F.D. Gnanam

Mullitisation behaviour of calcined clay-alumina mixtures

The mullitisation behavior of three south Indian calcined clays with three different alumina sources has been studied. The calcined clays were ground and then mixed with different alumina sources such as reactive alumina, gibbsite and boehmite. Rectangular bars were prepared and sintered at 1600 °C/3 h. The X-ray diffraction and microstructural analysis were carried out to understand the mullitisation behavior. Thermogravimetric and differential thermal analyses of the powders were also carried out. The physical and mechanical properties were measured and discussed. The calcined clay (meta kaolin) derived samples show better strength and density than the uncalcined clay derived sample. The microstructure also shows a higher aspect ratio of mullite crystals. Among the three clay combinations, the calcined Neyveli clay and fine reactive alumina mixture was found to exhibit a better mullitisation behavior compared to other combinations.

Powder characteristics, sintering behavior and microstructure of sol-gel derived ZTA composites

Abstract A series of alumina/zirconia composites of varying compositions of zirconia were prepared through the sol–gel technique. Precursors were calcined at different temperatures ranging from 300 to 1400°C and sintered at 1530°C for 3 h. Compacts made from the powder calcined at 950°C yielded density up to >99% of theoretical density by pressureless sintering. Pore size distribution and the densification behavior were explained with respect to calcination temperature. Microstructural analysis of the sintered compacts revealed the uniform distribution of the zirconia grains in the alumina matrix. It is also observed that the faceted intergranular zirconia grains are at the grain junctions and the corners of the alumina matrix.

Densification behaviour and microstructure of gel-derived phase-pure mullite in the presence of sinter additives

Gels with high pure mullite composition have been synthesized using aluminum nitrate nanohydrate and ethyl silicate. Pure mullite has been sintered to 88.5% of the T.D., when sintered at 1600°C for 3 h. Suitable sintering aids such as SrO (0.5 wt.%) and clay (2 wt.%) have tremendous effect in promoting the densification behavior of mullite to attain density to 98 and 95% of T.D. respectively at relatively low temperatures by pressureless sintering. SrO promotes the densification of pure mullite via liquid phase sintering and also does clay assist the viscous flow sintering. Clear differences found in the microstructures of mullite could be related to the different sintering aids, which have apparent effect on the densification behavior. SrO doped samples show a duplex microstructure of fine equiaxed and elongated grain morphology, while the clay doped samples show equiaxed and acicular morphology.

Sol–gel mullite as the self-bonding material for refractory applications

Abstract The physico-chemical properties of permanent binders used in refractories play a vital role in deciding high temperature mechanical properties. The presence of a glassy phase in the matrix of the refractory may not affect the low temperature properties significantly, however the high temperature properties are very much reduced. In the present study, the mullite precursor derived from the sol–gel route has been used as the self-bonding material to the fused mullite aggregates. The sol–gel mullite as the permanent binder yields density up to 2.48 g/cc. The room temperature flexural strength (172 kg/cm 2 ) and compressive strength (785 kg/cm 2 ) of the self-bonded fused mullite are comparable to that of the conventional clay/alumina bonded mullite products. However, superior high temperature mechanical properties such as Hot Modulus of Rupture (HMOR) (48 kg/cm 2 at 1300°C), the retention flexural strength of 116 kg/cm 2 after 10 thermal cycles of ΔT =1000°C and Refractoriness under load (RUL) (>1800°C) were obtained for the samples of fused mullite bonded by sol–gel mullite.

Study on Utilization of Neyveli Clay in the Production of Value Added Ceramics

The present study deals with the utilization of Neyveli clay for value added artifacts like, porous membrane support, dense mullite tubes and substrates for electronic applications. In the present investigation, Neyveli clay and reactive alumina (mean particle size of 0.7 wt%) were used as starting materials to prepare stoichiometric mullite. MgO (3 wt%) and boehmite (5 wt%) were added as sinter additives to increase the densification, whereas starch and B2O3 were added as pore formers. The sintered (1600°C for 3 h) porous membrane supports show that the addition of organic pore former (starch) leads to low strength with a wide pore size distribution. The simultaneous addition of organic (10 wt%) and inorganic pore formers (10%) to the clay and alumina yields porous membrane supports with high strength (61 MPa) and high open porosity (35%) having mean pore diameter of 5 pm with very narrow pore size distribution. The linear thermal expansion co-efficient (3.4 × 10−6oC−1 at 400°C) and dielectric constant (7.4) are comparable to the reported values for samples containing 3% MgO. Dense tubes prepared with the addition of 3% MgO give bulk density of 2.91 g.cm−3, strength of 78.2 MPa without any open porosity.

Characterization of Aluminous Clays from Tamilnadu

Plastic aluminous clays are abundantly available in Tamilnadu. However, at present the utilization of these clays is very low due to lack of detailed characterization. In the present work, the green and fired characteristics of clays obtained from three different locations in Tamilnadu (Udayarpalayam, Panruti and Neyveli) have been investigated to assess their potential use as raw materials. Physical properties, plasticity index, particle size analysis, chemical analysis, X-ray diffraction and thermal analysis of these clays have been carried out. Physical properties such as density, bending strength, X-ray diffraction and scanning electron microscopy of the fired samples have also been carried out. The properties of the above three clays are compared with each other. The results show that these clays are plastic in nature containing a moderate amount of impurities with high alumina content.

Densification and Mechanical Properties of Sol-Gel Alumina: Effect of Sinter Additives

Boehmite sol was prepared by hot water hydrolysis of aluminium iso-propoxide using nitric acid as a catalyst. Hydrolysis was carried out at 80°C, magnesium nitrate and silver nitrate were added so that peptization was complete at 80°C for 1 h. The sol was precipitated in ammonia, the precipitate was centrifuged at 9000 rpm for 20 min, dried at 120°C and calcined at 600°C for three hours. The amount of magnesium oxide was kept constant at 0.5 wt% and that of silver oxide was varied from 0.5 to 10 wt%. The powder was compacted into cylindrical pellets using uniaxial press at 180 MPa and sintered at 1400°C for 3 h. 94 % T.D. was achieved for 1.0 wt% silver oxide content. Mechanical properties such as hardness, fracture toughness for compacts containing various concentrations of silver oxide were studied. The results showed that there was an increase in fracture toughness of alumina ceramics with increasing silver oxide content.