Materials science communicationNovel method of powder-based processing of copper nanofoams for their potential use in energy applications
Graphical abstract
Introduction
In order to obtain a nanoporous structure that exhibits a high density of specific surface area for energy or functional applications, a majority of earlier studies have been confined to organic or inorganic materials [1], [2], because it is generally more difficult to produce a nanoporous metallic structure owing to the relatively poor oxidation and corrosion resistance of metallic materials. Despite such disadvantages, some successful attempts have recently been made to take advantage of the great potential that nanoporous metallic structures have for use in functional applications such as substrates for heat-exchanger applications, catalysts, sensors, actuators, fuel cells, and microfluidic flow controllers [3], [4], [5], [6]; nanoporous metallic structures generally have relatively better mechanical properties and long-term operational reliability. Indeed, recent studies have focused considerable attention on processing and characterizing high-performance nanoporous noble metals through dealloying; that is, the selective dissolution during anodic corrosion in which less noble metal is dissolved out of an alloy and more noble metal remains nanoporous [7]. Moreover, since the earlier reports on nanoporous Au foams dealloyed from silver–gold alloys [8], [9], [10], there have been numerous follow-up studies on the processing, characterization, and mechanical properties of nanoporous noble metals [11], [12]. For example, Chen et al. used X-ray nanotomography and microbeam diffraction to study the structural evolution of nanoporous gold produced from Ag–Au [13]; Qian and Chen produced ultrafine nanoporous Au from Ag–Au by dealloying at lower temperatures [14]; and Ateya et al. produced nanoporous Au from Cu–Au alloy [15].
The latest reports, on the other hand, have focused on methods of producing non-noble nanoporous metallic foams. For example, nanoporous Ti foams were produced using a new dealloying method based on an attractive force among the constituent elements in a metallic melt [16], nanoporous Ni foams were produced by leaching manganese in a single-phase solid solution of face-centered cubic (fcc) Ni-γMn [17], and nanoporous Cu alloy foams were produced by dealloying in an alkaline solution to extract Al from CuAl2 or Cu–Al–Zn alloy systems, namely Raney copper [18], [19], [20].
The previously reported methods of producing nanoporous metallic structures are all based on complete melting processing, whereas this paper presents a novel powder-based processing method of producing the precursor alloys, which is simpler because a much lower processing temperature is required and a near-net shape geometry can be achieved directly without an additional machining or deposition process. This is important because if the nanoporous metallic foams are to be used as an electrode in batteries, die-sensitized solar cells, or fuel cells, they must be prepared in the form of a thin film with a thickness of tens to hundreds of microns; for example, anode material to be used in a typical coin cell should be a foil with a thickness of ∼100 μm. Therefore, use of this powder-based dealloying method makes it possible that one can avoid a difficult and costly additional material shaping process by obtaining the precursor alloy with near-net shape geometry.
In addition, this paper demonstrates that the powder-based dealloying method can be used to produce nanoscale Cu foams with the pore size on the order of a few hundreds of nanometers and can also be slightly modified to produce multiscale Cu foams containing a mixture of nanopores and micropores from Cu–Al alloys precursors.
Section snippets
Experimental
Nanoscale copper foams were synthesized using a novel powder-metallurgy process and Cu and Al powders (Cu: 99.9%, mean particle size: 1 μm, Metal Chem Tech; Al: 99.0%, mean particle size: 1 μm, Alfa Aesar, USA). A powder mixture composed of 30 at.% Cu and 70 at.% Al was blended using a simple mixing machine (8000-D Mixer Mill, SPEX CertiPrep) for approximately 10–30 min prior to compaction at room temperature. The selected composition of 30 at.% Cu and 70 at.% Al corresponds to a composition of
Results and discussion
It is generally recognized that ideal nanoporous structures can be obtained by chemically dealloying single-phase solid-solution binary alloys [18], [19]. Nevertheless, to ensure a higher volume density of porosity and specific surface area, this study used a precursor alloy at a composition of 30 at.% Cu–70 at.% Al with a slightly higher Al at.% than the single-phase alloy, θ, at 33 at.% Cu–67 at.% Al [21]. It is possible to dealloy Al out of the Al–Cu system because of the large difference in
Conclusions
On the basis of the results of this study, the following conclusions can be made:
- 1.
A simple method of powder-metallurgy processing was used to produce Cu nanofoams that exhibited strut sizes on the order of several tens to hundreds nanometers. A uniform nanoporous structure was observed throughout the thickness of the specimen. Compared to the struts formed in the Cu nanofoam sintered at 700 °C, the coarser struts formed in the Cu nanofoam sintered at 900 °C lose a certain portion of their
Acknowledgments
This research was supported by the Pioneer Research Center Program through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science, and Technology (2011-0001684). HC also acknowledges support from the Basic Science Research Program (2010-0005775) and the Priority Research Centers Program (2009-0093814; 2010-0029106) through the National Research Foundation (NRF) of Korea funded by the Ministry of Education, Science, and Technology. YS also acknowledges
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