2, Korea Testing Laboratory, Seoul, , Korea (the Republic of)
MgO stabilized zirconia has been received attention due to excellent ionic conductivity, fracture toughness, and thermal shock resistance. Zirconia (ZrO2) has three different phase with the temperature such as monoclinic (RT to 1170 °C), tetragonal (1170 °C to 2370 °C), and cubic fluorite structure (above 2370 °C). The cubic in ZrO2 is the most useful phase for various applications, and controlling the ZrO2 phases is a main issue in research field. The cubic phase can be stabilized in RT by doping the lower-valence cations (Mg2+) than Zr4+ because of creation an oxygen vacancy. The stabilized cubic phase could be monoclinic and tetragonal by long-term use in a high temperature above 1500 °C.
In this study, the phase stabilization of MgO stabilized zirconia with Mn doping was studied in terms of the valence state of Mn and the local atomic structure of zirconia. Content of cubic phase was observed by morphology and crystallographic analysis in MgO partially stabilized zirconia (MgPSZ) with MnO2 addition (5 and 10 mol%) and MgO fully stabilized zirconia (MSZ) with 5 mol% MnO2 addition. Although 5 mol% Mn-doped MgPSZ exhibited 66.11% cubic phase fraction, 10 mol% Mn-doped MgPSZ and 5 mol% Mn-doped MSZ had 95.63 and 98.72 % cubic phase fraction, respectively. The conductivity of Mn-doped MgPSZ and MSZ were measured by DC 4-point probe method, and it increased with MSZ, 5 mol% Mn-doped MgPSZ, 5 mol% Mn-doped MSZ, and 10 mol% Mn-doped MgPSZ in order.
The local atomic structure of zirconia was identified through extended X-ray absorption fine structure around Zr K-edge and the coordination of Zr-O bonding was decrease by the oxygen vacancy generation with Mn doping. Valence state of doped Mn ion was reduced from Mn4+ to Mn2+, and it led to generation of oxygen vacancy due to charge compensation, which was identified by X-ray photoelectron spectroscopy. In addition, it indicates that oxygen vacancy formation by substitution of Mn2+ in the Zr4+ site in MgPSZ and MSZ increased cubic phase fraction. However, 10 mol% Mn doping caused electrical conductivity and it is the reason why 10 mol% Mn-doped MgPSZ had higher conductivity than that of 5 mol% Mn-doped MSZ in spite of cubic phase fraction. It indicated that the conductivity of 5 mol% Mn-doped MSZ increased only the ion conduction without electronic conduction.