Himansu Shekhar Tripathi

The effect of ZnO addition on the densification and properties of magnesium aluminate spinel

Abstract Stoichiometric magnesium aluminate spinel can be developed by solid oxide reactions of calcined magnesia and calcined alumina. The raw materials were mixed; attrition milled, compacted under a uniaxial pressure of 100 MPa and finally fired in the temperature range of 1500 to 1650°C. Up to 2 wt% ZnO was incorporated as an additive. In this investigation the effect of ZnO on the densification and properties of the magnesium aluminate spinel has been studied. It was found that 99% of theoretical density was achieved on firing at 1550°C with the addition of 0.5 wt% ZnO. The optimum properties in terms of bulk density, hot strength and thermal shock resistance was obtained with 1 wt% ZnO. All the ZnO containing samples retained their strength up to 6–8th cycle on thermal shock. ZnO containing samples are comparatively more resistant to thermal shock than ZnO free samples.

Synthesis and densification of magnesium aluminate spinel: effect of MgO reactivity

Stoichiometric magnesium aluminate spinel was synthesized by reaction sintering of alumina with caustic and sintered magnesia. The volume expansion of 5-7% during MgAl2O4 formation was utilized to identify the starting temperature of spinel formation and densification by high temperature dilatometry. The magnesia reactivity was determined by measurement of crystallite size and specific surface area. Caustic magnesia and sintered magnesia behave differently vis-A-vis phase formation and densification of spinel. Densification of stoichiometric Mag-Al spinel was carried out between 1650 and 1750 degreesC. Attempts were made to correlate the MgO reactivity with microstructure and densification of spinel

Effect of chemical composition on sintering and properties of Al2O3–SiO2 system derived from sillimanite beach sand

Abstract Reaction sintering of beach sand sillimanite and alumina is an innovative as well as inexpensive method of mullite formation. Beach sand sillimanite, a by-product generated during the separation of rare earth compounds, and calcined alumina were used as starting materials and were mixed in appropriate proportion and sintered in compacted form at 1500–1575°C. The study reveals that the use of submicron size powder enhances the sintering process. In this investigation, effect of chemical composition on sintering and mechanical/thermomechanical properties has been studied. It was found that the densification process is dependent on the Al 2 O 3 /SiO 2 ratio. Silica rich composition achieves highest density at 1525°C whereas the alumina rich composition requires 1550°C. Flexural strength measured at room temperature and at 1200°C initially de-creases with alumina content, with a minima at 71–74% Al 2 O 3 and then increases again. Microstructure of the sintered is also dependent on the Al 2 O 3 /SiO 2 ratio of the batch. Mullite made from silica rich composition is needle shape in nature whereas in alumina rich composition they are non-acicular in nature.

Synthesis and thermo-mechanical properties of mullite–alumina composite derived from sillimanite beach sand: effect of ZrO2

Abstract A mullite–alumina composite was developed by reaction sintering of sillimanite beach sand and calcined alumina. ZrO2 (2–6 wt.%) was added as additive. The raw materials and additive were mixed, attrition milled and sintered in compacted form at 1400–1600°C with 2 h soaking. The effect of ZrO2 on the densification behaviour, thermo-mechanical properties and microstructure was studied. It was found that addition of ZrO2 slightly retards the densification process. All the samples achieved their highest bulk density at 1600°C. Thermo-mechanical properties of the sintered samples are not effectively altered by the presence of ZrO2. ZrO2 containing samples always show better resistance to thermal shock than the ZrO2 free samples. Scanning electron micrography shows that ZrO2 occupies both an intergranular and intragranular position in the mullite matrix. The mullite formed at 1600°C is mostly equiaxed in nature that suggests densification mainly occurs through solid state sintering.

Thermal shock behaviour of high alumina aggregates derived from sillimanite beach sand with and without Fe2O3 doping

Abstract In this investigation, three types of high alumina aggregates namely sillimanite, mullite and alumina rich aggregates were developed from sillimanite beach sand and calcined Al2O3 by reaction sintering route. The various phases present in the sintered compacts were identified by X-ray diffraction study in conjunction with infrared study. Thermal shock behaviour of the aggregates with and without Fe2O3 doping were measured by the room temperature flexural strength retention on ambient air quenching from 1000°C against the number of cycles. The results obtained are presented and discussed in this paper. It was found that all Fe2O3 containing samples show a strength enhancement on quenching by means of tempering. Attempts have been made to correlate this parameter with the microstructure and phase assembly of the product. ©

Mullite-Corundum Composites from Bauxite: Effect of Chemical Composition

In situ mullite-corundum composites have been prepared from low grade Indian bauxite containing high percentage of impurities (SiO2, Fe2O3, TiO2, CaO) and silica sol to study the effect of different mullite proportions on the refractory properties. Formation of different phases has been identified by X-ray diffractometry and the microstructure has been studied by field emission scanning electron microscopy. XRD analysis shows the presence of mainly mullite and corundum peaks. The bulk density increases and apparent porosity decreases with decreasing amount of mullite phase proportion in the mullite-corundum sintered aggregate. Refractoriness under load of mullite-corundum composites increases with increasing corundum amount up to a certain level and then decreases marginally. The unit cell parameter and cell volume calculated from X-ray diffraction data for mullite also follows the similar trend.

Solid state sintering of lime in presence of La2O3 and CeO2

The sintering of lime by double calcination process from natural limestone has been conducted with La2O3 and CeO2 additive up to 4 wt.% in the temperature range 1500–1650° C. The results show that the additives enhanced the densification and hydration resistance of sintered lime. Densification is achieved up to 98.5% of the theoretical value with La2O3 and CeO2 addition in lime. Grain growth is substantial when additives are incorporated in lime. The grain size of sintered CaO (1600°C) with 4 wt.% La2O3 addition is 82 μm and that for CeO2 addition is 50 μm. The grains of sintered CaO in presence of additive are angular with pores distributed throughout the matrix. EDX analysis shows that the solid solubility of La2O3 and CeO2 in CaO grain is 2.9 and 1.7 weight %, respectively. The cell dimension of CaO lattice is 4.803 %C. This value decreases with incorporation of La2O3 and CeO2. The better hydration resistance of La2O3 added sintered lime compared to that of CeO2 added one, is related to the bigger grain size of the lime in former case.

Effect of sillimanite beach sand composition on mullitization and properties of Al2O3-SiO2 system

Mullite was developed by reaction sintering of sillimanite beach sand and calcined alumina. Two varieties of sillimanite beach sand viz. S and Z having different compositions were selected. Synthesis and properties of mullite were very much dependent on the sillimanite beach sand composition. Presence of higher amount of impurities in the Z-variety of sillimanite sand favours the densification by liquid phase formation. Presence of zircon in Z-variety increases the hardness and fracture toughness. Alumina addition improves the mechanical/thermomechanical properties of the samples. Mullite retains the usual orthorhombic habit of sillimanite. Rounded to sub rounded zirconia dispersed within the mullite matrix of the sample ZA is noticed.

Synthesis and mechanical properties of mullite developed from beach sand sillimanite: effect of Fe2O3

Mullite aggregate was developed from sillimanite beach sand and calcined alumina by the reaction sintering route using 1–4 wt% Fe2O3, as additive. Effect of Fe2O3, on the densification behaviour, microstructure and thermomechanical properties of the aggregate was discussed. It was found that Fe2O3 favours densification process by liquid phase formation, thereby deteriorating the thermomechanical properties particularly at higher sintering temperature. Mullite formed without Fe2O3 additive at 1600°C was mostly equiaxed in nature whereas it was needle shaped with few occasional equiaxed mullite grains within the matrix when formed in presence of Fe2O3 at 1500°C. Up to 4.08 wt% Fe2O3 enters into the mullite structure by solid solution formation at 1500°C.

Study of Densification Behaviour, Microstructure vis-à-vis High Temperature Properties of Commercially Available Indian Magnesites

ABSTRACT Indian magnesites from Salem and Almora regions were selected for the present study. Chemical analysis of these magnesites confirmed that Salem region magnesite (SM) contains CaO and SiO2, whereas Almora origin magnesite (AM) shows higher amount of Fe2O3 and CaO as major impurities. Crystalline phases developed in sintered Salem magnesite are periclase, forsterite and monticellite, while magnesioferrite and monticellite are found in sintered Almora magnesite apart from periclase as major phase. Microstructural analyses reveal that sintered SM has lower grain size than sintered AM . Although, no significant difference was noticed at room temperature flexural strengths of both the sintered samples, but sintered AM samples exhibit better flexural strength at elevated temperature (1200°C). GRAPHICAL ABSTRACT