Snoek effect

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The Snoek effect [ snuːk ], named after its discoverer, the Dutchman Jakob Louis Snoek , is a mechanical damping phenomenon that occurs in metals with a body-centered cubic crystal structure and partially occupied octahedral gaps . An important example of materials in which the Snoek effect occurs is carbon-containing α-iron , in which the carbon atoms are dissolved in octahedral holes in the iron lattice.

In the unloaded state of the crystal which are interstitially dissolved in octahedral foreign atoms randomly distributed on all the octahedral holes. The octahedral gaps in the body-centered cubic crystal lattice are anisotropic , in contrast to the face-centered cubic and hexagonal close packed crystal lattice , that is, the distance from the center of the octahedron to the two tips is smaller than the distance from the center to the other corners of the octahedron. The alignment of the tips of the octahedron gaps is evenly and regularly distributed in all three spatial directions. If the volume of the interstitial foreign atom exceeds the volume made available by the octahedral gap, the crystal lattice in the vicinity of the occupied octahedral gaps is elastically distorted.

If, for example, elastic tension is applied in the z-direction of the crystal , the two short atomic distances are stretched in octahedral gaps whose tips are aligned in the z-direction (z-octahedral gaps), while the four long atomic distances are compressed due to the transverse contraction that is, the anisotropy of these octahedral gaps is reduced. In contrast, the anisotropy of the octahedron gaps, the tips of which point in the x and y directions, increases. This means that the elastic distortion is now less when the foreign atoms are embedded in the z-octahedral gaps than when the foreign atoms are embedded in the x or y-octahedral gaps. Consequently, the foreign atoms are now preferentially embedded in z-octahedral gaps.

The rearrangement of the foreign atoms does not take place immediately, but rather through a relaxation process . The jumps from one octahedral site to the next are also responsible for the diffusion . When a mechanical tension is applied, there is a time-dependent change in the dimensions of the crystal - an anelastic behavior that is associated with mechanical damping. When the crystal is relieved, there is a return to the statistical occupation of the octahedral gaps.

Since the relaxation time of the Snoek effect at room temperature is in the order of magnitude of 1 second, mechanical pendulum oscillations, for example, are suitable for cyclical excitation of the sample. The activation energy for diffusion processes in the material can be determined via the dependency of the mechanical damping of a material sample on frequency and temperature . The strength of the damping allows conclusions to be drawn about the content of dissolved foreign atoms.

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