Physical Review B, 2009
Resistivity, Seebeck coefficient, and Hall measurements were performed on densified nanocrystalline composite materials of undoped and Ag-doped PbTe nanocrystals to investigate the physical mechanisms responsible for Seebeck coefficient enhancement in nanocrystalline systems. The unique temperature dependence of the resistivity and mobility for these PbTe nanocomposites suggests that grain-boundary potential barrier scattering is the dominant scattering mechanism. We propose that carrier trapping in the grain boundaries forms energy barriers that impede the conduction of carriers between grains, essentially filtering charge carriers with energy less than the barrier height. These nanocomposites therefore demonstrate an enhanced Seebeck coefficient as compared to single crystal or polycrystalline PbTe at similar carrier concentrations.
Journal of Applied Physics, 2010
We present an overview of the challenges and practices of thermoelectric metrology on bulk materials at high temperature (300 to 1300 K). The Seebeck coefficient, when combined with thermal and electrical conductivity, is an essential propertymeasurement for evaluating the potential performance of novel thermoelectricmaterials. However, there is some question as to which measurement technique(s) provides the most accurate determination of the Seebeck coefficient at high temperature. This has led to the implementation of nonideal practices that have further complicated the confirmation of reported high ZT materials. To ensure meaningful interlaboratory comparison of data, thermoelectricmeasurements must be reliable, accurate, and consistent. This article will summarize and compare the relevant measurement techniques and apparatus designs required to effectively manage uncertainty, while also providing a reference resource of previous advances in high temperature thermoelectric metrology.
Applied Physics Letters, 2007
Dense lead telluride (PbTe) nanocomposites were prepared from PbTe nanocrystals synthesized employing an aqueous solution-phase reaction. This approach reproducibly synthesizes 100 nm to 150 nm nanocrystals with a high yield of over 2g per batch. Densification using spark plasma sintering dimensionally integrated nanoscale grains within a bulk matrix, resulting in a uniform dispersion of nonconglomerated nanocrystals. Transport properties of PbTe nanocomposites were evaluated through temperature dependent resistivity, Hall, Seebeck coefficient, and thermal conductivity measurements. These nanocomposites show an enhancement in the thermoelectric properties compared to bulk polycrystalline PbTe with similar carrier concentrations. Our results also indicate a strong sensitivity to stoichiometry, surface oxygen adsorption, and porosity.
Journal of Applied Physics, 2007
We report the synthesis and chemical, structural, and transport properties characterization of Ba8Ga16SixGe30-x type I clathrates with similar Ga-to-group IV element ratios but with increasing Si substitution (4<x<14). Substitution of 20 at.% Si within the Ga-Ge lattice framework of the type I clathrate Ba8Ga16Ge30 results in thermoelectric performance enhancement. The unique dependences of carrier concentration,electrical resistivity, Seebeck coefficient, and carrier effective mass on Si substitution level, and the lack of variation in the Ga-to-group IV element ratios may imply a modified band structure with Si substitution. These results indicate an additional method for tuning the electronic properties of Ba8Ga16Ge30 for thermoelectric applications.
Review of Scientific Instruments, 2012
The Seebeck coefficient is a physical parameter routinely measured to identify the potential thermoelectric performance of a material. However, researchers employ a variety of techniques, conditions, and probe arrangements to measure the Seebeck coefficient, resulting in conflicting materials data. To compare and evaluate these methodologies, and to identify optimal Seebeck coefficient measurement protocols, we have developed an improved experimental apparatus to measure the Seebeck coefficient under multiple conditions and probe arrangements (300 K–1200 K). This paper will describe in detail the apparatus design and instrumentation, including a discussion of its capabilities and accuracy as measured through representative diagnostics. In addition, this paper will emphasize the techniques required to effectively manage uncertainty in high temperature Seebeck coefficient measurements.
Measurement Science and Technology, 2013
In Seebeck coefficient metrology, the present diversity in apparatus design, acquisition methodology and contact geometry has resulted in conflicting materials data that complicate the interlaboratory confirmation of reported high efficiency thermoelectric materials. To elucidate the influence of these factors in the measurement of the Seebeck coefficient at high temperature and to identify optimal metrology protocols, we measure the Seebeck coefficient as a function of contact geometry under both steady-state and transient thermal conditions of the differential method, using a custom developed apparatus capable of in situcomparative measurement. The thermal gradient formation and data acquisition methodology, under ideal conditions, have little effect on the measured Seebeck coefficient value. However, the off-axis 4-probe contact geometry, as compared to the 2-probe, results in a greater local temperature measurement error that increases with temperature. For surface temperature measurement, the dominant thermal errors arise from a parasitic heat flux that is dependent on the temperature difference between the sample and the external thermal environment, and on the various thermal resistances. Due to higher macroconstriction and contact resistance in the 4-probe arrangement, the measurement of surface temperature for this contact geometry exhibits greater error, thereby overestimating the Seebeck coefficient.