An Overview of Novel Tetragonal Oxides with Enhanced High-Temperature Capabilities for Structural Applications
Abstract
Tetragonal oxides possess great potential in structural applications because of the superior thermomechanical properties and corrosion resistance. This paper presents an overview of the structure and thermal properties of (Dy, Ce, Y)doped ZrO2. The tetragonal structure is enhanced by doping Ce4+ cations to YSZ, whereas doping Dy3+ stabilizes the cubic phase. In comparison to conventional 8 wt.% Y2O3 stabilized ZrO2 (equivalent to 4 mol% Y2O3 stabilized ZrO2, 4YSZ), the thermal conductivity of 3Dy5Ce-YSZ and 2Dy-YSZ is decreased and the coefficient of thermal expansion (CTE) is increased, which are desirable for thermal barrier materials. The potential of Dy and Ce doped 4YSZ as thermal barrier materials for high-temperature applications has been highlighted in the paper.
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Introduction
Structural oxides that exhibit low thermal conductivity, high corrosion resistance, high thermal shock resistance, high melting point, and high CTE are utilized as thermal barrier coatings (TBCs) in gas turbines for example, to protect the hotsections from high-temperature gases [1]. TBCs comprise of a bond coat, thermally grown oxide (TGO) layer (e.g. α-Al2O3) and ceramic top coat [2]. The harsh operating environments of TBCs include high temperatures, high pressures, oxidative and corrosive conditions, and thermal cycling process, bringing about challenging requirements for novel material compositions [3]. The deposition of TBCs on the hot-section components contributes to the decrease of their surface temperature and thus enhances the fuel efficiency of the high-temperature applications.
Tetragonal oxides, such as doped ZrO2 with rare earth elements (e.g. Dy3+ and Sc3+) are reported to have lower thermal conductivity and enhanced CTE, as compared with conventional 8 wt.% YSZ [4, 5]. In contrast to the cubic structure, tetragonal 4YSZ shows higher fracture toughness attributed to the tetragonal-monoclinic transformation, contributing to the superior erosion and impact resistance [6, 7]. The thermochemical compatibility between 4YSZ and Al2O3 is high during the thermal cycling process, while the pyrochlore oxide is reported to react with Al2O3 forming new phases at high temperatures [8].
Although the thermal conductivity and sintering characteristics of Dy-YSZ coatings have been studied [4, 9], to our best knowledge, no apparent work has been reported on the structure, thermal conductivity and CTE of (Dy, Ce)-YSZ. This paper investigates the correlation between compositional variation and crystal structure of Dy3+ and Ce4+doped YSZ. It was discovered that the addition of Ce4+ to ZrO2 stabilized the tetragonal structure, and the thermal conductivity was reduced by 29%. Moreover, Dy3+ incorporation to ZrO2 reduced the thermal conductivity and stabilized the cubic phase [4].
Conclusion
In conclusion, Dy-YSZ shows cubic and tetragonal structures, while (Dy, Ce)-YSZ exhibits the tetragonal structure. The oxygen displacement in the tetragonal lattice is increased by doping Ce4+ in contrast to the incorporation of Dy3+ cations to YSZ. The thermal conductivity of 2Dy-YSZ and 3Dy5Ce-YSZ is lower than that of 4YSZ. The CTE values of 2Dy-YSZ and 3Dy5Ce-YSZ are enhanced as compared with undoped 4YSZ. These novel tetragonal oxides show great potential in the high-temperature structural applications such as TBCs, ascribed to their lower thermal conductivity and improved CTE.