tailieunhanh - Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 2 Part 2

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 2 part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Quasielastic Neutron Scattering QENS 269 resulting Mossbauer spectrum is a Lorentzian 25 26 Q- X fDW r rt s 2 Al Q 2 2 hw 2 where Ar Q is the full peak-width at half maximum. This diffusional broadening depends on the relative orientation between radiation and crystal _ 2h Ỉ X -__ _ 1 Ar Q - I 1 y WjEj where Ej yy exp iQ rk . T j j k 1 Wj is the probability for a displacement to coordination shell j Ej the corresponding structure factor Nj denotes the number of sites in the coordination shell j and rk are the displacement vectors to sites in shell j. For diffusion mediated by vacancies successive jumps of an atom are correlated. An extension of Eq. for correlated diffusion has been developed by Wolf 20 on the basis of the so-called encounter model see Chap. 7 . The mean time between encounters is Tenc TZenc where Zenc is the average number of jumps performed by a Mossbauer atom in one encounter. Each complete encounter is treated as an effective displacement not correlated to the previous or following encounter. Wolf showed that the line broadening can be expressed as 2h Ar Q Z 11 wrEj I enc j where Wjenc is the probability for a displacement of rj by an encounter with a defect. For further details and for an extension to non-Bravais lattices the reader is referred to 25 29 . An important consequence of Eq. and of Eq. is that both ơ Q w and Ar depend on the relative orientation between Q and the jump vector r and hence on the orientation of the crystal lattice. This can be exploited by measurements on monocrystals. By varying the crystal orientation information about the length and direction of the jump vector is obtained. In that respect MBS and QENS are analogous. Examples for the deduction of elementary diffusion jumps will be given in the next section. Quasielastic Neutron Scattering QENS The scattering of beams of slow neutrons obtained from nuclear reactors or other high-intensity neutron sources .

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