金属Zr热力学性质的第一原理研究

应用基于密度泛函与密度泛函微扰理论的平面波赝势方法计算了一组不同晶格常数下hcp结构金属Zr的声子谱及相应的静态总能,由此得到不同晶格常数下的自由能,由准谐近似及自由能极小判据得到自由能与温度的关系,进而计算了金属Zr热膨胀系数、体弹性模量、定容及定压摩尔热容与温度的关系,计算结果在较宽的温度范围内与实验相符.对热膨胀系数,同时应用Debye-Grüneisen模型进行了计算,并与第一原理方法的结果进行了比较.对摩尔热容的计算考虑了电子的贡献,表明在高温区电子热容的贡献不能忽略.     

PVD导热涂层的研究综述

首先从导热涂层的应用背景出发,分析了导热涂层研究的必要性,其次探讨了导热涂层的导热机理和影响涂层导热的宏观和微观因素。在此基础上,阐述了PVD导热涂层的研究现状,重点分析了Si C、AlN、DLC三种常见的具有较大应用潜力的PVD导热涂层。声子散射是影响涂层热导率的直接原因,涂层内部同位素、杂质、缺陷及晶界等均会引起声子发生散射,而界面声子散射引起的界面热阻对涂层导热性能影响巨大,通过合理选择制备技术和精确控制工艺参数,在一定程度上能改善涂层的导热性能,提高热导率。在此基础上,笔者提出了离子源辅助高功率脉冲磁控溅射(HiPIMS)的工艺配合,提高涂层质量和致密度,优化界面结构,降低界面热阻,以期实现涂层的高导热性能。     

SnxBi0.5-xSb1.5Te3热电材料的组织与性能研究

通过真空熔炼、球磨制粉、冷压成形和常压烧结制备具有高热电优值的p型SnxBi0.5-xSb1.5Te3热电材料。研究了Sn含量对SnxBi0.5-xSb1.5Te3热电材料晶体结构、微观形貌和热电性能的影响。结果表明：SnxBi0.5-xSb1.5Te3热电材料晶体结构为R-3m空间群斜方晶系的六面体层状结构;添加合金元素Sn,Bi0.5Sb1.5Te3基热电材料产生大量的纳米结构缺陷。合金元素Sn含量增加, SnxBi0.5-xSb1.5Te3热电材料载流子浓度和DOS有效质量增加,有效地提高电导率和功率因子;同时声子散射增强,显著地降低晶格热导率。在300K时,Sn0.015Bi0.485Sb1.5Te3的功率因子达3.10 mW?m-1?K-2,晶格热导率为0.358 W?m-1?K-1,ZT值为1.25。并且在300～400 K温度范围内,Sn0.015Bi0.485Sb1.5Te3的ZT值为1.25～1.33。     

中子三轴谱仪的应用

中子三轴谱仪是最早被发明,同时也是应用最广泛的中子散射谱仪之一。它利用了单色器、样品、和分析器3个转动轴的原理,使测量时可以直观地观测倒空间和能量空间的某一个点的散射性质,因此非常适合研究固体中的各种元激发。简单介绍了三轴谱仪的基本原理,然后给出一些三轴谱仪应用的例子。在这些例子中,既包括弹性散射也包括非弹性散射,例如铜单晶中的声子谱,非常规超导体中的磁共振峰,PrOs4As12中的晶格场,以及单层锰氧化合物Pr1-xCa1＋xMnO4中的漫散射。     

W热力学性质和弹性性质的第一性原理研究

使用第一性原理方法结合准谐近似理论研究了立方结构钨的热力学性质,包括平衡体积V、体弹模量B0、线膨胀系数α、熵S、振动自由能F、等压热容CP和等容热容CV随温度的变化关系。在计算体系的线膨胀系数、熵、振动自由能、等压热容和等容热容时考虑了热电子和热振动对自由能的贡献。计算结果表明：考虑热电子对自由能贡献后得到的线膨胀系数、熵、振动自由能和等压热容在0~2000 K范围内均与实验值符合较好。在得到平衡体积随温度变化的基础上,计算了钨立方结构的弹性性质,得到了弹性常数、体积模量、剪切模量和杨氏模量随温度的变化关系,所得结果与实验测量值符合较好     

fcc过渡族金属晶格动力学的改进分析型EAM模型计算

应用本研究组发展的改进分析型EAM多体势 ,由准谐和近似计算方法具体计算了 8种fcc过渡族金属 (Ag ,Au ,Cu ,Ir ,Ni,Pd ,Pt,Rh)和Al的 [10 0 ],[110 ]和 [111]3个方向的声子谱和晶格摩尔热容 ,并将计算结果与实验结果进行了比较。结果表明 :声子谱在低频率处与实验结果符合得很好 ,在高频率处有偏差 ;计算的晶格摩尔热容与实验结果符合良好。所采用的EAM多体势能较好的反映fcc金属原子间的相互作用。     

晶界几何结构对纳晶ZnO材料导热过程的影响

为了探究晶界几何结构对纳晶ZnO材料导热性能的影响,将晶界表面抽象出几种典型几何形状,深入讨论了晶界表面粗糙度的计算以及声子入射角对镜面反射率的影响,改进了晶界镜面反射率的计算模型.采用PhonTS软件,用迭代法求解玻尔兹曼输运方程模拟计算得到了纳晶ZnO晶格热导率.基于分子动力学理论计算了ZnO完美材料的热导率,分析...     

D03结构Fe3Al合金晶格动力学的嵌入原子模型计算

应用改进分析型嵌入原子模型，系统地计算了D03结构Fe3Al合金的晶格动力学性能，包括晶格常数、形成热、弹性常数、声子谱、态密度、摩尔定容热容、德拜温度和热膨胀系数等，并将计算结果与已有实验数据和其他理论计算结果进行了比较。计算所得声子谱相对于其他理论计算结果而言，与实验值的符合程度更好；计算的热膨胀量在低温区与实验结果基本一致，而在高温区明显偏低于实验结果。     

Heusler化合物热电器件

<正>日本产业技术综合研究所三上祐史采用机械合金化法制造Fe2VAl合金粉末,用通电烧结法制出烧结体,其热导率降低到18W/mK,约为电弧冶炼法试块的60%,这是本方法制得试样的晶界声子散射使热导率降低之故。若采用重元素W置换V,可使热导率降低50%,对电导率也影响不大。Fe2VAl烧结体通过     

Quantized Acoustic-Phonon Modes in Metallic Nanowire Structures

对金属纳米线结构中量子化的声学声子模式进行了详细的理论研究。利用柱形纳米线结构来描述以氧化铝为模板制备的金属镍纳米线。考虑器件结构中弹性声波对应的边界及连续条件,求解弹性波波动方程,可以得到纳米线结构中量子化的声学声子模式。发现量子化的声子能量和不同声子态之间的能量差会随着纳米线半径的减小而增大。在色散关系中,允许的声子模式位于qzVAl2O3这条直线的下方,这一特征不随纳米线半径的变化而改变。研究结果表明,金属纳米线系统在高频超声器件中具有潜在的应用前景     

Thermal conductivity of electron-doped CaMnO3 perovskites: Local lattice distortions and optical phonon thermal excitation

The thermal transport properties of a series of electron-doped CaMnO₃ perovskites have been investigated. Throughout the temperature range 5–300 K, phonon thermal conductivity is dominant, and both electron and spin wave contributions are negligible. The short phonon mean free paths in this system result in the relatively low thermal conductivities. The strong phonon scatterings stem from the A-site mismatch and bond-length fluctuations induced by local distortions of MnO₆ octahedra. The thermal conductivity in the magnetically ordered state is enhanced as a result of the decrease in spin–phonon scattering. The results also indicate that above the magnetic ordering temperature, observable thermal excitation of optical phonons occurs. The contribution of optical phonons to thermal conductivity becomes non-negligible and is proposed to play an important role in the glass-like thermal transport behavior (i.e. positive temperature dependence of the thermal conductivity) in the paramagnetic state. These features can be understood in terms of an expression of thermal conductivity that includes both acoustic and optical phonon terms.     

Thermophysical properties of rare-earth stannates: Effect of pyrochlore structure

Compounds with an A₂B₂O₇-type pyrochlore structure have been identified as a class of materials with low thermal conductivity. To ascertain the effect of the pyrochlore structure, the thermophysical properties of rare-earth stannates which constitute the longest isostructural series with this unique crystal structure were measured and the thermal conduction behavior was investigated by analyzing the variation in the inverse phonon mean free path with temperature in terms of the phonon-scattering theory. The results show that the contraction of rare-earth ions and the accompanying deviation from the ideal pyrochlore structure result in a simultaneous increase in the Young’s modulus and the thermal expansion coefficient. Scattering due to the strain field fluctuation caused by the systemic displacement of 48f oxygen ions rather than scattering from the so-called 8a site “oxygen vacancies” and thermal defects predominates in reducing the phonon mean free path and consequently the thermal conductivity at relative low temperature. At a certain critical temperature, the thermal conduction behavior of the stannates undergoes a transition to a minimum phonon mean free path mechanism, which is responsible for the remarkable variation in temperature dependence of thermal conductivity across the compositions.     

Effects of Sn-doping on the electrical and thermal transport properties of p-type Cerium filled skutterudites

p-type Sn-doped CoSb₃-based skutterudite compounds have been prepared using melting-quenching-annealing method and spark plasma sintering technique. Sn atoms in our samples are completely soluted on Sb-site with a fixed charge state and non-magnetic feature, providing a better choice to ascertain the effect of element doping at the [Co₄Sb₁₂] framework on the electrical and thermal transport properties in p-type skutterudites. Doping Sn at the framework introduces additional ionized impurity scattering to affect the electron transport greatly. Similar electrical transport properties between Ce_0.2Co₄Sb_11.2Sn_0.8 and Co₄Sb₁₁Sn_0.6Te_0.4 suggest that Ce fillers contribute little to the valence band edge. Filling Ce into the voids and doping Sn at the framework introduce additional phonon resonant and point defect scattering mechanisms, thereby reducing lattice thermal conductivity remarkably. Moreover, our data suggest that combining these two effects is more effective to suppress lattice thermal conductivity through scattering broad range of phonons with different frequencies.     

Thermoelectric properties of the hot-pressed Bi2(Te0.85Se0.15)3 alloy with addition of BN powders

Thermoelectric properties of the hot-pressed Bi₂(Te_0.85Se_0.15)₃ alloy were investigated with the addition of BN powders as phonon scattering centers. The Seebeck coefficient and electrical resistivity of the alloy increased as the volume fraction of BN increased. Although the thermal conductivity of the alloy decreased as the volume fraction of BN increased due to the reduction of K_el, the lattice thermal conductivity varied little. The figure-of-merit of the alloys, 1.6×l0^-3/K without the addition of BN, decreased as the volume fraction of BN increased because the increment of the electrical resistivity was much larger than the decrement of the thermal conductivity due to grain refinement.     

The effects of temperature and composition on the thermal conductivities of [(ZrO2)1−x(CeO2)x]0.92(Y2O3)0.08 (0 ? x ? 1) solid solutions

The thermal conductivities of [(ZrO₂)_1−x(CeO₂)_x]_0.92(Y₂O₃)_0.08 (0 ? x ? 1) solid solutions are studied in this paper. The incorporation of ZrO₂ and CeO₂ in the solid solution decreases the thermal conductivity compared with their end members (YSZ and YDC). The thermal conductivities of the solid solutions show clearly different temperature dependences in the ZrO₂-rich (0 ? x ? 0.5) region and in the CeO₂-rich region (0.5 ? x ? 1). The composition and the temperature dependence of the thermal conductivities are discussed based on established phonon scattering theories. We have concluded that the composition dependence of the thermal conductivity of this system is mainly controlled by the mass difference between Zr⁴⁺ and Ce⁴⁺, while the thermal conductivity-temperature relationship is dominated by the randomness of the defect distribution.     

Influence of phonon focusing on the Knudsen flow of phonon gas in single-crystal nanofilms of spintronic materials

Influence of the anisotropy of elastic energy on the phonon transport has been investigated in single- crystal nanofilms of Fe, Cu, MgO, InSb, and GaAs materials used for spintronic instruments and devices in the Knudsen flow regime of phonon gas. The dependences of the lattice thermal conductivity and lengths of free paths of phonons for all acoustic modes on the geometric parameters of the films have been considered for low temperatures with the dominance of the diffuse scattering of phonons at the boundaries. Physical aspects of the propagation of phonon modes in the films have been analyzed. It has been shown that the anisotropy of phonon transport in single-crystal films is due to the features of the propagation of phonon modes in elastically anisotropic films with a different relationship of the geometric parameters. The directions of heat flow and orientations of the film planes that yield the maximum and minimum thermal conductivity of phonons in film planes have been determined.     

Electronic structure and thermal properties of doped CaMnO3 systems

The electronic and thermal properties of hole (Na) and electron (Ga) doped CaMnO₃ systems are investigated based on the first principle density functional theory calculations using plane wave basis and pseudo-potential method. A semiconductor-to-conductor transition and a distorted band structure are found for the doped systems; enhanced density of states near Fermi level is observed. The phonon transfer speed and the phonon mean free path are lowered; meanwhile, the phonon specific heat is heightened in comparison with that of the undoped CaMnO₃ system, resulting in enhanced phonon thermal conduction. The calculation results indicate that the doped systems should have improved thermoelectric performance.     

纳米BaTiO3复合Cu1.8S块体的热电性能和力学性能研究

Cu_1.8S作为一种P型半导体热电材料,具有环境友好、原料丰富、价格低廉等优点而受到广泛关注。本研究采用机械合金化(Mechanical Alloying, MA)结合放电等离子烧结(Spark Plasma Sintering, SPS)工艺制备了一系列Cu_1.8S-x wt%BaTiO₃ (x =0,0.075,0.1,0.15,0.2)块体材料,研究了复合纳米BaTiO₃对Cu_1.8S的相结构、微观形貌、热电性能及力学性能的影响。结果表明,纳米BaTiO₃的加入不影响Cu_1.8S的相结构、晶胞参数和载流子浓度;纳米BaTiO₃均匀分布在Cu_1.8S基体的晶界处产生钉扎效应进而细化晶粒并产生气孔。Cu_1.8S-0.2 wt%BaTiO₃样品在773 K时获得最低的热导率2.2 Wm^_-1K^_-1,所有样品的ZT值基本保持不变约0.39 (773 K)。同时Cu_1.8S-x wt%BaTiO₃块体样品的维氏硬度由82 (x = 0)增加到87 (x = 0.2)。本研究表明在Cu_1.8S中复合纳米BaTiO₃可以在不影响材料热电性能的前提下有效提升块体样品的力学性能,为后续Cu-S体系热电性能和力学性能的协同提升提供了思路,有利于制备高机械性能且稳定耐用的Cu-S体系的热电器件。     

Effect of lattice defects on thermal conductivity of Ti-doped,Y2O3-stabilized ZrO2

The evolution of lattice structure and thermal conductivity has been studied systematically for a range of Ti-doped, Y₂O₃-stabilized ZrO₂ (YSZ) solid solutions. The mechanism of reducing the thermal conductivity by Ti doping has been determined. Ti⁴⁺ mainly substitutes for Zr⁴⁺ below a critical composition factor (x ? 0.08), above which the interstitial Ti⁴⁺ need to be considered separately. The effect of lattice defects caused by mass and radius differences between Ti⁴⁺ and Zr⁴⁺ ions on the phonon scattering coefficient was discussed quantitatively. And the reduction of oxygen vacancy by interstitial Ti⁴⁺ ions which increases the thermal conductivity at high Ti doping content was also determined. Concerning the integrated phase stability and thermo-mechanical properties, Ti-doped YSZ is believed to be a promising candidate for thermal barrier coatings at higher temperature.