Low-pressure MOCVD of Al2O3 films using aluminium acetylacetonate as precursor: nucleation and growth

A study of the deposition of aluminium oxide films by low-pressure metalorganic chemical vapour deposition from the complex aluminium acetylacetonate, in the absence of an oxidant gas, has been carried out. Depositions on to Si(100), stainless steel, and TiN-coated cemented carbide are found to be smooth, shiny, and blackish. SIMS, XPS and TEM analyses reveal that films deposited at temperatures as low as 600 degreesC contain small crystallites Of kappa-Al2O3, embedded in an amorphous matrix rich in graphitic carbon. Optical and scanning electron microscopy reveal a surface morphology made up of spherulites that suggests that film growth might involve a melting process. A nucleation and growth mechanism, involving the congruent melting clusters of precursor molecules on the hot substrate surface, is therefore invoked to explain these observations. An effort has been made experimentally to verify this proposed mechanism. (C) 2002 Elsevier Science B.V. All rights reserved.

Crucible Steel in South India-Preliminary Investigations on Crucibles from Some Newly Identified Sites

European accounts from the 17th century onwards have referred to the repute and manufacture of “wootz’, a traditional crucible steel made especially in parts of southern India in the former provinces of Golconda, Mysore and Salem. Pliny's Natural History mentions the import of iron and steel from the Seres which have been thought to refer to the ancient southern Indian kingdom of the Cheras. As yet the scale of excavations and surface surveys is too limited to link the literary accounts to archaeometallurgical evidence, although pioneering exploratory investigations have been made by scholars, especially on the pre-industrial production sites of Konasamudram and Gatihosahalli discussed in 18th-19th century European accounts. In 1991–2 during preliminary surveys of ancient base metal mining sites, Srinivasan came across unreported dumps with crucible fragments at Mel-Siruvalur in Tamil Nadu, and Tintini and Machnur in Karnataka and she collected surface specimens from these sites as well as from the known site of Gatihosahalli. She was also given crucible fragments by the Tamil University, Tanjavur, from an excavated megalithic site at Kodumanal, dated to ca 2nd c. Bc, mentioned in Tamil Sangam literature (ca 3rd c. BC-3rd c. AD), and very near Karur, the ancient capital of the Sangam Cheras. Analyses of crucible fragments from the surface collection at Mel-Siruvalur showed several iron prills with a uniform pearlitic structure of high-carbon hypereutectoid steel (∼1–1.5% C) suggesting that the end product was uniformly a high-carbon steel of a structure consistent with those of high-carbon steels used successfully to experimentally replicate the watered steel patterns on ‘Damascus’ swords. Investigations indicate that the process was of carburisation of molten low carbon iron (m.p. 1400° C) in crucibles packed with carbonaceous matter. The fabric of crucibles from all the above mentioned sites appears similar. Preliminary investigations on these crucibles are thus reported to establish their relationship to crucible production of carbon steel and to thereby extend the known horizons of this technology further.

Tailoring of Structural Morphology of Silver Nanowires in Electrochemical Growth

Noble metal such as Ag normally exists in an fcc crystal structure. However as the size of the material is decreased to nanometer lengthscales, a structural transformation from that of its bulk state can be expected with new atomic arrangements due to competition between internal packing and minimization of surface energy. In many previous studies, it has been shown that silver nanowires (AGNWs) grown inside anodic alumina (AAO) templates by ac or dc electrochemical deposition from silver salts or complexes, adopt fcc structure and below some critical diameter ∼ 20 nm they may acquire hcp structure at low temperature. This is, however, critically dependant on the nature of confinement, as AgNWs grown inside nanotube confinement with subnanometer diameter have been reported to have fcc structure. Hence the question of the crystal structure of metal nanowires under combined influence of confinement, temperature and deposition condition remains open. In this abstract we show that the alternative crystal structures of AGNWs at room temperature can be achieved with electrochemical growth processes under specific conditions determined by the deposition parameters and nature of confinement. We fabricated AgNWs of 4H hexagonal structure with diameters 30 – 80 nm inside polycarbonate (PC) templates with a modified dc electrodeposition technique, where the nanowires were grown at deposition potentials as low as 10 mV in 2 M silver nitrate solution[1]. We call this low-potential electrodeposition (LPED) since the electrodeposition process occurs at potential much less than the standard Nernst potential (770 mV) of silver. Two types of electrodes were used – stainless steel and sputtered thin Pt film, neither of which had any influence on the crystal structure of the nanowires. EDS elemental analysis showed the nanowires to consist only of silver. Although the precise atomic dynamics during the LPED process is unclear at present, we investigated this with HRTEM (high-resolution transmission electron microscopy) characterization of nanowires grown over various deposition times, as well as electrical conductivity measurements. These experiments indicate that nanowire growth does not occur through a three-dimensional diffusion controlled process, as proposed for conventional over-potential deposition, but follow a novel instantaneous linear growth mechanism. Further experiments showed that, (a) conventional electrochemical growth at a small over-potential in a 2 mM AgNO3 solution yields nanowires with expected fcc structure inside the same PC templates, and (2) no nanowire was observed under the LPED conditions inside hard AAO templates, indicating that LPED-growth process, and hcp structure of the corresponding nanowires depend on deposition parameters, as well as nature of confinement.

Synthesis of cnt-metal oxide nano-composite electrode materials for supercapacitator by low-pressure mocvd

Homogeneous composite thin films of Fe2O3-carbon nanotube were synthesized in a novel, single-step process by metalorganic chemical vapor deposition (MOCVD) using ferric acetyl acetonate as precursor. The deposition of composite takes place in a narrow range of CVD conditions, beyond which the deposition either multiwall carbon nanotubes (MWNTs) only or hematite (α-Fe2O3) only takes place. The composite film formed on stainless steel substrates were tested for their supercapacitive properties in various aqueous electrolytes.

Solidification Microstructure and Texture in Grain-refined Titanium alloys

In the present study, solidification microstructure and texture evolution in grain-refined Ti-6Al-4V and γ-TiAl alloys via trace boron addition are compared with their baseline counterparts. Boron addition resulted in dramatic grain refinement by almost an order of magnitude. The texture developed in these alloys is also markedly different from the baseline alloys.

Mechanism of enhanced corrosion of alumina graphite refractories near slag/metal interface

All refractories show enhanced corrosion near the slag/metal interface due to Marangoni and convective flows. However, in the case of oxide refractories containing graphite flakes, corrosion is severe due to periodic oscillations in the contact angle at the slag/metal interface, resulting in cyclic dissolution of oxide and graphite into the slag and metal, respectively. Alumina--graphite (AG) refractories should be used only where they are not in simultaneous contact with slag (flux) and low carbon steel.

Prebreakdown current measurements in vacuum gaps stressed by alternating voltages

A method has been suggested for the measurement of prebreakdown currents under a.c. conditions. Measurements were made using hemispherical stainless steel electrodes and currents from 10~3 A down to 10~7 A have been measured.

Supercapacitor studies on globular polypyrrole microstructures developed by a facile electrochemical route

Micrometre-scale polypyrrole (PPy) structures are synthesised for electrochemical supercapacitor applications by a facile electrochemical route. Globular polypyrrole microstructures of size < 5 μm are grown on stainless steel (SS-304) substrate by electro-polymerisation of pyrrole on oxygen microbubble templates electrochemically generated and stabilised in the presence of surfactant/supporting electrolyte/ dopant b-naphthalene sulfonic acid (b-NSA). Microstructures obtained with scan range of 0??1.6 V (against Ag/AgCl) are uniformly distributed over the surface with high coverage density of 5 x 105 to 8 x 10 cm-2. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that the formed microstructures are of Β-NSA doped PPy. Scanning electron microscopy showed the uniform spread and good coverage of microstructures over the substrate. Supercapacitor properties of PPy films are investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge methods with 1.0 M KCl as electrolyte in a three-electrode electrochemical cell. Specific capacitance of 583 Fg-1 is obtained, which is greater than the values (350-400 Fg-1 highest) usually reported for this material. Electrochemical impedance spectroscopy proves the superc

Supercapacitor studies on globular polypyrrole microstructures developed by a facile electrochemical route

Micrometre-scale polypyrrole (PPy) structures are synthesised for electrochemical supercapacitor applications by a facile electrochemical route. Globular polypyrrole microstructures of size <5 mu m are grown on stainless steel (SS-304) substrate by electro-polymerisation of pyrrole on oxygen microbubble templates electrochemically generated and stabilised in the presence of surfactant/supporting electrolyte/dopant beta-naphthalene sulfonic acid (beta-NSA). Microstructures obtained with scan range of 0-1.6 V (against Ag/AgCl) are uniformly distributed over the surface with high coverage density of 5 x 10(5) to 8 x 10 cm(-2). Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy revealed that the formed microstructures are of beta-NSA doped PPy. Scanning electron microscopy showed the uniform spread and good coverage of microstructures over the substrate. Supercapacitor properties of PPy films are investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge methods with 1.0 M KCl as electrolyte in a three-electrode electrochemical cell. Specific capacitance of 583 Fg(-1) is obtained, which is greater than the values (350-400 Fg(-1) highest) usually reported for this material. Electrochemical impedance spectroscopy proves the supercapacitance behaviour and explains the special inductive component of impedance observed in the high-frequency regime because of the globular structures of PPy deposited

Influence of Tempered Microstructures on the Transformation Behaviour of Cold Deformed and Intercritically Annealed Medium Carbon Low Alloy Steel

This research is focused on understanding the role of microstructural variables and processing parameters in obtaining optimised dual phase structures in medium carbon low alloy steels. Tempered Martensite structures produced at 300, 500, and 650 degrees C, were cold rolled to varied degrees ranging from 20 to 80% deformation. Intercritical annealing was then performed at 740, 760, and 780 degrees C for various time duration ranging from 60 seconds to 60 minutes before quenching in water. The transformation behaviour was studied with the aid of optical microscopy and hardness curves. From the results, it is observed that microstructural condition, deformation, and intercritical temperatures influenced the chronological order of the competing stress relaxation and decomposition phase reactions which interfered with the rate of the expected alpha -> gamma transformation. The three unique transformation trends observed are systematically analyzed. It was also observed that the 300 and 500 degrees C tempered initial microstructures were unsuitable for the production of dual structures with optimized strength characteristics.

Growth of Polypyrrole-Horseradish Peroxidase Microstructures for H2O2 Biosensor

Conducting polymer microstructures for enzymatic biosensors are developed by a facile electrochemical route. Horseradish peroxide (HRP)-entrapped polypyrrole (PPy) films with bowl-shaped microstructures are developed on stainless steel (SS 304) substrates by a single-step process. Potentiodynamic scanning/cyclic voltammetry is used for generation of PPy microstructures using electrogenerated oxygen bubbles stabilized by zwitterionic surfactant/buffer N-2-hydroxyethylpiperazine N-2-ethanesulfonic acid as soft templates. Scanning electron microscopic images reveal the bowl-shaped structures surrounded by cauliflower-like fractal PPy films and globular nanostructures. Raman spectroscopy reveals the oxidized nature of the film. Sensing properties of PPy-HRP films for hydrogen peroxide (H2O2) are demonstrated. Electrochemical characterization of the sensor films is done by linear sweep voltammetry (LSV) and amperometry. LSV results indicated the reduction of H2O2 and linearity in response of the sensing film. The amperometric biosensor has a performance comparable to those in the literature with advantages of hard-template free synthesis procedure and a satisfactory sensitivity value of 12.8 mu A/(cm(2) . mM) in the range of 1-10 mM H2O2.

Enhanced Raman Spectroscopy of Molecules Adsorbed on Carbon-Doped TiO2 Obtained from Titanium Carbide: A Visible-Light-Assisted Renewable Substrate

Titanium carbide (TiC) is an electrically conducting material with favorable electrochemical properties. In the present studies, carbon-doped TiO2 (C-TiO2) has been synthesized from TiC particles, as well as TiC films coated on stainless steel substrate via thermal annealing under various conditions. Several C-TiO2 substrates are synthesized by varying experimental, conditions and characterized by UV-visible spectroscopy, photoluminescence, X-ray diffraction and X-ray photoelectron spectroscopic techniques. C-TiO2 in the dry state (in powder form as well as in film form) is subsequently used as a substrate for enhancing Raman signals corresponding to 4-mercaptobenzoic acid and 4-nitrothiophenol by utilizing chemical enhancement based on charge-transfer interactions. Carbon, a nonmetal dopant in TiO2, improves the intensities of Raman signals, compared, to undoped TiO2. Significant dependence of Raman intensity on carbon doping is observed. Ameliorated performance obtained using C-TiO2 is attributed to the presence of surface defects that originate due to carbon as a dopant, which, in turn,, triggers charge transfer between TiO2 and analyte. The C-TiO2 substrates are subsequently regenerated for repetitive use by illuminating an analyte-adsorbed substrate with visible light for a period of 5 h.

Microstructure and Texture Evolution in Interstitial-free (IF) Steel processed by Multi-Axial Forging

In this study, severe plastic deformation (SPD) of Ti-bearing interstitial-free steel was carried out by multi-axial forging (MAF) technique. The grain refinement achieved was comparable to that by other SPD techniques. A considerable heterogeneity was observed in the microstructure and texture. Texture of multi-axially forged steels has been evaluated and reported for the first time. The material exhibited a six-fold increase in the yield strength after four cycles of MAF.

Performances of single and two-stage pulse tube cryocoolers under different vacuum levels with and without thermal radiation shields

Single and two-stage Pulse Tube Cryocoolers (PTC) have been designed, fabricated and experimentally studied. The single stage PTC reaches a no-load temperature of similar to 29 K at its cold end, the two-stage PTC reaches similar to 2.9 K in its second stage cold end and similar to 60 K in its first stage cold end. The two-stage Pulse Tube Cryocooler provides a cooling power of similar to 250 mW at 4.2 K. The single stage system uses stainless steel meshes along with Pb granules as its regenerator materials, while the two-stage PTC uses combinations of Pb along with Er3Ni/HoCu2 as the second stage regenerator materials. Normally, the above systems are insulated by thermal radiation shields and mounted inside a vacuum chamber which is maintained at high vacuum. To evaluate the performance of these systems in the possible conditions of loss of vacuum with and without radiation shields, experimental studies have been performed. The heat-in-leak under such severe conditions has been estimated from the heat load characteristics of the respective stages. The experimental results are analyzed to obtain surface emissivities and effective thermal conductivities as a function of interspace pressure.