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Aligned MWCNTs as Thermal Conductors in Adhesive Joints

Thermal conductivities of joints can be increased substantially.

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

Topics:
Adhesives and sealants Coatings, colorants and finishes Coatings, colorants, and finishes Composite materials Conductivity Graphite Joining Manufacturing processes Materials Materials properties Nanomaterials Nanotechnology Polymers Resins Thermoplastics
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Aligned MWCNTs as Thermal Conductors in Adhesive Joints

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    Defense Tech Briefs Magazine

    This article first appeared in the August, 2008 issue of Defense Tech Briefs Magazine (Vol. 2 No. 4).

    Read more articles from the archives here.

    Overview

    The document titled "NANO ENABLED THERMO-MECHANICAL MATERIALS IN ADHESIVE JOINTS: A NEW PARADIGM TO MATERIALS FUNCTIONALITY" presents research focused on enhancing the thermal conductivity of adhesive joints using multi-wall carbon nanotubes (MWNT). Traditional adhesives have low thermal conductivity, typically around 0.3 W/mK, which limits their effectiveness in applications requiring efficient heat transfer. The authors propose a novel approach to improve through-thickness thermal conductivity by aligning MWNTs within the adhesive matrix.

    The study highlights the importance of ensuring thermal contact between the conductive phase (MWNTs) and the adherent surfaces to achieve significant improvements in thermal conductivity. The researchers conducted numerical studies and experimental demonstrations, showing that the incorporation of aligned MWNTs can dramatically increase the thermal conductivity of adhesive joints, achieving values over 250 W/mK. This enhancement is several orders of magnitude greater than that of conventional adhesives.

    The processing method described involves growing aligned MWNTs on a quartz substrate, followed by a series of steps to prepare the adhesive joint. The MWNTs are infused with a resin solution, cured, and then subjected to plasma etching to expose the tips of the nanotubes, ensuring they make direct contact with the adherent surfaces. The final assembly includes a sandwich structure where the MWNT-infused adhesive film is placed between conductive graphite face sheets, which are then fused together through heating.

    The findings suggest that this new material configuration not only improves thermal conductivity but also opens up opportunities for tailoring thermal properties in structural joints. The research indicates a significant advancement in the design of composite materials, particularly in applications where thermal management is critical, such as in aerospace and automotive industries.

    Overall, the document emphasizes the potential of nanotechnology to revolutionize adhesive joint performance, providing a pathway for developing advanced materials that meet the demanding requirements of modern engineering applications. The work is a collaborative effort involving researchers from the Air Force Research Laboratory and various academic institutions, and it contributes to the ongoing exploration of nano-enabled materials in engineering.

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