Solutions of inhomogeneity problems with graded shells and application to core-shell nanoparticles and composites

This paper first presents the Eshelby tensors and stress concentration tensors for a spherical inhomogeneity with a graded shell embedded in an alien infinite matrix. The solution is then specialized to inhomogeneous inclusions in finite spherical domains with fixed displacement or traction-free boundary conditions. The Eshelby tensors in the infinite and finite domains and the stress concentration tensors are especially useful for solving many problems in mechanics and materials science. This is demonstrated on two examples. In the first example, the strain distributions in core-shell nanoparticles with eigenstrains induced by lattice mismatches are calculated using the Eshelby tensors in the finite domains. In the second example, the Eshelby and stress concentration tensors in the three-phase configuration are used to formulate the generalized self-consistent prediction of the effective moduli of composites containing spherical particles within the framework of the equivalent inclusion method. The advantage of this micromechanical scheme is that, whilst its predictions are almost identical to the classical generalized self-consistent method and the third-order approximation, the expressions for the effective moduli have simple closed forms.     

Preparation and characterization of core-shell Al@PFHP with improving the combustion and ignition properties of aluminum powder

The dense alumina shell on the surface of aluminum powder will hinder the combustion of aluminum powder and increase its ignition temperature. In this study, the aluminum oxide shell layer on the surface of aluminum powder was removed with hydrofluoric acid by one-pot method, and 3-Perfluorohexyl-1, 2-epoxypropane (PFHP) (F₃C(CF₂)₅CH₂C₂H₃O) was coated to form a uniform and controllable core-shell Al@PFHP. The core-shell Al@PFHP showed better thermal reaction and ignition performance. The exothermic enthalpy of Al@0.15 PFHP was increased by about 1.9 times, with lower ignition temperature (reduced by about 140 °C) and longer burning duration (increased by about 1.5 times) after coating with PFHP, compared with raw aluminum powder. In addition, the formation of PFHP coating shell can effectively improve the hydrophobicity and corrosion resistance of aluminum powder.     

Dispersion Waves of Two Fluids in a Porous Medium

The dispersion of two fluids in a porous medium is analyzed as a wave process. The wave equations are derived, and for plane wave solutions a wave number versus frequency dispersion relation is obtained. Suitable choices for the saturation dependence of terms in the equations of motion and the dynamic pressure difference equation lead to physical solutions.     

Fabrication of magnetic nanosized γ-FeNi-coated ceramic core-shell microspheres by heterogeneous precipitation and thermal reduction

Precursors with NiCO3·2Ni(OH)2·2H2O-and Fe2O3·nH2O-coated alumina,graphite and cenosphere were synthesized by precipitation using ferrous sulfate,nickel sulfate,ammonium bicarbonate,alumina,graphite and cenosphere as the main starting materials.Magnetic γ-FeNi-coated alumina,graphite and cenosphere core-shell structural microspheres were subsequently prepared by thermal reduction of the as-prepared precursors at 600℃ for 2h.Precipitation parameters,e.g.concentration of ceramic micropowders(10g/L),sulfate solution(0.2mol/L),rate of adding reactants(3mL/min)and pH value were optimized by a trial-and-error method.Powders of the precursors and the resulting coating of γ-FeNi with grain size below 40nm on alumina,graphite and cenosphere microspheres were characterized by scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS)and X-ray diffraction(XRD).The magnetic properties of the nanosize γ-FeNi-coated alumina,graphite and cenosphere microspheres were measured by vibrating sample magnetometer(VSM).The results show that the core-shell structuralγ-FeNi-coated ceramic microspheres exhibited higher coercivity than pure γ-FeNi powders,indicating that these materials can be used for high-performance functional materials and devices.     

Dispersion properties of vortex-type monatomic lattices

The paper presents a systematic study of dispersive waves in an elastic chiral lattice. Chirality is introduced through gyroscopes embedded into the junctions of a doubly periodic lattice. Bloch–Floquet waves are assumed to satisfy the quasi-periodicity conditions on the elementary cell. New features of the system include degeneracy due to the rotational action of the built-in gyroscopes and polarisation leading to the dominance of shear waves within a certain range of values of the constant characterising the rotational action of the gyroscopes. Special attention is given to the analysis of Bloch–Floquet waves in the neighbourhoods of critical points of the dispersion surfaces, where standing waves of different types occur. The theoretical model is accompanied by numerical simulations demonstrating directional localisation and dynamic anisotropy of the system.     

Instabilities of core-shell heterostructured cylinders due to diffusions and epitaxy: Spheroidization and blossom of nanowires

The morphological instabilities of core-shell heterostructures consisting of an epitaxially stressed cylinder embedded in a finite shell are investigated. The contributions of the surface diffusion, interface diffusion and volume diffusion, and a combination of these processes to the mass transport along the surface of the shell and the interface between the cylinder and shell are examined, respectively. As the driving forces for the instabilities, the capillary terms and the mismatch strain energy are also taken into account. The governing evolution equations of the surface and interface are established in terms of a linear instability analysis of the longitudinal and radial variations of the surface and the interface positions. The critical conditions for the zero and maximum fluctuations of the surface and interface in the radial and longitudinal directions are given. For a core-shell cylinder of nickel, it is demonstrated that at a small size, the contribution of the surface/interface diffusion to the morphological evolution is larger than that of the volume diffusion, even at an elevated temperature. It is shown that the analysis of the instabilities of the longitudinal and radial fluctuations can be used to predict the spheroidization and blossom of core-shell nanowires.     

Turbulent Flow and Dispersion of Inertial Particles in a Confined Jet Issued by a Long Cylindrical Pipe

In this work we examine first the flow field of a confined jet produced by a turbulent flow in a long cylindrical pipe issuing in an abrupt angle diffuser. Second, we examine the dispersion of inertial micro-particles entrained by the turbulent flow. Specifically, we examine how the particle dispersion field evolves in the multiscale flow generated by the interactions between the large-scale structures, which are geometry dependent, with the smaller turbulent scales issued by the pipe which are advected downstream. We use Large-Eddy-Simulation (LES) for the flow field and Lagrangian tracking for particle dispersion. The complex shape of the domain is modelled using the immersed-boundaries method. Fully developed turbulence inlet conditions are derived from an independent LES of a spatially periodic cylindrical pipe flow. The flow field is analyzed in terms of local velocity signals to determine spatial coherence and decay rate of the coherent K–H vortices and to make quantitative comparisons with experimental data on free jets. Particle dispersion is analyzed in terms of statistical quantities and also with reference to the dynamics of the coherent structures. Results show that the particle dynamics is initially dominated by the Kelvin–Helmholtz (K–H) rolls which form at the expansion and only eventually by the advected smaller turbulence scales.     

Synthesis of iron oxide nanoparticles via sonochemical method and their characterization

Preparation of iron oxide ( -Fe 2 O 3 ) nanoparticles was carried out via a sonochemical process. The process parameters such as temperature, sonication time and power of ultrasonication play important roles in the size and morphology of the final products. The iron oxide nanoparticles were characterized by transmission electron microscopy, X-ray powder diffraction, and thermogravimetric and differential thermal analyses. From transmission electron microscopy observations, the size of the iron oxide nanopar...     

Dispersion characteristics of wave propagation in layered piezoelectric structures: Exact and simplified models

This article presents a study of the dispersion characteristics of wave propagation in layered piezoelectric structures under plane strain and open-loop conditions. The exact dispersion relation is first determined based on an electro-elastodynamic analysis. The dispersion equation is complicated and can be solved only by numerical methods. Since the piezoelectric layer is very thin and can be modeled as an electro-elastic film, a simplified model of the piezoelectric layer reduces this complex problem to a non-trivial solution of a series of quadratic equations of wave numbers. The model is simple, yet captures the main phenomena of wave propagation. This model determines the dispersion curves of PZT4-Aluminum layered structures and identifies the two lowest modes of waves: the generalized longitudinal mode and the generalized Rayleigh mode. The model is validated by comparing with exact solutions, indicating that the results are accurate when the thickness of the layer is smaller or comparable to the typical wavelength. The effect of the piezoelectricity is examined, showing a significant influence on the generalized longitudinal wave but a very limited effect on the generalized Rayleigh wave. Typical examples are provided to illustrate the wave modes and the effects of layer thickness in the simplified model and the effects of the material combinations.     

Rheological behavior of PS-melts containing surface modified PMMA-particles

The rheological properties of polystyrene (PS) compounds containing cross-linked PMMA particles are characterized by oscillatory shear experiments, while the amount of covalently grafted carboxylic-acid terminated polystyrene (CT-PS) on their surface is varied. All samples show an additional relaxation process with a long relaxation time in the frequency range where the matrix flows. The strength of this process depends strongly on the amount of particles, i.e., on the degree of filling and, to the same extent, on the different “coverage” of the particles surface, i.e., the degree of grafting. For the highest degrees of filling and in dependence on the degree of grafting a particle network is formed which is characterized by an evolving equilibrium modulus. Moreover, compatibilization by grafting CT-PS onto the PMMA particles' surface reduces the strength of the additional relaxation process remarkably. This surprising effect is related qualitatively to the balance of the interaction between particles and the interaction between the particles and the matrix due to PS grafts. Some of these results can be understood quantitatively on the basis of the sticky hard-sphere model. Received: 3 January 2000 Accepted: 14 August 2000     

Dispersion effects of extensional waves in pre-stressed imperfectly bonded incompressible elastic layered composites

The effect of an imperfect interface, on time-harmonic extensional wave propagation in a pre-stressed symmetric layered composite is considered. The bimaterial composite consists of incompressible isotropic elastic materials. The shear spring type resistance model employed to simulate the imperfect interface can accommodate the extreme cases of perfect bonding and a fully slipping interface. The dispersion relation obtained by formulating the incremental boundary-value problem and the use of the propagator matrix technique, is analyzed at the low and high wavenumber limits. For the perfectly bonded and imperfect interface cases in the low wavenumber region, only the fundamental mode has a finite phase speed, while other higher modes have an infinite phase speed when the dimensionless wavenumber approaches zero. However, for the fully slipping interface in the low wavenumber region, both the fundamental mode and the next lowest mode have finite phase speeds. In the high wavenumber region, when the dimensionless wavenumber tends to infinity, the phase speeds of the fundamental mode and the higher modes depend on the phase speeds of the surface and interfacial waves and on the limiting phase speed of the composite. An expression to determine the cut-off frequencies is obtained from the dispersion relation. Numerical examples of dispersion curves are presented, where when the material has to be prescribed either Mooney–Rivlin material or Varga material is assumed. The effect of the imperfect interface is clearly evident in the numerical results.     

Fe及SiO2对铜基刹车材料摩擦磨损性能的影响机制

通过加压烧结法制备出铜基粉末冶金航空刹车材料,采用模拟刹车制动试验方法考察了不同转速条件下Fe含量和添加SiO2对材料摩擦磨损性能的影响,利用光学显微镜和扫描电子显微镜观察材料的显微组织结构及其磨损表面形貌,分析了Fe和SiO2对材料磨损性能的影响机制.结果表明:由于高硬度及耐磨的Fe弥散分布于铜基体中,使得刹车材料的摩擦系数和耐磨性能有所提高;SiO2虽然能够更有效地增加材料的摩擦系数和提高高速条件下的耐磨性,但对低速下材料磨损性能的提高不利.这是由于在低速下,SiO2易凸出摩擦表面而增加材料的磨损,而在高速下由于硬质SiO2颗粒对摩擦膜起到很好的钉扎作用而使其摩擦系数增加,磨损率降低.     

Cooperative entry of nanoparticles into the cell

Interaction of nanoparticles (NPs) with cell membrane is a crucial issue in studying drug delivery, photodynamic therapy system and cytotoxicity. Single NP with relatively small size cannot be fully wrapped by the cell membrane, which prohibits its uptake. One feasible way is cooperative entry, i.e., recruiting and assembling multiple small NPs to form a larger NP cluster to enter into a cell. In this work, we present theoretical analysis about the cooperative entry of multiple NPs. Through free energy calculation we investigate how the NPs׳ size, shape, interval and NP/cell interfacial binding energy influence the feasibility of entry. Interestingly we find that the cooperative entry of oblate ellipsoidal NPs can get larger energy compensation than individual ones as well as spherical ones. We also propose that soft NPs have preference in cooperative entry of the cell. Our work can be used to actively design and transfer NPs in applications such as drug delivery as well as to understand the shape effect on toxic mechanism of ellipsoidal NPs.     

Shape and size effects of ceria nanoparticles on the impact strength of ceria/epoxy resin composites

Ceria nanoparticles with various shapes (rods, cubes, and plates) and sizes were controllably synthesized and then introduced into epoxy resin. Subsequently, we investigated correlations between the shape and size of ceria nanostructures and the mechanical performance of composites. The samples were characterized by transmission electron microscopy, scanning electron microscopy, and X-ray diffraction. Compared with commercial ceria filled composites, the composites made with morphology-controlled ceria nano...     

Comparison of schemes for preparing magnetic Fe3O4 nanoparticles

Magnetic Fe3O4 nanoparticles were prepared by means of coprecipitation using NH3·H2O in water and in alcohol, and using NaOH in water. A series of instruments such as SEM, TEM, HRTEM, FT-IR, XRD and VSM were used to characterize the properties of the magnetic nanoparticles.was the longest. The process using NaOH in water was the simplest and the reaction time was the shortest, but the particle characteristics were inferior to those of the other two methods. The mean size of magnetic Fe3O4 nanoparticles prepared by coprecipitation in alcohol was the smallest among the three, but the nanoparticles aggregated severely. The magnetic Fe3O4 nanoparticles were coated with oleic acid using saturated sodium coated successfully and thoroughly.     

Thermophoresis of nanoparticles hotter/colder than the surrounding dilute gases

Aerosol particles suspended in a diluted gas with non-uniform temperature distribution are expected to experience a thermophoretic force. In theoretical treatment of thermophoresis, it is usually assumed that the particle temperature is equal to the surrounding gas temperature. However, this might not always be the case. In some particular applications, the particle temperature can significantly differ from the gas temperature. In the present paper, we theoretically investigate the effect of the particle temperature on the thermophoresis of nanoparticles in the free molecule regime. Theoretical formulas for the thermophoretic force and thermophoretic velocity are obtained based on the gas kinetic theory. As examples, a spherical Ag nanoparticle suspended in a dilute He gas is considered, and the Rudyak–Krasnolutski potential is employed to model the gas–particle interaction. It is found that the influence of the particle temperature on the thermophoresis of nanoparticles can be significant. With increasing particle size, the error due to the equal gas–particle temperature assumption can be neglected.     

Dispersion in a Bingham plastic fluid: Effects of peripheral layer

The dispersion of a soluble matter in a plastic fluid flowing through a tube and a channel has been analysed by taking into account the variations of viscosity, diffusivity and yield stress. It has been shown that in the special case of a Bingham fluid, surrounded by a peripheral layer of a Newtonian fluid, the effective dispersion coefficient with which the solute disperses across a plane moving with the mean speed of the flow decreases with the viscosity of the peripheral layer fluid but increases as the molecular diffusion coefficient of this layer decreases. Further, the effective dispersion coefficient also decreases as the yield stress of the Bingham fluid increases.     

Controllable preparation of particles with microfluidics

This paper reviews recent development and achievements in controllable preparation of nanoparticles, micron spherical and non-spherical particles, using microfluidics. A variety of synthesis strategies are presented and compared, including single-phase and multiphase microflows. The main structures of microfluidic devices and the fundamental principles of microflows for particle preparation are summarized and identified. The controllability of particle size, size distribution, crystal structure, morphology, physical and chemical properties, is examined in terms of the special features of microfluidic reactors. An outlook on opinions and predictions concerning the future development of powder technology with microfluidics is specially provided.