Improved thermoelectric performance of highly-oriented nanocrystalline bismuth antimony telluride thin films

Improved thermoelectric performance of highly-oriented nanocrystalline bismuth antimony telluride thin films is described. The thin films are deposited by a flash evaporation method, followed by annealing in hydrogen. By optimizing the annealing conditions, the resulting thin films exhibit almost perfect orientation with the c-axis normal to the substrate, and are composed of nano-sized grains with an average grain size of 150 nm. The in-plane electrical conductivity and Seebeck coefficient were measured at room temperature. The cross-plane thermal conductivity of the thin films was measured by a 3ω method, and the in-plane thermal conductivity was evaluated by using an anisotropic factor of thermal conductivity based on a single crystal bulk alloy with almost the same composition and carrier concentration. The measured cross-plane thermal conductivity is 0.56 W/(m K), and the in-plane thermal conductivity is evaluated to be 1.05 W/(m K). Finally, the in-plane power factor and figure-of-merit, ZT, of the thin films are 35.6 μW/(cm K²) and 1.0 at 300 K, respectively.     

Optimization of the thermoelectric figure of merit in the conducting polymer poly(3,4-ethylenedioxythiophene)

Thermoelectric generators (TEGs) transform a heat flow into electricity. Thermoelectric materials are being investigated for electricity production from waste heat (co-generation) and natural heat sources. For temperatures below 200 °C, the best commercially available inorganic semiconductors are bismuth telluride (Bi(2)Te(3))-based alloys, which possess a figure of merit ZT close to one. Most of the recently discovered thermoelectric materials with ZT>2 exhibit one common property, namely their low lattice thermal conductivities. Nevertheless, a high ZT value is not enough to create a viable technology platform for energy harvesting. To generate electricity from large volumes of warm fluids, heat exchangers must be functionalized with TEGs. This requires thermoelectric materials that are readily synthesized, air stable, environmentally friendly and solution processable to create patterns on large areas. Here we show that conducting polymers might be capable of meeting these demands. The accurate control of the oxidation level in poly(3,4-ethylenedioxythiophene) (PEDOT) combined with its low intrinsic thermal conductivity (λ=0.37 W m(-1) K(-1)) yields a ZT=0.25 at room temperature that approaches the values required for efficient devices.     

Intercalation of copper into bismuth telluride

The intercalation and leaching of copper into and out of bismuth telluride was investigated using aqueous and solid state electrochemical methods. It was found that copper intercalates up to a concentration of 3 mg/g bismuth telluride. Copper may also react with bismuth telluride to form copper telluride (Cu₂Te). Boiling hydrochloric acid leaches, or controlled potential electrochemical leaches, were effective at removing intercalated copper from bismuth telluride, returning the bismuth telluride to its original, undoped state. E.m.f. measurements using a solid, copper-ion conducting electrolyte resulted in copper activities of between 0.000 73 and 0.044 for intercalated bismuth telluride. An effective copper diffusion coefficient of 4×10^–5 cm²s^–1 was measured for a sintered polycrystalline bismuth telluride pellet at 483 K.This work was conducted at the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3Q2, UK     

Seebeck and Figure of Merit Enhancement in Nanostructured Antimony Telluride by Antisite Defect Suppression through Sulfur Doping

Antimony telluride has a low thermoelectric figure of merit (ZT < ～0.3) because of a low Seebeck coefficient α arising from high degenerate hole concentrations generated by antimony antisite defects. Here, we mitigate this key problem by suppressing antisite defect formation using subatomic percent sulfur doping. The resultant 10-25% higher α in bulk nanocrystalline antimony telluride leads to ZT ～ 0.95 at 423 K, which is superior to the best non-nanostructured antimony telluride alloys. Density functional theory calculations indicate that sulfur increases the antisite formation activation energy and presage further improvements leading to ZT ～ 2 through optimized doping. Our findings are promising for designing novel thermoelectric materials for refrigeration, waste heat recovery, and solar thermal applications.     

Computer simulation of the load characteristics of low-temperature thermoelectric coolers

The load characteristics of low-temperature thermoelectric coolers have been studied by means of computer simulation using the differential equation of stationary thermal conductivity with temperature-dependent kinetic coefficients. Based on the obtained results, the branches of characteristics of the thermoelements employing bismuth telluride and a bismuth-based superconductor are compared.     

Carbon fiber polymer–matrix structural composites exhibiting greatly enhanced through-thickness thermoelectric figure of merit

The through-thickness thermoelectric behavior of continuous carbon fiber epoxy-matrix composites is greatly improved by adding tellurium particles (13 vol.%), bismuth telluride particles (2 vol.%) and carbon black (2 vol.%). The thermoelectric power is increased from 8 to 163 μV/K, the electrical resistivity is decreased from 0.17 to 0.02.Ω.cm, the thermal conductivity is decreased from 1.31 to 0.51 W/m.K, and the dimensionless thermoelectric figure of merit ZT at 70 °C is increased from 9 × 10⁻⁶ to 9 × 10⁻². Tellurium increases the thermoelectric power greatly. Bismuth telluride decreases the electrical resistivity and thermal conductivity. Carbon black decreases the electrical resistivity.     

Enhanced thermoelectric properties of solution grown Bi2Te(3-x)Se(x) nanoplatelet composites

We report on the enhanced thermoelectric properties of selenium (Se) doped bismuth telluride (Bi(2)Te(3-x)Se(x)) nanoplatelet (NP) composites synthesized by the polyol method. Variation of the Se composition within NPs is demonstrated by X-ray diffraction and Raman spectroscopy. While the calculated lattice parameters closely follow the Vegard's law, a discontinuity in the shifting of the high frequency (E(g)(2) and A(1g)(2)) phonon modes illustrates a two mode behavior for Bi(2)Te(3-x)Se(x) NPs. The electrical resistivity (ρ) of spark plasma sintered pellet composites shows metallic conduction for pure Bi(2)Te(3) NP composites and semiconducting behavior for intermediate Se compositions. The thermal conductivity (κ) for all NP composites is much smaller than the bulk values and is dominated by microstructural grain boundary scattering. With temperature dependent electrical and thermal transport measurements, we show that both the thermoelectric power S (-259 μV/K) and the figure of merit ZT (0.54) are enhanced by nearly a factor of 4 for SPS pellets of Bi(2)Te(2.7)Se(0.3) in comparison to Bi(2)Te(3) NP composites. Tentatively, such an enhancement of the thermoelectric performance in nanoplatelet composites is attributed to the energy filtering of low energy electrons by abundant grain boundaries in aligned nanocomposites.     

Enhanced thermoelectric figure-of-merit in p-type nanostructured bismuth antimony tellurium alloys made from elemental chunks

By ball milling alloyed bulk crystalline ingots into nanopowders and hot pressing them, we had demonstrated high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this study, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 in the temperature range of 75 and 100 degrees C has been achieved. This process is more economical and environmentally friendly than starting from alloyed bulk crystalline ingots. The ZT improvement is caused mostly by the lower thermal conductivity, similar as the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the increased grain boundaries of the nanograins, precipitates, nanodots, and defects. Our material also exhibits a ZT of 0.7 at 250 degrees C, similar to the value obtained when ingot was used. This study demonstrates that high ZT values can be achieved in nanostructured bulk materials with ball milling elemental chunks, suggesting that the approach can be applied to other materials that are hard to be made into ingot, in addition to its advantage of lower manufacturing cost.     

多掺杂协同调控碲化锡热导率和功率因子提升热电性能

碲化锡(SnTe)是一种碲化铅的无铅替代物, 在热电领域有广阔的应用前景。但是, 纯相碲化锡样品具有较高的热导率与较低的塞贝克系数, 导致热电性能较差。本研究通过多重掺杂可以显著降低热导率, 提升塞贝克系数, 从而提升热电性能。SnTe热压样品的晶格热导率随着Se和S的引入明显降低,比如SnTe_0.7S_0.15Se_0.15室温下晶格热导率仅为0.99 W•m^-1•K^-1。透射电子显微镜显示, SnTe掺杂样品内存在大量的纳米沉淀相与晶格位错。在此基础上, 掺杂In在价带顶引入共振态大幅提高了样品的塞贝克系数。实验表明通过多重掺杂可以有效提升碲化锡的热电性能, 其中样品Sn_0.99In_0.01Te_0.7S_0.15Se_0.15在850 K时峰值ZT值达到0.8, 这说明碲化锡的确是一种有应用前景的中温区热电材料。     

Some spectral response characteristics of ZnTe thin films

Zinc telluride thin films have been grown at room temperature and higher temperature substrates by thermal evaporation technique in a vacuum of 10^-6 torr. A main peak in the photocurrent is observed at 781 nm (1.58 eV) with two lower amplitude peaks on the lower wavelength side and one on higher wavelength side. The evaluated thermal activation energy is found to correspond well with the main spectral peak. From these studies it can be inferred that temperatures up to 453 K is still in the extrinsic conductivity region of the studied ZnTe thin films. The paper was presented at the 6th Asian Thermophysical Properties Conference (6th ATPC), held at Gauhati University, during 8–11 October 2001.     

Study of thermal conduction in bismuth-thallium specimens between 1.5 K and50 K

Three samples of bismuth-thallium, in which the thallium consent was low and varied between 2 to 9 %, were found to be superconducting with transition temperatures dependent on thallium concentration. The thermal conductivities of these samples have been measured over the temperature range from 1.5 to 50 K both in normal and superconducting states. Addition of low concentration of thallium in bismuth results in a gradual distortion of the bismuth lattice. This is evident from the Laue back reflection x-ray photographs taken on these samples. The thermal conductivity data on these specimens at low temperatures show considerable departure from the usual temperature dependence of thermal conductivity that is generally observed for an elemental or alloy superconductor. This deviation is particularly marked in the normal strte conductivity variation. The results have been discussed and analysed in order to identify the scattering mechanisms operative in these samples over the studied temperature range.     

Se掺杂量对n型Bi2Te3-xSex微结构及热电性能的影响

通过水热法合成不同Se掺杂量的Bi₂Te_3-xSe_x (0 ≤x ≤0.45)纳米粉体, 采用放电等离子烧结技术, 制备出致密度较高的块体材料。通过X射线衍射、扫描电镜、透射电镜等测试手段对材料的微结构进行了表征, 并重点研究了含有不同Se掺杂量块体材料的显微结构和热电性能。结果表明: Se元素的掺杂使得粉体XRD特征衍射峰向高角度偏移, 并且衍射峰出现宽化, 晶粒尺寸变小。随着Se掺杂量的增加, 块体材料的电导率先增大后减小; Se元素的掺杂有效地降低了材料的热导率, 并提高了材料的Seebeck系数。研究结果表明: 在整个测试温度区间, 所有经过Se掺杂的样品ZT值都高于未掺杂样品。当Se掺杂量为0.3时, 样品具有最大的ZT值, 平均约为0.51, 并在475 K时达到最大值0.57, 相比未经Se掺杂的Bi₂Te₃提高了159%。     

Thermal conductance of an individual single-wall carbon nanotube above room temperature

The thermal properties of a suspended metallic single-wall carbon nanotube (SWNT) are extracted from its high-bias (I-V) electrical characteristics over the 300-800 K temperature range, achieved by Joule self-heating. The thermal conductance is approximately 2.4 nW/K, and the thermal conductivity is nearly 3500 Wm(-1)K(-1) at room temperature for a SWNT of length 2.6 mum and diameter 1.7 nm. A subtle decrease in thermal conductivity steeper than 1/T is observed at the upper end of the temperature range, which is attributed to second-order three-phonon scattering between two acoustic modes and one optical mode. We discuss sources of uncertainty and propose a simple analytical model for the SWNT thermal conductivity including length and temperature dependence.     

SrBi_4Ti_4O_(15)陶瓷材料介电和导热性能的研究

用溶胶-凝胶法制备了SrBi_4Ti_4O_(15)陶瓷材料,研究了烧结温度、铋含量及掺杂Nd对SrBi_4Ti_4O_(15)陶瓷结构、热扩散率及介电性能的影响.结果发现,SrBi_4Ti_4O_(15)陶瓷材料的热扩散率和介电常数随烧结温度的升高而增大,最佳烧结温度为1100℃,铋含量过量达10%时,SrBi_4Ti_4O_(15)陶瓷的热扩散率和介电常数最大.随着掺杂量Nd的增加,SrBi_4Ti_4O_(15)陶瓷的热扩散率和介电常数随之增大.     

Investigation of thermal conductivity and thermal diffusivity of liquid bismuth within the temperature range of 545–970 K

Thermal conductivity and thermal diffusivity of liquid bismuth within the temperature range from 545 K up to 970 K are investigated by the laser flash method. The measurement errors are equal to (3.5–4.5)%. Approximating equations are obtained, and the reference tables are presented for the temperature dependencies of the properties. The measurement results are compared to the published data available. The temperature dependence of the Lorentz number is calculated up to 970 K.     

Reduction in the thermal conductivity of single crystalline silicon by phononic crystal patterning

Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.     

Effect of silicon and sodium on thermoelectric properties of thallium-doped lead telluride-based materials

Thallium (Tl)-doped lead telluride (Tl(0.02)Pb(0.98)Te) thermoelectric materials fabricated by ball milling and hot pressing have decent thermoelectric properties but weak mechanical strength. Addition of silicon (Si) nanoparticles strengthened the mechanical property by reducing the grain size and defect density but resulted in low electrical conductivity that was not desired for any thermoelectric materials. Fortunately, doping of sodium (Na) into the Si added Tl(0.02)Pb(0.98)Te brings back the high electrical conductivity and yields higher figure-of-merit ZT values of ～1.7 at 770 K. The ZT improvement by Si addition and Na doping in Tl(0.02)Pb(0.98)Te sample is the direct result of concurrent electron and phonon engineering by improving the power factor and lowering the thermal conductivity, respectively.     

Thermal conductivity of carbon nanotube cross-bar structures

We use non-equilibrium molecular dynamics (NEMD) to compute the thermal conductivity (κ) of orthogonally ordered cross-bar structures of single-walled carbon nanotubes. Such structures exhibit extremely low thermal conductivity in the range of 0.02-0.07 W m(-1) K(-1). These values are five orders of magnitude smaller than the axial thermal conductivity of individual carbon nanotubes, and are comparable to the thermal conductivity of still air.     

Correlation on the Electrical and Thermal Conductivity for Binary Mg–Al and Mg–Zn Alloys

In this work, the electrical resistivity and thermal conductivity of both as-solution binary Mg–Al and Mg–Zn alloys were investigated from 298 K to 448 K, and the correlation between the corresponding electrical conductivity and thermal conductivity of the alloys was analyzed. The electrical resistivity of the Mg–Al and Mg–Zn alloys increased linearly with composition at 298 K, 348 K, 398 K, and 448 K, while the thermal conductivity of the alloys exponentially decreased with composition. Moreover, the electrical resistivity and thermal conductivity for both Mg–Al and Mg–Zn alloys varied linearly with temperature. On the basis of the Smith–Palmer equation, the thermal conductivity of both binary Mg alloys was found to be correlated quite well with the electrical conductivity in the temperature range from 298 K to 448 K. The corresponding Lorenz number is equal to $2.162\times 10^{-8} \,\hbox {V}^{2}\cdot \hbox {K}^{-2}$ 2.162 × 10 - 8 V 2 · K - 2 , and the lattice thermal conductivity is equal to $5.111 \,\hbox {W}\cdot \hbox {m}^{-1}\cdot \hbox {K}^{-1}$ 5.111 W · m - 1 · K - 1 . The possible mechanisms are also discussed.     

Anomalous size dependence of the thermal conductivity of graphene ribbons

We investigated the thermal conductivity K of graphene ribbons and graphite slabs as the function of their lateral dimensions. Our theoretical model considered the anharmonic three-phonon processes to the second-order and included the angle-dependent phonon scattering from the ribbon edges. It was found that the long mean free path of the long-wavelength acoustic phonons in graphene can lead to an unusual nonmonotonic dependence of the thermal conductivity on the length L of a ribbon. The effect is pronounced for the ribbons with the smooth edges (specularity parameter p > 0.5). Our results also suggest that, contrary to what was previously thought, the bulk-like three-dimensional phonons in graphite make a rather substantial contribution to its in-plane thermal conductivity. The Umklapp-limited thermal conductivity of graphite slabs scales, for L below ～30 μm, as log(L), while for larger L, the thermal conductivity approaches a finite value following the dependence K(0) - A × L(-1/2), where K(0) and A are parameters independent of the length. Our theoretical results clarify the scaling of the phonon thermal conductivity with the lateral sizes in graphene and graphite. The revealed anomalous dependence K(L) for the micrometer-size graphene ribbons can account for some of the discrepancy in reported experimental data for graphene.