Spiral hollow core

From this expression we observe that R > / hc- The terrace connecting two spirals, one left-handed and the other right-handed, will grow indefinitely if the diameter of the critical 2D nucleus, 2Rc, is less than the distance between the emerging points of the two spirals (Frank, 1949). The minimum measured separation between coupled spirals is about 50 nm, so that R < 25 nm (see Section 5.1). Asymmetric hollow cores can be clearly seen in Fig. 5.13 and are in fact composed of two single hollow cores of the same sign separated by about 2/ hc, as can be inferred from the figure. 

That a hollow core is formed by the creation of a free surface along a dislocation core implies that the curvature of the spiral step is reversed due to the strain field along the dislocation core. The effect of a strain field upon the advancement of a step was theoretically treated by Cabrera and Levine [14], [15],... 

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. 

In the case of growth spirals originating from dislocations with large b, hollow cores with diameters of micrometer order are observed at the spiral center however, when a number of dislocations with small b concentrate in a narrow area, a basin-like depression appears at the central area of the composite spirals, since the curvature of advancement of the spiral steps is reversed near the center. A straight step may appear near the spiral center as an intermediate state in the reversal of step curvature. Several examples are shown in Fig. 5.11. 

Hollow fiber refers to a membrane tube of very small diameter (e.g., 200 pm). Such small diameters enable a large membrane area per unit volume of device, as well as operation at somewhat elevated pressures. Hollow-fiber modules are widely used in medical devices such as blood oxygenators and hemodialyzers. The general geometry of the most commonly used hollow-fiber module is similar to that of the tubular membrane, but hollow fibers are used instead of tubular membranes. Both ends of the hollow fibers are supported by header plates and are connected to the header rooms, one of which serves as the feed entrance and the other as the retentate exit. Another type of hollow-fiber module uses a bundle of hollow fibers wound spirally around a core. 

Another core cooling design is shown in Fig. 7-21. The core is hollowed out and a spiral grooved stainless steel insert of the same taper as the core is emplaced. The water enters the bottom, travels up the spiral, and drops down a hole drilled in the center of the out channel. Although more expensive, this design offers superior temperature control over the conventional bubbler. 

---