Energy Harvesting for Soft-Matter Machines and Electronics

A new class of soft multifunctional materials could be used to convert mechanical deformation from vibrations and stretching into electrical energy.

February 1, 2020

Air Force Research Laboratory, Arlington, Virginia

Air Force (AF) materials capable of dramatic changes in shape and rigidity require soft-matter electronics that support functionality without interfering with the mechanics of the host structure. This program introduced a new class of soft, multifunctional materials that can be used to power these systems by converting elastic strain energy from large deformations into electricity. These materials are composed of soft elastomers embedded with a suspension of liquid metal (LM) droplets that control the electrical properties of the composite.

Optical images of (a) EGaIn-PU (φ = 0.5) and (b) EGaIn-PDMS (φ = 0.2) composites; insets show results of image processing to identify LM inclusions and estimate dimensions of an ellipsoidal fitting. (c) 3D X-Ray Nano-CT image of a Galinstan-PDMS composite.

Depending on their composition and microstructure, these LM-embedded elastomers (LMEEs) can be tailored to exhibit exceptionally high electric conductivity, electric permittivity, and/or thermal conductivity. LMEEs with high permittivity can function as high-k dielectrics for storing and harvesting electrostatic energy. When integrated with an elastically deformable AF structure, they have the potential to generate electricity as the host structure stretches, twists, or bends under external loading. This external loading may arise from air drag, wind, ambient vibrations, collisions, etc. and represents mechanical work that would be otherwise dissipated through damping.

As stated, the electric properties of soft elastomers can be tailored by adding a suspension of liquid metal droplets. Depending on their composition, these LM-embedded elastomers can exhibit either high electric conductivity (σ ~ 10⁴ S/m) or permittivity (ε_r ~ 10-50). Such materials can be used as electrodes and dielectrics, respectively, in a soft-matter capacitive generator that converts mechanical work into electrostatic energy through changes in capacitance and electrical enthalpy. Because the inclusions are liquid phase, these LM-embedded elastomer composites exhibit the same mechanical properties of unfilled rubber – low elastic modulus (0.1-1 MPa), high strain limit (up to 600%), and low mechanical hysteresis. Such properties are required in order for the generator to support large elastic deformations and maximize electrical enthalpy change.

The dielectric composites are composed of either polydimethylsiloxane (PDMS) or polyurethane (PU) embedded with a non-percolating suspension of LM microdroplets. Gallium-based alloys such as Ga-In-Sn and Ga-In eutectic (EGaIn) are used as the liquid metal. Referring to the optical and Nano-CT images in the accompanying figure, the LM suspension is polydisperse and has a random but statistically uniform spatial distribution. Despite the high-volume fraction φ of LM, the droplets do not form a percolating network and instead function as an “artificial dielectric” that significantly increases the effective electric permittivity (ε_r) of the composite. In the case of φ = 0.5, ε_r is 4x greater than the permittivity of the unfilled elastomer (ε_m).

This work was done by Carmel Majidi of Carnegie Mellon University for the Air Force Research Laboratory. AFRL-0286