Mobility of interstitials

R. Kirchheim and U. Stolz. Modeling tracer diffusion and mobility of interstitials in disordered materials. J. Non-Cryst. Solids, 70(3) 323-341, 1985. 

Certain acid dyes [67] stabilize silver oxalate by forming surface compounds, while other dyestufis accelerate the decomposition because their redox properties enhance the ease of electron transfer from the oxalate ion to the silver. The influences of incorporated cadmium, copper and other ions on the rate of thermal decomposition, and on the concentration and mobility of interstitial silver ions, have been reviewed [46,68]. 

K. Goto, T. Hondoh, and A. Higashi, Experimental determinations of the concentration and mobility of interstitials in pure ice crystals in Point Defects and Defect Interactions in Metals, eds. J. Takamura, M. Doyama, and M. Kiritani, University of Tokyo Press, 1982, p. 174. 

Void swelling occurs because the vacancies and interstitials created by radiation are absorbed preferentially at different sinks (cf. Allen, 2004). Preferentially, interstitials are absorbed at dislocations due to stress fields excess vacancies cluster together to form voids. The kinetics of formation of such clusters are controlled by the physical and chemical nature of the material. In pure metals, the kinetics of cluster nucleation and growth are determined in terms of the mobility of interstitials (vacancies), the irradiation time, radiation flux, and the mobility of the defects. In the... 

Table 61. Concentrations and mobilities of interstitial ions and vacant positions in silver halides ...

Composition can affect the types of loops formed under radiation. In Fe-Cr alloys, the damage structures as a function of temperature are basically similar to those in pure Fe under Fe self-ion irradiation, but the loop size in Fe-Cr alloys is much smaller than that in pure Fe [14]. Moreover, the proportions of two loop types in Fe-Cr alloys are also affected by the alloy composition [15]. The fraction of V2 <111> loops increases with Cr content after neutron irradiation at 400—450°C to 15 dpa [16,17]. <100> dislocation loops were predominant in the microstructure in Fe-Cr alloys with Cr content less than 6% and a mixed <100> and V2 <111> loops formed in Fe-Cr alloys with higher Cr content under neutron irradiation at 400°C [15]. In addition, the presence of Cr in a Fe-Cr binary alloy can remarkably decrease the size of the interstitial loops after neutron irradiation [11,18]. The ratio and size of V2 <111> and <100> dislocations and/or dislocation loops have some effects (although moderate) on radiation hardening because higher mobility of V2 < 111 > dislocation loops could result in their almost complete disappearance via annihilation at the surface. In Fe-Cr alloys, the suppression of mobility of interstitial defects by Cr atoms results in a higher recombination rate and leads to delays in the formation of visible defects [10,11]. Similar effects on loop size and type may also come from other constituent elements, such as Ni as well as interstitial atoms such as C and N [19]. 

In pure metals, the recovery of point defects created or retained at low temperature follows a general scheme, evidenced by the study of isochronous annealing of irradiated materials. This is because the mobility of interstitials is much higher than that of vacancies. Typically, in a metal such as copper, interstitials become mobile around 50-100K ( ==0.1 eV), and at 100 K have either recombined with vacancies, or clustered in small dislocation loops, or bound to impurities. Vacancies become mobile above =250K and form clusters that dissociate at high temperature ( = 500 K). 

The vacancy is very mobile in many semiconductors. In Si, its activation energy for diffusion ranges from 0.18 to 0.45 eV depending on its charge state, that is, on the position of the Fenni level. Wlrile the equilibrium concentration of vacancies is rather low, many processing steps inject vacancies into the bulk ion implantation, electron irradiation, etching, the deposition of some thin films on the surface, such as Al contacts or nitride layers etc. Such non-equilibrium situations can greatly affect the mobility of impurities as vacancies flood the sample and trap interstitials. 

Another subsidiary field of study was the effect of high concentrations of a diffusing solute, such as interstitial carbon in iron, in slowing diffusivity (in the case of carbon in fee austenite) because of mutual repulsion of neighbouring dissolved carbon atoms. By extension, high carbon concentrations can affect the mobility of substitutional solutes (Babu and Bhadeshia 1995). These last two phenomena, quenched-in vacancies and concentration effects, show how a parepisteme can carry smaller parepistemes on its back. 

Fig. 26. A schematic diagram illustrating a possible hydrogen diffusion path, as in Fig. 25. The diagram indicates the Si—H bonds, the distribution of weak Si—Si bonds that can be broken by the insertion of a hydrogen atom, and mobile hydrogen interstitial states (Street etal., 1988). 

Determination of the persistence and mobility of organotin compounds — especially in aquatic abiotic materials, such as sediments, sediment interstitial waters, suspended particulates, and the water column — and on the partitioning of these compounds between the surface microlayer and subsurface waters (Wilkinson 1984 Thompson et al. 1985). 

Brannon and Patrick [129] reported on the transformation and fixation of arsenic V in anaerobic sediment, the long term release of natural and added arsenic, and sediment properties which affected the mobilization of arsenic V, arsenic III and organic arsenic. Arsenic in sediments was determined by extraction with various solvents according to conventional methods. Added arsenic was associated with iron and aluminium compounds. Addition of arsenic V prior to anaerobic incubation resulted in accumulation of arsenic III and organic arsenic in the interstitial water and the exchangeable phases of the anaerobic sediments. Mobilization of... 

A simple yet valuable starting point for treating ionic conductivity, tr, is as the product of the concentration, C(, of mobile species (interstitial ions or vacancies), their charge, q and their mobility, u, ... 

Occasionally, it is possible to vary the composition to such an extent that it is possible either to fill completely a set of interstitial sites or to empty completely a particular set of lattice sites. When this happens, random walk theory predicts that at the half-stage, when the concentrations of filled and empty sites are equal, the ionic conductivity should pass through a maximum because the product of the concentration of mobile species, c, and sites to which they may migrate (1 — c,) is at a maximum. 

Mobilization of edemas (A) In edema there is swelling of tissues due to accumulation of fluid, chiefly in the extracellular (interstitial) space. When a diuretic is given, increased renal excretion of Na and H2O causes a reduction in plasma volume with hemoconcentra-tion. As a result, plasma protein concentration rises along with oncotic pressure. As the latter operates to attract water, fluid will shift from interstitium into the capillary bed. The fluid content of tissues thus falls and the edemas recede. The decrease in plasma volume and interstitial volume means a diminution of the extracellular fluid volume (EFV). Depending on the condition, use is made of thiazides, loop diuretics, aldosterone antagonists, and osmotic diuretics. 

The feasibility of the solnte to stay in the moving mobile phase and in the stagnant pore liqnid is proportional to the volnme of moving mobile phase (interstitial volume) and to the volnme of the mobile phase present in the pores (pore volnme). 

Whereas the ion does not compete strongly with Fe " ions for the A sites, the Cu" ion does. Moreover, the Cu ion can be fairly mobile in close-packed anion arrays, and we will suggest that accommodation of Cu in interstitial positions may be a source of some confusion about valence states. Therefore, let us consider first what evidence there is for the possibility of interstitial ions associated with the spinel structure. 

A common reason for diuretic use is for reduction of peripheral or pulmonary edema that has accumulated as a result of cardiac, renal, or vascular diseases that reduce blood delivery to the kidney. This reduction is sensed as insufficient effective arterial blood volume and leads to salt and water retention and edema formation. Judicious use of diuretics can mobilize this interstitial edema without significant reductions in plasma volume. However, excessive diuretic therapy may lead to further compromise of the effective arterial blood volume with reduction in perfusion of vital organs. Therefore, the use of diuretics to mobilize edema requires careful monitoring of the patient s hemodynamic status and an understanding of the pathophysiology of the underlying illness. 

Meyer s Rule (30) for oxide semiconductors. As the concentration of interstitial zinc donors increases, the ionization energy decreases. The mobility, on the other hand, varies from sample to sample, from 0.6 to 30 cm.yvolt-sec. at room temperature. 

Chemical solid state processes are dependent upon the mobility of the individual atomic structure elements. In a solid which is in thermal equilibrium, this mobility is normally attained by the exchange of atoms (ions) with vacant lattice sites (i.e., vacancies). Vacancies are point defects which exist in well defined concentrations in thermal equilibrium, as do other kinds of point defects such as interstitial atoms. We refer to them as irregular structure elements. Kinetic parameters such as rate constants and transport coefficients are thus directly related to the number and kind of irregular structure elements (point defects) or, in more general terms, to atomic disorder. A quantitative kinetic theory therefore requires a quantitative understanding of the behavior of point defects as a function of the (local) thermodynamic parameters of the system (such as T, P, and composition, i.e., the fraction of chemical components). This understanding is provided by statistical thermodynamics and has been cast in a useful form for application to solid state chemical kinetics as the so-called point defect thermodynamics. 

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