Deuteride precipitation and decomposition in the single crystal palladium-deuterium system

The pressure-composition (PC) behavior of deuterium in single crystal Pd has been measured over a temperature range extending from room temperature to 393 K. These data allow the determination of (i) relative partial molar enthalpy and entropy without the contribution of the configuration entropy, for the dilute solid solution phase, (ii) enthalpy and entropy change characterizing the α → α′ phase transformation, and (iii) Gibbs free energy loss associated with the PC thermodynamic hysteresis. Comparison of these quantities with published data for polycrystal Pd and with our own polycrystal Pd measurements show good agreement, indicating grain boundary interfaces and second phase inclusions known to exist in polycrystal Pd do not influence these thermodynamic properties. The time dependence of deuterium absorption and desorption from the gas phase has been measured in both single and polycrystal Pd as well. The activation energy and diffusion constant of deuterium in the solid solution phase of Pd agree with the published values. The time dependence of deuterium absorption and desorption within the miscibility gap are reduced by approximately two orders of magnitude compared to solid solution behavior. The two-phase kinetic data exhibit an Arrhenius behavior with a factor of two increase in the activation energy compared to diffusion. This is attributed to the phase transformation process controlling the deuterium absorption/desorption kinetic behavior when the Pd-D system is within the miscibility gap.; The microstructural characteristics of deuteride precipitation and decomposition in single crystal Pd have been investigated in a series of in-situ small-angle neutron scattering (SANS) measurements. The purpose was to determine and compare the particle morphology during deuteride precipitation and reversion, with the overall goal of developing a model for the pressure hysteresis known to exist in the Pd-H system. The particle morphology along the absorption and desorption branches of the 353 K PC isotherm are consistent with a loss of particle coherency, leading to the formation of large, micron-thick plates. The loss of coherency coincides with the system entering the miscibility gap, an observation that suggests irreversible dislocation formation in part drives the hysteretic behavior of the Pd-D system. SANS analysis further indicates the decomposition process is characterized by a much higher particle dispersion, with a factor of 40 greater surface-to-volume ratio of the precipitating phase. This is attributed to a more hetergeneous transformation process, presumably at dislocations formed during initial deuteride formation.; The effect of dislocations on deuteride formation in the Pd-D system has also been investigated with SANS measurements. High densities of dislocations were introduced into the deformed sample by cycling across the deuteride miscibility gap. Deformation via cycling not only leads to a more finely dispersed microstructure during both absorption and desorption, but also alters the observed habit plane of the large deuteride plate; (100) for the well-annealed Pd matrix and (110) for the deformed Pd matrix.