Synthesis and characterization of SrCe0.95Y0.05O3-δ nano powders by low temperature combustion

RARE METALS Vol. 25, No. 1, Feb 2006, p . 79

Synthesis and characterization of SrCe0.95Y0.050M nano powders by low temperature combustion MENG Bo‘), TANXiaoyao”, ZHANG Baoyan2’,and YANG Naitao’) 1) College of Chemical Engineering and Technology, Shandong University of Technology, Zibo 255049, China 2) College of Science,Northeastern University, Shenyang 11OOO4, China

(Received 2004-10-08)

Abstract: Nanosized SrC~,95YO,0503-~ powders with homogeneous composition were synthesized by the low temperature combustion process based on the Pechini method. A polymeric precursor sol was formed by using citric acid and ethylene glycol as the chelating agents of metal ions. The perovskite-type S r C ~ . 9 ~ Y ~ . o 5 0 ~ ~ p owith w d euniform rs shape and smaller than 25 nm in size were obtained through the combustion of the polymeric precursor sol at the existence of nitric acid and ammonium hydroxide. It was found that modulating the quantities of nitric acid and ammonium hydroxide could control the particle size, and the quantities of residue carbonate ions were also affected by the quantities of citric acid and ethylene glycol. Key words: inorganic nonmetal material; perovskite-type oxides; SrCe0.95Y0.0503-& combustion synthesis [This work was financially supported by the National Natural Science Foundation of China (No. 20076025).]

1. Introduction ABO3 perovskite-type oxides doped with some trivalent cations are now well known as high temperature type proton conductors with high transport number of protons. Especially, these rare earth element-doped AB03 perovskite oxides with A = Ca, Sr, and Ba, B = Ce, Zr,and Ti, etc. have been studied in detail [l-41. Such perovskite materials have received great interest from many application areas such as fuel cells, sensors, steam electrolyzers, hydrogen separation from hydrogen containing gas mixtures, and membrane reactors for the transport of protons

[5-81. It is well known that most properties of ceramic powders depend on their methods of production. Solid state reactions are the most widely utilized process and they are good for mass-producing cost-efficientpowders, because the raw materials are simply calcined to obtain the products. However, it is difficult to obtain a homogenous composition and Corresponding author: MENG Bo

dense, fie-grained sintered bodies because of poor dispersion by physical mixing [9-lo]. Therefore, wet chemical methods, such as co-precipitation, sol-gel, the Pechini, glycine-nitrate and the citrate acid methods, were studied. These methods provide a mixing of the elements at an atomic scale, which is known to accelerate the reaction of the phase formation [ll]. The Pechini and glycine-nitrate methods have been used to obtain a variety of mixed metal oxides with precise stoichiometry. Liu et al. [12] employed the Pechini method in the preparation of SrCe03-based ceramic powders. Cong et al. [13] employed the glycine-nitrate process to prepare the materials for inter-temperature solid oxide fuel cells. In this work, the preparation of the powders of SrCe03 doped with yttrium with the composition SrCe0,95Y0.0503~ by the low temperature combustion process, i.e. citrate-nitrate-ammonium hydroxide combustion method, was studied with respect to grain size and particle distribution as well as phase Purity.

E-mail: mb1963 @ sdut,edu.Cn

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2. Experimental Nanocrystalline S ~ C Q . ~ ~ Y(SCY) ~ . ~ ~powders O~-~ were prepared by a citrate-nitrate-ammonium hydroxide combustion route. All of the chemical reagents used were of analytical grade. Citric acid monohydrate (CA) and ethylene glycol (EG) were used as a chelating agent and the fuel, respectively. Nitric acid was used in order to introduce oxidizing ions. Ammonium hydroxide was used to facilitate the chelating reaction and combustion, respectively. Stoichiometric amounts of barium nitrate (Sr(N03)2), cerium nitrate hexahydrate (Ce(N03)3.6H20), and yttrium nitrate hexahydrate (Y(N03)3.6H20)were fist dissolved in a minimum volume of distilled water to obtain a transparent aqueous solution. Then measured amounts of citric acid (CA) and ethylene glycol (EG) were added to the metal nitrate solution, which was subsequently heated and stirred to form the precursor complex solution. The amount of CA was determined by the molar ratio of CA to the total metal ions (defined as C ratio), and the amount of IEG was based on the molar ratio of EG to CA (defined as EG/CA ratio). The C ratio varied from 1 to 4 and the EG/CA ratio varied from 1 to 2. After that, measured amounts of nitric acid and ammonium hydroxide were added to adjust the oxidant fuel ratio of the system. The amount of HN03 was determined by the molar ratio of HN03 to the total organic molecular compounds (defined as HNOdOrg. ratio) and the amount of NH3.H2O was based on the molar ratio of NH3.HZO to HN03 (defined as NH3.H2O/ HN03 ratio). The solution containing the complex precursor at neutral pH was then stirred and heated on a hot plate at 80°C to remove the excess water. Then the solution led to the highly viscous liquid, i.e. the polymeric precursor sol. Neither phase separation nor precipitation was observed during the sol formation under these conditions. As soon as the viscous liquid was formed, the temperature of the hot plate was increased to about 230°C. At this stage, the viscous liquid swelled and auto-ignited, with the rapid evolution of large volume of gases to prodwe voluminous and fluffy powders termed here as precursors. These precursors were calcined at 900°C to remove traces of un-decomposed citric acid, nitrates

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and their decomposition products and obtain the desired phase. The structure of the as-combusted and calcined product was examined by X-ray powder diffraction (XRD) technique using a computerized Rigaku, D/Dmax 2200 (Japan) X-ray diffractometer with Nickel filtered Cu & radiation. A continuous scan mode was used to collect 2 0 data from 0" to 80" with a 0.02 sampling pitch and a 2"/min scan rate, while X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. In order to see whether there is any phase transition, thermal analysis of the as-combusted product was carried out on an STA 409 EP (Netzsch) in the temperature range of 25-1200°C at a heating rate of lO"C/min. The microstructure and morphology of the final sintered ceramic powder were investigated via scanning electron microscopy (SEM XL30 & DX-4i Philips) and transmission electron microscope (TEM H-800, Japan).

3. Results and discussion 3.1. Determination of the optimum amount of chelating agent, HN03,and NHyH20 Four process variables investigated in this study are the C ratio, the EG/CA ratio, the HNOdOrg. ratio and the NH3.H20/HN03ratio. For solutions with low C ratio (C < 1.5), it is difficult to keep the metal ions in solution and irreversible precipitations usually occur during evaporation of the mixed solution due to the lack of a sufficient amount of chelating agent molecules. For the C ratio greater than 1.5,the solutions formed are clear and can easily dissolve the metal ions. A variation in the EG/CA ratio from 1 to 2 does not have significant effect on the solution, but it influences the concentration rate. The higher the EG/CA ratio is, the faster the concentration rate is. The excessive EG is oxidized into oxalic acid, and the oxalic acid can take part in both esterification and chelation, as a result facilitating the formation of sol. On the other hand, since all organic components involved will be eventually burned off, the C ratio and the EG/CA ratio cannot be very high considering the cost and the subsequent combustion reaction. In this study, the values of the C ratio and

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Meng B. et al., Synthesis and characterizationof SrCe0.~~Y0.0~03 nano powders by ...

the EGKA ratio are 1.5 and 1.2, respectively. For combustion reaction, the amounts of HN03 and NH3.HZO have obvious effect on the particle size, purity, and appearance of the final powders as shown in Table 1. From Table 1, it can be seen that when the HNOdOrg. ratio varies from 4 to 6 and the NH3.H20/HN03ratio varies from 0.8 to 1.O, the ap-

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pearance of the as-combusted powders is fluffy and yellowish white, and the average agglomerate size of the as-combusted powders decreases with increasing amounts of HN03 and NH3.HzO. In other words, we may control the particle size and purity by modulating the quantities of nitric acid and ammonium hydroxide.

Table 1. Effect of the addition quantities of HNOJand NH3.H20on the production

HN03/Org.

NH3.H20/HN03

ratio

ratio

1-3 4 5

0.5-0.8

Volume of fog

0.8-1.0

Burst into flame

0.8-1.0

6 7

0.8-1.0

Burst into flame Burst into flame Explosiveprocess

>1.5

Combustion phenomena

Observations of as-combustion powders Grayish black and fluffy Light yellow and fluffy Light yellow and fluffy Light yellow and fluffy Light yellow and snowflake

Average particle size of as-combustion powders / pm 44.7 44.3

43.3 43.0 41.5

3.2. Analysis of the synthesis mechanism The solution chemistry and reactions occurring in the sol forming process were reported in Ref. [14]. The possible chemical reaction equations during the combustion process were as follows. + 0.05Y(N03)3 + 3C6H807 +3.6C2H602 +16.502 -+ S I - ( N O ~+0.95Ce(N03)3 )~ SrCe0.95Y0.0503-6 +25.2C02 +22.8HzO+2.5Nz

(1)

Sr(N03)2+0.95Ce(N03)3+0.05Y(NO3)3 +3C6H8o7+3.6C2H602 +xNH3 .H20+16.502 +1.5x02 + SrCe0,95Y0,0503-6 +25.2C02 +(22.8+2.5x)H20+(2.5+0.5x)Nz NH3.H20+HNO3

-+ Nz + 3 H 2 0 + -10 2

(m (W

2

Sr(N03)z+0.95Ce(N03)3+O.O5Y(NO3)3 +3C6H807 +3.6CzH602 +40NH3 .Hz0+40HN03 + SrCeo.95Yo.0503-6 + 25.2COz + 142.8H20+ 40.75Nz + 3.5N02 From Reaction (I), it can be seen that plenty of oxygen is necessary for the precursor sol to complete combustion under the condition without the addition of HN03 and NH3-HzO.From Reaction (n), when the NH3.HZO is added into the system, the amount of oxygen needed for combustion is increased. Therefore, it is impossible to make the precursor sol complete combustion and obtain the purity product in air. From Reaction (Ill), when the NH~-HzO/HNO~ ratio is 1, nitric acid oxidizes the NH3-Hz0,converting it to nitrogen and oxygen. The formation of oxygen facilitates the combustion of the precursor sol. Based on Reactions (I) and (m), if the oxygen in air is neglected, the molar ratio of

(W

HN03 to the total organic compounds should be 5: 1 to ensure the complete combustion of the precursor sol. In the experiment, it is found that the precursor sol can be completely combusted, when the HNOdOrg. ratio is 4, as the air provides more oxidant for combustion reaction. When the amount of HN03 and NH3.HzO is higher, the combustion reaction will produce the nitrogen dioxide gas such as shown in Reaction (IV).

3.3. STA, XRD,SEM, and TEM analyses Fig. 1 is the thermal analysis (STA) result of the as-combusted powders obtained under the condition that the C ratio, EG/CA ratio, HNOdOrg. ratio, and

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NH3.H2O/HNO3are 1.5, 1.2,5, and 1.0 respectively. The exotherms at -205°C may be explained as the removal of organic impurities left in the sample after combustion. The endothems at -1012°C may be due to the crystallization accompanied by the release of trapped carbon dioxide or carbon present in the amorphous oxide structure. These assignments are also consistent with the mass changes observed in DTG curve. The overall mass change in the sample is only 12.7%. The result of the thermal study implies that the decomposition is almost complete.

results, it can be concluded that the perovskite-type oxide SCY can be formed after self-ignition. The SEM micrograph of S I C ~ . ~ ~ Y O . ~ ~ O ~ - ~ ~ O ders calcined at 900°C for 0.5 h before milling is shown in Fig. 3. The plate-like morphology of SCY is clearly seen in the micrograph. The average agglomerate size is smaller than 1 pm and appears to be uniform. Fig. 4 shows TEM images of SCY nanoparticles after milling. It can be seen that the aggregate size is smaller than 25 nm. From Fig. 4 it can also be found that the morphology of the particles is almost homogenous. It indicates that the agglomerates are easily grounded into a less voluminous homogeneouspowder.

1012°C

n o

"

-.-6

-12.7%

20

40 201 (")

2500 20003. 1500-

60

v)

t

8

G

0

SCY

f

-14

0

400

800

1200

Temperature / "C

Fig. 1. Thermal analysis curves of the as-combusted powders.

The XRD patterns obtained for the as-combusted powders and the powders calcined at 900°C for 0.5 h are shown in Figs. 2(a) and 2(b). It is seen from Fig. 2(a) that perovskite SrCeo.95Y~.0503-~ (SCY), and SrC03 phases coexist. After calcination at 90O0C, all the peaks of the XRD spectra are almost in agreement with the reported data of SrCe03 in JCPDS (No. 23-1412) such as in Fig. 2(b). Considering that the change of &spacing is induced by the Y substituting for Ce, and based on Reaction (IV),it is determined to be the SCY phase. Small variations in the d-space data between SCY samples and the standard SrCe03 data further support the distortion of the SrCe03 structure by substituting 5 mol% Ce with 5 mol% Y. Combining the X R D data with STA

"

20

40 20 I (0)

60

Fig. 2. XRD patterns of the as-combusted powders (a) and the powders calcined at 900°C for 0.5 h (b).

Fig. 3. SEM micrograph of SCY powders calcined at 9OOOC for 0.5 h before miUing.

Meng B. et al., Synthesis and characterizationof SrCe0.95Y0.0503 nano powders by

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proton conductive perovskite oxides, Solid State Zonics, 1998,106: 155. Munch W., Kreuer K.D., and Seifert G, Proton diffusion in perovskites: comparison between BaCe03, B W 3 , SrTi03, and CaTi03 using quantum molecular dynamics, Solid State Zonics, 1996, 86/88 183. Kunstler K., Lang H.J., Maiwald A., and Tomandl G, Synthesis, structure and electrochemicalproperties of In-doped BaCe03, Solid State Zonics,1998,107: 221. Marnellos G., Anopoulou O.S., Rizou A., and Michael S., Electrolytesfor the use of proton conducting solid improved performance of hydro- and dehydrogenation reactors, Solid State Ionics, 1997,W: 375. Hamakawa S., Hibino T., and Iwahara H., Electrochemical hydrogen permeation in a proton-hole mixed conductor and its application to a membrane Fig. 4. TEM photo of SCY powders calcined at 9OOOC reactor, J. Electrochem. SOC.,1994,141(7): 1720. for 0.5 h after milling. Reichel U., Arons R.R., and Schilling W., Investigation of n-type electronic defect in the protonic con4. Conclusion ductor S I C ~ , - ~ Y ~Solid O ~ -State ~ Zonics, 1996,86/88 639. Pure nanoparticles of S ~ C Q . ~ ~ Y ~have . ~ ~been O~-S Iwahara H., Uchida H., Morimoto K., and Hosogi S. synthesized by the low temperature combustion High-temperature C1-gas fuel cells using proroute based on the Pechini method. The most critical ton-conducting solid electrolytes, J. Appl. Electroprocessing parameters are the ratio of chelating chem., 1989,19:448. agent (CA) to the total metal ions (C ratio), the ratio Takatori K., Tani T., Watanabe N., and Kamiys N., of HN03to the total organic compounds (HN03/Org. Preparation and characterization of nano-structured ratio), and the ratio of NH3.HzO to HN03 ceramic powders synthesized by emulsion combus(NH3.HzO/HN03ratio). Fine and pure powders of tion method, J. Nunopart. Res., 1999,l: 197. [lo] Majewski P., Rozumek M., Tas C.A., and Aldinger SrCe0.95Y0.0~03-~ are obtained only at the C ratio F., Processing of LaSGaMg03 solid electrolyte, J. greater than 1.5, HN03/Org. ratio and Electroceram., 2002,s: 65. NH3.HZ0/HNO3ratio in the range of 4-6 and 0.8-1.0, [Ill Lee D.W., Won J.H., and Shim K.B., Low temperarespectively. The X-ray diffraction analysis demonture synthesis of BaCe03 nano powders by the citrate strates that no carbonate ( C O P ) is observed in the process, Matel: Lett.,2003,57: 3346. synthesized powders after heat treatment up to [12] Liu S.M., Tan X.Y., Li K., and Hughes R., Synthesis 900°C in air. SEM and TEM observations indicate of strontium cerates-based perovskite ceramics via that the size of the synthesized S~CQ.~~YO.OSO~-S water-soluble complex precursor routes, Ceram. Znt., powders is smaller than 25 nm and the distribution is 2002,28 327. uniform. [13] Cong L.G, He T.M., Ji Y., Guan P.F., Huang Y.L., and Su W.H., Synthesis and characterization of IT-electrolyte with perovskite structure References Lao.8Sro,2Gao,85Mgo.l~03-s by glycine-nitrate combustion method, J. Alloys Compd., 2003,348 325. [l] Schober T., Krug F., and Schilling W., Criteria for the [14] Liu M.L. and Wang D.S., Preparation of application of high temperature proton conductors in Lal,SrzCol-~~033,thin films, membranes, and SOFCs, Solid State lonics, 1997,W: 369. coatings on dense and porous substrates, J. Mate,: [2] Matsunami N., Schimura T., and Iwahara H., Res., 1995,lO (12): 3210. Anomalous penetration of implanted deuterium in