Correlation between processing conditions, microstructure and charge transport in half-Heusler alloys

Journal Article · Wed May 01 00:00:00 EDT 2013 · Journal of Solid State Chemistry

DOI:https://doi.org/10.1016/J.JSSC.2013.02.030· OSTI ID:22306297

Makongo, Julien P.A. ^[1]; Zhou, Xiaoyuan ^[2]; Misra, Dinesh K. ^[3]; Uher, Ctirad ^[2]

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1. Laboratory for Emerging Energy and Electronic Materials, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109 (United States)
2. Department of Physics, University of Michigan, Ann Arbor, MI 48109 (United States)
3. The Advanced Materials Research Institute, University of New Orleans, New Orleans, LA 70148 (United States)

Five bulk samples of n-type Zr₀.₂₅Hf₀.₇₅NiSn₀.₉₇₅Sb₀.₀₂₅ half-Heusler (HH) alloy were fabricated by reacting elemental powders via (1) high temperature solid state (SS) reaction and (2) mechanical alloying (MA), followed by densification using spark plasma sintering (SPS) and/or hot pressing (HP). A portion of the sample obtained by SS reaction was mechanically alloyed before consolidation by hot pressing (SS–MA–HP). X-ray powder diffraction and transmission electron microscopy studies revealed that all SS specimen (SS–SPS, SS–HP, SS–MA–HP) are single phase HH alloys, whereas the MA sample (MA–SPS) contains metallic nanoprecipitates. Electronic and thermal transport measurements showed that the embedded nanoprecipitates induce a drastic increase in the carrier concentration (n), a large decrease in the Seebeck coefficient (S) and a marginal decrease in the lattice thermal conductivity (κ_l) of the MA–SPS sample leading to lower ZT values when compared to the SS–HP samples. Constant values of S are observed for the SS series regardless of the processing method. However, a strong dependence of the carrier mobility (μ), electrical conductivity (σ) and κ_l on the processing and consolidation method is observed. For instance, mechanical alloying introduces additional structural defects which enhance electron and phonon scattering leading to moderately low values of μ and large reduction in κ_l. This results in a net 20% enhancement in the figure of merit (ZT=0.6 at 775 K). HH specimen of the same nominal composition with higher ZT is anticipated from a combination of SS reaction, MA and SPS (SS–MA–SPS). - Graphical abstract: In half-Heusler alloys, thermopower values are insensitive to processing method, whereas carrier mobility (μ), electrical conductivity (σ), and κ_l strongly dependent on the microstructure which in turn is altered by the synthesis, processing and consolidation method. Highlights: • Phase composition of HH alloy strongly depends on the synthesis technique. • Mechanical alloying of elements yields bulk HH alloy with metallic impurity phases. • Thermopower, carrier density, and effective mass of HHs are insensitive to processing conditions. • Mechanical alloying decreases the carrier mobility and lattice thermal conductivity of bulk HH.

• OSTI ID: 22306297
• Journal Information: Journal of Solid State Chemistry, Vol. 201; Other Information: Copyright (c) 2013 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA); ISSN 0022-4596
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
77 NANOSCIENCE AND NANOTECHNOLOGY
CARRIER MOBILITY
CHARGE TRANSPORT
ELECTRIC CONDUCTIVITY
HEUSLER ALLOYS
HOT PRESSING
MICROSTRUCTURE
NANOSTRUCTURES
PLASMA
PROCESSING
SOLIDS
SYNTHESIS
THERMAL CONDUCTIVITY
TRANSMISSION ELECTRON MICROSCOPY
X-RAY DIFFRACTION