Silver self-diffusivity

He studied the sintering of copper particles in the diameter range 15-100 microns and of silver particles of diameter 350 microns. The results for the larger volume fraction of copper and for silver were shown to fit the volume diffusion mechanism and yielded the results for volume self-diffusion... 

Figure 4.42 Self-diffusion coefficient in single and polycrystalUne silver, illnstrating the effect of grain boundary diffnsion, especially at lower temperatnres. From K. M. RaUs, T. H. Conrtney, and J. Wulff, Introduction to Materials Science and Engineering. Copyright 1976 by John Wiley Sons, Inc. This material is nsed by permission John Wiley Sons, Inc.

Consider the diffusivity data presented in Figure 4.42 for the self-diffusion of silver in polycrystalline and single-crystalline form. Use the correlations provided to calculate the following quantities. 

Person 1 Calcnlate the self-diffusivity in polycrystalline silver (grain boundary diffusion), Dgb, at 500°C in m /s. What is the activation energy for this process in kJ/mol ... 

Self-diffusion of Ag cations in the silver halides involves Frenkel defects (equal numbers of vacancies and interstitials as seen in Fig. 8.116). In a manner similar to the Schottky defects, their equilibrium population density appears in the diffusivity. Both types of sites in the Frenkel complex—vacancy and interstitial— may contribute to the diffusion. However, for AgBr, experimental data indicate that cation diffusion by the interstitialcy mechanism is dominant [4]. The cation Frenkel pair formation reaction is... 

Diffusion occurs rapidly along grain boundaries, dislocations, and free surfaces. The most important of these diffusion paths are grain boundaries. Data for self-diffusion in silver are given in Table 9.4 These data are based on experiments on both polycrystals and single crystals of silver. 

Quantitative measurements of diffusion coefficients under bombardment, made recently, substantiate the existence of an enhancement. In lead (122a), a flux of 3 x lO a cm 2 sec i increased the self-diffusion coefficient by a factor of about 100 at 20° and 10 at 40°. The effect was a dynamic one, not the result of permanent damage, since the original rate was again observed after the radiation source was removed. Heit-kamp, Biermann, and Lundy (122b) found an increase in the diffusion coefficient of lead in silver under alpha bombardment, and the increase agrees satisfactorily with that for lead in lead. Thus a flux of 1 X lO i cm 2 sec i gave rise to an 8.8% increase in diffusion coefficient for lead in silver at 438°, and one of 3 x lO o to an increase of 3.3% for lead in lead at 84°. 

We have labeled all possible processes by a binary notation that is explained in Figure 3.2. According to Muller and Ibach [12], the exchange mechanism, in which an atom leaves the terrace and is simultaneously replaced by an adatom, is unfavorable on Ag(lOO) and was therefore not considered for silver, but this process was implemented later for the self-diffusion of Au on Au(lOO) for more details, see Section 3.6. 

The formation of (II) provides a quite selective spot test for palladium. Gold must be removed prior to the test because it will cause the development of a deep ruby red in the spot plate test and a diffused violet spot on the paper, apparently due to the reduction of the gold ions to the colloidal metal. Interference may also arise from 0s04 , Os+, Ru+, and RuCle ions because they have distinct self-colors. Mercurous ion causes partial interference by the reduction of part of the palladium to the elementary state, but a positive response can still be seen. It is possible to detect I part of palladium in the presence of 200 parts of platinum or 100 parts of rhodium. Less favorable ratios should be avoided because of the color of these salts. No interference is caused by mercuric and iridic chloride, but free ammonia, ammonium ions, stannous, cyanide, thiocyanate, fluoride, oxalate, and tetraborate ions do interfere. Lead, silver, ferrous, ferric, stannic, cobaltous, nickel, cupric, nitrite, sulfate, chloride, and bromide ions do not interfere. 

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