Himansu Sekhar Tripathi

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.

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.

Thermo-mechanical properties of mullite—zirconia composites derived from reaction sintering of zircon and sillimanite beach sand: Effect of CaO

Abstract Mullite—zirconia composites containing 20% zirconia (mass fraction) were prepared by reaction sintering route utilizing Indian coastal zircon flour and sillimanite beach sand. 4%–12% of CaO (mole fraction) with respect to zirconia was used as additive. The effect of additive on densification, microstructure as well as various mechanical and thermo-mechanical properties was studied. Incorporation of CaO reduced the densification temperature of the composites to 1550 °C compared to 1600 °C (for CaO free samples). CaO formed small amount of liquid phase (calcium aluminosilicate), which facilitated sintering. Average grain size of the composites decreased up to 4% CaO addition, afterwards grain size increased with further addition of CaO. Samples with 4% CaO exhibited ∼225 MPa of flexural strength, ∼6 MP·m 1/2 of fracture toughness and significant improvement in thermal shock resistance. CaO stabilized the tetragonal zirconia phase and thus improved the mechanical properties.