  First thermoelectric properties measurements on bulk nanostructured
Ba8Ga16Ge30 clathrate-I are presented. A sol-gel-calcination route was
developed for preparing amorphous nanosized precursor oxides. The further
reduction of the oxides led to quantitative yield of crystalline nanosized
Ba8Ga16Ge30 clathrate-I. TEM investigations show the clathrate nanoparticles
retain the size and morphology of the precursor oxides. The clathrate
nanoparticles contain mainly thin plates (approx. 300 nm x 300 nm x 50 nm) and
a small amount of nanospheres (diameter ~ 10 nm). SAED patterns confirm the
clathrate-I structure type for both morphologies. The powders were compacted
via Spark Plasma Sintering (SPS) to obtain a bulk nano-structured material. The
Seebeck coefficient S, measured on low-density samples (53% of {\delta}x-ray),
reaches -145 {\mu}V/k at 375 {\deg}C. The ZT values are quite low (0.02) due to
the high resistivity of the sample (two orders of magnitude larger than bulk
materials) and the low sample density. The trend of the temperature dependence
of S is in agreement with the values obtained from electronic structure
calculations and semi-classical Boltzmann transport theory within the constant
scattering approximation. The total thermal conductivity (1.61 W/mK), measured
on high density samples (93% of {\delta}x-ray), shows a reduction of 20-25% in
relation to the bulk materials (2.1 W/mK). A further shaping of the sample for
the Seebeck coefficient and electrical conductivity measurements was not
possible due to the presence of cracks. An improvement on the design of the
pressing tools, loading of the sample and profile of the applied pressure will
enhance the mechanical stability of the samples. These investigations are now
in progress.
