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Sensors and Materials, Vol. 29, No. 12 (2017) 1733–1747
MYU Tokyo
S & M 1466

Piezoelectric Thick Film Based on
Bonding Technologies for Energy Harvester
Gang Tang,1,2† Bufeng Bao,2† Zhiran Yi,2 Guimiao Li,2 Bin Yang,2* and Jingquan Liu2
1

Jiangxi Province Key Laboratory of Precision Drive & Control, Department of Mechanical and Electrical Engineering,
Nanchang Institute of Technology, Nanchang 330099, China
2
National Key Laboratory of Science and Technology on Micro/Nano Fabrication,
Department of Micro/Nano Electronics, Shanghai Jiao Tong University, Shanghai 200240, China(Received August 21, 2017; accepted November 19, 2017)

Keywords: piezoelectric thick film, bonding technologies, piezoelectric energy harvesters

With the development of low-power-consumption electronic devices and systems, thedevelopment of power supplies will be a major challenge. High-performance piezoelectric energy
harvesters (PEHs) are an alternative that convert mechanical deformation into electric charge.
For PEHs, it is highly desirable to utilize materials with a high piezoelectric charge coefficient.
Piezoelectric thick films based on bonding and thinning technologies meet this requirement due totheir high-temperature sintering process. In this paper we review the development of piezoelectric
thick film fabrication processes, including the direct bonding process and the transferring process.
Additionally, PEHs in substrates for different applications are described. Moreover, we summarize
and compare the output performance of thick-film-based PEHs from all recently published papers
in terms of normalized power density and resonant frequency.

1.

Introduction

With the rapid development of low-power electronics and systems, the development of power
supplies for these devices will be a major challenge. However, micro-energy harvesters are
an alternative to solve this problem. They convert external mechanical energy into electricalenergy by different conversion mechanisms, such as electrostatic,(1–3) electromagnetic,(4–6)
triboelectric,(7–9) thermoelectric,(10–12) and piezoelectric(13–16) mechanisms. Because piezoelectric
harvesters have a simple structure, a high electromechanical coupling coefficient, no requirement
for an external voltage source, and a long lifetime, they have been used in volume-limited wirelesssensor nodes.(17) The piezoelectric coefficients of piezoelectric materials affect the output
performance of generators. Various piezoelectric materials have been deployed widely in microelectromechanical system (MEMS) energy harvesters,(18–22) such as AlN thin films, piezoelectric
ceramic lead zirconate titanate (PZT) thin films, and ZnO and BiTO3 nanowires. PZT is applied
as a functional piezoelectric material in energy harvesters due to its high piezoelectric constant.
Many microfabrication processes for PZT thin or thick films for harvesters have been reported.(13,14,23)
The common deposition processes for PZT and AlN using sol-gel and sputtering methods limit
the thin-film thickness to within 1–2 μm.   Liu et al. demonstrated a MEMS microgenerator
with a 3 μm PZT film deposited by the sol–gel process, which generated a power of 19.4–51.3*

Corresponding author: e-mail: binyang@sjtu.edu.cn
These authors contributed equally to the work.
http://dx.doi.org/10.18494/SAM.2017.1727†

ISSN 0914-4935 © MYU K.K.

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Sensors and Materials, Vol. 29, No. 12 (2017)

nW from 30 to 47 Hz at 1.0g.(13)   A 1-μm-thick PZT film was deposited by spin coating and its
structure was analyzed by X-ray diffraction.(14)   A 5 μm PZT film for d31 and d33 mode MEMS
generators was prepared by an aerosol deposition process.(23) The fabricated device under the d31
mode generated a maximum output power of 2.765 μW at 255.9 Hz at an acceleration of 2.5g. In
addition, a hydrothermal method(24) and a screen printing process(25) have been developed for thickfilms over 5 μm in thickness.  Lei et al.(26) combined the thick-film screen printing technique anda micromachining process using a silicon on insulator (SOI) wafer to fabricate a microcantileverwith a unimorph PZT thick film.   The fabricated harvester device produces 14.0 μW with an
optimal resistive load of 100 kΩ from 1g input acceleration at its resonant frequency of 235
Hz.  A sintering temperature above 550 ℃ can provide high piezoelectricity,(27) which limits theapplication of the subsequent MEMS fabrication process. However, bulk PZT ceramics provide
higher electromechanical coupling and harvesting efficiency than deposited piezoelectric thinfilms. Recently, piezoelectric energy harvesters (PEHs) based on PZT thick films have developed
quickly.
In this paper, we review the development of PZT thick-film energy harvesters, including
bonding technologies and structural design. Finally, we compare the output performance of all
published PZT thick-film harvesters, including the operating conditions, output voltage and power,
and normalized power density. This paper will provide some novel routes to improve the output
performance of energy harvesters for researchers or engineers in this area.

2.

Bonding Technology of PZT Thick Film

2.1

Au as the bonding layer for direct bonding

In recent years, a useful technique for preparing PZT thick films on silicon has been
investigated, namely the wafer bonding of bulk PZT and silicon using an intermediate layer andthen using a PZT thinning technique.(28–30) There is no doubt that high-performance bulk PZT
may significantly improve the performance of piezoelectric microgenerators. Tanaka et al.(28)prepared PZT thick films by adopting this bonding technology using Au as the intermediate
layer and the mechanical lapping method, as shown in Fig. 1(a). The bond strength of polarized
PZT/Au/Si wafers for two different poling processes was investigated: (i) the PZT ceramics werepolarized by applying 250 V DC (0.69 kV mm−1) throughout the entire bonding process [solid lines
in Fig. 1(b)]; or (ii) the ceramics were polarized by applying 750 V DC (2.1 kV mm−1) only duringcooling from 300 ℃ to room temperature (dashed line).  With the ﬁrst process, a strength of about
13 MPa was obtained for a wafer bonded at 550 ℃ for 1 h or 450 ℃ for 4 h.  However, the bonding
process needs special fabrication processing and bonding equipment, thus its cost is high. At thesame time, because the bonding temperature was higher than 500 ℃, some problems appear, suchas the repolarization issue and the larger residual stress caused by the thermal mismatch between
the PZT ceramics and silicon substrate.
2.2

Conductive epoxy as the bonding layer for direct bonding

In order to reduce the fabrication cost of the intermediate layer of Au, the use of epoxy resins as
an intermediate layer and a wet-etching process for thinning PZT were demonstrated.(29) Because
the epoxy resins used were nonconductive, it was very difficult to expose the bottom electrodes1735

Sensors and Materials, Vol. 29, No. 12 (2017)

(a)

(b)

(c)

(d)

(e)

(f)
Fig. 1. (Color online) (a) Bonding procedure using Au as bonding layer. (b) Bond strength of polarized PZT–
Au–Si wafers for two different poling processes: poling throughout the entire bonding process (solid lines) andpoling during the cooling stage, from 300 ℃ to room temperature (dashed line).(28) (c) Bonding process ofconductive epoxy.  (d) Realized bond strength dependence on the bond pressure at the curing temperature of 175 ℃.(30)(e) Transferring process of PMN–PT thick film.(32) (f) Schematic illustration of the SSCG and fabrication processof the flexible PMN–PZT energy harvester.(38)1736

Sensors and Materials, Vol. 29, No. 12 (2017)to produce an output signal using the micromachining process. Wang et al.(27) developed theintegration of bulk PZT and silicon by Cytop, a patternable bond layer, and the thinning of bulk
PZT by the chemical mechanical polishing method.
Bonding processes must ensure the precision of the designed structure. Firstly, a lowtemperature bonding process is essential to reduce the detrimental effect of thermal mismatch
between the PZT ceramics and silicon, and to be compatible with subsequent processes. Secondly,
the thickness of the intermediate bonding layer should be less than several microns. Thirdly, thethickness uniformity of the intermediate bonding layer should be well controlled in order to reduce
the thickness of the bulk PZT and obtain uniformity. Fourthly, the intermediate bonding layer
should be conductive. Otherwise, the bottom electrodes will be very difficult to expose. Finally,
the bonding strength should be sufficiently high to obtain good reliability of the fabricated device
and remain stable under an operating temperature of approximately 150 ℃, which is required forlater MEMS processes. To satisfy these conditions, Tang et al.(30) proposed a conductive epoxy
organic adhesive material as an intermediate bonding layer to bond bulk PZT and silicon wafers
at temperatures as low as 175 ℃.   The conductive epoxy resins (DAD-91L, Shanghai ResearchInstitute of Synthetic Resins) have been used in bonding experiments.
Figure 1(c) shows the bonding process. Firstly, one or two layers of conductive epoxy resin
were screen printed on a silicon wafer. Then, the PZT and the silicon wafer were bonded together
under a specified pressure and heated at a suitable temperature gradient in a vacuum oven, which
was from 60 to 175 ℃ at 15 ℃ per 10 min, which was followed by holding for more than 1 h at 175 ℃.  
The temperature gradient and the vacuum oven were necessary to obtain a uniform epoxy layer
and remove air bubbles from the bonding layer. The pressure was removed when the temperaturereached room temperature after cooling. The etching rate of bulk PZT by the wet-etching method
is low and the thickness uniformity of bulk PZT is very difficult to control by a mechanicallapping machine when its thickness is lower than 30 μm.   Therefore, a hybrid method combing
the mechanical lapping and wet-etching processes is used to reduce the thickness of the bulk PZT.
Several-hundred-micron-thick PZT was thinned to about 30 μm using mechanical lapping.  Then, awet-etching process was used to reduce the thickness of the PZT ceramic to the required thickness
using 1BHF:2HCl:4NH4Cl:4H2O as the etchant.(29) Finally, an additional mechanical polishingprocess was applied on the PZT films to improve the surface roughness of the PZT for subsequent
fabrication processes.(29) Figure 1(d) shows the bonding strengths of the PZT–conductive epoxy–Si-bonded wafers as a function of applied pressure at the curing temperature of 175 ℃ during
bonding. It can be seen that the bonding strength was more than 10 MPa at an applied pressureof 0.1 MPa. The large bond pressure contributed to improving the bond strength. However,the bonded wafers are easily deformed or damaged under higher bond pressure. Therefore, theapplied pressure of 0.1 MPa during bonding is a relatively ideal value for use in this application.
In addition, the error bar in Fig. 1(d) shows multiple experimental results and good uniformity ofbonding strength.
2.3Transferring process of piezoelectric thick film

In order to realize a flexible energy harvester with a high output current, the stress-controlled
exfoliating process was proposed to transfer the lead magnesium niobate–lead titanate (PMN–PT)
thin film from a bulk substrate onto a flexible substrate without mechanical damage by utilizing
the inherent residual stress of a Ni film.(31) Figure 1(e) shows the transferring process of a PMN–

Sensors and Materials, Vol. 29, No. 12 (2017)1737

PT thick film.(32) The (001)-oriented PMN–PT crystal was polished to a mirror surface using a
diamond suspension. A Au bottom electrode deposited PMN–PT plate was completely bonded
on a (100) silicon wafer by adhesive epoxy and then thinned to a film thickness of 8.4 µm using
grinding and subsequent chemical mechanical polishing (CMP). This mechanical exfoliatingprocess was carried out to peel off the top piezoelectric portion of the PMN–PT metal-insulatormetal (MIM) layer (8.6 µm in thickness) from the silicon substrate using an electroplated tensile Ni
stressor. The directional stress mismatch between the top Ni film (intrinsic tensile stress) and theunderlying layers (against compressive stress) of the MIM/epoxy/wafer released the piezoelectricthin film from the mother substrate without mechanical damage such as cracking, delamination,
or wrinkling.(31) Moreover, the thickness of the exfoliated PMN–PT can be easily controlled bymodulating the residual stress of the Ni film.(33) The freestanding thin PMN–PT film on a thinplastic substrate was sufficiently flexible to conform to achieve conformal contact on a curved
subcutaneous layer and corrugated organs in the human body, thus making it possible to generateenergy in the human body by slight mechanical movements.(34)
Meanwhile, in order to enhance the conversion efficiency of energy harvesters, researchers havedemonstrated an inherently high-piezoelectric-coefficient perovskite thin film on a plastic substrateusing a soft-lithographic transfer technique, enabling high-quality thin-film materials on a flexible
substrate.(35–37) Figure 1(f) schematically illustrates the solid-state single crystal growth (SSCG)
of lead magnesium niobate–lead zirconate titanate (PMN–PZT) and flexible device fabrication.(38)The single-crystal PMN–PZT block was completely bonded on a glass substrate using an adhesive
wax with a melting temperature of 60 °C. The PMN–PZT bulk on glass was thinned to a film
thickness of 10 µm via a polishing process. The PMN–PZT thin film (area of 2 × 3 cm 2) on theglass substrate was placed on a polyethylene terephthalate (PET; thickness, 125 µm) receiver
substrate coated with ultraviolet (UV)-sensitive polyurethane (PU; Norland optical adhesive, No.
73) as an adhesive epoxy and then UV light was used to cure the PU adhesive. The sample washeated on a hot plate at the melting temperature of the adhesive wax to separate the entire area of
the PMN–PZT thin film from the glass. Park et al. demonstrated large-area PZT thin films on
flexible substrates via a laser lift-off (LLO) process and fabricated a thin-film nanogenerator (NG)to realize highly efficient, lightweight, and flexible piezoelectric energy harvesting devices.(39)3.

PZT Thick Film Based Energy Harvesters

3.1

Silicon-based energy harvesters

Aktakka et al.(40) fabricated a unimorph piezoelectric cantilever with a thinned-PZT layer of
20 μm thickness for vibration energy harvesting.   The bulk-PZT pieces were bonded to an SOI
wafer using an Au/In bonding layer of 5 μm thickness and were later thinned to about 20 μm.  Theunpackaged harvester with a tungsten proof mass produces a maximum power of 2.74 μW at 0.1g (167Hz) and 205 μW at 1.5g (154 Hz) at resonance.  
Recently, a novel process to fabricate low-frequency piezoelectric MEMS energy harvesters bybonding bulk PZT on an SOI structure was proposed by Tang et al.(30) The key techniques of the processinclude a low-temperature bonding technique using conductive epoxy resin, thinning of the bulk
PZT using a combined mechanical lapping and wet-etching method, and the micromachining ofbulk ceramics by dicing. As shown in Fig. 2(a), a unimorph PZT microcantilever array constructedwith Au/Cr/PZT/conductive epoxy/Cr/Au/SiO2/Si/SiO2 multilayer structure with a Ni proof mass

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Sensors and Materials, Vol. 29, No. 12 (2017)

(a)

(b)

(c)
Fig. 2. (Color online) SEM cross-section view and top view of the fabricated (a) 31-mode thick PZT
microcantilever,(30) (b) 33-mode thick PMN–PT microcantilever,(41) and (c) 31-mode PZT harvester with integrated
proof mass.(42)

was designed and fabricated. With an exicitation of 1.0g, the output voltage, power, and power
density of this device are 2.72 VP–P, 11.56 μW, and 28856.7 μW cm−3 at the resonant frequency of
514.1 Hz with a matched load resistance of 70 kΩ.  The output performance of this device is higher
than that of other reported MEMS devices using piezoelectric thin films. Therefore, this novel
technique has great potential to allow the fabrication of high-performance piezoelectric MEMS
energy harvesters. Using this fabrication technique, we have developed a high-performancepiezoelectric 33-mode MEMS energy harvester based on PMN–PT single-crystal thick film asshown in Fig. 2(b).(41) The PMN–PT piezoelectric thick film is used to improve the coupling
coefficient and electromechanical coefficient. Interdigitated electrodes (IDEs) contribute to theimproved energy conversion efficiency of the 33-mode harvester. The experimental results showSensors and Materials, Vol. 29, No. 12 (2017)

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that the fabricated microcantilever produced a maximum output voltage and power of 5.36 VP–P
and 7.182 μW, respectively, at the resonant frequency of 406 Hz.  The calculated power density of
17181.8 μW/cm3 is also very promising compared with other reported devices.
In the above-reported studies, the Ni proof mass was manually assembled at the tip of thecantilever, which resulted in the mismatch of the locations of the centers of the cantilever and proof
mass. As a result, the cantilever will twist under deformation. Therefore, the output performanceand conformance will be reduced. We proposed a high-performance vibration-based energy
harvester with an integrated silicon proof mass, as shown in Fig. 2(c), which was formed by the
deep reactive-ion etching (DRIE) process from the opposite side.(42) The maximum output voltage
and power of our device at the resonant frequency are 3.4 V and 20.2 μW, and 6.08 V and 57.6 μW
under input vibration accelerations of 0.5g and 1g, respectively. The power density of the harvester
under 0.5g and 1g is 24754.9 and 70588.2 μW/cm3, respectively. The energy conversion efficiencyof the harvester at 0.5g and 1g is 38.15% and 26.9%, respectively.
3.2

Flexible-metal-foil-based energy harvesters

Most of the above-reported PEHs based on silicon technology easily break down and only workat low vibration levels from ambient sources due to the brittle properties of silicon. Currently,
most reported high-performance PEHs operate at high frequencies. However, the frequencies
of environmental vibration sources are relatively low (normally less than 100 Hz).(43) Althoughfewer papers have reported low-frequency PEHs, they worked below 2.0g, and their output power
was very low. However, in the fields of aerospace and the military, the acceleration of a ballistic
missile or rocket during launch can reach from 4g to 15g. Meanwhile, in a domestic environment,
the acceleration of automobiles and hand tools can also reach 10.0g.(44–48) Phosphor bronze, analloy of copper, has good electrical conductivity and resistance to fatigue, so phosphor bronze
can replace silicon as the substrate of energy harvesters operating at low frequency and highvibration levels. Therefore, we have developed a piezoelectric microgenerator based on bulk PZTbonding on the phosphor bronze composite structure to convert vibration energy into electricity.(49)
Figure 3 illustrates the fabrication process of the microgenerator. The whole structure includes
phosphor bronze as the supporting layer, a bulk PZT thin film as the functional piezoelectric
layer, conductive epoxy as the low-temperature bonding layer, and a proof mass. The starting
wafer of 400-μm-thick silicon has a 2-μm silicon oxide layer that is used to bond with 100-μmthick phosphor bronze, as shown in Fig. 3(a). The phosphor bronze should be thinned to theappropriate thickness to reduce the resonant frequency and the top surface should be polished to
improve the bonding force with the bulk PZT [Fig. 3(b)].  Then, one side of the 400-μm-thick PZT
is polished and deposited with the 20 nm/180 nm Cr/Au bottom electrode layer. This side of thebulk PZT is bonded to the phosphor bronze with epoxy resin [Fig. 3(c)]. The PZT layer is thinned
down to the required thickness [Fig. 3(d)] by the combined mechanical lapping and wet-etching
method. Figures 3(e) and 3(f) illustrate the Cr/Au layer as the top electrode, which was sputteredand patterned by dry etching to form the top and bottom electrodes, respectively. Figure 3(g)shows that the handle Si layer was etched from underneath to release the cantilever with a silicon
proof mass by the DRIE process. Finally, Fig. 3(h) shows the rectangular tungsten proof mass cutby the dicing method is assembled at the end of the cantilever. Due to the elasticity and fatigue
resistance eﬀects of the phosphor bronze, the device can withstand a vibration acceleration of 7.0g

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Sensors and Materials, Vol. 29, No. 12 (2017)

(a)

(e)

(b)

(f)

(c)

(g)

(d)

(h)

Fig. 3. (Color online) Fabrication process of microgenerator. (a) Silicon oxide deposited on silicon wafer. (b)Bonding phosphor bronze and silicon wafer and thinning of the phosphor bronze by mechanical lapping. (c) Bulk
PZT with bottom electrode bonding with phosphor bronze. (d) Thinning of PZT by using the combined mechanicallapping and wet-etching method and polishing. (e) Top electrode deposition. (f) Top electrode patterning. (g)Underside etching by DRIE. (h) Tungsten mass assembled at the end of the cantilever.(49)and generate an output voltage of 61.2 V.  Figure 4(a) shows the output voltage at diﬀerent loading
resistances under diﬀerent accelerations from 0.5g to 3.0g.  The eﬀective volume of the fabricatedmicrogenerator is calculated as 0.03732 cm3, and as the acceleration was increased from 0.5g to 3.0g,
the power density increased 24.7 times, from 351.02 to 8664 μW∙cm−3, as shown in Fig. 4(b).
3.3Energy harvesters with flexible polymer substrate

A flexible and lightweight energy harvester on a single thin plastic substrate can scavenge
energy from extremely small movements into electric signals.(50,51) Hwang et al. reported a flexible
energy harvester based on a PMN–PT thick film on a plastic substrate that can significantly
increase output current.(32)  Figure 5(a) shows the flexible PMN–PT energy harvester being bent
by tweezers without notable damage, and the inset shows the device on a rounded glass vial witha curvature radius of 1.4 cm. Periodic bending/unbending of this harvester was performed by alinear motor with a strain of 0.36% at a strain rate of 2.3%∙s−1 (frequency of 0.3 Hz) to evaluate theoutput performance of this device. The output voltage and current are shown in Figs. 5(b)(i) and5(b)(ii), respectively.   The maximum voltage and rectified current were about 8 V and 100 μA,
respectively. Fifty parallel-connected green LEDs were lighted simultaneously with the generated
electric power. In addition, the same group proposed a large-area d33-mode PZT energy harvester
fabricated via a laser lift-off process, which is shown in Fig. 5 (c).(39)   Figure 5(d)(i) shows the
open-circuit voltage of 200 V and the short-circuit current of 1.5 μA at a strain of  ~0.386% witha strain rate of ~2.32 %s−1. To characterize the effective power outputs of the PZT harvester, thevoltage and current density signals were recorded as a function of loading resistance from 2 kΩ to
1 GΩ [Fig. 5(d)(ii)].  As a result, the instantaneous power density can be calculated by multiplying
the output voltage and current, reaching up to 17.5 mW·cm−2 at a resistance of 200 MΩ.

1741

Sensors and Materials, Vol. 29, No. 12 (2017)

(a)

(b)

Fig. 4. (Color online) (a) Open-circuit output voltage from 1g to 7g at resonant frequency. (b) The power densityfrom 0.5g to 3.0g at resonant frequency.(49)

(a)

(b)

(c)

(d)

Fig. 5. (Color online) (a) Photograph of flexible PMN–PT thin-film energy harvester on a PET substrate.   (b)Open-circuit voltage (i) and short-circuit current (ii) signals generated from the energy harvester during periodic
bending and unbending.(32)  (c) Final flexible PZT thin-film NG attached to a glass tube and bent by human fingers(inset) (d) (i) Open-circuit voltage and cross-sectional current density measured from PZT thin-film NG and (ii)
Measured output voltage and cross-sectional current density under different load resistances from 2 kΩ to 1 GΩ.(39)A Mn-doped PMN-PZT crystal has a high piezoelectric charge coefficient (d33 ≈ 1140
pC N−1)(52,53) and electromechanical coupling factor (k 33 ≈ 0.92), which are significantly greaterthan those of other PZT materials.(52) Kim et al. demonstrated a ﬂexible single-crystalline(1−x)Pb(Mg1/3Nb2/3)O3–(x)Pb(Zr,Ti)O3 with 0.5 mol% Mn doping energy harvester for scavenging

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Sensors and Materials, Vol. 29, No. 12 (2017)

in vivo biomechanical energy.(54)  This single-crystal harvester generated a short-circuit current of 1.75
μA and an open-circuit voltage of 17.8 V.  It was able to switch a light bulb on and off at a distanceof approximately 5 m without requiring any other power source.
3.4

Bimorph PEHs (B-PEHs)

Yeo et al. presented a high-efficiency bimorph PEH with a piezoelectric compliant mechanism
design via depositing PZT films on Ni foils based on RF magnetron sputtering.(55) We havefabricated a B-PEH based on bulk PZT thick films with a low resonant frequency over a wide rangeof acceleration, which was realized by utilizing low-temperature bonding and thinning technology
on flexible beryllium bronze substrates at a low cost.(56) Figure 6(a) illustrates the threedimensional structure and micromachining process of B-PEH. The structure includes beryllium
bronze as a supporting layer, upper and lower bulk PZT thick films as functional piezoelectric
layers, conductive epoxy as a low-temperature bonding layer, and a proof mass. The effective
volume of this device was 30.6 mm3. Polymethyl methacrylate was utilized for the package ofthis device, which is shown in Fig. 6(b). The impedance of the fabricated piezoelectric harvester
was measured using an impedance analyzer (Keysight 4990A). The results are shown in Fig. 6(c),and show the optimal resistive load to be 325 kΩ.  The upper and lower PZT layers were thinned
down to about 53 and 76 μm, respectively, and commercial beryllium bronze with a thickness ofapproximately 50 μm was used as the substrate, as shown in Fig. 6(d).  Figure 6(e) shows that the
maximum effective output powers generated from the upper, lower, and bimorph beams at 3.0g
were around 0.563, 0.396, and 0.741 mW at matched load resistances of around 200, 120, and 360
kΩ, respectively at 3.0g. The harvester can light up about twenty-one serial LEDs at resonanceunder an applied acceleration amplitude of 3.0g. The output power increased in a manner similar
to a quadratic curve, which followed the general model of a velocity-damped oscillator for theconversion of mechanical energy of the vibrating mass into electrical power.(57,58)
We have summarized the development of PEHs in recent years, as shown in Fig. 7. It can beobserved that the designed resonant frequency of harvesters has decreased to meet the natural
frequency of environmental vibration sources. Meanwhile, because the silicon substrate is brittle
and cannot work at high accelerations, the flexible copper foil has been selected as the bonding
substrate. It is clear that the bimorph energy harvester has improved the normalized power density.

4.

Performance Comparison of Piezoelectric Thick Film Energy Harvesters

A variety of PEHs based on PZT thick films have been developed and their affected factors aredifferent. In order to compare their output performance, the input conditions including substratematerials, volume, and vibration acceleration, and the output performance including the resonant
frequency, power density, and normalized power density are given in detail in Table 1. The earliest
developed energy harvester utilized a steel substrate and bulk PZT-5A bimorph in 2004.  However,the volume (about 494 mm3) was very large. To reduce the device volume, some micromachining
methods were proposed for the fabrication of energy harvesters, including the thinned bulk
PZT and micropatterning methods. Because silicon is easy to fabricate, some developed energy
harvesters are micromachined on silicon substrates.
However, the resonant frequency of silicon-based energy harvesters is very high, which resultsin high resonant frequencies larger than 150 Hz.   However, the frequencies of environmental

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Sensors and Materials, Vol. 29, No. 12 (2017)

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 6. (Color online) (a) Schematic illustration of the micromachining processes of the proposed B-PEH. (b)Photograph of the packaged device. (c) Impedance spectrum of the device measured under the free condition. (d)Cross-sectional SEM image of the bimorph PZT layer. (e) Output voltage (red) and power (green) as a functionof external load resistances for upper, lower, and bimorph PZT layers of the piezoelectric harvester. Label colors
correspond to the inner color. (f) Maximum output power versus acceleration amplitude.(56)1744

Sensors and Materials, Vol. 29, No. 12 (2017)(a)Fig. 7.

(b)(Color online) Development of energy harvesters in recent years.

Table 1
Output performance of recent PEHs based on piezoelectric thick films.
Authors

Piezoelectric Supporting Volume
materialmaterial(mm3)

Bulk PZT-5A
2004
Steel
494.472
bimorphRoundy et al.(44)2010
Bulk PZT
AuIn
12.1
Aktakka et al.(59)2011
Bulk PZT-5H
Brass
613
Reilly et al.(60)2011
ThinnedSilicon27
Aktakka et al.(40)bulk PZT
Thinned2012
Silicon0.4
bulk PZT
Tang et al.(30)
Thinned2013
Silicon0.407
bulk PMN–PT
Tang et al.(41)
Thinned2014
Silicon47.82
bulk PZT
Janphuang et al.(61)Thinned2014
Silicon
0.816
bulk PZTYang et al.(42)Thinned2014
Silicon1.24
bulk PMN–PT
Yang et al.(62)Thinned
Phosphor
2016
37.05
bulk PZT
bronzeTang et al.(49)
Thinned
Beryllium
2017
30.6
bulk PZT
bronzeYi et al.(56)

Power
(µW)

Acceleration Frequency Power density(g)(Hz)(µW·mm−3)

Normalized
power density(µW·mm−3g−2Hz−1)

375

0.255

120

0.758385

0.097191

10.2

2.0

252

0.842975

0.000836

4000

0.25

100

6.525285

1.044046

205

1.5

154

7.592593

0.021912

11.56

1.0

514.1

28.9

0.056215

7.182

1.5

406

17.64619

0.019317

82.4

1.0

96

1.723128

0.017949

57.6

1.0

523

70.58824

0.134968

0.139

2.0

200

0.112097

0.00014

321

3.0

100.8

8.663968

0.00955

979
125.2

3.5
1.0

77.2
83

31.99346405
4.091503268

0.033830458
0.04929522

vibration sources are relatively low (normally less than 200 Hz). For example, a small microwave
oven works at a resonant frequency of 121 Hz, a washing machine at 109 Hz, a kitchen blendercasing at 121 Hz, and a CD on notebook computer at 75 Hz.(49) Meanwhile, silicon is brittle and1745

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cannot withstand high acceleration. Phosphor and beryllium bronze, as alloys of copper, havegood electrical conductivity and resistance to fatigue, so phosphor bronze can replace silicon as thesubstrate of energy harvesters operating at low frequencies and high vibration levels. In 2016 and2017, phosphor and beryllium bronze-based energy harvesters operated at resonant frequencies of
100.8 and 77.2 Hz, respectively.
Because the output power is not only related with the volume, but also the resonant frequency
and input vibration conditions, the normalized power density is very useful for comparing energy
harvester output performance. As can be seen in Table 1, the aforementioned bulk PZT bonding
and thinning technique can increase the output power. But it is very difficult to obtain large
normalized power density at low frequencies. In future, improving the power density will be a
research hotspot for the application of energy harvesters.

5.

Conclusions

High-performance PEHs are a power supply option for wireless electronics. It is highly
desirable to utilize materials with a high piezoelectric charge coefficient for PEHs. Bulk PZT
ceramic is a good choice due to its higher electro-mechanical coupling coefficient and higher
conversion efficiency (the ratio between the output electrical energy and the input mechanicalenergy) than deposited piezoelectric films such as AlN or sol–gel PZT. Piezoelectric thick films
based on bonding and thinning bulk PZT technologies will realize high-performance PEHs. In thismanuscript, the development of piezoelectric thick-film fabrication processes including the directbonding process and the transfer process has been reviewed. PEHs on silicon, flexible metal foil,
and flexible polymer substrates have been described and their applications considered on the basisof their working conditions. Finally, the output performance of thick-film-based PEHs from all
published papers was compared in terms of the materials, working conditions, power density, andnormalized power density. This will give some design routes for researchers or engineers in thisarea.

Acknowledgments
This work was supported by the National Natural Science Foundation of China (51565038) theOpen Fund of Jiangxi Province Key Laboratory of Precision Drive & Control (PLPDC-KFKT-201603),
and the 863 Program (2015AA043503).  

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