High-Performance PZT-Based Stretchable Piezoelectric Nanogenerator

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High-performance PZT-based Stretchable Piezoelectric Nanogenerator Xushi Niu, Wei Jia, Shuo Qian, Jie Zhu, Jing Zhang, Xiaojuan Hou, Jiliang Mu, Wenping Geng, Jundong Cho, Jian He, and Xiujian Chou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04627 • Publication Date (Web): 15 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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ACS Sustainable Chemistry & Engineering

High-performance PZT-based Stretchable Piezoelectric Nanogenerator

Xushi Niu,† Wei Jia,‡ Shuo Qian,† Jie Zhu, † Jing Zhang, † Xiaojuan Hou,† Jiliang Mu,† Wenping Geng,† Jundong Cho,†, § Jian He, †,* and Xiujian Chou†,* † Science

and Technology on Electronic Test and Measurement Laboratory, North University of

China, Taiyuan 030051, China. E-mail: [email protected]; [email protected] ‡ Shanghai § School

Institute of Space Power-source, Shanghai 200245, China

of Information and Communication Engineering, Sungkyunkwan University, Suwon

440746, Korea KEYWORDS: Piezoelectric composite, stretchable nanogenerator, energy harvesting, PZT

ABSTRACT: Stretchable piezoelectric nanogenerators (SPNG) are highly desirable for power supply of flexible electronics. Piezoelectric composite material is the most effective strategy to render piezoelectric nanogenerators stretchable. However, the generated output performance is unsatisfactory due to the low piezoelectric phase proportion. Here we demonstrate a high-performance Pb(Zr0.52Ti0.48)O3 (PZT) -based stretchable piezoelectric nanogenerator (HSPG). The proposed HSPG exhibits excellent output performance with a power density of ~81.25W/cm3, dozens of times higher than previously reported results. Mixing technique, instead of conventional stirring technology, is used to incorporate PZT particles into solid silicone rubber. The filler distribution homogeneity in matrix is thus remarkably improved, allowing higher filler composition. The PZT proportion in composite can be

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increased to 92wt% with satisfactory stretchability of 30%. Based on the excellent electrical and mechanical properties, the proposed HSPG can be attached to human body to harvest body kinetic energy with multiple deformation modes. The obtained energy can be used to operate commercial electronics or be stored into a capacitor. Therefore, our HSPG has great potential application in powering flexible electronics. Introduction Stretchable electronics that can operate under large deformation have been in great demand in wearable devices, biomedical devices, and artificial electronic-skins.1-4 Elastic energy supply components are a fundamental requirement for conformal integration in these electronic systems.5-8 Likewise, it is very promising to integrate a self-powered system into a flexible electronic device to increase battery life.913 In this regard, piezoelectric nanogenerators are an excellent alternative to conventional external power

supplies for their ability to harvest mechanical energy from the environment and human motion.14-17 However, piezoelectric materials, which are typical solid materials, are generally not inherently stretchable.18-20 The common solutions for imparting flexibility to piezoelectric materials is to thin piezoelectric crystals through CMP (Chemical Mechanical Polishing) technology21,22 and to spall piezoelectric thin films produced by sol-gel method on a rigid substrate.23-25 Piezoelectric nanogenerators of these technologies can bend but still lack elasticity and can only accommodate very small bending strain. Therefore, such a piezoelectric nanogenerator cannot satisfy the demand of a flexible electronic device due to poor performance under large deformation.


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Another strategy for this task is to prepare piezoelectric composites by integrating rigid piezoelectric particles into a flexible polymer matrix that can combine piezoelectricity and mechanical flexibility.26-29 As a representative study, Chang Kyu Jeong et al.27 dispersed PMN-PT particles into a translucent silicone rubber to prepare a stretchable piezoelectric nanogenerator (SPNG). However, the resulting output performance is unsatisfactory due to the low percentage (20wt%) of the piezoelectric phase in the composite material. Theoretically, the output performance increases with the increasing proportion of the piezoelectric phase. But there is a negative correlation between piezoelectric phase proportion and device stretchability. So the portion of piezoelectric phase has to be low to guarantee device stretchability. Moreover, an inhomogeneous distribution of piezoelectric phase in polymer matrix will further aggravate this negative correlation. During conventional stirring process, this inhomogeneous distribution is inevitable because of the high viscosity of the matrix and the ubiquitous agglomeration of piezoelectric powders induced by the microscopic force among the particles (van der Waals force, electrostatic force and the adhesive force from the liquid bridge in humid environment). Therefore, in order to increase the output performance as much as possible, the PMN-PT piezoelectric particles with high piezoelectric coefficient has been used. However, this material has a complicated preparation process and high cost,30-32 which cause the material dependency of device, limiting the further application of SPNG. So, in order to increase the output performance on the premise of satisfactory stretchability, it is urgent to improve the dispersion uniformity and increase the proportion of the piezoelectric phase.



In order to develop a high-performance stretchable piezoelectric nanogenerator (HSPG), the solid silicon rubber and the corresponding mixing technology are adopted to improve the distribution uniformity and increase the proportion of piezoelectric phase in matrix. Moreover, the utilization of PZT breaks from the material dependency duo to the easier preparation process compared with PMNPT, which accounted for 92wt% in piezoelectric composite with the satisfactory stretchability (30%). The stretchable piezoelectric nanogenerator thus produced exhibits a power output tens of times better than previously reported results(about 81.25 W/cm3).27,33-35 The summary of the performances and related parameters of composite-based flexible piezoelectric nanogenerators is provided in Table S1. Due to its excellent electrical and mechanical properties, this piezoelectric nanogenerator can be conformally attached to the elbows to harvest kinetic energy, indicating a promising application in wearable devices. Results and discussion Figure 1(a) illustrates the fabrication process of HSPG. The mixing technology is used to incorporate PZT powder into the raw rubber matrix, which is a crucial step in the operation. To the best of our knowledge, this is the first application of the mixing technology in piezoelectric composite (PC) preparation. At the beginning of the mixing, PZT powder group into macro-agglomerates in the matrix duo to the microscopic force among particles and the high viscosity of polymer matrix. And macroaggregates are decomposed into micro-agglomerates by laminar shear force and pressure output on two rollers of different rotational speed. After a long time of mixing, the micro-agglomerates will be broken up completely, and PZT particles are dispersed uniformly into the polymer matrix. In order to eliminate


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the inside bubbles, the PC is compacted at great pressure and be molded at the same time. The conductive composite (CC), composed of the silver-coated glass microspheres embedded in the raw rubber are then prepared using the same mixing technology.

Next, the PC is sandwiched between two CCs, and the primary vulcanizing is performed at 175℃ for 15 minutes to transform the composites into an elastic material made of plastic material. In this process, a three-dimensional network in the matrix is formed by the cross-linking reaction between the polymer chain and the curing agent (Scheme S1). In order to get rid of the redundant curing agent in composite and stabilize physical property, the secondary vulcanizing process is then conducted at 200 ℃ for 2 hours. The cross section scanning electron microscopy (SEM) diagram (Figure 1(b)) of the interface between electrode and piezoelectric layers demonstrates that the two different layers are crosslinked to a unibody structure. The poling process is conducted subsequently with an external voltage of 25kV/mm, at 150℃ for 1 hour in an air environment to align the electric dipoles in the PZT particles. At this point, the high-performance stretchable and PZT-based piezoelectric nanogenerator is completed. The electric layer shows a good conductivity with an electric conductivity and square resistance of about 125s/cm and 120mΩ/□, respectively. Detailed fabrication process of the HSPG can be found in the Experimental section. As the SEM images (Figure 1(c)) show, the PZT particles are homogeneously distributed in matrix after mixing. The element mapping of the piezoelectric composite obtained by Energy Disperse Spectroscopy (EDS) is presented in Figure S1, further elucidating that the PZT particles get a homogeneous dispersion in matrix without aggregate. The proposed PC with 20wt% piezoelectric phase



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can be stretched up to 575% (Figure 1(d)), which is ~3 times higher than conventional PC for the identical filler ratio.27 In addition, the characterization of the PZT powder is conducted by the X-Ray Diffraction (XRD), laser Raman analysis and ferroelectric hysteresis measurement, as shown in the Figure S2. These results indicate that the PZT possesses excellent piezoelectric properties. In short, thanks to the excellent stretchability of the matrix and piezoelectricity of PZT, the proposed HSPG possesses the capacity to harvest ambient energy under large strain. After polarizing, the electric dipole moments in PZT particles are aligned with electric field. The equal polarization charges with opposite polarity are induced on the two surfaces perpendicular to electric field. To balance polarization field, the equal charges from the ambient environment will be accumulated onto the CC by electrostatic induction. At this point, the density of polarization field (P) in PC is constant, which is determined by the sum of dipole moment vector (p) per unit volume.

P

p V

(1)

While, as shown in Figure 2a, the surface polarization charge density (P) varies with the deformation of HSPG. As more stress () is applied to the piezoelectric element, more of the polarization charge will change, whose quality (Q) is given by 6 Q    jd 3 jA ( j  1, 2,3, 4,5, 6) j 1

(2)

Where d3j is the piezoelectric constant and A is the poling area of the piezoelectric layer. The change of polarization field forces electrons to transfer between two CCs via external circuit, converting mechanical energy into electrical energy. Therefore, the output current generated by piezoelectric


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nanogenerator can be ascribed to the internal displacement current. According to Maxwell’s theory, the displacement current JD is described as JD 

D E P   t t t

(3)

Where D is the displacement field,  is the dielectric constant and E is electric field. In this piezoelectric composite, the displacement field includes only a polarization field having no external electric field applied to the piezoelectric layer. So, this displacement field can be expressed as

DP

(4)

Considering that polarization field and polarization charge density are numerically equal, the displacement current of piezoelectric nanogenerator is expressed as

JD 

P  p  t t

(5)

The HSPG can be regarded as a capacitor when operating in external circuit. Therefore, the open-circuit voltage of the HSPG is given by 6  jd 3 jh VOC   ( j  1, 2,3, 4,5, 6) j 1 

(6)

Where h is the thickness of PC. While considering an external circuit resistance R, the equation of output current can be expressed as

 I h

6 j 1

 jd 3 j

R

( j  1, 2,3, 4,5, 6)


(7)


In the above analysis, it can be concluded that stronger stresses on the PZT particles lead to more electrons being transferred between the two CCs through the external circuit, resulting in high output performance of the HSPG. Note that different PZT proportion cause different distribution of PZT particles in matrix (sparsely or compactly), which has a significant impact on the corresponding stress applied on PZT particles under the identical external force. To further study this important relation, the finite element analysis (FEA) is conducted using the COMSOL Multiphysics software. A three-dimensional simplification model composed of 32 PZT spherical particles and rubber material as the matrix is established for steady state simulation. The detailed COMSOL simulation parameters is shown in Table S2. As shown in Figure S3, the displacement field simulation indicates that PZT particles compress each other under tensile deformation of HSPG. Due to the complicated mechanical transference in matrix and filler, the different stress will be produced on PZT particles. Figure 2b gives the stress distribution diagrams of the PZT particles distributed sparsely and compactly. It can be seen that the stress applied on PZT particles is about 18MPa for sparse distribution situation, which can reach 36.7MPa on the rim of particle in the stretching direction. For the compactly distributed PZT particles, the stress is about 70MPa in the most middle area of particle, which is ~4 times higher than sparse distribution(Figure 2b(ii)). Note that very large stress (1400MPa) is produced at the contact point between two particles. It is actually attributed to the strong compression among PZT particles. Subsequently, the further potential simulation based on the same three-dimensional model is conducted to analyze the output performance of HSPG under different PZT distributions in matrix.


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Figure 3b(i) and (ii) show the piezoelectric potential distribution diagrams with PZT particles distributed in matrix sparsely and compactly, respectively. The result indicates that ~4 times higher piezoelectric potential is generated with PZT distributed in matrix compactly than sparsely. Likewise, there is a positive correlation between piezoelectric potential and stress, which is in accordance with the theoretical result from Equation 6. Consequently, it can be inferred that the nanogenerator with compact PZT particle distribution will produce more stress and thus higher output performance. Therefore, increasing the piezoelectric phase ratio and compactly distributing it in matrix is a very effective and simple method for improving the output performance of piezoelectric composite nanogenerator. Based on the above theoretical analysis, the proportion of the PZT filler in matrix has been proved to be a key parameter for HSPG. Here, the HSPGs (5cm×4cm) with several different PZT proportions are fabricated for experimental investigation. A linear module, drove by stepper motor, is used to mimic the periodic stretching stimulation. At the meantime, a test system, composed of a Keithley 2611B system Source Meter and a Lab View based on data acquisition system, is employed to record the output electrical singles. By regulating the controller parameters in linear module, the stretch ratio and strain rate of the HSPG is set to 30% and 6.25cm/s, respectively (Figure 3a). In order to shed more light on the mechanical properties of the HSPGs with different PZT proportions, the elongation at break is also analyzed using the linear module. As shown in Figure 3b, the increasing weight ratio of PZT particles increases the voltage output of HSPG, but sacrifices the tensile properties. In fact, the polymer phase dominates the PC when piezoelectric phase is less than 50wt%. So there is no apparent change in mechanical properties and electrical properties with piezoelectric phase proportion. By contrast, when



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PZT filler is higher than 50wt%, the piezoelectric phase becomes dominant. So the mechanical and electrical properties of material are more sensitive to piezoelectric phase proportion, which is especially remarkable for the piezoelectric proportion higher than 80wt%. Moreover, more analysis about the mechanical behavior of the PZT/rubber composite, such as stress-strain curves, tensile strengths and Young’s modulus with different PZT filler contents are shown in supporting information (Figure S4-6). The results show that the tensile strength of PC increases first and then decreases as PZT filler content increases, but the elongation at break decreases continuously. The change of tensile strength may be ascribed to the fact that PZT particle serves as reinforce in rubber matrix in the beginning. The Young’s modulus can be obtained from the curve slope of stress-strain curve. From Figure S4, we can get that the curve slope increases as PZT filler content increases. However, these curves are not strictly linear, so we calculated the Young’s modulus by dividing tensile strength by elongation at break, and the result is shown in Figure S6. The result shows that the deformability of PC decreases with PZT content increasing, but generally is good (

High-Performance PZT-Based Stretchable Piezoelectric Nanogenerator

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