Abstract
Mullite being one of the most important aluminosilicate materials has a lot of unique properties and industrial applications. Hence the need to produce via cheap and available starting materials is paramount. In this present study, mullite was synthesized from sintering of Nigerian sourced kaolin–alumina (high purity from Almatis) after reacting samples have been mixed in a high speed ball milling machine in the presence of MgO and Cr2O3 additives. The samples were uniaxially pressed in a mould to form compact and sintered at temperature of 1400-1600°C. Bulk density, apparent porosity, and modulus of rupture were investigated as a function of sintering temperature and presence of dopant. Microstructural studies using scanning electron microscope (SEM) and X-ray diffraction (XRD) studies were carried out on the kaolin-alumina sintered compacts. Results from the study indicate that mullite was successfully synthesized, as XRD pattern showed the presence of mullite as major phases at 1600°C without any trace of cristobalite (unreacted silica) or quartz. SEM studies showed the presence of needle-shaped mullite. The doped samples showed little improvement in the physico-mechanical properties of the aggregate. However, the degree of mullitisation for the doped samples at 1600°C far exceeds that of the undoped samples. Cr2O3 doped has 94.48% mullitisation while MgO has 79.96% mullitisation. The undoped sample has 59.19% degree of mullitisation. MgO doped was better densified.
Highlights
Mullite ceramics (3Al2O3.2SiO2) have recently generated a lot of interest due to its outstanding properties, such as low thermal expansion, low dielectric constant, high melting point, high thermal shock resistance and excellent creep resistance which has useful applications in the refractory industry [1, 2]
Mullite was synthesized from sintering of Nigerian sourced kaolin–alumina after reacting samples have been mixed in a high speed ball milling machine in the presence of MgO and Cr2O3 additives
Results from the study indicate that mullite was successfully synthesized, as X-ray diffraction (XRD) pattern showed the presence of mullite as major phases at 1600°C without any trace of cristobalite or quartz
Summary
Mullite ceramics (3Al2O3.2SiO2) have recently generated a lot of interest due to its outstanding properties, such as low thermal expansion, low dielectric constant, high melting point, high thermal shock resistance and excellent creep resistance which has useful applications in the refractory industry [1, 2]. In the solid state sintering method of mullite synthesis, kaolin and all other materials are heated at an elevated temperature, to yield mullite and cristobalite as shown in the scheme 1. At the aforementioned temperature range of sintering, impurity oxides present in the clay forms a glassy phase together with the formed mullite In another of our previous work Aladesuyi et al [18] the processed kaolin combined with reactive alumina at composition of 50:50% showed mullite as major phase in the XRD and cristobalite as minor phases. This is an indication that the quantity of added alumina is sufficient enough for mullite formation. The effects of the two dopant ion employed as sintering additives are discussed
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