Seed crystallites

In agreement with the findings reported in another study (36). seeding proved efficient by considerably shortening the nucleation period of ZSM-20 "Figure 4". Moreover, the nucleation time is even more reduced when ZSM-20 seed crystallites (0.5 wt. % with respect to dry gel) were added after the complete evaporation of ethanol, thus confirming the inhibiting role of the latter in ZSM-20 crystallization. 

A higher density7 sol—gel abrasive, produced by the introduction of seed crystallites formed by wet-milling with high alumina media or by introduction of submicrometer a-alumina particles, was patented (28) and designated Norton SG. The micro structure of this abrasive consists of submicrometer a-alumina crystals (Fig. 1) and its bulk density7 approaches that of fused alumina. Norton SG has proven to be an exceptional performer in coated and bonded abrasive products it was awarded the 1989 ASM Engineering Materials Achievement Award (29). 

Chen Z, Wei Shi E, Zheng Y-Q, Xiao B, Zhuang J-Y. Hydrothermal synthesis of nanosized C0AI2O4 on ZnAl204 seed crystallites. J Am Ceram Soc 2004 86 1058-60. 

There are different concepts to bring seed crystals to the surface of the support. The usual way is to have different surface charges (zeta potentials) of the support and seed crystallite. By variation of the pH of the dipping solution, a pH is looked for, where support and seed crystallites have opposite charges so that they attract each other electrostatically (Fig. 14). The concept was developed by Mintova et al. [36,37]. If the surface chemistry of support and seed is too similar that no pH with opposite surface charges exist, the physical van-der-Waals adsorption of positively charged macromolecules can result in a charge reversal. 

Covalent attachment of seed crystals to a ceramic surface by di-isocyanate linker between seed crystallite and support [39]. 

What happens is that the crystallites melt and fuse into a small tip. If we do this carefully, we will have our "seed". The tip s small size limits regrowth of the remelted part to that of a single crystal. Then, when we return the seed to the melt, we can initiate the growth of a much Icurger single crystal, provided that growth-conditions are suitable. 

Synthesis using TPABr as structure-directing agent and ammonia, water, hexanediamine, n-butylamine, diethylamine, ethylenediamine, or triethanolamine as base (seeds of TS-1 were added to get smaller crystallites and 100% crystallinity) Synthesis of fibrous titanosilicate... 

These interrelations are consistent with the above model of high temperature deactivation by coke formation through a reaction of coke growth with methanol. However, this mechanism needs coke seeds provided as "olefin coke" on external acidic centers. Development of ZSM5-catalysts for high temperature application with long life time thus concerns minimizing of acid sites on crystallite surfaces. 

Kortan AR, Hull R, Opila RL, Bawendi MG, Steigerwald ML, Carroll PJ, Brus LE (1990) Nucleation and Growth of Cdse on Zns Quantum Crystallite Seeds, and Vice Versa, in Inverse Micelle Media. J Am Chem Soc 112 1327-1332... 

Seeding neither affects the final chemical composition of ZSM-20 nor the crystal size or morphology. In contrast to non-seeded systems, isolated individual crystallites of 0.45 pm diameter tend to form polycrystalline aggregates after long synthesis times (27 days). 

Since these hexacyano complexes are hydrated salts, aqueous solutions are used. To prepare the saturated Sm[Fe(CN)6] solution 10 mL of 1 M neutral SmCl3 solution is first added, followed by the same amount of 1 M K3[Fe(CN)6] solution, to 500 mL of distilled water kept at 65°. In this way a supersaturated solution is obtained that (if everything is clean) can remain in this metastable state for hours before the first crystallites separate. To accelerate the procedure, microcrystals of Sm[Fe(CN)6] 4H20 are prepared by mixing 1 M solutions of 1 mL of SmCl3 and 1 mL of K3[Fe(CN)6], The tiny seed crystals that are obtained are used to initiate precipitation in the growth solution. After 10 minutes, still at 65°, the solution is filtered and 50 mL of hot dis-... 

Intentional seeding is a common practice among chemists who wish to coax the crystallization of a compound from solution or from the melt small crystals or crystallites of the desired material (seeds) are added to the system (e.g. Pavia et al. 1988 Shriner et al. 1997). In this way, the rate-limiting nucleation step, which may be extremely slow, can be accelerated. For this method to be applied, it is of course necessary that a sample of the desired crystalline material is available that is, the compound must have been already crystallized in a previous experiment. When polymorphic forms of a substance are known to occur, intentional seeding with one of the polymorphs is a useful and often the most successful way of preferentially producing it rather than the other form(s). 

In general, the initiation of the precipitation process may result from the presence of particulate matter in the bulk water that seeds the crystallization. The process is usually termed heterogeneous nucleation. It is possible for homogeneous nucleation to occur when the nucleation is spontaneous. Once nucleation has occurred, crystals can grow, provided that the solution is supersaturated. Suitable nucleation points on the heat transfer surface facilitate deposit formation on the surface. In turbulent flow, it is possible that crystallites that are formed in the bulk fluid may be carried into regions, where they can redissolve. 

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