Rapid consolidation of powdered materials by induction hot pressing

A rapid hot press system in which the heat is supplied by RF induction to rapidly consolidate thermoelectric materials is described. Use of RF induction heating enables rapid heating and consolidation of powdered materials over a wide temperature range. Such rapid consolidation in nanomaterials is typically performed by spark plasma sintering (SPS) which can be much more expensive. Details of the system design, instrumentation, and performance using a thermoelectric material as an example are reported. The Seebeck coefficient, electrical resistivity, and thermal diffusivity of thermoelectric PbTe material pressed at an optimized temperature and time in this system are shown to agree with material consolidated under typical consolidation parameters

Low effective mass leading to high thermoelectric performance

High Seebeck coefficient by creating large density-of-states effective mass through either electronic structure modification or manipulating nanostructures is commonly considered as a route to advanced thermoelectrics. However, large density-of-state due to flat bands leads to large transport effective mass, which results in a simultaneous decrease of mobility. In fact, the net effect of such a high effective mass is a lower thermoelectric figure of merit, zT, when the carriers are predominantly scattered by phonons according to the deformation potential theory of Bardeen–Shockley. We demonstrate that the beneficial effect of light effective mass contributes to high zT in n-type thermoelectric PbTe, where doping and temperature can be used to tune the effective mass. This clear demonstration of the deformation potential theory to thermoelectrics shows that the guiding principle for band structure engineering should be low effective mass along the transport direction

Should we teach old dogs new tricks? the impact of community college retraining on older displaced workers

This paper estimates the returns to retraining for older displaced workers--those 35 or older--by estimating the impact that community college schooling has on their subsequent earnings. Our analysis relies on longitudinal administrative data covering workers who were displaced from jobs in Washington State during the first half of the 1990s and who subsequently remained attached to the state’s work force. Our database contains displaced workers' quarterly earnings records covering 14 years matched to the records of 25 of the state's community colleges. We find that older displaced workers participate in community college schooling at significantly lower rates than younger displaced workers. However, among those who participate in retraining, the per-period impact for older and younger displaced workers is similar. ; We estimate that one academic year of such schooling increases the long- term earnings by about 8 percent for older males and by about 10 percent for older females. These per-period impacts are in line with those reported in the schooling literature. These percentages do not necessarily imply that retraining older workers is a sound social investment. We find that the social internal rates of return from investments in older displaced workers' retraining are less than for younger displaced workers and likely less than those reported for schooling of children. However, our internal rate of return estimates are very sensitive to how we measure the opportunity cost of retraining. If we assume that these opportunity costs are zero, the internal rate of return from retraining older displaced workers is about 11 percent. By contrast, if we rely on our estimates of the opportunity cost of retraining, the internal rate of return may be less than 2 percent for older men and as low as 4 percent for older women.Displaced workers ; Education

A high temperature apparatus for measurement of the Seebeck coefficient

A high temperature Seebeck coefficient measurement apparatus with various features to minimize typical sources of error is designed and built. Common sources of temperature and voltage measurement error are described and principles to overcome these are proposed. With these guiding principles, a high temperature Seebeck measurement apparatus with a uniaxial 4-point contact geometry is designed to operate from room temperature to over 1200 K. This instrument design is simple to operate, and suitable for bulk samples with a broad range of physical types and shapes

Combination of large nanostructures and complex band structure for high performance thermoelectric lead telluride

The complexity of the valence band structure in p-type PbTe has been shown to enable a significant enhancement of the average thermoelectric figure of merit (zT) when heavily doped with Na. It has also been shown that when PbTe is nanostructured with large nanometer sized Ag_2Te precipitates there is an enhancement of zT due to phonon scattering at the interfaces. The enhancement in zT resulting from these two mechanisms is of similar magnitude but, in principle, decoupled from one another. This work experimentally demonstrates a successful combination of the complexity in the valence band structure with the addition of nanostructuring to create a high performance thermoelectric material. These effects lead to a high zT over a wide temperature range with peak zT > 1.5 at T > 650 K in Na-doped PbTe/Ag_2Te. This high average zT produces 30% higher efficiency (300–750 K) than pure Na-doped PbTe because of the nanostructures, while the complex valence band structure leads to twice the efficiency as the related n-type La-doped PbTe/Ag_2Te without such band structure complexity

Weak electron–phonon coupling contributing to high thermoelectric performance in n-type PbSe

PbSe is a surprisingly good thermoelectric material due, in part, to its low thermal conductivity that had been overestimated in earlier measurements. The thermoelectric figure of merit, zT, can exceed 1 at high temperatures in both p-type and n-type PbSe, similar to that found in PbTe. While the p-type lead chalcogenides (PbSe and PbTe) benefit from the high valley degeneracy (12 or more at high temperature) of the valence band, the n-type versions are limited to a valley degeneracy of 4 in the conduction band. Yet the n-type lead chalcogenides achieve a zT nearly as high as the p-type lead chalcogenides. This effect can be attributed to the weaker electron–phonon coupling (lower deformation potential coefficient) in the conduction band as compared with that in the valence band, which leads to higher mobility of electrons compared to that of holes. This study of PbSe illustrates the importance of the deformation potential coefficient of the charge-carrying band as one of several key parameters to consider for band structure engineering and the search for high performance thermoelectric materials

Long-term earnings losses of high-seniority displaced workers

Displaced workers

Reevaluation of PbTe_(1−x)I_x as high performance n-type thermoelectric material

Thermoelectric transport properties of n-type PbTe_(1−x)I_x with carrier concentrations ranging from 5.8 × 10^(18)–1.4 × 10^(20) cm^(−3) are reinvestigated from room temperature to 800 K. The electronic transport properties, resistivity and Seebeck coefficient in this study are effectively consistent with prior reports, however the thermal conductivity has been found to be historically overestimated. The reassessment of the thermal transport properties, in combination with careful control of the carrier density by iodine doping, reveals a significantly larger figure of merit, zT ~ 1.4, than often previously reported for n-type PbTe. The results and analysis of the data from this study lead to a redetermination of zT for this historical thermoelectric material and provide a renewed interest in n-type PbTe based materials

Optical band gap and the Burstein–Moss effect in iodine doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy

Optical absorption edge measurements are performed on I doped PbTe using diffuse reflectance infrared Fourier transform spectroscopy. The Burstein–Moss shift, an increase in the absorption edge (optical band gap) with increasing doping level, is explored. The optical gap increases on the order of 0.1 eV for doping levels ranging from 3 × 10^(18) to 2 × 10^(20) cm^(−3), relevant doping levels for good thermoelectric materials. Chemical potential is estimated from transport measurements—specifically, Hall effect and Seebeck coefficient—using a single band Kane model. In heavily doped semiconductors, it is well-known that the band gap shrinks with increasing doping level. This effect, known as band gap renormalization, is fit here using an n^(1/3) scaling law which reflects an electron–electron exchange interaction. The renormalization effect in these samples is shown to be more than 0.1 eV, on the same order of magnitude as the band gap itself. Existing models do not explain such large relative changes in band gap and are not entirely self-consistent. An improved theory for the renormalization in narrow gap semiconductors is required

High thermoelectric figure of merit in heavy hole dominated PbTe

Thermoelectric transport properties of p-type PbTe:Na, with high hole concentrations of approximately 10^(20) cm^(−3), are reinvestigated from room temperature to 750 K. The greatly enhanced Seebeck coefficient at these doping levels can be understood by the presence of a sharp increase in the density of states around the Fermi level. As a result, the thermoelectric figure of merit, zT, reaches ~1.4 at 750 K. The influence of these heavy hole carriers may contribute to a similarly high zT observed in related p-type PbTe-based systems such as Tl-doped PbTe and nanostructured composite materials