Compact Particles

To be specific let us have in mind a picture of a porous catalyst pellet as an assembly of powder particles compacted into a rigid structure which is seamed by a system of pores, comprising the spaces between adjacent particles. Such a pore network would be expected to be thoroughly cross-linked on the scale of the powder particles. It is useful to have some quantitative idea of the sizes of various features of the catalyst structur< so let us take the powder particles to be of the order of 50p, in diameter. Then it is unlikely that the macropore effective diameters are much less than 10,000 X, while the mean free path at atmospheric pressure and ambient temperature, even for small molecules such as nitrogen, does not exceed... 

Uniaxial pressing is the method most widely used to impart shape to ceramic powders (24). Binders, lubricants, and other additives are often incorporated into ceramic powders prior to pressing to provide strength and assist in particle compaction (25). Simple geometries such as rectangular... 

R. A. Waggoner, M. Nakagawa, S. J. Glass, M. Reese, E. Fukushima 1998, (Particle compaction as observed by MRI), in Spatially Resolved Magnetic Resonance, eds. P. Blunder, B. Bliimich, B. Botto, E. Fukushima, Wiley-VCH, Weinheim, (pp.) 299-304. 

Hollow sand casting is a relatively simple process for making cast metal objects that for thousands of years was the most widely used of all casting methods. For the most basic castings, the molds are made from ordinary silica sand mixed with water so as to keep the sand particles compacted together to maintain the required hollow-shaped form. The hot, fluid... 

As continuing sedimentation increases the depth of a sedimentary layer relative to the seafloor, the overlying pressure increases because of the increased weight of the additional particles. The increased pressure leads to particle compaction if the pore waters can escape upward. Under these conditions, sedimentation generates an upward advective flow of pore water. This flow has the potential to transport solutes. 

Fini, A., Fernandez-Hervas, M. J., Holgado, M. A., Rodriguez, L., Cavallari, C., Passerini, N., and Caputo, O. 1997. Fractal analysis [Pg.155]

Fig. 5.1. Schematic diagram of different mechanisms of particle compaction [1].

The particle compact (or concentrated suspension) formed on the substrate... 

Particles from a dispersion can be convected to the inner or outer surface of a porous substrate in contact with the dispersion due to fluid flow through the porous support. Also body forces due to centrifugal or electric fields can, in principle, be used to assist the particle transport towards the substrate. When the support is not permeable for the particles in the dispersion, the particle transport results in a more or less dense particle compact. The gravitational force on the particles can also contribute to the particle packing process when the gravitational force is in the same direction as the fluid flow. 

We use the term compact in the broad sense. Both a concentrated dispersion near close packing but with overall repulsion between the particles is called a wet compact as well as the case were the particles are trapped in an energy minimum. In the latter case the relaxation time for particle breakup from the compact, i.e. due to diffusion over an energy barrier (activated diffusion), can be much longer than the time-scale of practical interest and the particle compact is in fact consolidated. 

There are in principle three types of forces that might drive particle deformation. For dry particles, the operative force is associated with the polymer surface tension yp,- As indicated above, at temperatures above Tg surface tension forces are sufficient to drive densification of latex powders. Particle compaction rrught... 

However, bulk density does not measure the particle compaction which occurs when the particles rearrange themselves or are deformed. Thus, a second measure, compressibility, is the percent difference between the loosely packed bulk density and the packed bulk density. When the compressibility is less than 20 percent, particles are free flowing, whereas at values from 20 to 40 percent, the particles are prone to packing during storage. Compressibilities in excess of 40 percent indicate that the material compacts easily and will probably not flow from the hopper without assistance. In addi-... 

Figure 1. Pressure filtration data plotted as P/J (= Xp/k) so a lower slope is indicative of a more permeable particle compact. Note that the top two curves represent the same slurry filtered at constant pressure and constant rate, while the lower two curves are constant rate filtration of the higher salt samples.

The X-ray study of microstructures showed that the direction rate of kaolinite particles compacted in odometer is equal to 0.88 in the horizontal plane. The same index for particles in the sample compressed in stabilometer turned out to be equal to 0.52, which corresponds to virtually nonoriented structure. 

Mercury porosimetry provides a more quantitative method of characterizing the porosity in a particle compact. Bulk density, pore volume, mean pore size, and the pore size distribution of a powder compact can all be determined by mercury porosimetry. ... 

High-alumina refractory 40-300 Carbon refractory 130-160 Magnesite refractory 160-300 Silica refractory 200-300 Morphology. The study of size, shape and texture of particles and particle compacts. B.S. 2455,1958 defines a variety of particle shapes ... 

A. Fini, M.-J. Fernandez-Hervas, M.-A. Holgado, L. Rodriguez, C. Cavallari, N. Passerini and O. Caputo, Fractal analysis of 6-cyclodextrin-indomethacin particles compacted by ultrasound, 7. Pharm. ScL, 86 (11) 1303-1309, 1997. 

Single-size particles Compacts with coated Hexagonal configurations... 

In this study, 7 mm diameter rod shaped SiC reinforced composites produced by pressure infiltration technique were examined. A schematic of the pressure infiltration apparatus used in this study is illustrated in Figure 1. A fused silica tube was used as a compact holder. The mean diameter of the SiC particles compacted in the silica tube was 23 pm. Commercial pure Al and Al-8 wt% Mg alloy melts were infiltrated into the compacts under argon pressure of 765 kPa at a melt temperature of 750 "C. Details of the infiltration procedure were explained in [14]. 

---