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Piezoelectricity

Sara M. Jarrar
Dec 2, 202430 min read

Sara M. Jarrar, Al-Zahra’a Secondary Girls School           

             

Abstract


Piezoelectric materials are substances that can convert mechanical energy into electrical energy and vice versa. Piezoelectric crystals are asymmetrically arranged but have a balanced charge. When a piezoelectric material is exposed to external mechanical stress, the geometry of its atomic structure deforms, directing negative charges to one side and positive charges to another, resulting in an electric dipole. The piezoelectric material is put between metal plates to collect these charges, producing a voltage and resulting in an electrical current, known as the direct piezoelectric effect. The opposite of this operation can be done by applying a signal to the piezoelectric material, leading to a release of mechanical energy and a change in the shape of the material, this is referred to as the in-direct piezoelectric effect. Piezoelectric materials, which can be either natural or synthetically made, are used in a variety of applications that require coupling between an electrical field and mechanical strain.


Examples include piezoelectric sensors, ultrasound imaging, NDT, and sonars, which can contribute to advancing different technological fields. Piezoelectric materials are great for energy harvesting, which is the process of harnessing ambient energy in the environment and transducing it into usable electricity. This can be done through piezoelectric energy harvesters, which convert mechanical energy into electricity that can be used as a power source for circuits and some devices. Examples of recommended ideas include embedding piezoelectric tiles within streets, pavements, running tracks, and the floors of different facilities to collect energy from footsteps and turn it into electricity to help light up buildings. Additionally, piezoelectric energy harvesters can be used in airports to transport luggage, in elevators to decrease damage and harm resulting from unexpected power cuts, or in shoes to produce electricity and use it to charge hand-held devices and self-winding watches.


 

Introduction


In recent years, the world has been grappling with an escalating energy crisis, which has significantly impacted various facets of our daily lives, from heating our homes to powering our workplaces. According to a recent survey, an alarming 83.3% of respondents expressed concern over the energy crisis, highlighting the urgency of the situation. As fossil fuels—our primary energy source—continue to deplete, it becomes increasingly crucial to explore sustainable and environmentally friendly alternatives. The reliance on fossil fuels not only contributes to greenhouse gas emissions, worsening climate change, but also threatens our energy security as reserves dwindle. In contrast, renewable energy sources like solar, wind, and hydroelectric power offer cleaner, inexhaustible options that can help alleviate the crisis while promoting sustainability. With the world projected to exhaust its fossil fuel reserves within a few decades, transitioning to these alternatives is no longer just an option; it's an imperative for ensuring a stable future.


One of the most crucial energy forms for our lives is electrical energy, for which we need to find more sustainable methods for its production. Electricity has become an imperative part of everybody’s life since the time it was discovered in the 1700s; it is found everywhere, and the majority of human activities and machines require electricity to function.


To address these challenges, scientists have been looking for substitutes for fossil fuels to use in electricity production. Some of the most common sustainable methods of electricity production are solar panels and air turbans. Although these methods are extremely effective, strong winds and sunlight are not always accessible in all regions.


This research explores the potential of piezoelectric materials in generating sustainable electrical energy, positing that if we increase the range of uses for these materials, it will lead to significant technological advancements. Such developments could not only reduce the overall consumption of electricity and ambient energy but also provide a viable alternative to traditional fossil fuel-based methods, particularly addressing the specific needs of regions with limited access to conventional energy sources.



 

History of Piezoelectricity


The term "piezo" in the piezoelectric effect originates from the Greek word "piezen" (πιέζω), which translates to "to squeeze" or "to press” (NANOMOTION, 2024). This term aptly describes the fundamental principle behind the piezoelectric effect (Fxstageadmin, 2024), which involves the conversion of mechanical energy—such as pressure or vibration—into electrical energy (Engr D., 2023). This conversion is significant in various applications, from sensors to actuators, where mechanical forces are transformed into electrical signals. The relevance of this concept becomes clearer when we examine the pioneering work of Carl Linaeus and Franz Aepinus in the mid-1800s (Fxstageadmin, 2024). 


These scientists were among the first to identify the piezoelectric effect during their investigations into the pyroelectric effect (Nanomotion, 2024). The pyroelectric effect refers to the generation of an electrical charge in certain materials when they experience a change in temperature (Engr D., 2023). By leveraging their understanding of pyroelectricity, Aepinus and Linaeus  (Nanomotion, 2024) were able to make groundbreaking observations regarding the relationships between mechanical stress and electrical charge, paving the way for further studies in piezoelectric materials.


Carl and Franz Curie had a foundational understanding of crystal structures, yet they were unable to fully articulate a comprehensive theoretical framework for their observations until Gabriel Lippmann entered the scene. Initially, the Curie brothers did not predict that crystals exhibiting the direct piezoelectric effect would also demonstrate the converse effect. However, in 1881, Lippmann discovered that these crystals could vibrate when an electric current was applied, revealing the inverse piezoelectric effect. This breakthrough prompted the Curies to conduct experiments that quickly confirmed Lippmann’s findings. They ultimately provided quantitative proof of the complete reversibility of electro-elastic-mechanical deformations in piezoelectric crystals.


In the next 25 years, much work was done to find out more about their unique structure and how they work. In 1910, Woldemar Voigt's Lehrbuch der Kristallphysik (Textbook on Crystal Physics) published the names of 20 natural crystal classes (Fxstageadmin, 2024) in which piezoelectric effects occur.



Figure 1. The Curie brothers

(OnScale Inc., 2019)


The evolution of technology during and after the wars has a profound impact on many aspects of modern life, particularly in the field of piezoelectricity. This technological progress, initiated by wartime needs, has not only contributed to military advancements but has also significantly influenced civilian applications. Understanding this history provides insights into how the urgency of war can catalyze scientific advancements, which subsequently transition into peacetime innovations that benefit society broadly. The connection between these historical contexts illustrates the dual-edged nature of technological progress, showcasing both its transformative potential and the ethical considerations it necessitates (Onh, 2023).


WW1 Era


In World War I, the development of sonar by French scientist Paul Langévin marked a significant milestone in the application of piezoelectric materials. The sonar system utilized quartz crystals, known for their piezoelectric properties, to detect submarines by emitting high-frequency sound waves and measuring the echoes that returned (Wikipedia contributors, 2024). This invention not only revolutionized naval warfare but also sparked a growing interest in piezoelectric devices across various industries.


Pros:

  • Enabled effective submarine detection, enhancing naval strategies.

  • Laid the groundwork for subsequent developments in piezoelectric technology, leading to applications like microphones and accelerometers.


Cons:

  • Primarily focused on military applications, limiting initial exploration of civilian uses.

  • The complexity and cost of early piezoelectric systems may have restricted widespread adoption.


WW2 Era


During World War II, a new class of synthetic materials named ferroelectrics (Engr D., 2023) was discovered by separate research groups in the United States, Russia, Germany, and Japan. They found that the piezoelectric effect of natural materials such as quartz is relatively small compared to some synthetic materials (ferroelectrics) that exhibit piezoelectric constants many times higher than natural materials. This discovery marked a turning point, as researchers identified synthetic materials that showcased significantly higher piezoelectric constants than natural materials like quartz.


Pros:

  • Dramatically improved the efficiency and effectiveness of military electronics (Engr D., 2023).

  • Expanded the potential applications of piezoelectric materials beyond natural limitations.


Cons:

  • Continued focus on military applications detracted from the exploration of civilian technologies.

  • Increased reliance on synthetic materials raised.


Post-World War 2 era


Over time, the market for piezoelectric products expanded significantly, driven by the commercial success of Japanese companies. This success garnered attention from other nations, prompting them to explore new opportunities in the piezoelectric sector. Manufacturers have since developed advanced devices utilizing these products, including smoke and intrusion alarms (Fxstageadmin, 2024), automotive sensors, and ultrasonic transducers.


Pros:

  • Growing market potential for innovators and investors.

  •  Advancements in technology leading to improved product applications. 

  • Increased safety and convenience through enhanced sensor technology.


Cons:

  • Competition may lead to market saturation.

  • Dependence on constant technological innovation to stay relevant.

  • Potential production and development costs for advanced devices can be high.


Piezoelectricity Applications Today


In today's world, piezoelectricity is used everywhere. The global market for piezoelectric devices stood at about $27.6 billion (Fxstageadmin, 2024) in 2019 and is likely to rise.

Their applications are essential in many fields, like the medical industry, for tools like ultrasound probes and diagnostic devices (Engr D., 2023).

The security and defense industries (Fxstageadmin, 2024) often use piezo products in sound transducers, found in devices like accelerometers, pressure sensors, microphones, and force sensors. The auto industry uses piezo products in components like fuel injector sensors, pressure sensors, and proximity sensors in driving aids like backup cameras (Fxstageadmin, 2024).


Figure 2. Discovery of piezoelectricity through history.

Piezoelectric materials. (n.d.). Mainland CCTT.


 

Working Mechanism


Piezoelectricity is a linear electromechanical mechanism that links mechanical and electrical states. It can be found in certain dielectric materials, which we call piezoelectric materials. They are commonly used in applications that require coupling between an electrical signal and a mechanical strain. For example, in audio devices, piezoelectric speakers utilize this effect to convert electrical signals into sound (AUDIOWELL, 2020).


There are two types of piezoelectric effects: the direct piezoelectric effect and the indirect piezoelectric effect, which is also known as the inverse or converse piezoelectric effect. The direct piezoelectric effect occurs when mechanical stress is applied to a piezoelectric material, resulting in the generation of an electric charge. Conversely, the indirect or inverse piezoelectric effect takes place when an electric field is applied to a piezoelectric material, causing it to undergo mechanical deformation. Both effects are essential for various applications in sensors, actuators, and energy harvesting technologies.


The way the direct piezoelectric effect works is quite straightforward. Piezoelectric materials generate electricity in response to the application of external mechanical stress. This happens due to electric dipoles that appear and do not cancel each other out due to the lack of an axis of symmetry and the repeated pattern in the structure of the piezoelectric materials. For instance, quartz (SiO₂) is a widely recognized piezoelectric material due to its crystalline structure, which lacks a center of symmetry, a key requirement for the piezoelectric effect. When mechanical pressure is applied to a quartz crystal, such as by squeezing a small rod, it induces stress that shifts the atomic structure. This stress misaligns electric dipoles within the crystal lattice, preventing cancellation of charges and resulting in an accumulation of electrical charge on the surfaces. Consequently, this generates a measurable voltage.


Piezoelectric materials in their normal state are neutral and perfectly balanced substances. When mechanical energy is applied in its different forms, it causes a deformation in the shape of the substance, which is usually put between metal plates to collect the resulting charges, allowing positive charges to gather on one side and negative charges on another, leading to a difference in the voltage between the sides of the materials and a flow of current.


The following figure shows the structure of silicon dioxide (SiO2), also known as quartz, which is a well-known naturally occurring piezoelectric material that is used in a variety of applications that utilize both the direct and the in-direct piezoelectric effects.



Figure 3. Direct piezoelectric effect in quartz.

(Components101, 2020)


The direct piezoelectric effect has an extensive range of applications, for example, in sensors and in energy harvesting techniques. Some devices and machines need to use both the direct and the in-direct piezoelectric in order to function properly, which will be further discussed and explained in the applications of piezoelectricity and energy harvesting chapters.


The in-direct piezoelectric effect is simply the change in mechanical strain in response to an applied electric current. When a current passes through a piezoelectric substance, it causes a deformation in its shape and causes it to oscillate.


The indirect piezoelectric effect is commonly used in applications that require high precision, which is why it is used in clocks to keep time.


Piezoelectric Constants


There are several piezoelectric constants that each indicates a certain relationship between two or more variables. As described in the APC website:


  1. Piezoelectric Charge Constant


The piezoelectric charge constant, d, is the polarization generated per unit of mechanical stress (T) applied to a piezoelectric material or, alternatively, is the mechanical strain (S) experienced by a piezoelectric material per unit of electric field applied. The first subscript to d indicates the direction of polarization generated in the material when the electric field, E, is zero or, alternatively, is the direction of the applied field strength. The second subscript is the direction of the applied stress or the induced strain, respectively. Because the strain induced in a piezoelectric material by an applied electric field is the product of the value for the electric field and the value for d, d is an important indicator of a material's suitability for strain- dependent (actuator) applications. 

Expanding the range of piezoelectric materials directly enhances their performance characteristics, particularly in terms of their piezoelectric coefficient d. As d increases, the strain induced in the material for a given electric field also increases, allowing for more efficient energy conversion. This efficiency is critical in actuator applications, where maximizing strain response can lead to reduced input energy requirements. Furthermore, by diversifying the types of piezoelectric materials available, we can optimize for specific operational environments and load conditions, thereby reducing the energy wasted on ineffective actuation. 


Moreover, a broader selection of materials with varying d values can facilitate the design of hybrid systems that capitalize on the unique properties of each material. This can minimize energy losses associated with inefficient material transitions and allow for systems that consume substantially less electricity overall. Consequently, as we increase the range of suitable piezoelectric materials, we are likely to see a proportional decrease in electricity consumption across a wide array of applications, leading to more sustainable and energy-efficient technologies.


  1. Piezoelectric Voltage Constant


The piezoelectric voltage constant, g, is the electric field generated by a piezoelectric material per unit of mechanical stress applied or, alternatively, is the mechanical strain experienced by a piezoelectric material per unit of electric displacement applied. The first subscript to g indicates the direction of the electric field generated in the material, or the direction of the applied electric displacement. The second subscript is the direction of the applied stress or the induced strain, respectively. Because the strength of the induced electric field produced by a piezoelectric material in response to an applied physical stress is the product of the value for the applied stress and the value for g, g is important for assessing a material's suitability for sensing (sensor) applications.


  1. Electromechanical Coupling Factor


The electromechanical coupling factor, k, is an indicator of the effectiveness with which a piezoelectric material converts electrical energy into mechanical energy, or converts mechanical energy into electrical energy. The first subscript to k denotes the direction along which the electrodes are applied; the second denotes the direction along which the mechanical energy is applied, or developed.


 

Piezoelectric Materials


Piezoelectric materials are smart materials that have the ability to change their physical shape when a voltage is passed through them and also change their polarization state (a dipole moment is formed) as a result of mechanical stress. They belong to a broader classification called dielectric materials, also known as insulators or non-conducting substances. There are natural piezoelectric materials that can be found in the environment, and there are synthetic or man-made piezoelectric materials.

Piezoelectric materials can be divided into polar and non-polar piezoelectric substances, and both polar and non-polar substances lack an axis of symmetry.

In order to have an enhanced understanding of the internal structure of these substances, there are a few concepts that one needs to acknowledge.


  1. Electronegativity

Electronegativity is defined as the tendency of an atom to attract shared electrons in a molecule. When an atom is more electronegative, electrons are pulled closer towards it rather than the other atom. The atom with a higher electronegativity rate develops a partial negative charge, while the atom with a lower electronegativity develops a partial positive charge.


  1. Polar and non-polar bonds

A high difference in the electronegativity of the atoms making up the molecule results in opposite charges and unevenly shared electrons; this is what we call a polar bond. On the other hand, a non-polar bond is a bond in which electrons are shared evenly between atoms (the center of negative and positive charges is the same).


  1. Dipole moment (μ)

The dipole moment is a vector quantity used to measure the net polarity of a molecule. The SI unit for dipole moment is (columb.m), but it’s mostly measured in Debye units. And it’s given by the formula μ = q · r where μ is the net dipole, q is the magnitude of charges, and r is the distance between the atoms.


  1. Curie temperature

The temperature at which certain magnetic materials change their magnetic characteristics change remarkably. When the temperature of a piezoelectric material surpasses its Curie temperature, it depolarizes and no longer can work as usual. The table below shows the Curie temperature of some of the most common ferroelectric materials.


Material’s name

Curie temperature

Bismuth ferrite (BiFeO3)

830 °C

Barium titanate (BaTiO3)

120°C -130°C

Lead zicronate titanate (PZT)

350°C (approximately, but the maximum temperature recommended is between 150°C-

250°C)


The Creation of Single Piezoelectric Crystals:


The specific structure piezoelectric materials need to acquire makes the process of making synthetic piezoelectric materials very exact and precise.


According to the “Concise Encyclopedia of Advanced Ceramic Materials” by Richard Brook, the most common techniques in this process are the development of potassium dihydrogen phosphate (KDP) and ethylene diamine tartrate (EDT) from aqueous solution, Czochralski pulling from the melt for LiNbO3, and flux or vapor-phase reaction growth for BaTiO3. Quartz, the most common single-crystal piezoelectric material, exists naturally. For device usage, only a limited quantity of extensively tested natural quartz with no defects or twinned areas is permitted. The bulk of single-crystal quartz is formed via the hydrothermal process of pressure-induced growth from a hot H2O solution. Temperatures of 670 K (397 °C), pressures of 170 MPa, and the inclusion of a few percent NaOH or Na2CO3 in the solution serving as "mineralizers" to improve the solubility of the quartz are typical circumstances. Filling the vessel with the solution to roughly 80% of its capacity, and when the temperature is raised, the requisite pressure is immediately reached.


Types of Piezoelectric Materials


A diverse array of piezoelectric materials exhibits significant utility in various applications, ranging from electronics to biological systems. Among naturally occurring crystals, quartz stands out as a stable crystal utilized in both watch crystals and frequency reference crystals for radio transmitters. Other notable natural piezoelectric substances include sucrose (table sugar), topaz, and tourmaline, as well as Rochelle salt, which generates high voltage under relatively low pressure, making it suitable for early crystal microphones. An intriguing example within this group is Berlinite (AlPO4), a rare phosphate mineral that possesses an identical structure to quartz. 


In addition to naturally occurring materials, several man-made piezoelectric crystals have been engineered to replicate the properties of quartz. These include gallium orthophosphate (GaPO4) and langasite (La3Ga5SiO14). The category of piezoelectric ceramics features notable examples such as barium titanate (BaTiO3), recognized as the first piezoelectric ceramic discovered, alongside lead titanate (PbTiO3) and lead zirconate titanate (PZT), which is the most commonly used piezoelectric ceramic to date. Other ceramics in this category include potassium niobate (KNbO3), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), and sodium tungstate (Na2WO4).


In response to environmental concerns related to the use of lead, lead-free piezoceramics have been developed, showcasing materials such as sodium potassium niobate (NaKNb), which exhibits properties akin to PZT, as well as bismuth ferrite (BiFeO3) and sodium niobate (NaNbO3). Furthermore, the realm of biological piezoelectric materials encompasses a variety of natural substances, including tendon, wood, DNA, bone, silk, enamel, dentin, and collagen. These biological materials highlight the intrinsic piezoelectric properties present in living organisms, offering potential insights into their applications in biomedical fields and beyond.


Additionally, there are materials called piezoelectric polymers, which are typically characterized by being lightweight and small in size; thus, they are commonly used in technical applications. Moreover, polyvinylidene fluoride (PVDF), which is an organic material that demonstrates piezoelectricity that is several times larger than quartz, It is often used in the medical field, such as in different medical textiles.

Piezoelectric materials have two main sub-categories, which are pyroelectrics and ferroelectrics, as shown in figures 4 and 5.

 





Figure 4. Sub-categories of piezoelectric materials.

                                                                                           (Khanbareh, 2016) 


Figure 5. Comparison between piezoelectric, pyroelectric and ferroelectric materials.

                                                                                      (Difference Between, n.d.)

 



 

Piezoelectricity Applications


The distinctive atomic structure of piezoelectric materials has resulted in a variety of cogent properties that enable these materials to convert mechanical energy into electricity and vice versa, as well as the fact that they are also appraised as components with a high sensitivity to vibrations. Along with the huge and continuous technological advancements, the applications of piezoelectric materials have broadened, and their uses are not limited to energy harvesting only; they have become a vital part of the advancement of many machines and systems that are considered essential parts of our lives.


In this section, we will epitomize the core working principles of some of the most important machines, for which piezoelectricity is one of the main components needed for them to function correctly.


  1. Sensors

There are many types of piezoelectric sensors, some of the most common are:


A - Piezoelectric force sensors:

Piezoelectric force sensors -also known as piezoelectric force transducers- are a special types of sensors that are used in dynamic measurements, most commonly to measure oscillating forces.


Subsequent to the proportional relationship between pressure and the electrical charge - this type of sensors use the direct piezoelectric, the mechanical strain applied causes a deformation in the piezoelectric materials which is then converted into electrical signals to dynamically measure the amount of force applied.


B - Piezoelectric pressure sensors:

A crystal is distorted elastically when pressure is applied. This deformation causes an electric charge flow, which lasts for a short while. The electric signal that results can be used to calculate the pressure that was applied to the crystal.


C - Piezoelectric accelerometers:

Piezoelectric accelerometers are used to measure a variety of mechanical variables most commonly for vibration measurements. An object with a known mass is attached to the piezoelectric crystal that when an external force or pressure is applied, the mass stays back as a result of inertia which causes stretches deforms the piezoelectric crystal producing a charge, the relationship between force is directly proportional to the electrical charge that generated, therefore, the acceleration can be calculated using Newton’s second law of motion (F=M*A).


One of the most common and important uses of piezoelectric sensors is in engine knock sensors. Engine knock sensors play a huge role in adjusting the ignition timing and improving the performance of the engine by preventing the risks of engine detonations from occurring. Engine detonation happens when a combustion occurs between the fuel-air mixture inside a car’s engine, causing it to ignite instead of burning slowly. This can happen as a result of different factors, some of the most common ones are extreme heat and pressure, which can even cause greater damage to the engine when a fuel with a low octane rating is used (the octane rating is a number that measures the fuel’s ability to resist pressure and heat).


Engine detonation can cause serious damage, consequently, knock sensors are used to prevent this undesirable phenomenon. One of the main components of a knock sensor is a piezoelectric material. Piezoelectric materials are sensitive, so they make the perfect fit to be used to detect unwanted vibrations before this spontaneous act of combustion happens, and turn them into electrical signals that are then sent to the Electronic Car Unit (ECU) to perform the needed adjustments.


The following image illustrates the basic structure of a knock sensor.






  1. Micropumps

Figure 6. Engine knock sensor.

(AZoSensors, 2023)



Micropumps are any type of pump that transfers a small volume of fluid across a space. Many of the applications of micropumps require high precision, a low flow rate, and being lightweight, yet piezoelectric micropumps are ideal for that.


The chamber of a micropump is expanded and contracted by a piezoelectric disk attached to the membrane that deflects under the influence of the external electric field. This creates a difference in pressure that causes the fluid to flow in or out of the desired place.


Piezoelectric micropumps have a lot of applications in a variety of fields; for example, they are commonly used in the medical field for drug delivery, in portable nebulizers, and in cell cultures. They also play a major role in controlling the flow rate and the flow direction in fuel cells, yet they have resulted in a huge development in that field.




                                                     Figure 7. Piezoelectric micropump

                                                 (Ashrf, Tayyaba, & Afzulpurkar, 2011)

 


  1. Medical applications

Besides the huge role of piezoelectric pumps in a variety of medical devices, piezoelectric materials have a lot of other medical applications that include:


A - Ultrasound imaging:

Ultrasound waves are sound waves with a frequency above 20 kHz, due to the fact that they are high-pitched, they can’t be heard nor sensed by humans. Ultrasound waves are used widely, especially in ultrasound imaging, especially for medical purposes. The ultrasound device relies on the direct and inverse piezoelectric effects to operate using a piezoelectric transducer (a transducer is a device that converts one form of energy to another and vice versa).


The piezoelectric material oscillates as a result of the electrical signal being sent through it, therefore creating vibrations. An ultrasound device sends high-frequency vibrations into the body; these vibrations reflect and are then turned into electrical signals using a piezoelectric transducer, providing the information needed for an image to be produced and used, for example, to monitor the health of the fetus and to check for the thyroid gland.


B - Nanopositioning:

The term "nanopositioning" refers to the technology used to move, measure, and position an object with sub-micron levels of accuracy. This involves moving the object over incredibly small distances, frequently at high speeds. Nanopositioning takes advantage of the piezoelectric effect by energizing a stack of piezoelectric layers using an electrical signal, causing them to enlarge.


Nanopositioning technologies have resulted in huge advancements in multiple fields, specifically the medical field. Piezoelectric nanopositioning systems are used in endoscopy, which is a medical procedure in which an optical cable (endoscope) is put inside the body to examine an organ or the hollow of the body.


Piezoelectric nanopositioning systems are also used in Magnetic Resonance Imaging (MRI) due to the ability of piezoelectric materials to handle a strong magnetic field without being affected by it. As well as in ophthalmic surgeries.


C - Piezotomes:

A piezotome is a device used in dental applications, specifically in piezosurgery, which is a procedure commonly used to section tissues and remove bones without causing harm to adjacent teeth by emitting vibrations. Piezotomes take advantage of the in-direct piezoelectric effect to produce precise vibrations; they also drastically improved the process of tooth extraction and implant.


  1. Sonars

Sonars are one of the oldest applications of the piezoelectric effect and were first developed by the French physicist Paul Langevin and some of his workers, who built a submarine ultrasonic detector in which a piezoelectric transducer was used.

Sonars use both direct and in-direct piezoelectric effects. A piezoelectric crystal oscillates as a result of electrical energy, thus producing high- frequency mechanical vibrations (waves) in the shape of a cone that travel through water; when these waves are obstructed by an object, they reflect back (just like an echo) to reach another piezoelectric crystal that detects these vibrations and turns them into electrical signals that are mostly used to calculate the depth of the objects and to navigate safely underwater.


  1. Non-Destructive Testing (NDT)

Non-Destructive Testing (NDT) is a wide range of analyzing methods that are used to test and evaluate materials without causing damage to their serviceability. Piezoelectric materials are an active component in Non- Destructive testing and are widely used in a variety of inspection methods. For instance, the piezoelectric material is supplied with an alternative current (AC) that causes it to oscillate producing ultrasonic vibrations through the in-direct piezoelectric effect, these vibrations are placed towards an object, they pass through the object until they encounter a change in some properties that causes them to reflect back to the transducer.


  1. Gas lighters

When you press the button of a gas lighter (igniter), you cause a hammer inside to apply force to a piezoelectric crystal, which then deforms and converts that mechanical energy into an electrical signal that generates a spark that causes the gas to ignite, yet no battery is required to cause the ignition. Lead zirconate titanate (PZT) is mostly used due to its low price and high sensitivity, as well as other types of piezoelectric crystals.


  1. Inkjet printing

Inkjet printing is a sort of computer printing that recreates a digital image or a document by spraying ink droplets onto paper or other substances. Piezoelectric crystals are placed on the head of the printer and when a current is applied, they deform in certain ways causing the ink to drop creating the wanted picture or document.


  1. Speakers and buzzers

A piezoelectric speaker or buzzer is essentially a speaker that produces sound by a piezoelectric action. The initial mechanical motion is produced by delivering a voltage to a piezoelectric material. Additionally, resonators and diaphragms often transform the motion into audible sound. They are generally tolerant of overloads that destroy most high-frequency drivers, and their electrical properties allow them to be used without crossovers. Additionally, there are drawbacks. Some amplifiers have a tendency to oscillate while driving capacitive loads, like the majority of piezoelectric materials, which can cause distortion or amplifier damage. Additionally, they often have a lower frequency response than other technologies, particularly in the bass and midrange. They are used in cases where high pitch and volume are of greater significance than the quality of the sound.


  1. Quartz clocks

The battery powers the microchip circuit. A microchip circuit causes a quartz crystal to oscillate (vibrate) 32768 times per second. The oscillations of the crystal are then detected by a microchip circuit and are converted into regular electric pulses, one every second. A small electric stepping motor is supplied with power by electric pulses. This is how electrical energy is transformed into mechanical power then a stepping motor rotates gears to keep time, subsequently, gears spin around the clock face.


Energy Harvesting


As the human population is continuing to grow rapidly and with the increased demand for energy in its different forms, the world is witnessing an uncharacteristic diminution of energy sources that we are used to being almost completely dependent on. This catastrophe is known as the energy crisis. Although some communities had started experiencing its negative outcomes earlier than others, yet its consequences have negatively impacted the world as a whole, especially with the disastrous depletion of fossil fuels that are–in the current era- the main source of energy, specifically electricity that is running the wheel of human activities in the world, moreover, fossil fuels –on a global scale- are considered one of the economic requirements for the industry field to continue to produce in such large amounts that are commensurate to our needs, and despite of the fact that these sources are non-sustainable and extremely harmful to our mother earth, yet there is a pressing need to find alternatives for fossil fuels.


Scientists and engineers have been working on the development of methods to convert other energy forms into electricity without causing harm to the environment. Some of the most common ways are through solar panels, air turbans and tides, which are very efficient. However, there are other forms of energy present in the environment that can be transduced into electrical energy, such as mechanical energy and vibrations.


This process of harnessing and collecting the ambient energy present in the environment and converting it into another form of energy that is beneficial and suitable for different uses is known as "energy harvesting" or "energy scavenging". With the technological advancements in different fields, and aside from the great contribution piezoelectric materials have made to the Internet of Things (IoT) field, the development of piezoelectric composites can be utilized in highly effective ways—especially in micro energy harvesting, where piezoelectricity plays a huge role—to convert mechanical energy into electricity that we can use to power multiple handheld devices or to decrease electricity consumption for large facilities through a variety of innovative ways.


In a personal interview with Professor Asan Abdul Muthalif, a professor in the Department of Mechanical and industrial engineering at Qatar University, where I asked him a few questions on this topic, and he explained the working mechanism of piezoelectric energy harvesting as the following “The working principle of PEH is quite straightforward. When a pzt patch is subjected to strains, a charge difference appears across the top and bottom surfaces. The rate of the charge flow will produce current and voltage across a resistance”, and he added “The application of piezoelectric materials for energy harvesting is gaining momentum, this is hugely contributed by the advancement of MEMs and NEMs technology. The development of MEMs sensors or actuators with low power requirements has made it possible for piezoelectric to be used as a power source for them. The use of piezoelectric will naturally get attention and find its way when it meets the power requirements or demand of the intended applications” He also stated that “Piezoelectricity has a strong potential and can contribute to a sustainable energy source”. When I asked Professor Abdul Muthalif whether he encourages young people and engineers to dive deeper into the field of piezoelectricity, he replied “Research on piezoelectric-based applications is ongoing, and many research articles are being published. Yes, I would support young people and engineers to explore new applications and optimize the performance of piezoelectric effects”.


Meanwhile, Professor Aysar Yasin, a professor in the engineering department at Al-Najah National University, supported the development of piezoelectricity energy harvesting techniques describing it as “a multidisciplinary field that requires collaboration between engineers, materials scientists, physicists, and other experts. Young engineers working on piezoelectric techniques can engage in interdisciplinary research and collaborate with professionals from various disciplines, fostering knowledge exchange and promoting innovation”, as well as that it can create career opportunities and lead to new sustainable energy solutions and a technological impact. He also stated that “while piezoelectric energy harvesters may not be capable of replacing large-scale power generation from fossil fuels entirely, they can certainly complement existing renewable energy sources and contribute to a diversified energy mix. Their potential applications in energy harvesting, especially in localized and remote settings, make them valuable tools in the transition towards a more sustainable and less fossil fuel-dependent energy future" and piezoelectric energy harvesters seem to have a promising future.


I am deeply indebted to Professor Abdul Muthalif and Professor Aysar Yasin for the valuable information they provided me with.


Although the range of applications of energy harvesting is quite limitless, in this section, a clear explanation of this process will be provided and multiple ideas in different areas will be clearly introduced and discussed.


Definition


Energy harvesting or scavenging is defined as the process of harnessing the ambient energy present in the environment and converting it into other energy forms that are suitable for a variety of uses. In simpler words, it is the process of collecting and deriving energy that is present in the environment and not taken advantage of, and turning it into other energy forms that we can use in our daily life activities, it is in fact a lot simpler than most people assume it to be.


There are different types of energy harvesters and each used for certain energy transformations for instance photovoltaic energy harvesters, wireless energy harvesters, wind energy harvesters and piezoelectric energy harvesters that are a type of a group mainly known as micro harvesters consisting of piezoelectric harvesting nodes, electrostatic harvesting nodes and electromagnetic harvesting nodes which are all used in to convert mechanical –kinetic- energy (force, vibrations, pressure..) into electricity.

There are a few main differences that distinguish each type of micro harvesters from the others. Starting with electrostatic energy harvesters, these harvesters consist primarily of a variable capacitor and an energy transfer circuit in which a mechanical pulling force resulting from charged variable capacitor plates causes mechanical energy to transform into electricity, they require an external power source but they generate a high voltage. Electromagnetic energy harvesters are based on the principle that states that moving a magnet around a coil of wire, or a coil of wire around a magnet, causes electrons in the wire to flow, resulting in an electrical current, they do not need an external electrical force but they produce a pretty low voltage. Lastly, piezoelectric energy harvesters use the direct piezoelectric effect to convert mechanical strain to electrical energy, they are simply structured and produce a high output voltage and have a wide frequency range but a low output current, additionally, piezoelectric materials have a relatively high energy density compared to other energy harvester such as electrostatic and electromagnetic energy harvesters.


Each kind of harvester special properties make it suitable for certain uses. In this research we will focus on piezoelectric energy harvesters, their mechanism and some of the applications they can be utilized in.


Working Mechanism


In essence, motion energy collecting systems often employ a proof mass that may move relative to the device frame. Energy can be transferred by immediately applying an external force on the proof mass or the frame.


Using the device frame as a reference for motion, the force accelerates the proof mass, creating work that can be converted into electrical energy in the first example. In the second scenario, the force accelerates the frame, causing an inertial force to occur on the proof mass in relation to the frame. This inertial force's work is used to convert energy.


There are 3 main coupling modes for piezoelectric materials, the first one is d33 in which the direction of the stress is parallel to the direction of the electric field (induced polarization in direction z-axis per unit stress applied in z-axis), this mode is the most used since it produces the highest outcomes. On the other hand, in mode d31, the direction of the force applied is perpendicular to the polarization direction (induced polarization in z-axis per unit stress applied in x-axis). Lastly, there is the d15 mode, which is the least used mode due to its poor outcomes, in d15 the electrical field is horizontal in film (induced polarization in the x axis per unit shear force applied about the y axis).


Figure 8 illustrates the relationship between d33, d31 and d15 on the y-axis with temperature on the x-axis.




Figure 8. The relationship between d33, d31, d15 and temperature.

(Lazar et al., 2018)


Figure shows the direction of the stress applied compared to the polarization direction in modes d33 (figure A) and d31 (figure B).


Figure 9. The polarization stress direction and the polarization direction in d31 and d33.

(Li, Xu, Liu, & Gao, 2018)


There are a variety of ways of connecting a piezoelectric energy harvesting circuit, most of their basic components are schematically shown in figure below.


Figure 10. Connecting components in a piezoelectric energy harvesting circuit.

(Tengku Mohamad, Sampe, & Berhanuddin, 2017)


Applications of Energy Harvesting


There are lots of ways to utilize the piezoelectric effect; unfortunately, some of these applications were not given enough attention in the past, yet in the current era, scientists and engineers are doing much more research and are working on developing new techniques to take advantage of this phenomenon with the best outcomes in terms of both usability (increasing the output) and sustainability.


In this section, we will discuss some innovative ideas to utilize piezoelectric harvesters in many aspects of life, and all the ideas discussed below can play a great role in creating smart and clean cities and communities that are more aware of the negative impact of fossil fuels on the climate.


  1. Embedding piezoelectric energy harvesters under streets and pave ways: A way we can take advantage of piezoelectric energy harvesters is by embedding them under roads, streets, and pave ways, especially those with busy human or vehicle traffic, and using them to convert the mechanical energy—in the form of pressure applied on the harvesters by humans or transportation means that causes a deformation in the piezoelectric material—to electrical energy. According to “Green energy harvesting from human footsteps” by Putri Abadi and Denny Darlis,, the average human with a weight of 60 kilograms can generate up to 71.20 volts and an average of 67.20 volts (in AC) by just walking on 2–20 piezoelectric transducers. The electricity produced can be stored and used to power up different appliances or to reduce the electricity from usual harmful sources that we need to light the streets up by using it as a power source for street LEDs, thus reducing the electricity bills that governments need to pay.

  2. Embedding piezoelectric energy harvesters in gym grounds and running tracks: Similar to the methodology of the previous idea, embedding piezoelectric energy harvesters under running tracks can help reduce electricity bills in a sustainable way, by converting mechanical strain from the movement of humans—whether running or walking—and transforming it using the piezoelectric phenomenon into electrical energy. A gym in England used this technique, and the electricity bills were found to have decreased by 60%.

  3. Using piezoelectric energy harvesters in kids' playgrounds and parks: Piezoelectric energy harvesting can similarly be embedded within the ground of kids' spaces or normal parks to generate electricity that can either be stored and used to light up streets or can be utilized in fun and educational ways for kids and teenagers, for example, interactive ground activities and games that can encourage youth to go outside more, move more and spend less time on the front of screens which increase their risks of getting myopia and farsightedness as they get older, for instance, by creating on-ground pianos that light up and produce a sound when you stand on them, or an X-O lights game, or educational cooperative games, as well as many other effective ways that help the environment as well as kids and teenagers.

  4. Embedding piezoelectric energy harvesters in airports to take advantage of the busy ongoing movements to transport luggage.

  5. In malls and in different buildings and facilities, piezoelectric energy harvesters can play a variety of significant roles in different aspects, for example, they can be used to collect kinetic energy from the footsteps of humans and turn it into electricity that can be used to power escalators, another way which piezoelectric energy harvesters can be utilized in, is through adding them to the structure of elevators to collect mechanical energy from the continuous up-down movement of the elevator as well as from humans inside of it, and store it, so that if an electrical issue happens and the current is cut, the electricity stored can be used as an alternative way to slow down the drop of the elevator and provide a safer placement which can help to reduce the harm that people can get as a result of this sudden technical hitch.

  6. Adding piezoelectric materials (energy harvesters) to different types of shoes (e.g., sneakers and boots) in order to charge wearable and hand-held devices, such as cell phones, which only require 10V to charge up, and self- winding clocks, in addition to many other appliances that we use on a daily basis.

  7. In water pipes to generate electricity from its movement.

  8. As sensors in places where people should not walk over or cross. To warn stakeholders.



Figure 11. Expressive image.

(Cao et al., 2021)


 

Conclusion & Discussion


The piezoelectric phenomenon has a direct impact on high-tech industries, and can largely help reduce electricity consumption. Piezoelectric materials have a low carbon footprint, are small in size, and are relatively easy to handle, additionally, there is no need for an external power source, and they are extremely precise. With these characteristics and the on-going research that aims to study this phenomenon more clearly, we can expect to exceedingly maximize the generated output of these substances so they can be used in even more applications than they already are and help to end this energy catastrophe and create a green environment for the upcoming generations.



 

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