Diffusion oxides, oxygen transport

Oxygen diffusivity allows oxygen transport into deeper layers of the material. It is about three orders of magnitude higher in elastomers than in glassy polymers. Thus, at equal reactivity, glassy polymers appear much more stable than elastomers, because their superficial oxidized layer is considerably thinner. 

In the very early stages of oxidation the oxide layer is discontinuous both kinetic and electron microscope" studies have shown that oxidation commences by the lateral extension of discrete oxide nuclei. It is only once these interlace that the direction of mass transport becomes of importance. In the majority of cases the metal then diffuses across the oxide layer in the form of cations and electrons (cationic diffusion), or as with the heavy metal oxides, oxygen may diffuse as ions with a flow of electrons in the reverse direction (anionic diffusion). The number of metals oxidising by both cationic and anionic diffusion is believed to be small, since a favourable energy of activation for one ion generally means an unfavourable value for the other... 

One of the most important features of the heat and mass transport of the thermochemical conversion processes is the char combustion process, which can be divided into three oxidation regimes. The prevalent regime in PBC systems, labeled Regime III, is equivalent to conversion regime I and is controlled by interstitial gas diffusion of oxygen to the surface of the particle phase. 

Fig. 8.1 Oxidation of a carbon substrate beneath a protective film containing cracks. The maximum rate represents the oxidation rate beneath the crack and is based on the cross-sectional area of the crack. The average rate depends on the crack size and crack distribution and is shown here for two selected examples. For the 10 nm crack, oxygen transport occurs via Knudsen diffusion, and the oxidation rate is essentially independent of the total pressure. For the 1 pim crack, oxygen transport occurs via normal gas-phase diffusion, and the oxidation rate varies inversely with the total pressure the results shown here are for a total pressure of 10 bars.2...

Discussion of condensed-phase diffusion must first recognize the distinction between the tracer or the chemical diffusivity and the oxygen transport rate under an oxygen potential gradient. From an oxidation standpoint, it is the latter that is most relevant. Unfortunately, to calculate the transport rate, one requires a knowledge of the structure and/or concentration of defects responsible for oxygen transport, in addition to the tracer or the chemical diffusivity. 

T. Tokuda, T. Ito, and T. Yamaguchi, Self-Diffusion in a Glassformer Melt Oxygen Transport in Boron Oxide, Z. Naturforschung, 26A, 2058-2060 (1971). 

A present-day fuel cell depends for its oxidant kinetics on irreversible diffusion for the transport of oxygen, and sacrifices the potential difference generated by the oxygen circulator, as in Figures A.l and A.2. 

The rate of oxygen diffusion is proportional to the diffusivity of the waste-rock pile. Although the diffusivity of waste rock is high because of the low moisture content of the waste rock, diffusive transport of oxygen is sufficiently slow that this process limits the rate of sulfide oxidation. Advective transport of oxygen results from changes in gas pressure between the waste-rock pile and the atmosphere. Wind has the potential to drive oxygen deeper into the pile than would occur... 

Diffusion of oxygen and/or metal atoms near the surface then occurs and the initial oxide layer is formed. The further growth of this layer involves two steps (a) reaction at the metal/oxide and oxide/oxygen interfaces, and (b) transport of material through the oxide layer. As the oxide layer gets thicker, the rate of growth is controlled by (b) which becomes the slower of the two steps. If the volume of the oxide produced is less than that of the parent metal, the simple rate law for an unimpeded reaction is observed... 

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