Impedance-Matched Reduced Acoustic Cloaking with Realizable Mass and Its Layered Design

We present an impedance-matched reduced version of acoustic cloaking whose mass is in a reasonable range. A layered cloak design with isotropic material is also proposed for the reduced cloak. Numerical calculations from the transfer matrix methods show that the present layered cloak can reduce the scattering of an air cylinder substantially.     

Probing the Air Storage Failure Mechanism of Ni-Rich Layered Cathode Materials

Ni-rich layered oxide cathode materials,such as LiNi_0.83Co_0.12Mn_0.05O₂ （NCM811）,exhibit high specific capacity and low cost,and become cathode material preference of high-energy-density Li-ion batteries.However,these cathode materials are not stable and will form Li-poor reconstructed layers and alkaline compounds （Li₂CO₃,Li OH） on the surface during the storage and processing in humid air,resulting in serious deterioration of ele...     

Enhanced Thermal Invisibility Effect in an Isotropic Thermal Cloak with Bulk Materials

A thermal cloak is well known for hiding objects from thermal signature. A bilayer thermal cloak made from inner insulation layer and outer isotropic homogeneous layer could realize such thermal protection. However, its thermal protection performance can be suppressed for low-thermal-conductivity surrounding media. We propose a tri-layer thermal cloak model by adding a transition layer between the insulation layer and the outer layer.Numerical simulations and theoretical analysis show that, unde...     

Application of Fluorescence with Polarized Light to Evaluate the Orientation of Dyes Adsorbed in Layered Materials

A new method of fluorescence polarization is applied to evaluate the angle of the preferential orientation of Rhodamine 6G (R6G) dye adsorbed in supported thin films of Laponite (Lap) clay. The method is based in the determination of the fluorescence dichroic ratio, obtained from the recorded fluorescence spectra with the detection polarizer horizontally and vertically oriented, as a function of the twisted angle of the film around its vertical axis, keeping the excitation polarizer in a fixed direction. The validity of the method is checked by comparing the experimental results obtained with both vertically and horizontally polarized excitations to that previously provided by absorption spectroscopy with linearly polarized light. A preferential orientation angle with respect to the normal to the clay layer of 62° is derived for R6G monomers adsorbed in Lap films.     

LaO1-xFxBiS2 层状材料超导电性研究

层状材料一直是超导研究领域的热点材料, 陆续有数十种新型的层状超导材料被不断发现, 从至今为止超导转变温度最高的铜氧化物材料, 到铁基超导, 再到诸多新型的层状材料, 学者们关于超导微观机制的理解也在随之更新, 从取得成功的 Bardeen-Cooper-Schrieffer 经典理论到至今无法解释的非常规超导机制, 研究之路从未停止. 本文将研究利用助熔剂法合成的 LaO1 -xFxBiS2 材料的结构、 电阻、 磁化率等性质, 并与国际社会上已存在的研究成果进行对比, 同时从晶体结构、 压力效应、 体超导电性、 临界温度、 微观机制五个方面, 对近年发现的镧系铋硫基层状超导材料的研究情况进行讨论.     

分层复合材料光波导的非线性二次增强效应

通过对分层复合材料光波导ＴＭ模等效介电系数的分析，清楚地预言了无限扩展分层复合材料结构中的非线性增强将因光波导的边界作用而引起进一步的加强。     

相变材料模拟金属构件热特性的数值研究

通过理论分析和数值计算论证了用相变材料模拟多种常用金属构件热特性的可行性.利用相变材料模拟了厚度分别为20 mm,30 mm,50 mm和100 mm的金属板材,发现在模拟较厚的金属板材时,相变材料的低导热系数对模拟效果产生了不利影响.在此基础上,利用导热系数增强的相变材料对厚度大于100 mm金属板材进行模拟,取得了较好的模拟效果.并利用相变材料模拟了金属构件轮和管的热特性.     

Structure and Thermal Stability of Polystyrene/Layered Silicate Nanocomposites

Polystyrene/layered silicates nanocomposites were prepared by intercalation in solution method, using CHCl₃ and CCl₄ as solvents. The clay used was organically modified by hexadecyltrimethyl–ammonium bromide (CTAB) at various surfactant loadings. It was found that intercalated nanocomposite structure was obtained using CHCl₃ as solvent while exfoliated or partially exfoliated was the predominant form in the case of CCl₄. X-ray diffraction and thermogravimetric analysis were used to characterize the nanocomposite morphology and thermal stability, respectively. Enhancement in thermal stability was observed for PS-nanocomposites compared to that of pristine polymer as indicated by TGA measurements. This increment was more prevalent for exfoliated nanocomposites prepared with carbon tetrachloride as solvent.     

二维层状晶体材料层间剪切模和层间呼吸模的拉曼光谱研究

我们将四片布拉格体光栅技术集成到单光栅拉曼光谱仪中,并且成功地探测到2至19层二硫化钼(MoS2)的层间剪切模和层间呼吸模。我们根据对称性分析和偏振拉曼的结果对所有观察到的模式进行了指认。同时,我们利用简单的"单原子线性链"模型得到了适用于任何二维层状晶体材料层间振动模的频率随层数变化关系的解析形式。根据此模型,我们发现层间弱范德瓦耳斯力作用决定了剪切模和呼吸模的频率随层数的变化。我们将此结果推广到ABC堆垛的多层石墨烯材料,发现光学衬度方法无法鉴别的AB和ABC型石墨烯可以利用能否观察到剪切模来加以鉴别。此研究结果也提供了准确地确定二维层状材料的厚度的一种新方法,为一般二维层状材料基本性质和器件应用方面的研究奠定了基础。     

Suppression of the Resonant Scattering in Imperfect Acoustic Cloaking with a Lossy Medium in R^3

It has been realized that resonance frequencies of imperfect the transformation acoustics in R^2 are located near Dirichlet acoustic cloaking based on a small perturbation of eigenvalues of the cloaked region [Chin. Phys. Lett. 26 （2009） 014301; 29 （2012） 124301]. In this work, we study the performance of the three-dimensional approximate cloaking system based on the transformation acoustics and show that the cloaking effect may be deteriorated at zeroth order Neumann eigenvalues of the concealed region. In particular, transmitted fields into the concealed region can be extremely resonated at frequencies corresponding to the zeroth-order Neumann eigenvalues while scattered fields are suppressed well for any frequency. To enhance the cloaking effect at resonance frequencies, we introduce a lossy medium inside the cloaked region and show that the new proposal can reduce the intensity of transmitted fields significantly due to the lossy medium.     

Three-Dimensional Finite Element Simulations for the Thermal Characteristics of PCRAMs with Different Buffer Layer Materials

Simulation of the heat consumption in phase change random access memories （PCRAMs） is investigated by a three-dimensional finite element model. It is revealed that the thermal conductivity and electrical conductivity of the buffer layer are crucial in controlling the heating efficiency in RESET process. The buffer layer mater/Ms W, TiN, WOa, Ti02 and poly-germanium （poly-Ge） are applied in the simulation respectively, and compared with each other. The simulation results show that limitation of electrical conductivity is effective on heating efficiency and the limitation of thermal conductivity is important on the reliable RESET process.     

相变微胶囊的热导率测量

提出应用3ω谐波探测技术进行脲醛树脂-石蜡相变微胶囊的等效热导率测量方法。测试了跨越相变温度区间的微胶囊等效热导率,分析了等效热导率随温度的变化关系。在其相变温度区间内,热导率存在极大值,该极值点对应的温度与其相变温度峰值一致。同样温度下,降温时的等效热导率略小于升温,这主要是由降温时相变材料的过冷引起。     

Thermal Plasma Processing of Nanostructured Si-based Ceramic Materials

A novel thermal plasma process, based on Thermal Plasma Chemical Vapor Deposition (TPCVD) for producing nanostructured ceramics from liquid precursors is described. The process combines the rapid thermal decomposition of low-cost liquid precursors injected into an Inductively Coupled Plasma (ICP) with a fast gas phase condensation due to the high cooling rate and short residence time existing in such a plasma. Examples of synthesis of Si-based nanostructured ceramic materials (SiC, Si₃N₄) as powders or coatings are given. Deposition rates of up to 10 m/min can be achieved by the present technique.     

Thermal Properties of Materials Characterized by Scanning Electron-Acoustic Microscopy

A modified technique of scanning electron-acoustic microscopy is employed to determine thermal diffusivity of materials. Using the dependence of the electron-acoustic signal on modulation frequency of the electron beam, the thermal diffusivity of materials is characterized based on a simplified thermoelastic theory. The thermal diffusivities of several metals characterized by the modified scanning electron-acoustic microscopy are in good agreement with the referential values of the corresponding materials, which proves that the scanning electronacoustic microscopy can be used to characterize the thermal diffusivity of materials effectively. In addition, for micro-inhomogeneous materials, such as biological tissues, the macro-effective （average） thermal diffusivities are characterized by the technique.     

Origin在不同材料导热系数测定中的应用

针对导热系数测定实验中数据处理不准确的问题,对牛筋、橡胶、铝三种不同材料在自然冷却和风扇冷却条件下的导热系数进行了测量,利用Origin软件进行分析比较表明,用二项式拟合和指数拟合即可得到较高的相关度,并且材料的导热系数与降温方式无关。     

Engineering the Internal Structure of Magnetic Silica Nanoparticles by Thermal Control

Calcination of hydrated iron salts in the pores of both spherical and rod‐shaped mesoporous silica nanoparticles (NPs) changes the internal structure from an ordered 2D hexagonal structure into a smaller number of large voids in the particles with sizes ranging from large hollow cores down to ten nanometer voids. The voids only form when the heating rate is rapid at a rate of 30 °C min^?1. The sizes of the voids are controlled reproducibly by the final calcination temperature; as the temperature is decreased the number of voids decreases as their size increases. The phase of the iron oxide NPs is α‐Fe₂O₃ when annealed at 500 °C, and Fe₃O₄ when annealed at lower temperatures. The water molecules in the hydrated iron (III) chloride precursor salts appear to play important roles by hydrolyzing Si? O? Si bonding, and the resulting silanol is mobile enough to affect the reconstruction into the framed hollow structures at high temperature. Along with hexahydrates, trivalent Fe³⁺ ions are assumed to contribute to the structure disruption of mesoporous silica by replacing tetrahedral Si⁴⁺ ions and making Fe? O? Si bonding. Volume fraction tomography images generated from transmission electron microscopy (TEM) images enable precise visualization of the structures. These results provide a controllable method of engineering the internal shapes in silica matrices containing superparamagnetic NPs.     

Improved Biodegradation and Thermal Properties of Poly(lactic acid)/Layered Silicate Nanocomposites

Poly(lactic acid) (PLA)/layered silicate nanocomposites have successfully been prepared using the solution route. Two types of organically modified nanoclays, namely, MEE and MAE have been used. The nanostructure, as observed from wide angle X-ray diffraction, indicates an intercalated hybrid for both PLA–MEE and PLA–MAE, and depends on the type of organic modifier used. Intercalation is higher in PLA–MEE as compared to PLA–MAE system. Crystallite dimensions of nanoclays and nanocomposites have been calculated from the Scherrer equation. Crystallite size of nanocomposites is higher than that of pure nanoclay, which in turn affects the properties of the nanocomposites. Wide angle X-ray diffraction patterns also suggest that PLA and its nanocomposites are predominantly amorphous before annealing but, after annealing, PLA and its nanocomposites are fairly crystalline. The crystallinity of the nanocomposites has decreased in comparison to neat polymer suggesting some sort of interaction between organically modified nanoclay and polymer. The nanohybrids show significant improvement in the thermal properties of the matrix as compared to pristine polymer. The nature of interaction between nanoparticles and polymer is higher in PLA–MEE against PLA–MAE, as evident from the lower value of the heat of fusion in the case of PLA–MEE. The nanoparticles act as nucleating agent, and thereby, control the spherulite dimension of the matrix. The comparison of biodegradation of PLA and its nanocomposites has been studied in several media. Biodegradability of PLA has significantly been enhanced in the presence of nanoclays which has been explained on the basis of amorphous content in the polymer matrix. Finally, the regulated biodegradation has been discussed.     

随机结构多孔介质等效热导率数值计算

本文对二维颗粒弥散多孔介质的辐射导热耦合等效热导率进行了数值计算研究。首先采用随机生成结构方法(Random generate-growth method,RGGM)生成实际多孔材料的复杂结构。在此基础上,采用离散坐标法及有限容积法求解了复杂结构内部辐射导热耦合换热,进而计算得到材料的等效热导率。根据建立的材料随机结构模型及等效热导率数值计算模型,分别研究了衰减系数、弥散相体积分数、温度等因素对材料等效热导率的影响,并将数值计算结果与理论计算结果进行对比,吻合较好,反映出模型用于预测实际多孔材料等效热导率的有效性。     

Thermal Analysis of the Ferromagnetic Materials in the Region of Curie Temperature by Temperature-Modulated DTA

Thermal analysis has been employed to measure changes of the heat capacity of ferromagnetic materials such as alloys in the region of the Curie temperature. Thermal response for these materials is usually small in this region. The necessary increase in sensitivity was achieved by applying temperature modulation. Results obtained by this technique including temperature-modulated differential thermal analysis simultaneously with A.C. thermomagnetometry are presented.