Zn₄Sb₃: A High-Performance Thermoelectric Material

NASA's Jet Propulsion Laboratory, Pasadena, California

Zn₄Sb₃ has been identified as a high-performance thermoelectric material. In p-type, Zn₄Sb₃samples have exhibited the greatest dimensionless thermoelectric figure of merit ever observed for a p-type material at temperatures from 200 to 350 °C. In this respect, Zn₄Sb₃fills a gap in the thermoelectric-performance spectrum between the state-of-the-art thermoelectric materials like (a) p-type Bi₂Te₃-based alloys, which exhibit their greatest figures of merit at lower temperatures and (b) p-type Te/Ag/Ge/Sb ("TAGS") alloys and p-type PbTe-based alloys, which exhibit their greatest figures of merit at higher temperatures. Thus, Zn₄Sb₃ offers an important thermoelectric-performance advantage for generating electrical energy from heat sources in the temperature range from 200 to 350 °C. Zn₄Sb₃also costs less than do the state-of-the-art lower- and higher-temperature alloys.

The Values of the Dimensionless Figure of Merit of samples of β-Zn4Sb3 in the temperature range of 200 to 350 ° C were found to exceed those of other thermoelectric materials.

Zn₄Sb₃ exists in three phases; α(which is stable below -10 °C), β (which is stable from -10 to 492 °C), and γ (which is stable from 492 °C to the melting temperature of 566 °C). In the temperature range of interest, Zn₄Sb₃ thus manifests itself as β-Zn₄Sb₃, which has been reported in the literature to be characterized by a band gap of about 1.2 eV. Single crystals of β-Zn₄Sb₃were prepared by the Bridgman gradient-freeze technique. In addition, polycrystalline samples were prepared by melting and direct reaction of powders of Zn and Sb followed by regrinding of the resulting ingots into powder followed by hot pressing to consolidate the powders into solid pellets.

The thermoelectric properties of the crystalline and polycrystalline samples were measured and found to be similar. The results show that β-Zn₄Sb₃is a heavily-p-doped semiconductor. The dimensionless thermoelectric figure of merit, ZT is defined by ZT = α²T/rl, where αis the Seebeck coefficient, T is the absolute temperature, ρ is the electrical resistivity, and λis the thermal conductivity. The figure illustrates ZT as a function of temperature as calculated from the measurements on the β-Zn₄Sb₃samples, plus ZT as a function of temperature for the state-of-the-art thermoelectric materials mentioned previously. One of the most interesting features of β-Zn₄Sb₃that contributes to its relatively large ZT is its thermal conductivity, which reaches a low value of only 6 mW/(cm·K) at 250 °C. This is the lowest thermal conductivity of any thermoelectric material known thus far.

There are many potential applications for β-Zn₄Sb₃in thermoelectric generators, especially for recovering electrical energy from waste heat. Sources that generate waste heat in the temperature range of peak thermoelectric performance of β-Zn₄Sb₃include garbage incinerators, geothermal sources (including hot oil from oil wells), power plants, and automobiles.

This work was done by Thierry Caillat of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to