Piezoelectric effect
Squeeze a crystal and it generates voltage. Apply voltage and it deforms. The piezoelectric effect — where mechanical stress and electric charge are two faces of the same crystal asymmetry.
Pressure makes electricity, electricity makes shape
The direct piezoelectric effect, discovered by Jacques and Pierre Curie in 1880, is the generation of electric charge in certain crystals when mechanical stress is applied. Squeeze a quartz crystal and opposite faces accumulate positive and negative charges, producing a measurable voltage. The effect is linear: double the force, double the voltage.
The inverse (converse) piezoelectric effect, predicted by Gabriel Lippmann in 1881 and confirmed by the Curies, is the reverse: apply an electric field across the crystal and it physically deforms. The strain is proportional to the field. This bidirectional coupling between mechanical and electrical energy is the foundation of piezoelectric technology.
Both effects arise from the same physics — the asymmetric arrangement of ions in the crystal lattice. When the lattice lacks a center of inversion symmetry, mechanical deformation shifts the center of positive charge relative to negative charge, creating a net electric dipole moment. Conversely, an external field pushes positive and negative ions in opposite directions, distorting the lattice.
Why only certain crystals are piezoelectric
Of the 32 crystal classes, 20 are non-centrosymmetric — they lack inversion symmetry. Of these, all 20 are piezoelectric. The key requirement is that the crystal structure must not look the same when viewed through a point of inversion. In a centrosymmetric crystal, any stress-induced dipole in one region is exactly canceled by an equal and opposite dipole elsewhere.
α-Quartz (SiO₂) has a relatively small piezoelectric coefficient (d₁₁ ≈ 2.3 pC/N) but excellent temperature stability and mechanical quality factor, making it ideal for frequency references and oscillators. PZT (lead zirconate titanate) is a ceramic with a piezoelectric coefficient 100× larger (d₃₃ ≈ 374 pC/N), used in actuators, sonar, and ultrasound transducers. Barium titanate (BaTiO₃) was the first material in which piezoelectricity was discovered in a ceramic (1940s), with d₃₃ ≈ 190 pC/N.
Piezoelectricity is everywhere
Sensors: The direct effect converts mechanical signals to electrical ones. Piezoelectric accelerometers measure vibration in aircraft engines and earthquake detectors. Pressure sensors in automotive knock detectors use the same principle. Your gas grill igniter works by striking a piezoelectric crystal sharply enough to generate a spark across a gap.
Actuators: The inverse effect provides nanometer-precision positioning. Scanning tunneling microscopes use PZT tube actuators to position their tips with sub-angstrom resolution. Inkjet printers fire droplets by pulsing voltage across a piezoelectric element. Adaptive optics telescopes use piezoelectric deformable mirrors to correct for atmospheric turbulence in real time.
Resonators and transducers: Quartz crystal oscillators provide the timing reference in virtually every digital clock, watch, and computer. Sonar transducers use PZT discs to convert electrical pulses into acoustic waves and back. Medical ultrasound imaging depends entirely on arrays of piezoelectric elements that both transmit and receive ultrasound pulses.