Broadband Piezoelectric Transducers
What is a Broadband Piezoelectric Transducers?
Piezoelectric transducers are key elements of many broadband ultrasonic systems, either pulse-echo or through-transmission, used for imaging and detection purposes. In ultrasonic broadband applications such as medical imaging, or non-destructive testing, piezoelectric transducers should generate/receive ultrasonic signals with good efficiency over a large frequency range. This implies the use of piezoelectric transducers with high sensitivity, broad bandwidth and short-duration impulse responses. High sensitivity provides large signal amplitudes which determine a good dynamic range for the system and the short duration of the received ultrasonic signal provides a good axial resolution.
The most important and common type of piezoelectric transducer elements used in ultrasonic broadband applications is a thin piezoelectric plate, with lateral dimensions much greater than the thickness, driven in a simple thickness extensional mode of vibration. They usually operate in the frequency range 0.5-15 MHz. Different types of piezoelectric materials are used for the active transducer element. Ferroelectric ceramics, like lead zirconate titanate (PZT), lead metaniobate, etc., have a high piezoelectric coupling coefficient. Piezoelectric polymers like polyvinylidene difluoride (PVDF) and copolymers have useful low-acoustic impedances. Piezoelectric composites are mixtures of piezoceramics with nonpiezoelectric polymers.
When designing a broadband piezoelectric transducer or when finding optimal transducer system configurations, it is useful to be able to predict the global response by means of theoretical calculations, bearing in mind that there is a large number of materials and configuration parameters involved in the global system. The aim of this chapter is to summarize the basic modeling approaches describing the electrical and ultrasonic characteristics of broadband multilayer transducers. In the active piezoelectric plates, the length and width to thickness ratios are sufficiently large so that one-dimensional models are good approximations to predict the properties of the transducer.
Modeling the transducer as a two-port network permits the use of the transfer matrix formalism of the circuit theory. In this chapter, a general methodology for the treatment of all the components of a transducer system, including acoustic matching layers and electric matching components, as a set of cascade networks, is also described. A computer program for design and optimization of transducer systems can be easily developed.
There are a wide variety of applications where broadband piezoelectric systems are used, mainly in order to obtain ultrasonic information for detection or visualization of the internal parts in diverse structures. These applications require external inspections with ultrasonic waves and the use of an echo-graphic procedure. The main application areas are in industry and medicine.
Most broadband piezoelectric applications require the design of very specific interface electronic systems, since the conventional continuous wave (CW) electronic schemes and the conventional analysis methods are not applicable.
In order to obtain a good discrimination of internal structures, it is convenient to improve the signal to noise ratio. Therefore, it is necessary to guarantee a high efficiency in the ultrasonic process. In addition, short ultrasonic pulses must be used in order to obtain good axial resolution.
The above-mentioned practical questions impose some technological requirements over the electronic systems used for broadband ultrasonic applications :
- The use of a pulsed regime for the ultrasonic inspection process, which involves a transient electrical excitation of the piezoelectric transmitter.
- Sensitivity considerations determine that the transducer should be excited with very short electrical pulses (spikes) of several hundreds of volts at peak amplitude.
- A high efficiency in emitter and receiver piezoelectric sub-processes, under broadband conditions, is required in order to optimise the coupling between spike generators and piezoelectric emitters, as well as between piezoelectric and electronic receivers.
It is not easy to attain simultaneously all these requirements because parameters influencing the response amplitudes act in a crossed way. Specifically, the optimum setting of one of the parameters depends on the settings of the others.
As a typical example, to obtain a good dynamic range on the received echo-signals, by using broadband piezoelectric arrays in medical imaging, it is necessary to drive the array elements with electrical spikes of 300-400 volts and rise-times of less than 30 nanoseconds. In general, the time duration and frequency spectrum of the inspection signals are very relevant parameters for the quality of the result obtained in visualization processes.
In this chapter, the characteristics required for the high voltage stages needed for driving broadband piezoelectric transducers are explained. The dependence of the driving pulses with matching networks and the external loads is also analysed. A simple circuital block diagram for interfacing transducers with the reception stages, in ultrasonic applications, is also presented.
Finally, a detailed analysis of electrical responses in high-voltage pulsed driving of piezoelectric transducers is performed in the time domain, taken into account some typical working conditions. For this purpose, several expressions related to the driving waveforms are introduced and discussed from a series of analytical linear approaches associated with practical situations in medical and industrial ultrasonic applications.
The Electromechanical Impedance Matrix Of Broadband Piezoelectric Transducers
Figure below shows a simple diagram of a broadband piezoelectric transducer. A piezoelectric layer of thickness t, with very thin electrodes of area A at its surfaces, is embedded between an attenuating backing material and the irradiated medium (load). Usually, a high attenuating material (“backing”) is bonded to the back face of the transducer element in order to enlarge the emission/reception (two-way) frequency band and therefore to shorten the impulse response (at the expense of a loss on sensitivity and signal amplitude). One or more acoustic matching layers are bonded in the front face in order to optimize the transmission of energy to the explored medium.
Constructive scheme of a thin piezoelectric plate transducer: 1 matching layer; 2 piezoelectric active element; 3 casing; 4 backing; and 5 coaxial cable
Piezoelectric transducers convert electrical energy into mechanical energy and vice versa. The direct piezoelectric effect consist of the change in polarization in a material induced by an applied mechanical stress, and is used in the ultrasonic reception stage. The converse piezoelectric effect consists of the dimensional change (mechanical strain) in a material induced by an applied electric field, and is used in the ultrasonic emission stage.
The linear piezoelectric constitutive equations define the interrelationships among the electric isplacement D, the electric field E, the mechanical stress T, and the elastic strain S. The complexity of the equations involved depends on both the symmetry of the piezoelectric material and the particular geometry (mechanical and electrical boundary conditions) of the transducer element.
Most of piezoelectric materials used in the fabrication of broadband ultrasonic transducers, (i.e. ferroelectric ceramics, piezoelectric polymers, and piezoelectric composites) become broadband piezoelectric transducers by a process of electrical poling along the thickness direction (Z ≡ 3 axis).