Phase transformation rate

Fig. 7. Comp)arison of phase transformation rate (Ov) between specimens Na3.9Ya6Po3Siz709 (Ih-annealing (o) 3h-annealing ( )) and Na3.75Y0.65P0.3Si2.7o9 (Ih-annealing ( ), 3h-annealing ( )).

Fig. 8. Phase transformation rate (Uv) of N3- to N5-type NaYPSi on the specimen Naa.9Y a6P0.3Si2.7O9.

Stecura, S., 1965, Crystallographic Modifications and Phase Transformation Rates of Five Rare Earth Sesquioxides Lanthanum Oxide, Neodymium Oxide, Samarium Oxide, Europium Oxide and Gadolinium Oxide, in Bur. of Mines Rept. of Inv. 6616. 

Shoek-indueed phase transformations are often represented in terms of the mass fraetion w of the initial phase. Currently, the reaetion rate vv is treated as simply a funetion of w and the applied pressure. Displaeive transitions are likely to be a funetion of erystal orientation and the applied shear stress also, but we do not eurrently know how to represent this funetional dependenee. 

The solid product, BaO, was apparently amorphous and porous. Decomposition rate measurements were made between the phase transformation at 1422 K and 1550 K (the salt melts at 1620 K). The enthalpy and entropy of activation at 1500 K (575 13 kJ mole-1 and 200 8 J K"1 mole-1) are very similar to the standard enthalpy and entropy of decomposition at the same temperature (588 7 kJ and 257 5 J K-1, respectively, referred to 1 mole of BaS04). The simplest mechanistic explanation of the observations is that all steps in the reaction are in equilibrium except for desorption of the gaseous products, S02 and 02. Desorption occurs over an area equivalent to about 1.4% of the total exposed crystal surface. Other possible models are discussed. 

It has been found that both the anhydrous Form III and dihydrate phases of carbamazepine exhibit fluorescence in the solid state [78]. The fluorescence intensity associated with the dihydrate phase was determined to be significantly more intense than that associated with the anhydrate phase, and this difference was exploited to develop a method for study of the kinetics of the aqueous solution-mediated phase transformation between these forms. Studies were conducted at temperatures over the range of 18 40 °C, and it was found that the phase transformation was adequately characterized by first-order reaction kinetics. The temperature dependence in the calculated rate constants was used to calculate activation energy of 11.2 kCal/ mol (47.4 cal/g) for the anhydrate-to-dihydrate phase conversion. 

Secondary processing does not always lead to phase transformations, as was shown during studies of the polymorphs of ranitidine hydrochloride [92]. No solid-solid transformation could be detected during either the grinding or compression of metastable Form I, stable Form II, or of a 1 1 mixture of these forms. The dissolution rates of both forms were found to be equivalent, and the solution-mediated transformation of Form I to Form II was observed to be slow. 

When the heat exchange between the inner vessel and its surroundings, maintained at a constant temperature T0, occurs at an infinitely large rate isothermal calorimeter, 2 in Fig. 1), the temperature of the inner vessel also remains constant. The heat produced or absorbed is generally evaluated from the intensity of a physical modification occuring at a constant temperature in the surrounding medium (phase transformation). 

Phase Properties, Compositions, and Transport and Transformation Rates ... 

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