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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Role of chemically and thermally induced crystal lattice distortion in enhancing the Seebeck coefficient in complex tellurides

Composition and crystal structure of complex materials can significantly change the Seebeck effect, i.e., heat to electrical energy conversion, which is utilized in thermoelectric materials. Despite decades of studies of various thermoelectric materials and their application, the fundamental understanding of this effect still is limited. One of the most efficient groups of thermoelectric materials is based on GeTe, where Ge is replaced by [Ag + Sb], i.e., Ag x Sb x Ge 50-2x Te 50 alloys, traditionally shown as (GeTe) m (AgSbTe 2 ) 100-m (TAGS-m series). Here, in this article, we report on the discovery of two unique phenomena in TAGS materials attributed to the effects from [Ag + Sb] atoms: (i) a linear relation between the Seebeck coefficient and rhombohedral lattice distortion, and (ii) resonance-like temperature-induced behavior of the contribution to the Seebeck coefficient produced by [Ag + Sb] atoms. Finally, our findings show that heat to electrical energy conversion strongly depends on the temperature- and compositionally-induced rhombohedral to cubic transformation where [Ag + Sb] atoms play a crucial mediating role.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Spin and charge thermopower effects in the ferromagnetic graphene junction

Using wave function matching approach and employing the Landauer-Buttiker formula, a ferromagnetic graphene junction with temperature gradient across the system is studied. We calculate the thermally induced charge and spin current as well as the thermoelectric voltage (Seebeck effect) in the linear and nonlinear regimes. Our calculation revealed that due to the electron-hole symmetry, the charge Seebeck coefficient is, for an undoped magnetic graphene, an odd function of chemical potential while the spin Seebeck coefficient is an even function regardless of the temperature gradient and junction length. We have also found with an accurate tuning external parameter, namely, the exchange filed and gate voltage, the temperature gradient across the junction drives a pure spin current without accompanying the charge current. Another important characteristic of thermoelectric transport, thermally induced current in the nonlinear regime, is examined. It would be our main finding that with increasing thermal gradient applied to the junction the spin and charge thermovoltages decrease and even become zero for non zero temperature bias.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Effective Mass from Seebeck Coefficient

Abstract Engineering semiconductor devices requires an understanding of the effective mass of electrons and holes. Effective masses have historically been determined in metals at cryogenic temperatures estimated using measurements of the electronic specific heat. Instead, by combining measurements of the Seebeck and Hall effects, a density of states effective mass can be determined in doped semiconductors at room temperature and above. Here, a simple method to calculate the electron effective mass using the Seebeck coefficient and an estimate of the free electron or hole concentration, such as that determined from the Hall effect, is introduced here is the Seebeck effective mass, n H is the charge carrier concentration measured by the Hall effect ( n H = 1/ eR H , R H is Hall resistance) in 10 20 cm −3 , T is the absolute temperature in K, S is the Seebeck coefficient, and k B / e = 86.3 μV K −1 . This estimate of the effective mass can aid the understanding and engineering of the electronic structure as it is largely independent of scattering and the effects of microstructure (grain boundary resistance). It is particularly helpful in characterizing thermoelectric materials.

Snyder, Gerald Jeffrey↗
DOE OSTI2022

Controlling Antiferromagnetic Magnon Polarization by Interfacial Exchange Interaction

We demonstrate highly efficient control of antiferromagnetic (AFM) magnon spins by the interfacial exchange interaction in heterostructures of ferrimagnetic yttrium iron garnet (YIG) and AFM Cr 2 O 3 . At low temperatures, Cr 2 O 3 is antiferromagnetically ordered. The interfacial exchange interaction exerted by YIG lifts the degeneracy between the AFM magnon modes in Cr 2 O 3 , resulting in a net spin polarization and a spin current dominated by left-handed magnons, even at zero magnetic field, which is detected by the spin Seebeck effect with a 5-nm-thick Pt film. In the AFM magnon-dominated region, even if the magnetic field is not sufficiently strong to induce the spin-flop transition in Cr 2 O 3 , the total spin Seebeck signal polarity flips when the YIG magnetization switches. It clearly indicates that the Cr 2 O 3 magnon polarization is controlled by YIG magnetization through the interfacial exchange interaction. The demonstration of the efficient control of AFM magnon polarization opens a pathway to manipulate AFM magnon quantum states using FM-AFM heterostructures.

42 ENGINEERING↗
DOE OSTI2014

Spin Hall magnetoresistance at Pt/CoFe{sub 2}O{sub 4} interfaces and texture effects

We report magnetoresistance measurements on thin Pt bars grown on epitaxial (001) and (111) CoFe{sub 2}O{sub 4} (CFO) ferrimagnetic insulating films. The results can be described in terms of the recently discovered spin Hall magnetoresistance (SMR). The magnitude of the SMR depends on the interface preparation conditions, being optimal when the Pt/CFO samples are prepared in situ, in a single process. The spin-mixing interface conductance, the key parameter governing SMR and other relevant spin-dependent phenomena, such as spin pumping or spin Seebeck effect, is found to be different depending on the crystallographic orientation of CFO, highlighting the role of the composition and density of magnetic ions at the interface on spin mixing.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Record thermopower found in an IrMn-based spintronic stack

The Seebeck effect converts thermal gradients into electricity. As an approach to power technologies in the current Internet-of-Things era, on-chip energy harvesting is highly attractive, and to be effective, demands thin film materials with large Seebeck coefficients. In spintronics, the antiferromagnetic metal IrMn has been used as the pinning layer in magnetic tunnel junctions that form building blocks for magnetic random access memories and magnetic sensors. Spin pumping experiments revealed that IrMn Néel temperature is thickness-dependent and approaches room temperature when the layer is thin. Here, we report that the Seebeck coefficient is maximum at the Néel temperature of IrMn of 0.6 to 4.0 nm in thickness in IrMn-based half magnetic tunnel junctions. We obtain a record Seebeck coefficient 390 (±10) μV K -1 at room temperature. Our results demonstrate that IrMn-based magnetic devices could harvest the heat dissipation for magnetic sensors, thus contributing to the Power-of-Things paradigm.

42 ENGINEERING↗
DOE OSTI2017

High-resolution structural characterization and magnetic properties of epitaxial Ce-doped yttrium iron garnet thin films

Thin films of magnetic garnet materials, e.g. yttrium iron garnet (Y 3 Fe 5 O 12 , YIG), are useful for a variety of applications including microwave integrated circuits and spintronics. Moreover, substitution of rare earth ions, such as cerium, is known to enhance the magneto-optic Kerr effect (MOKE) as compared to pure YIG. Thin films of Ce 0.75 Y 2.25 Fe 5 O 12 (Ce:YIG) have been grown using the pulsed laser deposition (PLD) technique and their crystal structure examined using high resolution scanning transmission electron microscopy. Homogeneous substitution of Ce in YIG, without oxidation to form a separate CeO 2 phase, can be realized in a narrow process window with resulting enhancement of the MOKE signal. We investigated the thermally generated signal due to spin Seebeck effect for the optimally doped Ce:YIG films.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Transverse thermoelectric effect in La{sub 0.67}Sr{sub 0.33}MnO{sub 3}|SrRuO{sub 3} superlattices

Transverse thermoelectric effects in response to an out-of-plane heat current have been studied in an external magnetic field for ferromagnetic superlattices consisting of La{sub 0.67}Sr{sub 0.33}MnO{sub 3} and SrRuO{sub 3} layers. The superlattices were fabricated on SrTiO{sub 3} substrates by pulsed laser deposition. We found that the sign of the transverse thermoelectric voltage for the superlattices is opposite to that for La{sub 0.67}Sr{sub 0.33}MnO{sub 3} and SrRuO{sub 3} single layers at 200 K, implying an important role of spin Seebeck effects inside the superlattices. At 10 K, the magnetothermoelectric curves shift from the zero field due to an antiferromagnetic coupling between layers in the superlattices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spin-current-driven thermoelectric generation based on interfacial spin-orbit coupling

The longitudinal spin Seebeck effect (SSE) in Bi{sub 2}O{sub 3}/Cu/yttrium-iron-garnet (YIG) devices has been investigated. When an out-of-plane temperature gradient is applied to the Bi{sub 2}O{sub 3}/Cu/YIG device, a spin current is generated across the Cu/YIG interface via the SSE and then converted into electric voltage due to the spin–orbit coupling at the Bi{sub 2}O{sub 3}/Cu interface. The sign of the SSE voltage in the Bi{sub 2}O{sub 3}/Cu/YIG devices is opposite to that induced by the conventional inverse spin Hall effect in Pt/YIG devices. The SSE voltage in the Bi{sub 2}O{sub 3}/Cu/YIG devices disappears in the absence of the Bi{sub 2}O{sub 3} layer and its thermoelectric conversion efficiency is independent of the Cu thickness, indicating the important role of the Bi{sub 2}O{sub 3}/Cu interface. This result demonstrates that not only the bulk inverse spin Hall effect but also the spin–orbit coupling near the interface can be used for SSE-based thermoelectric generation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Thermoelectric properties of composites made of Ni{sub 0.05}Mo{sub 3}Sb{sub 5.4}Te{sub 1.6} and fullerene

About 18 g of Ni{sub 0.05}Mo{sub 3}Sb{sub 5.4}Te{sub 1.6} were prepared by heating the elements in the stoichiometric ratio at 1000 K. The product was divided into four parts, and then C{sub 60} was added to three of these four parts at 1, 2, and 3 mass%, respectively. Each part was hot-pressed at 150 MPa and 923 K. The sample with 1% C{sub 60} was characterized via a Rietveld refinement and TEM analyses. Measurements of the three thermoelectric key properties revealed that the Seebeck coefficient barely depends on the carbon amount added, while both the electrical and the thermal conductivity decrease with increasing amount of carbon. Depending on the amount of C{sub 60} used and on the temperature, the thermoelectric performance was either enhanced or decreased, depending on whether the electrical conductivity decreased less or more than the thermal conductivity. At the highest temperature measured, all carbon-containing samples performed better than the unmodified bulk sample, namely up to 14%. These improvements are within the error margin, however. - Graphical abstract: Figure-of-merit of various Ni{sub 0.05}Mo{sub 3}Sb{sub 5.4}Te{sub 1.6}/C composites. - Highlights: • Composites of Mo{sub 3}(Sb,Te){sub 7} materials with C{sub 60} were synthesized and characterized. • The carbon additions form amorphous nanoparticles with sizes of the order of 50 nm. • The Seebeck effect remains basically unaffected from the carbon additions. • Electrical and thermal conductivity decrease differently with increasing carbon content. • The figure-of-merit was slightly improved, most noteworthy at higher temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Spin caloritronics in graphene with Mn

We show that graphene with Mn adatoms trapped at single vacancies features spin-dependent Seebeck effect, thus enabling the use of this material for spin caloritronics. A gate potential can be used to tune its thermoelectric properties in a way it presents either a total spin polarized current, flowing in one given direction, or currents for both spins flowing in opposite directions without net charge transport. Moreover, we show that the thermal magnetoresistance can be tuned between −100% and +100% by varying a gate potential.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Experimental study of thermodynamics propagation fatigue crack in metals

This work is devoted to the development of an experimental method for studying the energy balance during cyclic deformation and fracture. The studies were conducted on 304 stainless steel AISE samples. The investigation of the fatigue crack propagation was carried out on flat samples with stress concentrators. The stress concentrator was three central holes. The heat flux sensor was developed based on the Seebeck effect. This sensor was used for measuring the heat dissipation power in the examined samples during the fatigue tests. The measurements showed that the rate of fatigue crack growth depends on the heat flux at the crack tip and there are two propagation mode of fatigue crack with different link between the propagation mode and heat flux from crack tip.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Dynamic generation of spin-wave currents in hybrid structures

Spin transport through the interface in a semiconductor/ferromagnetic insulator hybrid structure is studied by the nonequilibrium statistical operator method under conditions of the spin Seebeck effect. The effective parameter approach in which each examined subsystem (conduction electrons, magnons, phonons) is characterized by its specific effective temperature is considered. The effect of the resonant (electric dipole) excitation of the spin electronic subsystem of conduction electrons on spin-wave current excitation in a ferromagnetic insulator is considered. The macroscopic equations describing the spin-wave current caused by both resonant excitation of the spin system of conduction electrons and the presence of a nonuniform temperature field in the ferromagnetic insulator are derived taking into account both the resonance-diffusion propagation of magnons and their relaxation processes. It is shown that spin-wave current excitation is also of resonant nature under the given conditions.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Short-range thermal magnon diffusion in magnetic garnet

Here, using the spin Seebeck effect (SSE), we study the propagation distance of thermally induced spin currents inside a magnetic insulator thin film in the short-range regime. We disambiguate spin currents driven by temperature and chemical potential gradients by comparing the SSE signal before and after adding a heat-sinking capping layer on the same device. We report that the measured spin decay behavior near the heat source is well accounted for by a diffusion model where the magnon diffusion length is in submicron range, in other words, two orders of magnitude smaller than previous estimates inferred from the long-range behavior. Our results highlight the caveat in applying a diffusive theory to describe thermally generated magnon transport, where a single decay length may not capture the behavior on all length scales.

36 MATERIALS SCIENCE↗
DOE OSTI2014

The Third Way of Thermal-Electric Conversion beyond Seebeck and Pyroelectric Effects

Thermal-electric conversion is crucial for smart energy control and harvesting, such as thermal sensing and waste heat recovering. So far, people are aware of only two ways of direct thermal-electric conversion, Seebeck and pyroelectric effects, each with distinct working conditions and limitations. Here, we report the third way of thermal-electric conversion beyond Seebeck and pyroelectric effects. In contrast to Seebeck effect that requires spatial temperature difference, the-third-way converts the time-dependent ambient temperature fluctuation into electricity, similar to the behavior of pyroelectricity. However, the-third-way is also distinct from pyroelectric effect in the sense that it does not require polar materials but applies to general conducting systems. We demonstrate that the-third-way results from the temperature-fluctuation-induced dynamical charge redistribution. It is a consequence of the fundamental nonequilibrium thermodynamics and has a deep connection to the topological phase in quantum mechanics. Our findings expand our knowledge and provide new means of thermal-electric energy harvesting.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2015

New opportunities at the frontiers of spintronics

The field of spintronics, or magnetic electronics, is maturing and giving rise to new subfields. These new directions involve the study of collective spin excitations and couplings of the spin system to additional degrees of freedom of a material, as well as metastable phenomena due to perturbations that drive the system far from equilibrium. The interactions lead to possibilities for future applications within the realm of energy-efficient information technologies. Examples discussed herein include research opportunities associated with (i) various spin-orbit couplings, such as spin Hall effects, (ii) couplings to the thermal bath of a system, such as in spin Seebeck effects, (iii) spin-spin couplings, such as via induced and interacting magnon excitations, and (iv) spin-photon couplings, such as in ultra-fast magnetization switching due to coherent photon pulses. These four basic frontier areas of research are giving rise to new applied disciplines known as spin-orbitronics, spin-caloritronics, magnonics, and spin-photonics, respectively. These topics are highlighted in order to stimulate interest in the new directions that spintronics research is taking, and to identify open issues to pursue.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Design for a spin-Seebeck diode based on two-dimensional materials

Studies of the spin-Seebeck effect (SSE) are very important for the development of fundamental science and novel low-power-consumption technologies. The spin-Seebeck diode (SSD), in which the spin current can be driven by a forward temperature gradient but not by a reverse temperature gradient, is a key unit in spin caloritronic devices. Here, we propose a SSD design using two-dimensional (2D) materials such as silicene and phosphorene nanoribbons as the source and drain. Due to their unique band structures and magnetic states, thermally driven spin-up and spin-down currents flow in opposite directions. This mechanism is different from that of the previous one, which uses two permalloy circular disks, and the SSD in our design can be easily integrated with gate voltage control. Moreover, since the concept of this design is rather general and applicable to many 2D materials, it is promising for the realization and exploitation of SSDs in nanodevices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗