Traditional Thermoelectric Materials Research Articles

Effects of Ball-Milling Atmosphere on the Thermoelectric Properties of TAGS-85 Compounds

Inspired by the high ZT value lately attained in Ar-protected ball-milled nanocrystalline p-BiSbTe bulk alloy, we report herein an investigation of the effects of ball-milling atmosphere on the thermoelectric (TE) properties of the traditional TE material (GeTe) 85 (AgSbTe 2 ) 15 (TAGS-85). TAGS-85 samples were prepared via a melting-quenching-annealing process, and then ball-milled in different atmospheres and subsequently densified using a spark plasma sintering technique. The Seebeck coefficient, electrical conductivity, thermal conductivity, and Hall coefficient were measured as a function of temperature from 10 K to 310 K. It was found that different ball-milling atmospheres, i.e., air, liquid N 2 (LN 2 ), and Ar, profoundly affected the TE properties. A state-of-the-art figure of merit ZT ≈ 0.30 was attained at 310 K in the Ar-ball milled sample. The results are discussed in terms of the carrier concentration, mobility, crystallinity, and the grain boundary scattering.

Electrodeposition of Bismuth Telluride Nanowires for Thermoelectric Applications

AbstractBismuth telluride (Bi2Te3) and its alloys have long been held as the best bulk commercial thermoelectric (TE) materials. In recent years, significant enhancement of the TE figure of merit (ZT) of these traditional TE materials has been predicted through reduction of dimensions (i.e., nanostructures and nanoengineering). We are particularly interested investigating electrolyte composition variations to control the composition of nanowires to enable large ZT enhancement. We report here the constant current electrochemical deposition of BixTey nanowires of diameters of 35, 55, 73 and 200 nm and lengths up to 50 microns. We are able to obtain controlled, uniform growth of high quality n-type bismuth telluride nanowires. A design of experimental matrix investigating the effects of current density and solution pH values on the overall growth rate and nanowire crystalline quality has been performed. The effects of growth conditions on materials and structural characteristics of BixTey nanowires have been studied by SEM, High Resolution TEM, EDX, concentric beam electron diffraction patterns, and ICP. The TE properties of individual BixTey nanowires are currently being evaluated using micro/nano fabricated devices and UHV Scanning Thermoelectric Microscopy.

Modeling the thermoelectric transport properties of nanowires embedded in oriented microporous and mesoporous films

The efficiency of thermoelectric devices has not increased significantly during the last 40 years, and currently, the figure-of-merit (ZT) of the best materials is less than one. However, there has been a resurgence of interest in thermoelectric materials due to recent results that have shown that size-effects in nanostructured materials may enable much more efficient thermoelectric devices. Theoretical calculations for isolated single nanowires predict over a 10-fold increase in the figure-of-merit. In order to realize these size-effects, wires must be synthesized with diameters less than the thermal de Broglie wavelength (typically less than 10 nm) which is smaller than is currently accessible by lithographic techniques. Also, in order to transfer meaningful amounts of thermal energy, a practical device must consist of an array of a large number of nanowires in parallel. One way to potentially synthesize these nanowire arrays is to use the pores of microporous and mesoporous materials as a mold to template the diameter and orientation of wires of suitable thermoelectric materials. Here we provide a review of the relevant physics and derive a model to assess the feasibility of using microporous and mesoporous frameworks in the fabrication of thermoelectric devices. The model accounts for the possible deleterious effects of thermal conduction through the microporous or mesoporous framework and the presence of bulk thermoelectric material (that may result from defects in the microporous or mesoporous framework) in parallel with the wires on the thermoelectric figure-of-merit. Simulation results are reported for SBA-15, MCM-41, and zeolites VFI, LTL, and LTA embedded with Bi 2Te 3 nanowires. The results show that the microporous frameworks may yield figures-of-merit much larger than one while thermal conduction through most mesoporous frameworks reduces the figure-of-merit below the level of currently available using traditional thermoelectric materials.