High frequency magnetics and nanoporous silver for energy applications
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
In this study, soft magnetic materials/composites and nanoporous silver
materials are developed for improving efficiency in energy conversion processes,
namely direct-current-to-direct-current (DC-DC) voltage converters and alkaline fuel
cells.
Switch-mode power electronics operating above 1 megahertz (MHz) have
drawn a lot of attention due to the increasing demand for miniaturization and faster
transient response. Soft magnetic materials are often employed as core materials for
inductors in these electronic devices. During the operation, magnetizations of the core
materials are rapidly switched and/or rotated. The magnetization switching process is
irreversible, costing energy dissipation (core loss). To achieve efficient operation in
these devices, soft magnetic materials with low core loss, large permeability and high
saturation magnetization are in high demand. In this study, we propose to develop two
dimensional (2D) magnetic materials/composites for the above demands. First,
polymer composite materials consisting of aligned metallic iron-nickel (FeNi or
permalloy) flakes have been developed to take advantage of their magnetic softness,
limited eddy current losses, high saturation magnetization, and low cost fabrication
methods. The high energy ball milling method is used to deform permalloy powders
into flakes. A sol-gel method is adopted to coat the surface of each individual flake
with silica to ensure the insulation between flakes. A technique combining of tape
casting and hot press is developed to fabricate the flake/polymer composite. Second,
weakly crystallized iron-hafnium-boron (FeHfB) ribbons are synthesized by melt-spinning technique. Heat treatments in controlled oxygen atmosphere are carried out to
fabricate the iron-hafnium-boron-oxide (FeHfBO) materials. At 5 MHz and a peak
magnetic induction of 20 mT, the core losses of the flake/polymer composite and
FeHfBO material developed in this study show comparable values to those of
commercial low temperature co-fired ceramic (LTCC) ferrite materials. However,
higher saturation magnetizations and larger permeability are achieved in the current
study.
Electrochemical reduction of oxygen in alkaline environment is of
tremendous importance for many electrochemical devices that are directly related to
efficient energy conversion and storage applications. Among those applications,
alkaline fuel cells (AFC) are promising future power sources due to the recent progress
on hydrogen oxidation reaction (HOR) catalysts and hydroxide exchange membranes;
lithium (Li)-air and zinc (Zn)-air secondary batteries are advanced energy storage
devices for future electric vehicles and electrical grids of solar and wind power plants
providing much higher capacity than current state-of-the-art lithium ion batteries.
However, the successful large-scale implementation of these technologies relies on the
development of active and stable oxygen reduction reaction (ORR) catalysts. In this
work, nanoporous silver (np-Ag) material is fabricated through a two-step chemical
de-alloying method. The porous structure is proposed to improve the interactions
between oxygen molecules and the catalyst surface, promoting the reaction rate at low
overpotentials. The np-Ag catalyst exhibits higher ORR activity in the low
overpotential regime and shows better stability in the alkaline electrolyte than the
state-of-the-art Pt/C catalysts.