Neck formation stage

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... 

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. 

The time and temperature required to form the desired degree of porosity in the dense solid depend mainly on the particle size of the alumina powder. The usual sintering sequence is imagined to be (1) neck formation between powder particles, (2) formation of open porosity with a continuous solid phase (intermediate stage), and (3) removal of closed pores imbedded in the dense solid. In the usual practical sintering of alumina, stage one is rapid, and the final density or porosity is determined mainly by stage three. 

An example of a study conducted using a tensile stage in the SEM is the evaluation of the ductile failure of poly(vinyl chloride). Smith et al. [316] stamped dumbbell shaped pieces of polymer from 1 mm thick sheets and extended them to a neck in an Instron tester. The prestrained pieces were then strained in the SEM. Low accelerating voltage was used for imaging of the uncoated specimens. These experiments showed that, after neck formation, fracture occurs by crack propagation from a flaw or cavity within the surface craze. 

Figure 9.5 Neck formation at stage 3, in the stress strain curve figure 9.2.

FIGURE 8.12 Neck formation during initial stages of sintering. (From... 

During the initial stages of sintering, there will be a net flow of vacancies from the neck that forms between particles. This flow of vacancies is exactly equivalent to a flow of atoms in the opposite direction, thus resulting in neck formation. 

This model considers the drop formation to take place in two stages, the expansion stage when the drop inflates at the nozzle tip and the detachment stage when the drop rises, forms a neck, and finally gets detached from the nozzle. The first stage is assumed to end when the buoyancy becomes equal to the interfacial tension force. For the termination of the second stage two conditions have been used, which result in two values of time of detachment. The lower of the two values is employed for calculation. 

The initial stage of sintering is characterized by formation and growth of contact areas (necks) between the particles neither formation of closed pores nor crystal growth occur. The linear shrinkage due to sintering does not exceed about 5%. 

They found similar effects as described above, indicating two stages in droplet formation, one for intmsion of the to-be-dispersed phase into the main channel and one for squeezing the neck till it reaches a certain thickness (w) after which it breaks. The proposed scaling relation is... 

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