Browsing by Author "Pérez, Nicolás"

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    Europium Clustering and Glassy Magnetic Behavior in Inorganic Clathrate-VIII Eu8Ga16Ge30
    (Basel : MDPI, 2022) Pérez, Nicolás; Sahoo, Manaswini; Schierning, Gabi; Nielsch, Kornelius; Nolas, George S.
    The temperature- and field-dependent, electrical and thermal properties of inorganic clathrate-VIII Eu8Ga16Ge30 were investigated. The type VIII clathrates were obtained from the melt of elements as reported previously. Specifically, the electrical resistivity data show hysteretic magnetoresistance at low temperatures, and the Seebeck coefficient and Hall data indicate magnetic interactions that affect the electronic structure in this material. Heat capacity and thermal conductivity data corroborate these findings and reveal the complex behavior due to Eu2+ magnetic ordering and clustering from approximately 13 to 4 K. Moreover, the low-frequency dynamic response indicates Eu8Ga16Ge30 to be a glassy magnetic system. In addition to advancing our fundamental understanding of the physical properties of this material, our results can be used to further the research for potential applications of interest in the fields of magnetocalorics or thermoelectrics.
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    Grain Boundary Phases in NbFeSb Half-Heusler Alloys: A New Avenue to Tune Transport Properties of Thermoelectric Materials
    (Weinheim : Wiley-VCH, 2023) Bueno Villoro, Ruben; Zavanelli, Duncan; Jung, Chanwon; Mattlat, Dominique Alexander; Hatami Naderloo, Raana; Pérez, Nicolás; Nielsch, Kornelius; Snyder, Gerald Jeffrey; Scheu, Christina; He, Ran; Zhang, Siyuan
    Many thermoelectric materials benefit from complex microstructures. Grain boundaries (GBs) in nanocrystalline thermoelectrics cause desirable reduction in the thermal conductivity by scattering phonons, but often lead to unwanted loss in the electrical conductivity by scattering charge carriers. Therefore, modifying GBs to suppress their electrical resistivity plays a pivotal role in the enhancement of thermoelectric performance, zT. In this work, different characteristics of GB phases in Ti-doped NbFeSb half-Heusler compounds are revealed using a combination of scanning transmission electron microscopy and atom probe tomography. The GB phases adopt a hexagonal close-packed lattice, which is structurally distinct from the half-Heusler grains. Enrichment of Fe is found at GBs in Nb0.95Ti0.05FeSb, but accumulation of Ti dopants at GBs in Nb0.80Ti0.20FeSb, correlating to the bad and good electrical conductivity of the respective GBs. Such resistive to conductive GB phase transition opens up new design space to decouple the intertwined electronic and phononic transport in thermoelectric materials.
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    High-performance giant magnetoresistive sensorics on flexible Si membranes
    (Melville, NY : American Inst. of Physics, 2015) Pérez, Nicolás; Melzer, Michael; Makarov, Denys; Ueberschär, Olaf; Ecke, Ramona; Schulz, Stefan E.; Schmidt, Oliver G.
    We fabricate high-performance giant magnetoresistive (GMR) sensorics on Si wafers, which are subsequently thinned down to 100 μm or 50 μm to realize mechanically flexible sensing elements. The performance of the GMR sensors upon bending is determined by the thickness of the Si membrane. Thus, bending radii down to 15.5 mm and 6.8 mm are achieved for the devices on 100 μm and 50 μm Si supports, respectively. The GMR magnitude remains unchanged at the level of (15.3 ± 0.4)% independent of the support thickness and bending radius. However, a progressive broadening of the GMR curve is observed associated with the magnetostriction of the containing Ni81Fe19 alloy, which is induced by the tensile bending strain generated on the surface of the Si membrane. An effective magnetostriction value of λs = 1.7 × 10−6 is estimated for the GMR stack. Cyclic bending experiments showed excellent reproducibility of the GMR curves during 100 bending cycles.
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    Thermoelectric Characterization Platform for Electrochemically Deposited Materials
    (Weinheim : Wiley-VCH Verlag GmbH & Co. KG, 2020) Barati, Vida; Garcia Fernandez, Javier; Geishendorf, Kevin; Schnatmann, Lauritz Ule; Lammel, Michaela; Kunzmann, Alexander; Pérez, Nicolás; Li, Guodong; Schierning, Gabi; Nielsch, Kornelius; Reith, Heiko
    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, Weinheim