DIELECTRIC AND PIEZOELECTRIC BEHAVIOR OF LITHIUM-NIOBATE BICRYSTALS AND POLYCRYSTAL CERAMICS

In studies directed towards improving the basic understanding of the influence of grain boundaries upon the properties of electronic ceramics, studies are being made of artificial planar bicrystal boundaries created in single domain ferroelectric lithium niobate crystals by thermal diffusion bonding of carefully polished "clean" (001) surface under uniaxial compressive stress applied normal to the bonding surfaces.; Characterization of the bicrystal boundaries has been made by optical scanning electron microscopy and etching techniques, to reveal the boundary structure and domain configuration.; A major part of the effort in this study is concerned with evaluating modifications to the dielectric, pyroelectric, and piezoelectric characteristics by comparing single crystals and bicrystals with different polarity and twist orientation combinations of their component parts. Techniques used include transport studies, dielectric relaxation studies, pyroelectric and thermal stimulated current measurements, and piezoelectric resonance measurements.; From the dielectric and pyroelectric studies, models are developed for the charge-carrier transport, relaxation and trapping phenomena in the boundary region establishing an atomistic model from which the potential barrier structure has been estimated.; The piezoelectric resonance studies reveal that the lattice discontinuity of a bicrystal boundary does not terminate the in-phase-vibration in linked-type bicrystals but isolates the out-of-phase vibration in encountering types. Evidence for the existence of a piezoelectrically inactive layer in the boundary region of encountering-type bicrystals has also been adduced.; The preservation of much of the piezoelectric resonance character in the bicrystal suggests that the resonance model can be extended to explore individual grain resonances in polar dielectric ceramics. The dielectric dispersion around 10('8) HZ in both conventional sintered and uniaxially hot-pressed LiNbO(,3) ceramics is shown to have obvious resonant character.; A computer simulation technique is developed which permits a modeling of the full dielectric spectrum of the ceramic. Both real and imaginary components of the impedance are quantitatively described using equivalent circuit models for the coupled individual grain resonances. Parameters for the equivalent circuit are calculated from single domain crystal properties and grain dimensions with no adjustable parameters. The close agreement between calculated and measured spectra demonstrate unequivocally the major importance of the damped piezoelectric resonances to the dielectric loss processes in strongly polar ferroelectric ceramics.