Determination of Immiscibility Diagrams

Determination of immiscibility diagrams usually involves a large number of experimental measurements. The locus of the immiscibility boundary is often determined by heat treating a series of samples of constant bulk composition at different temperatures, quenching these samples, and determining if the sample is phase separated by visual observation of opalescence. The temperature of immiscibility is defined as the temperature bracketed by samples which either are or are not opalescent. The accuracy with which this temperature can be defined is determined by the temperature interval between treatments, and thus depends on the number of samples used. This process must be repeated for other compositions until the complete immiscibility boundary is well defined. 

A number of techniques have been used in the experimental determination of immiscibility boundaries. The method discussed above is often aided by use of clearing studies. If we consider the microstructure of a sample heat treated at a temperature just below the immiscibility boundary, we soon realize that the detection of phase separation may not be very easy. The lever rule predicts that only a very small quantity of the minor phase will be present, which may make detection difficult. A clearing study relies upon pretreatment of our samples at a lower temperature, where the extent of phase separation is much greater and hence more apparent. The pretreated samples are heated to the region of the immiscibility boundary, held for a specified time, and examined for the disappearance of the opalescence. The immiscibility temperature then brackets the temperatures where opalescence remains or disappears. 

The detection of opalescence by the naked eye can be aided by use of a concentrated beam of light such as that produced by a laser. The intensity of the scattered light is increased over that obtained from ambient room light and the sensitivity of the determination of the existence of light scattering is improved. The process can be made more quantitative, and a certain degree of experimental bias removed, if 

Figrue 4.7 Effect of heat treatment temperature on the scattering of light from a phase separated glass (Data supplied by P. B. McGinnis) 

Other methods for determination of immiscibility temperatures involve direct detection of phase separation using either x-ray small angle scattering or electron microscopy. Both methods have been successfully used in a number of studies, but are much more instrumentation-intensive than simple visual examination. Methods based on physical property measurements, which have also been used to a limited extent, will be discussed later in this text in chapters dealing with those properties. 

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