- Email: [email protected]
AlN-based flexible piezoelectric skin for energy harvesting from human motion
AlN-based flexible piezoelectric skin for energy harvesting from human motion Francesco Guido, Antonio Qualtieri, Luciana Algieri, Enrico Domenico Lemma, Massimo De Vittorio, Maria Teresa Todaro PII: DOI: Reference:
S0167-9317(16)30154-X doi: 10.1016/j.mee.2016.03.041 MEE 10224
To appear in: Received date: Revised date: Accepted date:
4 November 2015 21 March 2016 22 March 2016
Please cite this article as: Francesco Guido, Antonio Qualtieri, Luciana Algieri, Enrico Domenico Lemma, Massimo De Vittorio, Maria Teresa Todaro, AlN-based flexible piezoelectric skin for energy harvesting from human motion, (2016), doi: 10.1016/j.mee.2016.03.041
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
AlN-based flexible piezoelectric skin for energy harvesting from human motion
Istituto Italiano di Tecnologia (IIT), Center for Biomolecular Nanotechnologies, Via Barsanti, 73010 Arnesano (Lecce), Italy b
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a
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Francesco Guidoa, Antonio Qualtieria, Luciana Algieria,c, Enrico Domenico Lemma a,c, Massimo De Vittorioa,c, Maria Teresa Todaroa,b
Istituto di Nanotecnologia of CNR, Strada Provinciale Lecce-Monteroni – Campus Ecotekne 73100 Lecce, Italy Dip. Ingegneria dell’Innovazione, Università del Salento, Lecce, 73100, Italy
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e-mail: [email protected]
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Abstract
In this work we report on the development of a flexible energy harvester based on piezoelectric
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Aluminum Nitride (AlN) thin film able to scavenge electrical energy from human motion at very
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low frequencies. Flexible devices integrating thin films with controlled residual stress have been
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realized on polyimide substrate to obtain a pre-stressed structure (PSS). These devices show an enhancement of the generated output voltage, if compared to a flat-shape structure, when subjected to a deformation: the piezoelectric skin undergoing folding /unfolding states exhibits fast snapping
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transitions due to the buckling effect, which increases the mechanical stress of the piezoelectric structure, improving the generated output voltage. Experimental results demonstrate a maximum peak-to-peak voltage of 0.7 V for a PSS, about six times higher than the corresponding voltage obtained for flat structures. These results have been validated by Finite Element Method (FEM) simulations of the total elastic energy of deformation and the mechanical stress versus the deformation, demonstrating the buckling effect. Keywords: energy harvesting; biomechanical energy; flexible devices; wearable devices
ACCEPTED MANUSCRIPT 1.
Introduction
Energy harvesters exploiting vibrational and mechanical energy sources are promising building
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blocks for the realization of self-powered energy systems to be employed indoor and outdoor, in
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remote and harsh environments as well as in living systems and human body.
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In this context there is an increasing interest in the development of flexible and biocompatible devices able to scavenge biomechanical energy from human motion, such as arm swings, stretching or walking as well as from extremely tiny displacements inside the human body (e.g. respiration,
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heartbeat, blood flow, eye blinking or muscle stretching) and convert it into electricity for powering
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portable and wearable electronics or implantable biomedical devices, such as cardiac-tachometers, pacemakers and artificial retinas [1]. Piezoelectric transducers are a good choice for such application due to the ability of piezoelectric materials to directly convert applied strain energy into
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electric energy. Aluminum nitride (AlN), by virtue of its good piezoelectric and dielectric properties [2,3], silicon and CMOS compatibility and biocompatibility [4] is a good candidate to produce
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compact and efficient piezoelectric MEMS. It can be also effectively deposited on soft/flexible substrates [5,6] thus enabling the fabrication of compact and lightweight wearable/implantable
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microgenerators . The development of such generators could find applications for powering wireless sensor nodes thus overcoming problems related to the employment of conventional batteries including toxicity, limited lifetime and periodic replacing/recharging operations. In the literature two approaches have been exploited for the development of piezoelectric microgenerators/nanogenerators harvesting energy from random and tiny human body motions: the first one employs nanostructured materials [7-9] whereas the second one focuses on thin films [10,11]. Among devices based on inorganic piezoelectric thin films, Kwi-Il Park et al. reported on a flexible nanogenerator based on a biocompatible BaTiO3 thin layer [12] showing open circuit voltage values up to 1 V, with corresponding power densities up to 7 mW/cm 3. However the fabrication process of this device is complicated and additionally BaTiO3 thin film needs annealing and poling treatments to induce a piezoelectric behavior.
ACCEPTED MANUSCRIPT In this work we report on the development of a biocompatible, flexible, lightweight and wearable prototype of piezoelectric energy harvester based on MEMS technologies applied to a soft substrate.
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Flexible devices with controlled residual stress structures and based on AlN thin film have been
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electrical energy from human motion at very low frequencies.
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fabricated by microfabrication processes to realize an artificial piezoelectric skin able to scavenge
Representative flexible devices undergoing folding/unfolding states have been characterized to assess the voltage generation properties by providing a mechanical stimulus by a continuous low
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frequency finger motion. The device based on a pre–stressed structure (PSS) exhibits an
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enhancement of the output generated voltage, if compared to the device based on a flat structure. This result has been attributed to the buckling effect involving a fast transition during the structure deformation, from initial state snapping into a secondary equilibrium state when the limit point
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buckling is reached. Experimental results have been validated by Finite Element Method (FEM), exploited for the calculation of the total elastic energy accumulated under the deformation and the
Experimental
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2.
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mechanical stress generated around the limit point buckling.
Figure 1 sketches the flexible device fabrication process. As shown in Figure 1b the structures, consisting of a sequence of AlN (120 nm), Mo (130 nm), AlN (900 nm) and Mo (210 nm) layers have been deposited on a 25 µm thick polymeric foil (Kapton HN) attached by silicone (Polydimethylsiloxane, PDMS Sylgard 184) to a silicon wafer using a roll to roll system (URAI K printing proofer). Thin films have been deposited by DC magnetron sputtering technique (K.J. Lesker Lab 18 system) without heating of the substrate in a single run in order to minimize contaminations. An optimized thin AlN interlayer has been grown to improve adhesion properties of the Mo/AlN/Mo structure and the crystallographic quality of the piezoelectric AlN film on the amorphous Kapton surface. This strategy has been already exploited [13] demonstrating higher columnar orientation of the
ACCEPTED MANUSCRIPT piezoelectric AlN film on Mo layer grown on a thin AlN interlayer in comparison to the AlN/Mo films deposited directly on the substrate. The Mo layers have been sputtered from a high-purity
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(99.95%) Mo target, in Ar atmosphere at a pressure of 5*10-3 mbar and a power of 200 W. The AlN
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layer has been deposited from a high purity Al target (99.9995%) with a gas mixture of N2 (20
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sccm)/Ar (20 sccm) at a pressure of 1.5*10-3 mbar. The sputtering plasma was generated in pulsed mode with a frequency of 100 kHz and a power of 1250 W.
These deposition parameters of AlN/Mo/AlN/Mo multilayer on the soft substrate resulted in
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compressive PSS, involving a flexible device bending perpendicular to its length axis. Flat
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structures have been also grown by a careful optimization of the deposition parameters, in order to compare fabricated devices based on different residual stress. Devices have been realized by microfabrication technologies including photolithography and dry
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etching techniques by using an inductively coupled plasma (ICP) etching system (Figure 1c). The etching of both Mo top electrode and AlN active layer has been performed in a single step, using a
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gas mixture (20 sccm of SiCl4, 25 sccm of N2 and 7 sccm of Ar). The power applied to the platen and to the coil were 45 W and 100 W, respectively. Another process step consisted of
AC
photolithography and dry etching to pattern both the Mo bottom electrode and the AlN interlayer. Electrical passivation of the devices has been obtained by the deposition of 1 µm thick parylene insulating layer (Figure 1d). The patterning of the parylene film (figure 1e) and the patterning of long metal strips, realized by optical lithography, deposition of Al/Cr thin films and lift-off technique, have been carried out for electrical connections (Figure 1f). After the resist removal, the soft polymeric foil has been cut by a metal blade to define the geometries of the flexible devices. The release/detachment of the devices from the rigid support of Si has been performed by dipping the sample in isopropyl alcohol at room temperature to swell the PDMS adhesive layer under Kapton foil. Figure 1g reports pictures of a realized flexible device, with highlighting the bending of the PSS, shown in the inset.
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g
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Figure 1. a-f) The fabrication process of the flexible device for energy harvesting; g) pictures of a realized flexible
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device
Figure 2 reports images of a realized prototype of the device adapted to a flexible glove in order to
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follow its deformation during finger motions. It can be observed that the flexible device has an
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excellent conformability if employed as wearable energy harvester.
Figure 2. a) Images of a realized prototype fixed on a finger, b) following the skin deformation
3.
Results and discussion
The working principle of a flexible piezoelectric generator is based on the temporary electric charge generated by a deformation of the material due to an applied external stress.
ACCEPTED MANUSCRIPT In this work, representative AlN flexible devices with a 4 x 6 mm2 size, having a 2 x 2 mm2 piezoelectric area and based on PSS and flat structure have been tested. The devices, adapted to the
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glove, have been connected to an oscilloscope to evaluate the generated output voltage during
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folding/unfolding states providing a mechanical stimulus by a continuous finger motion.
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Figure 3 reports the generated open circuit voltage for realized flexible devices based on PSS (red line) and flat structure (black line) under a periodic motion around 1 Hz frequency. The device based on PSS exhibits a maximum peak-to-peak voltage of 0.7 V with an estimated maximum
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electrical power of 0.2 µW. This figure shows also an enlarged view of the boxed area for one cycle
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of deformation.
Figure 3. Generated open circuit voltage for realized flexible devices with a pre-stressed structure (PSS, red line) and a flat structure (black line) undergoing folding/unfolding states with an enlarged view of the boxed are for one cycle of deformation
The maximum power, reached when the device is closed on an optimal load matching the device electrical impedance [14], has been calculated taking into account a measured impedance value of 220 kΩ. The estimated power density was 0.4 mW/cm3. The device based on flat structure shows a maximum peak-to-peak voltage as low as 0.12 V, about six times lower than the device based on PSS.
ACCEPTED MANUSCRIPT Such experimental results can be attributed to the buckling effect: a deformation applied to a PSS results in a fast transition from initial unfolding state snapping into a secondary equilibrium state,
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when the limit point buckling is reached. This transition involves a large variation of the total
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elastic energy of deformation versus the PSS deformation. The buckling effect has been already
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described by Yang et al. [15], who integrate a PVDF piezoelectric layer with a pre-stressed flexible substrate, realizing a large size assembled device to scavenge energy by body movements. To validate the experimental results, the total elastic energy of deformation as a function of the
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deformation has been analyzed by finite Element Method (FEM) simulations using Comsol
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Multiphysics. The mechanical stress behavior around the limit point buckling has been also calculated The model parameters included the geometrical features of the fabricated devices and the mechanical properties of the materials.
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To evaluate the total elastic energy of deformation, the involved elastic materials have been considered as linear and finger motion has been approximated by a displacement of the substrate,
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applied on the tip of the device, up to 1.1 mm towards the z-axis. The residual stress of the PSS has been estimated by Stoney’s equation [16] taking into account the device curvature radius, calculated
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to be 5.5 mm, based on profile measurements (profilometer DektatXT, Bruker). The calculated residual stress resulted to be 80 MPa. This value has been imposed in the FEM simulation to generate the initial bending state, replicating the real curvature of the PSS. The mechanical stress has been calculated as the derivative of the total elastic energy with respect to the deformation [17]. The deformation is defined as a non-dimensional parameter, calculated as δ/L, where δ is the displacement applied to the structure and L is the total length of the substrate. Figure 4 reports representative states of the simulated PSS and the plot of the computational results concerning the elastic energy of deformation and the mechanical stress behavior . Figure 4a shows the initial bending state of the PSS; Figure 4b reports an intermediate state with highlighted the direction and the position of the applied displacement; Figure 4c shows the final deformed state of the structure. Figure 4d shows the simulated total elastic energy of deformation (red line) and the
ACCEPTED MANUSCRIPT mechanical stress (black line) as a function of deformation. The elastic energy curve confirms the existence of a secondary equilibrium state due to a local energy minimum during the folding of the
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flexible structure.
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d)
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Figure 4. a) the simulation result of the initial bending state of the PSS; b) an intermediate state of the deformation, with the green arrows representing the direction and the position of the applied displacement; c) the final deformed structure; d) the total elastic energy of deformation (red line) and the mechanical stress (black line) versus deformation
In the same plot an increase of the mechanical stress around the limit point buckling, where the total elastic energy experiences a fast transition, can be observed.
4.
Conclusion
In this work AlN-based piezoelectric flexible skin as wearable energy harvester has been successfully fabricated. Flexible devices integrating a pre-stressed structure (PSS) when subjected to a deformation have shown an enhancement of the generated output voltage, if compared to
ACCEPTED MANUSCRIPT devices based on a flat-shape structure. A representative device with a 4 x 6 mm2 size and having a 2 x 2 mm2 piezoelectric area undergoing finger motion at 1 Hz frequency exhibited a maximum
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peak-to-peak generated voltage of 0.7 V, about six times higher than the voltage generated by a
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device with a flat structure. These results have been attributed to the buckling effect, which implies
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a fast snapping transition increasing the mechanical stress and validated by FEM simulations. This approach results to be very promising for energy harvesting from human motion because of the
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simple fabrication process and the device conformability.
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Acknowledgements
This research has been funded by national Italian Project PON ITEM.
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References:
[1] Hwang, G. T., Byun, M., Jeong, C. K., & Lee, K. J. (2015). Flexible Piezoelectric Thin‐Film
4(5), 646-658.
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Energy Harvesters and Nanosensors for Biomedical Applications. Advanced healthcare materials,
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[2] Iriarte , J.F., Rodrıguez, J.G., & Calle, F. (2010). Synthesis of c-axis oriented AlN thin films on different substrates: A review. Materials Research Bulletin, 45, 1039–1045. [3] Tonisch, K., et al., (2006). Piezoelectric properties of polycrystalline AlN thin films for MEMS application. Sensors and Actuators A: Physical, 132 (2), 658-663. [4] Jackson, N., et al., (2013). Flexible-CMOS and Biocompatible Piezoelectric AlN material for MEMS Applications. Smart Materials and Structures, 22(11), 115033. [5] Petroni, S., Rizzi, F., Guido, F., Cannavale, A., Donateo, T., Ingrosso, F., & De Vittorio, M. (2015). Flexible AlN flags for efficient wind energy harvesting at ultralow cut-in wind speed. RSC Advances, 5(18), 14047-14052. [6] Petroni, S., et al., (2012). Tactile multisensing on flexible aluminum nitride. Analyst,137(22), 5260-5264.
ACCEPTED MANUSCRIPT [7] Fang, J., Wang, X., & Lin, T. (2011). Electrical power generator from randomly oriented electrospun poly (vinylidene fluoride) nanofibre membranes. Journal of Materials Chemistry,
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21(30), 11088-11091.
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harvesting using PZT nanofibers. Nano letters, 10(6), 2133-2137.
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[8] Chen, X., Xu, S., Yao, N., & Shi, Y. (2010). 1.6 V nanogenerator for mechanical energy
[9] Bai, S., Xu, Q., Gu, L., Ma, F., Qin, Y., & Wang, Z. L. (2012). Single crystalline lead zirconate titanate (PZT) nano/micro-wire based self-powered UV sensor. Nano Energy, 1(6), 789-795.
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[10] Pan, C. T., Liu, Z. H., Chen, Y. C., & Liu, C. F. (2010). Design and fabrication of flexible
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piezo-microgenerator by depositing ZnO thin films on PET substrates. Sensors and Actuators A: Physical, 159(1), 96-104.
[11] Pi, Z., Zhang, J., Wen, C., Zhang, Z. B., & Wu, D. (2014). Flexible piezoelectric nanogenerator
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made of poly (vinylidenefluoride-co-trifluoroethylene)(PVDF-TrFE) thin film. Nano Energy, 7, 3341.
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[12] Park, Kwi-Il, et al. "Piezoelectric BaTiO3 thin film nanogenerator on plastic substrates." Nano Letters 10.12 (2010): 4939-4943.
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[13] Kamohara, T., Akiyama, M., Ueno, N., & Kuwano, N. (2008). Improvement in crystal orientation of AlN thin films prepared on Mo electrodes using AlN interlayers. Ceramics International, 34(4), 985-989. [14] Priya, S. (2005). Modeling of electric energy harvesting using piezoelectric windmill. Applied Physics Letters, 87(18), 184101. [15] Yang, B., & Yun, K. S. (2012). Piezoelectric shell structures as wearable energy harvesters for effective power generation at low-frequency movement. Sensors and Actuators A: Physical, 188, 427-433. [16] Stoney, G. G., The Tension of Metallic Films Deposited by Electrolysis, 1909, Proc. R. Soc. London, Ser. A, 82, pp. 172–17. [17] Carpinteri A., Scienza delle costruzioni, 1, 1992, pp. 240-246
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ACCEPTED MANUSCRIPT
Graphical abstract
ACCEPTED MANUSCRIPT Highlights
Flexible AlN-based energy harvester scavenging electric energy from human motion is
Piezoelectric skin integrates a pre-stressed structure (PSS) improving the generation of
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developed
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output voltage
The increase of generated voltage is attributed to fast snapping transition during the skin deformation (buckling effect) of the PSS
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The experimental results are validated by FEM simulations demonstrating the buckling
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effect
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S0167-9317(16)30154-X doi: 10.1016/j.mee.2016.03.041 MEE 10224
To appear in: Received date: Revised date: Accepted date:
4 November 2015 21 March 2016 22 March 2016
Please cite this article as: Francesco Guido, Antonio Qualtieri, Luciana Algieri, Enrico Domenico Lemma, Massimo De Vittorio, Maria Teresa Todaro, AlN-based flexible piezoelectric skin for energy harvesting from human motion, (2016), doi: 10.1016/j.mee.2016.03.041
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
AlN-based flexible piezoelectric skin for energy harvesting from human motion
Istituto Italiano di Tecnologia (IIT), Center for Biomolecular Nanotechnologies, Via Barsanti, 73010 Arnesano (Lecce), Italy b
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a
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Francesco Guidoa, Antonio Qualtieria, Luciana Algieria,c, Enrico Domenico Lemma a,c, Massimo De Vittorioa,c, Maria Teresa Todaroa,b
Istituto di Nanotecnologia of CNR, Strada Provinciale Lecce-Monteroni – Campus Ecotekne 73100 Lecce, Italy Dip. Ingegneria dell’Innovazione, Università del Salento, Lecce, 73100, Italy
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c
e-mail: [email protected]
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Abstract
In this work we report on the development of a flexible energy harvester based on piezoelectric
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Aluminum Nitride (AlN) thin film able to scavenge electrical energy from human motion at very
TE
low frequencies. Flexible devices integrating thin films with controlled residual stress have been
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realized on polyimide substrate to obtain a pre-stressed structure (PSS). These devices show an enhancement of the generated output voltage, if compared to a flat-shape structure, when subjected to a deformation: the piezoelectric skin undergoing folding /unfolding states exhibits fast snapping
AC
transitions due to the buckling effect, which increases the mechanical stress of the piezoelectric structure, improving the generated output voltage. Experimental results demonstrate a maximum peak-to-peak voltage of 0.7 V for a PSS, about six times higher than the corresponding voltage obtained for flat structures. These results have been validated by Finite Element Method (FEM) simulations of the total elastic energy of deformation and the mechanical stress versus the deformation, demonstrating the buckling effect. Keywords: energy harvesting; biomechanical energy; flexible devices; wearable devices
ACCEPTED MANUSCRIPT 1.
Introduction
Energy harvesters exploiting vibrational and mechanical energy sources are promising building
T
blocks for the realization of self-powered energy systems to be employed indoor and outdoor, in
IP
remote and harsh environments as well as in living systems and human body.
SC R
In this context there is an increasing interest in the development of flexible and biocompatible devices able to scavenge biomechanical energy from human motion, such as arm swings, stretching or walking as well as from extremely tiny displacements inside the human body (e.g. respiration,
NU
heartbeat, blood flow, eye blinking or muscle stretching) and convert it into electricity for powering
MA
portable and wearable electronics or implantable biomedical devices, such as cardiac-tachometers, pacemakers and artificial retinas [1]. Piezoelectric transducers are a good choice for such application due to the ability of piezoelectric materials to directly convert applied strain energy into
TE
D
electric energy. Aluminum nitride (AlN), by virtue of its good piezoelectric and dielectric properties [2,3], silicon and CMOS compatibility and biocompatibility [4] is a good candidate to produce
CE P
compact and efficient piezoelectric MEMS. It can be also effectively deposited on soft/flexible substrates [5,6] thus enabling the fabrication of compact and lightweight wearable/implantable
AC
microgenerators . The development of such generators could find applications for powering wireless sensor nodes thus overcoming problems related to the employment of conventional batteries including toxicity, limited lifetime and periodic replacing/recharging operations. In the literature two approaches have been exploited for the development of piezoelectric microgenerators/nanogenerators harvesting energy from random and tiny human body motions: the first one employs nanostructured materials [7-9] whereas the second one focuses on thin films [10,11]. Among devices based on inorganic piezoelectric thin films, Kwi-Il Park et al. reported on a flexible nanogenerator based on a biocompatible BaTiO3 thin layer [12] showing open circuit voltage values up to 1 V, with corresponding power densities up to 7 mW/cm 3. However the fabrication process of this device is complicated and additionally BaTiO3 thin film needs annealing and poling treatments to induce a piezoelectric behavior.
ACCEPTED MANUSCRIPT In this work we report on the development of a biocompatible, flexible, lightweight and wearable prototype of piezoelectric energy harvester based on MEMS technologies applied to a soft substrate.
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Flexible devices with controlled residual stress structures and based on AlN thin film have been
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electrical energy from human motion at very low frequencies.
IP
fabricated by microfabrication processes to realize an artificial piezoelectric skin able to scavenge
Representative flexible devices undergoing folding/unfolding states have been characterized to assess the voltage generation properties by providing a mechanical stimulus by a continuous low
NU
frequency finger motion. The device based on a pre–stressed structure (PSS) exhibits an
MA
enhancement of the output generated voltage, if compared to the device based on a flat structure. This result has been attributed to the buckling effect involving a fast transition during the structure deformation, from initial state snapping into a secondary equilibrium state when the limit point
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buckling is reached. Experimental results have been validated by Finite Element Method (FEM), exploited for the calculation of the total elastic energy accumulated under the deformation and the
Experimental
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2.
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mechanical stress generated around the limit point buckling.
Figure 1 sketches the flexible device fabrication process. As shown in Figure 1b the structures, consisting of a sequence of AlN (120 nm), Mo (130 nm), AlN (900 nm) and Mo (210 nm) layers have been deposited on a 25 µm thick polymeric foil (Kapton HN) attached by silicone (Polydimethylsiloxane, PDMS Sylgard 184) to a silicon wafer using a roll to roll system (URAI K printing proofer). Thin films have been deposited by DC magnetron sputtering technique (K.J. Lesker Lab 18 system) without heating of the substrate in a single run in order to minimize contaminations. An optimized thin AlN interlayer has been grown to improve adhesion properties of the Mo/AlN/Mo structure and the crystallographic quality of the piezoelectric AlN film on the amorphous Kapton surface. This strategy has been already exploited [13] demonstrating higher columnar orientation of the
ACCEPTED MANUSCRIPT piezoelectric AlN film on Mo layer grown on a thin AlN interlayer in comparison to the AlN/Mo films deposited directly on the substrate. The Mo layers have been sputtered from a high-purity
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(99.95%) Mo target, in Ar atmosphere at a pressure of 5*10-3 mbar and a power of 200 W. The AlN
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layer has been deposited from a high purity Al target (99.9995%) with a gas mixture of N2 (20
SC R
sccm)/Ar (20 sccm) at a pressure of 1.5*10-3 mbar. The sputtering plasma was generated in pulsed mode with a frequency of 100 kHz and a power of 1250 W.
These deposition parameters of AlN/Mo/AlN/Mo multilayer on the soft substrate resulted in
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compressive PSS, involving a flexible device bending perpendicular to its length axis. Flat
MA
structures have been also grown by a careful optimization of the deposition parameters, in order to compare fabricated devices based on different residual stress. Devices have been realized by microfabrication technologies including photolithography and dry
TE
D
etching techniques by using an inductively coupled plasma (ICP) etching system (Figure 1c). The etching of both Mo top electrode and AlN active layer has been performed in a single step, using a
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gas mixture (20 sccm of SiCl4, 25 sccm of N2 and 7 sccm of Ar). The power applied to the platen and to the coil were 45 W and 100 W, respectively. Another process step consisted of
AC
photolithography and dry etching to pattern both the Mo bottom electrode and the AlN interlayer. Electrical passivation of the devices has been obtained by the deposition of 1 µm thick parylene insulating layer (Figure 1d). The patterning of the parylene film (figure 1e) and the patterning of long metal strips, realized by optical lithography, deposition of Al/Cr thin films and lift-off technique, have been carried out for electrical connections (Figure 1f). After the resist removal, the soft polymeric foil has been cut by a metal blade to define the geometries of the flexible devices. The release/detachment of the devices from the rigid support of Si has been performed by dipping the sample in isopropyl alcohol at room temperature to swell the PDMS adhesive layer under Kapton foil. Figure 1g reports pictures of a realized flexible device, with highlighting the bending of the PSS, shown in the inset.
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ACCEPTED MANUSCRIPT
g
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Figure 1. a-f) The fabrication process of the flexible device for energy harvesting; g) pictures of a realized flexible
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device
Figure 2 reports images of a realized prototype of the device adapted to a flexible glove in order to
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follow its deformation during finger motions. It can be observed that the flexible device has an
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excellent conformability if employed as wearable energy harvester.
Figure 2. a) Images of a realized prototype fixed on a finger, b) following the skin deformation
3.
Results and discussion
The working principle of a flexible piezoelectric generator is based on the temporary electric charge generated by a deformation of the material due to an applied external stress.
ACCEPTED MANUSCRIPT In this work, representative AlN flexible devices with a 4 x 6 mm2 size, having a 2 x 2 mm2 piezoelectric area and based on PSS and flat structure have been tested. The devices, adapted to the
T
glove, have been connected to an oscilloscope to evaluate the generated output voltage during
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folding/unfolding states providing a mechanical stimulus by a continuous finger motion.
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Figure 3 reports the generated open circuit voltage for realized flexible devices based on PSS (red line) and flat structure (black line) under a periodic motion around 1 Hz frequency. The device based on PSS exhibits a maximum peak-to-peak voltage of 0.7 V with an estimated maximum
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electrical power of 0.2 µW. This figure shows also an enlarged view of the boxed area for one cycle
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of deformation.
Figure 3. Generated open circuit voltage for realized flexible devices with a pre-stressed structure (PSS, red line) and a flat structure (black line) undergoing folding/unfolding states with an enlarged view of the boxed are for one cycle of deformation
The maximum power, reached when the device is closed on an optimal load matching the device electrical impedance [14], has been calculated taking into account a measured impedance value of 220 kΩ. The estimated power density was 0.4 mW/cm3. The device based on flat structure shows a maximum peak-to-peak voltage as low as 0.12 V, about six times lower than the device based on PSS.
ACCEPTED MANUSCRIPT Such experimental results can be attributed to the buckling effect: a deformation applied to a PSS results in a fast transition from initial unfolding state snapping into a secondary equilibrium state,
T
when the limit point buckling is reached. This transition involves a large variation of the total
IP
elastic energy of deformation versus the PSS deformation. The buckling effect has been already
SC R
described by Yang et al. [15], who integrate a PVDF piezoelectric layer with a pre-stressed flexible substrate, realizing a large size assembled device to scavenge energy by body movements. To validate the experimental results, the total elastic energy of deformation as a function of the
NU
deformation has been analyzed by finite Element Method (FEM) simulations using Comsol
MA
Multiphysics. The mechanical stress behavior around the limit point buckling has been also calculated The model parameters included the geometrical features of the fabricated devices and the mechanical properties of the materials.
TE
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To evaluate the total elastic energy of deformation, the involved elastic materials have been considered as linear and finger motion has been approximated by a displacement of the substrate,
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applied on the tip of the device, up to 1.1 mm towards the z-axis. The residual stress of the PSS has been estimated by Stoney’s equation [16] taking into account the device curvature radius, calculated
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to be 5.5 mm, based on profile measurements (profilometer DektatXT, Bruker). The calculated residual stress resulted to be 80 MPa. This value has been imposed in the FEM simulation to generate the initial bending state, replicating the real curvature of the PSS. The mechanical stress has been calculated as the derivative of the total elastic energy with respect to the deformation [17]. The deformation is defined as a non-dimensional parameter, calculated as δ/L, where δ is the displacement applied to the structure and L is the total length of the substrate. Figure 4 reports representative states of the simulated PSS and the plot of the computational results concerning the elastic energy of deformation and the mechanical stress behavior . Figure 4a shows the initial bending state of the PSS; Figure 4b reports an intermediate state with highlighted the direction and the position of the applied displacement; Figure 4c shows the final deformed state of the structure. Figure 4d shows the simulated total elastic energy of deformation (red line) and the
ACCEPTED MANUSCRIPT mechanical stress (black line) as a function of deformation. The elastic energy curve confirms the existence of a secondary equilibrium state due to a local energy minimum during the folding of the
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flexible structure.
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Figure 4. a) the simulation result of the initial bending state of the PSS; b) an intermediate state of the deformation, with the green arrows representing the direction and the position of the applied displacement; c) the final deformed structure; d) the total elastic energy of deformation (red line) and the mechanical stress (black line) versus deformation
In the same plot an increase of the mechanical stress around the limit point buckling, where the total elastic energy experiences a fast transition, can be observed.
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Conclusion
In this work AlN-based piezoelectric flexible skin as wearable energy harvester has been successfully fabricated. Flexible devices integrating a pre-stressed structure (PSS) when subjected to a deformation have shown an enhancement of the generated output voltage, if compared to
ACCEPTED MANUSCRIPT devices based on a flat-shape structure. A representative device with a 4 x 6 mm2 size and having a 2 x 2 mm2 piezoelectric area undergoing finger motion at 1 Hz frequency exhibited a maximum
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peak-to-peak generated voltage of 0.7 V, about six times higher than the voltage generated by a
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device with a flat structure. These results have been attributed to the buckling effect, which implies
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a fast snapping transition increasing the mechanical stress and validated by FEM simulations. This approach results to be very promising for energy harvesting from human motion because of the
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simple fabrication process and the device conformability.
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Acknowledgements
This research has been funded by national Italian Project PON ITEM.
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References:
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ACCEPTED MANUSCRIPT [7] Fang, J., Wang, X., & Lin, T. (2011). Electrical power generator from randomly oriented electrospun poly (vinylidene fluoride) nanofibre membranes. Journal of Materials Chemistry,
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[12] Park, Kwi-Il, et al. "Piezoelectric BaTiO3 thin film nanogenerator on plastic substrates." Nano Letters 10.12 (2010): 4939-4943.
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[13] Kamohara, T., Akiyama, M., Ueno, N., & Kuwano, N. (2008). Improvement in crystal orientation of AlN thin films prepared on Mo electrodes using AlN interlayers. Ceramics International, 34(4), 985-989. [14] Priya, S. (2005). Modeling of electric energy harvesting using piezoelectric windmill. Applied Physics Letters, 87(18), 184101. [15] Yang, B., & Yun, K. S. (2012). Piezoelectric shell structures as wearable energy harvesters for effective power generation at low-frequency movement. Sensors and Actuators A: Physical, 188, 427-433. [16] Stoney, G. G., The Tension of Metallic Films Deposited by Electrolysis, 1909, Proc. R. Soc. London, Ser. A, 82, pp. 172–17. [17] Carpinteri A., Scienza delle costruzioni, 1, 1992, pp. 240-246
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Graphical abstract
ACCEPTED MANUSCRIPT Highlights
Flexible AlN-based energy harvester scavenging electric energy from human motion is
Piezoelectric skin integrates a pre-stressed structure (PSS) improving the generation of
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developed
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output voltage
The increase of generated voltage is attributed to fast snapping transition during the skin deformation (buckling effect) of the PSS
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The experimental results are validated by FEM simulations demonstrating the buckling
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effect
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