Piezoelectric Materials for Energy Harvesting

5

Piezoelectric Materials for Energy Harvesting Deepam Maurya, Yongke Yan, and Shashank Priya

CONTENTS 5.1 Introduction................................................................................................... 144 5.2 Solar Energy Harvesting................................................................................ 144 5.3 Wind Energy Harvesting............................................................................... 148 5.4 Wave and Flow Energy Harvesting............................................................... 149 5.5 Thermal Energy Harvesting.......................................................................... 149 5.5.1 Thermoelectric-Based Energy Harvesting........................................ 149 5.5.2 Pyroelectric-Based Thermal Energy Harvesting.............................. 150 5.6 Vibration Energy Harvesting......................................................................... 151 5.6.1 Automobile Vibrations (Suspension Energy Harvesting).................. 152 5.6.2 Vibrations in Civil Structures............................................................ 153 5.6.3 Vibration Energy Harvesting from Railway Tracks.......................... 153 5.6.4 Energy Harvesting from Human Motion........................................... 153 5.7 Materials for Vibration Energy Harvesting................................................... 154 5.7.1 Electromagnetic Induction................................................................. 155 5.7.2 Electrostatic Energy Harvesting........................................................ 155 5.7.3 Electroactive Polymers...................................................................... 155 5.7.4 Piezoelectric Energy Harvesting....................................................... 156 5.8 Piezoelectric Materials.................................................................................. 157 5.8.1 Polycrystalline Piezoelectric Ceramics............................................. 157 5.8.2 Single-Crystal Piezoelectric Ceramics.............................................. 158 5.8.3 Textured Piezoelectric Ceramics....................................................... 159 5.8.3.1 Textured PMN–PT Ceramics............................................. 161 5.8.3.2 [001] Textured Pb(Mg1/3Nb2/3)O3–PbZrO3–PbTiO3 Ceramics............................................................................. 163 5.8.3.3 Cofired Textured Piezoelectric for Energy Harvesting....... 165 5.8.3.4 Lead-Free Piezoelectric Materials for EnvironmentFriendly Energy Harvesters................................................ 166 5.8.3.5 Piezoelectric Materials for High-Temperature Energy Harvesting.............................................................. 169 5.8.3.6 Piezoelectric Polymer Materials for Flexible Energy Harvesting.............................................................. 170

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5.9 Conclusions.................................................................................................... 174 Acknowledgments................................................................................................... 174 References............................................................................................................... 174

Recent emphasis on wireless sensor nodes and distributed low-power electronic network has placed demand on developing energy harvesters that can locally harvest the electric energy from environmental sources such as solar, wind, water flow, heat, and mechanical vibrations. Figure 5.1 shows the compatibility of several commonly used electronic devices with the harvester output power. Technological advances in improving the efficiency of low-power electronic devices coupled with the developments of energy harvesting solutions are expected to provide a significant reduction in the cost of maintenance and simplify the installation. In this chapter, first, we briefly discuss various environmental sources and then focus on vibration energy harvesting specifically using piezoelectric materials. A comprehensive overview of the technological advances in design of piezoelectric materials including lead-free piezoelectric materials is provided.

5.2 SOLAR ENERGY HARVESTING Solar energy has attracted significant attention in recent years due to its abundance. As is well known, the total amount of energy received by the earth from the sun in

Types of harvesting providing power without external charger

100 W

e

10 W

µP desktop µP laptop 1W ac electrolumines cent display Bicycle lighting GSM and mobile phone 100 mW small LCD and OLED displays y erg conventional WSN transmit En 10 mW Photovoltaic or thermoelectric PALM, MP3 ry LED indicator light ng Vibration harvester 1 mW hu Transceiver bluetooth r (electrodynamic or piezo) e ow sp 100 µW es Miniature FM receiver l RF beam e Advanced WSN/active RFID m Miniature co 10 µW be photovoltaic or s Hearing aid e linear vic de 1 µW electrodynamic ic Passive RFID (no battery) s n rie tro tte lec a 100 nW E b Electronic watch n or calculator tto 10 nW Bu 32 kHz quartz oscillator A

AA

La

rg er

ore sm me o c be ing est v r ha

ge ab l

Rotary electrodynamic

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le ab cap

AA

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5.1 INTRODUCTION

Standby

FIGURE 5.1 Energy harvesting device becoming increasingly efficient along with shrinking integrated circuit (IC) chip line geometries and lower consumption levels. (Adapted with permission from IDTechEx, Cambridge, MA.)

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one day is sufficient to satisfy the energy requirements for the whole year. The variety and variation in solar cell efficiency demonstrated over the years are depicted in Figure 5.2 (source NREL). This plot describes the advances made in enhancing the performance of various configurations of the solar cell. The power conversion efficiency of a solar cell can be given as1 η=

Pout J scVoc FF = (5.1) Pin Pin

where Pout is the electrical output Pin is the absorbed solar energy Jsc is the short circuit current density Voc is the open-circuit voltage FF is the fill factor (ratio of maximum obtainable power to the product of the Voc and Jsc1 High-efficiency multijunction cells require expensive fabrication processes and thereby find limited applications like deep space explorations. With respect to energy harvesting, flexible technologies such as dye-sensitized solar cells (DSSCs) and hybrid solar cells are gaining a lot of attention as they can provide decent efficiencies (>10%) in a conformal package.2 Recent research on solid-state DSSCs utilizing perovskite-based electrolyte has provided further boost to the field. In addition to being an excellent fundamental materials research topic, a solid-state DSSC overcomes the problem of packaging and provides a pathway for the integration with multiple platforms.2 Recently, ABO3-type ferroelectrics have attracted great attention as photovoltaic materials.3 Although photovoltaic properties of ferroelectric materials are known for the past 50 years, they did not receive much attention due to their reported low power conversion efficiency.3 Interestingly, recent findings have demonstrated that the low conversion efficiency can be improved by exploiting the physical characteristics of the perovskites.4–6 Some of the interesting features of ferroelectric photovoltaics include extremely large and above bandgap open-circuit voltage.4 This is fundamentally different from conventional semiconductor solar cells.3 The ferroelectric photovoltaic effect and conventional p–n junction photovoltaic effects are schematically shown in Figure 5.3a and b.1 The absorbed photons, in the semiconductor of conventional p–n junction solar cells, can pump the electrons from the valence band to the conduction band leaving holes in the valence band.7 Subsequently, the built-in electric field in the p–n junction separates the electrons and holes, which are collected by the respective electrodes.7 The value of the open-circuit voltage (Voc) in p–n junction solar cells can be theoretically estimated by the quasi-fermi energy difference of photogenerated electrons and holes.7 This is further controlled by the bandgap of the light-absorbing semiconductors.7 However, the experimental results on ferroelectric photovoltaic materials suggested that the output photovoltage is proportional to the electrode spacing and magnitude of the electric polarization.4,8,9 Thus, the output Voc

44 40 36

Efficiency (%)

32 28 24 20 16 12 8 4 0

Multijunction cells (2-terminal, monolithic) LM = lattice matched MM = metamorphic IMM = inverted, melamorphic Three-junction (concentrator) Three-junction (non-concentrator) Two-junction (concentrator) Two-junction (non-concentrator) Four-junction or more (concentrator) Four-junction or more (non-concentrator)

Thin-film technologies CIGS (concentrator) CIGS CdTe Amorphous Si:H (stabilized) Nano-, micro-, poly-Si

BoeingSolar Sharp Spectrolab Junction(IMM, 302x) Spire (LM, 364x) (LM, 942x) Spectrolab Fraunhofer ISE Semiconductor Soifec (MM, 299x) (MM, 454x) (MM, 406x) (4-J, 319x) Emerging PV Boeing-Spectrolab Boeing-Spectrolab (MM, 179x) Solar (MM, 240x) Dye-sensitized cells BoeingJunction Spectrolab (5-J) Perovskite cells NREL NREL (IMM) (LM, 418x) Organic cells (various types) (IMM, 325.7x) NREL Organic tandem cells Single-junction GaAs BoeingSharp (IMM) Spectrolab Inorganic cells (CZTSSe) BoeingSingle crystal Sharp (IMM) Spectrolab Quantum dot cells Concentrator BoeingSharp (IMM) Spectrolab NREL Spectrolab NREL (IMM) Thin-film crystal NREL/ FhG-ISE (467x) Spectrolab Crystaline Si cells Japan Spectrolab FhG-ISE NREL IES-UPM Single crystal (concentrator) Alta Energy NREL (1026x) (117x) Devices Single crystal (non-concentrator) NREL Spectrolab Varian Alta Radboud U. Multicrystalline Devices Varian (216x) FhG-ISE SunPower Amonix Thick Si film (232x) (205x) Panasonic (96x) (92x) NREL Silicon heterostructures (HIT) Alta FhGSunPower Stanford Thin-film crystal ISE Devices Kopin Radboud U. (large-area) (140x) UNSW Varian UNSW Panasonic Panasonic UNSW UNSW NREL Spire UNSW Sanyo Sanyo NREL (15.4x) (14.7x) IBM Sanyo Sanyo UNSW UNSW UNSW EMPA ZSW (T.J. Watson Stanford NREL Sanyo UNSW/ NREL (Flex poly) ZSW UNSW Research Center) ZSW Georgia Eurosolare (14x) First Solar FhG-ISE ARCO Georgia Tech Georgia GE Global ISFH NREL Tech Tech Research Solexel WestingSpire NREL NREL UNSW NREL NREL Varian NREL KRICT U. Stutigart Fraunhofer ISE NREL First house NREL Sandia First Solar Solar KRICT Sharp U. So. RCA GE Global NREL (large-area) No. Carolina Matsushita Florida U. Stuttgart Mitsubishi Research AstroPower NREL United Solar Mobil State U. SolarexSolarex EPFL LG Chem. (small-area) Euro-CIS Boeing (aSi/ncSi/ncSi) Solar United Solar United Solar NREL Sharp Electronics Boeing NIMS Kodak Kodak ARCO ARCO Boeing UCLASharp Kodak EPFL Photon Energy IBM Sumitomo IBM IBM AMETEK Kodak Matsushita Boeing EPFL Heliatek Boeing Kaneka Konarka ARCO UCLA Solarex Monosolar United Solar Solarmer MIT U. of Maine SumiNREL/Konarka Konarka Boeing RCA EPFL tomo U. Linz U. of Maine Groningen EPFL U. Toronto Plextronics Heliatek (PbS-QD) Siemens RCA U. Linz U. Dresden NREL RCA U. Linz RCA RCA (ZnO/PbS-QD) RCA RCA

1975

1980

1985

1990

1995

2000

2005

2010

44.4% 43.6%

38.8% 37.9%

34.1% 31.1% 29.1% 28.8% 27.6% 26.4% 25.6% 25.0% 23.3% 20.8% 20.4% 20.4% 20.1% 17.9%

13.4% 11.9% 11.1% 11.1% 10.6% 8.6% (Rev. 04-24-2014)

48

2015

FIGURE 5.2 Increase in efficiency as a function of time for different kinds of solar cells. (Adapted from Kurtz, S., Opportunities and challenges for development of a mature concentrating photovoltaic power industry, TP-520-43208, National Renewable Energy Laboratory, Golden, CO, 2009.)



146

50

147

Piezoelectric Materials for Energy Harvesting Electron Photon

Cathode Conduction band

p


Elp

Eln n

Valence band

(a)

(c)

+ –

Cathode + + – – Anode

+

+

+

+

–

–

–

–

–

+

+

+

+

+

–

–

–

–

–

Dipole moment

Anode

Hole

+ –

+

+ –

+ –

+ –

(b)

e hod + – + – + – + – + – ode An + – Ferroelec tric Substrate

Cat

+ – + + – – Dipole moment (d)

FIGURE 5.3 The working principle of (a) p–n junction solar cells and (b) ferroelectric photovoltaic devices. Ferroelectric photovoltaic device architectures, (c) vertical and (d) lateral, in which a large photovoltage proportional to the electrode spacing can be measured along the polarization direction (P). (Yuan, Y. et al., J. Mater. Chem. A, 2, 6027–6041, Copyright 2014. Adapted by permission of The Royal Society of Chemistry.)

can be several orders of magnitude larger than the bandgap of the ferroelectric materials.4,10,11 The configurations of typical thin-film ferroelectric photovoltaic devices are shown in Figure 5.3c and d.1 In a recent work, Cao et al. have reported a 72-fold increase in the efficiency of Pb(Zr, Ti)O3 (PZT) based ferroelectric thin-film solar cells by using an n-type Cu2O cathode buffer layer.12 They reported that an ohmic contact on Pt/Cu2O, an n+–n− heterojunction on Cu2O/PZT, and a Schottky barrier on PZT/indium tin oxide (ITO) provided a favorable energy-level alignment for efficient electron extraction on the cathode.12 In another interesting recent work, Grinberg et al. reported high photocurrent density in [KNbO3]1−x[BaNi1/2Nb1/2O3−δ]x (KBNNO)-based compositions.13 The photocurrent density was found to be 50 times higher than that of archetype (Pb,La) (ZrTi)O3 materials.13 This material was also found to absorb up to six times more solar energy than other existing ferroelectric materials.13 These interesting results on the ferroelectric photovoltaic effect clearly demonstrate the potential of piezoelectric materials in solar energy harvesting. Domain engineering and chemical modifications could be successfully employed to enhance the efficiency of ferroelectric-based photovoltaic devices.

148



5.3 WIND ENERGY HARVESTING Wind forms the major source of mechanical energy and is readily available in most places thereby providing an excellent opportunity to tap and power electronic components.14 Commonly, in order to harvest wind energy, wind turbines are used.14 There are broadly three ways to classify wind turbines: (1) on the basis of the orientation of axis of rotation (vertical or horizontal), (2) on the basis of the component of aerodynamic forces (lift or drag) that power the wind turbine, and (3) on the basis of the energy generating capacity (micro, small, medium, or large). Kishore and Priya15 have defined the nomenclature of horizontal-axis wind turbines based on the size of the wind turbine rotor as follows:

1. Microscale wind turbine (µSWT): rotor diameter ≤ 10 cm 2. Small-scale wind turbine (SSWT): 10 cm < rotor diameter ≤ 100 cm 3. Mid-scale wind turbine (MSWT): 1 m < rotor diameter ≤ 5 m 4. Large-scale wind turbine (LSWT): rotor diameter > 5 m

For energy harvesting applications, SSWT becomes the category of choice.15 Recently, Kishore et al. have demonstrated small-scale wind energy portable turbine (SWEPT, Figure 5.4) that operates near ground level and provides a significant magnitude of power at quite low wind speeds. SWEPT is a three-bladed, 40 cm rotor diameter, direct-drive, horizontal-axis wind turbine that operates in a wide range of

FIGURE 5.4 An experimental prototype of “SWEPT.”


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wind speeds between 1.7 and 10 m/s and produces a rated power output of 1 W at a wind speed of 4.0 m/s.15 This power is sufficient for many of the wireless sensing applications as can be seen in Figure 5.4. Kishore et al.16 have also reported an ultralow start-up speed piezoelectric windmill that consisted of a 72 mm diameter horizontal-axis wind turbine rotor with 12 alternating polarity magnets around its periphery and a 60 mm × 20 mm × 0.7 mm piezoelectric bimorph element having a magnet at its tip. This wind turbine was found to produce a peak electric power of 450 μW at a wind speed of 4.2 mph.16 An extremely low cut-in wind speed of 1.9 mph was achieved by operating the bimorph in the actuator mode for 0.9. In order to synthesize cost-effective 〈001〉-oriented PMN–PT ceramics with high performance comparable to single crystals, the TGG process was employed. Figure 5.10a shows the XRD patterns of PMN–PT–xBT samples. Here, PMN– PT–0BT represents a random ceramic without BT seeds (x = 0), and PMN–PT–xBT (x = 1, 3, 5) represents textured ceramics with different BT template contents.82 XRD patterns confirmed the formation of a perovskite structure without any noticeable secondary phase. Domination of (00l) peaks in PMN–PT–xBT (x = 1, 3, 5) samples indicated high 〈001〉 orientation. The Lotgering factor83 calculated for these samples was found to be almost the same ( f > 98%). Figure 5.10b displays the scanning electron microscopy (SEM) images of random and textured cross-sectional samples. Compared to the equiaxed grains in random ceramics (left), all the matrix grains in the textured sample (right) were well aligned with the brick-wall microstructure. Another feature of textured ceramics is the existence of an aligned platelike BT template inside the oriented PMN–PT matrix. The BT template microcrystals had length of 5–10 µm and thickness of 0.5–1 µm. Therefore, besides a large grain size, the textured ceramics had two other unique characteristics: 〈001〉 grain orientation and the existence of a heterogeneous BT “core.” Figure 5.10c shows the electron backscatter diffraction (EBSD) inverse pole figures of the random ceramic and textured ceramic clearly illustrating that the textured grains exhibit strong 〈001〉-preferred orientation. The piezoelectric response and dielectric loss tangent factor (tan δ) for PMN–PT– xBT are shown in Figure 5.11. The d33 value was found to increase with increasing BT content to achieve a maximum value of 1000 pC/N at x = 1.80,84 The enhanced piezoelectric response was attributed to the high degree of texturing and domain structure similar to that of single crystals.84 The trends of variation in the value of d31 were similar to that of d33. However, the value of tan δ showed a reverse trend as

162

(211)

(200) (210)

(111)

(110)

(100)


Intensity (a.u.)


–5 BT, f = 0.99

–3 BT, f = 0.98

–1 BT, f = 0.98

–0 BT, f = 0

(a)

20

30 40 50 2 theta (degree)

60

BT 5 µm

–0BT

–1BT

15 µm

(b) 111

001

20 µm × 20 µm

101

100 µm × 100 µm

(c)

FIGURE 5.10 (a) XRD patterns of PMN–PT–xBT samples; (b) cross-sectional SEM images of PMN–PT–0BT (left) and PMN–PT–1BT (right); and (c) EBSD images of PMN–PT–0BT (left) and PMN–PT–1BT (right) surfaces. (Adapted with permission from Yan, Y. et al., Appl. Phys. Lett., 103, 082906. Copyright 2013, American Institute of Physics.)

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Piezoelectric Materials for Energy Harvesting 3.0

1000

2.5

d33

2.0

800 600

tanδ

1.0

400 d31

200 0

1.5

0

1

2

3

4

Dielectric loss (%)

Piezoelectric coefficient (pC/N)


1200

0.5

5

0.0

Volume percentage of BT template(%)

FIGURE 5.11 Piezoelectric properties and dielectric loss of PMN–PT–xBT ceramics. (Adapted with permission from Yan, Y. et al., Appl. Phys. Lett., 100, 192905. Copyright 2012, American Institute of Physics.)

compared to the d33 and d31 values. Interestingly, the textured PMN–PT–1BT ceramics were found to exhibit loss tangent value as low as 0.6%, which is about 1/3 of the magnitude obtained for most of the soft piezoelectric ceramics (>2.0%). Therefore, grain-oriented PMN–PT–1BT ceramics, with high piezoelectric response and low dielectric loss, are ideal substitutes for currently used soft piezoelectrics in piezoelectric energy harvesting technologies. 5.8.3.2 [001] Textured Pb(Mg1/3Nb2/3)O3 –PbZrO3 –PbTiO3 Ceramics The domain engineered PMN–PT relaxor piezoelectric single crystals have been characterized by high functional response. However, low ferroelectric rhombohedral to tetragonal phase transition temperature (TR-T ~ 60°C–90°C) and low coercive fields (Ec ~ 2–3 kV/cm) were found to limit the stability of these relaxor-based single crystals.75 Many attempts have been made to grow PZT-based single crystals near MPB to achieve high and stable piezoelectric properties, but so far there has been only limited success in growing PZT-based single crystals85 due to inherent technical challenges. Yan et al. synthesized textured PMN–PZT ceramics with enhanced piezoelectric properties (d33 > 1000 pC/N, g33 > 50 × 10 −3 Vm/N, and tan δ < 1.2%).86 Generally, high d33 piezoelectric materials possess low g33 value and vice versa (Figure 5.12a) because the d33 is mostly proportional to the square root of the dielectric constant of the piezoelectric materials. Interestingly, the textured PMN–PZT ceramics exhibited both high d33 and high g33 values due to the template-controlled dielectric characteristics.86 Table 5.3 summarizes the piezoelectric and dielectric properties of randomly

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Advanced Materials for Clean Energy 60

Company 1 Company 3

Company 2 T-PMN-PZT

30 (ii)

20

0

4 3 2

(i)

10

(a)

5

(iii)

40

KNN-based T-PMN-PZT

BNT-based PMN-PT

6

d33(T)/d33(R)

g33 (×10–3 Vm N–1)


50

1 0

200

400

600

d33 (pC N–1)

800

1000

1200

(b)

0

300

600

900

1200

1500

1800

–1 d33(T) (pC N )

FIGURE 5.12 Comparison of (a) g33 and (b) d33 values of various piezoelectric ceramics. Different colored lines (i), (ii), and (iii) indicate the plots of the g33·d33 = c function: (i) c = 16,500 × 10 −15 m2 N−1, (ii) c = 30,000 × 10 −15 m2 N−1, and (iii) c = 50,000 × 10 −15 m2 N−1. (Adapted with permission from Yan, Y. et al., Appl. Phys. Lett., 102, 042903. Copyright 2013, American Institute of Physics.)

TABLE 5.3 Piezoelectric and Dielectric Properties of PMN–PZT Piezoelectrics: Randomly Oriented Ceramic (R-Ceramic), 〈001〉 Textured Ceramic (T-Ceramic), and 〈001〉 Single Crystal (S-Crystal) Properties Piezoelectric charge constant, d33 (pC/N) Electromechanical coupling constant, k Relative dielectric permittivity, ε33/ε0 Piezoelectric voltage constant, g33 (×10−3 Vm/N) d33·g33 (×10−15 m2/N) Remnant polarization, Pr (μC/cm2) Coercive field, Ec (kV/cm) Curie temperature, Tc (°C)

R-Ceramic

T-Ceramic

S-Crystal87

230 0.4 (kp) 915 28.4 6,532 30 8.2 233

1100 0.84 (kp) 2310 53.8 59,180 36 7.4 204

1530 0.93 (k33) 4850 35.6 54,468 29 4.5 211

Source: Adapted with permission from Yan, Y. et al., Appl. Phys. Lett., 102, 042903. Copyright 2013, American Institute of Physics.

oriented ceramic (R-ceramic), 〈001〉 textured ceramic (T-ceramic), and 〈001〉 single crystal (S-crystal) of the PMN–PZT composition. The textured ceramic exhibited giant d33 of 1100 pC/N, which was 4.8 times higher than that of its randomly oriented counterpart having d33~ 230 pC/N.86 This increased ratio of the d33 value between R- and T-ceramic (4.8) was much higher than that of other textured piezoelectric ceramics (usually