Arup Ghosh

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

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.

The effect of CuO addition on the sintering of lime

Abstract The sintering of lime was carried out with 1–4 wt.% CuO in the temperature range 1500–1650°C. A double calcination process was adopted in the study. The result showed that without additive the densification was 88% and with additive it maximised to 93% of the theoretical value at 1550°C with 1 wt.% CuO. The density decreased due to the presence of large closed pores with a higher percentage of CuO. Hydration resistance was measured at 50 o C in 95% relative humidity through the weight gain after 3 h. Addition of CuO up to 2 wt.% improved the hydration resistance, but it was not significantly high in comparison to that of 1 wt.% CuO. The use of a higher level of CuO in lime did not show any further improvement in hydration resistance. The CaO forms a low melting compound (2CaO.CuO) with CuO which helps liquid phase sintering of lime. When the liquid content increased in the sinterred lime grain growth takes place simultaneously along with pore growth

Densification of reactive lime from limestone

Sintering of lime from natural limestone was carried out by a single stage (from natural carbonate) and double stage (from limestone converted hydroxide) process in the temperature range 1500 to 1650°C. In double stage process hydroxides were activated by three different techniques through pre-calcination and hydration. Different techniques employed are air-quenching the powder after precalcination, furnace cooling and water quenching of powder. The air quenching process showed better densification. Incorporation of hydroxide into carbonate powder up to an extent of 25 wt.% showed maximum densification. Hydration resistance was related to densification and grain size of sintered lime.

Refractories of alumina-silica system

The inorganic phases belonging to the binary alumina-silica system exhibit a plethora of excellent high temperature properties that make them useful for refractory applications. Synthesized from abundant aluminosilicate minerals, these refractories are ubiquitous in high temperature industrial applications. The refractories of alumina-silica system can be engineered to generate a range of high temperature properties by varying the Al2O3/SiO2 ratio, presence of other oxides and texture. This versatility is unique to this system and is reflected in widely varied fields of applications of these refractories. Composed predominantly of mullite and corundum phases, the non crystalline phases also play important role in determining the property and end use of these refractories. This review covers different theoretical and practical aspects of refractories of aluminosilicate system spanning 30-100% alumina. Important role of microstructure in aluminosilicate refractory has been discussed in light of phase diagram, raw materials and thermo-chemical reactions. The applications of these refractories in different areas have been discussed in detail with structure-property correlation.

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 characterization of mullite–zirconia composites by reaction sintering of zircon flour and sillimanite beach sand

Mullite–zirconia composites containing 10–30 wt% zirconia were prepared by reaction sintering of zircon flour, sillimanite beach sand and calcined alumina. Raw materials were attrition milled, shaped into pellets and bars and sintered in the temperature range of 1450–1600∘C with 2 h soaking at peak temperature. Sintered products were analysed in terms of various physical, mechanical and thermo-mechanical properties. The analyses of phases developed and microstructural analyses were carried out by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. It was observed that the addition of ZrO2 up to 20 wt% significantly improves flexural strength and fracture toughness. The transformation of t → m zirconia was found to be the dominant mechanism for enhancement in mechanical properties. ZrO2 occupies both the intergranular as well as intragranular positions. However, intragranular zirconias are much smaller compared to intergranular zirconias.