Phase transformations, solidification

Driving forces for solid-state phase transformations are about one-third of those for solidification. This is just what we would expect the difference in order between two crystalline phases will be less than the difference in order between a liquid and a crystal the entropy change in the solid-state transformation will be less than in solidification and AH/T will be less than AH/T . 

As it turns out, there are pharmaceutical implications associated with the polymorphism of glycerol esters, since phase transformation reactions caused by the melting and solidification of these compounds during formulation can have profound effects on the quality of products. For instance, during the development of an oil-in-water cream formulation, syneresis of the aqueous phase was observed upon using certain sources of glyceryl monostearate [13]. Primarily through the use of variable temperature X-ray diffraction, it was learned that... 

Principles of skeletal structure formation of Raney catalysts are discussed, first from the perspective of phase transformation by chemical leaching. Some ideas are then proposed for making new Raney catalysts. Rapid solidification and mechanical alloying (MA) are described as potential processes for preparing particulate precursors. A rotating-water-atomization (RWA) process developed by the author and co-workers is shown as an example of rapid solidification. 

Nucleation and Growth (Round 1). Phase transformations, such as the solidification of a solid from a liquid phase, or the transformation of one solid crystal form to another (remember allotropy ), are important for many industrial processes. We have investigated the thermodynamics that lead to phase stability and the establishment of equilibrium between phases in Chapter 2, but we now turn our attention toward determining what factors influence the rate at which transformations occur. In this section, we will simply look at the phase transformation kinetics from an overall rate standpoint. In Section 3.2.1, we will look at the fundamental principles involved in creating ordered, solid particles from a disordered, solid phase, termed crystallization or devitrification. 

Let us first consider the liquid-solid phase transformation. At the melting point (or more appropriately, fusion point for a solidification process), liquid and solid are in equilibrium with each other. At equilibrium, we know that the free energy change for the liquid-solid transition must be zero. We can modify Eq. (2.11) for this situation... 

Solidification. When the ingot or casting solidifies, there are three main possible microstructures that form (see Figure 7.5). We will describe here only the final structures the thermodynamics of the liquid-solid phase transformation have been described previously in Chapter 2. The outside layer of the ingot is called the chill zone and consists of a thin layer of equiaxed crystals with random orientation. 

When any phase transformation such as condensation or solidification occurs in the temperature fall, an additional exergy As of the latent heat of the phase transformation takes part in the available energy as shown in Eq. 10.13 ... 

Sato, K. (1999). Solidification and phase transformation behavior of food fats— a review. Fett/Lipid, 101, 467-74. [28, 111]... 

Sato, K., Solidification and Phase Transformation Behaviour of Food Fats—A Review, Fett/Upid 101 467-474 (1999). 

Contrary to the case of solidification of simple substances, crystallization of polymers occurs at a wide range of temperatures. Moreover, the low thermal conductivites and large heat capacities, characteristic of many polymer substances, imply that, even the latent heat naturally released in the phase transformation sets up important temperature gradients in the melt, able to alter the crystallization process significantly. 

A wide variety of spectroscopic techniques [1-4] are used to provide information about the identities and amounts of reactant, the solid product(s), or the solid phase(s) present at intermediate stages of the decomposition. The cooling of reactant samples after partial or complete decomposition may be accompanied by changes such as crystallographic transformations, solidification of any molten material and particle disintegration. Interpretations of observations must take account of these possibilities. 

In the metastable region between the binodal and spinodal curves, phase separation has to occur by the mechanism of nucleation and growth. In this region, the one-phase-state is Indeed stable against small concentration fluctuations but unstable against separation into two phases of more different concentrations. Phase transformations in one-component systems like condensation, evaporation or solidification as well as the crystallization of solutes from solvents occur by the nucleation and growth mechanism. The well known phenomena of oversaturation and hindered-phase transformation can be explained by discussing the nucleation as an equilibrium reaction with the creation of the "critical nucleus" (6, 7). 

The analysis of solid-liquid phase transformation in small pores can be done by thermoporometry. For a liquid contained in a porous material the solid-liquid interface curvature depends closely on the size of the pore and the solidification temperature is therefore dependent on this size (16). From the solidification thermogram it is therefore possible to obtain both the size of the pores from the solidification temperature position and also the total volume of water involved in this transformation from the measurement of the energy corresponding to this phase transformation. 

We will consider three types of phase transformation (1) crystal —> crystal, (2) glass crystal, and (3) crystal -> amorphous. Transformations (1) and (2) are closely related to solidification from the melt and dissolution into a liquid. Solidification is a major theme in Chapter 29. There are two topics to address ... 

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