Dendrites, microstructure formation

Growth front instability during transformation can lead to cellular or dendritic microstructures, depending on the severity of the instability. Minor instability leads to the formation of primary protuberances, called cells, which advance perpendicular to the interface. If the instability increases, these primary protuberances can themselves spawn secondary protuberances perpendicular to the primary protuberances, and a dendritic microstmcture develops. Cellular and dendritic microstructures are most commonly observed in vapor-solid or liquid-solid phase transformations, although they can also be formed in solid-solid phase transformations. 

The phase-field simulations reproduce a wide range of microstructural phenomena such as dendrite formation in supercooled fixed-stoichiometry systems [10], dendrite formation and segregation patterns in constitutionally supercooled alloy systems [11], elastic interactions between precipitates [12], and polycrystalline solidification, impingement, and grain growth [6]. 

Connection between Transport Processes and Solid Microstructure. The formation of cellular and dendritic patterns in the microstructure of binary crystals grown by directional solidification results from interactions of the temperature and concentration fields with the shape of the melt-crystal interface. Tiller et al. (21) first described the mechanism for constitutional supercooling or the microscale instability of a planar melt-crystal interface toward the formation of cells and dendrites. They described a simple system with a constant-temperature gradient G (in Kelvins per centimeter) and a melt that moves only to account for the solidification rate Vg. If the bulk composition of solute is c0 and the solidification is at steady state, then the exponential diffusion layer forms in front of the interface. The elevated concentration (assuming k < 1) in this layer corresponds to the melt that solidifies at a lower temperature, which is given by the phase diagram (Figure 5) as... 

In the example given above, the formation of a material microstructure was seen to take place as a result of the deposition of atoms on a substrate. Another, equally important, route to solid microstructures is via the solidification process. During the solidification process, the baseline microstructure, which will have a significant impact on both the material s properties as well as its subsequent microstructural evolution, is created as the liquid is superseded by a solid. The nature of the microstructure in the resulting solid can be quite diverse, ranging from featureless equiaxed polycrystals, to microstructures riddled with dendrites. 

Microstructure is a key aspect for polymers in general, and for polyurethanes (PUs) in particular. The morphology of PUs is governed by the formation of hard and soft domains and their intercalation. Consequently, new microstructures can be developed using dendritic and hyperbranched (HB) polymers in PU systems. 

Zinc can form alloys with Li, and the alloys can act as negahve electrode materials for lithium-ion batteries. Microstructure such as particle size, orientation, component composition, and porosity can be controlled, and also affects its electrochemical performance. Since lithium moves very fast in Zn-based alloys, the formation of lithium dendrites or fibers is mostly inhibited. Furthermore, some inert components can also be introduced to modify their electrochemical performance. 

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