Nanostructuring of Ba8Ga16Ge30 clathrates

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.
Comment: 21 pages, 10 figures