Piezoelectricity and the Piezoelectric Effect


Piezoelectricity and Piezoelectric Effect
Piezoelectricity is the ability of some materials to produce an electric charge in response to mechanical stress.

Piezoelectricity is a property of certain materials that allows them to generate an electric charge in response to applied mechanical stress. The term originates from the Greek word “piezein,” which means to press or squeeze, aptly describing the process of generating electricity through pressure.

How Piezoelectricity Works

The piezoelectric effect occurs at a microscopic level, where the applied mechanical stress leads to a displacement of the positive and negative charge centers within the crystal structure of the material. This displacement creates an electric polarization and hence an electric potential (voltage) across the material. Conversely, when an electric field is applied to a piezoelectric material, it causes a mechanical deformation, known as the converse piezoelectric effect.

Piezoelectric Effect

The piezoelectric effect is the direct interaction between the mechanical and electrical states in crystalline materials with no inversion symmetry. The effect occurs in both natural and synthetic materials. Deformation of these materials generates an electrical charge. Conversely, the material changes shape when an electric field is applied.

Piezoelectric Materials

Examples of Piezoelectric Materials
Examples of piezoelectric materials include certain crystals, ceramics, and organic matter.

Piezoelectric materials fall broadly into the categories of crystals, ceramics, and polymers. Some natural organic crystals and polymers exhibit piezoelectricity.

  1. Crystals
    • Quartz (SiO₂): A naturally occurring crystal with a well-defined and strong piezoelectric effect.
    • Topaz
    • Tourmaline
    • Rochelle Salt (Potassium Sodium Tartrate, KNaC₄H₄O₆·4H₂O): Known for its strong piezoelectric properties but has limited industrial use due to its water solubility and low-temperature stability.
    • Gallium Orthophosphate (GaPO₄): Similar to quartz in its piezoelectric properties, but with a higher temperature stability.
    • Sucrose (C12H22O11, table sugar): Generates electrical charge in response to mechanical stress, both in pure and impure (cane sugar) forms.
    • Lead titanate (PbTiO3)
  2. Ceramics
    • Lead Zirconate Titanate (PZT, Pb[ZrₓTi₁₋ₓ]O₃): A synthetic ceramic that exhibits one of the most significant piezoelectric effects and is widely used in various applications.
    • Barium Titanate (BaTiO₃): Known for its use in capacitors and nonlinear optics in addition to its piezoelectric properties.
    • Zinc oxide (ZnO): The Wurtzite structure of single crystals is piezoelectric.
  3. Polymers
    • Polyvinylidene Fluoride (PVDF): A thermoplastic polymer with piezoelectric properties used in flexible sensors and actuators.
    • Polyvinylidene Fluoride-Trifluoroethylene (P(VDF-TrFE)): A copolymer of PVDF that enhances the piezoelectric effect.
    • Poly L-lactic Acid (PLLA): A biodegradable polymer used in medical applications for its piezoelectric characteristics.
    • Collagen: Found in bones and tendons, collagen exhibits natural piezoelectric properties.
    • Cellulose: Certain forms of cellulose, especially in its crystalline form, show piezoelectric effects.
    • Glycine: An amino acid that exhibits piezoelectricity in specific crystalline forms.
    • Polyurea: A polymer known for its piezoelectric response under specific conditions.
    • DNA: Displays slight piezoelectricity due to its helical shape.

History and Word Origin

The piezoelectric effect was first discovered in 1880 by the Curie brothers, Jacques and Pierre, in tourmaline, Rochelle salt, and quartz. They observed that pressure applied to crystals generated an electrical charge. This was intriguing because it suggested a direct link between mechanical stress and electricity. The term “piezoelectricity” was coined by them, deriving from the Greek word for pressure.

Applications of Piezoelectricity

Piezoelectricity serves many uses both commercially and in nature.

Uses

  • Sensors and Actuators: Used in accelerometers, vibration sensors, and precision motion actuators.
  • Medical Devices: An example is ultrasound imaging, where the piezoelectric effect helps in generating and detecting sound waves.
  • Consumer Electronics: In microphones, headphones, and quartz watches.
  • Energy Harvesting: Collecting ambient mechanical energy (like footfall or bridge vibrations) and converting it into usable electrical energy.
  • Automotive Industry: Used in knock sensors for advanced engine management systems.
  • Military and Aerospace: Applications in sonar, guidance systems, and vibration monitoring.

Biological Role

Piezoelectricity is a fundamental aspect of some biological processes. Here are a few key areas where biological functions of piezoelectricity are observed:

  • Bone Remodeling and Growth: One of the most well-known biological functions of piezoelectricity is in bone tissue. Bone is piezoelectric, which means it generates electrical potentials when subjected to mechanical stress. This property likely plays a role in bone remodeling and growth, where the electrical signals generated by piezoelectricity stimulates the formation or resorption of bone by osteoblasts and osteoclasts, respectively.
  • Tendon Movement and Function: Similar to bones, tendons also exhibit piezoelectric properties. When tendons are stretched or compressed, they generate electrical signals. This piezoelectric behavior may aid in the repair and growth processes of tendons and also play a role in signaling and communication within the tissue.
  • Dental Applications: The piezoelectric properties of dental tissues like dentin have various applications, such as understanding tooth mechanics and the development of better dental restorations.
  • Hearing Mechanisms: In the ear, certain biological materials exhibit piezoelectric properties that are crucial for hearing. For instance, the piezoelectric effect in the cochlea helps convert mechanical vibrations (sound waves) into electrical signals that the brain interprets as sound.
  • Cell and Tissue Mechanics: Some cellular processes involve piezoelectricity, especially in cell membranes and in tissues under mechanical stress. This influences cell behaviors like migration, division, and communication.
  • Electrical Signaling in Cartilage: Similar to bone, cartilage also shows piezoelectric properties, playing a role in its growth, repair, and response to mechanical stress.

Piezoelectricity, Ferroelectricity, Pyroelectricity, and Triboluminescence

Some materials exhibit multiple phenomena such as piezoelectricity, ferroelectricity, pyroelectricity, and triboluminescence, although it’s not always common for a single material to display all of these properties simultaneously. The coexistence of these properties in a material depends on its internal structure and the nature of its atomic or molecular bonds.

  • Piezoelectricity and Ferroelectricity: Many materials that are piezoelectric are also ferroelectric. Ferroelectricity is a property where materials exhibit a spontaneous electric polarization that can be reversed by the application of an external electric field. This is closely related to piezoelectricity, where mechanical stress leads to polarization. For example, Lead Zirconate Titanate (PZT) is both ferroelectric and piezoelectric.
  • Ferroelectricity and Triboluminescence: Some ferroelectric materials may also exhibit triboluminescence, which is the emission of light when a material is mechanically stressed or fractured. This is less common, but there are instances where these properties coexist due to the restructuring of charge distributions under mechanical stress.
  • Piezoelectricity and Triboluminescence: Materials that are both piezoelectric and triboluminescent are uncommon, as the latter occurs in materials that undergo some form of fracturing or bond breaking. Both quartz and sucrose demonstrate both piezoelectricity (when deformed) and triboluminescence (when fractured).
  • Piezoelectricity and Pyroelectricity: Pyroelectric materials generate a temporary voltage when they are heated or cooled. If the material is also piezoelectric, this means it generates an electric charge in response to both mechanical stress and changes in temperature. Quartz, tourmaline, and barium titanate are examples of materials that display both piezoelectric and pyroelectric properties.

References

  • Curie, Jacques; Curie, Pierre (1880). “Développement par compression de l’électricité polaire dans les cristaux hémièdres à faces inclinées” [Development, via compression, of electric polarization in hemihedral crystals with inclined faces]. Bulletin de la Société Minérologique de France. 3 (4): 90–93. doi:10.3406/bulmi.1880.1564
  • Damjanovic, Dragan (1998). “Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics”. Reports on Progress in Physics. 61 (9): 1267–1324. doi:10.1088/0034-4885/61/9/002
  • Gautschi, G. (2002). Piezoelectric Sensorics: Force, Strain, Pressure, Acceleration and Acoustic Emission Sensors, Materials and Amplifiers. Springer. ISBN 978-3-662-04732-3. doi:10.1007/978-3-662-04732-3
  • Heywang, Walter; Lubitz, Karl; Wersing, Wolfram, eds. (2008). Piezoelectricity : Evolution and Future of a Technology. Berlin: Springer. ISBN 978-3540686835.
  • Manbachi, A.; Cobbold, R.S.C. (2011). “Development and Application of Piezoelectric Materials for Ultrasound Generation and Detection”. Ultrasound. 19(4): 187–96. doi:10.1258/ult.2011.011027