B Mukherjee

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 yttria on mechanical and microstructural properties of reaction sintered mullite-zirconia composites

Abstract Mullite-zirconia composites containing 0 to 7mol% yttria were prepared by reaction sintering process from Indian coastal zircon flour and calcined alumina. The main raw materials were attrition milled. Samples were isostatically pressed and sintered at temperatures of 1400 to 1650 °C with 2 h soaking time. The resulting composites were characterised in terms of phase compositions, martensitic start (M S ) temperature, mechanical properties and microstructural features. The presence of yttria results in a dense composite material at a comparatively low temperature. Addition of yttria increases the tetragonal zirconia fraction and reduces the M S temperature. Yttria goes into solid solution which was detected by EDX analyses. The effect of yttria on thermal expansion coefficient, room temperature flexural strength, hardness, fracture toughness is discussed.

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.

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.

Kinetics of Non-Isothermal and Isothermal Decomposition of Limestone

The kinetics of decomposition of limestone was studied using both non-isothermal and isothermal methods. The mechanism and kinetics of thermal decomposition were studied using Arrhenius equation applied to solid state reactions. It was found that in both non-isothermal and isothermal methods limestone seemed to be decomposed via a zero order reaction mechanism. The energy of activation for the non-isothermal and isothermal decompositions of limestone was 157.79 and 181.43 kJ.mol−1respectively. Similarly, In A values for non-isothermal and isothermal decomposition were found to be 14.2 and 13.2(s−1) respectively.

Preparation and Characterization of Reaction Sintered Zirconia-Mullite Composites

Zirconia dispersed mullite composites have been prepared from zircon flour and calcined alumina by reaction sintering. The raw materials have been characterized by chemical analyses, particle size distribution and X-ray diffraction. The batched raw materials have been attritor milled, uniaxially pressed and fired between 1450° and 1650°C with 2 hrs soaking time.The effects of MgO and Y2O3 additives on bulk density, porosity, rate of shrinkage and thermal expansion have been studied. Results indicate that the rate of shrinkage is higher in composites with additives and this shrinkage commences at a lower temperature as compared to that without additive. The thermal expansion and shrinkage behaviour are discussed.

Mechanical properties and microstructural character of isopressed reaction sintered mullite-zirconia composites

Zircon flour and alumina mixtures, fabricated in the form of discs, rods and bars, were cold isostatically pressed and sintered. The resulting mullite-zirconia composites were characterized in terms of mechanical properties and microstructural features. The effect of yttria addition on the final properties is also reported.Fine zircon and alumina mixtures with yttria addition resulted in dense composites with uniformly distributed rounded zirconia particles. A sintering temperature of 1550°C with two hours soaking yielded the best results in terms of fired density, flexural strength, fracture toughness, microhardness and tetragonal (t-) ZrO2, content Addition of 1.86 and 3.0 wt% Y2O3, resulted in significant improvement in terms of strength and t-Zr02 content A drop in the t-ZrO2 content with relatively large grain size was observed when the sintering temperature was raised to 1650°C.