Thermoelectric effects in spin field-effect transistors

We investigate the thermoelectric effects in a spin field-effect transistor with ferromagnetic leads held at different temperatures. The thermopower S and thermoelectric figure of merit ZT oscillate with the increase of the Rashba spin-orbit coupling strength. The oscillation amplitude of ZT decreases with increasing the spin polarization. S and ZT are strongly influenced by the interfacial barrier strength Z, exhibiting a nonmonotonous change with Z. The thermoelectric effects are also manipulated by the magnetization configuration of the ferromagnetic leads. It is expected that the present study of the thermoelectric effects is helpful in the design of thermoelectric devices.     

Bi2Te3/Sb超晶格纳米线的外延生长和热电测量

通过脉冲电沉积,外延生长出小单元长度的Bi₂Te₃/Sb超晶格纳米线.借助哈曼方法,测量了超晶格纳米线阵列的热电性能,330 K时的ZT值可达0.15.研究了Bi₂Te₃/Sb超晶格纳米线阵列器件的制冷或者加热能力,发现器件的上下表面的最大温差可以达到6.6 K.     

Thermoelectric properties of gated graphene ribbons in the ballistic regime

We investigate the thermoelectric properties of gated graphene ribbons in the ballistic transport limit using linear response theory and the Landauer formalism. The dependence of the electronic conductance, thermopower as well as electronic thermal conductance on both Fermi level and temperature are clarified and the validity of Wiedemann-Franz law is examined. The electronic part of thermoelectric figure of merit _ZTel${\mathit{ZT}}_{\mathit{el}}$ which gives an upper bound for the thermoelectric efficiency of the gated ribbons, is also calculated. It is shown that _ZTel${\mathit{ZT}}_{\mathit{el}}$ of wide and short gated ribbons is directly related to geometric aspect ratio of the graphene ribbon and for very short ribbons can exceed unity at room temperature. Our results could be useful in the design of efficient graphene-based thermoelectric devices.     

Influence of flat milling on vortex matching effect in Bi2Sr2CaCu2O8+y with antidot array

Significant enlargements of antidot diameter by Ar-ion milling were observed in Bi₂Sr₂CaCu₂O_8+y single-crystal films with antidot arrays as well as the thinning of the films. In an original sample with triangular array of antidots, whose diameter is about 200 nm, a few dip structures by the matching effect were observed in the vortex-flow resistance as a function of magnetic field. With increasing the milling time, the number of the dips increases and the appearance of the flow resistance becomes periodic oscillations. These features can be explained mainly by the increase of the antidot diameter.     

Thermoelectric properties of nano-bulk bismuth telluride prepared with spark plasma sintered nano-plates

The pursuit for a high-performance thermoelectric n-type bismuth telluride-based material is significant because n-type materials are inferior to their corresponding p-type materials in highly efficient thermoelectric modules. Herein, to improve the thermoelectric performance of an n-type Bi₂Te₃, we prepared Bi₂Te₃ nano-plates with a homogeneous sub-micron size distribution and thickness range of about a few tens of nanometers. This was achieved using a typical nano-chemical synthetic method, and the prepared materials were then spark plasma sintered to fabricate n-type nano-bulk Bi₂Te₃ samples. We observed a significant enhancement of the anisotropic electrical transport properties for the nano-bulk sample with a higher power factor along the in-plane direction (24.3?μW?cm^?1?K^?2 at 300?K) than that along the out-of-plane direction (8.1?μW?cm^?1?K^?2 at 300?K). However, thermal transport properties were insensitive along the measured direction for the nano-bulk sample. We used a dimensionless figure of merit ZT to calculate the thermoelectric performance. The results showed that the maximum ZT value of 0.69 was achieved along the in-plane direction at 440?K for the nano-bulk n-type Bi₂Te₃ sample, which was however smaller than that of the previously reported n-type samples (ZT of 1.1). We believe that a further enhancement of the ZT value in the fabricated nano-bulk sample could be accomplished by effectively removing the surface organic ligand of the Bi₂Te₃ nano-plate particles and optimizing the spark plasma sintering conditions, maintaining the nano-plate morphology intact.     

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

We investigate the figure of merit of a quantum dot (QD) system irradiated with an external microwave filed by nonequilibrium Green?s 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.     

Investigation of the possibilities for increasing the thermoelectric figure of merit of nanostructured materials based on Bi2Te3-Sb2Te3 solid solutions

The transport coefficients and thermoelectric figure of merit ZT for bulk nanostructured materials based on Bi₂Te₃-Sb₂Te₃ solid solutions have been investigated theoretically. Similar materials prepared by rapid quenching of the melt with the subsequent grinding and sintering contain amorphous and nanocrystalline regions with different sizes of particles. According to the performed estimations, the thermoelectric figure of merit of the amorphous phase can exceed the value of ZT for the initial solid solution by a factor of 2?C3 primarily due to the significant decrease in the thermal conductivity. The effective transport coefficients of the medium as a whole have also been investigated as a function of the parameters of each phase, and the concentration range of the amorphous phase, which corresponds to the effective values ZT > 1, has been determined.     

Thermoelectric properties of tunnel junctions

The thermoelectric (Seebeck) coefficient α and thermoelectric quality factor (figure of merit) ZT are estimated for a tunnel junction in metals. It is shown that α can be of the order of hundreds of μV/K while ZT can approach values ∼0.1–1. The maxima of α(h) and ZT(h) correspond to a certain width h of the tunnel junction; such h is about a few nanometers. The results we obtained can find applications in the constructions of novel thermoelectric generators.     

Out-of-plane coercive field of Ni80Fe20 antidot arrays

The out-of-plane magnetic anisotropy and out-of-plane magnetization reversal process of nanoscale Ni₈₀Fe₂₀ antidot arrays deposited by magnetron sputtering technique on an anodic aluminum oxide (AAO) membrane are investigated. The angular dependence of out-of-plane remanent magnetization of Ni₈₀Fe₂₀ antidot arrays shows that the maximum remanence is in-plane and the squareness of the out-of-plane hysteresis loop follow a |cos θ| dependence. The angular dependence of out-of-plane coercivity of Ni₈₀Fe₂₀ antidot arrays shows that the maximum coercivity lies on the surface of a cone with its symmetric axis normal to the sample plane, which indicates a transition of magnetic reversal from curling to coherent rotation when changing the angle between the applied magnetic field and the sample plane.     

Multi-vortex versus interstitial vortices scenario in superconducting antidot arrays

In superconducting thin films, engineered lattice of antidots (holes) act as an array of columnar pinning sites for the vortices and thus lead to vortex matching phenomena at commensurate fields guided by the lattice spacing. The strength and nature of vortex pinning is determined by the geometrical characteristics of the antidot lattice (such as the lattice spacing a₀, antidot diameter d, lattice symmetry, and orientation) along with the characteristic length scales of the superconducting thin films, viz., the coherence length (ξ) and the penetration depth (λ). There are at least two competing scenarios: (i) multiple vortices sit on each of the antidots at a higher matching period and (ii) there is nucleation of vortices at the interstitial sites at higher matching periods. Furthermore, it is also possible for the nucleated interstitial vortices to reorder under suitable conditions. We present our experimental results on NbN antidot arrays in the light of the above scenarios.     

Improving the thermoelectric properties of carbon nanotubes through introducing graphene nanosprings

Carbon nanotube (CNT) is a typical one-dimensional nanomaterial containing sp² hybridization states. In this paper, we investigate the ballistic thermoelectric performance of CNTs incorporating graphene nanosprings by using non-equilibrium Green's function. The calculations reveal that the thermoelectric figure of merit could be obviously improved by introducing graphene nanosprings, which is about ten times of that of pristine CNTs at 700 K. Such enhancement is mainly attributed to the remarkable suppression of phononic and electronic thermal conductance and improvement of Seebeck coefficient. In addition, compared to the zigzag graphene nanospring, introducing of the armchair case possesses better thermoelectric performance. The results presented in this paper indicate that embedding graphene nanospring is a viable method to optimize the thermoelectric performance of CNTs and could provide useful theoretical guidance for design and fabrication of CNTs-based thermoelectric devices.     

Thermoelectric properties of unoxidized graphene/Bi2Te2.7Se0.3 composites synthesized by exfoliation/re‐assembly method

Nanocomposites of n‐type thermoelectric Bi₂Te_2.7Se_0.3 (BTS) and unoxidized graphene (UG) were prepared from the exfoliated BTS and UG nanoplatelets. Polycrystalline BTS ingots were exfoliated into nanoscoll‐type crystals by chemical exfoliation, and were re‐assembled with UG nanoplatelets. The composites were chemically reduced by hydrazine hydrate and sintered by a spark‐plasma‐sintering method. The thermoelectric properties of the sintered composites were evaluated and exhibited decreased carrier concentration and increased thermal conductivity due to the embedded graphene. The peak ZT values for the UG/BTS‐US and UG/BTS‐EX composites were ～0.8 at the UG concentration of 0.05 wt%. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical and thermal transport properties of CdO ceramics

High temperature electrical and thermal transport properties, that is, electrical conductivity, Seebeck coefficient and thermal conductivity, of CdO ceramics have been investigated. Because of the good electrical properties and low thermal conductivity, the dimensionless figure-of-merit ZT of the CdO ceramics reaches 0.34 at 1023 K. This value is comparable to the best reported ZT for the n-type oxide ceramic thermoelectric materials and remains as potential to be further improved by porosity controlling or nanostructuring.     

锯齿形石墨烯反点网络加工与输运性质研究

具有特定边界的石墨烯纳米结构在纳电子学、自旋电子学等研究领域表现出良好的应用前景.然而石墨烯加工成纳米结构时,无序的边界不可避免地会降低其载流子迁移率.氢等离子体各向异性刻蚀技术是加工具备完美边界石墨烯微纳结构的一项关键技术,刻蚀后的石墨烯呈现出规则的近原子级平整的锯齿形边界.本文研究了氮化硼上锯齿形边界石墨烯反点网络的磁输运性质,低磁场下可以观测到载流子围绕着一个空位缺陷运动时的公度振荡磁阻峰.随着磁场的增大,朗道能级简并度逐渐增大,载流子的磁输运行为从Shubnikov-de Haas振荡逐渐向量子霍尔效应转变.在零磁场附近可以观测到反点网络周期性空位缺陷的边界散射所导致的弱局域效应.研究结果表明,在氮化硼衬底上利用氢等离子体刻蚀技术加工锯齿形边界石墨烯反点网络,其样品质量会明显提高,这种简单易行的方法为后续高质量石墨烯反点网络的输运研究提供了新思路.     

Thermoelectric properties of Cr1−xMoxSi2

The thermoelectric properties of Mo-substituted CrSi₂ were studied. Dense polycrystalline samples of Mo-substituted hexagonal C40 phase Cr_1−xMo_xSi₂ (x=0–0.30) were fabricated by arc melting followed by spark plasma sintering. Mo substitution substantially increases the carrier concentration. The lattice thermal conductivity of CrSi₂ at room temperature was reduced from 9.0 to 4.5 W m⁻¹ K⁻¹ by Mo substitution due to enhanced phonon–impurity scattering. The thermoelectric figure of merit, ZT, increases with increasing Mo content because of the reduced lattice thermal conductivity. The maximum ZT value obtained in the present study was 0.23 at 800 K, which was observed for the sample with x=0.30. This value is significantly greater than that of undoped CrSi₂ (ZT=0.13).     

纳米线阵列横向输运的热电特性研究

利用包络函数的平面波展开法计算准二维纳米线阵列中的电子态,获得电输运系数表达式.同时,通过合理近似考虑边界散射对声子输运的影响,计算得到了晶格热导率.以Si/Ge体系为例,研究了纳米线阵列横向输运的热电特性.结果表明：结构优值与费米能级、纳米线直径及间距等参数相关.通过对结构参数的调整,纳米线阵列的横向输运可有效提高热电性能. 关键词： 热电性能 纳米线阵列 Seebeck系数 晶格热导率     

熔体旋甩法合成n型(Bi_(0.85)Sb_(0.15))_2(Te_(1-x)Se_x)_3化合物的微结构及热电性能

采用熔体旋甩结合放电等离子烧结(MS-SPS)技术制备了单相n型四元(Bi0.85Sb0.15)2(Te1-xSex)3(x=0.15,0.17,0.19,0.21)化合物,并对所得样品的微结构和热电传输性能进行了系统研究.样品自由断裂面的场发射扫描电子显微镜及抛光面的背散射电子成分分析表明:块体材料晶粒细小,晶粒排列紧密,成分分布均匀且相结构单一,样品中存在大量10—100nm的层状结构.随着Se含量x的增加,样品的电导率和热导率逐渐增加,而Seebeck系数逐渐降低.相比商业应用的区熔材料,MS-SPS方法合成的高Se组成的样品均在425K后表现出更高的ZT值,其中(Bi0.85Sb0.15)2(Te0.83Se0.17)3样品具有最高的ZT值,在360K可达到0.96,并在320—500K均保持较高的ZT值,500K时其ZT值相比区熔材料提高了48%.此外,通过调节Se的含量,可以有效地调控材料的ZT峰值出现的温度段,这对多级或梯度热电器件的制备具有重要意义.     

Thermoelectric properties of Ca3−xYxCo4O9+δ ceramics

The Ca_3?xY_xCo₄O_9+δ (x=0, 0.15, 0.3) ceramics were prepared by combining the polyacrylamide gel method and the spark plasma sinter (SPS) technology in order to improve the thermoelectric properties of Ca₃Co₄O_9+δ ceramics. The Seebeck coefficients and the resistivities of the Y-doped samples were obviously enhanced due to the decrease of carrier concentration, and their thermal conductivities were decreased due to the impurity scattering effect. The thermoelectric properties were improved at high temperature by Y-doping according to the power factor analysis and the thermoelectric figure of merit (ZT) data. The optimized figure of merit ZT=0.22 at 973 K was obtained for Ca_2.7Y_0.3Co₄O_9+δ.     

The Effect of the Heavy-Hole Band on the Thermoelectric Figure-of-Merit of Heavily Doped p-Type Lead Telluride

The thermoelectric properties of heavily doped p-type PbTe have been theoretically studied in the temperature range from 300 to 900 K. The calculations are based on a three-band model of the PbTe electron energy spectrum taking into account the transport of electrons and light holes in the L-extrema and heavy holes in the Σ-extrema. The thermoelectric quantities turned out to be very sensitive to the parameters of the heavy-hole band. The best agreement with the experiment is reached at m_hh = 5m₀ and E_gΣ = 0.5 eV, where all calculated thermoelectric quantities agree well with the available experimental data within the entire interval from 300 to 900 K. The calculation confirms a significant increase of the value of the thermoelectric figure-of-merit to ZT = 1.2 that has been recently observed experimentally in heavily doped p-PbTe samples. The maximum of ZT corresponds to the temperature at which the band edges of light and heavy holes coincide in energy so that the steepest singularity in the density of states in the valence band is formed.