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The current method for piezocomposite production is the dice-and-fill technique, which consists in cutting two sets of grooves in a block of piezoceramic at right angles each other, then a polymer is cast into these grooves and the solid ceramic base is ground off. Polishing and poling are the following steps in order to achieve the final thickness and properties. This method is expensive, time consuming and size limited.
As an alternative for the dice-and-fill technique, continuos green fibers obtained by the modified viscous-suspension-spinning process, can be bundled into a cottonball-like shape, then burned and sintered. The sintered bundle impregnated with epoxy resin can be sliced into discs and then polarized. Recent results have yielded 1-3 type composites with excellent piezoelectric properties.
On the other hand, an innovative process has been developed for Sr 2 (Nb 0.5 Ta 0.5 ) 2 O 7 /PVDF composites, in this new fabrication method, appropriate amounts of oxides are mixed, pressed and sintered. The porous resulting material is subsequently infiltrated with PVDF solution and then poled. This new method for composites preparation is simple and offers a lead-free alternative smart material.
Another kind of piezocomposites can be achieved by spinning films of piezoceramic onto metal alloys, such as TiNi. The resulting materials is a hybrid composite that can utilize the different active and adaptive properties of the individual bulk materials. Due to the shape memory nature of TiNi, a possible application for this new heterostructures could be smart active damping of mechanical vibrations. DC sputtering and spin coating are the techniques necessary for the smart thin film TiNi/piezoelectric heterostructures fabrication. However, eventhough the films had a fine grain structure and high mechanical qualities, the ferroelectric properties were poor compared to literature values.
In the future, the properties of piezocomposites will be tailored, by varying the ceramic, the polymer and their relative proportions. Adjustments in the material properties will lead to fulfillment of the requirements for a particular device. [link] shows a comparison among piezoelectric ceramics, polymers and composites parameters where Z is the impedance, ε t 33 is the dielectrical constant, and ρ is the density.
| Material parameter | Piezoceramics | Piezopolymers | Piezocomposites |
| k t (%) | 45 - 55 | 20 - 30 | 60 - 75 |
| Z (10 6 Rayls) | 20 - 30 | 1.5 - 4 | 4 - 20 |
| ε t 33 /ε 0 | 200 - 5000 | ~10 | 50 - 2500 |
| tan γ (%) | <1 | 1.5 - 5 | <1 |
| Q m | 10 - 1000 | 5 - 10 | 2 - 50 |
| ρ (10 3 kg/m 3 ) | 5.5 - 8 | 1 - 2 | 2 - 5 |
Many potential applications exist which require film thickness of 1 to 30 μm. Some examples of these macroscopic devices include ultrasonic high frequency transducers, fiber optic modulators and for self controlled vibrational damping systems.
ZnO and PZT have been used for piezoelectric fiber optic phase modulators fabrication. The piezoelectric materials have been sputter deposited using dc magnetron source and multimagnetron sputtering systems. Coatings of 6 µm thick of ZnO and 0.5 µm of PZT are possible to achieve using these systems. However, thickness variation of approximately 15% occurs between the center and the end of ZnO coatings, results on affected modulation performance. Although PZT coatings achieved by sputtering posses uniformity and do not exhibit cracking, the PZT is only partially crystallized and it is actually a composite structure consisting of crystalline and amorphous material, diminishing the piezoelectric properties.
Sol-gel technique for thick PZT films have been developed. It is now possible to fabricate PZT sol-gel films of up to 60 µm. The electrical and piezoelectrical properties of the thick films reported are comparable with ceramic PZT.
Piezoelectric polymer coatings for high-frequency fiber-optic modulators have been also investigated. Commercial vinylidene fluoride and tetrafluoroethylene copolymer has been used. The advantage of using polymer coatings is that the polymer jacket (coating) can be easily obtained by melt extrusion on a single-mode fiber. Thus, uniformity is easily achieved and surface roughness is not present. Furthermore, if annealing of the polymer is made prior poling, a high degree of crystallinity is enhanced, leading to better piezoelectric properties.
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