Apparent effective hydrogen diffusivity

D apparent effective hydrogen diffusivity based on gas phase... 

In dispersive electronic transport, the time dependence of the drift mobility leads to some unusual experimental observations, for example, an apparent thickness and field dependence of the mobility. These same effects enter in the relation between the dispersive hydrogen diffusion and the electronic relaxation time. Following Jackson, Marshall and Moyer (1989), the time-dependent diffusion coefficient is described by... 

Effective diffiisivity, permeation rate, and apparent solubility of hydrogen in a series of Fe-Al binary alloys at 25 °C were studied by Luu et al. [95] using electrochemical hydrogen permeation measurements. Both hydrogen diffusivity and permeation rate in these... 

Post et al. (1989) prepared a series of iron and cobalt-base catalysts. The studies were performed in a fixed-bed micro reactor system at temperatures in the range 473—523 K. Variation of catalyst particle size in the range 0.2-2.6 mm showed that the conversion of synthesis gas decreases considerably when the average particle size was increased. Under reaction conditions, the major part of the hydrocarbon product would be the liquid. The liquid would fill the pores of the catalyst so that transport of hydrogen and carbon monoxide to the reactive sites occurred by diffusion of these reactants through this Hquid medium inside the pores. The apparent effective diffusivity D, can be related to the molecular diffusivity, (H2) and solubility, H (H2) of hydrogen in the paraffinic liquid by Eq. (49) ... 

Figure 3 - Apparent effective diffusivity De as a function of the Reynolds number (tracer gas. hydrogen in nitrogen as carrier gas ambient temperature) [9], Reproduced by permission of the American Institute of Chemical Engineers,...

We see on Fig.5 that the apparent effective dif fusivity for hydrogen is higher than for argon and helium.However the ratio of apparent diffusivities for two tracer gases is lower than the ratio of the corresponding diffusion coefficients.This may be due to the influence of intraparticle convective velocity.In fact,let us take a... 

The apparent effective diffusivity D can be related to the molecular diffusivity Dm(H2) and solubility H(ll2) of hydrogen in the paraffinic liquid by the following expression ... 

The above data relate to very pure iron samples with low dislocation densities. In real steels the trapping effects result in much lower apparent diffusivities, which are dependent on the metallurgical state of the steel, as well as its chemical composition. Typical values for the apparent diffusion coefficient of hydrogen in high-strength alloy steel at room temperature are in the region of 10" mVs. 

When the hydrogen pressure is 1 atm, and the temperature is 77 °K, the experimentally observed (apparent) rate constant is 0.159 cm3/ sec-g catalyst. Determine the mean pore radius, the effective diffusivity of hydrogen, and the catalyst effectiveness factor. 

One of the most popular applications of molecular rotors is the quantitative determination of solvent viscosity (for some examples, see references [18, 23-27] and Sect. 5). Viscosity refers to a bulk property, but molecular rotors change their behavior under the influence of the solvent on the molecular scale. Most commonly, the diffusivity of a fluorophore is related to bulk viscosity through the Debye-Stokes-Einstein relationship where the diffusion constant D is inversely proportional to bulk viscosity rj. Established techniques such as fluorescent recovery after photobleaching (FRAP) and fluorescence anisotropy build on the diffusivity of a fluorophore. However, the relationship between diffusivity on a molecular scale and bulk viscosity is always an approximation, because it does not consider molecular-scale effects such as size differences between fluorophore and solvent, electrostatic interactions, hydrogen bond formation, or a possible anisotropy of the environment. Nonetheless, approaches exist to resolve this conflict between bulk viscosity and apparent microviscosity at the molecular scale. Forster and Hoffmann examined some triphenylamine dyes with TICT characteristics. These dyes are characterized by radiationless relaxation from the TICT state. Forster and Hoffmann found a power-law relationship between quantum yield and solvent viscosity both analytically and experimentally [28]. For a quantitative derivation of the power-law relationship, Forster and Hoffmann define the solvent s microfriction k by applying the Debye-Stokes-Einstein diffusion model (2)... 

Said this, we can let the reader to recall Fig. 1.15, where we depicted amorphous-like phase regions at grain boundaries as the pathways open for preferential diffusion of hydrogen atoms. Apparently, an alloy can benefit from some fraction of amorphous phase to improve kinetics of hydrogen absorption, but complete amorphization of crystalline lattice lowers capacity for storing hydrogen [156]. Mechanochemical activation is therefore a complex process where kinetic and thermodynamic effects must be firstly well understood, and then optimized. 

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