Temperature \textdegree C 3000 Liquid (L) 2500- 2000- T+F Trans- form- able 1500 letra- gonal (T) 1000 Cubic (F) YSZ L+F 1200 1000 0 1600\textdegree C Tie Line 1400\textdegree C Tie Line 500 Mono- clinic M+F (M) F Monoclinic Nontransformable Tetragonal (T) Cubic 10 15 20 Mole % YO
-1- Temperature (°C) 800 La
Sr
Ga
MgO
(Bi
O
) (Y
O
)
BaTh
Gd
O
(ZrO
)(Y
O
)
La
CaAlO
600 (Ce
O
)(Gd
O
)
(ZrO
)(Sc
O
)
Log
(S cm
) -2- -3 0.6 0.8 10
T
(K
) 1.0 Singhal, High-temperature Solid Oxide Fuel Cells, Elsevier (2004) 1.2 (ZrO
)(Y
O
)
(ZrO
)(CaO)
Temperature \textdegree C 3000 Liquid (L) 2500- 2000- T+F Trans- form- able 1500 letra- gonal (T) 1000 Cubic (F) YSZ L+F 1200 1000 0 1600\textdegree C Tie Line 1400\textdegree C Tie Line 500 Mono- clinic M+F (M) F Monoclinic Nontransformable Tetragonal (T) Cubic 10 15 20 Mole % YO
-1- Temperature (°C) 800 La
Sr
Ga
MgO
(Bi
O
) (Y
O
)
BaTh
Gd
O
(ZrO
)(Y
O
)
La
CaAlO
600 (Ce
O
)(Gd
O
)
(ZrO
)(Sc
O
)
Log
(S cm
) -2- -3 0.6 0.8 10
T
(K
) 1.0 Singhal, High-temperature Solid Oxide Fuel Cells, Elsevier (2004) 1.2 (ZrO
)(Y
O
)
(ZrO
)(CaO)
The Correct Answer and Explanation is:
The image you’ve described appears to combine two key phase and conductivity diagrams related to yttria-stabilized zirconia (YSZ) and other solid oxide fuel cell (SOFC) electrolytes, from the reference Singhal, High-Temperature Solid Oxide Fuel Cells, Elsevier (2004). Let’s break this into two main sections to provide the correct interpretation and an explanation.
✅ Correct Interpretation:
- First Diagram – YSZ Phase Diagram:
- This shows the phase stability of ZrO₂ doped with Y₂O₃ (yttria) versus temperature and mole % YO₁.₅ (i.e., Y₂O₃).
- Key phases:
- M: Monoclinic (unstable at high T)
- T: Tetragonal (can be transformable or non-transformable)
- F: Cubic fluorite structure (high temperature stable)
- L: Liquid
- The 15 mole % YO₁.₅ region is the basis for 8 mol% YSZ, where cubic phase is stable, used in SOFC electrolytes.
- The 1400°C and 1600°C tie lines indicate phase coexistence at different compositions.
- Second Diagram – Ionic Conductivity vs. Temperature (Arrhenius Plot):
- Log conductivity (S/cm) vs 1/T (K⁻¹) for various doped zirconia and ceria systems:
- (ZrO₂)(Y₂O₃): YSZ
- (ZrO₂)(Sc₂O₃): ScSZ (scandia-stabilized zirconia)
- (CeO₂)(Gd₂O₃): GDC (gadolinia-doped ceria)
- ScSZ and GDC show higher conductivity than YSZ, especially at lower temperatures (600–800°C).
- YSZ remains dominant at very high temperatures (800–1000°C) due to phase stability and mechanical robustness.
- Log conductivity (S/cm) vs 1/T (K⁻¹) for various doped zirconia and ceria systems:
Explanation:
Yttria-stabilized zirconia (YSZ) is a crucial ceramic used as the electrolyte in high-temperature solid oxide fuel cells (SOFCs). The phase diagram in the first figure plots temperature versus mole % YO₁.₅ (yttria), showing the stability regions of various zirconia phases: monoclinic (M), tetragonal (T), cubic fluorite (F), and liquid (L). Pure zirconia (ZrO₂) is monoclinic at room temperature but transforms to tetragonal and then cubic phases at elevated temperatures. By doping ZrO₂ with yttria, the high-temperature cubic phase can be stabilized down to room temperature. Typically, 8 mol% Y₂O₃ (equivalent to 15 mole % YO₁.₅) is used to create fully stabilized cubic YSZ. This region offers excellent oxygen ion conductivity and mechanical stability at SOFC operating temperatures (~800–1000°C).
The second diagram presents the ionic conductivity behavior of various doped zirconia and ceria systems as a function of temperature (Arrhenius plot). YSZ exhibits high ionic conductivity at high temperatures but is outperformed by scandia-stabilized zirconia (ScSZ) and gadolinia-doped ceria (GDC) at intermediate temperatures. However, YSZ remains the preferred material due to its superior long-term chemical and mechanical stability under SOFC conditions. GDC, while more conductive at lower temperatures, suffers from reduction under fuel-rich conditions, limiting its use as a sole electrolyte.
Together, these diagrams highlight the trade-off between ionic conductivity and phase stability. YSZ offers an optimal balance for high-temperature applications, whereas alternative materials are being explored for intermediate-temperature SOFCs to improve performance and durability.
