Increasing Packing Fraction Research Articles

Evident structural anisotropies arising from near-zero particle asphericity in granular spherocylinder packings.

With magnetic resonance imaging experiments, we study packings of granular spherocylinders with merely 2% asphericity. Evident structural anisotropies across all length scales are identified. Most interestingly, the global nematic order decreases with increasing packing fraction, while the local contact anisotropy shows an opposing trend. We attribute this counterintuitive phenomenon to a competition between gravity-driven ordering aided by frictional contacts and a geometric frustration effect at the marginally jammed state. It is also surprising to notice that such slight particle asphericity can trigger non-negligible correlations between contact-level and mesoscale structures, manifested in drastically different nonaffine structural rearrangements upon compaction from that of granular spheres. These observations can help improve statistical mechanical models for the orientational order transformation of nonspherical granular particle packings, which involves complex interplays between particle shape, frictional contacts, and external force field.

Effect of Microwave Radiation on Tobacco (Nicotiana tabacum) Using VSWR Method

The values of dielectric constant (ε'p), dielectric loss (ε"p), relaxation time ( p) and conductivity ( p) of the sample were measured for different packing densities at 9.85 GHz microwave frequency and different temperature (200C, 35°C and 500C). Complex permittivity of Tobacco leaves has been measured over the 9.85 GHz single High frequency range using X - band microwave bench. To measure the dielectric parameter by using Sample holder for pulverized sample. We have designed and fabricated the sample holder at Microwave Research Laboratory, NES, Science College, Nanded. The experimental and calculated values for dielectric constant ( ) and dielectric loss ( ) are found to be same at relative packing fractions δr = 1. The above fact indicates that there is a fair agreement between calculated values of dielectric parameters and measured values of solid bulk. The co-relation formulae of Bottcher can be used to provide accurate estimates of ( ) and ( ) of powder materials at known bulk densities. The values of ( p) obtained suggest that the dielectric loss factor ( p) increases with the increase of bulk density of the material. The increase in conductivity with increase of packing fraction suggests that, at higher compaction the particles are more packed, and no micro-cracks developed in the samples. As packing fractions (δr) and temperature increases the moisture content of sample decreases linearly. The variations of ( p), ( p), tanδ, and σp with relative packing fraction (δr), and temperature clearly indicates that, effect of density and temperature on the dielectric parameters. The result shows that large cohesion in the particles of Tobacco leaves powder under investigations.

Revealing structural role of ZrO2 in silicate glasses from macroscale property by rigid‐unit packing fraction method

AbstractThe structural role of an oxide as a former and modifier can have significant effects on the chemical durability and mechanical properties of the glass. Some oxides with high‐field strength cations, for example, MgO and ZrO2, are often labeled as a third group—intermediate, due to their either undetermined or dual structural roles dependent on the glass compositions. Based on our recent modification of the Makishima–Mackenzie (MM) model using the rigid‐unit Packing Fraction (RUPF), we analyzed a series of novel zirconia‐containing bioactive glasses. The RUPF‐based MM‐model provides better prediction of the elastic moduli of these new glasses in comparison to experimental measurements. At the same time, the structural role of zirconia can be determined by comparison with calculations by assuming various structural roles and those from experiments. We reveal that ZrO2 acts as the network former in phosphosilicate glasses, which leading to significant increase in packing fraction and consequent increase in Young's modulus. The recent experimental and atomistic simulation results support the glass former role of zirconia in silicate glasses. This method is general and applicable to other oxides in glasses.

Heterogeneity and Memory Effect in the Sluggish Dynamics of Vacancy Defects in Colloidal Disordered Crystals and Their Implications to High-Entropy Alloys.

Vacancy dynamics of high-density 2D colloidal crystals with a polydispersity in particle size are studied experimentally. Heterogeneity in vacancy dynamics is observed. Inert vacancies that hardly hop to other lattice sites and active vacancies that hop frequently between different lattice sites are found within the same samples. The vacancies show high probabilities of first hopping from one lattice site to another neighboring lattice site, then staying at the new site for some time, and later hopping back to the original site in the next hop. This back-returning hop probability increases monotonically with the increase in packing fraction, up to 83%. This memory effect makes the active vacancies diffuse sluggishly or even get trapped in local regions. Strain-induced vacancy motion on a distorted lattice is also observed. New glassy properties in the disordered crystals are discovered, including the dynamical heterogeneity, the presence of cooperative rearranging regions, memory effect, etc. Similarities between the colloidal disordered crystals and the high-entropy alloys (HEAs) are also discussed. Molecular dynamics simulations further support the experimental observations. These results help to understand the microscopic origin of the sluggish dynamics in materials with ordered structures but in random energy landscapes, such as high-entropyalloys.

Discharge of elongated grains in silos under rotational shear.

The discharge of elongated particles from a silo with rotating bottom is investigated numerically. The introduction of a slight transverse shear reduces the flow rate Q by up to 70% compared with stationary bottom, but the flow rate shows a modest increase by further increasing the external shear. Focusing on the dependency of flow rate Q on orifice diameter D, the spheres and rods show two distinct trends. For rods, in the small-aperture limit Q seems to follow an exponential trend, deviating from the classical power-law dependence. These macroscopic observations are in good agreement with our earlier experimental findings [Phys. Rev. E 103, 062905 (2021)2470-004510.1103/PhysRevE.103.062905]. With the help of the coarse-graining methodology we obtain the spatial distribution of the macroscopic density, velocity, kinetic pressure, and orientation fields. This allows us detecting a transition from funnel to mass flow pattern caused by the external shear. Additionally, averaging these fields in the region of the orifice reveals that the strong initial decrease in Q is mostly attributed to changes in the flow velocity, while the weakly increasing trend at higher rotation rates is related to increasing packing fraction. Similar analysis of the grain orientation at the orifice suggests a correlation of the flow rate magnitude with the vertical orientation and the packing fraction at the orifice with the order of the grains. Lastly, the vertical profile of mean acceleration at the center of the silo denotes that the region where the acceleration is not negligible shrinks significantly due to the strong perturbation induced by the moving wall.

The neutronics effect of TRISO duplex fuel packing fractions and their comparison with homogeneous thorium‐uranium fuel

To improve the performance of the thorium fuel, the duplex fuel potential for the micro-sized high-temperature reactor (HTR) using TRISO fuel has previously been investigated. The objective of this work is to expand on past research by delving deeper into several crucial components of the microheterogeneous thorium-uranium TRISO fuel compact. The MCNPX simulation was used to compare the neutronic performance of TRISO duplex fuel to that of homogeneous (Th,U)O2 fuel at various packing fractions and 235U enrichment. Variations in duplex fuel packing fraction and 235U enrichment were shown to have a significant effect on the neutronics of TRISO fuel. The fuel rod with 20% TRISO packing fraction has the highest kinf value at BOC, fastest burnup rate, fastest 235U depletion, and longest cycle length. As the packing fraction increases, these values however deteriorate in the opposite direction. Furthermore, the conversion ratio increases and the Doppler effect becomes stronger. The findings of evaluating duplex fuel rods at the fuel block level differed. The 40% packing fraction fuel block model achieves the lowest reactivity swing, the strongest Doppler effect, and the longest cycle length. As its packing fractions reduced to 30% and 20%, the values changed in the opposite direction. In comparison to the duplex fuel design, the homogenous TRISO fuel has a lower achievable discharged burnup and shorter cycle length. However, at the beginning of its cycle, it has a stronger doppler effect and lower pin power peaking. Despite the fact that the duplex's power peaking value drops with increasing burnup and becomes comparable to homogeneous fuel, these are some of the challenges and concerns that should be addressed in future research. Despite this, the overall result indicates that the duplex fuel design has promising potential for application in a micro-sized HTR.

3D analysis of TRISO fuel compacts via X-ray computed tomography

Low-enriched uranium oxycarbide (LEUCO) and surrogate tristructural isotropic (TRISO)- coated-particle compacts with particle volumetric packing fractions of 25%, 40%, and 48% were imaged utilizing X-ray-computed tomography. Subsequent 3D image analysis identified and further quantified kernel size, sphericity, and observed porosity. In addition, the spatial distribution, coordination number, and kernel-nearest neighbors were analyzed and compared for the different packing fractions. Metrics such as observed porosity and sphericity enabled quantification and screening for abnormal kernels within TRISO compacts. Assessment of TRISO particle location confirmed and further quantified a non-uniform distribution of TRISO particles with the spatial distribution in the radial direction being roughly described as a dampened sinusoidal function. The amplitude and frequency of this non-uniform distribution increased with increasing packing fraction. Measured kernel-nearest neighbor distances indicated two regions along the radial surfaces of compacts where TRISO particles are more likely to be in intimate contact with one another. These regions were found: (1) at upper and lower faces of compacts (i.e., corners); (2) offset ∼10% of a compact's length from the axial center near the exterior surface. Within these regions, small quantities of defective TRISO surrogate particles (48% packing fraction) and defective LEUCO TRISO particles (40% packing fraction) were found. No defective particles were found within 25% packing fraction LEUCO TRISO compacts.

Effect of roll press on consolidation and electric/ionic-path formation of electrodes for all-solid-state battery

This study investigated unpressed and pressed electrodes with the synchrotron radiation X-ray computed laminography (CL) technique to clarify the relationship between the packing structure formation of an electrode processed with a roll press and the performance of all-solid-state batteries. Additionally, we evaluated the length and thickness of percolation paths constructed by the electrode particles using the 3-dimensional structure obtained by the X-ray CL measurement. The smallest packing fraction was in the cathode layers in both the pressed and unpressed electrodes. The cathode packing fraction had a non-uniform distribution shape as a function of the layer thickness. A similar distribution shape was maintained after pressing, except near the surface in contact with the pressing roller. Pressing caused the packing fraction of the cathode layer to become much larger than the unpressed one, especially near the surface where it significantly increased. The thickness of the percolation paths in the cathode layer also increased after pressing. Furthermore, we discovered that the cathode local path thickness, measured by using regions segmented by packing fraction values, had a linear relationship with the packing fraction. Consequently, the performance bottle neck is caused by the local layer that has the smallest packing fraction. • An unpressed and pressed electrodes are visualized by X-ray CT technique. • Pressing increased packing fraction especially near the pressing roll. • Percolation-path analysis was conducted to segmented layers by its packing fraction. • The path thickness of each layer linearly related to packing fraction. • The segment whose packing fraction is the smallest is bottle neck of performance.

Micromechanical description of the compaction of soft pentagon assemblies.

We analyze the isotropic compaction of assemblies composed of soft pentagons interacting through classical Coulomb friction via numerical simulations. The effect of the initial particle shape is discussed by comparing packings of pentagons with packings of soft circular particles. We characterize the evolution of the packing fraction, the elastic modulus, and the microstructure (particle rearrangement, connectivity, contact force, and particle stress distributions) as a function of the applied stresses. Both systems behave similarly: the packing fraction increases and tends asymptotically to a maximum value ϕ_{max}, where the bulk modulus diverges. At the microscopic scale we show that particle rearrangements occur even beyond the jammed state, the mean coordination increases as a square root of the packing fraction, and the force and stress distributions become more homogeneous as the packing fraction increases. Soft pentagons experience larger particle rearrangements than circular particles, and such behavior decreases proportionally to the friction. Interestingly, the friction between particles also contributes to a better homogenization of the contact force network in both systems. From the expression of the granular stress tensor we develop a model that describes the compaction behavior as a function of the applied pressure, the Young modulus, and the initial shape of the particles. This model, settled on the joint evolution of the particle connectivity and the contact stress, provides outstanding predictions from the jamming point up to very high densities.

Local crystalline order features in disordered packings of monodisperse spheres

A new tetrahedral structure model was developed for disordered sphere packings, including not only regular tetrahedron (T), but also quartoctahedron (Q) and bcc simplex (B), the tetrahedral building blocks in fcc, hcp and bcc crystal structures, respectively. Both geometric frustrated configurations and local configurations associated with crystalline order in disordered packings can be comprehensively characterized. It is found that with increasing packing fraction, the population of T, Q, and B increases. Moreover, the crystal-type local configurations formed by face-adjacent T, Q and B increases as packing fraction increases, which is more prevalent than the geometric frustrated ones formed by face-adjacent T. In addition, as packing fraction increases, the local density of irregular simplexes is found to increase more quickly than regular ones, playing more important roles in the density increase in disordered packings. These structure features are found to be intrinsic in the jammed random sphere packings with different friction coefficients, which is independent of interparticle friction and only determined by the packing fraction. Our findings provide new understanding for the structural nature of disordered packings and the underlying structural basis of the random close packing.

Computer Experiments on Self-diffusion Coefficients of Some Liquid Metals

The available experimental self-diffusion coefficients of twenty-two liquid elements near their melting temperatures are compared with the calculated data by the first-principal dynamics simulation. The holistic evaluation shows that the calculated self-diffusion coefficients are in reasonable agreement with the experimental data. For liquid metals with loose-packed structure, the self-diffusion coefficient decreases in general with the increase of packing fraction. For liquid metals with close-packed structure in which the coordination number is larger than 11.5, the self-diffusion coefficient fluctuates around 2.0×10-9 m2s-1, except for liquid Al, Ti and Li of which the self-diffusion coefficients are anomalously large. The packing fraction can only partially account for the self-diffusion coefficient of liquid metals.

The role of collective elasticity on activated structural relaxation, yielding, and steady state flow in hard sphere fluids and colloidal suspensions under strong deformation.

We theoretically study the effect of external deformation on activated structural relaxation and aspects of the nonlinear mechanical response of glassy hard sphere fluids in the context of elastically collective nonlinear Langevin equation theory. This microscopic force-based approach describes activated relaxation as a coupled local-nonlocal event involving caging and longer range collective elasticity, with the latter becoming more important and ultimately dominant with increasing packing fraction under equilibrium conditions. The central new question we address is how this physical picture of activated relaxation, and the relative importance of local caging vs collective elasticity physics, depends on external deformation. Theoretical predictions are presented for deformation-induced enhancement of mobility, the onset of relaxation speed up at remarkably low values of stress, strain, or shear rate, apparent power law thinning of the steady state structural relaxation time and viscosity, a non-vanishing activation barrier in the shear thinning regime, an apparent Herschel-Bulkley form of the rate dependence of the steady state shear stress, exponential growth of different measures of a dynamic yield or flow stress with the packing fraction, and reduced fragility and dynamic heterogeneity under deformation. The results are contrasted with experiments and simulations, and qualitative or better agreement is found. An overarching conclusion is that deformation strongly reduces the importance of longer range collective elastic effects relative to the local caging aspect for most, but not all, physical questions, with deformation-dependent fragility and dynamic heterogeneity phenomena being qualitatively sensitive to collective elasticity. Overall, nonlinear rheology is predicted to be a more local problem than quiescent structural relaxation, albeit with deformation-modified activated processes still important.

Correlation between crystal structure and microwave dielectric properties of two garnet-type ceramics in rare-earth-free gallates

Two garnet-type rare-earth-free ceramics, Ca3M2SiGa2O12 (M = Sn and Zr), were prepared through a solid-state reaction method. The relationship between crystal structure and microwave dielectric properties was investigated. The larger deviation of εr from εtheo in Ca3Zr2SiGa2O12 could be ascribed to the rattling Zr4+. The increase in packing fraction and the decrease in FWHM enhance the Q × f value by substituting Zr4+ with Sn4+. The smaller oxygen bond valence in Ca3Zr2SiGa2O12 indicates a smaller τf value. Good microwave dielectric properties are obtained with εr = 9.14 ± 0.02, Q × f = 106,800 ± 1700 GHz and τf = -45.8 ± 1.8 ppm/°C for Ca3Sn2SiGa2O12 and εr = 11.98 ± 0.03, Q × f = 84,200 ± 1500 GHz, and τf = -32.8 ± 1.4 ppm/°C for Ca3Zr2SiGa2O12. Furthermore, near-zero τf values of +5.7 ± 1.9 ppm/°C and +4.5 ± 1.6 ppm/°C appear in 0.95Ca3Sn2SiGa2O12-0.05CaTiO3 and 0.96Ca3Zr2SiGa2O12-0.04CaTiO3, respectively.

Compaction of mixtures of rigid and highly deformable particles: A micromechanical model.

We analyze the isotropic compaction of mixtures composed of rigid and deformable incompressible particles by the nonsmooth contact dynamics approach. The deformable bodies are simulated using a hyperelastic neo-Hookean constitutive law by means of classical finite elements. We characterize the evolution of the packing fraction, the elastic modulus, and the connectivity as a function of the applied stresses when varying the interparticle coefficient of friction. We show first that the packing fraction increases and tends asymptotically to a maximum value ϕ_{max}, which depends on both the mixture ratio and the interparticle friction. The bulk modulus is also shown to increase with the packing fraction and to diverge as it approaches ϕ_{max}. From the micromechanical expression of the granular stress tensor, we develop a model to describe the compaction behavior as a function of the applied pressure, the Young modulus of the deformable particles, and the mixture ratio. A bulk equation is also derived from the compaction equation. This model lays on the characterization of a single deformable particle under compression together with a power-law relation between connectivity and packing fraction. This compaction model, set by well-defined physical quantities, results in outstanding predictions from the jamming point up to very high densities and allows us to give a direct prediction of ϕ_{max} as a function of both the mixture ratio and the friction coefficient.

Crystal structure, spectra analysis and dielectric characteristics of Ba4M28/3Ti18O54 (M= La, Pr, Nd, and Sm) microwave ceramics

High dielectric constant Ba4M28/3Ti18O54 (M = La, Pr, Nd, and Sm) ceramics were synthesized via standard solid-state reaction route. All the samples possessed single-phase orthorhombic tungsten-bronze structure, and the unit cell volumes decreased monotonously when rare-earth ion changed from La to Sm. With the decreasing radius of rare-earth ions, the dielectric constant (εr) gradually decreased, which was correlated with the decrease of total ionic polarizability and blue shift of Raman vibration modes. The increase of the quality factor (Q × f) was associated with the increase of packing fraction and the decrease of FWHM of Raman vibration modes (Ag and B1g). The temperature coefficient of resonant frequency (TCF) shifted in the negative direction, which was highly related to the decrease of the TiO6 octahedral tilt angle. Infrared reflection (IR) spectroscopy analysis indicated that the infrared vibration modes between TiO6 octahedron and A-site cation (M3+ and Ba2+) at low frequency dominated microwave dielectric polarization in the Ba4M28/3Ti18O54 ceramics.

Active topological glass

The glass transition in soft matter systems is generally triggered by an increase in packing fraction or a decrease in temperature. It has been conjectured that the internal topology of the constituent particles, such as polymers, can cause glassiness too. However, the conjecture relies on immobilizing a fraction of the particles and is therefore difficult to fulfill experimentally. Here we show that in dense solutions of circular polymers containing (active) segments of increased mobility, the interplay of the activity and the topology of the polymers generates an unprecedented glassy state of matter. The active isotropic driving enhances mutual ring threading to the extent that the rings can relax only in a cooperative way, which dramatically increases relaxation times. Moreover, the observed phenomena feature similarities with the conformation and dynamics of the DNA fibre in living nuclei of higher eukaryotes.

Optimization of the TRISO fuel particle distribution based on octahedral and icosahedral-based segmentation methods in the pebble-bed nuclear core

The random distribution of TRISO fuel particles in the fuel elements may form local hotspots, thereby increasing the thermal stress on TRISO particle layers that may cause particle failures. Optimization of the distribution of TRISO fuel particles can significantly decrease the peak temperature, reducing the stress, and thus improve the safety and economy of the reactor. In this study, two optimization methods, namely octahedral and icosahedral-based segmentation optimization methods, were introduced to decrease the maximum temperature of the pebble fuel with packing fractions from 5% to 40%. ANSYS 19.1 was used to calculate the temperature distribution of pebble fuel elements with optimized and random TRISO particle distributions. The results showed that the optimization for the distribution of TRISO particles dispersed in the pebble fuel element is essential and feasible. The necessity and effectivity of optimization increase as packing fraction increases. The peak temperatures of the fuel pebbles with optimized TRISO particle distributions were significantly decreased in comparison with the ones with randomly distributed TRISO particle. The icosahedral-based optimization method is recommended among the two kinds of optimization methods because the effectivity of the icosahedral-based optimization method is higher than the octahedral-based ones and the icosahedral-based optimization method can also get a higher target packing fraction.

Glass forming liquids in a quenched random potential.

The response of a model two-dimensional colloidal glass former to an externally imposed spatially random potential, which acts as a quenched disorder, is investigated using numerical simulations, motivated by recent experiments and also mean field predictions. The external potential induces the onset of the glassy dynamics at increasingly smaller field roughness, with increasing packing fraction of the particulate assembly, and the existence of aging processes within the glassy regime is also observed. Furthermore, along the axis of increasing field roughness, the dynamical slowdown is not correlated to the hexatic order within the supercooled regime.

Effect of Expanded Graphite on the Tribological Behavior of Tin–Bronze Fiber Brushes Sliding against Brass

Fiber brushes with expanded graphite (EG) as a lubricant were successfully prepared. Compression properties of brushes filled with unbent fibers and bent fibers with different packing fractions were compared. Moreover, the friction and wear behaviors of the brushes with the same fiber packing fractions (17.5 vol%) but different EG volume contents were investigated. The effect of EG content on the tribological behavior of the fiber brush was analyzed according to the microstructure, debris, and worn surface of the brush and counterface disc. Results showed that both the elastic constant and compression strength of the brushes filled with bent fibers increased with an increase in packing fraction. Brushes filled with 17.5 vol% bent fibers had similar compression strength (2.69 MPa) but a lower elastic constant (150 N/mm) compared to the brush filled with 17.5 vol% unbent fibers (2.62 MPa and 425 N/mm). The friction coefficients and wear rates of the brushes decreased from 1.02 and 1.6 × 10−5 mm3/Nm to 0.25 and 0.28 × 10−5 mm3/Nm as the EG increased to a critical value (23.7 vol%). The worn surface and wear debris demonstrate that the wear mechanism of tin–bronze fiber brushes with EG sliding against brass discs transforms from adhesive wear to abrasive wear due to the formation of an EG lubricating film.

Effects of size polydispersity on random close-packed configurations of spherical particles.

We analyze the packing properties of simulated three-dimensional polydisperse samples of spherical particles assembled by mechanical compaction with zero interparticle friction, leading to random close-packed configurations of the highest packing fraction. The particle size distributions are generated from the incomplete beta distribution with three parameters: A size span and two shape parameters that control the curvature of the distribution function. For each size distribution, the number of particles is determined by accounting for the statistical representativity of all particle size classes in terms of both the numbers and volumes of particles. Remarkably, the packing fraction increases, up to a small variability, with an effective size span, known as the coefficient of uniformity, that combines the three control parameters of the distribution. The local particle environments are characterized by the particle connectivities and anisotropies, which unveil the class of particles with four contact neighbors as the largest class with an increasing population as a function of size span, indicating the higher stability of particles trapped by four larger particles. As a result of increasing topological inhomogeneity of the packings, the force distributions get increasingly broader with increasing effective size span. Finally, we find that larger particles do not always carry stronger average stresses, in particular when the particle size distribution allows for a sufficiently large number of small particles.