Papers by Keyword: Spinel

Abstract: The main aim of this research was to investigate the effect of the addition of coal fly ash as a secondary waste material on the synthesis and properties of forsterite ceramics. The utilization of fly ash as a raw material, which contains flux oxides, reduce the necessary firing temperature for the synthesis of forsterite ceramics in comparison with forsterite ceramics synthetized from high purity or industrial materials, while preserving its refractory properties. The results revealed that forsterite was synthesized as a major crystalline phase in all samples. The optimal firing temperature for obtaining good physico-mechanical parameters was found to be between 1400-1600°C for high purity or industrial materials, while mixtures containing fly ash could achieve similar parameters at lower firing temperatures between 1200-1300°C. The decrease in refractoriness was found to be within acceptable limits for utilization as a refractory material. These findings demonstrate the potential for cost reduction and energy savings through the use of secondary waste materials and reduced firing temperature in the synthesis of forsterite ceramics.

Abstract: The influence of microstructure on the resulting physico-mechanical and refractory properties of refractory forsterite–spinel ceramics was investigated in this paper. The raw materials were milled, mixed into four different raw material mixtures and sintered for two hours at 1550°C. The microstructure of the samples was examined by scanning electron microscopy and X-ray diffraction analysis was used to determine the mineralogical composition of the sintered samples. Physico-mechanical properties such as porosity, water absorption, bulk density, and modulus of rupture were also determined. Thermomechanical characteristics were assessed by the determination of refractoriness, refractoriness under load, thermal shock resistance and corrosion resistance to various metals. The results showed that a higher amount of spinel leads to improved microstructure, thermal shock resistance and that all mixtures have high corrosion resistance to all tested materials.

Abstract: The (Zn_0.5Cu_0.5)Al₂O₄ pigments were synthesized by solid state processing with an aim to examine the kinetics of phase formation. The calcining temperature was 1100 °C with firing times of 2, 12, 24, 48 and 96 hours during which intermediate grinding was carried out. All five obtained pigment samples appeared to be single-phase as probed by XRD. However, lattice refinement revealed differences in the lattice constant which kept decreasing and reached constancy after 24 hours of firing. Such reduction in the lattice constant was associated with incorporation of smaller copper cations into the Zn-sites. FTIR spectra also supported this result as the relevant peaks still got broadened and shifted for the calcination times of less than 48 hours. Elemental analysis of the 2-hour sample showed small presence of starting precursor particles, though not detected by XRD, suggesting an incomplete reactivity at a minute scale. These structural changes were resultantly reflected by small but significant alterations in the color parameters. A higher degree of solid solution for the samples fired for longer times, as judged by lowering in the lattice constant, resulted in detectable changes in both a and b parameters. Longer calcination times yielded a brighter (higher L values) brownish tone likely due to both diminution in the averaged particle size from pulverizing and completion of solid solution. The first would be useful for good dispersion in glazes whereas the latter could be directly related to the degree of cationic substitution, confirming the expected “true” color for this specific doping level at a particular calcination temperature.

Abstract: This article examines the influence of fly ash on corrosion resistance of refractory forsterite-spinel ceramics by molten iron as a corrosive medium. Fly ash in comparison with alumina were used as raw materials and sources of aluminium oxide for synthesis of forsterite-spinel refractory ceramics. Raw materials were milled, mixed in different ratios into two sets of mixtures and sintered at 1550°C for 2 hours. Samples were characterized by X-ray diffraction analysis and thermal dilatometric analysis. Crucibles were then made from the fired ceramic mixtures and fired together with iron at its melting point of 1535°C for 5 hours. The corrosion resistance was evaluated by scanning electron microscopy on the transition zones between iron and ceramics. Mixtures with increased amount of spinel had higher corrosion resistance and mixtures with fly ash were comparable to mixtures with alumina in terms of corrosion resistance and refractory properties.

Abstract: This article examines the utilization of fly ash in comparison with alumina as raw materials and sources of aluminium oxide for synthesis of forsterite-spinel refractory ceramics. Raw materials were milled, mixed in different ratios into two sets of mixtures and sintered at 1500°C for 2 hours. Sintered samples were characterized by X-ray diffraction analysis and scanning electron microscopy. Samples were also subjected to determination of porosity, water absorption and bulk density. Thermal and thermomechanical properties were determined by thermal analyses, refractoriness, refractoriness under load, thermal shock resistance and thermal dilatometric analysis with determination of thermal expansion coefficient. Mixtures with 10 wt.% and 20 wt.% of fly ash had the most promising results compared to alumina mixtures. Thermal shock resistance and modulus of rupture were improving with increasing content of aluminium oxide in the mixture.

Abstract: Forsterite refractory ceramics is utilized in the metallurgical and cement industries as a lining of metallurgical furnaces and rotary kilns for its high refractoriness up to 1850°C and refractoriness under load above 1600°C. Another significant property of forsterite is its coefficient of linear thermal expansion utilized in the electrotechnical industry for ceramic-metal joints. Addition of aluminium oxide into the raw material mixture results in creation of magnesium-alumina spinel (MgO·Al₂O₃) which improves sintering, thermal shock resistance and mechanical properties in comparison with pure forsterite ceramics. Inexpensive source of aluminium oxide is fly ash. Utilization of fly ash, secondary energetic product of coal-burning power plants, is important for the environment and sustainable development. This paper evaluated properties of fly ash-based forsterite-spinel ceramics in comparison with alumina-based forsterite-spinel ceramics. Forsterite-spinel ceramics was synthesized from olivine, calcined magnesite and fly ash/alumina powders. XRD analysis was used to determine mineralogical composition, thermal analyses were used to determine the behaviour during firing and scanning electron microscopy to determine the morphology of crystal phases. Refractoriness of pyrometric cones, refractoriness under load, thermal shock resistance, coefficient of linear thermal expansion, water absorption, porosity and modulus of rupture were also determined on fired test samples.

Abstract: Magneto-electric composites of two distinct phases, ferrimagnetic-piezoelectric system with general compositional formula (x) ferromagnetic + (1-x) piezoelectric were synthesized using a hybrid technique, mechano-chemical method by sintering the mixtures of piezo-electric BaTiO3 (BTO) and ferri-magnetic Mg0.2Cu0.5Zn0.3Fe2O4 (MCZF). Here, ferri-magnetic phase component MCZF (Mg0.2Cu0.5Zn0.3Fe2O4) was prepared using auto-combustion method, whereas piezo-electric BTO was procured commercially from Sigma-Aldrich. Here, the general composition of composites is given by (x) Mg0.2Cu0.5Zn0.3Fe2O4+(1-x) BaTiO3(x=15%, 30% and 45%). Presences of two phases in these magneto-electric composites were probed using X-ray diffraction (XRD) studies. Peaks observed in the XRD spectrum indicated spinel cubic structure for MCZF ferrite and tetragonal perovskite structure for BTO and, both spinel and pervoskite structures for synthesized composites. Micro-structure of the samples has been investigated using Field Emission Scanning Electron Microscope (FESEM). Frequency dependent dielectric properties of synthesized composites were measured from 100 Hz to 1 MHz at room temperature using a precision HIOKI make LCR HI-TESTER. Dielectric dispersion was observed at lower frequencies for the synthesized composites.

Abstract: Comparative assessment of phase formation in the system ZnO-CeO₂-Fe₂O₃-Cr₂O_3, obtained using a number of technological methods, is carried out. Spinel phase formation is established for all materials studied, except ZnO-CeO₂. Synthesized materials are examined with X-ray phase analysis, low-temperature nitrogen adsorption, and scanning electron microscopy. It is shown that the synthesis in the presence of an organic precursor allows obtaining fine spinel samples. High catalytic activity of the synthesized materials, containing the spinel phase, is established in the process of methyl orange oxidative destruction in the presence of hydrogen peroxide. It is proved that with an increase in the number of chromium cations in the sample, the catalytic activity of the materials also increases. Zinc ferrite, despite the considerably more developed surface, and ZnO /CeO₂ are not very effective in this process. The data obtained is useful for the development of materials for wastewater treatment at industrial enterprises, which use organic dyes in their production cycles.

Abstract: Spinel-based ceramic color pigments were successfully synthesized from utilization of aluminum dross waste and relevant oxide precursors by solid-state processing. Cobalt ions were selected as a chromophore to produce blue pigments. The conventional oxide route was also carried out for comparison purposes. The spinel phase readily formed when fired at 1100 °C; longer duration yielded a higher degree of purity. No preferential orientation of XRD reflection was observed, indicating random crystallographic arrangement. Phase formation was also confirmed by Fourier Transformed Infrared Spectroscopy (FTIR) which displayed both Co-O tetrahedral and Al-O octahedral which are the main framework for a spinel crystal. Slightly sharper FTIR peaks for the dross route compared to those from the oxide route suggest a difference in crystallinity between the two with different precursors. The particle size distribution was relatively wide (5 – 30 micron), possibly due to a crude nature of the dross precursor. The UV-vis spectra showed absorption in the range of 450-550 nm which is associated with the blue color caused by a shift of the 3d⁷ electrons of Co2+. The obtained dross-route pigments possessed both a and b color parameters (a = -2.3 to-2.6; b = -3.4 to-4.0) in the negative territory, implying greenness and blueness respectively. The L values were in the 20-30 range. When incorporating into practical glazes, the b parameters unexpectedly became more negative, indicating an even deeper blue tone. This result suggested a high potential for utilization of this dross waste as an alternative precursor source for sustainable production of spinel ceramic pigments.

Abstract: In this work three dental ceramics were characterized according to ISO 6872: yttria-stabilized zirconia (ZrO₂-Y₂O₃), lithium disilicate (Li₂Si₂O₅) and the spinel-zirconia composite (MgAl₂O₄-ZrO₂). The zirconia ceramic and the zirconia-spinel composite were sintered at 1600°C-2h, while the lithium disilicate was thermally treated at 820°C-20min. These materials were characterized by relative density, X-ray diffraction, scanning electron microscopy, hardness, fracture toughness, chemical solubility and cytotoxicity. The XRD results showed for the stabilized zirconia only the tetragonal phase of ZrO₂, and to the composite only the phase MgAl₂O₄, Li₂Si₂O₅ was the only phase to lithium disilicate. Relative density results showed that the zirconia and the lithium disilicate showed high densification (> 99.5%) and the composite had a relative density of 75% (10% composite doped with ZrO₂) and 90% (50% doped with ZrO₂). Hardness and toughness showed 450HV and 3.2MPa.m^1/2 to ZrO₂-MgAl₂O₄ composites, 525HV and 1.8MPam^1/2 to lithium disilicate and 1280HV and 8.0MPa.m^1/2 to zirconia. The materials evaluated showed chemical solubility <30μg/cm² and the results of cytotoxicity tests indicated cell viability of the samples near 100% for all the materials, showing good chemical stability and potential for dental applications.