Refractory-Metal Diffusion Inhibitors Slow Erosion of Catalytic Metal Particles in the growth of Carbon Nanotubes
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.666
Refractory-Metal Diffusion Inhibitors Slow Erosion of Catalytic Metal Particles in the growth of Carbon Nanotubes Michael J. Bronikowski1 and Melissa King1 1 Department of Chemistry, Biochemistry and Physics; University of Tampa, 401 W. Kennedy Blvd., Tampa, FL 33606
ABSTRACT
Catalytic growth of substantial amounts of Carbon Nanotubes (CNTs) to lengths greater than 1 – 2 cm is currently limited by several factors, including especially the deactivation of the catalyst particles due to erosion of catalyst atoms from the catalyst particles at elevated CNT growth temperatures. Inclusion of refractory metals in the CNT growth catalyst has recently been proposed as a method to prevent this catalytic particle erosion and deactivation, allowing the CNT to grow for greater times and reach substantially greater lengths. Here are presented results of recent investigations into this method. The system investigated employs Molybdenum as the erosion inhibitor and Iron as the CNT growth catalyst. Results show that inclusion of Mo leads to substantially longer catalyst particle lifetimes. INTRODUCTION Carbon nanotubes (CNTs) are of interest for materials applications because of their high tensile strength, high stiffness and outstanding electrical and thermal conductivities [1 – 3]. However, many envisioned bulk applications exploiting these properties for manufacturing [4, 5] will require substantial amounts (kg or more) of CNT whose lengths are comparable to the dimensions of the manufactured items, many cm or more. Thus, much previous research has focused on bulk growth of CNTs to such lengths. CNTs can be grown using metal-catalyzed Chemical Vapor Deposition (CVD), in which carbonaceous gases flow over nanometer-sized metal catalyst particles of
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catalytic metals. Under appropriate conditions of temperature, pressure and flow, the gases decompose to release carbon, which forms CNTS by nucleating and growing nanotubes upon the metal particles. CNT growth in such circumstances normally stops after the nanotubes have reached lengths of a few hundred microns or less, making such nanotube unsuitable for many materials applications. As has been discussed previously [6], a primary mechanism of this growth cessation involves Ostwald Ripening [7,8], which causes the metal particles to either erode away or grow too large to support nanotube growth. This erosion and ripening substantially shortens the catalyst particle lifetimes, resulting in much shorter CNT than would otherwise be possible. Recently, a novel method was proposed to overcome this catalyst particle erosion and ripening by using two component catalysts that combined catalytic metals and high-melting-point refractory metals in a single catalyst material, in
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