A class of thermoelectric materials has been synthesized with a thermoelectric figure of meritZT (whereT is temperature andZ is a function of thermopower, electrical resistivity, and thermal conductivity) near 1 at 800 kelvin. Although these materials have not been optimized, this value is comparable to the bestZT values obtained for any previously studied thermoelectric material. Calculations indicate that the optimized material should haveZT values of 1.4. These ternary semiconductors have the general formula RM₄X₁₂ (where R is lanthanum, cerium, praseodymium, neodymium, or europium; M is iron, ruthenium, or osmium; and X is phosphorus, arsenic, or antimony) and represent a new approach to creating improved thermoelectric materials. Several alloys in the composition range CeFe_4−xCo_xSb₁₂ or LaFe_4−xCo_xSb₁₂ (0<x< 4) have large values ofZT.

Barium-filled skutteruditesBa y Co 4 Sb 12   with an anomalously large filling fraction of up toy=0.44 have been synthesized. The lattice parameters increase linearly with Ba content. Magnetic susceptibility data show thatBa 0.44 Co 4 Sb 12   is paramagnetic, which implies that some of the Co atoms inBa y Co 4 Sb 12   have acquired a magnetic moment. The presence of the two different valence states of Co(Co 3+   andCo 2+ )  leads to the anomalously large barium filling fraction even without extra charge compensation. All samples shown-type conduction. The electrical conductivity increases with increasing the Ba filling fraction. The lattice thermal conductivity ofBa y Co 4 Sb 12   is significantly depressed as compared to unfilledCo 4 Sb 12 .  The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature reaching a maximum value of 1.1 forBa 0.24 Co 4 Sb 12   at 850 K.

We present evidence of a relatively high dimensionless figure of merit (ZT) in a polycrystallineskutterudite partially filled with ytterbium ions. The small-diameter yet heavy-mass Yb atoms partially filling the voids of the hostCoSb 3   system exhibit low values of thermal conductivity while the quite favorable electronic properties are not substantially perturbed by the addition of Yb. This combination is ideal for thermoelectric applications exemplifying the “phonon-glass electron-crystal” concept of a thermoelectricmaterial, resulting inZT=0.3  at room temperature andZT∼1  at 600 K forYb 0.19 Co 4 Sb 12 . 

High-performance Sb excessYb 0.2 Co 4 Sb 12+y  (y=0,0.3,0.6,1.0)  bulk materials with nanostructure have been prepared by combining melt spinning technique with spark plasma sintering. Average grain size of the bulk samples is about150nm  wheny=0  and 0.3, and the grain size increases with the increase ofy  . Moderately superfluous Sb may improve the electrical transport properties effectively and the thermal conductivity of the bulk samples decreases markedly due to the nanostructure. The thermoelectric performance of the samples is significantly improved, and the maximum figure of the merit reaches 1.26 for theYb 0.2 Co 4 Sb 12.3   compound at800K  .

Abstract

n-type filled skutterudite Yb_0.26Co₄Sb₁₂ composites in whichp-type GaSb nanostructured inclusions (5–20 nm) were dispersed were fabricated by an in situ method involving the introduction of metastable void-filling impurity Ga and enrichment of Sb in the synthesis procedure. With the homogeneously dispersed GaSb nanoinclusions, which probably result from the scattering of low-energy electrons by the GaSb-related boundary energy barriers, the power factor is enhanced due to the significant enhancement of the Seebeck coefficient. The total thermal conductivity was decreased with the depression of electronic thermal conductivity. As a result, the dimensionless thermoelectric figure of meritZT value was improved over a wide working temperature range of 300–850 K, and the expected optimal thermal-electric conversion efficiencyη_opt 15.0% was obtained for the Yb_0.26Co₄Sb₁₂/0.2GaSb nanocomposite.

The oxidation behavior of CoSb₃ thermoelectric material in air has been studied from 500 °C to 650 °C. The present work showed that the growth of oxide scale on CoSb₃ material obeyed a parabolic law from 500 °C to 600 °C, while tremendous weight loss occurred at 650 °C owing to the sublimation of Sb₂O₃. The oxidation activation energy of CoSb₃ was estimated to be 37.4 kJ/mol. SEM and XRD results showed that the oxide scale of CoSb₃ was the mixture of Sb₂O₃ and CoSb₂O₆ when CoSb₃ was oxidized below 600 °C; whereas the oxide scale consisted of an outer layer of Sb₂O₃ and an inner layer, which was composed of Sb₂O₃ and CoSb₂O₆, when CoSb₃ was oxidized at 650 °C. Cracks appeared at the scale/CoSb₃ interface after a certain oxidation time. The oxidation mechanism of CoSb₃ material was discussed. After oxidation at 650 °C for 48 h, thermoelectric performance of CoSb₃ declined obviously andZT value decreased from 0.24 to 0.17 at 327 °C.

Abstract

The microstructure and thermoelectric properties of Ba and In double-filled Ba_0.3In_0.2Co_3.95Ni_0.05Sb₁₂ materials with a figure-of-merit (ZT) of 1.2 at 800 K were carefully investigated by periodic quenching from 723 K to room temperature. The quenching treatment caused enrichment of Ba and loss of Sb and Co on the grain boundaries but had no effect on In and Ni. The enhancement in |α| in the starting period is due to the reduction inσ induced by the secondary precipitates. The reductions inσ,κ andκ_E are attributed to the secondary precipitates. The increase inκ_L is assumed to be due to the separation of Ba filler from the Sb-icosahedron voids. A promisingZT value of ∼1.0 remained after quenching 2000 times. It is concluded that the double-filled skutterudite materials have excellent performance stability under periodically fluctuating environmental temperature.

Highlights

? Periodically quenching was employed to investigate the performance stability of Ba and In double-filled skutterudite materials. ? We examine changes in microstructure and composition on the grain boundaries. ? Secondary precipitates decreaseσ,κ, andκ_E and increase |α|. ? Separation of Ba from the Sb-icosahedron voids enables increase ofκ_L. ? The double-filled skutterudite materials remain excellent thermoelectric performance under periodically fluctuating environmental temperature.

Maximizing the thermoelectric figure-of-merit (ZT) is a challenge owing to the conflicting combination of material properties. We explored simultaneously enhancing the power factor and reducing the thermal conductivity through filling In in Ba-filled skutterudite (Ba_0.3Co₄Sb₁₂). Two large ZT values of 1.33 and 1.34 have been achieved for Ba_0.15In_0.16Co₄Sb_11.83 and Ba_0.14In_0.23Co₄Sb_11.84 at 850 K, respectively. The excellent thermoelectric transport properties for Ba_rIn_sCo₄Sb₁₂ are supposed to be due to the orbital hybridizations induced by In filler. It was found that the In filler made the [Sb₄]⁴⁻ rings become bigger and squarer because of an electron transition from Sb to In brought out by the orbital hybridization between In and Sb and that the Ba filler caused the [Sb₄]⁴⁻ rings to get smaller and squarer because of a reverse electron transition from Ba to Sb induced by a large difference in electronegativities between Ba and Sb. A model of how to form the rectangular [Sb₄]⁴⁻ ring is presented, and the five chemical states of Sb in CoSb₃ are reasonably assigned to different chemical bonds in the model.