Research On Synthesis, Characterization And Valence Electron Structure Of Sr- And Mg- Doped LaGaO₃ Materials

Strontium and magnesium doped lanthanum gallate (LSGM) with higher oxygen-ion-conducting properties was first reported by Ishihara and Goodenough in 1994 and has been expected to be potential candidate for an alternative electrolyte material for reduced-temperature operation. Since then, critical assessments regarding the basic material properties of these materials as the electrolyte in SOFCs have been discussed in the literature. The researches indicated that it is difficult to synthesize the single-phase perovskite. This impedes the usage of LSGM as electrolyte for solid oxide fuel cells.In this dissertation, solid-state reaction technique, glycine-nitrate combustion method and polyacrylamide gel method were used to synthesize the Sr- and Mg-doped LaGaO₃. Room-temperature X-ray diffractometry (XRD) was performed on the synthesized powders and sintered samples. The particle morphology and grain sizes were observed through transmission electron microscopy(TEM). The sintering behaviors were studied by observing the changes of density as a function of sintering temperature and time. The density of sample was measured using the standard Archimedes method with distillated water. The surface and fracture surface of specimen were observed through scanning electron microscopy(SEM), too.Firstly, twenty seven kinds of the Strontium- and Magnesium-doped LaGaO₃ with different contents were prepared by means of solid state reaction. The results of XRD showed that, when calcined at 1500℃for 6h, LSGM with some contents are composed of main phase(LaGaO₃) without secondary phase. The others are composed of main phase(LaGaO₃) and secondary phase(LaSrGaO₄ and LaSrGa₃O₇). The phase region chart for different contents of LSGM was also drawn. The results of XRD showed that the grain size of LSGM increase with the increase of sintering time.Secondly, La_0.85Sr_0.15Ga_0.80Mg_0.20O_2.825 were prepared by means of glycine-nitrate combustion method. The results showed that the purity of LSGM is highest when M_glycine/M_metal-ion is 2: 1. The main phase existed in the uncalcined powders, along with a large amounts of secondary phases(LaSrGaO₄å’ŒLaSrGa₃O₇). With the increase of calcining temperature, the amount of main phase was increased and the amounts of secondary phases were decreased. When calcined at 1400℃, LSGM is composed of main phase(LaGaO₃) without secondary phases. LSGM with higher purity can be obtained by glycine-nitrate combustion method than by solid-state reaction technique or polyacrylamide gel method. This is its advantage. Thirdly, La_0.85Sr_0.15Ga_0.80Mg_0.20O_2.825 were prepared by means of polyacrylamide gel method. The results showed that the main phase did not exist in the uncalcined powders. When calcined at 800℃, small amount of main phase were found in powders. With the increase of calcination temperature, the amount of main phase was increased and the amounts of secondary phases were decreased. There still existed some amount of secondary phases when calcined at 800℃. The powder sizes are greater than those prepared by solid-state reaction technique or polyacrylamide gel method. The average grain sizes are about 80nm. This showed advantage of this method in preparing finer powders.Fourthly, the ionic conductivities of LSGM with different contents prepared solid-state reaction technique were studied. The results showed that, with the increase of contents of Sr or Mg, the ionic conductivities increased firstly, reach a maximum, then decreased. With the increase of sintering time, the ionic conductivities increased, hut when sintering time exceeds 24h, the ionic conductivities decreased. The ionic conductivities increased with the increase of testing temperature.Finally, the valence electron structures of LSGM with different contents were calculated by EET theory. The complex function "p" was put forward. The contents of LSGM with higher ionic conductivity can be forecast by using "p".