Piezoelectric ceramics polycrystalline

Dunn, M L. 1995. Effects of grain shape anisotropy, porosity, and microcracks on the elastic and dielectric constants of polycrystalline piezoelectric ceramics. Journal of Applied Physics 78 [3] 1533-1541. 

Piezoelectric ceramic materials include titanates of barium and lead (BaTiOj and PbTiOj), lead zirconate (PbZr03), lead zirconate-titanate (PZT) [Pb(Zr,1i)03], and potassium niobate (KNb03). This property is characteristic of materials having comph-cated crystal structures with a low degree of symmetry. The piezoelectric behavior of a polycrystalline specimen may be improved by heating above its Curie temperature and then cooling to room temperature in a strong electric field. 

Dielectric, piezoelectric and pyroelectric properties of LiTa03 derived ceramics containing additives of LiF and MgF2 were investigated and reported on in [407]. The materials were prepared at 900°C by means of two methods Reaction sintering, yielding powdered polycrystalline material ... 

The piezoelectric coefficients are third rank tensors, hence the piezoelectric response is anisotropic. A two subscript matrix notation is also widely used. The number of non-zero coefficients is governed by crystal symmetry, as described by Nye [2], In most single crystals, the piezoelectric coefficients are defined in terms of the crystallographic axes in polycrystalline ceramics, by convention the poling axis is referred to as the 3 axis. 

Polycrystalline materials in which the crystal axes of the grains are randomly oriented all behave electrostrictively whatever the structural class of the crystallites comprising them. If the crystals belong to a piezoelectric class and their crystal axes can be suitably aligned, then a piezoelectric polycrystalline ceramic becomes possible. 

Not all the tensor components are independent. Between Eqs (6.29a) and (6.29b) there are 45 independent tensor components, 21 for the elastic compliance sE, six for the permittivity sx and 18 for the piezoelectric coefficient d. Fortunately crystal symmetry and the choice of reference axes reduces the number even further. Here the discussion is restricted to poled polycrystalline ceramics, which have oo-fold symmetry in a plane normal to the poling direction. The symmetry of a poled ceramic is therefore described as oomm, which is equivalent to 6mm in the hexagonal symmetry system. 

Sintered ceramics made of lead-zirconium titanate (PZT Pb(Tii jZr,)03 x S 0.5) are usually used for phoioacoustic experiments [105, 106]. The unit cell of the lead-zirconium titanate has a perovskite structure. Below the Curie temperature (328 °C for the PZT-4 (Vemitron) used by us [24]), the cells are tetragonally deformed, i.e., positive and negative charges are shifted and electric dipole moments are produced. In analogy to ferromagnetism, domains with randomly distributed polarization direction are formed. By the application of an electric field, these can be orientated in a preferred direction, and the sintered polycrystalline ceramic is then remanently polarized. The properties of these anisotropic piezoelectric materials are described by various parameters which depend on the polarization and deformation direction. In the common terminology, the < ordinate system shown in Fig. 3 is obtained for the cylindrical piezoelectric crystals [24]. 

An important point now emerges the requirement that the piezoelectric, pyroelectric and ferroelectric effects are restricted to non-centrosymmetric crystals implies that these physical phenomena should not be observed in a polycrystalline solid. This is because the individual grains of a polycrystalline body will polarise in random directions that will cancel overall. This was changed by the discovery, in 1945, of a way to endow polycrystalline ceramic articles with ferroelectric properties. 

Until the late sixties the only known ferroelectrics, piezoelectrics, and pyroelectrics were certain inorganic monocrystals, or polycrystalline ceramics like lead titanate zirconate perovskites. Other known materials with macroscopic polarization were electrets, (for example mixmres of beeswax and rosin) in which the polarization was produced by application of the electric field in the melted state and then by cooling and the solidification of the polarized material. 

Pertsev, N.A., Zembilgotov, A.G., and Waser, R. 1998. Aggregate linear properties of ferroelectric ceramics and polycrystalline thin films calculation by the method of effective piezoelectric medium. Journal of Applied Physics 84 [3] 1524-1529. 

In this method, acoustic waves are generated by a piezoelectric transducer, which converts an oscillating electric field to a mechanical oscillation. Detection of acoustic waves that have traveled through a polymer specimen is done with the same type of transducer. Depending on its use, a transducer is called a transmitting transducer (transmitter) or receiving transducer (receiver). Common transducer materials are quartz and various polycrystalline ceramics, such as lead zirconate titanate (PZT), polarized in a strong electrostatic field. 

Piezoelectric materials can be grouped into the class of natural crystals, such as quartz or tourmaline, into one of polymers, such as polyvinylidene fluoride (PVDF) or that of polycrystalline ceramics. 

Piezoelectricity appears in natural crystals such as quartz, tourmaline, rochelle salt as well as in artificially produced ceramics and polymers such as e. g. nylon or copolymers of vinylidenefluoride (VDF) with trifluoroethylene (TrFE) or with tetrafluorethylene (TeFE). Most of the piezoelectric materials used for commercial sensor applications are synthetically produced polycrystalline ferroelectric ceramics such as e.g. lead-zirconate-titanate (PZT). 

Within single crystals and ceramic crystallites, respectively, the dipole moments of neighbouring domains are either perpendicular or anti-parallel to each other. For polycrystalline materials the orientation of the crystallites and thus of the domains is randomly distributed. In the original state these materials do not exhibit a macroscopic polarization and thus no piezoelectric effect. However, the latter can be induced by applying a static electric field below the Curie temperature where the domains of uniform dipole moments arrange towards the polarization field (paraelectric polarization). The field strength applied should be between the saturation and the breakdown range. Due to this polarization the ferroelectric material becomes piezoelectric. 

Due to the reversibility of the piezoelectric effect, materials exhibiting such an electromechanical coupling may be used to handle actuation as well as sensing tasks. The different piezoelectric materials are able to provide these properties in a frequency spectrum ranging beyond the level of acoustics. On the one hand, there are several monocrystals and polycrystalline ceramics, which are hard and brittle and therefore are suitable only for relatively small strains. On the other hand, there are semicrystalline polymers, which are soft and elastic but show less pronounced coupling properties. Another kind of... 

Discoveries of new piezoelectric materials started a new boom of research in the 1940s when A.V. Shubnikov predicted that piezoelectric properties would be found in amorphous and polycrystalline materials. His predictions were confirmed soon by observing that ferroelectric ceramics are strongly piezoelectric. The existence of piezoelectricity for certain synthetic and biological polymers has also been known for a long time. In particular, piezoelectricity in bone and tendon has been extensively studied. ... 

A ceramic material is heterogenous it is polycrystalline and contains pores it is often multiphase, the phases being distributed at grain boundaries, as a result of a liquid phase sintering, or forming inclusions. We may ask ourselves in what measure it is still possible to speak of electrical conductivity, relative dielectric permittivity, piezoelectric constant, etc. of the material. Or, to put it plainly, does the notion of material have meaning The answer is experimental, as we will see in section 11.5.1. 

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