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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 6
2 4
Fig. 2 —
Low-temperature metallic gluing enabled by well separatedmetallic nanorods:
(a) Two sets of well separated nanorods, which have metallic cores and shell elements that
form a eutectic alloy, are brought together, (b) they interpenetrate under fingertip pressure,
(c) shell elements meet and form a eutectic alloy, which is liquid at room temperature, and
(d) mixing of eutectic liquid with a metallic core leads to formation of three-component
alloys that are solid at room temperature.
Fig. 3 —
Scanning electron microscope
image of well separated Cu nanorods. Cour-
tesy of X. Niu, et al.,
Phys. Rev. Lett.,
Vol 110,
136102, 2013.
Fig. 4 —
Metallic glue formed in air and under a small pressure of 9 MPa (a) at room tempera-
ture, and (b) at 100°C. Reprinted with permission from
Scientific Reports
[15]
.
so diffusion on the nanorod surface is
much faster than on flat surfaces
[16]
.
Contact of the sides of the nanorods
through interpenetration provides high
surface area contact, maximizing the ef-
fects of the fast surface diffusion.
While the use of eutectic materials
as shells shows preliminary results of
a room temperature bond at very low
pressure, it is possible to use simpler,
single element nanorods in place of the
core-shell structure required in the eu-
tectic. Silver was successfully used to
create such a bond, but requires higher
pressure for sealing
[15]
.
TECHNOLOGICAL IMPACTS
The impact on technology is clear,
even using only well separated silver
metallic nanorods without a shell. Fol-
lowing the processes in Figs. 2a and 2b,
the fast surface diffusion of nanorods
without the liquid formation of eutec-
tic alloys, gluing also occurs, although
with some voids (Fig. 4a)
[15]
. To reduce
void concentration, a higher process-
ing temperature is needed. As shown in
Fig. 4b, performing the gluing process
at 100°C largely eliminates voids. Using
core-shell nanorods, and therefore the
assistance of liquid from the eutectic
alloy, it is expected that the room tem-
perature gluing process will produce a
bond that is void free, as seen in Fig. 4b.
Even with voids, the metallic glue
shown in Fig. 4a has superior ther-
mal conductivity and leak resistance.
In tests running a simulated CPU at
moderate load with forced air cool-
ing, the metallic glue reduces the CPU
temperature by 8°C ±3°C compared to
the widely used thermal grease, Arctic
Silver 5, operating at 61°C. This is signif-
icant, as keeping the CPU 10°-15°C cool-
er can double its lifespan
[5]
. The leak
rate of the metallic glue shown in Fig. 4a
is three orders of magnitude lower than
that of polymeric glue. This leak resis-
tance meets the standard for organic
solar cell and organic light emitting
diode technologies
[15]
, allowing them
to survive long-term, which may lead
to a new generation of inexpensive so-
lar and lighting technology. Further, as
demonstrated in Fig. 1d, metallic glues
are also useful as a vacuum seal. Capi-
talizing on the superior leak resistance
of the metallic glue, MPF Manufacturing
is investigating using the technology
and licensing the patent
[17]
.
Looking forward, the core-shell na-
norod glue is expected to perform even
better. First, the use of eutectic alloys
through the core-shell nanorods will re-
duce or completely eliminate the voids.
As a result, leak resistance will further
increase, and heat conduction will be-
come even more effective. Second,
the presence of liquid alloys instead of
solids will likely reduce the processing
pressure from a few megapascals to a
fraction of one megapascal, equivalent
to fingertip pressure.
~AM&P
Formore information:
HanchenHuang
is professor and chair, department
(a)
(b)
(c)
(d)