Electromechanical materials ferroelectrics

More recent development of complex perovskite structures has resulted in a new class of electromechanical materials, electmstrictors. Purely electrostrictive materials are para-electric and centrosymmetric that is, they do not possess a polar axis and are typically cubic. The electrostrictive materials of most interest are ferroelectrics that are operated above or near their transition temperatures. The electrostrictive effect is a second-order phenomenon whereby an applied electric field results in a lattice distortion and mechanical distortion in the material. 

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). 

The semiconducting properties of the compounds of the SbSI type (see Table XXVIII) were predicted by Mooser and Pearson in 1958 228). They were first confirmed for SbSI, for which photoconductivity was found in 1960 243). The breakthrough was the observation of fer-roelectricity in this material 117) and other SbSI type compounds 244 see Table XXIX), in addition to phase transitions 184), nonlinear optical behavior 156), piezoelectric behavior 44), and electromechanical 183) and other properties. These photoconductors exhibit abnormally large temperature-coefficients for their band gaps they are strongly piezoelectric. Some are ferroelectric (see Table XXIX). They have anomalous electrooptic and optomechanical properties, namely, elongation or contraction under illumination. As already mentioned, these fields cannot be treated in any detail in this review for those interested in ferroelectricity, review articles 224, 352) are mentioned. The heat capacity of SbSI has been measured from - 180 to -l- 40°C and, from these data, the excess entropy of the ferro-paraelectric transition... 

The piezoelectric effect is an electromechanical effect in which mechanical evoke and reverse an electric reaction in a ferroelectric material and vice versa. The word piezo has been derived from the Greek piezein which means press . Compounds are composed of positive and negative ions and are electrically neutral as a whole. The fact that electrically charged particles are still present in the crystal can for example be demonstrated by means of the electric... 

The class of ferroelectric materials have a lot of useful properties. High dielectric coefficients over a wide temperature and frequency range are used as dielectrics in integrated or in smd (surface mounted device) capacitors. The large piezoelectric effect is applied in a variety of electromechanical sensors, actuators and transducers. Infrared sensors need a high pyroelectric coefficient which is available with this class of materials. Tunable thermistor properties in semiconducting ferroelectrics are used in ptcr (positive temperature coefficient... 

These materials have shown piezoelectric responses after appropriate poling [18]. Their piezoelectric actuation properties are typically worse than ceramic piezoelectric crystals however, they have the advantages of being lightweight, flexible, easily formed, and not brittle. Additionally, while ceramics are limited to strains on the order of 0.1%, ferroelectric polymers are capable of strains of 10% [91] and very high electromechanical coupling efficiencies [93]. 

Electrostriction, which is a change in sample dimensions in response to the application of an electric field to a dielectric, is a universal characteristic and provides another example of an electromechanical effect. Some materials get thinner while others get thicker in the direction of the electric field. This effect is not reversible and a deformation does not produce any polarisation. The effect is found in all materials, not just those that lack a centre of symmetry, including glasses and hquids. However, the electrostrictive effect is generally very small except for ferroelectric perovskites, especially relaxor ferroelectrics described in the following (Section 6.7). 

The broad application of relaxor ferroelectrics in the modern sensors, actuators, high performance electromechanical transducers and other electronic devices generates the constant interest in investigation and fabrication of these materials. 

An important aspect of the enhanced intrinsic response of ferroelectrics is anisotropy, the direction dependence of properties. By symmetry, ferroelectric crystals are anisotropic with respect to dielectric, mechanical, and electromechanical properties, and this issue is essential when designing devices that exploit the highest material responses by correctly orienting single crystals [8, 33-35]. The crystal axes, along which the highest material response occurs, may not coincide with the polar directions of spontaneous polarization for a given ferroelectric phase. This is the case... 

It should be noted that only the A- or B-site donor dopants cause softening of ABO3 ferroelectric materials. As shown in Ref. [87], the fluorine doping of PZT leads to a hardening of the dielectric and electromechanical response. This effect can be explained by ordering of point defects or their associates involving the charged... 

These normalized electromechanical equations are applicable to any chemistry, and allow you tailor the optimal single-crystal intrinsic response. Furthermore, any improvement on the chemistry of a ferroelectric material will asymptotically converge to an optimal orientation of 54.76° for materials with weak anisotropy, or to zero in the limit of A.. = 2/3. 

Relaxor-type electrostrictive materials, such as those from the lead magnesium niobate-lead titanate, Pb(Mgp 3Nb2/3)03-PbTi03 (or PMN-PT), solid solution are highly suitable for actuator applications. This relaxor ferroelectric also exhibits an induced piezoelectric effect. That is, the electromechanical coupling factor kt varies with the applied DC bias field. As the DC bias field increases, the coupling increases and saturates. Since this behavior is reproducible, these materials can be applied as ultrasonic transducers which are tunable by the bias field [12]. 

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