Volume Fraction Of Large Particles Research Articles

Revealing the percolation–agglomeration transition in polymer nanocomposites via MD-informed continuum RVEs with elastoplastic interphases

This contribution builds the concluding step of a multiscale approach to effectively capture the mechanical behavior of polymer nanocomposites (PNCs), in this case, silica-modified polystyrene. By introducing continuum-based representative volume elements (RVEs) that employ previously identified elastoplastic property gradients for the interphases surrounding the fillers, the effects of particle size, particle volume fraction, and agglomeration on the mechanical performance are investigated. Uniaxial tension tests are simulated with the respective finite-element RVEs, and stress–strain curves are derived. The elastic and plastic material properties of the RVE can then be extracted and analyzed quantitatively by fitting the stress–strain curves with a Voce-type elastoplasticity formulation.At small degrees of agglomeration, i.e., good particle dispersion, in combination with sufficiently large particle volume fraction, percolation bands form, leading to improved elastic and plastic properties. Higher degrees of agglomeration or particle clusters behave like large single particles, which has an adverse effect, i.e., the nanoscale size effect is thereby neutralized. Therefore, the precise MD-informed elastoplastic interphase representation of our RVEs enables the investigation of the transition from beneficial percolation to unfavorable agglomeration. Ultimately, this contribution establishes a link between the effects of particle size, particle volume fraction, agglomeration, and percolation, which have so far only been discussed separately in the literature.Our methodology offers new insights into the structure–property relations of PNCs and their resulting mechanical behavior. The underlying multiscale approach with a systematic transition from molecular to microscopic scales is required to complement experimental observations and exploit the full potential of PNCs.

Study on dynamic response mechanism of rice grains in friction rice mill and scale-up approach of parameter based on discrete element method

Rice milling is a key step that decides nutritional quality and economic benefits of rice in rice processing. Because of diverse requirements for the bran removal degree and rice handling capacity, rice milling effect must be accurately controlled. In this study, the milling process of rice in milling cavity was simulated through discrete element method, and simulation effectiveness was verified by experiments. Dynamic response characteristics of rice particles to different parameters were clarified. The more energy input and the larger particle volume fraction in shear zone, the higher milling degree of rice. A scale-up mathematical model involving structural and operating parameters was established for the first time. There is a linear relationship between the model and the energy loss. Experimental results showed that the model can accurately predict the milling degree of rice. This study can provide theoretical guidance for the design of rice mill or similar equipment.

Stratified and gradient films by evaporation-induced stratification of bimodal latexes. Potential of confocal and scanning electron microscopy for compositional depth profiling

In the field of bimodal latex film's formation, finding methods to quantify particle concentration profiles within the film thickness is a challenge. Herein the advantages and limitations of confocal fluorescence microscopy (CFM) and scanning electron microscopy (SEM) are presented. Films are prepared from a bimodal fluorescent acrylic mixture of large non-deformable particles and small deformable particles having high Péclet numbers. A fixed volume fraction of large particles (ΦL) in the dispersion is used with increasing volume fractions of small particles latex (ΦS). At low ΦS, a surface stratification of large particles occurs with thicknesses up to several micrometers, allowing detection by both CFM and SEM. Small particles form a stratified surface layer above a threshold ΦS value. Its thickness of less than 500 nm is evidenced only by SEM due to the resolution limit of the confocal scan. SEM is also able to reveal concentration gradients of large particles below the “small-on-top” stratified layer.

Dependent Scattering in Thick and Concentrated Colloidal Suspensions

This study demonstrates numerically and experimentally that dependent scattering occurs in colloidal suspensions and can, counterintuitively, cause their transmittance to increase with increasing particle volume fraction. Radiation transfer through colloidal suspensions has been modeled with the radiative transfer equation (RTE) assuming independent scattering. Then, the effective absorption and scattering coefficients of the disperse medium are predicted as the sum of the cross sections of all particles divided by the volume of suspension. However, this approach is not valid when the average interparticle distance is on the same order of magnitude as the wavelength, corresponding to large particle concentrations. The latter situation is referred to as dependent scattering. Rigorously accounting for dependent scattering requires solving Maxwell’s equations, but is limited to relatively thin suspensions. Here, we extend the Radiative Transfer with Reciprocal Transactions (R2T2) method to predict the normal-hemispherical transmittance of thick and concentrated plane-parallel slabs of nonabsorbing nanoparticle suspensions and to rigorously account for dependent scattering effects. The radiation characteristics of a large number of particle ensembles were estimated using the superposition T-matrix method and the RTE was solved using Monte Carlo method combined with strategies for sampling the previously computed radiation characteristics. A wide range of particle size parameter, volume fraction, and optical properties as well as colloidal suspension thickness were investigated. Dependent scattering effects were found to prevail for particle volume fractions as low as 1% depending on the particle size and refractive index. Evidences of dependent scattering were also observed experimentally in the visible normal-hemispherical transmittance of 10 mm thick colloidal suspensions of silica nanoparticles with diameter between 16 and 30 nm and particle volume fraction ranging from 2% to 15%. Moreover, good agreement was found between experimental measurements and numerical predictions from the R2T2 method. By contrast, assuming independent scattering underestimated systematically the normal-hemispherical transmittance, especially for large particle volume fraction. As such, this paper presents, for the first time, experimental validation of the R2T2 method and its ability to account for dependent scattering.

Gradient Structure of the Transfer Layer in Friction Stir Welding Joints.

Despite a thirty-year history of friction stir welding, some basic aspects still remain unclear. In particular, questions arise about mass transfer and the formation of transfer layers. It is not clear why there are visible boundaries between the layers. The structure of the transfer layer has been studied very little. These issues are not considerably important from the viewpoint of obtaining high-quality welds, but they can help to a better understanding of the welding process. In this paper, the structural evolution in the transfer layer of 2024 aluminum alloy welds produced under various loads and with ultrasonic assistance is discussed. Structural studies revealed a gradient structure in the transfer layer. The grain size, the volume fraction and size of large intermetallic particles decrease towards the center of the layer, while the volume fraction of semi-coherent secondary particles increases. As a result, the microhardness is higher in the center of the transfer layer. A mass transfer mechanism is proposed based on the experimental results: the rotating tool transfers the material back layer by layer during welding; the contacting layers rub against each other and generate heat, due to which the structure at the layer boundary changes. With increasing axial force on the tool, the grain size also increases due to higher heat generation. Ultrasound has almost no effect on the grain structure, but it reduces the volume fraction and size of secondary particles and microhardness.

Effects of Strength and Distribution of SiC on the Mechanical Properties of SiCp/Al Composites.

In this paper, considering the strength and geometric discrete distribution characteristics of composite reinforcement, by introducing the discrete distribution function of reinforcement, the secondary development of ABAQUS is realized by using the Python language, the parametric automatic generation method of representative volume elements of particle-reinforced composites is established, and the tensile properties of silicon carbide particle-reinforced aluminum matrix composites are analyzed. The effects of particle strength, particle volume fraction, and particle random distribution on the mechanical properties of SiCp/Al composites are studied. The results show that the random distribution of particles and the change in particle strength have no obvious influence on the stress–strain relationship before the beginning of material damage, but have a great influence on the damage stage, maximum strength, and corresponding failure strain. With the increase in particle volume fraction, the damage intensity of the model increases, and the random distribution of particles has a great influence on the model with a large particle volume fraction. The results can provide a reference for the design, preparation, and characterization of particle-reinforced metal matrix composites.

Bimodal particle distributions with increased thermal conductivity for solid particles as heat transfer media and storage materials

Solid particles are being considered in several high temperature thermal energy storage systems and as heat transfer media in a variety of advanced power generation systems, particularly in concentrated solar power plants. The downside of such an approach is the low overall heat transfer coefficients caused by the inherently low thermal conductivity values of the low-cost solid media when coupled to heat exchanger for the power cycle working fluid. Choosing the right particle size distribution, emittance, and material of the solid media can all make a substantial difference in packed bed thermal conductivity. Current research though exclusively focuses on continuous unimodal distributions of particles. Here, we propose the use of a binary particle system with a bimodal size distribution to significantly increase packed bed thermal conductivity by reducing packed bed porosity. This is the first study related to ceramic solid particle heat transfer that has considered the thermal conductivity of non-unimodal size distributions at room and elevated temperatures. The following study found that for the binary particle system using Carbo particles CP 16/30 – CP 70/140 where the large particle volume fraction was 50% there was an 17–47% increase in packed bed thermal conductivity when compared with a nearly unimodal particle size distribution of CP 16/30 between 50 and 300 °C. Two different porosity and effective thermal conductivity models were studied, with one providing better prediction of porosity but both effective thermal conductivity models providing less predictive capacity. Importantly this approach can have a substantial impact of thermal performance, with little to no impact on the particle cost.

DEM study of segregation degree and velocity of binary granular mixtures subject to vibration

This paper presents a study on the Brazil-nut segregation of binary granular mixtures under vibration by the discrete element method. The ascending velocity of large particles (Va) and the overall segregation degree (Dsep) are correlated to the vibration velocity amplitude (Vb) and the dimensionless frequency (fd). Both Dsep and the average Va increase linearly with Vb under a given fd, while the coefficient shows a critical change at fd =1. Va conforms to a single-peak distribution, and the shape parameters of the distribution converge after Vb> 1.7. Both Va and Dsep can be correlated to (A/ds)‧fd0.5 universally, where A is amplitude and ds is the small particles size. The increase of the volume fraction of large particles leads to the increase of Dsep and Va, which is probably linked with the increase in granular temperature and the difference between the vibration patterns of large and small particles.

Clarification of an improved cluster renewal model (ICRM) for heat transfer characteristics in a rolling circulating fluidized bed (RCFB)

This paper presents a novel method to predict the gas layer thickness δ′ in an improved cluster renewal model (ICRM) to calculate the heat transfer coefficient h′c for a rolling circulating fluidized bed (RCFB). Eulerian-Eulerian two-fluid model (TFM) with kinetic theory of granular flow is used to perform a numerical simulation. After comparing the pressure gradient −△p/△z′ and the simulated heat transfer coefficient between multiphase and the wall hgs with previously published experiment data, the correctness and reliability of the simulation are able to be verified. In conclusion, firstly, the variation of h′c calculated from δ′ in the ICRM is almost the same with hgs in the case that the RCFB undergoes the rolling motion. In order to quantitatively evaluate the proposed novel method, an error percentage α between h′c calculated from δ′ and hgs is 2.043% which is less than 5%. This certified that the novel method to predict δ′ in the ICRM has higher accuracy to calculate h′c. Secondly, h′c calculated from δ′ is mainly influenced by rolling amplitude Θ rather than by rolling period T. Specifically, with the increase of Θ, amplitudes of Ts are decreased, which is caused by the increased heat transfer and the decreased δ′ between cluster and wall. The decreased amplitudes of Ts are able to increase heat transfer efficiency, which eventually increases h′c. Thirdly, in the case that normalized rolling period t/T is changed from 0 to 0.5, higher h′c calculated from δ′ in Region Ⅰ results from the large local cross-sectional particle volume fraction clocal in Region Ⅰ with a smaller δ′, while lower h′c calculated from δ′ in Region III results from the smaller clocal in Region III with a larger δ′. Therefore, it is concluded that h′c calculated from δ′ is able to well predict the bed-to-wall heat transfer coefficient of the RCFB even if the rolling motion is considered.

Packing void ratios of very dense ternary mixtures of similar ellipsoids

The paper presents a numerical study on the void ratio of a ternary mixture of similar ellipsoids. The effect of particle size distribution on the void ratio was investigated through samples of various volume fractions of large, medium, and small particles. The particle size ratio of large, medium, and small particles is 16:4:1. The ratio between the major and minor axes of an ellipsoid is 1.5. Thirty-nine very dense ternary samples and sixteen samples of very dense binary samples have been created using the discrete element method. The void ratios of the ternary samples are similar to those of the round washed sands of broader particle sizes. The void ratios of these numerical samples were estimated by an existing model of ternary mixtures of spheres without adjusting the constants to examine the effect of particle shape. A novel phenomenological model is suggested for the void ratio of a ternary mixture. The model is applied to the ternary mixture of similar ellipsoids, and the ternary mixture of round washed sands. A good agreement is observed in both cases.

Manufacturing high strength aluminum matrix composites by friction stir processing: An innovative approach

Abstract Shape Memory Alloys (SMA), e.g., NiTi (nitinol), are good candidates of reinforcement agents for the improvement of mechanical properties of aluminum alloys. Friction Stir Processing (FSP) has been shown to be an appropriate manufacturing process for particulate Aluminum Matrix Composites (AMCs), due to the mitigation of critical intermetallic formation between particles and matrix. However, AMCs processed by FSP with the matrix material made of high strength alloys, in particular 7xxx series, have systematically shown severe defects, such as clusters and cavities that result from poor material flow. Herein, we present an innovative strategy to manufacture Al7075/NiTi composites via FSP avoiding this unacceptable particle clustering. The strategy involves the addition of 7xxx series Al powder in a groove or multi-holes to be stirred together with the NiTi particles, leading to dissociation of the latter. The X-ray computed tomography acquisitions allow for 3D quantitative assessment of the size and spatial distribution of the embedded reinforcements. It is revealed that the groove filling method results in a more homogenous distribution compared to the multi-holes filling method. Moreover, the distribution is found to be dependent on the particle size, as a higher volume fraction of large particles is observed at the upper part of the FSP stir zone. These new AMCs, with a good Al-NiTi interface, present a potential to tailor the composite mechanical properties by exploiting the shape memory effect of the NiTi reinforcements.

Modeling and simulation of a single-mode multiphase Richtmyer–Meshkov instability with a large Stokes number

This study focuses on the effects of a large Stokes number (St) on the perturbation growth in linear and nonlinear stages of a Richtmyer–Meshkov instability (RMI) in a gas-particle system, which to the best of our knowledge has not been previously reported. A linear growth model is developed by linear stability analysis and numerically verified by the compressible multiphase particle-in-cell (CMP-PIC) method. Additionally, the RMI growth characteristics in the nonlinear stage are also investigated by CMP-PIC. For the linear growth model, two major differences characterize the effects of a large St. The first one is that an RMI with a large St, which performs significantly different from the RMI with a small St, is induced and driven only by the density difference of the gas-phase and totally independent of particle density. Second, due to the significant momentum coupling effects between gas and particle phases, which govern the gas-particle flow, the growth rate experiences exponential decay, even in the linear RMI stage. The decay behavior performs markedly different from any previous RMI models, especially those of the original single-phase RMI and the gas-particle RMI with a small St. Notably, in the nonlinear stage of the RMI with a large particle volume fraction, the decay effects are much more pronounced and lead to a fall in the growth rate to almost zero, which is not found in any other type of RMI. These findings offer the possibility to develop a new method to control the development of hydrodynamic instability.

Effect of surface texture, size ratio and large particle volume fraction on packing density of binary spherical mixtures

Packing density is a key factor in governing the properties of materials such as concrete, asphalt, ceramic etc. Therefore, determination of packing density of a particulate mixture accurately, is of great importance. However, involvement of many external and internal factors such as surface texture, shape, method of packing etc. has made it very complicated and tedious to determine the packing density. The study investigated the combined effect of particle surface texture, size ratio and large particle volume fraction on packing density and developed a descriptive model to predict the packing density. Further, design graphs were also developed for the convenience. The study revealed that the British pendulum number value of the surface texture linearly varies with the packing density. The rougher the surface, the lower the packing density. The interparticle friction hinders the particle rearrangement. Hence, the ability to achieve a higher packing state is reduced. Further, irregularities in the surface boundary create void spaces, increasing the total voids in the mix. Thus, the packing density reduces. The increase of the size ratio decreases the packing density linearly. The packing density variation with a large particle volume fraction follows a 3rd order polynomial curve. The trend analysis was conducted to develop the descriptive model and design graphs to predict the packing density of binary mixtures.

High-performance magnetostrictive composites with large particles volume fraction

An innovative two-step method is used to produce high-performance magnetostrictive polymer composites based on Tb0.5Dy0.5Fe1.95 alloy crystallites. The alloy crystallites are dynamically magnetically oriented in liquid epoxy, and then concentrated by extruding excess epoxy before curing in a horizontal magnetic field. Composites with a large particle volume fraction of 57% and enhanced <111> particulate orientation were realized. The saturation magnetostriction and the energy density exceeded 1500 ppm and 16 kJ/m3, which were 15% and 60% greater than for static magnetically-oriented composites. The large alloy volume fraction results in a performance approaching that of the monolithic Tb-Dy-Fe alloy, but in a flexible shape and without the limited frequency range.

Magnetorheology of Bimodal Fluids in the Single–Multidomain Limit

In this manuscript we investigate the shear rheology, sedimentation stability and redispersibility characteristics of bimodal MR fluids with a large-to-small size ratio σL/σS ≈ 100 where the small-size population of particles is in the single–multidomain limit (σS ≈ 100 nm) to promote the formation of core–shell supraparticles (i.e., large particles surrounded by the smaller ones). We focus on the effect of mixing the two kinds of particles in different proportions while keeping either the large particle volume fraction or the total volume fraction constant. Five different nanoparticles, having different chemical compositions and shapes, are investigated in this work: barium ferrite, magnetite, iron, chromium dioxide, and goethite. The results demonstrate that nanoparticles fill the voids between microparticles, and this locally enhances the magnetic field. The on-state yield stress and effective enhancement may increase or decrease depending on the magnetization of the nanoparticles as compared to that o...

Effect of vibration frequency, size ratio and large particle volume fraction on packing density of binary spherical mixtures

The packing density of a particulate mixture is an important aspect of materials sciences and engineering. The packing density of a mixture depends on several parameters. This study considers the combined effect of vibration frequency, size ratio and large particle volume proportion on packing density of binary spherical mixtures.When all other parameters are constant, the increase of vibration frequency increases the packing density to a maximum and further increase of vibration frequency decreases the packing density of a mixture. The relationship found to be following a 3rd order polynomial curve. The increase of size ratio decreases the packing density linearly. The increase of large particle volume proportion also increases the packing density to a maximum and further increase of large particle proportion reduces the packing density rapidly. This variation also follows a polynomial pattern. These relationships were analyzed to develop a model incorporating combined effect and design graphs were developed from the derived model. The design graphs can be easily used to determine the packing density for a given vibration frequency and given size ratio.

A study of factors influencing the water absorption capacity of Canadian hard red winter wheat flour

The nature of variation in water absorption capacity in Canadian hard red winter (HRW) wheat genotypes was studied. Wheat and flour were evaluated for a range of physical and chemical properties including hardness indices, protein content, total and water-extractable arabinoxylan (WEAX) contents and flour particle size by laser diffraction. Absorption, as measured by farinograph (FA) ranged from 54 to 66%. The most highly correlated conventional parameters to FA were protein and WEAX contents, particle size index and starch damage, with correlations ranging from r = 0.37 to 0.56 in absolute values. The volume fractions of large and small flour particles had the highest correlations to FA (r∼0.67). Multivariate regression analysis produced 3 or 4 variable models of FA prediction with R2 ranging from 0.64 to 0.95, depending on the sample population. Results indicated that FA is a complex quality parameter with low values attributable to one or more of low levels of wheat hardness, protein content or AX content, depending on genotype. Improving FA in HRW wheat consistent with hard spring wheat appears to be feasible by increasing concentration of deficient parameters by breeding, especially WEAX which has high heritability and ample variation in the HRW wheat germplasm that was studied.

Experimental study on the packing densification of mixtures of spherical and cylindrical particles subjected to 3D vibrations

ABSTRACTTo identify the dense packing of cylinder–sphere binary mixtures (spheres as filling objects), the densification process of such binary mixtures subjected to three-dimensional (3D) mechanical vibrations was experimentally studied. Various influential factors including vibration parameters (such as vibration time t, vibration amplitude A, frequency ω, vibration acceleration Γ) as well as particle size ratio r (small sphere vs. large cylinder), composition of the binary mixtures XL (volume fraction of cylinders), and container size D (container diameter) on the packing density ρ were systematically investigated. The results show that the optimal vibration parameters for different binary cylinder–sphere mixtures are different. The smaller the size ratio, the less vibration acceleration is needed to form a stable dense packing. For each binary mixture, high packing density can be obtained when the volume fraction of large cylindrical particles is dominant. Meanwhile, increasing the container size can decrease the container wall effect and get higher packing density. The proposed analytical model has been proved to be valid in predicting the packing densification of current cylinder–sphere binary mixtures.

The Impotence of Non-Brownian Particles on the Gel Transition of Colloidal Suspensions.

The ability to predict transitions in the microstructure of mixed colloidal suspensions is of extreme interest and importance. The data presented here is specific to the case of battery electrode slurries whereby the carbon additive is reported to form strong colloidal gels. Using rheology, we have determined the effect of mixed particle systems on the critical gel transition . More specifically, we show that the introduction of a high volume fraction of large non-Brownian particles has little to no effect on . Although is unchanged, the larger particles do change the shape of the linear viscoelasticity and the nonlinear yielding behavior. There are interesting similarities to the nonlinear behavior of the colloidal gels with trends observed for colloidal glasses. A comparison of experimental data and the prediction from theory shows that the equation presented by Poon et al. is able to quantitatively predict the transition from a fluid state to a gel state.

Simulation of shear thickening in attractive colloidal suspensions.

The influence of attractive forces between particles under conditions of large particle volume fraction, ϕ, is addressed using numerical simulations which account for hydrodynamic, Brownian, conservative and frictional contact forces. The focus is on conditions for which a significant increase in the apparent viscosity at small shear rates, and possibly the development of a yield stress, is observed. The high shear rate behavior for Brownian suspensions has been shown in recent work [R. Mari, R. Seto, J. F. Morris and M. M. Denn PNAS, 2015, 112, 15326-15330] to be captured by the inclusion of pairwise forces of two forms, one a contact frictional interaction and the second a repulsive force often found in stabilized colloidal dispersions. Under such conditions, shear thickening is observed when shear stress is comparable to the sum of the Brownian stress, kT/a3, and a characteristic stress based on the combination of interparticle force, i.e. σ ∼ F0/a2 with kT the thermal energy, F0 the repulsive force scale and a the particle radius. At sufficiently large ϕ, this shear thickening can be very abrupt. Here it is shown that when attractive interactions are present with the noted forces, the shear thickening is obscured, as the viscosity shear thins with increasing shear rate, eventually descending from an infinite value (yield stress conditions) to a plateau at large stress; this plateau is at the same level as the large-shear rate viscosity found in the shear thickened state without attractive forces. It is shown that this behavior is consistent with prior observations in shear thickening suspensions modified to be attractive through depletion flocculation [V. Gopalakrishnan and C. F. Zukoski J. Rheol., 2004, 48, 1321-1344]. The contributions of the contact, attractive, and hydrodynamics forces to the bulk stress are presented, as are the contact networks found at different attractive strengths.