engleski

Self-organization of metal-oxide growth in the nonstationary (linearly increasing) temperature field

The study of Ta-oxidation in the nonstationary (linearly increasing) temperature field (variable parameter is the heating rage b, b=DT/Dt), has shown that various types of self-organized structures (SO) in 2D and 3D develop in the highly non-equilibrium and the non-linear process. The structure selection principle was found to be the principle of the fastest growth of Ben-Jakob, which assures the adaptive system reaction on external conditions. Once the 3D oxide growth start in the nonstationary thermal field a number of particles appear at places where the screw dislocations from the metal interior intersect its surface. Dislocations represent the channels of fast cation transport thus supporting the oxide particle growth by local, isolated kinetics as the cellular automatons, i.e. by discrete space-time rules. At low heating rate b (bŁ10 K/min) the cellular automaton governs the oxide particle development into a hemispherical, multilayered structure consisting of Archimedean spiral discs, the radius of which is shrinking in Z direction. The beginning of 2D spirals in multilayers is at the dislocation channel (which is extended from metal into the growing oxide particle), while their ends are at the hemispherical surface of the particle. The lattices of two successive layers may be commensurate or incommensurate, while the phase angles of the spirals in these layers may show matching or mismatching. Consequently, the morphology of particle may reveal, either the existence of 3D shrinking spiral embedded into the hemispherical surface, or the rippled structure, only. Thus, at low heating rate b, a very complex type of self-organization is dominant. Increasing the heating rate b to 20 K/min Ł b Ł 50 K/min, the SO structures on Ta become less complex. They appear as single-layer structures with the logarithmic spiral envelope showing the volcano type vertical cross-section, and consisting of ~60° polar segments. This particle structure reminds on the asymmetric 6-petal "flower". Particle characteristics are a direct consequence of the SO structure organization on 2D triangular lattice: a) the cell organization into triangular segments ("petals"), b) organization of triangular segments into a hexagonal superstructure ("flower"). This type of organization with the shrinking spiral envelope, can be generated by gradual stepping from the hexagonal periphery toward the center in the clockwise direction. Triangular petals are shown to form the 5, 5, 4, 4, 3, 2 staircase. Thus, by increasing the heating rate b, the principle of the fastest growth decreases the complexity level of particle self-organization.

metal-oxide growth; self-organization

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