[en] Dislocations and vacancy clusters are produced by pressing and recover during sintering. We find that the main decrease of defects is happening at the time when the intensive skrinkage stage begins. Hence sintering could possibly happen as a defect induced diffusion process. Considering powders of different particle sizes we find that with decreasing particle size there is an increasing part of positrons annihilating at surface states. We confirmed this by a Monte Carlo simulation of the positron diffusion. (orig.)