Low-temperature thermoelectric power factor enhancement by controlling nanoparticle size distribution

Coherent potential approximation is used to study the effect of adding doped spherical nanoparticles inside a host matrix on the thermoelectric properties. This takes into account electron multiple scatterings that are important in samples with relatively high volume fraction of nanoparticles (>1%). We show that with large fraction of uniform small size nanoparticles (～1 nm), the power factor can be enhanced significantly. The improvement could be large (up to 450% for GaAs) especially at low temperatures when the mobility is limited by impurity or nanoparticle scattering. The advantage of doping via embedded nanoparticles compared to the conventional shallow impurities is quantified. At the optimum thermoelectric power factor, the electrical conductivity of the nanoparticle-doped material is larger than that of impurity-doped one at the studied temperature range (50-500 K) whereas the Seebeck coefficient of the nanoparticle doped material is enhanced only at low temperatures (～50 K).     

Giant thermoelectric power of (La,Ce)Al2

We have measured the thermoelectric power of LaAl₂ and of (La, Ce)Al₂ alloys between 0.3 and 9 K. A large, negative peak caused by the Kondo effect of the Ce impurities has been observed at about 1 K. The peak value of about ?16 µV/K obtained from the Nordheim-Gorter rule suggestsV=+0.05 eV andJ=?0.11 eV for the interaction energies of the potential and exchange scattering of the conduction electrons by the Ce impurities.     

In‐Plane Anisotropic Thermal Conductivity of Few‐Layered Transition Metal Dichalcogenide Td‐WTe2

2D Td‐WTe₂ has attracted increasing attention due to its promising applications in spintronic, field‐effect chiral, and high‐efficiency thermoelectric devices. It is known that thermal conductivity plays a crucial role in condensed matter devices, especially in 2D systems where phonons, electrons, and magnons are highly confined and coupled. This work reports the first experimental evidence of in‐plane anisotropic thermal conductivities in suspended Td‐WTe₂ samples of different thicknesses, and is also the first demonstration of such anisotropy in 2D transition metal dichalcogenides. The results reveal an obvious anisotropy in the thermal conductivities between the zigzag and armchair axes. The theoretical calculation implies that the in‐plane anisotropy is attributed to the different mean free paths along the two orientations. As thickness decreases, the phonon‐boundary scattering increases faster along the armchair direction, resulting in stronger anisotropy. The findings here are crucial for developing efficient thermal management schemes when engineering thermal‐related applications of a 2D system.     

The effect of physical vapor deposition parameters on the thermoelectric power of thin film molybdenum-nickel junctions

The effect of various physical vapor deposition parameters on the thermoelectric power of thin film (10 000 Å) Mo-Ni junctions deposited by electron beam evaporation was investigated in this study. The deposition parameters of interest were the substrate temperature, the deposition rate and the vapor source purity. The thermoelectric power of the thin film Mo-Ni junctions was dependent on the structure of the Mo thermoelectric element of the couple, the characteristics of which were significantly altered by varying the deposition parameters. Varying the deposition parameters caused a change in lattice imperfections in the metal, which changed the mean free path of the conduction electrons and the thermoelectric power of the couple. The parameters having the greatest effect on the thermoelectric power were the deposition rate and the substrate temperature. Couples deposited at a high deposition rate (10 000 Å min^-1) and a high substrate temperature (? 300 °C) demonstrated e.m.f. characteristics closest to bulk values.     

Interface driven energy filtering of thermoelectric power in spark plasma sintered bi(2)te(2.7)se(0.3) nanoplatelet composites

Control of competing parameters such as thermoelectric (TE) power and electrical and thermal conductivities is essential for the high performance of thermoelectric materials. Bulk-nanocomposite materials have shown a promising improvement in the TE performance due to poor thermal conductivity and charge carrier filtering by interfaces and grain boundaries. Consequently, it has become pressingly important to understand the formation mechanisms, stability of interfaces and grain boundaries along with subsequent effects on the physical properties. We report here the effects of the thermodynamic environment during spark plasma sintering (SPS) on the TE performance of bulk-nanocomposites of chemically synthesized Bi(2)Te(2.7)Se(0.3) nanoplatelets. Four pellets of nanoplatelets powder synthesized in the same batch have been made by SPS at different temperatures of 230, 250, 280, and 350 °C. The X-ray diffraction, transmission electron microscopy, thermoelectric, and thermal transport measurements illustrate that the pellet sintered at 250 °C shows a minimum grain growth and an optimal number of interfaces for efficient TE figure of merit, ZT～0.55. For the high temperature (350 °C) pelletized nanoplatelet composites, the concurrent rise in electrical and thermal conductivities with a deleterious decrease in thermoelectric power have been observed, which results because of the grain growth and rearrangements of the interfaces and grain boundaries. Cross section electron microscopy investigations indeed show significant grain growth. Our study highlights an optimized temperature range for the pelletization of the nanoplatelet composites for TE applications. The results provide a subtle understanding of the grain growth mechanism and the filtering of low energy electrons and phonons with thermoelectric interfaces.     

The electron paramagnetic resonance of manganese in Mo1−x Ga4Mn x

We report the results of electron paramagnetic resonance experiments on managanese dissolved in MoGa ₄ over the temperature range 293-1.8 K. It is found that both theg shift and the line width show an anomalous sharp concentration independent increase as the temperature is decreased from 100 to 4.2 K. The experimental results for the relaxation rate andg shift are analyzed in terms of currently available theories and found to be unexplained by them. A model is proposed for the alloys invoking a bottleneck effect at high temperatures which breaks down at low temperatures due to the onset of a Kondo effect. When the bottleneck effect is broken down it is suggested that the theories for the unbottlenecked regime again become applicable.One of us (W.Z.) would like to acknowledge the support of a maintenance grant from the Fonds National Suisse.     

Resistivity and thermoelectric power of YbCu4.5 under very high pressure

Transport properties of YbCU_4.5 at zero pressure and under high pressure are reported. The resistivity is strongly perturbed by the disorder of the crystallographic structure, showing notably high residual resistivity and anisotropy at low temperature. Several samples have been measured at zero pressure and results are dominated by the Kondo effect: resistivity anomaly and giant thermoelectric power. Crystal field effect is taken into consideration to account for resistivity data. Under pressure, the decrease of the Kondo temperature have been observed with two experiments, up to 17 and 93 kbar. The possible appearance of a magnetic ground state under pressure is discussed.     

Mixed valency in rare-earth fullerides

Mixed-valence phenomena associated with the highly correlated narrow-band behaviour of the 4f electrons in rare earths are well documented for a variety of rare-earth chalcogenides, borides and intermetallics (Kondo insulators and heavy fermions). The family of rare-earth fullerides with stoichiometry RE2.75C60 (RE=Sm, Yb, Eu) also displays an analogous phenomenology and a remarkable sensitivity of the rare-earth valency to external stimuli (temperature and pressure) making them the first known molecular-based members of this fascinating class of materials. Using powerful crystallographic and spectroscopic techniques which provide direct indications of what is happening in these materials at the microscopic level, we find a rich variety of temperature- and pressure-driven abrupt or continuous valence transitions-the electronically active fulleride sublattice acts as an electron reservoir that can accept electrons from or donate electrons to the rare-earth 4f/5d bands, thereby sensitively modulating the valence of the rare-earth sublattice.     

Determination of the alloy scattering potential in modulation-doped In<Subscript>0.53</Subscript>Ga<Subscript>0.47</Subscript>As/In<Subscript>0.52</Subscript>Al<Subscript>0.48</Subscript>As heterojunctions from magnetotransport measurements

The results of magnetotransport measurements are used to investigate the scattering mechanisms and hence to determine the alloy disorder scattering potential in modulation-doped In_0.53Ga_0.47As/In_0.52Al_0.48As heterojunction samples with spacer layer thickness in the range from 0 to 400 Å. The experimental data for the temperature dependence of Hall mobility are compared with the electron mobility calculated for major scattering processes by using the theoretical expressions available in the literature. It is found that alloy disorder scattering and polar optical phonon scattering are the dominant scattering mechanisms at low and high temperatures, respectively. However, the effects of acoustic phonon scattering, remote-ionized impurity scattering, background-ionized impurity scattering, and interface roughness scattering on electron mobility are much smaller than that of alloy disorder scattering, at all temperatures. The alloy disorder scattering potential is determined by fitting the experimental data for low-temperature transport mobility of two-dimensional electrons in the first subband of the heterojunction sample with the calculated total mobility.     

Properties of the conduction electrons in the metals of group VIII

The behaviour of the conduction electrons in the eighth group elements was deduced from the results of experiments on vacuum-deposited films, by combining measurements of the electrical resistivity with thickness determinations and optical investigations. The mean free path of the electrons, their concentration, their effective mass, and the proportion of them specularly reflected at the film surfaces (known as the scattering parameter) were calculated by solving a system consisting of the Sondheimer equation for the size-dependence of the film conductivity, the known formulae giving the mean free path at the Fermi level and the plasma wavelength of the electron gas, and the Dingle relation for the scattering parameter in the spectral region of the anomalous skin effect.The data thus obtained are in reasonable agreement with the generally accepted band structure of the transition metals.     

The nonlinear thermoelectric effect in disordered metallic systems

The nonlinear thermoelectric effect in a disordered metal due to the nonlocal corrections to the thermoelectric current is investigated. We have found that the electron-electron interaction makes the thermoelectric response nonlinear at low temperatures even if the applied electric field or temperature gradient is constant. Both the heat current induced by the electric field and the electric current induced by the temperature gradient are discussed.The authors are grateful to A. V. Sergeev for valuable discussions.     

Description of electronic transport properties of monocrystalline films by a statistical model

Grain boundaries in monocrystalline films are represented by a two- dimensional array of scatterers. The effects of electronic scattering are calculated by means of Matthiessen's rule starting from the electronic mean free path related to a particular type of scattering (bulk, grain boundary, external surface).

Theoretical expressions for the film resistivity and its temperature coefficient of resistivity and thermoelectric power are proposed; comparison with previously reported experiments gives a satisfactory fit.     

多晶材质电子结构及热电性能的模拟

发展了一种研究多晶体系电子态以及热电性质的计算机模拟方法。首先采用相场动力学方法模拟多晶材质图案，再利用其模型序参量构造晶界的势函数，用近自由电子近似构造体系的哈密顿量。求解薛定谔方程得到体系的本征态。通过电荷密度的分布研究电子的限域特征,分析模拟结果发现对于晶界为势垒的情况，电子的基态出现在最大晶粒中；而对于晶界为势阱的情况，电子更容易限域在多个晶粒交叉的晶界附近，由得到的本征能级和波函数可以计算出温差导致的电位差，即得到赛贝克系数随温度的变化。结果表明具有导电晶界的多晶体的赛贝克系数要高于具有导电晶粒的多晶体。     

Improvement of Low‐Temperature zT in a Mg3Sb2–Mg3Bi2 Solid Solution via Mg‐Vapor Annealing

Materials with high zT over a wide temperature range are essential for thermoelectric applications. n‐Type Mg₃Sb₂‐based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain‐boundary effect has been limited due to loss of Mg, which hinders a sample's n‐type dopability. A Mg‐vapor anneal processing step that grows a sample's grain size and preserves its n‐type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon‐scattering‐dominated T^?3/2 trend over a large temperature range, further supporting the conclusion that the temperature‐activated mobility in Mg₃Sb₂‐based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm² V^?1 s^?1 in annealed 800 °C sintered Mg_{3 + δ}Sb_1.49Bi_0.5Te_0.01, the highest ever reported for Mg₃Sb₂‐based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n‐type Bi₂Te₃) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.     

Anderson impurities in a transition metal superconductor. Kondo effect

Using the approach of Matsuura, Ichinose, and Nagaoka to treat Anderson impurities dissolved in a transition metal superconductor, the Kondo effect in a two-band superconductor is studied. It is found that the two-particle propagators for the superconductor are coupled to each other through thes- andd-electron vertex functions, which are obtained as solutions to a set of coupled Bethe-Salpeter equations. By rearranging some of the terms in the two-particle propagators, the pair-breaking parameters for thes andd electrons are obtained. An expression for the decrease in the transition temperature due to the Kondo scattering is obtained.     

Thermal conductivity of nanocrystalline silicon: importance of grain size and frequency-dependent mean free paths

The thermal conductivity reduction due to grain boundary scattering is widely interpreted using a scattering length assumed equal to the grain size and independent of the phonon frequency (gray). To assess these assumptions and decouple the contributions of porosity and grain size, five samples of undoped nanocrystalline silicon have been measured with average grain sizes ranging from 550 to 64 nm and porosities from 17% to less than 1%, at temperatures from 310 to 16 K. The samples were prepared using current activated, pressure assisted densification (CAPAD). At low temperature the thermal conductivities of all samples show a T(2) dependence which cannot be explained by any traditional gray model. The measurements are explained over the entire temperature range by a new frequency-dependent model in which the mean free path for grain boundary scattering is inversely proportional to the phonon frequency, which is shown to be consistent with asymptotic analysis of atomistic simulations from the literature. In all cases the recommended boundary scattering length is smaller than the average grain size. These results should prove useful for the integration of nanocrystalline materials in devices such as advanced thermoelectrics.     

Temperature-Dependent Thermal Boundary Conductance at Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> interfaces

With the ever-decreasing size of microelectronic devices, growing applications of superlattices, and development of nanotechnology, thermal resistances of interfaces are becoming increasingly central to thermal management. Although there has been much success in understanding thermal boundary conductance at low temperatures, the current models applied at temperatures more common in device operation are not adequate due to our current limited understanding of phonon transport channels. In this study, the scattering processes in Al and Pt films on Al₂O₃ substrates are examined by transient thermoreflectance testing at high temperatures. At high temperatures, traditional models predict the thermal boundary conductance to be relatively constant in these systems due to assumptions about phonon elastic scattering. Experiments, however, show an increase in the conductance indicating potential inelastic phonon processes.     

Anomalous Resistivity and the Electron–Polaron Effect in the Two-Band Hubbard Model with One Narrow Band

We search for anomalous normal and superconductive behavior in the two-band Hubbard model with one narrow band. We analyze the influence of the electron?Cpolaron effect and the Altshuler?CAronov effect on effective mass enhancement and scattering times of heavy and light components in the clean case. We find anomalous behavior of resistivity at high temperatures $T > W_{h}^{*}$ both in 3D and 2D situations. The SC instability in the model is governed by an enhanced Kohn?CLuttinger effect for p-wave pairing of heavy electrons via polarization of light electrons.     

Proximity effect sandwiches containing nonmagnetic localized states

The effects of resonant scattering by nonmagnetic localized states in the normal layer on the critical temperatures of proximity effect sandwiches are studied through the renormalization of the electron propagators in the McMillan proximity effect model. By treating both the resonant scattering of the normal layer electrons and the Coulomb correlation between electrons of opposite spins located on the same impurity site self-consistently, an expression for the decrease in the critical temperature that shows explicitly the dependences on the parameters that describe the impurity atoms is obtained. The decrease in the critical temperatures as a function of impurity concentration and of normal layer thickness is shown.     

Electric-field effect of near-band-edge photoluminescence in bulk ZnO

We have studied the effect of electric fields on the near-band-edge (NBE) emissions in bulk zinc oxide (ZnO) by using photoluminescence and photocurrent (PC) spectroscopy simultaneously. The intensity-quenching and peak-shift effects of the free exciton spectra were observed with increasing electric field. From the PC result, we find out that the free excitons are disturbed by the PC carriers of the photo-created electrons and holes. This disturbance reduces the recombination ratio and the lifetime of free excitons. Therefore, the intensity-quenching effect was attributed to the decrease in the recombination of free excitons. Thus, the shift of the free exciton peaks was related to Stark effect induced by electric field. As a result, we have found that these phenomena are caused to the exciton–electron scattering due to a strong interaction between the excitons in the conduction band and the photo-generated electron carriers with increasing the applied electric field.