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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.

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 (εr ~ 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 (εr) of the composite. In the case of φ = 0.5, εr is 4x greater than the permittivity of the unfilled elastomer (εm).

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

Topics:
Aerospace Composites Elastomers Electric power Electronic equipment Energy Energy Harvesting Imaging Materials Materials properties Metals Metals Nanomaterials Nanotechnology Off-board energy sources Optical Components Optics Polymers Smart materials
This Brief includes a Technical Support Package (TSP).
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Energy Harvesting for Soft-Matter Machines and Electronics

(reference AFRL-0286) is currently available for download from the TSP library.

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    Aerospace & Defense Technology Magazine

    This article first appeared in the February, 2020 issue of Aerospace & Defense Technology Magazine (Vol. 5 No. 1).

    Read more articles from this issue here.

    Read more articles from the archives here.

    Overview

    The document titled "Energy Harvesting for Soft-Matter Machines and Electronics," authored by Carmel Majidi from Carnegie Mellon University, presents a final performance report on a research program funded by the Air Force Office of Scientific Research (AFOSR). The report covers the period from March 15, 2013, to March 14, 2016, and focuses on the development of innovative soft multifunctional materials capable of significant changes in shape and rigidity, which are essential for soft-matter machines and electronics.

    The core of the research involves the creation of liquid metal-embedded elastomers (LMEEs). These materials are designed to convert elastic strain energy from mechanical deformations—such as stretching, twisting, or bending—into electrical energy. This conversion is particularly valuable as it allows for energy harvesting from various external mechanical loads, including air drag, wind, ambient vibrations, and collisions, which would otherwise be lost as heat through damping.

    LMEEs are composed of soft elastomers that incorporate a suspension of liquid metal droplets. The presence of these droplets enhances the electrical properties of the composite materials, enabling them to exhibit high electric conductivity, electric permittivity, and thermal conductivity. Specifically, LMEEs with high permittivity can serve as high-k dielectrics, which are crucial for storing and harvesting electrostatic energy.

    The report highlights the potential applications of these materials in creating soft-matter electronics that can function effectively without interfering with the mechanical properties of the host structures. This capability is particularly relevant for the development of advanced systems that require both flexibility and functionality, such as soft robots, wearable electronics, and other adaptive technologies.

    In addition to the technical advancements, the report includes references to several archival publications that emerged during the research period, showcasing the broader impact of the findings on the field of materials science. The document concludes with a distribution statement indicating that the report is approved for public release, emphasizing the significance of the research in contributing to the understanding and development of energy harvesting technologies in soft-matter systems.

    Overall, this report encapsulates a significant step forward in the integration of energy harvesting capabilities into soft materials, paving the way for innovative applications in various technological domains.

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