Characterization and Modeling of Hydrogen Diffusion in Palladium-Coated Niobium Trilayer Membranes by Electrochemical Permeation Techniques.
Wojcik, Garrett.
2011
-
Abstract: Hydrogen
purification membranes are traditionally made from pure Pd and its alloys because these
materials offer high throughputs while remaining 100% selective and catalyzing the
dissociation/recombination reactions at the surface. However, the high cost of Pd has
necessitated the development of newer, cheaper purification systems. The Group V
refractory metals Ta, Nb, and V promise ... read moreeven higher permeability at a lower cost, but
they are susceptible to forming oxide layers that are impervious to hydrogen. One
proposed solution has been to remove these surface layers and apply thin, protective
coatings of Pd to form a metallic triple layer. The electrochemical permeation technique
was used to study hydrogen transport through pure Pd, Pd-Ag, and Nb substrates coated
with Pd or Pd-Ag. A multilayer diffusion model was developed, in accordance with
conditions of the experiments, which can extend to any n-layered system and account for
general non-equilibrium interface conditions. Exact solutions were obtained in the
Laplace domain and inverted numerically using an appropriate algorithm. Graphical
representations of concentration or flux were produced for various example systems to
demonstrate multilayer phenomena associated with different surface conditions or with
changes in relative layer thickness, diffusivity, and solubility. A discussion on the
effects of non-equilibrium includes further graphical examples and focuses on the
deviations from equilibrium behavior. The time lag tL for a general triple layer with
non-equilibrium interfaces is derived completely. The significance of each term is
discussed, and special limiting forms of tL are obtained and their physical implications
are explained. It is also shown how tL can be attained simply from knowledge of F(s).
Parametric analyses are performed with respect to both the time lag tL and the
breakthrough time tb, the latter having no analytical form for multilayered systems to
this point. Expressions for tb in a symmetric triple layer are derived that are accurate
in certain parametric regions. These regions are evaluated through an error analysis
comparing the analytical tb with the single layer approximation and the exact value,
which shows that Pd/Nb is a system for which the analytical formula applies. Formulas
for non-equilibrium cases are developed and discussed as well. Experimental data from
Pd/Nb/Pd specimens is described well by the equilibrium model. Calculated values of k
for Pd/Nb are in agreement with theory and past studies. Since k = KPd/KNb << 1
and DPd < DNb, the downstream Pd layer has a barrier effect that causes accumulation
in the Nb layer. The overall process is not diffusion-limited with respect to Nb, so DNb
is not evaluable from tL or the full transient. DNb was instead determined from the
breakthrough time tb and was consistent with previously reported values. Results suggest
that the equilibrium assumption was valid at the interfaces and that bulk behavior was
observed. The results are discussed in the context of some common concerns regarding
metallic composites, including: anomalous behavior in thin films, presence of discrete
intermixing layers at the interfaces, and interface disturbances resulting from
non-equilibrium conditions.
Thesis (M.S.)--Tufts University, 2011.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Daniel Ryder.
Committee: Jerry Meldon, and Christoph Borgers.
Keyword: Chemical engineering.read less - ID:
- g445cr718
- Component ID:
- tufts:21052
- To Cite:
- TARC Citation Guide EndNote
