Studies on nucleation in glasses
Fig. 1: Optical micrograph, image analyzed for particle count.
Fig. 2: Isothermal nucleation (red) and growth (blue) curves of (GeS2)0.9(Sb2S3)0.1 glass.
Nucleation, the process in which crystals are formed from a distinct phase has evoked interest from pre-historical times. The occurrence of nucleation and crystal growth, or lack of it forms the key to the countless applications in which glasses are involved. While it is necessary to have crystal-free glasses in most of the applications, there exist several devices such as data storage devices which depend on the nucleation and early growth phenomena for their operation. This group focusses on the theoretical and experimental studies of nucleation kinetics in deeply undercooled glasses.
There are two types of nucleation processes, homogeneous and heterogeneous. In the homogeneous type, nuclei are formed spontaneously upon reaching a thermodynamically and kinetically favorable state. On the other hand, in heterogeneous nucleation, the origin of crystallization is from foreign particles that catalyzes the nucleation process. The classical nucleation theory (CNT) has been used successfully in several systems to describe the nucleation phenomenon in glasses. CNT assumes that the nuclei formed are spherical and their energy can be described using macroscopic terms.
Experimental determination of nucleation rates can be done using direct or indirect methods such as microscopy, thermal analysis, X-Ray diffraction and spectroscopy. A direct approach, known as Tammann’s method is very popular for many glasses with distinct or weekly intersecting nucleation and growth curves. Tammann’s method involves two isothermal heating stages, one at a nucleation temperature, followed by a development heat treatment, in which crystals are grown to observable size. The number of crystals can then be counted automatically using image analysis. Figure 1 shows an image processed micrograph of a (GeS2)0.9(Sb2S3)0.1 glass sample showing the β-GeS2 crystals identified by a computer program. A rapid method to measure nucleation rates is by the use of thermal analysis.
Significant conclusions can be drawn when the data of nucleation is presented along with that of crystallization. Figure 2 shows the isothermal nucleation and growth rates in (GeS2)0.9(Sb2S3)0.1 glass plotted against reduced temperature (T/TM). In practice, isothermal processes are very rare and TTT diagrams are used to predict phase changes with arbitrary thermal histories.
Ongoing work focusses on studies of nucleation in chalcogenide glasses. Simulation of transient nucleation using CNT is being used to analyze the experimental results. Both optical microscopy and thermal analysis are being used for experimental determination and their results are compared with theoretical predictions.