Controlled synthesis of CNT-based nanostructures for enhanced boiling and wicking

Glen Allen Powell, Purdue University

Abstract

This thesis investigates a nanostructure of copper-coated carbon nanotubes (CNTs) fabricated on copper substrates that improves heat transfer in boiling and evaporation. Multi-walled CNTs are grown by microwave plasma chemical vapor deposition (MPCVD) on copper plates of 1 mm thickness and copper wicks prepared from spherical copper powder sintered in forming gas. Two CNT growth catalysts are considered: (i) a metal trilayer of titanium, aluminum and iron films deposited by e-beam evaporation, and (ii) a liquid solution of ferric chloride hexahydrate applied by spray coating onto substrates prepared with a titanium film. To fabricate CNT arrays reliably on Cu, catalyst is re-deposited following an initial growth run. The second growth results in arrays of CNTs of approximately 40 &mgr;m in length on Cu substrates and approximately 10 &mgr;m in length on Cu wicks. CNT arrays are functionalized to make them hydrophilic by e-beam evaporation of a nominal thickness of 150 nm Cu directly onto the CNT array. Cu metal conformally covers the CNTs to a depth of approximately 10 &mgr;m from the CNT tips. A multi-layer structure using these hydrophilic copper coated CNT (Cu-CNT) arrays is proposed for enhancing wick heat transfer and is produced by sintering Cu powder on Cu-CNT arrays and growing Cu-CNTs on the resulting composite. Sessile drop tests and capillary rise tests demonstrate that Cu-CNT arrays have superior wetting and wicking properties as compared to CNT arrays, and establish that they may be used with water without degrading wicking behavior. When such Cu-CNT arrays are integrated with powder wicks by sintering of Cu powder on top of the Cu-CNTs, a poor bond results, and the sintered wick often detaches from the underlying Cu-CNT layer. High-temperature processing of the Cu-CNTs causes Ostwald ripening of the Cu coating on the CNTs, and sintering shrinks the Cu powder layer as necks between adjacent particles grow; these two factors affect poor bonding to the Cu-CNTs. However, Cu-coated CNTs grown on top of the pre-sintered Cu powder are found to bond strongly, and the thermo-fluid behavior of these samples is reported. Using a wicked boiling apparatus, thermal performance of these enhanced wicks is tested to assess their ability to decrease thermal resistance and increase critical heat flux (CHF) of Cu powder wicks. The two-layer composites of Cu wicks with top-coated Cu-CNTs exhibit thermal resistance as low as 0.22 °C/W and critical heat flux greater than 500 W/cm2 in several tests.

Degree

M.S.M.E.

Advisors

Garimella, Purdue University.

Subject Area

Mechanical engineering|Nanotechnology|Materials science

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