Magnetoelectric Composites-Based Energy Harvesters

Abstract

Electrical energy generation from renewable resources has been a quest in the last few decades to meet the energy demand of electrical appliances and gadgets. More importantly, portable gadgets and devices, wireless sensors, etc., which rely on batteries require intermittent charging, and it is difficult to find an omnipresent continuous electrical energy source connected to a power station for these batteries. Alternate to these power stations connected to electrical energy sources is harvesting the energy from omnipresent mechanical and acoustic vibrations and AC magnetic field. Energy harvesting from these waste energy resources is possible using piezoelectric and magnetoelectric materials. This chapter would discuss in detail various mechanisms and stimuli, which may be synergistically used to harvest energy from piezoelectric materials-based energy harvesters.

1. Introduction

Harvesting electrical energy from various wasted forms of energy in the environment could be a way to develop sustainable energy sources. However, to do so, we need to develop smart materials and structures, which could convert various wasted forms of energy into electrical energy. These smart materials and structures may enable us to develop sustainable energy sources required for powering up low-power electronic devices [1, 2]. In the recent past, a lot of research attention has been devoted to converting omnipresent mechanical and acoustic vibration energy to electrical energy using electromagnetic [3, 4, 5], electrostatic [6, 7, 8], and piezoelectric [9, 10, 11] transductions. The most popular technique among the methods is energy harvesting using the piezoelectric effect, which is proved to be advantageous over other methods due to its easier execution and higher power density. These piezoelectric energy harvesters are capable of harvesting energy from various kinds of vibration sources involving translational and rotary motions [12, 13, 14], wind [15, 16], or some fluid flow induced vibrations [17, 18]. Another ubiquitous form of wasted energy is magnetic energy found around the electrical current-carrying wires and appliances. This energy can be harvested by using magnetoelectric (ME) composites. An ME composite comprises a ferromagnetic (FM) material and a piezoelectric material combined with various phase connectivity schemes. On application of a magnetic field across such a composite, FM material undergoes magnetostriction, which leads to an elastic strain in the material. This elastic strain is transferred to the connected piezoelectric phase via a mechanical coupling. Due to this strain in the piezoelectric material, an electric field is generated in a transverse direction across the piezoelectric phase. This electric field is harvested in the form of electric energy using these ME composites. In other words, the application of a magnetic field across these composites produces an electric field. This effect is termed as the direct magnetoelectric effect. The converse effect is also true in these ME composites, where the application of an electric field leads to the generation of a magnetic field. Both effects have been explored in numerous applications such as energy harvesting, magnetoelectric transformers, AC and DC magnetic field sensors, phase shifters, and resonators. [19]. However, in this chapter, we will be discussing the direct magnetoelectric effect in ME composites, which is useful for energy harvesting. This chapter is organized in the following way: We will begin with a discussion of the magnetoelectric effect in magnetoelectric composites, the materials requirements, and the properties of the ME effect. A list of candidate materials for ME composites has been provided to give the idea of specific materials to the readers. Then, applications of these ME composites, specifically energy harvesting, have been discussed.

2. Magnetoelectric effect

ME effect is the induction of electric polarization in a material under an applied external magnetic field or conversely, the generation of magnetization due to an applied external electric field [20]. This induced polarization P depends on the applied external magnetic field H, according to the following expression:

$$\mathit{P}=\mathit{\alpha H}$$E1

where α is called second rank ME-susceptibility tensor.

ME effect is quantified by calculating the ME voltage coefficient α_E using the following expression:

$${\mathit{\alpha }}_{\mathit{E}}=\frac{\mathit{\alpha }}{{\mathit{\epsilon }}_{\mathit{o}}{\mathit{\epsilon }}_{\mathit{r}}}={\left(\frac{\mathit{\partial E}}{\mathit{\partial H}}\right)}_{\mathit{S}}$$E2

Here, ε_o and ε_r are the electric permittivity of free space and the relative permittivity of the dielectric material, respectively. The ME effect was first realized in single-phase materials, the so-called multiferroics, possessing both ferromagnetic and ferroelectric orderings. ME effect is a consequence of the coupling between these ordered parameters. Landau’s theory describes this coupling by the free energy F of the system expressed in terms of applied field, that is, magnetic field H or electric field E [21]. The ME effect was theoretically predicted by Curie in 1894 [22]. However, it was first observed by the Russian scientist Astrov in 1960 in a single-phase material Cr₂O_3, which is antiferromagnetic [23]. Afterward, many more single-phase materials were found to show this effect. ME effect in these single-phase materials was found to be weak and rare due to the nonsimultaneous presence of two ferroic orders at the same temperature. Therefore, artificially engineered multiferroic magnetoelectric composites came into realization. The first bulk ME composite was reported by Van den Boomgaard in the 1970s. Ferroelectric BaTiO₃ combined with cobalt ferrite/nickel ferrite to form bulk ME composite produced a ME voltage coefficient, α_E of 130 mV/cm Oe. The constituent materials were synthesized using a solid-state reaction technique [24]. In recent years, different forms of ME composites, such as layered stacks of piezoelectric/magnetostrictive materials, polymer-ceramic matrix composites, and rare earth elements-based composites, which have shown relatively stronger intrinsic ME effect have found great research interest for applications in futuristic electronic devices.

ME effect in the composites of ferromagnetic and piezoelectric materials appears as a product tensor property, which was first proposed by van Suchtelen in 1972. None of the constituent materials used to form a composite shows the ME effect individually. However, on forming a composite, a relatively stronger ME effect results. In this composite, the mechanical deformation in ferromagnetic material due to magnetostriction results in an electrical polarization due to the piezoelectric effect in the piezoelectric material. The product of the magnetostrictive effect (magnetic/mechanical effect) in ferromagnetic material and the piezoelectric effect (mechanical/electrical effect) in piezoelectric material gives a direct ME effect [25]:

$$\text{Direct}\mathrm{ME}\text{effect}=\text{magnetic}/{\text{mechanical}}_{\times }\text{mechanical}/\text{electric}$$E3

$$\text{Converse}\mathrm{ME}\text{effect}=\text{electric}/{\text{mechanical}}_{\times }\text{mechanical}/\text{magnetic}$$E4

A schematic for the ME effect in composites is shown in Figure 1. Direct ME effect is observed when the ME composite is subjected to a magnetic field, which causes a change in the shape of the ferromagnetic phase due to magnetostriction. In other words, a strain is developed in ferromagnetic material due to the application of a magnetic field. This strain is then transferred via mechanical coupling to the adjacent piezoelectric phase, causing an electric polarization in it due to the piezoelectric effect.

Figure 1.

Schematic diagram of strain-mediated ME effect in an ME composite.

For the ferromagnetic phase, due to the application of a magnetic field to the ME composite,

$${\left(\frac{\mathit{\partial S}}{\mathit{\partial H}}\right)}_{\mathit{T}}=\mathit{q}$$E5

and for the piezoelectric phase

$${\left(\frac{\mathit{\partial P}}{\mathit{\partial S}}\right)}_{\mathit{E}}=\mathit{d}$$E6

where S is the strain, T is stress, and q (where q = dλ/dH, λ is magnetostriction) and d are pseudo piezomagnetic and piezoelectric coefficients, respectively. Pseudo piezomagnetic coefficient is defined as the slope of magnetostriction versus magnetic field curve. For an ME composite, electric polarization due to the application of a magnetic field is found as:

$${\left(\frac{\mathit{\partial P}}{\mathit{\partial H}}\right)}_{\mathit{S}}={\mathit{k}}_{\mathit{c}}\mathit{qd}=\mathit{\alpha }$$E7

where k_c is a mechanical coupling factor ($\mathbf{0}\mathit{\le }\left|{\mathit{k}}_{\mathit{c}}\right|\mathit{\le }\mathbf{1}$) for the two phases and α is the ME susceptibility of the composite, from which we can calculate the ME voltage coefficient as:

$${\mathit{\alpha }}_{\mathit{E}}=\frac{\mathit{\alpha }}{{\mathit{\epsilon }}_{\mathit{o}}{\mathit{\epsilon }}_{\mathit{r}}}={\left(\frac{\mathit{\partial E}}{\mathit{\partial H}}\right)}_{\mathit{S}}$$E8

where ε_o and ε_r are the electrical permittivity of free space and the dielectric constant of the piezoelectric phase, respectively.

Therefore, an entirely new property comes up in a composite of ferromagnetic and piezoelectric materials since neither of the constituent material shows a magnetoelectric effect. This product property-based ME response has resulted in due to the elastic coupling between the two constituent materials. This ME response of the composite, however, strongly depends on individual material characteristics such as the high pseudo piezomagnetic coefficient of ferromagnetic material and large piezoelectric coefficients of piezoelectric material along with strong mechanical coupling (large k_c) between the two materials. It can be said that the ME effect is an extrinsic property of these ME composites, which is the function of various extrinsic parameters such as the microstructure of the composite and how two phases couple magnetoelectrically at a ferromagnetic-piezoelectric interface. Moreover, this ME response in these composites is observed in ambient conditions, which makes these composites technically viable. Also, the ME response in these composites is several orders of magnitude larger when compared with single-phase magnetoelectric multiferroics. Various composites of ferromagnetic and piezoelectric materials have been investigated in recent years. These ME composites can become useful for practical applications by utilizing this extrinsic ME effect.

3. Candidate materials for ME composites

For an ME composite, the major requirements from a materials perspective are a high piezoelectric coefficient for the piezoelectric phase and large magnetostriction in the ferromagnetic phase. Figure 2 shows piezoelectric coefficients and magnetostriction some of the constituent phases investigated for ME composites.

Figure 2.

Piezoelectric coefficients and magnetostriction of some of the constituent phases.

Many more candidate materials have been used to form ME composites. A ferromagnetic material can be chosen from a list of metallic FM materials, such Ni, Fe, or Co, from FM alloys, such as Terfenol-D, Metglas, and Permendure, or FM ceramics, such as Magnetic spinel ferrites, Garnets, and Manganites. All these materials have shown a good ME effect when combined with a piezoelectric material. A piezoelectric material is either an oxide ceramic, such as BaTiO₃, PZT, KNN, and PMN-PT, or a polymer such as polyvinylidene fluoride (PVDF).

3.1 Phase connectivity schemes

As mentioned earlier, strong ME response in ME composites above room temperature makes them technologically viable in various applications. Therefore, ME composites have been investigated using various combinations of materials and phase connectivity schemes, which include (1) bulk ceramic ME composites formed using piezoelectric ceramics and ferrites, (2) Bi-phase ME composites by combining FM alloys and piezoelectric materials, (3) three-phase (two FM phases and one piezoelectric or otherwise) ME composites, and (4) nanostructured thin films of ferroelectric and magnetic phases-based ME composite [26]. Newnham et al. introduced the concept of phase connectivity schemes (Figure 3) in which we use the notations 0–3, 2–2, and 1–3 for describing the structure of two phases of a bulk composite [27]. In these notations, each number represents the connectivity of the respective phase. For instance, 0–3 notation is used for a particulate composite in which particles of the FM phase (represented by 0) are engrained in the matrix of the piezoelectric phase (represented by 3). In 2–2 type composites, which we also call laminate composites, alternate layers (in form of thin sheets/films) of one phase are bonded with layers of another phase. In a 1–3 type composite, fibers/rods/wires/tubes of the magnetostrictive phase are embedded in the matrix of the piezoelectric phase. Each of these structures has its advantages and disadvantages.

Figure 3.

Phase connectivity schemes of (a) 0–3 particulate bulk composite, (b) 1–3 fiber-matrix bulk composite, and (c) 2–2 laminate bulk composite.

Among these connectivity schemes, the most useful is the 2–2 type laminate composite due to its ease of fabrication and relatively better ME response. As most of the FM materials possess higher conductivity compared to piezoelectric materials (which are usually good dielectrics) in 0–3 type particulate composite, the inhomogeneous distribution of FM particles in piezoelectric matrix leads to more dielectric losses, which eventually reduces ME response. The issue of higher dielectric losses may be overcome in 1–3 type composites; however, the difficulty of fabrication makes these composites less viable. Moreover, their ME response is also relatively low compared to 2–2 type laminate composite. The connectivity schemes can also be employed to make ME composite thin films on suitable substrates. However, the ME response in these ME composite thin films is not as good as it is in their bulk counterpart. This may be attributed to the relatively lower piezoelectric effect in piezoelectric thin films.

4. Applications of ME composites

ME composites find numerous applications from energy harvesting to AC/DC magnetic field sensors, resonators, phase shifters, ME transformers, etc. Here, we will focus on the energy-harvesting aspect of these ME composites. As mentioned earlier, in an ME composite an electric field can be generated by the application of a magnetic field across the composite, which is termed the direct magnetoelectric effect. Now to produce this direct ME effect, both AC and DC magnetic fields need to be applied across the composite. DC magnetic field is the bias field that produces strain in the magnetostrictive phase through magnetostriction, while the AC field is required to lift the time-reversal symmetry of the magnetostrictive phase. The sweeping of the DC magnetic field initially increases the magnetostriction, which finally reaches saturation at a specific value of the magnetic field for a particular FM material. To generate continuous electrical output from piezoelectric material, we need to subject it to periodic stress. Therefore, in an ME composite, the application of an AC magnetic field leads to the production of periodic stresses in the piezoelectric phase, which is connected to the FM phase via strain-mediated magnetoelectric coupling. Thus, an ME composite needs to be subjected to a low-frequency AC magnetic field of small magnitude to generate continuous ME output. This low-frequency weak AC magnetic field is omnipresent in the proximity of power cables. A power cable carrying 50 A alternating current of the frequency 50–60 Hz will generate an alternating magnetic field of about 10 Oe at a distance of 1 cm [28]. This is a weak magnetic field that is usually considered noise and it is also detrimental to the human body. This unused and wasted magnetic field energy could be converted into electrical energy, which will be useful for powering low-power devices.

4.1 ME composites-based cantilever

There have been several designs proposed for effectively harvesting the electrical energy from the magnetic field and acoustic vibrations. The most useful design is to have an ME composite cantilever in which an FM layer is bonded with a piezoelectric layer as a 2–2 type laminate composite. In the case of a cantilever made from piezoelectric material only and subjected to vibrations, a tip mass is used to actuate it. In an ME composite cantilever, the tip mass is replaced with a permanent magnet. On application of an alternating magnetic field across this ME cantilever, the magnetic energy is transformed into vibrations and then into electricity through the piezoelectric effect. A schematic of such an ME cantilever along with a mechanism of the ME effect is shown in Figure 4.

Figure 4.

Working principle of magnetoelectric cantilever-based energy harvester.

Using the above design, Liu et al. [29] demonstrated a copper-based piezoelectric twin beam with a NdFeB permanent magnet of dimensions 30 × 6 × 0.3 mm³. A maximum output power density of 11.73 μW cm⁻³ Oe⁻¹ was attained with a 100 Hz alternating magnetic field. A similar design was used by Lu et al. [30] for a composite beam of piezoelectric single crystal and Ni. They achieved a power density of 270 μW cm⁻³ Oe⁻¹ in a 50 Hz AC magnetic field. This kind of design has attracted a lot of attention for energy harvesting applications due to its flexibility and ease of fabrication [31, 32].

4.2 Resonant condition ME composite cantilever

Further enhancement of conversion efficiency can be achieved using ME composites at resonant conditions where the frequency of the AC magnetic field is matched with the natural frequency of the ME composite cantilever. This is usually achieved by making light and slender cantilevers formed from a good mechanical property magnetostrictive material and a strong piezoelectric material. This offers the advantage that it is capable of not only converting vibration energy but also it can harness the magnetic field energy at a low frequency using the ME effect. A schematic of such a dual-phase energy harvester is shown in Figure 5. Zhou et al. [30] demonstrated a dual-phase energy harvester of Ni and piezoelectric macro-fiber composites, which produced a power density of 4.5 mW cm⁻³ G⁻¹ at resonance.

Figure 5.

A self-biased dual-phase ME energy harvester.

4.3 Magneto-mechano-electric (MME)-based ME composite energy harvester

As mentioned earlier, if the tip mass in an ME composite cantilever is replaced with a magnet, it is called a magneto-mechano-electric (MME) component. A schematic of an MME is shown in Figure 6.

Figure 6.

Schematic of an MME generator.

In this MME component, electrical energy generation is ascribed to three mechanisms: magnetostriction in the FM layer due to the magnets and the deformation of the piezoelectric layer due to magnetostrictive strain, and AC magnetic field-induced vibrations. An MME component of nickel and piezoelectric single-crystal fibers which could generate a power density of 46 mW cm⁻³ Oe⁻¹ in a magnetic field of 1.6 G was reported by Ryu et al. [33]. This MME component could ignite 35 LEDs. The output power density of ME composites mainly depends on constituent piezoelectric and magnetostrictive materials and also the extent of ME coupling between them. For example, Terfenol-D, which is known as the giant magnetostrictive material and has saturated magnetostriction of around 1500–2000 ppm under the driving magnetic field, was used in early ME composite studies [34]. However, its brittleness makes it inviable for low-frequency magnetic field environments due to its higher natural frequency. Moreover, Terfenol-D requires a DC magnetic field of 2–3 kOe to reach saturation magnetostriction [35]. A strong DC magnetic field often leads to bulky structures and usually causes electromagnetic interference as well. Therefore, ME composites requiring a small bias field caught attention [36]. Chu et al. [37] reported Metglas and piezoelectric single-crystal fibers ME composites, which were longitudinally magnetized and poled (L-T) in the transverse direction. A large ME coefficient of 22.92 V cm⁻¹ Oe⁻¹ for a 6 Oe DC bias field could be obtained. However, achieving resonance at low frequency is rather difficult. Therefore, it may be concluded that all these combinations come with certain advantages and disadvantages [38, 39].

5. Conclusions

The ME composites of a ferromagnetic material and a piezoelectric material use the magnetoelectric effect in conjunction with the piezoelectric effect in electrical energy harvesting. They have shown better conversion efficiency as compared to individual piezoelectric energy harvesters. However, the choice of constituent materials and their connectivity schemes play a crucial role in designing these ME composite energy harvesters. Further, enhancement in energy harvesting properties in these ME composites may be achieved by using different phase connectivity schemes. Although the 1–3 phase connectivity scheme has shown significant improvement in the ME effect, the difficulty in the fabrication of these 1–3 type composites is still a hindrance.

Acknowledgments

The authors would like to thank VIT management for their encouragement and support.

Conflict of interest

Authors have no mutual conflict of interest.

Appendices and nomenclature

FM	Ferromagnetic
FE	Ferroelectric
PE	Piezoelectric
ME	Magnetoelectric
MME	Magneto-mechano-electric
BST	Barium strontium titanate
YIG	Yttrium iron garnet
PZT	Lead zirconium titanate
PMN-PT	Lead magnesium niobate-lead titanate
PZN-PT	Lead zinc niobate-lead titanate