G Sundararajan

Bio: G Sundararajan is an academic researcher. The author has contributed to research in topic(s): Sialon & Ceramic. The author has an hindex of 1, co-authored 1 publication(s) receiving 5 citation(s).

Novel route to β‐SiAlON–SiO2 ceramic composites

Abstract: This paper reports a novel synthetic route to prepare dense β‐SiAlON–SiO2 ceramic composites. The stoichiometric β‐Si4Al2O2N6 extrudates prepared by the reaction sintering of α‐Si3N4, α‐Al2O3, AlN and Y2O3 precursor mixture at 1675°C for 4 h were fine ground and mixed with different amounts of commercial fused silica (20, 40, 50, 60 and 80 wt‐%) powder, dry pressed and sintered for 3–4 h at 1500–1750°C. These sintered materials were thoroughly characterised for bulk density, apparent porosity, water absorption capacity, phase composition, microstructure, hardness, dielectric constant and coefficient of thermal expansion. These characterisation results are presented and discussed in this paper.

Development of β-SiAlON based ceramics for radome applications

Abstract: This paper is a review article covering various methods reported on synthesis of β-SiAlON based ceramic materials and on their net-shape consolidation into radome structures. It also identifies a composition out of a wide-range β-Si 6-z Al z O z N 8-z (where z = 0–4.1) solid solution suitable for radome applications and discusses about various efficient methods reported on fabrication of radome structures out of these compositions. This article also covers the literature pertaining to β-SiAlON-SiO 2 ceramic composites, which are considered to be materials of choice for certain high speed radome applications. Further, successful techniques employed for passivation of AlN powder against hydrolysis are also covered as this powder is one of the starting materials for both β-SiAlON and β-SiAlON-SiO 2 ceramic composites. Surface passivation of AlN is necessary as it decomposes into alumina and ammonia, when it comes into contact with water during aqueous processing of SiAlON based ceramics, thereby not permitting formation of desired SiAlON phase. Finally, the important properties of various commercial radome materials together with those of β-SiAlON and β-SiAlON-SiO 2 ceramic composites are also reviewed and presented in this article.

Aluminum nitride shaping by non-aqueous gelcasting of low-viscosity and high solid-loading slurry

Abstract: Aluminum nitride (AlN) ceramics has been prepared by a modified non-aqueous gelcasting technique, and the ceramic slurries with low viscosity and high solid loading were obtained by using 1-methyl-2-pyrrolidinone as solvent and Solsperse® 24,000 as dispersant. The rheological behaviors of the AlN ceramic slurry and the densities of AlN ceramics were studied. Typically, the AlN ceramic slurry (Solsperse® 24,000 of 0.5 wt% and solid loading of 50 vol%) showed a viscosity of 0.09 Pa·s at shear rate of 100 s −1 . And more interestingly, when increasing solid loading to 55 vol%, the AlN slurry still kept a low viscosity of about 0.28 Pa·s, which was reduced by 30% compared to the reported values. As a result, the resultant green body exhibited a relative density of 65.5%, and a flexural strength of 42.3 MPa. After sintering at 1900 °C for 5 h, the AlN ceramics with an improved relative density of 99.4% was produced. Thus, this research would provide a new way to prepare low-viscosity and high solid-loading AlN slurry.

Novel composites of $$\upbeta $$ β -SiAlON and radome manufacturing technology developed at ARCI, Hyderabad, for hypervelocity vehicles

Abstract: Keeping the importance of developing suitable radome (a word derived from radar $$+$$ dome) materials and near-net shape consolidation technique for manufacturing radomes suitable for hypersonic (>mach 5) radar-guided missiles in India, the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad, has initiated an in-house R&D programme and successfully developed a complete process know-how for manufacturing defect-free prototype $$\upbeta $$ -SiAlON-based radome structures with all the desired properties. As a part of this R&D programme, total six separate sub-projects mentioned below were undertaken and executed: (i) identification of the best composition out of $$\upbeta $$ - $$\hbox {Si}_{6-z}\hbox {Al}_{z}\hbox {O}_{z}\hbox {N}_{8-z}$$ (0 $$\le z \le $$ 4.1) solid solution, which possesses a right combination of properties required for radome applications, (ii) designing of an AlN-free precursor mixture for consolidating $$\upbeta $$ - $$\hbox {Si}_{4}\hbox {Al}_{2}\hbox {O}_{2}\hbox {N}_{6}$$ ceramics by following aqueous colloidal processing routes, (iii) development of a process for passivating water-sensitive AlN powder against hydrolysis, (iv) development of aqueous gelcasting (GC) and hydrolysis-assisted solidification (HAS) powder processing routes for consolidating dense $$\upbeta $$ -SiAlON ceramics using highly solids loaded (>50 vol%) aqueous slurries, (v) development of an hydrolysis-induced aqueous gelcasting (GCHAS) process, a novel near-net-shape consolidation technique, to produce radomes with very high-production yields and (vi) development of an economic route for synthesizing the low-dielectric constant and high strength novel $$\upbeta $$ -SiAlON- $$\hbox {SiO}_{2}$$ ceramic composites. In this paper, (i) the basis for choosing $$\upbeta $$ -SiAlON-based ceramics for hypervelocity radome applications, and (ii) the various bottle-neck problems faced, while executing this entire R&D work and the way they were overcome have been critically analysed and discussed systematically, while citing all the relevant and important references.

High temperature ceramic radomes (HTCR) – A review

Abstract: An electromagnetically transparent, structurally robust and environmentally resistant enclosure of radar antenna for ground based systems to modern avionics in military aircraft and missiles is called as radome. Radome materials are classified based on: (i) type of function - surface-based or flight-mode and (ii) speed of operation - subsonic, supersonic to hypersonic. The desired properties of these materials are low dielectric constant and low loss factor in addition to its capacity to withstand the high temperature of operation. Composite laminates of glass or aramid fibre reinforced polymeric resins are radome material candidates for applications in subsonic range. However, ceramics are the only viable option for military aerospace applications such as a fighter jet travelling at Mach 3 or an advanced hypersonic missile speeding up to Mach 5. This review outlines the hand-full of ceramic materials already in application as radome materials like high-purity-alumina, pyroceram, slip-cast-fused-silica, their processing technology, electromagnetic and mechanical properties, advantages and disadvantages with respect to advanced military vehicles. Use of silicon nitride based radome materials, that has exceptional mechanical strength and thermal stability up to 1400 °C is illustrated with respect to reaction bonded silicon nitride, hot pressed silicon nitride, silicon oxynitride, sialon and their composites. Design of new generation radome materials was conceptualized and discussed as applicable to silicon nitride and related ceramics, wherein incorporation of varied degree of porosity improves electromagnetic properties, simultaneously, maintaining the required mechanical strength. Multilayer and graded porosity and its influence on electromagnetic properties were briefly discussed. Si3N4 ceramics having controlled porosity leading to optimum electromagnetic and mechanical properties produced through systematic processing is proposed as the futuristic high temperature radome material for supersonic applications.