Electroplating of Aluminum with Nickel Particle Codeposition
Authors: Coleman Jonathan Joseph , Apblett Christopher Alan , Atanassov Plamen , Yelton W. Graham 
Published: February 2020

Aluminum-nickel nanostructured thin films have many uses. The exothermic mixing of the phases may be implemented for localized heat generation wherever restricted welding, brazing or local energy release may be required. Current fabrication methods generally rely on vapor deposition processes that require high vacuum and are limited to low rates. New fabrication methods such as electroplating these films can result in significant improvement of energetic density and cost savings. To pursue this, we investigated aluminum deposition to determine the characteristics and mechanism of the complex reduction process, nickel particle incorporation into an aluminum matrix, and nickel deposition properties in a multicomponent bath.

Because aluminum has a lower atomic density than nickel and the desired mixing ratio is 1:1, the aluminum volume must be 60% of the total. The codeposition process is limited in its capability for particle incorporation by particle diffusion and geometric hindrances and therefore the particulate species is almost always a significantly lower volume percentage than the matrix. For this reason the aluminum is the matrix material that is deposited out of the electrolyte and the nickel component is incorporated as imbedded particles. Aluminum deposition must be carried out in aprotic solvents because the reduction potential is lower than that of hydrogen, making aluminum deposits using aqueous solutions inefficient and low quality. Electrolytes were characterized based on transport properties and deposition kinetics.

Some of the most promising aluminum deposition is done in Chloroaluminate RTILs (room temperature ionic liquids) 2. These solutions are high aluminum concentration and low vapor pressure but typically high viscosity. Additions of toluene have improved the dendritic morphology and at 30-50 volume% toluene, the bath produces a smooth uniform coating of aluminum 1. The dilutions reduce the viscosity, and participate in complexing with the reactants, modifying the reduction and transport properties.

These electrolytes are investigated with RDE and EIS to determine the reaction properties of aluminum deposition. The reported reduction of aluminum metal occurs from the complex Al2Cl7-. This bulky anion has very poor diffusion properties, there are kinetic limitations due to the complex multi-step reduction process and our experiments aim to analyze the reaction mechanism to optimize the electrolyte composition, speciation and applied deposition current and potential.

EIS conductivity measurements show that pure TMPAC:AlCl3 ionic liquids have low conductivity and the conductivity is greatly increased with dilution with toluene or dichlorobenzene, although a maximum is reached as the improved transport starts to be overcome by the reduced ionic concentrations. This maximum conductivity is reached at about 50-60 molar% of DCB to IL.

Although these aluminum deposition electrolytes have been used before, little fundamental understanding of mechanism has been undertaken; a better understanding of the reaction properties will allow for design of a more effective codeposition process. Codeposition with particulates in aluminum enables production of heterogeneous films with improved mechanical properties; and cost effective bilayer or alloy deposition will increase aluminum’s already large applicability. Bi-phasic films may also be produced by alternatively depositing Al and Ni layer, and Nickel properties in the electrolyte were studied to develop this fabrication method also.

Comparisons are made between the electrochemically produced film and the commercial PVD film. These are characterized by energy release, burn rate. The high interfacial area of these films is subject to atomic diffusion which can reduce the burn characteristics. TEM analysis of the interfacial layers seems to indicate that the lower temperature process associated with codeposition can reduce this pre-reaction mixing. Accelerated aging is being performed on film from both fabrication methods and increased temperature heavily influences interatomic mixing.