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SELECTED APPLICATION AREAS. 89 6 Selected Application Areas. ELECTROCERAMICS In considering the properties of electroceramic composites, relevant areas of interest are phase morphology, symmetry, and microstructural scale. Some of the morphological aspects have been discussed and need no further elaboration. Symmetry considerations have been extensively discussed in papers by Newnham and coworkers (1978). In what follows, the influence of scale and periodicity are examined, with emphasis on the need to develop appropriate nanoscale composite structures. A good deal has been written about the importance of scale in magnetic, optical, and semiconductor materials. Many of the same effects occur in ferroelectrics (critical domain sizes, resonance phenomena, electron tunneling, and nonlinear effects). In ferromagnetic materials, there are three kinds of magnetic structures for small particles. Multidomain structures are common for particles larger than a critical size; magnetization in large particles takes place through domain wall motion. Below this critical size, single domain particles are observed, and switching takes place by rotation rather than wall movement, thereby raising the coercive field. Very small particles exhibit a supermagnetic effect in which the spins rotate in unison under thermal excitation. Only modest magnetic fields are required to align the spins of adjacent particles. Analogous behavior in ferroelectric particles has yet to be fully established, but a variety of interesting experimental results are accumulating. In BaTiO3 ceramics, single domain behavior is observed in grains less than approximately 1 µm, while dielectric phenomena resembling those found in superparamagnetism are found in relaxor ferroelectrics. The fluctuating microdomains in this superparaelectric state are about 20 nm across. Composite materials made up of single domain and superparaelectric particles have yet to be investigated in a systematic way with proper control of the connectivity and surrounding environment. The
SELECTED APPLICATION AREAS. 90 controlled synthesis of submicrometer ferroelectric grains will do much to stimulate research in this area. Surface treatment of the ferroelectric phase allows control of the mechanical boundary conditions. Titanyl coupling agents are effective in bonding PZT to epoxy. Mechanical pull tests have been used to demonstrate the strength of the ceramic-polymer bond. Improved stress transfer and large piezoelectric coefficients in piezoelectric composites are obtained as a result of better bonding. Polymers are about a hundred times more compliant than ceramics. If a ceramic grain is surrounded by polymer, the mechanical constraints are relatively small. This means that more complete poling is possible, as demonstrated in ferroelectric composites. Electrical boundary conditions can also be controlled by adjusting the dielectric constant and conductivity of the surrounding phase. Periodicity and scale are important factors when composites are to be used at high frequencies where resonance and interference effects occur. When the wavelengths are on the same scale as the component dimensions, the composite no longer behaves like a uniform solid. An interesting example of unusual wave behavior occurs in composite transducers made from poled ferroelectric fibers embedded in an epoxy matrix. When driven in thickness resonance, the regularly spaced fibers excite resonance modes in the polymer matrix, causing the matrix to vibrate with much larger amplitude than the piezoelectric fibers. The difference in compliance coefficients causes the nonpiezoelectric phase to respond far more than the stiff ceramic piezoelectric. Composite materials are therefore capable of mechanical amplification from prepoled PZT fibers mounted in a polymer matrix. Domain-divided transducers operate on a similar principle. Multidomain crystals and ceramics have been used as acoustic phase plates and high-frequency transducers. The extension of this thinking to phenomena associated with optical excitations automatically focuses attention on equivalent nanoscale structures. Specifically for wavelengths of 400 to 800 nm (optical spectra), to avoid serious scattering the internal structures must be below the 10-to 20-nm scale. In ferroelectrics, the superparaelectric internal polar structures of the relaxor materials offer the possibility of tailoring optically isotropic or anisotropic behavior under external electric field control. The recent demonstrations of ferroelectricity in some species down to 20 nm indicate that these anisotropies can be exploited in suitable assembled nanostructures. A wide range of potential property modifications, including shape-induced optical birefringence, shape- controlled optical nonlinearity, and potential modes for inducing optical bistability, remain to be explored. It is clear that there will be corresponding magnetic nanocomposites and that for these materials additional versatility can be expected because of the nanoscale interaction with the transport phenomena and the associated optical
