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  • 1.
    Paul, Biplab
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Björk, Emma M.
    Linköping University, Department of Physics, Chemistry and Biology, Nanostructured Materials. Linköping University, Faculty of Science & Engineering.
    Kumar, Aparabal
    Materials Science Centre, Indian Institute of Technology, Kharagpur 721302, India.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Nanoporous Ca3Co4O9 Thin Films for Transferable Thermoelectrics2018In: ACS applied energy materials, ISSN 2574-0962, Vol. 1, no 5, p. 2261-2268Article in journal (Refereed)
    Abstract [en]

    The development of high-performance and transferable thin-film thermoelectric materials is important for low-power applications, e.g., to power wearable electronics, and for on-chip cooling. Nanoporous films offer an opportunity to improve thermoelectric performance by selectively scattering phonons without affecting electronic transport. Here, we report the growth of nanoporous Ca3Co4O9 thin films by a sequential sputtering-annealing method. Ca3Co4O9 is promising for its high Seebeck coefficient and good electrical conductivity and important for its nontoxicity, low cost, and abundance of its constituent raw materials. To grow nanoporous films, multilayered CaO/CoO films were deposited on sapphire and mica substrates by rf-magnetron reactive sputtering from elemental Ca and Co targets, followed by annealing at 700 C to form the final phase of Ca3Co4O9. This phase transformation is accompanied by a volume contraction causing formation of nanopores in the film. The thermoelectric propoperties of the nanoporous Ca3Co4O9 films can be altered by controlling the porosity. The lowest electrical resistivity is ~7 mO cm, yielding a power factor of 2.32 × 10-4 Wm-1K-2 near room temperature. Furthermore, the films are transferable from the primary mica substrates to other arbitrary polymer platforms by simple dry transfer, which opens an opportunity of low-temperature use these materials.

  • 2.
    Paul, Biplab
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Schroeder, Jeremy Leroy
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Kerdsongpanya, Sit
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    van Nong, Ngo
    Risö-DTU, Denmark.
    Schell, Norbert
    Helmholtz-Zentrum Geestacht, Germany.
    Ostach, Daniel
    Helmholtz-Zentrum Geestacht, Germany.
    Lu, Jun
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Birch, Jens
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Eklund, Per
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, The Institute of Technology.
    Mechanism of Formation of the Thermoelectric Layered Cobaltate Ca3Co4O9 by Annealing of CaO-CoO Thin Films2015In: Advanced Electronic Materials, ISSN 2199-160X, Vol. 1, no 3, article id 1400022Article in journal (Refereed)
    Abstract [en]

    The layered cobaltate Ca3Co4O9 is of interest for energy-harvesting and heat-conversion applications because of its good thermoelectric properties and the fact that the raw materials Ca and Co are nontoxic, abundantly available, and inexpensive. While single-crystalline Ca3Co4O9 exhibits high Seebeck coefficient and low resistivity, its widespread use is hampered by the fact that single crystals are too small and expensive. A promising alternative approach is the growth of highly textured and/or epitaxial Ca3Co4O9 thin films with correspondingly anisotropic properties. Here, we present a two-step sputtering/annealing method for the formation of highly textured virtually phase-pure Ca3Co4O9 thin films by reactive cosputtering from Ca and Co targets followed by an annealing process at 730 °C under O2-gas flow. The thermally induced phase transformation mechanism is investigated by in situ time-resolved annealing experiments using synchrotron-based 2D X-ray diffraction (XRD) as well as ex situ annealing experiments and standard lab-based XRD. By tuning the proportion of initial CaO and CoO phases during film deposition, the method enables synthesis of Ca3Co4O9 thin films as well as CaxCoO2. With this method, we demonstrate production of epitaxial Ca3Co4O9 thin films with in-plane electrical resistivity of 6.44 mΩ cm and a Seebeck coefficient of 118 μV K−1 at 300 K.

  • 3.
    Rawat, Pankaj K.
    et al.
    Indian Institute Technology, India; Science and Engn Research Board, India.
    Paul, Biplab
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Simple design for Seebeck measurement of bulk sample by 2-probe method concurrently with electrical resistivity by 4-probe method in the temperature range 300-1000 K2016In: Measurement, ISSN 0263-2241, E-ISSN 1873-412X, Vol. 91, p. 613-619Article in journal (Refereed)
    Abstract [en]

    The 4-probe method has so far been the most popular method for concurrent measurement of Seebeck coefficient and electrical resistivity of bulk samples. However, for Seebeck measurement with higher accuracy, the 2-probe method is becoming preferred over 4-probe method. The problem with the previous apparatus designs is that they do not allow 2-probe arrangement for Seebeck measurement simultaneously with linear 4-probe arrangement for electrical resistivity measurement. So, the challenge is find a design where two different probe arrangements become possible at the same time in a single measurement run. Here, we report design and fabrication of an apparatus that allows Seebeck measurement by 2-probe method concurrently with electrical resistivity by 4-probe method of bar and disc samples in the temperature range from 300 to 1000 K. The uniqueness of the present design is that it does not require heating the entire sample chamber for temperature dependent measurements. This is because a small cylindrical furnace inside the sample chamber is used to control the sample background temperature. This internal furnace arrangement results in readily achievable set temperature with desired uniformity. Thus, it allows faster thermoelectric evaluation of samples. The design includes several preventive steps to negate the effect of off-axial heat flows on the measurement accuracy. The measurement error in Seebeck coefficient and electrical resistivity of PbTe sample is estimated to be smaller than 5%. (C) 2016 Elsevier Ltd. All rights reserved.

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