Variations In Packing Density Research Articles

Experimental studies of particle packing and sintering behaviour of monosize and bimodal spherical silica powders

The packing arrangements and sintering characteristics of monosize and selected bimodal mixtures of spherical silica particles are reported. Gravitational sedimentation and simple drying from a droplet of suspension were used to form consolidated bodies of micron and sub-micron sized particles. Monosized sediments were composed of ordered domains of close packing, interspersed with regions of disordered packing. During the initial stages of sintering, preferential densification of the ordered domains was observed and this promoted pore growth at domain boundaries. Moving to bimodal mixtures containing at least 20 wt% of the minor component prevented the formation of ordered domains and allowed the production of sediments with a reduced spatial variation in packing density. The more even distribution of porosity in bimodal sediments led to more uniform sintered microstructures. Comparisons are made with the results of earlier work using computer simulation techniques to study packing geometry. Die Packungsanordnung und die Sintercharakteristika von sphärischen Siliziumdioxid-Teilchen mit einerseits einheitlicher Größe und andererseits bestimmten bimodalen Größenverteilungen werden beschrieben. Gravitationssedimentation und einfaches Trocknen von Suspensionstropfen wurden angewandt, um konsolidierte Körper aus Teilchen der Größenordnung weniger Mikrometer oder weniger herzustellen. Die Sedimente der Teilchen einheitlicher Größe bestanden aus geordneten Bereichen dichter Packung, zwischen denen Bereiche ungeordneter Packung angeordnet waren. In den Anfangsstadien des Sinterns konnte hevorzugtes Verdichten der geordneten Bereiche beobachtet werden, was zu Porenwachstum an den Bereichsgrenzen führte. Beim Übergang zu bimodalen Mischungen, die mindestens 20 Gew.% der kleineren Komponente enthielten, wurde die Bildung geordneter Bereiche vermieden. Auf diese Weise konnten Sedimente mit einer geringeren räumlichen Variation der Packungsdichte hergestellt werden. Die gleichmäßigere Verteilung der Porosität in bimodalen Sedimenten führte zu gleichförmigeren Gefügen. Um die Packungsgeometrie zu untersuchen, wurden die Ergebnisse mit früheren Arbeiten verglichen, indem Berechnungen mittels Rechnersimulationen durchgeführt wurden. Cet article présente le comportement au frittage d'empilements de particules sphériques de silice, soit de même dimension, soit en mélanges bimodaux, en fonction de leur arrangement topologique. Une technique consistant en une sédimentation et en un séchage d'une gouttellette de la suspension a été utilisée pour préparer des compacts de particules dont les tailles étaient de l'ordre du micron et submicroniques. Les sédiments de particules monodisperses etaient composés de domaines ordonnés d'empilement désordonné. Durant les phases initiales du frittage, on a constaté une densification préférentielle des domaines ordonnés. Ce phénomène s'accompagne d'une croissance des pores aux joints entre les domaines. Lorsque l'on examine les mélanges bimodaux contenant au moins 20% en poids de l'espèce mineure, il apparaît que la formation de domaines ordonnés est inhibée, ce qui autorise la production de sédiments dont la densité compactée, donc la porosité initiale, est beaucoup plus homogène. En conséquence, le frittage conduit à une microstructure plus uniforme. Enfin, les résultats sont comparés à ceux d'un travail précédent basé sur des techniques de simulation de la géométrie de compacts par ordinateur.

Optical properties of activated reactive evaporated TiO 2 films

This paper discusses the optical properties of single-layer TiO 2 films deposited using an activated reactive evaporation process. The combined effects of substrate temperature (in the range 70–200 °C) and discharge currents (0–400 mA) on refractive index, extinction coefficient and packing density of these films are investigated. Significant changes in refractive index values have been observed with increases in substrate temperature and discharge current. The change in refractive index is correlated with the variation in packing density. The variation in extinction coefficient was reduced using the combined effects of substrate temperature and discharge currents. A comparison with films deposited in neutral oxygen has also been made.

Influence of packing density and binder contents of the green sheet on the dimensional stability and warping of the sintered alumina substrates.

The relations among packing density in Al2O3 green sheet and shrinkage, demensional stability and warping were discussed theoretically and experimentally. Dimensional stability in Al2O3 substrate strongly depended on packing density of particles in the green sheet and was experimentally varied by 0.5% when the packing density was changed around 1 vol%. These experimental values were fairly agreed with those obtained by theoretical considerations. Moreover, it was found that the warping observed in the sintered substrate was caused by the difference of shrinkage in thickness direction, which was due to the variations of packing density and binder content in thickness direction. This variation could be controlled around zero using the green sheet with a packing density of above 60 vol%. We could get the Al2O3 substrate with high dimensional stability.

TRANSPORT PROPERTIES OF THE RARE EARTH DISORDERED SYSTEMS (R1−xYbx)Ba2Cu3O7 (R = Y, Sm)

Electrical and magnetic measurements have been carried out for the rare earth disordered superconducting copper oxide systems ( Y 1−x Yb x) Ba 2 Cu 3 O 7 (substitution with large rare earth mass difference) and ( Sm 1−x Yb x) Ba 2 Cu 3 O 7 (substitution with large rare earth ionic size difference). Effect of compositional variation upon room temperature electrical resistivity shows no disorder scattering contribution from the randomly distributed rare earth ions located on the (1/2, 1/2, 1/2) site of the space group Pmmm. This result indicates very low conduction electron density of states surrounding rare earth ions. On the contrary, negative deviation of electrical resistivity from the linear Vegard law was observed. This reduced conduction electron scattering was discussed through the variation of packing density, grain size and/or twin structure.

Studies of the adsorption of N 2 on the graphite basal plane by computer simulation

A molecular model for the physical adsorption of nitrogen on graphitized carbon black (gcb) has been utilized extensively in computer simulations of this system. After a brief review of the details of the model and of the simulation technique, a number of results produced in the course of these studies are summarized. These include the thermodynamic and structural properties of N 2 adsorbed on gcb at 73.6 K and at coverages ranging up to ~2 1 2 monolayers. Changes in molecular orientation relative to the surface are found to have a significant effect on the lateral interaction in a layer; variations in packing density in the first layer are discussed; and the rate of molecular interchange between first and second layers is characterized. Likely directions for future simulation studies of physisorbed films are proposed.

Radiation Damage in Amorphous Nb40Ni60

We have performed a study of displacement radiation damage in a relatively new type of material, the noncrystalline refractory-transition metal Nb40Ni60. This alloy has a crystallization temperature of ˜1000 K, and therefore exhibits excellent thermal stability; it also has a high microhardness and shows good ductility. This material was prepared by a splat-quenching method and properties of this alloy are discussed in detail in Ref. 1.We have simulated the effects of fast-neutron displacement-induced damage by a 3.5-MeV Ni+ beam that has a displacement per atom rate of ˜3 x 10-3 dpa/s. As shown in Fig. 1, the Ni+ ions have a range of ˜0.8 μm and a peak in induced damage at 0.65 μm as calculated by the Brice Code.2 This result is based on a weighted average of the elemental crystalline densities, and variations in packing density of about ±15% alter the location of peak damage only ±0.1 μm.

Effects of the cone-cell distribution on pattern-detection experiments.

At photopic luminance levels, the cone-cell variation of packing density across the retina provides a natural limit to the effective size of wide-field stimulus patterns. In some experiments, this eliminates the need for small test spots, which produce band-broadening effects in the spatial-frequency domain. Calculations of these effects are given, to aid in the design of such experiments.