Materials can be produced in thin/thick film form while maintaining film quality and stoichiometric balance.
Marshall Space Flight Center, Alabama
Stated generally, reducing the dimensionality of bulk-scale thermoelectric (TE) materials is theoretically and practically understood to be a viable route for maintaining/increasing phonon scattering, and maintaining/increasing electrical conductivity — necessary conditions for improving thermoelectric merit. Solution deposition of thin, anisotropic films of nanoscale particles of known TE materials is a route toward obtaining such low-dimensional materials with increased TE merit.
This innovation makes use of an intercalation/ exfoliation reaction of bulk Bi2Te3 (and/or Sb2Te3) via solvated electrons, understood from prior art. This work improves on prior art by mitigating side reactions that introduce impurities into the nanocrystals produced during the exfoliation. This allows the demonstration of a ZT = 0.19 for pure Bi2Te3. Additionally, this method includes solution casting, pressing, and annealing techniques to produce the materials in think/thick film form while maintaining film quality and stoichiometric balance. Three alloys have been developed in the Bi2xSb2(1 – x)Te3 system. Stoichiometries of x = 75%, 50%, and 25% were explored. Each experiment consisted of the synthesis (through Li intercalation, and subsequent exfoliation) of one gram of the desired alloy, which was hot-pressed into a 5-micron-thick film. Finely ground BiTe and SbTe were intercalated with Li, and exfoliated with pure ethanol. The nanoparticles (which are platelets) where then deposited on glass microscope slides for hot pressing and further annealing at 250 °C. The final thickness of the samples was 5 microns.
The thermal conductivity of these low dimensionality solids is reduced, owing to the fact that vibrations face greater difficulty propagating across a matrix of large numbers of NQDs (nano-quantum dots) than they do propagating through one continuous solid. This unique attribute of the technology allows increased performance, as measured by ZT, by more than a factor of 2. This performance increase is somewhat interesting in and of itself for thermoelectric applications, though there are other materials that exhibit similar performance. Additionally, thermal conductivity is reduced while maintaining electrical conductivity by using nanoscale solids pressed together. The combined thermal boundary and electrical conduction (through tunneling and hopping as well as conduction) is yet to be fully developed, and can yield materials in which ZT >1, at which point cooling and electrical generation costs become competitive with more traditional technologies that are not solid-state.