Electrodeposition of Gadolinium Metal from Organic Solvents
Authors: Gucik Mary Louise , Pillars Jamin R. , Strange Lyndi , Arrington Christian L. , Coleman Jonathan Joseph , Jau Yuan-Yu 
Published: December 2020

Neutron phase contrast imaging (NPCI) is a non-destructive analysis tool currently in use with thermal neutrons. It relies on neutron transmission gratings to detect the phase signal. These gratings are typically fabricated from gadolinium (Gd), as this element has the highest neutron absorption cross section at 250,000 barns. There is a wealth of literature documenting the fabrication process of these gratings, including depositing Gd via sputter coating, evaporation and particle settling techniques at dimensions appropriate to absorb thermal neutrons. However tighter packing density, less contamination and thicker Gd films are necessary for neutrons at higher energy levels ( >0.025 eV). With these requirements in mind, we investigated the electrodeposition of Gd on Si substrate gratings for use with higher energy neutrons.

Electrodeposition of Gd alloys is well documented in literature. With a very cathodic reduction potential of -2.28 V (acidic solution), depositing pure Gd metal is a tricky endeavor. In addition to excessive hydrogen evolution during plating, Gd undergoes rapid oxidation when exposed to air. This phenomenon is exacerbated with thin films due to their high surface area and can lead to pyrophoric events and brittle films that sluff off the substrate over time. We investigated aqueous solutions and with the use of many oxygen scavengers, we successfully plated gadolinium oxide (Gd2O3) thin films. However, this technique was not viable due to poor adhesion and rapid oxidation. Hoping for less oxygen contamination, we moved to two polar, aprotic organic solvents with promising potential windows: dimethylformamide (DMF) and dimethylsulfoxide (DMSO). The solubility of gadolinium(III) salts was investigated in the two solvents. At elevated temperatures, gadolinium(III) fluoride and gadolinium(III) chloride were soluble compounds, as well as gadolinium toluenesulfonic acid (GdTsOH). To increase conductivity, potassium chloride (KCl) and tetrabutylammonium tetrafluoroborate (TBATFB) were used as conducting salts. Pulsed plating techniques led to repeatable thin film deposition and EDS confirmed the presence of gadolinium uniformly spread over the surface. However, these films also suffered from poor adhesion, likely due to water contamination in the bath.

The limitations outlined above led to electroplating Gd in dimethyl sulfone (DMSO2). DMSO2 is used in industry as a high-temperature solvent; its large dielectric constant reflects its high polarity, indicating strong intermolecular interactions in liquid phase. A melting temperature of 109 °C eliminates the concern of water contamination in the bath. Solubility tests are being repeated in this new polar solvent and linear sweeps and EIS continue to indicate favorable plating conditions. Parameters, such as Gd metal loading and substrate pretreatments are being optimized for thick films with good adhesion.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
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