Improving Dispersion Plating of Nickel in Chloroaluminate Ionic Liquids

The alloying reaction of Nickel and Aluminum is exothermic and the heat output from this process can be utilized for many different applications from local brazing and welding to melting and heat generation as an energy source. One of the challenges associated with this reaction is that the components are in the solid state, thus mixing and reaction rates suffer from diffusion limitations. Structuring the separate elemental phases on the nanoscale is used to minimize diffusion distances and improve reactant mixing, increasing the overall rate. One method of fabricating these films with control over the nickel and aluminum phase structure is the electrochemical codeposition process. This technique allows for nickel particles on the nanoscale to be incorporated into an aluminum matrix, with control over the composition and size.  This method has manufacturing cost, speed, and safety advantages over traditional thin film vacuum techniques, if some of the challenges to dispersion plating can be overcome.

In the present work, the electrodeposition of Aluminum in 1-butyl 2-methylimidazolium based Ionic liquids was investigated as a means of incorporating disperse Nickel nanoparticles into an aluminum matrix material. While aluminum electrodeposition has been investigated thoroughly, modifications from normal parameters are required to compensate for the codeposition process. Chloroaluminate ionic liquids (ILs) are used for their desirable properties including room temperature operation, high purity and high efficiency aluminum deposition. 1-butyl 2-methylimidazolium cations were chosen specifically for their electrochemical stability. ILs are inherently high viscosity which affects the particulate mobility, the addition of co-solvents to the IL will significantly decrease the viscosity and allow for control of particle motion. This along with control of the convection of the electrolyte will allow for modification and optimization of transport of particles to the electrode surface.

Energetic output is influenced by several different properties, the most important being composition. The production of maximum heat is realized when the stoichiometry of the nickel and aluminum are 1 to 1 as this allows the chemical reaction, the production of AlNi, to go to completion without excess of either constituents. The stoichiometry of Ni to Al is optimized to approach the correct ratio by controlling the transport of the particles to the surface as well as the growth rate of the aluminum matrix material. These parameters cannot be decoupled as the matrix material is what secures the particles to the electrode, and therefore they must be optimized together. The aluminum deposition has been investigated in the absence of particles to determine how the surfactants and co solvent dilutions affect the matrix growth characteristics.

Migration and agglomeration of the particles in the electrolyte are very important as they determine the dispersion of the Ni phase in the deposited film. These properties are controlled by the surface chemistry of the particles in solution and by the particle-particle interaction in the solution. Pure nickel particles dispersed in the IL and co-solvent show loose agglomeration in the deposited film. Uniform distribution at the nanoscale is needed to ensure consistent burn properties once the reactions are triggered, so surface modification is used to decrease attraction between particles. These modifications include oxide layers as well as surfactants added to the electrolyte and results are shown in this work. Initial attempts of the high quantity incorporation show minor agglomeration of the as received nickel particles as can be seen in the back scattered SEM of a cross section of the deposited film, the lighter grey contrast is the nickel phase (figure 1).

Deposits were fabricated and analyzed with various techniques including electrochemical quartz crystal microbalance (EQCM) for particle incorporation and viscosity measurements, zetasizer for particle agglomeration and mobility, rotating disk electrode (RDE) for transport properties, scanning electron microscopy (SEM) for dispersion imaging, Energy-dispersive X-ray spectroscopy (EDS) for elemental analysis, and differential scanning calorimetry (DSC) for exothermic reaction properties.

^{Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.}