Ciystal growth

Crystal Formation There are obviously two steps involved in the preparation of ciystal matter from a solution. The ciystals must first Form and then grow. The formation of a new sohd phase either on an inert particle in the solution or in the solution itself is called nucle-ation. The increase in size of this nucleus with a layer-by-layer addition of solute is called growth. Both nucleation and ciystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, ciystals can neither form nor grow. Supersaturation refers to the quantity of solute present in solution compared with the quantity which would be present if the solution were kept for a veiy long period of time with solid phase in contac t with the solution. The latter value is the equilibrium solubility at the temperature and pressure under consideration. The supersaturation coefficient can be expressed... 

Critical Process Sten Ciystal growth Lapping,Grlnding,Pollshing Slicing... 

Mount the specimen on the holder, so that it is securely held without vibration or creep but is not strained by the mounting. This is achieved by fixing the specimen at one point only, using PVC adhesive tape (as supplied for ciystal growth laboratories). Kinematic location (resting under gravity with minimum constraints) may be used in some diffractometers. At all costs avoid over-constraint of the specimen. 

Figure 3. Response of the population density at 120 im (top) and of the ciystal growth rate (bottom) on a step in the heat input of the ciystallizer simulated with method of lines (line), Lax-Wendroff (dots), and the MCFS (dashes) algorithnt...

Ciystal Growth and Solid State Synthesis of Oxide Superconductors... 

TlSr2Can.iCun02n+3 ciystal growth, 273-80 crystal-chemical description, 198-9,259... 

FIGURE 6.12 Ciystal growth effectiveness factor as defined by equation (6.56). Redrawn, with permission from Ciarside [521... 

Byrappa K., Shekar K. V. K. and Rodriguez-Clemente R., Ciystal growth and morphology of hydrothermally grown lithium borates, J. Mater. Res. 8 (1993) pp. 2319-2326. 

Figure 13.15. Data on the CaC03 ciystal growth (20°C) carried out under conditions of constant pcoi- The data are plotted in line with Eq. (37) as (the last term on the right hand side can be neglected)...

When the attachment of the substrate to the precipitate to be formed is strong, the clusters tend to spread themselves out on the substrate and form thin surface islands. A special limiting case is the formation of a surface nucleus on a seed crystal of the same mineral (as in surface nucleation ciystal growth). As the cohesive bonding within the cluster becomes stronger relative to the bonding between the cluster and the substrate, the cluster will tend to grow three-dimensionally (Steefel and Van Cappellen, 1990). 

Variables. Polymorph solubilities, control of supersaturation, ciystal growth rate... 

Other reports (27-29) have focused on the role of citric acid, as a source of carboxylate anions, during precipitation of calcium phosphates from electrolyte solutions. It has been found that citrate anions inhibit the ciystal growth of calcium phosphates and hinder their transformation into hydroxyapatites. This was attributed to the adsorption of citrate anions into the crystals and the displacement of an equivalent amount of phosphate anions. Interestingly, Rhee and Tanaka (30) found that the presence of a collagen membrane in the medium changed the behavior of citrate anions from being an inhibitor to becoming a promoter of calcification, provided that the molar ratios of calcium to citric acid were between 2 and 12. 

Selective Crystal Removal or Size-Dependent Ciystal Growth, Chem. Eng. Common.. 6, 293 (1980). 

Baroimet A (1980) Polytypism in micas A survey with emphasis on the ciystal growth aspects. In Current Topics in Materials Science Vol. 5, E Kaldis (ed) North-Holland Publishing Company, p 447-548... 

Roy DM, Drafall LE, Roy R (1978) Ciystal chemistry, ciystal growth, and phase eqttihbria of apatites. In Phase Diagrams, Material Sciences and Technology 6-V. Alper AM (ed) Academic Press, New York, p 186-239... 

Muller-Kmmbhaar H, Burkhard TW, Kroll D (1977) A generalized kinetic equation for ciystal growth. J Ciyst Growth 38 13-22... 

Christoffersen J, Christoffersen MR, Kjaergaard N (1978) The kinetics of dissolution of calcium hydroxyapatite in water at constant pH. J Ciystal Growth 43 501-511 Codispoti LA (1989) Phosphoras vs. nitrogen hmitation of new and export production. In Berger WH, et al. 

Figure 6-18. Schematic illustration of an autoclave for hydrothermal single ciystal growth. (Reproduced with permission from ref. 125. Copyright 2000 John Wiley Sons Inc)...

In the next chapter the rates of nucleation and ciystal growth are discussed. Both mechanisms are decisive for the quality of crystalline products. 

Young s modulus decreases continuously from around 3,200 MPa for pure PEI to around 2,200 MPa for pure PTT. The addition of PEI affects the ciystallinity of PTT (decreases from around 27% for neat PTT to around 3% for 25 wt% blend), but the mechanism of ciystal growth is seen to be unaffected. The blends shows a syneigistic behavior in modulus of elasticity (which is attributed to a decrease in specific volume upon blending). Additionally syneigism is observed in the yield stress of PEI rich blends, and ductile nature. 

Figure 3.3 shows that two stage process of polymer ciystalhzation (nucleation and ciystal growth) has merits. Isotactic polypropylene spheralites grow as time of cooling progresses. 

The AE is the sum of the activation eneigy of the nucleation and ciystal growth. The lower the AE the faster the ciystallization rate. The AE was found to increase with the increasing relative ciystallinity of the nucleated PLA. Figure 6.8 shows that the activation energy is increased with increasing crystallinity, especially at the early stages of ciystallization. ... 

Crystallization kinetics. Expressions that describe ciystal growth and nucleation rates from solution. CSD. Crystal size distribution. 

Metastable zone. Zone on oonceniration-iemperatuie diagram in which homogeneous or heterogeneous nucleation will not immediately occur but in which ciystal growth will occur. 

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