Effect of Isovalent Ba-Site Substitutions on the Properties of (Ba_(l-x-y)M_yY_x)TiO_3 (M = Ca, Sr, Pb) PTCR Ceramics

摘要

An approach is proposed for fabricating fine-grained, low-resistivity BaTiO_3-based PTCR ceramics via partial isovalent substitutions on the Ba site. The grain size of the ceramics thus prepared is shown to decrease as the ratio of ionic radii r(Ba~(2+))/r(M~(2+)) (M = Ca, Sr, Pb) increases. Isovalent substitutions on the Ba site narrow down the range of donor dopant (yttrium) concentrations in which PTCR materials can be prepared. The experimental results agree well with thermodynamic calculations under the assumption that the materials contain the Y~(3+)Ti~(3+)O_3 phase, as suggested by ESR data, which point to the presence of Y~(3+)-Ti~(3+) associates. Partial calcium, strontium, and lead substitutions on the Ba site reduce the average grain size of PTCR ceramics, which is probably due to the lattice strain arising from the isovalent substitution. Partial replacement of Ba~(2+) with mixtures of different isovalent elements (e.g., Sr~(2+) and Pb~(2+)) offers the possibility of obtaining finegrained, low-resistivity PTCR ceramics, without changing the phase transition temperature.