Irakli Nakhutsrishvishvili

The surface reaction of single-crystal germanium with ammonia was studied by microgravimetry. The results indicate that the forming germanium nitride vaporizes and then deposits in the form of germanium oxynitride on a substrate located in the cold zone of the reactor. The formation of the oxynitride film is due to the presence of small amounts of water vapor in ammonia.

A formal rate equation is derived for the asymptotic growth of a vaporizing scale, which adequately describes the growth of a nitride layer on germanium in humid ammonia.

Based on the conventional theory of formation and growth of scales with the barrier layers formed by stabile oxides, a novel approach is developed for calculation of kinetic parameters of high-temperature oxidation of heat-resistant alloys forming Cr2O3 and A1 2O3. This approach enables one to determine easily both linear and parabolic constants of alloy oxidation using the experimental values of specific weight gain with time. A temperature dependence of diffusion flux decrease during oxidation and coalescence of voids and microchannels at internal boundaries of scale allows to suppose that competing processes take place during the increase of oxidation temperature. These processes influence in opposition on the character of the effective diffusion area variation of the alloys forming Cr2O3 and A12O3.

On the basis of Evans’ conceptual theory, a new equation of the growth of scale with the change ofthe effective area of diffusion has been tested and formulas obtained that allow constructing the kinetic curves of oxidation both for specific heat-resistant alloys and for possible, hypothetical subjects of research.

 The work considers the formal kinetics of growth of a scale with its simultaneous sublimation. The basis of the model is the power (parabolic, cubic) law of the mass gain of system “metal (alloy) – scale”, with the input of an additional parameter – rectilinear constant of the speed of the process. The gained equations are used to describe the growth of Ge3N4 on the surface of monocrystalline germanium in humid ammonia with simultaneous sublimation of nitride.

The paper deals with the research of air oxidation kinetics of silver-based composite. Oxidation kinetics of silver-based composite material, containing 10 weight percent of nickel, was studied. Changes of specific weight gain experimental curves in air in the 500-900°С temperature range are well described by the law of complex parabola. On the basis of this equation calculation formulas are obtained allowing constructing theoretical oxidation curves of the studied objects, which practically coincide with the experimental results. The temperature dependence of linear and parabolic constants of oxidation process is defined.

In this article the processes occurring when heating pure α-, β-Ge3N4 and their mixtures in a low dynamic vacuum, as well as in hydrogen and water vapor are discussed. Relative thermal stability of pure α- and β-modifications of Ge3N4 and their mixtures is studied. In vacuum α-Ge3N4

dissociates at temperatures of T ≥ 740°C, and β-Ge3N4 at T ≥ 780°C. At lower temperatures both modifications are sublimated. Sublimation heat of α-Ge3N4 is ~245 kJ/mol, and of β-Ge3N4 is ~290 kJ/mol. The predominant thermal stability of β-Ge3N4 is also evident in hydrogen and water vapor media. 

The paper gives a mathematical model of a thermogravimetric curve for the growth of scale on a metal surface with its simultaneous sublimation is presented and considers the case of the growth-sublimation of scale being preceded by the process of gas etching of the monocrystal surface. The obtained equations are used to describe the kinetic curve of the mass change of a germanium sample when a nitride layer is formed on it in a medium of hydrazine vapors.