Amplitude-independent mechanical damping in alkali halides
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At present there are two theories which are used to explain the observed amplitude-independent mechanical damping (internal friction) in alkali halides. In the theory of Granato and Lücke it is assumed that the damping constant, B, results from the dislocation interacting with phonons, while in the theory of Robinson and Birnbaum, the assumption is made that the dislocation drag is caused by charged dislocations, interacting with their compensating charge clouds. Both these theories predict a peak in the damping at a frequency of ∼ 10 MHz but the peak in the R-B theory is far less sharp than that in the G-L theory.
In this paper the MHz results of Suzuki, Ikushima, and Aoki for room temperature and the MHz results of Mitchell taken at 77 and 298 K are, together with some kHz results, compared with the two theories. It is shown that both the theories are able to explain the effect of temperature and irradiation but that the theory of Granato and Lücke does not fit the kHz results as well as the theory of Robinson and Birnbaum. The phonon damping analysis gives a change in B of a factor of 14 between the LiF specimens of Suzuki et al and Mitchell while for the charge cloud analysis the charges on the dislocations within the LiF specimen differ only by 20%.
It is concluded that the charge cloud damping theory with a damping constant as a function of frequency fits the experimental results better than the present phonon damping theory which has a damping constant not dependent on frequency.
KeywordsPolymer Halide Internal Friction Charge Cloud Cloud Analysis
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