T.R. Rama Mohan

Synthesis of nanocrystalline copper-tungsten alloys by mechanical alloying

Abstract Copper–tungsten exhibits total absence of solubility in both solid and liquid state. Mechanical alloying (MA) as a solid state, non-equilibrium process can be beneficial to the processing of such an immiscible system with the added features of refinement of structure. A study was undertaken to synthesise various Cu–W alloys and develop an ultrafine microcomposite structure of tungsten in copper matrix by mechanical alloying. Elemental powders of copper and tungsten were milled in high energy ball mills. The milling behaviour was found to depend on the composition, milling time and milling atmosphere. The milled powders were characterised for their particle size, microstructure and lattice parameters. Metastable mutual solid solubility in the system was confirmed. Crystallite sizes were found to be in the nanocrystalline regime. The conversion of milling energy effectively to generate deformed surfaces, which in turn led to metastable solid solubility and nanocrystalline structure, was aided by the presence of oxygen in the milling atmosphere.

Synthesis of nanosized titanium powder by high energy milling

Titanium powders of about 2 μ particle size were subjected to high energy attrition milling in an argon atmosphere. Selecting suitable milling parameters, nanosize (<100 nm) titanium powders were prepared after 15 h of milling. An average particle size of 35 nm was obtained at 30 h of milling after which the particle size stabilized with continuation of milling to 75 h. The powders after milling for various durations were characterized by TEM, ICP and XRD, and these results are reported and discussed.

A novel chemical solution technique for the preparation of nano size titanium powders from titanium dioxide

Abstract Nano size titanium powder was prepared by a novel chemical solution synthesis route from titanium dioxide (TiO 2 ). TiO 2 was allowed to form a complex, titanium catecholate precursor, in the presence of ammonium sulfate and concentrated sulfuric acid. The complex was filtered, washed with cold isopropyl alcohol and dried. Titanium hydride was prepared by heating the titanium catecholate precursor at 800°C in a hydrogen atmosphere. The product was then dehydrogenated in 8 × 10 −6 torr vacuum to get nano size titanium powders. The powder characteristics have been studied by X-ray diffraction, ICP, laser particle size analysis, and scanning and transmission electron microscopy. The latter indicated that the powders were 21.6 nm in size and selected area diffraction showed the powders to be crystalline.

Dilatometric sintering study of titanium-titanium nitride nano/nanocomposite powders

Abstract The sintering behaviour of titanium–titanium nitride nano/nanocomposite powders has been investigated by dilatometry. The nanosized titanium powders (40 nm) were produced by the attrition milling of micron sized titanium powders (12 μm) in Ar atmosphere while the nanosized titanium nitride powders (50 nm) were as procured. Two compositions of the nano/nanocomposite powders, i.e. Ti–8TiN and Ti–15TiN (wt-%) were produced by physical mixing and ultrasonification. Dilatometry was carried out at a constant rate of 10 K min−1 heating to various temperatures in the range of 450–1250°C followed by holding for 1 h. The effect of nanoTiN reinforcement on the sintering onset temperature, linear shrinkage, shrinkage rate, activation energy for sintering, microstructure and grain growth has been reported and discussed.

Structural and morphological studies of electrodeposited amorphous silicon thin films

Abstract Amorphous silicon thin films obtained from hydrofluosilicic acid using the electrodeposition method are analysed for structure and morphology. The chemical nature of the films is discussed using the data from IR spectroscopy. The electrical resistivity of these films is very high, of the order of 10 12 –10 13 Ω cm, under the present experimental conditions. X-ray diffraction spectra revealed that the films are not crystalline in nature. At low concentrations of the electrolyte, scanning electron microscopy photographs exhibited some microstructure with crystalline order of about 100 A. At high concentrations, the structure of the films changed widely to be homogenous.

Vibrational modes in electrodeposited amorphous silicon: FT-IR analysis

Infrared spectra of 13 samples of amorphous silicon bonded with hydrogen, fluorine and carbon, prepared by electrodeposition using a mixture of ethylene glycol and fluosilicic acid were analysed in the wave number region 4000-400 cm−1 with a Fourier transform infrared spectrometer. Strong absorption peaks were observed at 1000 cm−1 due to the SiFx stretching mode. Small peaks were seen around 2300 and 640 cm−1 due to SiH stretching and wagging modes of absorption. The number of bonded hydrogen atoms in the film deposited at 0.05 M, 50 mA cm−2 was calculated to be 6.2579×1021 and 1.2302×1020 atm cm−3 using integrated absorption of the CH and SiH stretching modes, respectively. The absorption coefficient around the SiFx stretch region was found to vary from 1300–2500 cm−1 as the molarity of the electrolyte was increased. Binding energy shifts in X-ray photoelectron spectrum were used as a cross check to confirm the silicon bonding with carbon, hydrogen, oxygen and fluorine atoms. The absence of columnar growth in SEM photographs indicates no polysilane formation in the films.

Polymeric encapsulation of 1-2-3 ceramic superconductors

YBa2Cu3O7−x pellets have been found to degrade and lose superconducting properties when exposed to the normal atmosphere for long durations. Hence, coating the superconducting pellets with a polymer to protect it from moisture and CO2 of the ambient atmosphere is highly desirable. In this paper the criteria of selection of a polymer and some preliminary studies on polymeric encapsulation of 1-2-3 pellets are reported. Fevitite epoxy and stycast have been found to show encouraging results.

Initial sintering kinetics of titanium-titanium nitride nano/nanocomposite powders

Abstract The present investigation deals with the mass transport mechanisms operating during the initial sintering of titanium–titanium nitride nano/nanocomposite powders. Two compositions of the nano/nanocomposite powders, i.e. nano Ti–8 wt-% nano TiN and nano Ti–15 wt-% nano TiN along with nano Ti were used in the present study. Sintering studies have been carried out by dilatometry of the powder compacts using a constant rate of heating (CRH) and employing model equations to the initial sintering portion. The shrinkage values obtained were analysed using Young and Cutlers equations and a model proposed by Johnson and Berrin. Based on the analysis of experimental data the effect of second phase TiN on the mass transport mechanisms of nano Ti was determined.

Effects of niobia and vanadia additions on the mechanical properties of alumina

Abstract The effect of niobia and vanadia additions on the mechanical properties of alumina has been studied extensively and reported. It was observed that sintered density values of niobia doped samples increase with an increase in temperature. The optimum density has been obtained at 1500°C, 500 min soaking with 0·5 wt% dopant concentration. The modulus of rupture values have been determined and correlated with a densification parameter. Vanadia doped samples have shown a different tendency in their sintered density values. In this case, the sintered density increases with an increase in temperature, but decrease with an increase in dopant concentration.

Shape-forming and densification of ceramic superconductors

Processing studies on varied shape-forming and densification of bulk ceramic superconductor, YBa1·5Ca0·5Cu3O7\t-\gd, are reported in this paper. Polyvinyl butyral-polyethylene glycol-trichloroethylene has been found to be the best binder-plasticizer-solvent system in plastic shape-forming. The effect of initial particle morphology on final densification has been the most sensitive single parameter as compared to compaction pressure and final sintering durations at ∼930°C. 1-2-3 powders of mean particle size ∼ 1·94 µm have yielded sintered densities ∼92% T.D. albeit with lower oxygen intake O6·7.