Hybrid Materials for Piezoelectric Energy Harvesting and Conversion E-Book

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Veröffentlichungsjahr: 2024

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Hybrid Materials for Piezoelectric Energy Harvesting and Conversion Power small devices more efficiently and practically with these essential materials Piezoelectric energy harvesting is an increasingly widely-deployed technique to generate electricity from mechanical energy. Reliability, ease of use, and cleanliness make piezoelectric energy harvesting in small electronic devices a potentially valuable alternative to the practical challenges and waste production of disposable or even reusable batteries. However, piezoelectric materials have their own challenges, advantages, and limitations, and choosing between them is a difficult engineering problem in itself; hybrid piezoelectric materials, which can be used to compensate the weaknesses of individual piezoelectric materials (like ceramic or polymer), are the emerging solution. Hybrid Materials for Piezoelectric Energy Harvesting and Conversion offers a systematic analysis of these hybrid piezoelectric materials and their applications. Each hybrid piezoelectric material is analyzed for its fundamentals, structural requirements, and applications, and the result is a significant contribution to materials science and electronic engineering. Hybrid Materials for Piezoelectric Energy Harvesting and Conversion readers will also find: * Comprehensive coverage of piezoelectric materials to provide the best fit for any set of engineering needs * Detailed discussion of inorganic, organic, and hybrid piezoelectric materials * Surface modification of piezoelectric filler in composite based piezoelectric materials * Importance of semiconductive and conductive materials in enhancing piezoelectric response of hybrid piezoelectric materials * In depth analysis of bio-based hybrid piezoelectric materials Hybrid Materials for Piezoelectric Energy Harvesting and Conversion is ideal for researchers in materials sciences, polymers, textiles, green and renewable energy, and all related fields.

Leseprobe

Hybrid Materials for Piezoelectric Energy Harvesting and Conversion

Edited by

S. Wazed AliIndian Institute of Technology DelhiNew Delhi, India

Satyaranjan BairagiUniversity of GlasgowGlasgow, UK

Shahid Ul IslamJamia Millia IslamiaNew Delhi, India

Copyright © 2024 by John Wiley & Sons, Inc. All rights reserved.

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List of Contributors

Ramasamy Alagirusamy

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

S. Wazed Ali

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

and

School of Interdisciplinary Research (SIRe)

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Akshaya Aliyana

Research Institute for Flexible Materials, School of Textiles and Design

Heriot‐Watt University

Edinburgh

UK

Arunachalakasi Arockiarajan

Department of Applied Mechanics

Indian Institute of Technology Madras (IIT Madras)

Chennai, Tamil Nadu

India

and

Ceramic Technology Group‐Center of Excellence in Materials and Manufacturing Futuristic Mobility

Indian Institute of Technology Madras (IIT Madras)

Chennai, Tamil Nadu

India

Aiswarya Baburaj

Department of Electronics

Mangalore University

Mangaluru

India

Satyaranjan Bairagi

Materials and Manufacturing Research Group

James Watt School of Engineering, University of Glasgow

Scotland

UK

Sourav Banerjee

School of Interdisciplinary Research (SIRe)

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Swagata Banerjee

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Dipankar Chattopadhyay

Department of Polymer Science and Technology

University of Calcutta

Kolkata, West Bengal

India

and

Center for Research in Nanoscience and Nanotechnology

Acharya Prafulla Chandra Roy Sikhsha Prangan

University of Calcutta

Kolkata

India

Ipsita Chinya

Department of Chemical Engineering

Dr. V R Godhania College of Engineering and Technology

Porbandar, Gujarat

India

and

Academy of Scientific and Innovative Research

CSIR‐CGCRI

Kolkata, West Bengal

India

and

Functional Material and Device Division

CSIR‐Central Glass & Ceramic Research Institute

Kolkata, West Bengal

India

Anupam Chowdhury

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Srijan Das

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Sheer Khanyisile Dhlamini

Department of Chemical Sciences

University of Johannesburg

Johannesburg

South Africa

and

DST‐CSIR National Centre for Nanostructured Materials

Council for Scientific and Industrial Research

Pretoria

South Africa

Sahal Ebrahim

Department of Nanoscience and Technology

University of Calicut

Kerala

India

Adrija Ghosh

Department of Polymer Science and Technology

University of Calcutta

Kolkata, West Bengal

India

Priyanka Gupta

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Mukta Nitin Mirlekar

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Satya Narayan Naik

Centre for Rural Development and Technology

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

and

School of Interdisciplinary Research (SIRe)

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Naveen Kumar Sindhughatta Kalaiah

Department of Electronics

Mangalore University

Mangaluru

India

Jonathan Tersur Orasugh

Department of Chemical Sciences

University of Johannesburg

Johannesburg

South Africa

and

DST‐CSIR National Centre for Nanostructured Materials

Council for Scientific and Industrial Research

Pretoria

South Africa

Deepalekshmi Ponnamma

Materials Science and Technology Program

Department of Mathematics, Statistics and Physics

College of Arts and Sciences

Qatar University

Doha

Qatar

Suprakash Sinha Ray

Department of Chemical Sciences

University of Johannesburg

Johannesburg

South Africa

and

DST‐CSIR National Centre for Nanostructured Materials

Council for Scientific and Industrial Research

Pretoria

South Africa

Abhishek Sasmal

Functional Materials and Devices Division

CSIR‐Central Glass & Ceramic Research Institute

Kolkata, West Bengal

India

and

Department of Applied Mechanics

Indian Institute of Technology Madras (IIT Madras)

Chennai, Tamil Nadu

India

Shrabanee Sen

Functional Materials and Devices Division

CSIR‐Central Glass & Ceramic Research Institute

Kolkata, West Bengal

India

Chirantan Shee

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

Huaz Khas, New Delhi

India

Ankur Shukla

Indian Institute of Technology Delhi

Hauz Khas, New Delhi

India

Mayuri Srivastava

School of Interdisciplinary Research

Department of Textile and Fibre Engineering

Indian Institute of Technology Delhi

New Delhi

India

Preface

The field of materials science has continually evolved to meet the ever‐growing demands of modern technology. As we stand on the precipice of a new era characterized by the fusion of diverse disciplines and the relentless pursuit of sustainability, it is increasingly evident that the future of materials lies in their ability to be both versatile and environmentally friendly. In this context, the realm of piezoelectric materials has emerged as a cornerstone of innovation, offering a unique combination of properties that make them exceptionally promising for a wide range of applications.

Hybrid Materials for Piezoelectric Energy Harvesting and Conversion represents a comprehensive exploration of the cutting‐edge developments and transformative potential of piezoelectric materials in the context of hybrid materials and sustainable technologies. Piezoelectric materials, known for their ability to convert mechanical energy into electrical energy and vice versa, have found their way into an array of applications, from sensors and energy harvesters to actuators and medical devices. Their versatility is undeniable, but it is their integration into hybrid systems and the ingenious combinations with other materials that hold the key to unlocking new horizons in science and engineering.

As the world faces unprecedented challenges related to energy generation, environmental sustainability, and the quest for multifunctional materials, the fusion of piezoelectric materials with other novel materials becomes imperative. The amalgamation of piezoelectricity with materials such as polymers, composites, nanoparticles, and even biological substances has paved the way for revolutionary breakthroughs. The resulting hybrid materials not only inherit the piezoelectric properties but also acquire additional functionalities, such as enhanced mechanical properties, biocompatibility, and tailored electrical conductivities.

In this book, we embark on a captivating journey through the diverse landscape of hybrid piezoelectric materials. We delve into the fundamental principles governing piezoelectricity, exploring the physics and chemistry that underpin these exceptional materials. We also examine the synthesis and fabrication techniques that enable the creation of hybrid structures, shedding light on the delicate balance required to harness the synergies between different materials.

The book then takes readers on a tour of the myriad applications where hybrid piezoelectric materials have begun to make their mark. From their use in sustainable energy generation to the development of smart materials for healthcare and aerospace, these materials are driving innovation across multiple domains.

Throughout these pages, we have endeavored to bring together contributions from experts in the field, each offering their unique perspective and insights. The chapters encompass a wide spectrum of topics, from theoretical considerations to practical implementations, providing a holistic view of the field's current state and prospects.

As editors of this book, our goal is to inspire and inform researchers, students, and practitioners in the fields of materials science, engineering, and beyond. We hope that the content presented here will ignite new ideas and contribute to the continued evolution of hybrid piezoelectric materials as a driving force for technological advancement.

We extend our heartfelt gratitude to all the authors who have contributed their expertise to this endeavor as well as to the readers who embark on this exciting exploration of hybrid piezoelectric materials. The journey ahead promises a world of possibilities, and we invite you to join us on this captivating expedition.

Editors

1Introduction to Hybrid Piezoelectric Materials

Sheer Khanyisile Dhlamini1,2, Jonathan Tersur Orasugh1,2, Suprakash Sinha Ray1,2, and Dipankar Chattopadhyay3,4

1Department of Chemical Sciences, University of Johannesburg, Johannesburg, South Africa

2DST‐CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria, South Africa

3Department of Polymer Science and Technology, University of Calcutta, Kolkata, West Bengal, India

4Center for Research in Nanoscience and Nanotechnology, Acharya Prafulla Chandra Roy Sikhsha Prangan, University of Calcutta, Kolkata, India

1.1 Introduction

The exhaustion of conventional fossil fuels and pollution resulting from nonrenewable energy sources have created a global energy crisis. Another problem resulting from the energy crisis is that long‐lasting batteries have a limited lifespan, requiring replacement every few years; replacing these becomes expensive when hundreds of sensors are in secluded areas [1]. This has spurred significant interest in energy harvesting as an alternative. The development of green energy harvesters becomes more and more important as the world's demand for alternative energy increases. Energy garnering is a favorable technology that could help solve the global energy crisis with no depletion of natural resources. Energy garnering technologies allow low‐power equipment to operate for an unlimited period, eliminating the need to replace batteries where it would be impractical, hazardous, or too costly [2]. Using energy‐harvesting technologies would also enable running self‐sustained remote sensors, powering portable devices, and wireless electronics [3]. Piezoelectric energy‐harvesting technology offers several advantages over other energy‐harvesting methods, comprising smaller sizes, higher output voltage, a straightforward mechanism, and high sensitivity to applied strain [4]. In the recent decade, piezoelectric hybrid materials have received considerable attention due to their advantages over traditional organic and inorganic piezoelectric materials, as shown in Figure 1.1. They are one of the most emerging material classes at the forefront of technological advancement.

Figure 1.1 The number of articles on hybrid, organic, and inorganic piezoelectric materials published from 2002 to 2023 in ScienceDirect. (Search keywords used: Hybrid piezoelectric materials; Organic piezoelectric materials; and Inorganic piezoelectric materials. The search was performed on 12 June 2023).

In many high‐performance applications, traditional piezoelectric materials such as quartz and lead zirconate titanate (PZT) ceramics still exhibit excellent piezoelectric properties and are advantageous [5]. Both synthetic and naturally occurring materials can produce piezoelectric effects. Cane sugar, quartz, topaz, tendon, Rochelle salt, tourmaline, DNA, and bone are examples of naturally occurring materials that exhibit piezoelectricity [6]. Piezoelectric materials play an essential role in numerous applications in our everyday life, as shown in Figure 1.2. At home, the piezo buzzer in our alarms wakes us up in the morning, and the piezoelectric speakers in our headphones allow us to listen to music, and on our mobile phones, we use the piezo motor to adjust the focus of the camera. When driving to work, the car uses piezoelectric fuel injection to enhance fuel efficiency. In case of an accident/collision, the airbags used to protect us use a piezoelectric acceleration sensor [7]. By combining piezoelectric materials with two or more other materials leads to the development of hybrid materials that have improved properties and can be applied to novel applications [8]. Hybrid piezoelectric materials offer additional flexibility and tailored functionality, allowing them to offer unique solutions in certain situations. With hybrid piezoelectric materials, existing technologies can be enhanced, and new devices and systems can be developed, ranging from healthcare, ultrasonic transducers, energy storage, smart fabrics, sensors and actuators, energy‐harvesting systems, and robotics [9–11].

Figure 1.2 Selected devices that make use of the piezoelectric effect (self‐drawn).

In hybrid piezoelectric materials, two or more piezoelectric materials are combined to enhance the piezoelectric properties of the resulting material [8]. The combination of these materials results in enhanced piezoelectric performance and can be tailored to have specific properties. The choice of the inorganic components, along with the synthesis routes, conditions, and connectivity, may allow one to tailor the properties of the hybrid materials to meet specific requirements [12]. Due to the special combination of qualities that may be obtained from the inorganic and organic components, hybrid materials are a great choice for usage in a variety of industries, making them superior in the majority of facets to traditional one‐function materials. A recent review of these materials showed several advantages: improved performance, strong optical absorption, superconductivity, adjustable bandgaps, easy solution‐based processing, increased piezoelectric coefficients, high photoconversion efficiency, enhanced flexibility, tailored mechanical properties, high charge mobility, and light‐harvesting properties for solar devices [13].

1.2 The Concept of Piezoelectricity

1.2.1 History of Piezoelectricity

Carolus Linnaeus and Franz Aepinus studied the pyroelectric effect in the mid‐eighteenth century, when a temperature change caused a material to generate an electric potential. Based on this knowledge, both René Just Haüy and Antoine César Becquerel proposed that mechanical stress and electric charge were related; however, their experiments were inconclusive [14]. These scientists observed the piezoelectric effect, but it was not fully understood until two French physicists, Jacques and Pierre Curie, demonstrated it in 1880 [15]. While studying quartz, tourmaline, and Rochelle salt crystals, they discovered that applying pressure on these materials generates electrical charges on their surface [15]. Upon mechanical pressure, all of these materials exhibited characteristic crystal structures composed of a lattice of molecules having asymmetric dipole moments; as a result, they are commonly referred to as piezoelectric crystals. In 1881, Wilhelm G. Henkel named this phenomenon “piezoelectricity” after the Greek words piezein, which means to press or squeeze, and elektron, meaning amber, which refers to substances that attract other substances when rubbed like amber [16]. However, the Curie brothers did not predict the converse piezoelectric effect. In 1881, Gabriel Lippmann mathematically demonstrated the converse effect from fundamental thermodynamic principles; he deduced that if crystals exhibit the direct piezoelectric effect (electricity produced by applied stress), they would also exhibit the converse piezoelectric effect (stress caused by applied electric field) [17]. Immediately after discovering the converse effect, the Curie brothers obtained quantitative proof of the complete reversibility of electro‐elasto‐mechanical deformations in piezoelectric crystals. With the publication of Woldemar Voigt's Lehrbuch der Kristallphysik (Textbook on Crystal Physics), Voigt was able to describe the 20 natural crystal classes that exhibited piezoelectricity and define the piezoelectric constants using tensor analysis [18]. In 1893, Lord Kelvin developed an atomic model to describe the piezoelectric effect [19].

In 1917, during World War I, Paul Langevin and his French colleagues developed sonar, which was the first application of piezoelectricity. They built an ultrasonic submarine detector using a quartz crystal transducer between two steel plates and a hydrophone [20]. In 1935, the quest for understanding new synthetic piezoelectric materials led to the discovery and development of potassium dihydrogen phosphate (KDP) and ammonium dihydrogen phosphate (ADP) crystals [21]. A breakthrough in ferroelectrics with a perovskite structure was achieved in the early 1940s. During World War II, researchers from three countries (United States, Japan, and Russia) independently discovered the ferroelectric ceramic material barium titanate (BaTiO3) in 1946. Since then, piezoelectric materials have made rapid progress [22]. In 1952, physicists at the Tokyo Institute of Technology developed the first piezoelectric material PZT, which exhibits better piezoelectricity than BaTiO3 and operates at a higher temperature [23]. In the search for ferroelectric materials suitable for high temperatures, Aurivillius discovered bismuth layer‐structured ferroelectrics (BLSFs) in 1949. Piezoelectric activity in BLSFs can be further improved by suitable doping. The BLSFs are promising candidates for applications in high‐temperature sensors. Around 1950, the first evidence of piezoelectricity in wood was reported, followed shortly afterward by evidence of it in collagenous tissues such as tendons and bones. This research was carried out by Eiichi Fukada. In 1969, Heiji Kawai discovered piezoelectric effects in the synthetic polymer poly(vinylidene fluoride) (PVDF) polymers, which can be used for new sensors that cannot be realized with piezoceramics or single crystals [24]. For the purpose of simplistic understanding, the whole history is presented in Figure 1.3.

Figure 1.3 The history of piezoelectricity (self‐drawn).

1.2.2 The Piezoelectric Effect

The piezoelectric effect transpires when a material experiences mechanical stress that generates a small electrical potential across its sides. When a piezoelectric material is compressed, electricity is produced, and there is a conversion of kinetic or mechanical energy due to crystal deformation into electrical energy, as shown in Figure 1.4a. An electric field is generated when the positive and negative charge centers of piezoelectric materials shift under mechanical stress; this is referred to as direct piezoelectric effect. Whenever a voltage is applied to opposite sides of a material, it will undergo the opposite effect, known as the converse piezoelectric effect. On the other hand, converse piezoelectric effect takes place whenever an electric voltage is applied across a material, as shown in Figure 1.4b, causing its balanced electric charges to become unbalanced. In order to rebalance the electric charges, the material's atoms must be rearranged, and this causes the material to deform slightly.

Figure 1.4 The two varieties of piezoelectric phenomenon: (a) direct, (b) converse piezoelectric effect (self‐drawn).

Piezoelectric materials behave and perform differently depending on how many electric dipoles they contain. The piezoelectric property is greatly affected by crystal symmetry. A crystal is any solid with constituents (atoms or molecules) arranged orderly based on repetitions of the same basic atomic building block (the unit cell). In most crystals, such as your monocrystals, the polar axes of all the dipole moments lie in one direction, as shown in Figure 1.5a, and they are symmetrical. In contrast, in piezoelectric crystals, which are polycrystalline, as shown in Figure 1.5b, there are different regions with different polar axes causing irregular alignment of dipoles and asymmetry. Cutting the polycrystal results in the two remaining pieces having different polar axes. The material, at this stage, does not exhibit piezoelectricity, as all domains need to be polarized in one direction through a process known as poling in order to obtain a net polarization [25]. Poling is the procedure by which an electric field is applied to a piezoelectric material, and thus creates an instinctive polarization; as a result, due to the electric field, the molecules are freer to move, and the dipoles in the crystal are forced to align and face in nearly the same direction (the electric field's applied path), as shown in Figure 1.6. Changing the poling conditions can easily tailor the degree of polarization [26].

Figure 1.5 Dipole arrangement in (a) monocrystalline and (b) polycrystalline materials (self‐drawn).

Figure 1.6 Poling process to generate polarization of a polycrystalline material transduction principle of piezoelectronics (self‐drawn).

1.3 Comparison between Piezoelectric Materials

As shown in Figure 1.7, hybrid materials have several advantages over ceramics and polymers, since they combine the best properties of both. However, depending on the targeted application, ceramics or polymers can be preferred over hybrid materials. A typical ceramic piezoelectric material is used in microelectromechanical system (MEMS) devices and transducers, such as sonar, which require piezoelectric response without requiring flexibility [8]. The majority of piezoelectric ceramic materials have excellent piezoelectric characteristics, including strong electromechanical coupling coefficients and a high piezoelectric constant. However, because of their tremendous stiffness, these materials have high resonant frequencies. Due to the brittleness of ceramic materials, repetitive cycles of deformation can result in early failure [27]. Piezoelectric polymers/polymer materials are especially suitable for biomedical applications due to their compatibility with the human body; they are also flexible and lightweight. However, in general, their piezoelectric coefficients are not very high. They are also incapable of withstanding high temperatures [28]. Due to their weak mechanical properties, piezoelectric polymers are not recommended for applications in which they will be subjected to heavy loads [29].

Figure 1.7 The different classes of piezoelectric materials (self‐drawn).

Piezoelectric hybrid materials offer several advantages over both ceramics and polymers as they combine the properties of these two materials, with the possibility of tailoring properties by modifying their structure and composition [30]. A hybrid material made of both ceramic and polymer piezoelectric components can provide a product with a good balance of piezoelectric performance and acoustic match that can be tailored to meet the requirements of the intended application. By choosing the right combinations and amounts of constituent materials, the adoption of composite materials toward the design and fabrication of their hybrid counterparts enables the tuning of specific features. The mechanical, thermal, electrical, and other properties of the composite can be tailored for a particular application by carefully selecting the kind of orientation and volume fraction of the reinforcing components [31]. Comparing composites to ceramics and polymers alone, they also provide more design flexibility. They can be created to have precise geometries and optimal performance by being developed to have complex shapes and structures, resulting in lightweight and high‐performance systems [8]. Because hybrid materials contain diverse materials with complementing properties, they can display greater strength and stiffness than ceramics and polymers alone. They can be strengthened using a polymer matrix that contains ceramic fibers or particles, which can considerably improve the composite's mechanical characteristics and make it stronger and stiffer [28]. Compared to polymers alone, composites and/or hybrids can provide improved thermal stability. Hybrid materials can resist greater temperatures without significant degradation by adding thermally stable reinforcing elements, increasing their suitability in high‐temperature environments [8]. When used in a hybrid structure, the piezoceramics' acoustic match to tissue or water is substantially improved, since the stiff and dense ceramic is swapped out for a less rigid and more pliable polymer [8]. The advantages of hybrids can vary depending on the exact application, the choice of constituent materials, and the production process. To choose the best material, it is important to carefully analyze both the application's needs and the desired features.

1.4 Piezoelectric Material Types

Due to their ability to directly convert mechanical energy into electrical energy, piezoelectric materials are an essential part of energy‐harvesting systems, sensors, and actuators. The investigations on piezo‐materials have resulted in a wide variety of piezoelectric materials available today. Approximately 200 distinct categories of piezoelectric materials are employed in energy‐harvesting applications [32]. The main categories of piezoelectric materials are natural and synthetic, which are also further classified into ceramics, single crystals, polymers, and nanocomposites, as depicted in Figure 1.8. In this section, we will focus more on hybrid materials. The development of piezoelectric nanostructures has allowed the exploitation of these materials, which have been shown to have enhanced properties [33, 34].

Figure 1.8 Categorization of piezoelectric materials (self‐drawn).

1.4.1 Inorganic Piezoelectric Materials

1.4.1.1 Single Crystal‐Based Piezoelectric Materials

Piezoelectric crystals are single crystals that grow in a long‐range order based on the crystal matrix dot space. The single crystal structure's regular ordering of positive and negative ions causes the dipoles to be properly aligned, which is essentially what causes their piezoelectric action [35, 36]. Due to their greater piezoelectric coefficients, these materials are used in a variety of electromechanical fields, including sonar transducers, medical ultrasonic devices, actuators, hydrophones, accelerometers, and sensors [37]. Rochelle salt and quartz are naturally occurring piezoelectric single crystals; however, to enhance their piezoelectric properties, they must be aligned and cut in a particular crystallographic direction. Lithium niobate (LiNbO3), lead magnesium niobate–lead titanate solid solution (PMN–PT), as well as lead zinc niobate–lead titanate (PZN–PT), are the most used single‐crystal piezoelectric materials.

1.4.1.1.1 Lithium Niobate (LiNbO3) LiNbO3 is a single‐crystal oxide material with an ilmenite crystal structure, a high electromechanical coupling coefficient, spontaneous polarization coefficient, low‐temperature resistance, electro‐optic coefficient, corrosion resistance, and piezoelectric properties [38]. LiNbO3 is also nontoxic, in contrast to lead‐containing materials, making it suitable for biomedical applications [39]. The crystal was first synthesized using the flux method by Remeika and Matthias in 1949, who also discovered that this crystal has ferroelectric properties [40]. LiNbO3 single crystals have extensively been used in acoustics and electro‐optics due to their strong and stable electro‐mechanical coupling capabilities [41]. LiNbO3 has a very high Curie temperature (1142 °C); this high Curie temperature makes it possible to be used as a high‐temperature acoustic transducer, such as an accelerometer for jet aircraft. As a single crystal with a high Curie temperature, LiNbO3 can sustain stable piezoelectricity over a long period of time and a wide range of temperatures without experiencing problems with grain size and porosity [42]. LiNbO3 is an excellent material for surface acoustic wave (SAW) devices. About 70% of radio‐frequency (RF) filters, based on SAW, are fabricated using LiNbO3 single crystals [43]. LiNbO3 films need to be highly crystalline, possess a pure LiNbO3 phase and a smooth surface, be twin‐free and domain‐free, and have a good texture quality in order to be used in electrically active acoustic and optical devices and to ensure high performance [43]. There are a number of crystal cuts in LiNbO3 that are now widely used in piezoelectric applications, including 36° and 163° rotated Y‐cuts [5].

1.4.1.1.2 Lead Magnesium Niobate–Lead Titanate Solid Solution (PMN–PT) PMN–PT is a single crystal composed of a solid‐solution material, made of lead magnesium niobate (PMN) and lead titanate (PT). Each processing method for PMN–PT material, including single crystal, polycrystalline ceramic, thick film, and thin film, has unique functional properties that are appropriate for particular applications. For instance, PMN–PT single crystals have the best functional properties for actuator applications [44]. The PMN–PT single crystal of the composition on the morphotropic phase boundary (MPB) can have piezoelectric coefficient as high as 2820 pC/N. PMN–PT‐based materials exhibit high piezoelectricity, high electrostriction, and high dielectric permittivity and can be used to make multilayer capacitors, actuators, sensors, and electro‐optical devices [45]. The strong piezoelectric capabilities of PMN–PT solid solutions are related to the polarization rotation between adjacent rhombohedral and tetragonal phases via one (or more) low‐symmetry intermediate phases, that is, a monoclinic (orthorhombic or triclinic) phase [44]. Most PMN–PT crystals can be grown using the high‐temperature flux technique or the Bridgeman method. The Bridgman technique has proven to be a promising growth technology for PMN–PT crystals [37].

1.4.1.2 Ceramic‐Based Piezoelectric Materials

Piezoelectric ceramics refer to polycrystalline materials having a perovskite crystal structural architecture, made of large number of identically chemically constituted single crystal “grains.” However, because each grain's orientation varies from the other, the spacing between the ions also varies slightly. The most commonly used piezoelectric ceramics are potassium niobate (KNbO3), BaTiO3, PZT, and potassium sodium niobate (KNN).

1.4.1.2.1 Pb(Zr,Ti)O3 (PZT) PZT is a perovskite‐structured ceramic material that possesses superior piezoelectric properties as it is a solid solution of lead zirconate (PbZrO3) and lead titanate (PbTiO3). PZT is regarded as the major piezoelectric ceramic that is broadly utilized in applications, such as energy harvesting, actuators, transducers, sensors, and sonar systems [8]. These materials are distinguished by their exceptional piezoelectric characteristics. PZT has a high piezoelectric constant (500–600 pC/N), a high electromechanical coupling coefficient (0.69), good mechanical stability, and low dielectric loss, which results in a higher conversion from mechanical to electrical energy [46, 47]. PZT can be employed in a variety of applications since it is mechanically robust, possesses excellent chemical stability, and does not degrade in humid conditions [8]. PZT thick and thin films have been utilized extensively in a range of piezoelectricity as well as piezo‐resistivity‐based sensors, including pressure sensors and biosensing cantilevers. This newly found material's acceptance as a piezoelectric material was substantially accelerated by Sir Bernard Jaffe's study on it. The material also has high Curie temperatures of 350 °C and retains its piezoelectric properties at high temperatures [8]. Because of its high density and high Young's modulus (50 GPa), PZT is too brittle and unable to undergo significant deformation; hence, it cannot be used in applications that require flexibility [48]. Bulk PZT has a high resonance frequency due to its stiffness, making it unsuitable for capturing energy from low‐frequency ambient vibrations [8]. Another disadvantage of PZT is that it cannot be used in some applications, such as devices implanted into the body, because lead is dangerous to both human health and the environment [48]. Due to their cost‐efficiency, good piezoelectric properties, and ease of integration into energy‐harvesting devices, PZT has been extensively used for energy‐harvesting as well as storage applications [49].

1.4.1.2.2 Barium titanate (BaTiO3) Another significant ceramic‐based piezoelectric material is BaTiO3. During World War II, scientists found that BaTiO3 was the first perovskite‐structured ferroelectric substance. It was extensively used in phonograph needles and sonar detection [50]. BaTiO3 is a ferroelectric substance, which means that when an electric field is applied, it can switch between several polarization states. Pure, unmodified BaTiO3 has a low piezoelectric constant (190 pC/N) and a low Curie temperature (130 °C) [5]. However, BaTiO3 can be modified to have good piezoelectric properties; for example, it has been shown using different processing techniques that result in dramatic improvements in piezoelectric properties. For instance, Karaki et al. [51] successfully fabricated high‐density BaTiO3 piezoelectric ceramics while maintaining the average grain size by a two‐step sintering method from hydrothermally synthesized BaTiO3 nanoparticles of 100 nm, which showed improved piezoelectric properties compared to unmodified BaTiO3. This ceramic material showed excellent piezoelectric properties, with a piezoelectric coefficient of 460 pC/N, an electromechanical coupling factor of 42%, and a dielectric constant of 5000. It has also been shown that the introduction of fine domains into these ceramics leads to a high piezoelectric constant. Takahashi et al. [52] reported that the introduction of fine domains with a size of 50 nm into BaTiO3 ceramics fabricated by microwave sintering results in a high piezoelectric coefficient of 350 pC/N. Wada et al. [53] also showed that all piezoelectric‐related constants of single‐crystal BaTiO3 increased with the decrease in ferroelectric domain size. They prepared BaTiO3 grain‐oriented ceramics by the templated grain growth (TGG) method and obtained a high piezoelectric coefficient of 788 pC/N. BaTiO3 can be used in several applications, such as sonar systems, sensors, capacitors, and piezoelectric transformers [50].

1.4.1.2.3 Potassium Sodium Niobate (KNN) KNN is a lead‐free ceramic piezoelectric material made up of NaNbO3 and KNbO3; it is a promising alternative to lead‐based piezo ceramics [49]. Undoped KNN has a very high Curie temperature (above 400 °C). However, it has a low piezoelectric constant (80–160 pC/N) and is difficult to fabricate due to low sintered densification [54]. Modified KNN structures can result in higher piezoelectric properties closer to those of PZT materials; for instance, Saito et al. [55] created highly textured polycrystals with a high piezoelectric constant (416 pC/N) using the reactive template grain generation technique. Guo et al. [56] also obtained very high piezoelectric constant ranging from 200 to 235 pC/N in KNN ceramics by doping LiNbO3 or LiTaO3 using traditional solid‐state methods. The biggest benefit of KNN is that it does not contain lead, making it suitable for use in energy harvesting and medical implants [54].

1.4.2 Organic Piezo Materials

Piezoelectric polymers are carbon‐based substances with lengthy polymer chains that have mechanical flexibility; as a result, they can withstand high strains, making them more suitable for piezoelectric energy‐harvesting applications. Piezoelectric polymers include materials such as PVDF, polylactic acid (PLA), polyamides (PA), copolymers, cellulose, and its derivatives. In order for polymers to exhibit considerable piezoelectricity, mechanical stretching and poling are typically required. The applied electrical field alters the orientation of positive and negative charges and dipoles, which in turn causes piezoelectricity [48, 57]. In comparison to inorganic piezoelectric materials, polymers have different properties, making them uniquely qualified to fill niches that inorganic materials cannot. The properties of polymers include high biocompatibility, excellent flexibility, environmental friendliness, and ease of processing. Since polymers have a lower piezoelectric strain constant (d31) than inorganic materials, they tend to make better sensors. It is easier to make sensors from these materials because they are lightweight and can be cut and shaped into complex shapes. Due to their high strength, high impact resistance, low elastic stiffness, low density, and low dielectric constant, they have the advantage of having high voltage sensitivity, low acoustic impedance, and high mechanical impedance. Numerous energy‐related applications, including medical transducers, vibrometers, audio transducers, displays, shock sensors, and pressure sensors, have found these materials to be acceptable. Nylons, polyimide (PI), PLA, PA, cellulose, as well as its derivatives, polyurea, and polyurethane (PU) are examples of other polymers that exhibit some degree of piezoelectric effect.

1.4.2.1 PVDF

The semicrystalline polymer PVDF is composed of chains aligned to form dipoles, and it was discovered by Kuwaii. In polycrystalline PVDF, polymer chains are arranged in zigzag patterns in the crystal grains, with an amorphous matrix surrounding the crystalline structure and it is usually 35–70% crystalline [48]. Due to its non‐centrosymmetric crystal structure, PVDF exhibits piezoelectric activity, and it is preferred over other polymer materials [58]. Additionally, it has a low acoustic impedance, great electromechanical response, and good chemical stability. PVDF exhibits a negative piezoelectric constant, in contrast to other widely used piezoelectric materials such as PZT; this means that PVDF compresses rather than expands when exposed to an electric field [59]. The material is mechanically stretched to align the molecular chains and then poled under tension to give it its piezoelectric properties [59]. PVDF exists in three main polymorphic phases: alpha (α), beta (β), and gamma (γ). The β phase is the most piezoelectrically active phase of PVDF and has the strongest ferroelectric properties; however, a high amount of crystallinity must be attained initially in order for the phase to develop [60]. The β‐phase content and degree of crystallinity are influenced by stretching ratio and temperature, which in turn will affect the electroactive properties of the polymer [61]. To induce and stabilize the desired phase, a variety of techniques can be used, such as mechanical stretching, electrical poling, or annealing. Moreover, PVDF is biocompatible, which makes it ideal for biomedical applications. Among its applications are biosensors, implants, and scaffolds for tissue engineering. One of the most promising PVDF copolymers investigated for its piezoelectric properties is trifluoroethylene (PVDF–TrFE). As PVDF–TrFE contains an extra fluorine, it increases the possibility of forming the β phase [48]. PVDF–TrFE has a higher piezoelectric power density (312.5 μW/cm3) compared to PVDF with a piezoelectric power density of 81.3 μW/cm3[48].

1.4.2.2 Polylactic Acid (PLA)

PLA is a biodegradable and biocompatible polymer with very low piezoelectric activity; however, it can be modified to exhibit applicable piezoelectric activities. The alignment and orientation of Poly‐L‐lactic acid (PLLA)'s polymer chains are thought to be the cause of its piezoelectric characteristic. Stretching or poling are two examples of processing methods that can be used on PLLA to align the polymer chains in a desired orientation and produce a non‐centrosymmetric structure that supports piezoelectricity. PLLA is suitable for applications that require flexible and lightweight piezoelectric materials. PLLA is biocompatible and it has potential to be used in biomedical applications, such as medical implants, biomedical sensors, and tissue engineering scaffolds.

1.4.3 Hybrid Piezoelectric Materials

Hybrid/nanocomposite materials have been developed to enhance the properties of different piezoelectric materials. Nanocomposites combine piezoelectric ceramic nanoparticles with a polymer matrix [28]. The ceramic nanoparticles contribute unique properties such as high mechanical strength, piezoelectricity, and thermal stability to the nanocomposite; the most used ceramic materials include PZT, ZnO, and BaTiO3 nanoparticles. The polymer matrix provides flexibility, processability, and mechanical properties to the nanocomposite, with the most used polymer matrices used are PVDF, PU, and polystyrene (PS). Incorporating other components to construct hybrid systems can improve the piezoelectric performance of pure piezoelectric materials by compensating for the shortcomings of each component and exploiting synergistic effects [62]. In comparison to ceramics and polymers, nanocomposites have been shown to possess improved properties [63]. The most explored hybrid piezoelectric nanocomposite materials are PVDF–PZT, BaTi03/PVDF, and ZnO–PVDF, among others [28].

1.4.3.1 PVDF–PZT systems

One of the most researched polymer–ceramic piezoelectric hybrids/composites is PVDF–PZT [8]. The 0–3 types of polymer–ceramic piezoelectric composites were first reported by Kitayama and Sugawara [41]. A PVDF–PZT composite is a type of hybrid piezoelectric material that combines PVDF, which is the organic polymer material that acts as the organic matrix, with PZT, which is the inorganic ceramic material that acts as the ceramic filler [41]. PZT is too stiff to be employed in fluids such as water or blood, and PVDF has a comparatively low piezoelectric constant and low stiffness on its own [8]. Compared to pure PVDF or PZT materials alone, PVDF–PZT composites have improved piezoelectric properties. PZT ceramic fibers or particles can improve charge generation and mechanical‐to‐electrical energy conversion by increasing the piezoelectric response. PVDF–PZT composites benefit from the mechanical properties of PVDF and the high stiffness of PZT ceramics. The presence of PZT fillers improves the stiffness and mechanical strength of the composite, making it more robust and suitable for applications that require both piezoelectricity and mechanical integrity [28]. PVDF–PZT composites also exhibit enhanced mechanical properties and flexibility. The composite can be bent or shaped without losing its piezoelectric properties due to the flexibility of PDVF. This allows the composite to be used in flexible electronics and wearable technology, while PZT fillers increase the composite's stiffness and mechanical strength, making it more resilient [28].

PVDF–PZT composites can be used in various applications such as sensors, medical imaging, actuators, sonar systems, energy harvesting, and ultrasound transducers. It has been shown that the piezoelectric properties of the composite can be improved by making use of fibers through the electrospinning (ES) technique. Chang et al. [64] synthesized PZT–PDVF nanocomposite; the PZT nanofibers were prepared via ES, and the modified PZT nanofibers and PVDF were dispersed in dimethylformamide (DMF) to fabricate the PZT/PVDF nanocomposites. The results showed improved piezoelectric properties, with a piezoelectric coefficient of 87.4 pm/V with only 2.4 vol% of PZT nanofibers. Chamankar et al. [65] prepared flexible pressure sensors based on nanocomposite fibers with enhanced dielectric and piezoelectric properties. The flexible nanocomposite fibers were prepared from PZT ceramic particles and PVDF polymer. PZT ceramic particles were synthesized by the sol–gel process method, after which the prepared solution was electrospun to give PZN nanoparticles with a size of 85.85 nm. PVDF–PZT nanocomposite fibers were prepared through the ES method with a different volume fraction of PZT. The PVDF–PZT nanocomposite fibers were 338.9 nm. To fabricate the power‐generating modules, the nanocomposite fibers were cut into a square (4 cm × 4 cm). A paper sheet was employed as a spacer between the top and bottom of the nanocomposite fiber web, and aluminum foils were positioned at the top and bottom of the web as the electrode. The results showed that increasing the PZT volume fraction increased the crystallinity and electroactive β phase, piezoelectric constant, dielectric constant, piezoelectric sensitivity, and output voltage of PVDF–PZT nanocomposite fibers.

1.4.3.2 BaTiO3/PVDF

A BaTiO3/PVDF composite is a type of hybrid piezoelectric material that combines BaTiO3, which is the inorganic ceramic material that acts as the ceramic filler, and PVDF, which is the organic polymer material that acts as the organic matrix. Just like PZT–PVDF, BaTiO3/PVDF composites also result in enhanced piezoelectric properties. The use of self‐poled, highly crystalline PVDF and BaTiO3 nanoparticles, both of which have positive piezoelectricity, gives the composite outstanding piezoelectric performance [50]. BaTiO3, a ferroelectric material with a high dielectric permittivity, when combined with PVDF, creates a composite with increased dielectric characteristics, enhancing electrical performance and energy storage capacity. The mechanical characteristics of the composite are enhanced; the composite is made stiffer and stronger by the use of BaTiO3 ceramics, which makes the composite more appropriate for applications that require both mechanical integrity and piezoelectricity [8].

The composites can be prepared using various methods, including ES, melt blending, and solution casting [66]. They can be developed into different materials, such as films, fibers, or coatings, depending on the targeted application. Dashtizad et al. [67] synthesized a BaTiO3–PVDF nanocomposite with improved piezoelectric properties. PVDF–BaTiO3 and PVDF–BaTiO3–Ag nanocomposite fibers were prepared by the ES method. By using the sol–gel method, BaTiO3 particles were synthesized, and BaTiO3–Ag were subsequently irradiated with ultraviolet light for 2, 6, and 10 minutes in silver nitrate solution to precipitate silver on the BaTiO3 particles. Using a specific force and a defined frequency, piezoelectric properties of each composite were measured. Based on the results, the nanocomposite showed better piezoelectric properties than PVDF fiber. PVDF–BaTiO3–Ag composites showed a higher output voltage than PVDF–BaTiO3 owing to the greater extent of β‐phase formation. Kong et al. [68] fabricated low‐cost, flexible piezoelectric pressure sensors using BaTiO3–PVDF nanofibers by ES method. PVDF was dissolved in a mixture of first mix acetone and DMSO; the solution was stirred until a clear solution was obtained. BaTiO3 was then added to the clear solution, and the solution was electrospun using near‐field ES method. To obtain the hierarchical structured film, the barium titanate (BTO)/PVDF membrane was placed between molds and dried at a constant temperature as shown in Figure 1.9a. As a result, the fabricated flexible pressure sensor was able to detect human activity successfully; this shows it can be used in wearable bioelectronics, as shown in Figure 1.9c,d. The morphology of the membrane is depicted in Figure 1.9b. When a periodic load of 2 N at 2.5 Hz was applied to the 2 mm bulge high‐entropy alloy (HHS) sensor, it produced a maximum output voltage of 2.32 V.

Figure 1.9 BaTiO3–PVDF: (a) the schematic of the fabrication process, (b) the SEM image of the five‐layer fiber membrane and a single fiber, (c) the photographs of the HHS pressure sensor integrated into the soles of shoes, and (d) the output voltage generated by walking.

Source: Reproduced with permission from Kong et al. [68], © 2023/John Wiley & Sons, Inc.

1.4.3.3 ZnO–PVDF

In ZnO–PVDF composites, PVDF, which is the organic material, is combined with ZnO metal particles or fibers. The ZnO provides additional conductivity or mechanical reinforcement, while the PVDF acting as the organic matrix may contribute to flexibility and processability [69]. ZnO nanoparticles or microparticles improve the composite's overall piezoelectric responsiveness. Under mechanical stress, ZnO particles produce charges, and the PVDF matrix aids in charge separation, producing a stronger piezoelectric effect [8]. Because ZnO and PVDF are compatible, ZnO particles can be effectively dispersed throughout the PVDF matrix [70]