Scattering Centers For Phonons Research Articles

Enhancement of thermoelectric performance in n-type Si90Ge10-based alloy by metallic Zn doping

Silicon–germanium (SiGe) alloy has become one of the representative high-temperature thermoelectric (TE) materials due to its advantages of stability, non-toxicity, oxidation resistance, and high mechanical strength. However, the high thermal conductivity and expensive Ge greatly limit the enhancement of zT value and its application. In this paper, n-type Si90Ge10P2Znx nanocomposites were prepared by ball milling and spark plasma sintering. By adjusting the Zn content and sintering time, multiple phonon-scattering centers, such as Zn precipitates, nano-pores, and layered structures, have been introduced into the SiGe matrix. The thermal conductivity was significantly reduced to 2.59 W m−1 K−1 without deteriorate power factor (PF), thus leading to a high zT value of 1.23 at 873 K. At 323–873 K, the average zT value (zTavg) also reached 0.6, increased by approximately 25% in comparison to the reported value using the same ratio of Si90Ge10. Compared with the conventional radioisotope TE generator with Si80Ge20 composition, the zTavg value increased by nearly 30% with only half of Ge, giving strong impetus to the application of SiGe-based TE materials.

Raising the thermoelectric performance in Pb/In-codoped BiCuSeO by alleviating the contradiction between carrier mobility and lattice thermal conductivity

Owing to the unavoidable electron scattering, introducing phonon-scattering defects to suppress the phonon diffusion in thermoelectric materials will always deteriorate the carrier transport. Therefore, exploring new methods to counterpoise the contradiction between carrier transport and phonon scattering is urgently needed. Herein, present work highlights that Pb/In codoping on BiCuSeO can suppress the phonon propagation while preserving the carrier transport, which alleviates the contradiction between carrier mobility (μ) and lattice thermal conductivity (κl), and improves the thermoelectric performance in Pb/In-codoped BiCuSeO. Multi-scale phonon-scattering centers, such as point defects, dislocations, In2O3/Cu2Se nano-precipitates and phase/grain boundaries, were introduced to promote full-wavelength phonon scattering, which brought about much reduced κl and substantially low total thermal conductivity, and a minimum κl of 0.30 W m−1 K−1 was achieved in Bi0·91Pb0·06In0·03CuSeO at 873 K. More importantly, Pb/In codoping can compensate the increased density of state effective masses caused by Pb single-doping, which resulted in improved μ and the obvious increased μ/κl. In addition, the improved μ also led to considerably increased electrical conductivity and power factor. Finally, the maximum ZT of 1.23 at 873 K and a high ZTavg of 0.72 between 300 and 873 K are achieved in Bi0·93Pb0·06In0·01CuSeO, which is superior to both the pristine and Pb-single-doped counterparts. Our findings elucidate the importance of balancing the carrier transport and phonon scattering during defect modulation of the thermoelectric performance.

Enhanced Thermoelectric Properties of p-Type Bi0.5Sb1.5Te3-Cu8GeSe6 Composite Materials.

Bismuth-telluride-based thermoelectric materials have been applied in active room-temperature cooling, but the mediocre ZT value of ∼1.0 limits the thermoelectric (TE) device's conversion efficiency and determines its application. In this work, we show the obviously improved thermoelectric properties of p-type Bi0.5Sb1.5Te3 by the Cu8GeSe6 composite. The addition of Cu8GeSe6 effectively boosts the carrier concentration and thus limits the bipolar thermal conductivity as the temperature is elevated. With the Cu8GeSe6 content of 0.08 wt %, the hole concentration reaches 5.0 × 1019 cm-3 and the corresponding carrier mobility is over 160 cm2 V-1 s-1, resulting in an optimized power factor of over 42 μW cm-1 K-2 at 300 K. Moreover, the Cu8GeSe6 composite introduces multiple phonon-scattering centers by increasing dislocations and element and strain field inhomogeneities, which reduce the thermal conductivity consisting of a lattice contribution and a bipolar contribution to 0.51 W m-1 K-1 at 350 K. As a consequence, the peak ZT of the Bi0.5Sb1.5Te3-0.08 wt % Cu8GeSe6 composite reaches 1.30 at 375 K and the average ZT between 300 and 500 K is improved to 1.13. A thermoelectric module comprised of this composite and commercial Bi2Te2.5Se0.5 exhibits a conversion efficiency of 5.3% with a temperature difference of 250 K, demonstrating the promising applications in low-grade energy recovery.

Achieving High Thermoelectric Performance of Eco-Friendly SnTe-Based Materials by Selective Alloying and Defect Modulation.

Recently, rock-salt lead-free chalcogenide SnTe-based thermoelectric (TE) materials have been considered an alternative to PbTe because of the nontoxic properties of Sn as compared to Pb. However, high carrier concentration that originated from intrinsic Sn vacancies and relatively high thermal conductivity of pristine SnTe lead to poor TE efficiency, which makes room for improving its TE properties. In this study, we present that the Na incorporation into the SnTe matrix is helpful for modifying the electronic band structure, optimization of carrier concentration, introducing dislocations, and kink planes; benefiting from these synergistic effects obviates the disadvantages of SnTe and makes a significant improvement in TE performance. We reveal that Na favorably impacts the structure of electronic bands by valence, conduction band engineering, leading to a nice enhancement in the Seebeck coefficient, which exhibits the highest power factor value of 37.93 μWcm-1 K-2 at 898 K, representing the best result for the SnTe material system. Moreover, a broader phonon spectrum is introduced by new phonon-scattering centers, scattered by dislocations and kink planes which suppressed lattice thermal conductivity to 0.57 Wm-1 K-1 at 898 K, which is much lower than that of pristine SnTe. Ultimately, a maximum ZT of 1.26 at 898 K is achieved in the Sn1.03Te + 3% Na sample, which is 97% higher than that of the pristine SnTe, suggesting that SnTe-based materials are a robust candidate for TE applications specifically, an ideal alternative of lead chalcogenides for TE power generation at high temperatures.

Enhancing the Thermoelectric Performance of Mg2Sn Single Crystals via Point Defect Engineering and Sb Doping.

Mg2Sn is a potential thermoelectric (TE) material that exhibits environmental compatibility. In this study, we fabricated Sb-doped Mg2Sn (Mg2Sn1-xSbx) single-crystal ingots and demonstrated the enhancement of TE performance via point defect engineering and Sb doping. The Mg2Sn1-xSbx single-crystal ingots exhibited considerably enhanced electrical conductivity because of the donor-doping effect in addition to high carrier mobility. Moreover, the Mg2Sn1-xSbx single-crystal ingots contained Mg vacancy (VMg) as a point defect. The introduced VMg and doped Sb atoms formed nanostructures, both acting as phonon-scattering centers. Consequently, lower lattice thermal conductivity was achieved for the Mg2Sn1-xSbx single-crystal ingots compared with polycrystalline counterparts. Owing to the significant enhancement in the electrical conductivity and the reduction in the lattice thermal conductivity, the maximum power factor of 5.1(4) × 10-3 W/(K2 m) and the maximum dimensionless figure of merit of 0.72(5) were achieved for the Mg2Sn0.99Sb0.01 single-crystal ingot, which are higher than those of single-phase Mg2Sn1-xSb polycrystals.

Effect of oxidation degree on the thermal properties of graphene oxide

The introduction of graphene oxide into both metals and polymers has yielded materials with enhanced thermal properties. Of particular interest is functionalized graphene oxide containing substantially heterogeneously distributed functional groups. The thermal properties of graphene oxide were investigated using non-equilibrium molecular dynamics to understand mechanism of thermal transport at the nanoscale. The mechanism of phonon transport in the functionalized graphene sheet was discussed, and the structure factors limiting phonon transport were determined. The results indicated that the oxidation level influences the thermal conductivity greatly. There is a need to reduce the oxidation level in order to enhance phonon transport and reduce phonon scattering in the functionalized graphene sheet. Phonon transport in the functionalized graphene sheet at a high oxidation level is governed by the phonon mean free path associated with phonon-defect scattering. Oxygen-containing functional groups adversely influence the thermal properties due to enhanced phonon-scattering arising from the increased number of phonon-scattering centers. The intrinsic thermal conductivity of about 2480W/(mK) at room temperature agrees with existing data on single layer pristine graphene. At room temperature, the intrinsic thermal conductivity is around 72W/(mK) at an oxidation level of 0.35 and around 670W/(mK) at an oxidation level of 0.05. The phonon mean free path is limited mainly by interior defects and decreases with increasing the oxidation level due to enhanced phonon-defect scattering.

Silicon carbide particles induced thermoelectric enhancement in SnSeS crystal

Solid-state thermoelectric composites consisting of promising SnSeS and SiC ceramic particles were fabricated, and their thermoelectric properties were examined. The SiC particles were randomly distributed into the SnSeS matrix, leading to the formation of the SiC/SnSeS composites. The introduction of SiC particles to the SnSeS improved the power factor which is mainly due to the effective enhancement of the Seebeck coefficient arising from the formation of phonon-scattering centers. Consequently, a maximum power factor of 215 µW m−1 K−2 was obtained for the composite with a SiC content of 1 wt.% which was greater than that of the pristine SnSeS. This result indicated that the introduction of SiC particles to the SnSeS is an efficient route to achieve high performance for widespread applications.

Structure and Thermoelectric Properties of Nanostructured Bi1−xSbx Alloys Synthesized by Mechanical Alloying

We report on the synthesis of Bi1−xSbx alloys and the investigation of the relationship between their structural and thermoelectric properties. In order to produce a compound that will work efficiently even above room temperature, Bi1−xSbx alloys were chosen, as they are known to be the best suited n-type thermoelectric materials in the low-temperature regime (200 K). Using a top–down method, we produced nanostructured Bi1−xSbx powders by ball-milling in the whole composition range of 0 < x < 1.0. Nanostructuring of Bi1−xSbx alloys increases the band gap and thus results in an enlargement of the semiconducting composition region (0 ≤ x ≤ 0.5) compared to its bulk counterpart (0.07 ≤ x ≤ 0.22). The enhancement of the band gap strongly affects the transport properties of the alloys, i.e. the electrical conductivity and the Seebeck coefficient. Moreover, nanostructuring reduces the thermal conductivity through the implementation of grain boundaries as phonon-scattering centers, leading to a significant enhancement of the thermoelectric properties. The highest figure-of-merit observed in this study is 0.25 which was found for Bi0.87Sb0.13 at 280 K.

A Practical Approach to Evaluate Lattice Thermal Conductivity in Two-Phase Thermoelectric Alloys for Energy Applications.

Modelling of the effects of materials’ microstructure on thermal transport is an essential tool for materials design, and is particularly relevant for thermoelectric (TE) materials converting heat into electrical energy. Precipitates dispersed in a TE matrix act as phonon-scattering centers, thereby reducing thermal conductivity. We introduce a practical approach to tailor a definite precipitate size distribution for a given TE matrix, and implement it for PbTe. We evaluate vibrational properties from first principles, and develop an expression for phonon relaxation time that considers both matrix vibrational properties and precipitate size distribution. This provides us with guidelines for optimizing thermal conductivity.

Thermal conductivity reduction by embedding nanoparticles

Reduction of thermal conductivity is important to enhance the performance of thermoelectric materials. One possible way to achieve this goal consists in embedding nanoparticles in the material, since they act as extrinsic phonon-scattering centers. In this paper, we study the effects of this embedding by means of a new formula for thermal conductivity obtained on the base of a hierarchy of hydrodynamical models which describe the transport of acoustic phonons in semiconductors. These models use as state variables suitable moments of the phonon occupation number, evolution equations of which are found starting from the Boltzmann---Peierls transport equation, and are closed by means of the maximum entropy principle. All the main interactions of phonons among themselves, with isotopes, nanoparticles, and boundaries are taken into account. Numerical results relative to the case of germanium nanoparticles embedded in a Si$$_{0.7}$$0.7Ge$$_{0.3}$$0.3 alloy crystal show that the thermal conductivity can be significantly reduced even at room temperature.

Effect of SiC ceramics on thermoelectric properties of SiC/SnSe composites for solid-state thermoelectric applications

Tin selenide (SnSe) based thermoelectric materials with varying amounts of embedded silicon carbide (SiC) particles were fabricated, and their thermoelectric properties were investigated. The SiC particles were evenly distributed in the SnSe matrix, thereby leading to the formation of the SiC/SnSe composite samples. The introduction of SiC into the SnSe matrix improved the power factors, owing mainly to an increase in the Seebeck coefficient, and a decrease in the thermal conductivity arising from the formation of phonon-scattering centers. Consequently, a ZT of 0.125 (at 300K) was obtained for the SiC/SnSe composite with a SiC content of 1wt%; this value was larger than that of the pristine SnSe. The results of this study indicate that the introduction of SiC particles into the SnSe matrix constitutes an efficient strategy for achieving thermoelectric enhancement for solid-state applications.

Effect of Nano-ZrW2O8 on the Thermoelectric Properties of Bi85Sb15/ZrW2O8 Composites

In this study, Bi85Sb15/x wt.% ZrW2O8 (x = 0, 0.1, 0.5, 1) thermoelectric nanocomposites were prepared successfully by ball milling and spark plasma sintering. The effect of ZrW2O8 nanoparticles on the thermoelectric properties of the Bi85Sb15/ZrW2O8 composite was investigated. Thermal conductivity, Seebeck coefficient, and electrical conductivity were measured between 77 K and 300 K. x-Ray diffraction and scanning electron microscopy were adopted for microstructure characterization of the composites. The electrical transport properties are mainly discussed with regard to the microstructures. The results show that nanoinclusions did not grow during sintering. It is found that the thermal conductivity decreases with the addition of a small amount of ZrW2O8 nanoparticles, which serve as additional phonon-scattering centers. The obtained thermal conductivity is 0.5 W/m K for the Bi85Sb15/1 wt.% ZrW2O8 composite at 80 K, which is just half of the value for the Bi85Sb15 matrix. However, the electrical transport properties are degraded with increasing content of ZrW2O8. The calculated ZT is also degraded due to the poor electrical properties.

The thermoelectric performance of ZrNiSn/ZrO 2 composites

The effects of ZrO 2 nano particles addition to the ZrNiSn matrix on the thermoelectric properties were investigated. Thermal conductivity, Seebeck coefficient and electrical resistivity were measured. The X-ray powder diffraction, EPMA and FESEM were adopted for the microstructure characterization of the composites. The transport properties are mainly discussed with regard to the microstructures. The lattice contribution to thermal conductivity can be reduced by addition of small amount of ZrO 2 nano particles, which serve as additional phonon-scattering centers. The dimensionless figure of merit ZT of ZrNiSn half-Heusler thermoelectric material was improved by introducing ZrO 2 nano particles.

Thermoelectric Properties of Hot-Pressed Ultra-Fine Particulate Sige Powder Alloys with Inert Additions

ABSTRACTThe objective of the work reported here is to reduce the thermal conductivity of thermoelectric materials in order to improve their figureof- merit and conversion efficiency. Theory predicts that the addition of ultra-fine, inert, phonon-scattering centers to thermoelectric materials will reduce their thermal conductivity [1]. To investigate this prediction, ultra-fine particulates (20Å to 120Å) of silicon nitride have been added to boron doped, p-type, 80/20 SiGe. All of the SiGe samples produced from ultra-fine powder have lower thermal conductivities, than that for standard SiGe, but high temperature heat treatment increases the thermal conductivity back to the value for standard SiGe. However, the SiGe samples with silicon nitride, inert, phonon-scattering centers, retained the lower thermal conductivity after several heat treatments. A reduction of approximately 25% in thermal conductivity has been achieved in these samples.