Research Article www.acsami.org
Dielectric Elastomer Generator with Improved Energy Density and Conversion Efficiency Based on Polyurethane Composites Guoling Yin,† Yu Yang,† Feilong Song,† Christophe Renard,† Zhi-Min Dang,*,‡ Chang-Yong Shi,§ and Dongrui Wang*,† †
Department of Polymer Science and Engineering, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China ‡ Department of Electrical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China § Elements Department, Beijing Institute of Fashion Technology, Beijing 100029, People’s Republic of China S Supporting Information *
ABSTRACT: Dielectric elastomer generators (DEGs), which follow the physics of variable capacitors and harvest electric energy from mechanical work, have attracted intensive attention over the past decade. The lack of ideal dielectric elastomers, after nearly two decades of research, has become the bottleneck for DEGs’ practical applications. Here, we fabricated a series of polyurethane-based ternary composites and estimated their potential as DEGs to harvest electric energy for the first time. Thermoplastic polyurethane (PU) with high relative permittivity (∼8) was chosen as the elastic matrix. Barium titanate (BT) nanoparticles and dibutyl phthalate (DBP) plasticizers, which were selected to improve the permittivity and mechanical properties, respectively, were blended into the PU matrix. As compared to pristine PU, the resultant ternary composite films fabricated through a solution casting approach showed enhanced permittivity, remarkably reduced elastic modulus, and relatively good electrical breakdown strength, dielectric loss, and strain at break. Most importantly, the harvested energy density of PU was significantly enhanced when blended with BT and DBP. A composite film containing 25 phr of BT and 60 phr of DBP with the harvested energy density of 1.71 mJ/cm3 was achieved, which is about 4 times greater than that of pure PU and 8 times greater than that of VHB adhesives. Remarkably improved conversion efficiency of mechano-electric energy was also obtained via cofilling BT and DBP into PU. The results shown in this work strongly suggest compositing is a very promising way to provide better dielectric elastomer candidates for forthcoming practical DEGs. KEYWORDS: dielectric elastomer generator, composite, polyurethane, energy density, energy conversion
1. INTRODUCTION Since the pioneering report of Pelrine et al.,1 dielectric elastomer generators (DEGs), which can harvest electric energy from mechanical work, have recently attracted intense attention.2−5 DEGs are, in essence, variable capacitors in which dielectric elastomers (DEs) sandwiched between two compliant electrodes can convert part of the mechanical energy into electric energy during stretching−releasing cycles. As illustrated in Scheme 1, the stretched and charged DEs sandwiched between two compliant electrodes would increase in thickness (d) and decrease in area (A) during the release of the in-plane stress. Consequently, the electric energy stored in the DE is raised, which can be calculated according to the following equation by assuming that the charge is constant. WE =
0.5(C2V22
−
C1V12)
=
⎛
C 0.5C1V12⎜ 1 ⎝ C2
⎞ − 1⎟ ⎠
Scheme 1. Schematic Illustration of the Work Mechanism of a Dielectric Elastomer Generatora
a
A dielectric elastomer sandwiched between two compliant electrodes is stretched and charged (a). When in-plane stress is released, the elastomer reduces the area and increases the thickness, resulting in the improvement of charge density and energy density (b).
given mass/volume is determined by its maximum strain and electrical breakdown strength. It has been widely accepted that DEGs could exhibit energy densities as high as 400 J/kg, which is more than an order of magnitude greater than those of
(1)
where WE is the generated electric energy, C and V are the capacitance and the voltage of the DE, and the subscripts 1 and 2 represent the states of the DE before and after the relaxation, respectively. Therefore, the max energy output of a DEG with a © 2017 American Chemical Society
Received: October 27, 2016 Accepted: January 25, 2017 Published: January 25, 2017 5237
DOI: 10.1021/acsami.6b13770 ACS Appl. Mater. Interfaces 2017, 9, 5237−5243
Research Article
ACS Applied Materials & Interfaces piezoelectric ceramics and electromagnetic generators.6 Thus, various prototype DEGs have been developed and demonstrated to harvest electric energy from human walking, ocean waves, water current, wind, etc.2−5 To maximize the output energy density and mechano-electric conversion efficiency of DEGs, great efforts have been focused on optimizing the device configurations and energy harvesting operations of acrylic-based elastomers (3M VHB adhesives) over the past decade. For example, McKay and co-workers have integrated the charge/harvest circuit elements into the DEG and achieved an energy density of 10 J/kg with an energy conversion efficiency of 12%.7 By optimizing the charging manner to the DEG, Wang and co-workers have enhanced the energy density up to 18.9 J/kg with an efficiency of 18.3%.8 In 2013, Huang et al. have demonstrated that the energy density of VHB-based DEG can even be enhanced to 560 J/kg with an efficiency of 27% by using an equi-biaxial stretching manner.9 Very recently, the energy density of VHB has been pushed to a record-high value of 780 J/kg by further optimizing the energy harvesting operation scheme.10 All of these findings and achievements confirm the brilliant future of DEGs as innovative power generator systems. However, the widely investigated VHB adhesives suffer from the loss of tension that severely affects the long-term service of consequent DEGs. Moreover, the cross-linking structure of these acrylic elastomers locks the modification toward better material candidates for DEGs. As compared to the activities on commercial available acrylicbased elastomers, less attention has been paid to developing novel elastomers for DEGs. Silicone rubbers have been recognized one of the most promising alternatives to acrylicbased elastomers, due to their good mechanical and electrical properties, low sensitivity to moisture, and great reliability over time.4 Maas et al. have reported a prototype DEG by using a commercially available silicone rubber, showing a harvested energy density of 630 J/kg.11 Recently, silicone rubbers incorporated with various fillers of high permittivity have been proposed as better candidates for DEGs. Bortot et al. have demonstrated, based on simulation, that the harvested energy density of silicone rubber can be improved by 60% via adding 10% volume of magnesium niobate-lead titanate (PMN-PT).12 There are also some examples showing, in theory, that the performance of silicone rubber as DEGs could be enhanced by blending with core−shell silver/silica nanoparticles13 or introducing ionic interpenetrating networks.14 Unfortunately, the experimentally measured energy density of the modified silicone rubbers was not reported in these articles.12−14 The development of elastomers with high permittivity, high electrical breakdown strength, low dielectric/viscous loss, and large strain at break is highly desired for practical DEGs.2−5,15 However, research on the fabrication and energy conversion behavior of DEs other than VHBs is relatively rare to date. In this Article, a novel type of polyurethane-based ternary nanocomposites is fabricated, and their performance as DEG candidate is carefully evaluated. The physically cross-linked thermoplastic polyurethane (TPU) was chosen as the elastic matrix due to its merits including relatively high permittivity (∼8) over other polymers, easy modification and processing, and cost-efficiency. In fact, PUs have long been proposed as one superior candidate for dielectric elastomer actuators, which convert electric energy into mechanical work due to their intrinsically high permittivity and fast response rate. Recently, the electric-field driven deformation of PUs has been substantially improved through constructing percolative con-
ducting networks with the help of carbon nanomaterials.16−18 In light of the investigation on novel elastomer systems for DEGs, herein we report the mechano-electric conversion performance of PU-based composites for the first time. To further improve the harvested energy density, barium titanate (BT) nanoparticles and dibutyl phthalate (DBP) plasticizer were blended into the TPU matrix via solution mixing to optimize its dielectric and mechanical properties. The asprepared ternary composite films, after being assembled into donut-shaped generators, showed the remarkably enhanced ability to harvest electricity as compared to pure PU and the well-known VHB adhesives under given operation conditions. The dielectric, mechanical, and mechano-electric conversion behaviors of those ternary composites are discussed in detail.
2. EXPERIMENTAL SECTION 2.1. Materials. The polyether-based TPU, Elastollan 1185A10, was purchased from BASF. The material has a melt flow index (MFI) of 1−10 g/10 min at 190 °C according to the supplier. The chemical structure of the PU was examined by using 1H nuclear magnetic resonance spectroscopy (1H NMR). The spectrum is shown in Figure S1. DBP (99%, Beijing Chemical Industry Co.) was used as the plasticizer. BT nanoparticles with the average diameter of 100 nm were obtained from Aladdin Reagents Co., China, and used as received. The size and crystalline structure of BT nanoparticles were characterized by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results are shown in Figure S2. All other chemicals were commercially available products and used without further purification. 2.2. Preparation of Composite Elastomer Films. The elastomer films were prepared by a solution casting method. First, a certain amount of BT nanoparticles was dispersed into a solution of PU in N,N-dimethylformamide (DMF) with the concentration of 10 wt % under sonication. For the preparation of ternary composite, DBP was further added into the dispersion. The resultant slurry was degassed, coated onto precleaned glass substrates, and dried under vacuum. The thicknesses of elastomer films were adjusted by controlling the gap of film applicator. The obtained films were denoted as PU/BTx/DBPy, where x and y represent the weight parts (phr, parts per hundred rubber) of BT nanoparticles and DBP plasticizer, respectively. As an example, the elastomer film PU/BT15/ DBP40 consists of 100 g of PU matrix, 15 g of BT nanoparticles, and 40 g of DBP. 2.3. DEG Configuration and Harvesting Circuit. A donutshaped DEG, similar to that reported by Lee and co-workers,19 was adopted in this study. As shown in Figure 1, elastomer films with the thickness of ca. 200 μm were fixed between two plastic rings with the inner diameter of 60 mm. The central part of the elastomers was sandwiched by another two plastic disks (diameter of 20 mm), which were connected to a tensile machine (Shimadzu AG-IC, Japan) to apply areal strain on elastomers and record the strain/stress data. Conductive carbon grease was coated onto the top and bottom surfaces of elastomer films as compliant electrodes. A typical circuit including an input power supply, the DEG, a diode, a load (R = 100 MΩ), and an oscilloscope (Tektronix, U.S.) was set up to record the harvested electric energy (illustrated as Figure 1d). The measurements were carried out through four subsequent steps: (i) stretching the DEG by using the tensile machine under a constant speed of 1000 mm/min to a displacement of 30 mm; (ii) closing switch 1 to charge the DEG for 20 s; (iii) opening switch 1 and releasing the DEG under a speed of 1000 mm/min to the displacement of 0; and (iv) closing switch 2 to measure the electrical energy stored in the DEG by using the oscilloscope. The input bias voltage (V1) was controlled by the power supply, and the capacitance of elastomers at stretched state (C1) was measured by using a multimeter. In this study, the relatively low bias voltage of 500, 700, and 900 V was applied to avoid any danger to human and the materials. The voltage of the DEG at relaxed state (V2) was recorded by the oscilloscope. Thus, the generated electric energy can be calculated through eq 1. The input mechanical energy was 5238
DOI: 10.1021/acsami.6b13770 ACS Appl. Mater. Interfaces 2017, 9, 5237−5243
Research Article
ACS Applied Materials & Interfaces
where ε0 is the vacuum permittivity and εr, A, and d represent the relative permittivity, area, and thickness of the dielectric, respectively. Thus, eq 1 can be rewritten as WE = 0.5V12
⎞ ε0εrA1 ⎛ A1d 2 ε εA − 1⎟ = 0.5V12 0 r 1 (λ 2 − 1) ⎜ d1 ⎝ A 2 d1 d1 ⎠ (3)
where λ = A1/A2 = d2/d1 is defined as the size change ratio during a stretching−relaxation cycle. It can be concluded that the harvested energy of a DEG is determined by εr of the elastomer, the bias voltage V1, and the strain at the stretched state. Therefore, the synergistic improvements in εr, Eb, and tensile strain at break are highly desired for ideal DEGs. Taking the service life of DEGs into account, the elastomers should be operated under conditions far below their Eb and strain at break. In this situation, the electric energy that can be harvested is mainly determined by the εr of elastomers under given stretched strains and bias voltages. By considering that the εr of PU (∼8) is higher than that of VHB adhesives (∼4.5, as shown in Figure S3),2 more energy should be harvested in PU-based DEGs under the same operation conditions. Another important parameter of DEGs is the mechano-electric conversion efficiency (η) that is determined by the ratio of harvested electric energy to consuming mechanical energy over a cycle.3 Apparently, decreasing the consuming mechanical energy through reducing modulus and viscous loss of elastomers should give benefit to the improvement of conversion efficiency.15 Therefore, in the point of view of materials, dielectric elastomers that combine high εr, high Eb, high strain at low stress, and low dielectric and viscous losses should be ideal candidates for DEGs. To further enhance the εr of the TPU, ferroelectric BT nanoparticles with the εr as high as 1700 at room temperature21 were incorporated into the elastomer matrix. As shown in Figure S4, dense composite films without any gas holes were obtained through the solution casting approach. It can also be observed that many aggregates of BT nanoparticles with the size of several micrometers were uniformly dispersed throughout the PU matrix, demonstrating the relatively weak interfacial interactions between PU matrix and BT nanoparticles. The dielectric properties of pure PU film and PU/BT binary composite films with different BT contents are shown in Figure 2. As expected, εr of PU is remarkably improved by blending with BT. The εr at 1 kHz reached 9.0, 9.5, 11.0, and 13.6 for the composites containing BT 5, 15, 25, and 50 phr, respectively. It should also be noted that the dielectric loss of resultant composite films remained at relatively low values (
