Pressure-induced crystallization of liquids: Some new developments

As the result of numerous studies, it has been established that the properties of various materials are significantly affected by the shape, volume fraction, orientation, size distribution and the degree of dispersion of the various phases formed during their synthesis. Narrowing down to glass, crystallization is quite vital in glass fabrication processes. The rates of crystal nucleation and growth of glass-forming melts are of particular significance as they determine whether a given liquid can be vitrified or is likely to crystallize on cooling. As noted long ago by one of the founders of glass science, G. W. Morey, “Devitrification is the chief factor which limits the composition range of practical glasses, and it is an ever-present danger in all glass manufacture and working, and takes place promptly with any error in composition or technique”. On the other hand, in a variety of cases controlled crystallization is of the essential importance to produce glass-ceramics with desired properties. Therefore, in order to control the attributes of the newly evolving phases developing and the characteristics of the resulting material, an in-depth comprehension of the mechanisms of nucleation and crystal growth is imperative.

So far, these processes have been studied mainly by varying external temperature. New perspectives in this direction can be expected if alternatively pressure is chosen as the factor controlling crystallization. By this reason, based on the knowledge accumulated in the analysis of crystallization caused by temperature variation and existing extensions to pressure-induced crystallization, recently, Drs. Jürn W. P. Schmelzer from the Institute of Physics of the University of Rostock and Dr. Alexander S. Abyzov from the Kharkov Institute of Physics and Technology derived a set of equations that could be used to determine the pressure at which growth, nucleation and overall crystallization rates of glass-forming liquids achieve their maximum values. In this analysis, they utilized the classical theories of nucleation and growth that have been proven overtime to possess the capability to adequately reflect the basic features of the processes under consideration. In addition, they improved the degree of accuracy of the theoretical predictions by formulating more correct expressions for the thermodynamic driving force of crystallization, the dependence of the surface free energy on temperature and pressure and, in an alternative form, for the curvature dependence of the surface tension. In analogy to the widely discussed Kauzmann temperature, the notion of the Kauzmann pressure was introduced and analyzed. Their work is currently published in International Journal of Applied Glass Science.

In particular, the research method employed commenced with the formulation of the basic equations for the description of crystal nucleation and growth. Next, the formulated relations were then used to determine the location and the magnitude of the maxima of nucleation, growth, and overall crystallization rates in dependence on pressure. In this way, the two advance, in close cooperation with other colleagues, a more precise description of the concept of pressure to be one of the basic parameters for determining the location and the magnitude of the maxima of nucleation, growth, and overall-crystallization rates.

In a nutshell, the Schmelzer-Abyzov study presented the development of the relations that describe the main characteristics of crystallization processes in dependence on pressure. In general, a set of equations for determining the pressures and magnitudes of the maximum nucleation, growth, and overall crystallization rates of glass-forming liquids were derived and analyzed. Altogether, pressure fragility, defined appropriately, has been shown to be one of the basic parameters for determining the location and the magnitude of the maxima of nucleation, growth, and overall-crystallization rates as a function of pressure.