Neutral ambipolar diffusion

Equation (1.57a) implies that in the locally electro-neutral ambipolar diffusion concentration of both ions evolves according to a single linear diffusion equation with an effective diffusivity given by (1.57b). Physically, the role of the electric field, determined from the elliptic current continuity equation... 

The most important diffusion coefficient for chemistry and materiab science is the chemical diffusion coefficient which characterizes the diffusion kinetics of composition changes. This is formally a diffusion of neutral components, and, for ionic compounds, a charge-neutral ambipolar diffusion of at least two chemically different charged particles . A relevant example is the cheinge in stoichiometry of the oxide M2O (Fig. 6.17c) in the sense of... 

Ambipolar diffusion involves the transport of charged species, and in such cases overall electric charge neutrality must be maintained during diffusion. Moreover, during ambipolar diffusion the difference in the mobilities of the diffusing species sets up a field, the Nernst field, that influences the rates of motion of the particles. 

Polymerization Mechanism in Region III. In region III, all the electrons cannot be transported to the anode in a half cycle of the discharge frequency. A possible charge transportation mechanism is an ambipolar diffusion of ion and electron pairs which will cause polymerization. The diffusion of free radicals may also contribute to the polymerization. In our experiment, the contribution of these two mechanisms cannot be distinguished because the ion and electron pairs behave as neutral gases. 

We will discuss next the ambipolar diffusion, that is, electro-diffusion of two oppositely charged ions in a solution of a univalent electrolyte with local electro-neutrality. Assume the dimensionless ionic diffusivities are constant. Then the relevant version of (1.9) is... 

Another important case relates to ambipolar - diffusion, when a flux of neutral species is driven by a chemical potential gradient In this case, the thermodynamic factor is usually written in similar form (e.g., d[iB/dcB or d In aB/dlncB), and may comprise additional multipliers depending on particular formulae the concentration... 

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 to a chemical potential gradient of one of its components results in a flux of that component through the binary compound as a neutral species. The process, termed ambipolar diffusion, is characterized by a chemical diffusion coefficient Z)chem which is related to the defect and electronic diffusivities by... 

Neutral species within a plasma undergo diffusive and convective transport identical to that in a gas, however, charged species have an additional restriction as a result of the conservation of charge neutrality over distances which are greater than the Debye length of the plasma (of the order of 100 pm). Therefore, in a plasma which contains an equal number of positive ions and electrons, their transport is coupled through the coulombic interactions forcing them to have an equal diffusivity called the ambipolar diffusivity, DA3). 

Figure 1.1a shows schematically the operation of a membrane that is permeable to hydrogen molecules (corresponding to a porous membrane or a dense material in which molecules dissolve and diffuse) or to neutral hydrogen atoms (corresponding to a material in which hydrogen dissolves dissociatively, as in a metal). Figure 1.1b shows schematically how a mixed proton-electron conductor performs the same process by so-caUed ambipolar diffusion of both protons and electrons in the same direction to maintain electroneutrality and zero net current. 

In cases where a continuous and coherent layer of oxide film is present, further reaction can proceed only by diffusion of some of the reactants across the film. There are several possible mechanisms for this transport of material. In many solids, the passage of neutral atoms is less likely than the transport of charged particles, ions and electrons. In such cases, called ambipolar diffusion, the concentration gradient is not the only constraint on the system. In addition, and at all times, overall charge neutrality needs to be maintained. 

Rather limited and sporadic use has been made of the stationary afterglow technique. Historically, it played an important role as the first method to produce data on reactions of aeronomic interest. Typically, a gaseous sample is subjected to a pulse of radiation or excitationf and the subsequent history of the decaying plasma is followed by sampling from it into a mass spectrometer. The phenomena governing this history, such as ambipolar diffusion of the ions and electron-ion recombination, are complex, and ion-neutral reactions constitute only one part of this. As always, the whole must be understood before reliable quantitative information can be obtained for a part, and the deciphering of the history to yield quantitative information on reaction rates is thus difficult. 

Expressions for the reverse saturation current I are calculated from the ambipolar diffusion equation which governs the transport of the thermally generated minority carriers in the homogeneous, quasi-neutral regions of the cell. For the one dimensional case, this equation has the form... 

Effects arising from the partial decoupling of the flows of the ionised and neutral fluids were subsequently investigated by Mullan (1971) and by Draine (1980). The more general case of a magnetic field which is inclined relative to the shock front has been studied recently by Wardle and Draine (1987). The most important consequence of a magnetic field which is transverse to the flow is ion-neutral velocity drift, or ambipolar diffusion. 

In the literature on diffusion and diffusion-controlled reactions or processes one encounters many different terms that describe the diffusional behaviour under different experimental conditions tracer and self-diffusion of atoms and ions, diffusion of defects, chemical diffusion, ambipolar diffusion, a.o. Many of these are used for treating diffusion in compounds, and in the following chapters these phenomena and terms will be described in more detail. Here we will start out with a few simple phenomenological descriptions, and for simplicity we look only at diffusion of neutral, independent particles. 

Multiple steady states in one-dimensional electro-diffusion with local electro-neutrality [14]. This section is concerned with the construction and study of multiple steady states occurring in onedimensional ambipolar electro-diffusion with local electro-neutrality and... 

The Debye radius td is a plasma parameter characterizing the quasi neutrality. It represents the characteristic size of the charge separation and plasma polarization. If the electron density is high and the Debye radius is small (ro R), then the deviation from quasi neutrality is small, electrons and ions move together, and diffusion is ambipolar. If, vice versa, electron density is relatively low and the Debye radius is large (rp > R), then the plasma is not quasi neutral, and electrons and ions move separately and diffusion is free. For calculations of the Debye radins it is convenient to use the following numerical formula ... 

The method fails to distinguish between ambipolar motion and diffusion of neutral defects. Thus in the case of YBa2Cu306+x it is not certain that the measurement determines the ionic conductivity. It can very well be that one measures the rate of diffusion of neutral oxygen interstitials. It was shown that Oj is present in large concentrations in YBa2Cu30g+x, and diffusion of Oj cannot be excluded. ... 

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