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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 2 M etallic glues can serve as excel- lent conductors for heat dissi- pation and electrical current in METALLICGLUE FOR AMBIENT ENVIRONMENTS MAKINGSTRIDES Advancements in nanoscience are making it possible to metallically glue two solids together at room temperature, in air, and under a small amount of mechanical pressure. Stephen Stagon and Alex Knapp, University of North Florida, Jacksonville Paul Elliott and Hanchen Huang,* Northeastern University, Boston

brazing also involve high-temperature melting, where brazing refers to joining through added molten metal at even higher temperatures than soldering, and welding involves melting or fusing the members to be joined, often un- der an inert environment [1] . The joining from such high temperature processes, as compared to polymer glue, is me- chanically strong, effectively conducts electricity and heat, and degrades slow- ly (if at all) in ambient environments. Further, its leak resistance to air and moisture goes from good to better with time due to oxidation [1] . Metallic gluing refers to the pro- cess of joining two solids with metal as the connecting party, which operates at room temperature, in air, and under low pressure. Metallic glues feature the combined advantages of the ambi- ent condition of gluing and the supe- rior properties of the joint from high- temperature soldering (or welding and brazing), making them beneficial to many advanced technologies. As an example, consider desktop and laptop computers. The core of com- puting is the central processing unit (CPU), and connecting the CPU to exter- nal components for heat dissipation or electrical conduction is necessary. The process of making the connection, if it requires high temperature, can damage

the CPU by exceeding the thermal bud- get [4] . For heat dissipation [5] , an ideal connection conducts heat efficiently, which makes metals with high thermal conductivity desirable. However, if sol- der is used, the temperatures necessary to create a good bond can damage the CPU. Also, solder bonds can be relative- ly thick, resulting in reduced heat trans- fer. Further, the thermal conductivity of most solders is low, conducting roughly 5%-20% as effectively as a pure metal such as copper [6,7] . Thermal grease is often used as an interface material, filling the space be- tween the heat sink and CPU. However, the thermal conductivity of this grease is only a fraction that of copper—a mere 1%-2% [7,8] . This low conductivity limits the amount of heat that can be dissi- pated from the CPU and is a significant barrier to further miniaturization and reliability of devices such as tablets and computers. Thermal greases also suffer from problems such as pump out, where grease is forced out of the interface during thermal cycling, and dry out [5] . Figure 1a shows the config- uration of a CPU with a heat sink in a laptop computer, for simplicity. Desk- top computers often contain an addi- tional protective and heat transferring plate between the CPU and heat sink with two separate interfaces requiring

electronic devices and also as leak-resis- tant seals for vacuum environments. The potential market for these applications is extensive and growing rapidly. TECHNOLOGICAL RELEVANCE It is common practice to join two solids together using a third substance for gluing or soldering. Gluing usual- ly refers to the joining process that is made in ambient conditions—at room temperature, in air, and without pres- sure, or with a small amount of mechan- ical pressure [1] . Sealing an envelope with polymer glue is a good example. Despite this process being easy and inexpensive, it often produces prop- erties that make it unsuitable for use in high-tech environments. For exam- ple, polymer glue—unlike metallic sol- der—is permeable to air and moisture, degrades fast in ambient temperature or environment, has low mechanical strength, does not effectively conduct electricity or heat, and does not retain its function at high temperatures [2,3] . In contrast, soldering usually refers to the joining process that uses added molten metal at increased tempera- tures, generally much higher than room temperature [1] . Similarly, welding and

*Member of ASM International