Chemical Synthesis of Pure and Doped LaGaO 3 Powders of Oxide Fuel Cells by Amorphous Citrate/EG Method
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00*
1340* 1200°
1000* 850' 700*
500°
350° 200° 100°C
1340°C 1200"C 1000,C 850°C 700°C 500"C 350'C 200C 100"C
LaGaO•3
1200"C 1000TC 850"C 7fl0°C 500°C 350oC 200"C 100.C
Ill
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2 theta
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80
Fig. 1 XRD spectra of precursor powders of LG, LSG and LSGM samples
238
with the previously reported values [3]. LSG samples (after calcination at 1340 0 C for 6 h) were also orthorhombic with the lattice parameters ofa=5.491, b=5.523, c=7.764 A. Although the single-phase LG was formed from the starting X-ray amorphous resins at about 850*C, single-phase LSG could only be produced after calcination at 13400 C for 6 h. The resins of the LSG stoichiometry, after calcination at 1200'C, still contained the secondary phases of La 4 Ga2 0 9 and SrLaGa 3O7 . The resins of the LSGM stoichiometry, on the other hand, contained about 3-4 wt% of the above-mentioned secondary phases even after calcination at 1400 0C for 8 h. The crystal structure of the LSGM samples of this study were found to be non-cubic (i.e., orthorhombic), in sharp contrast to many earlier reports [2-4, 61, and this finding about the crystal structure of LSGM is in accordance with the recent study of Ishihara, et al. [9]. The results of the simultaneous TG/DTA analyses are given in Fig. 2. The DTA traces of LSG and LSGM resins showed exotherms at -4000, 4900, and 605 0 C. The first exotherm was associated with charring of the polymer, the second with the pyrolysis of the organics, and the last one resulted from char burnout. TG analysis showed that most of the weight loss occurred at temperatures between 2500 and 500°C. This corresponded to the range where polymer burnout occurred. The results of residual carbon analyses (as a function of calcination temperature) are shown in Table 1. The nitrogen content of the 100°C-calcined precursor samples were found to be in the range of 1.05 to 1.2 wt%, but with increasing calcination temperatures (starting from 200*C) it decreased to levels below the reliable detection level (i.e., 100 ppm) of the equipment used. ICP-AES analysis results (in terms of mole ratios) of the 1340*C-calcined samples of LSG (i.e., La/Sr=8.984, La/Ga--0.912) and LSGM (La/Sr=3.990, La/Ga=0.958, La/Mg--4.693) compositions confirmed the theoretical mole ratios of elements present. Amorphous citrate/EG method, therefore, is shown to be able to yield LSG and LSGM ceramics of high elemental uniformity. Table I. Results of residual carbon analyses (wt%)
Temp. (0 C)
LaGaO 3
La0.9Sr 0.IGaO2.95
LaolSro2Cao.s3Mgo.1702.815
100
31.7(3)
33.3(2)
350
10.3 (3)
10.4 (1)
32.7(6) 13.8 (2)
700
0.530 (3)
0.550 (5)
0.280 (4)
850 1000 1340
0.059 (1) 0.042 (2) 0.010
0.143 (9) 0.050 (3) 0.0124 (2)
0.168 (3) 0.060 (2) 0.0143 (4)
FTIR plots (as a function of calcination temperature) of the LG, LSG, and LSGM samples are given in Fig. 3. The broad band at 3500-2500 cm-' is due to O-H stretching. The presence of the citrate ion was detected by the band at 2990-2874 cmn' at low calcination temperatures. Dissolved
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