Concentration diffusion coefficient

Material modulus Tear strength Gas pressure Gas concentration Diffusion coefficient Temperature... 

In practice two methods are used for stationary planar electrodes in quiescent solution chronoamperometry and chronopotentiometry. By use of an electroactive species whose concentration, diffusion coefficient, and n value are known, the electrode area can be calculated from the experimental data. In chronoamperometry, the potential is stepped from a value where no reaction takes place to a value that ensures that the concentration of reactant species will be maintained at essentially zero concentration at the electrode surface. Under conditions of linear diffusion to a planar electrode the current is given by the Cottrell equation [Chapter 3, Eq. (3.6)] ... 

In crystals, impurities can take simple configurations. But depending on their concentration, diffusion coefficient, or chemical properties and also on the presence of different kind of impurities or of lattice defects, more complex situations can be found. Apart from indirect information like electrical measurements or X-ray diffraction, methods such as optical spectroscopy under uniaxial stress, electron spin resonance, channelling, positron annihilation or Extended X-ray Absorption Fine Structure (EXAFS) can provide more detailed results on the location and atomic structure of impurities and defects in crystals. Here, we describe the simplest atomic structures more complicated structures are discussed in other chapters. To explain the locations of the impurities and defects whose optical properties are discussed in this book, an account of the most common crystal structures mentioned is given in Appendix B. 

Diffusing component 2" Mixture component (diffusion medium) (solvent) Pressure (bar) Tempera- ture ( C) Concentration (%) Diffusion coefficient (m /h)... 

Physicochemical, process [566, 572, 606, 607], or stmctural [27, 610-612] modeling links measured impedances with physicochemical parameters of the electrochemical process (e.g., kinetic parameters, concentrations, diffusion coefficients, sample geometry, hydrodynamic conditions). 

The diffusion constant obtained by tracing the selected particle among many is the marker diffusion constant. The marker diffusion constant is indicated by the labeling symbol, as D. In contrast, the diffusion constant in Pick s law is defined for the many particles involved in the local concentration, and is called the concentration diffusion coefficient. In dilute solutions where particles move independently of each other, these two diffusion constants are the same. In concentrated solutions, the assumption of independent motion of the particles breaks down by molecular interaction, so that the two diffusion coefficients are not identical. 

To study the concentration diffusion coefficient, let us focus on a solute particle in solution. Its average velocity u is decided by the balance condition between the thermal driving force - V/r and the viscous resistance force f u. 

Carrier concentrations, diffusion coefficients and shapes of the Zn profiles in InP layers were compared for various annealing conditions. The Zn-implanted lnP S (4 X 1 o S/cm of active dopant) bulk samples were investigated by implanting them to a fluence of 10 l6/cm2 at an energy of 150keV and then... 

The kinetic analysis of a complicated electrochemical process involves two crucial steps the validation of the proposed mechanism and the extraction of the kinetic parameter values from experimental data. In cyclic voltammetry, the variable factor, which determines the mass transfer rate, is the potential sweep rate v. Therefore, the kinetic analysis relies on investigation of the dependences of some characteristic features of experimental voltammograms (e.g., peak potentials and currents) on v. Because of the large number of factors affecting the overall process rate (concentrations, diffusion coefficients, rate constants, etc.), such an analysis may be overwhelming unless those factors are combined to form a few dimensionless kinetic parameters. The set of such parameters is specific for every mechanism. Also, the expression of the potential and current as normalized (dimensionless) quantities allows one to generalize the theory in the form of dimensionless working curves valid for different values of kinetic, thermodynamic, and mass transport parameters. 

Gonzalez-Vazquez JP, Anta JA, Bisquert J (2009) Random walk numerical simulation for hopping transport at finite carrier concentrations diffusion coefficient and transport energy concept. Phys Chem Chem Phys 11 10359-10367... 

The barrier properties of the PCL-based composites were investigated. The transport properties, sorption and diffusion, were measured by a microgravimetric method . The studied model permeants were methylene chloride and water vapour for which the zero concentration diffusion coefficient Dq was determined. The presence of clay (hydrophilic platelets) in the composite gives rise to specific sites on which water molecules can be entrapped and immobilized, thus the water sorption increases on increasing the clay content, particularly for microcomposites containing Cloisite Na It was found out that the microcomposites as well as the intercalated nanocomposites have diffusion parameters for water vapour very near to those of pure PCL. 

As expected, all intercalated nanocomposites show very similar "gas barrier" behaviour, whether they have been prepared by melt blending or by in-situ intercalative polymerization. In contrast, the exfoliated samples obtained by in-situ polymerization of e-CL in presence of Cloisite 30B show enhanced barrier properties, as evidenced by determining the zero concentration diffusion coefficient (Do) by microgravimetry (Figure 6). In... 

Figure 6. Dependence of water vapour diffusion log Do (Do = zero concentration diffusion coefficient) on the clay content of microcomposites, intercalated nanocomposites, and exfoliated nanocomposites.

To summarize its applications, Eq. 8.2 describes most sets of measurements accurately. Root-mean-square fractional errors of 6-18% are found. The molecular weight dependence of the small-concentration diffusion coefficient is determined by the exponent a, which is consistently —0.5. The concentration exponent v is in the range 0.5-0.75, except for Brown, et al. s measurements, which lead to V 0.93(2). The molecular weight exponent y is modestly less than 0.5, namely between 0.32 and 0.46, except that Brown, et al. s measurements require y 0.6. The joint stretched exponential in c and M provides a good description for each set of measurements, with one consistent systematic deviation the approximation of using the same v for all polymer molecular weights is imperfect. 

Diffusion Moisture concentration Diffusion coefficient Moisture source... 

---