Crystals spiral growth

Figure 5.5 Development of a crystal growth spiral staring from a screw dislocation...

Figure 3.23. A growth spiral on a silicon carbide crystal, originating from the point of emergence of a screw dislocation (courtesy Prof, S, Amelinckx).

FIG. 4 Experimental observation of growth spirals on a salt crystal. (Courtesy of K. Reichelt, Jiilich.)... 

The concept of dislocations was theoretically introduced in the 1930s by E. Orowan and G. I. Taylor, and it immediately played an essential role in the understanding of the plastic properties of crystalline materials, but it took a further twenty years to understand fully the importance of dislocations in crystal growth. As will be described in Section 3.9, it was only in 1949 that the spiral growth theory, in which the growth of a smooth interface is assumed to proceed in a spiral step manner, with the step serving as a self-perpetuating step source, was put forward [7]. 

Figure 5.4. Relationship between normal growth rate R of a crystal lace and the step height h, the advancing rate v, and the step separation A, of a growth spiral.

Only a few crystals exhibit hollow cores at the centers of growth spiral layers. However, on the (0001) faces of SiC, which has a large fx value, hollow cores due to growth have often been observed. According to the summary by Sunagawa and Bennema [16], various degrees of the effect of the strain associated with dislocation cores have been observed depending on the sizes of b and the concentration of dislocations. 

Sunagawa,/. Narita, P. Bermema, and B. van der Hoek, Observation and interpretation of eccentric growth spirals,/. Crystal Growth, 42,1977,121-6... 

K. Maiwa, K. Tsukamoto. and 1. Sunagawa. Activities of spiral growth hillocks on the (111) faces of barium nitrate crystals growing in an aqueous solution,/. Crystal Growth,... 

The prismatic faces of natural rock-crystal are characterized by the development of striations parallel to the edges between m, r, and z faces (perpendicular to the c-axis). Natural rock-crystal showing no distinct striations is almost exceptional. In industrially mass-produced synthetic quartz using NaOH or KOH as mineralizers, no striations are observable on 1010 faces. As shown in Fig. 10.5(a), five-sided growth spiral hillocks are generally observed. However, if quartz crystals are synthesized in hydrothermal solution with NaCl as the mineralizer, the prismatic faces exhibit similar striations to those observed on natural crystals [5]. 

The ratio is up to ten times higher in NaCl solution than the ratios seen in NaOH or KOH solutions. From this, it is deduced that the striations are due to the remarkable anisotropy involved in the step advancing rate of the growth spirals developing on the m faces The main reason why this anisotropy occurs is understood to be due to the NaCl, which is added as a mineralizer in H O. The hydrothermal solution in which natural rock-crystal grows is, in general, NaCl aqueous solution. 

Elemental growth spiral step patterns are observed on all (0001), 1011, and 1010 faces of hematite crystals grown by post-volcanic action. 

The relevance of crystal faces to the subject of electrociystalhzation comes up as follows Each of the crystal faces just described contains all the microfeatures that have been described in previous sections, steps, kinks, etc. Further, the same phenomena of deposition—the ions crossing the electrified interface to form adions, the surface diffusion, lattice incorporation of adions, screw dislocation, growth spirals, etc.—occur on all the facets. 

C. Burton, F. Cabrera, and C. Frank, Nature 163 398 (1949). Nucleation unnecessary crystal growth can occur by rotation of spirals originating in screw dislocations. 

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