Thermodynamic Investigation of La0.8Sr0.2MnO3±δ Cathode, including the Prediction of Defect Chemistry, Electrical Conductivity and Thermo-Mechanical Properties

Authors

Shadi Darvish, Florida International UniversityFollow

Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Major/Program

Materials Science and Engineering

First Advisor's Name

Yu Zhong

First Advisor's Committee Title

Committee Chair

Second Advisor's Name

Surendra K. Saxena

Second Advisor's Committee Title

Committee Member

Third Advisor's Name

Arvind Agarwal

Third Advisor's Committee Title

Committee Member

Fourth Advisor's Name

Zhe Cheng

Fourth Advisor's Committee Title

Committee Member

Fifth Advisor's Name

Jiuhua Chen

Fifth Advisor's Committee Title

Committee Member

Sixth Advisor's Name

Jean Andrian

Sixth Advisor's Committee Title

Committee Member

Keywords

CALPHAD, Thermodynamic, SOFC, LSM cathode, Electrical Properties, Phase Stability, Thermo-mechanical Properties

Date of Defense

2-12-2018

Abstract

Fundamental thermodynamic investigations have been carried out regarding the phase equilibria of La_0.8Sr_0.2MnO_3±δ (LSM), a cathode of a solid oxide fuel cell (SOFC), utilizing the CALculation of PHAse Diagram (CALPHAD) approach. The assessed thermodynamic databases developed for LSM perovskite in contact with YSZ fluorite and the other species have been discussed. The application of computational thermodynamics to the cathode is comprehensively explained in detail, including the defect chemistry analysis as well as the quantitative Brouwer diagrams, electronic conductivity, cathode/electrolyte interface stability, thermomechanical properties of the cathode and the impact of gas impurities, such as CO₂ as well as humidity, on the phase stability of the cathode. The quantitative Brouwer diagrams for LSM at different temperatures are developed and the detailed analysis of the Mn³⁺ charge disproportionation behavior and the electronic conductivity in the temperature range of 1000-1200°C revealed a good agreement with the available experimental observations. The effects of temperature, CO₂ partial pressure, O₂ partial pressure, humidity level and the cathode composition on the formation of secondary phases have been investigated and correlated with the available experimental results found in the literature. It has been indicated that the CO₂ exposure does not change the electronic/ionic carriers’ concentration in the perovskite phase. The observed electrical conductivity drop is predicted to occur due to the formation of secondary phases such as LaZr₂O₇, SrZrO₃, SrCO₃ and Mn oxides at the LSM/YSZ interface, resulting in the blocking of the electron/ion transfer paths. For the thermo-mechanical properties of LSM, a new weight loss Mechanism for (La_0.8Sr_0.2)_0.98MnO_3±δ using the La-Sr-Mn-O thermodynamic database is modeled with respect to the compound energy formalism model. This newly proposed mechanism comprehensively explains the defect formation as a result of volume/weight change during the thermal cycles. According to the proposed mechanism the impact of cation vacancies regarding the volume change of cathode was explained.

Identifier

FIDC006589

ORCID

0000-0003-3351-6217

Recommended Citation

Darvish, Shadi, "Thermodynamic Investigation of La0.8Sr0.2MnO3±δ Cathode, including the Prediction of Defect Chemistry, Electrical Conductivity and Thermo-Mechanical Properties" (2018). FIU Electronic Theses and Dissertations. 3653.
https://digitalcommons.fiu.edu/etd/3653