(729d) Liquid Metal Composites As Dielectric Materials for Extreme Conditions

Truly soft and stretchable electronic materials are of potentially enormous value as they present the opportunity to maintain electrical performance (conductivity, inductance, capacitance, etc.) while deforming or conforming to curved or irregular surfaces. Achieving these two behaviors, however, has proven to be a challenge as materials that are soft rarely have electrical behavior that can rival traditional rigid materials and traditional rigid materials are brittle, non-conformable, and oftentimes heavy. To access the best of both of these worlds, composites of soft polymers and conductive materials have been developed. Among these materials, composites of gallium-based room temperature eutectic liquid metals have proven to have exceptional performance in achieving metallic conductivity or ceramic-like permittivity while maintaining a low modulus. Previous work by the PI has shown that composites of galinstan (gallium-indium-tin) in polydimethylsiloxane can achieve permittivity values up to 170 without a significant increase in material modulus. This makes the LM/PDMS composite valuable for a wide range of applications including position and motion sensors (pressure/tension), high impact capacitors, and dielectric actuators/generators.

Previous work has focused on achieving high performance at standard bench-top conditions. While these experiments are a good start with respect to proving the potential of LM/PDMS composites, they do not describe the full extent of conditions the composites are expected to be exposed to. For example, dielectric actuators and dielectric generators require materials with not only high permittivity but high dielectric breakdown as high voltages are typically necessary to achieve actuation. While PDMS, as well as other neat polymers, may have relatively high dielectric breakdown strength the methods that are typically used to improve conductivity or permittivity significantly degrade that breakdown strength. Capacitors, which are commonly the first point of breakdown in onboard electronics in rugged conditions, need to maintain their electrical performance at high temperatures and under mechanical impact. Traditional capacitor dielectric materials are either brittle (e.g., tantalum) or not thermally stable at high enough temperatures (e.g., thermoplastics).

In this work, we look at LM/PDMS composites, as well as model PDMS composite systems, under these extreme conditions, specifically high temperatures and high voltages, to understand what impact liquid metal has on the composite dielectric breakdown strength and permittivity. With the unique, metallic filler, it was expected that galinstan/PDMS composites would have some blend of previously described rigid filler and polymeric electrical behavior. It was found that dielectric breakdown strength was a function of galinstan concentration beyond a threshold concentration, which was an improvement upon the model conductive, rigid fillers. Multi-material composites were also studied to determine whether mixed conductive/capacitive rigid/deformable fillers could improve the dielectric breakdown strength of galinstan/PDMS composites without dramatically increasing the composite modulus. It was found that mixed composites of galinstan and conductive fillers showed an improvement in breakdown strength as compared to the single material composite of either while capacitive filler blends had a more complicated relationship. With respect to temperature, PDMS/galinstan composites were able to successfully maintain thermal, mechanical, and electrical integrity above 150^oC (a temperature that most thermoplastic dielectrics cannot achieve), making them attractive to address defense and commercial shipping and infrastructure challenges.

The work presented demonstrates a significant jump forward in the real-world application of liquid metal/PDMS composites for use in sensors, actuators, and electrical systems. While soft and stretchable electronics are an exciting and dynamic area of research, work like this allows us to fully understand the capabilities of such materials to ensure reliability, durability, and high performance.