Diffusion extrinsic

An investigation was made of diffusion into intrinsic n-type crystals which were doped with P to 5 x lO /cm, and into B pre-diffused extrinsic p-type crystals, by using a closed-tube diffusion technique. Profiles were determined by means of neutron activation and Ga radioactive tracer and sectioning techniques. The overall results for 900 to 1050C could be described by ... 

Information on ionization energies, solubiUties, diffusion coefficients, and soHd—Hquid distribution coefficients is available for many impurities from nearly all columns of the Periodic Table (86). Extrinsic Ge and Si have been used almost exclusively for infrared detector appHcations. Of the impurities,... 

Frequently, adsorption proceeds via a mobile precursor, in which the adsorbate diffuses over the surface in a physisorbed state before finding a free site. In such cases the rate of adsorption and the sticking coefficient are constant until a relatively high coverage is reached, after which the sticking probability declines rapidly. If the precursor resides only on empty surface sites it is called an intrinsic precursor, while if it exits on already occupied sites it is called extrinsic. Here we simply note such effects, without further discussion. 

Diffusion in the extrinsic region can readily be modified by doping, although knowledge of the mechanism by which the diffusion takes place is important if this is to be immediately successful. For example, sodium chloride structure materials that conduct by a vacancy mechanism can have the cation conductivity enhanced by doping with divalent cations, as these generate compensating cation vacancies. The inclusion of cadmium chloride into sodium chloride can be written ... 

The first four chapters introduce basic concepts that are developed to build up a framework for understanding defect chemistry and physics. Thereafter, chapters focus rather more on properties related to applications. Chapter 5 describes diffusion in solids Chapter 6, ionic conductivity Chapters 7 and 8 the important topics of electronic conductivity, both intrinsic (Chapter 7) and extrinsic (Chapter 8). The final chapter gives a selected account of magnetic and optical defects. 

Quenching of fluorescence of tryptophan residues, coenzyme fluoro-phores, or extrinsic probes buried in the interior of proteins by colli-sional quencher molecules diffusing through the protein matrix/7,25 27)... 

Basically, whenever isotopic exchanges occur between different phases (i.e., heterogeneous equilibria), isotopic fractionations are more appropriately described in terms of differential reaction rates. Simple diffusion laws are nevertheless appropriate in discussions of compositional gradients within a single phase— induced, for instance, by vacancy migration mechanisms, such as those treated in section 4.10—or whenever the isotopic exchange process does not affect the extrinsic stability of the phase. 

One can employ linearly polarized light to excite selectively those fluorophores that are in a particular orientation. The difference between excitation and emitted light polarization changes whenever fluorophores rotate during the period of time between excitation and emission. The magnitude of depolarization can be measured, and one can therefore deduce the fluorophore s rotational relaxation kinetics. Extrinsic fluorescence probes are especially useful here, because the proper choice of their fluorescence lifetime will greatly improve the measurement of rotational relaxation rates. One can also determine the freedom of motion of the probe relative to the rotational diffusion properties of the macromolecule to which it is attached. When held rigidly by the macromolecule, the depolarization of a probe s fluorescence is dominated by the the motion of the macromolecule. 

Many published results on electronic transport properties of organic materials, where metal contacts are usually made by evaporation of metals, do not describe the quality of the organic/metal interface, and some exotic observed features may perhaps be ascribed to extrinsic effects such as metal diffusion. The relatively simple contact lamination technique may become an alternative, since it provides a means for establishing electrical contacts without the potential disruption of the organic material associated with metal evaporation. The method consists in bringing the organic layer into mechanical contact with an elastomeric element coated with a thin metal film, which can also be patterned. The contacts are robust and reversible... 

When the point defect relaxation is diffusion controlled, we can use Eqn. (5.89) to determine k. After setting rAB = aAX (= unit cell dimension), it is found that at even moderate temperatures (= 100°C), x is on the order of a millisecond or less. This r is many orders of magnitude shorter than relaxation times for nonstoichiometric compounds where the point defect pairs equilibrate at external surfaces (Section 5.3.2). In other words, intrinsic defects equilibrate much faster than extrinsic defects if, during the defect equilibration, the number of lattice sites is conserved. 

Diffusion in ionically bonded solids is more complicated than in metals because site defects are generally electrically charged. Electric neutrality requires that point defects form as neutral complexes of charged site defects. Therefore, diffusion always involves more than one charged species.9 The point-defect population depends sensitively on stoichiometry for example, the high-temperature oxide semiconductors have diffusivities and conductivities that are strongly regulated by the stoichiometry. The introduction of extrinsic aliovalent solute atoms can be used to fix the low-temperature population of point defects. 

Extrinsic Crystal Self-Diffusion. Charged point defects can be induced to form in an ionic solid by the addition of substitutional cations or anions with charges that differ from those in the host crystal. Electrical neutrality demands that each addition results in the formation of defects of opposite charge that can contribute to the diffusivity or electronic conductivity. The addition of aliovalent solute (impurity) atoms to an initially pure ionic solid therefore creates extrinsic defects.10... 

For example, the self-diffusivity of K in KC1 depends on the population of both extrinsic and intrinsic cation-site vacancies. Extrinsic cation-site vacancies can be created by incorporation of Ca++ by doping KC1 with CaC and can be considered a two-step process. First, two cation vacancies and two anion vacancies form as illustrated in Fig. 8.12.11 Second, the single Ca++ cation and two Cl anions from CaCl2 are inserted into the cation and anion vacancies, respectively electric neutrality requires that each substitutional divalent cation impurity in KC1 be balanced... 

The extrinsic case applies at low temperatures or large doping levels. The site fraction of cation vacancies is equal to the solute-atom site-fraction and is therefore temperature independent. In the extrinsic regime, no thermal defect formation is necessary for cation self-diffusion and the activation energy consists only of the activation energy for cation vacancy migration. 

The expected Arrhenius plot for cation self-diffusion in KC1 doped with Ca++ is shown in Fig. 8.13. The two-part curve reflects the intrinsic behavior at high temperatures and extrinsic behavior at low temperatures. 

An Arrhenius plot of the cation self-diffusivity will then possess two linear regions. In the high-temperature intrinsic regime, the slope will be — Hg/3 + Hm)/k in the low-temperature extrinsic regime, the slope will be simply Hm/k, where Hm is the migration enthalpy of a cation vacancy. 

For chromophores that are part of small molecules, or that are located flexibly on large molecules, the depolarization is complete—i.e., P = 0. A protein of Mr = 25 kDa, however, has a rotational diffusion coefficient such that only limited rotation occurs before emission of fluorescence and only partial depolarization occurs, measured as 1 > P > 0. The depolarization can therefore provide access to the rotational diffusion coefficient and hence the asymmetry and/or degree of expansion of the protein molecule, its state of association, and its major conformational changes. This holds provided that the chromo-phore is firmly bound within the protein and not able to rotate independently. Chromophores can be either intrinsic—e.g., tryptophan—or extrinsic covalently bound fluorophores—e.g., the dansyl (5-dimethylamino-1-naphthalenesulfonyl) group. More detailed information can be obtained from time-resolved measurements of depolarization, in which the kinetics of rotation, rather than the average degree of rotation, are measured. For further details, see Lakowicz (1983) and Campbell and Dwek (1984). 

Figure 7. Donor impurity diffusion coefficient (Di) vs. electron concentration (electrons per cm3) showing regions of intrinsic and extrinsic diffusion. (Reproduced with permisssion from reference 119. Copyright 1981 Academic...

This dimensionless rate constant contains typical parameters of the process (i.e., the heterogeneous rate constant k°, the diffusion coefficient, and the experiment time), thus reflecting that the behavior of the process is the result of a combination of intrinsic (kinetics and diffusion) and extrinsic (time window) effects. The effect of Kplane in the voltammograms obtained when both species (a) or only oxidized species O (b) are initially present can be seen in Fig. 3.3. 

The gastrointestinal epithelium forms an extrinsic and an intrinsic barrier against diffusion of toxins and pathogens. The extrinsic barrier is characterized by secretion of mucus, which hinders colonization and accelerates clearance of pathogenic organisms. The importance of mucus as a barrier to drug absorption is discussed in Chapter 2 of this book. In addition, the intestinal immune... 

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