Spherulitic Crystal Growth Velocity in Selenium Supercooled Liquid

Abstract

The spherulitic crystal growth velocity in selenium supercooled liquid has been measured by infrared microscopy in isothermal conditions for rapidly heated samples of a-Se from temperatures well below T g . These data are compared and analyzed along with previously published data obtained on samples quenched from a temperature well above the melting point. The spherulites grew linearly over a course of time that corresponds to crystal growth controlled by crystal−liquid interface kinetics. The crystal growth velocity data obtained for these two different thermal histories can be described by the normal growth model for moderate supercoolings (ΔT <60 K). The screw dislocation growth provides a better description for larger supercoolings (ΔT > 90 K) that is also consistent with morphological observations. However, the prediction based on this model still significantly deviates for intermediate supercoolings. It is shown that all experimental data in the whole temperature range (T g < T < T m ) can be described by a combined approach including both these models, taking into account actual viscosity scaling of crystal growth η −1 . The kinetic information captured in the DSC curve is analyzed, providing evidence that the non-isothermal crystallization process can be described by the Johnson-Mehl-Avrami model. Two distinct regions characterized by different values of apparent activation energy were found. The transition between these regions coincides with morphological changes of spherulitic crystals and other properties. These regions are characterized by distinct apparent activation energies whose values are consistent with those obtained from microscopic measurement of crystal growth velocity.

Dynamics at the crystal-melt interface in a supercooled chalcogenide liquid near the glass transition

“…The polycrystalline diffraction ring observed at the two different q-values correspond to the reflections from (001) and (101) lattice planes of Se 18 , indicating the formation of Se crystallites near Tg in both Se and AsSe 4 samples. The calculated lattice parameters of a = 0.446 nm and c = 0.499 nm, agree well with the literature values reported for trigonal Se 6 . The spotty polycrystalline ring observed for Bragg reflections in both samples shows that the distribution of Se crystallites is not completely random and points towards the presence of preferred orientation.…”

“…This behaviour has been related to the nonlinearities in Kardar-Parisi-Zhang (KPZ) model of interfacial growth processes and scaling behaviour due to side-ways or lateral growth 36,37 . Screw dislocation assisted growth has been observed for Se crystallites, where the growth of the crystal results in the development of a spiral growth pattern or a screw dislocation and corresponds to lateral crystal growth 6 . The observation of a compressed exponential decay for the large crystallites may thus be indicative of such a situation where lateral growth becomes important and needs to be taken into account.…”

“…This stress removal proceeds with structural relaxations through processes that are closely related to diffusion and viscous flow, with similar activation energy, at temperatures close to the glass transition. On the other hand, for glass-ceramics processing, the kinetics of nucleation and growth of crystals is controlled by the diffusion of atoms or larger structural units from the supercooled liquid to the crystal-liquid interface [6][7][8] . The standard kinetic models treat crystallization of a liquid as a macroscopic phenomenon and the applicability of these models is often hampered by the lack of a direct microscopic understanding of the atomic scale processes that accompany crystallization [9][10][11] .…”

Kinetic Processes in Amorphous Materials Revealed by Thermal Analysis: Application to Glassy Selenium

“…Temperature-dependent data for crystal growth velocity u [9,10] and viscosity η [10,11,12] in a selenium supercooled liquid are shown in Figure 4. These two kinetic processes are closely bound together as will be discussed later.…”

“…This difference is negligible up to ~365 K but then rapidly increases with the exponential increase of the correction term f p . Note that the melting entropy of crystalline selenium is ΔS m ≈ 1.5R [10]. Such a relatively small value in combination with low supercooling causes relatively large difference between ξ and ξ a at higher temperatures.…”