Theoretical and Experimental Study of Pressure Hysteresis in the Palladium Hydride Phase Transformation

Citation

Moorthy, Aadith (2018) Theoretical and Experimental Study of Pressure Hysteresis in the Palladium Hydride Phase Transformation. Senior thesis (Major), California Institute of Technology. doi:10.7907/mems-1g53. https://resolver.caltech.edu/CaltechTHESIS:06072018-182938429

Abstract

A unique phenomenon occurring with metal hydride systems that presents a loss in hydrogen storage efficiency and has received little scientific attention is the hysteresis behavior observed during H2 absorption and desorption. As an ambient H₂ pressure is introduced into a metal hydride, the material undergoes a phase transformation from a hydrogen-poor phase to a hydrogen-rich phase during absorption and the reverse during desorption. However, the phase transformation is hysteretic as it occurs at a much higher H2 pressure for absorption than for desorption.

In this work, the thermodynamics of the metal hydride phase transformation with hydrogen uptake are experimentally studied using the palladium-hydride system with in-situ x-ray diffraction. The in-situ x-ray diffraction has enabled the study of the thermodynamic evolution of the microstructure of the palladium through lattice parameters, phase fractions, strain analyses and other information. The diffraction data has then been compared to the predictions from existing theories on hysteresis in metal hydrides, such as the Schwarz-Khachaturyan and Flanagan-Clewley theories. Finally, these theories are extended and combined to form a new general theory of metal hydride phase transformation thermodynamics that incorporates new attributes of importance for practical metal hydride systems, such as phase interface coherency and changes in dislocation formation energies due to work hardening. This new theory is very effective in explaining the prominent trends in the experimental data and provides a highly general approach for the analysis of phase transformations from hydriding in real metals.

Thesis Files