Thermofluidics with nanoparticles and carbon nanotubes:
On their physics and related novel technologies

Dimos Poulikakos
Professor and Director, Laboratory of Thermodynamics in Emerging Technologies, Institute of Energy Technology, ETH Zurich, Switzerland
email dimos.poulikakos@ethz.ch      
www.ltnt.ethz.ch

Abstract
In this lecture, the topic of nanoparticles in thermofluidics will be addressed.  The focus will be carbon nanotubes, with many unique properties and gold nanoparticles, which possess significantly lower melting temperatures compared to the melting temperature of bulk gold. Using molecular dynamics simulations, we demonstrate and quantify “thermophoretic” motion of solid gold nanoparticles inside carbon nanotubes subject to wall temperature gradients ranging from 0.4 to 25 K/nm. The particles move “on tracks” in a predictable fashion as they follow unique helical orbits depending on the geometry of the carbon nanotubes. The observed thermophoretic motion correlates with the phonon dispersion exhibited by a standard carbon nanotube and, in particular, with the breathing mode of the tube. An increased static friction for gold nanoparticles confined inside a zig-zag carbon nanotube when increasing the length of the nanoparticles is found. An unexpected, opposite trend is observed for the same nanoparticles inside armchair tubes. The issue of functionalization of solid liquid interfaces to facilitate phonon transport and increase the interfacial thermal conductance will also be addressed. On the experimental front, the measurement of the  thermal conductivity of individual multiwalled carbon nanotubes with a novel four-point-probe third-harmonic method will be discussed. A microfabricated device composed of four metal electrodes was modified to manufacture nanometer-sized wires by using a focused ion beam source. A carbon nanotube could then be suspended over a deep trench milled by the focused ion beam, preventing heat loss to the substrate. The multiwalled carbon nanotube was modelled as a one-dimensional diffusive energy transporter and its thermal conductivity was measured at room temperature under vacuum to be 300 ± 20 W/mK.

Moving on to gold nanoparticles, a novel process of direct writing and low temperature annealing of electrical conductors with nanoparticle inks on sensitive organic substrates  will be presented and the complex multiscale and multiphase physics of the underlying processes will be discussed. Combining nanoparticles and nanotubes, a flexible polymer field effect transistor (FET) with a nanoscale (40 nm) carbon nanotube channel was conceptualized and realized. The device was manufactured by direct-writing and spincoating of polymers and gold nanoink. Carbon nanotubes  were dispersed on a polyimide substrate and then marked in an SEM-FIB apparatus such that they could be contacted with gold nanoink. The CNTs were divided into two by a focused ion beam such that they can form the source and drain of the transistor. Poly(3-hexylthiophene) (P3HT) was direct written as an active layer. After fabrication the flexible transistors can be simply peeled off the substrate.     

Thursday, October 25th
Chemistry Laboratories: 1973 Wing, Room 102
2:00 – 3:00 p.m.