High temperature efforts at Los Alamos National Laboratory Page: 5 of 10
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0.015
0
0 0.010
--- H20
0.005 -.. - 2-propanol
--- 1-propanol
0
0.000
-0.005-
0.00 0.25 0.50 0.75 1.00
P/PO
Figure 3 The relative amounts of gaseous components adsorbing onto the PLD coated QCMs
as a function of partial pressure.
II. Composite Metal Membrane
While the PLD zeolitic films are porous structures, capable of separation of molecules
based on size, the metla membranes are dense structures, allowing only hydrogen to pass through
its lattice. The most popular metal currently used for pure hydrogen separation has been palladium
or one of its alloys. Several problems have limited its more widespread use. First, palladium or
palladium/silver alloys are expensive. The cost issue alone has greatly limited its usefulness for
many large scale processes. Even for applications where the materials investment can be justified,
the embrittlement problems associated with extended use have further restricted its use. Finally, a
membrane structure consisting only of palladium or palladium/silver does not pass hydrogen very
rapidly. A membrane that is thick enough to provide the necessary mechanical stability will
inherently have low permeation rates. The composite metal membrane is an attempt to effectively
address all three major issues restricting use of palladium based membranes.
Palladium is not unique in allowing atomic hydrogen to pass through its lattice. Indeed,
other metals such as the group 5 materials can provide substantially higher fluxes of hydrogen14.
They, however, suffer from other hindrances. Not only do these metals not catalyze the
dissociation of molecular hydrogen at their surfaces, they also form oxide layers, impeding the
flow of hydrogen. The composite structure mates the surface catalysis of palladium with the
structural integrity and improved flow rates of a group 5 refractory metal to form a hybrid
membrane that resists embrittlement, has high gas fluxes, and is relatively inexpensive. While this
concept is certainly not unique15,16, the application of vacuum thin film technology in the
fabrication of the structure has resulted in improved membrane performance.
Fabrication of the membrane began by mounting a refractory metal such as tantalum,
niobium, or vanadium in a vacuum chamber. The metal surface was cleaned by ion beam
sputtering. This removed any surface impurities along with the ever-present metal oxide layer.
Following the sputter cleaning, the metal foil was immediately coated on both sides with palladium4
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Peachey, N. & Dye, R. High temperature efforts at Los Alamos National Laboratory, article, December 1995; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc627315/m1/5/: accessed May 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.