Temperature-dependent Transport Research Articles

Diavik Waste Rock Project: Simulation of the geochemical evolution of a large test pile using a scaled temperature and sulfide-content dependent reactive transport model

The Diavik Waste Rock Project (DWRP) project included four principal components focused on the development of techniques for assessing the environmental impacts of waste rock at mine sites. These components were small-volume laboratory experiments, intermediate- and large-volume field experiments, and assessment of the operational-scale waste-rock stockpiles, which facilitated characterization of waste-rock weathering at different scales. The heavily instrumented large-scale field experiments (test piles) were constructed to replicate, as closely as practicable, the temperature, water flow, and gas transport regimes of a waste-rock pile that is exposed to annual freezing and thawing cycles and to facilitate characterization of the long-term weathering of a low-sulfide waste rock. An integrated conceptual model of sulfide-bearing waste-rock weathering, developed at the small scale, was applied to assess the capacity of the conceptual model to capture the geochemical evolution of the waste rock at the large field-scale test-pile experiment. The integrated conceptual model was implemented using reactive transport code MIN3P, taking into account scale-dependent mechanisms. The test-pile mineralogy was similar to the small-scale laboratory experiments and included low-sulfide waste rock with an S content of 0.053 wt% (primarily pyrrhotite). The flow regime of the test pile was simulated using parameters measured as part of other DWRP investigations, including temporally variable infiltration estimates that represented the measured precipitation events at the site. The temporally and spatially variable temperature of the test pile was interpolated from values measured using instrumentation installed at the beginning of the experiment and was included in the simulation to refine the temperature dependence of the geochemical reactions. To allow continuous, multi-year simulation, freezing was also simulated to represent the conditions experienced at the test-pile experiment. Normalized root mean square error analysis of the large-scale field experiment simulation results indicated most parameters compare well to measured daily mass flux (i.e., the fraction of the range of annual values encompassed in the residual was less than 0.5 for SO4, Fe, Ni, Si, Ca, K, Mg, Na, and pH and 1.0 or less for all parameters except Cu). The method of using an integrated conceptual model developed from the results of humidity cell experiments to implement a mechanistic approach for assessing the primary geochemical processes of waste-rock weathering on a large scale was shown to provide reasonable results; however, the results are specific to the study site and the approach requires application to various sites under different geological and climatological conditions to facilitate further refinement.

Investigation of novel quaternary Heusler alloys XRuCrZ (X = Co, Ni, Rh, and Pd; Z = Si and Ge) via first-principles calculation for spintronics and thermoelectric applications

The structural, electronic, magnetic, and transport properties of LiMgPdSb-type novel quaternary Heusler alloys XRuCrZ (X = Co, Ni, Rh, and Pd; Z = Si and Ge) have been studied employing the first-principles calculation derived from density functional theory. From the total energy calculations, it is found that all these alloys are stable in the cubic α phase, and spin-polarized calculations show that all the alloys favor a ferromagnetic state. In addition, it is ascertained that XRuCrZ (X = Co and Rh; Z = Si and Ge) exhibits half-metallic ferromagnetic property. In addition, the temperature-dependent transport properties have been studied. The calculated dimensionless figure of merit for CoRuCrSi is 0.43. Our results show that these alloys will be suitable for data storage and energy conversion applications.

Reduction in Urbach energy and density of states for pyrite (FeS2) thin films: Healing of sulfur vacancies during hematite to pyrite transformation

Pyrite, an inexpensive, earth-abundant photovoltaic material has potential to compete with silicon. Unfortunately, pyrite based devices suffer from low power conversion efficiencies and open-circuit voltages, which can be overcome by improved understanding of electronic and optical properties of pyrite. In the electronic and optical properties of pyrite role of sulfur vacancies is very crucial but less explored and still ambiguous. In the present study, effect of sulfur vacancies on the optical and charge transport properties of phase pure pyrite films has been investigated where the sulfur vacancies have been tamed by increasing the sulfurization pressure from subatmospheric to overatomspheric value. Lower Urbach energy in the pyrite films indicates a decrease in density of localized states, which is believed to be due to reduced sulfur vacancies at higher sulfurization pressure. Temperature dependent transport properties reveal the presence of Mott-variable range hoping conduction through the high density of localized states arising from sulfur vacancies. Decreased density of localized states reflects in increased hoping energy associated with longer hoping distance in the pyrite films.

Trap assisted charge transport and Schottky like conduction in CVD-grown amorphous SiC[formula omitted]O[formula omitted]-Si heterojunction

Silicon oxycarbide (SiCxOy), commonly used as low-κ inter layer dielectric in technological applications, is limited in performance due to dielectric breakdown in the presence of defect states. In this work, chemical vapour deposition technique has been used to fabricate carbon rich SiCxOy−n-silicon heterojunction using liquid polycarbosilane as precursor. The heterojunctions showed Schottky diode like behaviour with high current rectification (28k at ± 5 V) and high breakdown voltage (−100 V). Localised defect states originated from free residual carbon in deposited films, arising between valance band maxima and Fermi level of the oxycarbide, were found to be responsible for Schottky diode nature of the fabricated devices. A comprehensive band diagram of the heterojunction, alongside an insight of charge transport process, is presented based on valance band edge spectroscopy and temperature dependent transport measurements. A crossover between Zener and avalanche like soft breakdown was observed in our devices from which effective band gap of the oxycarbide films have been estimated. The devices showed excellent performance stability and reproducibility against rapid cyclic voltage sweep in ambient condition. Stability and performance reproducibility of the heterojunctions were remarkably maintained even after two years of storage in ambient condition that has long term implication in power electronics.

Experimental and computational approaches to study the high temperature thermoelectric properties of novel topological semimetal CoSi

Here, we study the thermoelectric properties of topological semimetal CoSi in the temperature range 300–800 K by using combined experimental and density functional theory (DFT) based methods. CoSi is synthesized using arc melting technique and the Rietveld refinement gives the lattice parameters of a = b = c = 4.445 Å. The measured values of Seebeck coefficient (S) shows the non-monotonic behaviour in the studied temperature range with the value of ∼−81 μV K−1 at room temperature. The |S| first increases till 560 K (∼−93 μV K−1) and then decreases up to 800 K (∼−84 μV K−1) indicating the dominating n-type behaviour in the full temperature range. The electrical conductivity, σ (thermal conductivity, κ) shows the monotonic decreasing (increasing) behaviour with the values of (12.1 W m−1 K−1) and (14.2 W m−1 K−1) Ω−1 m−1 at 300 K and 800 K, respectively. The κ exhibits the temperature dependency as, κ ∝ T 0.16. The DFT based Boltzmann transport theory is used to understand these behaviour. The multi-band electron and hole pockets appear to be mainly responsible for deciding the temperature dependent transport behaviour. Specifically, the decrease in the |S| above 560 K and change in the slope of σ around 450 K are due to the contribution of thermally generated charge carriers from the hole pockets. The temperature dependent relaxation time (τ) is computed by comparing the experimental σ with calculated σ/τ and it shows temperature dependency of 1/T 0.35. Further this value of τ is used to calculate the temperature dependent electronic part of thermal conductivity (κ e) and it gives a fairly good match with the experiment. Present study suggests that electronic band-structure obtained from DFT provides a reasonably good estimate of the transport coefficients of CoSi in the high temperature region of 300–800 K.

Plasmon-Phonon Coupling in Electrostatically Gated β-Ga2O3 Films with Mobility Exceeding 200 cm2 V-1 s-1.

Monoclinic β-Ga2O3, an ultra-wide bandgap semiconductor, has seen enormous activity in recent years. However, the fundamental study of the plasmon-phonon coupling that dictates electron transport properties has not been possible due to the difficulty in achieving higher carrier density (without introducing chemical disorder). Here, we report a highly reversible, electrostatic doping of β-Ga2O3 films with tunable carrier densities using ion-gel-gated electric double-layer transistor configuration. Combining temperature-dependent Hall effect measurements, transport modeling, and comprehensive mobility calculations using ab initio based electron-phonon scattering rates, we demonstrate an increase in the room-temperature mobility to 201 cm2 V-1 s-1 followed by a surprising decrease with an increasing carrier density due to the plasmon-phonon coupling. The modeling and experimental data further reveal an important "antiscreening" (of electron-phonon interaction) effect arising from dynamic screening from the hybrid plasmon-phonon modes. Our calculations show that a significantly higher room-temperature mobility of 300 cm2 V-1 s-1 is possible if high electron densities (>1020 cm-3) with plasmon energies surpassing the highest energy LO mode can be realized. As Ga2O3 and other polar semiconductors play an important role in several device applications, the fundamental understanding of the plasmon-phonon coupling can lead to the enhancement of mobility by harnessing the dynamic screening of the electron-phonon interactions.

First-Principles Calculations of Thermoelectric Transport Properties of Quaternary and Ternary Bulk Chalcogenide Crystals.

Chalcogenide crystals have a wide range of applications, especially as thermoelectric materials for energy conversion. Thermoelectric materials can be used to generate an electric current from a temperature gradient based on the Seebeck effect and based on the Peltier effect, and they can be used in cooling applications. Using first-principles calculations and semiclassical Boltzmann theory, we have computed the Seebeck coefficient, electrical conductivity, electronic thermal conductivity, power factor, and figure of merit of 30 chalcogenide crystals. A Quantum Espresso package is used to calculate the electronic properties and locate the Fermi level. The transport properties are then calculated using the BoltzTraP code. The 30 crystals are divided into two groups. The first group has four crystals with quaternary composition (A2BCQ4) (A = Tl; B = Cd, Hg; C = Si, Ge, Sn; Q = S, Se, Te). The second group contains 26 crystals with the ternary composition (A’B’Q2) (A’ = Ag, Cu, Au, Na; B’ = B, Al, Ga, In; Q = S, Se, Te). Among these 30 chalcogenide crystals, the results for 11 crystals: Tl2CdGeSe4, Tl2CdSnSe4, Tl2HgSiSe4, Tl2HgSnS4, AuBSe2, AuBTe2, AuAlTe2, AuGaTe2, AuInTe2, AgAlSe2, and AgAlTe2 are revealed for the first time. In addition, temperature-dependent transport properties of pure and doped AgSbSe2 and AgSbTe2 crystals with dopant compositions of AgSb0.94Cd0.06Te2 and AgSbTe1.85Se0.15 were explored. These results provide an excellent database for bulk chalcogenides crucial for a wide range of potential applications in renewable energy fields.

Exploring the phase diagram of InAs/GaSb/InAs trilayer quantum wells

We study topological insulators based on InAs/GaSb/InAs trilayer quantum wells with different InAs thicknesses ranging from 8.0 to 13.5 nm to experimentally ascertain the phase diagram in this material system. Gate-voltage and temperature-dependent transport measurements reveal distinct transport signatures between samples designed from the trivial-insulating, the topological-insulating, up to the semimetallic phase. Two-carrier transport together with pronounced Van Hove singularities are observable only for the samples grown in the topological-insulating phase and if camelback-like dispersions are present in the hybridized valence or conduction band. The different shaping and the amount of Van Hove singularities in our samples deliver information about a turning point inside the topological-insulating phase. Temperature-dependent measurements allow to further confirm the gap energy values for the trivial- and topological-insulating samples. Furthermore, the indirect inverted gaps are found to be more temperature insensitive in comparison to the trivial gap.

Dirac dispersions, lattice dynamics and thermoelectric properties of quaternary Heusler alloys LiMgXY(X = Pt, Pd, Au; Y= Sb, Sn)

This manuscript reports the structural, mechanical, electronic, and thermoelectric properties of the LiMgYZ system Y = t, Pd, Au; Z = Sb, Sn) computed within the first-principle density functional theory combined with semi-classical Boltzmann transport equations. Comparing the relative energetic stability of type-I, type-II, and type-III atomic arrangement of basis atoms, the type-I arrangement is found to be the most stable among these. The lattice dynamics also confirm that the type-I phase is stable for all the studied alloys except LiMgAuSn. It is interesting to note that the electronic band structure of these materials exhibits the Dirac point. The temperature-dependent transport coefficients of these materials have been calculated within semi-classical Boltzmann transport formalism within constant scattering time approximation (CSTA). The number of states at the Fermi level has successfully depicted the behavior of electrical conductivity. Among all the studied alloys, LiMgPtSb shows the highest temperature coefficient of resistivity which indicates that this material can be a good candidate for temperature sensing applications.

A novel combination of mathematical modelling approaches for simulating nearly hypersonic, viscous, reacting magnetogasdynamic flows

This work focuses on the mathematical combination of magnetogasdynamics (MGD) and the Dunn and Kang chemical kinetic model with a multi-component Harten–Lax–van Leer Contact (HLLC) local Riemann solver using high-order interpolation schemes. Due to the nature of nearly hypersonic, unsteady, viscous, reacting MGD flows, the gas dissociates which is taken into account with 11 species and 26 chemical reactions along with the temperature dependent transport properties through the Wilke model. In the present work, the electromagnetic effects are considered by solving a Poisson equation simultaneously with the chemical reaction equations. The non-linear convective terms are treated numerically with third-, fifth- and seventh-order Weighted Essentially Non-Oscillatory (WENO) interpolation schemes along with second-, fourth- and sixth-order central finite difference schemes for the viscous terms. The combination of these numerical and physical models are analysed for stability and validated for benchmark problems such as (a) Hartmann flow as MGD validation test case and (b) the shock-tube problem for testing the chemical reaction model in which case the reaction rate controlling temperature suggested by Park has also been taken into account. Furthermore, a complex physical problem of an external aerodynamic flow over the cube for the Mach number varying from 1.25 to 3 have also been investigated, because a limited number of studies is available for understanding the flow behaviour of sharp cornered blunt bodies such as cubes. The knowledge of shock structures of these objects has become imperative to predict the atmospheric re-entry trajectories of de-orbiting CubeSats, space debris or fragments of launch vehicles. The main physical finding is that the shock structures are observed to be much more resolved compared to previous works, especially in the wake region. The knowledge gap is bridged by detailing the shock structures and showing the dependence of the shock standoff distance on total enthalpy of the flow.

Temperature dependent carrier transport in few-layered MoS2: from hopping to band transport

Understanding the carrier transport mechanisms is critical for electronic devices based on 2D semiconductors. Here, using a two-terminal device configuration, we show that the carrier transport behaviours in chemical vapour deposited few-layer MoS2 transition from resonant tunnelling to hopping, and eventually to band transport as the temperature increases from 5 K to 370 K. Specifically, the transport in the channel is dominated by resonant tunnelling when T < 30 K is reflected in the temperature-independent conductance. At 50 K < T < 110 K, the channel conductance exhibits a dependence of exp(T 1/2), a signature of Efros–Shklovskii type variable range hopping (VRH). At 110 K < T < 160 K, carrier transport behaves in a transition region with potential attribution to Mott-type VRH. At 160 K < T < 210 K, the nearest neighbour hopping mechanism is confirmed by the linear dependence from the resistance curve derivative analysis. For VRH, the localization length, hopping distance and energy, Coulomb gap energy and density of states are extracted. At T > 210 K, the carrier transport is dominated by thermally activated band transport based on AC conductance and mobility analysis. These findings are significant for revealing the material properties for future 2D semiconductor device applications.

Microstructure and morphology related electrical characterization and dielectric relaxation studies of nanocrystalline Sb2Te3 synthesized by mechanical alloying

This article explores a rapid and facile mechanical alloying method for synthesizing nanocrystalline Sb2Te3 at room temperatures with the least number of ingredients under an inert atmosphere. XRD, FESEM, and TEM have characterized the structure and microstructure. The rhombohedral Sb2Te3 phase formation without any impurity has been ascertained after 3 h of milling. A minimal size-dependent band gap is revealed from the FTIR spectra. The ionic conductivities measured in the temperature range 300–473 K increases with rising temperature, confirming the semi-metallic character of samples. The impedance spectra analysis reveals the electrical relaxation process and temperature-dependent transport properties. The non–Debye type behavior is confirmed from the Cole-Cole plot. The frequency-dependent ac conductivity obeys the universal Jonscher power law, and a small polaron transaction is used to explain the temperature dependence of the frequency exponent. A reduced value of dielectric loss factor at higher frequencies makes the material applicable in higher frequencies.

Enhanced channel modulation in Aluminum- and Hydrogen-Doped Zinc-Oxide-Based transistors by complementary Dual-Gate operation

This study elucidates the enhanced channel modulation by the dual-gate operation of the ultrathin ZnO-based field-effect transistors (FETs). Bottom-gate atomic-layer-deposited zinc oxide (ZnO) transistors exhibit typical n-type enhancement-mode transfer characteristics. However, when equipped with the top Al2O3 layer, the FETs exhibit conductive transfer characteristics. These electrical property changes are attributed to Al2O3-induced hydrogen and aluminum doping effects, which increase carrier concentration. Although Al2O3-induced doping increased field-effect mobility by a factor of 2, a high off current and degraded transconductance swing were observed in FETs. However, operation in dual-gate mode after the deposition of the top-gate electrode resolved these degradations and led to achieving high-performance ZnO FETs: the off current significantly decreased, and noteworthy, field-effect mobility increased by a factor of 5. Temperature-dependent charge transport analysis revealed that the complementary dual-gate operation efficiently filled the localized trap states in the ZnO films, resulting in band-like transport. Thus, it is deduced that the complementary dual-gate operation enhanced channel modulation in the Al2O3-induced hydrogen- and aluminum-doped ZnO channel, resulting in high-performance ZnO FETs.

Temperature Dependent Carrier Transport in Hydrogenated Amorphous Semiconductors for Thin Film Memristive Applications

This paper demonstrates the transport of electron and hole carriers in two distinct hydrogenated amorphous semiconductor materials at different temperatures. Compared to crystalline materials, the amorphous semiconductors differ structurally, optically and electrically, hence the nature of carrier transport through such amorphous materials differ. Materials like hydrogenated amorphous silicon and amorphous IGZO have been used for the study of temperature dependent carrier transport in this paper. Simulation results have been presented to show the variation of free electron and hole concentration, trapped electron and hole concentration with energy at 300K for both the materials. The change in mobility with a change in the Fermi level has been plotted for different temperatures. The effect of temperature on Brownian motion mobility of electrons and holes in hydrogenated amorphous silicon and amorphous IGZO has been demonstrated towards the end of this paper.

Doping dependent electronic and magnetic ordering in mixed-valent La1−x Sr x MnO3 thin films

We have investigated the collective electronic and magnetic orderings of a series of La1−x Sr x MnO3 thin films grown epitaxially strained to (001) oriented strontium titanate substrates as a function of doping, x, for 0 ≤ x ≤ 0.4. We find that the ground states of these crystalline thin films are, in general, consistent with that observed in bulk crystals and thin film samples synthesized under a multitude of techniques. Our systematic study, however, reveal subtle features in the temperature dependent electronic transport and magnetization measurements, which presumably arise due to Jahn-Teller type distortions in the lattice for particular doping levels. For the parent compound LaMnO3 (x = 0), we report evidence of a strain-induced ferromagnetic ordering in contrast to the antiferromagnetic ground state found in bulk crystals.

Solution-based synthesis of wafer-scale epitaxial BiVO4 thin films exhibiting high structural and optoelectronic quality.

We demonstrate a facile approach to solution-based synthesis of wafer-scale epitaxial bismuth vanadate (BiVO4) thin films by spin-coating on yttria-stabilized zirconia. Epitaxial growth proceeds via solid-state transformation of initially formed polycrystalline films, driven by interface energy minimization. The (010)-oriented BiVO4 films are smooth and compact, possessing remarkably high structural quality across complete 2′′ wafers. Optical absorption is characterized by a sharp onset with a low sub-band gap response, confirming that the structural order of the films results in correspondingly high optoelectronic quality. This combination of structural and optoelectronic quality enables measurements that reveal a strong optical anisotropy of BiVO4, which leads to significantly increased in-plane optical constants near the fundamental band edge that are of particular importance for maximizing light harvesting in semiconductor photoanodes. Temperature-dependent transport measurements confirm a thermally activated hopping barrier of ∼570 meV, consistent with small electron polaron conduction. This simple approach for synthesis of high-quality epitaxial BiVO4, without the need for complex deposition equipment, enables a broadly accessible materials base to accelerate research aimed at understanding and optimizing photoelectrochemical energy conversion mechanisms.

Remediation Potential of Borehole Thermal Energy Storage for Chlorinated Hydrocarbon Plumes: Numerical Modeling in a Variably-Saturated Aquifer

Underground thermal energy storage is an efficient technique to boost the share of renewable energies. However, despite being well-established, their environmental impacts such as the interaction with hydrocarbon contaminants is not intensively investigated. This study uses OpenGeoSys software to simulate the heat and mass transport of a borehole thermal energy storage (BTES) system in a shallow unconfined aquifer. A high-temperature (70 C) heat storage scenario was considered which imposes long-term thermal impact on the subsurface. Moreover, the effect of temperature-dependent flow and mass transport in a two-phase system is examined for the contaminant trichloroethylene (TCE). In particular, as subsurface temperatures are raised due to BTES operation, volatilization will increase and redistribute the TCE in liquid and gas phases. These changes are inspected for different scenarios in a contaminant transport context. The results demonstrated the promising potential of BTES in facilitating the natural attenuation of hydrocarbon contaminants, particularly when buoyant flow is induced to accelerate TCE volatilization. For instance, over 70% of TCE mass was removed from a discontinuous contaminant plume after 5 years operation of a small BTES installation. The findings of this study are insightful for an increased application of subsurface heat storage facilities, especially in contaminated urban areas.

Temperature-dependent thermal transport of single molecular junctions from semiclassical Langevin molecular dynamics

Thermal conductance of single molecular junctions at room temperature has been measured recently using picowatt-resolution scanning probes. However, fully understanding thermal transport in a much wider temperature range is needed for the exploration of energy transfer at single-molecular limit and the development of single-molecular devices. Here, employing a semiclassical Langevin molecular dynamics method, a comparative study is performed on the thermal transport of an alkane chain between Au and graphene electrodes, respectively. We illustrate the different roles of quantum statistics and anharmonic interaction in the two types of junctions. For a graphene junction, quantum statistics is essential at room temperature, while the anharmonic interaction is negligible. For a Au junction, it is the other way. Our study paves the way for theoretically understanding thermal transport of realistic single-molecular junctions in the full temperature range by including both quantum statistics and anharmonic interaction within one theoretical framework.

Optimal design of CH4 pyrolysis in a commercial CVD reactor using support vector machines and Nelder-Mead algorithm

Chemical vapour deposition (CVD) of pyrocarbon (PyC) effectively fabricates advanced carbon materials. Controlling the nanotexture of PyC is critical for the desired application. Reactor operating conditions, including temperature, pressure, inlet flow rate, reactant concentration, govern thickness, and film uniformity. The optimal film performance can be obtained by selecting appropriate process conditions. However, the optimisation of the CVD reactor is challenging due to the highly nonlinear and multi-variable nature of the process. In this study, the support vector machine, a robust supervised learning algorithm and Nelder-Mead algorithm are coupled to optimise the CH4 pyrolysis in a commercial CVD reactor. To this end, a comprehensive CFD model for CH4 pyrolysis in a commercial CVD reactor is first constructed. The model accuracy is improved by considering temperature-dependent transport properties of gas mixture and incorporating the detailed gas and surface chemistry (14 species and 32 reactions). The deposition rate and film uniformity are then obtained using a parametric study. Subsequently, the support vector machine (SVM) is employed to deduce the correlation between the PyC growth rate/film uniformity and operating variables. It has been found that the accuracy of SVM is better than the linear regression model. Finally, SVM coupled with the Nelder-Mead algorithm is proposed to optimise the CVD process parameters to maximise the PyC film quality.

Effect of buffer termination on intermixing and conductivity in LaTiO3/SrTiO3 heterostructures integrated on Si(100)

The control of chemical exchange across heterointerfaces formed between ultrathin functional transition-metal oxide layers provides an effective route to manipulate the electronic properties of these systems. By determining the layer-resolved structural profile across the interface between the Mott insulator, LaTiO3 (LTO) grown epitaxially on SrTiO3 (STO)-buffered silicon by molecular beam epitaxy, we find that interfacial cationic exchange depends on the surface termination of the strained STO buffer. Using a combination of temperature-dependent transport and synchrotron x-ray crystal truncation rods and reciprocal space mapping, an enhanced conductivity in STO/LTO/SrO-terminated STO buffers compared to heterostructures with TiO2-terminated STO buffers is correlated with La/Sr exchange and the formation of metallic La1−xSrxTiO3. La/Sr exchange effectively reduces the strain energy of the system due to the large lattice mismatch between the nominal oxide layers and the Si substrate.