3.4 Piezoelectric materials synthesis  (Page 2/6)

Selective parameters for piezoceramic materials are given in [link] , where Q _m is the mechanical quality factor, T _c is the Curie point, d ₃₁ is the the transverse charge coefficient, and k _p , k _t and k ₃₁ are the electromechanical coupling factors for planar, thickness, and transversal mode respectively.

Recently, sol-gel processing has been used to prepare ceramics, making possible the preparation of materials that are difficult to obtain by conventional methods. Both, inorganic and organic precursor have been reported. Additionally, new techniques for the production of ceramic fibers have been developed. Better processing and geometrical and microestructural control are the main goals in the production of fibers.

The latest development in piezoceramic fibers is the modification of the viscous-suspension-spinning process (VSSP) for the production of continuos piezoelectric ceramic fibers for smart materials and active control devices, such as transducers, sensor/actuators and structural-control devices. The VSSP utilizes conventional synthesized ceramic powders and cellulose, as the fugitive carrier, to produce green ceramic fiber at a reasonable cost. [link] shows the schematic representation of the VSSP.

Synthesis of reactive PZT precursor powder by the oxalate coprecipitation technique has also been developed. The precursor transforms to phase pure PZT at or above 850 °C the PZT obtained by this technique showed a Curie temperature of 355 °C. The advantages of the coprecipitation technique are the lack of moisture sensitive and special handling precursors.

Although new materials have been investigated with the purpose of create replacements for ceramics, there has been a great improvement in their properties and, current research is focused in the development of new techniques for both synthesis and processing.

Piezoelectric single crystals.

The recent progress of the electronic technology requires new piezoelectric crystals with a high thermal stability and large electromechanical coupling factors. Single-crystal materials have been considered as replacements for polycrystalline ceramics. Ideally single-crystals of lead zirconate titanate (PZT) itself would be the main choice as it is the most prevailing piezoelectric material, but it is difficult to grow large single crystals. On the other hand, the fact that single-crystals offer many advantages over polycrystalline systems has been recognized. Materials such as lithium niobate present essentially no aging, no mechanical creep and excellent performance in high temperature conditions.

New piezoelectric single crystals grown by conventional RF-heating Czochralski (CZ) technique have been synthesized. High purity starting materials, mainly oxides powders, and Ar atmosphere are required. La ₃ Ga ₅ SiO ₁₄ , La ₃ Nb _0.5 Ga _5.5 O ₁₄ and La ₃ Ta _0.5 Ga _5.5 O ₁₄ single crystals have been grown by using this method. However, the CZ technique can be applied only to materials that can be synthesized by ordinary solid-state reaction and can undergo the pulling method.