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First thermoelectric properties measurements on bulk nanostructuredBa8Ga16Ge30 clathrate-I are presented. A sol-gel-calcination route wasdeveloped for preparing amorphous nanosized precursor oxides. The furtherreduction of the oxides led to quantitative yield of crystalline nanosizedBa8Ga16Ge30 clathrate-I. TEM investigations show the clathrate nanoparticlesretain the size and morphology of the precursor oxides. The clathratenanoparticles contain mainly thin plates (approx. 300 nm x 300 nm x 50 nm) anda small amount of nanospheres (diameter ~ 10 nm). SAED patterns confirm theclathrate-I structure type for both morphologies. The powders were compactedvia Spark Plasma Sintering (SPS) to obtain a bulk nano-structured material. TheSeebeck 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 tothe high resistivity of the sample (two orders of magnitude larger than bulkmaterials) and the low sample density. The trend of the temperature dependenceof S is in agreement with the values obtained from electronic structurecalculations and semi-classical Boltzmann transport theory within the constantscattering approximation. The total thermal conductivity (1.61 W/mK), measuredon high density samples (93% of {\delta}x-ray), shows a reduction of 20-25% inrelation to the bulk materials (2.1 W/mK). A further shaping of the sample forthe Seebeck coefficient and electrical conductivity measurements was notpossible due to the presence of cracks. An improvement on the design of thepressing tools, loading of the sample and profile of the applied pressure willenhance the mechanical stability of the samples. These investigations are nowin progress.

Nanostructuring of Ba8Ga16Ge30 clathrates