Stjepan Lugomer

Comparative in situ study of tantalum thin film oxidation in isothermal and nonisothermal conditions

A comparative study has been made of the isothermal (T = 700 K) and nonisothermal (T from 300 to 700 K) oxidation of Ta-films in air. Parabolic oxidation kinetics were observed for the isothermal study, whilst the nonisothermal oxidation exhibited logarithmic kinetics instead of the expected mixed logarithmic-parabolic characteristics. The isothermal kinetics was fitted to the parabolic equation: d=21(T−9)12−19 (T in min, d in nm). The nonisothermal kinetics for the nucleation period (T 50 min) was fitted to the equation: d=49 In(T−46) + 5, where t0 = RTaK0; a and K0 are constant. The nonisothermal oxidation behaviour is attributed to the relative decrease of importance of oxygen dissolution in comparison to that in isothermal oxidation. Transmission electron microscopy and Auger electron spectroscopy depth profile studies show that thin films are composed of an amorphous solid mixture of Ta2O5, TaO and TaOx (x < 1) during isothermal as well as nonisothermal oxidation.

Laser-induced three-modal breakdown of a tungsten surface

Abstract The complex surface breakdown pattern induced by the laser-material interaction was analysed and three response regions of plastic, superplastic and hydrodynamic breakdown located in the specific domains have been found. In the plastic regime, the breakdown structures are the cracks with the morphology of the wedge, isosceles and equilateral triangular crack, grown by the cavitation process in unidirectional, bidirectional and three-directional cavity aggregation. In the superplastic regime, the breakdown structures are the cracks of polygonal (delthoid and rhomboid) morphology accompanied the arc-deformation of the grain boundaries. The model of the superplastic deformation (leading also to the polygonal crack formation), based on the three kinetic stages of the superplastic deformation, is suggested. In the hydrodynamic response regime, the breakdown structure is the frozen bubble-antibubble structure.

Growth of Ta-oxide on thin tantalum films containing grain boundary particles

Abstract The effect of grain-boundary particles on the oxidation process of Ta thin films and on Ta-oxide morphology, has been investigated. Oxidation was performed in air, in a temperature interval between 300 and 700 K. Grain boundary particles mostly of spherical form, of size ∼0.2 μm, dislocations, and dislocation loops have been found by means of SEM. The increased oxidation temperature caused movement of dislocations during the oxidation process. The analysis indicates that a grain boundary dislocation can pass the particles by either climbing over them, or bowing between them. A dislocation passing the particles strongly affects both oxidation rate and morphology of the oxide through the stress concentration at the particles. The result is a roughening of the surface of the Ta-oxide and, in some cases, a highly increased oxidation rate in the stress-affected area.

Modification of the surface properties of Ti by explosion of laser-plasma containing Coulomb particles

This paper continues the study of the effects on a Ti surface caused by the explosion of a laser-plasma containing Coulomb particles. (Generation of such a system and particle self-organization in laser-material interaction (L-M I), was described in: S Lugomer and G Bitelli, Vacuum, 47, 13 (1996). The laser-plasma explosion in an O2 atmosphere, and the characteristic surface morphology by accelerated Coulomb particles, was reported in S Lugomer and G Bitelli, Vacuum, 47, 271 (1996).) Modification of the surface properties of titanium by the Coulomb particles in the exploding plasma was studied by Vickers microhardness tests, X-ray diffraction and Raman backscattering. A large jump of microhardness from the background value of 288 Kp/mm2 to ≈ 700–7200 Kp/mm2 was observed at the places of gassy Ti-oxide; the jump is smaller (to ≈450–700 Kp/mm2) on the other oxidized zones. Finally, a decrease of microhardness on the pure Tisurface occurs, because of plasma thermal effects (annihilation of dislocations in the surface zone). Raman backscattering studies have shown the existence of a partially crystalline Ti-oxide and also an amorphous (glassy) one. The contribution of the crystalline phases, anatase and the rutile (the low- and the high-temperature phases), changes with the number of laser pulses, i.e. the low-temperature phase is transferred into the high-temperature one, by repetitive laser pulsing. The broad background band (Bose peak) was interpreted on the basis of the fraction model using the Yakubos relation for fitting the real spectra. The phonon-fracton crossover frequency ωCO1, was found to be sensitive to the O2 pressure for P < 5 atm, and quite insensitive for P ≥ 5 atm. Correlation of the results of Raman backscattering and the X-ray diffraction, indicates that the laser plasma explosion in O2 generates the glassy matrix of Ti-oxide, into which the crystallites of TiO2 (the anatase and the rutile phases, of size ranging from <10 A to tens of nm) are embedded.

Laser-induced Rayleigh-Taylor instability frozen in a tungsten surface-breakdown pattern

Abstract Rayleigh-Taylor (R-T) instability frozen in the breakdown pattern of a W-surface, after treatment by a Q -switched Nd:YAG laser, at v = 5 kHz , v = 5 mm s −1 , Q ∼ 1.12 GW cm −2 , was studied. The R-T instability was found to appear in a form of chaotically distributed bubble-antibubble pairs in the hydrodynamic response zone, with the low density fluid layer over the high density one. Bubble-antibubble pairs are top-rounded with circular cross-section, showing the scale-induced bifurcation on the small and the large bubbles, i.e. those dominated by 2D effects, and those dominated by 3D ones.

Morphologic bifurcation observed in the nonisothermal oxidation of molybdenum

Abstract The non-isothermal oxidation of molybdenum produces the driving forces which because of the microstructural inhomogenities are not distributed homogenously, but show spatial variations. The spatial variation of the driving forces causes the spatial variation of nucleation density and variation of cation transport mechanism and of cation aggregation process. The final consequence is the trimodal morphologic bifurcation of Mo-oxide: ramified dendrite like, columnar, and whisker.