Thermoelectric Figure Of Merit ZT Research Articles

Weak Bonding Effect on the Ultralow Thermal Conductivity of Germanium Nanodot Arrays in Silicon

ABSTRACTThe thermoelectric figure of merit ZT depends on the thermal conductivity inverse. uperlattices with periodic thin layers were studied to obtain ZT > 1 due to phonon confinement between their layers. Unfortunately, their synthesis with ZT higher than 1 is hazardous due to lattice mismatches forming dislocations and cracks. Nanowires with low dimensionalities were also proposed. However, as the superlattices, they decrease the thermal conductivity in only one propagation direction. In experiments, these one-dimensional insulating materials usually fail to beat the lowest limit of amorphous Si (+/- 1 W/m/K). In this theoretical study, three-dimensional Ge quantum dot arrays in Si are proposed to obtain an extreme thermal-conductivity reduction. Two decrease effects are shown from a molecular supercrystal model. First, low phonon group velocities are computed by lattice dynamics. Second, near-field scattering is exalted assuming weak interface bonding. This prediction can lead to a significant ZT increase. Indeed, a thermalconductivity global minimum λ* = 0.009 W/m/K is predicted for a Si/Ge supercrystal with nanodot spacing of +/- 30 nm and Ge concentration of +/- 12.5 Ge at.%. This ultralow λ* is computed at 300 K assuming that all Ge nanodots are weakly bonded and scatter the phonons at the Si-Ge interfaces in the geometrical limit. Thermal conductivity evolution is analyzed with respect to the weakly-bonded Ge nanodot density.

A high thermoelectric figure of merit ZT > 1 in Ba heavily doped BiCuSeO oxyselenides

A high ZT value of ∼1.1 at 923 K in the BiCuSeO system is achieved via heavily doping with Ba and refining grain sizes (200–400 nm), which is higher than any thermoelectric oxide. Excellent thermal and chemical stabilities up to 923 K and high thermoelectric performance confirm that the BiCuSeO system is promising for thermoelectric power generation applications.

アーク溶融法と放電プラズマ焼結法を併用して合成したBa<SUB>8</SUB>Al<SUB>16</SUB>Si<SUB>30</SUB>系クラスレートの熱電気的特性

We investigated the thermoelectric properties of Ba8Al16Si30 clathrate samples prepared by combining arc melting and spark plasma sintering methods under two different arc melting conditions. The x-ray diffraction, scanning electron microscopy, and energy dispersive analysis of x-ray studies indicate that Al composition is close to 15, which is higher than that of a single crystal reported, for both arc melting conditions. The carrier concentration of samples decreases to about 1×1021 cm-3, which is lower than reported values so far in Ba8Al16Si30 system. The Seebeck coefficient and the electrical conductivity alter slightly with the arc melting condition due to the slight difference in the carrier concentration. The lattice thermal conductivity (RT) is as low as about 1.3 Wm-1K-1, which is comparable to Bi2Te3-based bulk alloy, independently of the arc melting condition. The thermoelectric figure of merit ZT is about 0.4 at 900 K. An estimation indicates that ZT reaches 0.8 (1200 K) at the optimum carrier concentration of about 3×1020 cm-3 in Ba8Al16Si30 system.

Crystal Structure and Thermoelectric Properties of a Novel ternary AgIn5Se8 alloy

A novel ternary Ag-In-Se semiconductor was prepared using spark plasma sintering technique, and its crystal structure and thermoelectric properties were evaluated. XRD analysis and bandgap measurement indicated that this ternary compound crystallizes in a single phase AgIn5Se8 with its bandgap of 1.11 eV, smaller than that of In2Se3, it therefore is of higher electrical conductivity than In2Se3. The highest power factor and thermoelectric figure of merit ZT, thermal conductivity at 863 K are 2.74x10-4 W.K-2.m-1, 0.61 and 0.39 W.K-2.m-1, respectively.

Thermoelectric properties of Li-doped Cu0.95-xM0.05LixO (M=Mn, Ni, Zn)

ABSTRACTThermoelectric properties of the Li-doped Cu0.95-xM0.05LixO (M=divalent metal ion; Mn, Ni, Zn) were investigated at the temperature up to 1273 K. In the doped divalent metal ions, Zn2+ ion was the most effective to reduce the thermal conductivity, and the Ni2+ substitution was preferable to decrease the electrical resistivity. For the Cu0.95-xNi0.05LixO sample at x=0.03, the maxima of the dimensionless thermoelectric figure of merit ZT and the power factor P at 1246 K were 4.2×10-2 and 1.6 ×10-4 W/K2m, respectively. The enhancement of the thermoelectric properties of the Li-doped Cu0.95-xM0.05LixO system was discussed.

Enhanced thermoelectric properties of Bi2(Te1−xSex)3-based compounds as n-type legs for low-temperature power generation

The abundance of low-temperature waste heat necessitates the development of reliable and scalable thermal-to-electric energy conversion technology. The thermoelectric device is one viable option. Commercially available Bi2Te3-based materials are optimized for near room temperature cooling applications. Currently there are no mass-produced materials available for 400 K to 650 K thermoelectric power generation. We report the successful realization of high performance n-type Bi2(Te1−xSex)3-based materials for the temperature range of interest, using a commercial zone-melting technique. The introduction of Se effectively increases the band gap, which significantly suppresses the “turn-over” in Seebeck coefficient and the appearance of a pronounced bipolar effect, shifting the corresponding temperature of the optimum thermoelectric figure of merit ZT towards a higher temperature range. Furthermore, we demonstrate that the electron concentration of Bi2(Te0.5Se0.5)3 can be effectively adjusted by iodine doping. The samples with electron concentrations between 3 × 1019 and 4.5 × 1019 cm−3 display optimal thermoelectric performances. The highest ZT value reaches 0.86 at 600 K for the sample with the electron concentration of 4.0 × 1019 cm−3, whose average ZT between 400 K and 640 K is 0.8, making this scalable zone-melted low-Te content Bi2(Te0.5Se0.5)3 compound a promising candidate for low-temperature power generation.

Thermoelectric properties of silicon carbide nanowires with nitride dopants and vacancies

The thermoelectric properties of cubic zincblend silicon carbide nanowires (SiCNWs) with nitrogen impurities and vacancies along [111] direction are theoretically studied by means of atomistic simulations. It is found that the thermoelectric figure of merit ZT of SiCNWs can be significantly enhanced by doping N impurities together with making Si vacancies. Aiming at obtaining a large ZT, we study possible energetically stable configurations, and disclose that, when N dopants locate at the center, a small number of Si vacancies at corners are most favored for n-type nanowires, while a large number of Si vacancies spreading into the flat edge sites are most favored for p-type nanowires. For the SiCNW with a diameter of 1.1 nm and a length of 4.6 nm, the ZT value for the n-type is shown capable of reaching 1.78 at 900K. The conditions to get higher ZT values for longer SiCNWs are also addressed.

Lead‐Free Thermoelectrics: High Figure of Merit in p‐type AgSnmSbTem+2

AbstractThermoelectric materials based on Pb‐free compositions are of considerable current interest in environmentally friendly power‐generation applications derived from waste‐heat sources. Here, a new study of the thermoelectric properties of the tin‐based compositions with the general formula AgSnmSbTem+2 (m = 2, 4, 5, 7, 10, 14, 18) is presented, where the m value is used as the tuning parameter of the thermoelectric properties. The electrical conductivity, Seebeck coefficient, and thermal conductivity are measured from 300 K to 723 K and the resulting thermoelectric figure of merit is determined as a function of the SnTe/AgSbTe2 ratio. A thermoelectric figure of merit ZT ≈1 is obtained at 710 K for m = 4, indicating that the system AgSnmSbTem+2 holds great promise as an alternative p‐type, lead‐free, thermoelectric material.

Thermoelectric Properties of Ba<sub>x</sub>Co<sub>4</sub>Sb<sub>12</sub> Prepared by High Pressure and High Temperature

Polycrystalline filled Skutterudite compounds BaxCo4Sb12(0-x-0.5) are synthesized by high pressure and high temperature （HPHT） technique. The thermal conductivity for CoSb3is depressed significantly by Ba-filling combined HPHT technique. The value of 1.25 Wm-1K-1for Ba0.372Co4Sb12is obtained at 633K. The dimensionless thermoelectric figure of merit ZT, increases with temperature increasing and reaches a maximal value of 1.01 at 663 K.

Stoichiometry Controlled, Single‐Crystalline Bi2Te3 Nanowires for Transport in the Basal Plane

AbstractThermoelectric Bi2Te3 based bulk materials are widely used for solid‐state refrigeration and power‐generation at room temperature. For low‐dimensional and nanostructured thermoelectric materials an increase of the thermoelectric figure of merit ZT is predicted due to quantum confinement and phonon scattering at interfaces. Therefore, the fabrication of Bi2Te3 nanowires, thin films, and nanostructured bulk materials has become an important and active field of research. Stoichiometric Bi2Te3 nanowires with diameters of 50–80 nm and a length of 56 μm are grown by a potential‐pulsed electrochemical deposition in a nanostructured Al2O3 matrix. By transmission electron microscopy (TEM), dark‐field images together with electron diffraction reveal single‐crystalline wires, no grain boundaries can be detected. The stoichiometry control of the wires by high‐accuracy, quantitative enegy‐dispersive X‐ray spectroscopy (EDX) in the TEM instrument is of paramount importance for successfully implementing the growth technology. Combined electron diffraction and EDX spectroscopy in the TEM unambiguously prove the correct crystal structure and stoichiometry of the Bi2Te3 nanowires. X‐ray and electron diffraction reveal growth along the [110] and [210] directions and the c axis of the Bi2Te3 structure lies perpendicular to the wire axis. For the first time single crystalline, stoichiometric Bi2Te3 nanowires are grown that allow transport in the basal plane without being affected by grain boundaries.

Thermoelectric properties and microstructures of AgSbTe2‐added p‐type Pb0.16Ge0.84Te

AbstractThe alloys in the AgSbTe2GeTe system and the AgSbTe2PbTe system are known as excellent thermoelectric (TE) materials, i.e. TAGS and LAST, respectively. Although the TE properties of the AgSbTe2(Pb,Ge)Te system have not been reported. Here, we show the high‐temperature TE properties of the alloys in the AgSbTe2(Pb,Ge)Te system. The samples of (Ag0.5Sb0.5Te)100−x(Pb0.16Ge0.84Te)x (x = 75, 80, 85, and 90) were prepared and the electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were examined in the temperature range from room temperature to 720 K. The samples prepared by hot‐press were composed of two phases: the GeTe‐based matrix phase and the PbTe‐based precipitate. These samples exhibited the moderate ρ and S as well as relatively low κ, leading to high thermoelectric figure of merit ZT (= S2T/ρκ, where T is the absolute temperature). The maximum ZT value was 0.84 at 720 K obtained in the sample of x = 80.

Thermoelectric properties of p-type (Bi2Te3)x(Sb2Te3)1−x prepared by spark plasma sintering

High performance (Bi2Te3)x(Sb2Te3)1−x bulk materials have been prepared by combining fusion technique with spark plasma sintering, and their thermoelectric properties have been investigated. With the increase of Bi2Te3 content, the electrical resistivity and Seebeck coefficient increase greatly and the thermal conductivity decreases significantly, which lead to a great improvement in the thermoelectric figure of merit ZT. The maximum ZT value reaches 1.33 at 398 K for the composition of 20% Bi2Te3–80%Sb2Te3 with 3 wt% excess Te.

Enhanced Thermoelectric Figure of Merit of Zintl Phase YbCd2–xMnxSb2 by Chemical Substitution

AbstractSamples of Zintl phase YbCd2–xMnxSb2 (CaAl2Si2‐type, space group P$\bar {3}$m1) were prepared by a solid‐state reaction followed by a suitable annealing and spark plasma sintering (SPS) for densification. Investigations were carried out on chemical substitution of Cd by Mn in order to optimize the thermoelectric figure of merit ZT in the solid solution system YbCd2–xMnxSb2 (x = 0, 0.05, 0.1, 0.15, 0.2, 0.4, 0.6, 0.8, 1.0, 1.5 and 2.0). Seebeck coefficients, electrical and thermal conductivities of all the samples were measured in the temperature range 300–650 K. The presence of Mn (x ≤ 0.2) substantially reduces the thermal conductivity and enlarges the Seebeck coefficient, meanwhile decreasing the electrical conductivity to a certain degree. As the concentration of Mn (x) becomes higher than 1.0, the semimetallic nature ofYbCd2–xMnxSb2 changes to that of a semiconductor with the majority the carriers switching from holes to electrons. Of all the samples, polycrystalline YbCd1.85Mn0.15Sb2 (p‐type charge carriers) shows the highest figure of merit ZT of 1.14 at 650 K.

Improving thermoelectric properties of p-type Bi2Te3-based alloys by spark plasma sintering

AbstractHigh-performance (Bi2Te3)x(Sb2Te3)1−x bulk materials were prepared by combining fusion technique with spark plasma sintering, and their thermoelectric properties were investigated. The electrical resistivity and Seebeck coefficient increase greatly and the thermal conductivity decreases significantly with the increase of Bi2Te3 content, which leads to a great improvement in the thermoelectric figure of merit ZT. The maximum ZT value reaches 1.33 at 398 K for the composition of 20%Bi2Te3-80%Sb2Te3 with 3% (mass fraction) excess Te.

Multiterminal single-molecule–graphene-nanoribbon junctions with the thermoelectric figure of merit optimized via evanescent mode transport and gate voltage

We study thermoelectric devices where a single 18-annulene molecule is connected to metallic zigzag graphene nanoribbons (ZGNR) via highly transparent contacts that allow for injection of evanescent wave functions from ZGNRs into the molecular ring. Their overlap generates a peak in the electronic transmission, while ZGNRs additionally suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with suppression of phonon transport through ZGNR-molecule-ZGNR structure, yield the thermoelectric figure of merit ZT ~ 0.5 at room temperature and 0.5 < ZT < 2.5 below liquid nitrogen temperature. Using the nonequilibrium Green function formalism combined with density functional theory, recently extended to multiterminal devices, we show how the transmission resonance can also be manipulated by the voltage applied to a third ZGNR electrode, acting as the top gate covering molecular ring, to tune the value of ZT.

C60‐doping of nanostructured Bi–Sb–Te thermoelectrics

AbstractNew bulk nanocomposite thermoelectric materials composed from nanocrystallites of Bi–Sb–Te alloys covered by C60 molecules have been synthesized and studied. The fullerene molecules provide thermal phonons blocking and particular charge transfer in the nanocomposite. The molecules act as electron traps, and thus decrease the density of free electrons in n‐type semiconductor and generate holes in p‐type materials. The capture of electrons is not accompanied by shifts of the Raman bands in C60 spectra. The density of free charge carriers and their Hall mobility change nonmonotonically with the increase of fullerene content in Bi–Sb–Te–C60 nanocomposites. The maximum value of thermoelectric figure of merit ZT = 1.15 was obtained in p‐type nanocomposite that is 30% higher than in the undoped starting material. The observed phenomenon provides new ways for transport properties optimization of thermoelectric materials and thermoelectric devices efficiency increase.

Thermoelectric effects in spin field-effect transistors

We investigate the thermoelectric effects in a spin field-effect transistor with ferromagnetic leads held at different temperatures. The thermopower S and thermoelectric figure of merit ZT oscillate with the increase of the Rashba spin–orbit coupling strength. The oscillation amplitude of ZT decreases with increasing the spin polarization. S and ZT are strongly influenced by the interfacial barrier strength Z, exhibiting a nonmonotonous change with Z. The thermoelectric effects are also manipulated by the magnetization configuration of the ferromagnetic leads. It is expected that the present study of the thermoelectric effects is helpful in the design of thermoelectric devices.

Multiple-Filled Skutterudites: High Thermoelectric Figure of Merit through Separately Optimizing Electrical and Thermal Transports

Skutterudites CoSb(3) with multiple cofillers Ba, La, and Yb were synthesized and very high thermoelectric figure of merit ZT = 1.7 at 850 K was realized. X-ray diffraction of the densified multiple-filled bulk samples reveals all samples are phase pure. High-resolution scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) analysis confirm that multiple guest fillers occupy the nanoscale-cages in the skutterudites. The fillers are further shown to be uniformly distributed and the Co-Sb skutterudite framework is virtually unperturbed from atomic scale to a few micrometers. Our results firmly show that high power factors can be realized by adjusting the total filling fraction of fillers with different charge states to reach the optimum carrier density, at the same time, lattice thermal conductivity can also be significantly reduced, to values near the glass limit of these materials, through combining filler species of different rattling frequencies to achieve broad-frequency phonon scattering. Therefore, partially filled skutterudites with multiple fillers of different chemical nature render unique structural characteristics for optimizing electrical and thermal transports in a relatively independent way, leading to continually enhanced ZT values from single- to double-, and finally to multiple-filled skutterudites. The idea of combining multiple fillers with different charge states and rattling frequencies for performance optimization is also expected to be valid for other caged TE compounds.

Promising thermoelectric properties in AgxMo9Se11 compounds (3.4≤x≤3.9)

Polycrystalline Mo9 cluster chalcogenides AgxMo9Se11 (3.4≤x≤3.9) have been prepared by powder metallurgy techniques, sintered by spark plasma sintering and characterized by x-ray diffraction. Their thermoelectric properties (electrical resistivity, thermopower, thermal conductivity) have been determined in the 300–800 K temperature range. The AgxMo9Se11 compounds show p-type conduction characteristics. The outstanding low lattice thermal conductivities give rise to a rather high value of the dimensionless thermoelectric figure of merit ZT of ∼0.65 at 800 K for x=3.8–3.9, making this family of materials particularly promising for thermoelectric power generation applications.

Structure and Thermoelectric Properties of Te- and Ge-Doped Skutterudites CoSb2.875−x Ge0.125Te x

n-Type CoSb2.875−xGe0.125Tex (x = 0.125 to 0.275) compounds with different Te contents have been synthesized by a melt–quench–anneal–spark plasma sintering method, and the effects of Te content on the structure and thermoelectric properties have been investigated. The results show that all specimens exhibited n-type conduction characteristics. The solubility limit of Te in CoSb2.875−xGe0.125Tex is found to be x = 0.25. The solubility of Te in CoSb3 is increased through charge compensation of the element Ge. The room-temperature carrier concentration Np of CoSb2.875−xGe0.125Tex skutterudites increases with increasing Te content, and the compounds possess high power factors. The maximum power factor of 3.89 × 10−3 W m−1 K−2 was obtained at 720 K for the CoSb2.625Ge0.125Te0.25 compound. The thermal conductivity decreases dramatically with increasing Te content due to strong point defect scattering. The maximum value of the thermoelectric figure of merit ZT = 1.03 was obtained at 800 K for CoSb2.625Ge0.125Te0.25, benefiting from a lower thermal conductivity and a higher power factor. The figure of merit is competitive with values reported for single-filled skutterudites.