Researchers have observed that the performance of some thermoelectric materials can be improved by combining different solid phases, intermixing materials like the clumps of fat and meat in a slice of salami. This could boost thermoelectrics’ energy efficiency, but scientists still lack the tools to fully understand how the bulk properties arise out of combinations of solid phases.

Now a research team based at the California Institute of Technology (Caltech) has developed a new way to analyze the electrical properties of thermoelectrics that have two or more solid phases. Because it’s sometimes difficult to separately manufacture the pure components that make up multi-phase materials, researchers can’t always measure the pure phase properties directly. The Caltech team developed a way to calculate the electrical properties of individual phases while only experimenting directly with the composite through a modification of effective medium theory.

The theory is traditionally used to predict the properties of a bulk composite based on the properties of the individual phases. “What’s new about what we did is we took a composite, and then backed-out the properties of each constituent phase,” says Tristan Day, a graduate student in Snyder’s Caltech laboratory.

The key to making the “reverse engineering” work lies in the different way that each part of a composite thermoelectric material responds to a magnetic field. By measuring certain electrical properties over a range of different magnetic field strengths, the researchers were able to tease apart the influence of the two different phases.