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ABSTRACT Recently, piezoelectric energy harvesters (PEHs) have been paid a lot of attention by many researchers to convert mechanical energy into electrical and low level vibration.

Currently, most of PEHs worked under high frequency and low level vibration. In this paper, we propose a micro cantilever generator based on the bonding of bulk PZT wafer and phosphor

bronze, which is fabricated by MEMS technology, such as mechanical chemical thinning and etching. The experimental results show that the open-circuit output voltage, output power and power

density of this fabricated prototype are 35 V, 321 μW and 8664 μW cm−3 at the resonant frequency of 100.8 Hz, respectively, when it matches an optimal loading resistance of 140 kΩ under the

excitation of 3.0 g acceleration. The fabricated micro generator can obtain the open-circuit stable output voltage of 61.2 V when the vibration acceleration arrives at 7.0 g. Meanwhile, when

this device is pasted on the vibrating vacuum pump, the output voltage is about 11 V. It demonstrates that this novel proposed device can scavenge high vibration level energy at low

frequency for powering the inertial sensors in internet of things application. SIMILAR CONTENT BEING VIEWED BY OTHERS MULTIMODAL MEMS VIBRATION ENERGY HARVESTER WITH CASCADED FLEXIBLE AND

SILICON BEAMS FOR ULTRALOW FREQUENCY RESPONSE Article Open access 23 March 2023 TUNING ZNO-BASED PIEZOELECTRIC NANOGENERATOR EFFICIENCY THROUGH N-ZNO/P-NIO BULK INTERFACING Article Open

access 24 May 2024 DESIGN AND OPTIMISATION OF MAGNETICALLY-TUNABLE HYBRID PIEZOELECTRIC-TRIBOELECTRIC ENERGY HARVESTER Article Open access 24 February 2021 INTRODUCTION In recent years,

energy harvesting has been investigated not only to response to the global energy shortage, but also to realize potential energy supply for micro-actuators, micro-sensors, especially the

devices in civilian medical, and military fields. Wasted mechanical energy from ambient vibrations can be transformed into useful electrical power. Therefore, a variety of harvesters have

been proposed to convert ambient energy into electrical energy under different mechanisms, including electromagnetic1,2,3,4,5, electrostatic6,7,8,9,10, triboelectric11,12,13,14,15,16,17 and

piezoelectric18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34 mechanisms. However, the output power of these devices based on the first three mechanisms fabricated with complex fabrication

processes cannot satisfy the power supply of electronic devices. A lot of piezoelectric energy harvesters (PEHs) specifically fabricated by MEMS technology18,19 have been explored recently

due to its advantages of simple configuration and high output performance. Currently, most investigations for power generation of vibration-based PEHs focus on the configuration of

cantilever unimorph beam with or without proof mass under _d_31 mode and _d_33 mode20. 5 μm PZT film for _d_31 and _d_33 modes MEMS generators was prepared by an aerosol deposition

process21. In addition, a hydrothermal method22 and screen printing23 process have been developed for thick films over 5 μm in thickness. A sintering temperature larger than 550 °C can

provide high piezoelectricity24, which limits the application of the following MEMS fabrication process. In order to improve the output performance of piezoelectric devices, bulk PZT bonding

and thinning techniques have been developed35,36,37,38. In our previous work, the energy harvester of 14 μm PZT films was reported39, which was realized by the low temperature (175 °C)

bonding and thinned-down techniques based on Si substrate. The mostly above reported piezoelectric energy harvesters based on silicon technology can be easily breaking down and only work at

low vibration level from ambient sources due to the brittle properties of silicon. Currently, most of reported PEHs worked under high frequency. However, the frequencies of environmental

vibration sources are relatively low (normally less than 200 Hz)40. For example, a small microwave oven works at the resonant frequency of 121 Hz, a washing machine at 109 Hz, a kitchen

blender casing at 121 Hz and a CD on notebook computer at 75 Hz40. Liu _et al_. demonstrated a low-frequency silicon piezoelectric energy harvester, which generated the output power from

19.4 nW to 51.3 nW within the operation frequency bandwidth ranging from 30 Hz to 47 Hz at 1.0 g41. Then a S-shaped MEMS micro generator with 2.5 μm PZT film deposited by sol-gel process was

reported, which generated the power of 42 mV at low frequency (<30 Hz) at 0.06 g42. Although fewer papers have reported low-frequency piezoelectric energy harvesters, they worked below

2.0 g. Moreover, their output power is very low. However, in the fields of aerospace and military, the working acceleration of the ballistic missile and rocket during launching can reach

from 4 g to15 g. Meanwhile, in our environment, the acceleration of automobile and hand tool also can arrive at 10.0 g43,44,45,46,47,48. Phosphor bronze as an alloy of copper has good

electrical conductivity and resistance to fatigue, so the phosphor bronze can replace silicon as the substrate of the energy harvester worked at the low frequency and high level vibration.

Here in this paper, we have developed a micro piezoelectric generator based on bulk PZT bonding on the phosphor bronze composite structure to convert vibration energy into electricity, which

can realize a low resonant frequency and scavenge the vibration energy at the high acceleration of 7.0 g. The key techniques for fabricating micro generators are described, including the

bulk PZT-Cu-Si bonding, bulk PZT thinning and patterning. In order to reduce the resonant frequency of this device, apart from the integration of silicon proof mass, the mass of tungsten is

assembled on the end of top surface from the composite cantilever. This proposed energy harvester has lightened nine LED bulbs in series. Moreover, it can effectively scavenge vibration

energy from the vacuum pump. RESULTS STRUCTURE AND FABRICATION Figure 1 illustrates the fabricating process of micro designed generator. The whole structure includes phosphor bronze as

supporting layer, bulk PZT thin film as functional piezoelectric layer, conductive epoxy as low temperature bonding layer and proof mass. The started wafer of 400 μm silicon with 2 μm

silicon oxide layer is used to bond with 100 μm thick phosphor bronze, as shown in Fig. 1(a). The phosphor bronze should be thinned to the appropriate thickness for reducing the resonant

frequency and the top surface should be polished to improve the bonding force with bulk PZT, as shown in Fig. 1(b). Then, one side of 400 μm thick PZT is polished and deposited with the 20 

nm/180 nm Cr/Au bottom electrode layer. This side of bulk PZT was bonded together with phosphor bronze by epoxy resin, as shown in Fig. 1(c). PZT layer was thinned down to the required

thickness by means of the mechanical lapping and wet-etching combined method, as shown in Fig. 1(d). Figure 1(e) and (f) illustrate the Cr/Au layer as the top electrode was sputtered and

patterned by dry etching to form the top and bottom electrode, respectively. Figure 1(g) shows that the handle Si layer was etched from the backside to release the cantilever with silicon

proof mass structure by deep reactive ion etching (DRIE) process. Finally, Fig. 1(h) shows that the rectangle tungsten proof mass cut by dicing method is assembled on the top of the

cantilever. The designed dimension of the piezoelectric micro generator is listed in Table 1. The resonant frequency of this device is simulated by finite element software of ANSYS. The

frequency of the first resonant mode is calculated as about 102.59 Hz, as shown in Fig. 2. Generally, it is difficult to thin down the ceramic plate to less than 50 μm due to its

brittleness. Therefore, the hybrid method of mechanical lapping and wet-etching processes is deployed to thin down 400 μm thickness bulk PZT to about 57 μm by mechanical lapping. An

additional mechanical polishing process on PZT film was applied to improve the surface roughness of PZT for subsequent fabrication processes. Moreover, good surface roughness contributes to

improve the adhesive force of following deposited metal layer. SEM image of the surface the PZT after grinding and polishing technique is shown in Fig. 3(a). It is seen that there are no any

big holes in Fig. 3(b). The roughness of about 0.82 μm is measured by digital microscope. Figure 3(c) shows the cross-sectional view with the high-quality bonding interface after thinning

down PZT. The conductive epoxy resin between PZT and phosphor bronze can be seen clearly. The thickness of the PZT layer after thinning is uniform throughout the diaphragm. Figure 4(a) shows

the bending status of the micro fabricated cantilever under the small compression force applied by fingers. It demonstrates that the composite cantilever based on the bonding of PZT and

phosphor bronze wafer has a good flexibility. Then, the fabricated device is fixed on PCB with an acrylic spacer of 2000 μm thickness to make this device freely move even at high

acceleration level. The top and bottom electrodes on PZT functional layer are connected with two bonding pads in PCB, respectively, as shown in Fig. 4(b). Figure 5 shows the measured phase

angle of the fabricated device versus the exciting frequencies using an impedance analyser (KEYSIGHT E4990A). The peak of the phase-angle curve is at about 105 Hz. There is a good agreement

between theoretical and experimental results. OUTPUT PERFORMANCE In order to investigate the output performance of the fabricated generator, the corresponding output average power was

calculated by the following equation: where _U__peak_−_Peak_ is the loading AC peak-peak voltage of close circuit, and _R_ is the loading resistance. Figure 6(a) shows the output maximum

open-circuit voltages versus resonant vibration frequency at different vibration levels. It is observed that the output voltages of the micro fabricated generator at 1.0 g, 2.0 g, 3.0 g, 4.0

 g, 5.0 g, 6.0 g, and 7.0 g acceleration are 16.5 V, 27.1 V, 36.8 V, 42.6 V, 46.8 V, 52.1 V, and 61.2 V, respectively. The maximum output voltage increases with increasing vibration

acceleration value. It is found that at high input acceleration of 7.0 g, the output voltage of is still stable. In order to obtain its output performance of the generator in detail, the

output voltages at the range of 85 Hz to 120 Hz are conducted at the accelerations from 0.5 g to 3.0 g. As seen from Fig. 6(b), for input accelerations of 0.5 g, 0.7 g, 1.0 g, 1.5 g, 2.0 g,

2.5 g, 3.0 g, the open circuit output voltages can achieve 10 V, 12.4 V, 16.5 V, 21.7 V, 26.3 V, 30.4 V, 35 V at the frequencies of 104.9 Hz, 104.4 Hz, 104.0 Hz, 103.2 Hz, 102.3 Hz, 101.9 

Hz, 100.8 Hz, respectively, which gradually decrease with the increase of the acceleration amplitude. The difference is mainly because of the nonlinear change in Young’s modulus of PZT39.

When the resonant vibration was achieved, the output power of the micro generator was highly dependent on the loading resistance and the maximum output power can be obtained under certain

loading resistance. Figure 7(a) shows the output voltage at different loading resistance under different excited accelerations from 0.5 g to 3.0 g. Under a certain acceleration exciting

condition, the output voltage increased with increasing loading resistance. The increasing trend at the range of lower matching resistance is larger than that under the condition of higher

resistance. Under each excitation conditions, the voltage output curve trends to level and eventually starts close to the open circuit voltage output value. Based on equation (1), the

corresponding output power is shown in Fig. 7(b). With the increase of loading resistance, the output power firstly increased and then decreased at certain acceleration. There is a peak

value of output power under each excited acceleration condition at a certain load resistance, which is named the optimal-matched resistance. When the loading resistance is matched with the

internal resistance of the micro generator, the maximum output powers are 13.1 μW, 24.4 μW, 45.8 μW, 91.9 μW, 148.6 μW, 225.8 μW and 321 μW at 0.5 g, 0.7 g, 1.0 g, 1.5 g, 2.0 g, 2.5 g and

3.0 g, respectively. In order to compare the output performance of the fabricated micro generator with different micromachining processes of bonding PZT with phosphor bronze wafer, the

output power density is deployed to be evaluated. The effective volume of the fabricated micro generator is calculated as 0.03732 cm3, along with the acceleration from 0.5 g to 3.0 g, 24.7

times the power density increased, from 351.02 μW cm−3 to 8664 μW cm−3, as shown in Fig. 7(c). Compared to the power density reported by39, this device has good output performance working

under high acceleration. Some reported devices operated in the frequency range of 10 Hz to 300 Hz are listed in Table 2 for comparison. This indicates that the power density of the

piezoelectric device reported in this paper demonstrates comparable performance at low frequency range. However, the thickness of phosphor bronze and bulk PZT can be reduced further to

shrink the whole dimension for improving the output power density. DISCUSSION To validate the capacity of the fabricated micro generator in practical application, the micro generator was

tested on vacuum pump with the rotation speed of 1400 r min−1 (type 2XZ-2), which was used in vacuum drying oven. Figure 8(a) shows the photograph of the fabricated micro generator prototype

fixed on the pump and an oscilloscope (Agilent DSO-X) was used to record the output voltage. When the vacuum pump works, the vibration of the pump drives the cantilever of the device to

oscillate. Subsequently, the micro generator will generate electrical voltage due to the oscillation of the cantilever. Figure 8(b) shows the output voltage of the micro generator fixed on

the pump. The open-circuit output voltage arrives at 11 Vp-p in 2 s. As shown in Fig. 8(c), the output electrical signal of this generator is stable and sustainable in 20 s, which can be

used to power a lot of micro electrical equipments or charge the battery. Figure 8(d) shows the output voltage is stable with wave period of 0.01 s. Therefore, the working frequency of this

device is calculated as approximately 100 Hz, which is close to the resonant frequency of this generator. Meanwhile, the micro generator can light LED bulbs under working on the vacuum pump

and the vibration acceleration of 3.0 g. Figure 9(a) shows that one light emitting diode (LED) bulb is lighten up by the micro generator worked on the vacuum pump due to its vibration level.

While we put this device on the shaker with the acceleration 3.0 g, as shown in Fig. 9(b), this generator lighten nine LED bulbs configured in series. Figure 10 shows the diagram of the

charging voltages and times of two capacitors with 3.3 μF and 10 μF at 1.0 g, 2.0 g, and 3.0 g, respectively. By using a full-wave rectifier made by four diodes, at the acceleration of 1.0 

g, the micro generator can charge 3.3 μF and 10 μF capacitors to 3.6 V and 2.4 V after 2.3 s and 2.4 s, respectively. With the increasing of vibration acceleration, the chargeable voltage

becomes higher and the time needs less. When the acceleration increases to 3.0 g, 3.3 μF and 10 μF capacitors arrive at 5.2 V and 4.5 V after 1.8 s and 1.9 s, respectively. It is concluded

that the charging time of the capacitor with 3.3 μF capacitor is much faster than that of 10 μF. With the advanced development of micro-nano fabrication technologies and microelectronics

circuits, some devices with low-power consumption are developed to work for a long time. Low power consumption of sensors has quickly developed, such as the wireless body temperature sensor

system controlled by a microcontroller unit with a power consumption of 27 μW in active mode49. The working voltage of some monitoring sensors such as the digital tri-axial vibration sensor

is 3.0 V. This confirmed that this developed micro generator can provide power autonomy for some low consumption electronic devices. In summary, this paper has demonstrated the design,

fabrication, and characterization of a novel MEMS-based piezoelectric energy harvester device based on PZT and phosphor bronze bonding technique, which operates at a low frequency and high

acceleration vibration. In this design, a tungsten proof mass is assembled with the cantilever beam of phosphor bronze and piezoelectric wafer to realize a resonant frequency as low as 105 

Hz. Due to the elasticity and fatigue resistance effect of the phosphor bronze, the device can withstand vibration acceleration of 7.0 g and generate the output voltage of 61.2 V. As a

result, for input accelerations from 0.5 g to 3.0 g, the output voltages are from 10 V to 35 V. In the case of 3.0 g acceleration, the output power of the micro generator could reach 321 μW.

Due to the high output performance, the micro generator could lighten one LED tested on the vacuum compression pump and nine LEDs at the acceleration of 3.0 g. This device can be applied in

the environments of low-frequency and high-level vibrations, such as aerospace and military fields. METHODS BONDING AND THINNING PROCESS OF COMPOSITE STRUCTURE Phosphor bronze wafer

(thickness of 100 μm) was bonded with silicon wafer first, then thinned to the required thickness and polished to add adhesive force with the followed PZT wafer. Finally, the polished bulk

PZT (thickness of 400 μm) was bonded with the prepared wafer with conductive epoxy by facing each other. The bonded wafer was put in a vacuum environment, and with 0.1 MPa pressure and the

curing temperature constant at 175 °C for 3 hours. METHOD OF OUTPUT PERFORMANCE TESTING The output performance of the micro generator was evaluated by a vibration testing system. It consists

of an electromagnetic shaker (SINOCERA JZK-5), a power amplifier (SINOCERA YE2706A), a function waveform generator (Agilent 33220A), and an accelerometer (SINOCERA YE5932A). The fabricated

micro generator and the accelerometer were assembled onto the shaker during experiments. The vibration frequency and amplitude of the shaker were controlled by the function generator and the

power amplifier, and the vibration conditions were monitored by the accelerometer. The output signal of the device was connected with different loading resistances to optimize the output

power at different vibration conditions. The output voltages from the testing sample were recorded by the oscilloscope (Agilent DSO-X). ADDITIONAL INFORMATION HOW TO CITE THIS ARTICLE: Tang,

G. _et al_. A piezoelectric micro generator worked at low frequency and high acceleration based on PZT and phosphor bronze bonding. _Sci. Rep._ 6, 38798; doi: 10.1038/srep38798 (2016).

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Systems (MEMS), _2011 IEEE 24th International Conference_. 125–128 (2011). Download references ACKNOWLEDGEMENTS This work was supported by 863 Program (2015AA043503). AUTHOR INFORMATION

AUTHORS AND AFFILIATIONS * Department of Micro/Nano Electronics, National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Shanghai Jiao Tong University, Shanghai, 200240,

China Gang Tang, Bin Yang, Guimiao Li, Jingquan Liu, Xiang Chen & Chunsheng Yang * Department of Mechanical and Engineering, Nanchang Institute of Technology, Nanchang, 330099, China

Gang Tang & Cheng Hou Authors * Gang Tang View author publications You can also search for this author inPubMed Google Scholar * Bin Yang View author publications You can also search for

this author inPubMed Google Scholar * Cheng Hou View author publications You can also search for this author inPubMed Google Scholar * Guimiao Li View author publications You can also

search for this author inPubMed Google Scholar * Jingquan Liu View author publications You can also search for this author inPubMed Google Scholar * Xiang Chen View author publications You

can also search for this author inPubMed Google Scholar * Chunsheng Yang View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS G.T., B.Y. and

C.H. designed the micro generator and wrote the main manuscript text. C.H. and G.T. characterized the device’s performance. C.H. and G.M.L. did the fabrication process. J.Q.L., X.C. and

C.S.Y. analyzed the experimental data. All authors contributed to reviewing the manuscript. ETHICS DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests.

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http://creativecommons.org/licenses/by/4.0/ Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Tang, G., Yang, B., Hou, C. _et al._ A piezoelectric micro generator worked at low

frequency and high acceleration based on PZT and phosphor bronze bonding. _Sci Rep_ 6, 38798 (2016). https://doi.org/10.1038/srep38798 Download citation * Received: 28 September 2016 *

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