Thermally activated phase transitions in Fe-Ni core-shell nanoparticles

Fe-Ni core-shell nanoparticles are versatile functional materials, and their thermal stabilities are crucial for their performances in operating conditions. In this study, the thermodynamic behaviors of Fe-Ni core-shell nanoparticles are examined under continuous heating. The solid–solid phase transition from body centered cubic (bcc) to face centered cubic (fcc) in the Fe core is identified. The transition is accompanied with the generation of stacking faults around the core-shell interface, which notably lowers the melting points of the Fe-Ni core-shell nanoparticles and causes even worse thermal stability compared with Ni ones. Moreover, the temperature of the structural transformation is shown to be tuned by modifying the Ni shell thickness. Finally, the stress distributions of the core and the shell are also explored. The relevant results could be helpful for the design, preparation, and utilization of Fe-based nanomaterials.     

具有本征低晶格热导率的硫化银快离子导体的热电性能

硫化银(Ag_2S)是一种典型的快离子导体材料,前期关于Ag_2S的研究主要集中在光电和生物等领域.最近的研究表明, a-Ag_2S具有和金属一样的良好延展性和变形能力.但是, Ag_2S的热电性能尚无公开报道.本工作合成了单相Ag_2S化合物,系统研究了其在300—600 K范围的物相变化、离子迁移特性和电热输运性质.研究发现, Ag_2S在300—600 K温度区间表现出半导体的电输运性质.由于单斜-体心立方相晶体结构转变, Ag_2S的离子电导率、载流子浓度、迁移率、电导率、泽贝克系数等性质在455 K前后出现急剧变化.在550 K, Ag_2S的功率因子最高可达5μW·cm~(–1)·K~(–2). Ag_2S在300—600 K温度区间均表现出本征的低晶格热导率(低于0.6 W·m~(–1)·K~(–1)). S亚晶格中随机分布的类液态Ag离子是导致b-Ag_2S体心立方相具有低晶格热导率的主要原因.在573 K, Ag_2S的热电优值可达0.55,与Ag_2Se, Ag_2Te, CuAgSe等已报道的Ag基快离子导体热电材料的性能相当.     

Anisotropy and twinning in cubic zinc sulfide crystals

The structure of zinc sulfide single crystals grown from the vapor has been studied by a unique combination of X-ray crystallography and thermal conductivity measurements. Weissenberg patterns reveal the face centered cubic structure with rotational twinning in the [111] direction, which could be ascribed to two overlapping lattices. These could be explained by normal stacking faults or by inverted twins. A strong anisotropy in thermal conductivity was measured and interpreted as favoring the inverted twin model.     

The influence of structure on the thermal conductivities of low-dimensional carbon materials

The influence of structure on the thermal conductivities of single-wall carbon nanotubes (SWCNTs) and graphene nanoribbons (GNRs), is investigated by using molecular dynamics (MD) method. The calculated results demonstrate that, both edge/chirality and width/diameter affect the thermal conductivities of these two low-dimensional carbon materials in nanoscale. However, the effects of structure on the thermal conductivities of SWCNTs and GNRs are different from each other. It is concluded that the influence of structure on the thermal conductivities of SWCNTs and GNRs is not stronger than that on their electric conductivities.     

基于团聚理论的纳米制冷剂导热系数预测方法

纳米流体中纳米颗粒以团聚体的形式存在.为预测纳米流体的导热系数,模拟了流体中纳米颗粒团聚体的三维空间结构并用热阻网络法计算了团聚体的导热系数.在得到团聚体的导热系数与考虑液体分子吸附层后团聚体的体积分数后,预测纳米流体的导热系数.用实验数据验证了该预测方法并运用该方法预测了铜-R22纳米制冷剂的导热系数.     

A Ubiquitous Thermal Conductivity Formula for Liquids,Polymer Glass,and Amorphous Solids

The microscopic mechanism of thermal transport in liquids and amorphous solids has been an outstanding problem for a long time.There have been several approaches to explain the thermal conductivities in these systems, for example, Bridgman's formula for simple liquids, the concept of the minimum thermal conductivity for amorphous solids, and the thermal resistance network model for amorphous polymers.Here, we present a ubiquitous formula to calculate the thermal conductivities of liquids and amorphous solids in a unified way, and compare it with previous ones.The calculated thermal conductivities using this formula without fitting parameters are in excellent agreement with the experimental data.Our formula not only provides a detailed microscopic mechanism of heat transfer in these systems, but also resolves the discrepancies between existing formulae and experimental data.     

Rhombohedral photonic crystals by triple-exposure interference lithography: Complete photonic band gap

The band structure of 3D photonic crystal that could be synthesized by means of interference lithography with triple-exposure two-beam interference technique has been investigated. As a result of the geometry optimization the optimal conditions for maximal band gaps with different refractive index contrast have been obtained. The refractive index threshold for gap opening equaled to 2.14 has been predicted for this method of photonic crystals synthesis. This value is close to the refractive index thresholds of the best known structures. The continuous transition between simple cubic, face centered cubic and bulk centered cubic symmetries has been considered.     

Thermal conductivity and expansion of PbF2 single crystals

In order to check a phenomenon of the negative correlation between ionic and thermal conductivities of solid substances, we studied the thermal conductivity and expansion of cubic PbF₂ single crystals at 50–300 and 5.6–317 K, respectively. We found that lead difluoride had a thermal expansion coefficient α that was equal to (28.5 ± 0.3)10⁻⁶ K⁻¹ at 300 K, and a thermal conductivity coefficient k(T) was equal to 1.40 ± 0.07 W/(m K) at the same temperature. Thus, the thermal conductivity for PbF₂ is the lowest among fluorite-type MF₂ (M = Ca, Sr, Ba, Cd, Pb) thermal conductivities, whereas its fluoride-ion conductivity is the highest one among MF₂ (M = Ca, Sr, Ba, Cd, Pb) ionic conductors.

     

用扩展的Enskog理论预测正构烷烃的导热率

1引言在现代工业特别是化学工业和石油工业中，广泛涉及到物质传热过程。因此，作为表征物质热传导强弱的导热率是科研与生产必不可少的重要物性数据之一。作为物质导热率研究的一个方面，液体导热率的研究一直受到人们的关注。现在已有许多液体导热率关联方程发表，但这些方程往往适用范围较窄，是纯经验或半经验的，其理论性不强。只有Enskog公式＊中各参数的物理意义明确，所以此公式的实际应用有待进一步研究。本文用热压代替znsx。g公式中的体系压力，其中热压用un状态方程回来计算，找出正构烷烃分子硬球直径的变化规律，对正构烷烃…     

THE MACQM CALCULATION OF THE TOTAL ENERGY CURVE OF THE BODY-CENTERED CUBIC STRUCTURE OF THE H_9~- CLUSTER

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The thermal conductivities of some low melting materials relevant to energy storage

The thermal conductivities of seven candidate materials for solar-energy storage were measured as a function of temperature in the solid and liquid states. A drop in thermal conductivity was observed near the melting point. It was shown that the ratio of thermal conductivities in the liquid and solid phases depended on the structure of the material investigated. The vibrational conductivity contribution was calculated and compared with the experimental values for the thermal conductivities.     

由球形原子构成的多种周期性介电结构的光子能带计算

采用矢量平面波方法计算了由球形空气原子构成的体心立方结构和六角密堆结构的光子能带，两种结构下都看到了贯通整个布里渊区的禁带。本文预言周期性介电结构的最大折射率调制度和最大禁带宽度对应于同样的原子体积分数，而且这个最佳的原子体积分数是结构无关的。文中还计算了由各向异性介电小球构成的面心立方结构的光子能带并看到了禁带，这与各向同性介电小球构成的面心立方结构很不相同。     

Investigation on characteristics of thermal conductivity enhancement of nanofluids

It has been shown that a nanofluid consisting of nanoparticles dispersed in base fluid has much higher effective thermal conductivity than pure fluid. In this study, four kinds of nanofluids such as multiwalled carbon nanotube (MWCNT) in water, CuO in water, SiO₂ in water, and CuO in ethylene glycol, are produced. Their thermal conductivities are measured by a transient hot-wire method. The thermal conductivity enhancement of water-based MWCNT nanofluid is increased up to 11.3% at a volume fraction of 0.01. The measured thermal conductivities of MWCNT nanofluids are higher than those calculated with Hamilton–Crosser model due to neglecting solid–liquid interaction at the interface. The results show that the thermal conductivity enhancement of nanofluids depends on the thermal conductivities of both particles and the base fluid.     

Phononic gaps induced by inertial amplification in BCC and FCC lattices

In this study, infinite and finite periodic body centered cubic (BCC) and face centered cubic (FCC) lattices with and without inertial amplification mechanisms are investigated. These three-dimensional lattices are modeled with mass and spring elements that are parametrically varied to observe their effect on phononic gap (stop band) limits. When inertial amplification mechanisms are used in both of the infinite periodic lattices, wide low frequency band gaps are generated. Moreover, wide and deep phononic gaps are obtained by using moderate amount of unit cells in the case of finite periodic lattices.     

Derivation of governing equation for predicting thermal conductivity of composites with spherical inclusions and its applications

A governing differential equation for predicting the effective thermal conductivity of composites with spherical inclusions is shown to be simply derived by using the result of the generalized self-consistent model. By applying the equation to composites including spherical inclusions such as graded spherical inclusions, microballoons, mutiply-coated spheres, and spherical inclusions with an interphase, their effective thermal conductivities are easily predicted. The results are compared with those in the literatures to be consistent. It can be stated from the investigations that the effective thermal conductivity of composites with spherical inclusions can be estimated as long as their conductivities are expressed as a function of their radius.     

Evolution of magnetism in iron from the atom to the bulk

The evolution of the magnetic moment in iron clusters containing 20-400 atoms is investigated using first-principles numerical calculations based on density-functional theory and real-space pseudopotentials. Three families of clusters are studied, characterized by the arrangement of atoms: icosahedral, body-centered cubic centered on an atom site, and body-centered cubic centered on the bridge between two neighboring atoms. We find an overall decrease of magnetic moment as the clusters grow in size towards the bulk limit. Clusters with faceted surfaces are predicted to have magnetic moment lower than other clusters with similar size. As a result, the magnetic moment is observed to decrease as function of size in a nonmonotonic manner, which explains measurements performed at low temperatures.     

Radiation ordering in quenched alloy sobserved “in situ” in the high voltage microscope

Different alloys with a face centered cubic disordered structure have been electron irradiated in the quenched or short range order state under direct observation in a high voltage electron microscope. Ordering due to 1 MeV irradiation has been observed in Au₄Mn, Ni₄Mo and Cu₃Pd. Care has been taken to avoid ordering due to the thermal effect of the electron beam. It has been demonstated that although similar states of order can be achieved by thermal and irradiation ordering, the path followed can be different however.     

Thermodynamically stable phases of carbon at multiterapascal pressures

Phases of carbon are studied up to pressures of 1 petapascal (PPa) using first-principles density-functional-theory methods and a structure searching algorithm. Our extensive search over the potential energy surface supports the sequence of transitions diamond → BC8 → simple cubic under increasing pressure found in previous theoretical studies. At higher pressures we predict a soft-phonon driven transition to a simple hexagonal structure at 6.4 terapascals (TPa), and further transitions to the face centered cubic electride structure at 21 TPa, a double hexagonal close packed structure at 270 TPa, and the body centered cubic structure at 650 TPa.     

介孔二氧化硅基导电聚合物复合材料热导率的实验研究

本文首先制备并表征了介孔二氧化硅SBA-15、 填充导电聚合物的复合材料PANI/SBA-15和复合材料PPy/SBA-15, 并建立双流计实验台开展了材料压片情况下的热导率研究. 在测量得到复合材料热导率的基础上, 引入当量孔径, 结合测量孔径对 PANI/SBA-15和PPy/SBA-15复合材料热导率随填充量的变化进行了定性分析. 分析表明: PANI/SBA-15和PPy/SBA-15复合材料的热导率比基材SBA-15的热导率大得多; 在相同的测量孔径和当量孔径情况下, PANI/SBA-15复合材料的热导率比PPy/SBA-15复合材料的热导率大; 导电聚合物填充到复合材料孔道内和孔道外都有助于热导率的提高, 填充到孔道内比填充到孔道外对热导率提高的贡献更大.     

离散发热器件基于(火积)耗散率最小和最高温度最小的构形优化比较

建立了导热基座上圆柱体离散发热器件的三维湍流散热模型,基于构形理论,考虑空气变物性及可压缩性和黏性耗散,研究了器件材料的热导率、热源强度和流体流速对器件最高温度、基于(火积)耗散定义的当量热阻和平均Nu数的影响.结果表明:在总发热功率一定的条件下,以器件最高温度和当量热阻为性能指标进行热设计,均存在最优热源强度分布使得散热性能最优.当各热源强度相同且热源热导率小于基座热导率时,提高热源热导率可明显改善散热性能;将热源热导率沿流动方向从低到高布置可降低器件最高温度,而将热源热导率均匀布置可使当量热阻最小.所得结果可为实际热设计中不同材质和不同发热率的电子器件最优布置提供理论支撑.