Hydrogen diffusion barriers

Several authors " have suggested that in some systems voids, far from acting as diffusion barriers, may actually assist transport by permitting a dissociation-recombination mechanism. The presence of elements which could give rise to carrier molecules, e.g. carbon or hydrogen , and thus to the behaviour illustrated in Fig. 1.87, would particularly favour this mechanism. The oxidant side of the pore functions as a sink for vacancies diffusing from the oxide/gas interface by a reaction which yields gas of sufficiently high chemical potential to oxidise the metal side of the pore. The vacancies created by this reaction then travel to the metal/oxide interface where they are accommodated by plastic flow, or they may form additional voids by the mechanisms already discussed. The reaction sequence at the various interfaces (Fig. 1.87b) for the oxidation of iron (prior to the formation of Fe Oj) would be... 

Another example of an industrially relevant CVD process is the production of tungsten (W) films on various substrates for (a) wear and corrosion protection and (b) diffusion barriers in electronic devices. One CVD method for depositing tungsten is through the reduction of WFs with hydrogen, which follows the overall reaction ... 

Fig. 1 Energy diagrams for hydrogen bonds between groups of equal pK. (a( Weak hydrogen bond 0-0 distance 2.8 A. (b) Low-barrier hydrogen bond (2.55 A) the hydrogen diffusely distributed, (c) Single-well hydrogen bond (2.29 A). Horizontal lines are zero point energy levels for hydrogen (upper) and deuterium (lower).

Glucose sensors are mainly used to determine blood glucose in clinical chemistry and diabetes home monitoring. The detection limit is around 1 pM and increases by additional diffusion barriers for undiluted whole-blood measurement and in-vivo application. Typical response times are below one min. Screen-printed electrodes are often made for single use. Entrapment of GOD in polyurethane and amperometric indication of hydrogen peroxide may result in a massively stabilized glucose sensor which may be reused more than 1000 times. 

The mathematical expectation of the activation barrier was calculated to be AE = 1.7 eV. It is significantly less thmi the barrier AE = 2.5 eV for the diffusion of O atom in Si bulk crystal. The calculation of prefactor (Do) was carried out as in [6] to be Do=4.5-10" cm S . The analysis of hydrogen diffusion in SiNT has shown, that, unlike O2 molecule, the H2 molecule is stable in SiNT. For H2 there exist several conformations, differing orientation of the molecular axis with respect to the SiNT axis. The calculated barrier is close to the diffusion barrier of H2 in bulk Si crystal. The calculated prefactor for diffusion of H2 molecule in SiNTisDo=3.9-10- cm -s. ... 

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