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Current Applied Physics 14 (2014) 1486e1491

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Current Applied Physics

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High pyroelectric power generation of 0.7Pb(Mg1/3Nb2/3)O3e0.3PbTiO3 single crystal

Young Joon Ko, Yong Keun Park, Byung Kil Yun, Minbaek Lee, Jong Hoon Jung*

Department of Physics, Inha University, Incheon, 402-751, Republic of Korea

a r t i c l e i n f o

Article history:Received 20 May 2014Received in revised form16 July 2014Accepted 12 August 2014Available online 2 September 2014

Keywords:Relaxor ferroelectricPMNePTHigh-power pyroelectric generation

* Corresponding author. Tel.: þ82 32 860 7659; faxE-mail addresses: jhjung@inha.ac.kr, qfml.jhjung@

http://dx.doi.org/10.1016/j.cap.2014.08.0171567-1739/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

We report the pyroelectric power generation of a relaxor ferroelectric 0.7Pb(Mg1/3Nb2/3)O3e0.3PbTiO3

(PMNePT) single crystal. For a temperature variation rate of 0.5 K/s, the PMNePT generated a closed-circuit current of 10 nA and an open-circuit voltage of 1.1 V at room temperature. The pyroelectriccurrent and voltage of the PMNePT single crystal were proportional to the temperature variation rateand were sharply enhanced near the structural phase-transition temperature. Additionally, the PMNePTpyroelectric generator produced stable power for excessive thermal cycling and was highly sensitive torandom thermal fluctuations. Our results indicate the potential importance of PMNePT for high-powerpyroelectric generator applications.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Recently, energy harvesting from ambient energy sources suchas vibration, light, and heat has received a great deal of attentiondue to the lack of global energy resources [1]. Although the har-vested energy from ambient sources is small, a reduction in thepower consumption of modern devices may provide an opportu-nity to realize emerging applications, i.e., self-powered electronicdevices [2]. Inversion symmetry broken crystals, such as ZnO andPb(Zr,Ti)O3, are considered an important class of materials formechanical energy harvesting due to their large piezoelectricity[3e5]. In fact, Wang and his colleagues have improved the piezo-electric voltage from 9 mV to 10e50 V based on ZnO nanowires invarious configurations and have demonstrated the ability to powersmall devices without an external power source [6].

Another promising energy-harvesting technique using theinversion symmetry broken materials is pyroelectricity, where thetemperature dependence of the spontaneous polarization canresult in the flow of electrons for electric output [7e10]. When thedimension/volume of amaterial is fixed, electric polarization can bemodified by changes in temperature. When the dimension/volumeof a material changes due to a change in temperature, strain isinduced due to the anisotropic deformation of the material,resulting in an additional contribution of piezoelectrically inducedcharges. For both cases, the screened free carriers in the electrodes

: þ82 32 872 7562.gmail.com (J.H. Jung).

can be altered, and the flow of free carriers, i.e., current, shouldoccur, as shown in Fig. 1(a). Note that the directions of the currentscreated by the change in polarization and dimension/volume areopposite. To date, there have been only a few reports on pyro-electric energy harvesting [11e13]. Yang et al. demonstrated thefirst pyroelectric nanogenerator based on ZnO nanowire arrays forharvesting thermoelectric energy. They obtained pyroelectric cur-rent and voltage of 0.12 nA and 5.8 mV, respectively, at roomtemperature [11]. Lee et al. developed the first fully stretchable,flexible piezoelectricepyroelectric hybrid nanogenerator based onthe P(VDF-TrFE) polymer. They demonstrated stable pyroelectricoutput voltage of 0.4 V at room temperature for stretchable modes[12]. Although the above results are outstanding, new materialsand/or devices are needed to further increase pyroelectric powergeneration.

In this paper, we report high pyroelectric power generationusing a relaxor ferroelectric 0.7Pb(Mg1/3Nb2/3)O3e0.3PbTiO3(PMNePT) single crystal. At room temperature, we obtained py-roelectric current and voltage of 10 nA and 1.1 V, respectively, forthe temperature variation rate of 0.5 K/s; both of which were quitelarger than previous reports. The pyroelectric current and voltage ofPMNePT were stable over 90 iterations of thermal cycling and alsohighly sensitive to thermal fluctuations. We successfully stored thepyroelectric voltage using a diode bridge and capacitor circuit topower a light-emitting diode (LED). The pyroelectric coefficient ofPMNePT was estimated to be ~104 and ~235 nC/cm2 K at 300 and370 K, respectively. The large enhancement of the pyroelectric co-efficient near 370 K was attributed to the structural-transition-induced rotation of the electric polarization.

Fig. 1. (a) Schematic diagram of the mechanism for pyroelectric power generation. Thick green and thin red arrows denote the electric polarization and projected electric po-larization along the c-axis, respectively. Pyroelectric current occurs when the electric polarization or dimension/volume of the crystal changes with respect to temperature vari-ations (dT/dt > 0). (b) Schematic illustration of the measurement set-up for pyroelectricity. To control the temperature of the sample precisely, we used a Peltier heater, electric fan,and Teflon container. (c) Temperature-dependent dielectric constant of 0.7Pb(Mg1/3Nb2/3)O3e0.3PbTiO3 (PMNePT) at 10 kHz for heating (a red line) and cooling (a blue line) runs. Inthe inset, we show the frequency dependences of the dielectric constant near 405 K. (For interpretation of the references to color in this figure legend, the reader is referred to theweb version of this article.)

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2. Experimental details

A solid solution PMNePT (001) single crystal was purchasedfrom I-Blue Photonics Company [14]. Thin plates of PMNePT(thickness: 30 mm; area: 0.12 cm2) were prepared via mechanicalpolishing [15]. We deposited Pt on both sides of a thin plate andapplied an electric field of 80 kV/cm at 150 �C for electric poling[16]. A frequency-dependent dielectric constant was obtained usingan LCR meter over a temperature range of 300e420 K.

Fig. 1(b) shows a schematic diagram of the set-up for pyro-electric current/voltage measurements. A thermoelectric Peltierheater was used to adjust the temperature of the PMNePT. A K-typethermocouple recorded the temperature during themeasurements.

Voltage pulses having various magnitudes and time intervals wereapplied to control the temperature variation rate. To block thermalfluctuations from the outside, the PMNePT crystal and Peltierheater were surrounded with a Teflon container. The output per-formance was measured using low-noise voltage and currentpreamplifiers.

3. Results and discussion

The dielectric and structural properties of the PMNePT singlecrystal were characterized by the temperature-dependent dielec-tric constant, as shown in Fig. 1(c). The dielectric constant ofPMNePT changed significantly with changes in the temperature

Y.J. Ko et al. / Current Applied Physics 14 (2014) 1486e14911488

and exhibited strong hysteresis with respect to heating/coolingruns. Near 360 and 405 K, the dielectric constant showed two peaksassociated with rhombohedral to tetragonal (ReT) and tetragonalto cubic (TeC) structural phase transitions [17]. The TeC transitiontemperature, corresponding to the ferroelectriceparaelectric tran-sition temperature, showed little driving frequency dependencesprobably due to our large poling voltage [18] (inset of Fig. 1(c)).

Fig. 2 shows the pyroelectric performance of PMNePT nearroom temperature. For clarity, we simultaneously showed thetemperature, temperature variation rate, pyroelectric current, andvoltage. The pyroelectric current/voltage observed only when thetemperature variation rate (dT/dt) was not zero, and reached itsmaximum value for peaks in the temperature variation rate(Fig. 2(a)). Notably, the pyroelectric current and voltage obtainedare very large, i.e., ~10 nA and ~1.1 V, respectively. Note that, thereported pyroelectric current and voltage of ZnO were only ~0.1 nAand ~5.8 mV for a similar temperature variation rate of 0.5 K/s [11].The significant enhancement in the pyroelectric current/voltage ofPMNePT compared with ZnO was attributed to the large change inthe electric polarizationwith respect to temperature changes in theformer [19]. Fig. 2(d) shows the durability test results for thePMNePT-based pyroelectric generator. For up to 90 iterations ofthermal cycling, the output current/voltage was nearly constant,

Fig. 2. Pyroelectric power generation of PMNePT near room temperature. (a) Cyclic changes(I), and open-circuit voltage (V). Cyclic changes in temperature with different dT/dt by adjucurrent. (d) Long-time stability of current and voltage for dT/dt ¼ 0.5 K/s.

indicating stable pyroelectric current/voltage generation forPMNePT.

We examined the effect of the temperature variation rate on theoutput current/voltage of the PMNePT. By changing the tempera-ture (T) and the time interval (t), we modified the temperaturevariation rate (dT/dt) to 0.4 and 0.2 K/s, as shown in Fig. 2(b) and (c),respectively. The pyroelectric current of 10.0 nA at dT/dt ¼ 0.5 K/sdecreased significantly to 5.7 and 2.0 nA for dT/dt ¼ 0.4 and 0.2 K/s,respectively. These results suggest that the pyroelectric currentshould be proportional to the temperature variation rate [7].

To demonstrate the application of a PMNePT-based pyroelectricgenerator for powering a small electronic device, we designed acircuit with two independent loops [20,21] (Fig. 3(a)). For anenergy-harvesting loop, the PMNePT-based pyroelectric generatorwas connected to rectifying diode bridges, and the charge stored ina 47-mF capacitor. For a discharge loop, a commercial LED wasconnected to the capacitor.

A step increase in the stored energy was evident in thermalcycles (Fig. 3(b)). As the charging process continued, the voltage ofthe capacitor became saturated at 1.5 V. The stored voltage waslower than the output voltage of the pyroelectric generator (~2.4 Vfor dT/dt ¼ 1.0 K/s) due to the voltage drop in the rectifying diodesand possible leakage of the capacitor. Fast discharge of the stored

in temperature (T), the corresponding differential curves (dT/dt), closed-circuit currentsting (b) the time interval and (c) temperature, and their corresponding closed-circuit

Fig. 4. Pyroelectric current and voltage for the applied voltage pulses of variousmagnitudes/intervals; (a) 0.5 V/10 s, (b) 0.3 V/30 s, (c) 1.3 V/50 s, (d) 1.6 V/40 s, and (e)1.0 V/20 s.

Y.J. Ko et al. / Current Applied Physics 14 (2014) 1486e1491 1489

voltage (~1.5 V) was enough to power a commercial LED, as shownin Fig. 3(c).

Fig. 4 shows sensitivity test results for the PMNePT-based py-roelectric generator. To mimic an outdoor setting, we randomlychanged the temperature by applying voltage pulses that varied inmagnitude and time to create a fluctuating dT/dt. The pyroelectriccurrent/voltage of PMNePT responded sharply to random tem-perature fluctuations and followed the temperature variation ratecurve. This result showed that the PMNePT-based pyroelectricgenerator was very sensitive over a wide range of thermalfluctuation.

Fig. 5 shows the pyroelectric performance of PMNePT atelevated temperatures. In Fig. 5(a) and (b), the pyroelectric current/voltage of PMNePT is shown at 324 and 367 K, respectively, for dT/dt ¼ 0.5 K/s. The pyroelectric current/voltage at 367 K was almostfour times larger than that for 324 K. To better understand thedifference in the response of PMNePT at 324 and 367 K, we per-formed pyroelectric current measurements over a wide tempera-ture range (300e370 K) and obtained the pyroelectric coefficient(p) using the relationship I ¼ pA(dT/dt), where I, A, and dT/dtrepresent the pyroelectric current, sample area, and temperaturevariation rate, respectively. Note that due to the limitations of ourPeltier heater, we were unable to increase the temperature beyond370 K. The pyroelectric coefficient showed little temperature vari-ation dependence below 355 K; however, it increased sharplyabove 360 K, as shown in Fig. 5(c). We note that the onset tem-perature of the sharp increase in the pyroelectric coefficient issimilar to the temperature behavior associated with the ReTtransition (Fig. 1(c)). Near the ReT transition temperature, the di-rection of the electric polarization should be significantly altered,accompanied by a change in dimension/volume. Therefore, thepyroelectric current/voltage should be significantly enhanced nearthe ReT transition temperature of PMNePT.

Fig. 3. (a) Energy harvesting, charging, and discharging circuit diagram composed of a pyroVoltage of the storage capacitor during charging. (c) Snapshot of the LED as it powers up u

In (1 � x)PMNexPT, a structural transition successively occurswith increasing temperature (i.e., ReT and TeC transition forx > 0.25, and a rhombohedral to cubic (ReC) transition for0� x� 0.25) [17]. The electric polarization of PMNePT is parallel tothe <111> and <001> directions for R and T, respectively, andvanishes for the C structure [22]. Additionally, the TeC and ReCtransition temperatures are known to decrease with a decreasing x,approaching room temperature for x ¼ 0.05. Therefore, we expect

electric generator, rectifying bridges, a capacitor, and a light-emitting diode (LED). (b)sing the stored charge in the capacitor.

Fig. 5. Pyroelectric power generation of PMNePT near (a) 324 and (b) 367 K. For each temperature, we show the cyclic changes in temperature, the corresponding differentialcurves, closed-circuit current, and open-circuit voltage. (c) Pyroelectric coefficient of PMNePT for the heating run.

Y.J. Ko et al. / Current Applied Physics 14 (2014) 1486e14911490

that significantly enhanced pyroelectric power could be generatednear x ¼ 0.05 at room temperature, which is expected to be usefulfor high-power pyroelectric generator applications [23].

4. Conclusions

In summary, we investigated the pyroelectric power generationof PMNePT single crystal. Using voltage pulses with various mag-nitudes and time intervals, we controlled the temperature variationrate and obtained pyroelectric current/voltage over a wide tem-perature range. At room temperature, PMNePT generated a pyro-electric current/voltage of 10 nA/1.1 V, which was almost ten timeslarger than that for ZnO for the same temperature variation rate of0.5 K/s. For temperature variation rates ranging of 0.2e1.0 K/s, thePMNePT pyroelectric generator produced stable power andexhibited high sensitivity to fluctuations in temperature. Near thestructural phase-transition temperature, the pyroelectric current/voltage was greatly enhanced, i.e., ~104 nC/cm2 K at 300 K and~235 nC/cm2 K at 370 K, due to the temperature-induced direc-tional change of the electric polarization.

Acknowledgments

This research was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) funded

by the Ministry of Education, Science and Technology (NRF-2013R1A2A2A01015734).

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