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A STUDY OF PHASE INTERACTIONS AND SINTERING IN TIO2-ZRO2-BASED CERAMICS WITH SINTERING AIDS OF DIFFERENT NATURE
Abstract
Ceramic materials based on titania and zirconia hold much promise in modern nuclear energy applications. Such materials due to their chemical and mineralogical similarity to nature-derived minerals may be used as matrices for radioactive waste immobilization as well as inert matrix fuels in the ‘burn and bury’ fuel cycle. However, sintering of such ceramics into a dense body is still a challenging issue. This paper addresses phase formation processes and sintering patterns of ceramics in TiO2 – Al2O3 – ZrO2 – MgO (TAZM) system doped with sintering aids with different behavior, i.e. CeO2 (C) and MnO (M). Ceria is known to readily form solid solutions with zirconia, thus promoting solid-phase sintering, while manganese oxide has been reported to form eutectic melts with TiO2, especially in presence of alumina, which would result in liquid-phase sintering. Experimental batches were made by co-milling of oxides obtained by calcination of respective salts (save for TiO2 that was added as-is) and further heat treatment in temperature range 1300-1500 °C. Ceria and MnO content varied from 3 to 9 % wt. and from 3 to 12 % wt respectively. Next, these batches were disintegrated in a ball mill and dice-shaped samples were produced by cold uniaxial pressing and fired in temperature range and 1200-1300 °C for TAZM-C and TAZM-M respectively. Phase composition and microstructure of both powder and sintered samples were described with X-ray diffraction (XRD), petrography and scanning electron microscopy (SEM), and materials sinterability was investigated by constant shrinkage method up to 1300 C. Without additives, TAZM ceramics demonstrated a complex phase composition and mediocre sinterability. Key crystalline phase here was a ternary compound Mg4Al2Ti9O24, which remained stable within the studied temperature range; increase in heating temperature resulted in formation of magnesia-stabilized zirconia (Mg0.2Zr0.8O1.8). Ceramic bodies even at maximum temperature showed a considerate value of open porosity (7.3 %). Both TAZM-C and TAZM-M ceramics followed expected patterns, without much difference to pure TAZM phase composition. TAZM-C samples had well-defined crystalline structure, with no traces of melt-like phases. The increase in ceria content primarily lead to zirconia phase evolutions (from cubic to tetragonal) and promoted formation of zirconia containing phases (ZrTiO4 and at elevated temperatures - Mg0.25Zr0.38Ti0.38O1.75). In TAZM-M samples the increase in MnO content resulted in improved reactivity of TiO2 and MgO promoted by melt formation, especially at high end concentrations. Both additives promoted ceramics sintering, shifting densification interval to lower temperatures, however, the effect was more pronounced in TAZM-M rather TAZM-C.
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