Chemical Expansivity in Ceramic Oxygen Transport Materials

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CHEMICAL EXPANSIVITY IN CERAMIC

OXYGEN TRANSPORT MATERIALS

ANDREW CAI

Submitted in partial fulfillment of the requirements for the degree of

Master of Science

Department of Materials Science and Engineering

CASE WESTERN RESERVE UNIVERSITY

August, 2020

CASE WESTERN RESERVE UNIVERSITY SCHOOL OF GRADUATE STUDIES

We hereby approve the thesis of Andrew Cai

Candidate for the degree of Master of Science*.

Committee Chair Professor Mark R. De Guire

Committee Member Professor Alp Sehirlioglu

Committee Member Professor K. Peter D. Lagerlöf

Committee Member Doctor Hoda Amani Hamedani

Date of Defense July 2, 2020

*We also certify that written approval has been obtained for any proprietary material contained therein. ACKNOWLEDGEMENTS

I would like to thank my advisor, Professor Mark R. De Guire for his professional guidance and assistance. I would like to also thank the American Chemical Society

Petroleum Research Fund for funding this project, as well as Doctor Ajit Sane of Volt

Research LLC for extensive guidance on experimental work and OTM applications.

Furthermore, I would like to thank my project co-workers from Case Western Reserve

University, John Bradley, Sipei Li, and Mirko Antloga. I would like to personally thank

Doctor John Gibbons, Chenxin Deng, and Doctor Benjamin Palmer from Case Western

Reserve University, Sergio Medina from the Edward Orton Jr Ceramic Foundation, Eileen

De Guire from the American Ceramic Society for their additional assistance. Moreover, I would like to thank Doctor Hisao Yamada, Cerone Inc., for donating several pieces of ceramic processing equipment that were invaluable to the materials synthesis aspects of this project. Finally, I would like to thank my respected committee members, Professor Alp

Sehirlioglu, Professor K. Peter D. Lagerlöf, and Doctor Hoda Amani Hamedani.

Table of Contents

Table of Contents ...... i

List of Figures ...... v

List of Appendix Figures...... ix

List of Tables ...... x

List of Appendix Tables ...... xii

Abstract ...... xiii

Chapter 1: Literature Review ...... 1

1.1. Introduction ...... 1

1.1.1. Technical background ...... 1

1.1.2. Project objectives ...... 4

1.2. Literature review ...... 5

1.2.1. Perovskite oxides ...... 5

1.2.2. Mixed electronic-ionic conducting oxygen transport membranes ..... 7

1.2.3. Lanthanum strontium ferrite-based membranes ...... 9

1.2.4. Chemical expansion studies ...... 11

1.2.5. Dilatometric measurements of oxygen transport membranes ...... 12

Chapter 2: Experimental Procedures ...... 14

2.1. Dilatometer calibration ...... 14

2.1.1. Alumina reference sample calibration run ...... 14

2.1.2. Experimental runs on alumina reference sample deviation ...... 14

2.2. Purchased and synthesized powders for study ...... 15

2.2.1. Compositions ...... 15

2.2.2. Teraoka amorphous malic acid precursor method ...... 16

2.3. Sample preparation ...... 18

2.3.1. Pellet pressing...... 18

2.3.2. Sintering ...... 19

2.4. Materials characterization: XRD, density, and SEM/EDS ...... 19

2.5. Dilatometer operation and atmosphere control ...... 21

2.5.1. Gas setup ...... 21

2.5.2. Oxygen step changes ...... 22

2.6. Data acquisition and reporting ...... 23

2.7. Calculations ...... 24

2.7.1. Chemical linear strain ...... 24

2.7.2. Coefficient of chemical expansion ...... 25

Chapter 3: Results ...... 26

3.1. Density ...... 26

3.2. X-ray diffractometry ...... 29

3.2.1. (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1) ...... 30

3.2.2. (La0.80Sr0.20)0.95 FeO3-δ (FCM2) ...... 31

3.2.3. (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) ...... 33

3.2.4. (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2) ...... 34

3.2.5. (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1) ...... 35

3.2.6. (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2) ...... 37

3.2.7. (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) ...... 38

3.2.8. (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) ...... 39

3.3. Scanning electron microscopy ...... 41

3.4. Energy dispersive x-ray spectroscopy ...... 50

3.5. Linear strain versus oxygen step changes (800 °C, 900 °C, 1000 °C) ...... 53

3.5.1. (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1) ...... 53

3.5.2. (La0.80Sr0.20)0.95 FeO3-δ (FCM2) ...... 54

3.5.3. (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) ...... 54

3.5.4. (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2) ...... 55

3.5.5. (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1) ...... 55

3.5.6. (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2) ...... 56

3.5.7. (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) ...... 56

3.5.8. (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) ...... 57

3.6. Coefficient of chemical expansion ...... 58

3.7. Summary tables ...... 59

Chapter 4: Discussion of Results ...... 63

4.1. Densification and sintering effects ...... 63

4.2. X-ray diffractometry ...... 64

4.3. Scanning electron microscopy ...... 69

4.4. Energy dispersive x-ray spectroscopy ...... 70

4.5. Effect of pO2 and temperature on chemical linear strain ...... 71

4.6. Coefficient of chemical expansion ...... 79

4.7. Reproducibility ...... 79

Chapter 5: Conclusion ...... 81

Chapter 6: Future Work ...... 83

6.1. Scanning electron microscopy ...... 83

6.2. Elemental analysis ...... 83

6.3. Reproducibility ...... 83

iii

6.4. Oxygen diffusion coefficient and flux measurements ...... 84

6.5. Porous-dense-porous architecture ...... 84

6.6. Ruddlesden-Popper perovskites...... 85

Appendix A: Procedures ...... 86

A.1. Dilatometer calibration ...... 86

A.2. Alumina reference sample deviation ...... 87

A.3. Starting powder weights ...... 89

A.4. Amorphous malic acid precursor powder synthesis ...... 90

A.5. Pellet pressing ...... 92

A.6. Archimedes density technique ...... 92

A.7. Experimental testing in dilatometer...... 93

A.8. pO2 calculations and gas mixtures ...... 93

A.9. Unit-cell density calculations ...... 95

A.10. Elemental analysis calculations ...... 97

A.11. Coefficient of thermal expansion...... 98

Appendix B: Supplemental Additions ...... 105

B.1. Defect chemistry ...... 105

B.2. Phase diagrams ...... 106

B.3. Reproducibility ...... 108

Appendix C: Glossary of Terms ...... 109

Works Cited ...... 110

List of Figures

Figure 1.1: a) A uniformly dense ceramic membrane exposed to a fuel-air pO2 gradient. The gradient-related stress appears entirely across the dense membrane. b) A porous/dense/porous design reduces the pO2 gradient, and therefore the stress, across the dense layer ...... 3

Figure 1.2: a) Schematic of perovskite structure b) Oxygen transport mechanism through the perovskite Membrane...... 5

Figure 1.3: Thermo-chemical expansion coefficient predication from data in air as a function of temperature and oxygen non-stoichiometry (Left) and chemical strain as a function of oxygen content with pO2 = 0.21 atm (air) as reference point for zero expansion. Republished with permission of John Wiley & Sons – Books, from Bishop et al. [ref. 33]; permission conveyed through Copyright Clearance Center, Inc...... 12

Figure 2.1: Percent linear change versus dilatometer time data in the Orton software...... 14

Figure 2.2: Drying of La0.50Sr0.50Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) with AMP route. Left: Early stage of drying. Right: Final stage of drying ...... 17

Figure 2.3: Box furnace used for calcination ...... 18

Figure 2.4: Dimensions of experimental sintered pellets ...... 18

Figure 2.5: MoSi2 furnace used for firing of green pellets ...... 19

Figure 2.6: Orton 1600D Dilatometer, showing the components of the atmosphere-control system ...... 21

Figure 2.7: Percent linear change (PLC) versus time (min) data from Orton 1600D dilatometer experimental runs in Python 3.7 ...... 24

Figure 3.1: XRD patterns of (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1). Top: As-received; peaks indexed for cubic LaCo0.4Fe0.6O3 (PDF 40-0224, red), rhombohedral La0.33Sr0.67Fe3O8.94 (PDF 80-1037, blue), and rhombohedral La0.3Sr0.7FeO3 (PDF 82-1964, green). Bottom: Sintered + tested; peaks indexed for cubic LaCo0.4Fe0.6O3 (PDF 40-0224, red) and rhombohedral La0.4Sr0.6FeO3 (PDF 82-1963, green) ...... 30

Figure 3.2: XRD patterns of (La0.80Sr0.20)0.95 FeO3-δ (FCM2). Top: As-received; peaks indexed for cubic LaFeO3 (PDF 75-0541, red), orthorhombic La0.7Sr0.3FeO3 (PDF 89-1269, blue), and orthorhombic La0.6Sr0.4FeO3 (PDF 40-0285, green). Bottom: Sintered + tested; peaks indexed for cubic LaFeO3 (PDF 75-0541, red), orthorhombic La0.8Sr0.2FeO3 (PDF 35-1480, blue) and orthorhombic LaFeO3 (PDF 88-0641, green) ...... 32

Figure 3.3: XRD patterns of La0.20Sr0.80Cr0.20Fe0.80O3-δ (PRAX1). Top: As-received; peaks indexed for cubic LaCrO3 (PDF 75-0288, red), rhombohedral La0.5Sr0.5FeO3 (PDF 82-1962, blue), and rhombohedral LaCrO3 (PDF 33-0702, green). Bottom: Sintered + tested; peaks indexed for cubic LaCrO3 (PDF 75-0288, red), orthorhombic La0.7Sr0.3Co0.3Fe0.7O3 (PDF 89- 1268, blue), and rhombohedral La0.6Sr0.4FeO3 (PDF 82-1261, green) ...... 33

Figure 3.4: XRD patterns of (La0.20Sr0.80) Co0.10Cr0.20Fe0.70O3-δ (PRAX2). Top: As-received; peaks indexed for cubic La0.8Sr0.2CrO3 (PDF 74-1980, red), SrCO3 (PDF 82-1962, blue) and rhombohedral La0.6Sr0.4Co0.4Fe0.6O3 (PDF 49-0284, green). Bottom: Sintered + tested; peaks indexed for cubic LaCrO3 (PDF 75-0288, red), orthorhombic La0.7Sr0.3Co0.3Fe0.7O3 (PDF 89- 1268, blue), and rhombohedral La0.6Sr0.4FeO3 (PDF 82-1961, green) ...... 35

Figure 3.5: XRD patterns of La0.50Sr0.50Cr0.20Fe0.80O3-δ (CWRU1). Top: As-calcined; peaks indexed for cubic LaCr1.01O3 (PDF 44-0333, red), SrCO3 (PDF 84-1778, blue), and rhombohedral La0.4Sr0.6FeO3 (PDF 82-1963, green). Bottom: Sintered + tested; with peaks indexed for orthorhombic La0.7Sr0.3FeO3 (PDF 89-1269, red), orthorhombic LaCrO3 (PDF 83-0256, blue), and rhombohedral La0.6Sr0.4FeO3 (PDF 49-0285, green) ...... 36

Figure 3.6: XRD patterns of (La0.20Sr0.80) Co0.10Cr0.10Fe0.80O3-δ (CWRU2). Top: As-calcined; peaks indexed for cubic LaCo0.4Fe0.6O3 (PDF 40-0224, red), SrCO3 (PDF 84-1778, blue), and cubic LaCrO3 (PDF 74-1961, green). Bottom: Sintered + tested; peaks indexed for cubic LaCrO3 (PDF 75-0288, red), orthorhombic La0.7Sr0.3Co0.3Fe0.7O3 (PDF 89-1268, blue), and rhombohedral La0.6Sr0.4FeO3 (PDF 82-1961, green) ...... 37

Figure 3.7: XRD (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ as-calcined (top) with indexing peaks for cubic LaCo0.4Fe0.6O3 (PDF 40-0224, red), SrCO3 (PDF 74-1491, blue), and rhombohedral La0.3Sr0.7FeO3 (PDF 82-1964, green). Bottom: Sintered + tested; peaks indexed for cubic LaFeO3 (PDF 75-0541, red), orthorhombic LaFeO3 (PDF 88-0641, blue), and orthorhombic La0.3Sr0.7FeO3 (PDF 82-1269, green) ...... 38

Figure 3.8: XRD patterns of (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4). Top: As-calcined; peaks indexed for cubic LaCrO3 (PDF 75-0288, red), SrCO3 (PDF 84-1778, blue), and rhombohedral La0.6Sr0.4FeO3 (PDF 49-0285, green). Bottom: Sintered + tested; peaks indexed for LaCrO3 (PDF 75-0288, red), orthorhombic La0.7Sr0.3FeO3 (PDF 89-1269, blue), and rhombohedral La0.6Sr0.4FeO3 (PDF 49-0285, green) ...... 39

Figure 3.9: SEM images of pellets prior to measurements of thermal and chemical expansion ...... 43

Figure 3.10: SEM images of pellets after measurements of thermal and chemical expansion.. 45

Figure 3.11: EDS analysis of sintered pellets ...... 51

Figure 3.12: Chemical strain versus log pO2 of (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1). Left: Specimens pressed without binder. Right: Specimens pressed with methyl cellulose binder, except for the 1,000 °C specimen (pressed without binder)...... 53

Figure 3.13: Chemical strain versus log pO2 of (La0.80Sr0.20)0.95 FeO3-δ (FCM2). Left: Specimens pressed without binder. Right: Specimens pressed with methyl cellulose binder...... 54

Figure 3.14: Chemical strain versus log pO2 of (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) ...... 54

Figure 3.15: Chemical strain versus log pO2 of (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2). Left: Specimens pressed without binder. Right: Specimens pressed with methyl cellulose binder ...... 55

Figure 3.16: Chemical strain versus log pO2 of (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1) ...... 55

Figure 3.17: Chemical strain versus log pO2 of (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2) .. 56

Figure 3.18: Chemical strain versus log pO2 of (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3). Left: Specimens pressed without binder. Right: Specimens pressed with methyl cellulose binder ...... 56

Figure 3.19: Chemical strain versus log pO2 of (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) .. 57

Figure 4.1: Properties of La0.6Sr0.4Co0.2Fe0.8O3-δ relevant to OTM applications: a) electronic conductivity b) oxygen permeation flux c) ionic conductivity as a function of temperature and sintering temperature. Republished with permission of Elsevier Science & Technology Journals, from Zeng et al. [ref. 43]; permission conveyed through Copyright Clearance Center, Inc...... 64

Figure 4.2: Isothermal chemical expansion as a function of pO2 at different temperatures using pO2 = 0.21 atm (air) as a zero expansion reference point. Republished with permission of John Wiley & Sons – Books, from Bishop et al. [ref. 33]; permission conveyed through Copyright Clearance Center, Inc...... 72

Figure 4.3: Linear strain versus Log pO2 of experimental compositions at 800 °C ...... 73

Figure 4.4: Linear strain versus Log pO2 of experimental compositions at 900 °C ...... 73

Figure 4.5: Linear strain versus Log pO2 of experimental compositions at 1000 °C ...... 74

Figure 4.6: Lattice constants in (La0.6Sr0.4)1-zCo0.2Fe0.8O3-δ as a function of A-Site deficiency ...... 76

Figure 4.7: Reproducibility of experimental compositions. Secondary measurements labeled (2) used methyl cellulose as a binder during the powder processing steps except for FCM1 1,000 °C (2) ...... 80

Figure 6.1: Apparatus to measure oxygen flux through OTM disk ...... 85

vii

List of Appendix Figures

Appendix A.1: Figure A.1: Orton software menu item locations ...... 87

Appendix A.1: Figure A.2: Orton software calibration display ...... 87

Appendix A.2: Figure A.3: Alumina reference sample tested at 800 °C under experimental atmospheres ...... 89

Appendix A.11: Figure A.4: Average PLC (~40 data points) at each pO2 level between 797 °C and 803 °C for (La0.80Sr0.20)0.95 FeO3-δ ...... 99

Appendix A.11: Figure A.5: PLC in gas mixture C at 800±3 °C for (La0.80Sr0.20)0.95FeO3-δ (FCM2) ...... 100

Appendix A.11: Figure A.6: PLC in gas mixture C at 900±3 °C for (La0.80Sr0.20)0.95FeO3-δ (FCM2) ...... 100

Appendix A.11: Figure A.7: PLC in gas mixture C at 1,000±3 °C for (La0.80Sr0.20)0.95FeO3-δ (FCM2) ...... 101

Appendix B.2: Figure B.1: Phase diagram of SrO-Fe2O3-La2O3. Republished with permission of American Ceramic Society, from Gavrilova et al. [ref. 38]...... 106

Appendix B.2: Figure B.2: Phase diagram of SrO-Fe2O3-La2O3. Republished with permission of American Ceramic Society, from Fossdal et al. [ref. 39] ...... 106

Appendix B.2: Figure B.3: SrFeO3-δ - LaFeO3 phase diagram. Republished with permission of American Ceramic Society, from Sasamoto et al. [ref. 68] ...... 107

Appendix B.2: Figure B.4: SrO-Cr2O3-La2O3. Republished with permission of American Ceramic Society, from Yokokawa et al. [ref. 50] ...... 107

Appendix B.3: Figure B.5: Reproducibility runs of FCM2 and PRAX2. Secondary measurements labeled (2) used methyl cellulose as binder...... 108

viii

List of Tables

Table 1.1: Chemical expansion coefficients for similar perovskite oxides as a function of chemical strain and change in oxygen non-stoichiometry. * testing was done in δ = 0.05 .. 12

Table 2.1: Deviation of the coefficients of thermal expansion (CTE) in the Orton alumina reference sample ...... 15

Table 2.2: Base chemical formula: LaxSr1-x(FeaCrbMgcCod)O3-δ, where a + b + c + d = 0.99 to 1.00...... 16

Table 2.3: Gas mixtures and respective calculated partial pressure of oxygen levels ...... 23

Table 3.1: Densities of pellets of the indicated compositions, before dilatometer measurements at the indicated temperature. Asterisks (*) indicate duplicate measurements on different pellets of the same composition and test temperature ...... 27

Table 3.2: Oxygen nonstoichiometry for La0.20Sr0.80Cr0.20Fe0.80O3-δ as a function of temperature and pO2 ...... 58

Table 3.3: Chemical expansion coefficients of experimental compositions ...... 58

Table 3.4: Average values of immersion density and % unit-cell density ...... 59

Table 3.5: Identified perovskite crystal structures of experimental compositions from XRD . 59

Table 3.6: Summary of pre-test and post-test SEM results ...... 60

Table 3.7: Summary of EDS results: deviations of elemental compositions from nominal values ...... 60

Table 3.8: Experimental chemical strain measurements (10–3) as a function of pO2 at 800 °C ...... 61

Table 3.9: Experimental chemical strain measurements (10–3) as a function of pO2 at 900 °C ...... 61

–3 Table 3.10: Experimental chemical strain measurements (10 ) as a function of pO2 at 1,000 °C ...... 62

Table 3.11: Coefficients of thermal (air to mixture D) and chemical expansion of experimental compositions 800-1,000 °C ...... 62

Table 4.1: Thermal decomposition temperatures of potential experimental cation carbonates ...... 66

Table 4.2: Ideal experimental compositions with their chemical strains at lowest pO2 ...... 75

Table 4.3: Reduction potentials of experimental B-Site cations ...... 77

List of Appendix Tables

Appendix A.8: Table A.1: Calculated ∆G° values for 2CO (g) + O2 (g) ↔ 2CO2 (g) as a function of temperature ...... 94

Appendix A.8: Table A.2: Calculated pO2 levels of gas mixtures as a function of Temperature ...... 94

Appendix A.8: Table A.3: Oxygen impurity in CO2 tank versus variation in pO2 at 1000 °C ...... 95

Appendix A.9: Table A.4: XRD volume and unit-cell data...... 96

Appendix A.10: Table A.5: Cation atomic fractions: Nominal versus EDS (single run analyses) ...... 97

Appendix A.11: Table A.6: Coefficients of linear thermal expansion ( ) of the studied materials as a function of temperature and atmosphere. All 𝑇𝑇1−𝑇𝑇2 values of CTE include experimental uncertainty (± one𝛼𝛼 standard deviation), are in units of 10–6 [°C–1] and were measured in the given atmospheres* between the temperatures T1 and T2 (in °C) indicated in the subscripts ...... 103

Chemical Expansivity in Ceramic Oxygen Transport Materials

Abstract

ANDREW CAI

Chemical expansion of eight lanthanum strontium ferrite (LSF)-based oxygen transport materials (four purchased, four synthesized) was assessed isothermally at 800 °C,

-0.678 -20.3 -15.9 900 °C, and 1,000 °C at oxygen potentials from 10 atm (air) to 10 –10 atm

(depending on temperature). The following compositions were tested:

(La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ, (La0.80Sr0.20)0.95 FeO3-δ, (La0.20Sr0.80) Cr0.20Fe0.80O3-δ,

(La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ, (La0.50Sr0.50)Cr0.20Fe0.80O3-δ, (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ,

(La0.20Sr0.80)Co0.10Cr0.10Mg0.05Fe0.75O3-δ, and (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ. Powder synthesis

utilized aqueous nitrate solutions with malic acid added as a complexing agent. These

exhibited good agreement with their target compositions. All eight materials sintered to

≥95% density in 16 hours at 1250–1350 °C. At the lowest partial pressure of oxygen (10-20.3,

10-17.9, 10-15.9) at their respective isothermal temperatures (800 °C, 900 °C, 1000 °C), three

compositions demonstrated the lowest chemical expansion: (La0.60Sr0.40)0.995Co0.20Fe0.80O3-δ,

(La0.20Sr0.80) Cr0.20Fe0.80O3-δ, and (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ. Among these materials,

(La0.60Sr0.40)0.995Co0.20Fe0.80O3-δ had a higher coefficient of chemical expansion compared to

(La0.20Sr0.80) Cr0.20Fe0.80O3-δ, which is attributed to chromium being less reducible and therefore causing less lattice distortion compared to cobalt. It was concluded that

(La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ had low chemical expansion due to the A-site deficiency, and both (La0.20Sr0.80) Cr0.20Fe0.80O3-δ and (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ due to their less

reducible chromium and magnesium content.

xii

Chapter 1: Literature Review

1.1. Introduction

1.1.1. Technical background

Oil and natural gas are used for transportation and for heating, or as fuel for

conventional power plants. Natural gas generates less CO2 per unit of energy released than

any other carbon-based fuel. Converting natural gas to syngas (a mixture of hydrogen and carbon monoxide), as opposed to just burning it, can generate hydrogen for zero-emission power. Syngas is also a first step in a path toward synthesizing higher-value hydrocarbons.

As byproducts of the oil extraction process, methane and other natural gases may be released in large volumes. If the wellhead is not equipped to capture the gas, it is often either just burned (“flared”) or released into the atmosphere. Converting natural gas to syngas is economical only at very large scales. If the pipeline infrastructure needed to aggregate gas from multiple sources at a large-scale reforming plant (with oxygen cryogenically separated from air) is not in place, the gas is flared. In 2012, natural gas equivalent to 3.5% of the world’s gas supply was simply flared. In 2017, 141 billion cubic meters of natural gas was flared (worth about $25 billion).1,2 Flaring is a significant source of carbon dioxide emissions,

and the energy of combustion is simply wasted.

Portable and reliable methane reformers based on mixed electronic-ionic conducting

(MEIC) ceramic oxygen transport membranes (OTMs) would allow the natural gas to be

instead converted to syngas at wellheads with the potential to be converted to higher-value

hydrocrabons or hydrogen fuel. Because the OTM-based reformer separates pure oxygen

from air directly, and can be scaled to handle the output from individual wells, reforming can

happen in the field, without the need for additional pipelines and a cryogenic oxygen plant.

Figure 1.1a illustrates the operating principle of an OTM reformer using MEIC

ceramics. The oxygen in air combines with the electrons in the membranes to form

negatively charged oxygen ions. The driving force for the diffusion of oxygen is the

differential in partial pressures of oxygen (pO2) across the ceramic. In an OTM-based natural

gas reformer, the pO2 difference results from having air on one side of the membrane and

natural gas on the other side. 3,4 The oxygen gradient drives oxygen ions from the air side to

the natural gas side without having to apply electric power, while (at elevated temperature)

the mixed ionic and electronic conductivity of the ceramic allows the necessary transport of

electrons and oxygen ions, in effect creating a short circuit within the ceramic. Charge

neutrality in the overall system is maintained by a flux of electrons to offset the flux of

oxygen ions in the opposite direction.5

One problem with these membranes is that gradients of pO2 necessary for

conversion reactions create stress gradients that can fracture the membrane.6 Not only does

this render the device useless; the resulting sudden direct contact of air and methane at high

temperature can cause an explosion.

A way to fix this is to introduce porous layers of the same MEIC ceramic on both sides of the primary membrane. Such an approach has been used in industrial research (e.g. reference 7). A patent7 by Sane and Cable emphasized the potential of this architecture to

reduce stress on the membrane. The porous layers lower the overall stiffness of the ceramic,

making the membrane experience lower stresses. The gradient in the partial pressure of

oxygen is also reduced in the dense membrane, further reducing the stress in the dense layer.

However, the difference in pressures must still be large enough to create an oxygen flux of at

least 10 sccm/cm2, a benchmark that will provide an adequate rate of reforming of the

natural gas.8 The lifetime goal for these membranes is 40,000 hours or more.8

Chemical expansivity (the volume change from a change in oxygen vacancy

concentration) is the cause of the stress gradients that occur in the presence of a pO2

10 gradient. Adler’s group measured the chemical expansivity in La0.60Sr0.40Co0.20Fe0.80O3-δ. This

group was able to measure chemical strain as a function of temperature and oxygen vacancy

concentration which came out linear. They observed the chemical strain increased as the

temperature increased and as the oxygen vacancy concentration increased. The slope of this

graph yielded a chemical expansion coefficient of 0.11. This coefficient will be explained in

§4.6.

Other than mechanical stability, the porous layers can increase the surface area to

expose the reacting gases to catalysts for the oxygen reduction reaction on the air side and

for the methane oxidation reaction on the natural gas side (Figure 1.1a).7–9,11

Overall, specific ceramic OTM materials that have sufficient oxygen flux struggle

with chemical stability when exposed to reducing environments. Weaker bonds between

metal ions and oxygen ions allow for better oxygen flow, but this allows metal cations to be

reduced to a lower valence at high temperatures and low pO2, leading to formation of

different phases and corresponding phase transformation stresses. The result is a need for trade-offs between oxygen diffusivity, phase stability, and mechanical stability.

There are other applications for MEIC OTMs besides natural gas reforming. Oxygen gas is one of the most used chemicals in the world. More than 100 million tons per year is produced (separated from air). Oxygen gas can be used in oxy-fuel combustion processes, oxygen-blown gasification conversion of natural gas, and conversion of coal and natural gas to syngas (a fuel gas mixture consisting mainly of hydrogen and carbon monoxide). These applications differ in their atmospheres on the output side, and different carbon monoxide and carbon dioxide levels as well. For example, in the process of producing oxygen as the final product, the pO2 goes from 0.21 to 1 atm. For a natural gas reformer, the pO2 goes from 0.21 to 10-15 atm or lower. All of these differences place different demands on the

ceramic.3,12

1.1.2. Project objectives

The objective of this research is to expose perovskite OTM materials based on

(La,Sr)FeO3 (LSF) to changes in partial pressures of oxygen at elevated temperature, in

ranges relevant to OTM reformer applications. During these exposures, chemical expansions

will be measured. X-ray diffraction (XRD) and scanning electron microscopy (SEM) with

energy-dispersive x-ray spectroscopy (EDS) will be used to determine the crystalline phases

and chemical compositions of the materials before and after these exposures. The chosen

compositions will allow the effect of cation substitutions (Co, Cr, Mg) on thermal and

chemical expansion in the LSF materials to be explored.

1.2. Literature review

1.2.1. Perovskite oxides

Materials that have been considered for ceramic OTMs include fluorites,

brownmillerites, pyrochlores, and Ruddlesden-Popper series, but perovskites have been the

5 most frequently studied. Perovskites have an ABX3 chemical formula where A and B are

cations. In the unit cell (Fig. 1.2) the larger cation (A) occupies the corners, cation B occupies the center, and the anion X (O2– in oxide perovskites) occupies the centers of the

faces. This arrangement gives the A cation a coordination number of 12. The B cation has a

coordination number of 6, filling one-fourth of the octahedral sites formed by the X anions.

Within these constraints, perovskites can exhibit cubic (Fig. 1.2), tetragonal, orthorhombic, or rhombohedral symmetry.13 Most perovskite oxide OTMs have alkaline earth and

lanthanide ions on the A-site and transition metal ions on the B-site. Figure 1.2 also

illustrates an oxygen vacancy diffusion path in a perovskite oxide for OTM applications.

Oxygen migration will always involve crossing through a triangle of cations (two A cations

on adjacent corners, and the B cation in the center of a unit cell).14

Figure 1.2: Schematic of perovskite structure with oxygen vacancy path.

The oxygen vacancy concentrations and oxide ion mobility in oxide perovskites can be high enough at temperatures greater than 700 °C to give good ionic conductivity. The oxygen transport rate k follows Arrhenius behavior (ln k = -(Ea/R)(1/T) + ln A) where R is the universal gas constant, T is the absolute temperature, and A is the pre-exponential factor.

The activation energy Ea depends on the bond energy between the metal cations and oxygen anions and on the size of the opening between the A-sites and B-site through which the

5 2 oxygen ions must pass. The dependence can be seen in Coulomb’s Law (F = ke (q1q2)/r ) where ke is the Coulomb’s law constant. The force of attraction or repulsion F varies with charges q1 and q2 and the distance r between the charges. The force (and bond energy) gets stronger with increasing charge on the cations and decreasing cationic radius (assuming constant radius and charge of the oxygen ion).

From studies by Teraoka et al.15 at 800 °C, ionic conductivity increases two and a half orders of magnitude when Sr replaced La in La1-xSrxCo1-yFeyO3-δ, and decreases by an order of magnitude as Fe replaced Co in La1-xSrxCo1-yFeyO3-δ while electronic conductivity rises, then declines. The ionic conductivity results from substitution of Sr for La, which forms oxygen vacancies (Eq. 1.1):

La2O3 × •• 2SrO 2SrLa + 2OO + VO (1.1) ′ ⟶ The electronic charge carriers mainly come from the transition metal substitutions

(Eqs. 1.2 and 1.3) (with reference to the B-site–O sublattice in the LaFeO3 structure):

× •• 2CoO 2CoFe + 2OO + VO (1.2) ′ × →2C oFe 2CoFe + 2 (1.3) ′ ′ ⇌ 𝑒𝑒

Cations with a lower oxidation state (Ln+3 → Sr+2) will lower the activation barrier

necessary for oxygen ions to move past the cations due to lowering of the coulombic

16 attraction. Teroaka et al. in 1988 also experimented with Ln0.6Sr0.4CoO3 OTMs by

substituting other trivalent lanthanide cations Ln on the A-site, but with increasing ionic

radii (Gd+3 < Sm+3 < Nd+3 < Pr+3 < La+3).17 They found that as the ionic radii of the cation

substitution decreases, the oxygen flux increases with increasing temperature.

Strontium cobaltite/ferrite and barium strontium cobaltite/ferrite perovskites are

prevalent in OTM literature because they have high oxygen flux. However, cobalt is less

chemically stable at low temperatures and in the presence of certain gases such as carbon

dioxide or sulfur dioxide.5 The instability results in the formation of cobalt precipitates that

degrade the lifetime of a membranes. Furthermore, since for any metal M that has 2+ and

3+ states M2+ is larger than M3+, the large lattice expansion caused by the reduction of cobalt

results in potential phase transformations that lowers ionic conductivity. Partial substitution

of the B-site (cobalt and iron) cation with less reducible metal cations can improve the

chemical stability.5 However, Babakhani et al. found that substituting Ni for Fe in barium

strontium cobaltite/ferrite increased oxygen permeability and phase stability at high temperatures even though Ni is more reducible than Fe.18

1.2.2. Mixed electronic-ionic conducting oxygen transport membranes

Several recent reviews cover the properties and applications of ceramic OTM

materials.5,8,12,19 To be a good material for oxygen transport, the membrane must meet multiple, sometimes competing, requirements such as high oxygen semi-permeation (>1 sccm/min), suitable mechanical properties between 700 °C – 1,000 °C, and good chemical stability under 10–16 atm to 0.21 atm between 700 °C and 1,000 °C.20

The mechanism of oxygen transport in MEIC OTMs consists of the following steps

(in this process, the mass transfer resistances are small and negligible):5

• Bulk-to-surface mass transfer of oxygen gas on feed side.

• Oxygen molecules on surface dissociate into O2– ions (Eq. 1.4) by combining

with conduction electrons ( ) flowing from the opposite side of the ′ membrane (Fig. 1.1) and filling𝑒𝑒 oxygen vacancies ( ••):

𝑉𝑉𝑂𝑂 + 2 + •• × (1.4) 1 ′ 2 𝑂𝑂2 𝑒𝑒 𝑉𝑉𝑂𝑂 ⇌ 𝑂𝑂𝑂𝑂 • The activity gradient between the feed side and permeate side causes oxygen

ions to diffuse through the membrane via oxygen vacancies.

• Oxygen ions reach to the permeate side surface and react with methane

(Eq. 1.5):

+ + 2 (1.5) 1 2 𝑂𝑂2 𝐶𝐶𝐶𝐶4 ⇌ 𝐶𝐶𝐶𝐶 𝐻𝐻2 • The partial oxidation of methane occurs resulting in a mixture of carbon

monoxide and hydrogen gas (syngas).

Takahashi et al. conducted early studies in 1976 with bismuth oxide and barium

oxide MEIC systems. They found that a rhombohedral perovskite, BaBiO3-δ, had high ionic

conductivity and good electronic conductivity at normal oxygen levels and intermediate

21 16 temperatures (500 °C). In 1985, Teraoka et al. showed that La1-xSrxCo0.4Fe0.6O3-δ (x = 0.0,

0.4, 0.8, and 1.0) perovskites could be used for oxygen transport applications.16 Oxygen

2 permeation exceeded 1.0 sccm/cm above 1,000 K in SrCo0.4Fe0.6O3-δ. Even though the

permeation was low for methane reforming requirements (10 sccm/cm2), this work helped lay the foundation for MEIC materials as oxygen transport membranes.

In 2000, Hendriksen et al. reviewed lanthanum strontium cobaltite/ferrite

membranes and concluded that delamination of the membrane from the support, and high

expansion of the membrane up to 0.35%, were significant shortcomings.22

Others have attempted to achieve mixed conductivity using dual-phase materials. Cai et al. in 2016 found that dual-phase membranes of samarium-doped ceria (largely an oxygen- ion conductor) and samarium strontium chromite/ferrite (an electronic conductor), have good electronic conductivity and high oxygen flux (7.6 mL cm-2 min-1 at 950 °C).23

1.2.3. Lanthanum strontium ferrite-based membranes

As mentioned previously, lanthanum strontium ferrite (LaxSr1–xFeO3-δ, LSF) is a promising single-phase OTM material with high oxygen conductivity as high as 0.5 S/cm and electronic conductivities (100 S/cm) at high temperature (1,000 °C).5 They have the

potential to maintain their structure in low oxygen partial pressures. They also are good

catalysts for the surface reactions on both sides of the membrane. Moreover, LSF materials

have relatively low thermal expansion coefficients.

In stoichiometric LaFeO3, the oxidation state of iron is 3+, whereas it is 4+ in

24 SrFeO3 from experiments done by Falcón et al. Both of these phases form in the

+3 +4 perovskite structure, as do their solid solutions LaxSr1–xFeO3-δ, with mixed Fe and Fe in

proportions that depend on the values of x and δ. This mixed valency allows LSF ceramics

to have high electronic conductivity. The number of oxygen vacancies per perovskite unit

cell is δ = cvVM, where cv is the molar concentration of oxygen vacancies per unit volume, and VM is the molar volume (i.e. volume per perovskite formula unit). The value of x has a

strong influence on the oxygen vacancy concentration, which likewise has a strong influence

on the ionic conductivity.9,25,26 (Usually ionic conductivity increases when the concentration

of oxygen vacancies increases.) For a given value of 1–x, the limiting cases are δ = (1–x)/2 if

none of the Fe is 4+, and δ = 0 if the fraction of Fe4+ = 1–x, with low partial pressure of

oxygen also tending to increase δ and decrease the fraction of Fe4+.9,25,26

Stevenson et al. and Li et al. both found that as the strontium substitution increased in LSF, the oxygen flux increased.27 The Sr-substituted material had a higher free volume allowing oxygen atoms to move freely. However, Li et al. showed that the material’s thermal

stability and phase stability decreased at higher temperatures (900 °C) in air due to the low

average bond energy between Sr–O (larger lattice expansion) compared to La–O.29

Studies by Sunarso et al.19 summarize effects on oxygen flux of substitutions for Fe

in LSF: LaxSr1–xFe1–yMyO3-δ where M is a transition metal or a mixture of transition metals.

Even though their fluxes are lower than what is needed for methane reforming applications

(10 mL/cm2/min), qualitatively–Sunarso et al. found that Ga and Ni doping tended to

decrease oxygen flux, and Co doping led to higher oxygen flux.

Kharton et al. partially substituted iron with gallium in LSF materials and found that

the thermal and chemical expansion coefficients had increased.29 Shen et al. found that as the

Cr content increased on the B-site, the electronic conductivity increased, but oxygen diffusivity decreased.30 Deka et al. substituted Ni for Fe and found that Ni changed the anion sub-lattice symmetry and improved the electrical and ionic conductivity.31 Furthermore,

Deka et al. found that Co doping, along with Ni, causes changes in anion sub-lattice

symmetry and saw the same trend with increasing electrical and ionic conductivity.31 Cobalt

doping on the B-site leads to higher oxygen permeability, making LSCF compositions

attractive for OTM applications. Teroaka et al. in 1988 found that as the cobalt content

increased, the ionic conductivity increased as well when it was made as the primary B-site

cation.17 However, cobalt substitution raises thermal expansion coefficients, and the ease for

Co to reduce to 2+ or to Co metal makes heavily Co-substituted LSCF susceptible to degradation and incompatibility with other materials. Along with the other material issues, cobalt has the disadvantage of being expensive.26

1.2.4. Chemical expansion studies

MEIC oxide materials not only undergo thermal expansion, but also chemical

expansion, which can lead to mechanical stresses5 as explained in §1.1.1. Chemical expansion

is due to the increase in oxygen vacancy concentration and chemical reduction of the

transition metal cation with increasing temperature and lower oxygen potential. This leads to

the increase of the cationic radii, repulsion between the cations that surround oxygen

vacancies, and the expansion of lattice parameters.10

In perovskite oxides, below certain intermediate temperatures, thermal expansion

dominates. Once a sufficient temperature is reached, chemical expansion becomes an

appreciable fraction of the total material expansion. Choi et al.32 studied the thermal and

chemical expansion properties of barium strontium cobaltite ferrite materials for solid oxide

fuel cell applications. They found that in air the expansion increased non-linearly above

500 °C.

Bishop et al.33 observed thermo-chemical expansion in strontium-doped lanthanum

cobalt iron oxides. Chemical expansion caused a significant increase in expansion above

500 °C (Fig. 1.3, left). Varying the oxygen potential isothermally showed that as the oxygen content decreased, the chemical expansion increased (Fig. 1.3, right). Table 1.1 shows chemical expansion coefficients for several perovskite oxides.

Figure 1.3: Thermo-chemical expansion coefficient predication from data in air as a function of temperature and oxygen non-stoichiometry (Left) and chemical strain as a function of oxygen content with pO2 = 0.21 atm (air) as reference point for zero expansion. Republished with permission of John Wiley & Sons – Books, from Bishop et al.33; permission conveyed through Copyright Clearance Center, Inc.

Table 1.1: Chemical expansion coefficients for similar perovskite oxides as a function of chemical strain and change in oxygen non-stoichiometry. * testing was done in δ = 0.05.33

Composition x Temp. (°C) Coeff. of chemical expansion La1-xSrxCoO3-δ 0.2-0.7 600–900 0.023-0.024* La0.6Sr0.4Co0.8Fe0.2O3-δ – 800 0.022 Ba0.6Sr0.4Co0.8Fe0.2O3-δ – 600–900 0.026-0.016 La0.597Sr0.398Co0.2Fe0.8O3-δ – 700–900 0.031 La0.6Sr0.4Co0.2Fe0.8O3-δ – 700–890 0.032* La0.25Sr0.75FeO3-δ – 650–875 0.017-0.047 SrTi1-xFexO3-δ 0.3-0.75 23 0.03 La1-xSrxCrO3-δ 0.16-0.3 1000 0.024

1.2.5. Dilatometric measurements of oxygen transport membranes

To analyze the effects of chemical expansivity, a dilatometer can be used. A

dilatometer is an instrument that measures dimensional changes caused by chemical or physical processes. The precision of the dilatometer will be optimal if the ambient environment has minimal temperature and humidity fluctuations. It is designed to measure

both the percent linear change (PLC) and temperature of the test sample under controlled

heating and cooling rates. A linear variable differential transformer (LVDT) can convert the

12 motion of an object (or the expansion or contraction of a material) to a corresponding electrical signal.34,35 Most dilatometers are mounted horizontally to allow for optimal temperature uniformity within the furnace.

Bayraktar et al.36 investigated lanthanum strontium ferrite/titanite materials using dilatometry. At 900 °C in air, they found higher values of chemical diffusion and surface exchange coefficients of their samples with Ti doping, La0.5Sr0.5Fe0.8Ti0.2O3-δ, compared to

La0.5Sr0.5FeO3-δ.

Chapter 2: Experimental Procedures

2.1. Dilatometer calibration

2.1.1. Alumina reference sample calibration run Measurements of chemical and thermal expansion were conducted in an Orton

1600D Dilatometer. Because of the very small elongations associated with these experiments, a calibration was conducted before the first run, and whenever the LVDT, the pushrod, sample holder, or thermocouple were removed or replaced to mitigate potential errors from moving the LVDT, the pushrod, sample holder, thermocouple, and furnace. A

1.00-inch-long alumina reference sample was used for calibration. The calibration run had a ramp rate of 10 °C/min from room temperature to 1000 °C and a tuning constant of air

(default parameter in the software). §A.1. gives details of the calibration procedure.

2.1.2. Experimental runs on alumina reference sample deviation To find the error and deviation of the reference sample, experimental runs in air were taken on the reference sample. The parameters were set to 10 °C/min from room temperature to 600 °C, 700 °C, 800 °C, 900 °C, and 1,000 °C with a hold time of 60 min for each temperature. This was done three times. §A.2 gives details of these measurements.

Figure 2.1 and Table 2.1 show the reproducibility of the alumina reference sample.

Figure 2.1: Percent linear change versus dilatometer time data in the Orton software.

Table 2.1: Deviation of the coefficients of thermal expansion (CTE in 10-6/°C) in the Orton alumina reference sample. The uncertainties in the reference specimen are smaller than the measured specimen-to-specimen variations in otherwise identical OTM pellets (§3.5). Test 600 °C 700 °C 800 °C 900 °C 1,000 °C 1 7.398 7.502 7.649 7.852 7.991 2 7.400 7.502 7.650 7.851 7.987 3 7.399 7.501 7.650 7.851 7.992

2.2. Purchased and synthesized powders for study

2.2.1. Compositions

Two compositions were obtained from Fuel Cell Materials (Lewis Center, OH):

• (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1)

• (La0.80Sr0.20)0.95 FeO3-δ (FCM2)

Two compositions were obtained from Praxair Specialty Ceramics (Woodenville,

WA):

• (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1)

• (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2)

Four compositions were synthesized in-house using the amorphous malic acid precursor method (§2.2.2):

• (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1)

• (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2)

• (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3)

• (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4)

Table 2.2 summarizes the target ranges of compositions for this work and gives qualitative descriptions of the roles of each cation in the formula, as described in the literature.

Table 2.2: Base chemical formula: LaxSr1-x(FeaCrbMgcCod)O3-δ, where a + b + c + d = 0.99 to 1.00.5,9 See §B.1 for defect chemistry. Element Range Reasoning Lanthanum 0.1 < x < 0.5 • Stability in natural gas conditions5 • Creates oxygen vacancies9 Strontium 0.5 < 1-x < 0.9 • Dominates ionic conductivity9 Iron a < 0.9 • Enhances electronic conductivity5 • Promotes liquid-phase densification8 Chromium b < 0.5 • Expands phase stability temperature range5 • Not multivalent which applies additional Magnesium c < 0.05 constraints on other cations in B-site that might mitigate chemical expansion9 • Enhances electrical conductivity5 Cobalt d < 0.2 • High oxygen permeability5 • Perovskite anion; sub-stoichiometry Oxygen 3-δ (oxygen vacancies) needed for oxygen transport5

2.2.2. Teraoka amorphous malic acid precursor method37

The amorphous malic acid precursor method (AMP) entails dissolving metal nitrate

salts in water with ammonium hydroxide and malic acid, and heating (while stirring) to

obtain a dry amorphous precursor to the intended multicomponent oxide. This method is

reported to produce oxides with values of specific surface area. The process uses hydroxy

acids like malic acid (C4H6O5), a bidentate, to complex with the metal cations in solution, and help maintain compositional homogeneity in the resulting amorphous precursor. This allows

for the formation of perovskite-type oxides at lower temperatures that result in high homogeneity, purity, and catalytic activity for combustion reactions and electrochemical reduction of oxygen.

Amounts of N2O6Sr, LaN3O9·6H2O, FeN3O9·9H2O, CoN2O6·6H2O,

CrN3O9·9H2O, and H12MgN2O12 (Fisher Scientific) were weighed in amounts required to

obtain 10-50 grams of the desired compositions (§A.3). The proportion of distilled water

(Fisher Scientific) was 1.5 L per 52.5 mmol of total starting powder.37 The molar ratio of

16 malic acid to metal ions is 3/n where n is the number of carboxylic acid groups (n = 2 for malic acid.

Metal nitrates, water, malic acid, and ammonium hydroxide components were mixed to create a solution that had pH = 2-3. The solution was dried/evaporated for 3-5 days as shown in Figure 2.2 below. The remaining precipitates were grounded using a pestle and mortar to form a powder. The powder was then placed into a large crucible with Fiberfrax®

(Unifrax Company) covering the opening and calcined at 850 °C for 6 hours in a

Thermolyne 48000 furnace (Figure 2.3).37 Afterwards, the calcined powder was again ground using a pestle and mortar. §A.4 provides details of the amorphous malic acid precursor powder synthesis route.

Figure 2.2: Drying of La0.50Sr0.50Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) with AMP route. Left: Early stage of drying. Right: Final stage of drying.

Figure 2.3: Box furnace used for calcination.

2.3. Sample preparation

2.3.1. Pellet pressing

Calcined powders were dry-pressed into disks with 0.5-inch diameter (d) using a steel

punch and die set in a Carver Press Hydraulic Unit Model 3912. The thickness t of the disks

after sintering satisfied the condition 10t < d) (Figure 2.4). §A.5. gives details of the pellet

pressing. The duplicate runs in FCM1, FCM2, PRAX2, and CWRU3 had pellets that used

0.5 grams of methyl cellulose as a binder, except FCM1 duplicate run at 1,000 °C.

Figure 2.4: Dimensions of experimental sintered pellets.

2.3.2. Sintering

The pressed disks were put in crucibles into a high temperature furnace capable of

reaching 1,700 °C (Fig. 2.5). The disks were sintered at 1,250°C – 1,350 °C with a ramp rate of 20°C/min for 16 h. The samples that did not contain chromium were fired at 1,250 °C and the samples with chromium were fired at 1,350 °C.

Figure 2.5: MoSi2 furnace used for firing of green pellets.

2.4. Materials characterization: XRD, density, and SEM/EDS

The as-calcined powders underwent x-ray diffraction (Bruker Discover D8 with

VÅNTEC-500 solid-state detector) to identify phases. These characterizations were done at

the Swagelok Center for Surface Analysis of Materials (SCSAM) at Case Western Reserve

University (CWRU).

X-ray diffraction settings were 5-minute frames at 30° with detector scan (2 )

between 15° and 105° using Cu Kα radiation with a nickel filter. The XRD analysisƟ was

done using the DIFFRAC.SUITE EVA 4.2.1 software from the Bruker Corporation. Given

the elements in the respective compositions (lanthanum, strontium, cobalt, chromium,

magnesium, iron, and oxygen; carbon was added if the pattern displayed non-perovskite

peaks) the software matched peaks to phases in the powder diffraction file (PDF). B-site

cations were included or excluded in the filter from the search depending on the

composition. The top 2–3 patterns with the best fit as identified by the software were

selected as the potential phases for our samples.

Before measurement of thermal and chemical expansion, as-sintered pellets

underwent density measurements using the Archimedes density technique. An immersion

density apparatus (Mettler AE 200) was used to measure dry weight (w1), submerged weight

(w2), and unsubmerged wet weight (w3) to obtain the density of the sintered pellets using

Equation 2.1. §A.6 contains details of the Archimedes density technique.

ρsample = ( ) ( 2.1 ) 𝑤𝑤1

𝑤𝑤3−𝑤𝑤2 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 Then fragments of those pellets𝜌𝜌 of 𝑇𝑇each composition were prepared by sputtering of

a palladium film to undergo scanning electron microscopy (SEM) (FEI Quanta 3D

environmental scanning electron microscope with focused ion beam (FIB) and energy-

dispersive x-ray spectroscopy (EDS) or FEI Helios 650 field emission scanning electron

microscope with EDS). The imaging conditions were typically voltages of 15-20 kV, current of 1 nA, and taken in secondary electron imaging mode.

After expansion testing, the different samples of each composition were prepared to undergo SEM/EDS along with XRD to identify changes in microstructure, compositional homogeneity, and phase stability. As with the pre-testing characterization, these analyses were done at SCSAM.

2.5. Dilatometer operation and atmosphere control

2.5.1. Gas setup

Figure 2.6 shows the modifications made to the dilatometer (Orton 1600D;

Westerville, Ohio) to enable measurements of expansion under controlled atmosphere. The

gases used in this project were nitrogen (99.995% with ≤ 1 ppm CO, and CO2), carbon

dioxide (99.99% with ≤ 20 ppm O2 and ≤ 50 ppm N2), and carbon monoxide (99.3%). The gases were purchased from Airgas. An alumina atmosphere control tube extended out of both sides of the furnace. The chosen gas or gas mixture flowed through this tube from left to right.

Figure 2.6: Orton 1600D Dilatometer, showing the components of the atmosphere-control system.

There were two Alicat Scientific M-2SLPM-D/GAS:N2, 10M mass flow controllers.

One mass flow controller handled the carbon monoxide flow and the other handled nitrogen or carbon dioxide as needed; a valve between the gas tanks prevented nitrogen and carbon dioxide from mixing. The inlet tubing was made of high-pressure/vacuum polyethylene, and the outlet tubing was high-temperature silicone rubber (both purchased form McMaster-Carr). The outlet tubing vented the gas exiting the furnace into the upper exhaust of a fume hood. A carbon monoxide detector (Macurco CM-6) was mounted just

21 outside the fume hood. The nitrogen and carbon dioxide compressed-gas cylinders and regulators (not shown) were secured to the lab bench to the right of the dilatometer. The

(smaller) carbon monoxide cylinder was secured in the fume hood.

The mass flow controllers used the same calibration constant for nitrogen, carbon dioxide, and carbon monoxide so using the same MFC for these three gases was straightforward. There is a T-fitting attached to the dilatometer that allowed for mixing CO2

and CO in predetermined proportions set by the mass flow controllers. The dilatometer

came with fittings that seal the left end of the atmosphere control tube to a gas inlet, where

the T-fitting was attached. Before running the gases, all fittings were tightened and the

output pressure from the pressure gauges on the gas tanks were adjusted to roughly 70 psi.

The mass flow controllers can only handle a maximum of 145 psig. The mass flow

controllers had flow units in sccm (standard cubic centimeters per minute). The dilatometer

was then heated up from room temperature to the chosen isothermal temperature point at a

ramp rate of 10 °C/min with a hold time of 720 minutes. §A.1 describes the calibration of

the dilatometer, and §A.7 gives additional details of the heat treatments.

2.5.2. Oxygen step changes

The dilatometer recorded percent linear change (PLC) of the pellet thickness, relative

to its value in air at the temperature of the furnace (near room temperature) at the start of

the test. Once the dilatometer reached the target testing temperature (800, 900, or 1,000 °C),

readings were taken in air for an additional 10–15 minutes. Once the furnace temperature

stabilized, the dilatometer took PLC readings every 20 seconds. The atmosphere was then

-4 –9.4 –4.9 changed sequentially to nitrogen (pO2 = 10 atm), carbon dioxide (pO2 = 10 to 10 atm

depending on the temperature), and four CO2/CO gas mixtures of progressively lower pO2

(Table 2.3). §A.8 gives details on the pO2 calculations. Each atmosphere was held for about

30 minutes. The PLC reading usually reached a stable value in ~2 minutes. When readings at

the last atmosphere were completed, the gas flow was stopped, the furnace was turned off,

and the specimen cooled at the passive cooling rate of the furnace (roughly 10 °C/min) for

about 6 hours.

Table 2.3: Gas mixtures and respective calculated partial pressure of oxygen levels. Air (sccm) 1000 0 0 0 0 0 0

N2 (sccm) 0 1000 0 0 0 0 0

CO2 (sccm) 0 0 1000 990.1 901 505 109 CO (sccm) 0 0 0 9.9 99 495 891

log pO2 (800 °C) –0.678 –4 –9.4 –14.4 –16.5 –18.4 –20.3

log pO2 (900 °C) –0.678 –4 –7.0 –12.1 –14.2 –16.1 –17.9

log pO2 (1,000 °C) –0.678 –4 –4.9 –10.1 –12.2 –14.1 –15.9

2.6. Data acquisition and reporting

When each experimental run was completed, the Orton Dilatometer Version 5.2.1

software produced a data file. The data files were exported into text files with temperature

(degrees Celsius), time (minutes), and percent linear change values. Raw PLC data were

processed by Python 3.7 at the set isothermal temperature. Figure 2.7 shows four examples

of typical raw data files.

Figure 2.7: Percent linear change (PLC) versus time (min) data from Orton 1600D dilatometer experimental runs in Python 3.7.

2.7. Calculations

2.7.1. Chemical linear strain

The precision of the dilatometer can measure PLC only to the fourth decimal place.

The thermal expansion of the specimens was large compared to the expansion from

changing the gas atmosphere, and fluctuations in temperature were occurring during the

equilibration to each new atmosphere, so precisely measuring chemical strain (ε C) at

constant temperature, separately from thermal strain was difficult. Furthermore, literature

values of the oxygen deficiency parameter δ and coefficient of chemical expansion (αC) were unavailable for most of the chosen compositions. Equation 2.2 was used to calculate

–0.678 chemical strain where and εpO2′ is the strain in air (pO2 = 10 ) and εpO2″ is the strain at pO2″

< 10–0.678.

" = C ( 2.2 ) 𝜀𝜀𝑝𝑝𝑝𝑝2 −𝜀𝜀𝑝𝑝𝑝𝑝2′

𝜀𝜀𝑝𝑝𝑝𝑝2′ 𝜀𝜀 The PLC values from the dilatometer were converted into a strain, PLC/100 = ε, at

the respective pO2. During calibration runs with just the alumina spacer/reference sample at

each isothermal temperature, the atmosphere changes had no net effect on the strain

(Fig. A.3, §A.2). This ensured that in runs with OTM samples, any change in the strain due

to the atmosphere would be solely in the OTM sample.

2.7.2. Coefficient of chemical expansion

Coefficients of chemical expansion αC can be computed from experimental

measurements of chemical strain ε C for some of the compositions tested if the difference in

oxygen non-stoichiometry ∆δ are known in the atmospheres at which the chemical strain

was measured using Equation 2.3.

= ( 2. 3 ) 𝐶𝐶 𝜀𝜀 𝐶𝐶 ∆𝛿𝛿 𝛼𝛼

Chapter 3: Results

3.1. Density

Tables 3.1a–h present density data (§2.4) of pressed and sintered pellets of each of the eight experimental compositions before and after dilatometer measurements at the indicated temperatures. The unit-cell density calculations are described in §A.9. No systematic trends were observed between the density values before and after dilatometer measurements, between pellets pressed with and without a binder, nor as a function of the temperature of the dilatometer measurements, so an average density of all specimens of a given composition is given (with standard deviation, s.d.) at the bottom of each of Tables

3.1a–h.

The spreads of densities among pellets made from the powders synthesized in this work (CWRU1–CWRU4) were comparable to those of the pellets made from commercial powders. Some of the pellets had some minor surface cracks after testing. It was believed that the cracking was due to the processing, so methyl cellulose was used as a binder during pellet pressing. It was observed that the small amount addition of methyl cellulose caused the pellets to stop cracking after testing. Samples that used methyl cellulose are described in

§3.6.

Table 3.1 (Start): Densities of pellets of the indicated compositions, before dilatometer measurements at the indicated temperature. Asterisks (*) indicate duplicate measurements on different pellets of the same composition and test temperature.

a. (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1) Temperature (°C) Immersion density (g/cm3) % of unit-cell density As-Sintered 6.125 96.6 800 6.166 96.6 96.9 800* 6.120 97.3 900 6.142 96.6 97.0 900* 6.149 96.9 1000 6.152 97.0 96.9 1000* 6.137 97.1 average ± s.d. 6.14 ± 0.02 96.9 ± 0.3

b. (La0.80Sr0.20)0.95FeO3-δ (FCM2) Temperature (°C) Immersion density (g/cm3) % of unit-cell density As-Sintered 6.025 97.4 800 6.330 102.4 102.0 800* 6.283 101.6 900 6.163 99.7 98.8 900* 6.058 98.0 1000 6.194 100.2 99.2 1000* 6.071 98.2 average ± s.d. 6.2 ± 0.1 100 ± 2

c. (La0.20Sr0.80)Cr0.20Fe0.80O3-δ (PRAX1) Temperature (°C) Immersion density (g/cm3) % of unit-cell density As-Sintered 5.721 98.0 800 5.671 97.1 900 5.627 96.4 1000 5.836 99.9 average ± s.d. 5.7 ± 0.1 98 ± 2

d. (La0.20Sr0.80)Co0.10 Cr0.20Fe0.70O3-δ (PRAX2) Temperature (°C) Immersion density (g/cm3) % of unit-cell density As-Sintered 5.872 99.0 800 5.782 97.5 900 5.776 97.4 98.2 900* 5.873 99.0 1000 5.810 98.0 97.9 1000* 5.804 97.9 average ± s.d. 5.82 ± 0.04 98.1 ± 0.7

Table 3.1 (Concluded): Densities of pellets of the indicated compositions, before dilatometer measurements at the indicated temperature. Asterisks (*) indicate duplicate measurements on different pellets of the same composition and test temperature.

e. (La0.50Sr0.50)Cr0.20Fe0.80O3-δ (CWRU1) Temperature (°C) Immersion density (g/cm3) % of unit-cell density As-Sintered 5.921 96.3 800 5.977 97.2 900 5.978 97.2 1000 5.895 95.9 average ± s.d. 5.94 ± 0.04 96.6 ± 0.7

f. (La0.20Sr0.80)Co0.10 Cr0.10Fe0.80O3-δ (CWRU2) Temperature (°C) Immersion density (g/cm3) % of unit-cell density As-Sintered 5.734 97.0 800 5.615 95.0 900 5.718 96.7 1000 5.599 94.7 average ± s.d. 5.67 ± 0.07 96 ± 1

g. (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) Temperature (°C) Immersion density (g/cm3) % of unit-cell density As-Sintered 5.655 96.4 800 5.646 96.3 98.2 800* 5.874 100.2 900 5.683 96.9 97.8 900* 5.787 98.7 1000 5.743 97.9 98.6 1000* 5.828 99.4 average ± s.d. 5.75 ± 0.09 98 ± 2

h. (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) Temperature (°C) Immersion density (g/cm3) % of unit-cell density As-Sintered 5.833 96.9 800 5.873 97.6 900 6.013 99.9 1000 5.903 98.1 average ± s.d. 5.91 ± 0.08 98 ± 1

3.2. X-ray diffractometry

Diffraction peaks for these perovskites do not vary strongly with composition, occurring (to zero decimal places) at 2 = 27°, 38°, 47°, 55°, 62°, 69°, 81°, 87°, 99°, 100°, and 105° for all materials studied here.Ɵ The unit-cell volumes that were used to calculate the unit-cell densities of the experimental compositions (§A.9) came from the entries in Powder

Diffraction File (PDF) that best matched the experimental powder XRD patterns.

The EVA Bruker software for XRD analysis was used to match the PDFs to the experimental diffraction patterns. In the software, the PDF matches were restricted to compounds that contained La, Sr, Fe, O, and Co, Cr, or Mg depending on the composition.

Furthermore, C was added as a filter to determine whether carbonates were still present in the samples after sintering. A list, mainly of perovskites (but might have some other structures, like Ruddlesden-Popper or pyrochlores), matched by the software were ranked by

"goodness of fit.” The quality of the fits were designated by a green bar going from left to right. The more the green bar shifted right, the better the fit was. The matches were excluded if they were not oxides or not the right stoichiometry. The top 3-5 matched PDF files were then selected. The PDF records for rhombohedral phases indexed the patterns as hexagonal cells (a = b ≠ c, α = β = 90°, γ = 120°) using 3-index (hk) notation, a practice also common in the literature on perovskite crystallography.

Three criteria were used to determine which of the PDF records best fit the experimental data: first, the best match of high-angle peak positions (2 ); then the best match of the intensity and the location of the strongest peak; finally, a Ɵcomparison to the literature to determine whether or not the PDF phase determination was reasonable.

The following subsections present the powder XRD patterns from as-received or as- calcined specimens, and from specimens of each material after measurement at 900 °C of

thermal and chemical expansion under the sequence of low-pO2 exposures from §2.5.2.

3.2.1. (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1)

The XRD patterns of FCM1 (Fig. 3.1) show only peaks for a single perovskite phase,

cubic LaCo0.4Fe0.6O3 (PDF 40-0224), in both the as-received and sintered + tested materials.

The software also identified rhombohedral phases as strong matches: (La0.33Sr0.67)FeO2.98

(PDF 80-1037) and (La0.3Sr0.7)FeO3 (PDF 82-1964) in the as-received powder, and

(La0.4Sr0.6)FeO3 (PDF 82-1963) in the tested material.

Of the matching records listed in Figure 3.1, the ones that best match the FCM1

composition and XRD patterns are rhombohedral perovskites: La0.3Sr0.7FeO3 (PDF 82-1964)

for as-received powder, and La0.4Sr0.6FeO3 (PDF 82-1963) for sintered+tested material.

The peak intensities from the as-received powder (Fig. 3.1, top) were noticeably stronger than from the other seven as-calcined powders (Figs. 3.2 – 3.8). In general, higher peak intensities among materials with similar compositions and crystal structures might result from larger (>~100 nm) and/or more chemically homogeneous crystals, giving narrower and taller peaks.

The peaks in the pattern from the sintered + tested material were sharper, but lower in intensity, than those in the as-received material. The heights of the 200 and 211 peaks changed slightly. The peak splitting at 69°, 81°, and 94° could indicate that both a cubic and rhombohedral phase are present. Comparing the present results to Co-free materials, this would be partly compatible with the findings of Gavrilova et al. (Figure B.2 in §B.2) that

La0.60Sr0.40FeO3 at 1,100 °C is two-phase (though they report orthorhombic and rhombohedral), whereas Fossdal reported that it is single-phase cubic at 1,000 °C.38,39

3.2.2. (La0.80Sr0.20)0.95 FeO3-δ (FCM2)

Three PDF records of perovskites matched the pattern of the as-received material

(Fig. 3.2, top): cubic LaFeO3 (PDF 75-0541), orthorhombic La0.7Sr0.3FeO3 (PDF 89-1269),

and rhombohedral La0.6Sr0.4FeO3 (PDF 40-0285), with orthorhombic La0.7Sr0.3FeO3 (PDF 89-

1269) best matching the experimental composition. The material after sintering and testing

at 900 °C (Fig. 3.2, bottom) had three matching PDF records: cubic LaFeO3 (PDF 75-0541),

orthorhombic La0.8Sr0.2FeO3 (PDF 35-1480) and orthorhombic LaFeO3 (PDF 88-0641), with

31 the orthorhombic phase best matching the experimental composition. The peaks were sharper, and the baseline less noisy, than in the pattern from the as-received specimen.

Both the as-received powder and the sintered+tested material of FCM2 agreed best in diffraction patterns and composition with PDF records of orthorhombic (La,Sr)FeO3

38 perovskites. Gavrilova et al. also reported that La0.8Sr0.2FeO3 was orthorhombic at 1,100 °C.

None of the top PDF matching candidates had A-site deficiency, which was 5% in FCM2.

3.2.3. (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1)

Three PDF records of perovskites match the experimental pattern of as-received

PRAX1 material (Fig. 3.3, top): rhombohedral La0.5Sr0.5FeO3 (PDF 82-1962), cubic LaCrO3

(PDF 75-0288), and rhombohedral LaCrO3 PDF 33-0702). The pattern from the sintered +

tested material (Fig. 3.3, bottom) matched three PDF records: cubic LaCrO3 (PDF 75-0288),

orthorhombic La0.7Sr0.3Co0.3Fe0.7O3 (PDF 89-1268), and rhombohedral La0.6Sr0.4FeO3 (PDF

82-1261). The differences in peak intensities compared to the PDF reference patterns (such

as the peak at 2 = 81°) was likely due to insufficient grinding of the sintered pellet.

Of the matching phases listed in Figure 3.3, the ones that best match the PRAX1

composition are both rhombohedral perovskites: La0.5Sr0.5FeO3 (PDF 82-1962) for the as-

received powder, and La0.6Sr0.4FeO3 (PDF 82-1261) for the sintered+tested material (the

greenphase match on the zoomed in graph is being blocked by others).

3.2.4. (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2)

An XRD pattern provided by Praxair indicated that this powder was multiphase. The

pattern obtained at CWRU (Fig. 3.4, top) on the as-received powder shows that SrCO3 is

present. The remaining peaks correspond to perovskite, matching PDF records for cubic

La0.8Sr0.2CrO3 (PDF 74-1980) and rhombohedral La0.6Sr0.4Co0.4Fe0.6O3 (PDF 49-0284). The

pattern from the sintered + tested (900 °C) specimen (Fig. 3.4, bottom) showed only

perovskite peaks. Three matching PDF records were cubic LaCrO3 (PDF 75-0288), orthorhombic La0.7Sr0.3Co0.3Fe0.7O3 (PDF 89-1268), and rhombohedral La0.6Sr0.4FeO3 (PDF

82-1961).

Of the matching records (Fig. 3.4), the ones that best match the PRAX2 XRD patterns and composition are both rhombohedral perovskites: La0.6Sr0.4Co0.4Fe0.6O3 (PDF 49-

0284) for as-received powder, and La0.6Sr0.4FeO3 (PDF 82-1961) for sintered+tested material.

3.2.5. (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1)

The XRD pattern of the as-calcined material (Fig. 3.5, top) shows perovskite peaks compatible with cubic LaCr1.01O3 (PDF 44-0333), rhombohedral La0.4Sr0.6FeO3 (PDF 82-

1963) (both perovskites) and SrCO3. Strontium carbonate decomposes to SrO and CO2

between 1,100 °C and 1,250 °C,40 above the calcination temperature of this material

(850 °C). The XRD pattern from the sintered + tested material (Fig. 3.5, bottom) matched

orthorhombic La0.7Sr0.3FeO3 (PDF 89-1269), orthorhombic LaCrO3 (PDF 83-0256), and

rhombohedral La0.6Sr0.4FeO3 (PDF 49-0285). The strontium carbonate peaks are no longer

evident. The sintering temperature was 1,350 °C, high enough to decompose SrCO3.

Of the matching phases listed in Figure 3.5, a rhombohedral perovskite La0.4Sr0.6FeO3

(PDF 82-1963) best matches the CWRU1 composition for the as-calcined powder, and orthrhombic perovskite La0.7Sr0.3FeO3 (PDF 89-1269) for the sintered+tested material. 36

3.2.6. (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2)

The as-calcined XRD pattern (Fig. 3.6, top) shows peaks compatible with two cubic

perovskites: LaCo0.4Fe0.6O3 (PDF 40-0224) and LaCrO3 (PDF 74-1961). SrCO3 was also

clearly detected (peaks labeled in blue). The pattern from the sintered + tested material (Fig.

3.6, bottom) matched cubic LaCrO3 (PDF 75-0288), orthorhombic La0.70Sr0.3Co0.3Fe0.7O3

(PDF 89-1268) and rhombohedral La0.6Sr0.4FeO3 (PDF 82-1961). Strontium carbonate was no longer present. The sintering temperature was at 1,350 °C which was high enough to thermally decompose SrCO3.

Of the matching phases listed in Fig. 3.6, the ones that best match the CWRU2 composition are cubic LaCo0.4Fe0.6O3 (PDF 40-0224) for the as-calcined powder and rhombohedral La0.6Sr0.4FeO3 (PDF 82-1961) for the sintered+tested powder.

3.2.7. (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3)

The XRD pattern (Fig. 3.7, top) from the as-calcined material shows perovskite peaks compatible with cubic LaCo0.4Fe0.6O3 (PDF 40-0224) and rhombohedral

(La0.3Sr0.7)FeO3 PDF 82-1964). SrCO3 was also detected (peaks labeled in green) but was less prominent than in the patterns shown in Figures 3.5 and 3.6. The XRD pattern from the sintered + tested material (Fig. 3.7, bottom) was compatible with cubic LaFeO3 (PDF 75-

0541, red), orthorhombic LaFeO3 (PDF 88-0641, blue), and orthorhombic La0.3Sr0.7FeO3

(PDF 82-1269). The strontium carbonate peaks were no longer present. The sintering temperature of 1,350 °C was high enough to decompose SrCO3.

Figure 3.7: XRD patterns of (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) as-calcined (top) with indexing peaks for cubic LaCo0.4Fe0.6O3 (PDF 40-0224, red), SrCO3 (PDF 74- 1491, blue), and rhombohedral La0.3Sr0.7FeO3 (PDF 82-1964, green). Bottom: Sintered + tested; peaks indexed for cubic LaFeO3 (PDF 75-0541, red), orthorhombic LaFeO3 (PDF 88-0641, blue), and orthorhombic La0.3Sr0.7FeO3 (PDF 82-1269, green).

Of the matching phases listed in Fig. 3.7, the ones that best match the CWRU3 composition were different cubic phases: LaCo0.4Fe0.6O3 (PDF 40-0224) for the as-calcined

powder and LaFeO3 (PDF 75-0541) for the sintered+tested powders (the red phase match is being covered by other matches).

3.2.8. (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4)

The XRD pattern (Fig. 3.8, top) of the as-calcined material shows perovskite peaks

compatible with cubic LaCrO3 (PDF 75-0288) and rhombohedral La0.6Sr0.4FeO3 (PDF 49-

0285.) SrCO3 was detected (peaks labeled in green) but was less prominent than in the

patterns shown in Figures 3.5, 3.6, and 3.7.

The pattern of the sintered + tested material (Fig. 3.8, bottom) is compatible with cubic LaCrO3 (PDF 75-0288), orthorhombic La0.7Sr0.3FeO3 (PDF 89-1269) and

rhombohedral La0.6Sr0.4FeO3 (PDF 49-0285). Strontium carbonate is barely detectable. The sintering temperature of 1,350 °C was high enough to thermally decompose SrCO3.

Out of the top PDF matching candidates listed in Fig. 3.8, both the as-calcined powder and sintered+tested powder best matched in composition and diffraction pattern with rhombohedral La0.6Sr0.4FeO3 (PDF 49-0285). There was no clear advantage for any of

the three phases in the XRD fit, the compositional agreement of rhombohedral

La0.6Sr0.4FeO3 (PDF 49-0285) made it the preferred choice. 40

3.3. Scanning electron microscopy

All of the SEM images were of the pressed pellet surface of the samples unless otherwise noted.

The surface of the sintered, untested pellet surface of composition (La0.60Sr0.40)0.995

Co0.20Fe0.80O3-δ (FCM1) (Fig. 3.9a) had grain sizes of 0.5–1.5 µm. The lighter particles,

typically 0.2 µm in diameter, were too small for their compositions to be identified by EDS;

even at low accelerating voltage (5-10 kV) the composition of the material beneath the bright spots dominated the x-rays being detected by EDS. However, the particles are suspected to be palladium (from the sputter coating) or cobalt-rich based on the SEM images of other compositions. A few pores, 0.3–1.2 µm in size, can be seen. This pellet was sintered at

1,250 °C and was 96.3% dense (Table 3.1a).

Composition (La0.80Sr0.20)0.95 FeO3-δ (FCM2) (Fig. 3.9b) showed grains mostly in the 1–

2 µm range. The rounded, unfaceted appearance of the grains suggests that a liquid phase

formed during sintering. The SrFeO3-δ–LaFeO3-δ phase diagram (§B.2, Figure B.3) estimates that the melting point of SrFeO3-δ may be near 1,250 °C, so Sr-rich regions of the powder

might have been susceptible to local formation of liquid. A few pores, 0.1–1 µm in size, can be seen. This pellet was 97.4% dense (Table 3.1b).

The pellet of (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) (Fig. 3.9c) was sintered at 1,350 °C.

Although the grain sizes spanned a range (0.5–2 µm) similar to that of FCM1 and FCM2, this composition appeared to have finer grain size overall. A few pores, 0.1–0.3 µm in size, can be seen. This pellet was 98.0% dense (Table 3.1c).

The pellet of (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2) (Fig. 3.9d) was sintered at

1350 °C. The grain size is relatively coarse, 2-6 µm. This is comparable to CWRU3. There are several distinct nodules, 100–200 nm in size, on the grain boundaries which were

identified as being cobalt-rich. No pores were evident on the surface. This pellet was 99.0% dense (Table 3.1d).

The pellet of (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1) (Fig. 3.9e) was sintered at 1,350

°C. This composition had markedly coarser grains overall, 2–5 µm. than the previous

specimens. A few pores, less than 1 µm in size were observed. This pellet was 96.3% dense

(Table 3.1e).

The pellet of (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2) (Fig. 3.9f) was sintered at

1350 °C. This composition had finer grains overall, 0.1–0.6 µm, comparable to FCM1 and

PRAX1, and a few nodules, ~20–30 nm in size, at grain boundaries. (Attempts to identify

the compositions of the nodules via EDS mapping were unsuccessful. In comparison to

Figure 3.9d, the nodules might be cobalt-rich.) Pores were not evident on the surface. This

pellet was 97.0% dense (Table 3.1f).

Composition (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) (Fig. 3.9g) was sintered

at 1,350 °C. The grain size is relatively coarse, 1-6 µm. A few pores, 0.5–1 µm in size, can be

seen. This pellet was 96.4% dense (Table 3.1g). As in Figure 3.9b, the rounded, unfaceted

appearance of the grains suggests that a liquid phase might have formed during sintering.

Composition (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) (Fig. 3.9h) was sintered at

1,350 °C. The grain sizes range from 0.5 to 2 µm, comparable to FCM1, FCM2, and PRAX1.

As in Figures 3.9b and 3.9g, the rounded, unfaceted appearance of the grains suggests that a

liquid phase might have formed during sintering. The lighter particles were suspected to be

charging areas where the palladium sputtering did not cover. A few pores, 0.2–0.5 µm in

size, can be seen. This pellet was 96.9% dense (Table 3.1h).

0.60 0.40 0.995 0.20 0.80 3-δ a. (La Sr ) Co Fe O (FCM1) b. (La0.80Sr0.20)0.95 FeO3-δ (FCM2)

c. (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) d. (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2)

e. (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1) f. (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2)

g. (La0.20Sr0.80)Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) h. (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) Figure 3.9: SEM images of pellets prior to measurements of thermal and chemical expansion.

The SEM images in Figure 3.10 are of samples after their thermal and chemical

expansions were measured at 900 °C. Methyl cellulose was used as a binder during pressing

of FCM1, FCM2, PRAX2, and CWRU3. PRAX1, CWRU1, CWRU2, and CWRU4, which

were pressed without binder, had been fragmented to do both SEM and XRD.

Some images in Figure 3.10 show larger or smaller grains that the images of as-

sintered pellets of the same composition in Figure 3.9. These differences probably result

from specimen-to-specimen or region-to-region differences in grain size, not from changes occurring during the dilatometer runs.

In FCM1 (Fig. 3.10a) the size of the grains (visible beneath a filamentary surface deposit) was ~2–3 µm (versus 0.5–1.5 µm as sintered). The EDS elemental mapping

(Fig. 3.10a, right side), taken at a lower accelerating voltage to obtain preferentially a surface analysis, identified the web-like features as palladium, probably from momentary instability in the deposition system during sputter coating. Pores were not observed on the surface of the sample (sintered at 1,250 °C, and 96.7% dense after thermal/chemical expansion measurement at 900 °C, Table 3.1a).

FCM2 (Fig. 3.10b) had a somewhat bimodal distribution of grain sizes: 0.5–1 µm and

0.25–0.5 µm, whereas the SEM image of the as-sintered specimen showed more uniform grain sizes (1–2 µm). Pores were not observed on the surface of the sample (sintered at

1,250 °C and 98.0% dense, Table 3.1b).

PRAX1 (Fig. 3.10c) had grain sizes ranging from 0.5–1 µm. EDS was not able to identify the string-like features on the surface of the grains using the applied elemental filters.

Pores were not observed on the surface of the sample (sintered at 1,350 °C and 96.4% dense, Table 3.1c).

PRAX2 (Fig. 3.10d) had cobalt-rich nodules on grain boundaries (Fig. 3.10d, right).

Pores were not observed on the surface of the sample (sintered at 1,350 °C and 99.0% dense, Table 3.1d).

CWRU1 (Fig. 3.10e) had no pores observed on the surface of the sample (sintered at

1,350 °C and 97.2% dense, Table 3.1e).

CWRU2 (Fig. 3.10f) had grain sizes of 0.1–0.8 µm. Pores 0.2-0.3 µm in diameter can be observed on the surface of the sample (sintered at 1,350 °C and 96.7% dense, Table 3.1f).

CWRU3 (Fig. 3.10g) had nodules, 50–100 nm in size, on the surface of the grains

(rather than nodules of similar size at the grain boundaries in PRAX2). These nodules were shown to be cobalt-rich by EDS elemental mapping (Fig. 3.10g, right). Pores were not observed on the surface of the sample (sintered at 1,350 °C and 98.7% dense, Table 3.1g).

CWRU4 (Fig. 3.10h) had grain sizes of 0.4-0.7 µm. As with PRAX1, EDS did not identify the string-like particulates on the grain boundaries when the elemental filter was applied. Pores 0.1–0.2 micron in size were observed on the surface of the sample (sintered at

1,350 °C and 99.9% dense, Table 3.1h).

a. (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1) Figure 3.10 (Start): SEM images of pellets after measurements of thermal and chemical expansion.

b. (La0.80Sr0.20)0.95 FeO3-δ (FCM2)

c. (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) Figure 3.10 (Continued): SEM images of pellets after measurements of thermal and chemical expansion.

d. (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2)

e. (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1) Figure 3.10 (Continued): SEM images of pellets after measurements of thermal and chemical expansion.

f. (La0.20Sr0.80) Co0.10 Cr0.10Fe0.70O3-δ (CWRU2)

g. (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) Figure 3.10 (Continued): SEM images of pellets after measurements of thermal and chemical expansion.

h. (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) Figure 3.10 (Concluded): SEM images of pellets after measurements of thermal and chemical expansion.

3.4. Energy dispersive x-ray spectroscopy

Chemical compositions were obtained via EDS for all the as-sintered and post-test

(tested at 900 °C) specimens, i.e. two different pellets of each composition. The software provided quantitative concentrations of La, Sr, Co, Fe, Cr, and Mg from the EDS spectra obtained from the whole region of images shown in Figures 3.9 and 3.10. The nominal and

EDS atomic fraction calculations are described in §A.10.

In FCM1 (Fig. 3.11a) the analyzed La, Sr, Co, and Fe contents in the as-sintered sample agreed with the nominal values to within 4 cat%, and within 2 cat% in the post-test

sample.

In FCM2 (Fig. 3.11b), the analyzed La, Sr, and Fe contents for the as-sintered sample

agreed with the nominal values to within 2 cat%, and within 4 cat% in the post-test sample.

In PRAX1 (Fig. 3.11c), the analyzed La, Sr, Fe, and Cr contents for the as-sintered sample agreed with the nominal values to within 3 cat%, and within 5 cat% in the post-test sample.

In PRAX2 (Fig. 3.11d) the analyzed La, Sr, Co, Fe, and Cr contents for the as- sintered sample agreed with the nominal values to within 3 cat%. In the post-test sample, the analyzed La, Co, Cr contents agreed with the nominal values to within 3 cat%, but the analyzed Sr content was 10 cat% lower, and the Fe content was 16 cat% higher, than the nominal values.

In CWRU1 (Fig. 3.11e) the analyzed La, Sr, Fe, and Cr contents for the as-sintered sample agreed with the nominal values to within 4 cat%, and within 2 cat% in the post-test sample.

In CWRU2 (Fig. 3.11f) the analyzed La, Co, and Cr contents for the as-sintered and the post-test samples agreed within 2 cat%. The as-sintered and post-test samples showed

5 cat% more Fe, and 2–8 cat% less Sr, than the nominal values.

In CWRU3 (Fig. 3.11g) the analyzed La, Sr, Co, Fe, Cr, and Mg contents for the as-

sintered sample agreed with the nominal values to within 3 cat%, and within 4 cat% in the

post-test sample.

In CWRU4 (Fig. 3.11h) the analyzed La, Sr, Fe, Cr, and Mg contents for the as- sintered sample agreed with the nominal values to within 2 cat%, and within 4 cat% in the

post-test sample.

a. (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ b. (La0.80Sr0.20)0.95 FeO3-δ

c. (La0.20Sr0.80) Cr0.20Fe0.80O3-δ d. (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ Figure 3.11 (Start): EDS analysis of sintered pellets.

f. (La0.20Sr0.80) Co0.10 Cr0.10Fe0.70O3-δ e. (La0.50Sr0.50) Cr0.20Fe0.80O3-δ

g. (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ h. (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ

Figure 3.11 (Concluded): EDS analysis of sintered pellets.

3.5. Linear strain versus oxygen step changes (800 °C, 900 °C, 1,000 °C)

Chemical strains due to changes in oxygen partial pressure were measured in each composition isothermally at 800 °C, 900 °C, and 1,000 °C. In Figures 3.12–3.19, the zero of strain is chosen to be in air (log pO2 = –0.678). For each composition, a different pellet was

used for each temperature. In these isothermal runs, 30–60 measurements of PLC (chemical

strain) were recorded in each atmosphere. The values plotted in Figures 3.12 – 3.19 are the

averages of these readings. The standard deviations of these averages (given in summary

tables 3.8 – 3.10) were in the third decimal place of absolute strain in units of 10–3 (e.g. for

CWRU4 at 800 °C in nitrogen, was 0.441 ± 0.002 (10–3)). The range of the readings was larger than the standard deviation, typically differing from the averages by ±0.007 (10–3).

The changes in chemical strain with each new atmosphere were large compared to the

standard deviations and the ranges in readings. §4.5 provides further discussion of the

chemical strain versus oxygen partial pressure.

Note that for each gas mixture (N2, CO2, A, B, C, D) the pO2 increases with

increasing temperature (see calculations in §A.8). Typically chemical expansion increased

with decreasing pO2 and with increasing temperature for all of the materials studied here.

3.5.1. (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1)

Figure 3.12 shows chemical strain with changing pO2 in FCM1 pellets that had been

sintered at 1,250 °C for 16 h. The specimens in the left graph, and the 1,000 °C specimen in

the right graph, had been pressed without binder before sintering; the 800 and 900 °C

specimens in the right graph had been pressed with methyl cellulose binder.

3.5.2. (La0.80Sr0.20)0.95 FeO3-δ (FCM2)

Figure 3.13 shows chemical strain with changing pO2 in FCM2 pellets that had been

sintered at 1,250 °C for 16 h.

Figure 3.13: Chemical strain versus log pO2 of (La0.80Sr0.20)0.95 FeO3-δ (FCM2). Left: Specimens pressed without binder. Right: Specimens pressed with methyl cellulose binder.

3.5.3. (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1)

Figure 3.14 shows chemical strain with changing pO2 in PRAX1 pellets that had been

sintered at 1,350 °C for 16 h.

2.0E-03 (La0.20Sr0.80) Cr0.20Fe0.80O3-δ

800 °C 900 °C 1.5E-03 1000 °C

1.0E-03 Linear Strain

5.0E-04

0.0E+00 -20 -10 0

Log pO2 Figure 3.14: Chemical strain versus log pO2 of (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1).

3.5.4. (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2)

Figure 3.15 shows chemical strain with changing pO2 in PRAX2 pellets that had been sintered at 1,350 °C for 16 h. The specimens in the left graph had been pressed without binder before sintering; the specimens in the right graph had been pressed with methyl cellulose binder.

Figure 3.15: Chemical strain versus log pO2 of (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2). Left: Specimens pressed without binder. Right: Specimens pressed with methyl cellulose binder.

3.5.5. (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1)

Figure 3.16 shows chemical strain with changing pO2 in CWRU1 pellets that had been sintered at 1,350 °C for 16 h.

(La0.50Sr0.50) Cr0.20Fe0.80O3-δ 3.0E-03 800 °C 900 °C 1000 °C

2.0E-03 Linear Strain

1.0E-03

0.0E+00 -20 -15 -10 -5 0

Log pO2 Figure 3.16: Chemical strain versus log pO2 of (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1).

3.5.6. (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2)

Figure 3.17 shows chemical strain with changing pO2 in CWRU2 pellets that had been sintered at 1,350 °C for 16 h.

3.0E-03 (La0.20Sr0.80) Co0.10Cr0.10Fe0.80O3-δ

800 °C 900 °C 1000 °C 2.0E-03 Linear Strain 1.0E-03

0.0E+00 -20 -15 -10 -5 0 Log pO2 Figure 3.17: Chemical strain versus log pO2 of (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2).

3.5.7. (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3)

Figure 3.18 shows chemical strain with changing pO2 in CWRU3 pellets that had been sintered at 1,350 °C for 16 h. The specimens in the left graph had been pressed without

57 binder before sintering; the specimens in the right graph had been pressed with methyl cellulose binder.

Figure 3.18: Chemical strain versus log pO2 of (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3). Left: Specimens pressed without binder. Right: Specimens pressed with methyl cellulose binder.

3.5.8. (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4)

Figure 3.19 shows chemical strain with changing pO2 in CWRU4 pellets that had been sintered at 1,350 °C for 16 h.

La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ 2.0E-03 800 °C 900 °C 1.5E-03 1000 °C

1.0E-03 Linear Strain

5.0E-04

0.0E+00 -20 -15 -10 -5 0

Log pO2 Figure 3.19: Chemical strain versus log pO2 of (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4).

3.6. Coefficient of chemical expansion

41 Bandopadhyay et al. measured values of δ for La0.20Sr0.80Cr0.20Fe0.80O3-δ (PRAX1)

from air to nitrogen between 800 °C and 1,000 °C (Table 3.2). Using the chemical strain data

from §3.6.3 (Fig. 3.14) for PRAX1, a value of αC was calculated using Equation 2.3 and

tabulated in Table 3.3.

Table 3.2: Oxygen nonstoichiometry for La0.20Sr0.80Cr0.20Fe0.80O3-δ as a function of 41 temperature and pO2. Gas Oxygen Non- Oxygen Non- Oxygen Non- Stoichiometry Stoichiometry Stoichiometry δ (1,000 °C) δ (900 °C) δ (800 °C) Air 0.20 0.17 0.15 Nitrogen 0.30 0.28 0.24

Table 3.3: Chemical expansion coefficients of experimental compositions. 800 °C 900 °C 1,000 °C 10 La0.60Sr0.40Co0.20Fe0.80O3-δ 0.032 – – 33 La0.60Sr0.40Co0.20Fe0.80O3-δ 0.031 0.031 – 42 La0.60Sr0.40Co0.20Fe0.80O3-δ 0.022 0.022 – FCM1 (this work) 0.0028 0.0028 – PRAX1 (this work) 0.0019 0.0013 0.0073

Adler et al.10 (700 °C – 890 °C), Bishop et al.33 (700 °C – 900 °C), Kuhn et al.42

(500 °C – 900 °C) reported values for αC for La0.60Sr0.40Co0.20Fe0.80O3-δ measured in nitrogen

–4 (pO2 = 10 ) (Table 3.3). This composition can be compared to FCM1, which differs only in the 0.5% A-site deficiency. Using values from the present work of chemical strain for FCM1 under nitrogen at 800 °C (ɛC = 0.00026) and 900 °C (0.00029), and values from Bishop et

33 al. for δ under nitrogen at 800 °C (δ = 0.092) and 900 °C (δ = 0.105), give a value of αC at

both temperatures of 0.0028. This value is about an order of magnitude lower than the

literature values (Table 3.3). This arises from the fact that the chemical strains measured in

this work in nitrogen were about an order of magnitude lower than the values shown in refs.

10, 33, and 42.

3.7. Summary tables

Table 3.4: Average values of immersion density and % unit-cell density. 3 Composition ρavg (g/cm ) Average % unit-cell FCM1 6.14 ± 0.02 96.9 ± 0.3 FCM2 6.2 ± 0.1 100 ± 2 PRAX1 5.7 ± 0.1 98 ± 2 PRAX2 5.82 ± 0.04 98.1 ± 0.7 CWRU1 5.94 ± 0.04 96.6 ± 0.7 CWRU2 5.67 ± 0.07 96 ± 1 CWRU3 5.75 ± 0.09 98 ± 2 CWRU4 5.91 ± 0.08 98 ± 1

Table 3.5: Identified perovskite crystal structures of experimental compositions from XRD. Composition As-received/calcined phase Sintered + tested phase FCM1 Rhombohedral Rhombohedral FCM2 Orthorhombic Orthorhombic PRAX1 Rhombohedral Rhombohedral PRAX2 Rhombohedral Rhombohedral CWRU1 Rhombohedral Orthorhombic CWRU2 Cubic Rhombohedral CWRU3 Cubic Cubic CWRU4 Rhombohedral Rhombohedral

Table 3.6: Summary of pre-test and post-test SEM results. Grain size Pore size Notes Composition (µm) (µm) Pre- Post- Pre- Post- Pre- Post- Light 0.5-1.5 2-3 0.3-1.2 – Pd-strings FCM1 particles FCM2 1-2 0.25-1 0.1-1 – Liq. phase – PRAX1 0.5-2 0.5-1 01.-0.3 – – Strings Co-nodules at Co- 2-6 – – – grain PRAX2 nodules boundaries CWRU1 2-5 – ≤ 1 – – Nodules at CWRU2 0.1-0.6 0.1-0.8 – 0.2-0.3 grain – boundaries Co-nodules on 1-6 – 0.5-1 – Liq. phase CWRU3 surface Liq. phase, Strings at CWRU4 0.5-2 0.4-0.7 0.2-0.5 0.1-0.2 light grain particles boundaries

Table 3.7: Summary of EDS results: deviations of elemental compositions from nominal values. Composition Pre-test cat% Post-test cat% FCM1 4 2 FCM2 2 4 PRAX1 3 5 3, but 10 cat% less Sr and 16 cat% 3 PRAX2 more Fe CWRU1 4 2 CWRU2 5, but 6 cat% less Sr 2, but 8 cat% less Sr CWRU3 3 4 CWRU4 2 2

–3 Table 3.8: Experimental chemical strain measurements (10 ) as a function of pO2 at 800 °C. For FCM1, FCM2, and CWRU3, the tabulated values are the average of the values from two runs on different pellets (one with binder, one without binder).

Specimen Air N2 CO2 Mix A Mix B Mix C Mix D 0.340 ± 0.510 ± 0.680 ± 0.850 ± 1.020 ± 1.189 ± FCM1 0.000 0.001 0.002 0.002 0.002 0.002 0.002 0.340 ± 0.509 ± 0.849 ± 1.019 ± 1.189 ± 1.358 ± FCM2 0.000 0.003 0.002 0.001 0.002 0.002 0.002 0.171 ± 0.343 ± 0.515 ± 0.686 ± 0.858 ± 1.201 ± PRAX1 0.000 0.002 0.003 0.001 0.002 0.002 0.001 0.338 ± 0.506 ± 0.675 ± 0.844 ± 1.012 ± 1.349 ± PRAX2 0.000 0.002 0.002 0.002 0.002 0.002 0.002 1.352 ± 1.521 ± 1.689 ± 2.027 ± 2.196 ± 2.365 ± CWRU1 0.000 0.002 0.001 0.001 0.001 0.001 0.001 0.511 ± 0.681 ± 1.192 ± 1.703 ± 1.873 ± 2.044 ± CWRU2 0.000 0.002 0.002 0.002 0.002 0.001 0.002 0.170 ± 0.339 ± 0.509 ± 0.678 ± 0.848 ± 1.107 ± CWRU3 0.000 0.002 0.002 0.001 0.0001 0.002 0.002 0.440 ± 0.609 ± 0.947 ± 1.117 ± 1.286 ± 1.456 ± CWRU4 0.000 0.002 0.002 0.002 0.001 0.002 0.003

–3 Table 3.9: Experimental chemical strain measurements (10 ) as a function of pO2 at 900 °C. For FCM1, FCM2, PRAX2, and CWRU3, the tabulated values are the average of the values from two runs on different pellets (one with binder, one without binder).

Specimen Air N2 CO2 Mix A Mix B Mix C Mix D 0.293 ± 0.510 ± 0.879 ± 1.025 ± 1.171 ± 1.318 ± FCM1 0.000 0.002 0.001 0.002 0.002 0.003 0.002 0.586 ± 0.879 ± 1.025 ± 1.318 ± 1.464 ± 1.610 ± FCM2 0.000 0.002 0.002 0.002 0.002 0.002 0.001 0.148 ± 0.442 ± 0.737 ± 0.885 ± 1.032 ± 1.180 ± PRAX1 0.000 0.003 0.003 0.002 0.002 0.001 0.001 0.344 ± 0.490 ± 0.710 ± 0.930 ± 1.149 ± 1.441 ± PRAX2 0.000 0.002 0.002 0.002 0.002 0.002 0.002 0.586 ± 1.463 ± 1.903 ± 2.049 ± 2.342 ± 2.635 ± CWRU1 0.000 0.001 0.001 0.002 0.003 0.002 0.002 0.738 ± 0.885 ± 1.180 ± 1.328 ± 1.918 ± 2.508 ± CWRU2 0.000 0.001 0.003 0.001 0.001 0.001 0.002 0.146 ± 0.292 ± 0.395 ± 0.585 ± 0.877 ± 1.168 ± CWRU3 0.000 0.002 0.002 0.002 0.002 0.003 0.001 0.742 ± 0.891 ± 1.039 ± 1.187 ± 1.335 ± 1.632 ± CWRU4 0.000 0.002 0.002 0.001 0.002 0.002 0.002

–3 Table 3.10: Experimental chemical strain measurements (10 ) as a function of pO2 at 1,000 °C. For FCM1, FCM2, PRAX2, and CWRU3, the tabulated values are the average of the values from two runs on different pellets (one with binder, one without binder). Except for FCM1 where the secondary run did not use binder.

Air N2 CO2 Mix A Mix B Mix C Mix D 0.193 ± 0.451 ± 0.708 ± 0.901 ± 1.094 ± 1.286 ± FCM1 0.000 0.002 0.001 0.002 0.002 0.002 0.002 0.516 ± 0.710 ± 0.967 ± 1.096 ± 1.547 ± 1.870 ± FCM2 0.000 0.002 0.001 0.002 0.002 0.002 0.001 0.733 ± 0.990 ± 1.247 ± 1.375 ± 1.504 ± 1.632 ± PRAX1 0.000 0.001 0.002 0.002 0.001 0.002 0.002 0.708 ± 0.902 ± 1.095 ± 1.288 ± 1.480 ± 1.866 ± PRAX2 0.000 0.003 0.003 0.002 0.002 0.001 0.002 0.516 ± 0.903 ± 1.420 ± 2.194 ± 2.452 ± 2.839 ± CWRU1 0.000 0.002 0.002 0.001 0.002 0.002 0.002 1.033 ± 1.163 ± 1.292 ± 1.886 ± 2.196 ± 2.584 ± CWRU2 0.000 0.002 0.002 0.003 0.001 0.002 0.002 0.388 ± 0.646 ± 0.840 ± 1.034 ± 1.163 ± 1.357 ± CWRU3 0.000 0.002 0.001 0.002 0.002 0.002 0.002 1.038 ± 1.168 ± 1.427 ± 1.556 ± 1.686 ± 1.945 ± CWRU4 0.000 0.001 0.001 0.002 0.001 0.003 0.004

Table 3.11: Coefficients of thermal (air to mixture D) and chemical expansion of experimental compositions 800-1,000 °C.

αC (εC/∆δ) Composition 800 °C 900 °C 1,000 °C FCM110,33,42 0.022–0.032 0.022–0.031 – FCM1 (this work) 0.0028 0.0028 – FCM2 – – – PRAX1 0.0019 0.0013 0.0073 PRAX2 – – – CWRU1 – – – CWRU2 – – – CWRU3 – – – CWRU4 – – –

Chapter 4: Discussion of Results

4.1. Densification and sintering effects

The immersion density measurements indicated that the sintered samples achieved the densification (95% or more) needed to separate air from methane in the dense layer of an OTM-based reformer. Therefore, these materials advanced to dilatometric and pO2 exposure testing. Figures 3.9b, 3.9g, 3.9h showed evidence of liquid phase sintering in

FCM2, CWRU3, and CWRU4. These compositions had the highest sintered densities (as percentage of unit cell density, Table 31.b, g, and h) of the materials studied here (CWRU3 and CWRU4 being tied with PRAX2), suggesting that the apparent presence of liquid assisted in densification.8

Sintering temperature not only affects the densification of the samples, but also has effects on the chemical properties of the composition. Zeng et. al.’s work43 with

La0.6Sr0.4Co0.2Fe0.8O3-δ (very close to FCM1) showed that increasing the sintering temperature from 1,000 to 1,300 °C yielded significantly better oxygen permeation and electronic and ionic conductivity (Fig. 4.1). In this work, Cr-free compositions (FCM1 and FCM2) were sintered at 1,250 °C, and Cr-containing compositions were sintered at 1,350 °C.

a b

Figure 4.1: Properties of La0.6Sr0.4Co0.2Fe0.8O3-δ relevant to OTM applications: a) electronic conductivity b) oxygen permeation flux c) ionic conductivity as a function of temperature and sintering temperature. Republished with permission of Elsevier Science & Technology Journals, from Zeng et al.43; permission conveyed through Copyright Clearance Center, Inc.

4.2. X-ray diffractometry

The XRD patterns for the as-calcined compositions made at CWRU (CWRU1–

CWRU4), and for as-received commercial powder PRAX2, showed the presence of SrCO3.

(Figs. 3.4-3.8) Table 4.1 lists the thermal decomposition temperatures for all the potential

cation carbonates in the compositions studied here. Calcining was carried out at 850 °C, high enough to thermally decompose all of the other potential carbonates, except SrCO3. The

SrCO3 peaks were missing from after sintering and expansion testing, demonstrating that the

sintering temperature (1250-1350 °C) was sufficient to thermally decompose SrCO3.

Table 4.1: Thermal decomposition temperatures of potential experimental cation carbonates. Carbonate Thermal decomposition temperature (°C) La 700-75044 Sr 1100-125040 Co 350-40045 Cr 550-60046 Mg 500-60047 Fe 400-50048

The diffraction patterns for (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1), as-received and

after expansion testing, matched best to rhombohedral phases ((La0.3Sr0.7)FeO3 (PDF 82-

1964) and (La0.4Sr0.6)FeO3 (PDF 82-1963) respectively) (Table 3.5). This is consistent with

the results of Kuhn et. al. on La0.6Sr0.4CoyFe1−yO3−δ (y = 0, 0.2, 0.4, 0.5, 0.6, 0.8, and 1.0), who

reported that, for all of these Co-doping levels, the primary phase was rhombohedral.42

The composition and diffraction patterns for (La0.80Sr0.20)0.95 FeO3-δ (FCM2), as-

received and after expansion testing, matched best to orthorhombic (La0.7Sr0.3FeO3 (PDF 89-

1269) and La0.8Sr0.2FeO3 (PDF 35-1480) respectively) phases (Table 3.5). This is also

consistent with the findings of Fossdal et. al. on La1-xSrxFeO3−δ, who found that increasing

the Sr content transforms the crystal structure from orthorhombic (0 ≤ x ≤ 0.2), to

rhombohedral (0.4 ≤ x ≤ 0.7), and to cubic (0.8 ≤ x ≤ 1.0).39

Both FCM1 and FCM2 were nominally A-site deficient, FCM1 by 0.5% and FCM2 by 5%. Commercial LSCF-based perovskites for SOFC and OTM applications are sometimes intentionally made to be A-site-deficient, to suppress reaction with other ceramic phases with which they are in contact. An example is (La,Sr)-based perovskites forming lanthanum zirconate or strontium zirconate when in contact with zirconia. Making the perovskite A-site-deficient reduces the activity of the La and Sr, making them less likely to react with other non-perovskite components in the system. Almost all of the matching PDF

reference materials were nominally stoichiometric perovskites (A/B = 1.00), so exact

compositional matches to FCM1 and FCM2 were not possible in the PDF available at

CWRU, even though the structural matches were very strong. Again, this can be taken as

evidence that the lattice parameters of the (La,Sr)(Co,Cr,Fe,Mg)O3 family of perovskites are

not highly sensitive to exact compositions, including slight A/B non-stoichiometry.

(La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) and (La0.20Sr0.80) Co0.10Cr0.20Fe0.70O3-δ (PRAX2)

were matched to different rhombohedral phases identified in the XRD data (Table 3.5), for

PRAX1, LaCrO3 (PDF 33-0702) for the as-received powder, and La0.6Sr0.4FeO3 (PDF 82-

1261) for the sintered+tested material, for PRAX2, La0.6Sr0.4Co0.4Fe0.6O3 (PDF 49-0284) for the as-received powder, and La0.6Sr0.4FeO3 (PDF 82-1961) for the sintered+tested material.

As mentioned with FCM2, a high Sr content in La1-xSrxFeO3−δ results in a rhombohedral (0.4

≤ x ≤ 0.7) or cubic (0.8 ≤ x ≤ 1.0) structure. Ignoring the B-site minority Cr and Co in

PRAX1 and PRAX2 puts them (with x = 0.8) on the border between the rhombohedral-to-

49 cubic transition. However, Gupta et. al. studied (La0.8Sr0.2)0.95Cr1-xFexO3 (x = 0.1-0.3) and

found them to be rhombohedral. Though those materials had a lower Sr content, the Cr

content made the structure rhombohedral. A phase diagram (Figure B.4) by Yokokawa et

50 al. shows that (Sr0.25La0.75)CrO3 was found to be rhombohedral which provides further

evidence that Cr on the B-site favors a rhombohedral structure. Thus there is support in the literature (though it is not conclusive) that PRAX1 and PRAX2 have a rhombohedral structure. More careful XRD work, including slower scans across multiple peaks at the higher range of 2θ, might give more conclusive evidence of the structure of these materials.

The compositions synthesized at CWRU, with four, five, or six cations, do not belong to compositional families whose structures have been studied in detail. Table 3.5 shows that (La0.50Sr0.50)Cr0.20Fe0.80O3-δ (CWRU1) matched a rhombohedral phase as calcined,

and an orthorhombic phase in the sintered + tested material (La0.4Sr0.6FeO3 (PDF 82-1963)

and La0.6Sr0.4FeO3 (PDF 49-0285) respectively). The as-calcined XRD is consistent with the

rhombohedral symmetry in FCM2, PRAX1, and PRAX2. However, only one other of the

post-test materials shares the orthorhombic symmetry. Hansen et al. also worked with

La1-xSrxFeO3−δ (x = 0.05, 0.15, 0.25, 0.35, 0.50, 0.70) and found that all their perovskites were

orthorhombic.51 It is possible that the sintered + tested material is two-phase (orthorhombic and rhombohedral). The EDS result (Fig. 3.11e) for this composition showed that both the pre-test and post-test materials were low in Sr compared to the nominal composition: the value of x in La1-xSrx is 0.31 for the pre-test specimen and 0.27 for the post-test specimen,

bringing the Sr content closer to the orthorhombic/rhombohedral phase boundary observed

39 by Fossdal et al. in La1-xSrxFeO3.

(La0.20Sr0.80) Co0.10Cr0.10Fe0.80O3-δ (CWRU2) best matched a cubic phase

(LaCo0.4Fe0.6O3, PDF 40-0224) as calcined, and a rhombohedral phase (La0.6Sr0.4FeO3, PDF

82-1961) for the sintered + tested specimen (Table 3.5). Similar to PRAX1 and PRAX2, the

Sr content of CWRU2 may also be near a rhombohedral/cubic phase boundary. As mentioned previously, the Cr content may also favor rhombohedral symmetry. Therefore, a mixture of a cubic and a rhombohedral phase is possible.

(La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) had a cubic structure (LaCo0.4Fe0.6O3

(PDF 40-0224) and LaFeO3 (PDF 75-0541) respectively) for both the as-calcined and sintered + tested XRD (Table 3.5). The Sr content is high enough that a cubic structure is

39 possible, based on the findings of Fossdal et al. in (LaSr)FeO3 cited above. The Mg content

might favor cubic symmetry rather than a rhombohedral structure from the Cr content, but

the same speculation does not appear to apply to CWRU4.

(La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4) had a rhombohedral (La0.6Sr0.4FeO3 (PDF

49-0285)) structure for both the as-calcined and sintered + tested XRD (Table 3.5). Even

though the Sr content would put CWRU4 into the rhombohedral phase based on the

findings of Fossdal et al, it is possible that a mixture of orthorhombic and rhombohedral

phases is present in the sintered + tested XRD. Alternatively, the Sr content, being close to

the orthorhombic and rhombohedral transition, could be more towards the orthorhombic

side. The role of Mg in determining the preferred symmetry of these phases has not been

explored in the literature, and more systematic study would be needed.

For five of the materials studied here (FCM2, PRAX1, PRAX2, CWRU2, and

CWRU4), the peaks in the XRD pattern of the sintered + tested material were sharper, and the baseline less noisy, than in the pattern from the untested specimen. This may indicate that some grain growth took place during the dilatometer runs of these materials.

The best-matching PDF reference materials did not have identical compositions to those studied here (Table 3.5). The PDF matching algorithm often came up with dozens of matching (La,Sr)(Co,Cr,Fe,Mg)O3 perovskite phases for each of the experimental patterns in this study. That indicates that the lattice parameters of this family of perovskites are not highly sensitive to composition. The phase assignments made here were based on quality of match to the experimental XRD patterns, and closest available agreement with the experimental compositions, to give a reasonable basis for the unit cell volume and the unit- cell-density calculations.

More relevant than an exact compositional match was finding the crystal system of each material, and checking for evidence that the materials underwent irreversible phase transformations on exposure to the low-oxygen atmospheres of the expansion measurements. The rhombohedral or orthorhombic distortions from cubic symmetry in this

family of perovskites are small; it is common in the older literature for all of the phases to be

indexed as cubic for simplicity. Nevertheless, phase stability is an important advantage for

OTM materials, because phase transformations can generate stresses in these ceramics

sufficient to cause fracture, independent of any thermal or chemical expansions.

CWRU1 and CWRU2 were the only compositions that displayed different crystal structures

in their as-calcined versus sintered + tested samples. These materials also showed the highest

total chemical expansion of the materials studied here (Figs. 4.3, 4.4, 4.5). A phase

transformation could have occurred during atmospheric testing: the chemical expansion for

CWRU2 in particular increased sharply below –15 < logpO2 < 10 at all three temperatures;

and chemical expansion rose strongly in CWRU1 with the onset of reducing atmospheres at

800, 900, and 1,000 °C, and again at logpO2 of ~–10 at 1,000 °C (Figs. 4.3, 4.4, 4.5).

However, the XRD was conducted on two different sample types, the as-calcined powder

versus the sintered powder. So it is possible that a phase change could have occurred during

the sintering steps also.

4.3. Scanning electron microscopy

Pre-test and post-test PRAX2 and post-test CWRU3 samples showed small cobalt-

rich nodules (Fig. 3.9 and 3.10). This could mean that these compositions are near a

solubility limit for cobalt, but their Co contents are lower (both 0.1) than FCM1 (0.2) which

showed no Co-rich nodules; and La1–xSrxCoO3 perovskites exist with 100% Co on the B-site

and 0 < x < 0.80.52

Since the SEM images were taken on the pressed faces of the specimens, the

presence of the nodules may be a surface effect. Lein’s group observed high cation mobility

of Fe and Co in La0.5Sr0.5Fe1−xCoxO3−δ (x = 0, 0.5 and 1), allowing these components to move

toward the high-pO2 surface (the opposite direction of oxygen ion diffusion) of OTMs

exposed to an oxygen gradient at temperatures of 900 °C or greater.53

4.4. Energy dispersive x-ray spectroscopy

The cation fractions of the pre-test and post-test specimens, as determined in EDS,

agreed with the intended compositions within 2 to 5%. The largest discrepancies came from

post-test PRAX2 and pre- and post-test CWRU2 (Figs. 3.11d and 3.11f). Both had greater iron content and lower strontium content than the nominal values. This could be due to the respective pellets having iron-rich areas rather than a homogeneous distribution. On the other hand, during powder processing, some contamination could have occurred like the small amount of cobalt detected in CWRU1. It is recommended that these samples/powders undergo inductively coupled plasma spectroscopy to get an independent, overall chemical analysis.

Cr-rich perovskites are prone to Cr volatilization.54 This was not observed in most of

the present EDS results on Cr-containing compositions (Figs. 3.11c–h), with only CWRU2 being low on Cr in the post-test pellets.

4.5. Effect of pO2 and Temperature on Chemical Linear Strain

In all of the materials studied here, chemical expansion increased as the temperature

increased and as pO2 decreased (§3.6). Similarly, a chosen value of chemical expansion was

reached at higher pO2 (less reducing conditions) as temperature increased. Results

10 33 42 of Adler (Fig. 1.3), Bishop et al. (Fig. 4.2), and Kuhn et al. with La0.6Sr0.4Co0.2Fe0.8O3-δ

–4 (like FCM1 but without A-site deficiency) at 700 °C–900 °C and between pO2 =1.0 and 10

bar showed qualitatively similar trends. However, the present results for chemical strains

in FCM1 were an order of magnitude lower than the literature values at the same pO2. Those

–3 studies reported chemical strains of 10 in nitrogen (log pO2 = –4) whereas in FCM1 in this

study, strains of that magnitude required log pO2 of –13 (at 1,000 °C), –14 (at 900 °C), or

–18 (at 800 °C) (Fig. 3.12).

Figure 4.2: Isothermal chemical expansion as a function of pO2 at different temperatures using pO2 = 0.21 atm (air) as a zero expansion reference point. Republished with permission of John Wiley & Sons – Books, from Bishop et al.33; permission conveyed through Copyright Clearance Center, Inc.

CWRU1, CWRU2, and CWRU4 reached chemical strain of 10–3 or higher in nitrogen

at 1,000 °C (Fig. 4.5), and PRAX1 and PRAX2 (repeat) came close (Figs. 3.14,

3.15). CWRU1 also exceeded 10–3 chemical strain at 800 °C (Fig. 4.3). In this work, no other

compositions reached chemical strain of 10–3 in nitrogen at 900 °C or 800 °C (Figs. 4.3, 4.4;

Tables 3.8, 3.9), i.e. in the temperature range studied in references 10, 33, and 42.

(La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ [FCM1] 2.5E-03 800°C (La0.80Sr0.20)0.95 FeO3-δ [FCM2] (La0.20Sr0.80) Cr0.20Fe0.80O3-δ [PRAX1]

(La0.20Sr0.80) Co0.10Cr0.20Fe0.70O3-δ [PRAX2]

(La0.50Sr0.50) Cr0.20Fe0.80O3-δ [CWRU1]

2.0E-03 (La0.20Sr0.80) Co0.10Cr0.10Fe0.80O3-δ [CWRU2]

(La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ [CWRU3]

(La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ [CWRU4]

1.5E-03

1.0E-03 Linear Strain

5.0E-04

0.0E+00 -20 -15 -10 -5 0 Log pO 2 Figure 4.3: Linear strain versus Log pO2 of experimental compositions at 800 °C.

3.0E-03 900°C (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ [FCM1] (La0.80Sr0.20)0.95 FeO3-δ [FCM2]

(La0.20Sr0.80) Cr0.20Fe0.80O3-δ [PRAX1] 2.5E-03 (La0.20Sr0.80) Co0.10Cr0.20Fe0.70O3-δ [PRAX2] (La0.50Sr0.50) Cr0.20Fe0.80O3-δ [CWRU1]

(La0.20Sr0.80) Co0.10Cr0.10Fe0.80O3-δ [CWRU2] (La Sr ) Co Cr Mg Fe O [CWRU3] 2.0E-03 0.20 0.80 0.10 0.10 0.05 0.75 3-δ (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ [CWRU4]

1.5E-03 Linear Strain 1.0E-03

5.0E-04

0.0E+00 -20 -15 -10 -5 0

Log pO2

Figure 4.4: Linear strain versus Log pO2 of experimental compositions at 900 °C.

(La Sr ) Co Fe O [FCM1] ° 0.60 0.40 0.995 0.20 0.80 3-δ 1000 C (La Sr ) FeO [FCM2] 3.0E-03 0.80 0.20 0.95 3-δ (La0.20Sr0.80) Cr0.20Fe0.80O3-δ [PRAX1]

(La0.20Sr0.80) Co0.10Cr0.20Fe0.70O3-δ [PRAX2]

2.5E-03 (La0.50Sr0.50) Cr0.20Fe0.80O3-δ [CWRU1]

(La0.20Sr0.80) Co0.10Cr0.10Fe0.80O3-δ [CWRU2]

(La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ [CWRU3]

2.0E-03 (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ [CWRU4]

1.5E-03 Linear Strain 1.0E-03

5.0E-04

0.0E+00 -15 -10 -5 0

Log pO2 Figure 4.5: Linear strain versus Log pO2 of experimental compositions at 1000 °C.

Thermal expansion coefficients obtained in this work (§A.11) were lower than literature values by about 50% (8–12 × 10–6 °C–1, compared to 15–20 × 10–6 °C–1). This, combined with the preceding discussion of chemical strains, suggests that readings of PLC in this work are consistently low. This could be due to the use of the alumina reference sample as a spacer, which is not part of the Orton standard operating procedure. The total PLC values reported here will in general contain contributions from the pellet and the alumina reference. As discussed in §A.11, the alumina reference is 25.4 mm long, whereas the experimental pellets were typically 1.3 mm thick. The CTE of alumina is about half that of

OTM perovskites (7.4–8.0 x 10–6/°C, Table 2.1).

Thermal strains of the alumina will therefore be about 10 times larger than those of the pellet. However, the alumina reference showed no systematic change in dimension in the atmosphere changes used in this study (§A.2, Fig. A.3). Therefore, the chemical strains

measured in this work should be due entirely to the perovskite pellets, though they might have been overwhelmed by thermal strains in the alumina reference (because of the observed thermal fluctuations during the nominally isothermal runs).

On the other hand, the reproducibility in the measurements of chemical strain

(Figs. 3.12, 3.13, 3.15, 3.18, and 4.7) was high. This indicates that trends between specimens

are still reliable and can be discussed qualitatively with regard to compositional differences

between the materials, as done below.

At 800, 900, and 1,000 °C, (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1), (La0.20Sr0.80)

Cr0.20Fe0.80O3-δ (PRAX1), and (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) had the lowest

linear strains at the lowest pO2 (Figs. 4.3–4.5; Table 4.2).

Table 4.2: Ideal experimental compositions with their chemical strains at lowest pO2. –3 –20.3 –17.9 –15.9 Composition (εC x 10 ) 800 °C (10 ) 900 °C (10 ) 1000 °C (10 ) FCM1 1.27 1.32 1.29 PRAX1 1.20 1.18 1.63 CWRU3 1.11 1.17 1.36

The current results suggest that A-site deficiency in (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ

(FCM1) contributed to lower chemical strain. Kostogloudis et al. demonstrated that the A- site deficiency generally leads to a lattice expansion (in a rhombohedral system) in this composition (Figure 4.6) but lowers the thermal expansion coefficient at 700 °C.55 The latter

effect suggests an increase in overall bond strength, which would be compatible with lower

chemical expansion. FCM1 is similar in composition to CWRU1 and CWRU4, except with

Co instead of Cr on the B-site. Perhaps the lower Sr content in FCM1 compared to the

other compositions resulted in lower chemical expansion since Sr content determines the

oxygen vacancy concentration.

55 Figure 4.6: Lattice constants in (La0.6Sr0.4)1-zCo0.2Fe0.8O3-δ as a function of A-Site deficiency.

This group observed that the electronic conductivity peaked between 500 and

600 °C, and decreased at all temperatures (100–1,000 °C) as the A-site deficiency increased.55

Thus even though (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1) had one of the lowest chemical strains at the lowest pO2 level (§4.5), the A-site deficiency might cause an undesired drop in electronic conductivity.

The low linear strain in the (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) and

(La0.20Sr0.80)Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) compositions may result from their significant concentrations of (Cr+Mg). These are the least reducible B-site cations used in this work (Table 4.3). So as a first consideration, these cations will be less likely to reduce to lower oxidation states at low pO2. Since reducing oxidation states increases cationic radii, the greater stability of Cr and Mg against reduction may have contributed to the low chemical strain of these two materials.

Table 4.3: Reduction potentials of experimental B-Site cations. Half-Reaction Reduction Potential (V) Co+3 + e- → Co+2 1.8256 Fe+4 + e- → Fe+3 0.9057 Fe+3 + e- → Fe+2 0.7756 Cr+3 + e- → Cr+2 -0.9156 Mg+2 + 2e- → Mg -2.3656

+4 In SrFeO3, there is substantial Fe . PRAX1, PRAX2, CWRU2, and CWRU3 had high values of Sr content (1-x = 0.80). High Sr content with Fe+4 compositions means that

+4 they could have higher chemical strain since Fe reduces easily. However, PRAX1 and

CWRU3 had among the lowest chemical expansions, while PRAX2 was in the middle of the group of eight. CWRU2 consistently had high chemical strains (Figs. 4.3-4.5).

PRAX2 had the fourth-lowest chemical strains, very close to those of PRAX1 at all temperatures. The composition of PRAX2 differed from that of PRAX1 only in a partial replacement of 0.1 of the Fe with Co. The argument for the greater reducibility of Co versus

Fe is compatible with the small increase of chemical strain in PRAX2 compared to PRAX1.

Using Coulomb’s Law, the attractive force (Eq. 4.1) and potential (Eq.4.2) between the cations and oxygen ions can be calculated (Table 4.5). ke is the Coulomb’s law constant

(8.988×109 N m2 C–2), q is the oxidation number of the respective ion, and r is the center-to- center distance between ions. It is assumed that oxygen ions have an oxidation state of –2 and ionic radius of 126 pm58, that r is the sum of the ionic radii between an oxygen ion and the respective cation, and La, Sr, and Mg ions are not multivalent.

| | (4.1) = 1 2 𝑞𝑞 𝑞𝑞 𝐹𝐹 𝑘𝑘𝑒𝑒 2 | 𝑟𝑟 | (4.2) = 1 2 𝑒𝑒 𝑞𝑞 𝑞𝑞 𝐸𝐸 𝑘𝑘 𝑟𝑟 77

From Table 4.4, the PRAX1 sample would most likely have the lower expansions since it contains Cr that has a high attractive force and potential to O, and Cr is not readily reduced (Table 4.3). However, as is often the case when bonds to oxygen are strong, this

might decrease oxygen flux.

Table 4.4: Attractive force and potential values between oxygen ions and experimental cations. Radii assuming O–2 radius of 126 pm, coordination number of 6 on B sites and 12 on A sites, from Shannon.58 Cationic Radii Force Potential Cation Oxidation State 58 (10–12 m) (10-8 N) (10-18 J) Sr (II) +2 158 1.6 4.6 Fe (II) +2 (3d6) 92 (high spin) 2.7 6.0 Fe (II) +2 (3d6) 75 (low spin) 3.2 6.5 Co (II) +2 88.5 2.8 6.1 Mg (II) +2 86 2.9 6.1 Cr (II) +2 (3d4) 94 (high spin) 2.7 5.9 Cr (II) +2 (3d4) 87 (low spin) 2.9 6.1 La (III) +3 150 2.6 7.1 Fe (III) +3 78.5 4.7 9.5 Co (III) +3 (3d6) 75 (high spin) 4.8 9.7 Co (III) +3 (3d6) 68.5 (low spin) 5.2 10.0 Cr (III) +3 75.5 4.8 9.7 Fe (IV) +4 72.5 6.6 13.1

4.6. Coefficient of Chemical Expansion

Literature values of the oxygen deficiency parameter δ were necessary to calculate

the coefficient of chemical expansion (Eq. 4.2). This coefficient is dimensionless, being a

ratio of chemical strain to change in δ, which are also dimensionless. Values of δ from

41 -0.678 -15 Bandopadhyay et al. (Table 3.2) from 10 atm to 10 atm pO2 between 750 °C -1,040 °C

were available for one of the three compositions that exhibited the lowest chemical strains,

33 (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1). Bishop et al. measured the coefficient of chemical

expansion value for (La0.60Sr0.40) Co0.20Fe0.80O3-δ (a close match to the composition of FCM1)

–4 from 1 atm to 10 atm pO2 between 700 °C and 900 °C . The coefficient of chemical expansion of (La0.20Sr0.80)Cr0.20Fe0.80O3-δ (PRAX1) was 0.0013-0.0073, lower than for

(La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ with value of 0.031.

4.7. Reproducibility

Figure 4.7 shows reproducibility of chemical strain measurements for CWRU3 and

FCM1, two of the three compositions that exhibited the lowest chemical strains. The low

chemical expansion was closely reproduced (and in some cases was even lower) in the

repeated measurements at all three temperatures. FCM2 and PRAX2 also showed a similar

level of reproducibility (§B.3). The second measurements labeled (2) used methyl cellulose as

a binder during the powder processing steps except for FCM1 1,000 °C (2).

1.5E-03 1.5E-03

(La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1)

800 °C 800 °C 1.0E-03 800 °C (2) 1.0E-03 800 °C (2) Linear Strain 5.0E-04 Linear Strain 5.0E-04

0.0E+00 0.0E+00 -20 -15 -10 -5 0 -20 -15 -10 -5 0 Log pO2 Log pO2

1.5E-03 1.5E-03 (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1) (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) 900 °C 900 °C 900 °C (2) 900 °C (2) 1.0E-03 1.0E-03 Linear Strain Linear Strain 5.0E-04 5.0E-04

0.0E+00 0.0E+00 -20 -15 -10 -5 0 -20 -15 -10 -5 0

Log pO2 Log pO2

1.5E-03 (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1) (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) 1.5E-03 1000 °C 1000 °C 1000 °C (2) 1000 °C (2) 1.0E-03

1.0E-03 Linear Strain Linear Strain 5.0E-04 5.0E-04

0.0E+00 0.0E+00 -15 -10 -5 0 -15 -10 -5 0 Log pO2 Log pO2

Figure 4.7: Reproducibility of experimental compositions. Second measurements labeled (2) used methyl cellulose as a binder during the powder processing steps except for FCM1 1,000 °C (2).

Chapter 5: Conclusion

The chemical strains of eight lanthanum strontium ferrite-based oxygen transport

materials, most substituted with Co, Cr, and/or Mg, were evaluated at 800, 900, and

1,000 °C over wide ranges of oxygen partial pressure (10–0.6–10–20) relevant to natural gas

reforming applications. Common behaviors exhibited by all materials were increasing

chemical expansion with increasing temperature and decreasing pO2.

At the lowest partial pressures of oxygen (10-20.3, 10-17.9, 10-15.9) and their respective

isothermal temperatures (800 °C, 900 °C, 1,000 °C), the compositions that demonstrated the lowest chemical expansion were (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1), (La0.20Sr0.80)

Cr0.20Fe0.80O3-δ (PRAX1), and (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3). Proposed

explanations for the low chemical expansion of these compositions include the presence of

A-site deficiency, Sr content, the stability of Cr and Mg against reduction, and Cr having a

high attractive force and potential to O.

Other compositional effects were observed. Both FCM1 and PRAX1 had low and

similar chemical strains. However, FCM1’s coefficient of chemical expansion was

significantly greater than PRAX1’s. PRAX1 has chromium while FCM1 had cobalt; so the stronger chromium-oxygen bond helps strengthen the lattice despite PRAX1 likely having more oxygen vacancies because of its higher strontium content (by a factor of 2).

Another compositional effect observed was that CWRU2 is the same as CWRU3, except it lacks the small replacement of Fe with 0.05 Mg. CWRU3 had chemical strains significantly lower than CWRU2 at all three temperatures. This is also seen in CWRU1 and

CWRU4, with CWRU4 having a small replacement of Fe with 0.05 Mg, and a much lower chemical strain than CWRU1. The effect of the Mg substitution was unexpectedly large.

EDS data of experimental pellets matched the nominal EDS relatively well. Sintering

temperatures of 1250-1350 °C removed strontium carbonate secondary phases based on the

XRD data of the compositions, and provided sufficient densification of 95% or more. In the

SEM data of the post-tested pellets, cationic diffusion may account for the cobalt nodules observed on the surfaces in (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (PRAX2, before and after

the low-pO2 testing) and (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3, post-testing only).

This behavior could adversely affect long-term chemical stability (and possibly mechanical

stability) of these materials as membranes. (PRAX2 had the fourth-lowest chemical

expansion, and CWRU3 was one of the three lowest). Duplicate runs demonstrated that the

chemical expansion data overall were very reproducible at all three temperatures of testing.

Chapter 6: Future Work

6.1. Scanning electron microscopy

The SEM and EDS imaging and data showed interesting results such as formation of

Co-rich precipitates at grain boundaries in PRAX2. Lower magnifications of the SEM

images could provide more reliable average chemical composition data, less subject to

localized inhomogeneity.

The SEM and EDS work done here was of the pressed surfaces of the pellets. It

would be of interest to observe the microstructures and EDS elemental analysis for the

interiors of these specimens, to check for consistency and for possible volatilization of

components like Mg and Cr. Unfortunately, due to constraints of time and access to the

microscope, additional SEM and EDS were not possible.

6.2. Elemental analysis

EDS on the SEM at SCSAM was used to determine the elemental analysis in the experimental compositions. These analyses may be affected by region-to-region differences within specimens, and are subject to uncertainties unless reference standards are used. A more accurate way for the elemental analysis of the powders would be to use inductively coupled plasma mass spectroscopy. Many groups use this method for LSF-based ceramic materials.59–61

6.3. Reproducibility

The chemical linear strain values for the duplicate runs mostly showed very good

agreement. However, not all compositions had duplicate runs such as,

(La0.20Sr0.80)Cr0.20Fe0.80O3-δ (PRAX1), (La0.50Sr0.50)Cr0.20Fe0.80O3-δ (CWRU1), (La0.20Sr0.80)Co0.10

Cr0.10Fe0.80O3-δ(CWRU2), and (La0.50Sr0.50)Cr0.20Mg0.05Fe0.75O3-δ(CWRU4). Duplicate runs of

these specimens, especially PRAX1 (one of the three compositions with the lowest chemical

expansion) should be carried out in future work. Runs with specimens about 25 mm in

length, such as those carried out by Adler10 and Bishop et al.33, without the alumina spacer,

should be carried out. This will determine whether the absence of the spacer will bring the

measurements of thermal and chemical strains into better agreement with the literature.

6.4. Oxygen diffusion coefficient and flux measurements

This project was able to observe the effects of chemical expansion. However, to convert chemical strain to coefficients of chemical expansivity (chemical expansion coefficients), measurement of the oxygen non-stoichiometry is necessary, which can be done using thermogravimetric analysis.62,63 To measure oxygen diffusivity as well as chemical

expansion during step changes in pO2, a dilatometer with higher precision and faster data

acquisition rate than the current one used in this project is needed.

To measure oxygen permeation into the membrane, 18O tracer diffusion studies can

be conducted to measure the intrinsic and grain-boundary oxygen diffusion coefficients of

the compositions. To measure the oxygen flux through the membrane, an apparatus like the

one shown in Figure 6.1 (proposed by Dr. Ajit Sane and Skip Robinson of Volt Research

LLC) can be used with air on one side and humidified natural gas on the other side.

Measuring the CO and H2 content using mass spectroscopy and mass balance can determine

the oxygen flux of the respective disk.64

6.5. Porous-dense-porous architecture

As explained previously in the introduction, porous-dense-porous architecture of a membrane could be utilized to improve the mechanical and chemical stability. These specimens can be created by using the Teroaka AMP method described in this project, along

with the sinter procedures, to create the dense pellets. Spray pyrolysis, or the use of fugitive

pore formers like polymer microbeads or carbon black, can be used to fabricate the porous layers.64

Figure 6.1: Apparatus to measure oxygen flux through OTM disk.64

6.6. Ruddlesden-Popper perovskites

An alternative phase structure for these materials are Ruddlesden-Popper oxides that

follow the An+1BnO3n+1 formula. Ruddlesden-Popper materials have demonstrated lower

5 thermal expansion coefficients compared to the more commonly known ABO3 perovskites.

The lower thermal expansion coefficients may aid in improving the mechanical properties of

OTMs.

A. Appendix A: Procedures

A.1. Dilatometer calibration34

1. Put the reference sample into the sample holder and set up the dilatometer as if an

experiment were running (connect the atmospheric control tube, and adjust the

furnace to the proper position).

2. On the main screen (Figure A.1), click on the “Dilatometer” pull down menu and

choose “Setup a Calibration Run”.

3. Provide a name for the calibration file and the reference sample length

4. The tuning constant is “Air”

5. Enter the starting temperature (this will be displayed on the dilatometer).

6. Set the H/C rate is set to 10 °C / minute and the segment temperature is 1000 °C.

7. Check that the Safety Shutoff PLC is set to 450%.

8. The temperature range will adjust automatically.

9. The reference file is the theoretical expansion data for the reference sample.

“REF=Alumina@” should be selected.

10. Press “Apply” and finally press “Run”.

11. Data are now being collected which displays Percent Linear Change versus

Temperature. The black curve is what the dilatometer is directly measuring. The red

curve is the theoretical reference curve (Figure A.2).

12. Once the calibration run is complete, select “Correct” to correct the measured data

to the theoretical curve (Figure A.2).

13. Once the correction has been made, a calibration file with the name that was given in

step 3 will be created. Subsequent experimental runs will use this calibration file.

Figure A.1: Orton software menu item locations.34

Figure A.2: Orton software calibration display.34

A.2. Alumina reference sample deviation

1. On the main menu page, select “Dilatometer” and in the pulldown menu; select

“Setup an Experimental Run.”

2. In the Setup Page, label the sample and write detailed notes in the “Remarks” box.

3. Input the starting temperature, starting length, and Air tuning constant.

4. There will be 5 Ramp settings.

• Ramp 1: 10 °C/minute Segment Temp-600 °C Hold-60 min

• Ramp 2: 10 °C/minute Segment Temp-700 °C Hold-60 min

• Ramp 3: 10 °C/minute Segment Temp-800 °C Hold-60 min

• Ramp 4: 10 °C/minute Segment Temp-900 °C Hold-60 min

• Ramp 5: 10 °C/minute Segment Temp-1000 °C Hold-60 min

5. Once those ramp settings are in place, check the “Safety Shutdown PLC” is 450.

6. The temperature range will adjust accordingly to the ramp settings.

7. Set the delay start time to 0.

8. Press “Apply”. You will be taken back to the main menu. Select “Run”.

9. The dilatometer will start taking Percent Linear Change versus Temperature Data.

10. Once the experimental run is complete, select “Data” then “Export”.

11. Export the text file produced during the run.

12. Once the data have been retrieved, repeat the previous steps (1-11) two more times.

13. The result will be three new data files for the reference sample. The software

analyzes the PLC and CTE data in these files, and presents statistics showing how

much the obtained values of CTE deviate from each other at the 5 hold

temperatures from step 4.

The purpose of the runs on the alumina reference sample is to see the effects of the intended atmosphere changes on alumina. The defect concentrations in alumina are so low that chemical expansion in this material is expected to be negligible.

Figure A.3 shows that there was no systematic effect of the changes in atmosphere on the PLC of the alumina reference sample at 800 °C. The average values of PLC in each atmosphere were all 0.5879. This applied at 900 °C and 1,000 °C also, but just with different respective average PLC values (0.6793 and 0.7886 respectively).

Figure A.3: Alumina reference sample tested at 800 °C under experimental atmospheres.

A.3. Starting powder weights

1. Most of the metal nitrate powders were 98%-100% pure which can be assumed to be

100% for calculation purposes.

2. Find the molecular weight of the starting powders (including the nitrate and

hydrates) in grams per mole.

3. Find the number of moles of cation present in the starting powders.

4. Find the molecular mass to moles of cation ratio of the starting powders.

5. Calculate the molecular weight of the typical oxide formed by the cation of the

starting powders in grams per mole.

6. Calculate the number of moles of cation present in the typical oxide.

7. Calculate the molecular mass to moles of cation ratio of the oxides.

8. For the compositions that are being observed, find the moles of each cation present

respectively.

9. Calculate the molecular weight of the target composition by multiplying the moles of

cation in the target composition from Step 8 with their respective molecular mass to

moles of cation ratio of oxides from Step 7.

10. Calculate the moles of product by taking the target yield (in grams) and dividing that

by the molecular weight of the target composition from Step 9.

11. To calculate the weight of starting metal nitrate needed, multiply the moles of

product from Step 10 by the molecular mass to moles of cation ratio of starting

powders from Step 4 and the moles of cation from Step 8.

12. Repeat step 11 for all cations present in the target powder compositions to find the

all the weights of starting metal nitrate powders needed.

A.4. Amorphous malic acid precursor powder synthesis

1. To find the malic acid needed in grams, multiply the molar ratio of acid to metal ions

(3/2 = 1.5) by the sum of the moles of cations in the target composition, the target

yield (in grams) wanted, and the molecular weight of malic acid (134.987 g/mol). The

take the product and divide it by the molecular weight of the target composition.

2. To find the amount of distilled water, take the target yield (in grams) and divide it by

the molecular weight of the target composition. This results in the moles of the

product attained which is then converted into millimols (1 mole = 1000 mmols).

Multiply the conversion with the 1.5 L / 52.5 mmols ratio to attain the amount of

water needed in liters.

3. In a 2-liter beaker, pour the previously calculated starting metal nitrates and malic

acid amounts.

4. Add the calculated water to the mixture and stir.

5. Take pH measurements on the starting solution which starts out to be very acidic

(~pH = 1).

6. Put ammonium hydroxide into a smaller beaker and slowly add it to the starting

solution until the pH = 2-3 (~15-17 mL of ammonium hydroxide) while stirring.

7. Put the 2-liter beaker starting solution onto a hot plate with the heat settings to 3.

8. Let the starting solution dry and evaporate (3-5 days)

9. Once the drying process is done, precipitate salts are leftover.

10. Extract the salt from the beaker and grind it with a pestle and mortar into a fine

powder.

11. Weigh the powder (this powder should be larger than the target yield because of the

nitrate still present).

12. Put the powder into a large crucible with fiberfrax covering the opening (to prevent

violent exothermic reactions) and then into a Barnstead International Type 48000

Furnace for calcining.

13. The ramp rate of the furnace should be 10 °C / minute and the calcination

temperature is 850 °C for 6 hours.

14. Once the calcination process is done, grind, with a pestle and mortar, the powder

again and put it into a separate container.

15. Weigh the powder (this should be close to the target yield).

A.5. Pellet pressing

1. Add sufficient calcined powder mixed with methyl cellulose as binder so that a layer

is formed that is not too thick into a pellet die (a combined weight of 0.5-0.8 grams).

2. To use the Carver Press, adjust the upper limit as needed.

3. Put the die onto the press platform and turn the valve on the press clockwise until it

cannot be turned (not too tightly).

4. Use the lever until the die is pressed and the pressure gauge reads 5000-6000 lbs.

5. Let the die sit at that pressure for 30-45 seconds for the powder to relax.

6. Carefully, while holding the bottom of the platform, turn the pressure valve

counterclockwise to release the pressure.

7. Gently remove the disk and place into a high temperature crucible.

8. Wash the die and its components with alcohol.

9. Repeat the previous steps if additional disks are needed.

A.6. Archimedes density technique

1. Set up the density kit which consists of the scale, beaker, and thermometer

2. Fill the beaker with water with the thermometer inside the density set

3. Take the mass of the sample (dry weight) using the top scale (above the water)

4. Take the sample out and put it into a small beaker of water

5. Put the beaker into a Branson 1200 sonicator and use it to fill in the open pores on

the surfaces of the sample

6. Put the wet sample on the scale below the water in the density kit and take the

weight

7. Take the wet sample off the scale below water and put the sample back onto the dry

scale (above water) to get the unsubmerged wet sample dry weight

8. 3 weights are now present; w1 = dry weight, w2 = the submerged wet weight, and w3

= unsubmerged wet weight

9. The temperature of the water must be taken because the density of water is a

function of temperature

10. The density of the sample can be calculated with the Equation 4 in §2.4

A.7. Experimental testing in dilatometer

1. Put sample between push rod tip and alumina reference sample.

2. Adjust the LVDT dial until dilatometer displays 0.0000 PLC.

3. Attach the atmospheric setup.

4. Move furnace all the way to the left.

5. Provide a sample name, run name, and operator name.

6. Input starting temperature and sample length.

7. Ramp at 10 °C per minute to set point (800 °C, 900 °C, or 1000 °C) with a hold time

of 720 minutes in total.

8. Start dilatometer run and wait for the dilatometer to reach set point temperature.

A.8. pO2 calculations and gas mixtures

The pO2 for each gas mixture and temperature was determined by first using

Equation A.1:

° = (A.1)

∆𝐺𝐺 ∆𝐻𝐻 − 𝑇𝑇∆𝑆𝑆

Then the following Gibbs Free Energy Equations A.2-A.465 were necessary for the 2CO (g)

+ O2 (g) ↔ 2CO2 (g) reaction and Table A.1 are the calculated values for ∆G° at each experimental temperature:

2CO (g) → 2C (g) + O2 (g) 2∆G° = 2(111,700 + 87.65T) (A.2)

2C (g) + O2 (g) → 2CO2 (g) 2∆G° = 2(-394,100 – 0.84T) (A.3)

2CO (g) + O2 (g) ↔ 2CO2 (g) ∆G°total = 2(111,700 + 87.65T) + 2(–394,100 – 0.84T) (A.4)

Table A.1: Calculated ∆G° values for 2CO (g) + O2 (g) ↔ 2CO2 (g) as a function of temperature

Temperature (°C) ∆G°total (kJ) 800 –378.5 900 –361.1 1000 –343.8

Using the equilibrium constant (Eq. A.5) the pO2 can be calculated using Equation

A.6. Table A.2 gives the calculated pO2 values from the carbon monoxide and carbon dioxide gas mixtures.

° [ ] = = 2 −∆𝐺𝐺 𝐶𝐶 𝐶𝐶2 (A.5) 𝑅𝑅𝑅𝑅 𝑝𝑝 2 𝑘𝑘 𝑒𝑒 𝐶𝐶𝐶𝐶 𝑂𝑂2 𝑝𝑝 𝑝𝑝 = 2 ° (A.6) 𝐶𝐶𝐶𝐶2 exp 𝑂𝑂2 𝑝𝑝 𝑝𝑝 2 𝐶𝐶𝐶𝐶 −∆𝐺𝐺 𝑝𝑝 � 𝑅𝑅𝑅𝑅 � Table A.2: Calculated pO2 levels of gas mixtures as a function of temperature.

Gas Composition Log pO2 at 800 °C Log pO2 900 °C Log pO2 1000 °C

CO2 99.99% CO2 + 0.003% CO –9.4 –7.0 –4.9

A 99.01% CO2 + 0.99% CO –14.4 –12.1 –10.1

B 90.1% CO2 + 9.9% CO –16.5 –14.2 –12.2

C 50.5% CO2 + 49.5% CO –18.4 –16.1 –14.1

D 10.9% CO2 + 89.1% CO –20.3 –17.9 –15.9

-0.678 -4 The pO2 for air (10 atm) and nitrogen gas (10 atm) were assumed constant

across 800 °C - 1000 °C.66

The CO2 tank provided by Airgas (CD I200) was given that at 25°C, the tank

contained 99.99% CO2, N2 impurity measurement of ≤50 ppm (0.005%), and O2 impurity

measurement of ≤20 ppm (0.002%). The assumption was made that rest is carbon monoxide

(0.003%) and the nitrogen impurity had a negligible effect on the overall pO2 for the gas

tank. From this, the pO2 level was calculated, for the carbon dioxide gas tank since the partial

pressure varies with temperature. Table A.3 demonstrates the variation of the oxygen

impurity in the Airgas carbon dioxide gas tank and its effect on the calculated pO2 at 1000

°C. The amount of oxygen impurity was believed to have negligible effect as seen by the

variation of pO2.

Table A.3: Oxygen impurity in CO2 tank versus variation in pO2 at 1000 °C.

Oxygen (ppm) Impurity in CO2 Gas Tank at 25 °C Calculated pO2 (atm) at 1000 °C 10 0.00001 20 0.00001 30 0.00001

A.9. Unit-cell density calculations

The unit-cell densities of the compositions (Table A.4) were computed using

Equation A.7

w = × (A.7) A c 𝑛𝑛 𝑚𝑚 𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷 𝑍𝑍 where n is the number of formula units per𝑁𝑁 𝑉𝑉 unit cell, mw is the formula mass of the

composition, and NA is Avogadro’s number. The volume of the unit cell (VC) and the

number of formula units per cell (Z) were taken from the PDF records for the phases that best matched the experimental XRD patterns (§3.2).

Table A.4: XRD volume and unit-cell data. Unit-cell Volume Composition PDF entry density (Å3) (g/cm3) (La0.3Sr0.7)FeO3 (82-1964) (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ and 349.30* 6.338 La0.60Sr0.40Co0.20Fe0.80O3-δ42 La0.8Sr0.2FeO3 (PDF 35- (La0.80Sr0.20)0.95 FeO3-δ 242.86 6.183 1480) La0.6Sr0.4FeO3 (PDF 82- (La0.20Sr0.80) Cr0.20Fe0.80O3-δ 342.87 5.839 1261) La0.6Sr0.4Co0.4Fe0.6O3 (PDF (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ 338.16 5.930 49-0284) La0.4Sr0.6FeO3 (PDF 82- (La0.50Sr0.50) Cr0.20Fe0.80O3-δ 350.51 6.149 1963) LaCo0.4Fe0.6O3 (PDF 40- (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ 56.64 5.912 0224) LaCo0.4Fe0.6O3 (PDF 40- (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ 56.64 5.865 0224) La0.6Sr0.4FeO3 (PDF 49- (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ 355.57 6.018 0285)

* Averaged for FCM1 between PDF 82-1964 and Kuhn et al.’s 42 XRD analysis of La0.60Sr0.40Co0.20Fe0.80O3-δ.

A.10. Elemental analysis calculations

The atomic fraction for each element for both the nominal and EDS values was calculated by taking each element fraction and dividing it by the sum of the total number of cations. The calculated values are displayed in Table A.5.

Table A.5: Cation atomic fractions: Nominal versus EDS (single run analyses).

Composition Cation Total fLa fSr fCo fFe fCr fMg FCM1 (Nominal) 1.995 0.299 0.199 0.100 0.401 0.000 0.000 FCM1 (EDS) 0.558 0.332 0.186 0.059 0.423 — — FCM1-Tested (EDS) 0.469 0.314 0.179 0.105 0.402 — — FCM2 (Nominal) 1.950 0.390 0.097 0.000 0.513 0.000 0.000 FCM2 (EDS) 0.456 0.410 0.096 — 0.493 — — FCM2-Tested (EDS) 0.327 0.427 0.087 — 0.486 — — PRAX1 (Nominal) 2.000 0.100 0.400 0.000 0.400 0.100 0.000 PRAX1 (EDS) 0.438 0.114 0.368 — 0.400 0.119 — PRAX1-Tested (EDS) 0.524 0.112 0.349 — 0.422 0.117 — PRAX2 (Nominal) 2.000 0.100 0.400 0.050 0.350 0.100 0.000 PRAX2 (EDS) 0.426 0.086 0.351 0.030 0.498 0.036 — PRAX2-Tested (EDS) 0.436 0.073 0.307 0.031 0.514 0.075 — CWRU1 (Nominal) 2.000 0.250 0.250 0.000 0.400 0.100 0.000 CWRU1 (EDS) 0.487 0.307 0.204 0.002 0.369 0.118 — CWRU1-Tested (EDS) 0.448 0.269 0.208 0.002 0.415 0.106 — CWRU2 (Nominal) 2.000 0.100 0.400 0.050 0.400 0.050 0.000 CWRU2 (EDS) 0.441 0.098 0.374 0.043 0.363 0.122 — CWRU2-Tested (EDS) 0.454 0.114 0.313 0.028 0.520 0.025 — CWRU3 (Nominal) 2.000 0.100 0.400 0.050 0.375 0.050 0.025 CWRU3 (EDS) 0.464 0.119 0.366 0.045 0.403 0.045 0.022 CWRU3-Tested (EDS) 0.521 0.143 0.395 0.062 0.331 0.051 0.018 CWRU4 (Nominal) 2.000 0.250 0.250 0.000 0.375 0.100 0.025 CWRU4 (EDS) 0.466 0.273 0.236 — 0.365 0.105 0.021 CWRU4-Tested (EDS) 0.314 0.259 0.285 — 0.315 0.117 0.024

A.11. Coefficient of thermal expansion

A standard definition of the coefficient of linear thermal expansion, αT, is

= = (A.8) ( ) ⁄ 0 2 1 𝑇𝑇 ∆𝑙𝑙 𝑙𝑙 𝑙𝑙 − 𝑙𝑙 𝛼𝛼 0 2 1 where l 0 is the length of the∆𝑇𝑇 specimen𝑙𝑙 𝑇𝑇 at− a𝑇𝑇 reference temperature T0 (usually room

temperature) and l 1 and l 2 are the lengths of the specimen at temperatures T1 and T2 in °C.

In the Orton dilatometer, percent linear change PLCT at some temperature T2 is defined as

100( ) = = 100 (A.9) 2 0 𝑇𝑇2 𝑙𝑙 − 𝑙𝑙 𝑇𝑇2 𝑃𝑃𝑃𝑃𝑃𝑃 0 𝜀𝜀 where is engineering strain𝑙𝑙 at a temperature T2. If the reference temperature and T1 are

𝑇𝑇2 both room𝜀𝜀 temperature (TR in °C), then l 1 = l 0, and Equation A.8 gives

= ( ) 𝑙𝑙2 − 𝑙𝑙0 𝛼𝛼𝑇𝑇 This “overall” coefficient of thermal expansion𝑙𝑙0 𝑇𝑇 can2 − be𝑇𝑇𝑅𝑅 thought of as representing the net

change in dimension between TR and T2 as a line of slope α T. In terms of PLC T,

= = (A.9b) 100( ) ( ) 𝑃𝑃𝑃𝑃𝐶𝐶𝑇𝑇2 𝜀𝜀𝑇𝑇2 𝛼𝛼𝑇𝑇 If instead one is𝑇𝑇2 interested− 𝑇𝑇𝑅𝑅 in𝑇𝑇 the2 − rate𝑇𝑇𝑅𝑅 of change in specimen dimensions with temperature at a temperature far from room temperature, say at 800 °C, and given values of the specimen length over some narrow range of temperature above and below 800 °C, say

797 to 803 °C, then Equation A.8 can be rewritten to obtain the coefficient of linear thermal expansion in the vicinity of 800 °C as ′ 𝛼𝛼𝑇𝑇 = (A.10) (803 797) ′ 𝑙𝑙803 − 𝑙𝑙797 𝛼𝛼𝑇𝑇 𝑙𝑙0 −

Note that here l 0 is still the length of the specimen at room temperature. A series of

manipulations can re-express in terms of PLC as given by the dilatometer: ′ 𝛼𝛼𝑇𝑇 ( ) ( ) ( ) ( ) = = 803 0 797 0 (803 797) 𝑙𝑙 −(803𝑙𝑙 797𝑙𝑙 )− 𝑙𝑙 ′ 𝑙𝑙803 − 𝑙𝑙0 − 𝑙𝑙797 − 𝑙𝑙0 𝑙𝑙0 − 𝑙𝑙0 𝛼𝛼𝑇𝑇 𝑙𝑙0 100−( ) 100( − )

= 803 0 797 0 𝑙𝑙 100− (𝑙𝑙803 797𝑙𝑙) − 𝑙𝑙 ′ 𝑙𝑙0 − 𝑙𝑙0 𝛼𝛼𝑇𝑇 − = (A.10b) 100(803 797) ′ 𝑃𝑃𝑃𝑃𝐶𝐶803 − 𝑃𝑃𝑃𝑃𝐶𝐶797 𝛼𝛼𝑇𝑇 While each pellet was− being held under a steady flow of gas of fixed composition (or under static lab air) at a nominally fixed setpoint temperature (800, 900, or 1,000 °C) for typically 25 to 30 minutes, the thermocouple readings fluctuated ± 5 °C around the setpoint.

PLC readings at those larger fluctuations were more erratic, but the PLC readings tracked temperature fluctuations of ±3 °C reproducibly (Fig. A.4).

Figure A.4: Average PLC (~50-70 data points) at each pO2 level and temperature between 797 °C and 803 °C for (La0.80Sr0.20)0.95 FeO3-δ (FCM2)

Typically ~50-70 PLC measurements were obtained within ± 3 °C of each set point in each

gas mixture. For example, Figures A.5–A.7 show PLC versus temperature for (La0.80Sr0.20)0.95

FeO3-δ (FCM2) within ± 3 °C at 800. 900, and 1,000 °C respectively. The variations in PLC

with temperature were larger than the variations in PLC at each temperature. This enabled

the coefficient of thermal expansion in the vicinity of 800, 900, and 1,000 °C (± 3 °C) to ′ 𝑇𝑇 be computed from the slope of the line𝛼𝛼 ar fit to plots like Figures A.5–A.7 using Equation

A.10b under the local temperature fluctuations in each atmosphere.

Figure A.5: PLC in gas mixture C at 800±3 °C for (La0.80Sr0.20)0.95 FeO3-δ (FCM2).

Figure A.6: PLC in gas mixture C at 900±3 °C for (La0.80Sr0.20)0.95 FeO3-δ (FCM2).

100

Figure A.7: PLC in gas mixture C at 1,000±3 °C for (La0.80Sr0.20)0.95 FeO3-δ (FCM2).

Experimentally, the values of and will in general be numerically different; they ′ 𝑇𝑇 𝑇𝑇 can be equal if the thermal expansion 𝛼𝛼is truly linear𝛼𝛼 from any T2 to TR, or if the slope at

(T2 + T1)/2 coincidentally equals the slope between l 0 and l 2.

Table A.6 gives values obtained in this work of the overall coefficients of linear thermal expansion between room temperature and 800, 900, and 1,000 °C (Equation

𝑇𝑇 A.9b) and values of𝛼𝛼 in the vicinity of 800, 900, and 1,000 °C (Equation A.10b). ′ 𝑇𝑇 The values for𝛼𝛼 CTE in Table A.6 are about half to two-thirds of those reported for

LSF perovskites in the literature. Factors that might contribute to this discrepancy include:

• The dilatometer runs carried out in this work included the alumina reference sample

along with the sintered pellet in the sample holder (§A.7.1). Otherwise, the pellets

were too thin to be in contact with both the push rod and the opposite end of the

sample holder.

• The pellets were much thinner (1.3 mm, vs. 25.4 mm) than the alumina reference, so

the measured PLCs may have been dwarfed by the PLC of the alumina reference.

101

We cannot currently account for the discrepancies between the present CTE values and the literature. This effect might also extend in a systematic way to the values of chemical strain obtained here. Nevertheless, the absence of repeatable dimensional changes of the alumina reference sample under changes in atmosphere (§A.2, Fig. A.3), and the good reproducibility of the measurements of chemical expansion (§3.5.1, 3.5.2, 3.5.4, 3.5.7) suggest that the trends in the present values of chemical strain are still reliable.

102

Table A.6 (start): Coefficients of linear thermal expansion ( ) of the studied materials

as a function of temperature and atmosphere. All values of 𝑇𝑇CTE1−𝑇𝑇2 include experimental uncertainty (± one standard deviation), are in units of 10–6 [°C𝛼𝛼 –1] and were measured in the given atmospheres* between the temperatures T1 and T2 (in °C) indicated in the subscripts.

a) (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ (FCM1) air N2 CO2 A* B* C* D* 9.4 ± 0.5 11.1 ± 0.2 9.7 ± 0.5 9.3 ± 0.3 9.0 ± 0.3 9.0 ± 0.3 9.0 ± 0.4 9.0 ± 0.5 11 ± 1 9.5 ± 0.3 9.2 ± 0.2 8.9 ± 0.3 8.9 ± 0.2 8.9 ± 0.2 31−800 𝛼𝛼′ 10.1 ± 0.4 10.8 ± 0.3 9.1 ± 0.4 9.1 ± 0.3 9.1 ± 0.3 9.2 ± 0.4 9.5 ± 0.5 797−803 𝛼𝛼 9.8 ± 0.4 10.7 ± 0.4 9.0 ± 0.2 9.0 ± 0.1 9.0 ± 0.2 9.1 ± 0.2 9.4 ± 0.2 𝛼𝛼30−900 ′ , 11.3 ± 0.3 11.1 ± 0.5 10.1 ± 0.4 8.1 ± 0.8 11.1 ± 0.2 10.1 ± 0.2 10.1 ± 0.5 𝛼𝛼897−903 29−1 000, 11.0 ± 0.7 11.0 ± 0.8 10.0 ± 0.3 8.0 ± 0.7 11.0 ± 0.2 10.0 ± 0.2 10.0 ± 0.5 𝛼𝛼′ 997−1 003 𝛼𝛼 b) (La0.80Sr0.20)0.95 FeO3-δ (FCM1)

air N2 CO2 A* B* C* D* 9.4 ± 0.3 9.2 ± 0.5 8.8 ± 0.3 9.3 ± 0.2 8.8 ± 0.8 9.0 ± 0.3 9.4 ± 0.3 9.0 ± 0.4 9.1 ± 0.2 8.7 ± 0.2 9.2 ± 0.2 8.7 ± 0.2 8.9 ± 0.2 9.3 ± 0.3 30−800 𝛼𝛼′ 10.4 ± 0.5 9.4 ± 0.4 9.1 ± 0.4 9.4 ± 0.3 9.4 ± 0.4 9.2 ± 0.8 9.0 ± 0.1 797−803 𝛼𝛼 10.1 ± 0.5 9.3 ± 0.3 9.0 ± 0.3 9.3 ± 0.2 9.3 ± 0.1 9.1 ± 0.2 8.9 ± 0.2 𝛼𝛼30−900 ′ , 9.0 ± 0.6 11.4 ± 0.2 10.9 ± 0.7 8.8 ± 0.6 8.4 ± 0.3 8.6 ± 0.1 8.1 ± 0.3 𝛼𝛼897−903 29−1 000, 8.7 ± 0.5 11.3 ± 0.9 10.8 ± 0.8 8.7 ± 0.4 8.3 ± 0.3 8.5 ± 0.3 8.0 ± 0.3 𝛼𝛼′ 997−1 003 𝛼𝛼 c) (La0.20Sr0.80) Cr0.20Fe0.80O3-δ (PRAX1) air N2 CO2 A* B* C* D* 9.4 ± 0.4 9.2 ± 0.4 8.5 ± 0.4 9.0 ± 0.4 9.1 ± 0.4 9.0 ± 0.2 8.8 ± 0.2 8.9 ± 0.3 9.1 ± 0.4 8.4 ± 0.2 8.8 ± 0.2 9.0 ± 0.2 8.9 ± 0.2 8.7 ± 0.2 37−800 𝛼𝛼′ 10.0 ± 0.5 10.6 ± 0.5 9.8 ± 0.5 9.9 ± 0.3 9.3 ± 0.2 9.1 ± 0.4 9.1 ± 0.4 797−803 𝛼𝛼 9.6 ± 0.5 10.5 ± 0.5 9.7 ± 0.5 9.8 ± 0.3 9.2 ± 0.2 9.0 ± 0.2 9.0 ± 0.2 𝛼𝛼35−900 ′ , 8.2 ± 0.3 9.1 ± 0.6 9.0 ± 0.4 11.1 ± 0.5 9.0 ± 0.5 8.1 ± 0.3 9.1 ± 0.2 𝛼𝛼897−903 29−1 000, 7.9 ± 0.3 9.0 ± 0.3 8.9 ± 0.7 11.0 ± 0.4 8.9 ± 0.4 8.0 ± 0.5 9.0 ± 0.5 𝛼𝛼′ 997−1 003 𝛼𝛼 d) (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ (PRAX2) air N2 CO2 A* B* C* D* 9.0 ± 0.1 9.0 ± 0.3 8.2 ± 0.3 8.1 ± 0.2 9.2 ± 0.1 8.8 ± 0.3 8.9 ± 0.3 8.7 ± 0.2 8.9 ± 0.1 8.1 ± 0.1 8.0 ± 0.2 9.1 ± 0.2 8.7 ± 0.2 8.8 ± 0.1 26−800 𝛼𝛼′ 8.4 ± 0.2 10.7 ± 0.3 10.0 ± 0.3 9.1 ± 0.2 9.5 ± 0.3 9.3 ± 0.3 9.3 ± 0.4 797−803 𝛼𝛼 8 ± 1 10.6 ± 0.8 9.9 ± 0.2 9.0 ± 0.2 9.4 ± 0.2 9.2 ± 0.2 9.2 ± 0.2 𝛼𝛼28−900 ′ , 9.1 ± 0.5 9.9 ± 0.3 9.9 ± 0.3 9.9 ± 0.4 8.9 ± 0.6 10.9 ± 0.4 8.9 ± 0.4 𝛼𝛼897−903 26−1 000, 8.8 ± 0.2 9.8 ± 0.2 9.8 ± 0.3 9.8 ± 0.4 8.8 ± 0.8 10.8 ± 0.4 8.8 ± 0.7 𝛼𝛼′ *) Gas𝛼𝛼997 −mix1 003 A = 99.01% CO2 + 0.99% CO, Gas mix B = 90.1% CO2 + 9.9% CO, Gas mix C = 50.5% CO2 + 49.5% CO, Gas mix D = 10.9% CO2 + 89.1% CO.

103

Table A.6 (end): Coefficients of linear thermal expansion ( ) of the studied materials

as a function of temperature and atmosphere. All values of𝑇𝑇 CTE1−𝑇𝑇2 include experimental uncertainty (± one standard deviation), are in units of 10–6 [°C𝛼𝛼 –1] and were measured in the given atmospheres* between the temperatures T1 and T2 (in °C) indicated in the subscripts.

e) (La0.50Sr0.50) Cr0.20Fe0.80O3-δ (CWRU1) air N2 CO2 A* B* C* D* 9.4 ± 0.2 9.0 ± 0.9 8.6 ± 0.4 8.8 ± 0.2 8.6 ± 0.4 8.9 ± 0.3 9.0 ± 0.3 9.1 ± 0.6 8.9 ± 0.2 8.5 ± 0.2 8.7 ± 0.2 8.5 ± 0.2 8.8 ± 0.2 8.9 ± 0.2 26−800 𝛼𝛼′ 7.5 ± 0.4 11.9 ± 0.9 10.2 ± 0.6 9.1 ± 0.4 9.3 ± 0.4 9.1 ± 0.2 9.4 ± 0.3 797−803 𝛼𝛼 7 ± 2 11.7 ± 0.7 10.1 ± 0.4 9.1 ± 0.2 9.2 ± 0.2 9.0 ± 0.2 9.3 ± 0.2 𝛼𝛼28−900 ′ , 9.1 ± 0.6 8.8 ± 0.5 9.0 ± 0.4 9.0 ± 0.4 8.8 ± 0.5 8.6 ± 0.6 9.1 ± 0.4 𝛼𝛼897−903 29−1 000, 8.9 ± 0.8 8.8 ± 0.3 8.9 ± 0.2 8.9 ± 0.5 8.7 ± 0.8 8.5 ± 0.7 9.0 ± 0.4 𝛼𝛼′ 997−1 003 𝛼𝛼 f) (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ (CWRU2)

air N2 CO2 A* B* C* D* 8.9 ± 0.2 9.3 ± 0.7 9.6 ± 0.3 8.4 ± 0.2 8.9 ± 0.3 8.7 ± 0.3 8.9 ± 0.4 8.6 ± 0.5 9.2 ± 0.5 9.5 ± 0.3 8.3 ± 0.2 8.8 ± 0.2 8.6 ± 0.3 8.8 ± 0.2 32−800 𝛼𝛼′ 10.1 ± 0.3 10.0 ± 0.4 10.3 ± 0.3 9.5 ± 0.3 9.5 ± 0.4 9.0 ± 0.6 9.0 ± 0.3 797−803 𝛼𝛼 9.7 ± 0.5 9.8 ± 0.5 10.2 ± 0.3 9.4 ± 0.2 9.4 ± 0.2 8.9 ± 0.2 8.9 ± 0.3 𝛼𝛼35−900 ′ , 10.1 ± 0.6 10.8 ± 0.5 11.0 ± 0.3 8.9 ± 0.4 8.7 ± 0.3 8.9 ± 0.2 8.9 ± 0.5 𝛼𝛼897−903 30−1 000, 9.8 ± 0.7 10.7 ± 0.8 10.9 ± 0.3 8.8 ± 0.8 8.7 ± 0.8 8.8 ± 0.4 8.8 ± 0.5 𝛼𝛼′ 997−1 003 𝛼𝛼 g) (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ (CWRU3) air N2 CO2 A* B* C* D* 9.5 ± 0.2 10.0 ± 0.8 8.3 ± 0.7 9.5 ± 0.2 8.9 ± 0.2 9.1 ± 0.2 9.0 ± 0.3 9.1 ± 0.4 9.9 ± 0.7 8.2 ± 0.4 9.4 ± 0.3 8.8 ± 0.2 9.0 ± 0.2 8.9 ± 0.2 29−800 𝛼𝛼′ 10.5 ± 0.1 9.4 ± 0.2 9.3 ± 0.4 9.3 ± 0.2 9.1 ± 0.2 9.5 ± 0.3 9.3 ± 0.3 797−803 𝛼𝛼 10.1 ± 0.4 9.3 ± 0.3 9.2 ± 0.2 9.2 ± 0.2 9.0 ± 0.2 9.4 ± 0.2 9.2 ± 0.1 𝛼𝛼28−900 ′ , 11.1 ± 0.5 9.8 ± 0.4 9.0 ± 0.5 10.9 ± 0.3 10.7 ± 0.8 10.4 ± 0.4 9.3 ± 0.6 𝛼𝛼897−903 , 10.8 ± 0.6 9.7 ± 0.6 8.9 ± 0.5 10.8 ± 0.3 10.6 ± 0.7 10.3 ± 0.4 9.2 ± 0.2 𝛼𝛼29−1 000 ′ 𝛼𝛼997−1 003 h) (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ (CWRU4)

air N2 CO2 A* B* C* D* 8.7 ± 0.8 9.3 ± 0.2 8.6 ± 0.3 9.0 ± 0.6 9.0 ± 0.1 9.2 ± 0.8 9.0 ± 0.5 8.4 ± 0.8 9.2 ± 0.4 8.5 ± 0.3 8.9 ± 0.3 8.9 ± 0.2 9.1 ± 0.2 8.9 ± 0.2 28−800 𝛼𝛼′ 10.3 ± 0.2 9.0 ± 0.5 9.7 ± 0.4 9.4 ± 0.8 9.2 ± 0.2 9.3 ± 0.1 9.3 ± 0.5 797−803 𝛼𝛼 9.8 ± 0.6 8.9 ± 0.2 9.6 ± 0.3 9.3 ± 0.2 9.1 ± 0.3 9.2 ± 0.2 9.2 ± 0.2 𝛼𝛼41−900 ′ , 9.2 ± 0.3 8.9 ± 0.6 9.0 ± 0.7 8.9 ± 0.5 8.7 ± 0.2 8.5 ± 0.3 8.4 ± 0.3 𝛼𝛼897−903 35−1 000, 8.9 ± 0.4 8.8 ± 0.7 8.9 ± 0.4 8.8 ± 0.2 8.7 ± 0.4 8.4 ± 0.8 8.3 ± 0.7 𝛼𝛼′ 997−1 003 *) Gas𝛼𝛼 mix A = 99.01% CO2 + 0.99% CO, Gas mix B = 90.1% CO2 + 9.9% CO, Gas mix C = 50.5% CO2 + 49.5% CO, Gas mix D = 10.9% CO2 + 89.1% CO

104

B. Appendix B: Supplemental Additions

B.1. Defect chemistry

The compositions in this project are acceptor-doped perovskites. If the electronic

charge compensation prefers the change in oxidation states of Fe+3 to Fe+4 over cobalt ions,

67 the substitution of SrO into the La(Fe,Co)O3 lattice can be written as:

1 ( , ) 2 + 2 + + 2 + 2 + 2 2 𝐿𝐿𝐿𝐿 𝐹𝐹𝐹𝐹 𝐶𝐶𝐶𝐶 𝑂𝑂3 (B.1) ′ ∙ 𝑆𝑆𝑆𝑆𝑆𝑆 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 𝑂𝑂2 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 �⎯⎯⎯⎯⎯⎯⎯� 𝐿𝐿𝐿𝐿2𝑂𝑂3 𝑆𝑆𝑆𝑆𝐿𝐿𝐿𝐿 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹

+3 +3 . +4 +3 ’ +2 Where the LaLa is La , FeFe is Fe , Fe Fe is Fe in a Fe site, and Sr La is Sr in a

La+3 site. However, the assumption in this project is that or LSF-based materials, the

following defect reaction occurs to produce oxygen vacancies for charge compensation:67

( , ) 2 + 2 + + 2 + 𝐿𝐿𝐿𝐿 𝐹𝐹𝐹𝐹 𝐶𝐶𝐶𝐶 𝑂𝑂3 (B.2) ′ ∙∙ 𝑆𝑆𝑆𝑆𝑆𝑆 𝐿𝐿𝐿𝐿𝐿𝐿𝐿𝐿 𝑂𝑂𝑂𝑂 �⎯⎯⎯⎯⎯⎯⎯� 𝐿𝐿𝐿𝐿2𝑂𝑂3 𝑆𝑆𝑆𝑆𝐿𝐿𝐿𝐿 𝑉𝑉𝑂𝑂 Pseudo-chemical reactions are also occurring between oxygen vacancies and the surrounding gas adding to the oxygen-nonstoichiometry:4

1 2 + + + 2 2 (B.3) ∙ ∙∙ 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑂𝑂𝑂𝑂 → 𝑂𝑂2 𝑉𝑉𝑂𝑂 𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹

1 2 + + + 2 2 (B.4) ∙ ∙∙ 𝐶𝐶𝐶𝐶𝐹𝐹𝐹𝐹 𝑂𝑂𝑂𝑂 → 𝑂𝑂2 𝑉𝑉𝑂𝑂 𝐶𝐶𝐶𝐶𝐹𝐹𝐹𝐹

105

B.2. Phase diagrams

Figure B.1: Phase diagram of SrO-Fe2O3-La2O3. Republished with permission of American Ceramic Society, from Gavrilova et al.38

Figure B.2: Phase diagram of SrO-Fe2O3-La2O3. Republished with permission of American Ceramic Society, from Fossdal et al.39

106

Figure B.3: SrFeO3-δ - LaFeO3 phase diagram. Republished with permission of American Ceramic Society, from Sasamoto et al.68

Figure B.4: SrO-Cr2O3-La2O3. Republished with permission of American Ceramic Society, from Yokokawa et al.50

107

B.3. Reproducibility

1.5E-03 (La0.80Sr0.20)0.95 FeO3-δ (FCM2) (La0.20Sr0.80) Co0.10Cr0.20Fe0.70O3-δ (PRAX2) 800 °C 1.5E-03 800 °C (2) 900 °C 900 °C (2) 1.0E-03

1.0E-03 Linear Strain 5.0E-04 Linear Strain 5.0E-04

0.0E+00 0.0E+00 -20 -10 0 -20 -15 -10 -5 0 Log pO2 Log pO2

(La Sr ) FeO 0.80 0.20 0.95 3-δ (FCM2) (La Sr ) Co Cr Fe O 0.20 0.80 0.10 0.20 0.70 3-δ (PRAX2) 2.0E-03 1.5E-03 900 °C 1000 °C 900 °C (2) 1000 °C (2)

1.5E-03

1.0E-03

1.0E-03 Linear Strain Linear Strain 5.0E-04 5.0E-04

0.0E+00 0.0E+00 -20 -15 -10 -5 0 -15 -10 -5 0

Log pO2 Log pO2

(La0.80Sr0.20)0.95 FeO3-δ (FCM2) 2.0E-03 1000 °C 1000 °C (2) 1.5E-03

1.0E-03 Linear Strain

5.0E-04

0.0E+00 -15 -10 -5 0

Log pO2

Figure B.5: Reproducibility runs of FCM2 and PRAX2. Secondary measurements labeled (2) used methyl cellulose as binder.

108

C. Appendix C: Glossary of Terms

FCM1 (La0.60Sr0.40)0.995 Co0.20Fe0.80O3-δ

FCM2 (La0.80Sr0.20)0.95 FeO3-δ

PRAX1 (La0.20Sr0.80) Cr0.20Fe0.80O3-δ

PRAX2 (La0.20Sr0.80) Co0.10 Cr0.20Fe0.70O3-δ

CWRU1 (La0.50Sr0.50) Cr0.20Fe0.80O3-δ

CWRU2 (La0.20Sr0.80) Co0.10 Cr0.10Fe0.80O3-δ

CWRU3 (La0.20Sr0.80) Co0.10Cr0.10Mg0.05Fe0.75O3-δ

CWRU4 (La0.50Sr0.50) Cr0.20Mg0.05Fe0.75O3-δ

LSF Lanthanum Strontium Ferrite

OTM Oxygen Transport Membrane

pO2 Partial Pressure of Oxygen

MEIC Mixed Electronic-Ionic Conducting sccm Standard Cubic Centimeters per Minute

LVDT Linear Variable Differential Transformer

AMP Amorphous Malic Acid Precursor

SEM Scanning Electronic Microscopy

FIB Focused Ion Beam

EDS Energy Dispersive X-Ray Spectroscopy

SCSAM Swagelok Center for Surface Analysis of Materials

CWRU Case Western Reserve University

PLC Percent Linear Change

XRD X-Ray Diffraction

CTE Coefficient of Thermal Expansion

109

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