Piezoelectric Fabrics for Energy Harvesting F06-GT05

Size: px

Start display at page:

Download "Piezoelectric Fabrics for Energy Harvesting F06-GT05"

Derek Hines
6 years ago
Views:

1 Piezoelectric Fabrics for Energy Harvesting F06-GT05 Project Leader: Principal Investigators: François M. Guillot, Georgia Institute of Technology, ME Haskell W. Beckham, Georgia Institute of Technology, PTFE Johannes Leisen, Georgia Institute of Technology, PTFE Goal Statement The goal of this project is to demonstrate the feasibility of integrating piezoelectric materials into fabrics, and to use these fabrics as the basis for the construction of systems capable of harvesting mechanical energy from the environment and delivering usable electrical power. Abstract We aim to develop textile fabrics capable of converting mechanical energy into electrical energy. The energy can come either from the environment (wind), or directly from the user of the textile fabric (mechanical energy from motion). For example, piezoelectric fabrics could be used in the construction of wind-harvesting banners or in the design of clothing capable of collecting some of the mechanical energy associated with walking or running. The harvested energy would be used to either directly power a device, or to recharge a battery without having to remove it from its device. Examples of such devices include wireless sensor networks, wearable instruments, and light-emitting diodes. This report presents a brief overview of energy harvesting technologies based on piezoelectric materials. It then describes the experimental testing techniques that have been used to assess the energy-harvesting capabilities of various sensors, and closes with the presentation of preliminary results. I. Introduction Harvesting waste mechanical energy is an attractive alternative to strict reliance on traditional batteries with limited lifetimes. Low-power wireless sensors are used in hundreds of commercial and military applications, where battery replacement is often impractical for a number of reasons (e.g., cost, inaccessible locations, etc.). On the other hand, wearable devices will undoubtedly multiply in the years to come, due to the constant decrease both in size and power requirements of complex digital systems. Batteries for these devices also present a number of disadvantages, such as their bulky size and their need to be replaced or recharged periodically.

2 To address these battery-related problems, a number of researchers have recently proposed using energy harvesting systems, i.e. systems capable of transforming mechanical energy into usable electrical energy. Most of these systems rely on piezoelectric materials incorporated in some kind of structure subjected to mechanical excitations. Typically, these structures consist of simple, classical mechanical systems, such as cantilever beams or compressible rubber patches. Recent advances in the development of piezoelectric ceramic fibers offer promising alternatives to the reliance on bulk piezoelectric materials, and has opened the way for innovative solutions to energy harvesting. These fibers have already been used in composite structures, where they combine the advantages of the large piezoelectric constants of ceramics with the mechanical flexibility of polymers. This projects aims at studying the possibility of integrating piezoelectric fibers directly into fabrics and to assess whether these piezoelectric fabrics would be capable of harvesting sufficient mechanical energy from the environment (human motion, wind, ) to power small electronics. Recently, much has been written about the integration of electronics into textiles or "electrotextiles". For the most part, however, these novel textiles act as passive elements that simply conduct electricity between power sources (e.g., batteries) and various devices such as high-brightness light-emitting diodes (HBLEDs) in an integrated and flexible substrate. Even when the phrase "energy-harvesting textile" has been used, the textile is a simply a passive component that conducts electricity generated by solar cells or by thermoelectric generator chips (i.e., convert body heat to electricity) integrated into the fabric. In contrast, we propose to develop fabrics that are active elements for energy harvesting; the mechanical motion of the fabric itself generates the electricity. II. Background Piezoelectric materials are capable of converting mechanical excitations into electrical outputs and vice-versa. They are widely used in numerous applications, ranging from acoustic transducers to mechanical actuators. Two common types of piezoelectric materials are ceramics (such as lead zirconate-titanate, or PZT) and polymers (such as polyvinylidene fluoride, or PVDF). Polymers have the advantage of being soft and flexible, but they have lower dielectric and piezoelectric constants than ceramics. Recently, however, a method called the Viscous Suspension Spinning Process (VSSP) has made it possible to produce ceramic fibers with diameters as small as 5 microns. PZT fibers made with this low-cost technology can be incorporated into composite structures with non-piezoelectric polymers, or even directly woven into traditional fabric, thereby combining the flexibility of PVDF with the large piezoelectric coefficients of PZT. Studies have shown that, in the case of ceramic/polymer composites, when the fiber volume fraction exceeds a critical value, the effective piezoelectric coefficients of the composite become comparable to, or even larger than the bulk ceramic material. A. Energy harvesting with monolithic piezoelectric elements Figure illustrates the basic principles of energy harvesting with a bulk piezoelectric material as the transducing element. In this example, two continuous electrodes of surface area A are deposited on opposite sides of the slab of piezoelectric material. These electrodes, made of

3 3 an electrically conductive material, have been used to pole the piezoelectric element in the thickness (referred to as 3 ) direction (poling consists in applying a large DC electric field to the material in order to permanently align its molecular dipoles; this process is a necessary step to render the material piezoelectrically active, and defines the geometry of this activity). The poling direction is represented by the polarization vector P r. If a stress X is applied to the material along its length (referred to as the direction), then a voltage V will appear across the electrodes. This configuration is usually designated as the 3- mode, in reference to the piezoelectric coefficient that relates the electrical variables to the mechanical ones: V d 3 = X, () ε t X 33 X ε 33 where t is the thickness of the sensor, is the (thickness) dielectric permittivity coefficient of the material, and is its (longitudinal) piezoelectric strain coefficient. d 3 3 Electrodes - V t X P + Poling direction Monolithic piezoelectric material FIG.. Illustration of energy conversion in the 3- mode: conventionally poled monolithic piezoelectric material subjected to a longitudinal mechanical stress X. Furthermore, the electrical power generated by the sensor under the mechanical excitation defined by X is given by d3 Π = At X f X, () ε 33 where f if the frequency of the mechanical excitation. Thus, one can see that the available power from such a system is proportional to the material s piezoelectric coefficient and is inversely

4 4 proportional to its dielectric constant. The power can also be expressed in terms of the capacitance C of the sensor: Π = CV f, (3) illustrating the proportionality between these two quantities. B. Energy harvesting with piezofibers The main drawback of using monolithic ceramic piezoelectric materials is their mechanical rigidity and brittleness. In some applications, they can be replaced with piezoelectric polymers, which offer a much greater flexibility, but at the expense of the energy conversion efficiency (polymers have much lower piezoelectric coefficients than ceramics). Composite sensors, made of piezoelectric ceramic fibers embedded in an appropriate compliant polymer matrix, have the potential to overcome these limitations, in the sense that they exhibit the same level of piezoelectric activity as bulk ceramics, combined with the flexibility of soft polymers. In contrast with the configuration described in the previous section, piezoelectric devices incorporating ceramic fibers often rely on a different type energy conversion mode, namely the 3-3 mode. This mode is achieved by using a more complex electrode pattern, referred to in the literature as interdigitized or interdigitated, sometimes abbreviated IDE (InterDigitated Electrode). This type of device is illustrated in Figure, which shows the top view of a composite sensor made with PZT fibers inside a polymer matrix. It also shows a typical IDE pattern, which may also be present at the bottom of the device (some applications require electrodes on one side only). Polymer matrix - + PZT fibers Interdigitated electrodes FIG.. Piezofiber composite structure.

5 5 Figure 3 represents a side view of the sensor depicted in Figure, without the polymer matrix, and with a double-sided electrode configuration. When the IDEs are used to pole the PZT fibers, they create a relatively complex electric field inside the material (as represented by the field lines in Fig. 3), resulting in a net, permanent polarization along the fiber axis. When this sensor is subjected to a stress along its length, this mechanical excitation is now parallel to the polarization vector, corresponding to the so-called 3-3 mode (the 3-3 nomenclature comes from the usual conventions of the theory of piezoelectricity, to which we adhere in this document; note however that, in Figure 3, the length of the fiber is aligned with the direction). Therefore, in this configuration, the voltage V measured across the electrodes is related to the stress X by the d 33 coefficient, instead of the d 3 one. The motivation for doing this is to be found in the fact that, in most ceramics, d 33 is about twice as large as d 3, resulting in a potentially higher energy conversion factor. 3 - Electric field lines Piezoelectric fiber material + - t X P IDE electrodes Poling direction FIG. 3. Illustration of energy conversion in the 3-3 mode: piezoelectric material poled along its length with interdigitated electrodes and subjected to a longitudinal mechanical stress X. Computing the available electrical power in this type of IDE device is not trivial, however, and one cannot obtain a simple, exact expression analogous to Eq. (). Approximate expressions can be derived, based on assumptions made to compute the electric field []. These approximations depend, among other parameters, on the number of electrode digits, their width, as well as their separation. Theoretical analyses have shown that, in fact, the 3-3 IDE configuration is less efficient than the traditional 3- configuration, for the same volume of

6 6 active piezoelectric material [3]. These theoretical results were also confirmed by an experimental study [4] which demonstrated that the harvested power in the 3- mode was about 3 times larger than in the 3-3 mode. These results can be explained by the fact that, even though d 33 is larger than d 3, the sensor capacitance in the latter case is larger than in the former. Therefore, the 3-3 IDE configuration does not appear to offer significant advantages. Finally, one should note that, even though the 3- mode has been used traditionally with monolithic (bulk) materials, and that the 3-3 IDE mode has been mostly used with fiber composites, they are interchangeable, and the 3- mode can be used with fibers, for example. Based on the above discussion, the next two sections outline our approach to PZT fiber integration into fabrics, and to performance evaluation. III. Fiber Integration PZT fibers are commercially available in two different forms: green material (not fired), and unpoled, fired material. Green PZT fibers are flexible and can be easily manipulated (they can be bent with small radii of curvature) without damage. Once fired, however, they become rigid and brittle, and therefore extremely delicate to handle. Embedding several fibers inside a polymer matrix, as is commonly done, is not a viable option in this project, since our goal is to integrate the fibers into traditional fabrics, which need to retain as many of their original fabriclike characteristics as possible, including breathability. We therefore propose to coat individual fired PZT strands with a polymer jacket to produce a flexible piezoelectric fiber. The resulting structure would be analogous to that of an optical fiber cable, where the fragile glass core is encased inside a protective, flexible plastic jacket. These coated fibers can then be woven into regular fabrics, with the proper orientation, as determined by the use of the final product. The PZT fiber pattern can be mono- or bi-directional, depending on the type of mechanical solicitation that it will undergo. The next step is the poling of the PZT material. For reasons of simplicity and greater energy conversion (as discussed above), we propose to rely on the 3- mode of piezoelectricity. We therefore need to apply a large DC voltage across the thickness of the fabric, in order to create a permanent polarization within the PZT fibers. This can be accomplished in two ways. The first approach is to place the fabric between two conductive plates and to apply the poling electrical field across these plates. The second approach is to have the permanent electrodes installed on the sensor, and to use these both to apply the poling field and later to collect the electrical power when the fabric sensor is used in its energy harvesting mode. In addition to the dielectric breakdown limit of the coating polymer, both its thickness and its dielectric constant will have to be known, in order to estimate the actual electric field inside the PZT material. Among the anticipated challenges to overcome are the application of the poling field without producing electrical arcing between the electrodes, and the need to prevent fiber rotation once they have been poled. Figure 4 illustrates the concepts described above and represents a schematics of a mono-directional PZT fabric. We will also experiment with the nature of the conductive electrodes. We anticipate that initial laboratory tests will be performed with relatively simple procedures, such as the deposition of conductive ink. Real-life systems will involve more practical features, such as commercially available conductive fabrics. Anticipated issues to be addressed include prevention of electrode shorting, electrical insulation for the user/wearer and weatherproofing.

7 7 3 P PZT fiber Polymer jacket Electrode Fabric FIG. 4. Schematic representation of an energy harvesting textile sensor with mono-directional, coated PZT fibers embedded in a piece of fabric IV. Experimental A. Fiber Coating In order to improve the mechanical flexibility of the ceramic fibers, which are rigid and brittle, we coated them with a UV-cured acrylate oligomer, as illustrated in Figure 5..5 cm UV lamp Cure twice top and bottom FIG. 5. Schematic representation of UV curing process The resulting product is shown in Figure 6, which is a cross-sectional photograph of a coated fiber. Future work on this aspect of the project will include better control of the jacket thickness uniformity, as well as optimization of the composite mechanical properties.

8 kv/mm was applied to the fibers for one hour, at 0 C.

8 8 FIG. 6. Cross-sectional view of a PZT fiber coated with a heated acrylate oligomer at 70ºC for 30 minutes. B. Polarization For testing purposes, we have been polarizing fibers by sandwiching them between two copper plates immersed in a heated oil bath. A polarizing electric field of.8 kv/mm was applied to the fibers for one hour, at 0 C. The silicone oil was used because of its superior electrical insulating properties; the oil bath was degassed by the application of a vacuum overnight. The resulting polarization is along the thickness direction of the fibers. C. Experimental apparatus for sensor testing We believe that our thickness-polarized fibers offer the most promising potential for our intended application, and we decided to test their energy-harvesting performance against that of two commercially available sensors: one polymer sensor and one fiber-based sensor. The three sensors were glued to an aluminum beam, as shown in Figure 7; their piezoelectric operating mode is indicated in brackets. The polymer sensor has two uniform electrodes covering most of its surface area on both sides; the composite sensor has double-sided interdigitated electrodes. PVDF polymer film sensor [d 3 ] (Measurement Specialties, Inc., DT4-05K) Piezo fiber / polymer matrix composite sensor [d 33 ] (Advanced Cerametrics, Inc., PFC) 30 thickness-polarized fibers [d 3 ] FIG. 7. Energy-harversting sensors operating in different modes.

9 9 The detailed structure of the sensor made with our thickness-polarized fibers is illustrated in Figure 8.; it uses the aluminum beam as its bottom electrode and a layer of silver ink as its top electrode. Silver ink Epoxy Silver ink / conductive epoxy Al. beam FIG. 8. Thickness-polarized fibers and associated electrodes on an aluminum beam. The fundamental vibration mode of the beam, characterized by a resonance frequency of 4.6 Hz, was excited, and the resulting vibrations were measured by a laser Doppler vibrometer, as illustrated in Figure 9. This configuration had the advantage of providing the same welldefined mechanical excitation to each sensor, whose electrical response was measured by connecting the electrodes directly to an oscilloscope. Laser vibrometer Clamped, cantilever beam FIG. 9. Experimental setup to measure the vibration-induced electrical response of energy-harvesting sensors.

10 0 The capacitance of each sensor was measured and was used in conjunction with the measured voltage output to compute the available energy G, according to the following formula (see Equation (3)): CV G =. (4) These experimental results are summarized in the table below: Polymer Sensor PFC Sensor PZT Fibers Capacitance (nf) Active volume (mm 3 ) Voltage output (mv) Energy (nj) Energy / Volume (mj/m 3 ) Our PZT fibers generated about 7 times more energy than the polymer sensor, but about 7 times less energy than the composite sensor. However, the capacitance measurement of our fibers also revealed that it is lower than the theoretical value by a factor of 0. This is due to the fact that air pockets as well as some epoxy are present between the two electrodes. The theoretical capacitance value can be approached by completely filling the space between the electrodes with the piezoelectric material only. In this case, the available energy from our fibers should increase by a factor of more than 000, clearly outperforming the current piezoelectricbased energy harvesting technology. V. Future Work In order to achieve this projected performance, we propose to deposit both electrodes on individual fibers, taking care not to short them. This should greatly enhance the amount of harvested electrical power, as discussed above, and will also eliminate the requirement of maintaining the proper fiber orientation with respect to the electrodes. Furthermore, the fibers will be coated with the flexible polymer jacket after the electrode deposition, which will solve the issues of electrode shorting and electrical insulation at the fabric level. Graduate Student Contributor: Sunghyun Nam Project Website:

Highly piezoelectric, thermal stable ferroelectrets from cyclic olefin copolymer. Yan Li, Hui Wang, Changchun Zeng ANTEC 2015, Orlando

Highly piezoelectric, thermal stable ferroelectrets from cyclic olefin copolymer Yan Li, Hui Wang, Changchun Zeng ANTEC 2015, Orlando Content 1. Introduction 2. COC ferroelectret 3. Hybrid COC-PDMS ferroelectret