Ag composite foams: Synthesis, morphology and compressive property

Scripta Materialia 117 (2016) 68–72

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Ultralight Co/Ag composite foams: Synthesis, morphology and compressive property Bin Jiang a,b,c, Xudong Yang d, Weifei Niu b, Chunnian He a, Chunsheng Shi a, Naiqin Zhao a,c,⁎ a

School of Materials Science and Engineering, Tianjin Key Laboratory of Composites and Functional Materials, Tianjin University, Tianjin 300072, China Tianjin Special Equipment Inspection Institute, Tianjin 300192, China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China d Sino-European Institute of Aviation Engineering, Civil Aviation University of China, Tianjin 300300, China b c

a r t i c l e

i n f o

Article history: Received 14 January 2016 Received in revised form 22 February 2016 Accepted 22 February 2016 Available online 10 March 2016 Keywords: Electroless plating Composites Foams Compression test Ultralight

a b s t r a c t Ultralight monolithic Co/Ag composite foams were synthesized by a novel and facile method based on the traditional electroless plating. The resultant foams had remarkably low densities down to 12.7 mg/cm3 or 99.9% porosity. Morphology of the Co/Ag composite foams was studied and compressive properties of the foam were investigated in light of its relative density. The densification strain of the foam can reach 87% because of the ultralow density. The compressive yield strength was measured and found to be in agreement with the value predicted by the cellular solids theory. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Metal foams are a new class of materials with low densities and novel physical, mechanical, thermal, electrical and acoustic properties. They offer potential for lightweight structures, for energy absorption, and for energy conversion [1–2]. The current understanding of their production, properties and uses was assembled in several publications [3–4]. The achievable porosity of the metal foams was usually below 97.5% [4]. Techniques for making ultralight metal foams with porosity above 99% to date have been somewhat limited in scope yielding relatively high density compared to foam monoliths of other materials such as aerogel. Approaches to the synthesis of ultralight non-metallic foams are not well-suited for metals [5–9]. The ability to form ultralight metal foams is difficult and sometimes elusive with the use of conventional methodology. Electroless plating on polymer templates have been applied to produce porous structures [10]. The porosity characteristics can be tuned by selecting the appropriate copolymer. The coating should be as thin as possible in order to synthesize the metal foams with ultralow density. However, the films are too weak to stand as a monolithic structure on their own in isolation from the template if the coating is too thin [11]. Therefore, it is difficult to synthesize the

ultralight metal foams with the density below 20 mg/cm3 or the porosity above 99% by conventional electroless plating process. Ultralow-density (b 20 mg/cm3) metal foams have only been reported by T.A. Schaedler [12]. An ultralight nickel phosphorus alloy based on periodic hollowtube microlattices was fabricated by starting with a template formed by self-propagating photopolymer waveguide prototyping, coating the template by electroless nickel plating, and subsequently etching away the template. The resulting metallic microlattices exhibit ultralow density (0.9 mg/cm3). In this paper, a novel and facile method based on the electroless plating to obtain ultralight monolithic Co/Ag composite foams was reported. The resultant Co/Ag composite foams had remarkably low densities down to 12.7 mg/cm3 or 99.9% porosity. The compressive properties of the foams were investigated in light of its relative density. 2. Material and methods A schematic illustration of the production is shown in Fig. 1. The specimen preparing process consists of the electroless silver plating, electroless cobalt plating and removal of the template stages. 2.1. Electroless silver plating method

⁎ Corresponding author at: School of Materials Science and Engineering, Tianjin Key Laboratory of Composites and Functional Materials, Tianjin University, Tianjin 300072, China. E-mail address: [email protected] (N. Zhao).

http://dx.doi.org/10.1016/j.scriptamat.2016.02.024 1359-6462/© 2016 Elsevier Ltd. All rights reserved.

The template used in this study is a polymer foam which is used as a household cleaning eraser [13]. Polymer foam with dimensions of 8 mm × 8 mm × 30 mm was cut, and no extra pretreatments for the

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Fig. 1. Schematic illustration of the production of the ultralight Co/Ag composite foams.

foam were performed prior to use. The electroless silver plating bath contained silver nitrate, ammonia and glucose. First, an ammonia solution (2 wt.%) was added dropwise to 10 ml silver nitrate solution (2 wt.%) until the precipitate completely dissolved to form [Ag(NH3)2]+. Then, 5 ml of glucose solution (10 wt.%) was added. The polymer foam sample was immediately dipped into the electroless silver plating bath for 10 min at 50 °C. Subsequently, the sample was rinsed in distilled water and completely dried in a drier at 120 °C for 30 min.

2.3. Synthesis of ultralight Co/Ag composite foams The sample acquired after electroless cobalt plating was heated to 700 °C in an air atmosphere in a muffle furnace to burn away the polymer template, and the ultralight monolithic Co/Ag composite foams were synthesized. Microstructural characterizations of the foams were performed by using the field-emission scanning electron microscopes (Hitachi S-8010 and Zeiss Gemini 500). The compression tests were performed on a servo-electric INSTRON 5848.

2.2. Electroless cobalt plating method 3. Results and discussion The polymer foam acquired after electroless silver plating was coated by electroless cobalt plating. The samples were immersed in electroless cobalt plating solution with cobalt sulfate (14 g/l) as the cobalt source, sodium hypophosphite (21 g/l) as the reducing agent, and sodium citrate (60 g/l) and boracic acid (30 g/l) as complexing agents. The electroless cobalt plating bath was kept at pH 8.0 by addition of sodium hydroxide and plating was performed at 90 °C. Subsequently, the sample was rinsed in distilled water and completely dried in a drier at 120 °C for 30 min.

3.1. Structures of polymer foam and electroless Co/Ag coating Fig. 2a displays a scanning electron microscopy (SEM) image of the polymer foam. The polymer foam is a flexible, open-cell foam made from melamine resin, a thermoset polymer. The foam's characteristic feature is its three-dimensional network structure consisting of slender filaments. The traditional method of electroless plating on dielectric templates such as polymer involves two, often three, stages [14]. The

Fig. 2. (a) SEM image of the polymer foam. (b) The polymer foam with Ag coating in three-dimension. (c) SEM image of electroless Ag coating. (d) SEM image of electroless Co coating.

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polymer templates must be catalytically activated prior to the electroless plating to provide a surface that can interact with metal ions in solution causing their reduction on the surface and growth of the coating. SnCl2 and PdCl2 were most commonly used as activators for this purpose [15–17]. However, tin ions may introduce unexpected impurities and PdCl2 is expensive and toxicant. In this study, a traditional and practical method, electroless silver plating, was successfully used to fabricate continuous Ag coating in three-dimension on the polymer template (Fig. 2b). Fig. 2c shows that the Ag coating which is composed of silver particles with the sizes of 200–400 nm was synthesized. The polymer foam with the continuous Ag coating as the catalytic activator prior to the electroless plating was coated by electroless cobalt plating. SEM image of electroless Co coating is shown in Fig. 2d and the Co coating is composed of cobalt nanoparticles with the sizes of 20– 30 nm. 3.2. Structures of ultralight Co/Ag composite foams A piece of ultralight Co/Ag composite foam with the porosity of 99.9% (ρ = 12.7 mg/cm3) stands on a dandelion (Fig. 3a). A lowmagnification SEM image of the foam is shown in Fig. 3(b–c). The foam has a three-dimensional network structure consisting of uniform slender filaments which is similar with the original polymer foam template (Fig. 2a). Fig. 3d shows that the filament is tubular in structure and the cross section of the filament of the Co/Ag foam is triangular,

analogous to the filament of the polymer template. A double layer structure was clearly observed on the corresponding backscattered electron (BSE) images (Supplementary Fig. S1). The structure of the film of the Co/Ag foam is shown in Fig. 3e. The film is a double layer structure (Co/Ag) and the Co layer is only 146 nm in thickness (Supplementary Fig. S2). The thickness of the Co layer can easily be controlled by changing the time of the electroless cobalt plating (Supplementary Fig. S3) and the Co/Ag foams with different densities can be produced. The EDS analysis of the film which is shown in Fig. 3e shows that Ag, Co, P and O are present (Fig. 3f). The distribution of Ag, Co, P and O elements is shown in Supplementary Fig. S4 respectively. In the double layer structure, the Ag coating was produced by the electroless silver plating and the Co films should remain as a supersaturated solid solution of phosphorous in a crystalline face-centered cubic cobalt lattice after deposition. Because the foam was heated at 700 °C in air in order to remove the polymer template and the Co film is only about 100 nm thick, the cobalt was oxidized. 3.3. Compressive properties of ultralight Co/Ag composite foams Uniaxial compressive tests were performed on the ultralight Co/Ag composite foams at a strain rate of 1 mm/min. The compressive stress–strain curves for the foams exhibited characteristics typical of open cell metal foams [18–20], including yielding (i.e. plastic deformation) followed by a plateau region and then densification (Fig. 4a).

Fig. 3. (a) A piece of ultralight Co/Ag composite foam with the porosity of 99.9% stands on a dandelion. (b–c) Low-magnification SEM image of the Co/Ag composite foam. (d) SEM image of a filament of the Co/Ag foam. (e) The microstructure of the film of the ultralight Co/Ag composite foam. (f) Energy dispersive spectrometer (EDS) analysis of the film of the Co/Ag foam.

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Fig. 4. (a) A representative compressive stress–strain curve for the ultralight Co/Ag composite foams (foam relative density 0.15%). (b) Comparison of the relative yield strength as a function of relative density of the Co/Ag composite foams.

J. Lian et al. [21] showed that hollow nanocrystalline Ni cylinders differing only in wall thicknesses, 500 and 150 nm, exhibit strikingly different collapse modes: the 500 nm sample collapses in a brittle manner, via a single strain burst, while the 150 nm sample shows a gradual collapse, via a series of small and discrete strain bursts. The ultralight nickel phosphorus alloy with wall thickness below 150 nm can complete recovery after compression exceeding 50% strain [12]. These Ni films remain as a supersaturated solid solution of phosphorous in a crystalline facecentered cubic nickel lattice after deposition. Compared with the Ni film, the Co film which was produced by electroless cobalt plating is also composed of solid Co–P solutions [22]. The filament of Co/Ag foam is also hollow in structure and the Co layer is only 146 nm in thickness. A similar stress–strain behavior of the ultralight nickel phosphorus alloy was expected. However, the present foams showed a similar stress–strain behavior in contrast to other metallic foams, such as aluminum foams. Because the foam was heated at 700 °C in air and the Co film is only about 100 nm thick, the cobalt was oxidized. On the other hand, the cell wall of the foams in our study contains a lot of pure silver. Therefore, the samples collapsed in a brittle manner, via a single strain burst. The relatively brittle nature of the cobalt coating results in the collapse plateau being a serrated curve and the Co/Ag foam sample became powders after the compression test. The Co/Ag composite foams exhibited a longer, flatter plateau because the structure affords more opportunity for cell walls to collapse and deform. Ideal energy absorbers have a long flat stress–train curve. The absorber collapses plastically at a constant nominal stress, called the plateau stress, σpl, up to a limiting nominal strain, εD. Energy absorbers for packaging and protection are chosen so that the plateau stress is just below that which will cause damage to the packaged object. The best choice is then the one which has the longest plateau, and therefore absorbs the most energy before reaching εD. The area under the curve, roughly σplεD, measures the energy that the foam can absorb, up to the end of the plateau. Foams which have a long flat stress–strain curve perform well in this function. Hollow tubes, shells, and metal honeycombs have the appropriate type of stress–strain curves [23]. The Co/Ag foams have hollow tubes, so the densification strain εD of the Co/Ag foam with the porosity of 99.8% (density 15.5 mg/cm3) can reach 87%. The energy per unit volume absorbed by the foam up to densification is 1.39 mJ/cm3. With the similar relative density, the Ni–P microlattices [12] with density of 14 mg/cm3 have a higher energy absorption (4.6 mJ/cm3) than the foams of our study. The base architecture of Ni–P microlattices consists of a periodic array of hollow tubes that connect at nodes, forming a hierarchical cellular architecture at three distinct length scales. Therefore, the Ni–P microlattices exhibit a higher plateau stress and energy absorption. On the other hand, the

cell wall of the foams in our study contains a lot of pure silver resulting in the yield stress of the cell wall being dramatically lower. The normalized compressive yield strength (defined as the yield strength of the foam, σ, divided by the yield strength of the parent solid, σys) was measured from the stress–strain curves of samples with a range of densities (Fig. 4b). Theoretical relationships for the normalized yield strength of an open cell foam have been reported by Gibson and Ashby  3=2 σ ρ ≈ 0:3 σ y;s ρs

ð1Þ

where ρ is the density of the foam, ρs is the density of the parent solid [24]. Nominal yield strength for the fully dense pure silver is: σy.s = 35 MPa [25]. The σy.s of the electroless cobalt thin film is 150– 200 MPa [25]. The σy.s of the cell wall material (Co/Ag thin films) is chosen to be 80 MPa. According to Eq. (1), the theoretical compressive yield strength with different relative densities is shown in Fig. 4b (the dotted line). There was agreement of the experimental data with the yield strength. 4. Conclusions The ultralight Co/Ag composite foams with porosity above 99% were synthesized based on the traditional electroless plating. The Co/Ag foams had a three-dimensional network structure consisting of uniform slender filaments. The structure of the film of the Co/Ag foam is a double layer structure. The resultant ultralight monolithic Co/Ag foams have remarkably low densities down to 12.7 mg/cm3 or 99.9% porosity. The densities of the foams can be easily controlled by changing the time of electroless plating. Compressive properties of the foams were investigated in light of its relative density. Contrast to other metal foams, the present ultralight Co/Ag foams showed a similar stress–strain behavior. The metal foams have a long flat stress–train curve in the compression test and the densification strain of the Co/Ag foam with a porosity of 99.8% can reach 87%. The compressive yield strength was measured and found to be in agreement with the value predicted by the cellular solids theory. Acknowledgments This work was supported by the State Key Program of National Natural Science of China (Grant No. 51531004), China–EU Science and Technology Cooperation Project SQ2013ZOA100006, and the National Natural Science Foundation of China (NSFC, No. 51301198),

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