Whisker like structures

The extensive morphology studies conducted by Smith and his co-workers [23] enabled him to classify eutectic microstructures in terms of the parameters that influence the growth process. Some of the main features of this classification scheme are illustrated in Fig. 41, which shows the influence of solution entropy and volume fraction on the microstructure at a growth velocity of 5 X 10-4 cm/sec. The vertical line at ASa — 23 J/mol K 1 divides normal and anomalous structures. Whisker-like... 

Although some patterning approaches (e.g., solid ink nanoprinting) can be also characterized as additive techniqnes, additive techniques mainly refer to deposition techniques. The common deposition techniques include physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD), which typically require energy (thermal or electron/ion energies) input and vacuum environment. These techniques have been successfully used to fabricate nanoscale thin films, fibers, whiskers, and tubular or rod-like structures. 

There are other fiber-like structures, such as carbon nanotubes (single-walled, SWCNT, and multiwalled, MWCNT) and different types of whiskers, which are used as reinforcing structures in polymer composites. These composites are discussed in Section 6.3 ( Nanocomposites ). 

Some large multiwalled CNT-like structures were also observed on the basal plane of the whiskers with a broken tip that has sufficiently large flat area (Eigure 4.8). Owing to the irregular shape of the whisker and its relatively small diameter, the shape of the nanotubes was highly distorted and the average wall thickness of the formed structures noticeably exceeded that of CNTs obtained by Kusunoki et al. [19]. 

FIGURE 4.8 Tip of a broken SiC whisker annealed in vacuum at 1700°C for 30min (a) TEM and (b) HRTEM of the framed section of the whisker demonstrating growing MWCNT-like structures. 

Some of the FIB cross-section micrographs created as part of the Galyon and Palmer study appear to show voids at the Sn-Cu interface. Any theory of whisker-formation and growth must explain the mass transport (i.e., where the Sn comes from) that forms the whisker-nodule structure. The micrographs in Figs. 28-34 do not show any obviously missing Sn in the immediate vicinity of the whisker nodule, so it was reasoned that the Sn comes from a remote region, most likely the Sn-Cu interface layer where voids were evident. 

Whiskers are monocrystaUine fibers of very small diameters (down to 0.1 pm) typically grown by gas phase processes. Due to their nearly ideal structure and the absence of crystalline imperfections like grain boundaries or dislocations, SiC whiskers may possess outstanding mechanical properties for example. 

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