The influence of microstructure on the sintering process in crystalline metal powders investigated by positron lifetime spectroscopy: I. Electrolytic and spherical copper powders

Abstract

We investigate the influence of microstructure (dislocations, and grain and subgrain boundaries) on the sintering process in compacts of electrolytic and spherical copper powders by means of positron lifetime spectroscopy. We compare the lifetime data obtained to the kinetics of the annealing out of vacancy clusters after low-temperature electron irradiation, and the kinetics of recovery and recrystallization after plastic deformation. The change of powder-particle and grain sizes with temperature is determined in a complementary study by metallography and x-ray line-profile analysis. At the intensive-shrinkage stage, the effective powder-particle size in electrolytic copper powder is and the grain size is . Due to the dendritic morphology of the powder, the effective powder-particle size is much smaller than that determined by particle-size analysis . Because of the small powder-particle and grain sizes, a measurable fraction of positrons annihilate at grain boundaries and in surface states, i.e. at inner pore surfaces. At higher temperatures , grain boundaries are, besides a small surface component for compacts of electrolytic powder, the only detectable lattice defects in both powders. We find that the observed shrinkage rates can be explained - at least qualitatively - by Coble creep, while Nabarro-Herring and Kosevic (dislocation) creep seem to play only a minor role in the systems investigated.

Vacancy-type defects in electroplated Cu films probed by using a monoenergetic positron beam

“…The lifetime spectrum of positrons for this sample was found to be 116Ϯ1 ps. This value agrees with the lifetime of positrons annihilated from the free state, [12][13][14][15][16][17] suggesting that the S value obtained for the annealed Cu͑5N͒ sample is close to the characteristic value of S for defect-free Cu (S f ). In Fig.…”

“…This lifetime agrees with that for deformed Cu reported previously. 12,16 It is well known that plastic deformation of Cu introduces two types of defects: dislocations, and vacancies introduced by dislocation movements, such as dislocation intersection and jog dragging. These vacancies are considered to locate adjacent to dislocation lines.…”

“…13,16,17,24 The major defect reaction in stages III and IV is the migration of vacancies and the resultant recombination with interstitial clusters and formation of vacancy agglomerates. Stage V ͑ϳ300°C͒ is the final annealing stage of defects introduced by the irradiation.…”

“…11 Point defects in Cu have been investigated by this method. [12][13][14][15][16][17] In conventional positron annihilation experiments, high-energy positrons ͑р0.54 MeV͒ emitted from 22 Na are implanted into samples. Because the maximum penetration depth of such positrons is on the order of 0.1 mm, it is possible to obtain information about the bulk properties of the material.…”

Defect identification in GaAs grown at low temperatures by positron annihilation

“…These results are similar to those obtained in other metals after sintering. 47 The first lifetime is attributed to annihilation inside grains, i.e., in material which is mostly free of defects whereas the long second lifetime is due to large open volume associated with grain boundaries. According to the theoretical calculations ͑Table I͒, the lifetime corresponds to an open volume of at least four vacancies in As.…”