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Carbon nanotubes (CNTs) have been not attractive for thermal energy conversion applications because of their high thermal conductivities and low Seebeck coefficient. In other words, traditional thermoelectric materials are basically low bandgap semiconductors such as bismuth telluride and antimony telluride, which have shown outstanding performance. In this report, we studied the effect of a network consisting of CNTs as an impurity in a matrix of bismuth telluride (Bi 2 Te 3 ) particles. We dispersed Bi 2 Te 3 particles and CNTs together in a solvent and fabricated composite samples under vacuum filtration. Seven different CNTs concentration, 0, 10, 20, 50, 70, 90, 100 vol% were used to compare the influence of CNTs on electrical conductivity and thermopower of the composites. At the low contents (10 and 20 vol%) of CNTs, the electrical conductivity and the thermopower were a little increased, which can be attributed to p-typed doping effect of CNTs on Bi 2 Te 3 particles. At the high contents (50, 70 and 90 vol%) of CNTs, the electrical conductivity and the thermopower were decreased, which can be attributed to hindered carrier mobility.

We present the effects of In4Se3 addition on thermoelectric properties of n-type Bi2Te2.7Se0.3. In this study, polycrystalline (In4Se3)x-(Bi2Te2.7Se0.3)1−x pellets were prepared by mechanical alloying followed by spark plasma sintering (SPS). The thermoelectric properties such as Seebeck coefficient and electrical and thermal conductivities were measured in the temperature range of 300 K to 500 K. Addition of In4Se3 into Bi2Te2.7Se0.3 resulted in segregation of In4Se3 phase within Bi2Te2.7Se0.3 matrix. The Seebeck coefficient of the (In4Se3)x-(Bi2Te2.7Se0.3)1−x samples exhibited lower values compared with that of pure Bi2Te2.7Se0.3 phase. This reduction of Seebeck coefficient in n-type (In4Se3)x-(Bi2Te2.7Se0.3)1−x is attributed to the formation of unwanted p-type phases by interdiffusion through the interface between (In4Se3)x and (Bi2Te2.7Se0.3)1−x as well as consequently formed Te-deficient matrix. However, the decrease in electrical resistivity and thermal conductivity with addition of In4Se3 leads to an enhanced thermoelectric figure of merit (ZT) at a temperature range over 450 K: a maximum ZT of 1.0 is achieved for the n-type (In4Se3)0.03-(Bi2Te2.7Se0.3)0.97 sample at 500 K.

Indium-selenium-based compounds have received much attention as thermoelectric materials since a high thermoelectric figure of merit of 1.48 at 705 K was observed in In4Se2.35. In this study, four different compositions of indium-selenium compounds, In2Se3, InSe, In4Se3, and In4Se2.35, were prepared by mechanical alloying followed by spark plasma sintering. Their thermoelectric properties such as electrical resistivity, Seebeck coefficient, and thermal conductivity were measured in the temperature range of 300 K to 673 K. All the In-Se compounds comprised nanoscaled structures and exhibited n-type conductivity with Seebeck coefficients ranging from −159 μV K−1 to −568 μV K−1 at room temperature.

To investigate the effect of annealing in controlled atmosphere on the thermoelectric properties of Bi-Te film, Te-deficient Bi-Te film was deposited by sputtering, and then annealed with various Bi-Te alloy powders with different Te concentrations in a closed system at 250°C for 24 h. Bi-Te phases other than Bi2Te3 in the as-deposited film could be removed when the film was annealed with Bi-Te source powder containing 62 at.% or higher content of Te. At the same time, the values of Seebeck coefficient and carrier concentration of the films approach −105 μV/K and 3 × 1019 cm−3 to 6 × 1019 cm−3, respectively. This result indicates that mass transport of Te to the film takes place, resulting in the formation of Bi2Te3 phase and reduction of the amount of p-type carriers due to compositional change of the film from Te-deficient to stoichiometric. Annealing in controlled Te-vapor atmosphere is an effective method to improve the thermoelectric properties of Bi-Te film by changing the composition and phase of Te-deficient film to stoichiometric Bi2Te3 film.

The solidification of alloys in the Bi2Te3-PbTe pseudobinary system at off- and near-eutectic compositions was investigated for their microstructure and thermoelectric properties. Dendritic and lamellar structures were clearly observed due to the phase separation and the existence of a metastable ternary phase. In this system, three phases with different compositions were observed: binary Bi2Te3, PbTe, and metastable PbBi2Te4. The Seebeck coefficient, electrical resistivity, and thermal conductivity of ternary alloys as well as binary compounds were measured. The phonon thermal conductivities of Pb-Bi-Te alloys were lower than those in binary PbTe and Bi2Te3, which could have resulted from the increased interfacial area between phases due to the existence of the metastable ternary phase and the resultant phase separation.

In order to design nano structured materials with enhanced thermoelectric properties, the alloys in the pseudo-binary Bi2Te3-PbTe system are investigated for their micro structure properties. For this synthesis, the liquid alloys are cooled by the water quenching method. Micro structure images are obtained by using an electron probe micro analyzer(EPMA). Dendritic and lamellar structures are clearly observed with the variation in the composition ratio between Bi2Te3 and PbTe. The increase in the Bi2Te3 composition ratio causes to change of the structure from dendritic to lamellar. The Seebeck coefficient of sample 5, in which the mixture rate of Bi2Te3 is 83%, is measured as the highest value. In contrast, the others decrease with the increase of the Bi2Te3 composition ratio. Meanwhile, p-type characteristics are observed in sample 6, at 91%-Bi2Te3 mixture rate. The power factors of the all samples are calculated with the Seebeck coefficient and resistivity.