Mullite–β-Spodumene Composites from Aluminosilicates |
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Authors: | Ammini Yamuna Sankaranarayanan Devanarayanan Malathi Lalithambika |
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Affiliation: | Department of Physics, University of Kerala, Kariavattom, Thiruvananthapuram 695 581, India;Regional Research Laboratory (CSIR), Industrial Estate, Thiruvannthapuram 695 019, India |
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Abstract: | Sintering studies were conducted using kaolin, metakaolin, zeolite 4A, and various synthetic mixtures of Al2O3 and SiO2 in the presence of Li2CO3 and LiCl as fluxing agents. Various compositions of the above were prepared, and conventional sintering studies were conducted at temperatures of 900°–1450°C with soaking periods of 1–3 h. Kaolin, metakaolin, and amorphized kaolin in the presence of Li2CO3 showed nucleation centers of β-spodumene as pink specks, whereas synthetic mixtures of Al2O3 and SiO2 failed to behave in the same manner. To determine whether the pink specks formed were color centers or F centers, the samples were subjected to UV, IR, and X-ray irradiation; however, the samples showed no tenebrescence properties. External addition of iron as an impurity in a nonlayered system also resulted in pink speck formation. This observation indicated that impurities present in the natural kaolin were the cause of this phenomenon. Moreover, the LiCl-based samples did not result in pink specks, even though the kaolinitic samples contained iron as an impurity. Therefore, although β-spodumene was formed in aluminosilicates in the presence of Li2CO3 and LiCl, the pink variety of β-spodumene (kunzite) formation occurred only in the presence of lithium-rich aluminosilicates and in the presence of iron as an impurity. The phase identification and microstructure were explained based on XRD, DTA, and SEM studies. |
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Keywords: | aluminosilicates sinter/sintering lithium |
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