(Invited) Fabrication of Copper/Carbon Nanotube Composite Films Using Plating Techniques
Authors: Arai Susumu 
Published: February 2020

Carbon nanotubes (CNTs) [1,2] have excellent thermal conductivity [3], extremely high current-carrying capacity (ampacity), i.e., stable electrical resistance in the presence of high currents [4], and superior mechanical properties [5,6]. Metal/CNT composites are therefore expected to be potential functional materials, and there have been many investigations concerning the fabrication of metal/CNT composites [7,8]. Composite plating is one promising process for the fabrication of metal/CNT composite films, and its application to such has been investigated [9]. In particular, Cu/CNT composite films are expected to be used for electronics applications.
CNTs are generally categorized into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs). CNTs have anisotropic electrical and thermal conductivity, i.e., they have higher electrical and thermal conductivity in the axis direction than in the direction perpendicular to the axis direction.
In this study, Cu/MWCNT and Cu/SWCNT composite films were fabricated using composite plating techniques, including electroplating and electroless plating. The dispersibility of CNTs in the plating baths is extremely important for composite plating with CNTs that are hydrophobic. Polyacrylic acid (mean molecular weight 5000) is an effective dispersant for MWCNTs in an acidic copper sulfuric bath [10], and Cu/MWCNT composite films with a homogeneous MWCNT distribution have been formed by electroplating using such a Cu/MWCNT composite bath [11,12]. Metal/MWCNT composite films, including the Cu/MWCNT composite film, intrinsically have a tendency toward a bumpy surface morphology due to the (anisotropic) electrical conductivity of MWCNTs [13]. However, a relatively flat surface morphology could be obtained using appropriate current densities (cathode overpotentials) [14] or surfactants [15]. In addition, reverse current electrodeposition could be used to increase the MWCNT content in Cu/MWCNT composite films [16].
Cu/MWCNT composite films have also been obtained by electroless plating using an alkaline bath. Ethylenediaminetetraacetic acid (EDTA), glyoxylic acid and sodium dodecyl sulfate (SDS) + hydroxypropyl cellulose (HPC) were used as a complexing agent, a reducing agent and surfactants for MWCNTs, respectively [17]. The Cu/MWCNT composite films were also successfully fabricated on an acrylonitrile butadiene styrene resin substrate [18].
SWCNTs have the smallest diameter among CNTs and have a tendency to form a characteristic aggregate known as a bundle. This bundle cannot be disintegrated to a primary particle, i.e., single SWCNTs, with the addition of dispersants alone. The use of both a powerful mechanical disintegration method and a dispersant is effective for the disintegration of SWCNT bundles. The selection of dispersants for SWCNTs is also important. In this study, trimethyl stearyl ammonium chloride (TMSAC), which is a cationic surfactant, was selected to prepare a Cu/SWCNT composite electroplating bath [19]. Cu/SWCNT composite films with relatively homogeneous SWCNT distribution were formed by electroplating. Cu/SWCNT composite films with homogeneously dispersed SWCNTs were also fabricated by electroless plating using a bath containing EDTA, glyoxylic acid, SDS+HPC (Fig. 1) [20].
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Figure 1