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Nanoparticles (NPs) of Ba8Ga16Ge30 clathrate-I were synthetized via
sol-gel-calcination/chemical-reduction route. A solution of the metallic
cations was used to prepare an acryl-amid-based gel. The gel is dried
and calcined to obtain nanocrystalline powders of precursor oxides. The
oxides are reduced by reacting with CaH2 to produce the clathrate, which
is embedded in a CaO matrix. CaO is removed by a washing step to obtain
the clathrate NPs. The shape and size of the precursor oxide NPs can be
modified by addition of complexing agents, surfactants or by varying the
pH or the metal and surfactant concentration in the gel. Powder X-ray
diffraction and SAED patterns confirm the clathrate-I-type crystal
structure of the products. SEM/TEM investigations show that the size and
morphology of the oxides are retained in the clathrate NPs after the
reduction. The clathrate NPs exhibit morphology of thin plates similar
to 300 x similar to 300 nm(2) and thickness of similar to 50 nm, or
sphere-like morphology with similar to 200 nm diameter, depending on the
sol gel synthesis conditions. The clathrate NPs were compacted via spark
plasma sintering (SPS) to pellets with 53-93 % of crystallographic
density. The total thermal conductivity (kappa) of the pellet with 93 %
density shows a reduction of 25 % in comparison to the reported K in
bulk clathrate. Preliminary characterization of the Seebeck (S) and
electrical resistivity (R) of the low density sample (53 %) indicates
n-type conduction and semiconductor behavior of the Ba8Ga16Ge30
clathrate-I. The transport properties of Ba8Ga16Ge30 clathrate-I with
3-, 4- or 5-layer slabs and [100] surface termination as well as of the
bulk material were calculated by using the semi-classical Boltzmann
transport theory within the constant scattering approximation. Our
results show an increase in S for the geometries with reduced dimensions
in agreement with the experimental observations. (C) 2015 WILEY-VCH
Verlag GmbH & Co. KGaA, Weinheim
