Influence of organic solvent on solvothermal synthesis of samaria and gadolinia doped ceria nickel oxide composites

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1 Influence f rganic slvent n slvthermal synthesis f samaria and gadlinia dped ceria nickel xide cmpsites Alexander Rdrig Arakaki 1,a, Sandra Maria Cunha 1,b, Walter Kenji Yshit 1,c, Valter Ussui 1,d, Dlres Ribeir Ricci Lazar 1,e 1 Institut de Pesquisas Energéticas e Nucleares Av. Prf. Lineu Prestes, 2242 Cidade Universitária, SP, SP, Brasil a alexander@ipen.br, b smcunha@ipen.br, c wyshit@ipen.br, d vussui@ipen.br, e drlazar@ipen.br Keywrds: Dped-ceria / nickel xide, synthesis, cprecipitatin, slvthermal treatment Abstract. The synthesis and ceramic prcessing f samaria and gadlinia dped ceria - nickel xide cmpsites, mainly applied as Slid Oxide Fuel Cell andes, were studied in this wrk. Pwders with cmpsitin Ce 0.8 (SmGd) 0.2 O 1,9 /NiO and mass rati f 40/60%, were synthesized by hydrxide cprecipitatin with CTAB surfactant, fllwed by slvthermal treatment in n-butanl, ethanl and n-prpanl at 150 ºC fr 16 hurs, calcinatin at 600 C fr 1 hur and milling. Sintering f cmpacted samples was perfrmed at 1300 C fr 1 hur. The pwders were analyzed by X-ray diffractin, scanning electrn micrscpy, nitrgen adsrptin methd (BET), laser beam scattering (Cilas) and TG/DTA. The ceramics were analyzed by scanning electrn micrscpy, EDS, XRD and density measurements by Archimedes methd. The results shwed that the pwders have a high specific surface area (42-85 m 2 /g). The ceramic characterizatins shwed a high chemical hmgeneity and density value f 99% TD fr pwders treated with ethanl and prpanl. Intrductin The (CeO 2 ) based ceramics have been widely studied due t its high inic cnductivity allwing their use as Slid Oxide Fuel Cell electrlyte. The dpants that prvide highest imprvement in the inic cnductivity are the trivalents rare earths samaria and gadlinia. The andic materials that present high chemical, physical cmpatibility and highest electrnic cnductivity with ceria based electrlytes are the cermets cmpunds cntaining metallic nickel and the same electrlyte material [1,2]. Metallic nickel can be frmed by reductin in hydrgen atmsphere frm the nickel xide prduced by cprecipitatin with the gadlinia and samaria dped ceria (GSDC) [3]. The andic cmpsite perfrmance depends n the pwder characteristics such as particle size, surface area, and chemical hmgeneity, as well as n ceramic micrstructure, including triple phase bundaries (TPB) and perclatin chains [1-3]. It is well knwn that these prperties can be adjusted during the pwder synthesis and gel precipitatin is a usual emplyed methd. The assciatin with slvthermal technique was recently intrduced t allw the preparatin f crystalline nanpwders at lw temperatures [4]. This prcess invlves the use f an rganic slvent in an autclave under autgenus pressure f atm generated by temperatures frm 100 t 250ºC. These cnditins prmte the grwth f the particles in preferred directins, making pssible the frmatin f particulate frms f nancubes, nanrds, nantubes and nanplates[5]. The shape f the particles can als be changed with the emplyment f a surfactant in precipitatin, which perates in the nucleatin, particle grwth, cagulatin and flcculatin[6]. Fr this purpse parameters like ph f precursrs slutin, reactin temperature and time, slute cncentratin and type f slvent have t be cntrlled[5,6]. In this wrk samaria and gadlinia dped ceria - nickel xide pwders have been synthesized by cprecipitatin, in the presence f CTAB (hexadecyltrimethylammnium brmide) surfactant, fllwed by slvthermal treatment. 1834

2 Materials and Methds Chlrides f cerium, samarium, gadlinium and nickel, btained by respective xide disslutin, (purity > 99,9%, Aldrich) were used as starting materials in the Ce 0,8 (SmGd) 0,2 O 1,9 / NiO synthesis, with mass rati 40/60%. Precipitatin synthesis was perfrmed by heating the slutin cntaining the metallic ins (Me) and CTAB, with rati Me/CTAB = 2, drpwised t a stirred ammnia slutin 7.0M at 98 ºC. Ni 2+ ins cncentratin was fixed at 0.1M t avid nickel lss. The final ph was 7 and the supernatant was light blue. The btained gels were submitted t washing with water t remve the chlride ins, and then slvthermically treated at 150 ºC fr 16 hurs in butanl, ethanl r prpanl. The resulting pwders were divided in tw prtins: the first ne was vendried at 80ºC fr 24 hurs and then ball-milled fr 16 hurs. The secnd fractin was submitted t calcinatin at 600ºC fr 1 hur, ball milling in ethanl and ven-drying at 80ºC fr 24 hurs. Fr cmparisn purpses, tw thers experiment were perfrmed: withut CTAB and slvthermal synthesis and withut CTAB and with slvthermal treatment in butanl. The sample cdes are explained in Table 1. The additin f the cde C600 identifies calcined samples and the cde 1300 the sintered samples. Table 1. Experiments cdes accrding t synthesis prcedure. Azetrpic Slvthermal Sample cde CTAB Distillatin Treatment GSDC/NiO E YES NO Ethanl GSDC/NiO P YES NO Prpanl GSDC/NiO D NO YES NO GSDC/NiO B NO NO Butanl GSDC/NiO B2 YES NO Butanl The resulting pwders were pressed by uniaxial cmpactin at 100MPa in a metallic matrix. The green pressed pellets were sintered in air at 1300ºC fr 1 hur using an electrical bx furnace (Lindberg/Blue M). The pwders were characterized by X-ray diffractin (Rigaku, Multiflex), scanning electrn micrscpy (Philips, XL30), agglmerate size distributin by laser scattering (CILAS, 1064), thermal analysis (TG/DTA) (Setaram Labsys Instrumentatin, TG-DTA/DSC) and specific surface area by gas adsrptin (B.E.T.) (Quantachrme, Nva 1200). Ceramic sinterability was evaluated by density measurements using Archimedes methd and the ceramic pellets were characterized by X-ray diffractin, scanning electrn micrscpy and EDS (Hitachi, TM 3000). Results and Discussins Figure 1 shws XRD patterns f ceria-nickel based pwder after slvthermal treatment and calcinatin. After slvthermal synthesis pwders are frmed by ceria flurite phases structure (JCPDS ) and nickel hydrxide (JCPDS ). The calcinatin step keeps ceria flurite phase and prvides the frmatin f mnclinic nickel xide structure (JCPDS ). The particle size distributin curves presented in Fig. 2 shw the similar distributin and mean agglmerate size f μm fr the nt calcined pwders and μm fr the calcined pwders. It is wrth nting just the GSDC/NiO-P higher agglmerate average size f 5.1 μm and different distributin curve. 1835

3 Cumulative mass % GSDC/NiO EC 600 GSDC/NiO PC 600 GSDC/NiO DC 600 GSDC/NiO BC 600 GSDC/NiO E GSDC/NiO P GSDC/NiO D GSDC/NiO B 0 Figure 1. X-ray diffractin patterns f Ce 0,8 (SmGd) 0,2 O 1,9 / NiO synthesized pwders. 0,01 0, Agglmerate size (μm) Figure 2. Agglmerate size distributin curves f Ce 0.8 (SmGd) 0.2 O 1.9 /NiO synthesized pwders. Specific surface area values f prepared pwders are presented in Table 2. It can be bserved that all pwders have high reactivity, arund m 2.g -1 fr nt calcined pwders, and the calcinatin step decreases this reactivity. Higher specific surface area was fund fr the pwders synthesized by slvthermal treatment in prpanl withut calcinatins (GSDC/NiO-P sample). Amng calcined pwders GSDC/NiO-PC600 has the higher surface area. Table 2. Specific surface area values f Ce 0.8 (SmGd) 0.2 O 1.9 /NiO synthesized pwders. Sample cde Specific surface Specific surface area (m 2 Sample cde /g) area (m 2 /g) GSDC/NiO-E 72.4 GSDC/NiO-EC GSDC/NiO-P 97.5 GSDC/NiO-PC GSDC/NiO-D 81.6 GSDC/NiO-DC GSDC/NiO-B 85.4 GSDC/NiO-BC GSDC/NiO-B GSDC/NiO-B2 C SEM micrgraphs (Fig.3 and 4), shw the slvthermal treatment influence n the agglmerate size and mrphlgy. It can be seen that GSDC/NiO-E and GSDC/NiO-P pwders (Fig.3 a. and b.) are frmed by smaller plates and are less agglmerated than GSDC/NiO-D GSDC/NiO-B pwders (Fig.3 c. and d.). As seen in the Table 3, GSDC/NiO-E and GSDC/NiO-P pwders after calcinatin and pressing, prduce ceramics with higher densities, prbably due t the dispersin prmted by CTAB and/r the cmplete frmatin f nickel xide after calcinatin. (Fig. 4). Cnsidering the theretical cmpsite density f 6.9 g.cm -3, the ceramic GSDC/NiO PC reached 99% f theretical density. The TG/DTA results f the thermal decmpsitin prcess f synthesized samples after precipitatin, slvthermal treatment in butanl and drying, as a functin f CTAB additin, are shwn in Fig.5. The weight lss arund 15% is due t the remain remval f free water until 200ºC and after that, structural OH - grups f hydrated xides. Endthermic peaks are prduced by the weight lss up t 300ºC. Exthermic peak crrespnds t the imprvement f xide crystallizatin and the xidatin f Ce 3+ [6]. The samples synthesized with and withut CTAB presented the same behavir, althugh the CTAB use causes slight faster decmpsitin and smaller weight lss. 1836

4 Eighth Internatinal Latin American Cnference n Pwder Technlgy, Nvember 06 t 09, Cstã d Santinh, Flrianóplis, SC, Brazil (c) (d) Figure 3. SEM micrgraphs f Ce0.8(SmGd)0.2O1.9 / NiO synthesized pwders: GSDC/NiO-E, GSDC/NiO-P, (c)gsdc/nio-d and (d)gsdc/nio-b. (c) (d) Figure 4. SEM micrgraphs f Ce0.8(SmGd)0.2O1.9 / NiO calcined pwders: GSDC/NiO-EC600, GSDC/NiO-PC600, (c)gsdc/nio-dc600 and (d)gsdc/nio-bc

5 100 N CTAB Me/CTAB = 2 Weight lss (%) N CTAB Me/CTAB = 2 DTA (μv) Endthermic Temperature ( C) Temperature ( C) Figure 5. TG and DTA analysis f Ce 0.8 (SmGd) 0.2 O 1.9 /NiO pwders synthesized by cprecipitatin, slvthermal treatment in butanl and drying, as a functin f CTAB presence. The dped ceria and nickel xide ceramics hmgeneity verificatin was made by plished and thermally etched GSDC/NiO-EC600 samples using scanning electrn micrscpy with secndary (SE) and backscattered electrns (BSE) detectin, and the micrgraphs are shwn in Fig.6. This images present grain sizes f apprximately 100nm and hmgeneus distributin f cnstituent cmpnents. NiO grains appear darker and larger than the Ce 0,8 (SmGd) 0,2 O 1,9 grains. Figure 6. SEM micrgraphs f Ce 0.8 (SmGd) 0.2 O 1.9 / NiO ceramics: (SE)GSDC/NiO-EC , (BSE) GSDC/NiO-EC In rder t verify the elements distributin in the micrstructure f GSDC/NiO-PC samples, SEM-EDS analysis was perfrmed n the yellw circled area shwn in Fig.7a. This analysis indicates the presence f all precipitated elements in this area. The estimated EDS chemical analysis (semi quantitative) indicates that the desired stichimetry was reached. Figure 7. SEM-EDS results fr the GSDC/NiO-EC ceramics: SEM micrgraph, EDS spectrum f the yellw circled area. 1838

6 The X-ray diffractin patterns f Ce 0,8 (SmGd) 0,2 O 1,9 / NiO ceramics presented in Fig.8, shw the frmatin f crystalline ceria and nickel xide. - CeO NiO GSDC/NiO-B2C Intensity GSDC/NiO-BC GSDC/NiO-DC GSDC/NiO-PC GSDC/NiO-EC Θ (Degree) Figure 8. X-ray diffractin patterns f Ce 0,8 (SmGd) 0,2 O 1,9 / NiO ceramics. Table 3. Density f the Ce 0.8 (SmGd) 0.2 O 1.9 /NiO ceramic pellets. Sintered density ceramic (g.cm -3 ) Sample cde Withut calcinatin Calcined at 600 C GSDC/NiO - E GSDC/NiO - P GSDC/NiO - D GSDC/NiO - B GSDC/NiO B Cnclusins Slvthermal treatment with rganic slvents f ceria-nickel xide based gels cprecipitated in the presence f CTAB surfactant prvides a hmgeneus cmpsite frmatin. This is an imprtant factr t materials applied as Fuel Cells Andes, cnsidering that this cmpnent depends n a high inic and electrnic cnductivity, and therefre n the metallic phase perclatin. The use f CTAB surfactant in Ce 0,8 (SmGd) 0,2 O 1,9 /NiO cprecipitatin rute synthesis, cntributes t the increase f specific surface area f pwders, and als the imprvement f ceramic density and micrstructure, especially fr pwders treated with prpanl. The calcinatin step is necessary in rder t the cmplete nickel xide frmatin during synthesis. Acknwledgments The authrs wuld like t thank the CNPq (Prject /2009-0) and FINEP (PaCOS netwrk) fr the financial supprt and the schlarship. Thanks als t ur clleagues frm CCTM/IPEN and CCCH/IPEN, especially frm the Labratries f Micrscpy, XRD, Cilas and Thermal Analysis. References [1] S.M Haile, Acta Mater., Vl.51 (2003), p [2] Minh, N.Q. J.Am. Ceram.Sc., v. 76, n.3, p , [3] Chen, M.; Kim, B.H.; Xu, Q.; Nam, O.J.; K, J.H. J. Eur. Ceram. Sc., v. 28, p , [4] Arakaki, A. R.; Yshit, W.K.; Ussui, V.; Lazar, D.R.R. Mater. Sci. Frum, v , p , [5] B. Djuricic, S. Pickering, J. Eur. Ceram. Sc. Vl.19 (1999), p [6] Pan, C.; Zhang, D.; Shi, L. J. Slid State Chem., v.181, p. 1298,