Cell models for the thermodynamics and kinetics of interstitials in metallic solutions
Wasz, Margot Lancaster
Doctor of Philosophy
This study investigates the thermodynamics and kinetics of hydrogen in a binary palladium-based metal solution using classical jump rate theory applied to the cell model approach. An electrochemical technique is used to measure the diffusivities of hydrogen in palladium alloys containing Er concentrations up to 8 atomic percent at temperatures ranging from 273 to 340 K. The activation energy for hydrogen diffusion was found to increase with Er content to a maximum at five atomic percent, then decrease. A second-order model is presented which reconciles the thermodynamic behavior of the system against the observed diffusivities in the Pd-Er-H system. Related topics for cell models and interstitial systems are also presented. Solubilities for high nitrogen concentrations in liquid iron-based solutions are explained using the cell model approach. The thermodynamic and kinetic behavior of interstitial-vacancy interactions are examined in light of experimental data for carbon in iron, cobalt, and nickel. In these studies, vacancy-interstitial binding energies as high as 1 eV for Fe-C are found to have no perceptible influence in the solubility data. Enthalpy data suggest an upper interstitial-vacancy binding energy limit of 0.4 eV in the Co-C system. When extended to second-order terms and lattice dilation, the cell model is in agreement with observed non-linearities for solvent diffusion for the fcc Fe-C and Ni-C systems. Finally, high temperature deviations in the Arrhenius behavior for interstitial diffusion are explained in terms of magnetic disordering for the Fe-C system, and dual interstitial site occupation for the Co-C system.
Engineering; Materials science