Ferroelectric ceramic

The physical phenomenon of ferroelectricity-initially termed Seignette electric-ity-was first discovered in sodium potassium tartrate tetrahydrate (Rochelle or Seignette salt), and later in analogy to ferromagnetic behavior coined ferroelectricity by Valasek (1924). Its history is listed in Table 8.4, which shows the impressive change from a curious isolated property to a widespread and economically enormously important and promising ceramic engineering material (Cross and Newnham, 1987). 

Milliwaves Visible light Es Space charge and Ultraviolet 

Early barium titanate era high-K capadtors developed 

Age of high sdence soft modes and order parameters 

Age of miniaturization size effects, manipulated modes and dipoles 

From the standpoint of crystal structure, ferroelectric materials are a further subset of pyroelectric and piezoelectric materials. As with pyroelectrics, ferroelectrics also display a spontaneous polarization in the absence of an applied electric field. The 

Another distinguishing feature of ferroelectric behavior is the polarization versus electric field P—B) hysteresis loop. The hysteresis loop results from the domain reorientation which occurs as the electric field direction is varied. The size and shape of the loop is determined by the magnitude of the dipole moment of the unit cell and the domain-switching characteristics of the material. Hysteresis loop behavior is measured using either a Sawyer—Tower circuit or a Diamant—Pepinsky bridge. Details of the construction and operation of a Sawyer—Tower circuit are given in Reference 24. Thin film properties have also been measmed with these two devices, and in addition, a commercially available measurement system has been widely used.  

Relatively few applications have utilized the ferroelectric effect in ceramics. Ferroelectric ceramics have been widely employed because of the other properties that they display, however. Their dielectric, piezoelectric, and pyroelectric properties have led to their use in capacitor, actuator and other piezoelectric applications, and infrared detection devices. Again, the most widely used materials are the lead-based ABO perovskite compounds. 

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Ceramics. The properties of ferroelectrics, basically deterrnined by composition, are also affected by the microstmcture of the densifted body which depends on the fabrication method and condition. The ferroelectric ceramic process is comprised of the following steps (10,24,25) (/) selection of raw oxide materials, (2) preparation of a powder composition, (J) shaping, (4) densification, and (5) finishing. 

Ferroelectric Ceramic—Polymer Composites. The motivation for the development of composite ferroelectric materials arose from the need for a combination of desirable properties that often caimot be obtained in single-phase materials. For example, in an electromechanical transducer, the piezoelectric sensitivity might be maximized and the density minimized to obtain a good acoustic matching with water, and the transducer made mechanically flexible to conform to a curved surface (see COMPOSITE MATERIALS, CERAMiC-MATRix). 

Bauer, F. (1982), Behavior of Ferroelectric Ceramics and PVF2 Polymers Under Shock Loading, in Shock Waves in Condensed Matter—1981 (edited by W.J. Nellis, L. Seaman, and R.A. Graham) American Institute of Physics, New York, pp. 251-267. 

Other strongly ferroelectric crystals have been discovered and today, PZT -Pb(Ti, Zr)03 is the most widely exploited of all piezoelectric (ferroelectric) ceramics. 

Haertling, G. H. 1992. Current status of thin/thick film ferroelectrics. Ceram. Trans. 25 1-18. 

Composites. See also Composite materials Composites. See also Laminates aluminum-filled, 10 15-28 carbon fiber, 26 745 ceramic-filled polymer, 10 15-16 ceramic-matrix, 5 551-581 conducting, 7 524 from cotton, 8 31 ferroelectric ceramic-polymer,... 

Approximately ten years ago, it was first reported by Haertling and Land (jj that optical transparency was achieved in a ferroelectric ceramic material. This material was, in reality, not just one composition but consisted of a series of compositions in the lanthanum modified lead zirconate-lead titanate (PLZT) solid solution region. The multiplicity of compositions, each with different mechanical, electrical and electrooptic properties has led to a decade of study in defining the chemical and structural nature of these materials in understanding the phenomena underlying their optical and electrooptic properties and in evaluating the practicality of the large number of possible applications (2-12),... 

It has recently been shown that organic photoconductor-liquid crystal sandwich cells can in theory act as dynamic scattering devices 164> and the technical possibilities ought to be tested. In this context, it should be noted that dyes can be used in two-layer photocondensers (consisting e.g. of phthalocyanine and a ferroelectric ceramic), which are very sensitive to light and have a response time of lO-4 to 10-3 Sec 165). 

Kitayama, T., Nakayama, H. Piezoelectricity of composite systems of polymer and powdered ferroelectric ceramics. 18 th Meeting on Appl. Phys. Japan (Apr. 1971) Tokyo. 

Before the phenomenon piezoceramics can be discussed, some attention should be paid to ferroelectric ceramics. 

An ideal single crystal shows a P E) behavior as depicted in Figure 1.6. The non-ferroelectric dielectric ionic and electronic polarization contributions are clearly linear, and are suposed by the spontaneous polarization Ps (dashed curve in Figure 1.6). To reverse the polarization an electrical field with an amplitude E > Ec is required. In opposite to single crystals in polydomain ferroelectric ceramics, the remanent polarization Pr is smaller than the spontaneous one Ps due to backswitching even for opposite fields as shown in Figure 1.6. In that case Ps can be estimated by extrapolation of (non-switching) P-values to E — 0. 

Two types of contributions to dielectric and piezoelectric properties of ferroelectric ceramics are usually distinguished [6], [9-12], One type is called an intrinsic contribution, and it is due to the distortion of the crystal lattice under an applied electric field or a mechanical stress. The second type is called an extrinsic contribution, and it results from the motion of domain walls or domain switching [8], To provide an understanding of material properties of pzt, several methods to separate the intrinsic and extrinsic contributions were proposed. These methods are indirect, and are based on measurements of the dielectric and piezoelectric properties of ferroelectric ceramics [8], [10-12], In the experiments reported in this paper a different approach is adopted, which is based on measurements of high-resolution synchrotron X-ray powder diffraction. The shift in the positions of the diffraction peaks under applied electric field gives the intrinsic lattice deformation, whereas the domain switching can be calculated from the change in peak intensities [13,14],... 

Figure 13.1 Examples of the field dependence of piezoelectric coefficients (a) direct effect in ferroelectric ceramics, measured with a dynamic press (b) converse effect in rhombohedral 60/40 pzt thin films with different orientations, measured with an optical interferometer [1], correspond to pseudocubic axes.

Investigation of the piezoelectric relaxation in ferroelectric ceramics using dynamic press 257... 

Figure 13.6 Piezoelectric coefficient d and tan<5p of a single phase Bi4Ti30i2 ferroelectric ceramic with highly anisotropic grains, at room temperature. On the right are shown charge-pressure hysteresis loops at selected frequencies, with a clockwise hysteresis at 0.07 Hz and counter-clockwise hysteresis at 70 Hz. See [17] for details.

Because a ceramic is composed of a large number of randomly oriented crystallites it would normally be expected to be isotropic in its properties. The possibility of altering the direction of the polarization in the crystallites of a ferroelectric ceramic (a process called poling ) makes it capable of piezoelectric, pyroelectric and electro-optic behaviour. The poling process - the application of a static electric field under appropriate conditions of temperature and time -aligns the polar axis as near to the field direction as the local environment and the crystal structure allow. 

The detailed discussion of the prototype ferroelectric ceramic barium titanate in Section 2.7.3 provides the essential background to an understanding of the later discussion in the text. 

Fig. 2.44 Schematic illustrating the changes accompanying the application of electrical and mechanical stresses to a polycrystalline ferroelectric ceramic (a) stress-free - each grain is non-polar because of the cancellation of both 180° and 90° domains (b) with applied electric field - 180° domains switch producing net overall polarity but no dimensional change (c) with increase in electric field 90° domains switch accompanied by small ( 1%) elongation (d) domains disorientated by application of mechanical stress. (Note the blank grains in (a) and (b) would contain similar domain structures.)...

Ferroelectric ceramics find applications in capacitors, infrared detection, sound detection in air and water, the generation of ultrasonic energy, light switches, current controllers and small thermostatic devices. In all these cases... 

Arlt, G. (1998) Strong ultrasonic microwaves in ferroelectric ceramics. IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45, 4-10. 

The grain size of a ferroelectric ceramic has a marked effect on the permittivity for the size range 1-50 /mi (see Fig. 2.48). Below about 1 /mi the permittivity falls with decreasing grain size. An important factor leading to this behaviour is the variation in the stress to which a grain is subjected as it cools through the Curie point. 

It should be noted that a poling process is often necessary with single-crystal ferroelectric bodies because they contain a multiplicity of randomly oriented domains. There is therefore a sequence of states of increasing orderliness polycrystalline ferroelectric ceramics, poled ferroelectric ceramics, single-crystal ferroelectrics and single-domain single crystals. 

Because ferroelectric ceramics have non-linear characteristics the effects are more correctly described by the equations,... 

The uncooled thermal imaging technology exploiting ferroelectric ceramics is being challenged by other technologies, significantly by one based upon resistance bolometer materials such as vanadium oxide (VOv) and amorphous silicon. 

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