The thermoelectric properties of (GeTe)(x)(AgSbTe(2))(100-x) compounds (x = 75, 80, 85 and 90; TAGS-x) have been studied as a function of temperature from 300 to 720 K. At 720 K the dimensionless figure of merit ZT reaches the state-of-the-art value of 1.53 for TAGS-75 and 1.50 for TAGS-80 and TAGS-85 samples, respectively. But the ZT value of the TAGS-90 sample is only 0.50 at 720 K due to the high carrier concentration. Utilizing high-resolution transmission electron microscope and selected area electron diffraction techniques, we identify a considerable number of nanoscale domains with typical size ∼10 nm in the samples that show high ZT values. It is suggested that the presence of nanoscale domains, like the situation in PbTe-AgSbTe(2) compounds, should make a slight contribution to the low lattice thermal conductivity of TAGS compounds due to the enhanced mid-frequency phonon scattering.

Recrystallization induced in situ nanostructure formation was used as a new means to obtain high performance polycrystalline thermoelectric materials, which was realized by a simple hot forging process to the coarse-grained Bi 0.5 Sb 1.5 Te 3 alloys. The pole figure measurement showed that the oriented textures were weakened or eliminated after hot forging of the alloys with a quasi-layered crystal structure, implying the presence of deformation recrystallization. Transmission electron microscopy observation revealed the recrystallization induced in situ nanostructures and high density of defects in the hot forged samples. Transport property measurements indicated that the hot forged samples had both increased electrical power factor and reduced thermal conductivity, compare to the initial alloys without hot forging. The maximum ZT values of >1.3 at room temperature were reproducibly obtained for the hot forged samples, suggesting that the simple new method can be applied for large scale production of high performance polycrystalline thermoelectric materials with in situ nanostructures.

Polycrystalline higher manganese silicides (HMS) with Ge additions were prepared by induction melting followed by hot-pressing. The phase structures and microstructure of the pellets were investigated, and their thermoelectric properties were measured from room temperature to 650°C. It was found that the solubility of Ge in HMS was limited to around 1.6%, beyond which an extra phase of Si y Ge 1Ày appeared. The electrical conductivity was continuously enhanced by Ge additions, while the Seebeck coefficient was slightly decreased. The thermal conductivity showed first a decreasing then an increasing relationship with increasing Ge additions. The HMS cells, mainly along the c-axis, were remarkably enlarged by the substitution of Ge, which probably resulted in the enhancement of phonon scattering due to an increased number of defects, reducing the phonon thermal conductivity. The dimensionless figure of merit of the optimized HMS polycrystals was improved by more than 30% compared with the pure HMS material.

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Temperature and size effects on the behavior of nanoscale water molecule clusters are investigated by molecular dynamics simulations. The flexible three-centered (F3C) water potential is used to model the inter-and intramolecular interactions of the water molecule. The differences between the structural properties for the surface region and those for the interior region of the cluster are also investigated. It is found that as the temperature rises, the average number of hydrogen bonds per water molecule decreases, but the ratio of surface water molecules increases. After comparing the water densities in interior regions and the average number of hydrogen bonds in those regions, we find there is no apparent size effect on water molecules in the interior region, whereas the size of the water cluster has a significant influence on the behavior of water molecules at the surface region.

Half-Heusler thermoelectric materials Hf(/Zr)NiSn were prepared by levitation melting followed by melt-spinning to refine the boundary structures, and then they were consolidated by spark plasma sintering. X-ray diffraction analysis and scanning electron microscopy showed that single phased halfHeusler compounds without compositional segregation had been obtained. It was found that the thermoelectric properties, especially the thermal conductivity, depended strongly on the boundary structures. The melt-spinning samples with refined boundary structures had a lower thermal conductivity but a power factor comparable to that of the sample prepared by levitation melting, thus providing good thermoelectric properties.

Thermoelectric (TE) materials (GeTe) 80 (Ag y Sb 2Ày Te 3Ày ) 20 (y = 0.6, 0.8, 1.0, 1.2, and 1.4) were prepared, and their TE properties and microstructure studied in this work. Due to their relatively low thermal conductivity and proper carrier concentration, high ZT values were obtained for all samples except for y = 1.4. Using transmission electron microscopy, twins, antiphase domains, and low-angle grain boundaries were observed throughout the sample with y = 1.2. Nanoscale regions with double atomic spacing were detected. These regions and the matrix were coherent without obvious mismatch. The relationship between high ZT values and microstructure is discussed.