| Formation of core-shell grains in BaTiO3 based dielectric ceramics is essential to achieve stable dielectric temperature characteristic. This grain core-shell structure consists of a grain core, which is high purity BaTiO3, ferroelectric, and a paraelectric grain shell, where there is a concentration gradient of dopant. Most commonly core-shell additives are ZrO2, CeO2, Nb2O5+CO 2O3. As a result, internal stress is generated due to this dopant inhomogeneity within the grain and the normally sharp phase transition at Curie point becomes diffused. Up to now understanding and control of core-shell grains in BaTiO3 ceramics is still far from satisfying, which is the basis of this thesis work.; In this work, processing influences on core-shell grain formation were investigated in the terms of powder size, stoichiometry, particle surface modification by slurry pH and leached Ba2+, pressing pressure and additive effects such as ZrO2, Nd2O3, BaO and flux CuO, BaCuO2, followed by multiple regression analysis to identify significant variables so as to form core-shell grains in a controller manner.; It is found that ZrO2 amount; powder size, stoichiometry and temperature/time are most significant in control of core-shell characteristic (low Δϵ% < 30%). The desired core-shell characteristic was obtained with the maximum internal stress, which was achieved by the fine and uniform grains along with maximum amount ZrO2 solid solution in the BaTiO3 lattice and Ba2(Ti, Zr)5O 12 ∼ Ba2(Ti, Zr)9O20 second phase developed in the grain shell region; the desirable shell/grain thickness ratio was estimated to ∼0.05. Any variation in processing, which favors such structural and micro structural development, facilitates the coreshell formation. It is found that, Ba/Ti surface stoichiometry of 0.5∼0.6 facilitated core-shell characteristic, and this chemistry of BaTiO3 particle surface and optimum additive dispersion can be achieved by neutral slurry pH; with adequate eutectic liquid phase, uniformly distributed fine grain morphology was achieved with high sintered densities. A dielectric formulation of 0.8∼1.3 wt% ZrO2, and 3 m/o BaCuO2 and 0.1∼0.3 m/o Nd2 O3 was successfully developed to be sintered at 1055∼1085°C/45∼90 min with Δϵ% < 25%. This formulation has the potential to be co-fired with base metal electrodes. |
