Method for reducing contact resistivity of carbon nanotube-containing epoxy adhesives for aerospace applications Iosif D. Rosca, Suong V. Hoa ⇑ Concordia University, Mechanical and Industrial Engineering, Concordia Center for Composites, Center for Research in Polymers and Composites (CREPEC), 1455 De Maisonneuve Blvd. W., QC, Canada H3G1M8 article info Article history: Received 18 May 2010 Received in revised form 8 September 2010 Accepted 27 October 2010 Available online 13 November 2010 Keywords: A. Adhesive joints Carbon nanotubes B. Electrical properties abstract Bonded joints prepared with conductive epoxy adhesive based on carbon nanotubes (CNTs) display an unusually high resistance mainly due to the high contact resistivity at the adhesive metal interface. A new method is proposed to reduce efficiently the contact resistivity by forcing a controlled amount of electric current through the bonded joint. Current treatment at 0.5 A/cm 2 of current densities for 30 s typ- ically reduces to up to 10 times the contact resistivity. Apart from noble metals, all other metals are usu- ally covered by a more or less conductive oxide layer. This oxide layer is the main cause for high contact resistivity. During the current treatment large gradients of electric field developed around the highly curved CNTs are capable of breaking down locally the oxide layer generating conductive channels. Static shear strength and fatigue resistance of the bonded joint are not affected by the current treatment. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Electrically conductive adhesives (ECAs) based on organic matrices and various inorganic fillers are used in the electronic industry as die attach materials [1] or as an alternative to tin/lead solder [2]. All commercially available structural adhesives are elec- trical insulators, therefore ECAs need to be developed to bond con- ductive structural parts that will withstand high loads, high current densities and to assure electrical continuity that is to elim- inate inter-panel jumper cabling [3,4]. Commercially available ECAs are usually filled with silver particles at very high loadings (>80 wt.%) which assures high conductivity but very low shear strength. High aspect ratio, high electrical conductivity and excel- lent oxidation resistance recommend carbon nanotubes (CNTs) as filler for ECAs. Their high aspect ratio allows low nanotube load- ings with minimal effect on the mechanical performance of the bond. Recently, Li et al. [5] investigated CNT-filled adhesives as a replacement for tin/lead solder for aerospace applications, and Wu et al. [6] obtained highly conductive ECAs based on silver coated CNTs. Individual carbon nanotubes display high contact resistance when contacted with metal electrodes [7,8]. This contact resistance is expected to manifest itself also in joining metal with ECA con- taining CNTs. In our previous work [9] we have obtained highly conductive epoxy composites based on multiwall carbon nano- tubes (MWCNTs). Knowing the resistivity of the adhesive we have predicted very low resistance for lap joints made with aluminum adherents. Contrary to our expectations, the lap joints have shown resistances up to four orders of magnitude higher than the pre- dicted values. This implies much higher contact resistances than those reported in the literature [5]. In order to exploit efficiently highly conductive adhesives it is paramount to significantly reduce the contact resistance such that the adhesive joints can withstand high current densities without excessive heating. These facts led us to investigate in detail the causes for the high contact resistance between adhesives containing CNTs and metal adherents. For short carbon fiber composites, Chekanov et al. [10] have found that upon discharging a high voltage through the composite the resistivity of the composite decreased due to resin breakdown between carbon fibers. However, keeping longer than necessary the high voltage, the composite may be damaged as at the moment of the breakdown the resistance suddenly drops and the current intensity surges, resulting in burning of the composite. Instead of applying voltage to the bonded joint, in this paper we propose a new method to reduce efficiently the contact resistance by apply- ing a controlled current density for a determined time. 2. Materials and methods 2.1. Materials Industrial grade MWCNTs were produced by Nanolab Inc. with a measured diameter and length of 11.5 nm and 3.34 lm respec- tively [9]. The epoxy resin Epon 862 and the curing agent Epikure W were purchased from Hexion Speciality Chemicals. Aluminum 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.10.016 ⇑ Corresponding author. Tel.: +1 514 848 2424; fax: +1 514 848 3175. E-mail address: hoasuon@alcor.concordia.ca (S.V. Hoa). Composites Science and Technology 71 (2011) 95–100 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech