热电材料β-Cu2-xSe热传导性能模拟研究

“声子液体”热电材料β-Cu2-xSe具有优异的热电转换效率,采用分子动力学模拟的方法研究其热传导性能,分析了类“液态”离子的扩散能力和材料导热性能的相关性,并探究了材料加工处理手段(掺杂和空位)对材料热传导性能的影响。结果表明:类“液态”离子扩散能力和导热系数存在极强的相关性,β-Cu2-xSe中Cu^+移动能力的增强会增加晶格的非简谐振动,从而强化了声子散射,导致材料导热系数的降低。掺杂和空位对“声子液体”热传导性能有不同的影响:材料内部存在空位时,Cu^+倾向于在晶格缺陷中移动,降低了与Se构成的固定亚晶格碰撞概率,造成声学支声子频率的下降,有效地降低了导热系数,提高了材料的热电转换效率;相比空位,掺杂对导热系数的影响不明显。     

硼掺杂球磨SiGe合金的热电性能

SiGe合金热电材料作为一种传统的高温热电材料一直以来受到广泛关注。本研究通过B在球磨SiGe合金中的P型掺杂,有效增加了材料的载流子浓度,优化材料的电学性能。通过球磨降低材料的晶粒尺寸,增强晶界对声子的散射,降低材料的晶格热导率。另外,B掺杂使点缺陷散射和载流子-声子散射得到增强,材料的晶格热导率进一步降低。在室温时,Si_(0.8)Ge_(0.2)B_(0.04)的晶格热导率为~4Wm~(-1) K~(-1)。由于掺杂后电导率提高,热导率降低,因此热电优值zT得到了提高。在850K时,Si_(0.8)Ge_(0.2)B_(0.04)的最大热电优值为0.42,与Si_(0.8)Ge_(0.2)B_(0.002)的样品相比,其优值提高了2.5倍左右。     

新型多共振腔声子晶体声屏障的带隙机理及其隔声性能研究

为控制道路噪声水平,设计了一种新型多共振腔的声子晶体声屏障。该设计先采用理论和仿真计算分析多腔声子晶体的能带结构,研究不同共振腔数量和晶胞形状对声子晶体禁带特性的影响情况,发现随着共振腔个数的增加,带隙的数量和宽度也在增加;针对同种多腔散射体模型还对不同晶格排列方式进行能带特性分析,阐明不同排列方式对带隙范围的影响。为进一步验证带隙的准确性,利用有限元法和边界元法分别仿真计算六共振腔声子晶体声屏障的传递损失,得到的传递损失曲线与其带隙特性能较好的匹配上。最后利用室外试验分别验证其在三角晶格和正方形晶格排列下的隔声效果。试验证明了该新型多共振腔声子晶体声屏障在带隙范围内具有良好的降噪性能。     

一维嵌套结构声子晶体透射特性

设计新型周期嵌套结构一维声子晶体,由采用传递矩阵法所得纵波与横波透射率谱图获得较传统一维晶格结构宽数倍的巨带隙与局域模。通过研究结构参数与带隙关系,获得有利于宽带隙产生条件及对局域模影响最大因素。通过控制掺杂材料厚度及位置,能较好实现局域模的精确调控,可显著增强声子晶体低频滤波效应。通过详述研究结果的物理机理,提出准缺陷耦合及缺陷共振等概念。     

杂质吸收对一维掺杂声子晶体缺陷模的影响

为了研究杂质吸收对一维掺杂声子晶体缺陷模的影响,引入复波数,推导出一维掺杂声子晶体的转移矩阵,计算了一维掺杂声子晶体的透射系数和反射系数随衰减系数的变化特征.得出杂质的衰减系数对一维掺杂声子晶体透射波中和反射波中的缺陷模都有显著的影响.在透射波中,随着衰减系数的增加缺陷模峰高迅速降低,κ在0～0.002k范围内缺陷模比较明显.当衰减系数增加到0.005k时,缺陷模几乎消失.反射波中的缺陷模随着衰减系数的变化也有相同的特征.     

ZAO纳米粉体的制备及其在红外隐身中的应用研究

采用溶胶-凝胶法制备了铝掺杂氧化锌(ZAO)纳米粉体.使用X射线衍射仪(XRD)研究了Al2O3掺杂对ZnO物相结构和晶粒度的影响,结果表明,Al离子的掺杂并没改变ZnO的晶体结构,Al抖对Zn2 的替位掺杂使晶格发生畸变,衍射角偏离,随Al离子掺杂浓度的增加,晶体的平均粒径减小.对ZAO纳米粉体样品的红外吸收特性分析表明:随Al离子掺杂浓度的增加,红外吸收出现蓝移.理论分析得出适当调节ZAO栽流子浓度可以制备低红外发射率材料.     

杂质吸收对一维声子晶体滤波器设计的影响

为了研究杂质吸收对一维声子晶体滤波器设计的影响,引入复波数并推导出一维掺杂声子晶体的转移矩阵,计算了一维掺杂声子晶体的透射系数随衰减系数的变化特征。得出：滤波透射峰的峰值随杂质的衰减系数增加而迅速减小,滤波透射峰的半高宽随衰减系数增加而增大。滤波透射峰的峰值和半高宽都随吸收杂质的厚度的增加而减小。在设计声子晶体滤波器时,必须考虑杂质吸收这一重要因素,应选择衰减系数小于0.0005k的掺杂材料,并且杂质的厚度应小于一个波长。     

镧掺杂Sr0.4Ba0.6TiO3材料的导热性能

研究了不同镧掺杂浓度下Sr0.4Ba0.6TiO3材料的导热性能,研究镧元素对其导热性能的影响,得到了不同掺杂量下钛酸锶钡材料的热导率,材料密度随烧结温度与成型压力的增加而增大;同时发现少量镧元素的掺杂,可以增加钛酸锶钡材料的热导率;当镧元素掺杂量较大时,降低了材料的热导率.钛酸锶钡材料的电子热导率随材料烧结温度的升高基本不变,声子热导率随材料烧结温度的升高而增加.     

单元结构尺寸对不锈钢/空气二维声子晶体声波禁带的影响

使用平面波展开法,讨论了单元结构尺寸对正方形排列的不锈钢／空气二雏声子晶体声波禁带的影响。结果表明,当填充率(晶格常数)增大时,最低禁带宽度增大(减小);最低禁带中心频率基本不受散射体半径的影响,但随晶格常数的增大而降低。     

异层等价离子双掺杂策略优化BiCuSeO的热电性能

选取BiCuSeO双亚层超晶格热电材料为研究对象, 通过La、Ag单掺杂和双掺杂两种方式等价取代[Bi₂O₂]²⁺亚层和[Cu₂Se₂]^2-亚层中的Bi、Cu位点, 并对其热电输运性能和缺陷调控机理进行研究, 结果发现：La-Ag双掺杂可以结合两种单掺杂的优势, 在适度提升载流子浓度的同时保持与纯样相当的载流子迁移率, 从而使电导率得到大幅度的提升。与此同时, La-Ag双掺杂可以引发能带收敛效应, 有助于同步获得较高的载流子迁移率和Seebeck系数, 最终使PF得到了优化; 另一方面, 由于点缺陷对载热声子的强烈散射作用, 样品的晶格热导率和总热导率进一步降低, 使最终ZT值也得到了优化。结果, La-Ag双掺杂样品的ZT值在755 K下达到0.46, 高于原始纯样(ZT=0.27)和单掺杂样品。该项研究表明La、Ag异层等价双掺杂策略可以实现BiCuSeO热电输运参数的协同调控与优化。     

Vibronic Transitions of Atomic Bubbles in Condensed 4He

Laser spectroscopy of atomic dopants can be used as a tool for the investigation of elementary excitations in quantum fluids and solids. Here we present results of a laser-spectroscopic study of transition-metal (Au, Cu) atoms in liquid and solid ⁴He. In particular we observe transitions of inner shell electrons that have not been studied before. Such transitions are weakly perturbed by the interaction with the helium matrix and display a characteristic structure composed of a sharp zero-phonon line and a relatively broad phonon wing.     

Electron and phonon renormalization near charged defects in carbon nanotubes

Owing to their influence on electrons and phonons, defects can significantly alter electrical conductance, and optical, mechanical and thermal properties of a material. Thus, understanding and control of defects, including dopants in low-dimensional systems, hold great promise for engineered materials and nanoscale devices. Here, we characterize experimentally the effects of a single defect on electrons and phonons in single-wall carbon nanotubes. The effects demonstrated here are unusual in that they are not caused by defect-induced symmetry breaking. Electrons and phonons are strongly coupled in sp(2) carbon systems, and a defect causes renormalization of electron and phonon energies. We find that near a negatively charged defect, the electron velocity is increased, which in turn influences lattice vibrations locally. Combining measurements on nanotube ensembles and on single nanotubes, we capture the relation between atomic response and the readily accessible macroscopic behaviour.     

Investigation of the influence of lattice anharmonicity on the heat capacities of diamond,silicon, and germanium

The isochoric heat capacities of diamond, silicon, and germanium have been calculated ab initio using the quantum-statistical method. The phonon energy has been calculated with the Morse potential. The anharmonicity of lattice atomic vibrations is shown to occur at temperatures below the Debye temperature. Experimental data on the temperature dependences of the heat capacities of diamond, silicon, and germanium can be interpreted more correctly by taking into account the lattice anharmonicity.     

Breakdown of a gold nanowire between electrodes

Deformation behavior of an atomic Au wire placed between Au electrodes was investigated by using a generalization of the method of linear muffin-tin orbitals (LMTO) within the local-density approximation (LDA). We studied the dynamical motion of the atoms in the wire. Soft phonon modes were studied. It has been found that collective motions are dominated by motions perpendicular to the wire axis. Large displacements which resulted in wire breakage were also studied. The energy barrier associated with this process has been calculated.     

Lattice Distortions and Multiple Valence Band Convergence Contributing to High Thermoelectric Performance in MnTe

Here, a new route is proposed for the minimization of lattice thermal conductivity in MnTe through considerable increasing phonon scattering by introducing dense lattice distortions. Dense lattice distortions can be induced by Cu and Ag dopants possessing large differences in atom radius with host elements, which causes strong phonon scattering and results in extremely low lattice thermal conductivity. Density functional theory (DFT) calculations reveal that Cu and Ag codoping enables multiple valence band convergence and produces a high density of state values in the electronic structure of MnTe, contributing to the large Seebeck coefficient. Cu and Ag codoping not only optimizes the Seebeck coefficient but also substantially increases the carrier concentration and electrical conductivity, resulting in the significant enhancement of power factor. The maximum power factor reaches 11.36 µW cm⁻¹K⁻² in Mn_0.98Cu_0.04Ag_0.04Te. Consequently, an outstanding ZT of 1.3 is achieved for Mn_0.98Cu_0.04Ag_0.04Te by these synergistic effects. This study provides guidelines for developing high-performance thermoelectric materials through the rational design of effective dopants.     

Calculation of the electronic structure and exchange interaction in the InSb and GaAs semiconductors codoped with Mn and Ni

Density functional theory calculations have been used to study the electronic structure of Mn-doped, Ni-doped, and Mn/Ni-codoped InSb and GaAs semiconductors. The ferromagnetic transition energy has been calculated using a multiscale method in which exchange interaction is calculated by the Hartree–Fock exact atomic method and is then included as a Hubbard parameter in calculation of the electronic structure of the material. The present calculation results demonstrate that, in all cases, there is hybridization of the impurity d states with the valence band of the host semiconductor. The contributions of the Ni and Mn dopants are approximately additive.     

One- and two-phonon Raman scattering from hydrogenated nanocrystalline silicon films

One- and two-phonon Raman scattering from intrinsic and boron as well as phosphorus doped hydrogenated nanocrystalline silicon films prepared by plasma enhanced chemical vapor deposition technique were investigated. With regard to one-phonon Raman measurements of intrinsic films, redshifts attending by asymmetrical broadening of one-phonon transverse optical (TO) mode with diminishing mean dimension of Si nanocrystals can be ascribed to incorporating effects of phonon confinement and tensile strain. Photoluminescence behavior of these intrinsic specimens can be interpreted by a consistent way when recombination of quantum-confined excitons in Si nanocrystals is assumed. As to one-phonon Raman signals of doped nc-Si:H materials, besides joint effects of phonon confinement and tensile strain, additional redshifts accompanying with asymmetrical broadening of one-phonon TO band with increasing doping level can be assigned to carrier effect and disorder from doping. With diminishing average size of Si nanocrystals or increasing dopants, the decay of two-phonon Raman amplitudes of intrinsic and doped samples can be attributed to disorder. Raman experiments indicate that all the energies of the two-phonon TO branches are different from twice the energies of the one-phonon TO active bands, which reveal that the two-phonon TO modes do not come from the Raman active phonons at wavevector k=0. The peak shift of two-phonon transverse optical (2TO) modes relates to phonon confinement and disorder. Negligible peak shift in TO (2TO) modes of intrinsic and doped films on temperature indicates that the interface strain in nc-Si:H/c-Si can be ignored.     

Lattice dynamics of Ga1−xAlxAs studied by ab initio calculations

We have calculated the phonon dispersion and phonon density of states of superlattices of Ga_1−xAl_xAs for x=0.0, 0.25, 0.75 and 1.0 using the ab initio method within the supercell approach and calculating the Hellmann–Feynman forces. A noticeable difference of force constants between Ga–As and Al–As atomic pairs has been found. In any case, Al atoms vibrate in well-separated high-frequency optic modes.     

Large bipolarons and superconductivity

Superconductivity has long been speculated to result from charge carriers paired as mobile charged bosons. Although the pairing of carriers as small (single-site) bipolarons is known, small bipolarons readily localize. By contrast, large (multi-site) bipolarons, in analogy with large polarons, should be mobile. It is shown that large bipolarons can form in solids with very displaceable ions, e.g., many oxides. Large-polaronic (but not small-polaronic) carriers produce absorption spectra like the carrier-induced absorptions observed in cuprates. Redistribution of the self-trapped carriers of large bipolarons among sites of carriers' molecular orbitals in response to atomic motions lowers phonon frequencies. The dependence of the phonon zero-point energy on the spatial distribution of large bipolarons produces a phonon-mediated attraction between them. This dynamic quantum-mechanical attraction fosters the condensation of large bipolarons into a liquid. Superconductivity can result when the large-bipolarons' groundstate remains liquid rather than solidifying.     

From thermal conductive to thermal insulating:Effect of carbon vacancy content on lattice thermal conductivity of ZrCx

Lattice thermal conductivities of zirconium carbide(ZrCx,x=1,0.75 and 0.5)ceramics with different car-bon vacancy concentrations were calculated using a combination of first-principles calculations and the Debye-Callaway model.The Grüneisen parameters,Debye temperatures,and phonon group velocities were deduced from phonon dispersions of ZrCx determined using first-principles calculations.In addi-tion,the effects of average atomic mass,grain size,average atomic volume and Zr isotopes on the lattice thermal conductivities of ZrCx were analyzed using phonon scattering models.The lattice thermal con-ductivity decreased as temperature increased for ZrC,ZrC0.75 and ZrC0.5(Zr2C),and decreased as carbon vacancy concentration increased.Intriguingly,ZrCx can be tailored from a thermal conducting material for ZrC with high lattice thermal conductivity to a thermal insulating material for ZrC0.5 with low lattice thermal conductivity.Thus,it opens a window to tune the thermal properties of ZrCx by controlling the carbon vacancy content.