Postgrowth Microwave Treatment to Align Carbon Nanotubes

We show that a commercial microwave oven can be used after growth to increase alignment of carbon nanotubes (CNTs) and reduce their resistance as thermal and electrical interface materials. Forests of multiwall CNTs were grown directly on both sides of aluminum foils by thermal chemical vapor deposition (CVD) and subsequently exposed to a microwave treatment in air. Scanning electron micrographs revealed enhanced vertical alignment of CNTs after postgrowth microwave treatment. The microwave treatment creates an electric field near the CNT growth substrate that aligns the CNTs orthogonally to the growth substrate. Microwaved CNT forests produced increased mechanical stiffness by approximately 58%, and reduced thermal and electrical contact resistances by 44% and 41%, respectively, compared to as-grown forests. These performance changes are attributed to an increase in the real contact area established at the CNT distal ends because of the enhanced forest alignment. This conclusion is consistent with several prior observations in the literature. This work demonstrates a facile method to enhance the alignment of CNTs grown by thermal CVD without the use of in situ plasma or electric field application.