Oxygen diffusion effects

There is an international standard, ISO 691455 which covers both the continuous and intermittent procedures plus the simplified intermittent method. Strip test pieces are used, 1 mm thick to minimise oxygen diffusion effects. Measurement at a series of temperatures is recommended and results are presented in graphical form but no consideration is given to interpretation. British Standards did not accept this revision of ISO 6914 and BS 903 Part A5256 is identical to the 1985 ISO method. The revision was not accepted in the UK because mistakes in handling comments resulted in inconsistencies. As an example, the title is now stress relaxation but a note says that this term is avoided ... 

Although tga studies of oxidation are relevant to combustion, it must be emphasized that studying degradation imder conditions of rapid weight loss from liquid samples at over 200°C has no relevance for degradation of solid polymers in service use. Furthermore, unless exceedingly thin sample are used, such studies are almost certainly influenced by oxygen diffusion effects discussed below. 

Firstly, they might be expected to have an effect when corrosion occurs under conditions of active (film-free) anodic dissolution and is not limited by the diffusion of oxygen or some other species in the environment. However, if the rate of active dissolution is controlled by the rate of oxygen diffusion, or if, in general terms, the rate-controlling process does not take place at the metal surface, the effect of crystal defects might be expected to be minimal. 

In low density reactant compacts, the reaction is believed to involve gas phase oxygen diffusion whereas under conditions of improved contact, in high-density material, the mobile species is identified as Fe2+. The metal catalyzes decomposition of the oxidant (KMn04), an effect that is inhibited by small quantities of certain additives (e.g. NaF). There is a large and specialist literature devoted to self-heating reactions. 

Subczynski, W. K., E. Markowska, and J. Sielewiesiuk. 1991. Effect of polar carotenoids on the oxygen diffusion-concentration product in lipid bilayers. An EPR spin label study. Biochim. Biophys. Acta 1068 68-72. 

The degradation profile can be detected by measuring a property, such as microhardness, as a function of depth after ageing so that the magnitude of any effect from the limitation of oxygen diffusion could be measured for any temperature and material combination. The effect of a degradation profile on a bulk property will depend on the particular... 

It should be noted that temperature can have a significant effect on the degradation as it controls both the rate of oxygen diffusion into the material and the rates of reaction of the products of the irradiation. The diffusion of oxygen is a limiting factor (as it is with heat ageing) and oxidation is directly connected to the dose rate effect. 

A microhardness method allows thin test pieces and can also be used to examine degradation as a function of thickness to detect any effect of limitation in oxygen diffusion. 

The assumption can be made that degradation is dependent on total radiation dose but is independent of dose rate (see Section 6.13). However, acceleration levels can be very high and this is a prime reason why, in practice, it is often found that the effect of a given dose decreases with increased dose rate. The limiting factor is the rate of oxygen diffusion. Recommended practice is to test at two or more dose rate levels to determine the magnitude of this effect. 

Giletti BJ (1986) Diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks. Earth Planet Sci Lett 77 218-228... 

Eor illustration purposes, we consider here a simple scenario of this interplay. We evaluate the effectiveness factor at a fixed cell voltage and thus at a fixed rig. We can express the corresponding current density as a two-variable function, jg =f f, Sqi), where the reaction penetration depth, CL/ depends on rjg. This function can be used to determine the effectiveness factor, rcL- In the case of severely limited oxygen diffusion, the following relations for local oxygen partial pressure and current density can be obtained ... 

Figure 3-13 (a) The shape of a "perfectly" shaped quartz crystal, and (b) the effective "shape of quartz crystal with respect to oxygen diffusion (after coordinate transformation using Equation 3-70d). The length (thickness) to diameter ratio is about 2 1 in (a) and 1 5 in (b). 

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