A review on three‐dimensional graphene: Synthesis,electronic and biotechnology applications‐The Unknown Riddles

In the last decade, carbon‐based nanostructures such as buckyball (C₆₀), carbon nanotube (CNT), graphene and three‐dimensional (3D) graphene have been identified as promising materials for electronic, electrochemical energy storage (batteries and supercapacitors), optical and sensing applications. Since the discovery of graphene in 2004, scientists have devised mass production techniques and explored graphene as a promising material for a wide range of applications. Most of the electronic and solar cell applications require materials with good electronic conductivity, mobility and finite bandgap. Graphene is a zero bandgap material which prevents it from the mainstream applications. On the other hand, 3D graphene has good electronic conductivity, mobility, bandgap and electrochemical properties. This review article will focus on the synthesis of the 3D graphene, its structure‐property relationships, biotechnology and electronic applications and the hidden properties that are yet to be explored fully.     

导热高分子材料研究进展

讨论了提高聚合物导热性能的途径－合成高导热系数的结构聚合物，用高导热无机填料对聚合物进行填充复合。综述了导热高分子材料的研究成果：聚合物导热的基本概念和影响其导热性能的因素及导热系数的预测理论；聚合物基导热复合材料的选材、复合技术及其应用。指出了导热高分子材料的研究方向－－纳米导热填料的研究和开发；聚合物树脂基体的物理化学改性；聚合物基体与导热填料复合新技术的研究和开发；复合材料导热模型的建立、导热机理（特别是聚合物基体与导热填料界面的结构与性能对材料导热性能的影响）及导热通路的形成等；探索高导热本体聚合物材料的制备方法和途径等。对导热高分子材料的研究和开发有重要意义。     

Strain-engineering of band gaps in piezoelectric boron nitride nanoribbons

Two-dimensional atomic sheets such as graphene and boron nitride monolayers represent a new class of nanostructured materials for a variety of applications. However, the intrinsic electronic structure of graphene and h-BN atomic sheets limits their direct application in electronic devices. By first-principles density functional theory calculations we demonstrate that band gap of zigzag BN nanoribbons can be significantly tuned under uniaxial tensile strain. The unexpected sensitivity of band gap results from reduced orbital hybridization upon elastic strain. Furthermore, sizable dipole moment and piezoelectric effect are found in these ribbons owing to structural asymmetry and hydrogen passivation. This will offer new opportunities to optimize two-dimensional nanoribbons for applications such as electronic, piezoelectric, photovoltaic, and opto-electronic devices.     

温差发电用BiCuSeO基热电材料的研究进展

BiCuSeO基热电材料由于具有较低的热导率和较高的Seebeck系数,热电性能优异,且原料储藏丰富、价格低廉、安全无毒,被认为是一种具有潜在应用前景的新型热电转换材料。首先介绍了BiCuSeO基材料的晶体结构、电子结构、热电性能等基本特征,随后综述了近年来国内外关于BiCuSeO基热电材料的研究进展,评述了提高其热电性能的手段,包括Na、Ag、Mg、Ca、Sr、Ba等低价元素掺杂,铜空位,双空位,带隙调整,晶粒细化,织构化和调制掺杂等。通过电热输运特性的协同调控,可使其ZT值从未掺杂样品的0.4左右提高到1.5。最后从实际应用的角度出发提出了今后BiCuSeO基热电材料的研究方向及研究重点。     

Pressure infiltration of boron nitride preforms with molten aluminum for low density heat sink materials

Cubic boron nitride (cBN) has outstanding mechanical and thermal properties. The previous research focused on mechanical properties, to data, the thermal property of cBN has rarely been reported. In this work, a wide range of aluminum/cubic boron nitride (Al/cBN) composites were fabricated by pressure infiltration at 5.0 GPa and 960–1600 °C. The microstructure, phase composition, thermal conductivity and coefficient of thermal expansion of the Al/cBN composites were investigated. The results showed that a maximum thermal conductivity of 266 W/mK and the coefficient of thermal expansion of 4–6 × 10^?6 K^?1 which matches well to semiconductors, indicating that the Al/cBN composites are promised heat sink materials of high efficiency for the wide band gap semiconductors.     

(001)面双轴应变锗材料的能带调控

本文基于形变势理论构建(001)面双轴应变Ge材料的能带结构模型。计算结果表明(001)面双轴应变可以将Ge的能带从以L能谷为导带底的间接带半导体调控到以Δ4能谷为导带底的间接带半导体或者以Г能谷为导带底的直接带半导体。同时室温下Ge的带隙与应变的关系可用四段函数来表示:当压应变将Ge材料调控为以Г能谷为导带底的间接带半导体后,每增加1%的压应变,禁带宽度将线性减小约78.63meV;当张应变将Ge材料调控为直接带半导体后,张应变每增加1%,禁带宽度将线性减小约177.98meV;应变介于-2.06%和1.77%时,Ge将被调控为以L能谷为导带底的间接带半导体,禁带宽度随着压应变每增加1%而增加11.66meV,随着张应变每增加1%而线性减小约88.29meV。该量化结果可为研究和设计双轴应变Ge材料及其器件提供理论指导和实验依据。     

新一代高导热金属基复合材料界面热导研究进展

热物理性质不同的材料之间存在界面热阻,界面热阻对热传输过程产生极大的影响,并在很大程度上决定了复合材料的导热性能。金刚石颗粒增强金属基复合材料(Metal matrix composites,MMCs)充分发挥了金刚石的高热导率和低热膨胀系数的优点,有望获得高的热导率以及与半导体相匹配的热膨胀系数,可满足现代电子设备在散热能力上提出的越来越高的要求,作为新一代电子封装材料已引起广泛关注。界面热导(界面热阻的倒数)既是决定复合材料导热能力的关键因素,也是研究的难点,复合材料制备工艺、界面改性方式(金属基体合金化或金刚石表面金属化)以及改性金属种类均会影响界面热导。详细论述了界面热导理论及实验研究的最新成果,并对金刚石/金属复合材料在未来研究中面临的主要问题进行探讨。     

Electronic structure and stability of semiconducting graphene nanoribbons

We present a systematic density functional theory study of the electronic properties, optical spectra, and relative thermodynamic stability of semiconducting graphene nanoribbons. We consider ribbons with different edge nature including bare and hydrogen-terminated ribbons, several crystallographic orientations, and widths up to 3 nm. Our results can be extrapolated to wider ribbons providing a qualitative way of determining the electronic properties of ribbons with widths of practical significance. We predict that in order to produce materials with band gaps similar to Ge or InN, the width of the ribbons must be between 2 and 3 nm. If larger bang gap ribbons are needed (like Si, InP, or GaAs), their width must be reduced to 1-2 nm. According to the extrapolated inverse power law obtained in this work, armchair carbon nanoribbons of widths larger than 8 nm will present a maximum band gap of 0.3 eV, while for ribbons with a width of 80 nm the maximum possible band gap is 0.05 eV. For chiral nanoribbons the band gap oscillations rapidly vanish as a function of the chiral angle indicating that a careful design of their crystallographic nature is an essential ingredient for controlling their electronic properties. Optical excitations show important differences between ribbons with and without hydrogen termination and are found to be sensitive to the carbon nanoribbon width. This should provide a practical way of revealing information on their size and the nature of their edges.     

Band Engineering of Thermoelectric Materials

Lead chalcogenides have long been used for space‐based and thermoelectric remote power generation applications, but recent discoveries have revealed a much greater potential for these materials. This renaissance of interest combined with the need for increased energy efficiency has led to active consideration of thermoelectrics for practical waste heat recovery systems—such as the conversion of car exhaust heat into electricity. The simple high symmetry NaCl‐type cubic structure, leads to several properties desirable for thermoelectricity, such as high valley degeneracy for high electrical conductivity and phonon anharmonicity for low thermal conductivity. The rich capabilities for both band structure and microstructure engineering enable a variety of approaches for achieving high thermoelectric performance in lead chalcogenides. This Review focuses on manipulation of the electronic and atomic structural features which makes up the thermoelectric quality factor. While these strategies are well demonstrated in lead chalcogenides, the principles used are equally applicable to most good thermoelectric materials that could enable improvement of thermoelectric devices from niche applications into the mainstream of energy technologies.     

薄膜材料导热行为及其测试和预测

薄膜材料在集成电路,光电子技术,微结构传感器等微电子元件的应用日益广泛,其导热性能直接影响元器件的热噪声,进而对其可行一和使用性能产生明显影响,薄膜材料导热性能及其测试研究愈益受人瞩目,为此,本文对薄膜材料导热性能及各种测试方法进行了综述,并在分析薄膜微结构模型的基础上,对计算薄膜有效热导率的不同预测议程进行了评述,从而可为薄膜材料的制备工艺和性能变化提供技术判据。     

Thermal properties of graphene and nanostructured carbon materials

Recent years have seen a rapid growth of interest by the scientific and engineering communities in the thermal properties of materials. Heat removal has become a crucial issue for continuing progress in the electronic industry, and thermal conduction in low-dimensional structures has revealed truly intriguing features. Carbon allotropes and their derivatives occupy a unique place in terms of their ability to conduct heat. The room-temperature thermal conductivity of carbon materials span an extraordinary large range--of over five orders of magnitude--from the lowest in amorphous carbons to the highest in graphene and carbon nanotubes. Here, I review the thermal properties of carbon materials focusing on recent results for graphene, carbon nanotubes and nanostructured carbon materials with different degrees of disorder. Special attention is given to the unusual size dependence of heat conduction in two-dimensional crystals and, specifically, in graphene. I also describe the prospects of applications of graphene and carbon materials for thermal management of electronics.     

Strain-driven electronic band structure modulation of si nanowires

One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.     

Lattice Softening Significantly Reduces Thermal Conductivity and Leads to High Thermoelectric Efficiency

The influence of micro/nanostructure on thermal conductivity is a topic of great scientific interest, particularly to thermoelectrics. The current understanding is that structural defects decrease thermal conductivity through phonon scattering where the phonon dispersion and speed of sound are assumed to remain constant. Experimental work on a PbTe model system is presented, which shows that the speed of sound linearly decreases with increased internal strain. This softening of the materials lattice completely accounts for the reduction in lattice thermal conductivity, without the introduction of additional phonon scattering mechanisms. Additionally, it is shown that a major contribution to the improvement in the thermoelectric figure of merit (zT > 2) of high‐efficiency Na‐doped PbTe can be attributed to lattice softening. While inhomogeneous internal strain fields are known to introduce phonon scattering centers, this study demonstrates that internal strain can modify phonon propagation speed as well. This presents new avenues to control lattice thermal conductivity, beyond phonon scattering. In practice, many engineering materials will exhibit both softening and scattering effects, as is shown in silicon. This work shines new light on studies of thermal conductivity in fields of energy materials, microelectronics, and nanoscale heat transfer.     

Thermo-mechanical fatigue resistance characterization and materials ranking from heat-flux-controlled tests. Application to cast-irons for automotive exhaust part

In a context where the mass, the cost and the mechanical strength of materials must be jointly optimized, it is necessary to have experimental data quickly available and sufficiently robust to make efficient conception choices. For thermomechanical fatigue, standard tests usually allow comparing material for the same temperature and strain ranges although differences in thermal properties such as conductivity or thermal expansion could make significant deviations when the same thermal flux is applied particularly for structure with forced heat flux operating regimes. A new protocol is then proposed in order to compare the specific resistances of metallic materials against thermomechanical fatigue. It can easily rank materials according to their lifetime under thermomechanical loadings where strain range and temperature amplitude are determined by the heat flux applied on an industrial part. The method is based on a complete numerical analysis to determine experimental loading conditions as a temporal evolution of temperature and mechanical strain representative of thermomechanical loading observed in TMF critical areas for the part. TMF tests on hollow specimens are carried out to rank the materials: temperature and strain amplitude are different for each alloys whereas heat flux is identical. A materials ranking list based on TMF resistance is then determined according to their lifetimes under “heat-flux-controlled” tests. The method is tested for exhaust parts and demonstrates the superiority of some cast irons over others, whereas the intrinsic isotherm mechanical properties suggested an alternative classification. The obtained ranking is confirmed by experimental tests on industrial structures.     

Low Thermal Conductivity through Dense Particle Packings with Optimum Disorder

Heat transport plays a critical role in modern batteries, electrodes, and capacitors. This is caused by the ongoing miniaturization of such nanotechnological devices, which increases the local power density and hence temperature. Even worse, the introduction of heterostructures and interfaces is often accompanied by a reduction in thermal conductivity, which can ultimately lead to the failure of the entire device. Surprisingly, a fundamental understanding of the governing heat transport processes even in simple systems, such as binary particle mixtures is still missing. This contribution closes this gap and elucidates how strongly the polydispersity of a model particulate system influences the effective thermal conductivity across such a heterogeneous system. In a combined experimental and modeling approach, well‐defined mixtures of monodisperse particles with varying size ratios are investigated. The transition from order to disorder can reduce the effective thermal conductivity by as much as ≈50%. This is caused by an increase in the thermal transport path length and is governed by the number of interparticle contact points. These results are of general importance for many particulate and heterostructured materials and will help to conceive improved device layouts with more reliable heat dissipation or conservation properties in the future.     

Electronic and Electrical Conductivity of AB and AA-Stacked Bilayer Graphene with Tunable Layer Separation

This humble work attempts to study the electronic and electrical conductivity characteristics of AB and AAstacked bilayer graphene (BLG) sheet, by using ab initio calculation. The electronic transport coefficient was calculated by using Boltzmann transport equations implanted in Boltztrap package at various temperatures from 80 to 380 K. First, this study will begin to experiment an interlayer spacing from 3.55 and 3.35 Å respectively for AA and AB-BLG. If the distance between the layers is more than 5.00 Å, the increase or decrease of energy does depend on the interlayer spacing so band gap is equal to zero. The electrical conductivity of AA and AB-BLG is compared to the experimental electrical conductivity of graphene monolayer under increasing spacing between layers to 4.00 Å. Band gap decreases with the increasing space while conductivity increases with increasing space. AA-BLG electrical conductivity shows a value near to the experimental electrical conductivity of graphene ribbon for 4.00 Å at 380 K. Nevertheless, the distance variation does not much affect the electronic and electrical characteristics of AB-BLG. In addition, the increase between the interlayer distances does not influence so much the electrical conductivity. Therefore, increasing the distance between interlayers decreases the electrical conductivity due to the increasing of band gap.     

Numerical and analytical calculation of the orthotropic heat transfer properties of fibre reinforced materials

This paper is on the investigation of the orthotropic heat transfer properties of unidirectional fibre reinforced materials. The orthotropic effective thermal conductivity of such composite materials is investigated based on two different approaches: the finite element method as a representative for numerical approximation methods and an analytical method for homogenised models based on the solution of the respective boundary value problem. It is found that fibre reinforced composites possess strong orthotropic heat transfer properties, which are getting more distinctive with increasing deviation of the thermal conductivities of matrix and reinforcements. Furthermore, the effect of small perturbations of the periodic configuration of fibres in the matrix on the thermal conductivity is investigated.     

Alkaline earth lead and tin compounds Ae2Pb,Ae2Sn,Ae = Ca,Sr, Ba,as thermoelectric materials

Abstract

We present a detailed theoretical study of three alkaline earth compounds Ca₂Pb, Sr₂Pb and Ba₂Pb, which have undergone little previous study, calculating electronic band structures and Boltzmann transport and bulk moduli using density functional theory. We also study the corresponding tin compounds Ca₂Sn, Sr₂Sn and Ba₂Sn. We find that these are all narrow band gap semiconductors with an electronic structure favorable for thermoelectric performance, with substantial thermopowers for the lead compounds at temperature ranges from 300 to 800 K. For the lead compounds, we further find very low calculated bulk moduli—roughly half of the values for the lead chalcogenides, suggestive of soft phonons and hence low lattice thermal conductivity. All these facts indicate that these materials merit experimental investigation as potential high performance thermoelectrics. We find good potential for thermoelectric performance in the environmentally friendly stannide materials, particularly at high temperature.     

Adiabatic shear banding in a bimetallic body

Summary Thermomechanical deformations of a body made of two different materials and under-going simple shearing deformations are studied with the objectives of finding out when and where adiabatic shear bands will initiate and how they will subsequently grow. Each material is modeled as strain and strain-rate hardening but thermally softening. A shear band is presumed to have formed if the introduction of a temperature perturbation centered around the common interface between the two materials results in an eventual localization of the deformation into a region of width considerably smaller than the width of the initial temperature bump. For a fixed set of material properties the effect of the applied overall strain-rate, and for a fixed applied strain-rate the effect of varying the shear modulus, thermal conductivity, and the coefficient of thermal softening of one material relative to the other have been examined. It is found that a shear band forms in the material that softens more rapidly.With 8 Figures     

Thermally insulative thermoelectric argyrodites

Ag/Cu-based argyrodites have recently received extensive attention as promising thermoelectric materials. This can be largely understood by the hierarchical chemical bonding structure that allows the existence of high-concentration and highly mobile Ag⁺/Cu⁺ ions distributed in the rigid polyanionic framework, since the diffusive cations enable an effective phonon scattering for phononically insulating as that in woods while the rigid framework ensures an electronic charge transport as that in trivial thermoelectric semiconductors. This review focused firstly on the synthesis of both single-crystalline and poly-crystalline argyrodites as well as the chemical compositions, phases and crystallographic features; secondly on the origins of the low lattice thermal conductivity including crystal structure complexity, weak bonded ions, strong lattice anharmonicity and low sound velocity; and thirdly on the electronic band structure characteristics including effective mass and band degeneracy for an evaluation on the electronic transport properties, as well as performance stability. These together lead a perspective on future development of thermoelectric argyrodites to emphasize the enhancement of electronic performance such as by carrier concentration and by manipulating the electronic band structure.