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https://hdl.handle.net/2142/25455
Description
Title
Hydrogen in niobium-tantalum superlattices
Author(s)
Miceli, Paul Frank
Issue Date
1987
Doctoral Committee Chair(s)
Zabel, Hartmut
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
hydrogen
niobium-tantalum superlattices
H-metal systems
Curie-Weiss type temperature dependence
Language
en
Abstract
Hydrogen has been dissolved in Nb/Ta superlattices for
which the structural and thermodynamic properties of this
new H-metal system were studied, in situ, by x-ray
diffraction. It is found that H induces a strain modulation
exhibiting a Curie-Weiss type temperature dependence, thus,
providing a first example of a strained layer superlattice
which can be strained after the growth of the structure. In
addition, a lattice-gas lattice-liquid phase transition was
observed where critical fluctuations exist only for
wavelengths longer than the superlattice periodicity. The
absence of short wavelength fluctuations prohibit phase
separation and constitutes a· novel manifestation of a
coherent phase transition. A mean field lattice gas model
is presented, which, combined with x-ray data from the
superlattice satellites, provides insight to the fundamental
properties of a lattice gas in the presence of modulated
interactions. Motivated by these experiments, a generalized
theoretical framework from which to view coherent phase
transitions is discussed.
Due to the epitaxial constraints imposed by the
substrate on the film, the lattice expansion due to H occurs
only normal to the plane of the film, up to a maximum
lattice expansion of Delta d / d ~ .016, and beyond which,
precipitation of oriented phases occurs. Also observed is
an enhanced solubility for H where it takes two orders of
magnitude less gas pressure to obtain the same H
concentration as compared to bulk Nb. It is demonstrated
that this enhancement is directly related to the one
dimensional lattice expansion.
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