Comparison of crystal structure and thermoelectric properties in Ba8CuxSi46-x clathrate compounds synthesized by the Czochralski method and spark plasma sintering

In this study, Ba8CuxSi46-x clathrate samples synthesized by the Czochralski (CZ) method and the spark plasma sintering (SPS) technique were compared to evaluate how the synthesis process influences the structure, electrical transport properties, and power factor. Based on the XRD patterns, it was confirmed that both samples exhibited the characteristic clathrate crystal structure. In contrast, BSE and SEM observations revealed that the CZ-grown sample consisted of a homogeneous clathrate phase (Ba8Cu4Si42), whereas the SPS sample also contained Ba-rich secondary phases along with numerous cracks and voids. Additionally, EBSD analysis confirmed that the CZ sample had substantially larger crystal grains than the SPS sample, with grain size exceeding 50 μm within the observed area. Thermoelectric characterization indicated that the CZ-grown and SPS samples showed closely similar Seebeck coefficients (−49.9 μV K−1 and -46.1 μV K−1 at 550 °C, respectively). In contrast, the electrical resistivity of the CZ sample was significantly lower (0.56 mΩ cm) than that of the SPS sample (1.80 mΩ cm), which can be attributed to reduced grain boundary scattering. Consequently, the power factor of the CZ sample reached up to 4.33 μWcm−1K−2, which was 3.7 times higher than that of the SPS sample (1.17 μWcm−1K−2) at 550 °C. The PF value of Ba–Cu–Si clathrate synthesized by the CZ method in this study did not reach the practical benchmark of 10 μWcm−1K−2. Nevertheless, these results suggest that the CZ method is considered a promising approach for improving the thermoelectric performance of clathrate compounds.