Bonding between particles

If most of the particles are less than ca 0.6 cm in size, flow obstmctions can occur by physical, chemical, or electrical bonds between particles. This cohesiveness is characterized by the bulk material s flow function. The forces acting to overcome a cohesive arch and cause flow are described by a hopper s flow factor, which can be obtained from the design charts (see Fig. 7). The minimum opening size required to prevent a cohesive arch from forming can be calculated from the comparison of the flow factor and flow function. 

All products require a volume and velocity of air to break the cohesive bonds between particles, wet, or dry, and to permit the batch to become fully fluidized. Laboratory trials will yield values for process air volume for the various stages of the process. Using this volume and the dimensions of the product container, a face velocity through the distributor plate can be estimated (permeability of the plate is not considered). It is reasonable to assume that approximately the same velocity will be needed in scale-up. In Table 4, estimates for process air volume are derived from the cross-sectional areas of the base of the product container for various sizes of fluid bed dryers. 

For sintering to proceed, two conditions must be met. First, there must be a means or mechanism for materials transport that will allow material to flow to fill pores and create bonds between particles. Second, there must exist a source of energy to drive or activate that materials transport. There are several different forms of sintering, each with its own combination of conditions, as shown in Table 7.6. We examine solid-state sintering in depth as one of the most general methods, but similar principles can be seen to apply to all sintering methods. 

The thixotropic characteristics provided by fumed sihca are due to its ability to develop a loosely woven, latticelike network by hydrogen bonding between particles. This network raises the apparent viscosity of the system, increases the cohesive forces, and contributes to the suspension of the solid. Because the hydrogen bonds themselves are relatively weak, they are easily disrupted through the action of an apphed stress or shearing force and quickly reform when the stress or shearing force is removed. 

When a solid dissolves in a liquid, energy is needed to break the strong bonds between particles in the solid. At higher temperatures, more energy is present. Thus, the solubility of most solids increases with temperature. For example, caffeine s solubility in water is only 2.2 g/100 mL at 25°C. At 100°C, however, caffeine s solubility increases to 40 g/100 mL. 

The bonds between particles in a liquid are not as strong as the bonds between particles in a solid. When a liquid dissolves in a liquid, additional energy is not needed. Thus, the solubility of most liquids is not greatly affected by temperature. 

On the other hand, the internal bond strength of benzylated boards manufactured using particles with 51 WPG was about two times higher than that of a conventional board (Fig. 22). The bonding between particles in conventional particleboard is via point adhesion, whereas in hot-melt and self-bonded boards it occurs by plain adhesion. Accordingly, the internal bond strength... 

Colloidal zeolites have been used as building blocks to fabricate hierarchical porous materials. Infiltrating ethanol sol of zeolite nanoparticles into an ordered array of polystyrene spheres resulted in macroporous zeolites, which involves a self-assembly process. After ethanol evaporation, zeolite nanoparticles were aggregated by capillary forces. High concentration of external silanol groups favored the formation of hydrogen bonds between particles and eventually Si-O-Si bonds after calcination. The method has been further developed to produce transparent and self-standing zeolite membranes with controlled mesoporosity. Concurrently, the preformed zeolite-coated polystyrene spheres have been... 

Bonds between particles in flocculated sulfamerazine suspension. 

In commercial applications of submicron powdered materials as additives, fillens, and pigments, nonagglomerated or weakly agglomerated primary particles are usually desired. Quantitative, predictive methods for describing neck formation and the nature of the bonds between particles are not available. In the ab,sence of proven methods, we di.scu.ss some guidelines that may serve as a starting point for future research. 

Dry granulation involves the aggregation of particles by high pressure to form bonds between particles by virtue of their close proximity. Two approaches to dry granulation are used in the pharmaceutical industry slugging and roller compaction. In either method, the material can be compacted with a binder to improve the bonding strength. 

Mechanical stimulation triggers the chemical dissolution of certain single-crystal metals, as has been reported by Dickinson.15 Therefore, in cleaning, the continued mechanical impact through brush or liquid on particles will activate the bonds between particles and the wafer surface. 

Fig. 9 REVS assay. A An AT-cut quartz crystal was coated successively with a chromium layer, a gold layer, a chemical linker layer, and a receptor that mediated specific attachment of the particle. The crystal was then transversely oscillated by applying a RE voltage at the main resonant frequency to the gold electrodes on either side of the disc. B Increasing the applied voltage results in a transverse oscillation of greater amplitude which, in principle, leads to greater inertial forces between the particle and the surface and concomitant deformation of the surface and the particle. Ultimately the bond between particle and receptor surface breaks and vibrations in the quartz are excited. These vibrations can be detected by using the quartz as a sensitive microphone. Note that the figures are not to scale...

Clark et al. (12) studied the sintering of MgO at high temperatures. They found a sudden increase in strength at a temperature well below that at which sintering proper with densification sets in, and they interpreted this increase to result from destruction of adsorbed moisture films on the surfaces of the particles—i.e., the replacement of hydrogen or hydroxyl bonding between particles by primary ionic bonds. 

The formation of surface ethers essentially changes the mechanism of formation of oxyhydroxides at their heat treatment. Arising at high-temperature processing the surface zirconium ethers prevent from formation of bonding between particles of oxyhydroxides, and at their approach to each other there is a steric repulsion of surface hydrocarbonaceous radicals. 

It should be pointed out however, that the green bonds of such a product are not as strong as sodium stabilized products. The sodium catalyzes the formation of bonds between particles and other oxide surfaces and without in bond strength is cut by roughly 40-50%. 

Particles held together by the wedge liquid form eapillary spaees. The forces acting in these spaces flood the capillaries with liquid. A plastie state is reaehed eharacterised by a definite density and consistency, which allows the paste not only to be spread uniformly onto grids but also to be retained there. If the added water exceeds a certain eritieal amount, the liquid film between particles grows in thickness, the bonding between particles is weakened, and the entire system is transformed into a suspension. 

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