Neck formation

Fig. 4. Scanning electron micrograph of 5-p.m diameter Zn powder. Neck formation from localized melting is caused by high-velocity interparticle coUisions. Similar micrographs and elemental composition maps (by Auger electron spectroscopy) of mixed metal coUisions have also been made.

Although the DMT theory attempts to incorporate distance-dependent surface interactions into the adhesion problem, it does not take into account the effect surface forces have on the elastic deformation. In other words, it does not predict the neck formation predicted by JKR. 

Stage III Plastic deformation of the fibrillar structure after neck formation is complete. 

The drop formation is considered to proceed exactly in the same fashion as the bubble formation under constant flow conditions, viz. the two step (the expansion and detachment) mechanism. The tensile force does not arise in the expansion stage because there is no neck formation. 

Two mechanisms were proposed for the strong upward and downward flows [135], The first was based on interplay of the liquid-liquid and solid-liquid surface tensions and the gravitational and inertia forces. The second is correlated with the neck formation, the droplet break-up and the retossing of the fluid. 

It has been reported [45] that supported Pd is substantially less thermally stable than other noble metals in reducing environments. Data of Baker et al. [46] for titania-supported Pd and Pt in hydrogen show Pd to be Jess thermally stable over a wide range of temperatures. These results are consistent with those of Sermon [47] showing that neck formation starts at about 60°C in Pd black compared to >200°C in Pt black [39). 

Here, we shall consider several macroscopic features of the plastic deformation of glassy epoxy-aromatic amine networks. Mostly, the tensile or compression deformation has an inhomogeneous character. Usually, diffuse shear zones (or coarse shear bands) are clearly seen at room temperature deformation. Shear zones start from the defects on the sample boundaries or voids (dust) in the bulk. At higher temperatures, the samples are homogeneously deformed with neck formation (DGER-DADPhS, P = 1) 34>. 

Particle surface smoothing and rounding of pores Grain boundaries form Neck formation and groivth... 

The initial stage of sintering [20—23] is fiequently referred to as the neck formation stage, as is shown in Figure 16.1. The sintering driving force for the initial powder compact is due to the curvature difference between particle surface and that of the neck, see Figure 16.5. The six... 

Finally, stress whitening after neck formation in tensile bars of crystalline polymers imder plane stress conditions may be associated with some kind of craze-II-formation, in analogy to the corresponding observations in amorphous PC. 

Thus, it seems to be of interest to examine the influence of stress-induced polymorphic changes on the microhardness. While in the case of f-PP two samples comprising the a or phase were characterized, here we wish to follow the microhardness behaviour during the a-j6 polymorphic transition caused by a mechanical field. For this purpose PBT has been selected as a suitable material because of its ability to undergo stress-induced polymorphic transition from the a (relaxed) to the P (strained) form. Bristles of commercial PBT with a diameter of about 1 mm were drawn at room temperature via neck formation (final diameter about 0.5 mm and draw ratio of 3.4) and thereafter annealed in vacuum at 200°C for 6 h with fixed ends (Fakirov etal., 1998). 

Neck formation between spherical particles (a) without shrinkage but with a decrease in particle radius, r (b) with shrinkage [34]. 

Consolidation of the microstructure by neck-formation is the goal in the sintering step. Microstructural changes here are minor compared to the changes in pore size and porosity occurring in the wet-dry layer transition. 

Fig. 8.12. Schematic picture of contact points (a) and neck formation and sintering between particles (b), with radius R and neck width x. Path 1 represents matter transport from the grain boimdary to the neck surface with curvature radius p.

Coalescence and neck formation may continue for particles within the agglomerate stmctures if the particles are not cooled and collected. 

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. 

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