Critical evaluation and thermodynamic modeling of the Mn–Cr–O system for the oxidation of SOFC interconnect

doi:10.1016/j.ssi.2006.01.012

Solid State Ionics

Volume 177, Issues 7–8, 15 March 2006, Pages 765-777

https://doi.org/10.1016/j.ssi.2006.01.012 Get rights and content

Abstract

A complete critical evaluation and thermodynamic modeling of the phase diagrams and thermodynamic properties of the Mn–Cr–O system at 1 bar total pressure are presented. Optimized equations for the thermodynamic properties of all phases are obtained, which reproduce all available and reliable thermodynamic and phase equilibrium data within experimental error limits from 25 °C to above the liquidus temperatures at all compositions and oxygen partial pressures. As results of optimization, the Gibbs energy function of MnCr₂O₄ is for the first time properly estimated and the discrepancies of the phase diagram experiments of the Mn–Cr–O system are resolved. In particular, unexplored phase diagrams and thermodynamic properties of the Mn–Cr–O system of importance for the oxidation of SOFC interconnect are predicted on the basis of the optimized model parameters. The database of the model parameters can be used along with software for Gibbs energy minimization in order to calculate any type of phase diagram sections and thermodynamic properties.

Introduction

Solid oxide fuel cell (SOFC) has been extensively studied as an efficient power generation system. Since unit cell voltage is approximately 1 V, unit cells are connected electrically in series by interconnects in SOFC stacks to generate the desired power output. There are two major candidates for the interconnect of SOFC, LaCrO₃-based ceramic and Fe–Cr ferritic heat resistance alloy. The interconnect materials have to meet very severe demands: oxygen partial pressure of air side (cathodic condition) is about 0.21 bar and that of fuel side (anodic condition) is 1.2 × 10^− 27–1.2 × 10^− 18 bar when fuel gas is H₂–H₂O gas mixture in the temperature range of 600 to 1000 °C [1].

Nowadays, alloy interconnect is attractive for commercial use of SOFC due to gas-tightness, machinability and other advantages of alloys than ceramics. Alloy interconnect can be used below 800 °C where Fe–Cr alloys show good oxidation resistance. Oxidation behavior of Fe–Cr alloys in reduction and oxidation conditions of SOFC operation atmospheres has been intensively studied. In the case of Fe–16Cr alloy with less than 0.3 mass% of Mn (SUS430) candidate, the Mn–Cr–O (+ Fe) oxides are usually observed [1], [2] after oxidation in air side. However, in spite of its importance, the fundamental researches on the phase diagram of the Mn–Cr–O system are rare and there are still certain discrepancies for the phase equilibria of the system.

The main goal of the present study is to perform a critical assessment and optimization of the thermodynamic properties and phase equilibria at 1 bar total pressure of oxide phases of the Mn–Cr–O system. In the thermodynamic “optimization” of a chemical system, all available thermodynamic and phase equilibrium data are evaluated simultaneously in order to obtain one set of model equations for the Gibbs energies of all phases as functions of temperature and composition. From these equations, all of the thermodynamic properties and the phase diagrams can be back-calculated. In this way, all the data are rendered self-consistent and consistent with thermodynamic principles. Thermodynamic property data, such as activity data, can aid in the evaluation of the phase diagram and phase diagram measurements can be used to deduce thermodynamic properties. Discrepancies in the available data can often be resolved, and interpolations and extrapolations can be made in a thermodynamically correct manner.

In the present study, the thermodynamic properties of the Mn–Cr–O system were optimized on the basis of the previously optimization of the Mn–O [3] and Cr–O [4] systems. The present optimization covers the range of oxygen partial pressures from equilibration with pure oxygen to metal saturation and temperatures from 25 °C to above the liquidus. All calculations in the present study were performed with the FactSage [5] software.

Section snippets

Phases and thermodynamic models

The calculated phase diagram of the Mn–Cr–O system at 1 bar total pressure is presented in Fig. 1. The following solution phases are found in the Mn–Cr–O system:

Cubic spinel (encompassing cubic-Mn₃O₄, MnCr₂O₄ and Cr₃O₄): (Mn²⁺,Cr²⁺,Cr³⁺)^T[Mn²⁺,Mn³⁺,Mn⁴⁺,Cr³⁺,Va]₂^OO₄
Tetragonal spinel (limited solution extended from tetragonal-Mn₃O₄): (Mn²⁺,Mn³⁺,Cr²⁺,Cr³⁺)^T[Mn²⁺,Mn³⁺,Cr³⁺,Va]₂^OO₄
Monoxide: MnO rich solution containing small amount of CrO_1.5 and MnO_1.5
Corundum: Cr₂O₃ rich solution containing small

Gibbs energy of MnCr₂O₄

The thermodynamic properties of stoichiometric MnCr₂O₄ spinel have been rarely studied. No low-temperature and high-temperature heat capacities were measured and, therefore, the entropy at 298 K (S_298.15°) is not known. No enthalpy measurements were performed and no oxygen partial pressure measurements for the three-phase equilibria like MnCr₂O₄ + Cr + MnO were conducted.

MnCr₂O₄ is known to be normal spinel with Mn²⁺ occupying the tetrahedral sites and Cr³⁺ solely occupying the octahedral sites.

Conclusions

The Mn–Cr–O system at a total pressure of 1 bar was critically evaluated and optimized based on all available and reliable thermodynamic and phase diagram data. As a result of the optimization, one set of model parameters has been obtained which reproduces all the data within experimental error limits in the temperature range from 25 °C to above the liquidus and for oxygen partial pressures up to 1 bar. Many unexplored phase diagrams for the Mn–Cr–O system are predicted by the thermodynamic

References (39)

H. Kurokawa et al.
Solid State Ion.
(2004)
C.W. Bale et al.
Calphad
(2002)
I.-H. Jung et al.
Acta Mater.
(2004)
I.-H. Jung et al.
J. Phys. Chem. Solids
(2004)
A. Naoumidis et al.
J. Eur. Ceram. Soc.
(1991)
A.D. Pelton
Calphad
(2001)
P. Papaiacovou et al.
React. Solids
(1990)
E. Pollert et al.
J. Phys. Chem. Solids
(1977)
E. Pollert et al.
Mater. Res. Bull.
(1980)
Yu.V. Golikov et al.
J. Phys. Chem. Solids
(1985)

Yu.V. Golikov et al.

J. Solid State Chem.

(1987)

P. Holba et al.

Mater. Res. Bull.

(1975)

J.-H. Jun, private communication....

Y.-B. Kang, I.-H. Jung and H.-G. Lee, in...

S. Degterov et al.

J. Phase Equilib.

(1996)

S.E. Dorris et al.

J. Am. Ceram. Soc.

(1988)

I.-H. Jung et al.

J. Am. Ceram. Soc.

(2005)

M. Hillert et al.

Z. Met.kd.

(1988)

S. Degterov et al.

Metall. Mater. Trans., B

(2001)

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Critical evaluation and thermodynamic modeling of the Mn–Cr–O system for the oxidation of SOFC interconnect

Abstract

Introduction

Section snippets

Phases and thermodynamic models

Gibbs energy of MnCr2O4

Conclusions

Solid State Ion.

Calphad

Acta Mater.

J. Phys. Chem. Solids

J. Eur. Ceram. Soc.

Calphad

React. Solids

J. Phys. Chem. Solids

Mater. Res. Bull.

J. Phys. Chem. Solids

J. Solid State Chem.

Mater. Res. Bull.

J. Phase Equilib.

J. Am. Ceram. Soc.

J. Am. Ceram. Soc.

Z. Met.kd.

Metall. Mater. Trans., B

Gibbs energy of MnCr₂O₄