Terry M. Tritt

High figure of merit in partially filled ytterbium skutterudite materials

We present evidence of a relatively high dimensionless figure of merit (ZT) in a polycrystalline skutterudite partially filled with ytterbium ions. The small-diameter yet heavy-mass Yb atoms partially filling the voids of the host CoSb3 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 thermoelectric material, resulting in ZT=0.3 at room temperature and ZT∼1 at 600 K for Yb0.19Co4Sb12.

Identifying the Specific Nanostructures Responsible for the High Thermoelectric Performance of (Bi,Sb)2Te3 Nanocomposites

Herein, we report the synthesis of multiscale nanostructured p-type (Bi,Sb)(2)Te(3) bulk materials by melt-spinning single elements of Bi, Sb, and Te followed by a spark plasma sintering process. The samples that were most optimized with the resulting composition (Bi(0.48)Sb(1.52)Te(3)) and specific nanostructures showed an increase of approximately 50% or more in the figure of merit, ZT, over that of the commercial bulk material between 280 and 475 K, making it suitable for commercial applications related to both power generation and refrigeration. The results of high-resolution electron microscopy and small angle and inelastic neutron scattering along with corresponding thermoelectric property measurements corroborate that the 10-20 nm nanocrystalline domains with coherent boundaries are the key constituent that accounts for the resulting exceptionally low lattice thermal conductivity and significant improvement of ZT.

Unique nanostructures and enhanced thermoelectric performance of melt-spun BiSbTe alloys

We report a melt spinning technique followed by a quick spark plasma sintering procedure to fabricate high-performance p-type Bi0.52Sb1.48Te3 bulk material with unique microstructures. The microstructures consist of nanocrystalline domains embedded in amorphous matrix and 5–15 nm nanocrystals with coherent grain boundary. The significantly reduced thermal conductivity leads to a state-of-the-art dimensionless figure of merit ZT∼1.56 at 300 K, more than 50% improvement of that of the commercial Bi2Te3 ingot materials.

High figures of merit and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials

Mg2(Si,Sn) compounds have shown great promise for thermoelectric applications due to good thermoelectric properties, nontoxicity, and abundantly available constituent elements. Herein we report on the thermoelectric properties and microstructure of high performance Mg2Si0.4−xSn0.6Sbx (0⩽x⩽0.015) alloys. The state-of-the-art ZT value of ∼1.1 has been attained in the samples with x=0.0075 due to the relatively low thermal conductivity. In light of the simple cubic structure and mostly light constituent elements, the reduction in lattice thermal conductivity has been discussed in connection with a fairly large amount of in situ formed nanostructures in these samples.

Effect of substitutions on the thermoelectric figure of merit of half-Heusler phases at 800 °C

The merit of thermally stable MNiSn (M=Ti, Zr, Hf) half-Heusler phases, as n-type thermoelectric materials, for high-temperature power generation has been examined. Sb doping at the Sn site is shown to increase both the figure of merit, ZT, and the temperature at which ZT is maximized. The benefits of increased alloying at the M and Ni sites, on the thermal conductivity and thermoelectric transport properties, have also been investigated. The thermoelectric figure of merit, ZT∼0.8 at T∼800°C, for select Sb-doped MNiSn alloys was found to meet or exceed the industry benchmark set by SiGe alloys.

High figure of merit in Eu-filled CoSb3-based skutterudites

We report measurements of electrical resistivity, thermopower, thermal conductivity, and Hall coefficient of polycrystalline Eu-doped CoSb3-based skutterudites with compositions Eu0.20Co4Sb12, Eu0.43Co4Sb11.59Ge0.31, and Eu0.42Co4Sb11.37Ge0.50. The relatively high mobility of these compounds, as compared to that of La- and Ce-filled skutterudites, may play a role in the large thermoelectric figure of merit (ZT>1 at 700 K) of Eu0.42Co4Sb11.37Ge0.50. We discuss the significant potential of these compounds for thermoelectric applications.

High thermoelectric performance BiSbTe alloy with unique low-dimensional structure

We report a detailed description of an innovative route of a melt spinning (MS) technique combined with a subsequent spark plasma sintering process in order to obtain high performance p-type Bi0.52Sb1.48Te3 bulk material, which possesses a unique low-dimensional structure. The unique structure consists of an amorphous structure, 5–15 nm fine nanocrystalline regions, and coherent interfaces between the resulting nanocrystalline regions. Measurements of the thermopower, electrical conductivity, and thermal conductivity have been performed over a range of temperature of 300–400 K. We found that MS technique can give us considerable control over the resulting nanostructure with good thermal stability during the temperature range of 300–400 K and this unique structure can effectively adjust the transport of phonons and electrons, in a manner such that it is beneficial to the overall thermoelectric performance of the material, primarily a reduction in the lattice thermal conductivity. Subsequently, this results...

Effect of Sb doping on the thermoelectric properties of Ti-based half-Heusler compounds, TiNiSn1−xSbx

Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T≈650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.

Recent Advances in Nanostructured Thermoelectric Half-Heusler Compounds

Half-Heusler (HH) alloys have attracted considerable interest as promising thermoelectric (TE) materials in the temperature range around 700 K and above, which is close to the temperature range of most industrial waste heat sources. The past few years have seen nanostructuing play an important role in significantly enhancing the TE performance of several HH alloys. In this article, we briefly review the recent progress and advances in these HH nanocomposites. We begin by presenting the structure of HH alloys and the different strategies that have been utilized for improving the TE properties of HH alloys. Next, we review the details of HH nanocomposites as obtained by different techniques. Finally, the review closes by highlighting several promising strategies for further research directions in these very promising TE materials.

High temperature Seebeck coefficient metrology

We present an overview of the challenges and practices of thermoelectric metrology on bulk materials at high temperature (300 to 1300 K). The Seebeck coefficient, when combined with thermal and electrical conductivity, is an essential property measurement for evaluating the potential performance of novel thermoelectric materials. However, there is some question as to which measurement technique(s) provides the most accurate determination of the Seebeck coefficient at high temperature. This has led to the implementation of nonideal practices that have further complicated the confirmation of reported high ZT materials. To ensure meaningful interlaboratory comparison of data, thermoelectric measurements must be reliable, accurate, and consistent. This article will summarize and compare the relevant measurement techniques and apparatus designs required to effectively manage uncertainty, while also providing a reference resource of previous advances in high temperature thermoelectric metrology.