Synthesis of Nanostructured Metallic Foams By Plasma Electrolysis Deposition

Abstract

Within the framework of fundamental physics studies at CEA, laser target experiments are conducted to investigate laser-matter interactions. Some of these experiments require metallic foams samples in a milimetric scale with an apparent volumetric mass within few hundreds milligrams per cube centimeters. Metallic foams materials are developed and studied since few years for their unique properties offered by their aerated structure. For example, due to a high specific surface, metallic foams could find applications in batteries, supercapacitors, as well as chemical sensors. However, all the metallic foams obtained by the processes referenced in the literature do not meet the specifications needed for laser experiments. This is why, the CEA has developed a new and innovative technique to synthetize metallic foams by plasma electrolysis deposition [1].This process consists in applying a voltage within the 20-100V range at a cathode immerged in a solution containing metallic ions. Under these specific conditions, a uniform gaseous sheath is created around the cathode which is then isolated from the liquid. Due to the high electrical field applied at the electrode surface, several electrical plasma discharges are formed between the cathode and the gas/liquid interface. When the electrical discharges meet the solution, metallic cations are reduced to form metallic strands about 100 nm in diameter. The formed strands are all interconnected together to form metallic foam with micrometric porosities [2]. A few millimeters wide foam with a volumetric mass up to 100mg/cc is obtained in about 10 seconds which is very fast compared to classical electrochemical deposition techniques. The foams synthetized can be made up of pure metals (Cu, Au, Pt ...) or alloys (AuCu ...) in accordance to the metallic ions diluted in the solution.In order to control the shape and homogeneity of the synthetized foams, a better understanding of the mechanisms involved in the foams’ growth is required. By direct camera observation, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), we can show that the growth behavior and the structure of the foams vary with the applied voltage. We observe three main growth patterns which seem to correspond to the different propagation modes of streamer type electrical discharges reported in the literature on plasma discharges in dielectric liquids [3,4]. The first one obtained at “low” voltage is composed of very thin strands (Ø < 100 nm) growing linearly with few ramifications. The second one is formed by highly branched strands about 100 to 600 nm in diameter. TEM observations of first and second mode strands show that their structure is monocrystalline and field oriented. A third mode obtained at the highest voltages is made up of strands above 1 µm thick with a shape very similar to the aggregates obtained in diffusion limited conditions [5]. Voltammetry studies conducted on the electrolytes show that the reduction of metallic species are indeed diffusion limited. Moreover a lower limit current corresponds to a more predominant germination phenomenon on strands. The mass transportation plays a major role alongside the applied voltage in defining the growth mode of the metallic foams and the microstructure of the strands. The structure of the metallic strands and it’s evolution with the synthesis parameters seems in accordance to the electrodeposition theory [6]. However if our results indicate that the reduction and crystallization of the strands may result from an electrochemical process, the field oriented crystalline structure obtained is unlikely to be possible inside an aqueous electrolyte with the speed of growth observed. We conclude this work by proposing an original mechanism for the growth of the foam involving electrochemistry within a plasma medium.