Ammini Yamuna

Bio: Ammini Yamuna is an academic researcher from University of Kerala. The author has contributed to research in topics: Kaolinite & Mullite. The author has an hindex of 2, co-authored 2 publications receiving 54 citations.

Phase‐Pure Mullite from Kaolinite

Abstract: Sintering of kaolinite in the presence of certain carbonate mineralizers, viz., CaCO 3 , Na 2 CO 3 , and K 2 CO 3 , has been conducted at 950°-1350°C. The influence of these mineralizers at the above temperatures is evaluated using XRD and SEM techniques. A comparative study of phase formation of the above compositions shows that the sodium- and calcium-fluxed samples give rise to multiphase systems, while K 2 CO 3 -incorporated samples give phase-pure mullite. The observation that kaolinite in the presence of K 2 CO 3 can act as a precursor material for phase-pure mullite is of great industrial significance.

Mullite–β-Spodumene Composites from Aluminosilicates

Abstract: Sintering studies were conducted using kaolin, metakaolin, zeolite 4A, and various synthetic mixtures of Al2O3 and SiO2 in the presence of Li2CO3 and LiCl as fluxing agents. Various compositions of the above were prepared, and conventional sintering studies were conducted at temperatures of 900°–1450°C with soaking periods of 1–3 h. Kaolin, metakaolin, and amorphized kaolin in the presence of Li2CO3 showed nucleation centers of β-spodumene as pink specks, whereas synthetic mixtures of Al2O3 and SiO2 failed to behave in the same manner. To determine whether the pink specks formed were color centers or F centers, the samples were subjected to UV, IR, and X-ray irradiation; however, the samples showed no tenebrescence properties. External addition of iron as an impurity in a nonlayered system also resulted in pink speck formation. This observation indicated that impurities present in the natural kaolin were the cause of this phenomenon. Moreover, the LiCl-based samples did not result in pink specks, even though the kaolinitic samples contained iron as an impurity. Therefore, although β-spodumene was formed in aluminosilicates in the presence of Li2CO3 and LiCl, the pink variety of β-spodumene (kunzite) formation occurred only in the presence of lithium-rich aluminosilicates and in the presence of iron as an impurity. The phase identification and microstructure were explained based on XRD, DTA, and SEM studies.

Elaboration and characterization of tubular macroporous ceramic support for membranes from kaolin and dolomite

Abstract: A low cost macroporous support for ceramic membranes was prepared by in situ reaction sintering from local natural mineral kaolin with dolomite as sintering inhibitor. The characterization focused on the phase evolution, microstructure, pore structure, mechanical strength and water permeability at various compositions and sintering temperatures. The sintering of kaolin was improved with 5 wt% dolomite, but clearly inhibited with ≥10 wt% dolomite. For the 20 wt% dolomite samples, the crystalline phases were mainly composed of mullite, cordierite and anorthite after sintering between 1,150 and 1,300 °C. Moreover, both mean pore size and mechanical strength increased with increasing sintering temperature from 1,100 to 1,300 °C, but the water permeability and porosity decreased. The 1,250 °C sintered macroporous support with 20 wt% dolomite exhibited good performances such as porosity 44.6%, mean pore size 4.7 μm, bending strength 47.6 MPa, water permeability 10.76 m3 m−2 h−1 bar−1, as well as good chemical resistance. This work provides opportunities to develop cost-effective ceramic supports with controllable pore size, porosity, and high strength for high performance membranes.

Processing of Microcellular Mullite

Abstract: A new processing route for manufacturing partially interconnected open-cell, microcellular mullite ceramics has been developed. The strategy adopted for making microcellular mullite ceramics entailed the following steps: (i) fabricating a formed body from combining polysiloxane, Al 2 O 3 (a reactive filler), polymer microbeads (used as sacrificial templates), and Y 2 O 3 (a sintering additive); (ii) cross-linking the polysiloxane in the formed body; (iii) transforming the polysiloxane by pyrolysis into SiO 2 ; and (iv) synthesizing mullite by reacting SiO 2 and Al 2 O 3 . By controlling the sintering temperature and the micro-bead and additive contents, it was possible to adjust the porosity so that it ranged from 38% to 85%. The compressive strengths of the microcellular ceramics with ∼40% and ∼70% porosities were ∼90 and ∼10 MPa, respectively. The superior compressive strengths were attributed to the homogeneous distribution of small (≤ 20 μm), spherical cells with dense struts in the microcellular ceramics.

Sintering and reactive crystal growth of diopside–albite glass–ceramics from waste glass

Abstract: Diopside–albite glass–ceramics were fabricated by sintering the powder mixtures of crystallization promoters and waste glass. Two kinds of promoters were synthesized using kaolin clay, talc and chemical reagents. The crystalline phases were formed by a reactive crystallization between promoters and glass during sintering. The effect of promoter components, additions and sintering temperatures on the crystallizing and densifying behavior, microstructures and mechanical properties of glass–ceramics was investigated. The results showed that the higher densities and better mechanical properties were obtained for the glass–ceramics with 12–15% crystallization promoters sintered at 950 °C for 2 h.