Diffusion coefficients ambipolar

The movement of the fast electrons leads to the fonnation of a space-charge field that impedes the motion of the electrons and increases the velocity of the ions (ambipolar diffusion). The ambipolar diffusion of positive ions and negative electrons is described by the ambipolar diffusion coefficient... 

From photocarrier grating measurements at low laser intensities (100 mW cm-2), the value of the ambipolar diffusion coefficient of meso PS is deduced to be about... 

As for the permeability measurements, most techniques based on the analysis of transient behavior of a mixed conducting material [iii, iv, vii, viii] make it possible to determine the ambipolar diffusion coefficients (- ambipolar conductivity). The transient methods analyze the kinetics of weight relaxation (gravimetry), composition (e.g. coulometric -> titration), or electrical response (e.g. conductivity -> relaxation or potential step techniques) after a definite change in the - chemical potential of a component or/and an -> electrical potential difference between electrodes. In selected cases, the use of blocking electrodes is possible, with the limitations similar to steady-state methods. See also - relaxation techniques. 

Wagner enhancement factor — describes usually the relationships between the classical - diffusion coefficient (- self-diffusion coefficient) of charged species i and the ambipolar - diffusion coefficient. The latter quantity is the proportionality coefficient between the - concentration gradient and the - steady-state flux of these species under zero-current conditions, when the - charge transfer is compensated by the fluxes of other species (- electrons or other sort(s) of -> ions). The enhancement factors show an increasing diffusion rate with respect to that expected from a mechanistic use of -> Ficks laws, due to an internal -> electrical field accelerating transfer of less mobile species [i, ii]. 

Here ct, ce and cp are the concentrations of the positive ions, electrons and the positron probability density at a point r measured from the center of the blob at time t. Dp is the diffusion coefficient of the positron, Di = De = Damb 0 is the ambipolar diffusion coefficient of the blob, a2 o2 ss a2 is the dispersion of the intrablob species, and a2 is the dispersion of the positron space distribution by the end of its thermalization. Decay rate Te-1 = 1/t + kescs is the sum of the electron solvation rate and possible capture by solute molecules t 2 = 1 /t2 + l/r + kpscs accounts for the free e+ annihilation, solvation and reaction with S. Similarly, t 1 = l/rjmr + hscs, where T r is the rate of the ion-molecule reaction. 

Since diffusion of the electrons and ions tends to spread the discharge radially, one expects that constriction effects will set in at higher pressures for gases which have larger ambipolar diffusion coefficients as is observed (39, 40) for helium relative to the other rare gases. 

At this point, it is a useful exercise to recast Eq. (7.73) in terms of Pick s first law, in order to get a different perspective on the so-called ambipolar diffusion coefficient. It can be shown (see App. 7C) that... 

Since this equation is in the form of Pick s first law, it follows that the chemical or ambipolar diffusion coefficient responsible for oxidation is... 

Ambipolar diffusion coefficients Z ambi Z ambi- These diffusion coefficients are special cases of Z)chem and reflect the fact that in ionic compounds, the fluxes of the ions and defects are by necessity coupled, in order to maintain charge neutrality. 

Exposing a binary compound as a whole to a chemical potential, i.e., for diiy[x/dx 7 0, results in the ambipolar migration of both constituents of that compound down that gradient. The resulting ambipolar diffusion coefficient for an MO oxide is given by... 

Since it was assumed that the rate-limiting step was the diffusion of the anions, it follows that the ambipolar diffusion coefficient, /)ambi x-Make that substitution in Eq. (10C.4) and note that the total volume of matter transported per unit time is J S fl X or... 

The electric field is of the form and magnitude to produce equal loss of electrons and positive ions (ambipolar diffusion). The ambipolar diffusion coefficient is the same for the electrons and the positive ions in the case of only one kind of positive ion with constant mobility and no negative ions. The ambipolar diffusion coefficient for this case is obtained by eliminating the electric fields from Eqs. (1) and (2) and rearranging with n+ = =... 

The ambipolar diffusion coefficient is the coefficient of the concentration gradient in Eq. (3). 

The ambipolar diffusion coefficient for H30 has thus been estimated to be 60 cm sec" at 298°K and 1 Torr, assuming oc... 

If the lattice and grain boundary coefficients are additive, then for a pure binary compound the effective (or ambipolar) diffusion coefficient is given by (11)... 

The corresponding terms transformed to diffusivities are called the ambipolar diffusion coefficients for oxygen ... 

Miyatani and Tabuchi os extended the Haynes-Shockley method for determining D and Dy, (or drift mobility) in a MIEC. A pulse of material is introduced into the MIEC through a SE. The response is measured at a remote site on the MIEC rrsing another SE and a reference electrode. A voltage applied to the MIEC forces the excess rrraterial to drift. The difiusion broadens the measured signal. Both the ambipolar drift arrd ambipolar diffusion coefficients can be determined. This method was applied to aAg2S. 

Employing equation (3.1.222) to determine an effective velocity U for all ions in ambipolar diffusion, develop a relation between the ambipolar diffusion coefficient D for a highly ionized gas and the diffusion coefficients of electrons (D ) and positively charged ions (D+) and any other necessary quantities, e.g. ionic mobilities fi" and of positive ions and electrons, respectively, in the gas phase. 

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