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How to Optimize Carbon Nanotube Arrays

When designing devices, engineers often must join together materials that expand and contract at different rates as temperatures change. Such thermal differences can cause problems if, for instance, a semiconductor chip is plugged into a socket that can’t expand and contract rapidly enough to maintain an unbroken contact over time.

The potential for failure has intensified as devices have shrunk to the nano-scale, causing strains that are difficult to observe and to avoid.

A team of engineers at Stanford University say they have discovered how to create carbon nanotube structures that remain strong and supple at these critical interfaces where thermal stress is intrinsic to the design. Currently, materials like solder and gels are used at such junctions. But as electronics continue to shrink, more electrical power will be pushed through smaller circuits, putting materials under ever increasing thermal stress.

The Stanford research focuses on nanotubes, infinitesimally thin strands of carbon atoms that have the potential to efficiently conduct heat, are strong for their size, and can be flexible depending upon their fabrication. They say that their experiments and simulations revealed how to create carbon nanotube structures (CNTs) with the optimal blend of all three characteristics, strength, flexibility, and heat conductivity, needed to protect against thermal stress.

The team worked to assemble CNTs with different structural characteristics, then measured the flexibility, and thermal conductivity of each structure to look for the optimal structure. Left alone, the carbon atoms that form CNTs will create structures that resemble a bowl of spaghetti. But the engineers collaborated with others at the University of Tokyo to create CNTs that grow relatively straight, like grasses. In addition, longer CNTs, grown less densely together, seem to have the best combination of flexibility, heat conductivity, and strength, for use in electronics and other industrial applications where thermal stress is expected.

The results showed that as CNT strands grew longer, they tended to grow straighter and were less tangled, which increased the flexibility of the structure. However, they can not grow perfectly straight due to van der Waals forces, weak attractions that exist between molecules. While these forces may not be critical in other types of structures, carbon nanotubes are so thin that minute forces could still fundamentally affect them, and engineers will have to accept some bending and irregularity as they strive to create workable, though less than ideal, structures for dissipating heat.

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