Particle packing densities

It is striking that the isotherms in Figure 10.4 change first from Type II to IV and finally to Type I. Thus, the results of Avery and Ramsay (1973) have clearly demonstrated that mesopores and micropores can be produced by the progressive compaction of a non-porous powder. However, a drastic loss of surface area accompanied this change (from 630 to 219 m2 g"1) and Avery and Ramsay point out that this was associated with a marked increase in the particle packing density. 

Comparing the type of information obtained on suspensions with that obtained on composites gives useful insight into the types of mechanisms that control creep of ceramic matrix composites. The very large increase in creep resistance of dense particulate composites, i.e., more than 65vol.% particles, suggests that the particle packing density is above the percolation threshold. Creep of particulate composites is, therefore, controlled by direct interparticle contract, as modified by the presence of relatively inviscid matrices. Mechanisms that control such super-threshold creep are discussed in Section 4.5. 

The value of error grows significantly with the increase of the air velocity and particles packing density. That is why it seems incoirea to use conventional computational instruments in the given situation. 

Velamakanni, B.V. and Lange, F.F., Effect of interparticle potentials and sedimentation on particle packing density of bimodal particle distributions during pressure filtration, J. Am. Ceram. Soc., 74, 166, 1991. 

One hundred fully hydroxylated silica samples prepared by different processes and having different structural characteristics such as specific surface area, type or size distribution of the pores, particle packing density, and structure of the underlying silica skeleton were analyzed two or more times each. Eighty-five percent of the 231 measurements were from samples with a specific surface area less than 400 m2/g. The OH groups in ultramicropores, into which only water molecules can penetrate, were not classified as surface silanol groups but as structurally bound water. 

The microstructure of various amorphous silicas was first discussed in a comprehensive manner by Iler in his early book The Colloid Chemistry of Silica and Silicates (I). Iler drew attention inter alia to the importance of the dense silica particle size and particle packing density in controlling the surface and colloidal properties of sols, gels, and precipitates. In particular, he showed how a change in coordination number of these globular... 

Very small particles of fume silica were compacted in the work of Avery and Ramsay (8), who found that high compaction pressures resulted in the conversion of the isotherm type from IV to I. A drastic loss of BET area accompanied this change (from 630 to 219 m2/g), and Avery and Ramsay concluded that this change was associated with a marked increase in particle packing density. The shape and reversibility of this Type I isotherm was a clear indication that the effective pore width had been reduced to below 2 nm, that is, that the compact had become microporous. 

That the optical properties of such 2D/3D colloid arrays will depend on particle packing densities has been demonstrated by Dusemund et al., who measured the reflectivity of gold colloid films as a function of colloid volume fraction [47]. Although the films were not ordered they showed a clear shift in surface plasmon position with increasing particle volume fraction. The only compromise was that the volume fraction could not be directly determined on the samples from which spectra were taken, but had to be measured on separately prepared TEM grids. Several groups have found that clustered quantum dots exhibit red-shifted fluorescence, a question recently reviewed by Weller [48]. 

Figure 9.7. The effect of particle size and the repulsive range on the particle packing density. (From ref. (15) with permission of The American Ceramic Society)...

Consolidated structure The consolidated structure (i.e, powder compact) that forms after the liquid is removed from the suspension depends on the state of the dispersion. Well-dispersed suspensions tend to form consolidated structures with a higher particle packing density (lower total porosity) when compared to samples prepared from suspensions with flocs/agglomerates (Figure 10.26) (12). In addition, the samples prepared from well-dispersed suspensions tend to have a smaller average pore size and a narrower pore size distribution. 

Particle shape Shape affects particle packing density and specific surface, which has a marked effect on pressure rise in constant rate filtrations (or filtrate flux decline in constant pressure processes). Needle shaped particles lead to lower pressure losses than equiaxed particles. Platelet shaped particles can be difficult to wash and dewater. 

Particle packing The particle packing density in the granule is controlled by the particle size distribution of the powder as well as on the particle concentration and colloidal stability of the slurry prior to spray drying (25). Particle packing densities of —45-55% are common for granules. 

The reflected irradiance of the powder does change with particle packing density however, the change is not substantial ( > 1%) unless the powder is highly compressed. Packing by hand using a flat surface, for example, is sufficient. It is important that the placement of the specimen holder on the integrating sphere be uniform from specimen to specimen. The machined front of the holder should correspond in placement to the diameter of the... 

Case 1 T pressure work buildup excess pore pressure. Interparticle contacts become impermanent and the effective normal stress, representing the sohd-supported part of total normal stress in bed, eventually vanishes. Mud rigidity decreases even as the particle packing density (i.e., wet bulk density) may remain largely imaffected. However, the bottom is now in a fluidlike transitory state, which reverts to bed almost as soon as wave action ceases and interparticle bonds are reestablished. For transitory fluid mud to change to a... 

The value of x is determined by system geometry, primarily by the particle size (radius r, if the particles are spherical) and the particle packing density. In the case of moderately dense structures, such as those with a primitive cubic lattice with the coordination number of 6, one can approximately write that... 

Uniformity (bore shape) Particle packing density Surface morphology (smooth/rough) Precursor morphology (dense/porous)... 

Pressure filtration experiments have been conducted with AI2O2 powder in which conventional methods (controlled pH) are used to alter interparticle forces Conditions that produce the maximum interparticle repulsive forces result in an extremely high particle packing density (relative densities up to 70%) which is relatively insensitive to the applied pressure. On the other hand, the packing density for conditions that produce the... 

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