PZT
/ pee-zee-TEE /
If barium titanate is the champion dielectric, PZT is the champion piezoelectric — the ceramic inside most ultrasound probes, sonar arrays, precision actuators, buzzers, and ultrasonic cleaners. PZT stands for lead zirconate titanate, a perovskite solid solution written Pb(Zr,Ti)O3: the A-site holds lead, and the B-site is shared between zirconium and titanium in a ratio the maker chooses.
The magic is in that Zr-to-Ti ratio. Written Pb(Zr_x Ti_(1-x))O3, the material changes crystal symmetry as x varies, and near x about 0.52 it sits right on the morphotropic phase boundary — the composition where the rhombohedral (Zr-rich) and tetragonal (Ti-rich) phases meet. At that boundary an unusually large number of allowed polarization directions is available at once, so the material is exceptionally easy to pole and its piezoelectric and dielectric coefficients peak sharply. This is why commercial PZT is formulated close to that 52/48 line, reaching piezoelectric charge coefficients d33 of roughly 200 to over 600 picocoulombs per newton. Doping tunes it into two families: 'soft' PZT (donor-doped, for example with niobium) has high sensitivity and large strain but higher loss and is easy to depole — good for sensors and actuators; 'hard' PZT (acceptor-doped, for example with iron) has lower coefficients but low loss, high mechanical quality factor, and resists depoling — good for high-power transmitters and ultrasonic welding.
PZT has dominated piezoelectrics for more than half a century because nothing else combines such large coefficients, a high enough Curie point (around 350 degrees C), and cheap ceramic processing. But it carries an honest liability: it is roughly 60 percent lead by weight. That has made it a target of restrictions such as RoHS, under which piezoelectric PZT currently survives only on a specific exemption, because the leading lead-free candidates — potassium sodium niobate (KNN) and bismuth sodium titanate (BNT) systems — still underperform it. A large global research effort is aimed at closing that gap, but as of today PZT remains the material to beat.
A diesel common-rail fuel injector uses a stack of hundreds of thin PZT layers: a voltage pulse makes the stack lengthen a few microns, and that motion, amplified hydraulically, snaps the injector open and shut thousands of times a minute with the speed and precision no solenoid can match.
PZT's edge is the morphotropic phase boundary near 52/48 Zr/Ti, where many polarization directions meet and the piezoelectric response peaks.
PZT is about 60 percent lead. It persists in devices only under specific regulatory exemptions; the much-sought lead-free piezoceramics exist but still cannot match its combination of coefficient, Curie point, and cost.