Diffusion of hydrogen, into metals

In metals, the distance between the individual atoms in the lattice is of the order of 0-4 nm and only atoms of very small size are able to penetrate interstitially. This takes place, for instance, in the diffusion of hydrogen into iron, and of carbon into austenite, etc. This type of interstitial diffusion is usually rapid, since the inward movement of the solute atoms is relatively unhampered. 

Endothermic occlusion takes place by diffusion of hydrogen into a metal lattice which is very little changed by the process. In exothermic occlusion by palladium, however, the face-centred cubic lattice (a phase) of palladium, with lattice constant 3.88 A, will accomodate, below 100°C, no more than about 5 at. % hydrogen, and then undergoes a transition to an expanded phase ( 3 phase), with lattice constant 4.02 A and H/Pd = 0.5—0.6. The H—Pd system thus splits into a and 3 phases in the manner familiar for two partially miscible liquids. The consolute temperature (rarely observable for solid phases) is about 310°C at H/Pd = 0.22. The phase diagram is, however, not well established because formation of the... 

Hydrogen can permeate selectively dense metal membranes, behaviour that permits the separation of hydrogen from gas mixtures. The mass transfer mechanism consists of several steps dissociation of hydrogen molecules into atoms, interaction of hydrogen atoms with the metal surface and their adsorption, diffusion of hydrogen into the metal lattice, and desorption of hydrogen atoms from the other metal surface and their recombination into molecules.The overall transport process through the metal wall is called permeation and is ruled by the expression ... 

The ZrHi 7 layer is made by gaseous diffusion of hydrogen into a zirconium metal lattice at high temperature. Hydrogen atoms exist as interstitials in the zirconium metal lattice stmcture. The density of ZrHj 7 varies according to its hydrogen contents x in ZrH [25]. 

CHX and hydrocarbon wax are, respectively, the active intermediates formed by the hydrogenation of surface carbide and products of FTS formed by chain growth and hydrogenation of CHX intermediates. The hydrocarbon wax can contain molecules with the number of carbon atoms in excess of 100. Bulk carbide refers to a crystalline CoxC structure formed by the diffusion of carbon into bulk metal. Subsurface carbon may be a precursor to these bulk species and is formed when surface carbon diffuses into an octahedral position under the first surface layer of cobalt atoms. 

Also, the hydrogen evolution overpotential may be decreased when using this type of MFE. The mercury film thickness can be easily regulated by electrolytic deposition with a coulometric control. When the film is relatively thick, its thickness along the MFE surface may not be uniform. MFEs are not stable in time when the mercury film thickness is very low, due to the diffusion of mercury into the metallic support. 

The alkali metal is deposited at the electrode surface and diffuses at a certain velocity into the interior of mercury. The course of the electrochemical process merely depends upon the composition of the phases at the boundary between the electrolyte and the electrode. The quicker the diffusion of the alkali metal from the surface of the mercury to the interior, the less risk there is of too concentrated amalgam being formed at the surface of the cathode and, therefore, less probability of hydrogen deposition. 

Examples of Ihe deterniinalioii of self-diffusion coefficients in solids are Ihe diffusion of hydrogen ions and water molecules (labelled with T and O, respectively) in alums, of Cl (labelled with Cl) in AgCl, and of 1 (labelled with l) in Agl. Besides self-diffusion, many other diffusion coefficients of trace elements in metals, oxides, silicates and other substances have been determined by application of radio-tracers. Investigation of the migration of trace elements from solutions into glass revealed fast diffusion of relatively small monovalent ions such as Ag+. 

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