Nanomechanics and Testing of Core-Shell Composite Ligaments for High Strength, Light Weight Foams
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Nanomechanics and Testing of Core-Shell Composite Ligaments for High Strength, Light Weight Foams Aiganym Yermembetova1, Raheleh M. Rahimi1, Chang-Eun Kim1, Jack L. Skinner2,3, Jessica M. Andriolo3, John P. Murphy3, and David F. Bahr1 1
School of Materials Engineering, Purdue University, West Lafayette, IN 47907 U.S.A. Mechanical Engineering, Montana Tech, Butte, MT 59701 U.S.A. 3 Montana Tech Nanotechnology Laboratory, Butte, MT 59701 U.S.A. 2
ABSTRACT Composite nanostructured foams consisting of a metallic shell deposited on a polymeric core were formed by plating copper via electroless deposition on electrospun polycaprolactone (PCL) fiber mats. The final structure consisted of 1000-nm scale PCL fibers coated with 100s of nm of copper, leading to final core-shell thicknesses on the order of 1000-3000 nm. The resulting open cell, core-shell foams had relative densities between 4 and 15 %. By controlling the composition of the adjuncts in the plating bath, particularly the composition of formaldehyde, the relative thickness of copper coating as the fiber diameter could be controlled. As-spun PCL mats had a nominal compressive modulus on the order of 0.1 MPa; adding a uniform metallic shell increased the modulus up to 2 MPa for sub-10 % relative density foams. A computational materials science analysis using density functional theory was used to explore the effects pretreatment with Pd may have on the density of nuclei formed during electroless plating. INTRODUCTION Metallic foam materials can provide exceptional strength versus density properties that in combination with high specific surface area have a potential to produce catalyst materials, fuel cells, filters, and other applications [1-3]. One of the most important features of foams is the relative density (the density of the cellular structure divided by density of the monolithic material that comprises the whole structure). For example, polymeric foams used for packaging, insulation, and cushioning have relative densities between 0.05 and 0.2 [2]. Metallic foams, in comparison, more commonly have relative densities on the order of 0.2-0.3; this occurs both in bulk form when foams are formed via replication [4] and at the nanoscale when formed via dealloying [5]. There is a need for foams with metallic surfaces (providing catalyst or filtering properties not viable in polymeric structures) at relative densities closer to those in the open cell polymeric foam regime. In this report metallic foams were manufactured using electrospun polymer fiber networks as a template and then subsequently coated with copper by electroless deposition. This current work used both randomly distributed and aligned PCL networks as a template that can be regarded as open-cell foams, due to material contained in cell edges only [2]. EXPERIMENT Fiber mats of PCL were created through electrospinning a solution of 9 wt% PCL in trifluoroethyl alcohol (TFE) in an electrospinning tool with dual control electrodes to enforce fiber alignment [6] as shown in figure 1. The spinneret, contro
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