Growth Mechanisms of Low Density Metallic Foams Synthetized By Plasma Electrolysis Deposition
Authors: Pinot Julien , Durut Frédéric , Botrel Ronan , Chicanne Cédric , Theobald Marc , Vignal Vincent 
Published: May 2020

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 envelop 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 and Scanning Electron Microscopy (SEM), 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 electrical discharges reported in the literature on plasma discharges in dielectric liquids [3,4]. The first one observed 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. And the third one 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]. Furthermore, we observe that the potential predominance range of each growing mode can be shifted by modifying the concentration or the nature of the metallic ions. Our investigations show that the mass transportation plays a major role with the applied voltage in defining the growth mode of the metallic foams. Electron BackScatterred Diffraction (EBSD) and X-ray Diffraction (XRD) analyses have been conducted to study the crystalline structure of each mode and thus providing information on the conditions of electro-crystallization of the strands. This work concludes by proposing an electrochemical model of the foam growth.

References:

[1] R.Botrel, US patent n° US 10,400,345 B2 (2019)
[2] S.Rocher et al., Eur. Phys. J. Appl. Phys. 81, 10803 (2018
[3] R.E.Hebner, Series B: Physics. 193 (1987)
[4] O. Lesaint, J. Phys. D: Appl. Phys., 49, 144001 (2016)
[5] V.Fleury et al., Phys. Rev. A, 43, 12 (1991)

Figure 1