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At least 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Giant Transport Anisotropy in ReS 2 Revealed via Nanoscale Conducting-Path Control

The low in-plane symmetry in layered 1T'-ReS 2 results in strong band anisotropy, while its manifestation in the electronic properties is challenging to resolve due to the lack of effective approaches for controlling the local current path. In this work, we reveal the giant transport anisotropy in monolayer to four-layer ReS 2 by creating directional conducting paths via nanoscale ferroelectric control. By reversing the polarization of a ferroelectric polymer top layer, we induce conductivity switching ratio of >1.5×10 8 in the ReS 2 channel at 300 K. Characterizing the domain-defined conducting nanowires in an insulating background shows that the conductivity ratio between the directions along and perpendicular to the Re-chain can exceed 5.5×10 4 in monolayer ReS 2 . Theoretical modeling points to the band origin of the transport anomaly, and further reveals the emergence of a flat band in few-layer ReS 2 . Our work paves the path for implementing the highly anisotropic 2D materials for designing novel collective phenomena and electron lensing applications.

2D van der Waals materials↗
DOE OSTI2019

Computational prediction of lattice thermal conductivity: A comparison of molecular dynamics and Boltzmann transport approaches

The predictive modeling of lattice thermal conductivity is of fundamental importance for the understanding and design of materials for a wide range of applications. Two major approaches, namely molecular dynamics (MD) simulations and calculations solving approximately the Boltzmann transport equation (BTE), have been developed to compute the lattice thermal conductivity. We present a detailed direct comparison of these two approaches, using as prototypical cases MgO and PbTe. The comparison, carried out using empirical potentials, takes into account the effects of fourth order phonon scattering, temperature-dependent phonon frequencies (phonon renormalization), and investigates the effects of quantum vs classical statistics. We clarify that equipartition, as opposed to Maxwell-Boltzmann, govern the statistics of phonons in MD simulations. We find that lattice thermal conductivity values from MD and BTE show an apparent, satisfactory agreement; however such an agreement is the result of error cancellations. As a result we also show that the primary effect of statistics on thermal conductivity is via the scattering rate dependence on phonon populations.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Understanding the lattice thermal conductivity of SrTiO 3 from an ab initio perspective

Here, we present a detailed analysis of the structure dependence of the lattice thermal conductivity of SrTiO 3 . We have used both ab initio molecular dynamic simulations and density functional theory calculations to decouple the effect of different structural distortions on the thermal conductivity. We have identified two main mechanisms for tuning the thermal conductivity when a distortion is applied. First, the modification of the acoustic-modes energy dispersion when a change in the lattice parameters is imposed and second, the low energy polar modes. In particular and counterintuitively, we have found that an increase in the angle of the oxygen octahedral rotations increases the thermal conductivity due to its coupling to these polar modes.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Thermal conductivity of the quasi-one-dimensional materials TaSe 3 and ZrTe 3

The high breakdown current densities and resilience to scaling of the metallic transition metal trichalcogenides TaSe 3 and ZrTe 3 make them of interest for possible interconnect applications, and it motivates this study of their thermal conductivities and phonon properties. These crystals consist of planes of strongly bonded one-dimensional chains more weakly bonded to neighboring chains. Phonon dispersions and the thermal conductivity tensors are calculated using density functional theory combined with an iterative solution of the phonon Boltzmann transport equation. The phonon velocities and the thermal conductivities of TaSe 3 are considerably more anisotropic than those of ZrTe 3 . The maximum LA velocity in ZrTe 3 occurs in the cross-chain direction, and this is consistent with the strong cross-chain bonding that gives rise to large Fermi velocities in that direction. The thermal conductivities are similar to those of other metallic two-dimensional transition metal dichalcogenides. At room temperature, a significant portion of the heat is carried by the optical modes. In the low frequency range, the phonon lifetimes and mean free paths in TaSe 3 are considerably shorter than those in ZrTe 3 . Here, we find that the shorter lifetimes in TaSe 3 are consistent with the presence of lower frequency optical branches and zone-folding features in the acoustic branches that arise due to the doubling of the TaSe 3 unit cell within the plane.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Robust Nodal Behavior in the Thermal Conductivity of Superconducting UTe 2

The superconducting state of the heavy-fermion metal UTe 2 has attracted considerable interest because of evidence of spin-triplet Cooper pairing and nontrivial topology. Progress on these questions requires identifying the presence or absence of nodes in the superconducting gap function and their dimension. In this article, we report a comprehensive study of the influence of disorder on the thermal transport in the superconducting state of UTe 2 . Through detailed measurements of the magnetic-field dependence of the thermal conductivity in the zero-temperature limit, we obtain clear evidence of the presence of point nodes in the superconducting gap for all samples with transition temperatures ranging from 1.6 to 2.1 K obtained by different synthesis methods, including a refined self-flux method. This robustness implies the presence of symmetry-imposed nodes throughout the range studied, further confirmed via disorder-dependent calculations of the thermal transport in a model with a single pair of nodes. In addition to capturing the temperature dependence of the thermal conductivity up to 𝑇 𝑐 , this model provides some information about the locations of the nodes, suggesting a 𝐵 1⁢𝑢 or 𝐵 2⁢𝑢 symmetry for the superconducting order parameter. Additionally, comparing the new, ultrahigh conductivity samples to older samples reveals a crossover between a low-field and a high-field regime at a single value of the magnetic field in all samples. In the high-field regime, the thermal conductivity at different disorder levels differs from each other by a simple offset, suggesting that some simple principle determines the physics of the mixed state, a fact which may illuminate trends observed in other clean nodal superconductors.

spin-triplet pairing↗
DOE OSTI2015

Fabrication of Conductive Paths on a Fused Deposition Modeling Substrate using Inkjet Deposition

Inkjet deposition is one of the most attractive fabrication techniques for producing cost efficient and lightweight electronic devices on various substrates with low environmental impact. Fused Deposition Modeling (FDM) is one of the most used and reliable additive manufacturing processes by extrusion of wire-shaped thermoplastic materials, which provides an opportunity for embedding printed electronics into mechanical structures during the building process and enables the design of compact smart structures that can sense and adapt to their own state and the environment. This paper represents one of the first explorations of integrating inkjet deposition of silver nanoparticle inks with the FDM process for making compact electro-mechanical structures. Three challenges have been identified and investigated, including the discontinuity of the printed lines resulting from the irregular surface of the FDM substrate, the non-conductivity of the printed lines due to the particle segregation during the droplet drying process, and the slow drying process caused by the skinning effect . Two different techniques are developed in this paper to address the issue of continuity of the printed lines, including surface ironing and a novel thermal plow technique that plows a channel in the FDM substrate to seal off the pores in the substrate and contain the deposited inks. Two solutions are also found for obtaining conductivity from the continuous printed lines, including porous surface coating and using a more viscous ink with larger nanoparticle size. Then the effects of the printing and post-processing parameters on the conductivity are examined. It is found that post-processing is a dominant factor in determining the conductivity of the printed lines.

42 ENGINEERING↗
DOE OSTI2019

Impact of Ultra-Conducting Winding on the Power Density and Performance of Non-Heavy Rare Earth Traction Motors

The emergence of carbon nanotube-copper composite conductors has opened a new degree of freedom in the electric machine design to improve the machine's performance: the conductivity of the winding. The goal of the present study is: 1) to determine how much improvement in power per unit volume and performance can be achieved with the increase in winding conductivity; and 2) to investigate any physical limitations on the effectiveness of this degree of freedom in improving the power per unit volume and performance of electric traction motors. Conventional electric motor topologies affected by ferromagnetic saturation as well as slotless topology that is not affected by saturation are analyzed. It was found that the ferromagnetic saturation of the lamination materials reduces the effectiveness of highly conductive winding in improving the power per unit volume. For machine topologies that are not affected by ferromagnetic saturation, the power density improvement reaches the theoretical expectation. In both cases, highly conductive winding improves the efficiency map by enlarging high efficiency operating area into light load as well as high speed operations.

Raminosoa, Tsarafidy↗
DOE OSTI2026

A Unified Off-Board Charger for Three-Phase Inductive and Single-Phase Conductive EV Charging

This paper presents a unified off-board charger integrating a three-phase inductive power transfer (IPT) system and a single-phase conductive (plug-in) charger for electric vehicles (EVs). The proposed charger shares a high-frequency inverter and an integrated magnetic structure that enables three operating modes: conductive charging, inductive charging, and simultaneous charging. The transmitter coils for the three-phase IPT and the high-frequency transformer windings for the conductive charger are arranged on a common ferrite pad while maintaining magnetic decoupling between the two power transfer paths. The proposed unified architecture effectively increases charger utilization and power density without a proportional increase in cost, weight, or volume. Finite-element analysis using Ansys Maxwell confirms negligible coupling between the inductive and conductive coils, and the extracted parameters are employed in PLECS simulations to verify the independent operation of each charging mode.

Jo, Cheolhui [ORNL] (ORCID:0000000322692434)↗
DOE OSTI2019

Maximizing the Performance of a 3D Printed Heat Sink by Accounting for Anisotropic Thermal Conductivity During Filament Deposition

Newly developed high thermal conductivity polymer composite 3D printing filaments are used to characterize the thermal properties as a function of print orientation. The thermal conductivity of a printed part is anisotropic and varies by 2-6 times depending on the print direction - demonstrating higher conductivity in the deposition direction (in-plane) than in the two directions perpendicular to the deposition direction (cross-plane and through-plane). Therefore, deposition path planning greatly affects the overall heat dissipation rate and the performance of the heat sink. Traditionally, 3D printing slicers generate deposition paths based solely on geometric constraints. This work investigates a new approach of deposition path planning assisted by computational predictions of the heat sink thermal performance. The proposed approach uses a thermal simulation of a 3D-printed part, accounting for the anisotropic thermal properties, and the orientation of the local material properties are assigned based on the deposition path in multiple print orientations. The performances predicted via the simulations are compared, and the optimal deposition path is determined. For the highest thermal conductivity 3D printing filament (~ 12 W/m- K in-plane), a heat sink printed with the print direction parallel to the fins z-axis had ~20% improved performance in comparison to a heat sink with print direction perpendicular to the fins z-axis. Moreover, a plastic 3D printed heat sink was able to perform within 7% of an extruded Aluminum heat sink with similar geometry under natural convection. The computational predictions show the same trend as experimental measurements using 3D printed heat sinks.

Smith, Matthew↗
DOE OSTI2025

Machinable, high‐conductivity NaSICON through mitigation of humidity effects during solid‐state synthesis

The Na + super ion conductor (NaSICON, Na 1+x Zr 2 Si x P 3-x O 12 ) is a solid electrolyte well-known for fast, selective Na + transport at low temperatures, uniquely enabling sodium-based batteries. Producing high-quality NaSICON from solid-state methods, especially when cost-effective, potentially hygroscopic precursors are used, is not trivial. To understand and eliminate the influence of humidity during processing, a scheme was developed to reproducibly yield a high Na + conductivity (3.75 mS/cm at 25°C, 81.7 mS/cm at 150°C), high density (97%), and machinable NaSICON without the use of binders, sintering aids, or dopants. Controlled humidity studies over 20%–50% RH coupled with thermal, structural, and electrical analysis reveal that calcination temperatures < 1000°C leave NaSICON processing susceptible to water absorption at > 20% RH due to the presence of hygroscopic Na 3 PO 4 and Na 2 CO 3 during shaping, pressing, and sintering. Water absorption results in NaSICON with lower densities, machinability, and Na+ conductivity, due to impaired intergranular Na+ transport. At the other extreme, fully converting precursor to the NaSICON phase at 1230°C before pressing and sintering leads to poor conductivity and density. By calcining at 1000°C, excellent quality NaSICON may be produced under a range of laboratory environments, enabling low-cost production of high-conductivity, machinable NaSICON necessary the ever-growing energy storage market.

NaSICON↗
DOE OSTI2014

Mechanical and transparent conductive properties of ZnO and Ga-doped ZnO films sputtered using electron-cyclotron-resonance plasma on polyethylene naphtalate substrates

Transparent conductive ZnO and Ga-doped ZnO (GZO) films were deposited on polyethylene naphtalate (PEN) sheet substrates using electron cyclotron resonance plasma sputtering. Both ZnO and GZO films were highly adhesive to the PEN substrates without inserting an intermediate layer in the interface. When compared at the same thickness, the transparent conductive properties of GZO films on PEN substrates were only slightly inferior to those on glass substrates. However, the carrier concentration of ZnO films on PEN substrates was 1.5 times that of those on glass substrates, whereas their Hall mobility was only 60% at a thickness of 300 nm. The depth profile of elements measured by secondary ion mass spectroscopy revealed the diffusion of hydrocarbons out of the PEN substrate into the ZnO film. Hence, doped carbons may act as donors to enhance carrier concentration, and the intermixing of elements at the interface may deteriorate the crystallinity, resulting in the lower Hall mobility. When the ZnO films were thicker than 400 nm, cracks became prevalent because of the lattice mismatch strain between the film and the substrate, whereas GZO films were free of cracks. The authors investigated how rolling the films around a cylindrical pipe surface affected their conductive properties. Degraded conductivity occurred at a threshold pipe radius of 10 mm when tensile stress was applied to the film, but it occurred at a pipe radius of 5 mm when compressive stress was applied. These values are guidelines for bending actual devices fabricated on PEN substrates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Hydrogen induced electric conduction in undoped ZnO and Ga-doped ZnO thin films: Creating native donors via reduction, hydrogen donors, and reactivating extrinsic donors

The manner in which hydrogen atoms contribute to the electric conduction of undoped ZnO and Ga-doped ZnO (GZO) films was investigated. Hydrogen atoms were permeated into these films through annealing in an atmospheric H{sub 2} ambient. Because the creation of hydrogen donors competes with the thermal annihilation of native donors at elevated temperatures, improvements to electric conduction from the initial state can be observed when insulating ZnO films are used as samples. While the resistivity of conductive ZnO films increases when annealing them in a vacuum, the degree of increase is mitigated when they are annealed in H{sub 2}. Hydrogenation of ZnO crystals was evidenced by the appearance of OH absorption signals around a wavelength of 2700 nm in the optical transmittance spectra. The lowest resistivity that was achieved by H{sub 2} annealing was limited to 1–2 × 10{sup −2} Ω cm, which is one order of magnitude higher than that by native donors (2–3 × 10{sup −3} Ω cm). Hence, all native donors are converted to hydrogen donors. In contrast, GZO films that have resistivities yet to be improved become more conductive after annealing in H{sub 2} ambient, which is in the opposite direction of GZO films that become more resistive after vacuum annealing. Hydrogen atoms incorporated into GZO crystals should assist in reactivating Ga{sup 3+} donors.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Phonon localization in heat conduction

Nondiffusive phonon thermal transport, extensively observed in nanostructures, has largely been attributed to classical size effects, ignoring the wave nature of phonons. We report localization behavior in phonon heat conduction due to multiple scattering and interference events of broadband phonons, by measuring the thermal conductivities of GaAs/AlAs superlattices with ErAs nanodots randomly distributed at the interfaces. With an increasing number of superlattice periods, the measured thermal conductivities near room temperature increased and eventually saturated, indicating a transition from ballistic to diffusive transport. In contrast, at cryogenic temperatures the thermal conductivities first increased but then decreased, signaling phonon wave localization, as supported by atomistic Green's function simulations. The discovery of phonon localization suggests a new path forward for engineering phonon thermal transport.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Giant isotope effect on phonon dispersion and thermal conductivity in methylammonium lead iodide

Lead halide perovskites are strong candidates for high-performance low-cost photovoltaics, light emission, and detection applications. A hot-phonon bottleneck effect significantly extends the cooling time of hot charge carriers, which thermalize through carrier–optic phonon scattering, followed by optic phonon decay to acoustic phonons and finally thermal conduction. To understand these processes, we adjust the lattice dynamics independently of electronics by changing isotopes. We show that doubling the mass of hydrogen in methylammonium lead iodide by replacing protons with deuterons causes a large 20 to 50% softening of the longitudinal acoustic phonons near zone boundaries, reduces thermal conductivity by ~50%, and slows carrier relaxation kinetics. Phonon softening is attributed to anticrossing with the slowed libration modes of the deuterated molecules and the reduced thermal conductivity to lowered phonon velocities. Our results reveal how tuning the organic molecule dynamics enables control of phonons important to thermal conductivity and the hot-phonon bottleneck.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On Splitting the Conduction Band of Liquid Mercury

Electron characteristics of the contributions to the mercury optical conductivity at room temperature have been calculated based on comparison of the model spectrum of the optical conductivity with the experimental data. It is shown that the experimental spectrum of the liquid-mercury optical conductivity can be described only taking into account the possibility of splitting the conduction band.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Conduction of the three-dimensional model of a composite with structural anisotropy

The electric conduction of the model of a structurally anisotropic composite that represents an isotropic matrix with disk-shaped inclusions (oblate spheroids) is analytically studied. The disks have the same orientation and their centers are randomly distributed in the composite volume. The cases of dielectric and ideally conducting inclusions are considered. The conduction of the model in a broad concentration range is described by the approximate effective medium method. Conduction in the critical region (in the vicinity of the metal–dielectric phase transition point) is studied using the similarity hypothesis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Conductance distribution near the Anderson transition

Using a modification of the Shapiro approach, we introduce the two-parameter family of conductance distributions W(g), defined by simple differential equations, which are in the one-to-one correspondence with conductance distributions for quasi-one-dimensional systems of size L{sup d–1} × L{sub z}, characterizing by parameters L/ξ and L{sub z}/L (ξ is the correlation length, d is the dimension of space). This family contains the Gaussian and log-normal distributions, typical for the metallic and localized phases. For a certain choice of parameters, we reproduce the results for the cumulants of conductance in the space dimension d = 2 + ϵ obtained in the framework of the σ-model approach. The universal property of distributions is existence of two asymptotic regimes, log-normal for small g and exponential for large g. In the metallic phase they refer to remote tails, in the critical region they determine practically all distribution, in the localized phase the former asymptotics forces out the latter. A singularity at g = 1, discovered in numerical experiments, is admissible in the framework of their calculational scheme, but related with a deficient definition of conductance. Apart of this singularity, the critical distribution for d = 3 is well described by the present theory. One-parameter scaling for the whole distribution takes place under condition, that two independent parameters characterizing this distribution are functions of the ratio L/ξ.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Universal Frequency Dependence of the Hopping AC Conductance in p-Ge/GeSi Structures in the Integer Quantum Hall Effect Regime

The hopping ac conductance, which is realized at the transverse conductance minima in the regime of the integer Hall effect, has been measured using a combination of acoustic and microwave methods. Measurements have been made in the p-GeSi/Ge/GeSi structures with quantum wells in a wide frequency range (30–1200 MHz). The experimental frequency dependences of the real part of ac conductance σ{sub 1} have been interpreted on the basis of the model presuming hops between localized electronic states belonging to isolated clusters. At high frequencies, dominating clusters are pairs of close states; upon a decrease in frequency, large clusters that merge into an infinite percolation cluster as the frequency tends to zero become important. In this case, the frequency dependences of the ac conductance can be represented by a universal curve. The scaling parameters and their magnetic-field dependence have been determined.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗