Piezo buzzer elements and ultrasonic transducers sit at the output end of the piezoelectric supply chain. The material has already been selected, and what decides whether the device works is the element geometry, the electrode and, above all, the poling. Poling is the step that makes a sintered ceramic piezoelectric at all, and it is the process most often left out of a purchase specification, which is why two suppliers can quote the same grade and deliver very different parts. This guide covers the element types, the parameters to specify and how to verify poling on arrival.
The element types
- Buzzer elements — a thin piezoelectric disc bonded to a metal plate, the unimorph, or two ceramic layers bonded together, the bimorph. They vibrate in bending and are used in alarms, sounders and appliance and automotive modules.
- Transducer discs and rings — thickness poled discs for cleaning, welding, atomisation and ranging, or radially poled rings for cylindrical transducers and hydrophones.
- Stack actuators — many thin ceramic layers co-fired with internal electrodes, giving large displacement at low voltage, used in precision positioning, dosing and micro pumps.
- Bimorph and unimorph benders — for low voltage actuation, micro positioning and haptic devices.
What poling actually does
After sintering, the domains in the ceramic are randomly oriented and the body has no net piezoelectric effect. Poling applies a strong direct electric field, usually between two and four kilovolts per millimetre for PZT, at elevated temperature and for a defined time, to align the domains; the alignment is then locked in when the field is removed. Three consequences matter to a buyer:
- Poling direction is a specification. A thickness poled disc and a radially poled ring of the same material behave differently, and an element poled in the wrong direction cannot be corrected afterwards.
- Poling quality sets the coefficients. Under-poled material reports low and inconsistent d33 and coupling, while over-poling risks dielectric breakdown and scrap. The supplier poling condition is part of the product.
- Poling is undone by heat. Exceeding the depolarisation temperature, well below the Curie temperature, permanently reduces the piezoelectric effect. Any assembly step involving soldering, curing or overmoulding must be checked against this limit.
The parameters to specify
1. Resonance frequency and impedance
For buzzers and transducers the resonant frequency and the impedance at that frequency are the working specifications. Specify the measurement method and the drive condition, since both shift the result.
2. Capacitance and permittivity
Capacitance at one kilohertz, converted to relative permittivity against the element dimensions, is the cheapest and most sensitive incoming screen for material and poling consistency.
3. Coupling coefficient and quality factor
Specify the coupling factor for the mode you will use, kp for a radial disc, kt for a thickness mode element, and the mechanical quality factor for any continuous high drive application.
4. Sound pressure level and displacement
For buzzers, specify the sound pressure level at a defined distance and drive voltage; for actuators, specify free displacement and blocking force, both with the drive condition.
5. Electrode material and attachment
Silver fired electrodes suit wire or epoxy attachment; nickel or nickel gold suits soldering and multilayer co-firing. Electrode adhesion and solderability are specifications, and the electrode choice interacts with the assembly process and with the poling in adjacent zones.
6. Geometry and tolerances
Diameter, thickness, concentricity and flatness all shift the resonance, so they are functional tolerances and not cosmetic ones. For bimorphs and buzzers, specify the metal plate material, thickness and bond layer.
7. Ageing and stability
Piezoelectric coefficients drift downward with time after poling, most rapidly in the first hours and days. For precision actuators and for sensors that must hold calibration, specify the ageing allowance and ask for data over the first thousand hours.
Incoming inspection and assembly verification
- Measure capacitance on a sample per lot and convert to permittivity, as the primary screen for material and poling consistency.
- Run an impedance sweep and extract resonance and anti-resonance frequencies to calculate coupling and quality factor in the intended mode.
- Measure d33 with a Berlincourt meter and record it per lot, since a drop against previous lots is the clearest sign of a poling change.
- Confirm poling state by comparing capacitance before and after a controlled heating cycle, or by checking the manufacturer test report against the part.
- Control assembly heat during soldering and curing, and measure capacitance after assembly on a sample to confirm that no depoling has occurred.
- Inspect electrode coverage, chipping and edge damage, since these concentrate stress and cause cracking in stacks and benders.
Common sourcing mistakes
- Buying on d33 alone, without coupling, loss and poling consistency, then fighting lot to lot variation in the finished device.
- Specifying a buzzer by frequency only, without sound pressure level and impedance, then discovering the alarm is too quiet.
- Ignoring the depolarisation limit and discovering during reflow or overmoulding that the parts have lost output.
- Accepting an element poled in a direction that suits the supplier manufacturing rather than the design vibration mode.
- Treating lead-free KNN as a drop-in for PZT: KNN is harder to pole, needs higher fields and shows different temperature behaviour.
- Comparing suppliers on coefficients measured in different vibration modes or at different drive conditions.
Sourcing piezo elements from Hunan and China
China supplies a large share of the world piezoelectric element volume, from low cost buzzer discs to high power transducer elements and multilayer stacks, supported by a mature domestic powder and sintering base. Hunan hosts advanced ceramics manufacturers serving instrument, ultrasonic and export customers, within the province new materials industrial base. LiiFoo maintains verified supplier profiles in the advanced ceramics cluster with documented material capability and certification.
For poling characterised samples with measured coefficients, a second source qualification or a datasheet cross reference, see the cluster above or contact our sourcing team.
Related reading: soft versus hard PZT grade selection covers the material choice upstream of the element.
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