Seoul National University · Materials Science
Professor Han-Il Yoo's research lab specializes in the fundamental understanding of defect chemistry, ion transport, and electrical properties in complex oxides, with a focus on functional ceramics for energy and electronic applications. Key research directions include oxygen nonstoichiometry, chemical diffusion, and conductivity relaxation in perovskite-type oxides such as BaTiO₃, SrCeO₃, and La₂NiO₄₊δ, particularly under varying oxygen and water vapor activities. The lab employs advanced electrochemical techniques, including conductivity relaxation and defect thermodynamics modeling, to investigate defect equilibria, vacancy mobility, and the role of dopants in modulating ionic and electronic transport. Their work bridges materials science and solid-state ionics, contributing to the development of advanced dielectrics, proton conductors, and oxide semiconductors.
Figures are computed from collected data and may differ slightly.
The electrical-insulation degradation of BaTiO3 is now of growing interest as the BaTiO3-based dielectric layers of multilayer ceramic capacitors are getting thinner to submicron thicknesses. The degradation is understood to be due to the electrotransport of oxygen vacancies and may be monitored by the colors emanating from the cathode and/or anode. In the case of single crystal BaTiO3, a brown color emanates from the anode and a blue color from the cathode. We will experimentally review the gen
The oxidation and hydration kinetics of a proton conductor oxide, SrCe(0.95)Yb(0.05)O(2.975), were examined via conductivity relaxation upon a sudden change of oxygen activity in a fixed water-activity atmosphere, and vice versa, in the ranges of -4.0 < log a(O(2)) < or = 0.01 and -5.0 < log a(H(2)O) < -2.0 at 800 degrees C. It was found that under an oxygen-activity gradient in a fixed water-vapor-activity atmosphere, the conductivity relaxation with time is monotonic with a single relaxation t
Oxygen (nonstoichiometry) re‐equilibration kinetics was examined, via a conductivity relaxation method, on 1 m/o donor (La)‐doped BaTiO 3 (nominal composition, Ba 0.99 La 0.01 Ti 0.9975 O 3 ) against oxygen partial pressure in the range of −16<log ( P O2 / atm)≤ 0 at 1200°C. The kinetics has been found to be onefold with a single relaxation time or twofold with two different relaxation times depending on oxygen activity. It is attributed to the relative contributions depending on oxygen activ
The chemical diffusivity of 1.8 mol% aluminum‐doped BaTiO 3−δ was measured on single‐crystal specimens, as a function of ambient oxygen partial pressure, in the range 10 −18 atm ≤ P O2 ≤ 1 atm and at temperatures of 800°≤ T ≤ 1100°C, via a conductivity‐relaxation technique. As in the polycrystalline, undoped BaTiO 3−δ described in Part II of this work, the chemical diffusivity exhibited a maximum, of thermodynamic origin, approximately at the stoichiometric composition (δ= 0). The measured diffu
Electrical conductivity relaxations were measured at a fixed temperature on proton-conducting upon sudden changes of oxygen activity in fixed water-vapor activity atmospheres and vice versa, in the ranges of and , respectively. It has been confirmed that the conductivity relaxes generally with two different relaxation times or twofold, but always nonmonotonically upon hydration/dehydration and monotonically upon oxidation/reduction. This twofold relaxation is attributed to the superposition of t
La(2)NiO(4+delta) is an oxygen excess compound (delta > 0) with oxygen interstitials (O(i) and holes (h*) in majority, which deviates positively from the ideal-dilute-solution behavior of defects. It was earlier attempted to interpret this positive deviation by taking into account the activity coefficients of both O(i) and h*. In this work, we examined the nonstoichiometry, electrical conductivity, and thermopower against oxygen activity in the entire stability range of the oxide at 800 degrees
Article Cross Effect Between Electronic and Ionic Flows in Semiconducting Transition Metal Oxides was published on February 1, 1990 in the journal Zeitschrift für Physikalische Chemie (volume 168, issue 2).
Insulation resistance degradation of dielectric BaTiO(3) is expected to be closely correlated to its defect structure frozen in from elevated processing temperatures. For BaTiO(3), respectively doped with variable-valence (Mn(Ti)) and fixed-valence acceptors (Al(Ti)), their defect structures were frozen in by quenching at different equilibrium oxygen activities in the range of -18 < log a(O(2))< or = 0 at 1000 and 900 degrees C, respectively, and their electrical conductivities were measured aga
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