Transport Properties Of Materials Research Articles

Preface: CEST 2023

The Central European Symposium on Thermophysics (CEST) 2023 is a 5th scientific meeting whose scopes are thermophysical properties of materials, heat transfer, and the measurement of thermophysical and other transport properties of materials. It is a successor of the international conference Thermophysics that was organized annually since 1996. While Thermophysics was well-established and of long tradition, CEST provides a novel format in which one of the four co-chairs (R. Černý, Á. Lakatos, Z. Suchorab, and A. Trník) acts each year as the CEST chair and organizes the symposium in a different country. In 2023 it is jointly organized by• Department of Physics, Faculty of Natural Sciences and Informatics, Constantine the Philosopher University in Nitra, Slovakia.List of Committees, Organizers are available in the pdf.

The Impact of Polymorphism on Charge Transport Properties for Pyrene-FxTCNQ Cocrystals

Research on the charge transport properties of organic semiconductor materials is crucial for the development of organic sensors. In this paper, we investigated the charge transport properties of pyrene-TCNQ, pyrene-F2TCNQ, and pyrene-F4TCNQ cocrystals using the quantum nuclear tunneling model and Monte Carlo simulations to calculate charge mobility. At room temperature, we observed hole mobility values is 0.658 cm2V-1s-1 and electron mobility values is 1.492 cm2V-1s-1, respectively, for pyrene-TCNQ. As the number of F atoms in TCNQ increases, we observed an increase in both hole and electron mobility. Our results showed that the electron to hole mobility ratio was approximately 2.2 for pyrene-TCNQ, 0.6 for pyrene-F2TCNQ, and 0.5 for pyrene-F4TCNQ. Our findings suggest that the introduction of F atoms causes changes in the TCNQ molecular structure, cocrystal stacking mode, adjacent molecular distance, and intermolecular polarization. These changes lead to a modification of the reorganization energy and transfer integrals, ultimately affecting charge transport. Additionally, our results showed that transport along the π-π direction in the three cocrystals exhibited strong anisotropic characteristics, which were much larger than those observed in the other two directions.

Reactive transport modeling for assessing coupled hydrogeochemical processes at interfaces in deep geological repositories: from the laboratory to the real world

Abstract. Deep geological repositories with a multi-barrier concept are foreseen by various countries for the disposal of high-level radioactive waste. Simulation tools for a close-to-reality description of repository evolution scenarios are required, especially to resolve the challenging task of comparing and assessing the long-term safety of different repository concepts in different host rocks within the German site-selection process. Chemical, thermal, and pressure gradients at the interfaces of the different barriers in a repository can lead to mineral dissolution and precipitation, generating non-linear responses in transport and mechanical properties of barrier materials and host rocks. Reactive transport modeling (RTM) can be applied to investigate these perturbations and processes across temporal and spatial scales to assess subsurface evolution. Nevertheless, implementing RTM at the continuum scale while accounting for pore-scale heterogeneities and geometry evolution remains a challenge. Pore-scale simulations offer the potential to capture the complex evolution of porous media over a broad range of Peclet and Damköhler numbers, and they can be utilized to improve the RTM by using upscaling methods (Prasianakis et al., 2020). In this context, we developed “lab-on-a-chip” experiments, combining time-lapse high-resolution optical microscopy and confocal Raman spectroscopy to test extended constitutive equations to classically employed Archie's law to improve the description of changes in transport properties (e.g., diffusivity) in evolving porous media in RTM. The 3D Raman tomography of the porous media combined with pore-scale modeling enabled the derivation of upscaled transport parameters. Our results highlight the importance of calibrating pore-scale models with quantitative experiments prior to simulations over a wide range of Peclet and Damköhler numbers, whose results can be further used for the derivation of upscaled modeling parameters. The derived and parameterized constitutive equations based on simple systems have been tested in reactive transport models as a sensitivity case study to evaluate uncertainties in the predictions of the evolution of real subsurface systems such as clay–cement interfaces in deep geological repositories.

Reliable Lab-Scale Evaluation for Enhanced SOFCs Electrode Performance - Influence of Electronic Transport

Understanding and optimizing electrochemically active zone for oxygen reduction reaction (ORR) in solid oxide fuel cells (SOFCs) cathodes are indispensable to maximize device’s performance. For a mixed ionic and electronic conducting electrode, the range of this zone depends on the oxygen surface exchange and solid-state oxygen/electron transport properties of electrode materials and the oxygen gas transport through porous electrode. This implies that electrode microstructure, including surface area, porosity, and tortuosity, needs to be carefully designed and controlled to maximize electrochemically active electrode volume. This aspect is also critical to realize valid characterization of the electrode’s electrochemical activities. For instance, when a material’s electronic conductivity is low or the electrical contact points are spaced beyond a critical distance, the electrode polarization resistance measured by electrochemical impedance spectroscopy will be controlled by the sheet resistance not the oxygen exchange resistance. In this presentation, we will discuss our synthesis approach of lanthanum strontium cobalt ferrite (LSCF) and spinel powders by using co-precipitation with a continuous stirred-tank reactor, in order to precisely control powder’s chemistry and morphology (ultimately, electrode microstructure). And, we will discuss the influence of electrode microstructure and current collecting method on electronic conduction in porous electrode and, eventually, on electrochemical performance and its reliable evaluation in a lab-scale button cell configuration.This work was supported by the US Department of Energy (DOE), Office of Fossil Energy, (Office of Fossil Energy & Carbon Management), “Solid Oxide Fuel Cell Manufacturing in Support of Office of Fossil Energy” program through Argonne National Laboratory under FWP No. 27327.1.

KMCpy: A python package to simulate transport properties in solids with kinetic Monte Carlo

Understanding ion transport in functional materials is crucial to unravel complex chemical reactions, improving the rate performance of materials for energy storage and conversion, and optimizing catalysts. To model ion transport, atomistic simulations, including molecular dynamics (MD) and kinetic Monte Carlo (kMC) have been developed and applied. However, kMC simulations are utilized to a lower extent than MDs due to a lack of systematic workflows to construct models for predicting transition rates. Here, we present kMCpy, a lightweight, customizable, and modular python package to compute the ionic transport properties in crystalline materials using kMC. kMCpy is remarkably versatile and user-friendly, making it a powerful code for studying materials′ kinetics in crystalline systems. kMCpy can be combined with (local) cluster expansion Hamiltonians derived from first-principles calculations. kMCpy is versatile with respect to any type of crystalline material, bearing any dimensionality, i.e., 1D, 2D, and 3D. kMCpy provides (i) a comprehensive workflow to enumerate all possible migration events in crystalline systems, (ii) to derive transition rates efficiently and at the accuracy of first-principles calculations, and (iii) a robust kMC solver to study kinetic phenomena in materials. The workflow implemented in kMCpy provides a systematic way to compute highly accurate kinetic properties. Hence, kMCpy can be used in high-throughput simulations for the discovery and optimization of novel functional materials.

Stochastic and mixed density functional theory within the projector augmented wave formalism for simulation of warm dense matter.

Stochastic density functional theory (DFT) and mixed stochastic-deterministic DFT are burgeoning approaches for the calculation of the equationof state and transport properties in materials under extreme conditions. In the intermediate warm dense matter regime, a state between correlated condensed matter and kinetic plasma, electrons can range from being highly localized around nuclei to delocalized over the whole simulation cell. The plane-wave basis pseudopotential approach is thus the typical tool of choice for modeling such systems at the DFT level. Unfortunately, stochastic DFT methods scale as the square of the maximum plane-wave energy in this basis. To reduce the effect of this scaling and improve the overall description of the electrons within the pseudopotential approximation, we present stochastic and mixed DFT approaches developed and implemented within the projector augmented wave formalism. We compare results between the different DFT approaches for both single-point and molecular dynamics trajectories and present calculations of self-diffusion coefficients of solid density carbon from 1 to 50eV.

A semi-analytic method for the computation of the effective properties of composites of two isotropic constituent materials

We develop a computational framework for the semi-analytic representation of the effective transport properties of two-component composite materials, in the spirit of Bergman’s spectral representation. The components of the effective permittivity (or conductivity) tensor are expanded as power series in a contrast parameter between the two phases. The coefficients (the moments of a spectral measure) of these expansions are determined recursively through the numerical evaluations of integrals defined solely on the boundary between the constituent phases, and discretized by high-order (possibly exponentially converging) quadrature schemes. The high precision reached by these coefficients allows the series representation to be recast into Padé approximants (or continued fractions) to provide analytical expressions valid over large (possibly infinite) regions of the parameter complex plane. Examples, thus far limited to two-dimensional rhombic unit cells, demonstrate that the method is reliable for microgeometries and material parameters for which other methods are far less dependable. The analytical representations are particularly useful to study the optical or electrostatic resonant behaviour of some composite materials.

High Thermoelectric Performance in Cu2SnS3 by Control Over Phase‐Dependent Mobility Edge

AbstractComplex thermoelectric materials composed of multiple phases with order/disorder mixing feature have drawn intense attention due to their high performance. However, some physical mechanisms relevant to thermoelectric properties including the relationship between carrier mobility and disorder extent in these materials are still elusive. Here, a phase‐dependent mobility edge in (Cu2Sn1‐yS3)0.85(CuGaS2)0.15 solid solutions is demonstrated. By control over the mobility edge in these solid solutions by increasing the ratio of tetragonal phase with less disorder, a weakened localization of electronic states and promoted carrier mobility are realized. Combined with reduced lattice thermal conductivity, a record high zT value of 1.22 is attained at 823 K for (Cu2Sn0.92S3)0.85(CuGaS2)0.15 with the lowered mobility edge. The findings shed light on a new route to tuning the electrical and thermal transport properties for complex thermoelectric materials with multiple phases.

Sub-Micrometer Phonon Mean Free Paths in Metal-Organic Frameworks Revealed by Machine Learning Molecular Dynamics Simulations.

Metal-organic frameworks (MOFs) are a family of materials that have high porosity and structural tunability and hold great potential in various applications, many of which require a proper understanding of the thermal transport properties. Molecular dynamics (MD) simulations play an important role in characterizing the thermal transport properties of various materials. However, due to the complexity of the structures, it is difficult to construct accurate empirical interatomic potentials for reliable MD simulations of MOFs. To this end, we develop a set of accurate yet highly efficient machine-learned potentials for three typical MOFs, including MOF-5, HKUST-1, and ZIF-8, using the neuroevolution potential approach as implemented in the GPUMD package, and perform extensive MD simulations to study thermal transport in the three MOFs. Although the lattice thermal conductivity values of the three MOFs are all predicted to be smaller than 1 W/(m K) at room temperature, the phonon mean free paths (MFPs) are found to reach the sub-micrometer scale in the low-frequency region. As a consequence, the apparent thermal conductivity only converges to the diffusive limit for micrometer single crystals, which means that the thermal conductivity is heavily reduced in nanocrystalline MOFs. The sub-micrometer phonon MFPs are also found to be correlated with a moderate temperature dependence of thermal conductivity between those in typical crystalline and amorphous materials. Both the large phonon MFPs and the moderate temperature dependence of thermal conductivity fundamentally change our understanding of thermal transport in MOFs.

Deterministic Current‐Induced Perpendicular Switching in Epitaxial Co/Pt Layers without an External Field

AbstractCurrent‐induced spin‐orbit torques (SOTs) have emerged as a powerful tool to control magnetic elements and non‐uniform magnetic textures such as domain walls and skyrmions. SOT‐induced switching of perpendicular magnetization generally requires an external field to break the rotational symmetry of the spin‐orbit effective fields responsible for the deterministic reversal. The proposed mechanisms to eliminate this requirement often rely on complex multilayer structures that necessitate laborious optimization in the material and spin transport properties, making them less attractive for applications. Herein, current‐induced, external field‐free switching of an epitaxial MgO/Pt/Co trilayer with an extremely large perpendicular anisotropy in excess of 3 Tesla is reported. It is found that switching occurs due to the interplay of strong SOTs, local anisotropy fluctuations, and the Dzyaloshinkii‐Moriya interaction inherent to this epitaxial system. Given that these layers constitute the base stack of a magnetic tunnel junction, this switching mechanism offers the most technologically viable path toward devices such as field‐free SOT‐based magnetic random‐access memories.

Antiferromagnetic spintronics: From metals to functional oxides

Antiferromagnetic spintronics exploits unique properties of antiferromagnetic materials to create new and improved functionalities in future spintronic applications. Here, we briefly review the experimental efforts in our group to unravel spin transport properties in antiferromagnetic materials. Our investigations were initially focused on metallic antiferromagnets, where the first evidence of antiferromagnetic spin-transfer torque was discovered. Because of the lack of metallic antiferromagnets, we then shifted towards antiferromagnetic Mott insulators, where a plethora of transport phenomena was found. For instance, we observed a very large anisotropic magnetoresistance, which can be used to detect the magnetic state of an antiferromagnet. We also observed reversible resistive switching and now provide unequivocal evidence that the resistive switching is associated with structural distortions driven by an electric field. Our findings support the potential of electrically controlled functional oxides for various memory technologies.

Thermal Hall effect and the Wiedemann–Franz law in Chern insulator

Thermal Hall effect, where a transverse temperature difference is generated by implementing a longitudinal temperature gradient and an external magnetic field in the perpendicular direction to systems, is a useful tool to reveal transport properties of quantum materials. A systematic study of the thermal Hall effect in a Chern insulator is still lacking. Here, using the Landauer–Büttiker formula, we investigated the thermal Hall transport of the Harper–Hofstadter model with flux φ = 1/2 and its generalizations. We demonstrated that the Wiedemann–Franz law, which states that the thermal Hall conductivity is linearly proportional to the quantum Hall conductivity in the low temperature limit, is still valid in this Chern insulator, and that the thermal Hall conductivity can be used to characterize the topological properties of quantum materials.

A data-driven modeling approach to quantify morphology effects on transport properties in nanostructured NMC particles

We present a data-driven modeling approach to quantify morphology effects on transport properties in nanostructured materials. Our approach is based on the combination of stochastic modeling of the 3D nanostructure and numerical modeling of effective transport properties, which is used to investigate process-structure–property relationships of hierarchically structured cathode materials for lithium-ion batteries. We focus on nanostructured LiNi1/3Mn1/3Co1/3O2 (NMC) particles, the nanoporous morphology of which has a crucial impact on their effective transport properties (i.e, effective ionic and electric conductivity) and thus on the performance of the cell. First, we develop a parametric stochastic model for the 3D morphology of the nanostructured NMC particles based on excursion sets of so-called χ2-fields. This model, which has only two parameters, is then fitted to FIB-SEM image data of the NMC particles manufactured with different calcination temperatures and different particle sizes. This way it is possible to generate digital twins of the NMC particles. In a second step, measured 3D image data and corresponding digital twins are used as input for the numerical simulation of effective transport properties. Based on the results obtained by these simulations, we can quantify process-structure–property relationships. Overall, we present a methodological framework that allows for an efficient optimization of the fabrication process of nanostructured NMC particles.

Electronic structure, elastic and transport properties of new Palladium-based Half-Heusler materials for thermoelectric applications

Electronic structure with reference to bandgap energies play a significant role in deciding the dimensionless figure of merit (ZT), which in turn is used to identify the thermoelectric performance of a material. Slater Pauling arrangement of valence electrons and band orientations along k-space decide the effective mass of electrons/holes in the corresponding band valleys of PdMX (M= Sc, Y, and X = P, As, Sb) semiconductors. A detailed picture of the electronic structure and transport properties is rigorously established in this report. Calculations of elastic constant, phonon dispersion and formation (cohesive) energy reveal that these alloys are mechanically, dynamically, and chemically stable materials. Substantially, low lattice thermal conductivity is observed in the range 4.57 W/mK for PdScAs and 11.51 W/mK for PdYSb alloy. The relaxation time and effective mass are calculated with Deformation potential theory. The maximum calculated ZT value for PdScP, PdScAs, PdScSb, PdYP, PdYAs and PdYSb are 0.28, 0.33, 0.19, 0.43, 0.44 and 0.27 respectively.

Atomic motifs govern the decoration of grain boundaries by interstitial solutes

Grain boundaries, the two-dimensional defects between differently oriented crystals, tend to preferentially attract solutes for segregation. Solute segregation has a significant effect on the mechanical and transport properties of materials. At the atomic level, however, the interplay of structure and composition of grain boundaries remains elusive, especially with respect to light interstitial solutes like B and C. Here, we use Fe alloyed with B and C to exploit the strong interdependence of interface structure and chemistry via charge-density imaging and atom probe tomography methods. Direct imaging and quantifying of light interstitial solutes at grain boundaries provide insight into decoration tendencies governed by atomic motifs. We find that even a change in the inclination of the grain boundary plane with identical misorientation impacts grain boundary composition and atomic arrangement. Thus, it is the smallest structural hierarchical level, the atomic motifs, that controls the most important chemical properties of the grain boundaries. This insight not only closes a missing link between the structure and chemical composition of such defects but also enables the targeted design and passivation of the chemical state of grain boundaries to free them from their role as entry gates for corrosion, hydrogen embrittlement, or mechanical failure.

Structure Evolution and Bonding Inhomogeneity toward High Thermoelectric Performance in Cu2CoSnS4-xSex Materials.

Lightweight diamond-like structure (DLS) materials are excellent candidates for thermoelectric (TE) applications due to their low costs, eco-friendly nature, and property stability. The main obstacles restricting the energy-conversion performance by the lightweight DLS materials are high lattice thermal conductivity and relatively low carrier mobility. By investigating the anion substitution effect on the structural, microstructural, electronic, and thermal properties of Cu2CoSnS4-xSex, we show that the simultaneous enhancement of the crystal symmetry and bonding inhomogeneity engineering are effective approaches to enhance the TE performance in lightweight DLS materials. Particularly, the increase of x in Cu2CoSnS4-xSex makes the DLS structure with the ideal tetrahedral bond angles of 109.5° favorable, leading to better crystal symmetry and higher carrier mobility in samples with higher selenium content. In turn, the phonon transport in the investigated DLS materials is strongly disturbed due to the bonding inhomogeneity between anions and three sorts of cations inducing large lattice anharmonicity. The increase of Se content in Cu2CoSnS4-xSex only intensified this effect resulting in a lower lattice component of the thermal conductivity (κL) for Se-rich samples. As a result of the enhanced power factor S2ρ-1 and the low κL, the dimensionless thermoelectric figure of merit ZT achieves a high value of 0.75 for Cu2CoSnSe4 DLS material. This work demonstrates that crystal symmetry and bonding inhomogeneity play an important role in the transport properties of DLS materials and provide a path for the development of new perspective materials for TE energy conversion.

Measurement of the conduction band spin-orbit splitting in WSe2 and WS2 monolayers

The conduction band spin-orbit splitting ∆${}_{c}$ has a strong impact on optical, transport, and spin-valley properties of 2D materials based on transition metal dichalcogenide monolayers. Here, the authors study charge tunable devices based on WSe${}_{2}$ and WS${}_{2}$ monolayers. In addition to the well-known radiative recombination of neutral and charged excitons, photoluminescence measurements highlight a weaker intensity optical transition. This line is interpreted as an impurity-assisted recombination of the negatively charged exciton (triplet trion), whose radiative recombination energy differs by ∆${}_{c}$ from that of the trion's direct optically active recombination.

Anisotropic Phonon Bands in H-Bonded Molecular Crystals: The Instructive Case of α-Quinacridone

Phonons play a crucial role in the thermodynamic and transport properties of solid materials. Nevertheless, rather little is known about phonons in organic semiconductors. Thus, we employ highly reliable quantum mechanical calculations for studying the phonons in the α-polymorph of quinacridone. This material is particularly interesting, as it has highly anisotropic properties with distinctly different bonding types (H-bonding, π-stacking, and dispersion interactions) in different spatial directions. By calculating the overlaps of modes in molecular quinacridone and the α-polymorph, we associate Γ-point phonons with molecular vibrations to get a first impression of the impact of the crystalline environment. The situation becomes considerably more complex when analyzing phonons in the entire 1st Brillouin zone, where, due to the low symmetry of α-quinacridone, a multitude of avoided band crossings occur. At these, the character of the phonon modes typically switches, as can be inferred from mode participation ratios and mode longitudinalities. Notably, avoided crossings are observed not only as a function of the length but also as a function of the direction of the phonon wave vector. Analyzing these avoided crossings reveals how it is possible that the highest frequency acoustic band is always the one with the largest longitudinality, although longitudinal phonons in different crystalline directions are characterized by fundamentally different molecular displacements. The multiple avoided crossings also give rise to a particularly complex angular dependence of the group velocities, but combining the insights from the various studied quantities still allows drawing general conclusions, e.g., on the relative energetics of longitudinal vs transverse deformations (i.e., compressions and expansions vs slips of neighboring molecules). They also reveal how phonon transport in α-quinacridone is impacted by the reinforcing H-bonds and by π-stacking interactions (resulting from a complex superposition of van der Waals, charge penetration, and exchange repulsion).

Parametric Study of the Vacuum Permeator for the Tritium Extraction eXperiment

Tritium breeding is a critical component of any self-sustaining future fusion reactor. The liquid-metal eutectic PbLi is of particular interest as a tritium breeder material due to its favorable thermophysical and neutronic properties. One of the several remaining challenges facing PbLi breeder blankets is the need to design and validate a highly efficient tritium extraction system. The vacuum permeator is a promising extraction concept that utilizes tritium permeation through a highly permeable metal membrane. The Tritium Extraction eXperiment (TEX) is a forced-convection PbLi loop constructed to investigate tritium extraction from PbLi with vacuum permeators. Accurate thermal-hydraulic and tritium transport models are required to establish appropriate test matrices, predict experiment outcomes, and analyze data. However, the hydrogen transport properties of PbLi and permeator materials have large uncertainties. A database is collected and a parametric analysis is conducted on the effect of hydrogen transport material properties, including diffusivity of H in PbLi and the permeator, solubility of H in PbLi and the permeator, and the permeator surface recombination constant, on the expected tritium extraction efficiency for a vacuum permeator installed in TEX. Herein, we observe that the solubility of H in PbLi and the permeator and the recombination constant of the permeator have the largest effect on extraction efficiency.

A systematic ab initio study of vacancy formation energy, diffusivity, and ionic conductivity of Ln2NiO4+δ (Ln=La, Nd, Pr)

This study systematically investigates the vacancy formation energy, diffusivity, and ionic conductivity of the Ln2NiO4+δ (Ln = La, Nd, Pr, and δ = 0.125) compound using the ab initio approach. Specifically, the impact of thermal expansion on the oxygen transport properties is considered, using a combination of quasi-harmonic approximation (QHA) and a linear regression model to study and reproduce the temperature-dependent properties of Ln2NiO4+δ. Overall, the predictions are in excellent agreement with previous theoretical studies in the literature. It is confirmed that the ionic transport properties of the Ln2NiO4+δ are not dominated by oxygen vacancy diffusion due to the high vacancy formation energy. Additionally, the interstitialcy mechanism, which involves interstitial O2- hopping parallel to the a-b plane with the rocksalt layer, is determined to be the more favorable diffusion path. Meanwhile, the predicted energy barrier, diffusion coefficient, and ionic conductivity of Ln2NiO4+δ show reasonable agreement with experimental data, with Pr2NiO4+δ exhibiting the lowest activation energy barrier (ΔEb = 0.722 eV) and the highest thermal expansion, diffusivity, and ionic conductivity. Overall, this study presents an efficient and computationally facile tool for predicting ionic transport properties in materials where thermal expansion is the main driving force for temperature-dependent properties.