Abstract
The sublimation mechanism of bubble migration in solids in a temperature gradient has been investigated by microscopic observation of artificial bubbles in ammonium chloride single crystals. The dependence of the bubble velocity on temperature, temperature gradient, and bubble size corresponds with the theory, which takes into account the influence of the evaporation coefficient. Changes in the shape can be understood qualitatively by means of the calculated temperature fields. The shrinking velocity of bubbles, not being in equilibrium of forces, is evaluated and compared with experiments. The polyhedral form of the bubbles and the lenticular shape of pores in UO2 is explained by the low value of the evaporation coefficient for certain crystallographic planes.

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