Interparticle Contact Area Research Articles

Contact radius and curvature corrections to the nonlocal contact formulation accounting for multi-particle interactions in elastic confined granular systems

We present contact radius and curvature corrections to the nonlocal contact formulation that account for multi-particle interactions in elastic confined granular systems. The nonlocal contact formulation removes the classical assumption of independent contacts by taking into account the interplay of deformations due to multiple contact forces acting on a single particle. The contact radius correction considers the components of these deformations that contribute to the inter-particle contact area. The curvature correction improves the description of contacting surface profiles by including higher order terms in their Taylor series expansions. To validate the corrected formulation, we restrict attention to rubber spheres under different loading conditions, in the absence of gravitational forces, adhesion or friction. Specifically, we show that the predictions of contact force and radius are in remarkable agreement with finite-element simulations and experimental observations up to levels of deformation at which contact impingement occurs, which was not possible with the original elastic nonlocal contact formulation. Convergence of the curvature corrected formulation is observed at a four-term correction.

Sintering shrinkage of uniaxial cold compacted iron: influence of the microstructure on the anisothermal and isothermal shrinkage of uniaxial cold-compacted iron

ABSTRACTThe influence of the microstructure of uniaxial cold-compacted green iron on the sintering shrinkage was investigated. Pores in the green parts are very slightly oriented, while the dimension of the interparticle contact areas is anisotropic. A large and anisotropic anisothermal shrinkage in alpha iron was measured, greater than isothermal shrinkage at the 1120°C. The results were interpreted on the basis of the geometrical and of the structural activity, and the effective diffusivity responsible for neck growth was determined.

Reactive Forging − Pressure Assisted Thermal Explosion Mode of SHS for Processing Dense In Situ Composites and Structural Parts: A Review

Over the last decades, there has been a decisive shift from basic science to applied research that brought about the development of a number of unconventional combustion processes that allow for simultaneous synthesis and consolidation of otherwise difficult‐to‐process high temperature materials. External pressure application during thermal explosion (TE) mode of self‐propagating high‐temperature synthesis (SHS) can produce dense products provided the combination temperature and pressure are sufficient for consolidation. In the developed TE‐based reactive forging (RF) method, a self‐sustained reaction is ignited by rapid heat transfer from press die and rams, and a moderate consolidation pressure is applied when the combustion product still contains a sufficient amount of liquid or very soft phase. Plastic flow of the product results in filling the shape of the die thus producing a near‐net‐shape part. Reactive forging (RF) is especially efficient in combination with the developed short distance infiltration (SDI) approach based on the presence of a low melting phase component (e.g., Al, Mg, Ti–Ni eutectics) in the non‐dense reactant blend that is squeezed into the pores under the applied pressure thereby increasing the interparticle contact area and enhancing exothermic heat release rate. Compared to traditional reactive melt infiltration, SDI has the advantage of the considerably shorter infiltration distances (μm vs. mm–cm). Close temperature monitoring during RF and RF/SDI processing provides a deeper insight into the SHS reaction mechanisms. The “heat sink” action of the pressure die after TE results in rapid cooling of the consolidated products and correspondingly fine microstructures and high mechanical properties. Examples of successful application of the RF/SDI processing of homogeneous and functionally graded in situ composites based on binary (Al2O3, TiB2, TiC, TiN) and ternary (MgAl2O4, Ti3SiC2, Ti3AlC2) ceramics, as well as intermetallic–ceramic and metal–ceramic interpenetrating phase composites are described. Nanostructuring of reactant blends results in lower SHS ignition temperatures, and thus can be utilized for two‐stage ignition of multi‐component composite materials and structures with tunable ignition delay. Pressure assisted SHS is a “green,” cost‐effective, and thus perspective manufacturing route of near‐net‐shape structural parts. Examples of successful application of RF and RF‐SDI approach to fabrication of ceramic matrix–diamond grinding wheels, light armor tiles and high melting temperature nozzles are presented.

Ultrasonic investigation of granular materials subjected to compression and crushing

Ultrasonic wave propagation measurement has been used as a suitable technique for studying the granular materials and investigating the soil fabric structure, the grain contact stiffness, frictional strength, and inter-particle contact area. Previous studies have focused on the variations of shear and compressional wave velocities with effective stress and void ratio, and lesser effort has been made in understanding the variation of amplitude and dominant frequency of transmitted compressional waves with deformation of soil packing. In this study, continuous compressional wave transmission measurements during compaction of unconsolidated quartz sand are used to investigate the impact of soil layer deformation on ultrasonic wave properties. The test setup consisted of a loading machine to apply constant loading rate to a sand layer (granular quartz) of 6 mm thickness compressed between two forcing blocks, and an ultrasonic wave measurement system to continuously monitor the soil layer during compression up to 48 MPa normal stress. The variations in compressional wave attributes such as wave velocity, transmitted amplitude, and dominant frequency were studied as a function of the applied normal stress and the measured normal strain as well as void ratio and particle size. An increasing trend was observed for P-wave velocity, transmitted amplitude and dominant frequency with normal stress. In specimen with the largest particle size (D50 = 0.32 mm), the wave velocity, amplitude and dominant frequency were found to increase about 230%, 4700% and 320% as the normal stress reached the value of 48 MPa. The absolute values of transmitted wave amplitude and dominant frequency were greater for specimens with smaller particle sizes while the normalized values indicate an opposite trend. The changes in the transmitted amplitude were linked to the changes in the true contact area between the particles with a transitional point in the slope of normalized amplitude, coinciding with the yield stress of the granular soil layer. The amount of grain crushing as a result of increase in the normal stress was experimentally measured and a linear correlation was found between the degree of grain crushing and the changes in the normalized dominant frequency of compressional waves.

Insight Into a Novel Strategy for the Design of Tablet Formulations Intended for Direct Compression

In a recent study, it was demonstrated that improving flow of a model poorly flowing and poorly compactable drug substance, acetaminophen, via dry coating while using fine excipients, may promote direct compression. To validate this novel strategy, particularly for high drug-loading formulations, this study investigates the effect of microcrystalline cellulose (MCC) particle size and dry coating on powder tabletability and flowability. It was determined that blends containing fine-sized MCC (20 μm) resulted in the highest tablet tensile strength and best tabletability because it provides higher interparticle contact area compared with coarse-sized MCC. Although tabletability can be improved using fine MCC, flowability is poor but can be improved through dry coating, a process that coats glidants (nano-sized silica) onto particle surfaces. To retain the tabletability, which was adversely affected because of the presence of glidant in the blend, while simultaneously enhancing flowability via dry coating, separately blending the drug substance with glidant is shown to be the best method of processing. The combined use of fine excipients and selective dry coating offers a novel and advantageous formulation strategy in comparison with the conventional use of coarse excipients, such as Avicel PH 102, that have been designed and marketed for direct compression.

Particle shape effect on thermal conductivity and shear wave velocity in sands

This study presents the correlations between quantified shape parameters and geotechnical properties for nine sand specimens. Four shape parameters, sphericity, convexity, elongation and slenderness, were quantified with two-dimensional microscopic images with the aid of image processing techniques. An instrumented oedometer cell is used to measure compressibility, thermal conductivity and shear wave velocity during loading, unloading and reloading stages. As the particle shape inherently determines the initial loose packing condition, initial void ratio and shape parameters are well correlated with compressibility. The applied stress in soils increases the interparticle contact area and contact quality; round particles tend to achieve higher thermal conductivity and shear wave velocity during stress-induced volume change. Multiple linear regression is implemented to capture the relative contributions of involved variables, revealing that the thermal evolution is governed by the initial packing density and particle shape. The experimental observations underscore the predominant effect that particle shape has on the geomechanical and physical properties upon stress-induced soil behavior.

Microstructure characteristics of high borated stainless steel fabricated by hot-pressing sintering

The present study investigated the microstructure of powder metallurgy (P/M) high borated stainless steel through hot-pressing sintering in a temperature range of 1000–1150 °C within 30 min under 30 MPa. Microstructure and phase examinations were carried out by applying scanning electron microscope, electron backscatter diffraction and X-ray diffraction analysis. The results of as-atomized powders demonstrated that many powders kept egg-type structure with an austenite outer layer and the eutectic borides were much finer than those in traditional cast products. Microstructure studies revealed that borides suffered Ostwald ripening and were significantly influenced by the sintering temperature. Orientation maps indicated that the inter-particle contact areas consisted of equiaxed grains and the regions consisting of large elongated grains partly inherited the microstructure characteristics of as-atomized powder particles. Furthermore, the mechanisms governing the morphological changes in microstructure were discussed.

Polystyrene colloidal crystals: Interface controlled thermal conductivity in an open-porous mesoparticle superstructure

Colloidal crystals typically consist of sub-micron sized monodisperse particles, which are densely packed on a face centered cubic lattice. While many properties of this material class have been studied over the past decades, little is known about their thermal transport properties. The high amount of interfaces and their small interparticle contact area should result in efficient thermal insulation. Using laser flash analysis we report for the first time on the temperature dependent thermal conductivity of a freestanding 366nm polystyrene (PS) colloidal crystal. Macroscopic monoliths of these samples were fabricated by colloidal self-assembly. We demonstrate a very low thermal conductivity κ of 51mWK−1m−1 (κ of bulk PS∼140mWK−1m−1). Remarkably, this low thermal conductivity is reached at a comparatively high density of 750kgm−3. It can be further increased by almost 300% upon film formation and loss of the colloidal mesostructure. Additionally, this open porous structure is largely independent of the surrounding atmosphere. This can be rationalized by the small size (∼100nm) of the pores present within this colloidal crystal.

Semi-solid Powder Rolling of AA7050 Alloy Strips: Densification and Deformation Behaviors

Semi-solid powder rolling (SSPR) is a novel strip manufacturing process which combines the features of semi-solid rolling and powder rolling. SSPR involves rolling procedure of metallic alloy powders when both liquid and solid phases coexist. In this study, densification process and deformation mechanisms of AA7050 powder in the presence of liquid phase were investigated. The relationships between the apparent plastic Poisson’s ratio and the relative density as well as the thickness reduction ratio and relative density were calculated. The relationship between deformation strain and relative density was calculated as well. The experimental data of relationship between rolling forces and relative density were achieved. The results show that the densification process of SSPR also can be divided into three stages and liquid phase mainly affects stage I. In stage I, semi-solid particles’ rearrangement and restacking are the main densification mechanisms when the initial liquid fraction is lower than 20 pct. When the initial liquid fraction is higher than 20 pct, flowing and filling of liquid, semi-solid particles rearrangement and restacking are the densification mechanisms. In stage II, severe deformation of particles associated with a rapid increase in the apparent plastic Poisson’s ratio and interparticle contact area contributes to the densification process. In stage III, the densification mechanisms are the deformation and collapse of isolated pores. The calculated deformation strain increases gradually with the same slopes as the relative density increases in stages II and III no matter how much the liquid fraction is. Higher liquid needs less deformation degree that contributes to the densification.

Discussions of Some Issues for Wind Blown Sand Flow Simulation

The transport of dust and sand by wind can provide a major mechanism conducive to land form changes due to aeolian activity, including sandy coasts, deserts and agricultural fields. Meanwhile, a strong electric field and electrical discharge phenomena could exist and be associated with the generation of dust storms. The wind-blown sand blown flow is a kind of gas-solid two-phase flow. Nowadays, numerical modeling methods are usually employed by researchers for its convenience and speed in forecasting and assessments on pollutants dispersions and variations. Generally, a CFD-DEM method is used for wind-blown sand blown flow simulation, in which large eddy simulation (LES) is a method of computational fluid dynamics (CFD) to simulate the dynamic flow of gas or air and discrete element method (DEM) is often used to simulate the particle-particle collision. Giving the initial and boundary conditions is the key to get the appropriate simulation results. In this study, some issues for wind blown sand flow simulations are discussed, 1) the effect of time-series inlet wind velocity attributed to the atmospheric boundary layer movements that induced by regional climate and local meteorological process; 2) the role of water content adsorbed on sand surface for the tribo-electric charging of wind-blown sand; 3) the evaluation of the inter-particle contact time and contact area in the real heat transfer process when the softening treatment used in the DEM method to track the movement of each particle.

Shape separation of gold nanoparticles using a pH-responsive amphiphilic dendrimer according to their shape anisotropy distinction

We present a novel method for shape-selective separation of nanoparticles in aqueous media utilizing an amphiphilic cationic dendrimer (C18N5) through regulation of the pH of the dispersion medium. This amphiphilic dendrimer (C18N5) bears a poly(amidoamine) dendron with four terminal amine groups and one hydrophobic long alkyl chain, and it can strongly adsorb to the surface of gold nanoparticles at low pH and promote their aggregation at high pH. During aggregation, isotropic NPs with smaller inter-particle contact area have much better solution stability than anisotropic NPs. Therefore, isotropic NPs remain stable in the supernatant, while anisotropic NPs are subject to aggregation and subsequent precipitation. As a result of this simple separation strategy, gold nanoplates with a purity level of 97% (in number density) can be rapidly and conveniently achieved.

Effect of consolidation pressure up to 1.8GPa on the sintering of nanocrystalline nc-Y2O3

Abstract Sintering of nanocrystalline (nc) monoclinic yttrium oxide (Y 2 O 3 ) was studied in the homologue temperature range of 0.4–0.7 T M . Samples were isostatically consolidated at super high pressure (SHP), up to 1.8 GPa. The combined effects of consolidation pressure and sintering temperature on the properties and microstructure are explored. The physical properties of the samples, mainly elastic modulus follow in general the density changes, or the interparticle contact area, but are also affected by sintering temperature. The effect of compaction pressure and sample density on the phase transition is studied and discussed.

Spark-plasma sintering efficiency control by inter-particle contact area growth: A viewpoint

An ambiguity in the available experimental data on the presumably faster spark-plasma sintering (SPS) densification kinetics compared with conventional hot pressing of powders is pointed out. A hypothesis of the major impact of the evolution of the inter-particle contact area on the densification rate is put forward. It is argued that properly controlled interplay between surface diffusion and external pressure-imposed creep provides better efficiency of the SPS process. The formulated ideas can be used for the SPS pressure–temperature cycle optimization.

High-purity gold nanobipyramids can be obtained by an electrolyte-assisted and functionalization-free separation route

Gold nanobipyramids (NBPs) have attracted intensive attentions because they exhibit more advantageous plasmonic properties than comparable gold nanorods. However, unlike that short gold nanorods can be synthesized routinely in a high yield (around 99% of the total particles), current syntheses of gold NBPs generally receive a low yield (around 30% of the total particles) and co-produce spherical impurities difficult to separate. Thus an effective purification route of gold NBPs is desirable for optimizing their performances. In this study, we demonstrated that the spherical nanoparticles with smaller inter-particle contact area can be separated from the NBPs that undergo gradual precipitation by electrolyte-induced electrostatic screening. During this procedure, no special surface-functionalization of the NBPs was needed. As a result of this simple separation strategy, NBPs at a level of purity to above 90% is achieved in a single purification round. In particularly, the precipitates of cetyltrimethylammonium bromide (CTAB)-capped NBPs can be easily converted into colloidal state due to the strong steric constraint of CTA+ bilayer, facilitating further investigations.

Paper-based nanosilver conductive ink

Monolayer-protected silver nanoparticles sized 5 nm were directly synthesized in relatively high concentration (0.093 M) in water phase using cheap chemicals. The ink (20 % silver) prepared with the silver nanoparticles was written on sulfuric paper by a facile pen-on-paper paradigm. The effect factors on the formation and conductive mechanism of the nanosilver films from the silver ink were studied, including the microstructure, size evolution and interaction of particles and so on. Results show that the silver nanoink allows for a low sintering temperature and with a relatively high conductivity. The grown particle, increased inter-particle contact area and off dodecanoic acid layer were the three dominating factors responded to the conductivity of the nanosilver films.

하중 조건이 지반의 열전도도에 미치는 영향: 입자 스케일에서의 연구

지반 물질의 열전도도는 경험식이 제안하는 단위 중량, 간극률 등의 영향 인자 이외에도 하중조건에 따라 크게 좌우된다. 본 논문에서는 개별요소법에 의해 생성된 입자상 지반재료의 열 전달 특성을 열 네트워크 모델로 해석하여 하중이 열전도도에 미치는 영향을 평가하였다. 하중의 변화에 의한 개별 입자들간의 접촉수 및 간극률, 간극수의 전도도에 따른 열전도도를 산출하여 영향 요소들간의 관계를 분석하였다. 전도도의 변화 양상은 전단강성도 분석과 유사하게 열전달 방향 및 하중 크기에 따른 멱함수 형태로 회귀분석이 가능하였다. 해석 결과 하중에 따른 입자간 접촉 면적의 증가 및 간극수의 전도도가 전체 입자상 물질의 열 흐름에 큰 영향을 미침을 알 수 있었다. 열전도도의 이방성은 하중 방향에 의해 좌우되며 입자 스케일에서의 매커니즘이 열 흐름을 좌우하는 중요한 인자임을 보였다. The stress condition mainly dominates the thermal conductivity of soils whereas governing factors such as unit weight and porosity suggested by empirical correlations are still valid. The 3D thermal network model enables evaluation of the stress-dependent thermal conductivity of particulate materials generated by discrete element method (DEM). The relationship among dominant factors is analyzed based on the coordination number and porosity determined by stress condition and thermal conductivity of pore fluid. Results show that the variation of thermal conductivity is strongly attributed to the enlargement of inter-particle contact area by loading history and pore fluid conductivity. This study highlights that the anisotropic evolution of thermal conductivity depends on the directional load and that the particle-scale mechanism mainly dictates the heat transfer in soils.

Modeling and simulation of compact strength due to particle bonding using a hybrid discrete-continuum approach

Abstract The compaction of powder beds into solid bodies occurs by virtue of the formation of inter-particle bonds. The mechanical strength of the compact depends on the type of bonding interaction, as well as, the inter-particle contact area created in the compression process. A hybrid quasi-continuum computational approach has been implemented to study the bonding occurring in compressed granular assemblies. The approach resolves the powder bed on the particle level, allowing for the tracking of contact area generated by particle deformation and the computation of history-dependent inter-particle bonding forces. The magnitude of the bonding force is calculated using a synthetically constructed potential aiming to mimic the shape of typical molecular-type interactions. The uniaxial compaction and subsequent relaxation of powder beds, representative of pharmaceutical excipients have been simulated. Due to the bonding occurring between the individual particles the compressed beds acquire tensile strength. Post-relaxation tensile loading of the compacts is used to quantify the magnitude of this tensile strength. To validate the computational results, tablets are prepared using a compaction simulator under conditions closely resembling the simulated scenarios, where subsequently the tablets are subjected to tensile loads until failure. The predicted values for the tablet strength utilizing the present methodology capture the general trends exhibited by the experimental record.

Three-dimensional random network model for thermal conductivity in particulate materials

This paper describes a three-dimensional random network model to evaluate the thermal conductivity of particulate materials. The model is applied to numerical assemblies of poly-dispersed spheres generated using the discrete element method (DEM). The grain size distribution of Ottawa 20–30 sand is modeled using a logistic function in the DEM assemblies to closely reproduce the gradation of physical specimens. The packing density and inter-particle contact areas controlled by confining stress are explored as variables to underscore the effects of micro- and macro-scales on the effective thermal conductivity in particulate materials. It is assumed that skeletal structure of 3D granular system consists of the web of particle bodies interconnected by thermal resistor at contacts. The inter-particle contact condition (e.g., the degree of particle separation or overlap) and the particle radii determine the thermal conductance between adjacent particles. The Gauss–Seidel method allows evaluation of the evolution of temperature variation in the linear system. Laboratory measurements of thermal conductivity of Ottawa 20–30 sand corroborate the calculated results using the proposed network model. The model is extended to explore the evolution of thermal conduction depending on the nucleation habits of secondary solid phase as an anomalous material in the pore space. The proposed network model highlights that the coordination number, packing density and the inter-particle contact condition are integrated together to dominate the heat transfer characteristics in particulate materials, and allows fundamental understanding of particle-scale mechanism in macro-scale manifestation.

Characterisation of conduction phenomena in soils at the particle-scale: Finite element analyses in conjunction with synthetic 3D imaging

This paper describes an innovative method to characterise conduction parameters in geomaterials at the particle-scale. The technique is exemplified using 3D synthetic grain packing generated with discrete element approaches. This creates a geo-mechanically viable user-defined 3D granular image through which the particle skeleton and the corresponding pore network are constructed. Images are then imported into the finite element analyses to solve the governing equations of hydraulic and thermal conduction. Navier–Stokes equation is uniquely upscaled to Darcy’s law to assess hydraulic conductivity in soils, while a similar approach implements the Fourier equation to evaluate thermal conduction through grain chains and pore network. High performance computing is used to meet demanding numerical calculations of 3D meshed geometries. Packing density (i.e., porosity) and inter-particle contact areas are explored as variables to highlight the effects of pore volume and inter-particle contact condition in hydraulic and thermal conduction. This emerging technique allows not only characterising the macro-scale behaviour of conduction phenomena in soils but also quantifying and visualising the preferential and local conduction behaviour at the particle-scale. Laboratory measurements of hydraulic and thermal conductivities support numerically obtained results and validate the viability of the new methods used herein. This study introduces an alternative way to determine physical parameters of soils using emerging technology of rigorous numerical simulations in conjunction with 3D images, and to enable fundamental observation of particle-scale mechanisms of macro-scale manifestation.

Depletion-Induced Shape and Size Selection of Gold Nanoparticles

For nanoparticle-based technologies, efficient and rapid approaches that yield particles of high purity with a specific shape and size are critical to optimize the nanostructure-dependent optical, electrical, and magnetic properties, and not bias conclusions due to the existence of impurities. Notwithstanding the continual improvement of chemical methods for shaped nanoparticle synthesis, byproducts are inevitable. Separation of these impurities may be achieved, albeit inefficiently, through repeated centrifugation steps only when the sedimentation coefficient of the species shows sufficient contrast. We demonstrate a robust and efficient procedure of shape and size selection of Au nanoparticles (NPs) through the formation of reversible flocculates by surfactant micelle induced depletion interaction. Au NP flocculates form at a critical surfactant micelle molar concentration, C(m)* where the number of surfactant micelles is sufficient to induce an attractive potential energy between the Au NPs. Since the magnitude of this potential depends on the interparticle contact area of Au NPs, separation is achieved even for the NPs of the same mass with different shape by tuning the surfactant concentration and extracting flocculates from the sediment by centrifugation or gravitational sedimentation. The refined NPs are redispersed by subsequently decreasing the surfactant concentration to reduce the effective attractive potential. These concepts provide a robust method to improve the quality of large scale synthetic approaches of a diverse array of NPs, as well as fine-tune interparticle interactions for directed assembly, both crucial challenges to the continual realization of the broad technological potential of monodispersed NPs.