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Influence of Cobalt and Iron Additions on the Electrical and ThermalProperties of (La,Sr)(Ga,Mg)O32d

J. W. Stevenson,*,z K. Hasinska,**,a N. L. Canfield, and T. R. Armstrong*,b

Pacific Northwest National Laboratory, Richland, Washington 99352, USA

(La,Sr)(Ga,Mg)O32d (LSGM) perovskite compositions doped with Co or Fe were prepared by a combustion synthesis technique.Small doping levels of Co or Fe (#10 mol % on the B-site) resulted in increased electrical conductivity at low temperatures (dueto the introduction of electronic charge carriers into the lattice), but at high temperatures the electrical conductivity was dominat-ed by ionic conduction and was similar in magnitude to that of the base LSGM composition. Higher additions of Co or Fe result-ed in much higher electrical conductivity in which the electronic component was dominant. A reversible weight loss at elevatedtemperatures was observed in all of the compositions as the lattice oxygen stoichiometry decreased with increasing temperature.This loss of lattice oxygen correlated with a reduction in the rate of increase (or even a net decrease) in electrical conductivity withincreasing temperature. This behavior was attributed to the elimination of electronic charge carriers (and a reduction in valence ofsome of the Co or Fe cations in the material) as lattice oxygen vacancies were formed. The thermal expansion of the materialsincreased substantially with increasing Co content, while only slight increases in thermal expansion were observed with increas-ing Fe content.© 2000 The Electrochemical Society. S0013-4651(00)01-088-0. All rights reserved.

Manuscript submitted January 24, 2000; revised manuscript received May 13, 2000.

Lanthanum gallate, LaGaO3, doped with strontium and magne-sium, (La,Sr)(Ga,Mg)O32d (LSGM), is a promising electrolyte mate-rial for intermediate temperature solid oxide fuel cells (SOFCs) dueto its high ionic conductivity and high ionic transference number.1-9

For example, at 8008C, the oxygen ion conductivity ofLa0.9Sr0.1Ga0.8Mg0.2O2.85 is 0.15 S/cm (which exceeds the conduc-tivity of yttria-stabilized zirconia by a factor of 3).10 However, ifLSGM is to be successfully utilized in SOFC applications, satisfac-tory electrode materials must be developed that are compatible withLSGM and allow high power densities to be achieved. Previous stud-ies on LSGM-based cells have focused primarily on doped lanthanumcobaltite and lanthanum manganite as cathode materials.11-14 Lan-thanum cobaltite appears to be superior to lanthanum manganite dueto its very high mixed (electronic and ionic) conductivity relative tolanthanum manganite. Unfortunately, the linear thermal expansioncoefficient (TEC) of doped lanthanum cobaltite is much higher(>20 ppm 8C21) than that of LSGM (<12 ppm 8C21). Huang et al.15

reported that partial replacement of Co with Ni in Sr-doped lan-thanum cobaltite resulted in high electrical conductivity and a some-what lower TEC (15.6-17.6 ppm 8C21). In the same study, substitu-tion of Ni for Fe in Sr-doped lanthanum ferrite resulted in reasonablyhigh conductivity and TEC values (11.7-12.8 ppm 8C21) similar tothat of LSGM.

The present study was undertaken to determine the effect of sub-stituting Co or Fe for Ga and Mg in LSGM. It has been reported that,at low doping levels, the addition of Co or Fe enhances the ionic con-ductivity of the material.16,17 Also, it was desirable to determinewhether higher doping levels might yield lanthanum gallate-basedcompositions exhibiting adequate electronic conductivity and appro-priate thermal expansion behavior for use as the cathode in LSGM-based SOFCs.

ExperimentalThe desired compositions were prepared from nitrate solutions

using the glycine/nitrate combustion synthesis technique describedelsewhere.9 The synthesized powders were calcined at 10008C for2 h in air prior to compaction in steel dies (55 MPa). The compact-ed powders were isostatically pressed (138 MPa) and then sinteredin air for 2 h at 1300-16008C. The bulk densities of the sintered sam-

** Electrochemical Society Active Member.** Electrochemical Society Student Member.*a Present address: NexTech Materials Ltd., Worthington, Ohio, USA.*b Present address: Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA.

z

09-375-2186

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ples were determined by the Archimedes method using ethanol.Thermal expansion measurements were carried out on sintered sam-ples (<25 mm long) using a vertical pushrod dilatometer; the sam-ples were heated to 12008C at a rate of 28C/min in air. The phase dis-tributions in the synthesized and sintered materials were determinedby X-ray diffraction (XRD). Sintered samples were prepared forpowder XRD analysis by grinding the samples in a mortar and pes-tle. The experimental diffraction patterns were collected at roomtemperature using Cu Ka radiation. Estimates of the crystallinephase concentrations in the materials were based on comparison ofthe observed peak heights after background subtraction. The sum ofthe heights of the selected peaks for all phases present in any givenspecimen was taken as the total diffracted intensity. The reportedestimated concentrations (in wt %) are equal to the ratio of the heightof the selected peak for each individual phase vs. the total diffractedintensity. Thermogravimetric analysis (TGA) was performed in airon powder specimens using heating and cooling rates of 28C/min.Buoyancy corrections were calculated using an alumina standard.The electrical conductivity of sintered bars (nominal dimensions:25 3 3 3 3 mm) was measured in air as a function of temperatureusing a four-point dc method with platinum electrodes.

Results and Discussion

In this paper, La12xSrx(Ga0.8Mg0.2)12yCoyO32d compositions arereferred to as LSGMC. La12xSrx(Ga0.8Mg0.2)12yFeyO32d compo-sitions are referred to as LSGMF. The numbers following the abbre-viation refer to the relative proportions of Sr (x) and Co or Fe (y) inthe material. For example, La0.9Sr0.1(Ga0.8Mg0.2)0.8Co0.2O32d is des-ignated LSGMC-1020, while La0.9Sr0.2(Ga0.8Mg0.2)0.6Fe0.4O32d isdesignated LSGMF-2040.

Phase development.—The compositions studied are listed inTable I. Two LSGM compositions, La0.9Sr0.1Ga0.8Mg0.2O2.85(LSGM-10) and La0.8Sr0.2Ga0.8Mg0.2O2.85 (LSGM-20) were select-ed as the base compositions in which various amounts of Co or Fewere substituted for Ga and Mg. All of the compositions were ana-lyzed by XRD after heat-treatment at 14008C for 2 h. The results ofthis analysis, also shown in Table I, indicated that all of the compo-sitions contained at least 97 wt % of the desired perovskite phase.The lattice symmetry of the perovskite phase is also shown for eachcomposition. Cubic symmetry prevails at low doping levels. Higherdoping levels resulted in lower symmetry structures (orthorhombicor hexagonal), except for the LSGMF-10xx compositions, whichremained cubic.

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TGA.—The results from TGA in air are shown in Fig. 1-4. All ofthe compositions exhibited a reversible weight loss when heated inair. This weight loss results from the loss of lattice oxygen as the ma-terial becomes increasingly oxygen deficient with increasing temper-ature. Similar behavior has been observed in other perovskites con-taining cobalt and iron.18 For the LSGMCs with low Co doping (upto 20%), the TGA curves exhibit a similar shape, with the magnitudeof the oxygen loss at high temperatures increasing with increasing Cocontent. At higher Co levels (30-60%), the magnitude of the oxygenloss was similar at high temperatures, but the onset temperature (atwhich significant oxygen loss begins) increased with increasing Cocontent. The LSGMF compositions exhibited similar trends, exceptthat at high Fe levels (30-60%) the magnitude of the oyxgen loss athigh temperature decreased with increasing Fe content.

Since electroneutrality must be maintained within a crystallinematerial, the oxygen vacancies created by the loss of lattice oxygenmust be charge compensated by other changes within the crystal. Coand Fe adopt a number of valence states, while La, Sr, Ga, and Mg areessentially fixed-valence cations, so the oxygen vacancies are almostcertainly compensated through a reduction in valence of some of theCo or Fe cations. (This conclusion is supported by the TGA results forLSGM, without any Co or Fe, which contains only the fixed-valence

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cations. In the absence of variable-valence cations, the oxygen stoi-chiometry remained constant as a function of temperature.)

Using the LSGMC compositions as an example, the overall elec-troneutrality relation, including the acceptor dopants, can be writtenas (using Kroger-Vink notation)19

[Sr9La] 1 [Mg9Ga] 1 [Co9Ga] 5 [Co•Ga] 1 2[VO

••] [1]

Each oxygen vacancy formed is charge compensated by the reduc-tion in valence of two Co cations, e.g., from Co41 to Co31

[2]

or from Co31 to Co21

[3]

Similarly, in LSGMF compositions, charge compensation is accom-plished by the reduction in valence of Fe cations.

Since this temperature-dependent oxygen stoichiometry has aneffect on the electrical behavior of these materials, these TGA resultsare discussed further in the following section.

O Co V Co OO G O Ga a3 31 1 12 2

1

2 2r•• ′

O Co V Co OO G O Ga a3 31 1 12 2

1

2 2• ••

r

Table I. Phases present after heat-treatment to 14008C for 2 h in air.Symmetry of

Composition Code Phases present perovskite lattice

La0.9Sr0.1Ga0.76Mg0.19Co0.05O3 LSGMC-1005 Single phase (perovskite) CubicLa0.9Sr0.1Ga0.72Mg0.18Co0.1O3 LSGMC-1010 97% perovskite Cubic

2% MgO1% Co3O4

La0.9Sr0.1Ga0.64Mg0.16Co0.2O3 LSGMC-1020 99% perovskite Orthorhombic1% Sr3Ga4O9

La0.9Sr0.1Ga0.56Mg0.14Co0.3O3 LSGMC-1030 Single phase (perovskite) OrthorhombicLa0.9Sr0.1Ga0.48Mg0.12Co0.4O3 LSGMC-1040 Single phase (perovskite) OrthorhombicLa0.9Sr0.1Ga0.32Mg0.08Co0.6O3 LSGMC-1060 Single phase (perovskite) HexagonalLa0.9Sr0.1Ga0.76Mg0.19Fe0.05O3 LSGMF-1005 Single phase (perovskite) CubicLa0.9Sr0.1Ga0.72Mg0.18Fe0.1O3 LSGMF-1010 Single phase (perovskite) CubicLa0.9Sr0.1Ga0.64Mg0.16Fe0.2O3 LSGMF-1020 99% perovskite Cubic

1% La2Sr2O5La0.9Sr0.1Ga0.56Mg0.14Fe0.3O3 LSGMF-1030 Single phase (perovskite) CubicLa0.9Sr0.1Ga0.48Mg0.12Fe0.4O3 LSGMF-1040 99% perovskite Cubic

1% MgOLa0.9Sr0.1Ga0.32Mg0.08Fe0.6O3 LSGMF-1060 99% perovskite Cubic

1% SrGa2O4La0.8Sr0.2Ga0.76Mg0.19Co0.05O3 LSGMC-2005 98% perovskite Cubic

2% La2O3La0.8Sr0.2Ga0.72Mg0.18Co0.1O3 LSGMC-2010 99% perovskite Cubic

1% La2O3La0.8Sr0.2Ga0.64Mg0.16Co0.2O3 LSGMC-2020 99% perovskite Orthorhombic

1% La2O3La0.8Sr0.2Ga0.48Mg0.12Co0.4O3 LSGMC-2040 98% perovskite Hexagonal

1% La4SrO71% MgO

La0.8Sr0.2Ga0.32Mg0.08Co0.6O3 LSGMC-2060 98% perovskite Orthorhombic 1% La2O31% Ga2O3

La0.8Sr0.2Ga0.76Mg0.19Fe0.05O3 LSGMF-2005 99% perovskite Cubic1% La2O3

La0.8Sr0.2Ga0.72Mg0.18Fe0.1O3 LSGMF-2010 98% perovskite Cubic 1% La2O31% Ga2O3

La0.8Sr0.2Ga0.64Mg0.16Fe0.2O3 LSGMF-2020 Single phase (perovskite) OrthorhombicLa0.8Sr0.2Ga0.48Mg0.12Fe0.4O3 LSGMF-2040 98% perovskite Hexagonal

1% FeO1% MgO

La0.8Sr0.2Ga0.32Mg0.08Fe0.6O3 LSGMF-2060 Single phase (perovskite) Orthorhombic

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Electrical conductivity.—The results from four-point dc meas-urements on sintered bars of LSGMC-10xx compositions are shownas Arrhenius plots in Fig. 5. The sintering conditions and density(expressed as a percentage of theoretical density) of the specimensused for conductivity (and thermal expansion) measurements aresummarized in Table II. The conductivity of the base composition,LSGM-10, is also shown. The observed electrical conductivitieswere consistent with values reported by Keppeler et al.20 for threesimilar Co-doped LSGM compositions. At low temperatures, theconductivities of the compositions with 5 and 10 mol % Co(LSGMC-1005 and LSGMC-1010) are higher than that of LSGM-10, and their Arrhenius plots have a lower slope. The lower sloperesults from a lower activation energy (0.30-0.35 eV), which sug-

Figure 1. Change in mass as a function of temperature for the indicatedLSGMC-10xx compositions. Open symbols refer to heating data; closedsymbols refer to cooling data.

Figure 2. Change in mass as a function of temperature for the indicatedLSGMF-10xx compositions. Open symbols refer to heating data; closed sym-bols refer to cooling data.

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gests that electronic conduction is dominant in the low temperatureregion. However, at higher temperatures, the slope changes, and theplot essentially overlays that of LSGM-10, as ionic conduction,which increases rapidly with increasing temperature due to a higheractivation energy, becomes dominant. The conductivities ofLSGMC-1005 and LSGMC-1010 at high temperatures were actual-ly slightly less than that of LSGM-10, but this may be attributable tothe fact that the densities of these specimens were only 90-93% oftheoretical, while the LSGM-10 specimen was 97% dense. The acti-vation energies of LSGMC-1005 and LSGMC-1010 at high temper-atures were 0.80 and 0.60 eV, respectively; these values are similarto that of LSGM-10 at high temperatures (0.71 eV).

Figure 3. Change in mass as a function of temperature for the indicatedLSGMC-20xx compositions. Open symbols refer to heating data; closedsymbols refer to cooling data.

Figure 4. Change in mass as a function of temperature for the indicatedLSGMF-20xx compositions. Open symbols refer to heating data; closed sym-bols refer to cooling data.

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As expected (given the very high conductivity of acceptor-dopedlanthanum cobaltite),21 increased substitution of Co for Ga and Mg(LSGMC-1020, -1030, -1040, and -1060) resulted in substantial in-creases in the total electrical conductivity. The same trend was ob-served in the simpler La(Ga,Co)O3 system.22 The Arrhenius plotsare linear in the low temperature region; this linearity, the high mag-nitudes of the conductivities, and the low magnitudes of the calcu-lated activation energies (shown in Table III) are consistent withelectronic conduction via the small polaron mechanism, which iscommon among oxides containing cations which readily adopt mul-tiple valences.23 Note that the activation energies decrease with in-creasing Co content. This may be a result of an increasing fractionof adjacent B-cation sites (in the ABO3 structure) being occupied by

Table II. Sintering conditions and density of specimens used inelectrical conductivity and thermal expansion measurements.

% of Composition Sintering conditions theoretical density

LSGMC-1005 13008C, 2 h 93LSGMC-1010 15008C, 2 h 90LSGMC-1020 15008C, 2 h 96LSGMC-1030 15008C, 2 h 95LSGMC-1040 15008C, 2 h 95LSGMC-1060 13008C, 2 h 98LSGMF-1005 13008C, 2 h 88LSGMF-1010 14008C, 2 h 90LSGMF-1020 14008C, 2 h 91LSGMF-1030 14008C, 2 h 90LSGMF-1040 14008C, 2 h 95LSGMF-1060 13008C, 2 h 95LSGMC-2005 13008C, 2 h 95LSGMC-2010 13008C, 2 h 95LSGMC-2020 14008C, 2 h 97LSGMC-2040 13008C, 2 h 99LSGMC-2060 14008C, 2 h 98LSGMF-2005 13008C, 2 h 92LSGMF-2010 13008C, 2 h 91LSGMF-2020 14008C, 2 h 90LSGMF-2040 13008C, 2 h 92LSGMF-2060 14008C, 2 h 92

Figure 5. Arrhenius plots of electrical conductivity in air for the indicatedLSGMC-10xx compositions.

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Co cations, so that conduction occurs increasingly by transfer ofelectrons between nearest neighbor sites instead of next-nearestneighbor sites.

The Arrhenius plots for the compositions with higher Co contentdeviate from linearity in the high temperature region, as the total con-ductivity failed to continue to increase at the rate which would be ex-pected if the concentration of carriers remained constant. (For somecompositions, the conductivity began to decrease with increasing tem-perature.) Also, as noted in the section on TGA, these compositionsexhibited weight loss at elevated temperatures due to the loss of lat-tice oxygen. Both of these phenomena have been observed in otherperovskite systems containing cobalt and iron, such as(La,Sr)(Co,Fe)O32d (LSCF).18 For LSCF, the deviation from lineari-ty in the Arrhenius plots resulted from a reduction in the number ofelectronic charge carriers (electron holes) as the concentration of oxy-gen vacancies increased. (For electroneutrality to be maintained in thematerial, two electron holes are eliminated for each lattice oxygenvacancy formed.) The same mechanism may occur in the materials inthe present study, but that explanation must be considered tentative,since comparison of the TGA data and the electrical conductivity datashows that, for some of these compositions, the onset temperature atwhich significant oxygen loss begins to occur is substantially lowerthan the temperature at which the deviation from linearity in the Ar-rhenius plots begins to occur. Further studies may explain why someof these materials can experience substantial oxygen loss before anyeffect on the electrical properties is observed, while others respondimmediately to a change in the oxygen stoichiometry.

Arrhenius plots for the LSGMC-20xx compositions are shown inFig. 6. Overall, the behavior of this system is similar to that of theLSGMC-10xx compositions. The LSGMF compositions (Fig. 7 and8) also exhibited behavior similar to that of the LSGMC composi-tions. For example, compositions with relatively low Fe additions(<10 mol % on the B-site) showed enhanced conductivity (relative toLSGM) at low temperatures (presumably due to electronic conduc-tion), but at high temperatures the conductivity appeared to be dom-inated by the ionic conduction. No increase in the ionic conductivi-ty relative to the base LSGM composition was observed. Ishiharaet al.16,17 reported enhanced ionic conductivity (relative to LSGM)for similar compositions with low doping levels of Co or Fe. In thosestudies, the conductivity was measured in an atmosphere having anoxygen partial pressure of 1025 atm. In the present study, in whichthe conductivity measurements were performed in air, no enhance-

Table III. Activation energy for electrical conduction calculatedfrom linear (low temperature) portions of Arrhenius plots.

Composition Activation energy (eV)

LSGMC-1005 0.35LSGMC-1010 0.30LSGMC-1020 0.21LSGMC-1030 0.17LSGMC-1040 0.15LSGMC-1060 0.14LSGMF-1005 0.53LSGMF-1010 0.46LSGMF-1020 0.41LSGMF-1030 0.38LSGMF-1040 0.34LSGMF-1060 0.29LSGMC-2005 0.35LSGMC-2010 0.30LSGMC-2020 0.22LSGMC-2040 0.14LSGMC-2060 0.13LSGMF-2005 0.58LSGMF-2010 0.49LSGMF-2020 0.43LSGMF-2040 0.35LSGMF-2060 0.35

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ment of ionic conductivity relative to LSGM was observed for lowlevel doping of either Co or Fe.

For higher doping levels of Fe (>10 mol %), the electrical con-ductivity continued to increase with increasing Fe content, but themagnitudes of the electrical conductivity were substantially lowerfor these LSGMF compositions than for the corresponding LSGMCcompositions. This is consistent with the fact that iron-based per-ovskites exhibit substantially lower conductivity relative to compa-rably doped cobalt-based perovskites.21,24

Thermal expansion.—The thermal expansion behavior of theLSGMC and LSGMF compositions was not linear; several repre-sentative plots are shown in Fig. 9 and 10. The average values ofthermal expansion over the temperature ranges 25-1200 and 25-

Figure 6. Arrhenius plots of electrical conductivity in air for the indicatedLSGMC-20xx compositions.

Figure 7. Arrhenius plots of electrical conductivity in air for the indicatedLSGMF-10xx compositions.

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8008C are shown in Table IV. The thermal expansion of the LSGMCcompositions increased rapidly with increasing Co content, so thatcompositions exhibiting high electrical conductivity (e.g., LSGMC-xx40, -xx60) also had TECs which were much higher than the baseLSGM electrolyte material. The trend toward higher thermal expan-sion with increasing Co content may be at least partially explainedby the increasing magnitude of oxygen loss at high temperatures(see Fig. 1 and 9). This behavior is consistent with previous studieswhich have observed expansion in perovskite materials as latticeoxygen is removed.25-28 The expansion is generally attributed to anincrease in the average cation radius as some of the cations in theperovskite are reduced in valence (to maintain electroneutrality)when oxygen ions are removed from the structure.

However, the thermal expansion of the LSGMF compositionsincreased only slightly with increasing Fe content, and tended to

Figure 8. Arrhenius plots of electrical conductivity in air for the indicatedLSGMF-20xx compositions.

Figure 9. Thermal expansion in air for the indicated LSGMC compositions.Heated at 28C/min.

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decrease in the highest Fe compositions. As for the LSGMC compo-sitions, the trend in thermal expansion tended to correlate with themagnitude of oxygen loss at high temperatures (see Fig. 2 and 10).While the LSGMFs exhibited thermal expansion behavior whichmatched reasonably well to LSGM, their electrical conductivities(with the possible exception of LSGMF-xx60) were too low(<20 S/cm) for them to be considered attractive candidates for SOFCcathode applications.

Conclusions(La,Sr)(Ga,Mg)O32d doped with Co or Fe on the B-site exhibited

a wide range of electrical and thermal behavior. For relatively minoradditions of Co or Fe, the electrical conductivity increased at lowtemperatures (due to the introduction of electronic charge carriersinto the lattice), but the high temperature conductivity was dominat-ed by ionic conduction and was similar in magnitude to that ofLSGM. Higher additions of Co or Fe resulted in much higher electri-cal conductivity in which the electronic component was dominant.All of the compositions exhibited a reversible weight loss at elevatedtemperatures due to a decrease in lattice oxygen stoichiometry. Thisloss of lattice oxygen resulted in a reduction in the rate of increase ofelectrical conductivity with increasing temperature. This behaviorwas attributed to a reduction in the concentration of electronic chargecarriers as lattice oxygen vacancies were formed. The addition of Fehad little effect on the thermal expansion behavior, but the addition ofCo resulted in a substantial increase in the thermal expansion.

AcknowledgmentsPacific Northwest National Laboratory is operated for the U.S.

Department of Energy by Battelle Memorial Institute under contractDE-AC06-76RLO 1830. This work was funded by the Federal Ener-gy Technology Center under contract 28024.

Pacific Northwest National Laboratory assisted in meeting the publica-tion costs of this article.

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Figure 10. Thermal expansion in air for the indicated LSGMF compositions.Heated at 28C/min.

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Table IV. Average TEC.

Average TEC Average TEC Composition (ppm 8C21, 25-12008C) (ppm 8C21, 25-8008C)

LSGM-10 11.9 10.8LSGMC-1005 12.7 11.6LSGMC-1010 14.2 12.9LSGMC-1020 16.5 14.7LSGMC-1030 19.4 16.1LSGMC-1040 19.8 17.3LSGMC-1060 21.2 18.5LSGMF-1005 11.6 10.8LSGMF-1010 12.4 11.8LSGMF-1020 12.5 11.4LSGMF-1030 12.9 11.2LSGMF-1040 13.1 11.4LSGMF-1060 12.1 10.9LSGM-20 12.4 11.1LSGMC-2005 13.3 12.1LSGMC-2010 14.7 13.1LSGMC-2020 17.3 15.0LSGMC-2040 19.9 16.9LSGMC-2060 21.8 18.2LSGMF-2005 11.7 11.2LSGMF-2010 12.4 11.6LSGMF-2020 13.2 12.8LSGMF-2040 14.4 13.0LSGMF-2060 13.7 11.6

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