Temperature dependent solubility of Yb in Yb–CoSb3 skutterudite and its effect on preparation, optimization and lifetime of thermoelectrics

Abstract

Although filled skutterudites are actively being developed for automotive waste heat recovery the compositions considered available are limited. Typically synthesis conditions are chosen with slight excess filling element to produce filled skutterudites at the 'filling fraction limit', traditionally considered to be a single value that depends only on the elements involved (e.g. filling element, Co/Fe ratio). The filling fraction limit is often debated and thought perhaps to vary with processing method. This work opens up a new dimension of available compositions by showing that the 'filling fraction limit' is a thermodynamic quantity that for Yb varies by a factor of 5 depending on annealing temperature and nominal composition. As the filling element controls the electronic doping in these semiconductors this study not only enables optimization of thermoelectric properties using thermodynamic control rather than non-equilibrium processing conditions (reaching zT ¼ 1.3 at 850 K without nanostructures) but it also predicts dopant precipitation effects after extended use. The novel phase diagram approach used here should be easily applied to other ternary thermoelectric materials to uncover similar phenomena. Thus skutterudite material with the optimized thermoelectric composition can be produced from a range of nominal compositions with appropriate annealing.

Promising Thermoelectric Bulk Materials with 2D Structures

“…BiCuSeO also exhibits an intrinsically low thermal conduc tivity, although the power factor for pristine BiCuSeO is not that striking compared to state of the art TE materials [116][117][118][119] [44,46,55,114,115] and fitting curves from the single parabolic band model. Reproduced with permission.…”

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

“…12,[15][16][17][18]21 These experimental results all seem to indicate a strong dependence on the synthetic conditions, which most likely relates to the temperature dependence of the FFL or thermodynamically controlled chemical reaction processes. 22 Dilley et al 18 experimentally derived a Yb FFL of 0.22 from the lattice parameters vs nominal composition, when the samples were synthesized via a combination of induction melting and annealing. Using a non-equilibrium melt-spinning method, Li et al 17 observed a high Yb-filling fraction of 0.28-0.29 (electron probe microanalysis results) for the sample with nominal composition Yb 0.3 Co 4 Sb 12+y .…”

“…Special attention should be paid to the fact that the Yb FFLs determined from many of these previous studies lack sufficient evidence to unambiguously conclude that such high FFLs are true, and none of them show combined experimental results for lattice parameters and experimentally derived Yb compositions as assessed by electron probe microanalysis or energy-dispersive X-ray spectroscopy (EDS). Most recently, Tang et al 22 studied the Yb-Co-Sb ternary phase diagram and argued that Yb FFL is a thermodynamic parameter that depends on the annealing temperature and nominal composition, which could possibly account for some of the FFL discrepancies. In addition, only a few studies have been devoted to the role of excess Yb (e.g., Yb 2 O 3 ) and its effects on the TE transport properties while exceeding the FFL, 23,24 particularly in light of the fact that such excess Yb may form the electrically insulating Yb 2 O 3 or other impurity phases such as metallic YbSb or YbSb 2 .…”

Electrical and thermal transport properties of Y bxCo4Sb12 filled skutterudites with ultrahigh carrier concentrations

“…Therefore, as a promising TE material, the Yb-filled CoSb 3 system has been extensively studied since it was first reported in 2000. [14][15][16][17][18][19][20][21][22] Apart from single-filled skutterudites, Yb has been also used being the filling elements for the double-filled or multiple-filled skutterudites, which possesses higher TE performances. [23][24][25] For example, the maximum TE figure of merit (zT) of 1.7 and 2.0, have been reported for (Ba, La, Yb) x Co 4 Sb 12 and (Sr, Ba, Yb) x Co 4 Sb 12 , respectively.…”

“…However, although there has been numerous studies which relates to the Yb x Co 4 Sb 12 system, the maximum n value reported so far is only around 6 × 10 20 cm −3 because the real Yb filling fraction in CoSb 3 is limited around 0.20. [14][15][16][17][18][19][20][21][22] The lack of electrical and thermal transport properties for the Yb x Co 4 Sb 12 samples under higher carrier concentrations makes correlating optimal n value with the highest TE performance still unclear. Addressing this issue, the Yb x Co 4 Sb 12 samples with larger filling fractions are required.…”