Theoretical and computational approach of physical properties of materials

Abstract

Silicon carbide (SiC), Zinc oxyde (ZnO), graphite and molybdenum disulfide (MoS2) attract much interest as materials with technological applications for the development of new electronic devices, in particular the new generation of semiconductors known as Power Semiconductor Devices (PSDs) or Field Effect Transistors (FETs). One of the biggest challenges is to understand the mechanical failure that occurs in the manufacturing process of these materials as a result of the stresses induced during the heating cycles to which they are subjected. Therefore, the fundamental objective of this thesis is the evaluation and analysis in chemical-physical terms of the stress-strain relationships. From these relationships, the limit of mechanical stability of these systems can be determined. Computational simulation allow acces to these relationships in a quantitative way, thus providing information that is difficult to acces, sometimes experimentally.In this study, we present results fromfirstprinciples density functional theory calculations that quantitatively account for the response of selected covalent, ionic and layered materials to general stress conditions. In particular, we have evaluated the ideal strength along the main crystallographic directions of 3C and 2H polytypes ofSiC, hexagonal ABA stacking of graphite, ZnO and 2H-MoS2. Transverse superimposed stress on thetensile stress was taken into account in order to evaluate how the critical strength is affected by thesemulti-load conditions. In general, increasing transverse stress from negative to positive values leads to the expected decreasing of the critical strength. Few exceptions found in the compressive stressregion correlate with the trends in the density of bonds along the directions with the unexpectedbehavior. In addition, we propose a modified spinodal equation of state able to accurately describethe calculated stress–strain curves. This analytical function is of general use and can also be appliedto experimental data anticipating critical strengths and strain values, and for providing informationon the energy stored in tensile stress processes. The first part of this Doctoral Thesis will be devoted to the presentation of the theoretical and methodological bases of the computational tools that are used in the simulations of the mechanical behavior that will be investigated in these materials. In the second part, stressstrain relationships are evaluated along relevant crystallographic directions, the ideal voltage is calculated and the results are interpreted and explained in terms of the chemical bond and the thermodynamic stability limit using the spinodal equation. The thesis will conclude with a summary of the most relevant contributions of this study.