In situ monitoring of crystallographic changes in Pd induced by diffusion of D

Crystallographic changes in a palladium wire cathode were monitored in situ , as deuterium was electrochemically deposited on the surface and diffused radially into the wire. Initially, the win was pure Pd. A constant electrol ysis current density of 1 mA/cm(2) was maintained and D slowly diffused int o the wire. As the D concentration increased, the wire transformed from pur e Pd, to the alpha phase, and finally into the beta phase. This reversible phase transformation begins on the surface and progresses radially inward. During the experiment, x-ray-diffraction data were collected from a volume element of about 180 pi. This volume element was systematically moved in 50 -mu m steps from the edge to the center of a 1.0 mm diameter Pd wire. Throu ghout the course of the experiment, the bulk value of x in PdDx, as determi ned from simultaneous measurements of the electrical resistivity, increased from 0 to similar to 0.72. For each setting of the volume element, a monot onic increase in the volume of the a phase was observed, until the material entered the two-phase region. Once the beta phase appeared, the volumes of both phases decreased slightly with continued loading. The integrated inte nsities of diffraction peaks from each phase were used in conjunction with the known phase diagram to estimate the rate of compositional change within the volume element. The diffusion rate for the solute atoms was estimated to be 57 +/- 8 nm/s, based on the temporal and spatial dependence of the in tegrated intensities of the diffraction peaks from each phase. These data a lso were used to evaluate the time dependence of the concentration of the s olute atoms partial derivative c/partial derivative t and their diffusivity D. The value of partial derivative c/partial derivative t increased linear ly from 6.2x 10(-5) s(-1) at the surface, to 7.6X10(-5) s(-1) midway into t he wire. D was computed to be (3.1+/-1.0) x10(-11) m(2)/s when the transiti on began at r=250 mu m; 2 ks later it had decreased to (2.1+/-0.3) x 10(-11 ) m(2)/s. This may be due to the fact that the volume of the beta lattice a lso decreased during this period. [S0163-1829(98)01846-3].