Piezoelectric Ceramic Transducer

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Piezoelectric ceramic transducer

General information

Piezoelectric transducers or sensors, as well as actuators, are functional ceramic or polymer materials that are capable of converting mechanical energy, e.g. in the form of vibrations or forces, into electrical energy and vice versa. They are used in testing technology and monitoring of industrial plants as sensors for measuring forces, torques, strains, pressures or accelerations, as well as in ultrasound or sound emission testing with active or passive ultrasonic sensors.

The piezoelectric transducers used in piezo sensors usually have high mechanical stiffness and a wide operating temperature range. Due to their wide frequency range from a few Hz to several 100 kHz, they can be used for both dynamic and quasi-static measurement and testing tasks [1]. In addition to high load cycle counts (see: fatigue), high stiffness and strength, and temperature resistance, the technical application requirements include linearity of electrical properties, extensive hysteresis-free behaviour or proportionality between mechanical and electrical characteristic values, and good insulation resistance.

Application of the direct piezoelectric effect

In technical applications of piezoelectric transducers for sensors, both the direct and inverse piezoelectric effects are utilised (see also: piezo ceramics). In the direct piezoelectric effect, the mechanical deformation of the piezoelectric transducer as a result of an external force causes a shift in the electrical polarisation at the metallised surface, creating a surface tension or charge that can be used technically as measurable electrical signals for sensors. This effect is used in particular, for example, in passive sensors such as acceleration or sound emission transducers for registering external forces or accelerations as well as for detecting stress-induced structure-borne noise (sound emission). The compressive stress on the oscillator generates a positive voltage and the tensile stress generates a negative voltage.

In the inverse or reverse piezoelectric effect, an electric field is applied to the transducer, e.g. via capacitor plates or metallised surfaces, which leads to deformation inside the material and can be used technically for switching operations in actuators or for the generation of ultrasound [2]. This effect is used in active applications such as the generation of high sound power for cleaning systems or for processing materials (drilling, cutting or welding). If a positive voltage is applied to the piezoelectric transducer, it expands; if a negative voltage is applied, it contracts.

When these transducers are used in ultrasonic sensors, the direct piezoelectric effect is used to receive and the inverse effect to transmit ultrasound into a medium. In this case, i.e. the generation of ultrasound, the mechanical deformation of the applied alternating voltage follows almost without delay due to the low seismic mass of the transducer. When receiving ultrasound, the opposite is true, which is why these sensors (standard, angle, S/E or composite sensors) (Figs. 1 and 2) are suitable for transmitting and receiving ultrasound.

Fig. 1:

Schematic presentation (a) of the standard sensor, (b) of the angle beam sensor, (c) of the composite sensor and (d) of the transmitter (S)-receiver (E) sensor

Fig. 2:

Designs (a) of the standard sensor, (b) of the angle beam sensor, and (c) of the transmitter (S)-receiver (E) sensor from the company Fa. SONOTEC Ultraschallsensorik Halle GmbH, Halle (Saale)

Piezo sensors, e.g. in ultrasound testing, require electronics downstream of the transducer for signal processing and post-processing, which serve to amplify the signal and match the impedance. If this electronics is integrated into the sensor housing, then a voltage output is available at the sensor; otherwise, a charge output is available, which requires a corresponding charge amplifier [1].

Transducer materials and test sensors

Different materials are used for the manufacture of ultrasonic sensors in non-destructive testing, depending on the area of application. In addition to classic single-crystal vibrating materials such as quartz, triglycine sulphate (abbreviation: TGS) and lithium tantalate LiTaO3, mainly polycrystalline ceramic piezoelectric materials, plastic films such as polyvinylidene fluoride (abbreviation: PVDF) and composite materials (ultrasonic composite sensors) consisting of epoxy resin and piezoelectric ceramic material [3, 4].

Among the polycrystalline ceramic piezo materials, barium titanate BaTiO₃, lead zirconate titanate (abbreviation: PZT), lead titanate (abbreviation: PT) and lead metaniobate (abbreviation: PbNb₂O₆) are the materials most commonly used for the construction of transducers. Such PZT ceramics (see: piezoelectric ceramic) have a transition temperature, known as the Curie temperature T_c or Curie point, at which an energy-induced shift of individual ions in the crystal lattice occurs. Above the Curie point, no piezoelectric effect occurs. Below the Curie point, spontaneous polarisation occurs and electric dipoles form. Below the Curie point (T < T_c), lead zirconate titanate has a tetragonal or rhombohedral lattice, whereas at T > T_c, a cubic crystal lattice exists [8].

Lead zirconate titanate has a very high coupling factor and resonance frequency (Table 1), which means that only a small part of the sound wave is lost at the interface due to reflection R and high sound energies can be transmitted into the test piece. For this reason, these transducer materials are used for narrow-band frequency spectra and high sensitivities [4, 5].

Lead titanate exhibits only low cross-coupling, which is why most of the sound energy is generated with the thickness vibration. PT transducers are therefore particularly suitable for the construction of very small sensors (finger tips). Lead metaniobate has a very low acoustic wave impedance Z and, due to its properties (see: Table 1), can be easily dampened. These transducers are used, for example, in shock wave sensors, which operate with very short ultrasonic pulses (Dirac shock) and are ideal for wall thickness measurement.

In principle, piezoelectric ceramics have high operating temperatures (> 500 °C) and high acoustic impedance, which can result in high losses of sound energy at interface surfaces due to reflection. Although this problem cannot be avoided by using special coupling layers on the sensor, significantly higher transmission factors T can be achieved [3–7]. An overview of the acoustic properties of piezo ceramics is given in Table 1.

Table 1:

Properties of the most commonly used vibrating materials according to [4]

Eigenschaft	PZT	PT	PbNb2O6	PVDF	Composite
Akustische Impedanz Z (106 kg m-2s-1)	33,7	33,0	20,5	3,9	9,0
Resonanzfrequenz f (MHz)	< 25	< 20	< 30	10 – 160	< 10
Kopplungsfaktor Dicke kt (%)	0,45	0,51	0,3	0,2	0,6
Kopplungsfaktor Quer kp (%)	0,58	< 0,01	< 0,1	0,12	≈ 0,1
Relative Dielektrizitätszahl ε	1700	215	300	10	450
Temperaturlimit Tmax (° C)	365	350	570	80	100

Plastic films made of polyvinylidene fluoride (abbreviation: PVDF) or related copolymers have very low sound wave resistance Z or acoustic impedance and therefore low reflection losses (see: reflection sound waves). This high transmission T or D of the ultrasonic waves (see: transmission sound waves) allows the use of such sensors for immersion bath technique as well as non-destructive poplymer testing, although only low sound power levels can be transmitted into the test medium compared to piezoceramic sensors. A disadvantage is the low coupling factor for thickness vibrations, which is why PVDF is generally not sensitive enough for direct coupling to the test piece [3–6].

Piezo composites or composite transducers are composites of parallel piezoelectric ceramic rods and epoxy resin or filled epoxy resin (abbreviation: EP). The composite can be constructed in different geometric arrangements and with varying piezo ceramics. The combination with plastic results in lower sound wave resistance Z compared to pure ceramic piezo materials, but the test temperature is reduced to a maximum of 100 °C due to shrinkage and ageing processes (see: Table 1). Due to their low density ρ and low acoustic impedance, transducers with piezo composites are particularly suitable for testing using the immersion bath technique, for use in transducers for phased array testing, and for the manufacture of S/E and angle beam sensors [3–6]. Thanks to an adapted plastic protective layer, the use of polymethyl methacrylate or Plexiglas (abbreviation: PMMA) as a lead-in section and the special properties of the EP resin composite, these sensors can be used specifically in non-destructive plastic testing using ultrasound, as the test frequencies for these materials are comparatively low (0.5–5 MHz) and the attenuation and number of scatter centres are very high [9].

While the plastic films used as the basis for PVDF transducers and the composite sensors can be adapted for different applications by adjusting the geometry, packing density and angle of inclination of the ceramic rods, single crystals require special cuts oriented to the crystal axes of the single crystal (e.g. quartz) in order to achieve the same adaptation. In polycrystalline PZT ceramics, as in single crystals, the piezoelectric longitudinal, transverse, shear or bending effect occurs depending on the polarisation direction (Fig. 3) [8], which also characterises the main directions of vibration.

Fig. 3:

Vibration modes of piezoceramic transducers (a) bending transducer, (b) longitudinal transducer, (c) surface shear transducer, and (d) thickness shear transducer

The polarisation direction in piezo ceramics is determined by the manufacturing process (casting of the metal oxide mixture, firing, post-treatment and polarisation in an electric field). The charge generated on the surface of the PZT ceramic does not depend on the speed (strain rate basics) or acceleration of the deformation, but only on its absolute value. The resonance frequency of the transducers depends essentially only on the geometric dimensions, especially the thickness d.

See also

• Piezoelectric ceramic
• Piezoelectric force transducer
• Ultrasonic composite sensors
• Ultrasonic immersion bath sensors
• Servo-hydraulic testing machine

References

[1]	Laible, M., Bill, B., Gehrke, K.: Mechanische Größen, elektrisch gemessen – Grundlagen und Beispiele zur technischen Ausführung. Expert Publishing, Renningen, 8th Edition (2013); (ISBN 978-3-8169-3215-4)
[2]	Hering, E., Modler, K.-H. (Eds.): Grundwissen des Ingenieurs. Fachbuchverlag Leipzig. 13th Edition (2002), Carl Hanser, Munich Vienna (ISBN 3-446-21443-7, see AMK-Library under L 37)
[3]	Splitt, G.: Prüfköpfe mit Composite-Schwingern – ein Meilenstein für die Ultraschallprüfung. NDTnet – July (1996) Vol.1 No. 07: http://www.ndt.net/article/splitt/splitt.htm, (access on December 2, 2025)
[4]	Deutsch, V., Platte, M., Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer, Berlin (1997), (ISBN 3-540-62072-9; see AMK-Library under M 45)
[5]	Witzgall, M.: Ultraschallprüfköpfe. Wiki.ZfP.tum.de https://wiki.tum.de/pages/viewpage.action?pageId=244647027 (access on December 2, 2025)
[6]	Schuster, V., Lach, M., Platte, M.: Die Qual der Wahl: Welcher Prüfkopf für welchen Einsatz?. DACH-Jahrestagung, Salzburg 2004
[7]	Schmid, A. J.: Piezokeramik – Funktion, Bauarten und Anwendungen. Argillon GmbH Piezoproducts Redwitz a. d. Rodach, www.keramverband.de/keramik/pdf/05/sem05_04.pdf (access on December 2, 2025)
[8]	Schrüfer, E.: Elektrische Messtechnik. Carl Hanser, Munich, 9th Edition (2007); (ISBN 978-3-446-40904-0)
[9]	Busse, G.: Zerstörungsfreie Kunststoffprüfung. In: Grellmann, W., Seidler, S. (Eds.): Kunststoffprüfung. Carl Hanser, Munich (2025) 4th Edition, pp. 461–528 (ISBN 978-3-446-44718-9; E-Book: ISBN 978-3-446-48105-3; see AMK-Library under A 23)