Tõnu Laas

Armor materials’ behavior under repetitive dense plasma shots

Experiments on the plasma focus device PF-12 have been carried out to investigate changes in the structure of the surface and bulk of tungsten and tungsten doped with 1% lanthanum oxide after repeated powerful deuterium plasma shots (8, 25 and 100). The surface morphology of the targets exposed to plasma streams is analyzed by electron and optical microscopy. Due to the plasma effect, different surface structures, such as wave-like structures, a melted layer, a mesh of microcracks, droplets, craters, crevices and holes, appear. The change of cross-section hardness after a number of shots in different materials is investigated.

Stress Relaxation Mechanism by Strain in the Si-SiO2 System and its Influence on the Interface Properties

The results of the investigation of stresses relaxation by strain by means of EPR spectra, IR absorption spectra, SEM and samples deflection are presented. It has been shown that stresses relaxation mechanism depended on the oxidation conditions: temperature, cooling rate, oxide thickness. In the Si-SiO2-Si3N4 system the stresses relaxation by the strain occur due to the opposite sign of the thermal expansion coefficient of Si-SiO2 and Si3N4 on Si. Laser irradiation allows to modify the system stresses.

Interaction of Point Defects with Impurities in the Si-SiO2 System and its Influence on the Interface Properties

The results of an investigation of the point defects (PD) generation, redistribution and interaction with impurities in the Si-SiO2 system during the process of its formation in use of of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectra are presented. The type and density of the point defects that are generated in the Si surface layer during thermal oxidation depend on the oxidation conditions: temperature, cooling rate, oxidation time and impurity content. The difference between interface properties of n- and p-type wafers could be related to different Fermi level position at the interface and to different PD densities in volume.