Domain and Grain Boundary Structural Effects on the Resistivity Behavior in Donor Doped, PTCR Barium Titanate

201 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1993.Polycrystalline barium titanate (BaTiO$\sb3$) can be made n-type conducting and reveal a positive temperature coefficient of resistivity (PTCR) near the ferroelectric phase transition (T$\sb{\rm c}$). This is accomplished by development of a grain boundary potential barrier, the height of which is low at temperatures below T$\sb{\rm c}$ and high at temperatures above. The properties of the grain boundary region, its structure, chemical composition and related defect structure are not well understood. The purpose of this investigation is to address these deficiencies in the literature. This was achieved by characterizing the structure using TEM, SEM, HREM, EDS, and TEM as a function of temperature and relating the observed structure to the resistivity-temperature characteristics. Y$\sb2$O$\sb3$ and La$\sb2$O$\sb3$ doped BaTiO$\sb3$, PTCR systems were studied and compared.Electrical property analyses show that for the optimum samples in each system, a similar room temperature resistivity is obtained. The larger ionic radii doped materials produce a diffuse resistivity transition, while the Y$\sb2$O$\sb3$ doped materials show an abrupt change in resistivity near T$\sb{\rm c}$. SEM analysis on polished and etched sections revealed a uni-directional, columnar type domain structure which occurred in every grain, independent of donor dopant type. TEM analysis showed the interior and grain boundary domain structures to be 90$\sp\circ$ type. Non-annealed samples showed uniform and wide domain ($\sim$0.8 $\mu$m) growth from the interior up to the immediate grain boundary. In Y-doped samples, fine domain ($\sim$0.08 $\mu$m) and no-domain regions develop during annealing in the near grain boundary areas, each encompassing large areas, typically 1-2 $\mu$m$\sp2$.Structural characterization of annealed samples, using CBED, HOLZ pattern analysis and EDS, revealed the fine domain regions to be of a greater degree of tetragonality than the coarse interior domain structures, while the no-domain regions were highly pseudocubic. The structural differences are due to segregation effects.Domain switching studies revealed the fine domains disappear instantaneously at a lower transformation temperature than the coarse domain interior, leaving the near grain boundary regions cubic and void of domain structure and spontaneous polarization, establishing new grain boundary potentials. Only a small region of each grain need be closed to conduction for an abrupt increase in resistivity to occur.HREM images of grain boundary regions show coherency and are suggestive of easy electron pathways. The grain boundary core plus depletion region is narrow (1-4 nm). Quantitative analyses show that the small depletion widths, segregated defect concentrations within the grain boundary, and the high field dielectric constants associated with the grain boundary are highly correlated, giving only a narrow range of specific values in which good PTCR behavior can be obtained. (Abstract shortened by UMI.)U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD