Ferroelectric Ceramics for Dielectric Electromechanical and Pyroelectric Applications
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this area. Currently, sales of technical ceramics are about $4 billion, with capacitors accounting for $0.5 billion; all other ferroelectric ceramic devices p r o b a b l y account for only about 30% of that level. 1 The number of research publications is now the reverse of the product values, indicating the need to develop information for transducer, sensor, and optical communication applications. However, a number of interesting areas are currently being discussed in the research literature, ranging from basic considerations of the origin of dielectric constant in barium titanate to development of new dielectric systems.
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TEMPERATURE l'CI
Figure 1. Temperature dependence of dielectric constant in specimens with 0.6 at.% Dy with grain sizes indicated.15
One of the most intriguing problems when dealing with a very high dielectric constant ceramic is assessing mechanisms contributing to the low and moderate frequency response. The possible contributions to the low frequency permittivity may be divided into at least five categories: 1. Intrinsic, or the motion of ions and electrons in the crystal structure. 2. Domain wall motion, which could either be "90°" type or 180°. 3. Piezoelectric coupling, which could be electromechanical contributions from a poled ceramic shape, individual grains, or domains within a grain.
4. Defect dipole motion. 5. Phase wall motion (principally associated with relaxor dielectrics). Of these m e c h a n i s m s , the d o m i n a n t contribution to the dielectric constant in ferroelectric ceramics is usually the intrinsic ionic motion, which can be expected to remain stable to at least GHz frequencies. The present question is whether a large c o n t r i b u t i o n comes from d o m a i n wall motion. It has been suggested that domain wall motion could contribute up to 40% of the dielectric constant in coarse-grained barium titanate, 2 and this may be even higher in fine-grained b a r i u m t i t a n a t e . 3 T h e domain wall modal conflicts with the older and generally accepted modal of dielectric constant increasing with decreasing grain size (see Figure 1). The increase results because of the reduction of ferroelastic "90°" t y p e walls a n d the c o n s e q u e n t increase of internal stresses. 4 ' 5 Thermodynamic modeling6 s u p p o r t s the stress enhancement of the permittivity and does not consider wall motion. The principal difference in the two models lies in the dependence of domain size and occurrence with grain size in the region of one micron. Additional studies are required to resolve the domain configurations as a function of grain size. These studies are made particularly difficult by the sensitivity of the barium titanate domain structure to temperature, mechanical stress, and electrical field. The thinning of ceramic samples and the use of TEM techniques may strongly affect the intrinsic domain configurations in barium titanate. A third contribution to the overall dielectric constant is the piezoelectric respo
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