Aligned MWCNTs as Thermal Conductors in Adhesive Joints

Thermal conductivities of joints can be increased substantially.

August 1, 2008

Air Force Research Laboratory

A developmental method of increasing the thermal conductance of an adhesive joint in a composite-material structure (or between a composite-material structure and another structure) involves exploitation of the inherently large thermal conductances of multi-wall carbon nanotubes (MWCNTs) along their longitudinal axes. A composite material of the type to which the method applies is, somewhat more specifically, a polymer-matrix/fiber composite. Typically, the matrix polymer is an epoxy, and the adhesive used to bond the composite-material structure may not be an epoxy. In any event, the thermal conductivity of the adhesive is about 0.3 W/mK — a value that is insufficient for many applications in which there are requirements for efficient transfer of heat.

A Joint Between Two Structural Components contains not only an adhesive but also perpendicularly oriented MWCNTs in thermal contact with the components to increase the thermal conductivity of the joint. This view is greatly simplified and not to scale.

The basic idea of the method is to incorporate, into a joint between two structural components, MWCNTs having their longitudinal axes oriented substantially along the axis through the thickness of the joint and their ends in thermal contact with both structural components (see figure). In other words, the basic joint configuration is modified to incorporate a thin layer containing MWCNTs oriented through the thickness.

In principle, the layer of through-thickness-oriented MWCNTs could be grown on one of the structural components and infiltrated with the adhesive, and then the two structural components could be pressed together. This approach was deemed not to be practical for a proof-of-concept experiment because the nonuniformity of height and imperfect alignment of MWCNTs as grown makes it difficult or impossible to ensure adequate thermal contact between the structural components to be bonded and the ends of all or a substantial portion of the MWCNTs.

In an alternative approach that was followed in the proof-of-concept experiment, these difficulties were overcome by fabricating the MWCNT-containing layer separately, prior to placement between the components to be bonded, as an epoxy/MWCNT composite in which the epoxy served to keep the MWCNTs oriented along the through-the-thickness axis. The fabrication of the MWCNT-containing layer included growth of perpendicularly oriented MWCNTs on a quartz substrate, infiltration of the layer with an epoxy that was then cured, removal of the resulting epoxy/MWCNT film from the substrate, etching of the film in an oxygen plasma to expose the tips of the MWCNTs, and coating of both sides of the film with gold. For the purpose of the experiment, the structural components to be bonded were represented by pyrolitic graphite face sheets coated with gold and palladium. In preparation for bonding, the faying surfaces of the MWCNT-containing layer and the graphite face sheets were coated with thin layers of indium. Then the face sheets were placed on opposite faces of the MWCNT-containing layer and the resulting sandwich structure was heated to 175 °C to bond the layers together. In the experiment, the through-the- thickness thermal conductivity of the structure was measured to be 250 W/mK — hundreds of times the thermal conductivity of a typical adhesive or matrix.

This work was done by Ajit K. Roy, Sabyasachi Ganguli, Sangwook Sihn, Liangti Qu, and Liming Dai of the Air Force Research Laboratory.

AFRL-0090