Effective medium theory is typically employed to predict the macroscopic properties of multiphase composite materials, based on a knowledge of the properties of the individual components. Recently, however, a research team from the California Institute of Technology and the University of Southern California for the first time applied the theory to the opposite problem: calculating conductivity and mobility of individual phases from the overall bulk properties of Cu_1.97Ag_0.03Se, a multiphase thermo-electric composite material.

Thermoelectric materials transform electricity into heat and vice versa. Compared to traditional heating and cooling systems, thermoelectrics can be used to heat or cool on a smaller or more localized scale (think “car seats”). In most cases thermoelectric materials possess a complex stoichiometry. Phase-pure syntheses can be challenging and the presence of impurity phases makes it difficult to accurately assess the properties of the thermoelectric target material.

Under the lead of G. Jeffrey Snyder, the researchers studied the multiphase composite Cu_1.97Ag_0.03Se with magnetic-field dependent resistivity measurements and analyzed the results using effective medium theory.

As recently reported in Applied Physics Letters (DOI: 10.1063/1.4897435), both the magnetoresistance and the Hall effect of the different phases within Cu_1.97Ag_0.03Se exhibit unique dependence on an applied magnetic field, which makes them identifiable to the researchers. One impurity phase, for example, exists at temperatures below 390 K and displays high mobility values, thereby dominating the transport properties of the composite material despite taking up only 3% of the total sample volume.

“Cu_1.97Ag_0.03Se is a useful thermoelectric, which is intended to be used above the superionic phase transition temperature, at 390 K, where the material consists of a single phase,” said Snyder. “The research we carried out for this paper below the superionic phase transition temperature is more of an academic exercise, demonstrating that we can determine the contributions of individual phases to bulk properties. It is really interesting to see how even a small impurity phase can dominate the overall behavior of a composite material.”

Understanding the contribution of individual phases to the overall properties of thermoelectric composite materials will guide researchers in their quest to optimize this class of materials and to further the implementation of thermoelectrics in everyday applications. The newly discovered use of effective medium theory will certainly play an important role in future studies, as Tristan W. Day, who is the first author of this study, confirmed.

“What we have learned in this study, with regard to using effective medium theory in reverse, is useful for studying other composite materials as well. The methodology can be applied to our work on organic/inorganic thermoelectrics, for example,” said Day.