热电材料的热输运调控及其在微型器件中的应用

热电能量转换技术是一种利用材料实现热能和电能直接相互转换的清洁能源技术.热电材料的性能是决定热电转换效率的关键因素,而热电材料的性能优化涉及电输运和热输运性能的协同优化.文章介绍了近期热电材料中热输运的调控机制和方法,其中重点论述了基于原子分子层次调控热传导的宽频声子散射效应和横波阻尼效应,以及低维结构和复合材料中的基于纳米微米尺度的微结构调控.最后介绍了室温附近热传导的调控机制及其在高效微型热电器件研发中的应用和进展.     

低维微纳尺度体系声子热传导和热调控:来自芯片散热的非平衡统计物理问题

半导体芯片发展路线图上的一个障碍是“热死(heat death)”,也就是大量热量的产生而导致芯片被烧毁.所以散热问题成为进一步发展半导体工业亟待解决的关键问题.芯片里的热传导包含一维和二维材料中声子热传导以及声子通过不同材料间的界面热传导.本文总结了过去30年来一维、二维和界面声子热传导的主要理论和实验进展,重点介绍了一维体系声子热传导发散的物理机制以及反常热传导和反常扩散之间的关系.本文还简要讨论了与此相关的非平衡态统计物理的基本问题:从给定的哈密顿量出发是否能够推导出宏观输运行为.本文从微观图像的角度讨论了调控声子热传导的几种方法:纳米声子晶体,纳米热超材料,界面和声子凝聚等.为了让读者全面了解声子热传导,还简要地介绍了其他声子热输运现象,包括热导量子化、声子热霍尔效应、手性声子,以及声子与其他载流子之间的相互作用.最后,本文讨论了声子热传导研究面临的挑战和机遇,包括声子在量子信息和技术中的潜在应用.     

热整流效应的消失与翻转现象

研究了热整流器模型当中碰撞效应对其热传导性质,尤其是热整流效应的影响:碰撞会引起声子能带的交叠.在高温区,发现在考虑了粒子间的碰撞之后,声子能带会有明显的重叠,热整流效应会被削弱甚至消失.而在低温区,由于交换热源时,碰撞效应对声子能带交叠的贡献存在差别,因而出现了翻转的热整流效应.结果表明在设计热整流器件时,必须要考虑碰撞效应的影响.     

同济大学“声子学与热能科学研究中心”——人才招聘

"声子学"作为物理学前沿的一个新兴热门领域,正越来越多地吸引着人们的注意。作为全世界第一个声子学研究机构:"同济大学—新加坡国立大学声子学和热能科学中心"成立于2012年5月。本中心是按2011工程精神与国外知名大学协同创办的创新基地。中心以同济大学为基地,协同新加坡国立大学,为物理学、数学、生命科学、信息科学和工程、材料科学与工程、能源科学与工程等学科的科研工作者提供了进行跨学科协同研究的平台。本中心将从事声子/热的基本规律、声子与其他的热载体如电子、光子、磁化子等相互作用的理论、计算模拟,和实验研究。中心还将致力于研究调控声子/热的物理原     

低维纳米材料量子热输运与自旋热电性质 ——非平衡格林函数方法的应用

低维材料不断涌现的新奇性质吸引着科学研究者的目光. 除了电子的量子输运行为之外, 人们也陆续发现和确认了热输运中显著的量子行为, 如 热导低温量子化、声子子带、尺寸效应、瓶颈效应等. 这些小尺度体系的热输运性质可以很好地用非平衡格林函数来描述. 本文首先介绍了量子热输运的特性、声子非平衡格林函数方法及其在低维纳米材料中的研究进展; 其次回顾了近年来在 一系列低维材料中发现的热-自旋输运现象. 这些自旋热学现象展现了全新的热电转换机制, 有助于设计新型的热电转换器件, 同时也给出了用热产生自旋流的新途径; 最后介绍了线性响应理论以及在此理论框架下结合声子、电子非平衡格林函数方法进行的一些有益的探索. 量子热输运的研究对热效应基础研究以及声子学器件、能量转换器件的发展有着不可替代的重要作用.     

热驱动微光学

通过实例研究和近期报道的文献,论述了热驱动微光学的研究进展。驱动器主要通过应用在液体和液气界面的压力来调控光学微流体,包括微镜片和薄膜微透镜,热驱动器利用芯片上的温度变化产生所需要的压力差,从而对微透镜进行调控。同时还讨论了用于微光学结构的各种设备、结构、液体和膜材料,并提供了典型操作特性。     

燃气和电驱动热声发动机的对比研究

热声发动机利用热声效应把热能高效转化为声功,系统中不存在任何机械运动部件,应用前景光明.热声发动机的一个突出优点是直接采用热能驱动,可以提高低温位热能的品位.在原有电驱动热声发动机的基础上,制作了一台燃气驱动行波热声发动机,以氮气为工质,充气压力为1.2 MPa时,获得了1.34的最大压比.对电驱动和燃气驱动两种加热方式作了对比研究,得到了燃气驱动条件下热声发动机的起振状态、加热功率和直流抑制的规律.     

表面共振圆环结构对Si纳米线热传导的调控

利用声子的波动性,在纳米线表面引入共振结构,可以有效阻碍声子输运.为进一步优化共振结构,本文基于平衡态分子动力学(EMD)方法,研究表面共振圆环结构的高度和宽度对Si纳米线热输运性质的影响.结果表明：随着共振圆环高度的增加,Si纳米线的热导率逐渐减小,最大减幅可达61.9%.当高度达到2nm以后,热导率基本不再变化.共振圆环宽度则对热导率的影响较小.声子色散关系中出现的平带,证实了共振圆环引起的声子共振效应;最低共振频率的变化说明了共振圆环中的声子波长决定了共振圆环高度对纳米线热导率的最大影响程度.研究进一步发现,随着共振圆环高度的增加,声学支声子对热导率贡献的比重变小.本文研究结果对高效热电材料和隔热材料的微纳结构设计提供了一种新的思路.     

同济大学“声子学与热能科学研究中心”--人才招聘

“声子学”作为物理学前沿的一个新兴热门领域，正越来越多地吸引着人们的注意。作为全世界第一个声子学研究机构：“同济大学-新加坡国立大学声子学和热能科学中心”成立于2012年5月。本中心是按2011工程精神与国外知名大学协同创办的创新基地。中心以同济大学为基地，协同新加坡国立大学，为物理学、数学、生命科学、信息科学和工程、材料科学与工程、能源科学与工程等学科的科研工作者提供了进行跨学科协同研究的平台。
　　本中心将从事声子/热的基本规律、声子与其他的热载体如电子、光子、磁化子等相互作用的理论、计算模拟，和实验研究。中心还将致力于研究调控声子/热的物理原理和相关技术，以及如何用于提高热电转换效率。中心也将研究如何实现声子作为信息载体的处理技术和应用。     

热声发动机用加热器的设计与实验验证

热声发动机利用热声效应将热能转换为机械能,热能由核心部件加热器提供。加热器在热声发动机的能量传递和转化过程中起着举足轻重的作用。针对热声发动机的工作特点,介绍了一种采用时均流对流换热公式进行热声发动机用加热器设计的方法。基于该方法,设计了一种新型热声发动机用电加热器,并应用于自行研制的太型多功能行波热声发动机试验台。设计计算和实验结果表明,该加热器能够充分满足热声发动机的加热要求。本文对以振荡流体为特征的换热器设计具有一定参考价值。     

Chiral perturbation theory for fDS/FD and BBS/BB

The decay constants for D and D_s mesons, denoted f_D and f_DS respectively, are equal in the SU(3)_V limit, as are the hadronic amplitudes for and mixing. The leading SU(3)_V violating contribution to (F_DS/F_D) and to the ration of hadronic matrix elements relevant for and mixing amplitudes are calculated in chiral perturbatiion theory. We discuss the formalism needed to include both meson and anti-meson fields in the heavy quark effective theory.     

Near-infrared bands of S2: 3Πgi-3Σu+ system

Vibrational and rotational analyses of the near-infrared bands of S₂ lying in the region 7440–8085 Å are reported. They form a new band system involving a ${}^3\text{Π}{}_{\mathrm{gi}}\text{-}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ transition and arise from the same initial ³Π_gi state of the ³Π_gi-³Δ_ui band system reported earlier. The analyses of the bands of this system due to the isotopic molecules ³²S³⁴S and ³⁴S₂ are also reported.     

Magnetism in nanometer-thick TiOx/Co/TiOx/TiN/Si multilayered structures

Most studies on Co-doped TiO₂ system were focused on thin films grown by MBE-based methods. In this work we report the ferromagnetism of nanometer-thick-layered TiO₂/Co/TiO₂/TiN film grown on Si substrate by conventional magnetron sputtering. For the growth of TiO₂ on silicon, a non-oxide thermally stable material, TiN, was introduced to prevent Ti penetration into the Si substrate. Structural, magnetic, and transport measurements respectively by Raman, SQUID and Hall effect show that our samples are n-type semiconductors and exchange bias effect due to exchange coupling between Co and interfacial CoO. For the rapid vacuum annealed specimen, we found an enhanced loss and a Perminvar-type constricted hysteresis loop, which attributed to pinning of domain walls due to an induced anisotropy by the pair ordering in the metallic alloy of Co-Ti-Si.     

The decay process 11ΛB → π− + 11C

The branching ratios are calculated for ¹¹_ΛB decay to the ¹¹C ground and excited states below 8 MeV for two possible spin values of ¹¹_ΛB. It is found that the decay rate to the ¹¹C^★ state at E^★ = 6.48 MeV is comparable in magnitude to that leading to the ¹¹C ground state if $\text{J(}{}^{11}{}_{\mathrm{\Lambda }}\text{B}\text{) =}\text{5}\text{2}$ is assumed. This result, unlike the branching ratios calculated for the $\text{J(}{}^{11}{}_{\mathrm{\Lambda }}\text{B}\text{) =}\text{7}\text{2}$ case, is in accord with experiment and lends support to the assumption that $\text{J =}\text{5}\text{2}$ holds for ¹¹_ΛB. The necessity of the reinterpretation of some of the so-called ¹³_ΛC events in terms of ${}^{11}{}_{\mathrm{\Lambda }}\text{B}\text{→ π}{}^{\mathrm{?}}\text{+}{}^{11}\text{C}{}^1$ is indicated.     

The decay processes Λ8Li → π− + 8Be1 (∼ 17 MeV)

The branching ratio is calculated for _Λ⁸Li decay to the (2⁺) ⁸Be¹ states near 17 MeV, using intermediate coupling wave functions for _Λ⁸Li and for the relevant ⁸Be¹ states. It is pointed out that this ratio is sensitive primarily to a mixing angle ? in the _Λ⁸Li wave function. Within one standard deviation, the data allow two ranges (+0.05 to +0.25 rad and +1.10 to +1.25 rad) for the value of ?. The further requirement that there also be acceptable agreement between the angular distribution expected for the subsequent ${}^8\text{Be}{}^1\text{(? 17}\text{MeV → 2}{}^4\text{He}$ decay and the data, shifts these allowed ranges for ?, to (+0.13 to 0.40) rad and (+0.9 to +1.2) rad. It is predicted that the dominant transition should be to ⁸Be¹ (16.6 MeV), as is observed to be the case, rather than to ⁸Be¹ (16.9 MeV). The interpretation of these values for ? is discussed in some detail and their implications for intermediate coupling shell-model calculations of Λ-hypernuclear wave functions are considered.     

The 5T2g → 5Eg absorption in MgO: Fe2+

At helium temperatures two sharp lines at 9350 and 9510 cm^?1 have been observed for the first tune on the low-energy side of the broad double-peaked absorption corresponding to the ${}^5\text{T}{}_{\mathrm{2g}}\text{→}{}^5\text{E}{}_{\mathrm{g}}$ transition in Fe²⁺ at the octahedral site in MgO. The lower energy line has a half width of 4 cm^?1; Zeeman measurements show that it is of magnetic dipole origin. The Zeeman spectra are consistent with those expected for a pure electronic transition from the (⁵T_2g)T_2g ground state to the ⁵E_g excited state. The second line, with a halfwidth of ~ 35 cm^?1, a vibrational sideband.     

Electrical property of HfOxNy-HfO2-HfOxNy sandwich-stack films

The effects of HfO_xN_y on the electrical property of HfO_xN_y-HfO₂-HfO_xN_y sandwich-stack (signed as SS) films were investigated. Excellent electrical performances were achieved in SS films, with a high dielectric constant of 16 and a low leakage current of ∼2 × 10⁻⁸ A/cm² at 1 MV/cm. Schottky (SK) emission and Frenkel-Poole (PF) emission are found to be the dominant mechanisms for the current conduction behavior. After a long time stress, the flat-band voltage shift in the SS film is much smaller than that in a pure HfO_xN_y film indicating fewer charge traps existed in the SS film. Based on the experiments, the new SS structure is more favorable for the improvement of electrical performances than a pure HfO_xN_y or HfO₂ structure.     

Spinor techniques for calculating pp → W±/Z0 + jets

We present techniques which enable one to calculate quickly the amplitudes for many scattering processes in the high-energy limit. As an illustration of the method, these are applied to the diagrams for $\text{p}\text{p}\text{→}\text{V}\text{+ 0, 1}\text{or}\text{2}\text{jets}$, where V = W^± or Z⁰. The form of the results lends itself to immediate numerical evaluation.     

Etude par diffraction neutronique des solutions solides K1−xBixF1+2xetRb1−xBixF1+2x

Bragg neutron diffraction studies have been carried out on the fluorite type solid solutions K_1?xBi_xF_1+2x (0.50 ? × ? 0.70) and Rb_1?xBi_xF_1+2x (0.50 ? × ? 0.60). The distribution of the fluorine atoms between normal and interstitial sites is given as a function of substitution rate. A substition mechanism is proposed. Electrical and NMR results on one side and structural data on the other side are correlated. A study of the background as a function of temperature has allowed to determine the static origin of its modulation. By inelastic neutron diffusion, it has been shown that the number of carriers is weak, which involves a high mobility.     

Electronic and magnetic structures of V-doped zinc blende Zn1-xVxNyO1-y and Zn1-xVxPyO1-y

Electronic and magnetic structures of zinc blende ZnO doped with V impurities are studied by first-principles calculations based on the Korringa-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA).Calculations for the substitution of O by N or P are performed and the magnetic moment is found to be sensitive to the N or P content.Furthermore,the system exhibits a half-metallic band structure accompanied by the broadening of vanadium bands.The mechanism responsible for ferromagnetism is also discussed and the stability of the ferromagnetic state compared with that of the paramagnetic state is systematically investigated by calculating the total energy difference between them by using supercell method.