Engineering and characterization of dopant scale effects in advanced multiphase electroceramics

Improved methods for understanding, evaluating, and controlling particle mixedness dependent properties are desired and applicable to almost any multicomponent chemical system which uses particulate raw components, and in which chemical or phase homogeneity is limited by chemical diffusion or reaction kinetics. Two electroceramic materials whose properties are known to rely upon a critical minor component were investigated with respect to the dependence of a key figure of merit upon the mixedness of the critical components in the pre-sintered green state. Properties of positive temperature coefficient of resistivity (PTCR) barium titanate were shown to be highly dependent upon achieving a high level of homogeneity of the critical manganese component. The nonlinear current-voltage characteristic in zinc oxide based voltage dependent resistors (VDRs) was shown to depend strongly upon the degree of homogeneity of a transition metal dopant. In both materials, systematic dependences of the functional properties upon the mixedness of the critical dopant were demonstrated. Chemically coating the critical dopant onto the remaining particulate raw materials through use of chemical (solution) precursors was shown to significantly increase chemical mixedness and improve electronic device properties. The mixedness of each particulate system was quantified via numerical simulation models which use inputs including the particle size distributions, volume fractions, and molecular weights of the raw material components. Impedance spectroscopy was investigated and shown to be a highly effective characterization technique for quantifying the degree of homogeneity achieved in sintered ceramics. Strong correlations were demonstrated between the impedance spectroscopy depression angles and the desired electrical properties. Magnitudes of the depression angles were linked to the ceramic microstructure and to the chemical mixedness in the grain boundary regions by means of equivalent circuit models. Numerically solved network simulations were used to model current-voltage responses in varistor microstructures. Comparisons were made of the current-voltage responses and the current flux distributions for different model microstructures (for which the individual grain boundary properties varied between relatively narrow distributions of values to more broadly defined distributions). Significantly greater spatial disparity in current flux was observed as grain boundary distributions were increased, ultimately leading to higher leakage currents and poorer voltage clamping ability.