical phenomena in solids. However, the transition state results obtained at 0K are often utilized for the prediction or interpretation of dynamical processes at high temperatures, and the error bars of such an approximation are largely unknown. In this benchmark study, all the major temperature effects, including lattice expansion, lattice vibration, electron excitation, and band-edge shift, are evaluated with first-principles calculations for defect diffusion in solids. With the inclusion of these temperature effects, the notable discrepancies between theoretical predictions at 0K and the experimental diffusivities at high temperatures are dramatically reduced. In particular, we find that lattice expansion and lattice vibration are the dominant factors lowering the defect formation energies and hopping barriers at high temperatures, but the electron excitation exhibits minor effects. In sharp contrast to typical assumptions, the attempt frequency with lattice expansion and vibration varies significantly with materials: several THz for aluminum bulk but surprisingly over 500THz for 4H-SiC. For defects in semiconductors, the band-edge shift is also significant at high temperatures and plays a vital role in defect diffusion. We expect that this study would help accurately predict the dynamical processes at high temperatures.

Role of dynamic surface tension of silicone polyether surfactant-based silicone coatings on protein adsorption: An insight on the ‘ambiguous’ interfacial properties of Fouling Release Coatings

Non-toxic antibiofouling coatings are currently developed to avoid the negative impacts of the settlement of (micro)organisms on any water immersed surfaces. In some industries such as microalgae culture, marine sensors or even contact lenses, in addition to the need for antibiofouling efficiency, transparency and non-toxicity are prerequisite surface properties. In this work, seven amphiphilic transparent antifouling coatings have been elaborated using silicone polyether copolymer surfactants mixed in a polydimethylsiloxane-based elastomer matrix. Average molar masses (1.3–11.7 kDa) and molar mass distributions of each surfactant were assessed by pulse-field gradient spin-echo (PGSE) diffusion NMR. Surface and bulk properties of the designed coatings were characterized by measuring their surface roughness (0.03–0.07 μm), Young's modulus (1.4–2.2 MPa) as well as their wetting behavior. Results obtained from different wetting techniques have shown that coatings had initially a low surface free energy (around 20 mJ/m2) and that their surface became amphiphilic in contact with water (from 21 to 42 mJ/m2). Fitting of water contact angle measurements with the so-called O'Brien-Paranjape dynamic surface tension (DST) model enabled the calculation of the surfactant's diffusivities within the PDMS elastomer matrix (ranging from 9.96 × 10−15 to 1.77 × 10−13 m2·s−1) while providing a comprehensive insight on the kinetic interfacial variation of the overall amphiphilic coating. Coatings with the highest rates of water contact angle variation have been shown to positively affect the bovine serum albumin (BSA) protein adsorption.

Diffusivities and atomic mobilities in bcc Ti–V–Mo alloys

In pursuit of a comprehensive exploration into the diffusion kinetics of bcc Ti–V–Mo alloys, sixteen sets of diffusion couples were prepared within the bcc phase of Ti–V–Mo alloys. These diffusion couples were subjected to annealing at 1373 K and 1473 K. Composition profiles were obtained by means of the electron probe micro-analysis technique (EPMA). The calculation of diffusivities in bcc Ti–V–Mo alloys was performed using the Whittle-Green and Hall methods. Based on available thermodynamic descriptions, the atomic mobility parameters of the bcc Ti–V–Mo system were assessed by using the DICTRA module of the Thermo-Calc software. A comparison is made between the simulated composition-distance curve and the diffusion path with the experimental values, revealing a significant level of concordance.

Modelling of Diffusion Process in Vacuum Drying of Propellants

One crucial step in the propellant-making process that takes a lot of time and energy is drying. The study of inert propellants' vacuum drying behaviours – which mimics those of real propellants – is the main focus of this work. This study presents the development of analytical and numerical models for the vacuum drying of inert pro-pellants, which were validated through an in-house vacuum drying experiment. The calculation of effective diffusivity, a crucial parameter characterising the rate of solvent vapour transfer during the vacuum drying of inert propellant samples, is one of the outcomes of this work. It involves removing liquid solvents from the texture of a propellant. Conclusions and suggestions for technical practice are developed based on the studyʼs findings.

Improvement of electrolytes for aluminum ion batteries: A molecular dynamics study.

The aluminum ion battery (AIB) is a promising technology, but there is a lack of understanding of the desired nature of the batteries' electrolytes. The ionic charge carriers in these batteries are not simply Al3+ ions but the anionic AlCl4- and Al2Cl7-, which form in the electrolyte. Using computational analysis, this study illustrates the effect of mole ratios and organic solvents to improve the AIB electrolytes. To this end, molecular dynamics simulations were conducted on varying ratios forming acidic, neutral, and basic mixtures of the AlCl3 salt with 1-ethyl-3-methylimidazolium chloride (EMImCl) ionic liquid (IL) and an organic solvent electrolyte [dichloromethane (DCM) or toluene]. The data obtained from diffusion calculations indicates that the solvents could improve the transport properties. Both DCM and toluene lead to higher diffusion coefficients, and higher conductivity. Detailed calculations demonstrated solvents can effectively improve the formation of AlCl3⋯Cl (AlCl4-) and AlCl4-···AlCl4- (Al2Cl7-) especially in acidic mixtures. The densities, around 1.25 g/cm3 for electrolyte mixtures of AlCl3-EMImCl, were consistent with experiment. These results, in agreement with experimental findings, strongly suggest that DCM in acidic media with AlCl3 and EMImCl might provide a promising basis for battery development.

Mechanism and experimental study of desulfurizing agent based on Mn/Ce-Ca for sintering flue gas desulfurization (FGD)

A novel Mn/Ce-Ca-based desulfurizer was prepared by dipping-roasting method to improve the performance of single calcium hydroxide (Ca(OH)2) for sintering flue gas desulfurization (FGD) at 100–350 ℃. In this study, metal dopants iron oxide (Fe2O3) and magnesium oxide (MgO) are added to improve the microstructure of single Ca(OH)2 and enhance the utilization of active components. The experimental results on the fixed bed showed that the desulfurization performance got best when the ratio of Fe2O3 to MgO was 3:2, the dipping times of desulfurizer were two, the temperature was 350 ℃, and the oxygen content was 5%. Meanwhile, the effect of H2O content was also studied. Great desulfurization ability with a removal efficiency of 100%, sulfur capacity of 69.95 mg/g, and breakthrough time of 76 min was achieved by desulfurizer 3F2M–3MC-Ca. Even at a low temperature of 100 °C, the sulfur capacity of the desulfurizer is three times higher than that of single Ca(OH)2, confirming its advantages in low-temperature desulfurization. Moreover, the reaction mechanism is systematically elaborated via the kinetic, diffusion, and thermodynamic calculations, and the characterization of this desulfurizer is studied by XRD, SEM, BET, XPS, and ICP-OES. Results show that the addition of Mn/Ce significantly enhances the activity of the desulfurizer by providing abundant oxygen vacancies during the reduction reaction. The work presented here has profound implications for future studies of the low-temperature dry sintering flue gas desulfurization field. Finally, the potential application of this technology is discussed to demonstrate the industrial application value.

Evaluation of ambipolar diffusion coefficient in AlxGa1−xN semiconductor

Numerical approach to account for the influence of randomly fluctuating potential on carrier mobility in compound semiconductors with compositional disorder is developed and exploited for calculating the heavy hole-defined ambipolar diffusion coefficient in AlxGa1-xN layers. The rates of inelastic heavy hole-phonon scattering were obtained by integrating the quasi-classical hole-phonon scattering rates for the virtual crystal over the semiclassical confining potential obtained by using the local landscape method. The calculated dependence of the heavy-hole diffusion coefficient on the aluminum content in AlxGa1-xN crystal in a wide compositional range (x = 0 – 1) evidences a substantial influence of localization on the diffusivity of nonequilibrium carriers and is in a good quantitative correspondence with the experimental data obtained in this work by using the light-induced transient grating technique. The adopted Monte Carlo simulation technique also enables diffusivity calculations in other multicomponent compounds (e.g., high-In-content InGaN) and their structures, in particular, those for LEDs and µ-LEDs.

Application of the Thermal Diffusivity Standard for the Heat Transfer Parameter Control in Absorbing Materials

Metrological support creation and use of heat transfer etalons are important stages in the development of modern materials science. This is especially concerned to the emergence of new materials in the world with previously unattainable thermophysical parameters. The purpose of this work was to develop and experimentally verify the idea of joint application of the transient gratings method which is well-known in nonlinear optics and the single thermal diffusivity etalon of conventional type for the heat transfer metrological control in materials of a wide values range. The method proposed is based on thermal diffusivity etalon application as a source of calibrated optical signals that are excited in it by short laser pulses. Their lifetime is formed by the etalon thermal diffusivity and on the transient grating spatial period. The etalon linear graph of gratings lifetimes as a function of the gratings periods squared and grating lifetime of the material under study are used for the thermal diffusivity calculation. Thermal diffusivity of thin sub-surface layers of the samples under study – duraluminium, monocrystalline silicon and thermoelectric lead telluride film was measured. The results obtained are in close agreement with the reference values.

Experimental and numerical investigation of leakage and diffusion accidents based on nuclear energy hydrogen production system

The high-temperature gas-cooled reactor, as a fourth-generation nuclear power system, possesses a distinct advantage of high core outlet temperature, making it a viable option for large-scale, low-carbon hydrogen production. However, before this technology can be widely adopted, it is crucial to conduct a safety assessment of the nuclear hydrogen production system, considering the unique physical and chemical properties of hydrogen. This study aimed to address this requirement by implementing two experimental setups, utilizing helium as a substitute for hydrogen. Concentration distribution measurements were performed in both open spaces and inside a wind tunnel. The concentration decay rates were found to be 0.177 under windless conditions and 0.313 in windy conditions. Subsequently, numerical models were employed to validate the feasibility of leakage and diffusion calculations for subsonic leakage jets and high-pressure underexpanded leakage jets, respectively. Moreover, multiple simulation calculations were conducted to design leakage accident scenarios based on the nuclear hydrogen production system. The corresponding concentration decay patterns were summarized, and evaluation relational expressions were proposed to determine diffusion distance and explosion distance for the nuclear energy hydrogen production system, accounting for the influence of buoyancy effects and incorporating the average concentration of combustible hydrogen clouds. Lastly, considering the worst-case accident conditions of the current system, a fan reverse blowing scheme was suggested to mitigate safety risks. The findings indicated that this scheme could potentially reduce the separation distance from 479 m to 183 m. Overall, this research contributes valuable insights into the safety assessment and design of nuclear hydrogen production systems.

PROSPECULAR: A model for simulating multi-angular spectral properties of leaves by coupling PROSPECT with a specular function

Understanding the optical properties of leaves is of great importance for characterizing the relationships between leaf traits and remote sensing signals. The relationship between biochemical parameters and leaf spectral reflection has been widely established through PROSPECT, while surface structural parameters can be linked to leaf multi-angular reflection through Bidirectional Reflectance Distribution Function (BRDF) models. However, it's still challenging to characterize how leaf spectra response changes with different illumination and viewing geometries, in connection to both biochemical and surface structural traits. To address this challenge, we propose the model PROSPECULAR, which combines PROSPECT with a specular function determined by illumination-viewing geometry parameters, wavelength-dependent refractive index, and roughness. This model simulates spectral and multi-angular BRF and retrieves both leaf biochemical and surface structural parameters. The PROSPECULAR accounts for the wavelength dependence of the leaf surface refractive index in the specular function and separates the specular reflection from PROSPECT in the diffuse component calculation. The accuracy of PROSPECULAR's spectral simulation was assessed using a multi-angular dataset measured in the hemispherical space (42 viewing zenith angles (VZAs) at 20–50° source zenith angles (SZAs)). Then, the inversion accuracy of PROSPECULAR was validated using a multi-angular dataset measured in the principal plane (13 VZAs and 20–50° SZAs). The results showed that PROSPECULAR was able to simulate multi-angular BRF (RRMSE = 16.27%) when SZA was <50° and retrieve a unified set of biochemical parameters for each leaf sample using multi-angular BRF (RMSE of chlorophyll, water, dry matter, protein, and carbon-based constituents were 11.31 μg/cm2, 0.0029 g/cm2, 0.0022 g/cm2, 0.00096 g/cm2, and 0.0028 g/cm2, respectively). The agreement between the calculations and measurements of multi-angular BRF for various plant species indicated that PROSPECULAR is an effective leaf optical model. In contrast, the existing radiative transfer models (PROSPECT, PROCWT, PROCOSINE, and PROSDM) showed varying levels of accuracy in inverting biochemical parameters, depending on the VZA. The advantage of PROSPECULAR is its potential for characterizing both spectral and directional properties of leaves at arbitrary given illumination-viewing geometry and inverting leaf parameters using BRF data from several VZAs in the hemispherical space.

Thermal Design of the Conduction-Cooled High Temperature Superconducting Magnet for Pulsating Magnetic Field

A conduction-cooled high temperature super-conducting (HTS) solenoid is fabricated to produce pulsat-ing 4 T magnetic field for magnetic refrigeration. HTS con-ductor of 245 m is wound as a solenoid with 112 layers each of which consists of 12 turns of the conductor. For a stable ramping operation of the magnet, AC loss inside the wind-ing pack should be effectively rejected to the heat sink. The thermal drains are inserted between the layers and con-nected to the magnet flange which is cooled by a commercial two-stage Gifford-McMahon cooler. They are made of Ox-ygen-Free High Conductivity (OFHC) copper, and has a width of 6 mm and a thickness of 50 μm. Thermal analysis is conducted to confirm the effect of the thermal drain on the AC loss heat rejection. AC loss of the magnet is esti-mated by using T-A formulation (current vector potential T and magnetic vector potential A). The ramp rate of the mag-net and the heat sink temperature are set to 0.1 T/s and 4.5 K, respectively. In the thermal diffusion calculation, each winding layer is considered as the lumped capacitance. In addition, thermal conduction between the adjacent lay-ers and that through the thermal drains are considered. The thermal analysis confirms that the conductor temperature of the winding is maintained lower than the operating limit of 20 K. Based on the results of the thermal analysis, the stable pulsating operation of the conduction-cooled super-conducting magnet is predicted to be feasible and is also confirmed by the experiment.