Synthesis of Ce-Doped Zircon by a Sol-Gel Process
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Both gels were then abraded in a mortar and samples 1-5 grams in weight were sintered in a vacuum furnace (1.10. atm) at 1200-1500 0 C for 1 hour. The gel with cerium was also sintered without milling at 1200-1600 0 C for 1 hour in air. The heating rate used was 20-25 °C/min. The energy dispersive X-ray analyses (EDX) of resulting powders showed a slight lack of silica, probably caused by incomplete tetraethoxysilane hydrolysis. X-ray powder diffraction (XRD) measurements were made using CuKct and CoKot radiation on a Rigaku Denki diffractometer. For a semi-quantitative estimate of the phase content by intensity ratio estimate the original Rigaku software was employed. The lattice constants refinement of the synthesized zircon was carried out using quartz as an internal standard. Quantitative X-ray wave dispersive spectroscopy (WDS) analyses and high resolution transmission electron microscopy (HRTEM) using a JOEL 2010 FX with a LaB 6 filament and an accelerating voltage of 200 kV were used to characterize these materials.
RESULTS XRD analysis of the powders After calcination at 900'C the XRD patterns of (Zro, 9Ceo,,)Si0 4 and ZrSiO 4 gels are almost identical and gave rise in the range of 10-65' 20 three main broad peaks which corresponded to poorly crystallized tetragonal ZrO 2. Figure 1 shows the XRD patterns of the sintered in vacuum, pure and Ce-doped zircon precursors. Figure 2 shows the XRD patterns of Ce-doped gel precursors sintered without milling in air. The changes in phase compositions are described based on the phase contents in weight percent disregarding the amorphous matter from intensity ratio estimate (the error is -20 relative %). After sintering the milled ZrSiO 4 gel precursor in vacuum at 1200'C, the tetragonal zirconia modification (T-ZrO 2) composed -90 % of the crystalline phases, after 1300°C the significant part of T-ZrO 2 transforms to the monoclinic modification (M-ZrO 2), with almost no zircon formation (-5 %). In diffractograms of gel sintered at 1400 0 C, the contents of M-ZrO 2 and zircon increase with decrease of T-ZrO 2 . After sintering at 1500'C, a significant jump in zircon yield is observed simultaneously with a sharp decrease of M-ZrO 2 content and an insignificant decrease of T-ZrO 2 content. The intensity ratio estimation showed that the resulting powder contained -80% of zircon and -6% of unreacted cristobalite. For milled (Zr0. 9Ce0. 1)SiO 4 gel sintered in vacuum, the same picture is observed as a whole; however, the temperatures of the beginning of the zircon formation and active "expending" of MZrO 2 decreased by 100 0C. Thus, after sintering at 1400'C, the yield of Ce-doped zircon was about 85%. About 6% of the Ce3+ silicate, Ce2 Si 2O 7 was formed. Cristobalite was not detected in this sample. When sintered in air but not milled (Zro.9Ceo.1)SiO 4 gel, contents of M-ZrO 2 do not exceed -10% over the temperature range studied. The significant increase in the zircon yield is observed in the range 1400-1500'C, accompanied by practically complete loss of T-ZrO 2 . Afte
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