TECHNICAL PAPERS

SOLID-STATE SCIENCE AND TECHNOLOGY

Colloidal Processing of BaCeO3-Based Electrolyte FilmsVishal Agarwal and Meilin Liu**

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA

ABSTRACT

Preparation of high-quality electrolyte films on porous substrates is critical to the fabrication of high-performancesolid-state ionic devices such as solid oxide fuel cells and chemical sensors. In this study, a colloidal process has beeninvestigated for the preparation of BaCeO3-based electrolyte films on both dense and porous substrates for electrochem-ical applications. The important processing variables affecting the microstructures of green films are identified and opti-mized to obtain uniform, crack-free green films of BaCe68Gd,203 with high packing density of the electrolyte particles.Further, dense ceramic films of BaCe08Gd,203-based electrolyte have been successfully fabricated on different substratesby careful process control. In addition, observations indicate that small amounts of additives can dramatically influencethe densification behavior of barium cerate-based electrolyte films.

IntroductionSolid oxide fuel cells (SOFCs) and chemical sensors are

among the solid-state ionic devices whose performancecan be considerably enhanced by the use of thin-film elec-trolytes. In order to achieve high ionic conductivity, theelectrolyte must be dense and uniform, with a well con-trolled crystal structure and stoichiometry SOFCs basedon yttria-stabilized zirconia (YSZ) are operated at hightemperatures (800-1000°C) to insure sufficient ionic con-ductivity. High operating temperatures result in high oper-ating costs as well as system degradation due to reactionsat the interfaces between cell components.1 These prob-lems can be overcome by reducing the operating tempera-tures to intermediate temperatures (600-800°C). Solid elec-trolytes based on barium cerate have been reported to haveconductivities similar to those of YSZ at temperaturesabout 200°C lower.2-4 Barium cerates doped with 10-20% oflanthanide series elements have been widely reported tohave high ionic conductivity at intermediate temperaturesand thus hold good promise as electrolyte materials forSOFCs to be operated at these temperatures. In particular,BaCe,,Gd,203 (BCG) exhibits the highest ionic conductiv-ity at temperatures below 800°C among the electrolytesstable in a fuel cell environment,2 Thus, BaCe,,Gd,203is anattractive choice as an electrolyte material for high-per-formance, intermediate-temperature SOFCs, provided thatit can be fabricated in a thin-film form.

For efficient operation of a solid-state ionic device atintermediate temperatures, the electrolyte must be used ina thin-film form to reduce resistive losses in the elec-trolyte.1 Further, the electrolyte membranes must be con-tinuous and crack-free in order to prevent gas leakage,and must rest between two porous electrodes throughwhich gases can transport freely to or away from the elec-trode-electrolyte interfaces, where electrochemical reac-tions occur. Thus, the fabrication of high-quality elec-trolyte films on porous electrodes assumes significantimportance.

There are a number of techniques for depositing thinfilms of ceramics on porous and dense substrates, includ-

Electroehemical Society Student Member.** Electrochemleal Society Active Member.

ing chemical vapor deposition, electrochemical vapordeposition, sol-gel processes, and various sputteringprocesses using ion beam, magnetron, electron beam, andso forth.' The drawbacks of physical vapor deposition andsputtering techniques include difficulties in obtaininggood compositional homogeneity67 and high costs due tothe requirement of high vacuum conditions.7 Sol-gel tech-niques, on the other hand, overcome these problems andoffer many additional advantages. Films derived from asol-gel process have good compositional homogeneity andhigh purity, and can be prepared over a wide range of com-positions in a relatively simple manner.8'° Another advan-tage of a sol-gel process is that it requires much less equip-ment and is thus relatively inexpensive as compared to theother techniques." According to the precursors used or thegelation mechanism involved, sol-gel process is usuallyclassified to alkoxide method and colloidal method. In acolloidal method, a sol is prepared from fine powders anda substrate is then coated with the sol solution to form agreen film, which is subsequently heated to elevated tem-peratures to remove the organics and to densify the film.

The objective of this study is to fabricate films ofBaCeO3-based electrolytes on dense and porous substratesusing a colloidal process. The processing parametersaffecting the film microstructure are systematically inves-tigated. The key processing variables are subsequentlyidentified and their effect on the quality of films is studiedin detail. The effect of a "sintering aid" on the densifica-tion behavior of the electrolyte films is also observed. Theelectrochemical properties of the films and the perf or-mance of the solid-state ionic devices based on these filmswill be discussed in a subsequent communication.21

ExperimentalPreparation of BaCe,,Gdo203 electrolyte powders.—

Powders of BaCe,,Gd0203 (BCG) were prepared using thePechini process"'4 and conventional ceramic processing.In the conventional ceramic processing, barium carbonate,cerium oxide, and gadolinium oxide (all from AlfaAESAR) were mixed in stoichiometric amounts and ball-milled (in alcohol with zirconia balls as the grindingmedia) for 24 h to ensure good mixing of the powders andto reduce the particle size to 1-4 m. The mixtures werethen calcined at 1350°C for 8 h" and x-ray diffraction

J. Eloctrochem. Soc., Vol. 143, No. 10, October 1996 The EIectrochemcal Society, Inc. 3239

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3240 J. Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, Inc.

(XRD) was used to confirm the formation of BaCe0 8Gd02O3phase in the powder. In case of incomplete reaction, calci-nation at 1350°C and balimilling were repeated until pureperovskite phase (as verified by XRD) was obtained. Thereacted powders were then ballmilled for another 24 h tobreak up the particle aggregates and to reduce the particlesize to 1-4 tm, as determined using scanning electronmicroscopy (SEM) and Microtrac particle size analyzer. Inthe Pechini process,'24 stoichiometric amounts of bariumcarbonate, cerium nitrate, and gadolinium nitrate (allfrom Alfa AESAR) were dissolved in citric acid (Aldrich)and ethylene glycol (Fisher) to form a sol having a stablemetal cation dispersion. The sol was then dried to form agel, which was further heated to elevated temperatures toremove the organic constituents and to produce fine parti-cles of desired barium cerates. It was found that heat-treatment at 1000°C for 4 h was necessary to ensure purepervoskite phase (as determined using XRD); powdersfrom heat-treatment at temperatures below 1000°C oftencontained BaCO phase. The mean size of aggregates (asmeasured using microtrac particle size analyzer and SEM)in the powders derived from the Pechini process rangedfrom 0.1 to 0.3 p.m.

Preparation of colloidal sols.—Colloidal sols were pre-pared from each of the two types of powders by dispersingthe powders in deionized water. The range of solids load-ing was varied from 0.1 to 1.0 gram powder per ml waterin increments of 0.1 gram powder per ml of water.Dispersing agents were subsequently added and the solwas ultrasonicated for 10-15 mm to assist the dispersionof electrolyte particles. Three types of dispersing agentswere used in this study: 1-1-1-trichloroethane (type A), amixture of 50 volume percent (yb) 1-1-1-trichloroethaneand 50 yb isopropanol (type B), and sodium polyacrylate(type C).a Type A and B dispersants were obtained fromFisher Scientific Company and type C dispersant wasobtained from R.T Vanderbilt Company, Inc. At eachsolids loading, the amounts of the dispersants were varieduntil a visually homogeneous and stable sol was obtained.A sol was considered unstable if the particle sedimenta-tion took place in less than 2 h. For the type A and B dis-persants, the volume fractions of dispersant in water werevaried from 0.05 to 0.1, while for the type C dispersant thevolume fractions of the dispersant in water ranged from0.02 to 0.07. The pH of the sol was monitored using anOaktron electronic pH meter.

Preparation of substrates—The choice of a substrate isdetermined primarily by the application of the electrolytefilm. However, the wettability of the colloidal sol with thesubstrate and the thermal expansion mismatch betweenthe film and the substrate must be acceptable. In this study,pellets of BaCe08Gd.2O3 were used as dense substrates inorder to minimize problems due to chemical reactions andthermal mismatch between the film and substrates. Disksof a-A1203 (from Vesuvius McDanel Company) were alsoused as dense substrates since the thermal expansion coef-ficient of a-A1203 is similar to that of the electrolyte.Composite NiO-BaCe03Gd0203 pellets were used as conduc-tive, porous substrates in order to facilitate the electricalcharacterization of the electrolyte films. The powders ofBaCe05Gd0203 and NiO-BaCe08Gd02O3, prepared by theconventional ceramic processing as described earlier, wereuniaxially pressed to pellets in a die at a mechanical pres-sure of 35,000 psi. The BaCe05Gd0203 pellets were sinteredat 1500°C for 8 h and the NiO-BaCe0.8Gd0203 pellets weresintered at 1350°C for 3 h to achieve densities greater than95% of the theoretical values. The use of a dense substrateeliminates the problem of sol infiltration into the substratethat is usually encountered with porous substrates. Sincethe surface microstructure and the cleanliness of a sub-

The use of sodium polyacrylate will introduce some Na to theelectrolyte. XRD studies suggested that all Na doped into theelectrolyte and electrical measurements indicated that the dopedNa has no observable detrimental effect on the electncal proper-ties of the electrolyte.1°

strate are important factors affecting the wettability of asol, all sintered substrates were ground to 1200 grit,cleaned ultrasonically in methanol, and then dried in anoven at 65°C for 2 h before coating.

Dip coating, drying, and firing—Films of colloidal solswere deposited on the substrates by a dip-coating method.Substrates were typically submerged in a sol for about 5 sbefore they were pulled out at a rate of approximately20 cm/mm. The coated substrates were dried overnight toevaporate the solvent. Typically, the thickness of adeposited layer varied from 3 to 7 jim, depending on thesolids loading. Two types of drying conditions were stud-ied: one in an oven at 65°C and the other in the ambientatmosphere at about 2 7°C. After drying, the resultinggreen films were fired at temperatures ranging from 1100to 1500°C in 50°C intervals, and the firing time at eachtemperature was varied from 1 to 4 h in 1 h intervals.Firing rates were varied from 3 to 1 0°C/mm. Unless statedotherwise, all heat-treatments were carried out in a tubefurnace in air and sintered samples were cooled down at arate of 10°C/mm.

Characterization of films.—Thermogravimetric analysis(TGA) was used to characterize the thermal evolution ofthe films. X-ray diffraction was used to analyze the phas-es present in the films. The microstructures of green andfired films were revealed using an SEM equipped withEDS. The detailed characteristics of the microstructureswere further quantified using stereological techniquesY1The mean intercept method was used to estimate the par-ticle or grain sizes and pore sizes in green and fired filmsand the point count method was used to estimate theporosity of films and substrates. Typically, about 10 meas-urements were taken for each data point to improve theconfidence of the data.

Results and DiscussionCharacteristics of the colloidal sols.—The stabilities of

the sols were studied as a function of particle size and thetype of dispersant. When the powders derived from thePechini process were dispersed using dispersant A or B, themost stable sols had pH of about 6.5 and were stable forabout 24 h. When the same powders (from the Pechiniprocess) were dispersed using dispersant C, the most stablesols had pH of about 12.5 and were stable for 7-10 days.The dispersion mechanism of dispersant A and B may beelectrostatic whereas the dispersion mechanism of disper-sant C is perhaps a combination of steric and electrostatic(i.e., electrosteric)u because of the polymeric nature of dis-persant C and the polar nature of the solvent (water). Alsoobserved were the difficulties encountered in preparingstable sols using powders with larger particles (1-4 pim)obtained from conventional ceramic processing. Sedimen-tation usually took place in 5-6 h when type A or B disper-sant was used and in 24 h when dispersant C was used.

To form a stable dispersion, the minimum volume frac-tions of dispersant A or B in water were 0.07 for conven-tional powders and 0.06 for the Pechini powders. With dis-persant C, the minimum volume fractions were 0.05 forconventional powders and 0.04 for powders from thePechini process. The sol viscosities ranged from 5 to 7 cen-tipoise at 2 7°C. The optimum solids loading for an accept-able wettabilty was found to be 0.4 to 0.6 gram powder perml water for the three types of dispersants on bothBaCe0 8Gd0 2°3 and NiO-BaCe05Gd0203 substrates. A solidsloading of 0.5 gram powder per ml water was thus chosenfor further studies.

Microstructures of green filins.—In order to achieve highdensity of sintered films, it is necessary to start with uni-form, crack-free green films with high packing density ofthe electrolyte particles. Accordingly, the effect of variousprocessing parameters on the microstructures of greenfilms was studied, including particle size and distribution,nature and amount of dispersant, types of substrate, andthe drying conditions.

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j. iecrrocnem. .oc., VOl. 1 4i, NO. 10, uctoer 1 b I ne uectrocnemical society, Inc. I

Shown in Fig. la is the microstructure of a green filmobtained from a sol using type A or B dispersant and pow-ders from conventional processing. The sol was depositedon a BaCe0 8Gd0 203 substrate and dried in the ambientatmosphere at about 27°C. The microstructure showsextremely loose packing (nearly 50% porosity) of particleswith large amount of needle-like organics in the greenstate. These needle shaped organics may have formed dueto the recrystallization of the dispersants A and B duringdrying. The use of the same dispersant (type A or B) withsol-gel derived powders led to particle agglomerates in thegreen film as shown in Fig. lb. The use of dispersant Cwith conventional powders resulted in loose particle pack-ing along with agglomerate formation in the green film.The use of dispersant C for the sol-gel derived powders,however, gave stable dispersion and excellent packing(about 6% porosity) in the green state, as shown in Fig. ic.The particles are densely packed and the microstructure ofthe film is free of cracks and agglomerates. Although the

green film microstructure was very sensitive to the parti-cle size and the nature of dispersant, it was relativelyinsensitive to the different types of substrates and the dry-ing conditions studied. The optimum volume fraction ofdispersant C in water was 0.04 for crack-free green films.An increase in volume fraction of dispersant C to 0.05 orgreater resulted in cracks in the green state, as shown inFig. id, due to overdeflocculation of particles in the col-loidal sal. Overdeflocculation may occur when excessiveamount of polymer dispersant absorbs together a largernumber of individually dispersed particles to form clus-ters.'9 These clusters of particles can lead to cracks in thegreen films as seen in Fig. id. Thus, the amount of disper-sant C should be no more than just enough to form a thinpolymer coating on each individual particle.

Thermogravimetric analysis—Green films with opti-mized microstructure were then fired at elevated temper-atures to remove the organics and to form dense ceramic

Fig. 1. Surface views of greenfilms of electrolyte on denseBoCe08Gd02O3 substrates: (a)type A dispersant and powdersprepared using conventionalceramic processing, (b) type Bdispersant and powders pre-pared using the Pechini process,(c) type C dispersant and pow-ders from the Pechini process,and (d) excessive type C disper-sant and powders from thePechini process.

Fig. 2. (a) Surface view and(b) cross-sectional view of anelectrolyte film (on a denseBaCe08Gd02O3 substrate) aftersintering at 1450°C for 2 h. (c)Surface view of an electrafilm after sintering at 1450°C2 h when a mechanical pressureof 30,000 psi was applied in thegreen state.

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3242 J. Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, Inc.

films. TGA studies of a green film containing 24 yb (9.3weight percent (w/o)) dispersant C indicated that there isa continuous weight loss from about 125 to about 450°C.The weight loss above 450°C was negligible. The estimat-ed shrinkage due to the organics burn-off is about 24%.TGA studies of other green films indicated that most of theorganics were removed by 450°C. Accordingly, the use ofslow firing rates below 450°C is necessary to eliminatedelamination or spalling of films from substrates.Typically, the heating rate was kept at 5°C/mm from roomtemperature to 450°C and at 7°C/mm above 450°C.

Microstructures of sintered films on denseBaCe08Gd02O3 pellets.—Shown in Fig. 2a is the micro-structure of a film (with 24 yb dispersant C) derived froman optimized colloidal sol with powders from the Pechiniprocess. The film was fired at 145 0°C for 2 h and it appearsto be uniform with an average grain size of 0.75 p.m. It is,however, quite porous as seen from the cross-sectionalview in Fig. 2b. The film seemed to adhere well to the sub-strate. The porosity of the film, as determined using thepoint counting method, was about 30% and the pore sizeranged from 0.1 to 0.5 p.m. An increase in the firing tem-perature from 1450 to 1500°C and in the firing time from 2to 5 h resulted in an increase in grain size, but there wasno significant improvement in densification. The porosityis due primarily to the inefficient packing of particles andlarge volume fraction of organics (dispersant) in the green

films. The reduction in the amount of organics, however,was limited by the stability of the sols; further loweringthe amount of dispersant made the sols unstable.

In order to improve particle packing in the green state, agreen film on a dense BCG substrate was subjected to aumaxial pressure of about 30,000 psi (using a die and ahydraulic press). After firing at 1450°C for 2 h, the pressedfilms (Fig. 2c) showed some improvement in density (from70 to 85%) over the uripressed films (Fig. 2a). In practice,however, subjecting a green film to a mechanical pressureof 30,000 psi may be difficult because the electrolyte filmsare usually supported by a porous electrode structure. Aporous electrode may not have sufficient mechanicalstrength to withstand such pressure. It is possible, howev-er, to use osmotic pressure to improve the packing density2°of particles in a green film deposited on a porous substrate.

Electrolyte films derived from multiple coatings—An-other approach taken to improve film density was to applya second layer of sol film on the top of a fired film whichwas deliberately made porous. The rationale behind thisapproach was that the second layer would fill in the poresleft in the first layer. Also, the first porous layer may act asa good substrate by effectively absorbing the solventthrough the pores which improves the particle-particlecontact and hence the adhesion and packing of particles.Clearly, it is necessary to ensure that the particles aresmaller than the pores. Since the pore sizes in the filmsfired at 1450°C were between 0.1 and 0.5 p.m (Fig. 2a), theapplication of a second colloidal layer (with particles sizesof 0.1 to 0.3 p.m) on the top of this fired film would not beeffective in filling in the pores in the first layer. In order toincrease the porosity and the sizes of the pores in the firstlayer, the firing temperature of the layer was lowered.Shown in Fig. 3 is the surface view of a film (on a denseBCG substrate) after firing at 1100°C for 2 h. Stereologicalanalysis of this micrograph indicated that most pores werebetween 1 and 3 p.m in size, with some pores up to 6 p.m.The particles (0.1 to 0.3 p.m) in a sol film deposited on thetop of this porous layer will have high probability offalling into the pores in the first layer. Once inside thepores, the particles may also promote densification by act-ing as a "bridge" between the particles during sintering.Shown in Fig. 4a and b are the surface and cross-section-al views of a dense (greater than 97% of theoretical densi-ty), uniform, and crack-free film obtained by firing a firstlayer at 1100°C and then a second layer at 1450°C. Thethickness of the resulting film is about 7 to 8 p.m and thegrain size is about 1-1.5 p.m. The film seems to adhere wellto the substrate. As shown in Fig. 5, the x-ray diffractionpattern of the electrolyte film (on an a-A1303 substrate)matched well with that obtained from a BaCe0 8Gd0 203 pel-let,15 confirming the formation of pervoskite phase in thefilm. The result indicates that the pores in the first layerfired at 1100°C were filled by depositing a second layer ofcolloidal coating on the top of it. Thus, multiple coatingscan improve the density of derived films.

Fig. 4. (a) Surface view and(b) cross-sectional view of anelectrolyte film (on the top of adense BaCe08Gd02O3 substrate)prepared by the multiple coatitechnique. The film was sinterat 1450°C for 3 h.

Fig. 3. Surface view of on electrolyte film (on a BaCe08Gd0203substrate) fired at 1100°C for 2 h.

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J. Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, Inc. 3243

IWI

80

60

40

20

100 - [ film

Ii J8060

40

n ..pellet

I L.I0 20 40 60 80 100

Angle (two theta)Fig. 5. X-ray diffraction patterns of a BaCe03Gd02O3 pellet and an

elecfrolyte film (derived from colloidal processing) on the top of analumina substrate.

Electrolyte films on NiO-BaCe08Gd0O, composite elec-trodes.—In order to construct a solid-state ionic devicebased on an electrolyte film, such as an SOFC, the sub-strate must be electrically conductive and porous. Thus, wethen focused on fabricating dense electrolyte films on elec-tronically conductive and porous substrates. It is well

known that NiO-YSZ cermets have been widely used asanodes in YSZ-based fuel cells.'2122 Accordingly, NiO-BaCe08Gd02O3 composite would be a logical choice asanodes for BCG based fuel cells. In this case, an electrolytefilm could first be deposited on a dense NiO-BaCe0 8Gd, 203substrate, and the NiO could then be reduced to Ni metalto create porosity in the substrate. Figure 6a shows a sur-face view of a dense (greater than 97% of theoretical den-sity) NiO-BaCe0 3Gd 203 pellet. According to EDS analysis,the darker phase in the microstructure is NiO and thelighter one is BaCe08Gd02O3. The powder composition waschosen such that after reduction of NiO to Ni metal, thevolume fraction of Ni would be 40% of the composite. Thisvolume fraction of Ni in the substrate would ensure thatthe substrate is electronically conductive.23 Dense filmswere then deposited on the top of this substrate using themultiple coating approach. Figure 6b and c show thecross-sectional and surface view of a film (sintered at1500°C for 3 h) on the top of a NiO-BaCe08Gd02O3 sub-strate. The micrographs indicate that the film is dense(greater than 97% of the theoretical density) and the grainsize in the film is large (10-30 pm). An EDS analysis of thefilm indicated the presence of Ni in the film. It is believedthat Ni diffused from the substrate into the film and influ-enced the densification of the film, as evidenced from theobserved high density and large grain size.

In our next processing step, the samples were exposed tohydrogen (at 750°C) to reduce NiO to Ni in order to createsome porosity in the substrate. Figure 6d shows a cross-sectional view of the sample after reduction in hydrogen.The microstructure of the film was unaffected while poreswere formed in the substrate (about 15% porosity) due to

Fig. 6. (a) A surface view ofan NiO-BaCe08Gd02O3 substratefired at 1350°C for 3 h, (b)cross-sectional view and (c) sur-face view of an electrolyte film(sintered at 1500°C for 3 h) onthe top of an NiO-BOCe)8Gd(,203substrate, and (d) cross-sectionalview of the same sample afterreduction in hydrogen at 750°Cfor 2 h.

ti)

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3244 J. Electrochem. Soc., Vol. 143, No. 10, October 1996 The Electrochemical Society, Inc.

Fig. 7. A cross-sectional viewof an electrolyte film (sintered at1200°C for 2 h) on the top of onNiO-BaCe08Gd02O3 substrate:(a) before and (b) after reductionin hydrogen at 750°C for 2 h.The substrate was prefired at1000°Cfor3h.

the reduction of NiO. The prefiring temperature of theNiO-BaCe0 8Gd0 203 substrates was then lowered from 1350to 1000°C to introduce additional porosity (about 10%) inthe substrates.

To verify the role of Ni as a "sintering aid," the sinter-ing temperatures and time of the film were systematicallylowered from 1500°C to determine the lowest temperatureat which dense films could be formed. It was found that aminimum temperature of 1200°C and a sintering time of2 h was necessary to form an electrolyte film with densitygreater than 97%. Figure 7a and b show the cross sectionsof a film (sintered at 1200°C for 2 h) on the top of a NiO-BCG substrate before and after reduction in hydrogen.The film is dense (greater than 97% of the theoretical den-sity) and the grain size is much smaller (1-3 rim) in com-parison to the films fired at higher temperatures. It is thusclear that Ni has a dramatic effect on the densificationbehavior of the electrolyte films. Films on a BaCe08Gd0 203substrate could not be densified at temperatures below1450°C while films on a NiO-BaCe0 8Gd0 203 substrate werereadily densified at temperatures as low as 12 00°C. Theonly difference between the two substrates is the presenceof Ni in the latter substrate. Apparently, the diffusion ofNi into the film somehow accelerated the densification ofthe films and lowered the sinterrng temperatures from1450 to 1200°C. This observation has important implica-tion to tailoring the sintering temperature of the elec-trolyte films.

Although the dramatic influence of Ni on the sinteringbehavior of barium cerate-based films was observed, thedetailed mechanism of "sintering aid" effect is still un-known. Other transition metal oxides,24 such as the oxidesof Cu and Co, were also found to have similar effect on sin-tering behavior of BaCeO3-based electrolytes. The mech-anism of the observed sintering aid effect and the effect ofthese additives on the electrical properties will be dis-cussed in a subsequent communication.25

ConclusionsParticle size of BaCe04Gd02O3 powders as well as the

nature and the amount of dispersant in the colloidal sol areidentified as the main processing variables affecting thegreen film microstructure in the colloidal processing.Crack-free, dense, and uniform green films have been ob-tained by optimizing the processing variables. Denseceramic films of BaCe 8Gd02O3-based electrolyte have beensuccessfully fabricated on both dense and porous sub-strates using a "muhiple coating" approach through care-ful control of processing parameters. Results indicate thatNiO acts as a "sintering aid" and dramatically improvesthe densification behavior of the BaCefl8GdO: films.Accordingly, dense films of BaCe0 8Gd0 203-based elec-trolyte have been successfully prepared on porous Ni-BaCe08Gd,32O3 substrates at temperatures as low as 1200°C.

AcknowledgmentsThis work was supported by EPRI under Contract No.

RP 1676-19 and by NSF under Award No. DMR-9357520.The authors gratefully acknowledge EPRI and NSF fortheir financial support of this research.

Manuscript submitted Dec. 18, 1995; revised manuscriptreceived May 1, 1996.

The Georgia Institute of Technology asssisted in meetingthe publication costs of this article.

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