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

To 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 X_L (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.     

Assessment of powder flow characteristics in incoherent soup concentrates

Soup mixtures represent specific flow characteristics while particles of different sizes and properties form a homogeneous mixture. In such mixtures, particle–particle interactions differ with addition of different fat types. This study was done to present a characterization and comparison of the cohesion index, powder flow speed dependency and caking properties of four different aggregates of fat particles with three different moisture contents, used in various compositions of powdered soup concentrates. ESEM micrographs have shown that different fat types bind particles differently. The moisture content of cream soup concentrates has a significant influence on cake height ratio of all cycles – more moisture causes higher cake height ratios (from r_s = 0.86 to r_s = 0.76; p < 0.05). Cream soup concentrates cake strength values are also significantly influenced by the mixtures moisture content – higher moisture content samples showed higher cake strength values (r_s = 0.73, p < 0.05). There is a significant correlation between compaction coefficients of all speeds and cake height ratios of all cycles for the cream soup concentrates. Combination of measuring techniques (Powder Flow Analyzer, ESEM and Mastersizer) together with Spearman’s Rank Order Correlation, as a method of non-parametric statistics, provides parturient results in characterization of extremely non-homogenous powder mixtures.     

Effects of particle size ratio on the macro- and microscopic behaviors of binary mixtures at the maximum packing efficiency state

The effects of particle size ratio on the mechanical behaviors of binary mixtures are investigated using three-dimensional discrete element method. The samples with three types of particle size ratios (SR, \(\hbox {SR}=3.0\), 4.5 and 6.0) are prepared at the maximum packing efficiency state, which appears at 70 % of the large particle volume content. The results demonstrate that the maxima of peak deviator stress are obtained at the sample with \(\hbox {SR}=4.5\). The initial elastic modulus, \(E_{0}\), increases with increasing SR. The value of deviator stress increases with increasing SR at the softening stage, whereas an opposite trend is observed at the critical state. The evolutions of the effective particle ratio can capture the differences among the evolutions of the deviator stress of different samples during the softening stage and the critical state to some degree. In addition, different SRs generate different packing structures of the binary mixtures and have apparent influences on the force chain networks in the binary mixtures. With increasing SR, the strong force chains become stronger, and the weak force chains become weaker. The deviator stress contributed by the contacts between large particles and between large and small particles constitutes a major part of the overall deviator stress of the binary mixtures at the maximum packing efficiency state. Furthermore, the positions of the critical state lines of binary mixtures in the \(p-q\) plane and \(v-\ln p\) plane are sensitive to SR, and the lubrication effect of small particles in the binary mixtures is enhanced with increasing SR.     

牙科复合树脂中无机填料双峰混合体的堆积性能

通过测定用作牙科复合树脂中无机填料的粉体的振实密度 ,使用Mathematica和SAS/STAT软件 ,用统计学的方法得到了双组分粉体混合物的粒度分布与堆积体比容之间关系的经验公式。说明可以使用引入了粒度分布宽度作为变量的改良线性堆积模型预测双峰粉体的堆积密度。结论为双组分粉体大小粒径比相差越大 ,混合物堆积密度越高。该结论为优化牙科复合树脂的组成提供了理论依据     

Effect of particle size ratio and contribution of particle size fractions on micromechanics of uniaxially compressed binary sphere mixtures

This paper is an extension of the recent work of Wi?cek (Granul Matter 18:42, 2016), wherein geometrical parameters of binary granular mixtures with various particle size ratio and contribution of the particle size fractions were investigated. In this study, a micromechanics of binary mixtures with various ratio of the diameter of small and large spheres and contribution of small particles was analyzed using discrete element simulations of confined uniaxial compression. The study addressed contact normal orientation distributions, global and partial contact force distributions and pressure distribution in packings of frictional spheres. Additionally, the effect of particle size ratio and contribution of particle size fractions on energy dissipation in granular mixtures was investigated. The particle size ratio in binary packings was chosen to prevent small particles from percolating through bedding. The bimodality of mixtures was found to have a strong effect on distribution of contact normal orientation and distribution of normal contact forces in binary mixtures. Stress transfer in binary packing was also determined by both, particle size ratio and volume fraction of small particles. Dissipation of energy was higher in mixtures with higher particle size ratios and decreased with increasing contribution of small spheres in system.     

Predicting porosity of binary mixtures made out of irregular nonspherical particles: Application to natural sediments

Predicting porosity or packing density of sediments made of coarse and fine components of arbitrary geometry is critical to many science and engineering applications. Well-established analytical models for packing of spheres express porosity of the binary mixture as a function of fine-to-coarse particle size ratio. Nevertheless, the applicability of such models to natural granular materials is limited given the nonspherical and irregular nature of the particles whose packing depends on both particle size and shape. The objective of this study is to develop a model that predicts the porosity of binary mixtures made up of irregular nonspherical particles. We modified a previously developed linear sphere-packing model so that it takes into account the effect of both the particle size and shape. As an input, the modified model uses the coarse-to-fine particles specific surface area ratio instead of using the particle size ratio required by the sphere-packing model. We tested the modified model by predicting the porosities of a binary mixture composed of coarse and fine calcite aggregates. We further validate the model by using published data on the porosity of binary mixtures made of synthesized, cubical and cylindrical particles. Our model predictions show good agreement with the measured porosity.     

Effect of the particle size ratio on the structural properties of granular mixtures with discrete particle size distribution

In this study, the discrete element method was used to examine the structural properties and geometric anisotropy of polydisperse granular packings with discrete uniform particle size distributions. Confined uniaxial compression was applied to granular mixtures with different particle size fractions. The particle size fraction (class) was defined as the fraction of the sample composed of particles with a certain size. The threshold value of number of particle size fractions (i.e., the value above which structural properties of assemblies remain constant) was determined. The effect of heterogeneity in particle size on the critical value of number of particle size fractions was investigated for packings with different ratios between diameters of the largest and smallest grains. The threshold number of particle size classes decreased from five to three as the diameter ratio between the largest and smallest grains increased. Regardless of the diameter ratio, the critical number of particle size fractions (above which the packing density and coordination number of the granular mixtures remained constant) was determined to be five. The study has also shown an increase in packing density of binary mixtures with particle size ratio increasing up to 2.5, which was followed by decrease in density of mixtures with larger particle size ratios, which has not so far been reported in the literature.     

Investigation on the influence of granular packing on the flow properties of cementitious suspensions

Fresh concrete can be considered as a suspension of grains of various sizes in a continuous fluid phase. The rheological properties of fresh concrete greatly depend on the physical factors, chemical and mineralogical characteristics of the fine components. Interparticle interactions occur during flow and modify the apparent rheological behaviour. Therefore, a thorough understanding of the rheological behaviour of cementitious suspensions with respect to particle packing is required. The objective of this study is to characterise the interrelationship between the flow properties and the particle packing density of the cementitious suspensions. The experimental investigation included Puntke tests for determining the packing density, and rheological tests that were performed in a rheometer for the characterisation of cement and silica fume (C + SF) as well as cement and fly ash (C + FA) mixtures. The effect of the water to powder ratio (w/p ratio) and the packing density on the flow properties of the cementitious suspensions was studied. From the study, it was observed that a good correlation exists between the w/p ratio and the yield value (g) for both C + SF and C + FA mixtures. The packing density shows a marked influence on the value of g for both mixtures, but has less influence on the value of plastic viscosity (h) for C + FA mixture.     

Effects of multicomponent catalyzer on preparation of ultrafine α-Al2O3 at low sintering temperature

α-Al₂O₃ powder, with a purity of 99.95% and an average particle size of 80 nm, was prepared via the thermal decomposition of AACH precursor. During the AACH preparation, OP-10 (alkylphenol ethoxylates) was used as a dispersant and a deflocculating agent, and a self-confected multicomponent catalyzer (MC) was used to prevent powder agglomeration. Our experiments illustrated that: apart from the necking and agglomeration-preventing effect during the sintering process, MC had potential promotive effects on the reduction of α-Al₂O₃ phase transformation temperature by maintaining the powder particles in a comparatively dynamic-sintering state, thus significantly improving the dispersibility of the sintered powder. BET and ICP analyses indicated that MC was propitious for increasing the specific surface area of the formed α-Al₂O₃ powder and induced very tiny impurities in the sintered products.     

Effects of supersaturation on CdS film growth from dilute solutions on glass substrate in chemical bath deposition process

CdS films were prepared on a glass substrate and their growth was analyzed as a function of the supersaturation ratio in dilute solutions. Nanocrystalline CdS with a hexagonal (wurtzite) structure is present in the CdS films and aggregated into colloidal particles. The particle size increases from 117 nm to 357 nm when the relative supersaturation ratio (S_r) increases from 0.4 to 1.0, but particle size decreases to 149 nm and the absorption edge blue shifts from 488 nm to 456 nm when S_r increases further to 4.0. The evolution of the CdS film is explained on the basis of the proposed growth kinetic model.     

The role of various boron precursor on superconducting properties of MgB2/Fe

The superconducting properties of Fe sheathed MgB₂ wire has been studied as a function of precursor B powder particle size. The in situ processed MgB₂ samples were prepared by means of conventional solid state reaction method with magnesium powder (99.8%, 325 mesh) and three different types of amorphous boron powders (purity; 98.8%, >95% and 91.9%) from two sources, Pavezyum (Turkish supplier) and Sigma Aldrich. The particle sizes of Turkish boron precursor powder were selected between 300 and 800 nm. The structural and magnetic properties of the prepared samples were investigated by means of the X-ray powder diffraction (XRD) and ac susceptibility measurements. The XRD patterns showed that the diffraction peaks for our samples belong to the main phase of the MgB₂ diffraction patterns. The highest critical temperature, T_c = 38.4 K was measured for the MgB₂ sample which was fabricated by using the 98.8% B. The critical current density of this sample was extracted from the magnetization measurements and J_c = 5.4 × 10⁵ A cm⁻² at 5 K and B = 2 T. We found that the sample made by using the 98.8% boron showed almost 2 times higher J_c than that of obtained from 91.9% B powder.     

Mathematical modeling of coating time in dry particulate coating using mild vibration field with bead media described by DEM simulation

To theoretically understand the previously reported dry particulate coating process using a mild vibration field with a bead media, a mathematical analysis model of the dry coating system was developed. In this coating process, an ordered mixture with coarse host particles (drug-loaded ion exchange resin, diameter approximately 100 µm) and fine guest particles (acrylic polymer particle, primary particle size of approximately 100 nm) is formed using a vibrating a vessel. Second, the guest particles on the host particulate surface are firmly fixed using the collision of coated particles zirconia beads (diameter 1.5 mm). Our model assumes that the unfixed guest particles are fixed by particle-to-particle collisions (C_c) provided by the apparatus, thereby increasing the coating ratio. C_c was estimated using the discrete element method and some experimental results. The model includes parameters such as the number of C_c, host particles and unfixed guest particles. The coating time simulated by the established model equation in this study fits well with the experimental results of the dry process. It depends on the ratio of the number of collisions contributing to the increased coating ratio to the number of unfixed guest particles on the surface of host particles.     

Numerical study on compression processes of cohesive bimodal particles and their packing structure

In this study, the compression characteristics of bimodal cohesive particles were investigated using a discrete element method (DEM) simulation. The compression and packing processes were simulated under different conditions of size ratios of 1–4 and fine particle mixing ratios of 0–0.5. The cohesive force was expressed using the surface energy proposed by the Johnson-Kendall-Roberts (JKR) cohesion model having a surface energy of 0–0.2 J/m². The calculated results demonstrated that even in the case of cohesive particles, an increase in the particle size ratio reduced the void fraction of the powder bed during the packing and compression processes. In addition, it was found that the cohesive force decreased the contact number, especially the coarse-coarse contacts, although it had little impact on the void fraction. Our DEM simulations suggested that it is necessary to evaluate the contact numbers even under similar void fractions, which will be essential in the case of different material mixtures, such as all-solid-state batteries.     

The influence of densification temperature and the size of powder particles on the properties of isotropic Nd-Fe-B magnets made of quenched ribbons

The influence of densification temperature and the size of powder particles on magnetization of remanence J _r , coercivity _J H _c of hot-densified Nd-Fe-B magnets made of MQP-A quenched ribbons was investigated. A connection between magnetic properties, parameters of production process and microstructure of the magnet was confirmed. It was found that the remanence of isotropic magnet depends on temperature of hot densification process but is practically independent of the size of powder particles. On the other hand, the temperature of hot densification process and the size of powder particles have a substantial influence on coercivity.     

Optimization of the spray-drying process for developing aquasolv lignin particles using response surface methodology

Particle size and morphology are of high industrial importance due to the fact that product properties and performance can be affected by this factors. For instance, bulk properties, processability and appearance of the final product are given by particle size and shape. In this regard, the optimization of process parameters for particle development is required when targeting formulation of specific product and/or particular properties. Response surface methodology (RSM) was used to optimize the spray drying process for the development of aquasolv lignin particles with desired particle size and morphology. The inlet drying temperature X₁: 180–200 °C, atomization pressure X₂: 1.3–1.7 bar and feeding rate X₃: 65–75 mL min⁻¹ were kept as independent variables while the optimizing responses were_: Yield fine of particles with desired particle size and particle size (D₅₀). The quadratic part of the equation and statistical analysis showed substantial effect of the atomizing pressure and feeding rate on the responses and the optimized conditions validated the model. Optimum processing conditions for spray drying of aquasolv lignin were inlet temperature of 173 °C, 1.8 bar atomization pressure and 62 mL/min feeding rate. With this, desired responses of powder were 66% of yield and particle size of D₅₀ < 30 µm were obtained. The experimental values were found to be in agreement with the predicted values indicating the suitability of the model in predicting the particle size and yield of aquasolv lignin.     

Mixing narrow coarse and fine coal fractions – The maximum volume fraction of suspensions

A main objective of coal-water slurry fuel (CWSF) preparation is to achieve maximum loading of coal. In the absence of a strong colloidal attractive force, the maximum loading is determined by the packing density of the particles which in turn is a function of the particle size distribution. In this study, coal fractions of different mean sizes with a narrow distribution were separated by sieving. Mixtures of different mean coarse to fine size ratios were then prepared. For each size ratio, different amounts of coarse particle contents were prepared. With these different mixtures, water was added to produce the CWSF. The maximum volume packing density, φ_max, for each mixture was determined using a rheological vane yield stress technique. The determination of φ_max involving the direct yield stress measurements of extremely high concentration suspensions is an entirely new and accurate approach. It was found that the highest φ_max was obtained when the coarse to fine ratio was ～10 located at a coarse coal content of 70 wt%. This result is consistent with that obtained by theoretical modelling of bimodal mixing of monodisperse size spheres with a size ratio of 10. At lower size ratio, φ_max obtained at the optimum coarse coal content was lower.     

Synthesis of α-alumina powder with internal nanostructures from Al13-cluster

Nanostructured α-alumina powder was synthesized by precipitation and calcination of Al₁₃-clusters that were formed by the hydrolysis of Al³⁺ions with hydroxide. The Al₁₃-clusters were precipitated with oxalic acid by two-stage and one-stage precipitation techniques. The precipitates were calcined in air at 1100 °C. The resulting α-alumina particles were characterized using particle size analyzer, X-ray diffraction (XRD) and transmission electron microscope (TEM). The pH and precipitation technique were found to influence the microstructural features of the α-alumina powder produced. Alumina with more extensive nanostructures inside the grain can be produced through the two-stage precipitation technique.     

Structural and magnetization studies on nanoparticles of Nd doped α-Fe2O3

It has been observed that, compared to bulk form, the nanocrystalline α-Fe₂O₃ is finding application in various areas. Magnetic properties of α-Fe₂O₃ are found to be influenced by the size of particles and are also sensitive to synthesis method employed for sample preparation. In the present work we have prepared a series of Nd doped α-Fe_2−xO₃ samples (x = 0.0–0.5) by combustion method, without using any fuel. The analysis of room temperature neutron diffraction patterns shows that all the compounds of the series form in the hematite (α-Fe₂O₃) structure, space group R−3c. Magnetization measurements show that there is a broad distribution of particle size in the samples. We find that the increase in the Nd content results in the dilution of magnetism of α-Fe₂O₃. From results we believe that inclusion of Nd in α-Fe₂O₃ drastically modifies the magnetic properties.     

Reaction pathways,activation energies and mechanical properties of hybrid composites synthesized in-situ from Al–TiO2–C powder mixtures

In-situ aluminum matrix composites were fabricated from Al–TiO₂–graphitic C powder mixtures using exothermic dispersion method. The effects of C/TiO₂ molar ratio on the reaction processes, activation energies and mechanical properties of the resulting materials were investigated. When the C/TiO₂ molar ratio is 0, Al reacts with TiO₂ to produce fine α-Al₂O₃ particles and Ti, which then reacts with Al to form large rod-like Al₃Ti phase. By adding graphite C into the Al–TiO₂ system, the activation energy of the first reactive step increases; in addition, the resultant Ti preferentially reacts with C to form hard TiC particles. When the C/TiO₂ molar ratio increases to 1.0, the Al₃Ti phase disappears and the reinforcements consist of nano-sized α-Al₂O₃ and TiC phases. The tensile strength of the composites increases from 239.2 MPa to 351.8 MPa and the elongation increases from 4.1% to 5.6%, suggesting a marked increase in damage tolerance (i.e., toughness).     

Conductivity and percolation in epoxy resin/conductive filler composites

Abstract

Composites of the polymer/filler type, when the filler is a conductive material such as metal particles, exhibit electrical conductivity that increases with concentration of the conductive phase. These complex materials are considered to be chaotic mixtures of conductive particles randomly distributed in an insulating matrix. The conductivity of these materials in dc electric fields is studied in terms of percolation theory, where electrical conductivity σ is rapidly increased at a critical concentration, defined as P _c, of the conductive phase, according to σ≈(P­P _c)^α. In this work, various epoxy resin/conductive filler composites were prepared. The filler was metal powder of copper, aluminium, or zinc. The conductive behaviour of these materials was studied at temperatures varied from 20 to 140°C. Data obtained from these measurements are analysed using percolation theory and introducing new parameters β and σ_c into the above relation. A semiempirical algorithm is introduced for the determination of α, β, σ_c, and P _c.