Crystal growth container

The heated solution is passed into the vaporization container where the temperature is lowered to the operating temperature by vaporization of an equivalent amount ofthe solvent. The supersaturated solution thus produced, flows down a central pipe and upward through the crystal growth container. As the supersaturated liquor passes the fluidized crystals, the supersaturation is released to the surface of the crystals, allowing for uniform growth. 

The now saturated mother liquor is passed out of the crystal growth container into the circulation loop where it is again mixed with fresh feed liquor and the cycle repeated. 

In the crystal growth container a sufficient quantity of crystals is maintained in a fluidized bed to achieve almost complete release of supersaturation. The individual crystals must be kept in constant motion, as they are by the fluidization, to prevent their growing together, but the motion must not... 

The crystal growth container is similar to the other type crystallizers outlined above, but the supersaturated solution is produced differently. A vertically arranged shell-and-tube heat exchanger is used to remove the sensible heat of the feed and the heat of crystallization. 

The final step to be examined is the sizing of the crystal growth container. 

A sample calculation was shown to illustrate the basic approach to sizing a crystal growth container. 

Another link between thermodynamics and kinetics arises from the fact that most expressions for the rate of crystal growth contain concentrations (or gradients... 

Epitaxial crystal growth methods such as molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have advanced to the point that active regions of essentially arbitrary thicknesses can be prepared (see Thin films, film deposition techniques). Most semiconductors used for lasers are cubic crystals where the lattice constant, the dimension of the cube, is equal to two atomic plane distances. When the thickness of this layer is reduced to dimensions on the order of 0.01 )J.m, between 20 and 30 atomic plane distances, quantum mechanics is needed for an accurate description of the confined carrier energies (11). Such layers are called quantum wells and the lasers containing such layers in their active regions are known as quantum well lasers (12). 

The softened seawater is fed with dry or slaked lime (dolime) to a reactor. After precipitation in the reactor, a flocculating agent is added and the slurry is pumped to a thickener where the precipitate settles. The spent seawater overflows the thickener and is returned to the sea. A portion of the thickener underflow is recirculated to the reactor to seed crystal growth and improve settling and filtering characteristics of the precipitate. The remainder of the thickener underflow is pumped to a countercurrent washing system. In this system the slurry is washed with freshwater to remove the soluble salts. The washed slurry is vacuum-filtered to produce a filter cake that contains about 50% Mg(OH)2. Typical dimensions for equipment used in the seawater process may be found in the Hterature (75). 

Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequilihrium conditions, and hydrolysis can each play a role ia the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

In the flux-growth method, crystals of the desired ceramic are precipitated from a melt containing the components of the product phase, often in addition to additives used to suppress the melting point of the flux. These additives remain in solution after crystal growth is complete. Crystals are precipitated onto seeds by slowly cooling the melt or the seed, or occasionally by evaporating volatile components of the melt such as alkaH haHdes, depressing the solubiHty of the product phase. 

For crystal growth from the vapor phase, one better chooses the transition probability appropriate to the physical situation. The adsorption occurs ballistically with its rate dependent only on the chemical potential difference Aj.1, while the desorption rate contains all the information of local conformation on the surface [35,48]. As long as the system is close to equilibrium, the specific choice of the transition probability is not of crucial importance. 

When a precipitate separates from a solution, it is not always perfectly pure it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co-precipitation. We must distinguish between two important types of co-precipitation. The first is concerned with adsorption at the surface of the particles exposed to the solution, and the second relates to the occlusion of foreign substances during the process of crystal growth from the primary particles. 

The properties described above have important consequences for the way in which these skeletal tissues are subsequently preserved, and hence their usefulness or otherwise as recorders of dietary signals. Several points from the discussion above are relevant here. It is useful to ask what are the most important mechanisms or routes for change in buried bones and teeth One could divide these processes into those with simple addition of new non-apatitic material (various minerals such as pyrites, silicates and simple carbonates) in pores and spaces (Hassan and Ortner 1977), and those related to change within the apatite crystals, usually in the form of recrystallization and crystal growth. The first kind of process has severe implications for alteration of bone and dentine, partly because they are porous materials with high surface area initially and because the approximately 20-30% by volume occupied by collagen is subsequently lost by hydrolysis and/or consumption by bacteria and the void filled by new minerals. Enamel is much denser and contains no pores or Haversian canals and there is very, little organic material to lose and replace with extraneous material. Cracks are the only interstices available for deposition of material. 

Both reaction types are carried out in sealed tantalum containers at temperatures around 700 °C, above the melting point of Pr2ls to speed up the reaction and to assure crystal growth upon slow cooling. 

E)ven though it is evacuated, capsules have been known to explode because the quartz (metal) walls could not contain the internal vapor pressure of the material being grown as single crystal. Care must be exercised not to handle the hot ciy>sule before and after crystal growth. 

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