Quantifying the Role of an Interfacial Adhesive in a Magnetoelectric Coupling by Controlled Processing in Nickel–PVDF Laminates

Magnetoelectric composites build upon the electromechanical and magnetomechanical coupling of their respective piezoelectric and magnetostrictive constituents to provide an alternative electromagnetic coupling route. Devices are usually made by adjoining the piezoelectric and magnetostrictive components, often using epoxy adhesive. The adhesive is therefore essential and yet usually associated with losses and performance reduction, there is a clear need for deep physical understanding of its role in magnetoelectric performance. We studied the effect of different adhesives and their processing on the magnetoelectric output of polyvinylidene fluoride (PVDF)/nickel composites, 100 and 200 μm thick, correspondingly. Three epoxy adhesives were examined, used immediately after mixing, and alternatively following a set time, rendering the epoxy more viscous. The processing effects on the interfacial adhesive layer thickness and the on-resonance magnetoelectric coefficient, αE,ME = Eout/Hin, characteristics were measured, showing about 20% variance, peaking at 200 V/cm·Oe. This was followed by a functional fatigue test, where performance was sampled during about 24 h of continuous operation. An analytical model accounting for the adhesive layer was developed and used to examine its role in the magnetoelectric performance. In particular, we identified distinct mechanisms dominating the resonance frequency, the quality (Q) factor, and the value of αE,ME. The analysis points to a trade-off brought by increased adhesive thickness, between increased strain in the PVDF layer and reduced interfacial losses on the one hand, and increased viscoelastic losses in the adhesive on the other; this resulted in an optimal adhesive thickness of about 60 μm. This study advances the understanding of the physical processes dominating the ME coupling through the interface, and establishes that rational processing can bring improved performance.