3.4.3 Growth and coarsening

This chapter talks about the growth and coarsening of helium. At low temperatures and/or high helium production rates, helium bubbles grow by athermal process. After nucleation of bubbles a decrease of the bubble density and an increase of their size are observed. Two mechanisms for this coarsening have been discussed. They are bubble migration and coalescence and helium resolution and reabsorption. Bubble migration can be driven by stress and temperature gradients or can occur by random (Brownian) motion. The motion of a bubble is caused by atoms changing their position on the bubble surface due to various transport mechanisms such as surface diffusion, vacancy volume diffusion or vapour transport through the gas. Coarsening mechanisms (Migration and coalescence, Ostwald ripening) and Exponents n for non-ideal gas behaviour are listed. Ostwald ripening is caused by concentration gradients of helium dissolved in the surrounding of bubbles of different size. Examples of experimental results on bubble growth under external gas supply are given. In the case of external gas supply during the growth period bubble growth is controlled by the absorption of the newly supplied gas. In the presence of an applied mechanical tensile stress and/or an irradiation-induced vacancy supersaturation the growth processes is due to vacancy condensation. The formation and the properties of bubbles such as density are discussed. The discussions in this section show that both the nucleation and the growth and coarsening processes of helium bubbles after low temperature introduction are different from those in high temperature in-situ experiments.