tailieunhanh - Diffusion Solids Fundamentals Diffusion Controlled Solid State Episode 2 Part 4

Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 2 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 322 18 Diffusion of Interstitial Solutes in Metals bility of H in Ni other methods such as Gorsky effect NMR and QENS have limited applicability for the Ni-H system. The Arrhenius parameters below the magnetic transition Curie temperature TC 627K seem to be only slightly different than above see Fig. . Diffusion of hydrogen in iron has considerable importance due to the technical challenge of hydrogen embrittlement of iron and steels. The absolute values for H-diffusion in Fe are higher than those for Pd and Ni see Fig . In spite of the technological interest and the large number of studies for references see 19 the scatter of the data is rather large especially below room temperature. Several reasons for this scatter have been discussed in the literature surface effects trapping of hydrogen by impurities dislocations grain boundaries or precipitates and the formation of molecular hydrogen in micropores either already existing in the material or produced by excess loading for references see 18 . Hydrogen in niobium has been studied over a very wide temperature region see Fig. . Because of the high diffusivities and its relatively small changes with temperature the Gorsky technique could be used for both H and D in nearly the whole temperature range. Tritium diffusion has been studied in a more limited temperature range. Isotope effects of hydrogen diffusion in Nb are very evident from Fig. . Similar isotope effect studies are available for V and Ta see Table . Hydrogen Diffusion in Metals T K 323 1 T K 1 Fig. . Diffusion of H D and T in Nb according to 13 Non-Classical Isotope Effects Classical rate theory for the isotope effect in diffusion predicts if many-body effects are neglected for the ratio of the diffusivities 33 34 D Ẽ- D2 V mi where m1 and m2 are the isotope masses. If manybody effects are taken into account Eq. must be modified see Chap. 9 . Nevertheless the isotope effect is still completely attributed