Nucleation rate of solidification probed by x-ray absorption temperature scans in undercooled liquid metals

A new method to measure the nucleation rate of solidification I(T) in under cooled liquids is described. The method is based on the phase sensitivity o f the x-ray absorption coefficient above core-electron absorption edges. By tuning the photon energy to a high-contrast value, the liquid and solid sa mple fractions can be accurately measured as a function of temperature and time, during the crystallization phase of suitable sample temperature scans . The method can be applied to emulsions of liquid droplets or powder mixtu res at high temperatures. The samples are composed of a large number (10(4) -10(9)) of independent micrometric droplets which guarantee excellent stati stics. A theoretical treatment of the nucleation statistics, suitable for t he interpretation of the experimental data, is developed. The method is app lied to the crystallization of undercooled liquid palladium droplets disper sed in sintered Al2O3 powder. The Pd nucleation rate I(T) has been determin ed in the interval 1560 K < T < 1610 K covering a range of 6 orders of magn itude. The data indicate the occurrence of a time independent volume nuclea tion phenomenon. The results of this fundamental measurement are discussed in light of much interest from both thermodynamical and metallurgical point s of view. A fit of our data with a model from the classical theory of nucl eation gives a value of the exponential factor compatible with known thermo dynamic quantities such as the solid-liquid interfacial energy sigma and a preexponential factor K-v on the order of 3.4x10(31) m(-3) s(-1), suggestin g the occurrence of a heterogeneous nucleation process. (C) 2000 American I nstitute of Physics. [S0021-8979(00)01912-5].