Thermoelectric Properties of Ultralong Silver Telluride Hollow Nanofibers

Ultralong Ag_<i>x</i>Te_<i>y</i> nanofibers were synthesized for the first time by galvanically displacing electrospun Ni nanofibers. Control over the nanofiber morphology, composition, and crystal structure was obtained by tuning the Ag⁺ concentrations in the electrolytes. While Te-rich branched p-type Ag_<i>x</i>Te_<i>y</i> nanofibers were synthesized at low Ag⁺ concentrations, Ag-rich nodular Ag_<i>x</i>Te_<i>y</i> nanofibers were obtained at high Ag⁺ concentrations. The Te-rich nanofibers consist of coexisting Te and Ag₇Te₄ phases, and the Ag-rich fibers consist of coexisting Ag and Ag₂Te phases. The energy barrier height at the phase interface is found to be a key factor affecting the thermoelectric power factor of the fibers. A high barrier height increases the Seebeck coefficient, <i>S</i>, but reduces the electrical conductivity, σ, due to the energy filter effect. The Ag₇Te₄/Te system was not competitive with the Ag₂Te/Ag system due to its high barrier height where the increase in <i>S</i> could not overcome the severely diminished electrical conductivity. The highest power factor was found in the Ag₂Te/Ag-rich nanofibers with an energy barrier height of 0.054 eV

Thermal Transport Driven by Extraneous Nanoparticles and Phase Segregation in Nanostructured Mg₂(Si,Sn) and Estimation of Optimum Thermoelectric Performance

Solid solutions of magnesium silicide and magnesium stannide were recently reported to have high thermoelectric figure-of-merits (<i>ZT</i>) due to remarkably low thermal conductivity, which was conjectured to come from phonon scattering by segregated Mg₂Si and Mg₂Sn phases without detailed study. However, it is essential to identify the main cause for further improving <i>ZT</i> as well as estimating its upper bound. Here we synthesized Mg₂(Si,Sn) with nanoparticles and segregated phases, and theoretically analyzed and estimated the thermal conductivity upon segregated fraction and extraneous nanoparticle addition by fitting experimentally obtained thermal conductivity, electrical conductivity, and thermopower. In opposition to the previous speculation that segregated phases intensify phonon scattering, we found that lattice thermal conductivity was increased by the phase segregation, which is difficult to avoid due to the miscibility gap. We selected extraneous TiO₂ nanoparticles dissimilar to the host materials as additives to reduce lattice thermal conductivity. Our experimental results showed the maximum <i>ZT</i> was improved from ∼0.9 without the nanoparticles to ∼1.1 with 2 and 5 vol % TiO₂ nanoparticles at 550 °C. According to our theoretical analysis, this <i>ZT</i> increase by the nanoparticle addition mainly comes from suppressed lattice thermal conductivity in addition to lower bipolar thermal conductivity at high temperatures. The upper bound of <i>ZT</i> was predicted to be ∼1.8 for the ideal case of no phase segregation and addition of 5 vol % TiO₂ nanoparticles. We believe this study offers a new direction toward improved thermoelectric performance of Mg₂(Si,Sn)

Extraordinary Off-Stoichiometric Bismuth Telluride for Enhanced n‑Type Thermoelectric Power Factor

Thermoelectrics directly converts waste heat into electricity and is considered a promising means of sustainable energy generation. While most of the recent advances in the enhancement of the thermoelectric figure of merit (<i>ZT</i>) resulted from a decrease in lattice thermal conductivity by nanostructuring, there have been very few attempts to enhance electrical transport properties, i.e., the power factor. Here we use nanochemistry to stabilize bulk bismuth telluride (Bi₂Te₃) that violates phase equilibrium, namely, phase-pure n-type K_0.06Bi₂Te_3.18. Incorporated potassium and tellurium in Bi₂Te₃ far exceed their solubility limit, inducing simultaneous increase in the electrical conductivity and the Seebeck coefficient along with decrease in the thermal conductivity. Consequently, a high power factor of ∼43 μW cm^–1 K^–2 and a high <i>ZT</i> > 1.1 at 323 K are achieved. Our current synthetic method can be used to produce a new family of materials with novel physical and chemical characteristics for various applications