41 research outputs found
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Towards tellurium-free thermoelectric modules for power generation from low-grade heat
Thermoelectric technology converts heat into electricity directly and is a promising source of clean electricity. Commercial thermoelectric modules have relied on Bi2Te3-based compounds because of their unparalleled thermoelectric properties at temperatures associated with low-grade heat (<550âK). However, the scarcity of elemental Te greatly limits the applicability of such modules. Here we report the performance of thermoelectric modules assembled from Bi2Te3-substitute compounds, including p-type MgAgSb and n-type Mg3(Sb,Bi)2, by using a simple, versatile, and thus scalable processing routine. For a temperature difference of ~250âK, whereas a single-stage module displayed a conversion efficiency of ~6.5%, a module using segmented n-type legs displayed a record efficiency of ~7.0% that is comparable to the state-of-the-art Bi2Te3-based thermoelectric modules. Our work demonstrates the feasibility and scalability of high-performance thermoelectric modules based on sustainable elements for recovering low-grade heat
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Signatures of a Charge Density Wave Phase and the Chiral Anomaly in the Fermionic Material Cobalt Monosilicide CoSi
Materials with topological electronic states have emerged as one of the most exciting discoveries of condensed quantum matter, hosting quasiparticles with extremely low effective mass and high mobility. Weyl materials contain such topological states in the bulk and additionally have a non-trivial chiral charge. However, despite known quantum effects caused by these chiral states, the interplay between chiral states, and a charge density wave phase, an ordering of the electrons to a correlated phase is not experimentally explored. Indications for the formation of a charge density wave phase in the Weyl material cobalt monosilicide CoSi are observed. Furthermore, the typical signatures of the charge density wave phase together with typical signatures of Weyl fermions in magnetic field dependent electrical transport characterization are investigated. The charge density wave and the chiral contribution to the electrical magneto-transport are separated as well as a suppression of the charge density wave phase is observed in magnetic fields. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Transparent Power-Generating Windows Based on Solar-Thermal-Electric Conversion
Zhang Q, Huang A, Ai X, et al. Transparent Power-Generating Windows Based on Solar-Thermal-Electric Conversion. Advanced Energy Materials . 2021: 2101213.Integrating transparent solar-harvesting systems into windows can provide renewable on-site energy supply without altering building aesthetics or imposing further design constraints. Transparent photovoltaics have shown great potential, but the increased transparency comes at the expense of reduced power-conversion efficiency. Here, a new technology that overcomes this limitation by combining solar-thermal-electric conversion with a material's wavelength-selective absorption is presented. A wavelength-selective film consisting of Cs0.33WO3 and resin facilitates high visible-light transmittance (up to 88%) and outstanding ultraviolet and infrared absorbance, thereby converting absorbed light into heat without sacrificing transparency. A prototype that couples the film with thermoelectric power generation produces an extraordinary output voltage of approximate to 4 V within an area of 0.01 m(2) exposed to sunshine. Further optimization design and experimental verification demonstrate high conversion efficiency comparable to state-of-the-art transparent photovoltaics, enriching the library of on-site energy-saving and transparent power generation
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Thermoelectric Characterization Platform for Electrochemically Deposited Materials
Successful optimization of the thermoelectric (TE) performance of materials, described by the figure of merit zT, is a key enabler for its application in energy harvesting or Peltier cooling devices. While the zT value of bulk materials is accessible by a variety of commercial measurement setups, precise determination of the zT value for thin and thick films remains a great challenge. This is particularly relevant for films synthesized by electrochemical deposition, where the TE material is deposited onto an electrically conductive seed layer causing an in-plane short circuit. Therefore, a platform for full in-plane zT characterization of electrochemically deposited TE materials is developed, eliminating the impact of the electrically conducting seed layer. The characterization is done using a suspended TE material within a transport device which was prepared by photolithography in combination with chemical etching steps. An analytical model to determine the thermal conductivity is developed and the results verified using finite element simulations. Taken together, the full in-plane zT characterization provides an inevitable milestone for material optimization under realistic conditions in TE devices. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Influence of the substrate-induced strain and irradiation disorder on the Peierls transition in TTF-TCNQ microdomains
The influence of the combined effects of substrate-induced strain, finite
size and electron irradiation-induced defects have been studied on individual
micron-sized domains of the organic charge transfer compound
tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) by temperature-dependent
conductivity and current-voltage measurements. The individual domains have been
isolated by focused ion beam etching and electrically contacted by focused ion
and electron beam induced deposition of metallic contacts. The
temperature-dependent conductivity follows a variable range hopping behavior
which shows a crossover of the exponent as the Peierls transition is
approached. The low temperature behavior is analyzed within the segmented rod
model of Fogler, Teber and Shklowskii, as originally developed for a
charge-ordered quasi one-dimensional electron crystal. The results are compared
with data obtained on as-grown and electron irradiated epitaxial TTF-TCNQ thin
films of the two-domain type
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Doping High-Mobility Donor : Acceptor Copolymer Semiconductors with an Organic Salt for High-Performance Thermoelectric Materials
Organic semiconductors (OSCs) are attractive for fabrication of thermoelectric devices with low cost, large area, low toxicity, and high flexibility. In order to achieve high-performance organic thermoelectric devices (OTEs), it is essential to develop OSCs with high conductivity (Ï), large Seebeck coefficient (S), and low thermal conductivity (Îș). It is equally important to explore efficient dopants matching the need of thermoelectric devices. The thermoelectric performance of a high-mobility donorâacceptor (DâA) polymer semiconductor, which is doped by an organic salt, is studied. Both a high p-type electrical conductivity approaching 4 S cmâ1 and an excellent power factor (PF) of 7 ”W Kâ2 mâ1 are obtained, which are among the highest reported values for polymer semiconductors. Temperature-dependent conductivity, Seebeck coefficient and power factor of the doped materials are systematically investigated. Detailed analysis on the results of thermoelectric measurements has revealed a hopping transport in the materials, which verifies the empirical relationship: S â Ïâ1/4 and PF â Ï1/2. The results demonstrate that DâA copolymer semiconductors with proper combination of dopants have great potential for fabricating high-performance thermoelectric devices. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
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Temperature gradient-induced magnetization reversal of single ferromagnetic nanowires
In this study, we investigate the temperature- and temperature gradient-dependent magnetization reversal process of individual, single-domain Co39Ni61 and Fe15Ni85 ferromagnetic nanowires via the magneto-optical Kerr effect and magnetoresistance measurements. While the coercive fields (HC) and therefore the magnetic switching fields (HSW) generally decrease under isothermal conditions at elevated base temperatures (Tbase), temperature gradients (ÎT) along the nanowires lead to an increased switching field of up to 15% for ÎTââ= 300âK in Co39Ni61 nanowires. This enhancement is attributed to a stress-induced, magneto-elastic anisotropy term due to an applied temperature gradient along the nanowire that counteracts the thermally assisted magnetization reversal process. Our results demonstrate that a careful distinction between locally elevated temperatures and temperature gradients has to be made in future heat-assisted magnetic recording devices
A Tunable Strain Sensor Using Nanogranular Metals
This paper introduces a new methodology for the fabrication of strain-sensor elements for MEMS and NEMS applications based on the tunneling effect in nano-granular metals. The strain-sensor elements are prepared by the maskless lithography technique of focused electron-beam-induced deposition (FEBID) employing the precursor trimethylmethylcyclopentadienyl platinum [MeCpPt(Me)3]. We use a cantilever-based deflection technique to determine the sensitivity (gauge factor) of the sensor element. We find that its sensitivity depends on the electrical conductivity and can be continuously tuned, either by the thickness of the deposit or by electron-beam irradiation leading to a distinct maximum in the sensitivity. This maximum finds a theoretical rationale in recent advances in the understanding of electronic charge transport in nano-granular metals
Untersuchung von Size-Effekten thermischer Transportkoeffizienten von NanodrÀhten
In den letzten Jahren haben die ForschungsaktivitĂ€ten im Bereich Thermoelektrik stetig zugenommen. Das neu erweckte Interesse an der Thermoelektrik ist zurĂŒckzufĂŒhren auf neue nanostrukturierte Materialien, Quantenschicht-Strukturen und NanodrĂ€hte, welche
eine wesentliche Steigerung der thermoelektrischen EffektivitĂ€t Z im Vergleich zum Massivmaterial versprechen. FĂŒr NanodrĂ€hte ist die gröĂte Steigerung der thermoelektrischen EffektivitĂ€t zu erwarten. Zur BestĂ€tigung der Theorie bedarf es neuer Messmethoden zur Bestimmung des Seebeck-Koeffizienten S, der elektrischen LeitfĂ€higkeit Ï und der WĂ€rmeleitfĂ€higkeit λ, um hieraus eine Steigerung der thermoelektrischen EffektivitĂ€t Z = (Sexp2)Ï/λ experimentell zu bestĂ€tigen.
Der Schwerpunkt der Doktorarbeit lag in der Untersuchung thermoelektrischer Eigenschaften von NanodrÀhten. Hierzu wurden neueMessmethoden zur Bestimmung der elektrischen und thermischen LeitfÀhigkeit von NanodrÀhten entwickelt.
Die elektrische und thermische LeitfĂ€higkeit von Pt-NanodrĂ€hten wurden mit dem in dieser Arbeit entwickelten λ-Chip gemessen. Die elektrische LeitfĂ€higkeit der Pt-NanodrĂ€hte ist im Vergleich zum Massivmaterial entsprechend der klassischen Size-Effekt-Theorie reduziert. Ebenso wurde eine Abnahme der WĂ€rmeleitfĂ€higkeit beobachtet. Die Ergebnisse stimmen mit den im Rahmen der klassischen Size-Effekt-Theorie zu erwartenden Resultaten gut ĂŒberein, jedoch bedarf die Reduzierung der Lorenz-Zahl noch einer theoretischen ErklĂ€rung.
Im Weiteren wurde die elektrische LeitfĂ€higkeit von BixTe1-x und BixSb1-x-NanodrĂ€hten mit dem λ-Chip bestimmt. Hierzu wurden zunĂ€chst unterschiedliche Kontaktmaterialien getestet, um die Diffusion des Kontaktmaterials in den Nanodraht auszuschlieĂen. Als bewĂ€hrtes Kontaktmaterial stellte sich ein Schichtsystem aus Titan und Gold heraus. Die Ti-Schicht wirkt hierbei als Diffusionsbarriere und Haftvermittler-Schicht. Die WĂ€rmeleitfĂ€higkeit der Bi-haltigen NanodrĂ€hte konnte mit dem λ-Chip nicht gemessen werden, da die UnterĂ€tzung der NanodrĂ€hte mittels reaktivem IonenĂ€tzen die NanodrĂ€hte angriff. Als Alternative können die NanodrĂ€hte auf dem λ-Chip mit einem fokusierten Ionenstrahl unterĂ€tzt werden. Der Aufwand hierzu ist jedoch relativ hoch und diese Alternative wurde deshalb nicht weiter verfolgt. Als weitere Alternative wurde der Z-Chip entwickelt. Hierbei werden die NanodrĂ€hte auf den fertigen Chip aufgebracht und mittels Elektronenstrahl-induzierter Deposition an den elektrischen Kontakten fixiert. Der Chip ermöglicht die Messung der elektrische LeitfĂ€higkeit in 4-Punkt-Anordnung, der WĂ€rmeleitfĂ€higkeit und des Seebeck-Koeffizienten an
einem einzelnen Nanodraht. Somit ist die Bestimmung der thermoelektrischen EffektivitĂ€t an einem Nanodraht möglich. DesWeiteren wurden die theoretischen Grundlagen zur Bestimmung der WĂ€rmekapazitĂ€t an einzelnen NanodrĂ€hten mit dem Z-Chip prĂ€sentiert. Zum Zeitpunkt der DurchfĂŒhrung dieser Arbeit fehlte jedoch das notwendige Equipment zur AusfĂŒhrung der WĂ€rmekapazitĂ€tsmessung an einzelnen NanodrĂ€hten.
Des Weiteren wurde die Cross-Plane Methode zur Bestimmung der WĂ€rmeleitfĂ€higkeit an eingebetteten NanodrĂ€hten entwickelt. Analog der Messmethode, welche fĂŒr die Einzeldrahtmessungen verwendet wird, handelt es sich hierbei um eine stationĂ€re âJoule-Heatingâ Methode. Die Temperaturdifferenz wird aus der WiderstandsĂ€nderung einer auf die eingebetteten NanodrĂ€hte aufgebrachten Heizschicht bestimmt.Mit derMethode wurde die WĂ€rmeleitfĂ€higkeit von BixTe1-x-NanodrĂ€hten ermittelt.
Die elektrische LeitfĂ€higkeit wurde von BixTe1-x-NanodrĂ€hten unterschiedlicher Zusammensetzung und Herstellungsparameter mit dem λ- und dem Z-Chip bestimmt. Die gemessenen NanodrĂ€hte zeigen sowohl intrinsisches wie extrinsisches Leitungsverhalten verbunden mit einer, im Vergleich zum Volumenmaterial, reduzierten TemperaturabhĂ€ngigkeit der elektrischen LeitfĂ€higkeit infolge von OberflĂ€chen- und Korngrenzenstreuung der LadungstrĂ€ger. Die elektrischen LeitfĂ€higkeitsmessungen stimmen mit Beobachtungen anderer Gruppen gut ĂŒberein.
Die WĂ€rmeleitfĂ€higkeit konnte an einem einzelnen BixTe1-x-Nanodraht und an eingebetteten BixTe1-x-NanodrĂ€hten gemessen werden. Die WĂ€rmeleitfĂ€higkeit ist gegenĂŒber dem Massivmaterial reduziert. Die Ergebnisse sind in guter Ăbereinstimmung mit bisher publizierten Ergebnissen von Bismuttellurid-NanodrĂ€hten
Data publication: Integration of Multifunctional Epitaxial (Magnetic) Shape Memory Films in Silicon Microtechnology
Measured raw data (XRD, texture, SEM, PPMS and EDX