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Fabrication of flexible energy harvesting device based on K0.5Na0.5NbO3 nanopowders
Journal of Alloys and Compounds 629 (2015) 113–117
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication of flexible energy harvesting device based on K0.5Na0.5NbO3 nanopowders Yongyong Zhuang a,⇑, Zhuo Xu a,⇑, Fei Li a, Zhipeng Liao b, Weihua Liu b a b
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China Vacuum Microelectronic and Microelectronic Mechanical Institute, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 9 September 2014 Received in revised form 28 December 2014 Accepted 29 December 2014 Available online 8 January 2015 Keywords: Nanostructure materials Sol–gel processes Piezoelectricity Transmission electron microscopy, TEM
a b s t r a c t K0.5Na0.5NbO3 (KNN) powder was synthesized by a novel sol–gel method and used to fabricate an energy harvesting device. The precursor gel and KNN powder were studied by thermogravimetric analysis (TG), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The elemental composition of the KNN powder was characterized by energy dispersive X-ray analysis (EDAX). Well crystallized single phase perovskite KNN powder with an average particle size on the order of 450 nm was obtained from the gel after calcining at 750 °C for 2 h. A flexible and implantable energy harvesting device fabricated using the KNN nanopowder exhibited an output power of up to 0.13 lW with a load resistance of 100 MX. The Young’s modulus of the device was determined to be 10.4 GPa by atomic force microscopy (AFM). Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction High-quality power source, of microscale or even nanoscale size have been attracting increasing attention, owing to the recent miniaturization of various functional electronic devices. These nanodevices need to be independent, sustainable, maintain-free, and able to operate continuously through charging of their power source. Thus, the development of nanotechnology that can harvest energy from the environment is urgently required. Various such energy harvesting devices have been developed, such as nanogenerators [1–3], solar cells [4,5], thermoelectric cells [6], and hydrogen cells [7–9]. Zinc oxide (ZnO) [10,11] is a piezoelectric material reported to be suitable for harvesting energy to generate electricity from ambient mechanical energy, including heartbeats, blood flow, muscle stretching, and body movement. However, the low piezoelectric coefficient of ZnO material compared with those of other piezoelectric materials, such as lead zirconate titanate (PZT) [12,13], barium titanate (BT) [14], sodium potassium niobate (KNN) [15,16], and lead magnesium niobium titanate (PMNT) [17] limits its application. Among these piezoelectric materials, KNN is an environmentallyfriendly material (no lead content), and has a higher Curie temperature, which indicates a higher stability at higher temperatures. Therefore, K1xNaxNbO3 (KNN)-based materials have become promising candidates for mechanical energy harvesting. ⇑ Corresponding authors. Tel.: +86 15929945189. E-mail addresses: [email protected] (Y. Zhuang), [email protected]. cn (Z. Xu). http://dx.doi.org/10.1016/j.jallcom.2014.12.239 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
In this paper, we report the fabrication and characterization of piezoelectric K0.5Na0.5NbO3 (KNN) based nanomaterials. A flexible and implantable KNN-based energy harvesting device was developed. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDAX) were used to characterize the molar ratio of K/Na in the KNN samples. The output voltage and current of the fabricated device was measured, and the output power was calculated to be 0.13 lW with a load resistance of 100 MX. This value is slightly higher than that of the PZT materials (0.12 lW) reported by Wu et al. [18]. The low-cost, flexibility, implantability and wearablility of the present devices demonstrate their potential for application in the future energy harvesting technologies. 2. Experimental procedures 2.1. Synthesis of K0.5Na0.5NbO3 powders K0.5Na0.5NbO3 (KNN) nanopowder was synthesized using a sol–gel method. Commercial Na2CO3 (Analytical reagent grade (AR), 99.8%, Tianjin Tianli Chemical Reagent Co. Ltd., Tianjin, China), Nb2O5 (4 N, 99.99%, Sinopharm Chemical Reagent Co. Ltd., China), K2CO3 (AR, 99.0%), citric acid (AR, 99.5%), aqueous ammonia (AR, 25–28%,) (Tianjin Hedong District Hongyan Chemical Reagent Factory, Tianjin, China), HF (AR, 40.0%, Tianjin Fuyu Fine Chemical Co. Ltd., Tianjin, China), were used as the starting materials. Firstly, a suitable amount of Nb2O5 powder was dissolved in HF (40%) solution after heating in water at 90 °C for 12 h. Then, ammonia hydroxide solution was added dropwise, which lead to the precipitation of Nb5+ as hydroxide (Nb(OH)5) under basic conditions (ca. pH 9). To synthesize KNN, stoichiometric amounts of Na2CO3 and K2CO3 powder were mixed with the precipitated Nb(OH)5 powder in a Na+:K+:Nb5+ molar ratio of 1:1:2. After that, citric acid solution (2 mol/L) was added dropwise to the mixture until the powder was completely dissolved to form a colorless sol. The main chemical reactions that produced the niobium precursor can be expressed as follows:
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Nb2 O5 þ 2ðn þ 5ÞHF ! 2Hn NbF5þn þ 5H2 O;
2þ
þ
ð1Þ 3þ
NbF6 NbF5 þ F NbF4 þ 2F NbF3 þ 3F NbF2 þ 4F
NbF 2Nb
5þ
4þ
þ 5F Nb
5þ
þ 6F ;
þ 10OH þ 10NHþ4 ! 2NbðOHÞ5 # þ10NH3 " þ5H2 ";
3NbðOHÞ5 þ 5H3 C6 H5 O7 ! 3Nb
5þ
þ 15H2 O þ 5C6 H5 O3 7 :
ð2Þ ð3Þ ð4Þ
To obtain a dry gel, the precursor sol solution was heated at 120 °C for 12 h. The dry gels were finally calcined at 750 °C for 2 h to form KNN powder with perovskite structure. 2.2. Fabrication of flexible energy harvesting device with the K0.5Na0.5NbO3 nanopowder The energy harvesting device consisted of three layers as shown in Fig. 1. The copper clad laminate films (two copper layers and one intermediate Kapton layer, 120 lm total thicknesses) were chosen as the flexible substrate and bottom electrode. A mixture of the KNN nanopowder and poly(dimethylsiloxane) (PDMS) in a mass ratio of 1:10 were spin-coated on the clean substrate at 3000 rpm (rpm), and cured for 30 min at 80 °C to obtain the piezoelectric potential layer. A copper foil (25 lm) was placed on the mixture layer as the top electrode. 2.3. Characterization The suitable calcination temperature of the powder was confirmed by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) (NETZSCH STA 449C, Germany). The phase composition of the KNN powder was analyzed using XRD (D/max2200 Rigaku, Japan). The morphology and microstructure of the powder were characterized by scanning electron microscopy (SEM; Quanta FEG 250 FEI, USA) and transmission electron microscopy (TEM; JEM 2100 JEOL, Japan). The elemental composition of the powder was characterized by energy dispersive X-ray analysis (EDAX; Quanta FEG 250 FEI, USA). The Young’s modulus and the topography of the energy harvesting device under external force were measured by atomic force microscopy (AFM; INNOVA Veeco, Germany) with a probe radius of 2 nm. The sensitivity and spring constant of the probe were determined to be 0.0439 lm/V and 0.02785 N/m, respectively. The impedance of the device under different frequencies was characterized using a precision impedance analyzer (4294A Agilent, USA). The d33 value was measured by using a quasi-static d33 tester (ZJ-4A Institute of Acoustics, Chinese Academy of Sciences, China). The output current and voltage signals of the energy harvesting device were collected by using an oscilloscope (3014B Tektronix, USA) equipped with a low-noise current preamplifier (SR570 SRS, USA) and a low-noise voltage preamplifier (SR560 SRS, USA), respectively.
3. Results and discussion Fig. 2 shows the TG–DSC results obtained for the sol–gel derived precursor powder up to 900 °C. Endothermic peaks appeared at 120 °C and 227 °C, and one exothermic peak was observed at 603 °C. Four regions of mass loss were observed in the TG curve. The endothermic peak at 120 °C and the minor mass loss of about 6.8% between room temperature and 165 °C were attributed to the removal of moisture and alcohol. The endothermic peak at 227 °C accompanied by a mass loss of about 56.2% from 165 °C to 260 °C was caused by the removal of excess citric acid. The third region with a mass loss of about 15.7% between 260 °C and 550 °C was attributed to the decomposition of organic groups from citric acid. The mass loss of 14.5% in the temperature range of 550–690 °C was related to the formation of the KNN phase. The exothermic peak at 603 °C is considered to be the crystallization temperature of intermediate KNN phase [19]. A mass loss of about
Fig. 1. Schematic diagram of an energy harvesting device, where the three origin color layers are copper electrodes, the dark red color layer is Kapton, the white spots in the mediate layer are the KNN nanopowder, the gray layer is the PDMS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) curves of the gel precursor powder.
93.2% was observed from room temperature to 690 °C and no further mass loss occurred at higher temperatures. Thus, to ensure complete crystallization of KNN, the gel powder was calcined at 750 °C. Fig. 3 shows the XRD patterns of KNN powder obtained using the present sol–gel technique and the traditional solid state reaction method, annealed at 750 °C for 2 h. All the powder showed perovskite structure and no impurity phases were observed. The composition of the as-synthesized sol–gel KNN powder was similar to that of the K0.5Na0.5NbO3 powder obtained by the solid state reaction method. This confirms the retention of the 1:1:2 M ratios of the Na+/K+/Nb5+ species in the KNN powder. However, the characteristic peak splitting usually observed at around 45° for pseudocubic powder is not obvious in Fig. 3(a). This is attributed to broadening effects associated with the fine crystalline size of the KNN nanopowders [20]. Fig. 4 shows SEM and EDAX images of KNN nanopowder obtained by calcinations of the gel precursor at 750 °C for 2 h. The powder consisted of regular cubic nanoparticles, and had low agglomeration. The average particle size of the KNN powder was estimated to be 450 nm from Fig. 4(b). The distribution of O, Na, Nb, and K elements in the powder is shown in Fig. 4(c)–(f), respectively. The even color of the four images indicates the uniform distribution of the four elements in the sample. The quantitative results are shown in Fig. 4(g). The table inset in Fig. 4(g) indicated that the molar ratio of K:Na:Nb:O in the sample was
Fig. 3. XRD patterns of KNN powder obtained from (a) sol–gel method and (b) solid state reaction method. The powder was annealed at 750 °C for 2 h.
Y. Zhuang et al. / Journal of Alloys and Compounds 629 (2015) 113–117
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Fig. 4. SEM images and EDAX images of the KNN nanopowder: (a) low magnification image of KNN powder; (b) high magnification of (a); (c), (d), (e) and (f) are the distribution maps of O, Na, Nb and K elements of the rectangular region from (a); (g) is the EDAX image of KNN powder, the insert table is the quantitative result of KNN powder.
(0 1 1) crystal plane of orthorhombic K0.5Na0.5NbO3. This is coincident with the selected area diffraction pattern (SAED) shown in Fig. 5(d). The view direction of the diffraction pattern is along the ½1 1 1 crystal direction. The mechanical properties of the flexible energy harvesting device fabricated using the KNN powder is shown in Fig. 6. The width, length, and thickness of the device were 2.5 cm, 4.5 cm, and 80 lm, respectively. Fig. 6(a) shows a topographical image of the PDMS/KNN mixture layer. The surface roughness of the whole layer was on the order of 20 nm. An image of this layer under external force is shown in Fig. 6(b). The distribution of lateral force across the layer can be calculated using the following equation:
FL ¼ S K V L
Fig. 5. TEM images and SAED pattern of KNN nanopowder: (a) low magnification; (b) high magnification; (c) high resolution TEM image of (b); (d) SAED pattern from the rectangle area in (b).
nearly 1:1:2:6. The K:Na:Nb ratio was coincident with the designed molar ratio. The excess of O element was caused by the specimen holder and conductive adhesive. Accordingly, the chemical formula of the KNN powder obtained from the gel precursor should be K0.5Na0.5NbO3. Fig. 5 shows TEM images of the KNN powder and the selected area electron diffraction pattern (SAED) of a KNN nanoparticle. The particle size of the KNN nanopowder was found to be on the order of 200 nm. The high magnification image of the red rectangular area of Fig. 5(b) was shown in Fig. 5(c). The interplanar spacing of the direction marked with the blue arrow in Fig. 5(c) was determined to be 2.842 Å, which corresponded to
ð5Þ
where FL is the lateral force, S is the sensitivity (0.0439 lm/V), K is the spring constant of the probe (0.02785 N/m), and VL is the lateral signal. The measured lateral force was on the order of 1 nN. The force profiles are shown in Fig. 6(c). The AFM probe was controlled to repeatedly approach (a–b), contact (b–c), and retract (d–e–f) from the PDMS/KNN layer. Suitable mechanical properties are essential for the fabrication of durable and reliable devices. The Young’s modulus of the fabricated device was calculated from the contact (b–c) part using the Hertz model [21]:
F ¼ 4=3ER1=2 d3=2 ; 1 1 1 ¼ þ ; R R1 R2
ð6Þ ð7Þ
where F is the loading force, d is the indentation depth, E is Young’s modulus, R is the radius of the effective radius, R1 is the radius of the probe (2 nm), and R2 is the radius of the PDMS/KNN layer (1). The obtained average Young’s modulus was 10.4 GPa. The
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Y. Zhuang et al. / Journal of Alloys and Compounds 629 (2015) 113–117
Fig. 6. Atomic force microscopy (AFM) images of the energy harvesting device with (a) topography image; (b) lateral signal image; (c) force profiles on the PDMS/KNN layer.
harvesting device. The external force was applied to the device using fingers. When an external force was placed on the device, a voltage can be induced via the piezoelectric response (the d33 of the device was 6 pC/N). As a result, a current through an outer load was observed in the external circuit. This process generated output current and voltage signals, with positive/negative signals, as shown in Fig. 8. When the external force was removed, the strain transferred to the device caused by the external force disappeared, and the voltage between the two electrodes also disappeared. Therefore, the accumulated electrons flowed back through the outer load in the external circuit. This process generated reverse output current and voltage signals, as shown in Fig. 8. The largest output current and voltage signals observed for the KNN based device were 4.48 nA and 0.17 V, respectively. The power delivered to the outer load under an external force was estimated using the following equation [24]: Fig. 7. Impedance versus frequency loop of the energy harvesting device.
impedance properties of a device affect its energy harvesting efficiency. The impedance of the fabricated energy harvesting device under different frequencies is shown in Fig. 7. The impedance of the device was observed to decrease with increasing frequency. The maximum and minimum impedance values were 800 KX and 2.76 X at 10 kHz and 79 MHz, respectively. To investigate the performance of the flexible energy harvesting device, the short-circuit current and open-circuit voltage were measured under external force. Ni et al. [22] have verified that the current and voltage signals obtained under such conditions are related to the piezoelectricity of the piezoelectric materials. The power generation mechanism previously proposed [18,22,23] for similar energy generating devices based on piezoelectric materials can be applied to the present KNN nanopowder-based energy
PL ¼
1 T
Z
V 0 ðtÞ2 dt; RL
ð8Þ
where PL is the delivered power, V0(t) is the real-time voltage, RL is the resistance of the outer load, and T is the time period of external force application. A delivered power of up to 0.13 lW was obtained with a load resistance of 100 MX. 4. Conclusions Single phase and well crystallized K0.5Na0.5NbO3 (KNN) nanopowder was prepared using a sol–gel method, with a calcination temperature of 750 °C. The average grain size of the KNN powder was determined to be about 450 nm. The Young’s modulus of an energy harvesting device fabricated using the powder was about 10.4 GPa. The power delivered by the fabricated energy harvesting device was 0.13 lW with a load resistance of 100 MX. This work is
Fig. 8. Short-circuit current (a) and open-circuit voltage (b) of the flexible energy harvesting device when subject to external force. The inset of (a) is a photograph of the energy harvesting device, the size of the device is 2.5 cm 4.5 cm 80 lm. The inset of (b) is a SEM image of the PDMS/KNN layer, the distribution of the KNN powder in PDMS is uniform.
Y. Zhuang et al. / Journal of Alloys and Compounds 629 (2015) 113–117
expected to represent a general high-efficiency method of fabricating low-cost flexible, implantable, or wearable mechanical energy harvesting devices. Acknowledgements The work was supported by the National Nature Science Foundation of China (Grant Nos. 51102193 and 51372196), China Postdoctoral Science Foundation (Grant Nos. 2012M521761 and 2014T70914) and 111 Project B14040. References [1] A.F. Y u, M. Song, Y. Zhang, J.Z. Kou, J.Y. Zhai, Z.L. Wang, Nanotechnology 25 (2014). 455503-1-6. [2] L. Gu, N.Y. Cui, L. Cheng, Q. Xu, S. Bai, M.M. Yuan, W.W. Wu, J.M. Liu, Y. Zhao, F. Ma, Y. Qin, Z.L. Wang, Nano Lett. 13 (2013) 91–94. [3] D. Dhakars, Y. Grawli, S. Chhatre, P. Wadgaonkar, S. Ogale, Phys. Chem. Chem. Phys. 16 (2014) 22874–22881. [4] P.K. Mokhtar, S.N. Masoud, H.M.S. Mostafa, J. Alloys Comp. 617 (2014) 627–632. [5] A. Lamberti, A. Sacco, M. Laurenti, M. Fontana, C.F. Pirri, S. Bianco, J. Alloys Comp. 615 (2014) S487–490. [6] A.I. Boukai, Y. Bunimovich, J.T. Kheli, J.K. Yu, W.A. Goddard, J.R. Heath, Nature 451 (2008) 168–171.
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[7] C. Corgnale, T. Motyka, S. Greenway, J.M. Perez-Berrios, A. Nakano, H. Ito, T. Maeda, J. Alloys Comp. 580 (2013) S406–409. [8] L. Schlapbach, Nature 460 (2009) 809–811. [9] H.G. Pan, R. Li, Y.F. Liu, M.X. Gao, H. Miao, Y.Q. Lei, Q.D. Wang, J. Alloys Comp. 463 (2008) 189–195. [10] S. Xu, Y. Qin, C. Xu, Y. Wei, R. Yang, Z.L. Wang, Nat. Nanotechnol. 5 (2010) 366–373. [11] Q.L. Liao, Z. Zhang, X.H. Zhang, M. Mohr, Y. Zhang, H.J. Fecht, Nano Res. 7 (2014) 917–928. [12] S. Xu, B.J. Hansen, Z.L. Wang, Nat. Commun. 1 (2010) 93–97. [13] Y. Qi, N.T. Jafferis, K.L. Jr, C.M. Lee, H. Ahmad, M.C. McAlpine, Nano Lett. 10 (2010) 524–528. [14] S. H Shin, Y.H. Kim, M.H. Lee, J.Y. Jung, J. Nah, ACS Nano 8 (2014) 2766–2773. [15] L.Q. Cheng, K. Wang, J.F. Li, Chem. Commun. 49 (2013) 4003–4005. [16] X.P. Wang, J.G. Wu, D.Q. Xiao, J.G. Zhu, X.J. Cheng, T. Zheng, B.Y. Zhang, X.J. Lou, X.J. Wang, J. Am. Chem. Soc. 136 (2014) 2905–2910. [17] S. Xu, Y.W. Yeh, G. Poirier, M.C. McAlpine, R.A. Register, N. Yao, Nano Lett. 13 (2013) 2393–2398. [18] W. Wu, S. Bai, M.M. Yuan, Y. Qin, Z.L. Wang, T. Jing, ACS Nano 6 (2012) 6231–6235. [19] A. Chowdhury, J. Bould, J. Nanopart. Res. 12 (2010) 209–215. [20] A. Chowdhury, J. Bould, M.G.S. Londesborough, S.J. Milne, Chem. Mater. 22 (2010) 3862–3874. [21] M. Nikkhah, J.S. Strobl, E.M. Schmelz, J. Biomech. 44 (2001) 762–766. [22] X. Ni, F. Wang, A. Lin, Q. Xu, Z. Yang, Y. Qin, Sci. Adv. Mater. 5 (2013) 1781–1787. 7. [23] S. Bai, Q. Xu, L. Gu, F. Ma, Y. Qin, Z.L. Wang, Nano Energy 1 (2012) 789–795. [24] X. Chen, S.Y. Xu, N. Yao, Y. Shi, Nano Lett. 10 (2010) 2133–2137.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication of flexible energy harvesting device based on K0.5Na0.5NbO3 nanopowders Yongyong Zhuang a,⇑, Zhuo Xu a,⇑, Fei Li a, Zhipeng Liao b, Weihua Liu b a b
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China Vacuum Microelectronic and Microelectronic Mechanical Institute, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
i n f o
Article history: Received 9 September 2014 Received in revised form 28 December 2014 Accepted 29 December 2014 Available online 8 January 2015 Keywords: Nanostructure materials Sol–gel processes Piezoelectricity Transmission electron microscopy, TEM
a b s t r a c t K0.5Na0.5NbO3 (KNN) powder was synthesized by a novel sol–gel method and used to fabricate an energy harvesting device. The precursor gel and KNN powder were studied by thermogravimetric analysis (TG), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The elemental composition of the KNN powder was characterized by energy dispersive X-ray analysis (EDAX). Well crystallized single phase perovskite KNN powder with an average particle size on the order of 450 nm was obtained from the gel after calcining at 750 °C for 2 h. A flexible and implantable energy harvesting device fabricated using the KNN nanopowder exhibited an output power of up to 0.13 lW with a load resistance of 100 MX. The Young’s modulus of the device was determined to be 10.4 GPa by atomic force microscopy (AFM). Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction High-quality power source, of microscale or even nanoscale size have been attracting increasing attention, owing to the recent miniaturization of various functional electronic devices. These nanodevices need to be independent, sustainable, maintain-free, and able to operate continuously through charging of their power source. Thus, the development of nanotechnology that can harvest energy from the environment is urgently required. Various such energy harvesting devices have been developed, such as nanogenerators [1–3], solar cells [4,5], thermoelectric cells [6], and hydrogen cells [7–9]. Zinc oxide (ZnO) [10,11] is a piezoelectric material reported to be suitable for harvesting energy to generate electricity from ambient mechanical energy, including heartbeats, blood flow, muscle stretching, and body movement. However, the low piezoelectric coefficient of ZnO material compared with those of other piezoelectric materials, such as lead zirconate titanate (PZT) [12,13], barium titanate (BT) [14], sodium potassium niobate (KNN) [15,16], and lead magnesium niobium titanate (PMNT) [17] limits its application. Among these piezoelectric materials, KNN is an environmentallyfriendly material (no lead content), and has a higher Curie temperature, which indicates a higher stability at higher temperatures. Therefore, K1xNaxNbO3 (KNN)-based materials have become promising candidates for mechanical energy harvesting. ⇑ Corresponding authors. Tel.: +86 15929945189. E-mail addresses: [email protected] (Y. Zhuang), [email protected]. cn (Z. Xu). http://dx.doi.org/10.1016/j.jallcom.2014.12.239 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
In this paper, we report the fabrication and characterization of piezoelectric K0.5Na0.5NbO3 (KNN) based nanomaterials. A flexible and implantable KNN-based energy harvesting device was developed. X-ray diffraction (XRD) and energy dispersive spectroscopy (EDAX) were used to characterize the molar ratio of K/Na in the KNN samples. The output voltage and current of the fabricated device was measured, and the output power was calculated to be 0.13 lW with a load resistance of 100 MX. This value is slightly higher than that of the PZT materials (0.12 lW) reported by Wu et al. [18]. The low-cost, flexibility, implantability and wearablility of the present devices demonstrate their potential for application in the future energy harvesting technologies. 2. Experimental procedures 2.1. Synthesis of K0.5Na0.5NbO3 powders K0.5Na0.5NbO3 (KNN) nanopowder was synthesized using a sol–gel method. Commercial Na2CO3 (Analytical reagent grade (AR), 99.8%, Tianjin Tianli Chemical Reagent Co. Ltd., Tianjin, China), Nb2O5 (4 N, 99.99%, Sinopharm Chemical Reagent Co. Ltd., China), K2CO3 (AR, 99.0%), citric acid (AR, 99.5%), aqueous ammonia (AR, 25–28%,) (Tianjin Hedong District Hongyan Chemical Reagent Factory, Tianjin, China), HF (AR, 40.0%, Tianjin Fuyu Fine Chemical Co. Ltd., Tianjin, China), were used as the starting materials. Firstly, a suitable amount of Nb2O5 powder was dissolved in HF (40%) solution after heating in water at 90 °C for 12 h. Then, ammonia hydroxide solution was added dropwise, which lead to the precipitation of Nb5+ as hydroxide (Nb(OH)5) under basic conditions (ca. pH 9). To synthesize KNN, stoichiometric amounts of Na2CO3 and K2CO3 powder were mixed with the precipitated Nb(OH)5 powder in a Na+:K+:Nb5+ molar ratio of 1:1:2. After that, citric acid solution (2 mol/L) was added dropwise to the mixture until the powder was completely dissolved to form a colorless sol. The main chemical reactions that produced the niobium precursor can be expressed as follows:
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Nb2 O5 þ 2ðn þ 5ÞHF ! 2Hn NbF5þn þ 5H2 O;
2þ
þ
ð1Þ 3þ
NbF6 NbF5 þ F NbF4 þ 2F NbF3 þ 3F NbF2 þ 4F
NbF 2Nb
5þ
4þ
þ 5F Nb
5þ
þ 6F ;
þ 10OH þ 10NHþ4 ! 2NbðOHÞ5 # þ10NH3 " þ5H2 ";
3NbðOHÞ5 þ 5H3 C6 H5 O7 ! 3Nb
5þ
þ 15H2 O þ 5C6 H5 O3 7 :
ð2Þ ð3Þ ð4Þ
To obtain a dry gel, the precursor sol solution was heated at 120 °C for 12 h. The dry gels were finally calcined at 750 °C for 2 h to form KNN powder with perovskite structure. 2.2. Fabrication of flexible energy harvesting device with the K0.5Na0.5NbO3 nanopowder The energy harvesting device consisted of three layers as shown in Fig. 1. The copper clad laminate films (two copper layers and one intermediate Kapton layer, 120 lm total thicknesses) were chosen as the flexible substrate and bottom electrode. A mixture of the KNN nanopowder and poly(dimethylsiloxane) (PDMS) in a mass ratio of 1:10 were spin-coated on the clean substrate at 3000 rpm (rpm), and cured for 30 min at 80 °C to obtain the piezoelectric potential layer. A copper foil (25 lm) was placed on the mixture layer as the top electrode. 2.3. Characterization The suitable calcination temperature of the powder was confirmed by thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) (NETZSCH STA 449C, Germany). The phase composition of the KNN powder was analyzed using XRD (D/max2200 Rigaku, Japan). The morphology and microstructure of the powder were characterized by scanning electron microscopy (SEM; Quanta FEG 250 FEI, USA) and transmission electron microscopy (TEM; JEM 2100 JEOL, Japan). The elemental composition of the powder was characterized by energy dispersive X-ray analysis (EDAX; Quanta FEG 250 FEI, USA). The Young’s modulus and the topography of the energy harvesting device under external force were measured by atomic force microscopy (AFM; INNOVA Veeco, Germany) with a probe radius of 2 nm. The sensitivity and spring constant of the probe were determined to be 0.0439 lm/V and 0.02785 N/m, respectively. The impedance of the device under different frequencies was characterized using a precision impedance analyzer (4294A Agilent, USA). The d33 value was measured by using a quasi-static d33 tester (ZJ-4A Institute of Acoustics, Chinese Academy of Sciences, China). The output current and voltage signals of the energy harvesting device were collected by using an oscilloscope (3014B Tektronix, USA) equipped with a low-noise current preamplifier (SR570 SRS, USA) and a low-noise voltage preamplifier (SR560 SRS, USA), respectively.
3. Results and discussion Fig. 2 shows the TG–DSC results obtained for the sol–gel derived precursor powder up to 900 °C. Endothermic peaks appeared at 120 °C and 227 °C, and one exothermic peak was observed at 603 °C. Four regions of mass loss were observed in the TG curve. The endothermic peak at 120 °C and the minor mass loss of about 6.8% between room temperature and 165 °C were attributed to the removal of moisture and alcohol. The endothermic peak at 227 °C accompanied by a mass loss of about 56.2% from 165 °C to 260 °C was caused by the removal of excess citric acid. The third region with a mass loss of about 15.7% between 260 °C and 550 °C was attributed to the decomposition of organic groups from citric acid. The mass loss of 14.5% in the temperature range of 550–690 °C was related to the formation of the KNN phase. The exothermic peak at 603 °C is considered to be the crystallization temperature of intermediate KNN phase [19]. A mass loss of about
Fig. 1. Schematic diagram of an energy harvesting device, where the three origin color layers are copper electrodes, the dark red color layer is Kapton, the white spots in the mediate layer are the KNN nanopowder, the gray layer is the PDMS. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) curves of the gel precursor powder.
93.2% was observed from room temperature to 690 °C and no further mass loss occurred at higher temperatures. Thus, to ensure complete crystallization of KNN, the gel powder was calcined at 750 °C. Fig. 3 shows the XRD patterns of KNN powder obtained using the present sol–gel technique and the traditional solid state reaction method, annealed at 750 °C for 2 h. All the powder showed perovskite structure and no impurity phases were observed. The composition of the as-synthesized sol–gel KNN powder was similar to that of the K0.5Na0.5NbO3 powder obtained by the solid state reaction method. This confirms the retention of the 1:1:2 M ratios of the Na+/K+/Nb5+ species in the KNN powder. However, the characteristic peak splitting usually observed at around 45° for pseudocubic powder is not obvious in Fig. 3(a). This is attributed to broadening effects associated with the fine crystalline size of the KNN nanopowders [20]. Fig. 4 shows SEM and EDAX images of KNN nanopowder obtained by calcinations of the gel precursor at 750 °C for 2 h. The powder consisted of regular cubic nanoparticles, and had low agglomeration. The average particle size of the KNN powder was estimated to be 450 nm from Fig. 4(b). The distribution of O, Na, Nb, and K elements in the powder is shown in Fig. 4(c)–(f), respectively. The even color of the four images indicates the uniform distribution of the four elements in the sample. The quantitative results are shown in Fig. 4(g). The table inset in Fig. 4(g) indicated that the molar ratio of K:Na:Nb:O in the sample was
Fig. 3. XRD patterns of KNN powder obtained from (a) sol–gel method and (b) solid state reaction method. The powder was annealed at 750 °C for 2 h.
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Fig. 4. SEM images and EDAX images of the KNN nanopowder: (a) low magnification image of KNN powder; (b) high magnification of (a); (c), (d), (e) and (f) are the distribution maps of O, Na, Nb and K elements of the rectangular region from (a); (g) is the EDAX image of KNN powder, the insert table is the quantitative result of KNN powder.
(0 1 1) crystal plane of orthorhombic K0.5Na0.5NbO3. This is coincident with the selected area diffraction pattern (SAED) shown in Fig. 5(d). The view direction of the diffraction pattern is along the ½1 1 1 crystal direction. The mechanical properties of the flexible energy harvesting device fabricated using the KNN powder is shown in Fig. 6. The width, length, and thickness of the device were 2.5 cm, 4.5 cm, and 80 lm, respectively. Fig. 6(a) shows a topographical image of the PDMS/KNN mixture layer. The surface roughness of the whole layer was on the order of 20 nm. An image of this layer under external force is shown in Fig. 6(b). The distribution of lateral force across the layer can be calculated using the following equation:
FL ¼ S K V L
Fig. 5. TEM images and SAED pattern of KNN nanopowder: (a) low magnification; (b) high magnification; (c) high resolution TEM image of (b); (d) SAED pattern from the rectangle area in (b).
nearly 1:1:2:6. The K:Na:Nb ratio was coincident with the designed molar ratio. The excess of O element was caused by the specimen holder and conductive adhesive. Accordingly, the chemical formula of the KNN powder obtained from the gel precursor should be K0.5Na0.5NbO3. Fig. 5 shows TEM images of the KNN powder and the selected area electron diffraction pattern (SAED) of a KNN nanoparticle. The particle size of the KNN nanopowder was found to be on the order of 200 nm. The high magnification image of the red rectangular area of Fig. 5(b) was shown in Fig. 5(c). The interplanar spacing of the direction marked with the blue arrow in Fig. 5(c) was determined to be 2.842 Å, which corresponded to
ð5Þ
where FL is the lateral force, S is the sensitivity (0.0439 lm/V), K is the spring constant of the probe (0.02785 N/m), and VL is the lateral signal. The measured lateral force was on the order of 1 nN. The force profiles are shown in Fig. 6(c). The AFM probe was controlled to repeatedly approach (a–b), contact (b–c), and retract (d–e–f) from the PDMS/KNN layer. Suitable mechanical properties are essential for the fabrication of durable and reliable devices. The Young’s modulus of the fabricated device was calculated from the contact (b–c) part using the Hertz model [21]:
F ¼ 4=3ER1=2 d3=2 ; 1 1 1 ¼ þ ; R R1 R2
ð6Þ ð7Þ
where F is the loading force, d is the indentation depth, E is Young’s modulus, R is the radius of the effective radius, R1 is the radius of the probe (2 nm), and R2 is the radius of the PDMS/KNN layer (1). The obtained average Young’s modulus was 10.4 GPa. The
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Fig. 6. Atomic force microscopy (AFM) images of the energy harvesting device with (a) topography image; (b) lateral signal image; (c) force profiles on the PDMS/KNN layer.
harvesting device. The external force was applied to the device using fingers. When an external force was placed on the device, a voltage can be induced via the piezoelectric response (the d33 of the device was 6 pC/N). As a result, a current through an outer load was observed in the external circuit. This process generated output current and voltage signals, with positive/negative signals, as shown in Fig. 8. When the external force was removed, the strain transferred to the device caused by the external force disappeared, and the voltage between the two electrodes also disappeared. Therefore, the accumulated electrons flowed back through the outer load in the external circuit. This process generated reverse output current and voltage signals, as shown in Fig. 8. The largest output current and voltage signals observed for the KNN based device were 4.48 nA and 0.17 V, respectively. The power delivered to the outer load under an external force was estimated using the following equation [24]: Fig. 7. Impedance versus frequency loop of the energy harvesting device.
impedance properties of a device affect its energy harvesting efficiency. The impedance of the fabricated energy harvesting device under different frequencies is shown in Fig. 7. The impedance of the device was observed to decrease with increasing frequency. The maximum and minimum impedance values were 800 KX and 2.76 X at 10 kHz and 79 MHz, respectively. To investigate the performance of the flexible energy harvesting device, the short-circuit current and open-circuit voltage were measured under external force. Ni et al. [22] have verified that the current and voltage signals obtained under such conditions are related to the piezoelectricity of the piezoelectric materials. The power generation mechanism previously proposed [18,22,23] for similar energy generating devices based on piezoelectric materials can be applied to the present KNN nanopowder-based energy
PL ¼
1 T
Z
V 0 ðtÞ2 dt; RL
ð8Þ
where PL is the delivered power, V0(t) is the real-time voltage, RL is the resistance of the outer load, and T is the time period of external force application. A delivered power of up to 0.13 lW was obtained with a load resistance of 100 MX. 4. Conclusions Single phase and well crystallized K0.5Na0.5NbO3 (KNN) nanopowder was prepared using a sol–gel method, with a calcination temperature of 750 °C. The average grain size of the KNN powder was determined to be about 450 nm. The Young’s modulus of an energy harvesting device fabricated using the powder was about 10.4 GPa. The power delivered by the fabricated energy harvesting device was 0.13 lW with a load resistance of 100 MX. This work is
Fig. 8. Short-circuit current (a) and open-circuit voltage (b) of the flexible energy harvesting device when subject to external force. The inset of (a) is a photograph of the energy harvesting device, the size of the device is 2.5 cm 4.5 cm 80 lm. The inset of (b) is a SEM image of the PDMS/KNN layer, the distribution of the KNN powder in PDMS is uniform.
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