Liquid/crystal interface

In zeolite systems chosen for study diffusion in the liquid phase and crystal growth on the crystal-liquid interface were the two major steps in converting gels to mordenite, zeolites A and X, the former being the rate-determining step for mordenite and the latter for zeolite X crystallization. In the mordenite system the effect of seed crystals, with surface areas per unit mass different by an order of magnitude, demonstrated the mechanism of nucleation on the seed crystal surfaces. The data support the hypothesis that crystal growth of the zeolite occurs from the solution phase rather than in the gel phase. 

As the conversion rate to mordenite is progressively increased by using larger amounts of seed crystals, and as the nucleation process takes place on the seed crystal surfaces, the overall conversion process is limited by the rate at which the soluble species in the liquid phase is transported to the crystal-liquid interface. At the same initial conversion level, smaller... 

This study showed that the overall crystallization processes for mor-denite, zeolite X, and zeolite A were similar. However, the physical properties of the crystallizing system determine the rate-limiting step for a particular zeolite synthesis. In the case of mordenite in which both the viscosity of the batch composition and the morphology of seed crystals were varied, it was observed that diffusion in the liquid phase was the ratedetermining step. For zeolite X the actual growth rate on the crystal-liquid interface was the rate-limiting factor as shown by identical conversion rates for the seeded and unseeded systems. For zeolite A in the system chosen, both processes influenced the conversion rate. 

We start with dislocations and describe both glissile (conservative) and climb (nonconservative) motion in Chapter 11. The motion of vapor/crystal interfaces and liquid/crystal interfaces is taken up in Chapter 12. Finally, the complex subject of the motion of crystal/crystal interfaces is treated in Chapter 13, including both glissile and nonconservative motion. 

MOTION OF CRYSTAL/VAPOR AND CRYSTAL/LIQUID INTERFACES... 

Because experimental study of the structure of crystal/liquid interfaces has been difficult due to the buried nature of the interface and rapid structural fluctuations in the liquid, it has been investigated by computer simulation and theory. Figure B.3 provides several views of crystal/liquid (or amorphous phase) interfaces, which must be classified as diffuse interfaces because the phases adjoining the interface are perturbed significantly over distances of several atomic layers. 

Singular and Vicinal Interfaces. The crystal/liquid interface during crystal growth from an undercooled liquid can be singular, vicinal, or general, depending upon the type of material and the driving force [1]. Many types of crystals require... 

Note that this primitive single-jump model neglects the diffuse nature of the crystal/liquid interface. 

Crystal/crystal interfaces possess more degrees of freedom than vapor/crystal or liquid/crystal interfaces. They may also contain line defects in the form of interfacial dislocations, dislocation-ledges, and pure ledges. Therefore, the structures and motions of crystal/crystal interfaces are potentially more complex than those of vapor/crystal and liquid/crystal interfaces. Crystal/crystal interfaces experience many different types of pressures and move by a wide variety of atomic mechanisms, ranging from rapid glissile motion to slower thermally activated motion. An overview of crystal/crystal interface structure is given in Appendix B. 

On the other hand, a diffuse interface possesses a significantly wider core that extends over a number of atomic distances. A diffuse crystalline/amorphous phase interface is shown in Fig. B.3. Similar structures exist in crystal/liquid interfaces [5]. 

Interfaces may also be classified broadly into homophase interfaces and heterophase interfaces. A homophase interface separates two regions of the same phase, whereas a heterophase interface separates two dissimilar phases. Crystal/vapor and crys-tal/liquid interfaces are heterophase interfaces. Crystal/crystal interfaces can be either homophase or heterophase. Examples of crystal/crystal homophase interfaces are illustrated in Figs. B.2, B.4, and B.5. Examples of heterophase crystal/crystal interfaces are shown in Figs. B.6 and B.7. Figure B.6o shows an interface between f.c.c. and h.c.p. crystals where the small mismatch between close-packed lll fcc... 

The effect of the microscopic kinematic viscosity at the crystal-liquid interface is particularly important. This can be seen in Fig. 4.9, where the discrepancy between predicted and measured frequency is apparent. The discrepancy is not related to the added mass of the glucose the model accounts for that difference. The deviation... 

Jiang Q., Shi H X. and Zhao M., Free energy of crystal-liquid interface, Acta Mater. 47(1999) pp.2109-2112. 

As an application of this kind of calculation, we refer to the case of liquid crystals. Okan< et al. have discussed the role of excluded volume effect on the molecular orientation in th< interface involving liquid crystals. They used spherocylinder systems, but considered only limited Ccise of orientation. 

Once it has been ascertained that the hot solution is saturated with the compound just below the boiling point of the solvent, it is allowed to cool slowly to room temperature. Crystallization should begin immediately. If it does not, add a seed crystal or scratch the inside of the tube with a glass rod at the liquid-air interface. Crystallization must start on some nucleation center. A minute crystal of the desired compound saved from the crude material will suffice. If a seed crystal is not available, crystallization can be started on the rough surface of afresh scratch on the inside of the container. 

Fluid flow within the melt has a crucial effect on crystal quality. If the crystal is stationary, the dominant convection pattern is upward flow of material at the crucible walls and radial flow inward at the surface (type I). Rapid rotation of crystal causes material to be thrown radially outward at the surface, and opposes the thermal convective flow (type III). These flow patterns are shown in Figure 3. In the intermediate regime, where the two flows are of comparable rates, a more complex surface pattern is observed, labeled type II. The crystal-liquid interface is convex toward the melt in type I flow and planar in type II, a condition that is used for the growth of large crystals of gadolinium gallium garnet ... 

Free energy profiles are an important characteristic of ion behaviour in heterogeneous systems such as crystal/liquid interfaces. It is obvious that the... 

In addition to the vapor diffusion method described previously, other techniques such as the batch and micro-batch methods, bulk and micro dialysis, free interface diffusion, liquid bridge, and concentration dialysis have also been developed to produce crystals for x-ray diffraction analysis (see McPherson, 1982 and McPherson, 1999). 

The model we have just described is called the BCF surface diffusion model because diffusion on the crystal surface is considered to be the rate-controlling step. While this is true in vapor growth, it is often not true in solution growth where diffusion from the bulk solution to the crystal-liquid interface can often be the limiting rate step. 

---