BaCe0.9−xZrxY0.1O3−δ powders were synthesized by a solid-state reaction. Different contents of cerium and zirconium were studied. Pellets were sintered using either conventional sintering in air at 1700◦C or the Spark Plasma Sintering (SPS) technique. The density of the samples sintered by SPS is much higher than by conventional sintering. Higher values of ionic conductivity were obtained

for the SPS sample

Bonanos, Nikolaos

Caboche, Gilles

Estournès, Claude

Ricote, Sandrine

Journal of Nanomaterials

English

S. Ricote

G. Caboche

C. Estournes

N. Bonanos

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Synthesis, Sintering, and Electrical Properties of
                    
                      
                        
                          
                            BaCe

International audienceBaCe0.9−xZrxY0.1O3−δ powders were synthesized by a solid-state reaction. Different contents of cerium and zirconium were studied. Pellets were sintered using either conventional sintering in air at 1700◦C or the Spark Plasma Sintering (SPS) technique. The density of the samples sintered by SPS is much higher than by conventional sintering. Higher values of ionic conductivity were obtained for the SPS sample

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Synthesis, Sintering, and Electrical Properties of BaCe0.9−xZrxY0.1O3−δ

BaCe0.9−xZrxY0.1O3−δ powders were synthesized by a solid-state reaction. Different contents of cerium and zirconium were studied. Pellets were sintered using either conventional sintering in air at 1700◦C or the Spark Plasma Sintering (SPS) technique. The density of the samples sintered by SPS is much higher than by conventional sintering. Higher values of ionic conductivity were obtained for the SPS sample

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   Open Archive Toulouse Archive Ouverte (OATAO)  OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.  This is an author-deposited version published in: http://oatao.univ-toulouse.fr/  Eprints ID :  2412  To link to this article :  URL : http://dx.doi.org/10.1155/2008/354258 To cite this version : Ricote, Sandrine and Caboche, G. and Estournès, Claude and Bonanos, N. ( 2008) Synthesis, Sintering, and Electrical Properties of BaCe0.9−xZrxY0.1O3−δ. Journal of Nanomaterials (JNM), vol. 2008 . ISSN 1687-4110 Any correspondence concerning this service should be sent to the repository administrator: staff-oatao@inp-toulouse.fr   Hindawi Publishing CorporationJournal of NanomaterialsVolume 2008, Article ID 354258, 5 pagesdoi:10.1155/2008/354258Research ArticleSynthesis, Sintering, and Electrical Properties ofBaCe0.9−xZrxY0.1O3−δS. Ricote,1 G. Caboche,1 C. Estournes,2 and N. Bonanos31 Institut Carnot de Bourgogne, CNRS UMR 5209, Universite´ de Bourgogne, 9 avenue Alain Savary, 21078 Dijon Cedex, France2Plateforme Nationale de Frittage Flash (PNF2) et CIRIMAT UMR5085 CNRS/UPS/INPT, Universite´ Paul Sabatier,118 route de Narbonne, 31062 Toulouse Cedex 04, France3Risø National Laboratory for Sustainable Energy, Fuel Cells and Solid State Chemistry Division, Technical University of Denmark,P.O. Box 49, 4000 Roskilde, DenmarkCorrespondence should be addressed to S. Ricote, sandrine.ricote@risoe.dkReceived 26 October 2007; Revised 9 May 2008; Accepted 2 September 2008Recommended by Wanqin JinBaCe0.9−xZrxY0.1O3−δ powders were synthesized by a solid-state reaction. Different contents of cerium and zirconium were studied.Pellets were sintered using either conventional sintering in air at 1700◦C or the Spark Plasma Sintering (SPS) technique. The densityof the samples sintered by SPS is much higher than by conventional sintering. Higher values of ionic conductivity were obtainedfor the SPS sample.Copyright © 2008 S. Ricote et al. This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.1. INTRODUCTIONFuel cells can exhibit high-energy efficiency, and the onlyrelease is clean water when used with hydrogen. Types offuel cells are classified according to the electrolyte material.Solid electrolytes are used in solid oxide fuel cell (SOFC).In that case, the electrolyte conducts oxygen ions fromthe cathode to the anode where they react chemically withhydrogen to form water. Oxygen diffusion occurs at roughly800–900◦C for YSZ [1, 2]. In order to lower the operatingtemperature, it is interesting to use a proton-conductingceramic [2, 3]. This kind of fuel cell is called protonic ceramicfuel cell (PCFC). In that case, protons pass through theelectrolyte to combine with oxygen at the cathode to liberatewater. Hydrogen transport requires lower temperatures thanoxygen transport.Many rare-earth-doped perovskite materials seem tobe good candidates for this application, because of theirhigh proton conductivity at a temperature about 600◦Cand their stability in reducing atmosphere [4]. Bariumcerates are known for their high protonic conductivity butare not chemically stable; they react with carbon dioxideto form carbonates. Barium zirconates are chemically andmechanically stable but their protonic conductivity is quitelow [5–8]. An oxide with both cerium and zirconium couldexhibit the two required properties for a PCFC electrolyte:a satisfying protonic conductivity and a sufficient stability[9, 10]. Thus, this study deals with BaCe0.9−xZrxY0.1O3−δpowders (x = 0.3 (powder referred to BCZY63), x = 0.7(BCZY27), and x = 0.9 (BCZY09)). The minimal valuechosen for x was 0.3, because a compound with more than60% of cerium atoms is not stable after a CO2 treatment[10], and the lattice is orthorhombic. Kreuer [5] pointedout a decrease of the mobility of the protonic defectswhen the perovskite structures were deviating from cubic(orthorhombic). A trivalent dopant, yttrium, is used tocreate oxygen vacancies by charge compensation [2, 4, 11,12]. These oxygen vacancies are necessary for the protonconduction, as explained afterwards.Protons are incorporated in the material by the absorp-tion of water molecules in the oxygen vacancies (reaction 1)[13–15].Reaction 1H2O(g) + VO•• + OOx ←→ 2OHO•. (1)Proton transfer occurs by the Grotthuss diffusion mecha-nism. This is a two-step mechanism which involves both aproton-transfer and a reorientation step [3, 15–18].2 Journal of NanomaterialsCerates and Zirconates are pure proton conductors onlyat low temperature and under specific atmosphere. At high-oxygen partial pressure, p-type conduction is predominant(reaction 2).Reaction 212O2(g) + VO•• ←→ 2h• + OOx. (2)Thus, the conductivity is given by (3) where σi is the ionicconductivity and σ◦p is the p-type conductivity at an oxygenpartial pressure of 1 atm [4, 6, 19, 20] as follows:σtotal = σi + σ◦p·P(O2)1/4. (3)2. EXPERIMENTALBaCe0.9−xZrxY0.1O3−δ powders (x = 0.3, 0.7, and 0.9) weresynthesized by a solid state reaction. BaCO3 (Aldrich, 99+%),CeO2 (Aldrich, 99.9%, <5 microns), Y2O3 (Sigma-Aldrich,99.99%), and YSZ (Aldrich, submicron powder; 99.5%,5.3 weight % of yttria) and were mixed in stoichiometricproportions for 20 hours in a ball mill. The resulting powderswere calcined in air at 1400◦C for 24 hours.Crystallographic phases, lattice parameters, and the-oretical density were determined with a standard X-raydiffractometer INEL CPS 120 using CuKα1 radiation.Scanning Electron Microscope (SEM, Jeol JSM 6400F)experiments were achieved to assess the particles size andmorphology.2.1. Conventional sinteringA binder was added to the powders to facilitate the forming.Cylindrical pellets (10 and 20 mm diameter) were pressedusing a hydraulic press (350 MPa) and sintered in air at1700◦C for 6 hours.2.2. Spark plasma sinteringSPS was carried out in the equipment Sumitomo SPS 2080supplied by (Plateforme Nationale de Frittage Flash (PNF2),Universite´ Paul Sabatier, France). The precursors, withoutbinding material, were set in graphite die with an innerdiameter of 8 mm and sintered under vacuum (roughly1 Pa). The experiments were done in the temperature range1600–1700◦C for 5 minutes. The pressure was monitoredat 100 MPa during the process. The heating rate for all SPSexperiments was maintained at around 150◦C min−1, and thenatural cooling rate down to 800◦C was 100◦C min−1. Thetemperature was controlled by an optical pyrometer focusedon a small hole located at the surface of the die. Shrinkagewas recorded from the dilatometer provided with the SPSmachine.The DC conductivity was measured in a 4-point probearrangement. Pt paste was applied to each end of the barand contacted with Pt leads. Two Pt potential probes at afixed distance were set on one side of the bar, as shown inPt potential probesPt current collecting leadsFigure 1: DC equipment.15 25 35 45 55 65 752θ (degrees)Intensity(a.u.)[100][110][111][200][211][220][310][222]BCZY09BCZY27BCZY63Figure 2: X-Ray patterns for BCZY09, BCZY27 and BCZY63powders.Table 1: BCZY powders lattice parameter and density.Powder Lattice parameter (A˚) Density (g·cm−3)BCZY09 4.199 6.152BCZY27 4.242 6.207BCZY63 4.331 6.232Figure 1. Different oxygen pressures were obtained by mixingO2 and N2 (oxidizing atmosphere), or H2 and N2 (reducingatmosphere). The gas mixture was moisturized by bubblingthrough water held at 13◦C, giving a water-vapour partialpressure of ca. 0.015 atm. The isotope effect was measured byusing D2 and D2O in place of H2 and H2O. The conductivityvalues were not corrected for the porosity of the samples.3. RESULTS AND DISCUSSION3.1. Synthesized powdersThe BCZY powders are single-phased and the structure iscubic (Figure 2). A 24-hour dwell is necessary to completethe reaction. Previous syntheses were achieved with a 12-hour dwell, and peaks of the precursors were present in theX-Ray patterns.S. Ricote et al. 31μm 15 keVFigure 3: SEM picture of the BCZY27 powder.8 mmFigure 4: Picture of the BCZY09 SPS pellet, after polishing.The results of the lattice parameter (refined with Celrefsoftware) and density are summarized in Table 1. The grainsare polygonal and nanoscale (about 300 nm for each ceriumand zirconium content) (Figure 3).3.2. Sintered pelletsMeasurements of the sample geometry and Archimede’smethod were run to determine the density. For the SPSpellets, the two values are similar; the open porosity isnegligible. A heat treatment at 800◦C in air is required toremove graphite from the surface of the pellet, followed bypolishing. The BCZY09 pellets are nearly fully transparent(Figure 4), with a density percentage of 99%. An SEM picturewas obtained after a thermal etching at 1300◦C (Figure 5).The impurities are due to the polishing and the gold coating.The grain size is about 500 nm, and no crack is present onthe pellet. The BCZY27 and BCZY63 samples disintegratedtotally after a few days, revealing a difference of colorsbetween the bulk and the superficial layers. XRD patternswere recorded on these broken pellets, after grinding: onlythe BCZY perovskite phase is present in the diffractogram.One hypothesis is the partial reduction of cerium ionsbecause of the highly reductive atmosphere in the graphitedie at high temperature. Ce3+ ions are larger than Ce4+ions and mechanical stresses would be generated during thereduction.The pellets made by conventional sintering at 1700◦Cpresent a high-open porosity (Figure 6). Indeed, the density2μmFigure 5: SEM picture of the BCZY09 SPS pellet, after a thermaletching.Table 2: Fitted parameters (σi)H2O, (σi)D2O, and σ◦p for BCZY09bars (SPS and CS) at 800◦C.Sample(σi)H2O (σi)D2O σ◦p(mS/cm) (mS/cm) (mS/cm)BCZY09 (SPS) 4.33 2.66 58.25BCZY09 (CS) 1.85 1.21 25.3percentage obtained by geometric measurements is about80%, whereas the one obtained with Archimede’s methodis about 90%. Cracks and defects exist at the surface ofthe samples; the pressure is not homogeneously distributedduring uniaxial pressing. An SEM picture is shown inFigure 7: the grains are irregular and bigger than the ones inthe SPS pellet, between 500 nm and 1.5 μm. Furthermore, achange of the superficial color is obvious, due to an oxygennonstoichiometry. Polishing is necessary to remove it. TheX-ray patterns show the expected perovskite phase.Conventional sintering in different atmosphere (syn-thetic air or argon) was tried to avoid this superficialcoloration, but with no positive result.DC experiments were tried at 600◦C, but the really highdensity of the SPS sample required a very long equilibriumtime (more than one day for each gas composition). So theDC curves were obtained at 800◦C and are presented inFigure 8.Equation (3) was adjusted to the conductivity data inboth gas atmospheres and the parameters (σi)H2O, (σi)D2O,and σ◦p were determined [19, 20]. The results are summa-rized in Table 2. The difference between conductivity of D2Oand H2O exchanged states shows the protonic component ofthe conductivity.One can note a higher ionic contribution of the con-ductivity for the SPS sample, which can be partly relatedto the higher density. Grain boundaries in the SPS samplesdo not diffuse the light (transparent sample). These grainboundaries should present different properties than thoseof the pellet obtained by conventional sintering. Grain4 Journal of Nanomaterials1 cmFigure 6: Picture of the BCZY09 CS pellet, after polishing.2μmFigure 7: SEM picture of the BCZY09 CS pellet.boundaries contribute to the total conductivity of thesample. So, this could also explain the difference of ionicconductivity of the two samples.The p-type conductivity is also higher for the SPS sample.The highly reductive atmosphere at 1700◦C creates holes.The fast cooling does not let enough time for the reoxidationstep. Holes are trapped in the material, generating higher p-type conductivity.4. FUTURE WORKThree different sets of experiment will be run to completethis study. First, different temperature dwells, and coolingrates will be tested to investigate the influence on themechanical stress releases. Then, different atmospheres canbe used during the SPS sintering. This will modify thereductive property in the graphite die at high temperature.Finally, the influence of the grain boundaries on the totalconductivity will be determined by impedance spectra at lowtemperature with silver electrodes.5. CONCLUSIONSBCZY compounds can be synthesized by a solid-statereaction, with a 24-hour dwell at 1400◦C. The XRD analysesreveal a cubic perovskite structure. The sintering can beachieved by a conventional way, with a 6-hour dwell at−25 −20 −15 −10 −5 0Log {P(O2) atm}010203040506070σ(mS.cm−1)BCZY09 conventional sinteringBCZY09 SPS(a)−25 −20 −15 −10 −5 0Log {P(O2) atm}010203040506070σ(mS.cm−1)BCZY09 conventional sinteringBCZY09 SPS(b)Figure 8: DC curves at 800◦C for H2O exchanged state (a) and D2Oexchange state (b).1700◦C, followed by a fast surface polishing. To reduce thesuperficial defects, isostatic pressing should be achieved.To increase the density percentage, SPS experiments canbe run. When cerium is present in the sample, the pelletdisintegrates. Other SPS procedures (different temperaturedwells, cooling rates, and atmospheres) are now studied toobtain stable BCZY27 and BCZY63 pellets. The SPS methodis very promising because of the high-densified cracklesssamples obtained.Conductivity measurements were run on BCZY09 bars.The higher ionic component of the conductivity for the SPSsample is beneficial to use the material as electrolyte forPCFC. The p-type conductivity is also higher for the SPS bar.This can be changed with a slower cooling rate allowing areoxidation of the sample.REFERENCES[1] D. W. Lee, J. H. Won, and K. B. Shim, “Low temperaturesynthesis of BaCeO3 nano powders by the citrate process,”Materials Letters, vol. 57, no. 22-23, pp. 3346–3351, 2003.S. Ricote et al. 5[2] K. Ku¨nstler, H.-J. Lang, A. Maiwald, and G. Tomandl,“Synthesis, structure and electrochemical properties of In-doped BaCeO3,” Solid State Ionics, vol. 107, no. 3-4, pp. 221–229, 1998.[3] W. Mu¨nch, K.-D. Kreuer, G. Seifert, and J. Maier, “Proton dif-fusion in perovskites: comparison between BaCeO3, BaZrO3,SrTiO3, and CaTiO3 using quantum molecular dynamics,”Solid State Ionics, vol. 136-137, pp. 183–189, 2000.[4] N. Bonanos, “Oxide-based protonic conductors: point defectsand transport properties,” Solid State Ionics, vol. 145, no. 1–4,pp. 265–274, 2001.[5] K. D. Kreuer, “Aspects of the formation and mobility ofprotonic charge carriers and the stability of perovskite-typeoxides,” Solid State Ionics, vol. 125, no. 1–4, pp. 285–302, 1999.[6] N. Bonanos, K. S. Knight, and B. Ellis, “Perovskite solidelectrolytes: structure, transport properties and fuel cellapplications,” Solid State Ionics, vol. 79, pp. 161–170, 1995.[7] M. J. Scholten, J. Schoonman, J. C. van Miltenburg, and E. H.P. Cordfunke, “The thermodynamic properties of BaCeO3 attemperatures from 5 to 940 K,” Thermochimica Acta, vol. 268,pp. 161–168, 1995.[8] T. Shimada, C. Wen, N. Taniguchi, J. Otomo, and H.Takahashi, “The high temperature proton conductorBaZr0.4Ce0.4In0.2O3-α,” Journal of Power Sources, vol. 131, no.1-2, pp. 289–292, 2004.[9] K. H. Ryu and S. M. Haile, “Chemical stability and protonconductivity of doped BaCeO3-BaZrO3 solid solutions,” SolidState Ionics, vol. 125, no. 1–4, pp. 355–367, 1999.[10] K. Katahira, Y. Kohchi, T. Shimura, and H. Iwahara, “Protonicconduction in Zr-substituted BaCeO3,” Solid State Ionics, vol.138, no. 1-2, pp. 91–98, 2000.[11] T. Schober, F. Krug, and W. Schilling, “Criteria for theapplication of high temperature proton conductors in SOFCs,”Solid State Ionics, vol. 97, no. 1–4, pp. 369–373, 1997.[12] I. Kosacki and H. L. Tuller, “Mixed conductivity inSrCe0.95Yb0.05O3 protonic conductors,” Solid State Ionics, vol.80, no. 3-4, pp. 223–229, 1995.[13] H. Iwahara, “Proton conducting ceramics and their applica-tions,” Solid State Ionics, vol. 86–88, part 1, pp. 9–15, 1996.[14] Ph. Colomban, “Latest developments in proton conductors,”Annales de Chimie Science des Mate´riaux, vol. 24, no. 1, pp. 1–18, 1999.[15] A. S. Nowick and Y. Du, “High-temperature protonic conduc-tors with perovskite-related structures,” Solid State Ionics, vol.77, pp. 137–146, 1995.[16] T. Norby, “Solid-state protonic conductors: principles, prop-erties, progress and prospects,” Solid State Ionics, vol. 125, no.1–4, pp. 1–11, 1999.[17] R. Hempelmann, “Hydrogen diffusion mechanism in protonconducting oxides,” Physica B, vol. 226, no. 1–3, pp. 72–77,1996.[18] N. Agmon, “The Grotthuss mechanism,” Chemical PhysicsLetters, vol. 224, no. 5-6, pp. 456–462, 1995.[19] Y. Larring and T. Norby, “Protons in LaErO3,” Solid StateIonics, vol. 70-71, part 1, pp. 305–310, 1994.[20] D. Lybye and N. Bonanos, “Proton and oxide ion conductivityof doped LaScO3,” Solid State Ionics, vol. 125, no. 1–4, pp. 339–344, 1999.

Aspects of the formation and mobility of protonic charge carriers and the stability of perovskite-type oxides,”

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Criteria for the applicationofhightemperatureprotonconductorsinSOFCs,”

High-temperature protonic conductors with perovskite-related structures,”

Hydrogen diﬀusion mechanism in proton conducting oxides,”

Latest developments in proton conductors,”

o n ,a n dK .B .S h i m ,“ L o wt e m p e r a t u r e synthesis of BaCeO3 nano powders by the citrate process,”

Oxide-based protonic conductors: point defects and transport properties,”

Perovskite solid electrolytes: structure, transport properties and fuel cell applications,”

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Proton conducting ceramics and their applications,”

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Solid-state protonic conductors: principles, properties, progress and prospects,”

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The Grotthuss mechanism,”

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The thermodynamic properties of

BaCe0.9-xZrxY0.1O3-delta powders were synthesized by a solid-state reaction. Different contents of cerium and zirconium were studied. Pellets were sintered using either conventional sintering in air at 1700 degrees C or the Spark Plasma Sintering (SPS) technique. The density of the samples sintered by SPS is much higher than by conventional sintering. Higher values of ionic conductivity were obtained for the SPS sample. Copyright (C) 2008 S. Ricote et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited

Ricote, S.

Caboche, G.

Estournes, C.

Online Research Database In Technology

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Synthesis, Sintering, and Electrical Properties of

BaCe0.9−xZrxY0.1O3−δ

Synthesis, Sintering, and Electrical Properties of BaCe0.9−xZrxY0.1O3−δ

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