The effect of MnCO3 doped from 0 to 55 mol% into BaO-(Nd0.7Sm0.3)2O3-4TiO2 (BNST) sintered in a reducing atmosphere on the microstructure and electrical properties was studied. Mn3+ completely substituted into Ti4+-sites of BNST to form a solid solution, so there is no second phase until 42 mol% which is the maximum solubility. Mn (<42 mol%)-doped BNST sintered in a reducing atmosphere is in a semi-conducing state because the concentration of free electron is higher than that of the acceptors. On the other hand, when Mn content doped into BNST exceeds a critical value (>43 mol%), the second Mn-rich phase due to excess of Mn3+ substituted into Ti4+-site, corresponding to original BaO-(Nd0.7Sm0.3)2O3-4TiO2 (1 1 4) phase, is formed. Mn (>43 mol%)-doped BNST sintered in a reducing atmosphere is in an insulating state because the concentration of the acceptors is higher than that of liberated free electron, so the insulation resistance becomes high and tan δ becomes low. The formation of the second Mn-rich phase affects Q × f factor and temperature coefficient of capacitance (T.C.C.) of BNST significantly.
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