Solid-phase crystallization

An example of a binary eutectic system AB is shown in Figure 15.3a where the eutectic is the mixture of components that has the lowest crystallisation temperature in the system. When a melt at X is cooled along XZ, crystals, theoretically of pure B, will start to be deposited at point Y. On further cooling, more crystals of pure component B will be deposited until, at the eutectic point E, the system solidifies completely. At Z, the crystals C are of pure B and the liquid L is a mixture of A and B where the mass proportion of solid phase (crystal) to liquid phase (residual melt) is given by ratio of the lengths LZ to CZ a relationship known as the lever arm rule. Mixtures represented by points above AE perform in a similar way, although here the crystals are of pure A. A liquid of the eutectic composition, cooled to the eutectic temperature, crystallises with unchanged composition and continues to deposit crystals until the whole system solidifies. Whilst a eutectic has a fixed composition, it is not a chemical compound, but is simply a physical mixture of the individual components, as may often be visible under a low-power microscope. 

Fig. 3. Explosive crystallization in a-Si by rapid argon-laser scanning, (a) At a scan speed of 85 cm sec-1 both explosive solid-phase crystallization and liquid-phase crystallization are exhibited, (b) Two different types of solid phase crystallization are Observed at a scan speed of 900 cm sec 1, (c) The situation is the same as that in (b) but an even higher scan speed of 1000 cm sec 1 is used. [From Bensahel and Auvert (1983b).]...

Sample Al203/Si02 (gel) Synthesis time, (h) Solid phase (%crystal.) Composition Al Na b b per unit h2o c cell HM++ d a SiOR e Pore filling (%) f... 

Face-centered cubic argon is the stable solid phase of pure argon, but upon the addition of as little as 1 percent of N2 to Ar, the solid phase crystallizes with the close-packed hexagonal structure. A study of the sublimation pressures of solid solutions of Ar with small amounts of N2 would give valuable information concerning the hexagonal phase, and such a study could be made with the apparatus used in this experiment. 

Figure 1 Comparison of process time and temperature between the conventional solid phase crystallization and the alternating magnetic field crystallization...

T. Matsuyama, M. Tanaka, S. Tsuda, S. Nakano, and Y. Kuwano, Improvement of n-type poly-Si film properties by solid phase crystallization method, Jpn. J. Appl. Phys. 32, 3720, 1993. 

The model is based on the following considerations Due to the fact that 111 planes of crystalline Si feature the lowest specific surface energy these planes are preferentially formed [62,63]. This leads, for example, in the case of solid phase crystallization of a-Si, to a double-pyramid (octahedral) structure... 

The formation of a new phase inside a solid phase is very difficult, because the transition generally implies a change in density, hence in volume. This leads to a change in pressure, and thereby to an additional, generally large and positive, term in free energy for nucleus formation. Except for some solid —> solid transitions where the change in density is small, this tends to prevent nucleation any formation of a new phase will occur at the boundary of the system. Sublimation then does not need nucleation the new phase (gas) is already present. The same holds for a solid phase (crystals) in a solution. When crystals in air tend to melt, a liquid... 

The presence of solid solutions has some important implications, as will be illustrated for a highly simplified example of two components, depicted in Figure 15.24. Assume that we have a liquid fat of composition a3 at temperature T. Cooling it to T2, crystals will form (provided that nucleation occurs). Now a liquid phase of composition a2 will result and a solid phase (crystals) a5. The molar ratio of solid to liquid will be given by the ratio (a3 — a2)/(a5 — a3). Assume now that the mixture is cooled further to T3. The crystals of composition a5 remain, and the liquid a2 will separate into a liquid and crystals of composition ax and a5, respectively. Note that the composition of the system differs from that resulting from cooling directly to T3. In other words, there is no equilibrium (i.e., within the polymorphic form, probably p ). There is, however, a surface equilibrium the crystals a4 will probably be formed at and around the existing crystals... 

Olson, G.L., Roth, J.A. Kinetics of solid phase crystallization in amorphous silicon. Mater. Sci. Rep. 3, 1 (1988)... 

Nucleation plays a fundamental role whenever condensation, precipitation, crystallization, sublimation, boiling, or freezing occur. A transformation of a phase a, say, a vapor, to a phase p, say, a liquid, does not occur the instant the free energy of p is lower than that of a. Rather, small nuclei of p must form initially in the a phase. This first step in the phase transformation, the nucleation of clusters of the new phase, can actually be very slow. For example, at a relative humidity of 200% at 20°C (293 K), far above any relative humidity achieved in the ambient atmosphere, the rate at which water droplets nucleate homogeneously is about 10 54 droplets per cm3 per second. Stated differently, it would take about 1054 s (1 year is 3 x 107 s) for one droplet to appear in 1 cm3 of air. Yet, we know that droplets are readily formed in air at relative humidities only slightly over 100%. This is a result of the fact that water nucleates on foreign particles much more readily than it does on its own. Once the initial nucleation step has occurred, the nuclei of the new phase tend to grow rather rapidly. Nucleation theory attempts to describe the rate at which the first step in the phase transformation process occurs—the rate at which the initial very small nuclei appear. Whereas nucleation can occur from a liquid phase to a solid phase (crystallization) or from a liquid phase to a vapor phase (bubble formation), our interest will be in nucleation of trace substances and water from the vapor phase (air) to the liquid (droplet) or solid phase. 

I 6) Shishido, T., Wang, Y., Morioka, H., Kondo. M. and Takehira, K. (2002). Partial oxidation of methane over Ni/Mg-Al oxide catalysts prepared by solid phase crystallization method from Mg-Al hydrotalcitc-likc precursors. Applied Catalysis A General, 223, 35-42. 

The next two chapters deal with investigations concerning solid silicon monoxide. The application of thin films of this material is based on its unique mechanical, chemical, and dielectric properties. It is related to Si-Si systems in so far as solid SiO consists of small particles of Si and Si02. Depending on the conditions for synthesis, the material has different local structures. In the contribution of U. Schubert and T. Wieder (Chapter 18), the structure and reactivity of a special SiO modification (Patinal ) is described. This material consists of Si and Si02 regions of 0.25 - 0.5 nm in diameter, which are connected by a thin interface. Most of the SiO reactions are also observed for elemental silicon. H. Hofineister and U. Kahler (Chapter 19) show that thermal processing of solid SiO (from BALZERS) up to 1300°C leads to phase separation into Si nanocrystallites embedded in an SiOx matrix. Their internal structure is determined by solid-phase crystallization processes. 

Their internal structure is determined by solid-phase crystallization processes. 

The reaction of dehydrohalogenation typically carry out at presence of solutions of alkalis (B ) in ethanol with addition of polar solvents. At use of tetrahydrofiiran synthesis goes at a room temperature. This method allows avoiding course of collateral reactions. The amorphous phase only cumulene modification of carbyne is received as a result. Then, crystal of P-carbyne is synthesized from amorphous carbine by solid-phase crystal-ization. 

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