Thesis (M.Sc.)--Chulalongkorn University, 2003

The fine particle strontium and iron substituted lanthanum gallate perovskite oxides, La[subscript 1-x]Sr[subscript x]Ga[subscript 1-y]Fe[subscript y]O[subscript 3-delta] where Sr = 0.5, Fe = 0.8; Sr = 0.4, Fe = 0.6-0.8; Sr = 0.3, Fe = 0.3-0.8 and Sr = 0.2, Fe = 0.3-0.8 were synthesized by a modified citrate method and were calcined at 800 ํC. The single phase or some traces of secondary phase powder were selected for preparing the membrane discs using a uniaxial pressing machine. They were then sintered at 1,200-1350 ํC for 10 hours. Both types of powder and membrane were subjected to the single-phase determination by the X-ray diffraction. In addition, the surface and cross section of each membrane were investigated by scanning electron microscopy. It was found that LSGF 6428 (represented from La[subscript 0.6]Sr[subscript 0.4]Ga[subscript 0.2]Fe[subscript 0.8]O[subscript 3-delta]), LSGF 6437, LSGF 7328, LSGF 7337, LSGF 7346, LSGF 8228, LSGF 8237and LSGF 8246 showed the single phase or some traces of secondary phase but LSGF 5528, LSGF 7355, LSGF 7364, LSGF 7373, LSGF 8255, LSGF 8264 and LSGF 8273 showed the clearly observed secondary phase. It was obvious that the secondary phase increased when the amount of Sr or Ga increased because La and Fe were partially substituted by the larger ionic radii, Sr and Ga, respectively For the XRD patterns of membranes, it was found that the secondary phase tended to form at increasing sintering temperature. Therefore, it indicated that the phase transformation of La[subscript 1-x]Sr[subscript x]Ga[subscript 1-y]Fe[subscript y]O[subscript 3-delta] perovskite depended on the increasing of sintering temperature. This can be explained by the vaporization of Ga at high temperature that caused the formation of LaSrGaO[subscript 4] or secondary phase.