A composite theoretical model for the thermal conductivity of nanocrystalline materials

In order to study the thermal conductivity of nanocrystalline (NC) materials, a two-phase composite model consisting of grain interior (GI) regarded as an ordered crystal phase and plastically softer grain boundary-affected zone (GBAZ) phase was presented. The effects of GI and GBAZ on thermal conduction were considered, respectively. In this work, time independent Schrodinger’s wave equation (TISWE) was used to study the carriers’ transmission in a crystal particle, through which we can get the thermal conductivity of the GBAZ. The thermal conductivity of GI was calculated based on a kinetic theory. The whole effective grain thermal conductivity was simulated by a modified formula for composite materials. The results showed that as the grain size decreases to 80 nm, it has a strong size effect, and the thermal conductivity decreases with the decreasing of grain size.     

Study on the effect of temperature rise on grain refining during fabrication of nanocrystalline copper under explosive loading

Nanocrystalline (NC) copper was fabricated by severe plastic deformation of coarse-grained copper at a high strain rate under explosive loading. The feasibility of grain refinement under different explosive loading and the influence of overall temperature rise on grain refinement under impact compression were studied in this paper. The calculation model for the macroscopic temperature rise was established according to the adiabatic shock compression theory. The calculation model for coarse-grained copper was established by the Voronoi method and the microscopic temperature rise resulted from severe plastic deformation of grains was calculated by ANSYS/ls-dyna finite element software. The results show that it is feasible to fabricate NC copper by explosively dynamic deformation of coarse-grained copper and the average grain size of the NC copper can be controlled between 200～400 nm. The whole temperature rise would increase with the increasing explosive thickness. Ammonium nitrate fuel oil explosive was adopted and five different thicknesses of the explosive, which are 20 mm, 25 mm, 30 mm, 35 mm, 45 mm, respectively, with the same diameter using 20 mm to the fly plate were adopted. The maximum macro and micro temperature rise is up to 532.4 K, 143.4 K, respectively, which has no great effect on grain refinement due to the whole temperature rise that is lower than grain growth temperature according to the high pressure melting theory.     

爆炸载荷下纳米晶铜晶粒度分布及影响因素研究

 采用爆炸动态加载使粗晶铜发生高应变率塑性大变形的方法制备了纳米晶铜。利用X射线衍射法对其晶粒度进行了检测,借助于LS-DYNA3D非线性有限元程序对试样变形过程进行了数值模拟,在此基础上对应变和应变率进行了统计,分析了宏、细观应变对晶粒细化程度的影响。结果表明：采用爆炸加载法可制备出纳米晶铜,平均晶粒度范围可有效控制在100 nm以内;爆炸加载过程中应变率高达10⁴ s^-1,应变的提高有利于晶粒细化;在爆炸加载方向晶粒度成不均匀分布。     

First-principles investigations on elastic,thermodynamic and lattice thermal conductivity of topological insulator LaAs

Topological insulators are always a hot topic owing to their various peculiar physical effects, which are useful in spintronics and quantum information processing. Herein, we systematically investigate the elastic, thermodynamic and lattice thermal conductivity of a new typical topological insulator LaAs by combining the first-principles approach and an iterative solution of the Boltzmann transport equation. The obtained elastic constants and other lattice structural parameters of LaAs are well consistent with the experimental and other theoretical results. For the first time, the lattice thermal conductivity (5.46 W/(m?K)) and mean free path (14.4 nm) of LaAs are obtained,which manifests that the LaAs is more likely to be a desirable thermoelectric material. It is noted that the obtained mode-averaged Grüneisen parameters by different ab initio simulation packages are very similar, suggesting that our results are rather responsible. From the phonon scattering rates of LaAs, we speculate that the reduction of acoustic-optical gap and the larger phonon scattering may jointly result in reduction of thermal conductivity for LaAs. Meanwhile, the temperature dependence curves of the lattice thermal conductivity, heat capacity and phonon mean free path are also presented. We expect our work can provide more information for further experimental studies.     

CoSb3纳米热电材料的制备及热传输特性

以Sb，Co为起始原料，采用固相反应法合成了CoSb3.通过高能球磨制得CoSb3纳米粉末，用放电等离子烧结(SPS)方法制备出最小平均晶粒尺寸为150nm的块体材料.研究了晶粒尺寸与热传输性能之间的关系：CoSb3化合物结构纳米化对其晶格热导率κL有显著影响，当晶粒尺寸由微米尺度减小到纳米尺度，晶格热导率κL显著降低，但对载流子热导率κc的影响不甚显著.CoSb3化合物的热导率κ随晶粒尺寸的减小而降 低主要是由于晶格热导率κL随晶粒尺寸的减小而降低所致. 关键词： 纳米 Skutterudite 制备 热传输特性     

Thermally stable coherent domain boundaries in complex-structured Cr2Nb intermetallics

The strength and hardness of nanostructured materials are significantly enhanced owing to the large amount of grain boundaries (GB) produced by a reduced grain size. The thermal stability of the GB is a key to maintaining the grain size and thus the strength/hardness in nanostructured materials at high temperatures. In this work, coherent domain boundaries (DB) were introduced by compressive processing to sub-divide a complex-structured intermetallic Cr₂Nb into nanograins of size down to 2 nm. These DB persisted after an annealing of 10 h at 1273 K. The coherent DB have been investigated by aberration-corrected high-resolution transmission electron microscopy and first-principles calculations. The high thermal stability is evidently a result of low formation energies of the DB.     

纳晶铜晶粒尺寸对热导率的影响

用热压烧结法制备得到纳晶铜块体. 用激光法测定了不同温度下制备得到的纳晶铜块体的热导率, 并建立卡皮查热阻模型对样品热导率进行模拟. 通过对比, 模拟结果与实验数据基本一致. 随着热压烧结温度的升高, 纳晶铜晶粒尺寸也随之增大. 在900和700 ℃其热导率分别达到了最大和最小值且所对应的热导率分别为200.63和233.37 W·m^-1·K^-1, 各占粗晶铜块体热导率的53.4%和60.6%. 验证了纳晶铜热导率在一定的晶粒尺寸范围内具有尺寸效应, 随着晶粒尺寸的减小, 热导率逐渐减小.     

Microstructure and thermostability of a W–Cu nanocomposite produced via high-pressure torsion

A coarse-grained W–25% Cu alloy is subjected to high-pressure torsion (HPT) at room temperature to different strains. Evolution of the microstructure during HPT processing is studied using X-ray diffraction analysis, scanning and transmission electron microscopy. It is demonstrated that HPT processing results in fragmentation of the tungsten particles and the formation of a 5–15?nm grain size nanostructure at equivalent strains of ≥256 (saturation). It is shown that the nanostructured W–25% Cu is thermostable up to 500°C, with grain growth up to 50?nm at 720°C. During HPT processing, the lattice parameter of the copper and tungsten was found to increase and decrease, respectively, with increased level of equivalent strain. This is proposed to occur through the interdiffusion of copper atoms into tungsten grains and tungsten atoms into copper grains, as suggested by energy-dispersive X-ray analysis of the individual grains. The formation of a limited solid solution is considered and possible mechanisms for this effect discussed.     

晶粒尺寸对CoSb3化合物热电性能的影响

系统地研究了晶粒尺寸对CoSb₃化合物热电性能的影响规律，结果表明晶粒尺寸对CoSb₃化合物的晶格热导率κ_p、电导率σ、能隙宽度E_g和Seebeck系数α有显著影响.当晶粒尺寸由微米尺度减小到纳米尺度时，晶格热导率κ_p显著降低，Seebeck系数α有较大幅度的增加，能隙宽度E_g变宽，电导率σ有一定程度的下降.平均晶粒尺寸为200nm的CoSb₃化合物在温度为700K时，ZT值达到0.43，比平均晶粒尺寸为5000nm的试样增加了4倍.     

Effect of copper addition on some properties of rapidly solidified lead-free Sn–-10 wt% Zn alloys

This study aimed at investigating the effect of adding copper (Cu) on some properties of the lead-free alloys which rapidly solidified from melt. X-ray analysis, hardness, elastic modulus, electrical conductivity and resistivity were studied. The results indicated that the alloy hardness and elastic modulus improved by increasing the copper (Cu) content and decreasing the zinc (Zn) content. The electrical conductivity ranged from 0.250 to 0.847?×?10⁷ ohm^?1 m^?1 for the alloy under study. The electrical resistivity increases linearly with temperature until the melting point is reached. The residual resistivity results from disturbances in the lattice rather than caused by thermal vibration and the most drastic increases in the residual resistivity are caused by foreign atoms in solid solution with matrix metal. The electrical resistivity values ranged from 11.8 to 40?×?10^?8 ohm m, when the copper content changed from 0.0 to 2.0 wt% and zinc changed from 8.0 to 10.0 wt%.     

Sintering and electrical conductivity of gadolinia-doped ceria

Bulk specimens of Ce_0.9Gd_0.1O_2-δ prepared with powders within a range of specific surface area were sintered in oxidizing, inert, and reducing atmospheres. The aim of this work is to investigate the effects of the sintering atmosphere on the microstructure and grain and grain boundary conductivities of the solid electrolyte. The lattice parameter determined by Rietveld refinement is 0.5420(1) nm, and the microstrain was found negligible in the powder materials. Specimens sintered in the Ar/4 % H₂ mixture display larger average grain sizes independent on the particle size of the starting powders. The grain and grain boundary conductivities of specimens sintered under reducing atmosphere are remarkably lower than those sintered under oxidizing and inert atmospheres. The activation energy (～0.90 eV) for total electrical conductivity remains unchanged with both the initial particle size and the sintering atmosphere.     

The linear thermal expansion of bulk nanocrystalline Al and SS304 at low temperature

The linear thermal expansions (LTE) of bulk nanocrystalline (NC) Al, 304 stainless steel (SS304) and their conventional coarse-grained poly-crystalline counterparts (CCPC) were studied by the strain gage method in four directions within rolling plane (for bulk NC SS304 and Al) and one direction (for their CCPC) from liquid nitrogen temperature to 300 K. LTE of bulk NC Al and SS304 in four directions were equal to or less than those of their CCPC. This result was different from the fact that the smaller thermal expansions of many other bulk nanocrystalline materials were larger than those of their CCPC. We had to conclude that the less linear thermal expansions of bulk NC SS304 and Al were attributed to their larger defects at grain boundaries and residual strain. However, the larger linear thermal expansions of many other NC materials resulted from two factors.     

Quick preparation and thermal transport properties of nanostructured β-FeSi2 bulk material

This paper reports that the nanostructured β-FeSi2 bulk materials are prepared by a new synthesis process by combining melt spinning(MS) and subsequent spark plasma sintering(SPS).It investigates the influence of linear speed of the rolling copper wheel,injection pressure and SPS regime on microstructure and phase composition of the rapidly solidified ribbons after MS and bulk production respectively,and discusses the effects of the microstructure on thermal transport properties.There are two crystalline phases(α-Fe2Si5 and ε-FeSi) in the rapidly solidified ribbons;the crystal grains become smaller when the cooling rate increases(the 20 nm minimum crystal of ε-FeSi is obtained).Having been sintered for 1 min above 1123 K and annealed for 5 min at 923 K,the single-phase nanostructured βFeSi2 bulk materials with 200-500 nm grain size and 98% relative density are obtained.The microstructure of β-FeSi2 has great effect on thermal transport properties.With decreasing sintering temperature,the grain size decreases,the thermal conductivity of β-FeSi2 is reduced remarkably.The thermal conductivity of β-FeSi2 decreases notably(reduced 72% at room temperature) in comparison with the β-FeSi2 prepared by traditional casting method.     

Influence of grain size distribution on the lattice thermal conductivity of Bi2Te3-Sb2Te3-based nanostructured materials

The change of the lattice thermal conductivity of bulk nanostructured materials based on Bi₂Te₃-Sb₂Te₃ solid solutions with grain size distribution has been studied. These materials have a polycrystalline structure with grain sizes ranging from a few tens of a nanometer to a few micrometers. Large grains may contain inclusions or consist of several smaller parts which can be identified with coherent scattering regions seen in X-ray diffraction. The change of the lattice thermal conductivity mediated by additional scattering by inclusions and grain boundaries has been calculated. This calculation allows for the effect of nanoparticle size distribution. The calculated estimates are compared with the available experimental data.     

Anomalous strain effect on the thermal conductivity of borophene: a reactive molecular dynamics study

Borophene, an atomically thin, corrugated, crystalline two-dimensional boron sheet, has been recently synthesized. Here we investigate mechanical properties and lattice thermal conductivity of borophene using reactive molecular dynamics simulations. We performed uniaxial tensile strain simulations at room temperature along in-plane directions, and found 2D elastic moduli of 188 N m⁻¹ and 403 N m⁻¹ along zigzag and armchair directions, respectively. This anisotropy is attributed to the buckling of the borophene structure along the zigzag direction. We also performed non-equilibrium molecular dynamics to calculate the lattice thermal conductivity. Considering its size-dependence, we predict room-temperature lattice thermal conductivities of 75.9 ± 5.0 W m⁻¹ K⁻¹ and 147 ± 7.3 W m⁻¹ K⁻¹, respectively, and estimate effective phonon mean free paths of 16.7 ± 1.7 nm and 21.4 ± 1.0 nm for the zigzag and armchair directions. In this case, the anisotropy is attributed to differences in the density of states of low-frequency phonons, with lower group velocities and possibly shorten phonon lifetimes along the zigzag direction. We also observe that when borophene is strained along the armchair direction there is a significant increase in thermal conductivity along that direction. Meanwhile, when the sample is strained along the zigzag direction there is a much smaller increase in thermal conductivity along that direction. For a strain of 8% along the armchair direction the thermal conductivity increases by a factor of 3.5 (250%), whereas for the same amount of strain along the zigzag direction the increase is only by a factor of 1.2 (20%). Our predictions are in agreement with recent first principles results, at a fraction of the computational cost. The simulations shall serve as a guide for experiments concerning mechanical and thermal properties of borophene and related 2D materials.     

Optimal design of coating material for nanoparticles and its application for low-temperature interconnection

Metal nanoparticles need coating material so as to avoid aggregating to each other. On the contrary, there are occasions when the coating materials are required to be removed. Here, a theoretical model to relate removability of coating materials to their molecular structure is suggested. The model is used to find an optimum coating material, secondary amine, for use in low-temperature interconnection material. An interconnection made of methyloctylamine-coated silver nanoparticles was formed between a pair of copper electrodes by heating and pressurizing the nanoparticles and electrodes at 250 °C and 2.5 MPa, respectively, for 150 s. Shear strength and thermal conductivity of the formed interconnection were 17.8 MPa and 219 W/mK, respectively. This thermal conductivity value is greater than that obtained using Pb–Sn and silver solders.     

铝纳米颗粒的热物性及相变行为的分子动力学模拟

采用分子动力学方法模拟了纳米金属铝在粒径为0.8-3.2 nm 时的熔点、密度和声子热导率的变化, 研究了粒径为1.6 nm的铝纳米颗粒的密度、比热和声子热导率随温度的变化. 采用原子嵌入势较好地模拟了纳米金属铝的热物性及相变行为, 根据能量-温度曲线和比热容-温度曲线对铝纳米颗粒的相变温度进行了研究, 并利用表面能理论、尺寸效应理论对铝纳米颗粒熔点的变化进行了分析. 随着纳米粒径的不断增大, 铝纳米颗粒的熔点呈递增状态, 当粒径在2.2-3.2 nm时, 熔点的增幅减缓, 但仍处于递增趋势. 随着纳米粒径的增大, 铝纳米颗粒的密度呈单调递减, 热导率则呈线性单调递增, 且热导率的变化情况符合声子理论. 随着温度的升高, 粒径为1.6 nm的铝纳米颗粒的密度、热导率均减小. 该模拟从微观原子角度对纳米材料的热物性进行了研究, 对设计基于铝纳米颗粒的相变材料具有指导意义.     

高压烧结镀Cr、Ti膜金刚石/铜复合材料热导率研究

 采用磁控溅射方法,在金刚石表面镀覆活性金属Cr膜和Ti膜,在高温高压下合成了镀活性金属膜金刚石/铜复合材料。实验发现,活性金属的加入增强了金刚石与铜界面间的结合强度,减少了界面热阻,提高了复合材料的热导率。复合材料热导率随着金刚石体积分数的增加而降低,随着金刚石的粒度增大而提高。这主要是由界面热阻引起的,可以通过增大金刚石粒度和改善界面状态来提高复合材料热导率。     

晶体相场法模拟纳米晶材料反霍尔-佩奇效应的微观变形机理

采用晶体相场模型模拟获得了平均晶粒尺寸从11.61–31.32 nm的纳米晶组织, 研究了单向拉伸过程纳米晶组织的强化规律的微观变形机理. 模拟结果表明: 晶粒转动、晶界迁移等晶间变形行为是纳米晶材料的主要微观变形方式, 纳米晶尺寸减小, 有利于晶粒转动, 使屈服强度降低, 显示出反霍尔-佩奇效应.当纳米晶较小时, 变形量超过屈服点达到4%, 位错运动开启, 其对变形的直接贡献有限, 主要通过改变晶界结构而影响变形行为, 位错运动破坏三叉晶界, 引发晶界弯曲, 促进晶界迁移. 随纳米晶增大, 晶粒转动困难, 出现晶界锯齿化并发射位错的现象. 关键词： 晶体相场 纳米晶 反霍尔-佩奇效应 微观变形     

纳米结构碲化铋合金的制备及电热输运特性

采用惰性气体保护蒸发-冷凝法制备了纳米Bi及Te粉末, 结合机械合金化和放电等离子烧结技术, 在不同烧结温度下制备出了单一物相且具有纳米层状结构及孪晶亚结构的n型Bi₂Te₃块体材料, 并系统研究了块体材料的晶粒尺度、微结构及其对电热传输特性的影响. SEM, TEM分析结果表明, 以纳米粉末为原料, 通过有效控制工艺条件, 可以制备出具有纳米层状结构Bi₂Te₃合金块体材料, 同时纳米层状结构中存在孪晶亚结构; 热电性能测试结果表明, 具有纳米层状结构及孪晶亚结构的块体试样与粗晶材料相比, 热导率大幅度降低, 在423 K附近, 热导率由粗晶材料的1.80 W/mK降至1.19 W/mK, 晶格热导率从1.16 W/mK降至0.61 W/mK, 表明纳米层状结构与孪晶亚结构共存, 有利于进一步提高声子散射, 降低晶格热导率. 其中在693 K放电等离子烧结后的试样于423K附近取得最大值的无量纲热电优值(ZT), 达到0.74.