Question

10 points. What are the key differences between interstitial and substitutional diffusion? If you have Fe, name 3 elements that would diffuse interstitially and 3 that would diffuse substitutionally.

Answer 1

Diffusion mechanism (7 methods) 1. Interstitial diffusion (Na, Li) - fast processi Si Si - diffuses in interstitials. - does not depend upon vacancy concentration. Interstitial impurity Silicon atom Si Si atom 2. Substitutional diffusion Si Si Si Diffuse in vacancy. Substitutional Vacancy - Slow diffusion. impurity atom Si - Thus controlled. Si IC Technology Ms. Neha Singh Si Si Si 7

Interstitial Diffusion Migration from one interstitial site to another (mostly for small atoms that can be interstitial impurities: (e.g. H, C, N, and O) to fit into interstices in host. Position of interstitial atom before diffusion Position of interstitial om affer diffusion Interstitial diffusion is generally faster than vacancy diffusion because bonding of interstitials to the surrounding atoms is normally weaker and there are Carbon atom in Ferrite many more interstitial sites than vacancy sites to jump to. 18

Substitution-diffusion vacancies and interstitials • applies to substitutional impurities • atoms exchange with vacancies Number (or concentration*) rate depends on (1) number of vacancies; of Vacancies at T (2) activation energy to exchange. AE Motion of a host of substitutionalatom ch N Vacancy • KgT gives eV see web handout for derivation. Vacancy Position of interstitia atom before diffusion Position of interstitial atom after diffusion AE is an activation energy for a particular process (in J/mol, cal/mol, eV/atom). 6

For Fe

interstitial diffusion - Arsenic, germanium, cadmium

Substitutional diffusion- Silicon, Nickel, Copper

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