Cellulose-based magnetoelectric composites

Abstract

Since the first magnetoelectric polymer composites were fabricated more than a decade ago, there has been a reluctance to use piezoelectric polymers other than poly(vinylidene fluoride) and its copolymers due to their well-defined piezoelectric mechanism and high piezoelectric coefficients that lead to superior magnetoelectric coefficients of >1 V cm^-1 Oe^-1. This is the current situation despite the potential for other piezoelectric polymers, such as natural biopolymers, to bring unique, added-value properties and functions to magnetoelectric composite devices. Here we demonstrate a cellulose-based magnetoelectric laminate composite that produces considerable magnetoelectric coefficients of ≈1.5 V cm^-1 Oe^-1, comprising a Fano resonance that is ubiquitous in the field of physics, such as photonics, though never experimentally observed in magnetoelectric composites. The work successfully demonstrates the concept of exploring new advances in using biopolymers in magnetoelectric composites, particularly cellulose, which is increasingly employed as a renewable, low-cost, easily processable and degradable material.Magnetoelectric materials by converting a magnetic input to a voltage output holds promise in contactless electrodes that find applications from energy harvesting to sensing. Zong et al. report a promising laminate composite that combines a piezoelectric biopolymer, cellulose, and a magnetic material.

Fig. 1

Cellulose ME laminate and the experimental set-up of ME effect measurement. a Scheme of cellulose crystal II, the most common crystalline type in regenerated cellulose materials. The saccharide unit provides dipolar segments along the aligned fibril thus rendering the piezoelectric nature. b Illustration of cellulose fibril alignment at the cross-section of cellulose film. Part of the ordered structure provides crystalline properties. c Schematic of cellulose-based ME laminate structure. The thickness is measured as 19 ± 2 µm for hot-press film and 27 ± 3 µm for the control film by using a micrometer caliper. The cellulose films are first sputter coated with 50 nm thick gold electrodes on both sides and then glued with Metglas 2605 SA1, of which the thickness is 25 µm. To ensure even distribution, the epoxy is preheated to 60 °C to improve the liquidity. d Schematic view of the bulk system for ME voltage measurement. The output voltage is collected from the interface gold electrodes and monitored as root mean square values using a lock-in amplifier. The inset below shows the picture of the final cellulose ME laminate used in the measurements

Fig. 2

ME effect of cellulose-Metglas laminates. a, b ME voltage coefficient as a function of H _ac frequency when a H _dc = 10.8 Oe for hot-press sample and b H _dc = 5.9 Oe for the control sample. The experimental data points are fitted to a Lorentzian resonance model (the solid line) and the resonance peaks found at 56.1 kHz. c The resonant ME output voltage as a function of applied H _dc strength. The ME laminates were induced by H _dc = 10.8 Oe for hot-press sample and H _dc = 5.9 Oe for the control sample at which a Lorentzian resonance profile has been observed. d Resonance enhanced α _ME as a function of H _dc for hot press (red dotted line) and control (blue dotted line). All data are obtained under H _ac = 0.5 Oe

Fig. 3

Representation of anti-resonance effect of ME output voltage. a–c ME output voltage of hot-press sample as a function of H _ac frequency under a H _dc = 4.2 Oe, b H _dc = 10.8 Oe, and c H _dc = 12.8 Oe. d–f ME output voltage of air-dried sample as a function of H _ac frequency under d H _dc = 3.9 Oe, e H _dc = 5.9 Oe, and f H _dc = 12.7 Oe. The experimental data (dots) are fitted to a modified Maxwell Eq. 4 or Lorentzian function (shown as solid lines)

Fig. 4

The effect of treatment on cellulose crystallinity and morphology. a DSC thermograms of hot-press (red line) and control (blue line) films. The melting and oxidation temperatures are 302.9 and 345.5 °C for hot-press film; 281.3 and 299.0 °C for air-dried film. The endothermic peak in this range corresponds to melting process of cellulose. The difference of oxidation (decomposition) temperature of the two films has been verified by using TGA (Supplementary Fig. 5). b, c SEM cross-section images of b hot-pressed and c air-dried cellulose films. The magnification is ×15,000 for detailed view (scale bar, 1 µm) and ×5,000 for full view (inset, scale bar, 2 µm)

Fig. 5

Effect of treatment on piezoelectric response measured by PFM. a, b PFM amplitude images of a hot-press and b air-dried samples. A conductive tip is used to apply a constant bias of 9.4 V to induce local ME displacement while imaging. c, d Piezoelectric butterfly loops of c hot-press and d control cellulose films elucidated by using SS-PFM. The red and blue dots are the hysteresis loops representing the bias-induce amplitude displacement. The pink and cyan dots represent phase changes corresponding to the hysteresis loops. e Histograms of bias induced amplitude displacement at applied voltage of −25 V during SS-PFM measurements