Showing papers by "Zhifeng Ren published in 2012"
TL;DR: In this article, Minnich et al. reviewed the progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermiolectric figure of merit, with new discussions on device physics and applications.
Abstract: This review is an update of a previous review (A. J. Minnich, et al., Energy Environ. Sci., 2009, 2, 466) published two years ago by some of the co-authors, focusing on progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermoelectric figure of merit, with new discussions on device physics and applications, and assessing challenges on these topics. Understanding of phonon transport in bulk materials has advanced significantly as the first-principles calculations are applied to thermoelectric materials, and experimental tools are being developed. Some new strategies have been developed to improve electron transport in thermoelectric materials. Fundamental questions on phonon and electron transport across interfaces and in thermoelectric materials remain. With thermoelectric materials reaching high ZT values well above one, the field is ready to take a step forward and go beyond the materials' figure of merit. Developing device contacts and module fabrication techniques, developing a platform for efficiency measurements, and identifying applications are becoming increasingly important for the future of thermoelectrics.
1,049 citations
01 Jan 2012
TL;DR: In this article, Minnich et al. reviewed the progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermiolectric figure of merit, with new discussions on device physics and applications.
Abstract: This review is an update of a previous review (A. J. Minnich, et al., Energy Environ. Sci., 2009, 2, 466) published two years ago by some of the co-authors, focusing on progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermoelectric figure of merit, with new discussions on device physics and applications, and assessing challenges on these topics. Understanding of phonon transport in bulk materials has advanced significantly as the first-principles calculations are applied to thermoelectric materials, and experimental tools are being developed. Some new strategies have been developed to improve electron transport in thermoelectric materials. Fundamental questions on phonon and electron transport across interfaces and in thermoelectric materials remain. With thermoelectric materials reaching high ZT values well above one, the field is ready to take a step forward and go beyond the materials' figure of merit. Developing device contacts and module fabrication techniques, developing a platform for efficiency measurements, and identifying applications are becoming increasingly important for the future of thermoelectrics.
826 citations
TL;DR: In this paper, the dimensionless figure of merit (ZT) plays a key role in the conversion efficiency from thermal to electrical energy, which has been used in power supply of aeronautic and astronautic exploring missions, now showing notable advantages to harvest the widely distributed waste heat and convert the abundant solar energy into electricity at lower cost than Si-based photovoltaic technology.
631 citations
TL;DR: The experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods is reported, which is consistent with a coherent phononHeat conduction process.
Abstract: The control of heat conduction through the manipulation of phonons as coherent waves in solids is of fundamental interest and could also be exploited in applications, but coherent heat conduction has not been experimentally confirmed. We report the experimental observation of coherent heat conduction through the use of finite-thickness superlattices with varying numbers of periods. The measured thermal conductivity increased linearly with increasing total superlattice thickness over a temperature range from 30 to 150 kelvin, which is consistent with a coherent phonon heat conduction process. First-principles and Green’s function–based simulations further support this coherent transport model. Accessing the coherent heat conduction regime opens a new venue for phonon engineering for an array of applications.
508 citations
TL;DR: An alternative materials design is reported, using alloy Si(70) Ge(30) instead of Si as the nanoparticles and Si(95)Ge(5) as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 ± 0.1 at 900 °C.
Abstract: Modulation-doping was theoretically proposed and experimentally proved to be effective in increasing the power factor of nanocomposites (Si80Ge20)70(Si100B5)30 by increasing the carrier mobility but not the figure-of-merit (ZT) due to the increased thermal conductivity. Here we report an alternative materials design, using alloy Si70Ge30 instead of Si as the nanoparticles and Si95Ge5 as the matrix, to increase the power factor but not the thermal conductivity, leading to a ZT of 1.3 ± 0.1 at 900 °C.
465 citations
TL;DR: It is shown that a planar structure, consisting of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference film, can highly absorb electromagnetic radiation in the entire visible range, and thus can become a platform for high-efficiency solar cells.
Abstract: We show that a planar structure, consisting of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference film, can highly absorb (superabsorb) electromagnetic radiation in the entire visible range, and thus can become a platform for high-efficiency solar cells. The perforated metallic film and the ultrathin absorber in this broadband superabsorber form a metamaterial effective film, which negatively refracts light in this broad frequency range. Our quantitative simulations confirm that the superabsorption bandwidth is maximized at the checkerboard pattern of the perforations. These simulations show also that the energy conversion efficiency of a single-junction amorphous silicon solar cell based on our optimized structure can exceed 12%.
361 citations
TL;DR: By adding aluminium (Al) into lead selenide (PbSe), the authors successfully prepared n-type PbSe thermoelectric materials with a figure-of-merit (ZT) of 1.3 at 850 K.
Abstract: By adding aluminium (Al) into lead selenide (PbSe), we successfully prepared n-type PbSe thermoelectric materials with a figure-of-merit (ZT) of 1.3 at 850 K. Such a high ZT is achieved by a combination of high Seebeck coefficient caused by very possibly the resonant states in the conduction band created by Al dopant and low thermal conductivity from nanosized phonon scattering centers.
345 citations
TL;DR: It was found that Se increases the doping concentration of K in PbTe as a result of the balance of electronegativity and also lowers the lattice thermal conductivity because of the increased number of point defects.
Abstract: We present detailed studies of potassium doping in PbTe1–ySey (y = 0, 0.15, 0.25, 0.75, 0.85, 0.95, and 1). It was found that Se increases the doping concentration of K in PbTe as a result of the balance of electronegativity and also lowers the lattice thermal conductivity because of the increased number of point defects. Tuning the composition and carrier concentration to increase the density of states around the Fermi level results in higher Seebeck coefficients for the two valence bands of PbTe1–ySey. Peak thermoelectric figure of merit (ZT) values of ∼1.6 and ∼1.7 were obtained for Te-rich K0.02Pb0.98Te0.75Se0.25 at 773 K and Se-rich K0.02Pb0.98Te0.15Se0.85 at 873 K, respectively. However, the average ZT was higher in Te-rich compositions than in Se-rich compositions, with the best found in K0.02Pb0.98Te0.75Se0.25. Such a result is due to the improved electron transport afforded by heavy K doping with the assistance of Se.
343 citations
TL;DR: In this article, a thermoelectric figure-of-merit (ZT) of ∼1.6 at 700°C is achieved in β-phase copper selenide (Cu2Se) made by ball milling and hot pressing.
272 citations
TL;DR: In this article, a lower thermal conductivity in p-type half-Heuslers was achieved when Ti is used to replace Zr, i.e., Hf1−xTixCoSb0.8Sn0.2, due to larger differences in the atomic mass and size between Hf and Ti.
Abstract: High lattice thermal conductivity has been the bottleneck for further improvement of the thermoelectric figure-of-merit (ZT) of half-Heuslers (HHs) Hf1−xZrxCoSb0.8Sn0.2. Theoretically, the lattice thermal conductivity can be reduced by exploring larger differences in the atomic mass and size in the crystal structure, leading to higher ZT. In this paper, we experimentally demonstrated that a lower thermal conductivity in p-type half-Heuslers can be achieved when Ti is used to replace Zr, i.e., Hf1−xTixCoSb0.8Sn0.2, due to larger differences in the atomic mass and size between Hf and Ti compared with Hf and Zr. The highest ZT peak, ∼1.0 at 800 °C, in the Hf1−xTixCoSb0.8Sn0.2 (x = 0.1, 0.2, 0.3, and 0.5) system was achieved using Hf0.8Ti0.2CoSb0.8Sn0.2, which makes this material useful in power generation applications.
231 citations
Journal Article•
TL;DR: In this article, a lower thermal conductivity in p-type half-Heuslers was achieved when Ti is used to replace Zr, i.e., Hf1−xTixCoSb0.8Sn0.2, due to larger differences in the atomic mass and size between Hf and Ti.
Abstract: High lattice thermal conductivity has been the bottleneck for further improvement of the thermoelectric figure-of-merit (ZT) of half-Heuslers (HHs) Hf1−xZrxCoSb0.8Sn0.2. Theoretically, the lattice thermal conductivity can be reduced by exploring larger differences in the atomic mass and size in the crystal structure, leading to higher ZT. In this paper, we experimentally demonstrated that a lower thermal conductivity in p-type half-Heuslers can be achieved when Ti is used to replace Zr, i.e., Hf1−xTixCoSb0.8Sn0.2, due to larger differences in the atomic mass and size between Hf and Ti compared with Hf and Zr. The highest ZT peak, ∼1.0 at 800 °C, in the Hf1−xTixCoSb0.8Sn0.2 (x = 0.1, 0.2, 0.3, and 0.5) system was achieved using Hf0.8Ti0.2CoSb0.8Sn0.2, which makes this material useful in power generation applications.
TL;DR: With Tl doping, modification of the band structure around the Fermi level helped to increase the Seebeck coefficient, and the lattice thermal conductivity decreased, probably as a result of effective phonon scattering by both the heavy Tl(3+) ions and the increased grain boundary density after ball milling.
Abstract: Group IIIA elements (B, Ga, In, and Tl) have been doped into PbSe for enhancement of thermoelectric properties. The electrical conductivity, Seebeck coefficient, and thermal conductivity were systematically studied. Room-temperature Hall measurements showed an effective increase in the electron concentration upon both Ga and In doping and the hole concentration upon Tl doping to ∼7 × 1019 cm–3. No resonant doping phenomenon was observed when PbSe was doped with B, Ga, or In. The highest room-temperature power factor ∼2.5 × 10–3 W m–1 K–2 was obtained for PbSe doped with 2 atom % B. However, the power factor in B-doped samples decreased with increasing temperature, opposite to the trend for the other dopants. A figure of merit (ZT) of ∼1.2 at ∼873 K was achieved in PbSe doped with 0.5 atom % Ga or In. With Tl doping, modification of the band structure around the Fermi level helped to increase the Seebeck coefficient, and the lattice thermal conductivity decreased, probably as a result of effective phonon s...
TL;DR: 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.
Abstract: Thallium (Tl)-doped lead telluride (Tl0.02Pb0.98Te) 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 Tl0.02Pb0.98Te 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 Tl0.02Pb0.98Te sample is the direct result of concurrent electron and phonon engineering by improving the power factor and lowering the thermal conductivity, respectively.
TL;DR: In this paper, the authors proposed a new energy frontier research center for the U.S. Dept. of Energy (S3TEC Energy Frontier Research Center, award number DE-SC0001299).
TL;DR: The design, fabrication, and performance of a nanoporous, coaxial array capacitive detector for highly sensitive chemical detection is reported, shown to achieve parts per billion level detection sensitivity to a broad class of organic molecules.
Abstract: We report on the design, fabrication, and performance of a nanoporous, coaxial array capacitive detector for highly sensitive chemical detection Composed of an array of vertically aligned nanoscale coaxial electrodes constructed with porous dielectric coax annuli around carbon nanotube cores, this sensor is shown to achieve parts per billion level detection sensitivity, at room temperature, to a broad class of organic molecules The nanoscale, 3D architecture and microscale array pitch of the sensor enable rapid access of target molecules and chip-based multiplexing capabilities, respectively
Book•
05 Sep 2012
TL;DR: In this article, the authors describe the properties and applications of aligned carbon nanotube arrays and potential applications of Carbon Nanotube Arrays, as well as the technologies to achieve their alignment.
Abstract: Introduction to Carbon.- Introduction to Carbon Nanotubes.- Growth Techniques of Carbon Nanotubes.- Chemical Vapor Deposition of Carbon Nanotubes.- Physics of Direct Current Plasma-Enhanced Chemical Vapor Deposition.- Technologies to Achieve Carbon Nanotube Alignment.- Measurement Techniques of Aligned Carbon Nanotubes.- Properties and Applications of Aligned Carbon Nanotube Arrays.- Potential Applications of Carbon Nanotube Arrays.
TL;DR: In this paper, an all-dry fabrication method of the hybrid structure of pH-responsive hydrogel and carbon nanotube arrays using initiated chemical vapor deposition (iCVD) was reported.
Abstract: We report an all-dry fabrication method of the hybrid structure of pH-responsive hydrogel and carbon nanotube arrays using initiated chemical vapor deposition (iCVD). Vertically aligned carbon nanotube (VACNT) arrays with low site density were coated by a vapor-deposited poly(methacrylic acid-co-ethylene glycol diacrylate) hydrogel with complete retention of the aligned structure and precise control of the coating thickness on each nanotube. The hydrogel coating imparted pH-responsiveness to the nanotube arrays and significantly enhanced the surface wettability. With an ultrathin 50 nm hydrogel coating, the hybrid hydrogel–VACNT array structure exhibited superwettability in neutral buffer solutions, which was attributed to the synergistic effect of the structure porosity and the ionization of the pH-responsive hydrogel. Under the acidic condition, the wettability depended on the coating thickness, and the apparent contact angle of the hybrid structure was in agreement with the calculated results from the Cassie–Baxter model.
TL;DR: In this paper, the authors investigate phonon mean free path distribution inside fully unfilled (Co4Sb12) and fully filled (LaFe4S b12) bulk skutterudites by fitting their thermal conductivity to analytical models which employ different phonon dispersions.
Abstract: Experimental thermal conductivity of bulk materials are often modeled using Debye approximation together with functional forms of relaxation time with fitting parameters. While such models can fit the temperature dependence of thermal conductivity of bulk materials, the Debye approximation leads to large error in the actual phonon mean free path, and consequently, the predictions of the thermal conductivity of the nanostructured materials using the same relaxation time are not correct even after considering additional size effect on the mean free path. We investigate phonon mean free path distribution inside fully unfilled (Co4Sb12) and fully filled (LaFe4Sb12) bulk skutterudites by fitting their thermal conductivity to analytical models which employ different phonon dispersions. We show that theoretical thermal conductivity predictions of the nanostructured samples are in agreement with the experimental data obtained for samples of different grain sizes only when the full phonon dispersion is considered.
TL;DR: Parker et al. as discussed by the authors showed that n-type PbSe has large thermopowers characteristic of a high performance material, with values as high in magnitude as 250μV/K at 1000
Abstract: It was recently predicted [D. Parker D. J. Singh, Phys. Rev. B 82, 035204 (2010)], contrary to then-prevalent opinion, that p-type PbSe would be a good thermoelectric material. This prediction was confirmed in Wang et al. [Adv. Mater. 23, 1367 (2011)], and recent experimental work (Zhang et al. Energy Env. Sci. 5, 5246 (2012)] now indicates that n-type PbSe may show good thermoelectric performance. In light of this work, we now re-examine the thermoelectric performance of n-type PbSe with a revised approximation (not available at the time of the original work) which improves band gap accuracy. We now find that n-type PbSe has large thermopowers characteristic of a high performance material, with values as high in magnitude as 250 μV/K at 1000 K and 300 μV/K at 800 K. We further find that optimal 1000 K n-type doping ranges are between 2 × 1019 cm−3 and 7 × 1019 cm−3, while at 800 K, the corresponding range is from 7 × 1018 to 4 × 1019 cm−3.
TL;DR: In this article, Ni, Co, Fe, and Mn are intentionally doped into Cu0.7Se0.3 in order to understand their effects on the thermoelectric material.
Abstract: Bi2Te3 based thermoelectric devices typically use a nickel layer as a diffusion barrier to block the diffusion of solder or copper atoms from the electrode into the thermoelectric material. Previous studies have shown degradation in the efficiency of these thermoelectric devices may be due to the diffusion of the barrier layer into the thermoelectric material. In this work, Ni, Co, Fe, and Mn are intentionally doped into Cu0.01Bi2Te2.7Se0.3 in order to understand their effects on the thermoelectric material. Thermoelectric transport properties including the Seebeck coefficient, thermal conductivity, electrical resistivity, carrier concentration, and carrier mobility of Cu0.01Bi2Te2.7Se0.3 doped with 2 atomic percent M (M = Ni, Co, Fe, Mn) as Cu0.01Bi2Te2.7Se0.3M0.02 are studied in a temperature range of 5-525 K. It is seen that the introduction of Ni, Co, Fe, or Mn does not affect the overall figure of merit, and therefore demonstrates that the diffusion barrier is not leading to device degradation as pre...
TL;DR: The results of this approach clearly demonstrated the metal/semiconductor interface concept and confirmed the potential of strongly correlated material systems as promising thermoelectric materials.
Abstract: We present the figure-of-merit (ZT) improvement in nanostructured FeSb2−xAgx with Ag1−ySby nanoinclusions through a metal/semiconductor interface engineering approach Owing to the interfaces between FeSb2−xAgx and Ag1−ySby phases, as well as the identical work functions, both thermal conductivity and electrical resistivity of the nanocomposites were significantly reduced in the lower temperature regime compared with pure FeSb2 Overall, an improvement of 70% in ZT was achieved for the optimized nanocomposite FeSb1975Ag0025/Ag077Sb023 sample, in which Ag077Sb023 is about 10% by molar ratio The results of this approach clearly demonstrated the metal/semiconductor interface concept and confirmed the potential of strongly correlated material systems as promising thermoelectric materials
Posted Content•
TL;DR: In this article, the temperature dependence of thermoelectric transport properties of four FeSb2 nanocomposite samples with different grain sizes was studied and the results indicated the presence of substantial phonon drag effects in this system contributing to a large Seebeck coefficient at low temperature.
Abstract: We study the temperature dependence of thermoelectric transport properties of four FeSb2 nanocomposite samples with different grain sizes. The comparison of the single crystals and nanocomposites of varying grain size indicates the presence of substantial phonon drag effects in this system contributing to a large Seebeck coefficient at low temperature. As the grain size decreases, the increased phonon scattering at the grain boundaries leads to a suppression of the phonon-drag effect, resulting in a much smaller peak value of the Seebeck coefficient in the nanostructured bulk materials. As a consequence, the ZT values are not improved significantly even though the thermal conductivity is drastically reduced.
01 Jan 2012
TL;DR: In this article, the authors introduce various chemical vapor deposition methods, including thermal chemical vaporization and plasma-enhanced chemical vaporisation, as well as the mechanism of growth and alignment of carbon nanotubes.
Abstract: Chemical vapor deposition is a popular industrial method to grow carbon nanotubes because of the mass production at low cost. Such method is also a very important technique to in situ align carbon nanotubes. In this chapter, we introduce various chemical vapor deposition methods, including thermal chemical vapor deposition and plasma-enhanced chemical vapor deposition, as well as the mechanism of growth and alignment.
TL;DR: The simple concept of this technique is to self-assemble a layer (usually a monolayer) of polystyrene spheres (PS) on a substrate and employ NSL with a very shallow static evaporation, which is used to make Au and Fe nanoribbon gratings deposited on transparent substrates.
Abstract: The simple concept of this technique is to self-assemble a layer (usually a monolayer) of polystyrene spheres (PS) on a fl at substrate. The layer of PS is subsequently used as a shadow mask for evaporation of thin fi lms, e.g., metallic thin fi lms. In the simplest version, NSL employs the self-assembly of a densely packed hexagonal lattice of PS and perpendicular substrate evaporation, which leads after PS removal to a honeycomb array of nano-quasi-triangles. [ 1 ] More complicated patterns and nanoparticle shapes can be obtained if the evaporation angle is statically or dynamically modifi ed. [ 7 ] Here, we demonstrate fabrication of nanoribbon gratings by employing NSL with a very shallow static evaporation. The PS diameter is reduced by ionic etching after the assembly and controls the grating geometry. We use this shallow angle NSL (SANSL) technique to make Au and Fe nanoribbon gratings deposited on transparent substrates. Interaction of gratings with the electromagnetic radiation has been studied extensively since 1902. [ 8 ] Classically, subwavelength metallic gratings are polarizers of the transmitted waves; a wave with electric fi eld polarized perpendicular to the grating lines passes through the grating with little back refl ection. In turn, a wave polarized parallel to the lines is strongly back refl ected. This follows from simple boundary conditions at the perfect metal surface: [ 9 ] the parallel to the metal surface component of the electric fi eld must vanish, but the perpendicular one does not. Thus, the wave polarized parallel to the grating lines cannot pass since it would have to have wavelength λ = 2 d / n (where d is the interline distance and n = 1,2,3..) in order to assure the vanishing fi eld nodes at the metal surfaces of the neighboring grating lines. This cannot happen, since in the subwavelength limit λ > > d . In the optical range, most metals (e.g., Au, Ag) cannot be considered as perfect and thus the above analysis is no longer valid. In this range, some metals can support various plasmonic oscillations, including the surface plasmon polariton (SPP). [ 11 ] The extraordinary
01 Jan 2012
TL;DR: Carbon is the fourth most abundant chemical element in the universe by mass and is also the second abundant element by mass in human body as mentioned in this paper. But it is not the most abundant element in air.
Abstract: Carbon is the fourth most abundant chemical element in the universe by mass. It is also the second abundant element by mass in human body.
TL;DR: In this paper, the effect of Pb substitution for Te in La3Te4?xPbx on the thermoelectric properties in the temperature range 300?1100?K was studied on samples synthesized by the melting, ball milling and hot-pressing method.
Abstract: The effect of Pb in La3Te4?xPbx on the thermoelectric properties in the temperature range 300?1100?K was studied on samples synthesized by the melting, ball milling and hot-pressing method. The samples crystallize in the Th3P4structure. A small amount of Pb substitution for Te in La3Te4 reduced the electrical conductivity, but the impurity phase introduced by more Pb significantly enhanced the electrical conductivity. The electrical conductivity of the La3Te3.6Pb0.4 sample reached 2.06???105?S?m?1 at room temperature. The expansion of crystal lattice confirmed from x-ray diffraction patterns and the slight reduction in electrical conductivity all verify the fact that Pb substitutes a Te site when x is small. The Seebeck coefficient increases linearly with temperature for all samples. While the thermal conductivity varies slightly with temperature, the lattice thermal conductivity approaches the minimum value at high temperatures. For La3Te3.7Pb0.3, a power factor of 1.4???10?3?W?m?1?K?2 at 1100?K was achieved with a thermal conductivity of 1.86?W?m?1?K?1, giving a maximum figure-of-merit ZT???0.8 at 1100?K.
TL;DR: In this paper, the Seebeck coefficients, electrical resistivities, total thermal conductivities, and magnetization were reported for n-type Bi0.88Sb0.12 nano-composite alloys made by Ho-doping at the 0, 1, and 3 % atomic levels.
Abstract: The Seebeck coefficients, electrical resistivities, total thermal conductivities, and magnetization are reported for temperatures between 5 and 350 K for n-type Bi0.88Sb0.12 nano-composite alloys made by Ho-doping at the 0, 1, and 3 % atomic levels. The alloys were prepared using a dc hot-pressing method, and are shown to be single phase for both Ho contents with grain sizes on the average of 900 nm. We find the parent compound has a maximum of ZT = 0.28 at 231 K, while doping 1 % Ho increases the maximum ZT to 0.31 at 221 K and the 3 % doped sample suppresses the maximum ZT = 0.24 at a temperature of 260 K.
Journal Article•
TL;DR: By adding aluminium (Al) into lead selenide (PbSe), the authors successfully prepared n-type PbSe thermoelectric materials with a figure-of-merit (ZT) of 1.3 at 850 K.
Abstract: By adding aluminium (Al) into lead selenide (PbSe), we successfully prepared n-type PbSe thermoelectric materials with a figure-of-merit (ZT) of 1.3 at 850 K. Such a high ZT is achieved by a combination of high Seebeck coefficient caused by very possibly the resonant states in the conduction band created by Al dopant and low thermal conductivity from nanosized phonon scattering centers.
TL;DR: In this article, a structural modulation of ∼10 nm and the presence of non-stoichiometric ordered nanoparticles were found to be responsible for the low lattice thermal conductivity of the Bi2Te2.7Se0.3 bulk thermoelectric materials studied.
Abstract: N-type Bi2Te2.7Se0.3 bulk thermoelectric materials with peak ZT values up to ∼1 were examined by transmission electron microscopy and electron diffraction. Two nanostructural features were found: (i) a structural modulation of ∼10 nm, which consisted of nanorods with crystalline and nearly amorphous regions, having the rod axes normal to (0,1,5)-type planes, and wave vector normal to (1,0,10)-type planes and (ii) non-stoichiometric ordered Bi-rich nanoparticles. The presence of the structural modulation was not influenced by the ion milling energy or temperature in this study while the non-stoichiometric ordered nanoparticles were only observed when ion milling at low temperatures and low energy was used. It is proposed that both the structural modulation of ∼10 nm and the presence of non-stoichiometric nanoparticles are responsible for the low lattice thermal conductivity (∼0.6 W/mK) of the Bi2Te2.7Se0.3 bulk thermoelectric materials studied.
TL;DR: Inelastic neutron scattering measurements are utilized to explore relative changes in the generalized phonon density of states of nanocrystalline Si1 xGex thermoelectric materials prepared via ball-milling and hot-pressing techniques.
Abstract: Inelastic neutron scattering measurements are utilized to explore relative changes in the generalized phonon density of states of nanocrystalline Si1 xGex thermoelectric materials prepared via ball-milling and hot-pressing techniques. Dynamic signatures of Ge clustering can be inferred from the data by referencing the resulting spectra to a density functional theoretical model assuming homogeneous alloying via the virtual-crystal approximation. Comparisons are also presented between as-milled Si nanopowder and bulk, polycrystalline Si where a preferential low-energy enhancement and lifetime broadening of the phonon density of states appear in the nanopowder. Negligible differences are however observed between the phonon spectra of bulk Si andhot-pressed, nanostructured Si samples suggesting that changes to the single-phonon dynamics above 4 meV play only a secondary role in the modified heat conduction of this compound.