The electrical conductivity behavior of perovskite metal oxides at high temperature

This dissertation describes the study of the high temperature electrical conductivity behavior of perovskite metal oxides including CaCu₃Ti ₄O₁₂, SrTi_1−x−yIn_xRu _y O_3−δ (x = 0, 0.2, y = 0.2, 0.4), La_xSr _1−xFeO_3−δ (x = 0, 0.5), and La_0.2 Sr_0.8Fe_0.55Ti_0.45 O_3−δ .; The perovskite related oxide, CaCu₃Ti₄O₁₂ which is known to have a huge dielectric constant was prepared for further study of its electrical properties. The electrical conductivity measured by the DC four-probe method shows two different activation energy regions (E_a ∼ 0.67 eV and 0.1 eV) with a change in slope at ∼330°C. According to DC and AC results, CaCu₃Ti₄O₁₂ contains semiconducting grains and insulating grain boundaries. The DC and AC conductivities are consistent even though CaCu₃Ti₄O ₁₂ and quenched CaCu₃Ti₄O₁₂ have different dielectric constants (∼20,000 and ∼30,000, respectively). Dielectric constants were significantly dependent on the density of the sample. The average grain boundary thickness of CaCu₃Ti₄O₁₂ and quenched CaCu₃Ti₄O₁₂ obtained by Transmission Electron Microscopy (TEM) were 1.5 nm and 12 nm, respectively.; The perovskite oxides, SrTi_0.8Ru_0.2O_3−δ , SrTi_0.6Ru_0.4O_3−δ, and SrTi_0.6In_0.2Ru_0.2O_3−δ were prepared by solid-state synthesis. The electrical conductivity of SrTi _0.6In_0.2Ru_0.2O_3−δ shows a discontinuity at ∼300°C at low oxygen partial pressure (pO ₂ ∼ 10−4 atm) due to decomposition to SrTiO ₃ and ruthenium metal. The temperature and the pO₂ dependences of the electrical conductivity were investigated by using DC and AC four-probe methods. SrTi_0.8Ru_0.2O_3−δ and SrTi_0.6Ru_0.4O_3−δ have conductivities of 6.4 Scm−1 and 5.4 Scm−1, respectively at 900°C and at pO₂ = 1 atm. The conductivity of SrTi _0.6In_0.2Ru_0.2O_3−δ at the same conditions was two orders of magnitude lower compared with SrTi _1−xRu_xO_3−δ. For SrTi_0.6 In_0.2Ru_0.2O_3−δ, when the pO₂ was increased from 10−4 atm to 1 atm, the conductivity decreased indicating n-type conductivity. The pO₂ dependence of the conductivity shows that SrTi_0.6In_0.2Ru _0.2O_3−δ has significant ionic conductivity, σ _i ∼ 0.22 Scm−1 at 1000°C.; The electrical conductivities of SrFeO_3−δ, La _0.5Sr_0.5FeO_3−δ, and La_0.2Sr _0.8Fe_0.55Ti_0.45O_3−δ were studied at oxygen partial pressures, ∼10−17 ≤ pO₂ ≤ 0.3 atm and at 750 ≤ T ≤ 1000°C. Slow equilibrium kinetics were observed in the intermediate pO₂ region (∼10 −5 atm). All experimental conductivity results were obtained manually because long equilibration times were needed. The pressure dependence of the conductivity at constant temperature shows quite different behavior compared with previous report...