Medical Design Briefs - June 2024 - 26

Thermally Engineering Templates for Highly Ordered
Self-Assembled Materials
The material carries
almost no heat and
therefore stops
heat transfer.
University of Illinois
Urbana-Champagne
Urbana, IL
Self-assembled solidifying eutectic materials
directed by a template with miniature
features demonstrate unique microstructures
and patterns as a result of
diffusion and thermal gradients caused
by the template. Despite the template
trying to force the material to solidify
into a regular pattern, when the template
carries a lot of heat it also can interfere
with the solidification process and
cause disorder in the long-range pattern.
Researchers at the University of Illinois
Urbana-Champaign and the University
of Michigan Ann Arbor have developed
a template material that carries
almost no heat and therefore stops
heat transfer between the template material
itself and the solidifying eutectic
material. This was accomplished by
forming the template from a material
with very low thermal conductivity, ultimately
resulting in highly organized
self- assembled microstructures.
" The key novelty of this research is that
we carefully controlled the flow of heat.
By controlling the flow of heat, the pattern
becomes far better and more regular
than before because we're controlling
Lamellar
more of the parameters. Previously, the
template controlled the flow of atoms,
but the heat flows were uncontrolled, "
says Paul Braun, a professor of materials
science and engineering and director of
the Materials Research Laboratory, who
led this research along with postdoctoral
researcher Sung Bum Kang. The results
of this research were recently published
in the journal Advanced Materials.
Eutectic materials are a homogeneous
mixture that have a melting point
that is lower than the melting point of
either constituent. Common examples
of eutectic systems include solder (a
mixture of lead and tin) and mixtures
of salt (sodium chloride) and water.
When eutectic mixtures are cooled
from the liquid phase, they separate
into two materials that form a pattern at
the solidifying front. The material
doesn't separate into just two big layers.
Instead, it forms structures including a
multi-layered structure (lamellar), like
a tiered cake, a rod-like structure or
even more complex structures. The resulting
microstructure of the material,
however, is only well ordered over short
distances. Instabilities that arise in the
self-assembly process lead to defects in
the microstructure and affects the properties
of the resulting solid material.
For many applications, such as optics or
mechanics, very good order over long
distances is required.
The solidification process can be
controlled by a template consisting of
pillars that act as barriers to the moveStart
bulk structure (AgCl/KCL)
Rod
or
Peel-off and flip-over
ment of atoms and molecules. This
forces the structure to form a more
regular pattern when it solidifies. But
the issue, Braun explains, is that the
pillars carry a lot of heat and instead of
having a flat, solidifying front, the
shape of the front becomes complex.
This leads to irregular patterns and
long-range disorder. " We figured out
how to make the pillars so that they
were really good insulators, " Braun
says. " So all of the heat is only flowing
through the material that's solidifying.
The template is now only acting as a
barrier to the flow of atoms, but almost
no heat is moving between the solidifying
material and the template. "
The researchers explored template
materials with lower thermal conductivities
than the eutectic system and
found that low thermal conductivity
template material resulted in highly
organized microstructures with longrange
order. Specifically, they used porous
silicon (essentially a silicon foam)
that is at least 100 times less thermally
conductive than crystalline silicon.
The template material's low thermal
conductivity minimizes the flow of heat
in the " wrong " direction.
" The thermal conductivity of the template
is a critical factor in determining
the rate of heat transfer during the solidification
process, " Kang says. " The porous
silicon we used for the templates
have a low thermal conductivity and led
to about 99 percent uniformity of the
unit cells of the structure. " In compariFinal
structure
Schematic illustration of the template-directed eutectic solidification process. Liquid (gold) AgCl (cyan)-KCl (black) eutectic system solidifies through the pillar template.
www.medicaldesignbriefs.com
26
Medical Design Briefs, June 2024
Pillar
template
Molten eutectic

http://www.medicaldesignbriefs.com

Medical Design Briefs - June 2024

Table of Contents for the Digital Edition of Medical Design Briefs - June 2024