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Phase Stability Analysis of Lanthanum-Doped Alumina During Synthesis and Sintering
AuthorNgwa Nforbi, Lum-Ngwegia
AdvisorGraeve, Olivia A
Chemical and Materials Engineering
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The aim of this research was to study the phase stability during synthesis and consolidation of nanocrystalline lanthanum-doped γ-Al2O3 powders. We used solution combustion synthesis by dissolving precursor nitrate compounds and an organic fuel in a pre-heated muffle furnace at 500°C. Several preliminary syntheses were carried out in order to obtain the correct fuel-to-oxidizer ratio necessary for the production of the desired lanthanum-doped γ-Al2O3. The as-synthesized powders were then heat-treated at 1000°C for 2 hours in order to remove impurities and improve the crystallinity of the powders. Sintered circular specimens were made by pressing the heat-treated powders and subsequently annealing them at 1800°C for 4 hours. The use of this material in optical windows requires that the material have high strength and optical transparency. Elimination of all the pores during sintering is therefore crucial. In addition, preparing specimens of the γ-Al2O3 phase is optimal, since the crystal structure is cubic and transparency is more readily achievable. Several different samples with varying weight percents of La were attempted to determine how much of the La could effectively prevent the γ-Al2O3 phase from transforming into the more stable α-Al2O3 phase. The different phases of compounds produced with increasing amounts of La were also identified. The as-synthesized and heat-treated powders as well as the annealed circular discs were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The average particle sizes of the powders were determined by dynamic light scattering (DLS). XRD experiments showed that the γ-alumina phase was stabilized when the powders were calcined at 1000°C with 5 wt% La, 10 wt% La and 13 wt% La. Increasing the amount of La resulted in the formation of the La compounds LaAlO3 in the heat-treated powders containing 15 wt% La and above and LaAl11O18 in the sintered specimens. Crystallite sizes of the heat-treated powders were determined from the XRD line profile analysis of the peaks and were less than 50 nm for the heat-treated powders. Dynamic light scattering experiments were carried out to analyze the average particle sizes of the as-synthesized powders as well as the powders heat treated at 1000°C for two hours. In all the DLS plots for the as-synthesized powders, particle sizes were less than 500 nm, while some of the heat-treated powders had sizes greater than 500 nm, and some even greater than 1 µm. The average particle sizes are considerably larger than the crystallite sizes because particles are generally formed from the agglomeration of crystallites and will therefore have larger sizes. SEM was done to analyze the morphology of the heat-treated powders as well as the sintered specimens. Only the specimens that contained 5 wt% La and 44.4 wt% La (the lowest and highest amounts, respectively) were analyzed, since the microstructures of the specimens with intermediate La amounts do not change much from each other. It was seen from the micrographs that the heat-treated powders were agglomerated. The SEM pictures of the 5 wt% La and 44.4 wt% La specimens showed that both specimens had grains which adopted a more or less hexagonal shape, and this was more defined in the Al2O3:44.4 wt% La specimen since it contained a larger amount of the secondary phase LaAl11O18 which has a hexagonal structure. Both specimens contained pores at the triple points of the grains as well as internal porosity. The Al2O3:5 wt% La specimens had larger grain sizes ranging from ~2.0 µm to 17 µm compared to the ~2.0 µm to 3.0 µm grain sizes for the Al2O3:44.4 wt% La specimens. This observation could probably be due to the effect of particle pinning which occurs when a specimen has secondary particles in its microstructure that pins or hinders the growth of grains. Density measurements were done using the Archimedes’ method. The percent densities of all sintered specimens were above 80% of the theoretical densities. Theoretical densities were calculated for each specimen based on the amount of La that had been used as dopant. None of the specimens were fully dense because of the presence of pores. The transparency of these specimens was determined simply by how well they were able to transmit fluorescent light and green laser light. All the specimens were very translucent to green laser light but only barely translucent to fluorescent light.