Evolution of Microstructure During Solidification of an Aluminium Alloy Under Stirring

In the present work, the evolution of microstructure during solidification of A356 alloy under stirring is performed experimentally in a high temperature concentric viscometer. The stirring during solidification results a semisolid slurry in the annular space between the cylinders. This slurry is removed periodically during processing using a vacuum removal quartz tube and quenched in water for micrograph analysis. From the micrograph analysis, the shape, stacking arrangement and corresponding microstructural evolution of the suspended primary particles in the slurry are studied. The work also predicts the fraction of solid present in the extracted slurry. Finally, the effect of microstructure and the solid-fraction on the slurry viscosity is presented.

Direct ultrafast laser written C-band waveguide amplifier in Er-doped chalcogenide glass

This paper reports the fabrication and characterization of an ultrafast laser written Er-doped chalcogenide glass buried waveguide amplifier; Er-doped GeGaS glass has been synthesized by the vacuum sealed melt quenching technique. Waveguides have been fabricated inside the 4 mm long sample by direct ultrafast laser writing. The total passive fiber-to-fiber insertion loss is 2.58 +/- 0.02 dB at 1600 nm, including a propagation loss of 1.6 +/- 0.3 dB. Active characterization shows a relative gain of 2.524 +/- 0.002 dB/cm and 1.359 +/- 0.005 dB/cm at 1541 nm and 1550 nm respectively, for a pump power of 500 mW at a wavelength of 980 nm. (C) 2012 Optical Society of America

Fabrication of Highly Dense Nanoholes by Self-Assembled Gallium Droplet on Silicon Surface

We report the fabrication of nanoholes on silicon surface by exploiting the solubility of silicon in gallium by local droplet etching. Nanometer-sized gallium droplets yield nanoholes when annealed in ultra-high vacuum at moderate temperatures (similar to 500 degrees C) without affecting the other regions. High vacuum and moderate annealing temperatures are key parameters to obtain well-defined nanoholes with diameter comparable to that of Ga droplets. Self-assembly of Ga droplet leads to a nanohole density of 4-8 x 10(10)/cm(2).

Synthesis and supramolecular assembly of the bifunctional borinic acid 1,2-fcB(OH)](2)

Treatment of the chloro-substituted diboradiferrocene derivative 1 with Me3SiOMe and subsequent hydrolysis resulted in formation of the novel organometallic bis(borinic acid) derivative 3. The assembly of 3 into supramolecular structures via hydrogen bonding and reversible covalent boron-oxygen bond formation was explored. Upon crystallization from acetone or THF one-dimensional chains form in which molecules of 3 alternately serve as hydrogen bond donors and acceptors. The additional OH hydrogens that are not involved in hydrogen bonding within the polymeric chains undergo hydrogen bonding to the solvent molecules. Removal of the solvent was achieved at moderate temperature under high vacuum. While the polymeric chains remain intact, in the absence of the solvent as a hydrogen bond acceptor, short contacts to the Cp rings of neighboring polymer strands lead to a network-like structure. At higher temperatures, further dehydration occurs with formation of B-O-B linkages as confirmed by MALDI-TOF mass spectrometry. Oligomers with up to 15 repeating units (30 ferrocenes) were detected.

Microstructure and mechanical properties of oxidation resistant suction cast Nb-Si-Al alloy

In this paper, we report a significant improvement in mechanical and oxidation properties of near eutectic Nb-Si alloys by the addition of aluminum (Al) and control of microstructural length scale. A comparative study of two alloys Nb-18.79at%Si and Nb-12.3at%Si-9at%Al were carried out. The processing for microstructure refinements were carried out by vacuum suction casting in water cooled thick copper mould. It is shown that addition of Al suppresses Nb3Si phase and promotes beta Nb5Si3 phase under nonequilibrium solidification condition. The microstructural length scale and in particular eutectic spacing reduces significantly to 50-100 nm in suction cast ternary alloy. A detailed TEM study shows the presence of delta-Nb11Si4 phase in Nb matrix. The hardness of Nb solid solution can be increased as a consequence to a level observed in Nb3Si intermetallic due to the well oriented precipitates. Compression test yields the ultimate strength of 1.8 +/- 0.1 GPa and engineering strain of 2.3 +/- 0.03%. In comparison, the binary Nb-18.79 at% Si alloy possesses an ultimate strength of 1.35 +/- 0.1 GPa and strain of 0.2 +/- 0.01% when processed under identical conditions. The latter exhibits coarser microstructural length scale (300-400 nm) and a brittle behavior. The indentation fracture toughness of Al containing suction cast alloy shows a value of 20.2 +/- 0.5 MPa root m which represents a major improvement over bulk Nb-Si eutectic alloy. The detailed thermal studies confirm a multifold improvement in oxidation resistance up to 1000 degrees C. (C) 2012 Elsevier B.V. All rights reserved.

Optical properties change with the addition and diffusion of Bi to As2S3 in the Bi/As2S3 bilayer thin film

The role of Bi layer (thickness similar to 7 nm) on As2S3 film was extensively studied for different optical applications in which Bi (top layer) as active and diffusing layer and As2S3 as barrier (matrix) layer. Bilayer thin films of Bi/As2S3 were prepared from Bi and As2S3 by thermal evaporation technique under high vacuum. The decrease of optical band gap with the addition of Bi to As2S3 has been explained on the basis of density of states and the increase in disorder in the system. It was found that the efficient changes of optical parameters (transmission, optical band gap, refraction) could be realized due to the photo induced diffusion activated by the focused 532 nm laser irradiation and formation of different bonds. The diffusion of Bi into As2S3 matrix increases the optical band gap producing photo bleaching effect. The changes were characterised by different experimental techniques. (C) 2012 Elsevier B.V. All rights reserved.

Low-thermal-budget solution processing of thin films of zinc ferrite and other complex oxides

Further miniaturization of magnetic and electronic devices demands thin films of advanced nanomaterials with unique properties. Spinel ferrites have been studied extensively owing to their interesting magnetic and electrical properties coupled with stability against oxidation. Being an important ferrospinel, zinc ferrite has wide applications in the biological (MRI) and electronics (RF-CMOS) arenas. The performance of an oxide like ZnFe2O4 depends on stoichiometry (defect structure), and technological applications require thin films of high density, low porosity and controlled microstructure, which depend on the preparation process. While there are many methods for the synthesis of polycrystalline ZnFe2O4 powder, few methods exist for the deposition of its thin films, where prolonged processing at elevated temperature is not required. We report a novel, microwave-assisted, low temperature (<100°C) deposition process that is conducted in the liquid medium, developed for obtaining high quality, polycrystalline ZnFe2O4 thin films on technologically important substrates like Si(100). An environment-friendly solvent (ethanol) and non-hazardous oxide precursors (β-diketonates of Zn and Fe in 1:2 molar ratio), forming a solution together, is subjected to irradiation in a domestic microwave oven (2.45 GHz) for a few minutes, leading to reactions which result in the deposition of ZnFe2O4 films on Si (100) substrates suspended in the solution. Selected surfactants added to the reactant solution in optimum concentration can be used to control film microstructure. The nominal temperature of the irradiated solution, i.e., film deposition temperature, seldom exceeds 100°C, thus sharply lowering the thermal budget. Surface roughness and uniformity of large area depositions (50x50 mm2) are controlled by tweaking the concentration of the mother solution. Thickness of the films thus grown on Si (100) within 5 min of microwave irradiation can be as high as several microns. The present process, not requiring a vacuum system, carries a very low thermal budget and, together with a proper choice of solvents, is compatible with CMOS integration. This novel solution-based process for depositing highly resistive, adherent, smooth ferrimagnetic films on Si (100) is promising to RF engineers for the fabrication of passive circuit components. It is readily extended to a wide variety of functional oxide films.

Analysis of rectangular folded waveguide SWS with grating on the broad wall

An E-plane rectangular folded-waveguide slow-wave structure with metal grating on the broad wall of the waveguide along the direction of the electric field has been proposed and analyzed for the dispersion and interaction impedance characteristics through three dimensional electromagnetic modeling in CST Studio. The effects of the presence of grating on the bandwidth and interaction impedance are demonstrated.

High power microwave coupling with a buried twisted pair cable

This paper deals with the coupling of High Power Microwaves with a buried twisted pair cable. The electric field at a distance of 1km from the HPM antenna has been computed and is used for further computation of induced voltage and current. It is found that the peak of the induced current and voltage in a buried unshielded twisted pair cable at a distance of 1km from an HPM antenna of power level 10GW is 20A and 2kV respectively.

Growth and characterization of micro and nanostructures of lead telluride (PbTe) by thermal evaporation method

Lead telluride micro and nanostructures have been grown on silicon and glass substrates by a simple thermal evaporation of PbTe in high vacuum of 3 x 10(-5) mbar. Growth was carried out for two different distances between the evaporation source and the substrates. Synthesized products consist of nanorods and micro towers for 2.4 cm and 3.4 cm of distance between the evaporation source and the substrates respectively. X-ray diffraction and transmission electron microscopy studies confirmed crystalline nature of the nanorods and micro towers. Nanorods were grown by vapor solid mechanism. Each micro tower consists of nano platelets and is capped with spherical catalyst particle at their end, suggesting that the growth proceeds via vapor-liquid-solid (VLS) mechanism. EDS spectrum recorded on the tip of the micro tower has shown the presence of Pb and Te confirming the self catalytic VLS growth of the micro towers. These results open up novel synthesis methods for PbTe nano and microstructures for various applications.

Studies of cryocooler based cryosorption pump with activated carbon panels operating at 11K

Cryosorption pump is the only solution for pumping helium and hydrogen in fusion reactors. It is chosen because it offers highest pumping speed as well as the only suitable pump for the harsh environments in a tokamak. Towards the development of such cryosorption pumps, the optimal choice of the right activated carbon panels is essential. In order to characterize the performance of the panels with indigenously developed activated carbon, a cryocooler based cryosorption pump with scaled down sizes of panels is experimented. The results are compared with the commercial cryopanel used in a CTI cryosorption (model: Cryotorr 7) pump. The cryopanel is mounted on the cold head of the second stage GM cryocooler which cools the cryopanel down to 11K with first stage reaching about similar to 50K. With no heat load, cryopump gives the ultimate vacuum of 2.1E-7 mbar. The pumping speed of different gases such as nitrogen, argon, hydrogen, helium are tested both on indigenous and commercial cryopanel. These studies serve as a bench mark towards the development of better cryopanels to be cooled by liquid helium for use with tokamak.

Effect of composition on thermoelectric properties of polycrystalline CrSi2

Ingots with compositions CrSi2-x (with 0 < x < 0.1) were synthesized by vacuum arc melting followed by uniaxial hot pressing for densification. This paper reports the temperature and composition dependence of the electrical resistivity, Seebeck coefficient, and thermal conductivity of CrSi2-x samples in the temperature range of 300 K to 800 K. The silicon-deficient samples exhibited substantial reductions in resistivity and Seebeck coefficient over the measured temperature range due to the formation of metallic secondary CrSi phase embedded in the CrSi2 matrix phase. The thermal conductivity was seen to exhibit a U-shaped curve with respect to x, exhibiting a minimum value at the composition of x = 0.04. However, the limit of the homogeneity range of CrSi2 suppresses any further decrease of the lattice thermal conductivity. As a consequence, the maximum figure of merit of ZT = 0.1 is obtained at 650 K for CrSi1.98.

Performance limits of transition metal dichalcogenide (MX2) nanotube surround gate ballistic field effect transistors

We theoretically analyze the performance of transition metal dichalcogenide (MX2) single wall nanotube (SWNT) surround gate MOSFET, in the 10 nm technology node. We consider semiconducting armchair (n, n) SWNT of MoS2, MoSe2, WS2, and WSe2 for our study. The material properties of the nanotubes are evaluated from the density functional theory, and the ballistic device characteristics are obtained by self-consistently solving the Poisson-Schrodinger equation under the non-equilibrium Green's function formalism. Simulated ON currents are in the range of 61-76 mu A for 4.5 nm diameter MX2 tubes, with peak transconductance similar to 175-218 mu S and ON/OFF ratio similar to 0.6 x 10(5)-0.8 x 10(5). The subthreshold slope is similar to 62.22 mV/decade and a nominal drain induced barrier lowering of similar to 12-15 mV/V is observed for the devices. The tungsten dichalcogenide nanotubes offer superior device output characteristics compared to the molybdenum dichalcogenide nanotubes, with WSe2 showing the best performance. Studying SWNT diameters of 2.5-5 nm, it is found that increase in diameter provides smaller carrier effective mass and 4%-6% higher ON currents. Using mean free path calculation to project the quasi-ballistic currents, 62%-75% reduction from ballistic values in drain current in long channel lengths of 100, 200 nm is observed.

Composition, structure and electrical properties of DC reactive magnetron sputtered Al2O3 thin films

Thin films of alumina (Al2O3) were deposited over Si < 1 0 0 > substrates at room temperature at an oxygen gas pressure of 0.03 Pa and sputtering power of 60 W using DC reactive magnetron sputtering. The composition of the as-deposited film was analyzed by X-ray photoelectron spectroscopy and the O/Al atomic ratio was found to be 1.72. The films were then annealed in vacuum to 350, 550 and 750 degrees C and X-ray diffraction results revealed that both as-deposited and post deposition annealed films were amorphous. The surface morphology and topography of the films was studied using scanning electron microscopy and atomic force microscopy, respectively. A progressive decrease in the root mean square (RMS) roughness of the films from 1.53 nm to 0.7 nm was observed with increase in the annealing temperature. Al-Al2O3-Al thin film capacitors were then fabricated on p-type Si < 1 0 0 > substrate to study the effect of temperature and frequency on the dielectric property of the films and the results are discussed.

Fretting studies on self-mated stainless steel and chromium carbide coated surfaces under controlled environment conditions

Adhesive wear has been widely accepted as the type of wear which is most frequently encountered under fretting conditions. Present study has been carried out to study the mode of failure and mechanisms associated under conditions where strong adhesion prevails at the contact interface. Mechanical variables such as normal load, displacement amplitude, and environment conditions were controlled so as to simulate adhesion as the governing mechanism at the contact interface. Self-mated Stainless Steel (SS) and chromium carbide with 25% nickel chrome binder coatings using plasma spray and high-velocity oxy-fuel (HVOF) processes on SS were considered as the material for contacting bodies. Damage in the form of plastic deformation, fracture, and material transfer has been observed. Further, chromium carbide with 25% nickel chrome binder coatings using HVOF process on SS shows less fretting damage, and can be considered as an effective palliative against fretting damage, even under high vacuum conditions. (C) 2013 Elsevier B.V. All rights reserved.