Researchers
at Texas A&M University have developed the first known metallic gel. Unlike
everyday gels, like those used in hand sanitizers, hair products or soft
contact lenses, this new material is made entirely of metals and can withstand
extreme heat. The discovery could be a game changer for energy storage.
The work is published in Advanced
Engineering Materials.
The gel is created by mixing two metal powders. When heated, one metal melts into a liquid, while
the other stays solid and forms a microscopic scaffold. The liquid metal
remains trapped inside this structure, creating a gel-like material that looks
solid but contains liquid within.
Everyday gels are semi-solid materials
containing an organic backbone holding liquids in place at room temperature.
Unlike them, metallic gels require very high temperatures, which, depending on
the metals used, can be around 1,000 degrees Celsius or 1,832 degrees
Fahrenheit.
"Metallic gels have never been
reported before, probably because no one thought liquid metals could be
supported by an internal ultrafine skeleton," said Dr. Michael J.
Demkowicz, a professor in the Department of Materials Science and Engineering,
who led the research.
"What's surprising in this case is
that when the majority component—copper—was melted into liquid, it didn't just
collapse into a puddle. That's what pure copper would have done," he
explained.
Metallic gels made from highly reactive
metals with strong electrical attraction, known as electronegativity, can be
used as electrodes in liquid metal batteries (LMBs). In simple terms, these
metals are very reactive and easily bond with other materials, which helps the
battery work efficiently.
LMBs are special types of batteries that
store and release large amounts of electrical energy. Instead of using solid
materials like most batteries, they use layers of liquid metal. Because the
parts are liquid, they do not wear out as quickly as regular batteries.
So far, LMBs have mainly been used in
large stationary systems, such as backup power for building applications that
need to keep running during a power outage. They have not been used in moving systems because
the liquid inside shifts when the battery moves. This can cause a short
circuit, which means the battery loses electrical power.
That is where metallic gel electrodes
come in. By holding the liquid metal in place, they could make it possible to use
LMBs in things that move, such as powering large ships or heavy industrial
vehicles that can safely handle the heat of these batteries.
To test the idea, researchers built a
small lab version of the battery using two cube-shaped electrodes. One was made
from a mix of liquid calcium and solid iron, which acted as the anode, and the
other from liquid bismuth and iron, which acted as the cathode.
When placed in a molten salt, a hot
liquid that allows electrical charge to flow between the two, the battery
worked successfully. It produced electricity, and the mostly liquid electrodes
stayed in shape and kept working as intended.
The research was performed by a team led
by Demkowicz and doctoral student Charles Borenstein, who is the first author
on the paper.
Demkowicz and Borenstein said that what
began as an exploration of the behaviors of metal composites of copper and
tantalum resulted in this serendipitous discovery.
"We were just exploring different
methods of processing composites by heat," Demkowicz said. "All we
wanted to do, at first, was to see: Does this even survive until one of the
components melts?"
Borenstein originally put a composite of
25% tantalum and 75% copper into the furnace heated to copper's melting point.
"Nothing happened, which I found
kind of confusing," he said, noting that the copper didn't run out and
pool. "We were pretty surprised by these results."
After testing other percentages of both
metals, he found that any combination of the metals with a volume of tantalum
above 18 percent still retained the gel form.
The next step was to bring the new
structure to a lab with a very high-resolution micro-CT scanner to examine the
metallic gel's interior. Although copper and tantalum are not ideal candidates
for electrodes, they are for CT scanning. As anticipated, the tantalum formed a
solid scaffolding structure holding the liquid copper within its lacunae.
That's when the team shifted their
research to the battery materials of iron, bismuth and calcium, and
demonstrated the feasibility of the metallic gel LMB.
Demkowicz said that an LMB made for
transportable applications could also employ a gel-like composite electrolyte,
such as a molten salt supported by a ceramic backbone, through which
the electrode's ions could pass.
He highlighted other potential
applications for LMBs, including one that he said would be especially exciting
to work on: powering a hypersonic vehicle, like those under feasibility study
at the Texas A&M University Consortium for Applied Hypersonics. Hypersonic
vehicles operate at extremely high temperatures and could theoretically be
powered by a very hot LMB.
Co-authors on the paper are Dr. Brady G.
Butler and Dr. James D. Paramore, visiting professors at Texas A&M, and Dr.
Karl T. Hartwig, professor emeritus at the university.
The high-resolution CT scanning was
performed at the University of Texas High-Resolution X-ray Computed Tomography
Facility in Austin.
Source: Pure metallic gel opens door to more powerful liquid metal batteries
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