High-performance n-type YbxCo4Sb12: from partially filled skutterudites towards composite thermoelectrics

The filling fraction limit (FFL) of skutterudites, that is, the complex balance of formation enthalpies among different species, is an intricate but crucial parameter for achieving high thermoelectric performance. In this work, we synthesized a series of YbxCo4Sb12 samples with x=0.2–0.6 and systemically studied the FFL of Yb, which is still debated even though this system has been extensively investigated for decades. Our combined experimental efforts of X-ray diffraction, microstructural and quantitative compositional analyses clearly reveal a Yb FFL of ~0.29 in CoSb3, which is consistent with previous theoretical calculations. The excess Yb in samples with x>0.35 mainly form metallic YbSb2 precipitates, significantly raising the Fermi level and thus increasing the electrical conductivity and decreasing the Seebeck coefficient. This result is further corroborated by the numerical calculations based on the Bergman’s composite theory, which accurately reproduces the transport properties of the x>0.35 samples based on nominal Yb0.35Co4Sb12 and YbSb2 composites. A maximum ZT of 1.5 at 850 K is achieved for Yb0.3Co4Sb12, which is the highest value for a single-element-filled CoSb3. The high ZT originates from the high-power factor (in excess of 50 μW cm-K−2) and low lattice thermal conductivity (well below 1.0 W m-K−1). More importantly, the large average ZTs, for example, ~1.05 for 300–850 K and ~1.27 for 500–850 K, are comparable to the best values for n-type skutterudites. The high thermoelectric and thermomechanical performances and the relatively low air and moisture sensitivities of Yb make Yb-filled CoSb3, a promising candidate for large-scale power generation applications. Reducing the amount of rare-earths filling in nanovoids of a popular thermoelectric crystal could improve large-scale energy recovery. Minerals known as skutterudites are being investigated as additions to electric vehicles and power-generation stations because they convert heat gradients into electricity at temperatures up to 600 degrees Celsius. To improve the energy conversion efficiency, rare-earth metals such as ytterbium (Yb) are usually incorporated into the skutterudite framework. Jihui Yang from the University of Washington in the US and co-workers have now determined the optimal filling ratio of Yb in these materials. X-ray and microscopy techniques showed that overloading Yb past its thermodynamic equilibrium led to metallic precipitates that negatively impacted performance. When a slightly lower-than-equilibrium fill ratio was used, however, thermoelectric conversion efficiencies approached the best values recorded for skutterudites filled with a single element. High thermoelectric figure of merit ZT of 1.5 and a high average ZT >1.0 between 300 and 850 K can be achieved for Yb-filled CoSb3, which are superior to those of any single-element-filled skutterudite and comparable to the best in this class of materials. The long-term debate about the Yb-filling fraction limit in CoSb3 is clarified to be ~0.29, and the excess Yb mainly forms metallic YbSb2 precipitates. The transport properties of the x >0.35 samples with YbSb2 precipitates are quantitatively reproduced by the Bergman’s composite theory, providing new understanding of the role of Yb in CoSb3.