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A study of substrate effect on the thermoelectric (TE) properties of Bi2Te3 (BT) and Sb2Te3 (ST) thin films grown by plasma-enhanced chemical vapor deposition (PECVD) was performed. Graphene substrates which have small lattice mismatch with BT and ST were used for the preparation of highly oriented BT and ST thin films. Carrier mobility of the epitaxial BT and ST films grown on the graphene substrates increased as the deposition temperature increased, which was not observed in that of SiO2/Si substrates. Seebeck coefficients of the as-grown BT and ST films were observed to be maintained even though carrier concentration increased in the epitaxial BT and ST films on graphene substrate. Although Seebeck coefficient was not improved, power factor of the as-grown BT and ST films was considerably enhanced due to the increase of electrical conductivity resulting from the high carrier mobility and moderate carrier concentration in the epitaxial BT and ST films. [ABSTRACT FROM AUTHOR]

The effect of Sn doping on the thermoelectric properties of p-type BiSbTe (BST) thin films was studied. Sn-doped BST films were grown on 4° tilted GaAs (001) substrates by metal-organic chemical vapor deposition. To control the Sn ion concentration in the films, we systematically controlled the dose of the Sn precursor by varying the H flow rate from 0 sccm to 100 sccm. The hole carrier concentration increased as the H flow rate was increased. Interestingly, the Seebeck coefficient of the films simultaneously increased with the carrier concentration when the H flow rate was increased up to 60 sccm. This might be attributed to the formation of virtual bound states in the valence band by Sn doping. Consequently, the Sn ion doping contributed to the thermopower enhancement of the BST films. [ABSTRACT FROM AUTHOR]

In this work, p-type 20%BiTe-80%SbTe bulk thermoelectric (TE) materials were prepared by mechanical deformation (MD) of pre-melted ingot and by mechanical alloying (MA) of elemental Bi, Sb, and Te granules followed by cold-pressing. The dependence on annealing time of changes of microstructure and TE properties of the prepared samples, including Seebeck coefficient, electrical resistivity, thermal conductivity, and figure-of-merit, was investigated. For both samples, saturation of the Seebeck coefficient and electrical resistivity were observed after annealing for 1 h at 380°C. It is suggested that energy stored in samples prepared by both MA and MD facilitated their recrystallization within short annealing times. The 20%BiTe-80%SbTe sample prepared by MA followed by heat treatment had higher a Seebeck coefficient and electrical resistivity than specimens fabricated by MD. Maximum figures-of-merit of 3.00 × 10/K and 2.85 × 10/K were achieved for samples prepared by MA and MD, respectively. [ABSTRACT FROM AUTHOR]

The effects of mechanical deformation and subsequent annealing on the thermoelectric properties and microstructure have been investigated for p-type (BiSb)Te alloys prepared by melting followed by quenching. The mechanically deformed pellets were prepared by repetition of cold-pressing of quenched samples at room temperature. Cold-pressed pellets were then annealed at 300°C in vacuum, and the thermoelectric properties and microstructure were traced through the course of the heat treatment. For the heavily deformed samples, the Seebeck coefficient rapidly increased at the very early stage of annealing and did not change as the annealing time increased, due to recrystallization of a new δ-phase which equilibrated at the annealing temperature of 300°C (δ-phase). At the initial stage of annealing (recovery stage), the electrical resistivity sharply increased, probably due to the interaction of antistructural defects with vacancies produced during the cold-pressing treatment. However, for the lightly deformed samples, recrystallization occurred only at some portion of the grain boundaries, and the newly generated δ-phase slowly replaced the original, as-solidified δ-phase. [ABSTRACT FROM AUTHOR]

To investigate the effects of segmentation of thermoelectric materials on performance levels, n-type segmented BiTe/PbSeTe thermoelectric material was fabricated, and its output power was measured and compared with those of BiTe and PbSeTe. The two materials were bonded by diffusion bonding with a diffusion layer that was ∼18 μm thick. The electrical conductivity, Seebeck coefficient, and power factor of the segmented BiTe/PbSeTe sample were close to the average of the values for BiTe and PbSeTe. The output power of BiTe was higher than those of PbSeTe and the segmented sample for small Δ T (300 K to 400 K and 300 K to 500 K), but that of the segmented sample was higher than those of BiTe and PbSeTe when Δ T exceeded 300 K (300 K to 600 K and 300 K to 700 K). The output power of the segmented sample was about 15% and 73% higher than those of the BiTe and PbSeTe samples, respectively, when Δ T was 400 K (300 K to 700 K). The efficiency of thermoelectric materials for large temperature differences can be enhanced by segmenting materials with high performance in different temperature ranges. [ABSTRACT FROM AUTHOR]

We present the effects of InSe addition on thermoelectric properties of n-type BiTeSe. In this study, polycrystalline (InSe)-(BiTeSe) 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 InSe into BiTeSe resulted in segregation of InSe phase within BiTeSe matrix. The Seebeck coefficient of the (InSe)-(BiTeSe) samples exhibited lower values compared with that of pure BiTeSe phase. This reduction of Seebeck coefficient in n-type (InSe)-(BiTeSe) is attributed to the formation of unwanted p-type phases by interdiffusion through the interface between (InSe) and (BiTeSe) as well as consequently formed Te-deficient matrix. However, the decrease in electrical resistivity and thermal conductivity with addition of InSe 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 (InSe)-(BiTeSe) sample at 500 K. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

It is challenging to improve the thermoelectric figure-of-merit as its constituent terms such as Seebeck coefficient, electrical conductivity, and thermal conductivity, are inter-related in the way that the enhancement of one term leads to the degradation of others. Therefore, it is highly desirable to design a new synthesis process that allows us to independently control these terms. Here, we report a simple, two-step process combining spark plasma sintering (SPS) and post-annealing (PA) to separately control the carrier density and mobility in the p-type (Bi 0.2 Sb 0.8 ) 2 Te 3 . High-temperature SPS enables enhancing the carrier mobility by reducing scattering sites such as grain boundaries. Then, the following PA at a lower temperature allows tailoring the carrier density without the degradation of mobility. Beyond bismuth telluride-based, room-temperature thermoelectric materials, we believe that our result will provide an insight for the performance enhancement of other thermoelectric materials such as oxide and skutterudite. [ABSTRACT FROM AUTHOR]

Abstract: Mesoporous TiO2 films were prepared by using titanium tetraisopropoxide as the titania precursor and triblock copolymer as the structure directing agent. The synthesized mesoporous TiO2 film was confirmed to have the ordered pore structure with rutile phase by small angle and wide angle X-ray diffraction analyses. The mesoporous TiO2 film has the porosity range from 21.6 to 35.6%, and its Seebeck coefficient was changed according to its porosity, up to −88.6μV/K. From the obtained Seebeck coefficient, the ordered mesoporous TiO2 film was found to be a good candidate of thermal sensing layer of thin film thermal sensor. [Copyright &y& Elsevier]