Jan_AMP_Digital

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. (a) (b) (c) (d)

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.

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

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