Advanced Composites and Hybrid Materials (2018) 1:478 505 https://doi.org/10.1007/s42114-018-0046-1 REVIEW Recent progress on piezoelectric energy harvesting: structures and materials Leilei Li 1 & Jie Xu 1 & Junting Liu 1 & Feng Gao 1 Received: 10 April 2018 /Accepted: 4 June 2018 /Published online: 16 August 2018 # Springer International Publishing AG, part of Springer Nature 2018 Abstract With the rapid development of advanced technology, piezoelectric energy harvesting (PEH) with the advantage of simple structure, polluted relatively free, easily minimization, and integration has been used to collect the extensive mechanical energy in our living environment holding great promise to power the self-sustainable system and portable electronics. In this paper, attempts have been made to review the progress of piezoelectric materials and devices used for energy harvesting. The review focused on three parts: structure of piezoelectric devices (cantilever, cymbal, and stacks); theoretical mode, including vibration mode (d 31 and d 33 ) and its responding theories of different devices; and piezoelectric materials, among which the electric performance of Pb(ZrTi)O 3 -based, lead-free-based, and composites applied in bulk/micro/nano-peh devices were introduced in details. It is suggested that a new structure of PEH should be designed by combining advantages of cantilever, stacks, and cymbal structure. Besides, high d 33 g 33 value and low ε of piezoelectric materials are required in order to generate high electric output power for bulk/micro/nano-peh. Additionally, lead-free piezoelectric ceramics is a candidate for manufacturing PEH devices, and nano-composite materials should be developed to further improve the flexibility of piezoelectric materials. Keywords PEH. Structures. Theoretical mode. Piezoelectric materials Abbreviations T c Curie temperature d ij Piezoelectric constant d eff 33 Effective piezoelectric constant g ij Piezoelectric voltage constant g eff 33 Effective piezoelectric voltage constant k p Electromechanical coupling coefficient Q m Mechanical quality factor ε Dielectric constant ε o Dielectric constant of vacuum ε r Relative dielectric constant E Electric field T Temperature S Active area of nano-materials * Feng Gao gaofeng@nwpu.edu.cn 1 State Key Laboratory of Solidification Processing, MIIT Key Laboratory of Radiation Detection Materials and Devices, NPU-QMUL Joint Research Institute of Advanced Materials and Structures (JRI-AMAS), School of Materials Science and Engineering, Northwestern Polytechnical University, Xi an 710072, Shaanxi, People s Republic of China K P MAX R OPT ω l w t t p t s t m t h s r b r t r p H A n R η V op Q Amplification coefficient for cymbal Maximum output power Optimal resistor Angular frequency Length of beam Width of beam Time Thickness of piezoelectric element Thickness of cantilever metal substrate Thickness of metal endcap Height of the cavity Strain Radius of the bottom part for the endcap cavity Radius of the top part for the endcap cavity Radius of the piezoelectric element for the endcap cavity Bending degree of cantilever Area of piezoelectric element Number of piezoelectric layer Resistor load Energy efficiency Open-circuit voltage Charge
Adv Compos Hybrid Mater (2018) 1:478 505 479 P C o C i j AC DC Output power Vacuum capacitance of energy harvesting Capacitance of energy harvesting Polarization direction of piezoelectric materials Stain direction of the piezoelectric materials Alternating current Direct current 1 Introduction With the development of wireless sensors and communication node networks, researchers have concentrated a great deal of interest on harvest or scavenge ambient wasted energy over the past decade, such as solar, wind, thermal gradient, and ambient vibrations, as a substitution of battery [1, 2]. Among this, vibration energy with supernal power density is widely present in industrial production and our daily life, such as human walking and motor rotation. Besides, the vibration energy from large size machine is up to ~ 800 μw/cm 3,and the power of 1 W can be generated by the heel striking during walking [3, 4], which attracts great interest on energy harvesting. Now, there are three kinds of energy harvesting devices such as electromagnetic, electrostatic, and piezoelectric energy harvesting to scavenge the vibration energy in the surrounding environment. The basic mechanism of electromagnetic vibration energy harvesting is electromagnetic induction of Faraday s law. Under the influence of external vibration, relative movement between induction coil and magnets vibration derived from changing magnetic fluxes results the formation of induced voltage [5]. The electromagnetic vibration energy harvesting with the benefit of larger output power and dispense with additional power supply in use, however, is difficult to compatible with modern industrial production technology of higher output voltage due to the developing tendency for device miniaturization [5]. Electrostatic vibration energy harvesting, also called capacitor vibration energy harvesting, mainly forms the potential difference above the capacitor plate to generate a stable voltage under external power [6]. And charges will come into forming due to the change of capacitance between electrode plates. The device is compatible with MEMS technology with high output power and voltage, especially compared with electromagnetic vibration energy harvesting, higher output voltage can be achieved with the same dimension; however, the challenges are the requirement of additional power [7, 8].Owingtobothelectrostatic and electromagnetic vibration, energy harvesting needs an additional electric power, it limits the development and application of devices; therefore, the piezoelectric energy harvesting (abbreviated as PEH) is becoming more meaningful and attracts much attention. Utilizing the positive piezoelectric effect, PEH device can convert the useless mechanical energy into electrical energy, and internal polarization is generated from deformed piezoelectric materials under external force. The device is easy to compatible with MEMS process technology, showing the advantages of environment adaptability, compact structure, high voltage, and operation without additional electrical power supply [10]. Besides, according to the theoretical calculation by Priya et al. [9], the power density for PEH is three to five times higher than that of electrostatic and electromagnetic device, as shown in Fig. 1. Meanwhile, PEH device can effectively collect various mechanical energies in the surrounding environment, such as converted low speed wind energy, cycle load from piezoelectric backpack into electricity, etc. Consequently, PEH device and its related materials attracted a wide range of attention. However, the harvesting power from PEH devices is still low now, on the order of micro- to milliwatts. To wider the potential application of self-powering, wireless sensor networks, and low-power electronic equipment, much attention is given to further enhance the power density (output power per unit volume) of energy harvesting or energy conversion efficiency in PEH. The conversion process of vibration energy to electrical energy for PEH can be divided into three parts as seen in Fig. 2, including mechanical transfer under external Fig. 1 Power density of vibration energy harvesting [9]
480 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 2 Major conversion process of vibration energy to electrical energy for PEH excitation (relating with mechanical stability of the piezoelectric transducer under large stresses, and mechanical impedance matching), mechanical electrical energy transfer (relating with the electromechanical coupling factor in the composite transducer structure, and transduction coefficient of piezoelectric materials) and electrical energy transfer (relating with electrical impedance matching and the circuit design) [11]. A suitable DC/DC converter is required to accumulate the electrical energy from a high impedance piezo-device into a rechargeable battery (low impedance) [11]. So, factors of mechanical strength, mechanical impedance, damping factor, coupling factor, piezoelectric coefficient, electrical impedance, and circuit loss, etc., have important influence on the output power of PEH devices. Commonly, researchers utilize the following ways, such as choosing optimum vibration mode, modifying the structure type, and selecting better piezoelectric materials, etc. to ameliorate energy conversion efficiency of PEH to meet practical application. Figure 3 shows the numbers of published papers in the database of Web of Science using Bpiezoelectric energy harvesting^ as key words. It can be seen that the publication number continuous increases, and then, it is more than 600 papers per year during 2015 and 2016. In this work, development of PEH was reviewed from structures to its basic materials, and a comprehensive systematic analysis of electrical performance of piezoelectric materials applied for PEH is presented. It is organized as three sections, including overview on the development of PEH applied for scavenging vibration energy, effects of microstructure of cantilever, cymbal, stacks and micro-nano-structure on PEH, theoretical models for different vibration mode of PEH, fabrication process of micro-nano-peh, influence of piezoelectric ceramics, piezoelectric polymer, nano-piezoelectric materials in the performance of PEH, development of piezoelectric material applied in bulk- to nano-peh, potential applications in different fields, and remarks and future perspectives. This paper does not intend to summarize all field of PEH due to rapid growth in this field. The concluding remarks and further development only reveal the authors viewpoint. 2 Structure of PEH Due to higher Young s modulus of piezoelectric ceramics, it is not easy to generate piezoelectric effect by stretching the piezoelectric material. To generate larger strain, many scholars have designed and fabricated various types of PEHs with different bending ways, including cantilever, cymbal and stacks, etc. Figure 4 shows the papers number on typical structure of PEHs in the database of Web of Science using Bcantilever piezoelectric energy harvesting,^ Bcymbal piezoelectric energy harvesting,^ and Bstack piezoelectric energy harvesting^ as key words, respectively. Results show that researchers pay much attention on the cantilever PEHs as the simple fabrication process and relatively larger strain. Fig. 3 Published papers about PEH between 2000 and 2016 Fig. 4 Published papers number on the typical structure of PEHs from 2000 to 2016
Adv Compos Hybrid Mater (2018) 1:478 505 481 Fig. 5 Structure schematic of cantilever PEHs. a Unimorph. b Bimorph [12] 2.1 Cantilever PEH Cantilever structure of PEH is simple and easy to generate great strain under base excitation in vibration system. The main problem now is the lower energy conversion efficiency, which can be further enhanced via shape and structure modification for cantilever PEH. 2.1.1 Unimorph and biomorph cantilever Cantilever PEH commonly includes unimorph and bimorph etc., as shown in Fig. 5. Unimorph cantilever PEH is mainly composed with two electrodes and piezoelectric material in the middle, similar to a sandwich structure shown in Fig. 5a. Commonly, the side of unimorph is combined with metal and elastic film to enhance the structural stability of devices. Besides, unimorph can be consisted of biomorph cantilever and cantilever arrays PEH, which can achieve higher output power at low frequency and resistant load [12]. Researchers always utilize bimorph cantilever to further enhance the conversion efficiency of PEH in vibration energy environment, which mainly consists of two same piezoelectric layers between the intermediate substrate, as shown in Fig. 5b. Output power can be enhanced with piezoelectric layers in series or parallel, because the series can increase the output voltage while parallel with the high output current (in Fig. 6). However, the production processing for bimorph cantilever PEH is relatively complex. Later, multilayer cantilever PEH consisted of more than two layers piezoelectric unimorph has been fabricated, to further enhance the stability and life time. However, lower energy conversion efficiency and relative high natural frequency for cantilever PEH restrict its practical application in vibration environment. Consequently, researchers pay much attention on shape and proof mass of cantilever PEH to effectively reduce the natural frequency for low resonance frequencies. Besides, output voltage and power can also be adjusted by the electrode assembly. 2.1.2 Proof mass for cantilever Although cantilever PEH has the advantages of relatively simple structure and relatively no complex processing technology, the main drawbacks for the device is the narrow resonant frequency band. When natural frequency of the device is slightly deviated from the vibration frequency in the environment, there are no resonance phenomena that appeared, resulting in the lower power conversion efficiency, which is an urgent problem for cantilever PEH to be solved. Researchers employ two ways such as adding proof mass or increasing the length of beam to reduce the natural frequency. However, it is an unadvisable way to increase the length of cantilever length to obtain a lower resonance frequency, which will limit the miniaturization tendency of PEH. In order to modify the resonance frequencies to meet practical application, a great deal of research has focused on the enhancement of the bending beam because of inertial effect derived from a proof mass (shown in Fig. 7) near the free end of the cantilever. Under the external acceleration, piezoelectric effect is generated from the bending cantilever devices by cyclical bending of piezoelectric layers. The device has a higher quality factor with simple simulated structure, which results the wildly researching in the influence of proof mass for cantilever on the output power, such as types and shapes of proof mass [13, 14]. Besides, adding a proof mass to cantilever is an optimum way to reduce the resonance frequency for cantilever rather than the increase length of beam. In 2009, Li et al. [15] proposed a sandwiched PZT-Si-PZT rectangle cantilever PEH with a nickel proof mass (750 750 750 μm). The results (a) (b) V V Piezoelectric layer Substrate layer Fig. 6 Schematic diagram of a antiparallel polarization piezoelectric bimorphs and b parallel polarization piezoelectric bimorphs
482 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 7 Vibration schematic diagram of cantilever PEHs showed that the resonance frequency can be dramatically reduced by the accession of a proof mass with reasonable weight. However, with double length of beam increased, both of voltage and resonance frequency reduced. In practical application, cantilever PEH demands a protective casing to prevent its bending beyond the maximum allowed application, while the accommodation of the proof mass comes at the expense of active piezoelectric area, both of which has led to the waste of space above the beam in the conventional block mass design. Figure 8 shows an L-shaped tungsten alloy proof mass to induce more strain and mass on the piezoelectric beam without increasing the overall volume [16]. Lower fundamental frequency of 20 31%, average power of 350 μw and power density of 1.45 mw/cm 3 under input acceleration of 0.75 g was obtained, which was 68% higher than that of conventional block-shaped mass harvester. When curved L-shaped PEHs were installed into the shoes under the walking speed of 3.0 mph, an average power of 49 μwcanbe obtained. Jia et al. [17] investigated the optimal proof-massto-cantilever-length ratio for power maximization. The result showed that with 70% of length covered by Si end mass (5.0 mm 3 ), the device was able to generate 1.78 μw at0.6/ ms 2 and up to 20.5 μw at2.7/ms 2 at 210 Hz, which is not limited by nonlinear damping. 2.1.3 Shape for cantilever At the same time, altering the cantilever shape to trapezoid, triangle, and rectangular is another way to strengthen energy conversion efficiency for cantilever PEH. Both theory simulation and experiment have demonstrated that maximum output power can be obtained from trapezoid or triangular cantilever with the Fig. 8 Attachment of curved L-shaped cantilever PEH in shoes [16] smallest areas, meeting the given energy requirement [18, 19]. Kauret et al. [20] also simulated the influence of maximum power generation from four designed shapes named as Pi, E, Rectangular, and T cantilever (shown in Fig. 9), composed of piezoelectric material (PZT-5H, AlN, BaTiO 3 ) with thickness of 0.5 μm and silicon base material of 1.5 μm. The result showed that the cantilever with E-shaped geometry (PZT-5H) was considered as the best one with a generated power of 49.005 μwand a displacement of 0.6078 μm, which is potential substitution for traditional power sources. Table 1 summarizes the properties of typical cantilever PEH to decrease the resonance frequency by adjusting the cantilever length and proof mass. With the relatively lower output power, small size, simple structure, and larger strain, it is more suitably integrated with the MEMS processing technology. 2.2 Cymbal structure In 1990s, Newnham et al. [21] proposed a cymbal structure, intriguing researchers interest to further study the mechanism of cymbal PEH. The structure is composed of a piezoelectric disk sandwiched between two metal cymbal-shaped endcaps, as shown in Fig. 10. Metal cap of cymbal can strengthen the endurance of the ceramic under high loads, and the actual piezoelectric coefficient d 31 and d 33 is amplified due to the presence of cavity, which allows the metal end caps to serve as a mechanical transformer for converting and amplifying a portion of the incident axial stress into the radial and tangential stresses [22]. Maximum output energy can be obtained by constituting cymbal and other structure of PEH together, such as multilayer stacks, cantilever, or integrate with each other under high load. In 2004, Kim et al. [23]investigatedaBcymbal^ PEH with two same metal caps of 29-mm diameter and 1-mm thickness to harvest electrical energy from mechanical vibrations. The result showed that output power of 39 mw was generated across a 400-kΩ resistor at the frequency of 100 Hz under a force of 7.8 N. Later, higher output power of 52 mw was achieved under high force of 70 N with a pre-stress load of 67 N at 100 Hz frequencies [24]. They further found that the
Adv Compos Hybrid Mater (2018) 1:478 505 483 Fig. 9 Schematic diagram of a rectangular-shaped, b Pi-shaped, c T-shaped, and d E-shaped cantilever [20] generated power enhanced with the frequency increased under the same force and obtained ~ 100 mw at 200 Hz [25]. Zhao et al. [26] studied the efficiency and coupling effects on various dimensions of cymbal PEH from the pavement caused by vehicle and gravity via finite element analysis (FEA). Considering the storage electric energy, cost, bonding between end steel cap and PZT, and pavement surface displacement, the optimum one was with total diameter (32 mm), cavity base diameter (22 mm), end cap top diameter (10 mm), cap steel thickness (0.3 mm), cavity height (2 mm), and PZT thickness (2 mm). The output voltage and power were about 97.33 V and 1.2 mw at 20-Hz vehicle load frequencies, respectively. Palosaari et al. [27] designed a piezoelectric disk between two convex steel acting as a force amplifier delivering stress to the PZT and protecting the harvester and then embedded it into a shoe for energy harvesting during walking, as shown in Fig. 11. At 1.19 Hz, the power energy of 0.66 mw was obtained with 250-μm thick steel disks, which is potentially applied to some monitoring electronics or extended the battery life of a portable device. As high circumferential stresses of cymbal caused by flexural motion of metal endcaps, Fig. 12 shows a slotted cymbal including fringe radial and cone radial slots fabricated to avoid the loss of mechanical input energy. With number and length of radial slots increased, output voltage and power of cymbal increased subsequently. Maximum output power around 16 mw could be achieved with 18 cone radial slots across 500-kΩ resistive load from a cymbal, 0.6 times higher than that of the original ones [28]. Arnold et al. [29] fabricated a new cymbal combined with a unimorph circular piezoelectric disk between the metal end caps to deal with higher loads at low-frequency, asshowninfig.13. Unimorph cantilever cymbal harvester canwithstandhighloadwithanalternative steel substrate design. The maximum output voltage was 25.6 V under the force of 2100 N at 1 Hz. Another unimorph cymbal harvester fabricated by replacing the PZT monolayer with the PZT/steel composite between the metal end caps [30] can generate a power of 121.2 mw at 3.3 MΩ resistive load under load of 1940 N, which shows potential applications in higher load condition. Because the frequency in the surrounding environment is usually less than 300 Hz, the maximum energy output can be obtained at the low resonant frequency. However, the high resonant frequency of cymbal is a barrier for application. Considering the advantage of the cantilever structure with low resonant frequency, Ren et al. [31] presented a highperformance cantilever driving low-frequency energy harvester (CANDLE) consisting of a cantilever beam and cymbal, as shown in Fig. 14. Using this special structure, the resonance frequency is reduced dramatically with the increase of output Table 1 Properties of typical cantilever PEH Structure Proof mass (g) Proof mass size (mm) Output power (μw) Year Rectangle cantilever Nickel 0.75 0.75 0.75 2009 [15] L-shape cantilever Tungsten alloy 350 2010 [16] Rectangle cantilever Si 5 5 5 20.5 2015 [17] E-shape cantilever 49.005 2016 [20]
484 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 10 Structure of cymbal PEH proposed by Newnham et al. [21] power immensely compared with traditional cymbal PEH. Peak voltage of 38 V and maximum power of 3.7 mw were obtained at 102 Hz. Different with most PEH, output voltage and power of CANDLE have a nonlinear leap with the mechanical excitation force or acceleration increased, as shown in Fig. 15. It can be seen that the open-circuit voltage and output power increased linearly first, then appeared a nonlinear leap, and finally came to linearity again with the excitation acceleration, leading to the properties several times enhanced. Due to the higher excitation acceleration at the leap point (65 m/s 2 ), it is meaningful to reduce the nonlinear leap point to obtain more energy for CANDLE [32]. Nowadays, some applications on harvesting the vibrational wasted energy from the asphalt can be found in the market, such as the Israeli Company Innowattech (www.innowattech. co.il). Since asphalt pavement stiffness is between 1000 and 4000 MPa, PEH must have a stiffness coefficient among this range [33]. Moure et al. [34] integrated 30,000 cymbals (single cymbal can generate to 16 μw for the pass of one heavy vehicle wheel) to embed into 100-m asphalt pavement with distance to the surface of 2 cm. The mechanical excitation provided by the car wheels deformed the asphalt road, and the cymbals embedded in the pavement layers below the road layer were excited. The output voltage is recovered by the copper bands to fee the external circuit and provides the final electric signal, as shown in Fig. 16. This PEH could account for more than 65 MW/h in a year with low cost for an emerging technology (less than 2 /kw/h). Table 2 summarizes the properties of PEH with typical cymbals structure. Comparing with cantilever structure, cymbal structure shows larger output power (around 100 microwatt), which is suitable to fabricate as bulk-peh for harvesting the mechanical energy at the high load environment; however, the loss of mechanical input energy and high resonant frequency should be modified. Fig. 11 Structure of cymbal PEHs proposed by Palosaari et al. [27]. a structure before assembly b structure after assembly c cross section with dimensions of μm
Adv Compos Hybrid Mater (2018) 1:478 505 485 Fig. 12 a Schematic diagram of Fringe radial slots. b Cone radial slots in the cymbal endcaps edge [28] 2.3 Stack structure Stack PEH can effectively improve the energy output, which is arranged by large numbers of thin piezoelectric materials together and stacked along the direction of the electric field. It may be a good choice for higher load applications; however, sometimes the complex stacking process in limiting space could be a barrier [35]. Figure 17 shows the fabricated process for stacks structure. Firstly, homogeneous slurry is prepared by ceramics powders, dispersant, binder, plasticizers, and solvent. Then, the ceramics film is obtained by tape casting technique. And, dried tapes are screen-printed with electrode. Later, the ceramics layers are stacked and sintered. Stack PEH always assembles electrically in series or parallel to effectively improve the output power. The piezoelectric stacks in series can generate higher open circuit voltage capable for igniting devices as lighter. Stacks in parallel possessed higher electric current, and power can be used for supplying electricity with energy harvesting [36]. Compared with the cantilever PEH with similar weight and size, the electrical power and power density generated by stacks are significantly higher in both resonance and off-resonance modes due to the d 33 vibration mode [37]. Because stack PEH only utilizes series or parallel mode to effectively harvest energy, resulting in its structure depended on different installation and mounting ways of power system in industrial environment. In large force vibration environment, such as in a heavy manufacture equipment or large operating machinery, the piezoelectric harvester operating in d 33 mode would be more durable and suitable for energy conversion because it is more robust and can afford much higher stress than that in d 31 mode [38]. Furthermore, the d 33 mode should have a higher coupling coefficient k than d 31 mode [39]. It means that more electrical power can be scavenged by d 33 mode than d 31 model under the same strain condition. Panda et al. [40] compared the energy harvesting devices between PZT multilayer stacks in parallel and bimorphs, and the devices are shown in Fig. 18. Output voltage obtained from multilayer stacks (125 mv) was lower than PZT bimorphs (450 mv) under 3 N force at a fixed frequency of 24 Hz, as it is reported that the overall capacitance of an n-layer stacks is n 2 times greater than that of a single-layer, while the open-circuit voltage of a singlelayer generator is n times greater than that of an n-layer [41]. Platt et al. [42] reported that a multiple PZT piezoelectric stacks (~ 145 layers) embedded inside a prototype implant was assembled electrically in parallel. It showed that the effective capacitance (1 10 μf) increased and peak open circuit voltage (30 V) decreased, which reached a more controllable output voltage level and lower matching electrical load. Since stack PEH with advantages of bearing high load, large piezoelectric constant in d 33 mode, it always desires to apply into the pavement or bridge to store mechanical energy. Lv et al. [43] investigated the effect of different thickness, diameter, and parallel connection on the mechanical and electrical behavior of PZT stacks embedded in asphalt pavement (shown in Fig. 19), when the thickness and diameter of piezoelectric disk are 1 and 8 mm with the output voltage of ~ 150 V, while the thickness and diameter of packing box of nylon are 10 and 42 mm with the voltage of ~ 90 V, indicating that the stack device has great potential for energy harvesting in asphalt pavement. Fig. 13 Schematic of a new cymbal structure. a Cross section view. b Top view [29]
486 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 14 Structure diagram of CANDLE [31] It is common that the wafer-stack configuration PEH has been proposed to convert electrical power from large force vibration. Jiang et al. [44] fabricated a novel PEH consisted of 36 piezoelectric wafers with diameter of 20 mm and total height of 34 mm, as shown in Fig. 20. The piezoelectric wafer-stack is assembled mechanically in series but electrically in parallel, which can generate up to 45 mw AC and 16 mw DC of electrical power under 3040-N amplitude at 2-Hz frequency. To further enhance the power effect and practical application for stack PEH, the stack PEH has been assembled with other structures (such as cantilever and cymbal) to form a novel device with different ways for power output. Ansari et al. [45] designed a fan-folded structure pacemakers (2 cm 0.5 cm 1 cm) consisted of several piezoelectric cantilever beams stacked on top of each PZT multilayer stacks, as shown in Fig. 21. It can generate more than 10-μW power,higher than the required power of 1 μw forapacemaker. Figure 22 [46] showed a flex-compressive piezoelectric energy harvesting cell (F-C PEHC) with a large load capacity and an adjustable force transmission coefficient assembled into replaceable individual components. Maximum power output of 17.8 mw in enhanced mode at 120-kΩ resistive load was obtained, which was the modifying structure of cymbal PEH. Fig. 15 Acceleration dependence of the nonlinear output voltage and power of the CANDLE [32] Table 3 summarizes the output voltage and power for stacks PEH. Output voltage ranges from 30 to 175 V and output power ranges from 45 to 125 mw can be achieved. Stack PEH with electrodes integration can afford high load with increasing d 33 and k p compared with d 31 mode. However, there is still existed some impediment such as complexity of stacks construction and electrode assembly to overcome. Thus, learning from other structures strong points to offset its own weakness of PEH is a best way to improve the energy power. 2.4 Micro-nano-structure With the development of micro-electronic and integrated circuit technology, the miniaturization of PEH is becoming a main trend, leading to the emergence technology of the micro-piezoelectric energy harvesting (MPEH) and nanopiezoelectric energy harvesting (NPEH). 2.4.1 Micro-piezoelectric energy harvesting MPEH is a device combined with the MEMS technology, and this micro-processing technology is integrated with microsensors, micro-actuators, micro-electronic processing, and control unit structure in the micron scale (1 μm 1 mm). With the requirement of extremely low power electrical and MEMS devices, MPEH becomes more and more attractive. MPEH realizes the objection of minimization, which is always suitable for applying at high frequency corresponding to the high natural frequency. However, it is difficult to convert vibration energy into electricity in low-frequency environment. Therefore, among various structures combined with MEMS, cantilever MPEH has attracted much attention due to its lower structural stiff, simple structure, easy to process and generate strain [47]. Figure 23 shows two vibration modes on MPEH with d 31 in TBEs and d 33 with IDE. In 2003, Sood et al. [48] successfully fabricated a MPEH (smaller than 300 300 μm) in d 33 mode to store the electricity converted from high frequency sound energy, to provide electrical energy for minimize wireless sensor. The first layer of membrane (SiO 2 and/or SiN x ) can control stress and bow
Adv Compos Hybrid Mater (2018) 1:478 505 487 Fig. 16 Scheme of the elements acting for energy harvesting by piezoelectric cymbals [34] the cantilever structure; the second layer of diffusion barrier/ buffer ZrO 2 prevented the electrical charge diffusion from the piezoelectric layer; the remaining upgrade layer is PZT piezoelectric materials, top interdigitate Pt/Ti electrode, respectively, as shown in Fig. 24. The result showed that more than 1 μw power at 2.36 V DC was achieved. A highly Q m ~ 800 of PZT thick film microcantilever was designed by Zhao et al. [49] using bulk silicon micromachining technology, which is potentially used for the mass detection and dynamic scanning force microscopy application. 2.4.2 Nano-piezoelectric energy harvesting As it is known, piezoelectric nanowires can be stimulated by tiny physical movement and disturbance at the low frequency. The concept of nano-piezoelectric generator first addressed by Wang [50] and expressed as the PEH in nano-structure often referred as nanogenerator (NG), which is the smallest power generation facility in the world. Figure 25 shows the structure of NG. The device is consisted of piezoelectric zinc oxide nanowire (NW) arrays and converted the nanoscale mechanical energy to electric energy. The charge formation in semiconductor NW is derived from the bending NW by contacting atomic force tip as Schottky barrier, and then the output energy is transferred by conductive atomic force microscopy (AFM). Since then, many NG for selfpowered nanotechnology have sprung up [50]. In order to solve the problem of expensive equipment and complex operational use of AFM system, NG integrating a Pt coated serrated electrode with vertically aligned ZnO NWs to convert ultrasonic waves into electricity, as shown in Fig. 26 [51]. The Pt coating NG enhanced the electrode conductivity and meanwhile created Schottky contact at the interface with ZnO, which needed much more stress to deform because of thickness of 1 3 mm. Maximum output energy was induced both with special substrate and an array of aligned ZnO NWs. As mentioned above, bulk-peh mainly designed as cymbal or stacks is used under high stress and large power environment, such as road PEH, bridge PEH, etc. Micro-PEH is always designed as cantilever structure due to its low frequency for vibration energy harvesting, while nano-peh can be used for Table 2 Properties of typical cymbal PEH Structure Diameter (mm) PZT thickness (mm) Force (N) Load Resistor (Ω) Power (mw) Year Cymbal 29 1 7.8 Low 400 39 2004 [23] 29 1.8 70 High 400 52 2005 [24] 29 1 70 High 200 100 2006 [25] 32 2 25,000 High 1.2 2010 [26] 35 24.8 Low 3.6 M 0.66 2012 [27] Slotted-cymbal 35 2 30 500 16 2009 [28] Cantilever-cymbal Low 251 k 3.7 2012 [31] Unimorph-cymbal 25.4 0.19 1940 High 3.3 M 121.2 2013 [30]
488 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 17 Manufacture process of multilayer ceramics stacks scavenging tiny and irregular vibration energy such as human motion and muscle contraction, which is potential for medical science with better biology compatible. However, there still existed many problems that should be overcome, such as naturally frequency, low efficiency, etc. Especially, despite both of micro-/nano-peh that realize the trends of miniaturization, however, there are still some new challenges brought. For micro- PEH, high working frequency and relatively low output power from piezoelectric film materials leads to the researching trend on the decrease of intrinsic frequency, integration of intermediate electrode, and performance of piezoelectric film to enhance the vibration energy harvesting in environment. For nano-peh, it shows the disadvantages of high cost, complicated processing technology, poor stability of quantity production, and difficult to control assembly that hinders the practical application. Additionally, its limited output power introduces a new challenge to fabricate NG groups. Although there are many challenges before the practical application of NG, it is a valuable research area which is potential in biomedical, wireless communication and sensing fields. 3 Theoretical model for PEH Performance prediction and structural design for PEH can be significantly guided by its corresponding theoretical mode which gives a perfect way to keep gaining insight into operating mechanism of PEH structure. It is important to establish the mathematical model for researching and fabricating PEH. 3.1 Vibration mode of PEH When the external electric field acts on piezoelectric materials, there mainly existes two vibration mode for PEH, including d 31 and d 33 mode,asseenininfig.27.first index is the polarization direction. Another is stress direction [52]. The d 31 mode in Fig. 27a means the polarization direction perpendicular to stress direction, such as the cantilever structure, and the polarization direction is along the thickness, while the strain direction is the long axis direction of the cantilever beam. The fabrication of d 31 modes of PEH is including top and bottom electrodes (TBEs), whose structure and type can be modified to meet the requirement of the microstructure design for enhancing performance. The d 33 mode in Fig. 27b means the polarization direction parallel to stress direction, such as multilayer stack structure, as the compression of piezoelectric stacks causing the polarization on the up-down surface [53]. As the strain distribution in cantilever structure distributes unevenly, leading to the current congestion effect because different electro density exists at a different location, however, piezoelectric properties of d 33 mode PEH are much higher than that operating in d 31 mode, which results in the enhancement of the electromechanical coupling coefficient [54, 55]. In addition, the d 33 mode PEH also can fabricate an interdigital electrode (IDE) which has higher output voltage than d 31 mode because of the lower capacitance [56]. Thus, researchers can utilize the different piezoelectric modes to design PEH by interdigital electrode (IDE). Hagood and Wilkie et al. [57, 58]fabricatedthed 33 mode PEH with IDE, and higher voltage can be generated from Fig. 18 PZT multilayer stacks a, PZTbimorphsb [40]
Adv Compos Hybrid Mater (2018) 1:478 505 489 Fig. 19 FEA model of PEH embedded in asphalt pavement. a PZT stack transducer embedded in asphalt pavement. b Packing stack transducer without cover. c PZT disk stacks [43] d 33 mode since the finger spacing of IDE is generally wider than the distance between electrodes in TBE, as shown in Fig. 28. However, the disadvantage for PEH with IDE is that the only superficial piezoelectric materials can be polarized by the bending polarization field, resulting in the IDE of PEH that is always applied for piezoelectric film materials. Kim et al. [58] investigated the output power of d 31 and d 33 mode in the same dimensions in a cantilever structure PEHs. The result showed that the output power of d 33 mode can be larger than that of d 31 mode PEH when the finger width is decreased to 2 μm and finger spacing is between 8 and 20 μm, which is strongly related to the dimensions of IDE. Thus, it can be concluded that the piezoelectric properties are strongly related to the vibration mode of PEH. To modify the vibration mode by various electrodes, structure applied in a practical situation is also of importance. 3.2 Theoretical mode of bimorph cantilever Theoretical mode for cantilever PEH is based on the first piezoelectric equation. It is well-known that the output power is changed with resistor load. The maximum output power can be obtained when the resistor is equal to the optimal resistor load. Both of P MAX and R OPT are important in studying the output electrical energy for PEH. Here, the typical equations for different structures of PEH will be given. Figure 29 is the photograph of bimorph cantilever. For the serial electrodes in Fig. 29a, it consisted of two rectangle piezoelectric materials along the middle of metal substrate with one end fixed. And, the electrode was coated on the top and bottom surfaces. The free end of the cantilever beam was motivated under the external mechanical force (F) leading to Fig. 20 Longitudinal mode of the piezoelectric wafer-stack [44] Fig. 21 3D model of a five beam fan-folded structure [45]
490 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 22 Three-dimensional configuration of the flex-compressive mode PEH [46] the deformation of the beam and then outputs electric energy. The maximum output power P MAX and optimal resistor R OPT can be expressed as follows [51, 59]: P MAX ¼ 9wt p þ t 2 s d31 H 2 ωt p qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2l ð1þ 32 s E 3 11 ε 2 33 þ ε T 2 33 2t p R OPT ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi lwω ε 2 33 þ ε T 2 33 ε 33 ¼ ε T 33 1 k2 31 ð2þ ð3þ where ω is the frequency form external force, l and w are the length and width of the beam, t p is the thickness of piezoelectric element, t s is the thickness of cantilever metal substrate, s is the strain, ε is the dielectric constant and d is the piezoelectric constant, H is the bending degree of cantilever, ε 33 is the one-dimensional dielectric constant, k is the electromechanical coupling coefficient, and T is the temperature. For parallel electrodes in Fig. 29b, the electrode is through the top and bottom surfaces connected together as one output pole, and the middle metal substrate as another pole. The maximum output power and optimal resistor are shown as follows [36, 59]: P MAX ¼ 9wt p þ t 2 s d31 H 2 ωt p qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2l ð4þ 32 s E 3 11 ε 2 33 þ ε T 2 33 t p R OPT ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2lwω ε 2 33 þ ε T 2 33 ð5þ According to Eqs. (1)and(4), the maximum output power for cantilever PEH is both related with the performance of piezoelectric materials and the structure of cantilever, which is proportional to the piezoelectric constant, thickness of piezoelectric materials and cantilever structure, working frequency, and bending degree of the beam under external force, inversely proportion to the length of beam, dielectric constant, and elastic compliance constant of piezoelectric materials. But, the effect of frequency on the output power has been overlooked because PEH is always used to harvest the vibration energy in the environment at relatively low frequency. Thus, to enhance output power, the materials should have favorable piezoelectric constant with relatively low dielectric constant and elastic compliance constant. Besides, the larger thickness as well as the longer beam of cantilever will generate larger strain resulting in the enhancement of output power. Nevertheless, the thickness of long beam will hinder the minimization of cantilever PEH. Thus, choosing a better piezoelectric material is the key point. 3.3 Theoretical mode of stack Figure 30 is the stack PEH with the electrodes that are in serial and parallel connections, and the external force is vertical to the stacks. When the electrodes are in series, the distance between negative and positive electrodes of stack PEH is nt c, and then the maximum output power and optimal resistor in series can be expressed as follows: P MAX ¼ d 33 2 F 2 ωnt p p 4 ffiffiffi 2 Aε33 nt p R OPT ¼ pffiffiffi 2 ωaε33 ð6þ ð7þ where ω is the frequency form external force, A and t p are the area and thickness of piezoelectric element, n is the number of piezoelectric layer, R is the resistor load, F is applied force and F = F MAX sin (ωt), s is the strain, ε is the dielectric constant, and d is the piezoelectric constant of piezoelectric element, respectively [36]. Table 3 Properties of typical stack PEHs Structure Diameter (mm) Size (mm) Output voltage (V) Output power (mw) Year 145 PZT wafers stacks 10 10 10 18 30 2005 [42] 36 PZT wafer stacks 20 20 20 34 70 45 2014 [44] PZT disk stacks 8 8 8 1 2015 [43] PZT stacks 12 10 10 175 125 2015 [40] Cymbal stacks 17.8 2016 [46] Five beam fan-folded structure mode stacks 20 5 10 10 2016 [45]
Adv Compos Hybrid Mater (2018) 1:478 505 491 Fig. 23 d 31 and d 33 modes of micro-piezoelectric energy harvesting [47] For parallel electrodes of stack PEH, the external force is vertical to the stacks. The polarization direction is opposite with the adjacent piezoelectric elements. The distance between negative and positive electrodes of stack PEH is t p, and the total electrode area is nawith the electrodes in parallel. The maximum output power and optimal resistor in parallel can be expressed as follows: P MAX ¼ n2 d 2 33 F 2 ωt p p 4 ffiffi 2 Aε33 t p R OPT ¼ pffiffiffi 2 naωε33 ð8þ ð9þ According to Eqs. (6)and(8), the maximum output power for series and parallel stack PEH is proportional to the thickness of piezoelectric element, especially the d 33 square and external force, inversely proportion to the dielectric constant and area of piezoelectric element. Besides, the structure with larger thickness and big area can obtain high output power, but it is unfavorable for the minimization tendency. Thus, to obtain larger output power, choosing materials with excellent piezoelectric properties may be better under the vibration environment. It is noticed that the greater output power for parallel electrodes of stack PEH can be acquired due to the high capacitance. 3.4 Theoretical mode of cymbal The effective piezoelectric constant (d eff 33 ) for cymbal PEH is shown as following equation due to the special mechanism of metal endcaps: d eff 33 ¼ d 33 Kd 31 ð10þ K is the amplification coefficient ranges from 10 to 100 depending on the design of the caps, which indicates that the piezoelectric cymbal has a very high transduction rate and larger displacement, expressed as the following equation [23, 60]: K ¼ r bðr b r t Þ ð11þ 2t h t p þ 2t m Thus, the open-circuit voltage and output power can be calculated using the following equations [61]: V OP ¼ Q C ¼ deff 33 Ft p ε 33 ε 0 A ð12þ P ¼ 1 2 def f 33 g ef f F 2 33 A ð13þ where d is the piezoelectric constant, g is the piezoelectric voltage constant, and t p, t m,andt h are the thickness of piezoelectric element, metal endcap, and height of the cavity, respectively. r b and r t are the radii of the bottom and top part of the endcap cavity, A is the area of piezoelectric element, F is the applied force, and Q is the charge. According to Eq. (13), the larger maximum output power for cymbal PEH is proportional to the piezoelectric transduction coefficient (d g) and external force and inversely proportional to A. Both of materials and structure will influence the output power greatly. Consequently, the structure of PEH and piezoelectric performance of materials are two important parameters to influence the output power for energy harvesting devices. Based on the theoretical optimized mode of PEH device, deep learning the working mechanism of PEH can better understand the mechanical-electrical coupling and energy transferring processing. And then, it is possible to reduce the gap between the working frequency and inherit frequency to fabricate a new PEH device which could achieve the maximum output power at the resonant frequency in the vibration environment. Fig. 24 Structure of micropiezoelectric energy harvesting in d 33 [48]
492 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 25 Schematic diagram of ZnO NG. a ZnO nanowires. b Deformed ZnO nanowire [50] 4 Piezoelectric materials used for energy harvesting Through piezoelectric effect, the PEH can convert the mechanical energy to electric energy. And, voltage and piezoelectric voltage constant generated can be shown as follows [62, 63]: V ¼ g ij F t=a g ij ¼ d ij =ɛ o ɛ r C ¼ ɛ o ɛ r A t P ¼ 1 2 CV2 ¼ 1 2 g ij d ij F A ð14þ ð15þ ð16þ ðatþ ð17þ where d ij is the piezoelectric constant, g ij is the piezoelectric voltage constant, F is the applied force, t p and A are the thickness and area of piezoelectric element, ε o is the dielectric constant in vacuum, ε r is the relative dielectric constant, and i and j are the polarization and stain direction of the piezoelectric materials. C and f are the capacitance of energy harvesting and frequency of vibration. Thus, the output power of energy harvesting is proportional to the Btransduction coefficient^ assessed by g ij d ij. For example, the d 33 mode for PEH: d 33 g 33 ¼ d 33 d 33 ε T ¼ d2 33 33 ε T 33 ð18þ Meanwhile, the energy efficiency (η) of PEH by converting mechanical energy into electrical energy can be evaluated using quality factor (Q m ) and electromechanical coupling factor (k p ) as shown in the following equation [61]: k 2 η ¼ 21 k 2 = k p d ffiffi ε 1 Q m þ! k2 21 k 2 ð19þ ð20þ AccordingtoEqs.(18) and(19), choosing the materials with larger d and lower ε can achieve highly output energy for PEH. However, researchers generally focus on searching high piezoelectric constant materials due to dielectric constant that is proportional to the change of ε according Eq. (21), resulting in the rare report on lower dielectric constant materials [64]. d ij 2Q ij ε r ε 0 P r ð21þ Fig. 26 Schematic diagram of direct-current NG driven by ultrasonic waves [51]
Adv Compos Hybrid Mater (2018) 1:478 505 493 Fig. 27 Vibration mode of piezoelectric energy harvesting [53]. a d 31 mode, b d 33 mode In short, piezoelectric material is the most important factor influencing electric properties for PEH. Fundamentally, the piezoelectric materials used for energy harvesting with higher power density and larger d 33 g 33 are desirable. 4.1 Traditional PZT-based piezoelectric ceramics Figure 31 shows the published papers number on leadbased PEH in the database of BWeb of Science^ using Blead piezoelectric energy harvesting^ as key words. It can be seen that PZT-based piezoelectric materials occupied the main parts in PEH applications because of high piezoelectric constant, large energy density, and energy conversion efficiency. However, most researchers focused on the PZT harvesting materials (87.22%) about the device structure simulation and circuit simulation, and they paid relatively less attention on the modification of PZTbased materials. Nevertheless, in recent years, the development of piezoelectric materials applied for PEH has been emerging gradually. 4.1.1 Introduction to PZT Fig. 28 Photograph of fabricated d 33 mode PEH [57] Binary solid solution PZT is the typical perovskite-type piezoelectric and ferroelectric materials, which is consisted of ferroelectric PbTiO 3 and antiferroelectric PbZrO 3. The multicomponent solid solution system and number of compositions for PZT are possibly influenced by the change of Zr/Ti ratio; composition with the Zr rich is rhombohedral crystal structure (R3c), and composition with Ti rich is tetragonal crystal structure (P4mm), as shown in Fig. 32 [65]. Intermediate compositions with coexisting rhombohedral and tetragonal phases are called morphotropic phase boundary (MPB) and represented by a line in the phase diagram. MPB gives a total of 14 possible polarization directions (six for tetragonal 001 and eight for rhombohedral 111 ), resulting in anomalously high piezoelectric properties [66]. Thus, various PZT-based piezoelectric ceramics have been recently introduced as potential materials for energy harvesting. However, to obtain high energy density for piezoelectric devices to meet the requirement of practical applications, researchers have doped kinds of elements, formed multiphase ceramics with PZTbased ceramics, or improved the preparation technology to obtain a larger transduction coefficient. 4.1.2 Oxides doping to PZT-based ceramics Doping modification is the most common method to improve electrical properties of ceramics. As the Fe 2 O 3 doped into PZT ceramics can effectively create defect dipoles and increase the net polarization [67], Yoon et al. [68] prepared the 0.99Pb(Zr 0.53 Ti 0.47 )O 3-0.01Bi(Y 1 x Fe x )O 3 (PZT-BYF) ceramic which had significantly increased the electrical properties with the maximum g 33 of 54.2 10 3 Vm/N and d 33 g 33 value of 20,167 10 15 m 2 /N. Hou et al. [69] comparedthe effect of group VII metal oxides (Fe 2 O 3,Co 2 O 3, and NiO) with mixed valence of + 2 and + 3 in Pb(Zn 1/3 Nb 2/3 )O 3 - Pb(Zr 0.50 Ti 0.50 )O 3 (PZN-PZT) system to identify the substitution mechanism and piezoelectric properties. Adding these oxides to ceramics resulted in increased grain size and decreased electrical resistivity with the d 33 g 33 of 13,120 10 15 m 2 /N in 0.3 mol% Co 2 O 3 -doped PZN-PZT ceramics [70]. Later, they selected the alkaline-earth metal ion of Sr 2+ which has the same valence as Pb 2+ to optimize the electrical properties of PZN-PZT ceramics. Ferroelectric and piezoelectric properties were deteriorated as the Sr 2+ increases, which was ascribed to the synergy between the grain size effect and
494 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 29 Photograph of electrodes connection for bimorph cantilever. a serial connection b parallel connection (a) V (b) V F F Piezoelectric layer Substrate layer dilution of Pb-O covalency. Optimal electrical properties were achieved at the grain size of ~ 1.79 μm, with the high d 33 of 465 pc/n and d 33 g 33 of 11,047 10 15 m 2 /N [71]. 4.1.3 Compounds additive to PZT-based ceramics Meanwhile, the addition of relaxor-type ferroelectrics to PZT is another beneficial way to improve its electrical properties. Yoo et al. [72, 73] added the anti-ferroelectric material Pb(Mg 1/2 W 1/2 )O 3 (PMW) with orthorhombic phase into Pb(Ni 1/3 Nb 2/3 )O 3 -Pb(Zr,Ti)O 3 (PNN-PZT) ceramics and fabricated a cantilever-type PEH with the dimension of 80 12 1.5 mm 3 ; the excellent d 33 g 33 of 13,416 10 15 m 2 /N and maximum power of 0.853 μw were obtained. The maximum output voltage of this device was AC 2.19 V, and the output voltage of boost DC-DC converter exhibited DC 5.01 V, which indicated sufficient value for lightening LED. Later, Pb(Zn 1/2 W 1/2 )O 3 -Pb(Mn 1/3 Nb 2/3 )O 3 -Pb(Zr 0.48 Ti 0.52 )]O 3 ceramics (PZW-PMN-PZT) with high-quality factor (1254) and density of 7.905 g/cm 3 were obtained when sintered at 930 C, and its d 33 g 33 was 8880 10 15 m 2 /N [74]. However, Curie temperature of PZT-based ceramics is significantly deteriorated by adding the relaxor ferroelectric Piezoelectric element Electrode -------- F + + + + + + + + a Series materials, which limited its practical applications [75]. BiYO 3 with high T c of 373 C that acted as co-dopant was reported by Yoo et al., and the 0.01BiYO 3-0.99Pb(Zr 0.53 Ti 0.47 )O 3 (BY-PZT) ceramics showed an excellent d 33 g 33 of 18,549 10 15 m 2 /N [76]. Later, a higher T c of 450 C from BiScO 3 -PbTiO 3 (BS-PT) piezoelectric ceramics was obtained with relatively low d 33 g 33 of 13,230 10 15 m 2 /N, which was potential materials for application in high temperature vibration energy harvesting [77]. Recently, Pb(Zn 1/3 Nb 2/3 )O 3 -Pb(Ni 1/3 Nb 2/3 )O 3 -Pb(Zr 1/2 Ti 1/ 2)O 3 (PZN-PNN-PZT) system attracts much attention as their excellent piezoelectric properties at the MPB region, which possesses large piezoelectric coefficient over 500 pc/n. Yuan et al. [62] studied the phase structure and piezoelectric properties of PZN-PNN-PZT system, and the results showed that the higher d 33 g 33 of 21,026.3 10 15 m 2 /N was obtained with the composition located on the rhombohedral phase side near MPB. Because the ε T 33 =ε 0 value showed a faster decreasing rate on the rhombohedral side when the composition was away from MPB, it is the most potential composition to obtain high-density energy for PEH. And, Nahm et al. [78, 79] also studied PZN-PNN-PZT ceramics on the pseudo-cubic side of the MPB, and it had a Piezoelectric element Fig. 30 Diagram of stacks as the electrodes are serial (a) and parallel (b) connections [36] + + - + - + - + - + - F b Parallel + + - + - + - + - + - Electrode
Adv Compos Hybrid Mater (2018) 1:478 505 495 0.98% 0.74%0.25% 0.25% 2.46% 0.74% 7.37% 87.22% relatively low ε T 33 value, meanwhile with a high piezoelectric constant. The high output power densities ranging from 160 mw/cm 3 to 231 mw/cm 3 and larger d 33 g 33 of 21,500 10 15 m 2 /N were obtained. 4.1.4 Texturing to PZT-based ceramics As mentioned above, it can be concluded that the high energy conversion is always achieved from PZT-based piezoelectric ceramics applied for PEH. PZT relaxorbased ferroelectric single crystal found in 1997 had a larger piezoelectric constant (d 33 > 2500 pc/n), five times higher than PZT-based ceramics, and the electromechanical coupling factor was more than 90%, motivating the developing of piezoelectric single crystal materials used for various applications [64]. However, there is little investigation on PZT-based single crystal comparing with the commonly used ceramic for energy harvesters as the PZT solid solution presents congruent melting point only Fig. 32 Phase diagram of PZT [65] PZT harvesting Modified PZT PZT/polymer PMN-PT PMN-PZT PZT-PZN PMW-PNN-PZT Others Fig. 31 Published papers of PEHs using lead-based piezoelectric materials (2000 2016) for the end-component of PT phase and near the MPB melting is incongruent leading to the formation of PZT. Thus, the preparation process of PZT single crystal with large size is much more difficult due to its complex manufacturing processes with higher cost, which limited its industrial production and application. Texture ceramics with grain oriented growth has achieved much attention, as its extraordinarily high piezoelectric response can reach in 60 80% of the same component single crystal; the results indicate that texturing is a meaningful method to improve the ferroelectric and piezoelectric performance. KSr 2 Nb 5 O 15 [80], Bi 4 Ti 3 O 12 [81], and Sr 3 Ti 2 O 7 [82] textured ceramics have been successfully fabricated by reactive template grain growth (RTGG) method. Recently, Yan et al. [83] prepared a <001> textured Pb(Mg 1/3 Nb 2/3 )O 3 -PbZrO 3 -PbTiO 3 (PMN-PZT) piezoelectric ceramics (90% texture degree) with giant magnitude of d 33 g 33 (59,000 10 15 m 2 /N), which was three times larger than the value of piezoelectric ceramics with grain growth randomly. High d 33 g 33 coefficient is mainly due to the growth orientation of textured piezoelectric ceramic with suppressing ε from template BaTiO 3,which opens new area for choosing a lower ε to template materials to strengthen transduction coefficient. Recently, they successfully fabricated multilayer textured PMN-PZN piezoelectric ceramics benders with a high performance and low cost by combining the TGG and low temperature cofiring ceramics (LTCC) process. High output power and power density of tri-layer textured ceramics were found to be 903 μw and15.5mw/cm 3. This textured ceramics is a promising material for power sources and meets the requirement of sensor nodes in requisite package [84]. Table 4 summarizes the properties of typical PZT-based ceramics applied for PEH. For PZT-based systems, the d 33 israngingfrom350to1100pc/n,d 33 g 33 is ranging from 8880.3 10 15 m 2 /N to 59,180 10 15 m 2 /N, and k is ranging from 0.44 to 0.84. It should be noticed that the d 33 and d 33 g 33 of PZT-based texturing ceramics is two to three times and three to seven times larger than PZTbased polycrystalline ceramics, respectively. It can be concluded that excellent piezoelectric performance of PZTbased ceramics can be acquired by adjusting its composition and preparation technology, when PZT-based piezoelectric materials far away the tetragonal phase at MPB have relatively small influence on piezoelectric constant with low ε, while texturing technology can bring low ε anisotropy template to form orientation grain growth phenomenon for high d 33 g 33 value of piezoelectric materials. However, PZT-based materials are gradually substituted due to the toxicity lead element which seriously pollutes the environment and harms human health, resulting in the development of lead-free piezoelectric materials.
496 Adv Compos Hybrid Mater (2018) 1:478 505 Table 4 Properties of typical PZT-based ceramics for PEHs Composition k d 33 (pc/n) d 33 g 33 (m 2 /N) Year PZW-PMN-PZT ceramics 0.59 351 8880.3 10 15 2010 [74] PZNNT ceramics 0.66 600 21,500 10 15 2011 [79] Fe 3+ -, Co 3+ -, and Ni 2+ -doped 410 13,120 10 15 2013 [70] PZN-PZT ceramics BY-PZT ceramics 350 18,549 10 15 2013 [76] PZT-BYF ceramics 380 20,167 10 15 2014 [68] PMW-PNN-PZT ceramics 0.60 518 13,416 10 15 2015 [72] BS-PT ceramics 0.54 450 13,230 10 15 2016 [77] Sr-doped PZN-PZT ceramics 465 11,047 10 15 2016 [71] PMN-PZN textured ceramics 0.84 1100 59,180 10 15 2013 [83] 4.2 Lead-free piezoelectric ceramics Although PZT-based PEH has attracted much attention as for its excellent piezoelectric constant (250 600 pc/n) [85], large output voltage (10 30 V) [86], and high piezoelectric voltage constant (24.89 10 3 mv/n) [87], the toxicity lead element will pollute environment in the manufacturing and waste process, causing a great harm to human health and worsen living conditions. With the enhancement of environmental consciousness, lead-free piezoelectric ceramics has gradually caused extensive concern recently. However, there is little report on lead-free piezoelectric ceramics utilized for energy harvesting because its piezoelectric performance is still much lower than leadbased piezoelectric materials. A representative advantage for lead-free materials with high piezoelectric performance is the existence of MPB phase, as the larger piezoelectric coefficient can be obtained in ferroelectric materials by modulating the composition in the proximity of MPB, such as (K,Na)NbO 3 (KNN), (Bi 0.5 Na 0.5 )TiO 3 (BNT), and Ba(Zr 0.2 Ti 0.8 )O 3 -(Ba 0.7 Ca 0.3 )TiO 3 (BZT-BCT). Among these materials, KNN-based ceramics are one of the most promising candidates due to a suitable piezoelectric properties and higher Curie temperature. For KNN, special additives have to be doped to avoid the evaporation of K and Na [88]. BNT has a superior dynamic inverse piezoelectric constant of 214 930 pm/v and a strong remanent polarization (38 μc/cm 2 ). However, the coercive field of pure BNT is too high (approximately 7.3 kv/mm) to polarize [89]. And, the BZT-BCT ceramics has a high piezoelectric coefficient (620 pc/n) with low-temperature stability [90]. To compensate for its drawbacks, researchers have committed to continuous improvement of dielectric properties of these system by doping elements or fabricating composite ceramics, such as Bi 0.5 Na 0.5 TiO 3 -BaTiO 3 (BNT-BT) [87] and Na 0.47 K 0.47 Li 0.06 NbO 3 [91]. Because its performance is still out-performed compared with PZT-based materials, consequently, there is still a lot of work needed for replacing PZT-based ceramics. In 2015, Kch et al. [84] introduced the BaTiO 3 (BT) to BNT ceramics to fabricate a lead-free PEH. The optimal piezoelectric voltage constant of 47.03 10 3 Vm/N, output power of 18 nw, and output voltage of 8.95 V were obtained despite of its lower piezoelectric constant (164 pc/n). After that, they fabricated KNN-BNT lead-free piezoelectric materials with a high piezoelectric constant of 204 pc/n, the maximum output power density of 24.6 nw/cm 2, and voltage of 10.8 V which were obtained [92]. To further overcome the poling problems and improve the piezoelectric properties, Bi 0.5 K 0.5 TiO 3 with coercive field of 4 kv/mm doped into BNT ceramics generated the maximum output power density and d 33 g 33 of 37.49 nw/cm 2 and 2410 10 15 m 2 /N, respectively [93]. Kholkin et al. [94] dopedfe 2 O 3 into KNN ceramics with the enhanced Q m of 135 and d 33 g 33 of 2600 10 15 m 2 /N, and generated an output power density of 0.38 mw/cm 3 at a constant load resistance of 470 kω. Yoo et al. [95] prepared (Na 0.56 K 0.44 ) 0.96 Li 0.04 (Nb 0.90 Ta 0.10 ) 0.998 Zn 0.005 O 3 ceramics with the enlarged phase transition from O to T phase; the highest d 33 g 33 of 10,470 10 15 m 2 /N and d 33 of 261 pc/n were obtained due to the enhancement of temperature stability. Cheon et al. [96] studied the effect of excess (K,Na) 2 O alkali oxide on the phase structure of (K 0.44 Na 0.52 Li 0.04 )(Nb 0.86 Ta 0.1 Sb 0.04 )O 3 ceramics, and the tetragonal phase was changed to orthorhombic phase at lower sintering temperature (1020 C), with the d 33 g 33 value of 8720 10 15 m 2 /N as well as a relatively low dielectric constant (463). Hou et al. [97] found that the simple composition of (Na 0.5 K 0.5 ) 0.94 Li 0.06 NbO 3 ceramics doped with Mn 2+ can built room temperature orthorhombic-tetragonal (O-T) phase boundary with an optimal d 33 g 33 value as high as 9314 10 15 m 2 /N by a sol-gel method. Kim et al. [98] fabricateda Mn 2+ -doped KNN thin films applied in size-scaled MEMS with the maximum power and power density of 3.62 μw and 1800 μw/cm 3 due to the reduction of leakage current density. Yoo et al. [99] fabricated a 12 80 1.5 mm cantilever PEH with the optimum output power and d 33 g 33 of
Adv Compos Hybrid Mater (2018) 1:478 505 497 Table 5 Properties of lead-freebased piezoelectric materials applied in PEHs Composition d 33 (pc/n) d 33 g 33 (m 2 /N) Year CeO 2 -(Na 0.5 K 0.5 ) 0.97 (Nb 0.96 Sb 0.04 )O 3 159 8200 10 15 2014 [99] (Na 0.56 K 0.44 ) 0.96 Li 0.04 (Nb 0.90 Ta 0.10 ) 0.998 Zn 0.005 O 3 261 10,470 10 15 2014 [95] 0.97K 0.5 Na 0.5 NbO 3-0.03Bi 0.5 Na 0.5 TiO 3 204 8337 10 15 2015 [92] 0.8Bi 0.5 Na 0.5 TiO 3-0.2Bi 0.5 K 0.5 TiO 3 166 2410 10 15 2015 [93] Fe 2 O 3 -doped K 0.5 Na 0.5 NbO 3 100 2600 10 15 2015 [94] 0.94Bi 0.5 Na 0.5 TiO 3-0.06BaTiO 3 164 7713 10 15 2016 [86] (KNa) 2 O-(K 0.44 Na 0.52 Li 0.04 )(Nb 0.86 Ta 0.1 Sb 0.04 ) 3 189 8720 10 15 2016 [96] Mn-doped (Na 0.5 K 0.5 ) 0.94 Li 0.06 NbO 3 212 9314 10 15 2016 [97] 0.839 μw and 8200 10 15 m 2 /N, as well as high Q m of 588 by doping CeO 2 to nonstoichiometric (K 0.5 Na 0.5 ) 0.97 (Nb 0.96 Sb 0.04 )O 3 ceramics. Table 5 summarizes the properties of typical lead-freebased piezoelectric materials for PEH, which mainly focus on the KNN-based ceramics and BNT-based ceramics. For lead-free-based ceramics, the d 33 is ranging from 100 to 261 pc/n and d 33 g 33 is ranging from 2410 10 15 to 10,470 10 15 m 2 /N. Although the lead-free piezoelectric ceramics is environment friendly, the piezoelectric performance and the composition stability are relatively much lower than that of lead-based piezoelectric ceramics. Thus, it is necessary to enhance the piezoelectric performance in the future work. Figure 33 shows the material performance comparison between PZT-based and lead-free piezoelectric ceramics in PEH. It can be seen that the piezoelectric performance of PZT-based ceramics has significantly better performance than lead-free ceramics, among which the piezoelectric constant is larger than 400 pc/n, while the d 33 g 33 is larger than 10,000 10 15 m 2 /N. The d 33 and d 33 g 33 of lead-based piezoelectric ceramics is more than four times and five times, respectively, indicating that there is still a long way to improve the piezoelectric performance for lead-free materials. Besides, the d 33 g 33 of PMN-PZN texture ceramics is five times greater than that of PZT-based ceramics, demonstrating that the texturing is a good processing technology to improve the materials performance to meet its practical application. 4.3 Piezoelectric polymer As we all know, ceramics are too brittle to fabricate a flexible PEH device. Lee et al. [19] reported that under the cycle load at high frequency, the fatigue phenomenon of piezoelectric ceramics arising from the crack development becomes the main problem. Therefore, researchers are committed to searching for a flexible piezoelectric material to effectively avoid the brittleness of ceramics. Compared with piezoelectric ceramics, polymer has larger loading capability under high strain conditions, which can convert more mechanical energy into electrical energy without cracking. In 1969, Kawai [100] first reported that polyvinylidene fluoride (PVDF) had relatively larger piezoelectric effect after high temperature polarization, leading to a historical revolution for piezoelectric polymer. Although the mechanical coupling coefficient for polymer is lower than that of piezoelectric ceramics, it has attracted much attention due to its excellent toughness, excellent flexiblity, and high processing capacity, which is potential material applied in flexible PEH field. Fig. 33 d 33 and d 33 g 33 of piezoelectric ceramics for PEH application
498 Adv Compos Hybrid Mater (2018) 1:478 505 In 2004, Shenck et al. [101] fabricated a flexible piezoelectric sole to harness the walking energy which consisted of multi-laminar PVDF stave under the ball of the foot and a PZT dimorph under the heel in sneaker. The result showed that for the PVDF stave, the average power was 1.3 mw and 8.3 mw from heel, which was not as high as anticipated due to limited electromechanical conversion efficiency. As the parasitic effects generated from human motion will disrupt the user s gait or endurance, thus, it is desirable to construct a piezoelectric device to obtain electrical energy from differential forces between human and backpack during walking. Rome et al. [102] investigated the energy harvesting backpack that converted the mechanical energy from the vertical carried loads movement (20 to 38 kg) to electricity, which generated a maximum power of 7.37 W, higher than the previous shoe devices. To avoid the impair from user s dexterity and fatigue, a transparent PVDF power harvesting backpack was fabricated with no additional stress or load for soldier over that of a conventional backpack. The maximum electrical energy of 45.6 mw and d 31 of 30 pc/n was achieved from the differential forces between wearer and pack [103]. Since the excellent power density for PEH has a great relationship with d 33 g 33.Thus,furtherresearch is needed to enhance the piezoelectric properties for polymer with high piezoelectric constant. 4.4 Nano-piezoelectric materials Materials used for nano-peh can be classified into nonferroelectric materials and ferroelectric materials. Recently, to gradually achieve the application for NG devices, researchers pay much attention on the improvement of output voltage and output density to evaluate its performance. When the press applied on to the NG device, it will generate the electric charge according to Eq. (22). Q ¼ FSd 33 ð22þ where Q is the electric charge generated on the surface of piezoelectric materials, d 33 is the piezoelectric constant, F is the applied force, and S is the larger active area of nano-materials. And, output power (P) on the load resistance (R) of the NG under the periodically pressing can be calculated by Eq. (23) [104]: processing to various morphology. After the report of NG by ZnO nanowires from Wang s team in2006[49, 50], it opened a floodgate research on all kinds of ZnO materials to harvest small vibration energy, such as nanowires, nanorods, nanoparticles, micro/nano-belt, and film. ZnO nanowire is the most research material, as shown in Fig. 34. As the NWs can be utilized to fabricate a flexible, wearable, or implantable energy harvesting device because of high sensitivity to small, irregular mechanical disturbances [105, 106], meanwhile, this material can also be combined with polymer to desire a feasible NG, which is potential material in the application of implantable self-powered device in the medical area. Wang et al. [107]fabricatedanactiveorself-poweredsensor (total thickness 16 μm) to monitor sleeping behavior, brain activities, and spirit status of a person as well as any biologically associated skin deformation, as the high sensitivity of super-flexible nanowire ZnO NG enabled to measure a slight local deformation on one s eyelid caused by the motion of eye ball underneath, as shown in Fig. 35. Although various nano-zno with better biological compatibility have been utilized in NG, the low piezoelectric coefficient and electromechanical conversion efficiency are the major hindrances to increase the output voltage and current for energy harvesting devices, resulting in the development of ferroelectric perovskite-type NWs. 4.4.2 Ferroelectric materials It is well-known that piezoelectric ceramics show high piezoelectric properties; nevertheless, it is not suitable to be prepared as NG due to its brittleness. Thus, it is challenging to prepare PZT or BaTiO 3 nanowire. The shortcoming for piezoelectric nanowire fabricated by electrospinning method has the high calcining temperature and shrinkage, which may lead to the performance deterioration to badly attach to the flexible substrate [102, 108]. Qin et al. [109] developed a suspending sintering technique for electrospinning nanowires to fabricate a flexible, dense, and tough PZT textile with aligned parallel nanowires. The maximum output voltage (6 V) and current P ¼ 1 T U 2 ðþ t dt ð23þ R 4.4.1 Non-ferroelectric materials Non-ferroelectric materials applied for NG is mainly concentrated on nano-semiconductor ZnO materials, which have piezoelectric effects, excellent biological compatibility, and easy Fig. 34 Published paper about ZnO-based NG (2000 2016)
Adv Compos Hybrid Mater (2018) 1:478 505 499 Fig. 35 NG attached a right eyelid was driven by moving the eye ball from right (R), center (C), and to left (L) or from L, C, and R [107] (45 na) for NG can light a commercial LCD and power a UV sensor to detect UV light quantitatively. Wang et al. [110] utilized the first chemical epitaxial growth to synthesis PZT nanowire arrays with d 33 of 152 pc/n applied for NG. The peak output voltage of ~ 0.7 V, current density of 4 μa/cm 2, and average power density of 2.8 mw/cm 3 were obtained [52, 111]. The advantage for PZT nanowire single crystal is possibly transferred onto flexible substrates, highly elastic and resistant to fatigue, and largely enhanced energy conversion efficiencies and piezoelectric constants because of the absence of domain boundaries which can pin up the dipole moments [112 114]. A flexible semitransparent energy harvesting with highly open voltage and stable power density of 10 V and 0.27 μw/cm 2 was fabricated in visible and near-infrared light regions, composed with laterally aligned Pb(Zr 0.52 Ti 0.48 )O 3 single nanowires and planar interdigitated Pt/Ti electrodes to avoid the decrease of dielectric constant from conventional sandwich configuration [115]. Considering that lead-based piezoelectric materials limit their application in the future because of environmental protection, it is interesting to fabricate NG by using lead-free piezoelectric materials with high piezoelectric performance. Additionally, to improve the flexibility of NG, an anodized Ti-mesh substrate with better flexibility, transparency, and robust mechanical property has been applied to electrode materials. Kim and Hwang et al. [116] manufactured a bendable and mechanically robust NG that consisted of oriented tetragonal phase BaTiO 3 nanotube film on the Timesh substrate encapsulated in a polydimethylsiloxane (PDMS) elastomeric. The output voltage and current reached up to 10.6 V and 1.1 ma. method and applied it for high-performance flexible NG. It generated an output voltage, current, and power density of 3.25 V, 55 na, and 338 mw/cm 3, respectively, which was larger enough to light commercial LCD. Due to the complicated composition and symmetry structure of BZT-BCT, it is difficult to synthesize BZT- BCT NWs via the hydrothermal method. Recently, the H 2 Zr 0.1 Ti 0.9 O 3 NWs with high aspect ratio acted as templates to converse a high-yield BZT-BCT NWs, which was fabricated via two-step hydrothermal methods by Sodano et al. [119]. The inverse piezoelectric constant of 90 pm/v, open circuit voltage, and power density of 6.25 V and 2.25 μw/cm 3 were obtained from BZT-BCT/PDMS nanocomposite NWs. However, the flexible NG device made by single BZT-BCT nano-piezoelectric material has relatively low output performance, which limits their use for operating commercial electronic devices. Then, the BCZT nanoparticles and Ag nano-wires were chosen as filler materials with PDMS matrix to fabricate a flexible NG, which effectively generated an output voltage peak of ~ 15 V and a current signal of ~ 0.8 μa without timedependent degradation [120]. Wang et al. [121] fabricated a metal-insulator-metal structure NG composed of (K 0.48 Na 0.52 )(Nb 0.95 Sb 0.05 )O 3 -Bi 0.5 (Na 0.82 K 0.18 ) 0.5 ZrO 3 nanofibers/pdms nanocomposite with the ultrahigh inverse piezoelectric constant of 338 pm/v, and then the maximum output power of 0.5 μw and power density of 4.508 mw/cm 3 were obtained. Until now, the orientation effect of nanowires or nanofibers from ferroelectric materials on piezoelectric properties has 4.4.3 Nano-composite piezoelectric materials To avoid the relative brittleness of piezoelectric materials, a novel type of piezoelectric NG based on nanowirepolymer composite with relative simpler fabrication processing was developed. It is made by the piezoelectric nanostructure embedded in polymer possessing both of high piezoelectric properties and flexibility. Basically, this device consists of four layers including top and bottom electrodes, the flexible substrate, and nanowire-composite layer, as shown in Fig. 36 [117]. As BZT-BCT piezoelectric materials shows high piezoelectric constant, it has attracted much attention on NG area. Qin et al. [118] synthesized lead-free BZT-BCT NWs and BZT-BCT/PDMS nanocomposite by the electrospinning Fig. 36 Schematic of BT nanowire-polymer composite NG [117]
500 Adv Compos Hybrid Mater (2018) 1:478 505 Fig. 37 Schematic of fabrication procedure of the NG on BaTiO 3 nanofibers in three kinds of alignment modes (R random, H horizontally, and V vertically) [122] seldom been reported. Jeong et al. [122] manufactured a PDMS-based flexible nanocomposites including BaTiO 3 nanofibers with different alignment modes aligned vertically, horizontally, or randomly in the PDMS matrix, as shown in Fig. 37. The vertically BaTiO 3 nanofibers achieved the highest output power of 0.1841 μw, maximum voltage of 2.67 V, and current of 261.40 na, which was attributed the vertically connection of nanofibers between electrodes compliant to mechanical stress. Table 6 summarizes the properties of piezoelectric NW materials applied in NG, mainly prepared by vapor liquid solid process, chemical epitaxial growth, and electrospinning technology. Compared with the bulk PEH, the processing technology of piezoelectric NW materials applied for NG is relatively complex and high cost, accompanied with the low piezoelectric properties. Consequently, researchers always utilize the output energy and output voltage for judging its performance, and the output power is between mw and μw. Although the output power from NG is relatively low, it can effectively harvest the tiny vibration in the environment. Thus, it is worthy to enhance the output power for NG which is potentially applied for biomedical and wireless sensor networks. 5 Concluding remarks and future perspective Today, as the developing trends of miniaturization for electronic devices, PEH devices have gone through three stages from bulk-, micro-, to nano-peh. Besides, much effort has been focused on the enhancement of energy conversion efficiency, especially the output energy for PEH and future perspectives that are shown as follows, which is closely related with vibration mode, structures, and piezoelectric performance for piezoelectric materials: 1. Main structure for PEH includes cantilever, stack, and cymbal. Cantilever PEH with larger strain is more suitable to apply at low frequency environment for MEMS system. Cymbal PEH has excellent load-bearing with relatively high loss mechanical energy. Stack PEH can obtain Table 6 Properties of piezoelectric materials applied in NG Composition Synthesis method Output voltage (V) Output energy (mw/cm 3 ) Year ZnO NW Vapor liquid solid 0.8 10 3 10 10 6 2006 [50] process PZT NWs Chemical epitaxial 0.7 2.8 2010 [111] growth PZT NWs Electrospinning 6 200 2012 [109] BZT-BCT NWs/PDMS nanocomposite Electrospinning 3.25 338 2013 [118] BZT-BCT NWs/PDMS nanocomposite Hydrothermol 6.25 2.25 10 3 2016 [119] KNNS-BKZ nanofibers/pdms Electrospinning 10 4.508 2016 [121]
Adv Compos Hybrid Mater (2018) 1:478 505 501 largest output energy under high load and can be adjusted by different electrode connection modes with more complexity processing technology. Besides, some existence problems among them are high resonant frequency and low energy efficiency. Therefore, to exert respective superiority of each structure to meet practical application, new structure of PEHs should be designed and fabricated by utilizing these advantages. 2. According to electrode distribution and PEH structure, vibration mode for PEH mainly includes d 31 and d 33,especially much higher output energy that can be achieved in d 33 mode. In addition, larger output voltage from d 33 mode can also be designed by interdigital electrode (IDE), while d 31 mode is more useful in low-frequency environment. Therefore, researchers try to design a device both integrated with d 31 and d 33 modes to further improve the output energy, while theoretical model for PEH should also be developed to guide the devices design. 3. Piezoelectric materials applied in bulk-/micro-/nano-peh have been described in detail, including traditional PZTbased and lead-free piezoelectric ceramics, piezoelectric polymer, ZnO nanowires, ferroelectric nanowires, and nanocomposites, among which lead-based piezoelectric materials show the best piezoelectric coefficient. However, to realize the environment friendly in the future, lead-free piezoelectric materials become candidate materials. Meanwhile, texturing technology is an efficient method to promote the d 33 g 33 value of piezoelectric materials. Therefore, the fabrication of the textured lead-free materials maybe a good choice for PEH devices in the future. Besides, relative lower output power and highly processing cost became a new challenge for piezoelectric materials applied for NG, which should be paid more attention and needed to overcome. As mentioned above, both of high d 33 with low ε is beneficial for output energy for PEH. However, researchers generally focus on searching high piezoelectric constant materials. But, designing materials with high d 33 and low ε or adding lower dielectric constant template for textured piezoelectric materials is a potential way to enhance output energy applied for PEH to gradually realize the actual demand in the future. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51672219, 51702259), the International Cooperation Foundation of Shaanxi Province (2017KW- 025), the Basic Research Program of Shenzhen (No. JCYJ20170306155944271), the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China (No.137-QP- 2015), and the B111^ Project (No. B08040). Compliance with ethical standards Conflict of interest exists. The authors have declared that no conflict of interest References 1. Roundy S, Paul KW, Jan MR (2004) Energy scavenging for wireless sensor networks. Springer, Boston, pp 1 24 2. Bedekar V, Oliver J, Priya S (2010) Design and fabrication of bimorph transducer for optimal vibration energy harvesting. IEEE T Ultrason Ferr 57:1513 1523 3. Paradiso JA, Starner T (2005) Energy scavenging for mobile and wireless electronics. IEEE Pervas Comput 4:18 27 4. Starner T (1996) Human-powered wearable computing. IBM Syst J 35: 618 629 5. 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Zhu RJ, Jiang JY, Wang ZG, Cheng ZX, Kimura H (2016) High output power density nanogenerator based on lead-free 0.96(K 0.48 Na 0.52 )(Nb 0.95 Sb 0.05 )O 3-0.04Bi 0.5 (Na 0.82 K 0.18 ) 0.5 ZrO 3 piezoelectric nanofibers. Rsc Adv 6: 66,451 66,456. 122. Jing Y, Jeong YG (2016) High performance flexible piezoelectric nanogenerators based on BaTiO 3 nanofibers in different alignment modes. ACS Appl Mater Interfaces 8: 15700 15,709 Leilei Li wasborningansu, China, in 1987. She received her B.E. in Chemistry from Longdong University in 2010. After that, she received her M.S. in Materials Chemistry from Northwest University in 2013. She is currently pursuing her Ph.D. studying in School of Materials Science and Engineering of Northwestern Polytechnical University. Her research focuses on the piezoelectric materials and devices, particularly intended for the preparation and electrical properties of piezoelectric ceramics and the design of devices applied for piezoelectric energy harvesting (PEH). Jie Xu received degrees of B.E. and M.E. in Materials Science and Engineering from Northwesten Polytechnical University in 2007 and China Building Materials Academy in 2010. He received Ph.D. degree in Materials Science and Engineering from Tsinghua University in 2014. He did postdoctoral research in Tsinghua Universityfrom2014to2016 and was a visiting scholar at the University of Birmingham in UK from 2015 to 2016. He is now an Associate Professor at the School of Materials Science and Engineering in Northwestern Polytechnical University. His research focuses on synthesis of ceramic powders, colloidal processing of ceramics, and functionally porous ceramics.
Adv Compos Hybrid Mater (2018) 1:478 505 505 Junting Liu was born in Guilin, China, in 1996. He received the B.E. degrees in Material Science and Engineering from Northwestern Polytechnical University (NPU) in 2018. During undergraduate period, he participated a piezoelectric energy harvesting project and was mainly responsible for the circuit design and external excitation device for piezoelectric energy harvesting. At present, he is studying as a Master Degree Candidate in School of Materials Science and Engineering of NPU. His current research focuses on the preparation of textured piezoelectric ceramics applied for piezoelectric energy harvesting. Feng Gao received degrees of B.E., M.E., and Ph. D. in Materials Science and Engineering from Northwestern Polytechnical University (NPU) in Xi an from 1992 to 2002. Following postdoctoral work at Postdoctoral Moble Station of Aerospace Science and Technology in NPU from 2003 to 2005, he worked in Pennsylvania State University of America as visiting scholar. He is currently Professor in the School of Materials Science and Engineering of NPU and Visiting Professor in the School of Engineering and Materials Science of Queen Mary University of London. His main research interests are development of functional electric materials, particularly those intended for energy conversion materials such as piezoelectric ceramics, thermoelectric materials, and devices.