Piezoelectric Effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. The word Piezoelectric is derived from the Greek piezein, which means to squeeze or press. One of the unique characteristics of the piezoelectric effect is that it is reversible, meaning that materials exhibiting the direct piezoelectric effect (the generation of electricity when stress is applied) also exhibit the converse piezoelectric effect (the generation of stress when an electric field is applied). When piezoelectric material is placed under mechanical stress, a shifting of the positive and negative charge centers in the material takes place, which then results in an external electrical field. When reversed, an outer electrical field either stretches or compresses the piezoelectric material (http://www.nanomotion.com/piezo-ceramic-motor-technology/piezoelectric-effect/).
Per Dr. Kevin T. Zawilski, Chemfiles Volume 5 Article 13, piezoelectric materials have the ability to generate a voltage in response to an applied mechanical stress or conversely change shape in response to an applied voltage. High performance piezoelectric materials have a wide range of applications including sonar arrays, ultrasonic imaging devices, and fine motion controllers. A major breakthrough in high performance piezoelectric materials was made in 1997, when the exceptional piezoelectric properties of relaxor ferroelectric single crystals were first measured.1,2 When crystals such as lead magnesium niobatelead titanate, (1–x) PbMg1/3Nb2/3O3 – (x) PbTiO3 (or PMN–PT), were measured along the direction, the electromechanical coupling factor (k33) was found to be >90% with achievable strain levels of >1.5%. Previously, the best performing piezoelectric materials were PbZr(1–y)TiyO3 (PZT) ceramics with k33 ranging from 70% to 75% and achievable strain levels of 0.1%.
2.2 Piezoelectric Effect Applications
The direct piezoelectric effect was first seen in 1880, and was initiated by the brothers Pierre and Jacques Curie. By combining their knowledge of piezoelectricity with their understanding of crystal structures and behavior, the Curie brothers demonstrated the first piezoelectric effect by using crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt. Their initial demonstration showed that quartz and Rochelle salt exhibited the most piezoelectricity ability at the time. Over the next few decades, piezoelectricity remained in the laboratory, something to be experimented on as more work was undertaken to explore the great potential of the piezoelectric effect. The breakout of World War I marked the introduction of the first practical application for piezoelectric devices, which was the sonar device. This initial use of piezoelectricity in sonar created intense international developmental interest in piezoelectric devices. Over the next few decades, new piezoelectric materials and new applications for those materials were explored and developed. During World War II, research groups in the US, Russia and Japan discovered a new class of man-made materials, called ferroelectrics, which exhibited piezoelectric constants many times higher than natural piezoelectric materials. Although quartz crystals were the first commercially exploited piezoelectric material and still used in sonar detection applications, scientists kept searching for higher performance materials. This intense research resulted in the development of barium titanate and lead zirconate titanate, two materials that had very specific properties suitable for particular applications (http://www.nanomotion.com/piezo-ceramic-motor-technology/piezoelectric-effect/).
There are many materials, both natural and man-made, that exhibit a range of piezoelectric effects. Some naturally piezoelectric occurring materials include Berlinite (structurally identical to quartz), cane sugar, quartz, Rochelle salt, topaz, tourmaline, and bone (dry bone exhibits some piezoelectric properties due to the apatite crystals, and the piezoelectric effect is generally thought to act as a biological force sensor). An example of man-made piezoelectric materials includes barium titanate and lead zirconate titanate (http://www.nanomotion.com/piezo-ceramic-motor-technology/piezoelectric-effect/).
In recent years, due to the growing environmental concern regarding toxicity in lead-containing devices and the RoHS directive followed within the European Union, there has been a push to develop lead free piezoelectric materials. To date, this initiative to develop new lead-free piezoelectric materials has resulted in a variety of new piezoelectric materials which are more environmentally safe (http://www.nanomotion.com/piezo-ceramic-motor-technology/piezoelectric-effect/).
Students in some universities here in the Philippines conducted some study on how they can use human motion to develop a mechanical energy and convert it into electrical energy. One of the study was conducted by a mechanical engineering from Bataan Peninsula State University main campus, the project is designed to charge two (2) batteries, 12 VDC, 2.55 AH through an alternator. The 12V alternator is directly connected to the rear wheel of the bicycle. The motor has two stage Reduction Gear, the first stage reduction from the motor is 13:40 teeth while the second stage 16:44 teeth with an overall ratio of 8.46. Once the batteries are charged, a switch connected in a series connected in series from battery 1 to battery 2, they supply the needed voltage of the motor 24VDC, 180watts, and 2500rpm. This study uses alternator to charge two batteries with a switch connected in a series to supply the 24 Volts motor in operating the bicycle without cranking (Bagaoisan, 2011). Another study is from the electrical engineering students of Mapúa Institute of Technology batch 2013, they conducted a study about piezoelectric transducers as an alternative source of energy, and they produced a device that could collect energy by using the stairs in their buildings. The study will provide a substitute energy source for the light loading in Mapúa Institute of Technology which is renewable and environmental friendly (Atienza, Deocampo, Jovena and Trinidad, 2013). Another study was conducted by electrical engineering student from the Don Honorio Ventura Technological State University, the project as the kinetic energy emanates from the passing of the students, the turnstile rotates and produces mechanical energy that will cause the dynamo to rotate. The dynamo will then produce electrical energy that will flow all the way to charge controller. Then the charge controller regulates the voltage and current before it would flow the battery. The battery stores the energy and supply power to the inverter, the inverter converts direct current into alternating current before going to the load (Caparas Jr., 2015).
Due to the intrinsic characteristics of piezoelectric materials, there are numerous applications that benefit from their use: High voltage and power sources is one of them. An example of applications in this area is the electric cigarette lighter, where pressing a button causes a spring-loaded hammer to hit a piezoelectric crystal, thereby producing a sufficiently high voltage that electric current flows across a small spark gap, heating and igniting the gas. Most types of gas burners and ranges have a built-in piezo based injection systems. Another is the sensors, the principle of operation of a piezoelectric sensor is that a physical dimension, transformed into a force, acts on two opposing faces of the sensing element. The detection of pressure variations in the form of sound is the most common sensor application, which is seen in piezoelectric microphones and piezoelectric pickups for electrically amplified guitars. Piezoelectric sensors are used with high frequency sound in ultrasonic transducers for medical imaging and industrial non-destructive testing. And another example is piezoelectric motors, because very high voltages correspond to only tiny changes in the width of the crystal, this crystal width can be manipulated with better-than-micrometer precision, making piezo crystals an important tool for positioning objects with extreme accuracy, making them perfect for use in motors, such as the various motor series offered by Nanomotion. Regarding piezoelectric motors, the piezoelectric element receives an electrical pulse, and then applies directional force to an opposing ceramic plate, causing it to move in the desired direction. Motion is generated when the piezoelectric element moves against a static platform (such as ceramic strips). The characteristics of piezoelectric materials provx`ided the perfect technology upon which Nanomotion developed our various lines of unique piezoelectric motors (https://sites.google.com/site/piezoelectrichighway/project-information)
Using patented piezoelectric technology, Piezoelectric ceramic (such as PZT)-based devices have long been used for mechanical-to-electrical energy harvesting. However, those materials tend to be stiff and limited in mechanical strain abilities; so, for many applications in which low-frequency and large-stroke mechanical excitations are available (such as human movement), direct coupling of the piezoelectric ceramic to the excitation source yields a very low input mechanical energy. Organic materials, however, are softer and more flexible; therefore, the input mechanical energy is considerably higher under the same mechanical force. Piezoelectric polymers such as PVDF, unfortunately, have a much lower piezoelectric coefficient compared to piezoelectric ceramic materials. A study has shown that the energy harvesting is lower than with piezoelectric ceramic bimorphs. Electrostatic-based systems, such as dielectric elastomers, typically require a very high electric field intensity (1– 6 kV on a film thickness of 50 µm, which corresponds to 20–120 MV/m) to achieve significant energy harvesting. Electrostrictive polymers recently have been discovered that generate large strain (above 5%) under moderate electric field intensity (400–800 V on a 20 µm film, which corresponds to 20–40 MV/m), and the mechanical energy density is comparable to that of piezoelectric crystals. Less known is that these materials also can be used for mechanical-to-electrical energy harvesting. This process is based on electric field-induced molecule motion or phase transition, although these materials also experience an electrostatic effect, which also plays a minor role in the energy harvesting. Furthermore virtually all piezoelectric applications, with the exception of the very large business in quartz filters and oscillators, involve ferroelectric materials, mostly in ceramic form.
The application of the piezoelectric power was not given much attention in the past the electromechanical conversion efficiency was relatively low and the quantity of generating dand micro process technology, the new piezoelectric materials with high piezoelectricity performance are being developed continuously, electromechanical conversion efficiency of the piezoelectric materials was improved by a large margin. Over the past few years, vibration-based piezoelectric energy harvesting has been investigated by several. In 2005,a on a piezoelectric cymbal transducer for energy harvesting. At 100Hz, thoutput power can reach 52 mW when connected with a load of 400 kN under an cyclic force of 70 N with a pre-stress load of 67 N. In 2007, a study reported on a piezoelectric drum transducer for energy harvesting. Under a pre-stress of 0.15 N and a cyclic stress of 0.7 N, a power of 11 mW was generated at 590 Hz with an 18 ill resistor.