Ignition characteristics of hybrid Al–Ni–Fe 2 O 3 and Al–Ni–CuO reactive composites fabricated by ultrasonic powder consolidation Somayeh Gheybi Hashemabad, Teiichi Ando ⇑ Department of Mechanical and Industrial Engineering, Northeastern University, Boston, MA, United States article info Article history: Received 27 June 2014 Received in revised form 15 October 2014 Accepted 16 October 2014 Available online 14 November 2014 Keywords: Reactive composites Bimetallic–thermite composites Powder consolidation abstract Reactive composites of compositions 2Al–Fe 2 O 3 -x(Al–Ni), 2Al-3CuO-x(Al–Ni) and 2Al–Al 2 O 3 -x(Al–Ni) (x = 1–4) were fabricated from nano-thick Al and Ni flakes and Fe 2 O 3 and CuO nanoparticles by ultrasonic powder consolidation (UPC). Application of in-plane ultrasonic vibration for 1 s at 573 K under a uniaxial pressure of 100 MPa produced full-density consolidates. Upon continuous heating at 125 K/min, the composites began self-heating just before they ignited. The 2Al–Fe 2 O 3 -1(Al–Ni) and 2Al-3CuO-1(Al–Ni) composites ignited at the melting point of aluminum, but all the other composites ignited well below the melting point, indicating that the reaction(s) between the Al and Ni flakes produced hot spots where liquid formed and triggered the ignition. The hybrid bimetallic–thermite compositions, 2Al–Fe 2 O 3 -x(Al– Ni) and 2Al–3CuO-x(Al–Ni), combine the large heat output of the Al–metal oxide thermite reaction and the low ignition temperature of Al–Ni exothermic reactions in single reactive composites. Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. 1. Introduction Nanoheaters are defined as reactive composites made of materi- als, nano-sized at least in one dimension that output heat in calcu- lated amounts through exothermic reactions of the constituents [1]. Reactive composites are largely categorized into bimetallic (e.g., Al–Ni), thermite (e.g., Al–metal oxide) and metal–polymer (e.g., Al-Teflon™) types [2]. They have been used as heaters, propellants [3] and explosives, as well as for self-propagating high-temperature synthesis (SHS) [4–11]. Applications of reactive materials began in the welding of rail sections where Al–iron oxide thermites were tra- ditionally used [12,13] and have recently extended to microjoining [14,15] and soldering in microelectronics [16] as nanostructured reactive materials suitable for localized heating became available. For example, pressurized mixtures of Ti-2B and Ti-C between two Mo sheets have been used as heat sources for ceramic-to-metal join- ing [17]. Au-coated stainless-steel sheets brazed with self-heating nanoscale multilayer Al/Ni foils have exhibited shear strengths as high as 48 MPa, which substantially exceeds that of conventional solder joints which is typically about 38 MPa [14,18]. More recently, exothermic blends of 14Al–3CuO–Ni were used for joining 1100 aluminum alloy sheets [19]. Due to the metastable nature of reactive composite materials, their fabrication requires a method that does not employ high temperatures. Techniques investigated for preparing powder-based nanoheaters include cold isostatic pressing and sintering [20], high- pressure shock activation [21], and more recently cold spray (CS) [22] and ultrasonic powder consolidation (UPC) [1]. UPC is a new additive manufacturing technique in which powders, instead of sheets or wires, are metallurgically consolidated by the action of ultrasonic vibration at low to moderate temperatures, typically within a second [23], and as such provides an enabling technique for the consolidation of reactive powder mixes, as reported for Al– Ni bimetallic and Al–Fe 2 O 3 and Al–Cu 2 O thermite composites [24]. In the present work, hybrid bimetallic–thermite nanoheaters of compositions 2Al–Fe 2 O 3 -x(Al–Ni) and 2Al–3CuO-x(Al–Ni) and ref- erence composites of compositions 2Al–Al 2 O 3 -x(Al–Ni) were fabri- cated by UPC and characterized of their microstructure and ignitability during continuous-heating. The hybrid compositions were chosen as they provide an opportunity to combine the excel- lent ignitability of bimetallic Al–Ni nanocomposites and the large heat output of thermites in one reactive composite. The 2Al– Al 2 O 3 -x(Al–Ni) composites were added to investigate the ignition characteristics of composites with and without a thermite reaction. 2. Experimental procedure 2.1. Material preparation Thermal plasma-synthesized Fe 2 O 3 and CuO nanoparticles (20–40 nm and 30–50 nm in size, respectively) and fine Al 2 O 3 http://dx.doi.org/10.1016/j.combustflame.2014.10.006 0010-2180/Ó 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. ⇑ Corresponding author. Fax: +1 617 373 2921. E-mail addresses: s.gheybi@gmail.com (S. Gheybi Hashemabad), tando@coe.neu. edu (T. Ando). Combustion and Flame 162 (2015) 1144–1152 Contents lists available at ScienceDirect Combustion and Flame journal homepage: www.elsevier.com/locate/combustflame