Macrokinetics of oxidation of porous substances

Oxidation of porous nanocrystalline titanium nitride. I. Kinetics

“…The dependences of the mass on both temperature and oxidation time that we found (Fig. 1) are mainly consistent with the model for oxidation of porous materials in [5][6][7]. This model is based on the idea of a mechanism for reaction of oxygen not only with the outer surface of the specimens but also with the inner surface of the pores, followed by their closing up with oxides.…”

Oxidation of porous nanocrystalline titanium nitride. III. Oxidation mechanism

“…The oxidability was characterized by q = ΔP/S, where ΔP (mg) is the weight increment of the sample determined by continuous weighing; S is the area of external (geometrical) surface, which was equal to 0.78 cm 2 for all samples. It is shown that at t ≤ 800°C the oxidability dependence on time q = f(τ) and temperature q = f(t) complies with the porous material oxidation model [2][3][4]. The behavior of this dependence changes at t > 800°C.…”

“…1) decrease at the initial stage (τ = 2 to 4 h) with increasing temperature from 600 to 800°C. This dependence is observed in the oxidation of porous materials [2][3][4]. According to [4], the temperature dependence of the weight increment, because of internal surface oxidation, is described by a curve with a noticeable peak corresponding to a certain (low) temperature.…”

“…This phenomenon is peculiar to the oxidation of porous materials. It is shown in [2][3][4] that oxidation in early stages is determined by the chemical interaction of oxygen with the internal surface and is stabilized after oxides occlude pore throats. A solid external oxide film is formed then, the process is controlled by oxygen diffusion through the scale, and oxygen penetration into the pores slows down.…”

“…Note that the total reaction surface area includes both the external (geometrical) surface area of the sample and the internal surface area of the pore oxidized. Based on [2][3][4], the internal surface area is to change with temperature and oxidation time as pores are obliterated with oxides. As nanocrystalline TiN oxidizes, the total reaction surface area decreases with increasing temperature and oxidation time (Table 2), which can be attributed to the smaller internal surface area of the pores involved in oxidation since the external surface area of all samples is practically the same.…”