A STATISTICAL APPROACH TO UNDERSTANDING NUCLEATION PHENOMENA

A series of undercooling experiments using pure metal samples of niobi um and zirconium were performed using the Marshall Space Flight Center 105 meter drop tube, and for zirconium using both electromagnetic and electrostatic levitation in the laboratory. The results from a large number of experiments allowed nucleation frequencies to be measured, f ollowing the suggestion of Skripov. Using the nucleation frequencies, the nucleation probability distributions and the cumulative distributi ons, the critical free energy of nucleation and the magnitude of the p re-exponential in the classical nucleation equation were determined. T he results for the pre-exponential and critical free energy for zircon ium ranged from 10(8) to 10(13) and 15 kT to 24 kT respectively. Highe r purity zirconium stock was observed to cause significant shifts in t he undercoolings to higher values. For niobium the values were 10(31) and 73 kT for the pre-exponential and the critical free energy, respec tively.