Figure Of Merit ZT Research Articles

Thermoelectric properties of quaternary (Bi,Sb)2(Te,Se)3 compound

Abstract The quaternary (Bi,Sb) 2 (Te,Se) 3 compounds are investigated using first-principles study and Boltzmann transport theory. The energy band structure and density of states are studied in detail. The electronic transport coefficients are then calculated as a function of chemical potential. The figure of merit ZT is obtained assuming a constant relaxation time and an averaged thermal conductivity. Our theoretical result agrees well with previous experimental data.

Synergetic effect of Zn substitution on the electron and phonon transport in Mg2Si0.5Sn0.5-based thermoelectric materials.

Mg2Si1-xSnx alloys are a prospective material for thermoelectric generators at moderate temperatures. The thermoelectric properties of Mg2Si0.5Sn0.5-based thermoelectric materials with only Zn substitution or Zn/Sb co-doping were investigated. Isoelectronic Zn substitution did not affect the carrier concentration, but improved the carrier mobility. Zn atoms incorporated into a Sb-doped Mg2Si0.5Sn0.5 matrix simultaneously boosted the power factor and suppressed the lattice thermal conductivity, leading to an enhancement of the thermoelectric figure of merit ZT of the resulting bulk materials. The interplay between the electron and phonon transport of Mg2Si0.5Sn0.49Sb0.01 substituted with Zn at Mg sites results in an enhancement of the ZT by 25% at ∼730 K, from ZT≈ 0.8 in Mg2Si0.5Sn0.49Sb0.01 to ZT≈ 1.0 in Mg1.98Zn0.02Si0.5Sn0.49Sb0.01. Solid solutions in the Mg2Si-Mg2Sn system appear to be more promising for thermoelectric applications.

Thermoelectric performance of polyparaphenylene/Li0·5Ni0·5Fe2O4 nanocomposites

Polyparaphenylene/Li0·5Ni0·5Fe2O4 (PPP/LiNi-ferrite, Li0·5Ni0·5Fe2O4 and LiNi-ferrite were used interchangeably) nanocomposites were synthesised using an in situ compounding method, and their thermoelectric (TE) properties were measured. Compared with single phase LiNi-ferrite, nanocomposites have significantly enhanced electronic transfer capability and substantially lowered thermal conductivity. The low thermal conductivity of the added PPP and the very large boundary between the conductive polymer and the oxide nanoparticles play a dominant role on the thermal conductivity reduction. The figure of merit ZT of the PPP/LiNi-ferrite nanocomposite has been improved several orders in a wide temperature range from 300 to 850 K. Fabrication of nanocomposites consisting of electrically conductive polymer and oxide nanoparticles may provide a promising way for realising high ZT TE performance.

Thermoelectric properties of the (Bi,Sb)2Te3-based material obtained by spark plasma sintering

The dependence of the thermoelectric properties of the nanostructured bulk (Bi,Sb)2Te3 material on the composition and the spark plasma-sintering (SPS) temperature TSPS has been studied. It has been revealed that the Bi0.4Sb1.6Te3 solid solution sintered at a temperature of 450–500°C has a thermoelectric figure of merit ZT = 1.25–1.28. The dependence of thermoelectric properties on the sintering temperature TSPS above 400°C is correlated to the transformation of the fine structure of the material due to the rearrangement of point vacancy-donor defects in the process of repeated recrystallization. It has been established that point structural defects make a considerable contribution to the formation of the thermoelectric properties of nanostructured material.

Theoretical study of thermoelectric properties of MoS2

We systematically studied the thermoelectric properties of MoS2 with doping based on the Boltzmann transport theory and first-principles calculations. We obtained an optimal doping region (around 1019 cm−3) for thermoelectric properties along in-plane and cross-plane directions. MoS2 in the optimal doping region has a vanishingly small anisotropy of thermopower possibly due to the decoupling of in-plane and cross-plane conduction channels, but big anisotropies of electrical conductivity σ and electronic thermal conductivity κe arising from the anisotropic electronic scattering time. The κe is comparable to the lattice counterpart κ1 in the plane, while κ1 dominates over κe across the plane. The figure of merit ZT can reach 0.1 at around 700 K with in-plane direction preferred by doping.

Enhancement of the Thermoelectric Performance of Polycrystalline In4Se2.5 by Copper Intercalation and Bromine Substitution

Thermoelectric (TE) materials and devices, stimulated by increasing demand for electricity around the world, are being intensively investigated because of their capability to directly convert heat of automobile exhaust and various industrial processes into electricity. [ 1 ] Usually, the scale of the capability is determined by the dimensionless fi gure of merit ZT = S 2 σT / κ , in which S , σ , κ , and T are the Seebeck coeffi cient, the electrical conductivity, the thermal conductivity, and the absolute temperature at which the properties are measured, respectively. Therefore, the pursuance of high ZT has traced the path of lowering the thermal conductivity and improving the power factor ( S 2 σ ). [ 2–6 ]

Microstructure and thermoelectric properties of Bi0.5Na0.02Sb1.48−x In x Te3 alloys fabricated by vacuum melting and hot pressing

The Bi0.5Na0.02Sb1.48−x In x Te3 alloys (x = 0.02–0.20) were synthesized by vacuum melting and hot pressing methods at 753 K, 60 MPa for 30 min. Effects of Na and In dual partial substitutions for Sb on the thermoelectric properties were investigated from 300 to 500 K. Substituting Sb with Na and In can enhance the Seebeck coefficient effectively near room temperature. The electrical resistivity of the Na and In dual-doping samples is higher within the whole test temperature range. The Bi0.5Na0.02Sb1.48−x In x Te3 samples (x = 0.02, 0.06) play a great role in optimizing the thermal conductivity. As for the Bi0.5 Na0.02Sb1.46In0.02Te3 alloy, the minimum value of thermal conductivity reaches 0.53 W·m−1·K−1 at 320 K. The thermoelectric performance of the Na and In dual-doped samples is greatly improved, and a figure of merit ZT of 1.26 is achieved at 300 K for the Bi0.5Na0.02Sb1.42In0.06Te3, representing 26 % enhancement with respect to ZT = 1.0 of the undoped sample.

Effect of Chromium Doping on the Thermoelectric Properties of Bi2Te3: Cr x Bi2Te3 and Cr x Bi2−x Te3

The thermoelectric (TE) properties of Bi2Te3 compounds intercalated and substituted with Cr, namely CrxBi2Te3 and CrxBi2−xTe3, respectively, have been investigated to study the influence of chromium on the TE properties of Bi2Te3. The Seebeck coefficients were found to be positive for all the samples in the temperature range between 300 K and 550 K. Although no effective enhancement of the Seebeck coefficient was observed, doping with Cr by means of either substitution or intercalation clearly not only improved the electrical conductivity but also lowered the thermal conductivity of Bi2Te3. As a result of the improvement, the figure of merit ZT is increased up to 0.8 and 0.65 at 300 K for 1% intercalated and 1% substituted Bi2Te3, respectively.

Optimized thermal coupling of micro thermoelectric generators for improved output performance

There is a significant push to increase the output power of thermoelectric generators (TEGs) in order to make them more competitive energy harvesters. The thermal coupling of TEGs has a major impact on the effective temperature gradient across the generator and therefore the power output achieved. The application of micro fluidic heat transfer systems (μHTS) can significantly reduce the thermal contact resistance and thus enhance the TEG's performance. This paper reports on the characterization and optimization of a μTEG integrated with a two layer μHTS. The main advantage of the presented system is the combination of very low heat transfer resistances with small pumping powers in a compact volume. The influence of the most relevant system parameters, i.e. microchannel width, applied flow rate and the μTEG thickness on the system's net output performance are investigated. The dimensions of the μHTS/μTEG system can be optimized for specific temperature application ranges, and the maximum net power can be tracked by adjusting the heat transfer resistance during operation. A system net output power of 126 mW/cm2 was achieved with a module ZT of 0.1 at a fluid flow rate of 0.07 l/min and an applied temperature difference of 95K.It was concluded that for systems with good thermal coupling, the thermoelectric material optimization should focus more on the power factor than on the figure of merit ZT itself, since the influence of the thermal resistance of the TE material is negligible.

Thermoelectric Properties of Nanocrystalline Silicon from a Scaled‐Up Synthesis Plant

Adv. Eng. Mater, 2013, 15, 379 DOI: 10.1002/adem.201200233 The following corrections of the above manuscript should be noted: The figure of merit zT was calculated wrong in Figure 4 and 5e of the above manuscript. The correct calculation can be seen in Figure 1 below. Also the maximum zT value is rather 0.30 at 700 °C than 0.32 as mentioned in the abstract, introduction and Section 2.2. However, the wrong calculation doesn't affect the discussion nor the interpretation.

Driving Perpendicular Heat Flow: (p×n)-Type Transverse Thermoelectrics for Microscale and Cryogenic Peltier Cooling

Whereas thermoelectric performance is normally limited by the figure of merit ZT, transverse thermoelectrics can achieve arbitrarily large temperature differences in a single leg even with inferior ZT by being geometrically tapered. We introduce a band-engineered transverse thermoelectric with p-type Seebeck in one direction and n-type orthogonal, resulting in off-diagonal terms that drive heat flow transverse to electrical current. Such materials are advantageous for microscale devices and cryogenic temperatures--exactly the regimes where standard longitudinal thermoelectrics fail. InAs/GaSb type II superlattices are shown to have the appropriate band structure for use as a transverse thermoelectric.

Enhancement of the thermoelectric figure of merit in a quantum dot due to external ac field

Abstract We investigate the figure of merit of a quantum dot (QD) system irradiated with an external microwave filed by nonequilibrium Greenʼs function (NGF) technique. Results show that the frequency of microwave field influence the figure of merit ZT significantly. At low temperature, a sharp peak can be observed in the figure of merit ZT as the frequency of ac field increases. As the frequency varies, several zero points and resonant peaks emerge in the figure of merit ZT . By adjusting the frequency of the microwave field, we can obtain high ZT . The figure of merit ZT increases with the decreasing of linewidth function Γ . In addition, Wiedemann–Franz law does not hold, particularly in the low frequency region due to multi-photon emission and absorption. Some novel thermoelectric properties are also found in two-level QD system .

Half-Heusler (TiZrHf)NiSn Unileg Module with High Powder Density.

(TiZrHf)NiSn half-Heusler compounds were prepared by arc melting and their thermoelectric properties characterized in the temperature range between 325 K and 857 K, resulting in a Figure of Merit ZT ≈ 0.45. Furthermore, the prepared samples were used to construct a unileg module. This module was characterized in a homemade thermoelectric module measurement stand and yielded 275 mW/cm2 and a maximum volumetric power density of 700 mW/cm3. This was reached using normal silver paint as a contacting material; from an improved contacting, much higher power yields are to be expected.

A theoretical investigation on thermoelectric performance of ternary (Bi1−x Sb x )2Te3 compound

The electronic structure and thermoelectric properties of (Bi1-x Sb (x) )(2)Te-3 compound are investigated by using full-potential linearized augmented plane-wave method and semiclassical Boltzmann theory. The density of states and band structure are studied in detail. The transport coefficients are then calculated with the assumption of the constant relaxation time approximation. We investigated the thermoelectric properties at full range of Sb compositions from x = 0 to x = 1. By using the calculated thermoelectric properties and an averaged thermal conductivity, the figure of merit ZT is obtained. The theoretical investigations show a good agreement with the present experimental data.

Performance Limits of Microactuation with Vanadium Dioxide as a Solid Engine

Miniaturization of the steam engine to the microscale is hampered by severe technical challenges. Microscale mechanical motion is typically actuated with other mechanisms ranging from electrostatic interaction, thermal expansion, and piezoelectricity to more exotic types including shape memory, electrochemical reaction, and thermal responsivity of polymers. These mechanisms typically offer either large-amplitude or high-speed actuation, but not both. In this work we demonstrate the working principle of a microscale solid engine (μSE) based on the phase transition of VO2 at 68 °C with large transformation strain (up to 2%), analogous to the steam engine invoking large volume change in a liquid-vapor phase transition. Compared to polycrystal thin films, single-crystal VO2 nanobeam-based bimorphs deliver higher performance of actuation both with high amplitude (greater than bimorph length) and at high speed (greater than 4 kHz in air and greater than 60 Hz in water). The energy efficiency of the devices is calculated to be equivalent to thermoelectrics with figure of merit ZT = 2 at the working temperatures, and much higher than other bimorph actuators. The bimorph μSE can be easily scaled down to the nanoscale, and operates with high stability in near-room-temperature, ambient, or aqueous conditions. On the basis of the μSE, we demonstrate a macroscopic smart composite of VO2 bimorphs embedded in a polymer, producing high-amplitude actuation at the millimeter scale.

Enhancement of Thermopower due to Deficiency of Sb in FeSb<sub>2</sub>

FeSb2 was recently found to be a narrow-gap semiconductor with strong electronelectron correlation and a large thermopower at low temperatures. We report measurements of the electrical resistivity, Seebeck coefficient and thermal conductivity between 5 K to 300 K on polycrystalline samples of FeSb2 and FeSb1.9. We found that the deficiency of Sb in the parent compound leads to a giant anomalous peak in thermopower (S) at low temperatures, reaching ~ 426 μV/K at 20 K, resulting in a high thermoelectric power factor at low temperatures, achieving 10 μW/K2m at 27 K.. Consequently, a significantly enhanced thermoelectric figure of merit ZT ~ 0.0015 is achieved near room temperature. At low temperatures there is no improvement in ZT values due to the high thermal conductivity (phonon dominant region). Keywords: Seebeck coefficient, thermal conductivity, resistivity, thermoelectric figure of merit. PACS: 72.20.Pa, 71.27.+a, 71.28.+d

Synthesis and Thermoelectric Properties of Nd-Doped Uddlesden-Popper Phase SrO(SrTiO<sub>3</sub>)<sub>n </sub>(N=1,2) Oxides

The Nd-doped SrO(SrTiO3)n (n=1,2) bulk samples were prepared by combining a sol-gel method and spark plasma sintering (SPS). The microstructures of the precursor powders were characterized by X-ray powder diffraction (XRD) and scanning electron microscope (SEM), thermogravimetric (TG) and differential scanning calorimetry (DSC). The oxides of (Sr1-xNdx)n+1TinO3n+1(n=1,2;x=0.05, 0.1) were prepared by solid-state reaction of the precursor powders with post-spark plasma sintering for the first time and the thermoelectric properties showed that electrical resistivity ρ and the absolute values |S| of Seebeck coefficient increased with temperature and depended on the dopant concentration, indicating a n-type degenerate semiconductor behavior. Compared with the total thermal conductivity κ (4.1-5.2 Wm-1K-1) at room temperature, the estimated electronic thermal conductivity κe(0.2-0.7 Wm-1K-1) were very small, indicating that lattice contribution was predominant in the RP phase compounds. The largest dimensionless figure of merit ZT, 0.13 at 905K, was obtained the 10 at.% Nd-doped Sr3Ti2O7. This synthetic method provides a simple way to prepare thermoelectric oxides.

Effect of Fe Substitution on Thermoelectric Properties of Fe x In4−x Se3 Compounds

Starting from elemental powder mixtures of FexIn4−xSe3 (x = 0, 0.05, 0.1, 0.15), polycrystalline In4Se3-based compounds with homogeneous microstructures were prepared by mechanical alloying (MA) and hot pressing (HP). With the increase of x from 0 to 0.15, the electrical resistivity and the absolute value of the Seebeck coefficient increased, while the thermal conductivity first decreased and then increased. The maximal dimensionless figure of merit ZT of 0.44 was obtained for the FexIn4−xSe3 (x = 0.05) sample at 723 K.

Influence of Sn on the structural and thermoelectric properties of the type-I clathrates Ba8Cu5Si6Ge35-xSnx (0 ≤ x ≤ 0.6)

ABSTRACTOn the search for cost-competitive thermoelectric clathrates we have investigated the influence of Sn substitutions for Ge on the structural and thermoelectric properties of the type-I clathrate Ba8Cu5Si6Ge35. The solid solubility of Sn was found to be limited to 0.6 atoms per unit cell. A series of compounds with the nominal compositions Ba8Cu5Si6Ge35-xSnx (x = 0.2, 0.4, 0.6) was synthesized in a high-frequency furnace. The samples were annealed, and subsequently ball milled and hot pressed. The hot pressed samples were characterized by X-ray powder diffraction, energy-dispersive X-ray spectroscopy and transport property measurements. Our results show that the substitution of Ge by Sn introduces vacancies at the 6d site of the type-I clathrate structure and shifts the highest dimensionless thermoelectric figure of merit ZT from 570 °C for the Sn free sample to lower temperatures. The highest figure of merit ZT = 0.42 is reached at about 320 °C for the Sn-substituted sample Ba8Cu5Si6Ge35Sn0.6.

N-型碲化铅纳米片的电化学制备及其热电性能研究

PbTe是一类具有很高的热电转换效率的材料。本文以阴极化碲电极为碲源，3-巯基丙酸为保护剂，在水相中利用电化学方法制备了n-型PbTe纳米片。透射电子显微镜和X-射线衍射仪的结果表明合成产物是面心立方结构的PbTe纳米片，保护剂的存在会在加热过程中形成PbS，形成纳米尺寸的PbTe/PbS杂化材料；利用红外吸收光谱仪首次研究了合成PbTe的近红外吸收；用热电测量仪测定了PbTe纳米粒子的电阻率、Seebeck系数、热导率和品质因数ZT等热电性能参数，结果表明合成的PbTe纳米片的热导系数和品质因数得到较体相PbTe有明显的改善。该方法制备条件温和，操作简单，绿色环保。 PbTe is a kind of thermoelectric material of high thermoelectric conversion efficiency. In this papern-type PbTe nanoplates were prepared by using cathodic tellurium electrode as tellurium source and 3-mercaptopropionic acid as protective agent in the aqueous medium. The result of transmission electron microscopy and X-ray diffraction showed that as-prepared PbTe nanoplates were in face-centered cubic structure and PbS and nanoscale hybrid PbTe/PbS materials were formed during the heating process at presence of the protective agent. The near-infrared absorption of as-synthetic PbTe nanocrystals was firstly evaluated with infrared spectrophotometer. The thermoelectric performance including electrical resistivity, Seebeck coefficient, thermal conductivity and the figure of merit ZT was assessed with thermoelectric analyzer, which indicated that thermal conductivity and figure of merit of as-prepared PbTe nanocrystals were improved in contrast to the bulk materials. Furthermore, the synthesis strategy presented here could be considered as an effective methodology with mild conditions, simple operation and environmental safety.