Takuji Kita

Nanostructured AgPbmSbTem+2 System Bulk Materials with Enhanced Thermoelectric Performance

Nanostructured Ag0.8Pbm+xSbTem+2 (m = 18, x = 4.5) system thermoelectric materials have been fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) methods followed by annealing for several days to investigate the effect on microstructure and thermoelectric performance. It was found that appropriate annealing treatment could reduce both the electrical resistivity and the thermal conductivity at the same time, consequently greatly enhancing the thermoelectric performance. A low electrical resistivity of 2 x 10-3 Ohm-cm and low thermal conductivity of 0.89 W m-1 K-1 were obtained for the sample annealed for 30 days at 700 K. The very low thermal conductivity is supposed to be due to the nanoscopic Ag/Sb-rich regions embedded in the matrix. A high ZT value of 1.5 at 700 K has been achieved for the sample annealed for 30 days.

High-performance Ag0.8Pb18+xSbTe20 thermoelectric bulk materials fabricated by mechanical alloying and spark plasma sintering

Polycrystalline AgnPbmSbTem+2n thermoelectric materials, whose compositions can be described as Ag0.8Pb18+xSbTe20 were prepared using a combined process of mechanical alloying and spark plasma sintering. Electric properties of the sintered samples with different Pb contents were measured from room temperature to 700K. The maximum power factor of 1.766mW∕mK2 was obtained at 673K for the Ag0.8Pb22SbTe20 sample, which corresponds to a high dimensionless figure of merit, ZT=1.37. This best composition is different from that reported before.

Seebeck effect in PbTe films and EuTe/PbTe superlattices

Theoretical calculations of the Seebeck coefficients of bulk PbTe and PbTe based superlattices were described in the framework of Boltzmann equation, taking into account temperature dependent band gaps, nonparabolicity, and anisotropy of effective masses. It is shown that the temperature gradient along the superlattice layer works more effectively on the enhancement of the thermoelectric figure of merit than the temperature gradient normal to the superlattice layer. Calculated Seebeck coefficients were compared to the experimental values for n-type PbTe, p-type PbTe, and EuTe/PbTe superlattices. The Seebeck coefficient of p-type PbTe was higher than that of n-type PbTe. The relatively high Seebeck coefficient is explained by the contribution from other extrema in the valence band. The EuTe/PbTe [001] superlattice shows higher Seebeck coefficients than PbTe bulk owing to the large density of states.

Synthesis and thermoelectric properties of fine-grained FeVSb system half-Heusler compound polycrystals with high phase purity

Nearly single-phased FeVSb half-Heusler compound thermoelectric materials with fine grains of diameter 100–200 nm were prepared from their elemental powders by combining mechanical alloying (MA) and spark plasma sintering. The resultant bulk samples showed a relatively low room-temperature electrical resistivity on the order of 10 µΩm, and a moderate negative Seebeck coefficient with a maximum value of −175 µV K−1 at 300 °C. It was found that proper excessive addition of V relative to the stoichiometric composition (FeVSb) during MA enhanced the phase purity and hence the power factor of the spark plasma sintered samples, resulting in a large power factor value of 2480 µW m−1 K−2 when the elemental powders were mixed with the composition FeV1.15Sb. Its thermal conductivity was significantly reduced mainly due to refined grain sizes, resulting in a high dimensionless figure of merit ZT of 0.31 at a low-to-mid temperature (300 °C) as for undoped half-Heusler compounds.

Thermoelectric properties of directionally solidified half-Heusler compound NbCoSn alloys

Single- and multiphase samples of the n-type half-Heusler NbCoSn were prepared by directional solidification using the optical floating zone melting method, and the thermoelectric properties of these samples were evaluated. NbCoSn has an excellent thermoelectric power which exceeds −250μVK−1 at around 900K and a relatively high carrier concentration, 4.82×1026m−3. A metalliclike temperature dependence of the electrical resistivity indicates that NbCoSn is a degenerate semiconductor. NbCoSn also shows an excellent power factor, 2.5mWm−1K−2 at about 650K, even without any tuning of the electrical properties which are susceptible to coexisting metallic phases.

Electrical and thermoelectrical properties of SnTe-based films and superlattices

SnTe-based films and superlattices (SLs) were prepared and their electrical properties were measured. A EuTe/SnTe SL exhibited a hole mobility of 2720 cm2/V s, which is the highest value reported for any semiconductor material at room temperature. The SnEuTe film also exhibited high hole mobility in contrast to the PbEuTe system. These properties are explained in terms of the band offsets of EuTe/SnTe heterojunction and a decrease in the number of Sn vacancies. In addition, SnTe/PbSe and SnTe/PbS SLs with thin SnTe layers displayed n-type conduction with Seebeck coefficients comparable to those for PbSe and PbS. These properties reflect the type II heterostructures.

Enhanced Seebeck coefficient in EuTe∕PbTe [100] short-period superlattices

Theoretical and experimental studies on Seebeck effect in EuTe∕PbTe superlattices were performed. Theoretical calculations, which take into account temperature dependent band gap, nonparabolicity, and anisotropy of effective masses in the PbTe conduction band, were performed in the framework of Boltzmann equation in which enhancement of Seebeck coefficient in EuTe∕PbTe short-period superlattices grown in [100] direction was predicted. The EuTe∕PbTe short-period superlattices with few monolayers EuTe were prepared on KCl (100) substrate and an enhanced Seebeck coefficient was observed in these superlattices as expected by theoretical calculations.

Epitaxial Synthesis of Srn+1TinO3n+1 (n = 2-5) Ruddlesden-Popper Homologous Series by Pulsed-Laser Deposition

Ruddlesden–Popper homologous series of Srn+1TinO3n+1 (2≤n≤5) compounds were synthesized with accumulating SrO monolayers and n unit cells of SrTiO3 layers alternately in a pulsed-laser deposition. The successive deposition of these layers at a constant growth temperature (Tg) of 780 °C resulted in a solid-solution film. When we varied the Tg between the independently optimized 575 and 780 °C for SrO and SrTiO3 depositions, respectively, a persistent intensity oscillation of reflection high energy diffraction was observed. The film structures on (001) SrTiO3 substrates were evaluated by X-ray diffraction measurements and compared to previously reported films grown by molecular-beam epitaxy.

Thermoelectric enhancement at low temperature in nonstoichiometric lead-telluride compounds

Pb1.17Te thermoelectric polycrystalline materials were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). The property measurement and microstructural characterization showed that the present material has special features different from traditional Pb1+xTe ingots with secondary Pb phase. An attractive enhancement of the thermoelectric figure of merit ZT = 0.64 was obtained at 450 K, with a low thermal conductivity of 1.11 W m−1 K−1 at this temperature. Transmission electron microscopy observation showed the existence of randomly dispersed nano features that are responsible for such enhancement, some of which are similar to the nanostructures reported in the AgPbmSbTem+2 system. The origin of these regions is discussed and their influence on thermal conductivity is revealed. The results confirm the effectiveness of such a kind of nano feature in improving thermoelectric properties, especially in reducing thermal conductivity. They also indicate a new way of obtaining thermoelectric materials with such a kind of nano feature via MA and SPS.

High-Throughput Screening for Combinatorial Thin-Film Library of Thermoelectric Materials

A high-throughput method has been developed to evaluate the Seebeck coefficient and electrical resistivity of combinatorial thin-film libraries of thermoelectric materials from room temperature to 673 K. Thin-film samples several millimeters in size were deposited on an integrated Al2O3 substrate with embedded lead wires and local heaters for measurement of the thermopower under a controlled temperature gradient. An infrared camera was used for real-time observation of the temperature difference Delta T between two electrical contacts on the sample to obtain the Seebeck coefficient. The Seebeck coefficient and electrical resistivity of constantan thin films were shown to be almost identical to standard data for bulk constantan. High-throughput screening was demonstrated for a thermoelectric Mg-Si-Ge combinatorial library.