Jan Očenášek

Finite-thickness effect on crystallization kinetics in thin films and its adaptation in the Johnson–Mehl–Avrami–Kolmogorov model

The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model is widely used to quantify the isothermal crystallization kinetics. The present work reports an analytical solution for the crystallization kinetics in the special case of plate-shaped samples with a finite thickness. As a result, we obtained an adapted JMAK model revealing the thickness range which influences the crystallization kinetics mode significantly. The analytical solution also provides theoretical bounds for the film thickness, where the assumption of 2D or 3D kinetics is accurate. Finally, the conclusions related to amorphous silicon and amorphous nickel-titanium thin films are reported.

Continuum crystal plasticity analyses of the plastic flow features underneath single-crystal indentations

Continuum crystal plasticity finite element simulations are performed for archetypal pure and alloyed fcc crystals to investigate the role of the crystalline orientation, hardening response and dislocation interactions on the plastic flow patterns developing underneath spherical and pyramidal indenter tips. Following our prior analyses, the orientation of plastic features such as subsurface lobes and surface rosettes goes along that of specific in-plane and out-of-plane slip systems. Interestingly, however, we currently show that the activity of the closely oriented slip systems in such lobes and rosettes is, in general, unaccountable to their development. In highly symmetric (001), (011) and (111) indentations, it is found that the slip systems with a net out-of-plane slip direction may contribute to rosette formation at the surface, whereas in-plane slip directions lead to lobe formation in the subsurface. The present results also show that while the isocontours of maximum shear stress τmax from anisotropic elasticity analyses indeed provide an indication of the indentation-induced elastic field, it is the projection of the stress tensor in all slip systems that drives lobe formation. The isocontours of τmax may not therefore dictate the plastic zone shape, even though they are useful in explaining some of its features. Finally, a conical shear band shape is found to develop immediately underneath the imprint, dictating accumulation of shear strains and their spreading towards the thickness of the crystal. This feature varies depending on crystal orientation, hardening response and on whether or not the cross-section under analysis contains normal slip directions.

Raman thermometry: Effective temperature of the nonuniform temperature field induced by a Gaussian laser

Raman spectroscopy is a widely applied analytical technique with numerous applications that is based on inelastic scattering of monochromatic light, which is typically provided by a laser. Irradiation of a sample by a laser beam is always accompanied by an increase in the sample temperature, which may be unwanted or may be beneficial for studying temperature-related effects and determining thermal parameters. This work reports analyses of the temperature field induced by a Gaussian laser to calculate the Raman scattered intensity related to each temperature value of the nonuniform field present on the sample. The effective temperature of the probed field, calculated as an average weighted by the laser intensity, is demonstrated to be about 70% of the maximum temperature irrespective of the absorption coefficient or the laser focus. Finally, using crystalline silicon as a model material, it is shown that this effective value closely approximates the temperature value identified from the thermally related peak shift.

A Novel Approach to Model Moving Heat Sources

The solution of quasistationary heat-conduction problems with moving heat sources on the surface of a body is a demanding task and still a topic of ongoing research. The authors have shown in several contributions that an exact analytical solution exists for a constant velocity of the heat source and constant thermal properties of the body. Since also convection at the surface can be treated properly, cooling and quenching problems can be solved, accordingly. The main characteristic of the governing equation is that the partial derivative of the temperature with respect to time must be replaced by the convective term being the velocity of the heat source times the gradient of the temperature. In this contribution it is shown that the quasistationary problem can be transformed into a stationary one, if a modified heat-conduction coefficient is introduced. The application of this concept and its limits are demonstrated in comparison with existing analytical solutions. For this purpose a model is set up which consists of two parts. In the first part the concept of transforming to a stationary problem is employed, whereas in the second part fictitious heat sources are defined. The methodology is shown to be very efficient as it avoids difficulties arising from the necessity to use very dense meshes and proper time integration routines necessary when applying standard fine element codes for a spatially fixed configuration of a body with moving heat sources on its surface.