Phase Transformation Of Yttria-stabilized Zirconia Research Articles

Predictions on conductivity and mechanical property evolutions of yttria-stabilized zirconia in solid oxide fuel cells based on phase-field modeling of cubic-tetragonal phase transformation

Solid oxide fuel cell (SOFC) recently emerges as a promising power production technology with high efficiency. However, the degradation of yttria-stabilized zirconia (YSZ) electrolyte, brought by the cubic-tetragonal (c-t) phase transformation, remains a critical issue. Here, the c-t phase transformation of YSZ and its influences on the SOFC are quantitively investigated. First, micro-Raman spectroscopy characterization validates the occurrence of the c-t phase transformation during long-term operation. A microelasticity phase-field model is then built to simulate the phase transformation. The effects of variant nuclei and the misorientation angle between adjacent grains are investigated first based on both single-crystal and double-crystal systems. Subsequently, the phase-field model is applied on the 3D reconstructed real microstructure of polycrystalline YSZ. The results indicate that larger misorientation angles between adjacent grains suppress the development of the variants, and thus benefit in maintaining the ionic conductivity of the electrolyte and the mechanical strength of SOFCs.

Conductivity Degradation of Transition Metal (Mn, Fe, Co) Doped Yttria Stabilized Zirconia by Phase Transformation

Conductivity degradation behavior of 1 mol% transition metal (Mn, Fe, Co) doped yttria stabilized zirconia (YSZ) under SOFC operation atmospheres was examined by AC impedance technique under an OCV condition, using a test cell consisting of dry air, Pt / transition metal (Mn, Fe, Co) doped 8YSZ / Pt, 1.2 % H2O–H2, at 1173 K for 300 h. Phase transformation of YSZ from the cubic to the tetragonal phase was analyzed by Raman spectroscopy. The phase transformation occurs significantly in the area corresponding to the transition metal oxides reduction in the YSZ lattice. Cross section analysis of the tested electrolyte revealed that color changed into dark from the anode side surface to inside the electrolyte. Micro-Raman analysis indicated the phase transformation of transition metal (Mn, Fe, Co) doped 8YSZ from the cubic phase to the tetragonal phase. We have experimentally demonstrated that the conductivity degradation of transition metal (Mn, Fe, Co) doped YSZ is closely associated to the phase transformation under SOFC operation atmospheres at 1173 K. All the transition metal (Mn, Fe, Co) doped YSZ showed a lesser conductivity degradation and a lower phase transformation than the Ni doped YSZ. The Fe doped YSZ showed the least Conductivity degradation and lowest phase transformation during the transition metal (Mn, Fe, Co, Ni) doped YSZ under SOFC operation atmospheres at 1173K.

Conductivity Degradation of Transition Metal (Mn, Fe, Co) Doped Yttria Stabilized Zirconia by Phase Transformation

Conductivity degradation behavior of 1 mol% transition metal (Mn, Fe, Co) doped yttria stabilized zirconia (YSZ) under SOFC operation atmospheres was examined by AC impedance technique under an OCV condition, using a test cell consisting of dry air, Pt / transition metal (Mn, Fe, Co) doped 8YSZ / Pt, 1.2 % H2O–H2, at 900°C for 300 h. Phase transformation of YSZ from the cubic to the tetragonal phase was analyzed by Raman spectroscopy. Micro-Raman analysis indicated the phase transformation of transition metal (Mn, Fe, Co) doped 8YSZ from the cubic phase to the tetragonal phase. We have experimentally demonstrated that the conductivity degradation of transition metal (Mn, Fe, Co) doped YSZ is closely associated to the phase transformation under SOFC operation atmospheres at 900°C. All the transition metal (Mn, Fe, Co) doped YSZ showed a lesser conductivity degradation and a lower phase transformation than the Ni doped YSZ. The Fe doped YSZ showed the least conductivity degradation and lowest phase transformation during the transition metal (Mn, Fe, Co, Ni) doped YSZ under SOFC operation atmospheres at 900°C.

Critical stress map for ZrO2 tetragonal to monoclinic phase transformation in ZrO2-toughened glass-ceramics

Inspired by recent observation of compressive stress induced tetragonal to monoclinic (t-m) phase transformation in yttria-stabilized zirconia, we conduct a systematic study to investigate the critical stress map for ZrO2 transformation under various loading conditions. The results show that (1) uniaxial tensile and compressive loadings can trigger the transformation, but not hydrostatic tensile and compressive loadings; (2) shear and equal biaxial tensile loading are most favored to trigger the transformation. Additionally, simulation on a model ZrO2-containing SiO2-based glass-ceramic shows that the stress fields around the running crack front can help triggering ZrO2 phase transformation, and consequently change the stress in ZrO2 from tensile to compressive and deflect the crack propagation. The insights obtained from these simulations can help us better understand ZrO2 transformation toughening mechanism, and further leverage it to enhance the mechanical performance of materials in many real applications.

Sintering behavior and phase transformation of YSZ-LZ composite coatings

Sintering behavior and phase transformations in yttria-stabilized zirconia (YSZ)-based thermal barrier coatings (TBCs) control their applications in gas turbines operated at high working temperatures required for improved fuel efficiency. In this work, to control the sintering behavior and reduce phase transformations in YSZ-based TBCs, lanthanum zirconate (LZ) powder was blended with the YSZ feedstock powder, and YSZ-LZ composite coatings were fabricated using the air plasma spraying method. The influence of mixture weight ratio of YSZ to LZ (75:25, 50:50, and 25:75) on the sintering behavior and phase stability of the composite coatings was investigated through the isothermal exposure test at 1100, 1300, and 1400 °C. The as-coated composites showed the pyrochlore and tetragonal phases, indicating that the phases are LZ and YSZ, respectively. As the exposure temperature was increased, the phase transformation of YSZ from the tetragonal phase to the monoclinic phase was accelerated. The content of monoclinic phase was changed with the increasing LZ content after thermal exposure at 1300 and 1400 °C. In addition, the composites showed different sintering and bridging behaviors at the adjacent splats with the LZ content. The composites prepared with the blended feedstock powders of LZ and YSZ produced an obvious effect on the phase stability and mechanical properties.

Hot Corrosion of Stabilized Zirconia Thermal Barrier Coatings and the Role of Mg Inhibitor

The 3–4 mol% yttria‐stabilized zirconia (YSZ) is widely used as a material for thermal barrier coating; however, the corrosive constituents present in fuel typically result in mechanical disintegration of YSZ coatings. The 3–4 mol% YSZ coatings with respective porosity of ~3% and ~22% have been undertaken with the objective to compare the hot corrosion behavior in air and sulfur‐rich atmospheres. The coatings are kept in contact with V2O5 + MgO powder mixture at 750°C for different dwell times of 24 and 76 h. The samples kept in air have shown intact YSZ layer for both the coatings, whereas a delamination of YSZ layer is observed for high porosity sample kept in sulfur‐rich atmosphere. XRD patterns of all the samples treated in sulfur‐rich atmosphere have indicated a phase transformation in YSZ from tetragonal to monoclinic. However, no such phase transformation has been found for samples treated in air. The V2O5‐induced hot corrosion attack on YSZ coating in air has been successfully inhibited by MgO, which forms a thermally stable Mg3V2O8 compound. However, in sulfur‐rich atmosphere, MgO is partially consumed to form sulfates, which allows certain fraction of V2O5 to react with Y2O3 causing the degradation of top coat.

Phase Transformation in Spray Pyrolysis Yttria-stabilized Zirconia Thin Films

The phase transformation of yttria-stabilized zirconia (ZrO2)0.92(Y2O3)0.08 (8YSZ) thin films used as an electrolyte layer in micro-solid oxide fuel cells (µ-SOFCs) is investigated. Amorphous 8YSZ thin films are deposited by spray pyrolysis and their crystal symmetry, phase transformation and pore formation during heat treatment are analyzed. Both, microstructure and phase transformation can affect the thin film properties such as electrical conductivity and therefore the overall performance of the µ-SOFC.

Influence of Y2O3 distribution on the rate of tetragonal to monoclinic phase transformation of yttria-stabilized zirconia during hydrothermal aging

Tetragonal to monoclinic phase transformation of yttria-stabilized zirconia, namely plasma-sprayed coatings and sintered bodies containing 4–8 mass % Y2O3 during hydrothermal aging was investigated with respect to Y2O3 distribution using 1 μmφ area from electron probe microanalysis (EPMA) and 20 nmφ area from transmission electron microscopy (TEM) analysis. Phase transformation at 473 K was prevented only in plasma-sprayed coatings having more than 6.7 mass % Y2O3 in 20 nmφ microscopic area. Furthermore, it was confirmed influence of Y2O3 distribution on the rate constants of this phase transformation was observed at 368 K.

Phase Transformation of Yttria-Stabilized Zirconia (3 mol% Y<SUB>2</SUB>O<SUB>3</SUB>) in Fuel Cell Caused by Manganese Diffusion

The diffusion coefficient of Mn in yttria-stabilized zirconia (YSZ, 3 mol%Y 2 O 3 ) and the structural change in YSZ have been investigated. The diffusion coefficients of Mn were determined to be 7 x 10 -14 m 2 /s at 1573 K and 2 x 10 -13 m 2 / at 1673 K by analyzing the concentration profiles of Mn in YSZ/Mn 3 O 4 diffusion couples. The solubilities of MnO x were 7.4 mol% and 7.6 mol% at 1573 and 1673 K, respectively. Remarkable grain growth was observed on the YSZ side near the interface. The dissolution of MnO x almost up to the solubility limit of 7.6 mol% resulted in grain growth and the phase transformation from the tetragonal phase to the fluorite-type cubic phase. These results were discussed by comparing with the previous studies on the ZrO 2 -Y 2 O 3 -MnO x ternary system and on the ZrO 2 -MnO binary system.