Effect of Carbon Nanotube (CNT) Length on the Mechanical Milling of Ni-CNT Powders and Ni-CNT/Al Reactive Synthesis
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INTRODUCTION
THE remarkable properties of CNTs have made them a strong contender as reinforcements for metal matrix composite materials for improved mechanical and electrical properties. CNT’s outstanding mechanical properties include an elastic modulus of approximately 1 TPa, and a tensile strength as high as 150 GPa.[1] The challenge of incorporating CNTs lies in achieving good dispersion while maintaining the structural integrity of the CNTs. There are a number of novel milling techniques that have been utilized to achieve such dispersion such as plasma-assisted ball milling, conventional ball milling, and powder metallurgy.[2,3] Powder metallurgy has been one of the primary processing techniques used for the dispersion of CNTs in metallic powders to produce CNT-reinforced metal matrix composites. Within powder metallurgy, mechanical milling has been one of the most common methods for combining the CNTs with the metal powder.[4] Metals that have benefited from the incorporation of CNTs include Al and Cu, while only a few studies have been based on other metals such as Mg, Ti, Ag, Sn, and Ni.[4]
VANESSA BUNDY, MEHUL CHAUHAN, CYRUS FITCH, PRATHMESH MODI, and K. MORSI are with the Advanced Materials Processing Laboratory (AMPL), Department of Mechanical Engineering, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182. Contact e-mail: [email protected] Manuscript submitted April 17, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
For example, Chen et al.[5] investigated the effect of CNT aspect ratio in Al-1 wt pct CNT composites with the CNTs being dispersed via three techniques followed by spark plasma sintering. Much of the CNT-reinforced Ni (i.e., Ni-CNT) composite investigations have used electroless deposition as the processing technique to develop films or coatings.[4] Recently, Gharegozloo et al. utilized planetary ball milling at 400 rpm to uniformly disperse 60 to 80 nm diameter multi-wall CNTs in 2.2 to 2.8 lm Ni powder to evaluate the microstructure and catalytic performance of Ni-CNT composites with varying CNT concentrations (5 to 30 wt pct) and milling times (1 to 15 hours).[6,7] Although the work focused on the catalytic performance, they determined that CNTs had become embedded and well dispersed after 15 hours of milling. Borkar et al.[8] compared mechanical milling and molecular-level mixing as dispersion techniques prior to spark plasma sintering consolidation. Planetary ball milling of Ni and 10-nm-diameter multi-wall CNTs was carried out at 400 rpm for 24 hours with minimal structural damage to the CNTs reported; however, no detailed characterization of the as-milled Ni-CNT powders was conducted.[8] Despite the work so far, there remains a gap in the knowledge regarding the CNT dispersion (especially using different length CNTs) and characterization of Ni-CNT composites while considering the nanostructure, microstructure, particle size evolution, changes in the crystallographic defect densities and strain. Such a study can provide a holistic view of mechanical milling of Ni-CNT pow
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