A Proposal to Research Lead-Free Piezoelectric Materials for Energy Harvesting Applications

Transcription

1 A Proposal to Research Lead-Free Piezoelectric Materials for Energy Harvesting Applications Compiled by: Brian LaQua Materials Science & Engineering Undergraduate October 20 th, 2011 Prepared for: Christine Nicometo Instructor, EPD 397

2 Introduction According to the United States Department of Energy (DOE), more than 80% of total United States energy 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 One clean energy project that is gaining popularity-and funding- is piezoelectric materials for energy harvesting applications. Piezoelectricity is an ability of certain materials to convert mechanical motion or stress into usable electrical energy. Since the only input in this energy system is motion or stress, the potential applications for energy harvesting through these materials are nearly limitless. 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 electronic devices [2] [3]. Currently, devices such as these cannot produce power on the same scale solar or geothermal power can, but even small contributions from renewable energy sources will help the DOE accomplish their 2035 goal. In 1950, Lead Zirconate Titanate (PZT) was discovered and observed to have very favorable piezoelectric properties [4]. Since then, it has been extensively researched, and variations of it have produced an industry leading piezoelectric coefficient, d 33, of 580 pc/n [5]. This coefficient is a measure of the polarization observed per a given stress. Therefore, a material with a higher piezoelectric coefficient will produce more electricity for a given stress or strain, improving the efficiency of the piezoelectric device. Other variations of PZT have been tailored for high temperature applications while maintaining relatively high piezoelectric coefficients. For these reasons, PZT is the leading piezoelectric material, used extensively as sensors, actuators, and capacitors [4]. 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, due to the adverse effects on intellectual and neurological development caused by lead poisoning [4]. The same study also points out that lead can affect the genetic transcription of DNA. Furthermore, lead oxide is vaporized 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]. However, since there is no equivalent substitute for PZT, its use has continued [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. One experiment for BaTiO 3 produced an improved piezoelectric coefficient of 584 pc/n, exceeding that of PZT [5]. Another piezoelectric ceramic with the possibilities of interesting applications is Zinc Oxide. 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 capable of producing 2 mw cm -3 in 8 m/s of wind, enough to continuously light up a commercial LCD [3] [6]. These advancements mark great progress, but more improvements must be made in order to use these materials for energy harvesting applications. Statement of Problem Lead Zirconate Titanate (PZT) and other piezoelectric materials have long been known for their fascinating ability to convert mechanical motion or stress into electrical energy, and vice versa. 2

3 This general property of piezoelectric materials has thrust them into the world of renewable energy. Certainly, the demand for renewable energy is strong, as indicated by the DOE s Energy Strategic Plan [1]. PZT should no longer be used, however, due to the toxic effects of the lead oxide present in the material [4]. Luckily, investigations of lead-free piezoelectric materials have long been underway. I would like to determine which lead-free piezoelectric material is most likely to succeed for energy harvesting applications, based on the material properties and economic/manufacturing feasibility. Ultimately, a replacement for PZT must be found in order to eliminate pollution from hazardous lead, and provide opportunities for a new renewable energy resource to meet our growing energy demands. Objectives In this research, I will analyze the advantages and disadvantages of top lead-free piezoelectric materials. My research goals include: Compare competing lead-free piezoelectric materials by evaluating the material properties, including dielectric constant, density, and Curie Temperature Investigate properties of KNaNb (KNN) based piezoelectric materials Determine if different applications of energy harvesting from piezoelectrics require different material properties, and whether this could lead to several materials being successful alternatives to PZT Analyze the safety and availability of the elements in lead-free piezoelectric materials to predict future limitations Investigate the manufacturing methods that are in use or in development to determine if they are feasible and cost-effective for mass production. Since there are many applications for piezoelectric materials, I will focus my research on those specific to energy harvesting applications. Not only will this reduce the information I have to review, but this may also lead to more current information due to the recent surge in demand for renewable energy. I plan to complete the above tasks through detailed analysis of technical reports, journal articles, and patents regarding piezoelectric materials. Preliminary Research As my research began, I quickly realized how many different piezoelectric materials exist today. The reason for the large amount of materials is the numerous additives that have been explored, producing wide-ranging effects on the material properties. To manage all of the options, I grouped the piezoelectric materials into three distinct categories: Ceramics, Polymers, and Single Crystals. Single crystals, such as quartz, exhibit very low piezoelectric properties, and were therefore ignored in this research. One polymer and two ceramic piezoelectric materials, all of which are lead-free, were chosen for preliminary analysis based on recommendations from initial sources. These materials are discussed in detail below. Polyvinylidene Flouride (PVDF) is one piezoelectric polymer that has received a lot of attention recently. PVDF certainly has advantages which justify this attention-it is inexpensive, easy to process, light-weight, and flexible [7]. Flexibility is a critical advantage over ceramic-based piezoelectrics, which are usually rigid or brittle. This advantage allows PVDF to be used in applications in which ceramics cannot, such as backpack straps to charge personal electronic devices [7]. In a wind tunnel experiment, a long PVDF sample generated a maximum voltage of 3

4 61.6 V with a power density of µw cm -3 [7]. This power output exceeded that of a PZT sample of the same size, indicating that in certain applications, the flexible polymer piezoelectric materials may be preferred for energy harvesting applications [7]. While reviewing the ceramic piezoelectric options, it was apparent that large research efforts are based on Barium Titanate derivatives. A newly patented composition, Ba(Zr 0.2 Ti 0.8 )O 3-50%(Ba 0.7 Ca 0.3 )TiO 3, displayed a maximum piezoelectric coefficient, d 33, of 584 pc/n [5]. The inventors point out that this value exceeds the maximum piezoelectric coefficient of PZT by 4 pc/n. However, a limitation to this material is discussed in the patent application. Testing performed by the same inventors reveal that this maximum d 33 value occurs at room temperature, and decreases by a factor of two at 70 C. Therefore, the patent suggests this material is only useful near room temperature, limiting the applications. This patent shows that properties comparable to that of PZT can be achieved, but more research must be done to determine if the temperature range of this material can be expanded, and how easily it can be mass-produced. Another promising ceramic material that displays promising piezoelectric properties is Zinc Oxide. Zhu et al. fabricated a flexible substrate containing an array of parallel ZnO nanowires, producing a peak power density of 11 mw cm -3, enough energy to light up a commercial LED [3]. Furthermore, this device produced a higher voltage than a similar device using PZT nanowires [3]. This research showed possibilities to replace PZT are also available on the nanoscale, which is extremely important for biomedical energy harvesting applications where the lead in PZT would be particularly harmful. A discussion with a professor researching ZnO has led me to pursue another ceramic, KNaNb (KNN). This will be one area of research going forward. Qualifications My desire to research lead-free piezoelectric materials comes from two personal interests. I recently concluded a co-op with Hamilton Sundstrand, an aerospace engineering company, and a division of United Technologies. One of my roles as a member of the Materials & Processes Department was evaluating products for hazardous materials and chemicals, and determining practical alternatives. This work began my interest in the environmental impacts of commonly used materials, such as lead. Another interest that led me to pursue piezoelectric materials is renewable energy. These special materials have the fascinating ability to convert mechanical stress or strain into electrical energy, making them a completely renewable energy source. Four years of undergraduate coursework have provided me with the knowledge of material properties, chemistry, and energy necessary to understand the complex topics discussed in technical reports and scientific journals. Furthermore, I am currently in MSE 451: Ceramics, taught by a professor researching piezoelectric nanostructures and nanodevices for energy harvesting. My background knowledge and current coursework will be useful in the analysis of lead-free piezoelectric materials. Management Plan The following chart outlines a schedule for completion of this research project: 4

5 Timeline Week # / /9-10/15-10/18-10/23-10/30-11/5-11/6-11/13-11/20-11/27- Task Description Dates 10/15 10/18 10/22 10/29 11/5 11/6 11/12 11/19 11/26 12/3 1 MTR Proposal 10/ Research and Organize Information Find sources on Zinc Oxide and PVDF MS&T '11 Trip Discuss Zinc Oxide Research with Professor Appt: 10/13 Review peer Organize Information and choose sources revisions 1.2 Proposal Document during 10/ Submit Final MTR Proposal travel. 10/ Proposal Presentation 10/20 2 MTR Project 11/ Continue Research and Information Organization Pursue KNN based piezoelectrics Research Cost of Manufacturing Method Cost/Safety/Availability of raw Materials Gather efficiencies of current devices. Compile Material Property Data Table. Use formulas to fill gaps in data. Parents in Town. Expect no 2.2 MTR Document available 11/ Introduction & Body time Conclusions & Recommendations Submit 75% Rough Draft & Attend MTR Conference 11/ Revise Document & Submit Final MTR 11/ MTR Presentation 12/1 Weeks 1 and 2 will focus on finalizing the proposal, concluding with an in-class presentation on 10/20/11. This work will be interrupted by my MS&T Conference trip to Columbus, OH, 10/15-10/18. I hope to incorporate peer revisions during travel time. Next, more research will be done in order to effectively evaluate the factors that go into my final recommendation. While this research is being performed, the information will be organized into the MTR rough draft. Following the MTR conference, the rough draft will be revised and the presentation preparations will begin. Conclusion Since lead poses health hazards for humans, replacements for the leading piezoelectric material, Lead Zirconate Titanate (PZT), must improve. There are several lead-free piezoelectric materials, each with their own advantages and disadvantages. PVDF is cheap to produce, but has low energy efficiency [2]. Barium Titanate can compete with the efficiency of PZT, but only near room temperature [5]. Zinc Oxide devices utilize nanostructures, making them difficult to produce on a large scale [3]. Large efforts to research and improve these materials are underway. With your support, I hope to evaluate these materials to determine which is most likely to succeed for energy harvesting applications. The only input needed for these applications is motion or stress, making them an attractive source of renewable energy. If one material can be made effective and affordable, we will be one step closer to reducing our dependence on fossil fuels. 5

6 References [1] U.S. Department of Energy, U.S. Department of Energy Strategic Plan, May 2011 United States. May This technical report, written by the U.S. Department of Energy (DOE), describes the most recent initiatives and achievements of the DOE. Several aspects of the DOE s renewable energy plan are discussed, including segregation of funds and expected timelines for renewable energy. This source was particularly useful for demonstrating USA s desire for renewable energy in my first paragraph. [2] e. a. C Jean-Mistral, "Comparison of electroactive polymers for energy scavenging applications," Smart Mater. Struct., vol. 19, pp , This journal article provides in-depth analysis of piezoelectric polymers. It also elaborated on some of the more interesting applications of piezoelectrics for energy harvesting applications. I found it most useful for providing examples of future piezoelectric devices and getting statistics on PVDF properties. This article led me to look into some of these potential energy applications. [3] B. Kumar and S. Kim, "Recent advances in power generation through piezoelectric nanogenerators," J. Mater. Chem., Kumar and Kim provide a very current summary of the research behind piezoelectric nanogenerators. The pair provides details of some important discoveries in the field, and notes any great improvements in electrical properties. This article was very useful for data on Zinc Oxide properties and applications. They also made me wonder why Zinc Oxide has primarily been used on the nanoscale (instead of macroscale), which I later discussed with my professor. [4] P. K. Panda, "Review: environmental friendly lead-free piezoelectric materials," J. Mater. Sci., vol. 44, pp , 10, Although it was written in 2009, which is old relative to some of my other sources, this journal article is a very descriptive explanation of competing lead-free piezoelectric materials. It also discusses the basics of piezoelectricity, and why leadbased PZT is harmful to the environment and humans. It is a lengthy and very enlightening source of background information on my research topic. Since it listed out all of the competing lead-free technologies of 2009, it gave me a great list of piezoelectric materials to research and questions for each. [5] W. L. Xiaobing Ren, Lead-Free Piezoelectric Material, US 2011/ A1, Mar This patent discloses the process and composition used to create a Barium Titanatebased lead-free piezoelectric material. The authors claim to have created a piezoelectric with a dielectric constant greater than that of PZT. Furthermore, several quantitative comparisons are made to other competing lead-free piezoelectric materials. The major limitation of their new material, however, is the temperature 6

7 range at which it can be used. This source made me wonder what other patents have been filed for competing lead-free piezoelectric materials. [6] S. Li, J. Yuan and H. Lipson, "Ambient wind energy harvesting using cross-flow fluttering," J. Appl. Phys, vol. 109, pp , Yuan and Lipson describe piezoelectric materials, and their experiment with PVDF for energy harvesting through wind motion. Their device, a PVDF tree with flexible, piezoelectric leaves created a relatively high power density, which encouraged me to consider PVDF as a candidate for replacement of PZT. Written in 2011, this article provides a very up-to-date account of the abilities of PVDF. This article will lead me to pursue other experiments with PVDF. [7] D. Vatansever, R. L. Hadimani, T. Shah and E. Siores. An investigation of energy harvesting from renewable sources with PVDF and PZT, Smart Mater. Struct., vol. 20, pp This journal article is a great comparison of a lead-free piezoelectric material, and PZT, a lead-based piezoelectric. Several experiments were performed to test PVDF s piezoelectric ability in energy harvesting applications. The article also highlights some of the advantages and disadvantages of PVDF relative to other lead-free options. This is a great addition to the other sources I have found so far because it discusses a polymer-based piezoelectric, rather than a ceramic. It led me to find other research on PVDF as a piezoelectric. 7