Endotaxial Intergrowth of Copper Telluride in GeTe-Rich Germanium Antimony Tellurides Leads to High Thermoelectric Performance

In composite materials with nominal compositions Cu2GexSb2Tex+4 (11 <= x <= 18, i.e., between Cu6.7Ge36.7Sb6.7Te50 and Cu4.5Ge40.9Sb4.5Te50), precipitates consisting of copper tellurides are endotaxially intergrown in a matrix of Cu-doped germanium antimony tellurides. The precipitates as well as the matrix material undergo various phase transitions as shown by temperature-dependent X-ray diffraction and X-ray absorption contrast imaging. Eventually, the precipitates dissolve in the matrix at temperatures exceeding 580 degrees C. The temperature-dependent behavior was also traced by photoemission electron microscopy up to 460 degrees C. At high temperatures, the thermoelectric properties are superior to those of pure germanium antimony tellurides obtained by comparable syntheses; a maximal zT value of 1.83 for Cu2Ge16Sb2Te20 is reached at 500 degrees C. The application of an effective mass model reveals optimal charge carrier concentrations for all three compositions investigated. The p-type Cu2Ge16Sb2Te20 material was used in combination with PbTe:In (n-type) to construct a thermoelectric module. Concludingly, the measurement of the Hall effect that suggests no significant changes in Cu-doping levels of the matrix with temperature application of grain boundary optimization and a temperature-induced reset of the microstructure are proposed as strategies for overcoming material degradation upon applying electrical currents.

Automated summary

In this study, it was demonstrated that a new composite material can be formed by adding copper and doping material to a specific composition of copper germanium antimony tellurides. The composite material exhibits superior thermoelectric properties suitable for temperature-dependent applications. Furthermore, strategies such as optimizing grain boundaries and temperature-induced microstructure reset were proposed to overcome material degradation during electrical current application.