Zhigang Jiang, Georgia Institute of Technology
1. Enhance the thermoelectric figure of merit by materials engineering
The thermoelectric figure of merit (, where T is the temperature) is a quantitative measure of the thermodynamic efficiency of thermoelectric materials/devices, and it can be manipulated by increasing the Seebeck coefficient (S) and the electrical conductivity () of the material, and/or reducing its thermal conductivity (). To date, most high-efficiency thermoelectric materials are semiconductors, where the values of S and are determined by the electronic properties of the material, while is dominated by the lattice (phonon) conduction. In this work, we focus on how to improve the ZT value via materials engineering.
First we start with examining the thermoelectric properties of single-phase AgSbTe2, a well-known thermoelectric material, which has the highest figure of merit, at K , among all simple semiconductors (not including thin films). X-ray powder diffraction (XRD) and transmission electron microscopy (TEM) have found that the crystalline structure of AgSbTe2 is rocksalt, and thermal conductivity measurement has revealed an extremely small W/mK  at room temperature, which is largely responsible for the high ZT value of this material. Recently it has also been shown that such a low thermal conductivity is primarily due to the extreme anharmonicity of the lattice bonding forces , indicated by an unusually high Grüneisen parameter.
Here we explore the possibilities to further enhance the ZT of AgSbTe2, for instance, via engineering nanostructures. Specifically, we investigated two different types of AgSbTe2 samples with different stoichiometry: (1) Sb-rich, Ag16.7Sb30Te53.3 and (2) Ag-rich, Ag19Sb29Te52, grown by Dr. Peter A. Sharma at the Sandia National Laboratories. Although the range of the stoichiometry is not very big, Sb2Te3 and Ag2Te precipitates are found embedded within the single-phase AgSbTe2. It is known that properly embedded nanostructures would enhance the thermoelectric efficiency of the material; heterogeneous nucleation of the precipitates may also form a complex interconnected network through the grain boundaries and defect sites, which could lower the ZT . Great care need to be taken in the material synthesis process to avoid such heterogeneous nucleation.
Another route to achieve high ZT is to look for other candidate materials with strong anharmonicity. Towards this end, we measured the volumetric thermal expansion coefficients of Bi2Se3, Bi2Te3 and Sb2Te3 using variable temperature XRD and calculated the Grüneisen parameters of these materials being 1.4, 1.5 and 2.3, respectively. The figure below shows that the linear thermal expansion coefficients of Sb2Te3 follow the standard Debye model at low temperatures; but above 150 K considerable deviation occurs, which can be attributed to the higher-order anharmonic effects (it also explains the unusually large Grüneisen parameter of Sb2Te3). Recently, the highest ZT value of ~2.4 has been reported at room temperature in the Bi2Te3/ Sb2Te3 superlattices. The strong anharmonicity of Sb2Te3 may be an important reason.
2. Spin Seebeck effect and its thermoelectric figure of merit
Recently, much attention has been given to a new type of thermoelectric effect, the spin Seebeck effect. If the conventional (charge) Seebeck effect can be described as the formation of a battery, where S and s characterize the battery voltage and internal resistance, the spin Seebeck effect can then be understood as forming a spin battery that generates spin current. Previously several different systems were investigated for this effect, which include: (1) magnetized metal (Ni81Fe19) films , (2) magnetic insulators (LaY2Fe5O12) , and (3) magnetic semiconductors (GaMnAs) . Unfortunately, the corresponding ZT values of these systems were found impractically low.
More recently, new hope has been raised due to the observation of giant spin Seebeck effect in strongly spin-orbit coupled systems . Interestingly, many conventional thermoelectric materials, such as Bi2Se3, Bi2Te3, and Sb2Te3 we studied here, also possess strong spin-orbit coupling, therefore they are the natural candidates for studying the spin Seebeck effect. Our preliminary measurements show that the thermoelectric spin voltage generated in these materials vanishes at room temperature; low temperature measurement is currently underway.
3. Impacts of the PRF project to the participating postdoctoral fellow, graduate student, and the PI
The postdoctoral fellow and the graduate student working on this project have learned a variety of experimental techniques and data analysis skills in the characterization of thermoelectric materials. These techniques include XRD, electronic and thermal transport measurements, Raman spectroscopy, photo- and electron-beam-lithography, metallic thin-film deposition and lift-off, etc. The experience gained in this project will benefit them in their future career in basic science or private industry.
The experimental setups developed in this project also benefit the PI in several other areas of research. In particular, the low temperature setup for studying the spin Seebeck effect has been the most productive measurement platform in the PI's lab, heavily used by all the group members. Moreover, the collaboration with Sandia Labs has gone beyond the study of thermoelectric materials. External funding from Sandia Labs for other research topics of common interest has greatly impacted the PI's career development.