Gas pressure sintering of silicon nitride powder coated with Al(2)O(3) and TiO(2)

Gas pressure sintering kinetics of silicon nitride powder coated with 10 wt% (9:1) Al(2)O(3) and TiO(2) have been studied at 1850degreesC with a pressure schedule of 0.3 MPa in the first stage and I MPa in the second stage. The rates have been analyzed with a liquid-phase sintering model. Diffusion-controlled intermediate-stage kinetics have been observed. The role of second-step pressurization with nitrogen and argon has been determined by monitoring the kinetics. Pressurization at an earlier stage (similar to90% relative density) reduces the densification rate but produces a denser material at the final stage. Although final density is greater, a porous surface layer forms on samples sintered with argon pressurization at the second stage. No such porous layer is formed in the case of pressurization with nitrogen. The mechanism of the intermediate-stage kinetics has been discussed with respect to the nature of the product analyzed by XRD after sintering

Enrichment of Metallic Single-Walled Carbon Nanotubes with Simultaneous Purification by Nitric Acid Treatment

Controlled treatment of pristine single-walled carbon nanotubes (SWCNT) with HNO3 (11 M) for 20 hours at 50 degrees C with subsequent repeated cycles of washing, centrifugation and ultrasonication in aqueous media have shown enrichment of the product with pure and debundled nanotubes of metallic variety. On the other hand, no such enrichment was observed when the tubes were treated with a mixture of concentrated nitric acid and sulfuric acid. The electrical varieties of the tubes were ascertained from the analysis of Raman and UV-Vis spectra and dc-resistivities. The purity of the product was gauged from transmission electron microscope images, energy dispersive spectra, thermogravimetric analysis and Raman spectra. A mechanism involving preferential attack of NO3- on the semiconducting tubes under the condition of the adopted protocol has been proposed from the comparison of the results after the treatment of the same pristine SWCNT with mixed acid (conc. HNO3 and H2SO4)

Effect of alkaline-earth fluorides on the nitridation of silicon

Effects of one per cent of alkaline earth fluorides (BaF2, CaF2, SrF2) on the first stage nitridation of silicon have been studied. The fluorides enhances the reaction rate and changes the parabolic kinetics for silicon-nitrogen reaction to a linear kinetics. The accelerating effect being in the order BaF2>CaF2>SrF2 with activation energies of 96.23, 146.44 and 251.04 kJ/mole respectively as compared to 385.43 kJ/mole for silicon-nitrogen reaction without additive. Thermodynamic consideration reveal that the fluorides accelerates the reaction by removing the SiO2 layer formed on the silicon grain by the reaction SiO2 (s) + 2MF2 (g) � SiF4 (g) + 2MO (s). Evidences have been cited to show that the reaction Si (g) + MF2 (g) � SiF2 (g) + M (g) also occurs. © 1984

Encapsulation of Carbon Nanoparticles by Nonionic Surfactant (Tween 80)

The stability of the dispersion by encapsulation of carbon nanoparticles/agglomerates by a nonionic surfactant (Tween 80) has been studied. The sizes of the encapsulated particles have been measured by Dynamic Light Scattering method. Increase in hydrodynamic radius of the particles indicating an adsorption of Tween 80 on the surface of Carbon nanoparticles, has been observed

Oxidation of silicon-nitride sintered with yttria and magnesia containing nitrogen rich liquid

Oxidation of pressureless sintered Silicon Nitride densified with a Nitrogen rich liquid phase in Y2O3-AlN-SiO2 and MgO-AlN-SiO2 systems have been studied. Thermogravimetric studies indicate a parabollic kinetics. Activation energies are 188 and 430 kJ/mole for the oxidation of materials densified with 11.89 and 23.15%. additives containing Y2O3 and 390 kJ/mole for those densified with 21% additives containing MgO.XRD studies show that the oxidation products are SiO2(Cristobalite), Si2ON2,YAlO3 (above 1250C) and Y2Si2O7 in case of Yttria containing additives and SiO2(Cristoballite),Si2ON2, MgSiO3(Protoenstatite),Mg2SiO4(Forsterite) and MgAl2O4(Spinel) in case of Magnesia containing aditives. XPS study shows the presence of a silicate phase in the outermost laver of the oxidised surface. Oxidation of MgO containing material is higher than that of Y2O3 containing material

Nitride and oxy-nitride ceramics for high temperature and engineering applications

Nitrides and oxynitrides of silicon and aluminum occupy prominent positions among other non-oxide members of ceramics on account of their inherent unique combination of properties arising out of their directional covalent bonding in the condensed state. Since many reviews could be found in the literature on the materials either individually or combining some of them, the present article is devoted to the key areas of the present research on the developments of these materials. For example, α’ – SiAlON is now attracting significant attention and the article highlights how compositional variations could lead to easy densification and improvement of its mechanical properties. Experimental results from various authors show that the mechanism of stabilization of α’ – SiAlON is very complex and cannot be explained on the basis of the size of the metal cations and solubility in the transient glassy phase. Y, Yb, Dy and to some extent Ca are good for stabilization of the phase. Evolution of different phases during temperature rise and how these phases affect sintering of the α’ material has been presented in this article. In-situ formation of elongated β’ grains have been shown to increase the toughness of the material. Simultaneously, the development of AlN ceramics in regard to application of this ceramic in different areas apart from that of microelectronics has been discussed. +Improved mechanical and electrical properties of AlN and its composites have shown their promise in diverse applications such as armors, electron tube components and dielectric devices

Synthesis of spherical Al2O3 and AlN powder from C@Al2O3 composite powder

A novel approach has been taken to produce (1) spherical Al2O3 particles by decarbonisation and (2) spherical AlN particles by nitridation and subsequent decarbonisation of C@Al2O3 composite particles. C@Al2O3 composite particles have been prepared by heterogeneous nucleation and crystallisation of Al(NO3)(3) on surfactant encapsulated carbon nano particles followed by evaporative decomposition of the nitrate. Overpressure (0.4 MPa) of nitrogen and a temperature range (1723-1873 K) have been used for nitridation. Whiskers as well as spherical particles of AlN have been observed in the final product. The final product has been characterised by X-Ray Diffraction, Scanning Electron Microscope and Carbon-Hydrogen-Nitrogen content analysis by Elemental Analyser and the mechanism of the nitridation reaction has been analysed. The average size of the spherical AlN particles consisting of crystallites in nano-dimensions (30-50 nm) could be varied from 100 nm to 8 mu m by changing the composition of the sol

Effect of quenching and partitioning on the microstructure and mechanical properties of a medium carbon low alloy low silicon (0.3C-1.0Mn-0.4Si-0.6Cr-0.46Ni-0.26Mo) steel

The importance of advanced high strength steel having high strength with balanced ductility has led to immense researches all over the world for increasing the strength to weight ratio in different structural applications. The results of quenching and partitioning (QP) heat treatment process on a hot rolled advanced medium carbon steel (0.3C-1.0Mn-0.4Si-0.6Cr-0.46Ni-0.26Mo) have been analyzed in the present investigation. Hardness, tensile and yield strength of the steel show considerable improvement after the heat treatment process. Microstructural analyses by Optical, Scanning and Transmission Electron Microscopes show the creation of smaller units and sub-units of multiple phases containing martensite, retained austenite and bainite in the QP steel than those of as-received steel. Microstructure evolution was assessed by conducting the heat treatment with different isothermal partitioning times followed by x-ray Diffraction phase analyses. Austenite content has been found to increase with the duration of partition. Mechanical properties were related with the microstructures obtained at different stages of partitioning. Formation of thin retained austenite films in the periphery of individual bainite and martensite strips, blocks of austenite and the presence of overall small microstructural features have been proposed to be the reason for the improvement in hardness, tensile and yield strength and ductility of the as received hot rolled steel after the designed quench and partitioning treatment

Pareto optimization of electro discharge machining of titanium nitride-aluminium oxide composite material using Genetic algorithm

Titanium nitride-aluminium oxide (TiN-Al 2O 3) is a new generation ceramic composite material having potential for many industrial applications as it possesses high resistance to thermal degradation, anti-wear and anti-abrasion properties. In the present research the characteristic features of EDM process are explored through Taguchi methodology based experimental studies with various process parametric combinations. Finally the process has been optimized using Genetic algorithm based Pareto optimization search. From Pareto optimal search the technology guideline for optimal parameter settings have been selected. The present research approach is extremely useful for maximizing the productivity while maintaining the geometrical accuracy within desired limit