CHINA PARTICUOLOGY Vol. 4, No. 1, 9-12, 2006 SYNTHESIS OF STRONTIUM- AND MAGNESIUM-DOPED LANTHANUM GALLATE BY GLYCINE-NITRATE COMBUSTION METHOD Ning Liu 1 , Yupeng Yuan 1 , Min Shi 1, * , Yudong Xu 1 , P. Majewski 2 and F. Aldinger 2 1 Department of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, P. R. China 2 Max-Planck-Institut fűr Metallforschung, Pulvermetallurgisches Laboratorium, Heisenbergstr. 5, D-70569 Stuttgart, Germany *Author to whom correspondence should be addressed. E-mail: shimin@mail.hf.ah.cn Abstract Sr- and Mg-doped lanthanum gallate powders with the composition of La 0.85 Sr 0.15 Ga 0.85 Mg 0.15 O 2.85 were synthesized by a glycine-nitrate combustion method. Powders prepared under different fuel combustion conditions were investigated by XRD and TEM. The results show that, under slightly rich fuel condition, the product powders contain less impurity phases, and powders prepared by the glycine-nitrate combustion contain far less impurity phases and have smaller particle sizes than those prepared by solid-state reaction method or acrylamide polymerization technique. Keywords glycine-nitrate combustion method, solid-state reaction method, acrylamide polymerization technique, phase constitution 1. Introduction As lanthanum gallate (LaGaO 3 ) ceramics (denoted as LSGM) doped with strontium and magnesium are known to have superior oxygen-ion-conducting properties, they have become widely used as electrolytes for Solid Oxide Fuel Cells (SOFCs) (Ishihara et al., 1994; Feng & Goodenough, 1994). In general, LSGM powders are prepared by using the solid-state reaction method or the acrylamide polym- erization technique (Huang et al., 1996; Tas et al., 2000; Tarancón et al., 2003). The conventional solid-state reac- tion method involves intimate mechanical mixing of the oxides of La, Sr, Ga and Mg and repeated grinding and heating cycles to achieve complete reaction between the reagents. Despite its simplicity, this method has the clear disadvantages of producing large grains, repeated thermal treatment and grinding, and the presence of many impuri- ties such as LaSrGaO 4 , LaSrGa 3 O 7 , La 3 Ga 5 O 12 and La 4 Ga 2 O 9 . The acrylamide polymerization technique, too, requires expensive metal alkoxide precursors and calls for great care in mixing the precursors to obtain LSGM pow- ders with the desired stoichiometry. The new synthesis route involving glycine–nitrate combustion was developed by Chlik et al. (Stevenson et al., 1997; Cong et al., 2003; Sin & Sdier, 2000) and has now become an attractive synthesis method for preparing multiple component inor- ganic oxides. This method offers several distinct advan- tages: first, the homogeneous mixtures of several compo- nents at molecular or atomic levels can be achieved in solution, and ultra-fine powders can be obtained, and second, this synthesis process is time-saving and the final products contain less impurities. Research has focused on the sintering ability of the synthesized powder and the electrical conductivity of the sintered product. But work on the influence of combustion fuel condition on synthesized powders has not yet been reported. This paper investigated the effects of combustion fuel condition on the constitution of the powder product, its morphology and particle size. Solid-state reaction and acrylamide polymerization were also used for comparison. 2. Experimental Ga (99.95% purity), La 2 O 3 (>99.95% purity), Mg(NO 3 ) 2 (>99% purity) and Sr(NO 3 ) 2 (>99.5% purity) were used as the starting materials. A powder with the composition of La 0.85 Sr 0.15 Ga 0.85 Mg 0.15 O 2.85 (denoted as LSGM) was syn- thesized by the glycine-nitrate combustion method. Ap- propriate amounts of Ga, La 2 O 3 , Mg(NO 3 ) 2 and Sr(NO 3 ) 2 were first dissolved in strong HNO 3 to obtain corresponding nitrate solutions. According to the formula of LSGM, these nitrate solutions were then mixed together with water in a glass beaker, and glycine (as fuel and complexant) was added into the mixed nitrate solution at a molar ratio of n Me : n glycine =1:1.78. The glass beaker containing the above mixed glycine–nitrate solution was heated on a hot plate, to boil off sufficient water until the solution began to froth and catch fire at some instant. In this way, a homogeneous white powder product was eventually obtained in a matter of several minutes. By acrylamide polymerization or by solid-state reaction, the above process would take several hours or even days. The combustion gas consists of CO, CO 2, H 2 O and N 2 , according to the following reaction: 85La(NO 3 ) 3(aq) + 15Sr(NO 3 ) 2(aq) + 85Ga(NO 3 ) 3(aq) + 15Mg(NO 3 ) 2(aq) + 356 1 4 C 2 H 5 NO 2(aq) = 100La 0.85 Sr 0.15 Ga 0.85 Mg 0.15 O 2.85(s) + 178 1 8 CO (g) + 890 5 8 H 2 O (g) + 463 1 8 N 2(g) + 534 3 8 CO 2(g) , where, (aq), (s) and (g) mean liquid, solid and gas, re- spectively. For examining the effect of combustion fuel condition on the properties of the final powder, different combustion fuel conditions were designed by adding different amounts of glycine. For example, for rich fuel condition, twice the