Nanostructured thermoelectrics based on periodic composites from opals and opal replicas. I. Bi-infiltrated opals

We have developed a variety of templating processes for the fabrication of three-dimensionally periodic, nanostructured thermoelectrics from opal and inverse opal matrices. These opals and inverse opals are periodic at optical wavelengths and have extremely high interfacial area. It was hoped that scattering processes at the interface between opal and infiltrated thermoelectric material would increase the thermoelectric figure of merit (ZT) by having a greater effect on phonon-mediated (lattice) thermal conductivity than on electronic conductivity. We provide the first demonstration that this approach can increase ZT by evaluating a simple prototype system: bismuth infiltrated into porous SiO₂ opal. We find a larger fractional decrease in thermal conductivity than for electrical conductivity (relative to bulk polycrystalline Bi). Since the thermopower is little changed, the overall effect we observe is as much as a two-fold increase of ZT compared with that for polycrystalline bulk bismuth. However, the observed ZT is much smaller than for single crystal bismuth