Oxidation Fronts

After polarization to more anodic potentials than E the subsequent polymeric oxidation is not yet controlled by the conformational relaxa-tion-nucleation, and a uniform and flat oxidation front, under diffusion control, advances from the polymer/solution interface to the polymer/metal interface by polarization at potentials more anodic than o-A polarization to any more cathodic potential than Es promotes a closing and compaction of the polymeric structure in such a magnitude that extra energy is now required to open the structure (AHe is the energy needed to relax 1 mol of segments), before the oxidation can be completed by penetration of counter-ions from the solution the electrochemical reaction starts under conformational relaxation control. So AHC is the energy required to compact 1 mol of the polymeric structure by cathodic polarization. Taking... 

When rfc = 0, the polymeric structure is considered to be open enough (i = 0) that any subsequent oxidation will not occur under conformational relaxation control, hence P = 1. Every polymeric chain at the poly-mer/solution interface acts as a nucleus a planar oxidation front is formed that advances from the solution interface toward the metal/polymer interface at the diffusion rate. 

The spreading of the oxidation was followed in air atmosphere at 150 °C. In Fig. 7, the first picture has been taken with external light that shows the location of the PP-film. The following images show the location of the oxidation front at five different times. 

The location of the oxidation front, taken as the average of the two oxidation fronts caused by initiating the sample in the middle of the PP film, was then plotted versus time. The resulting curve shows that the spreading of oxidation accelerates with time (see Fig. 8). It has been reported that the volatile species produced during oxidation of PP can contribute to the spreading of oxidation in-between different samples of PP [139, 140]. A device was constructed that enabled the air flow to be directed over the oxidising... 

Fig. 8 Location of the oxidation fronts in Fig. 7 plotted against time...

Cowie, G.L., Hedges, J.I., Prahl, F.G., and de Lange, GL. (1995) Elemental and biochemical changes across an oxidation front in a relict turbidite an oxygen effect. Geochim. Cosmochim. Acta 59, 33 16. 

An example of recent achievement in this area is a flexible, thin film Cu(In,Ga)Se2 solar cell deposited on a titanium foil, which was combined with a TiC>2 photocatalyst layer and modified by a niobium-doped titanium oxide front electrode to function as a photoelectrochemical tandem cell/membrane for a direct light-driven hydrogen evolution from an aqueous solution [48], Under illumination with UV/vis light, the system produced up to 0.052 pLH2/scm2 (e.g. the hydrogen formation rate was approximately 7,250 pmol/h g relative to the amount of TiC>2 used). Several aspects of the operating principles of the photoelectrochemical devices, the materials requirements, main bottlenecks, and the various device concepts (in relation to H2... 

Figure IS. Time evolution of the composition of the percolating water in the alteration envelope upstream of the oxidation front (see Figure 13). = 10. ...

Cowie, G. L., J. I. Hedges, F. G. Prahl, and G. J. De Lange. 1995. "Elemental and major biochemical changes across an oxidation front in a relict turbidite An oxygen effect." Geochimica Cosmochimica Acta 59 33-46. 

The most direct field evidence that the extent of sedimentary organic matter preservation is affected by exposure to bottom-water oxygen comes from oxidation fronts in deepsea turbidites of various ages and depositional settings (Wilson et al., 1985 Weaver and Rothwell, 1987). One of these deposits in which the timing of the exposure to oxic and anoxic conditions is well documented, is the relict f-turbidite from the Madeira abyssal plain (MAP) —700 km offshore... 

Comparative elemental analyses of the upper and lower sections of two sediment cores collected on the MAP show that organic concentrations decreased at both locations from values of 0.93-1.02 wt.% OC below the oxidation front to values 0.16-0.21 wt.% within the surface-oxidized layer (Figure 9). Pollen abundances decreased in the same samples from —1,600 grains g below the oxidation front to zero above it. Overall, 80% of the organic matter and essentially all of the pollen that has been stable for 140 kyr in the presence of pore-water sulfate was degraded in the upper section of the MAP cores as a result of long-term exposure to dissolved O2. 

Transport phenomena during anodization were also studied by Mackintosh and Brown " In AljOj the halogens and alkah metals moved into the specimen, the former ones to greater, the latter ones to lesser depth than implanted rare gases. Ag, Ba, Ca, Co, Cu, Fe, Ga, Hg, In, Mn, Ni, Sr, Tl, Sb and V moved outwards. In elemental A1 the alkali and alkaline earth metals moved towards the solid-electrolyte interface, The remaining metals moved partly with the advancing oxide front outwards into the growing oxide. Fowler et al. implanted Bi, Sn, Pb, Tl, Ce, Kr, Ag, Cr,... 

Cu and Au to a depth of 400 A in Al. During anodization Bi, Sn, Pb and Tl led to an increased oxygen uptake, Kr showed no effect and Au, Cu, Cr and Hg reduced the oxidation. An analysis using the He backscattering method showed that the oxidation rate starts to decrease as soon as the oxide front reaches the implanted atoms. 

Wilson, T. R. S., J. Thomson, S. Colley et al. (1985) Early organic diagenesis the significance of progressive subsurface oxidation fronts in jtelagic sediments. Geochim. Cosmochim. Acta 49, 811-22. 

Selim. S. (1995). Absorption, Excretion, and Mass Balance of t Piperonyl But oxide front lwo Different Fonnuhttion.s alier Dermal Application to Healthy Volunteers. Unpublished report no. P0594006 from Biological Test Center. Irvine, California, USA. Undertaken for the PRO Task force, Washington DC, USA. 

Thomson, J., Higgs, N.C., Hydes, D.J., Wilson, T.R.S. and Sorensen, J., 1987. Geochemical oxidation fronts in NE Atlantic distal turbidites and their effects in the... 

Study of the BZ reaction in a thin unstirred layer of reacting solution demonstrates that concentric waves ("target patterns") or spiral waves are developed. The reacting solution is normally spread out as a thin film with a few millimeters thickness in a Petri dish (diameter ca. 10 cm). After a certain time blue oxidation fronts which propagate on the red background (reduced ferroin) develop. 

Use relation (10-54) to estimate the front velocity of solid-state (coal) oxidation at low temperatures. How does the oxidation front velocity depend on temperature Which effects determine the temperature limit for acceleration of the oxidation front In which way does nonequilibrium plasma stimulate the process ... 

At 1100°C, however, predominantly stacking faults are formed close to the surface and in the bulk /5,6/ (fig.5,6). Their length depends on oxidation time and atmosphere. Si self-interstitials formed in supersaturation at the oxidation front are absorbed by the stacking faults close to the wafer surface and do not migrate into the bulk of the wafer, while the stacking faults deep in the bulk grow due to incorporation of the interstitials emitted from... 

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