Determination of the best Lead-Free Piezoelectric Material for Energy Harvesting Applications

Transcription

1 Determination of the best Lead-Free Piezoelectric Material for Energy Harvesting Applications Compiled by: Brian LaQua Materials Science & Engineering Undergraduate December 1 st, 2011 Prepared for: Christine Grohowski Nicometo Instructor, EPD 397

2 Executive Summary As demonstrated by the U.S. Department of Energy (DOE) in their May 2011 report, there is a large desire for renewable energy sources. Therefore, many types of large-scale renewable energy are being explored and improved, such as solar, wind, and geothermal power. One renewable energy source aims to assist the solution on a much smaller scale by replacing batteries in small devices and reducing the power requirements of larger systems 1 [1]. This source, called piezoelectric energy harvesting, utilizes an atomic property of piezoelectric materials to convert mechanical motion to electrical energy. This energy conversion is accomplished through slight atomic deformations in a material, creating electric dipoles and electrical charge. The leading piezoelectric material is Lead Zirconate Titanate (PZT), which has been developed for over a half a century, explaining its superior piezoelectric properties 2 [2]. Nonetheless, lead is extremely harmful to humans, making it one of the most regulated substances in the world today 3 [3]. Due to these regulations, industry has been pressured to replace lead and lead compounds [3]. Four lead-free piezoelectric materials were considered in this comparison; Barium Titanate (BaTiO 3 ), Polyvinylidene Flouride (PVDF), Potassium Sodium Niobate (KNN), and Zinc Oxide nanowires (ZnO). Factors considered included density, piezoelectric properties, Curie temperature, manufacturing ease/cost, and durability. The piezoelectric properties include the piezoelectric coefficient, d, and the electromechanical coupling coefficient, k. Both are a measure of the effectiveness of energy conversion and have subscripts that indicate the coupling mode, such as d 33 or d 31. The Curie temperature is maximum temperature the material will exhibit piezoelectric properties, thereby indicating the sensitivity to temperature fluctuations. Barium Titanate (BaTiO 3 ) based compounds currently show the highest d 33 values of lead-free materials, around 584 pc/n 4 [4]. This is only observed at low temperatures, restricting applications of the material to room temperature. BaTiO 3 has the advantage of manufacture ease, but suffers from cracking and therefore short life due to its polycrystalline nature 56 [2, 5]. Therefore, BaTiO 3 is an adequate choice when a commercially available material is needed with high performance, and durability is not a major concern. Although PVDF is commercially available, cheap, and lightweight, it exhibits very low piezoelectric properties, resulting in low power densities [6]. For example, the d 33 and k 33 values of PVDF are 20 pc/n and 0.16, respectively [6]. This does not mean that it is bad for all applications though. If there is no size restriction, and a lightweight, commercially available material is desired, PVDF is a good option. Potassium Sodium Niobate (KNN) based piezoelectrics show the most promise to replace PZT on the macroscale given their relatively recent discovery and high piezoelectric properties at high Curie temperatures [2]. KNN materials exhibit d 33 values up to 416 pc/n and k 33 values around 0.6 with Curie [1] W. Hankle, Energy harvesting. American Ceramic Society Bulletin [Online]. 89(1), pp Available: [2] P. K. Panda, "Review: environmental friendly lead-free piezoelectric materials," J. Mater. Sci., vol. 44, pp , 10, [3] J. M. Shoenung, "Lead compounds," in Ceramics and Glass Materials: Structure, Properties and Processing, J. F. Shackelford and R. H. Doremus, Eds. 2008,. [4] W. L. Xiaobing Ren, Lead-Free Piezoelectric Material, US 2011/ A1, Mar [5] T. R. Shrout and S. J. Zhang, "Lead-free piezoelectric ceramics: Alternatives for PZT?" Journal of Electroceramics, vol. 19, pp , Sept., [6] C. Jean-Mistral, S. Basrour and J. Chaillout, "Comparison of electroactive polymers for energy scavenging applications," Smart Mater. Struct., vol. 19, pp , i

3 temperatures up to 460 C [2, 5]. Additionally, the single-crystal structure of KNN increases the durability of the final product but introduces manufacturing troubles [2]. Because KNN is currently limited by manufacturing troubles, it would be a great choice for many applications in future designs rather than immediate adaptations. On the microscale or nanoscale, ZnO nanosystems show great promise. Devices already fabricated have produced a higher voltage than a AA battery 7 [7]. ZnO has low d 33 and k 33 values, around 5.9 pc/n and 0.09, respectively 8 [8]. This is compensated by utilizing nanowires to maximize efficiency, but for that reason, will not be commercially available for quite some time 9 [9]. With more advancement in nanoscience, manufacturing costs are likely to reduce. These materials are all very different, so ultimately only factors most relevant to the particular application will be important in the material selection. Implementation of lead-free piezoelectric energy harvesters should begin, eliminating harmful lead and reducing the dependence on batteries. [7] R. F. Service, Nanogenerators tap waste energy to power ultrasmall electronics. Science 328pp [8] H. Kim, W. Lee, H. V. Rasika Dias and S. Priya. Piezoelectric Microgenerators Current status and challengesm, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 56(8), pp [9] Zhong Lin Wang, Nanogenerators for self-powered devices and systems, Georgia Institute of Technology, SMARTech digital repository, 2011 ( ii

4 Table of Contents 1. Introduction The Need for Renewable Energy and How Piezoelectric Materials Can Help Why the Leading Piezoelectric Material, Lead Zirconate Titanate (PZT), Must be Replaced Comparison of Lead-Free Alternatives to PZT Comparison of Material Properties Density Piezoelectric Coefficient: Polarization per a Given Stress or Strain Electromechanical Coupling Coefficient: Ratio of Mechanical and Electrical Energy General Remarks about Manufacturing and Durability The Current Status of Piezoelectric Energy Harvesting Devices Conclusion References List of Figures Figure 1: CO2 concentration in ice cores for the last 650,000 years Figure 2: Atomic and molecular basis for the piezoelectric effect Figure 3. Comparison of two practical piezoelectric coupling modes Figure 4: Density comparison of competing lead-free piezoelectric materials Figure 5: Room temperature values of the piezoelectric coefficient, d 31 (a) & d 33 (b), as a function of Curie temperature for various piezoelectric materials Figure 6: Room temperature values of the electromechanical coupling coefficient, k 31 (a) & k 33 (b), as a function of Curie temperature for various piezoelectric materials Figure 7.Schematic of Zinc Oxide Lateral Integrated Nanogenerator (LING) List of Tables Table 1: Sources of mechanical energy in everyday life which can be harvested for electrical energy Table 2. Summary of considerations in selection of piezoelectric materials for energy harvesting iii

5 1. Introduction According to the United States Department of Energy (DOE), more than 80% of total United States energy currently comes from fossil resources [1]. In the same May 2011 report, the DOE set a goal to provide 80% of America s electricity from clean energy sources by According to the DOE s report, There is compelling evidence that carbon-dioxide emissions from human activities are adversely affecting our climate. The human use of fossil resources is the major source of these emissions. One clean energy project with zero harmful emissions that is gaining popularity is the use of piezoelectric materials for energy harvesting applications. Piezoelectricity is an ability of certain materials to convert mechanical motion or stress into electrical energy, and vice versa. Because of this property, they have long been used as sensors, transducer, and actuators. In the case of sensors and transducers, stress, motion, or pressure is converted into an electrical signal. Piezoelectric actuators expand or contract when voltage is applied. Due to the increasing concern over climate change, a new application for these materials have emerged, aimed at harvesting electricity from mechanical motion or stress. Some interesting examples include: laying the material in roads to harvest energy from automobile weight, placing flexible piezoelectrics in waves or wind to generate electricity through natural motion, or creating nanodevices that can power personal electronics through body movement [2] [3]. These devices will likely never be able to produce power on the same scale solar or geothermal power can, but possess great potential for replacing batteries in portable devices or reducing the power needs of larger systems. Piezoelectricity is not a new idea. Nobel laureates Pierre and Jacques Curie discovered the property in 1880 while studying quartz [4]. It wasn t until 1950 that Lead Zirconate Titanate (PZT) was discovered, and observed to have very favorable piezoelectric properties [4]. Since then, it has been extensively researched, making it the leading piezoelectric material, used extensively as sensors, actuators, and capacitors [4]. Several variations of it have even been tailored for high temperature applications while maintaining relatively high piezoelectric properties. A 2009 estimate of the PZT market was tens of billions of dollars worldwide [4]. There is a very important disadvantage of PZT, however, due to the high lead content. Lead has long been considered an environmental health hazard because of the adverse effects on intellectual and neurological development [4]. The same study also points out that lead can affect the genetic transcription of DNA. Furthermore, lead oxide is volatilized during calcination and sintering, two manufacturing processes of PZT, causing environmental pollution [4]. For these health and environmental reasons, strict regulations have been enforced on lead [5]. Since there is no equivalent substitute for PZT, its use has been allowed to continue [4]. Environmental regulations have shifted the driving force of piezoelectric research to lead-free replacements of PZT, and some promising results have been produced. Also discovered in 1950, Barium Titanate (BaTiO 3 ) has been heavily developed in recent years. A more recent discovery is Potassium Sodium Niobate (KNN), which is considered one of the most promising candidates to replace lead-based piezoelectrics [4]. Another piezoelectric, Zinc Oxide, operates on a much smaller scale, making it very different from the other materials considered in this report. Nanowires of this material were capable of lighting a LED [3]. Lead-free piezoelectrics are not limited to ceramic materials, however. Another promising material is a polymer called Polyvinylidene Flouride (PVDF). A recent study fabricated a PVDF leaf that produced enough power to continuously light up a commercial LCD [3] [6]. 1

6 In this report, these competing lead-free piezoelectric materials will be compared to evaluate which will be most likely to succeed for energy harvesting applications. Properties to be considered include the density of the material, the piezoelectric coefficient, and the electromechanical coupling coefficient. A qualitative review of manufacturing cost and the expected life of the material will be provided. Lastly, the power output of some of the most effective devices made so far will be shown to demonstrate the current status of the industry. The properties of these materials vary greatly, so it is likely that different applications of piezoelectrics will require different materials. If one material can be made effective and affordable, energy harvesting devices can begin to replace batteries in devices used every day. 2. The Need for Renewable Energy and How Piezoelectric Materials Can Help To begin to understand energy harvesting piezoelectrics, we must first understand one of the driving forces for current research. According to the Intergovernmental Panel on Climate Change, Scientific evidence for warming of the climate is unequivocal [7]. Some of their evidence includes a global sea level rise of 17 inches over the last century and the Greenland Ice sheet losing 36 to 60 cubic miles of ice per year between 2002 and Other climate changes can be linked directly to human interaction. For example, surface ocean waters have increased in acidity 30% since the industrial revolution, a result of higher carbon dioxide, or CO2, absorption [7]. Carbon dioxide emissions into the environment are a direct result of human use of fossil fuels [1]. To defend against the argument that the climate naturally cycles over long periods of time, the National Oceanic and Atmospheric Administration has measured the carbon dioxide concentration in ice cores over the last 650,000 years, shown in Figure 1. This clearly shows current CO2 levels are higher than ever before, and spiked after the industrial revolution [7]. This link between carbon dioxide, fossil fuels, and climate change has led to large efforts for renewable energy sources. Figure 1: CO2 concentration in ice cores for the last 650,000 years. CO2 is believed to cause changes in climate by the greenhouse effect [7]. One can clearly see that since the industrial revolution, CO2 concentration has never been higher. Because of information like this, there have been large efforts from government agencies to reduce our dependence on fossil fuels, a major contributor to CO2 pollution [7]. Many sources of renewable energy exist, but piezoelectric energy harvesting is particularly interesting because the nature of its energy originates from the atomic structure. Piezoelectricity is the ability of a material to produce a voltage when stress is applied or expand/contract upon an applied voltage [5]. Simply put, piezoelectric materials can convert mechanical energy into electrical energy, and vice versa. This property stems from shifts in the atomic structure of the material. Figure 2 shows that when a stress is applied, distortions in the positions of atoms create electric dipoles, or an electrical charge. In a process called poling, the material is exposed to a strong electric field, orienting the dipoles in the same direction [4]. When several of these dipoles are aligned in the same direction, the result is a material with a voltage drop across it, producing electrical energy [8]. 2

7 Figure 2: Atomic and molecular basis for the piezoelectric effect. a) Atomic model of an unstressed piezoelectric material, where the atoms are arranged such that the positive and negative charges cancel one another. b) Atomic model of stressed piezoelectric material subjected to compressive force, F. Movement of positive and negative charges creates a net electric dipole, or charge. c) Molecular model of piezoelectric material that has been poled under an electric field to align the dipoles in the same direction. d) When connected to a circuit and stress is applied, the voltage across the piezoelectric material causes a flow of electric charge. Adapted from [8]. There are two practical coupling modes, meaning the way the mechanical energy and electrical energy are related [9]. These modes are called the -31 and -33 modes, shown in Figure 3. In the -31 mode, the force is perpendicular to the direction of the poled molecules and resultant voltage [9]. On the contrary, the force is applied in the same direction as the molecular poling and voltage in the -33 mode [9]. An example of the -33 mode would be the compression of a piezoelectric block in the sole of a shoe, whereas the -31 mode may be the bending of a PVDF sheet in wind [9]. The coupling mode is used as a subscript of many piezoelectric properties. The piezoelectric coefficient, d, and the Figure 3. Comparison of two practical piezoelectric coupling modes. a) -31 coupling mode, where the direction of the force in the material is perpendicular to the direction of the molecular poling and resultant voltage. b) -33 coupling mode, where the force, molecular poling, and resultant voltage are in the same direction. The -33 mode has higher piezoelectric properties, but is used less frequently for energy harvesting because higher forces are required to overcome the high compressive stiffness and efficiently produce electricity. Adapted from [8, 9]. electromechanical coupling coefficient, k, are two piezoelectric properties that evaluate how effective a material is at converting mechanical energy into electrical energy [10]. A comparison of these two coupling modes is essential to determine which material is best for a particular energy harvesting application. The -33 mode produces much better piezoelectric properties, but because 3

8 straining the material in compression is difficult due to high mechanical stiffness, this mode is only effective in a high force environment [9]. Although the -31 mode has lower piezoelectric properties, larger strains will result from smaller input forces in -31 loading, producing more power in low-force situations. Piezoelectric materials utilize both of these coupling modes in many applications of energy conversion. They are most widely used in actuators, sensors, and transducers [5]. In the case of sensors and transducers, mechanical motion or stress is converted to an electrical signal. One application of piezoelectricity that the general public would be familiar with are push-button lighters, such as those on a gas grill. When the lighter button is pushed, a hammer strikes a long piezoelectric element, mechanically stressing the material [10]. An open circuit voltage is created through the piezoelectric effect, creating a spark that ignites the fuel [10]. Now, these materials are being researched for applications in energy harvesting, with good reason. Compared to other energy converters, piezoelectrics produce a high energy density at relatively low mechanical stress-strain levels [10]. Because the nature of piezoelectricity is on the atomic level, energy harvesting devices can be made small enough to replace batteries, even in areas never thought possible, such as the human body [11]. Furthermore, since the size of many sensor and micro systems is only constrained by the battery that powers it, piezoelectrics have the potential to reduce the size of these systems [11]. Plus, sources of mechanical vibration/motion are not hard to find. The body, wind, structures, and machinery are some examples. Table 1 provides some more sources of mechanical energy where piezoelectrics can be implemented. Energy harvesting piezoelectric materials employed in these areas could power sensors, robots, personal electronics, and laptops, replacing the need for batteries, and thereby reducing or eliminating maintenance. Table 1: Sources of mechanical energy in everyday life which can be harvested for electrical energy. These examples are just of few of the potential applications for piezoelectric materials. The more sources of energy that are exploited in a system, the more the power requirements of that system are reduced. Furthermore, because the nature of piezoelectricity is on the atomic level, energy harvesting devices can be made small enough to replace batteries, even in areas never thought possible, such as the human body [11]. 4

9 3. Why the Leading Piezoelectric Material, Lead Zirconate Titanate (PZT), Must be Replaced What is preventing implementation of energy harvesting devices utilizing piezoelectricity? Currently, the leading piezoelectric material is Lead Zirconate Titanate (PZT), which has an estimated worldwide market in 2009 of tens of billions of dollars [4]. Although PZT exhibits exceptional piezoelectric properties, it has one major disadvantage; Lead is ranked second on the Priority list of Hazardous Substances reported by the CERCLA (Comprehensive Environmental Response, Compensation and Liability Act, 1980) [12]. The basis for this rank: Lead affects almost every organ and system in the human body, and is known as a possible human carcinogen by the International Agency for Research on Cancer (IARC) and the Environmental Protection Agency (EPA) [12]. Lead and lead compounds are especially harmful to the nervous system of children [12]. To make matters worse, two stages of PZT manufacturing, calcination and sinterization, volatilize lead oxide into the environment [4]. This is alarming because nearly all of inhaled lead is absorbed by the body [12]. For these reasons, lead and lead compounds are one of the most highly regulated substances in the world [12]. Products containing lead or lead compounds are classified as hazardous waste, and must be disposed of according to the Resource Conservation and Recovery Act (RCRA) and must be reported to the Toxic Release Inventory (TRI) [12]. These environmental regulations, as well as many more, are forcing industry to find replacements for lead [12]. 4. Comparison of Lead-Free Alternatives to PZT In order to determine which lead-free piezoelectric material is most likely to replace Lead Zirconate Titanate (PZT), the material properties of each must be considered. There are four main factors that have been considered in this research; the density, piezoelectric coefficient, electromechanical coupling coefficient, manufacturing ease/cost, and durability. Lastly, a review of the most effective energy harvesting devices will be presented, to show the current status of the industry Comparison of Material Properties Density The density of a material is an important consideration when trying to reduce the weight of an overall system. Although weight will not be a major consideration in the initial design of the energy harvesting devices, it may be an important area for improvement once these products have been commercialized. Therefore, the densities of the competing lead-free materials were compared in Figure 4. 5

10 Clearly, all materials can reduce the weight of a system involving PZT, however, PVDF is the obvious choice for lightweight applications. One application where weight may be the primary focus is with energy harvesting incorporated into clothing. For example, Kymissis et al developed a PVDF film embedded in a shoe capable of scavenging 1.3 mw [2]. Another example of PVDF in clothing comes from research at University of California-Berkeley, where Liwei Lin produced long PVDF fibers she hopes to weave into cloth, charging portable devices [14]. Similarly, Wang suggests his ZnO nanowires can be incorporated into the fabric of a soldier s uniform to reduce the huge amount of battery weight they are responsible for carrying on a day to day basis [11]. Figure 4: Density comparison of competing lead-free piezoelectric materials. Comparing only the density of the piezoelectric materials, it is clear that all lead-free piezoelectrics are lighter than PZT based systems. PVDF and ZnO, however, are a good choice for lightweight applications, such as clothing. It is important to note that since ZnO nanowires only account for a small portion of the device, density of ZnO systems is affected by the nanowire concentration and the substrate material. Adapted from [2, 4, 11, 13] Piezoelectric Coefficient: Polarization per a Given Stress or Strain. One measure of the effectiveness of a piezoelectric material is the piezoelectric coefficient, d. When the piezoelectric coefficient is expressed as d 31, the direction of the force is perpendicular to the direction of the molecular poling and resultant voltage [9]. If it is expressed as d 33, the force, molecular poling, and resultant voltage are in the same direction [9]. Both of these coefficients measure the polarization generated (columns of electrical charge), per unit of mechanical stress (Newtons of force) [8]. Therefore, the higher the piezoelectric coefficient, the better the material is at converting mechanical stress into electrical charge. Since charge is linearly related to power output, this coefficient is a good indication of the power capabilities of a system. Rather than simply comparing the materials solely by this coefficient, it is more useful to also compare the Curie Temperatures of the materials at the same time, shown in Figure 5. The Curie temperature, T c of a piezoelectric material is the temperature above which a phase transformation occurs, eliminating the piezoelectric properties of that material. A general rule is piezoelectric materials can be used at ½ of their Curie temperature without noticeable decrease in piezoelectric properties [15]. Therefore, the higher the T c, the wider the temperature range that material can be effectively used. Furthermore, by plotting several data points for similar materials, relationships can be observed between the Curie temperature and piezoelectric coefficient of that material. For example, in Figure 5b, variations of PZT show a linear decrease in d 33 as T c increases. 6

11 Figure 5: Room temperature values of the piezoelectric coefficient, d 31 (a) & d 33 (b), as a function of Curie temperature for various piezoelectric materials. Since the piezoelectric coefficient is a measure of the polarization per a given stress, the high d 31 & d 33 values for PZT-based materials have made it an industry leader for decades. However, recent advancements in BaTiO 3 based materials have increased d values to compete with PZT. KNN-based materials show similar d 33 values to PZT at high Curie Temperatures, or the maximum operating temperature of the piezoelectric material. PVDF, although appealing due to its flexibility, is limited by its low d values. The d 31 values of ZnO were not available, but d 33 values were low for this material. Adapted from [2, 4, 5, 13, 15, 16, 17]. Lead-based piezoelectric materials have always produced the highest piezoelectric coefficients, with a maximum d 33 value of 1000 pc/n [15]. The same study also notes that variations of PZT have been tailored for high-temperature applications by altering their T c, making PZT very versatile. A recent patent for a BaTiO 3 -based piezoelectric has produced a d 33 value of 584 pc/n, which begins to narrow the gap in performance between PZT and lead-free materials [5]. The disadvantage of this patented material, however, is a low T c of 110 C, which restricts the material to room-temperature applications. Furthermore, numerous phase transitions at relatively low temperatures create a strong temperature dependence of the piezoelectric coefficient, limiting the overall usefulness of BaTiO 3 [15]. Figure 5 also highlights the major disadvantage of PVDF and ZnO, their piezoelectric coefficients. With d 33 values of 20 and 5.9 pc/n, respectively, the electrical efficiency of PVDF and ZnO is a far cry from PZT, or other lead-free piezoelectrics [2]. This is slightly compensated by the flexibility of PVDF relative to its ceramic counterparts, which allows PVDF to be strained farther without cracking, producing more 7

12 electricity per force [2]. In ZnO, this is lack of efficiency is compensated by the nanostructure of the system. KNN based piezoelectrics are considered as one of the most promising candidates for lead-free piezoelectrics [4]. Although this is the most recently discovered material being considered in this study, this statement can be justified by initial results of testing. KNN based materials exhibit a very high Curie temperature, and relatively good d 31 and d 33 values [4]. With further research, many improvements can be made to this material. It is also interesting to point out that two data points for KNN in Figure 5b match very closely with the linear trend line of PZT, indicating possibilities of future properties of the material Electromechanical Coupling Coefficient: Ratio of Mechanical and Electrical Energy The electromechanical coupling coefficient, k 31 and k 33, is another measure of the effectiveness of a piezoelectric material to convert mechanical into electrical energy [10]. It differs in that it also takes the piezoelectric coefficient, Young s Modulus, and the voltage constant into account [10]. The Young s Modulus is a measure of the material stiffness, with a higher k 33 value resulting from a higher Young s Modulus [8]. The voltage constant measures the voltage produced per a given stress [8]. Materials which produce a high charge and low voltage will exhibit the highest k values [10]. The larger this coefficient, the more suitable the material is for energy harvesting. Similar to the piezoelectric coefficient, when the electromechanical coupling coefficient is expressed as k 31, the direction of the force is perpendicular to the direction of the molecular poling and resultant voltage [9]. If it is expressed as k 33, the force, molecular poling, and resultant voltage are in the same direction [9]. Also similar to the piezoelectric coefficient, it is helpful to compare the electromechanical coupling coefficient to the Curie temperature, T c of the material. This is shown in Figure 6. From Figure 6, it is apparent that of the lead-free materials, KNN demonstrates a better ability to convert mechanical energy into electrical energy. Furthermore, in Figure 6b, it is shown that two variations of KNN have exhibited higher Curie temperatures than PZT [15]. As previously mentioned, ZnO and PVDF suffer from a low piezoelectric coefficient, which is a factor in the calculation for the electromechanical coupling coefficient. Therefore, it is understandable that both also have low k values. 8

13 Figure 6: Room temperature values of the electromechanical coupling coefficient, k 31 (a) & k 33 (b), as a function of Curie temperature for various piezoelectric materials. The electromechanical coupling coefficient, k, uses the piezoelectric coefficient, voltage constant, and elastic compliance of a material to determine its efficiency at converting mechanical energy to electrical energy. Therefore, it is an important property in material selection. The k 31 values of ZnO were not available, but k 33 values were low for this material. KNN exhibits the highest k 31 and k 33 values of the lead-free piezoelectrics, even with high Curie temperature modifications. Adapted from [2, 11, 13, 15, 17] General Remarks about Manufacturing and Durability Two major factors in the commercialization of any material or device are the manufacturing cost/ease and the expected lifespan. Based on the material explored during the research of this report, detailed manufacturing costs are not readily available, so a more general exploration of available research and comparisons will be provided. The expected lifespan of a material can also be evaluated through general knowledge of the molecular structure of the material. These factors are considered in the following paragraphs for each of the four lead free piezoelectric materials discussed in this report. 9

14 Similar to PZT, Barium Titanate (BaTiO3) was also discovered in 1950, making it the oldest of the leadfree piezoelectrics [4]. Its long history has allowed it to be commercially available, manufactured easily and at a relatively low cost [4, 15]. Furthermore, it can be manufactured by several different ceramic techniques, allowing for several options of manufacturing cost and final applicability [4]. There are disadvantages to BaTi0 3, however, due to its polycrystalline nature. Polycrystalline simply means many crystals of different size and orientations are present in the material. For this reason, BaTi0 3 cracks much easier than KNN, which can be produced in single crystals [2, 4]. The polycrystalline structure of BaTi0 3 reduces manufacturing yield as well as the durability of the final product. As previously mentioned, one of the primary advantages of KNN based materials is the durability resulting from the single-crystal structure of the material, allowing for longer lifespans at higher stresses [4]. Another advantage of KNN is that it is biodegradable, making it a very appealing alternative to PZT. Producing single crystals of any material, however, is a complicated process. The manufacturing is further complicated by the evaporation of potassium during the sintering process, creating only partiallydensified ceramics [13]. Furthermore, Niobium, Tantalum and Lanthanum, elements added to KNN materials in significant quantities, are sensitive to moisture and expensive [7]. These reasons lead to a higher production cost of KNN than BaTi0 3 [7]. Since KNN is the newest of the lead-free materials, with more research it is likely that these manufacturing issues will be eliminated or reduced. Similar to BaTi0 3, PVDF is commercially available. PVDF is inexpensive and easy to process due it its polymeric nature [19]. Plus, it is very flexible, allowing it to deform greatly without cracking [19]. This increases the amount of mechanical energy converted and allows for a long life of the material [2]. PVDF is best suited for macroscopic devices in ambient frequencies because it cannot be manufactured well on a microscale, such as for integration into micro-electromechanical systems (MEMS) [2]. Zinc Oxide (ZnO) is the material best suited for MEMS devices because it can be manufactured into nanowires [11]. This give ZnO a unique set of small energy-harvesting applications, such as sensors implanted into the human body [11]. Producing nanowires of any material, however, is expensive and difficult. For this reason, they will not be commercially available in the near future. Plus, because of the small nature of ZnO energy harvesters, they are very sensitive to stress, possibly reducing the life of a device [11] The Current Status of Piezoelectric Energy Harvesting Devices Although the material properties play a large role in the efficiency of a piezoelectric material, there are many other factors within the device that can influence the final power output of the system. This includes factors such as the circuit design, capacitor, and vibration damping. The research and improvement of these factors is outside the scope of this report, however, it is interesting and relevant to examine examples energy harvesting devices that have already been created. First, some devices containing lead will be shown as a reference for comparison. Next, general remarks about the lack of BaTi0 3 in energy harvesting will be discussed. Then, examples of devices made from PVDF, ZnO, and KNN will be briefly explained. Several PZT-based energy harvesting devices are already commercially available, and in use. One piezoelectric energy harvester, made by Cedrat Technologies, can produce 95 mw of power in a device 50 x 32 x 22 mm in size [17]. It is important to note, however, that this energy harvester is currently only in the prototype phase, so cost is not available [17]. A device sold by MIDE for $399 can produce a max voltage of 44.4 volts and 18 mw of power [17, 18]. Similar devices have already found use powering wireless sensors on the pitch link of Sikorsky Blackhawk helicopters [16] 10

15 Although BaTi0 3 discovered the same year as PZT, it was long overshadowed by PZT due to lower piezoelectric properties, and inherently low Curie temperature [15]. No energy harvesting devices utilizing BaTi0 3 were found, likely because it has long been dominated by PZT. Desire to replace lead in materials has created a new surge in the research of BaTi0 3, and it is probable that researchers will try to use BaTi0 3 in future devices. PVDF devices have been created with the intent of harvesting energy from natural motion, such as wind or waves. A brief literature review shows the highest power output of one of these devices was 600 µw, with a power density of 2 mw/cm 3 [6]. This device was a PVDF leaf which fluttered in the wind [6]. By combining multiple leaves into an energy harvesting tree, enough power could be collected to power nearby stationary devices, such as a street lamp. Many energy harvesting devices utilizing Zinc-Oxide (ZnO) nanowires have been created. Wang showed that one of his most recent devices had an output of 11 mw/cm -3 and 2.4 volts, enough to replace the 1.5 volts of an AA battery [11, 14]. Several demonstrations of Wang s nanogenerators have powered commercial LEDs and LCDs, confirming the feasibility of nanogenerators for powering personal electronics [3]. A schematic of one device is shown in Figure 7. Figure 7.Schematic of Zinc Oxide Lateral Integrated Nanogenerator (LING). a) ZnO nanogenerator without mechanical stress or deformation b) Upon bending the device, tension is created in the ZnO nanowires, creating a charge through the piezoelectric effect. Through gold electrical contacts, this charge is transferred to a capacitor for energy storage. Because ZnO uses nanostructures, these devices are not limited by size restrictions. Adapted from [3, 11]. KNN based materials are the newest of the lead-free materials, therefore, not many devices have been created yet [4]. One device, however, produced 24.6 V rms and µw, with a power density of 8 µw/cm 3, when vibrated in 0.7g acceleration [13]. The researchers who created this device add that this was suitable as the electric power source of a sensor network [13]. 11

16 5. Conclusion The desire for renewable energy and environmental regulations of lead has created a surge in the research of lead-free piezoelectric materials. There are many factors to consider when comparing these materials, shown in Table 2.The piezoelectric coefficient and electromechanical coupling coefficient are measures of how effective the material can convert mechanical energy to electrical energy [10]. Other considerations include the density, manufacturing cost/ease, and durability. Table 2. Summary of considerations in selection of piezoelectric materials for energy harvesting. This table provides a comparison of all materials considered in this report. These materials differ greatly, so the application will often determine which factors are most important. Adapted from [2-5, 8, 10, 11, 13, 15, 17] Abbr. (g/cm 3 ) T c [ C] d 33 d 31 k 33 k 31 Lead Zirconate Titanate PZT Base Material Advantages Disadvantages Great performance, Lead is Toxic, Strict comercially available Regulations Barium Titanate BaTiO Commercially available Cracks easily Polyvinlidene Maximum Curie Density Temperature, Material Property Piezoelectric Coefficient [pc/n] Electromechanical Coupling Coefficient Commercially avaiable, lightweight Flouride PVDF Potassium Great performance, Sodium Niobate KNN durable, great potential Zinc Oxide ZnO Micro/nano-applications, lightweight Low performance, large size Manufacturing cost Manufacturing cost and difficulty Barium Titanate (BaTiO 3 ) based compounds currently show the high d values, but only at low Curie temperatures, restricting applications to room temperature. BaTiO 3 has the advantage of manufacture ease, but suffers from cracking and therefore short life due to its polycrystalline nature [4, 15]. Therefore, BaTiO 3 is a good choice when a commercially available material with high performance is needed and durability is not a major concern. Although PVDF is commercially available, cheap, and lightweight, it exhibits very low piezoelectric properties, resulting in low power densities [2]. This does not mean that it is bad for all applications though. If there is no size restriction, and a lightweight, commercially available material is desired, PVDF is a good option. Potassium Sodium Niobate (KNN) based piezoelectrics show the most promise to replace PZT on the macroscale given their relatively recent discovery and high piezoelectric properties at high Curie temperatures [4]. Additionally, the single-crystal structure of KNN increases the durability of the final product but introduces manufacturing troubles [4]. Because KNN is currently limited by manufacturing issues, it would be fitting for applications in future designs rather than immediate modification. On the microscale or nanoscale, ZnO nanosystems show great promise. Devices already fabricated have produced enough power to replace the common AA battery [14]. Zinc Oxide nanowires maximize efficiency and increase applicability through nanostructures, but for that reason, will not be commercially available for quite some time [11]. With more advancement in nanoscience, manufacturing costs will reduce. These materials are all very different, so ultimately only factors most relevant to the particular application will be important in the material selection. Implementation of lead-free piezoelectric energy harvesters should begin, eliminating harmful lead and reducing the dependence on batteries. 12

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