Cameron Massey

Reversible work by electrochemical intercalation of graphitic materials

Graphite intercalation compounds are a class of materials systems formed as ions diffuse into a host graphite structure. The volume expansion associated with this process has been shown to be capable of performing work up to 3.8 MJ/m3. To evaluate GICs for solid state actuation, this study explores some factors affecting the rate at which the volume expansion occurs. Given that diffusion length has an exponential effect on rate, we tested a graphite paper comprised of 7-micron diameter PAN fibers. We found that the paper had ultimate strain and loading properties comparable to HOPG. The paper was cycled under various loads and temperatures to examine the strain rate and repeatability of the material. Testing showed a strong correlation between rate and temperature, while pressure had relatively little effect.

Graphite Intercalation Compounds as Actuation Materials

The intrinsic electrochemical behavior of Graphite Intercalated Compounds (GICs) during formation offers the potential for high-force, high-strain solid-state actuation applications. To explore this behavior we submitted a “model” system, highly-oriented pyrolytic graphite (HOPG)/sulfuric acid (H2 SO4 ), to axial compressive loads from 0 to 8 MPa, and measured the intercalation response in terms of voltage and displacement. We observed strains greater than 30% between 2 and 6 MPa, confirming the potential of GIC formation as a viable actuating mechanism. Further studies are planned in order to perform more precise analysis and examine alternate GICs.Copyright

Solid-state actuation based on reversible Li electroplating

Reversible electrochemical compound formation has considerable potential to form the basis of a high-strain high-force multifunctional actuator technology. We present preliminary experimental demonstrations of the reversible work capability of solid-state electroplating. Our experimental test case is the volume expansion incurred during the reversible electrochemical formation of thin-film Li metal from a ceramic lithium ion storage medium, LiCoO2 as part of the standard operation of a state-of-the-art Li-ion battery. Reversible work is accomplished through the plating or stripping of the pure Li film against an external load. With the active portion of the structure as a basis, we observe ~10% strain against loads up to 2 MPa, with the load being limited by battery failure. No change in actuation characteristics is observed up to failure.

Towards High Temperature Actuation Using Graphite Intercalation Compounds

Electrochemically formed graphite intercalation compounds (GICs) have many intrinsic properties well-suited for compact actuation in applications at high temperatures. GICs using ionic liquids are of interest because of their good thermal stability at elevated temperatures, high ionic conductivity, and low volatility. In this study we observed the potential and strain behavior of highly oriented pyrolytic graphite and 1-ethyl-3-methylimidazolium hexafluorophosphate subjected to a light compressive load and constant current. In situ measurements of the anode during intercalation showed a reversible strain of 2.5% to 4.5% from 100°C up to 250°C.Copyright