High-temperature Transport Research Articles

High temperature stability and transport characteristics of hydrogen in alumina via multiscale computation

The impact of hydrogen charge states on the stability and transport characteristics of hydrogen interstitials in alumina polymorphs is evaluated by multiscale computational methods including density functional theory (DFT), ab initio molecular dynamics (AIMD) and machine learned force fields. Thermodynamic calculations show that the protonic Hi+1 interstitial is the most stable defect species for most values of the electronic bandgap in both α and amorphous alumina (Al2O3). Further, active learned Gaussian approximation potentials (GAP) were developed using AIMD data to study temperature dependent long time proton diffusion in alumina. Diffusivity calculations from GAP-MD simulations are found to be comparable with of the AIMD data, while being ∼340 times faster and scalable to larger systems. Comparisons with diffusivity values for other interstitial charge states (Hi0 and Hi−1) and published experimental literature indicate that Hi+1 diffusion is the likely mechanism of hydrogen transport. A good agreement is obtained between Hi+1 diffusivity calculated in α-Al2O3 from DFT: 5.05 × 10−3 exp(-0.81 eV/kB/T) cm2/s and reported experiment: 9.7 ×10−4 exp(-0.83 eV/kB/T) cm2/s. Computationally and experimentally calculated energy barriers (0.81 and 0.83 eV respectively) only differ by 2.5%. Similarly, the pre-exponential diffusion coefficients only differ by 0.5 orders of magnitude. Moreover, the diffusivity of Hi+1 in amorphous Al2O3 in the 1000–2000 K range is calculated to be 2.53 × 10−2 exp(-0.89 eV/k B/T), just one order of magnitude higher than the corresponding value in α-Al2O3. This suggests that local structural disorder does not significantly affect the energy landscape and diffusion behavior of Hi+1 in Al2O3. Overall, these results show promise for the application of alumina polymorphs as hydrogen permeation barriers.

Evidence of a room-temperature quantum spin Hall edge state in a higher-order topological insulator

Room-temperature realization of macroscopic quantum phases is one of the major pursuits in fundamental physics1,2. The quantum spin Hall phase3-6 is a topological quantum phase that features a two-dimensional insulating bulk and a helical edge state. Here we use vector magnetic field and variable temperature based scanning tunnelling microscopy to provide micro-spectroscopic evidence for a room-temperature quantum spin Hall edge state on the surface of the higher-order topological insulator Bi4Br4. We find that the atomically resolved lattice exhibits a large insulating gap of over 200 meV, and an atomically sharp monolayer step edge hosts an in-gap gapless state, suggesting topological bulk-boundary correspondence. An external magnetic field can gap the edge state, consistent with the time-reversal symmetry protection inherent in the underlying band topology. We further identify the geometrical hybridization of such edge states, which not only supports the Z2 topology of the quantum spin Hall state but also visualizes the building blocks of the higher-order topological insulator phase. Our results further encourage the exploration of high-temperature transport quantization of the putative topological phase reported here.

High-Temperature Electronic Transport Properties of PEDOT:PSS Top-Contact Molecular Junctions with Oligophenylene Dithiols

In this study, we investigated the high-temperature electronic transport behavior of spin-coated PEDOT:PSS top-contact molecular ensemble junctions based on self-assembled monolayers (SAMs) of oligophenylene dithiols. We observed irreversible temperature-dependent charge transport at the high-temperature regime over 320 K. The effective contact resistance and normalized resistance decreased with increasing temperature (320 to 400 K), whereas the tunneling attenuation factor was nearly constant irrespective of temperature change. These findings demonstrate that the high-temperature transport properties are not dominated by the integrity of SAMs in molecular junctions, but rather the PEDOT:PSS/SAMs contact. Transition voltage spectroscopy measurements indicated that the contact barrier height of the PEDOT:PSS/SAMs is lowered at elevated temperatures, which gives rise to a decrease in the contact resistance and normalized resistance. The high-temperature charge transport through these junctions is also related to an increase in the grain area of PEDOT cores after thermal treatment. Moreover, it was found that there was no significant change in either the current density or normalized resistance of the annealed junctions after 60 days of storage in ambient conditions.

High-temperature transport properties of BaSn1−xScxO3−δ ceramic materials as promising electrolytes for protonic ceramic fuel cells

Protonic ceramic fuel cells (PCFCs) offer a convenient means for electrochemical conversion of chemical energy into electricity at intermediate temperatures with very high efficiency. Although BaCeO3- and BaZrO3-based complex oxides have been positioned as the most promising PCFC electrolytes, the design of new protonic conductors with improved properties is of paramount importance. Within the present work, we studied transport properties of scandium-doped barium stannate (Sc-doped BaSnO3). Our analysis included the fabrication of porous and dense BaSn1−xScxO3−δ ceramic materials (0 ⩽ x ⩽ 0.37), as well as a comprehensive analysis of their total, ionic, and electronic conductivities across all the experimental conditions realized under the PCFC operation: both air and hydrogen atmospheres with various water vapor partial pressures (p(H2O)), and a temperature range of 500–900 °C. This work reports on electrolyte domain boundaries of the undoped and doped BaSnO3 for the first time, revealing that pure BaSnO3 exhibits mixed ionic-electronic conduction behavior under both oxidizing and reducing conditions, while the Sc-doping results in the gradual improvement of ionic (including protonic) conductivity, extending the electrolyte domain boundaries towards reduced atmospheres. This latter property makes the heavily-doped BaSnO3 representatives attractive for PCFC applications.

Second harmonic helicity and Faraday rotation in gated single-layer 1T′−WTe2

Single-layer of 1T$'$ phase of WTe$_2$ provides a rich platform for exotic physical properties such as nonlinear Hall effect and high-temperature quantum spin hall transport. Utilizing a continuum model and diagrammatic method, we calculate the second harmonic conductivity of monolayer 1T$'$-WTe$_2$ modulated by an external vertical electric field and electron doping. We obtain a finite helicity and Faraday rotation for the second harmonic signal in response to linearly polarized incident light in the presence of time-reversal symmetry. The second harmonic signal's helicity is highly controllable by altering the bias potential and serves as an optical indicator of the nonlinear Hall current. Our study motivates future experimental investigation for the helicity spectroscopy of two dimensional materials.

Nonideal defect structure and high-temperature transport properties of misfit-layered cobalt oxide

In the study, the oxygen nonstoichiometry (δ), electrical conductivity (σ), and thermal expansion of misfit cobalt oxide (Ca3Co4O9+δ) were measured at various temperatures (RT ​≤ ​T/°C ​≤ ​850) and oxygen partial pressures (3.16 ​× ​10−5 ​≤ ​Po2/atm ≤ 10°). The dense specimens used in the experiment were produced through spark plasma sintering. Generally, the defect structure of the nonstoichiometric compound exhibits a negative deviation as defect concentration increases owing to the Coulomb attraction of oppositely charged point defects. However, δ and σ deviated positively from the ideal defect structure. For a positive deviation, the nonideal defect structure and the degenerated holes could accurately explain the physical properties. The activity coefficient of holes from the Fermi-Dirac integration using the Joyce–Dixon approximation increased from 2.5 to 11 as δ increased from 0.09 to 0.175. The anisotropic nature was confirmed and the thermal expansion showed orientation dependence (in-plane: 12 ppmK-1, out-of-plane: 13.1 ppmK-1). Chemically induced expansion was not observed regardless of the value of δ or the orientation.

Spin dynamics of the one-dimensional double chain spin- 12 antiferromagnet KNaCuP2O7

We combine a muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) technique with thermodynamic measurements to explore the spin dynamics of one-dimensional (1D) $S=\frac{1}{2}$ antiferromagnetic double chain ${\mathrm{KNaCuP}}_{2}{\mathrm{O}}_{7}$. Static magnetic susceptibility and specific heat are well described by a uniform 1D spin chain model with the intrachain interaction $J/{k}_{B}\ensuremath{\approx}55\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and small interchain interactions. Spin excitations probed by zero-field $\ensuremath{\mu}\mathrm{SR}$ evince that high-temperature diffusive spin transport turns into ballistic behavior with decreasing temperature below 30 K. In addition, we observe that longitudinal-field $\ensuremath{\mu}\mathrm{SR}$ varies hardly with an external magnetic field. The field-independent dynamical spin susceptibility disagrees with diffusive or ballistic behaviors.

Performance Projection of a High-Temperature CO2 Transport Membrane Reactor for Combined CO2 Capture and Methane-to-Ethylene Conversion

Direct conversion of methane into ethylene through the oxidative coupling of methane (OCM) is a technically important reaction. However, conventional co-fed fixed-bed OCM reactors still face serious challenges in conversion and selectivity. In this paper, we apply a finite element model to simulate OCM reaction in a plug-flow CO2/O2 transport membrane (CTM) reactor with a directly captured CO2 and O2 mixture as a soft oxidizer. The CTM is made of three phases: molten carbonate, 20% Sm-doped CeO2, and LiNiO2. The membrane parameters are first validated by CO2/O2 flux data obtained from CTM experiments. The OCM reaction is then simulated along the length of tubular plug-flow reactors filled with a La2O3-CaO-modified CeO2 catalyst bed, while a mixture of CO2/O2 is gradually added through the wall of the tubular membrane. A 12-step OCM kinetic mechanism is considered in the model for the catalyst bed and validated by data obtained from a co-fed fixed-bed reactor. The modeled results indicate a much-improved OCM performance by membrane reactor in terms of C2-yield and CH4 conversion rate over the state-of-the-art, co-fed, fixed-bed reactor. The model further reveals that improved performance is fundamentally rooted in the gradual methane conversion with CO2/O2 offered by the plug-flow membrane reactor.

Exploring temperature dependent electron–electron interaction of rocksalt SnS and SnSe within Matsubara-time domain

Both experimental and theoretical studies show non-trivial topological behaviour in native rocksalt phase for SnS and SnSe and categorize these materials in topological crystalline insulators. Here, the detailed electronic structures studies of SnS and SnSe in the rocksalt phase are carried out using many-body GW based theory and density functional theory both for ground states and temperature dependent excited states. The estimated values of fundamental direct bandgaps around L-point using G 0 W 0 (mBJ) are ∼0.27 (∼0.13) eV and ∼0.37 (∼0.17) eV for SnS and SnSe, respectively. The strength of hybridization between Sn 5p and S 3p (Se 4p) orbitals for SnS (SnSe) shows strong k-dependence. The behaviour of (ω), which is the averaged value of diagonal matrix elements of fully screened Coulomb interaction, suggests to use full-GW method for exploring the excited states because the correlation effects within these two materials are relatively weak. The temperature dependent electronic structure calculations for SnS and SnSe provide linearly decreasing behaviour of bandgaps with rise in temperatures. The existence of collective excitation of quasiparticles in form of plasmon is predicted for these compounds, where the estimated values of plasmon frequency are ∼9.5 eV and ∼9.3 eV for SnS and SnSe, respectively. The imaginary part of self-energy and mass renormalization factor (Z k (ω)) due to electron–electron interaction (EEI) are also calculated along W–L–Γ direction for both the materials, where the estimated ranges of Z k (ω) are 0.70 to 0.79 and 0.71 to 0.78 for SnS and SnSe, respectively, along this k-direction. The present comparative study reveals that the behaviour of temperature dependent EEI for SnS and SnSe are the almost same and EEI is important for high temperature transport properties.

Thermophysical and Transport Properties of the BaCl2-CsCl High Temperature Ionic Liquid Mixtures

High temperature ionic liquids (HTILs) densities and transport properties for mixtures BaCl2‑CsCl, x(BaCl2) = 0-1, have been studied as a function of composition and temperature. In terms of Arrhenius theory, the temperature correlation of all measured properties was made and discussed. Thermodynamic properties (isothermal compressibility, molecular volume, lattice energy, heat capacity, molar Gibbs energy, enthalpy and entropy) were derived for all the studied HTILs from experimental data. The viscosity isotherms show negative deviations from linearity, while conductivity isotherms have positive deviations which may be related to the formation of highly negative changed ion associated species. The evolution of the excess quantities: viscosity deviation (Δη), excess molar viscosity (ΔEη), excess molar conductivity (ΔEκ), show a very good parallelism. The linear behavior between conductivity and viscosity was determined using the fractional Walden rule and the average slope was found far from unity.

Dual nature of high-temperature electronic transport in layered perovskite-like cobaltites: exhaustive consideration of experimental features observed.

Complex oxides with the general formula Pr1-xYxBaCo2-yNiyO6-δ (x = 0, 0.1, y = 0, 0.2) were successfully synthesized via combustion of organo-metallic precursors. The high-temperature dc conductivity of the obtained sintered materials was studied in a wide range of oxygen partial pressures and temperatures by means of the 4-probe method. The resulting dependencies were juxtaposed with the previously published data on oxygen non-stoichiometry for the oxides considered. The comprehensive analysis of these datasets in attempts to explain the observed trends has shown the large inadequacy of currently existing conduction models. Consequently, a new model approach was developed to account for the numerous experimental and theoretical peculiarities being characteristic for the cobaltites with a layered double perovskite structure. One of the key propositions made postulates mixed nature of the band structure for materials studied with spin states of Co ions acting as a spatial descriptor of a particular type of conductivity: semiconducting or a metallic one. The abovementioned hypothesis was validated by magnetic, thermodynamic and structural arguments obtained both theoretically and experimentally. The models suggested were shown to be adequate in describing large arrays of data collected. Additionally, the reasons behind doping and temperature/pressure influences on the respective conductivity changes in Pr1-xYxBaCo2-yNiyO6-δ were uncovered. Transport and thermodynamic parameters determined were used to evaluate transference numbers and mobilities of different charge carriers which revealed the dominating role of metallic conductivity under oxidative conditions and the superiority of semiconducting charge transport in reducing environments. The obtained conclusions were further supported by utilizing the derived model equations for successful description of conductivity/non-stoichiometry data for other layered cobaltites. Also, interesting correlations between cation composition, thermodynamic and transport properties were found. Finally, general review of the formulated approach was made and further research directions were proposed.

Long-Range Cationic Order Collapse Triggered by S/Cl Mixed-Anion Occupancy Yields Enhanced Thermoelectric Properties in Cu5Sn2S7

To investigate pathways to adjust the charge carrier concentration and optimize the thermoelectric properties, we characterized structural properties, thermal stability, and thermoelectric performance of pristine and Cl-doped Cu5+εSn2−εS7. We demonstrate that Cl doping in Cu5Sn2S7-type monoclinic compounds induces a collapse of the long-range cationic ordering, ultimately leading to a sphalerite-type cubic phase characterized by ordered [Sn(S,Cl)4]x clusters. The change in crystal structure symmetry upon Cl doping is analyzed by Rietveld refinements against X-ray powder diffraction data, transmission electron microscopy, Mössbauer and X-ray absorption spectroscopy, and low- and high-temperature transport property measurements. The thermoelectric properties of the so-obtained cubic sphalerite Cu5+εSn2−εS7–yCly (0 ≤ ε ≤ 0.133, y = 0.35, 0.70) are strongly enhanced with respect to the undoped Cu5Sn2S7: the power factor improves slightly while both electronic and lattice contributions to the thermal conductivity are reduced. Overall, single-phase Cl-doped Cu5.133Sn1.866S7–yCly (y = 0.35, 0.70) compounds exhibit high thermoelectric performance, reaching a maximum ZT of 0.45 at 670 K.

Preparation and Characterization of Pure SiC Ceramics by HTPVT Induced by Seeding with SiC Nanoarrays.

Dense SiC ceramics were fabricated by high-temperature physical vapor transport (HTPVT) growth process using SiC nanoarrays as the crystal seeds, which was obtained by vacuum heat treatment of amorphous SiC films prepared by plasma-enhanced chemical vapor deposition (PECVD) with a porous anodic aluminum oxide (AAO) template. In the HTPVT process, two-step holding was adopted, and the temperature at the first step was controlled at 2100 and 2150 °C to avoid SiC nanoarrays evaporation, and the grain size of SiC crystal increased with the increase in temperature and decrease in the pressure of Ar. The temperature of the second step was 2300 °C, and rapid SiC grain growth and gradual densification were achieved. The prepared SiC ceramics exhibited a relative density of more than 99%, an average grain size of about 100 μm, a preferred orientation along the (0 0 0 6) plane, a Vickers hardness of about 29 GPa, a flexural strength of about 360 MPa, and thermal conductivity at room temperature of more than 200 W·m−1·K−1.

Effects of boron doping on the fabrication of dense 6H-SiC ceramics by high-temperature physical vapor transport

In this paper, boron-doped dense 6H-SiC ceramics was fabricated by the high-temperature physical vapor transport (HTPVT) method. The effect of B doping on the crystal structure stability of 6H-SiC was investigated based on density functional theory (DFT). The results show that B doping can be realized even under thermodynamical equilibrium conditions. Nevertheless, it is found that the B doping effects on the (0001) of Si-plane and (000-1) of C-plane are significantly different. The doping experiments demonstrated that B can observably change the crystal growth morphology, leading to the formation of elongated 6H-SiC crystals.

Oxidative stress in broiler chicken and its consequences on meat quality

Oxidative stress is inevitable in poultry production, and it affects the physiological, behavioral and biochemical status of growing chicken which ultimately deteriorates meat quality. Appearance, texture, juiciness, tenderness and odor are responsible for the overall meat quality as they are essential perceptible features, which determine the consumer’s judgment. Overproduction of free radicals including reactive oxygen species (ROS) disturbs the mitochondrial function in living cells. During high-temperature mitochondrial substrate oxidation and electron transport chain (ETC) activity increases. This increased activity results in excessive production of superoxide that oxidizes protein and lipid contents in muscle tissues. By oxidizing protein and lipid, ROS spoils the nutritive quality of chicken meat. High ambient temperature is one of the major contributing factors that enhance oxidative stress. Poultry feed with anti-oxidant supplementation and innovative processing techniques can help the poultry industry to overcome oxidative stress.

Asymmetric misfit nanotubes: Chemical affinity outwits the entropy at high-temperature solid-state reactions

Asymmetric two-dimensional (2D) structures (often named Janus), like SeMoS and their nanotubes, have tremendous scope in material chemistry, nanophotonics, and nanoelectronics due to a lack of inversion symmetry and time-reversal symmetry. The synthesis of these structures is fundamentally difficult owing to the entropy-driven randomized distribution of chalcogens. Indeed, no Janus nanotubes were experimentally prepared, so far. Serendipitously, a family of asymmetric misfit layer superstructures (tubes and flakes), including LaX-TaX2 (where X = S/Se), were synthesized by high-temperature chemical vapor transport reaction in which the Se binds exclusively to the Ta atoms and La binds to S atoms rather than the anticipated random distribution. With increasing Se concentration, the LaS-TaX2 misfit structure gradually transformed into a new LaS-TaSe2-TaSe2 superstructure. No misfit structures were found for xSe = 1. These counterintuitive results shed light on the chemical selectivity and stability of misfit compounds and 2D alloys, in general. The lack of inversion symmetry in these asymmetric compounds induces very large local electrical dipoles. The loss of inversion and time-reversal symmetries in the chiral nanotubes offers intriguing physical observations and applications.

High-temperature phonon transport properties of SnSe from machine-learning interatomic potential

As a promising thermoelectric material, tin selenide (SnSe) is of relatively low thermal conductivity. However, the phonon transport mechanisms in SnSe are not fully understood due to the complex phase transition, dynamical instability, and strong anharmonicity. In this work, we perform molecular dynamics simulations with a machine-learning interatomic potential to explore the thermal transport properties of SnSe at different temperatures. The developed interatomic potential is parameterized using the framework of moment tensor potential, exhibiting satisfactory predictions on temperature-dependent lattice constants and phonon dispersion, as well as phase transition temperature. From equilibrium molecular dynamics simulations, we obtained the thermal conductivity tensor from 200 K to 900 K. The origins of temperature-dependent thermal conductivity anisotropy and the roles of four-phonon scatterings are identified. The obtained interatomic potential can be utilized to study the mechanical and thermal properties of SnSe and related nanostructures in a wide range of temperatures.

The impact of atomic defects on high-temperature stability and electron transport properties in Sr2Mg1−xNixMoO6–δ solid solutions

The computational modeling within a density functional theory was applied for simulations of electronic spectra and calculations of structural and energy characteristics of the cubic double perovskite oxides Sr2Mg1−xNixMoO6–δ, where x = 0, 0.5, and 1. The oxygen stoichiometric molybdates are antiferromagnetic semiconductors with an energy gap near 2 eV. The energy-based arguments show that anti-site cation disorder may contribute to the structural stability of the molybdates. It is found that nickel doping is favorable for mitigated chemical expansion. The replacement of magnesium by nickel is accompanied by the contribution of Ni3d states to the valence band while leaving hybrid Mo4d-O2p states in the conduction band virtually unchanged. It is shown that the compounds under study are thermodynamically unstable in heavily reducing conditions which is confirmed by experimental results. The appearance of oxygen deficiency in Sr2Mg1−xNixMoO6–δ results in the formation of oxygen vacancy associated donor states near the bottom of the conduction band and the transition from the intrinsic to degenerate semiconductor. It is suggested that the influence of nickel dopants on the energy and density of the donor states may help to explain variations of the conducting properties with doping level.

Investigating the effect of temperature dependent many-body interactions on electronic structures of SnTe in the Matsubara-time domain

Recently, SnTe has gained attention due to its non-trivial topological nature and eco-friendly thermoelectric applications. We report a detailed temperature dependent electronic structure of this compound using DFT and GW methods. The calculated values of bandgaps by using PBEsol and G 0 W 0 methods are found to be in good agreement with the experiment, whereas mBJ underestimates the bandgap. The averaged value of diagonal matrix elements of fully screened Coulomb interaction () at ω = 0 eV for Sn (Te) 5p orbitals is ∼1.39 (∼1.70) eV. The nature of frequency dependent reveals that the correlation strength of this compound is relatively weaker and hence the excited electronic state can be properly studied by full-GW many-body technique. The plasmon excitation is found to be important in understanding this frequency dependent . The temperature dependent electron–electron interactions (EEI) reduces the bandgaps with increasing temperature. The value of bandgap at 300 K is obtained to be ∼161 meV. The temperature dependent lifetimes of electronic state along W–L–Γ direction are also estimated. This work suggests that EEI is important to explain the high temperature transport behaviour of SnTe.

Thermoelectric Performance Enhancement of the Cost-Effective Phosphide ZnCu2P8

A facile bulk ball milling synthesis for the abundant and inexpensive compound ZnCu2P8 was developed. Stoichiometries of Zn1–xCu2+xP8 (x = −0.25, 0, 0.1, 0.25) were synthesized and characterized through X-ray diffraction, EDS, diffuse reflectance, and high-temperature transport properties. The series exhibits low thermal conductivities near 1.0 W m–1 K–1 at 673 K and an enhancement of zT up to 0.25, a 150% increase compared to the parent phase. The measured thermoelectric properties are within the range of the best performing phosphides; however, a more in-depth study on the control of the phosphorus stoichiometry is required to realize full thermoelectric potential.