Is thermoelectric conversion efficiency of a composite bounded by its constituents?

Appl. Phys. Lett. 102, 053905 (2013); http://dx.doi.org/10.1063/1.4791684

We analyze the conversion efficiency of a bilayered thermoelectric
composite, and conclude that thermoelectric conversion efficiency of a
composite is not bounded by its constituents, and can be higher than all
its constituents in the absence of size and interface effects.
Conditions on constituent phases for enhanced conversion efficiency are
also identified, and the upper bound on their conversion efficiency is
established. This points to a new route for high efficiency
thermoelectric materials.