Isothermal crystallization products under

The kinetics of the isothermal crystallization of polymers have been thoroughly studied. However, practical processes such as extrusion, molding, injection and film production usually proceed under non-isothermal crystallization conditions. In order to obtain products with better properties, it is necessary to have quantitative evaluations of the non-isothermal crystallization process. A few methods have been developed to study the kinetics of non-isothermal crystallization by DSC for polymers. 

The Avrami model (19,20) states that in a given system under isothermal conditions at a temperature lower than V. the degree of crystallinity or fractional crystallization (70 as a liinction of time (t) (Fig. 11) is described by Equation 5. Although the theory behind this model was developed for perfect crystalline bodies like most polymers, the Avrami model has been used to describe TAG crystallization in simple and complex models (5,9,13,21,22). Thus, the classical Avrami sigmoidal behavior from an F and crystallization time plot is also observed in TAG crystallization in vegetable oils. This crystallization behavior consists of an induction period for crystallization, followed by an increase of the F value associated with the acceleration in the rate of volume or mass production of crystals, and finally a metastable crystallization plateau is reached (Fig. 11). 

In this work, nickel oxides are prepared by dehydration under vacuum (10 torr) at moderate temperatures (200-300 ) of a pure nickel hydroxide. The hydroxide itself is prepared by the steam distillation of a solution of reagent grade Ni(NOs)2 in an excess of aqueous ammonia (22). As has been shown (22), this method yields Ni(OH)2 containing less than 0.08% of NHs and N2O6. When dried at about 60°, the product has the composition NiO, 1.05 H2O and its BET surface area amounts to 34 m /gm. The external aspect is that of a fine crystalline powder and not of a gel. There are no small-diameter pores in the hydroxide particles since the adsorption-desorption isotherm of nitrogen at —195° does not present an hysteresis loop. The X-ray diagram is that of a well-crystalized nickel hydroxide. 

It has been mentioned previously that some studies showed that slow depressurisation favoured the growth of larger crystals. In this context, Shlichta [101] published a patent for the production of crystals of high morphological quality by a slow, time-controlled reduction of pressure. During the pressure reduction the interchange of heat between the solution and the environment is controlled to be under either isothermal or adiabatic conditions. 

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