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Tham khảo tài liệu 'diffusion solids fundamentals diffusion controlled solid state episode 2 part 1', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 14.3 Techniques of Mechanical Spectroscopy 243 an Fig. 14.5. Internal friction Q 1 n tan Ỗ and frequency dependent modulus M as functions of rT In this case a thermally activated process manifests itself in a loss peak which shifts to higher temperatures as the frequency is increased. Information on the activation enthalpy is then obtained from the peak temperatures Tpeak shifting with frequencies w by using the equation AH -feB d n . 14.20 B d. 1 Tpeak In the Hz regime torsional pendulums operating at their natural frequencies can be used. A major disadvantage of this technique is that the range of available frequencies is very narrow often less than half a decade. This makes it difficult to determine accurate values of the activation enthalpies and to analyse frequency-temperature relations in detail. In order to overcome this limitation devices with forced oscillations are in use. The frequency window of this technique ranges approximately from 30Hz up to 105 Hz. At higher frequencies the mechanical loss of solids can be studied by resonance methods 14 15 . At even higher frequencies in the MHz and GHz regimes ultrasonic absorption and Brillouin light scattering can be used. However most mechanical loss studies have been done and are still done with the help of low-frequency methods. Starting in the 1990s there have been efforts to make use of commercially available instrumentation for dynamic mechanical thermal analysis DMTA These devices usually operate in the three-point-bending mode. Among other systems this technique has been applied to study relaxation processes in oxide glasses 16-18 . 244 14 Mechanical Spectroscopy Fig. 14.6. Octahedral interstitial sites in the bcc lattice 14.4 Examples of Diffusion-related Anelasticty 14.4.1 Snoek Effect Snoek Relaxation The Snoek effect is the stress-induced migration of interstitials such as C N or O in bcc metals. Although effects of internal friction in bcc iron were reported as early as the late 19th century this
