1-3 Piezo-composite materials

The structure of these materials, illustrated on the table 1, is made of thin ceramic rods (which present a mechanical continuity following one dimension of the space) embedded into a polymer matrix (which present a mechanical continuity following three dimensions of the space). Used as plates and equiped of electrodes on their main sides, these materials offer many advantages compared with piezo-electrical transducers generally used in the field of Non Destructive Testing.

Table 1 : Comparative chart of the electroacoustical properties for different piezo-electrical materials

The physical properties of the piezo-composite materials, particularly their dielectric, elastic and piezo-electric properties, depend on the properties of the constituent materials, on the relative proportion of these materials and also on geometrical parameters of the microstructure. In practice, the microstructure of the material is defined so that all the waves subject to excitation could have a wavelength higher than the microstructure itself. If this condition is respected, the piezo-composite material behaves like an homogeneous material which could be described with simple parameters comparable to classical material parameters :

Fig. 1 Schematic representation of a piezo-composite plate with a 1-3 structure - After W.A. SMITH

The variation laws of the main parameters of the piezo-composite materials can be modelized. The figure 2 presents these variation laws for the following parameters : dielectric permitivity , acoustical impedance Z, longitudinal wave velocity VL, electromagnetic coupling coefficient kt, according to the volume of ceramic material within the composite. It is particularly interesting to consider the variation of kt which constitutes the most important parameter in so far as it shows the capacity of the piezo-composite material to turn reciprocally electrical energy into acoustical energy. For a 1-3 piezo-composite, kt is higher than the one of the constituent ceramic material in a large range of volume. This surprising property comes from the effect of release of the lateral constraints within the ceramic bars which provides a best ability to transform energy than the ceramic in solid plate.

fig. 2 Variation of dielectric constant acoustical impedance , velocity VL, coupling coefficient kt as a function of volumic fraction of PZT ceramic - After W.A. SMITH

Characteristics of 1-3 Piezo-composite materials

The properties of these materials imply a certain number of specific characteristics as :

• acoustical impedance : located between the one of the polymer material and the one of the ceramic material. In practice, it is often adjusted between 8 and 12 MRayleigh, which allows a good tranfer of energy in a large frequency band when the impedance of the coupling medium is low.

• coupling coefficient : its high value also contributes to the increase of energy transfer and to the widening of the bandwidth. These materials, thanks to the double effect of a low acoustical impedance and a high coupling coefficient, make possible a quite important improvement of the ratio sensitivity/bandwidth compared to the performances of the traditionnal transducers.

• electrical impedance : the judicious choice of the constituents and their respective proportions allow to adjust the electrical impedance and to favour, in that way, the matching of the transducer to its electrical environment.

• shaping : it is linked to the thermal and mechanical properties of the polymer phase.

• lateral modes : they are strongly attenuated with this type of material and parasitic effects are consequently reduced.

• The arisotropy of these materials on mechanic, dielectric and piezoelectric properties facilitates the realization of array structures with low cross coupling between elements. A single deposite of electrodes having the required geometries is usually sufficient.

We have just listed the main properties of the transducers using piezo-composite technology. Specific properties have also been developped in the field of withstanding in power, temperature or for the realization of very small or very large transducers.