High-Temperature Internal Friction in Potassium Chloride

A theory for the strain‐amplitude‐independent damping due to electrically charged dislocations oscillating within their surrounding charge clouds and for the strain‐amplitude‐dependent damping caused by the dislocations ``breaking away'' from their charge clouds is discussed. Damping experiments on single crystals of KCl [〈100〉, 〈111〉, and 〈110〉 orientations] at temperatures of 450° to 760°C, frequencies ranging from 40 to 200 kc/sec, and strain amplitudes from 2.1×10⁻⁸ to 1.7×10⁻⁴ are reported. The damping φ in the amplitude‐independent region could be represented by

$${\mathrm{[open\; phi]=[open\; phi]}}_0\hspace{0.17em}\exp {\mathrm{-(E}}_{\mathrm{eff}}\mathrm{/kT),}$$

where E_eff is frequency‐dependent and orientation‐independent. At 750°C the amplitude‐independent damping rose extremely rapidly with temperature; this rise is attributed to approaching the intrinsic isoelectric temperature. Amplitude dependence was found above a strain amplitude of 3×10⁻⁶ and the damping at high strain amplitudes was observed to decrease with time. The strain amplitude at which amplitude dependence was first detected did not vary noticeably with temperature or frequency. The experimental results are shown to be in general agreement with the proposed theory.