PIEZOELECTRICITY The relevance of piezoelectric materials

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Piezoelectricity is an inherent property of certain materials. Strange as it sounds, piezoelectric materials generate current when they are mechanically stressed. Inverse piezoelectricity also exists, where materials change shape upon the application of an electric field. The article explores the role of piezoelectric materials in modern times.

In 1880, two French brothers, Pierre and Jacques Curie, discovered piezoelectricity through some crystals of quartz, topaz, and Rochelle salt. The discovery was celebrated across Europe. The scientific community focused on piezoelectricity for years, but no practical application became a reality for 37 years.

The piezoelectric effect was discovered a year after the demonstration of piezoelectricity through crystals. Another French scientist, Gabriel Lippmann, thanks to whom colored photographs and prints came into existence, theoretically proved the piezoelectric effect. Curie brothers quickly confirmed the inverse piezoelectric effect.

In 1917, some scientists developed a transducer, achieving their high-frequency goals of “SONAR” applications (Sound Navigation And Ranging). Forward to 145 years in the future, these interesting materials are used in energy harvesting, transformers, actuators, control systems, sensor manufacturing plants, and many more.

Piezoelectricity: What is it?

The word “piezoelectric” is derived from a Greek word called “piezo”, which means ”to push”. Piezoelectricity is a phenomenon based on the piezoelectric effect.

Piezoelectric effect: The piezoelectric effect describes voltage generation through physical phenomena. Piezoelectricity is defined as the electricity induced by mechanical stress.

Inverse piezoelectric effect: The Inverse piezoelectric effect holds true. Whenever a material is subject to a voltage, it undergoes mechanical deformation, either expansion or contraction.

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Types of piezoelectric materials

Some piezoelectric materials exist naturally, while others are manufactured in factories.

Natural piezoelectric materials: Some materials naturally exhibit piezoelectricity, like quartz, Rochelle salt, topaz, tourmaline, sugarcane, wood, silk, rubber, and many others. Even bones, dentin tissue, enamel, and hair exhibit piezoelectricity naturally!

Synthetic piezoelectric materials: Various crystals, ceramics, composites, and polymers such as PZT (Lead Zirconate Titanate), lead niobate, zinc oxide, and PVDF (Polyvinylidene Fluoride).

Physics behind piezoelectricity

Many materials exhibit a crystalline structure. Atoms are bonded to each other, forming a repeating structure known as a unit cell. Generally, unit cells are cubic and symmetrical, such as face-centered cubic (FCC) and body-centered cubic (BCC) arrangements. In most materials, positive and negative charges are aligned symmetrically. The net charge becomes zero.

In a more technical sense, pairs of equivalent positive and negative charge carriers inside a crystalline material are called dipoles. In simple words, a dipole is a pair of two charge carriers separated at a distance, with an equivalent magnitude but opposite polarity. The dipole moment defines the distance between positive and negative charges within the material. The higher the separation, the higher the dipole moment— the higher the net charge on the material.

Certain materials exhibit less symmetric crystalline structures. One of the arrangements is called a non-centrosymmetric unit cell. In simple words, the crystalline structures inside these materials lack a center of symmetry, unlike other structures mentioned above. However, symmetrically aligned dipoles balance to cancel out each other, leading to a zero net dipole.

When pressure is applied to FCC, BCC, and other centrosymmetric structures, dipoles shift uniformly. Net dipole remains zero. In the case of non-centrosymmetric structures, when pressure is applied, dipoles move randomly. The symmetry breaks down, and there is no preservation of net charge. As a result, lack of symmetry causes random dipole movement, generating a net charge on the material.

The net charge on the material means that it produces an electric field. In short, putting mechanical stress (pressure or force) on a non-centrosymmetric material makes it generate voltage. Polarization is another term that means “dipole moment per unit volume of a material”. It defines how positive and negative charge carriers align and at what distance. The amount of voltage produced depends on the magnitude of polarization.

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Applications of piezoelectricity

Piezoelectric technology has been commercial for years in military applications like sonar, radar, sensing, and many more. The first use emerged in the twentieth century and continues today.

Several properties limit its usage. For example, synthetic piezoelectric materials are toxic to the environment, as some contain lead. Piezoelectric polymers are not flexible enough and are difficult to process.

Some natural piezoelectric materials exhibit a low piezoelectric constant, meaning they do not have much piezoelectric strength. Modern applications of piezoelectric materials are listed below.

Transducers

A transducer is an electronic device that converts energy from one form at the input to another at the output. Piezoelectric transducers are applicable in monitoring systems, medical imaging, military applications, industrial facilities, semiconductor manufacturing, and many other fields.

Sensors

Piezoelectric sensors are advanced sensors used in health monitoring applications to inspect topography, materials, and structures. They measure changes in temperature, vibrations, acceleration, pressure, and other such phenomena, and convert them into electrical signals. A common example is a seismograph.

Industrial applications

Piezoelectric materials are commonly used as actuators, diesel fuel injectors, igniters, optical adjustments, ultrasonic cleaning, ultrasonic welding, motors, relays, and various other industrial applications.

Medical imaging

Piezoelectric materials are used in medical applications that involve ultrasound equipment. For example, they are used in sonography, ultrasound imaging, and many non-invasive treatments.

Portable electronics

Piezoelectricity proves to be a lightweight power source for small-scale wearables and portable electronics. One of the most common uses of piezoelectric materials is to convert electrical signals into sound and vice versa— speakers in cell phones, earbuds, watches, and many more. Other applications include piezoelectric toothbrushes, buzzers, printers, and watches.

Textile-based piezoelectric nanogenerators remain a subject of research. Such piezoelectric devices function as power sources, generating low voltages and currents for wearable electronics. A real-time example of such technology is “smart” clothing that may harvest mechanical movement like walking, stretching, or running to convert it into piezoelectricity.

MEMS applications

Piezoelectric thin films are essential for MEMS (Microscale electronic and mechanical systems) like sensors, actuators, energy harvesting in portables, and upcoming quantum technologies. Researchers have recently produced high-quality piezoelectric thin films on insulating and conductive substrates. They are suitable for low-temperature stable use.

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Piezoelectric FET

Piezoelectric materials can replace the gate dielectric in a field-effect transistor. The piezo-FET operation is based on both the piezoelectric and its inverse effect. The main part of an FET is the channel, whether p-type or n-type. In piezoelectric FETs, the stress of the gate material is modified to obtain the best results. The on-state involves stressing the channel and relaxing it during the off-state. However, piezoelectric FET manufacturing involves complex and thorough procedures.

Aquatic robotics

AUVs (Aerial Unmanned Vehicles) are submersible vehicles that can be operated without a human. Also known as underwater drones, AUV functions completely depend upon the promise of underwater electronics. AUVs use actuators made from piezoelectric materials like PZT. The piezoelectric actuators are MFC (macro-fiber composite), compatible with moderate stress and vibrations.

Quantum computing

Polarization is closely related to quantum computing. Each state of a qubit depicts either a polarized photon or the spin of an electron. In such a way, piezoelectric materials remain a subject of research with ferroelectric and non-ferroelectric materials for their applications in cryogenic environments like quantum and superconductivity.

References

• https://piezo.com/pages/history-of-piezoelectricity?srsltid=AfmBOorvWqAdd1kgs_4q6Y0vgun3Q_DqhXoXD1jJnebIlAvdx97bs55o

• https://www.autodesk.com/products/fusion-360/blog/piezoelectricity/

• https://cordis.europa.eu/article/id/435789-breaking-crystal-symmetries-in-pursuit-of-piezoelectricity

• https://www.britannica.com/science/electric-polarization

• https://www.americanpiezo.com/blog/top-uses-of-piezoelectricity-in-everyday-applications/

• https://www.sciencedirect.com/science/article/abs/pii/B9780081001486000032

• https://www.sciencedirect.com/science/article/abs/pii/B978008102061600001X

• https://www.sciencedirect.com/science/article/abs/pii/S0029801818313854

• https://www.sciencedirect.com/science/article/pii/S2211285523002513#sec0135

• https://www.empa.ch/web/s604/piezoelectric-thin-films

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