Broad Grain Size Distribution Research Articles

The Influence of Grain Size Distribution on Laboratory‐Generated Volcanic Lightning

AbstractOver the last decades, remote observation tools and models have been developed to improve the forecasting of ash‐rich volcanic plumes. One challenge in these forecasts is knowing the properties at the vent, including the mass eruption rate and grain size distribution (GSD). Volcanic lightning is a common feature of explosive eruptions with high mass eruption rates of fine particles. The GSD is expected to play a major role in generating lightning in the gas thrust region via triboelectrification. Here, we experimentally investigate the electrical discharges of volcanic ash as a function of varying GSD. We employ two natural materials, a phonolitic pumice and a tholeiitic basalt (TB), and one synthetic material (soda‐lime glass beads [GB]). For each of the three materials, coarse and fine grain size fractions with known GSDs are mixed, and the particle mixture is subjected to rapid decompression. The experiments are observed using a high‐speed camera to track particle‐gas dispersion dynamics during the experiments. A Faraday cage is used to count the number and measure the magnitude of electrical discharge events. Although quite different in chemical composition, TB and GB show similar vent dynamics and lightning properties. The phonolitic pumice displays significantly different ejection dynamics and a significant reduction in lightning generation. We conclude that particle‐gas coupling during an eruption, which in turn depends on the GSD and bulk density, plays a major role in defining the generation of lightning. The presence of fines, a broad GSD, and dense particles all promote lightning.

Microstructural analysis of sheared polydisperse polyhedral grains.

This article presents an analysis of the shear strength of numerical samples composed of polyhedra presenting a grain size dispersion. Previous numerical studies using, for instance, disks, polygons, and spheres, have consistently shown that microstructural properties linked to the fabric and force transmission allow granular media to exhibit a constant shear resistance although packing fraction can dramatically change as a broader grain-size distribution is considered. To have a complete picture of such behavior, we developed a set of numerical experiments in the frame of the discrete element method to test the shear strength of polydisperse samples composed of polyhedral grains. Although the contact networks and force transmission are quite more complex for such generalized grain shape, we can verify that the shear strength independence still holds up for 3D regular polyhedra. We make a particular focus upon the role of different contact types in the assemblies and their relative contributions to the granular connectivity and sample strength. The invariance of shear strength at the macroscopic scale results deeply linked to fine compensations at the microstructural level involving geometrical and force anisotropies of the assembly.

THE EFFECTS OF GRAIN SIZE AND TEMPERATURE DISTRIBUTIONS ON THE FORMATION OF INTERSTELLAR ICE MANTLES

ABSTRACT Computational models of interstellar gas-grain chemistry have historically adopted a single dust-grain size of 0.1 micron, assumed to be representative of the size distribution present in the interstellar medium. Here, we investigate the effects of a broad grain-size distribution on the chemistry of dust-grain surfaces and the subsequent build-up of molecular ices on the grains, using a three-phase gas-grain chemical model of a quiescent dark cloud. We include an explicit treatment of the grain temperatures, governed both by the visual extinction of the cloud and the size of each individual grain-size population. We find that the temperature difference plays a significant role in determining the total bulk ice composition across the grain-size distribution, while the effects of geometrical differences between size populations appear marginal. We also consider collapse from a diffuse to a dark cloud, allowing dust temperatures to fall. Under the initial diffuse conditions, small grains are too warm to promote grain-mantle build-up, with most ices forming on the mid-sized grains. As collapse proceeds, the more abundant, smallest grains cool and become the dominant ice carriers; the large population of small grains means that this ice is distributed across many grains, with perhaps no more than 40 monolayers of ice each (versus several hundred assuming a single grain size). This effect may be important for the subsequent processing and desorption of the ice during the hot-core phase of star formation, exposing a significant proportion of the ice to the gas phase, increasing the importance of ice-surface chemistry and surface–gas interactions.

Grain growth studies on nanocrystalline Ni powder

The microstructure of nanocrystalline Ni powder produced by ball-milling and its thermal stability were investigated by applying different methods of X-ray diffraction line-profile analysis: single-line analysis, whole powder-pattern modelling and the (modified) Warren–Averbach method were employed. The kinetics of grain growth were investigated by both ex-situ and in-situ X-ray diffraction measurements. With increasing milling time, the grain-size reduction is accompanied by a considerable narrowing of the size distribution and an increase in the microstrain. Upon annealing, initial, rapid grain growth occurs, accompanied by the (almost complete) annihilation of microstrain. For longer annealing times, the grain-growth kinetics depend on the initial microstructure: a smaller microstrain with a broad grain-size distribution leads to linear grain growth, followed by parabolic grain growth, whereas a larger microstrain with a narrow grain-size distribution leads to incessant linear grain growth. These effects have been shown to be incompatible with grain-boundary curvature driven growth. The observed kinetics are ascribed to the role of excess free volume at the grain boundaries of nanocrystalline material and the prevalence of an “abnormal grain-growth” mechanism.

Tensile Deformation Microstructures and Deformation Behaviours in Electrodeposited Nanocrystalline Ni with Broad Grain Size Distribution

The electrodeposited nanocrystalline (nc) Ni with an average grain size of 27.2 nm and with a broad grain size distribution (BGSD) ranging from 5 to 120 nm was prepared. The tensile strain rate () jump test was performed at room temperature to measure the strain rate sensitivity (m). The result shows that m increases with decreasing of . In particular, when is less than 2×10−5 s−1, m increases rapidly which reaches 0.054 at =5 × 10−6 s−1, indicating that the deformation mechanisms involved in grain boundary diffusion and sliding are possibly activated. The room temperature loading-unloading tensile test was carried out. The results show that the capability of storing dislocations in BGSD nc Ni is very limited and the dislocation density is saturated when the stress reaches 1052 MPa with a strain of 7.8%. From the observation of TEM microstructures in the vicinity of tensile fracture, it is confirmed that there is significant intragranular dislocation sliding similar to that in coarse-grained materials in the process of plastic deformation of BGSD nc Ni.

Deformation behaviour of electrodeposited nanocrystalline Ni with broad grain size distribution

In the present work, nanocrystalline Ni (nc-Ni) with a broad grain size distribution (BGSD) of 5–120 nm and an average grain size of 27·2 nm was prepared. The BGSD nc-Ni sample shows a similar strength and good ductility in comparison with electrodeposited nc-Ni with a narrow grain size distribution. The intracrystalline dislocation network was observed in the post-deformed microstructure confirming the conventional intracrystalline dislocation sliding mechanism in BGSD nc-Ni. The uniaxial tensile loading–unloading–loading deformation shows BGSD nc-Ni has the capability to store dislocations in the grain interior, which is very limited compared with that of coarse grained metals. For BGSD nc-Ni, the strain rate sensitivity of flow stress m enhances with decreasing strain rate. At the strain rate of 5 × 10−6 s−1, m was estimated to be 0·055. At the corresponding strain rate, the enhanced ductility along with the decreased strength was achievable, indicating activation of other deformation mechanisms, e.g. grain boundary sliding or diffusion.

Quantum confinement effects on Cd 1− xNi xTe sputtered films

By using the r.f. sputtering technique CdNiTe nanostructured layers were prepared onto glass substrates. The nanoparticles in the films appear to have spherical shape with broad grain-size distribution. The measured average diameters were 35, 30, and 26 nm corresponding to 5, 10, and 15 nickel percent atomic content in the film, respectively. Energy dispersion spectroscopy indicates that Ni enters into the CdTe lattice in Cd sites. The structure of the nanostructured layers was zincblende cubic that resembles the bulk CdTe semiconductor. Particle-size effects on optical absorption spectra were observed. As the Ni content increases, the grain-size diameter diminishes, and the band-gap energy ( E g) increases. E g shifts have been found higher than the predicted values from the strong and weak quantum confinement models. Disagreement was explained by considering extra E g shifts as an effect of the lattice shrinkage originated from the replacement of Cd 2+ by Ni 2+ with smaller ionic radius, into the CdTe lattice.

Single domain oxide particles as a source of thermoremanent magnetization.

Single domain (SD) particles have long been suggested as a potentially strong and stable source for the paleomagnetic signal, but their actual occurrence in rocks has been much questioned. Two possible modes of occurrence of SD Fe-Ti oxides can be distinguished. These are as magnetite rods bounded by ilmenite lamellae in intergrown grains (analogous to Alnico permanent magnet alloys), and as isolated ultrafine particles, perhaps representing part of a much broader grain-size distribution (analogous to the oxide coatings used in tape recording). The evidence for the presence in rocks of these two types of magnetic particle is reviewed. Where intergrown grains are present the SD ‘Alnico model’ seems to be valid, and where isolated titanomagnetite grains are involved the SD ‘tape-recorder model’ is satisfactory. Where pure Fe3O4 particles are involved the situation is less clear, and current thinking stresses the role of the so-called pseudo-single domain (PSD) moments.