Abstract
In the present investigation, 5 wt% chemical grade MnO2 was added in a mixture of 50:50 Nigerian sources of kaolin and superfine calcined alumina powder and effect of this transition metal ion on their physico-mechanical properties, phase and microstructural evolution were studied during heating in the temperature range of 1,400–1,650°C. The heated samples were found to be highly porous (>30%) in this entire temperature range. The doping of 5 wt% MnO2 in this mixture did not improved the densification, but resulted into higher flexural strength (26 MPa) at 1,400°C compared to 7 MPa in the undoped sample. At 1,650°C, the reverse trend was observed, the flexural strength of undoped sample was found to be higher (38 MPa) than doped sample (27 MPa). The XRD study revealed the formation of mullite as major and corundum as minor phases in both the samples. The microstructural study shown the presence of needle shaped mullite crystals and corundum grains. The presence of inter-granular and intra-granular pores in MnO2 doped samples might have reduced the strength at 1,650°C. The aspect ratio of mullite needle at 1,650°C was found to be higher in doped sample. The theoretical and experimental value of >4 for MnO2 ion has been validated.
Highlights
Mullite is one of the most important phases in both traditional and advanced ceramics it exists rarely in natural rocks
This paper studies the performance in terms of densification, strength development, phase and microstructural evolution of mullite formation when kaolin-alumina mixture is doped with Mn4+
In one of our earlier study, a Nigerian source of kaolinitic clay containing high SiO2 together with impurities Fe2O3 and TiO2 gives rise to needle shaped mullite on heat treatment at 1,400°C; itself and the mullite content increases with increase in heating temperature (Ajanaku, Aladesuyi, Pal, & Das, 2016)
Summary
Mullite is one of the most important phases in both traditional and advanced ceramics it exists rarely in natural rocks. Sintered-mullites are produced by heat treatment of starting materials via solid-state reactions. These mullites are normally stoichiometric having chemical formula 3Al2O3·2SiO2. The starting materials for the synthesis of sinter-mullite are primarily alumina plus silica, alumino-silicates of the composition Al2SiO5, clays, Al2O3-rich sheet silicates which are abundantly available in nature. In one of our earlier study, a Nigerian source of kaolinitic clay containing high SiO2 together with impurities Fe2O3 and TiO2 gives rise to needle shaped mullite on heat treatment at 1,400°C; itself and the mullite content increases with increase in heating temperature (Ajanaku, Aladesuyi, Pal, & Das, 2016). Aladesuyi, Pal, Das, and Ajanaku (2017) reported that when 25 wt% of very fine calcined alumina was added to this Nigerian clay powder, Al2O3 reacted with the excess SiO2 and enhanced mullite formation. A study on the mullitization behavior of calcined clay–alumina mixture with different sources of alumina viz: reactive alumina, gibbsite and boehmite has been carried out and was found that mixture of calcined clay and reactive alumina exhibited better mullitization behavior compared to other combinations (Viswabaskaran, Gnanam, & Balasubramanian, 2003)
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