2014-a novel method for fabricating fe-cr-al open-cell metallic and alloyed foams, lh bac, bk kim,...

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86 Current Nanoscience, 2014, 10, 86-88

A Novel Method for Fabricating Fe-Cr-Al Open-cell Metallic and Alloyed Foams

Luong-Huu Baca,b

, Byoung-Kee Kimaand Young-Min Kong

a*

a

School of Materials Science and Engineering, University of Ulsan, Ulsan 680-749, Republic of Korea;b

School of Engineering Phys-ics, Hanoi University of Science and Technology, Hanoi, Viet Nam

Abstract: A new fabrication process for generating open-cell metallic and alloyed foams was developed by combining electrical explo-sion of wire (EEW) and electrospray (ESP) techniques. Fe-Cr-Al alloy nano-powders prepared by EEW in ethanol were used as a sta rtingmaterial, and commercial polyurethane (PU) sponges were used as templates. Fe-Cr-Al foams were successfully fabricated with porosi-

ties greater than 90%. The porosity of the fabricated foams was controlled by spraying time during the ESP process. As spraying time in-creased from 1 to 5 h, porosity decreased from 97 to 90%. The sintered foam possessed a continuous open-cell structure, which was de-

pendent on the structure of the PU template. The proposed method may be useful in the future as a simple means to fabricate open-cellporous materials.

Keywords: Electrical Explosion of Wire (EEW), Electrospray (ESP), Foams, Metallic and alloyed, Open-cell, Porous materials.

INTRODUCTION

Owing to their excellent physical and mechanical properties,

metallic and alloyed foams have found a variety of applicationssuch as lightweight structural materials, impacting energy absorb-ers, sound absorption, heat exchangers, diesel particulate filters

(DPF) or catalyzed soot filter (CSF), and biomedical implants [1-4]. Such foams are categorized as either closed or open-cell, de-pending on the connectivity of cells. Open-cell foams contain pores

that connect to the surface from the opposite side, whereas closed-cell foams contain pores that are isolated from the surface of thefoam. Closed-cell foams have a higher degree of strength than

open-cell foams, and are used mainly as thermal insulators or struc-tural components. Conversely, open-cell foams are necessary for alarge number of industrial applications such as filtration, separation,

heat/mass exchange, and sound/energy absorption.

To date, a number of methods for fabricating open-cell metallicfoams have been demonstrated, including powder sintering [5],

rapid prototyping [6], replication [7], and deposition techniques [8-10]. Among them, deposition techniques have been successfully

applied to fabricate highly uniform open-cell metallic foams withlow density, high porosity, high specific surface area, and goodinterconnection of cells, properties that are highly desirable for top

quality porous materials. In the deposition technique, metal atomsare deposited on the surface of polymeric template foam and re-moved by thermal decomposition, resulting in the desired metallic

foam. In addition, sputter deposition, electro-deposition, vacuumevaporation plating, evaporation, and chemical vapor deposition arecommon methods currently in use. However, these methods have

limitations, including their production requirements for high vac-uum conditions and/or toxic chemicals.

To the best of our knowledge, ESP deposition [11] has not beenpreviously applied in the fabrication of metallic foam. In the present

study, we present a combination method for synthesizing nanoparti-cles by EEW, and subsequently fabricating metallic foams by ESPdeposition of nanoparticles on polymeric foam. This method ismore economic and simple than other deposition techniques, as it

does not require the use of toxic chemicals ora vacuum system.

*Address correspondence to this author at the 18-415, School of MaterialsScience and Engineering, University of Ulsan, Ulsan 680-749, Republic ofKorea; Tel: +82-52-259-2240; Fax: + 82-52-259-1688;E-mail: [email protected]

MATERIAL AND METHODS

Commercial 72.2Fe-22Cr-5.8Al wire with a diameter o

0.2 mm was used to fabricate nanoparticles by EEW in ethanol. Adescription of the fabrication of Fe-Cr-Al nanoparticles by EEW inliquid can be found elsewhere [12-13]. Fabricated Fe-Cr-A

nanoparticles were nearly spherical, with a mean diameter o15 nm, particle size in the range of 5-100 nm, and a size distributionconsistent with a log-normal distribution with a large, long tail. Th

resulting suspensions (~15 wt.%) were used for ESP deposition tocoat PU foam. Commercial PU foam with a mean pore size of 700m and thickness of 2 mm was used as the polymer template. ESPdeposition was carried out on a 55 cm

2area of PU foam template

using a ESP system (ESR200R2, NanoNC Co., Ltd., Korea). Aslurry flow rate of 3 ml/h and a voltage of 4.5kV were used to obtain a stable cone jet mode for electrospraying. After the PU foamtemplate was coated by metallic nano-powders, it was dried for 24hat room temperature. The dried metallic preforms were then heated

at 500 oC for 1 h at a heating rate of 2 oC/min with flowing H2gato burn out the PU template. Next, heating of the residual bodycontinued up to 1400

oC under a flow of hydrogen gas (200 ml/min

at a heating rate of 5oC/min and a holding time of 2 h for sintering

followed by cooling at a rate of 5oC/min. The dimensions of sin

tered foams were measured with digital vernier calipers andweighed to four-decimal accuracy for calculating porosity and density. The structures of the fabricated foams, including cell morphology and cell wall microstructure, were observed using a scanningelectron microscope (JSM-6500F, JEOL, Japan). The pore sizes o

the obtained foams was measured in the SEM images.

RESULTSANDDISCUSSION

Fig. (1) shows the SEM images of nanoparticle-coated polymeric and sintered metallic foams. The SEM figures clearly show tha

the sintered foam possessed a homogeneous structure with openpore interconnectivity (Fig. 1c). More specifically, the cell structureof the foam mirrored the structure of the polymeric template

mainly because it was successfully decomposed during the burn-ouprocess. The average cell size of the Fe-Cr-Al foam was 450 mwhich was significantly smaller than that of the template due to th

burn-out process, as the polymer network shrunk and the gaps between coated metallic particles closed. In addition, this process mayhave also been aided by the distribution of shrinkage during the

sintering process. Specifically, during the initial stages of the sinter-ing process, nanoparticles were in loose contact with each other, bu

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A Novel Method for Fabricating Fe-Cr-Al Open-cell Metallic Current Nanoscience, 2014, Vol. 10, No. 1 8

became consolidated to form solid bonds upon heating. Thus, sin-tering allowed for reduced pore volume, resulting in compactshrinkage. Furthermore, as grain size and shape change during sin-tering, grain growth is common [14]. Thus, in our study, the nearlyspherical shape of starting powders fabricated by EEW becamefaceted after the sintering process (Figs. 1b and1d). Likewise, thegrain size of the sintered foam grew dramatically larger comparedto that of coated preforms. Although there had been a reduction insize as a result of this shrinkage, the relics of the template structure

were well maintained. Measurement of dimension of metallicpreforms and sintered foam showed that the sintered foam underwent a remarkable 34% reduction in volume.

Fig. (1).SEM micrograph of (a) Fe-Cr-Al coated preform, (b) surface of the

coating layer, (c) metallic foam sintered at 1400 oC, and (d) surface of asintered strut.

Importantly, the porosity and density of the foam used in thisstudy were easily controlled by varying the spraying time in theESP process as shown in Fig. (2). In our hands, the porosity ofmetallic foam decreased steadily as the spraying time increased,which was attributed to an increase in the thickness of the coatinglayer on the PU sponge. Specifically, the foam density increasedwith spraying time, while the foam porosity was inversely propor-tional to spraying time. During the initial spraying time, whichlasted 1 h, the degree of porosity reached as high as ~ 97%. Asexpected, variation in spraying time altered the porosity, whichdecreased to ~ 90% after a spraying time of 5h. On the contrary, thedensity of the metallic foam increased as the spraying time in-creased. Specifically, the measured densities of the foams weredependent on the strut thickness, whereby increases in strut thick-ness caused an increase in foam density and a decrease in porosity.Therefore, the strut thickness was adjusted by altering the porosityby choosing a suitable starting metallic slurry concentration andspraying time. Further, the density was adjustable from a range

between 0.24 and 0.66 g/cm,3 corresponding to a spraying time of1 to 5h. As the functional characteristics of the resulting structures

were highly variable but subject to exquisite control, they are suit-able for various applications including catalysis, sound absorption,filtration, and heat transfer.

The pore size of sintered foams can be determined primarilyfrom the size of the polymer template cells. Fig. (3) shows an SEMmicrograph of Fe-Cr-Al foam fabricated using a PU template with alarger pore-size. The pore size of the PU foam was ca. 1.90 mm,while the pore size of as-obtained alloyed foams was ca.1.30 mm.The pore size of sintered foams was similarly much smaller thanthat of PU foam due to the shrinkage phenomena; the porosity ofthe sintered foam was 97% with a density of 0.23 g/cm

3.

Fig. (2). Porosity and density of the Fe-Cr-Al foams as a function ospraying time on PU foam.

Fig. (3).SEM micrograph of Fe-Cr-Al foams sintered at 1400 oC with a PUpore size of 0.92 mm.

In this study, we reported a novel and reproducible method forpreparing metallic and alloyed foams using a combination of EEWand ESP processes. The outstanding features of this method are: (1low cost, (2) ability to synthesize metallic and alloyed nanoparticlefor fabrication of open-cell foams, (3) control of physical characteristics such as porosity at different scales, and (4) production ofoam with high porosity(> 90%) and high pore interconnectivity(~ 100%).

CONCLUSIONS

We presented a novel method combining EEW in liquid with

ESP for fabrication of open-cell metallic and alloyed foam. Theopen-cell structure of our method was easily manipulated by varying the PU template and/or adjusting the spraying process variablesthereby providing significant freedom in choosing a structure de

sign. This technology has the advantage of reproducible pore geometry, and may be applicable for numerous porous materials. Thistrategy opens up a novel and cost-effective route to fabricate me-

tallic and alloyed foam with a large internal surface area and poros-ity.

CONFLICT OF INTEREST

The authors confirm that this article content has no conflicts ofinterest.


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ACKNOWLEDGEMENTS

This work was supported by the Fundamental R&D Program

for Core Technology of Materials funded by the Ministry ofKnowledge Economy (MKE) of Korea.

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Received: May 5, 2012 Revised: March 13, 2013 Accepted: April 30, 2013

2014-a novel method for fabricating fe-cr-al open-cell metallic and alloyed foams, lh bac, bk kim,...

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