Rice husk ash as an alternate source for active silica production

Review on the physicochemical treatments of rice husk for production of advanced materials

Magnesium aluminate (MgAl2O4) spinel (MAS) is a synthetic material with cubic crystal structure and excellent chemical, thermal, dielectric, mechanical and optical properties. These properties made MAS an indispensable material for optically transparent windows, domes and armours, and for certain refractory applications. High processing cost of dense MAS ceramics is in the main responsible for its limited usage in certain important applications despite its excellent performance in them. The volume expansion (∼8%) associated with MAS phase formation from alumina and magnesia does not allow obtaining dense MAS bodies in a single-stage reaction sintering process. Therefore, dense MAS bodies are made by following a double stage firing process, which is expensive. The existing literature suggests that the processing cost of dense MAS ceramics could be reduced to a great extent by decisive selection of starting raw materials, powder processing and densification conditions, and by understanding the under...

A review on magnesium aluminate (MgAl2O4) spinel: synthesis, processing and applications

Rice husk (RH) has now become a source for a number of silicon compounds, including silicon carbide, silica, silicon nitride, silicon tetrachloride, zeolite, and pure silicon. The applications of such materials derived from rice husks are very comprehensive. The methods of synthesizing these silicon-based materials from RHs and their applications are reviewed in this paper.

Silicon-Based Materials from Rice Husks and Their Applications

A review on the susceptor assisted microwave processing of materials

Stoichiometric MgAl 2 O 4 spinel powders were synthesized by microwave assisted solid-state reaction (MWSSR powders) in a domestic microwave (MW) oven (2.45 GHz frequency, 700 W power) from aluminum tri-hydroxide and caustic MgO in the presence of carbon black (10–50 wt.% with respect to total raw mix). 20% C was found to be sufficient to produce spinel powder with 82% spinel content within 100 min while 50% C was able to produce 93% spinel containing powder within the same time. In conventional solid-state reaction (CSSR), in order to obtain the powder with the same amount of spinel phase (i.e., ∼93%), the above raw mix was calcined at 1350 °C for 1 h and the total heating and cooling cycle time was more than 24 h. For the purpose of better comparison, three other stoichiometric MgAl 2 O 4 powders were also prepared following conventional combustion synthesis using urea (CUCS powder) or sucrose (CSCS powder) as a fuel, and microwave assisted combustion synthesis using urea as a fuel (MWUCS powder), respectively. Effects of synthesis route on the powder properties were assessed by means of XRD, SEM, BET surface area, and particle size analysis. Among the various powders studied, the MWSSR powder was found to be superior as far as saving in processing time and power is concerned in addition to good sintering characteristics in terms of bulk density (B.D), apparent porosity (A.P) and water absorption (W.A).

Microwave assisted solid state reaction synthesis of MgAl2O4 spinel powders

Raw rice husks without precoking were directly pyrolysed in a graphite resistance heating furnace at 1100–1400 °C in nitrogen atmosphere. The furnace chamber pressure was varied from atmospheric pressure (14.7 Psi or 101.3 KPa) to 24 Psi (165 KPa). Considerable quantities of SiC whiskers were formed at 1200–1400 °C. Small quantities of Si 2 N 2 O were also formed at 1100 and 1200 °C. No Si 3 N 4 was formed. The increase in chamber pressure has resulted in (i) an increase in the crystallization of silica and carbon in rice husks, (ii) decrease in the formation of SiC whiskers. Pyrolysis in nitrogen atmosphere resulted in the formation of good quality (needle type) SiC whiskers.

Conversion of raw rice husks to SiC by pyrolysis in nitrogen atmosphere

The paper overviews the fundamental and engineering aspects of the interfaces in discontinuously reinforced metal-matrix composites. The major findings of studies undertaken at the Defence Metallurgical Research Laboratory, India, and Northwestern University, USA, are highlighted in addition to a detailed survey of the literature. The various issues such as the nature of interfacial bond, chemical reaction at the interfaces, effect of alloying and processing on structure of the interfaces and the properties of the composites are examined in this paper. Strategies are suggested to exploit the full potential of reinforcing the metallic matrix.

Interfaces in Discontinuously Reinforced Metal Matrix Composites

Dense β-Si4Al2O2N6 ceramics were obtained from α-Si3N4, α-Al2O3, AlN, and Y2O3 upon sintering (for 4 h at 1675°C) green bodies consolidated by an aqueous gelcasting and a conventional dry-powder processing route. The as-purchased AlN powder was treated with H3PO4 and Al(H2PO4)3 and dispersed in aqueous-organic premix solution along with α-Si3N4, α-Al2O3, and Y2O3 powder with the help of a commercial amino alcohol-based cationic (Dolapix A88) dispersing agent to obtain suspensions with 45–50 vol% solids loading for gelcasting purpose. The gelcat β-Si4Al2O2N6 exhibited superior hardness (1423±6 Hv), fracture toughness (3.95±0.3 MPa·m1/2), and coefficient of thermal expansion (3.798 × 10−6/°C between 30° and 1000°C) in comparison with the ceramic consolidated by conventional dry-powder pressing route, which exhibited only 1317±5 Hv, 3.30±0.2 MPa·m1/2 and 3.532 × 10−6/°C between 30° and 700°C.

Yashwant R. Mahajan

Papers

Microwave assisted solid state reaction synthesis of MgAl2O4 spinel powders

Conversion of raw rice husks to SiC by pyrolysis in nitrogen atmosphere

Interfaces in Discontinuously Reinforced Metal Matrix Composites

Aqueous Gelcasting Process for β‐Si4Al2O2N6 Ceramics

Slip-Cast Fused Silica Radomes for Hypervelocity Vehicles: Advantages, Challenges, and Fabrication Techniques