Electron-beam weld microstructures and properties of aluminium-lithium alloy 8090

Lithium-containing aluminium alloys are of considerable current interest in the aerospace and aircraft industries because lithium additions to aluminium improve the modulus and decrease the density compared to conventional aluminium alloys. Few commercial aluminium-lithium alloys have emerged for use in the aerospace industry. One such candidate is 8090, a precipitation-hardenable Al-Li-Cu-Mg alloy. The influence of electron-beam welding on the microstructure and mechanical properties of alloy 8090 material has been evaluated through microscopical observations and mechanical tests. Microscopic observations of the electronbeam welds revealed an absence of microporosity and hot cracking, but revealed presence of microporosity in the transverse section of the weld. Mechanical tests revealed the electronbeam weld to have lower strength, elongation and joint efficiency. A change in microscopic fracture mode was observed for the welded material when compared to the unwelded counterpart. An attempt is made to rationalize the behaviour in terms of competing mechanistic effects involving the grain structure of the material, the role of matrix deformation characteristics, grain-boundary chemistry and grain-boundary failure.

Electron spectroscopic investigation of the semiconductor-metal transition in La1-xSrxMnO3

We study the electronic structure of La1-xSrxMnO3+δ, x=0, 0.1, 0.2, 0.3, and 0.4, across the semiconductor-metal transition, using various electron spectroscopy techniques. The negligible intensity seen at EF using ultraviolet photoemission spectroscopy and bremsstrahlung isochromat spectroscopy (BIS) indicate an unusual semiconductor-metal transition observed for x≥0.2, consistent with the resistivity data. The BIS spectra show doped hole states developing about 1.4 eV above EF as a function of x. Auger electron spectroscopy gives an estimate of the intra-atomic Coulomb energy in the O 2p manifold to be about 6.8 eV. The Mn 2p core-level spectrum of LaMnO3, analyzed in terms of a configuration-interaction calculation, gives parameter values of the charge-transfer energy Δ=5.0 eV, the hybridization strength between Mn 3d and O 2p states, t=3.8 eV, and the on-site Coulomb energy in Mn 3d states Udd=4.0 eV, suggesting a mixed character for the ground state of LaMnO3.

Fascinating chemistry of metal clusters and carbon clusters (fullerenes)

Investigations of a variety of transition metal clusters by means of high-energy spectroscopies including BIS show the occurrence of a metal-insulator transition with decrease in the cluster size. The chemical reactivity of the clusters also varies significantly with the size. Among the many fascinating properties of the fullerenes C60 and C70, a noteworthy one is the interaction between metal clusters and fullerenes. Phase transitions of fullerenes involving orientational disorder and pressure-induced decrease in the band gap of C60 are other novel features of interest.

Metal-Insulator Transitions in Anion-Excess LaMnO3+? Controlled by the Mn4+ Content

LaMnO3+? samples with Mn4+ content up to 50% have been prepared by different methods. The structure of LaMnO3+? changes from orthorhombic to cubic (via rhombohedral) with increase in the Mn4+ content. LaMnO3+? samples containing greater than 20% Mn4+ are ferromagnetic and show resistivity maxima at a temperature Tt which is close to the ferromagnetic Curie temperature. The resistivity maximum is due to the occurrence of a metal-insulator transition. In samples heated to the same temperature, the value of Tt increases with % Mn4+. For a given sample, Tt increases with the temperature of heat treatment due to the increase in particle size. The onset of ferromagnetism in LaMnO3+? accompanied by an insulator-metal transition is similar to that found in La1-xCaxMnO3 and La1-xSrxCoO3.

Missile battery placement for air defense: A dynamic programming approach

A procedure to evaluate surface-to-air missile battery placement patterns for air defense is presented. A measure of defense effectiveness is defined as a function of kill probability of the defense missiles and the nature of the surrounding terrain features. The concept of cumulative danger index is used to select the best path for a penetrating hostile aircraft for any given pattern of placement. The aircraft is assumed to be intelligent and well-informed. The path is generated using a dynamic programming methodology. The software package so developed can be used off-line to choose the best among a number of possible battery placement patterns.

Processing maps for hot-working of powder metallurgy 1100 Al-10 vol % SiC-particulate metal-matrix composite

The hot-working characteristics of the metal-matrix composite (MMC) Al-10 vol % SiC-particulate (SiCp) powder metallurgy compacts in as-sintered and in hot-extruded conditions were studied using hot compression testing. On the basis of the stress-strain data as a function of temperature and strain rate, processing maps depicting the variation in the efficiency of power dissipation, given by eegr = 2m/(m+1), where m is the strain rate sensitivity of flow stress, have been established and are interpreted on the basis of the dynamic materials model. The as-sintered MMC exhibited a domain of dynamic recrystallization (DRX) with a peak efficiency of about 30% at a temperature of about 500°C and a strain rate of 0.01 s�1. At temperatures below 350°C and in the strain rate range 0.001�0.01 s�1 the MMC exhibited dynamic recovery. The as-sintered MMC was extruded at 500°C using a ram speed of 3 mm s�1 and an extrusion ratio of 10ratio1. A processing map was established on the extruded product, and this map showed that the DRX domain had shifted to lower temperature (450°C) and higher strain rate (1 s�1). The optimum temperature and strain rate combination for powder metallurgy billet conditioning are 500°C and 0.01 s�1, and the secondary metal-working on the extruded product may be done at a higher strain rate of 1 s�1 and a lower temperature of 425°C.

Reduction behaviour of Fe3+/Al2O3 obtained from the mixed oxalate precursor and the formation of the Fe0-Al2O3 metal-ceramic composite

Reduction behaviour of Fe3+/Al2O3 obtained by the decomposition of the oxalate precursor has been investigated by employing X-ray diffraction (XRD), Mössbauer spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. Calcination of Fe3+/Al2O3 at or below 1070 K yields mainly a poorly ordered, fine particulate form of ?-Al2�xFexO3. Calcination at or above 1220 K yields ?-Al2�xFexO3. Reduction of Fe3+/Al2O3 samples calcined at or below 1070 K gives the FeAl2O4 spinel on reduction at 870 K; samples calcined at or above 1220 K give Al2-xFexO3 with a very small proportion of metallic iron. Fe3+/Al2O3 samples calcined at 1220 K or above yield metallic iron and a very small proportion of the spinel on reduction below 1270 K. In the samples reduced at or above 1270 K, the main product is metallic iron in both ferromagnetic and superparamagnetic forms. The oxalate precursor route yields more metallic iron than the sol�gel route.

Strong metal-support interaction in Ni/TiO2 catalysts: in situ EXAFS and related studies

In situ EXAFS and X-ray diffraction investigations of Ni/TiO2 catalysts show that NiTiO3 is formed as an intermediate during calcination of catalyst precursors prepared by the wet-impregnation method; the intermediate is not formed when ion-exchange method is used for the preparation. On hydrogen reduction, NiTiO3 gives rise to Ni particles dispersed in the TiO2(rutile) matrix. The occurrence of the anatase-rutile transformation of the TiO2 support, the formation and subsequent decomposition/reduction of NiTiO3 as well as the unique interface properties of the Ni particles are all factors of importance in giving rise to metal-support interaction. Active TiO2(anatase) prepared from gel route gives an additional species involving Ni3+.

Chromium-manganese iron alloy system design cast in metal and sand moulds for erosion resistance: a positron lifetime study

Erosion characteristics of high chromium (Cr, 16-19%) alloy cast iron with 5% and 10% manganese (Mn) prepared in metal and sand moulds through induction melting are investigated using jet erosion test setup in both as-cast and heat-treated conditions. The samples were characterised for hardness and microstructural properties. A new and novel non-destructive evaluation technique namely positron lifetime spectroscopy has also been used for the first time to characterise the microstructure of the material in terms of defects and their concentration. We found that the hardness decreases irrespective of the sample condition when the mould type is changed from metal to sand, On the other hand, the erosion volume loss shows an increasing trend. Since the macroscopic properties have a bearing on the microstructure, good credence is obtained from the microstructural features as seen from light and scanning electron micrographs. Faster cooling in the metal mould yielded fine carbide precipitation on the surface. The defect size and their concentration derived from positron method are higher for sand mould compared to metal mould. Lower erosion loss corresponds to smaller size defects in metal mould are the results of quicker heat transfer in the metal mould compared to the sand mould. Heat treatment effects are clearly seen as the reduced concentration of defects and spherodisation of carbides points to this. The erosion loss with respect to the defects size and concentration correlate very well.

Structural, dielectric and ferroelectric properties of multilayer lithium tantalate thin films prepared by sol-gel technique

Multilayer lithium tantalate thin films were deposited on Pt-Si Si(111)/SiO2/TiO2/Pt(111)]substrates by sol-gel process. The films were annealed at different annealing temperatures (300, 450 and 650 degrees C) for 15 min. The films are polycrystalline at 650 degrees C and at other annealing conditions below 650 degrees C the films are in amorphous state. The films were characterized using X-ray diffraction, atomic force microscopy (AFM) and Raman spectroscopy. The AFM of images show the formation of nanograins of uniform size (50 nm) at 650 degrees C. These polycrystalline films exhibit spontaneous polarization of 1.5 mu C/cm(2) at an application of 100 kV/cm. The dielectric constant of multilayer film is very small (6.4 at 10 kHz) as compared to that of single crystal. (C) 2010 Elsevier B.V. All rights reserved.

Coordination-Driven Self-Assembly of M3L2 Trigonal Cages from Preorganized Metalloligands Incorporating Octahedral Metal Centers and Fluorescent Detection of Nitroaromatics

The design and preparation of novel M3L2 trigonal cages via the coordination-driven self-assembly of preorganized metalloligands containing octahedral aluminum(III), gallium(III), or ruthenium(II) centers is described. When tritopic or dinuclear linear metalloligands and appropriate complementary subunits are employed, M3L2 trigonal-bipyramidal and trigonal-prismatic cages are self-assembled under mild conditions. These three-dimensional cages were characterized with multinuclear NMR spectroscopy (H-1 and P-31) and high-resolution electrospray ionization mass spectrometry. The structure of one such trigonal-prismatic cage, self-assembled from an arene ruthenium metalloligand, was confirmed via single-crystal X-ray crystallography. The fluorescent nature of these prisms, due to the presence of their electron-rich ethynyl functionalities, prompted photophysical studies, which revealed that electron-deficient nitroaromatics are effective quenchers of the cages' emission. Excited-state charge transfer from the prisms to the nitroaromatic substrates can be used as the basis for the development of selective and discriminatory turn-off fluorescent sensors for nitroaromatics.

Influence of water and thermal history on ion transport in lithium salt-succinonitrile plastic crystalline electrolytes

Important issues of water and thermal history affecting ion transport in a representative plastic crystalline lithium salt electrolyte: succinonitrile (SN)-lithium perchlorate (LiClO4) are discussed here. Ionic conductivity of electrolytes with high lithium salt amounts (similar to 1 M) in SN at a particular temperature is known to be influenced both by the trans-gauche isomerism and ion association (solvation), the two most important intrinsic parameters of the plastic solvent. In the present study both water and thermal history influence SN and result in enhancement of ionic conductivity of 1 M LiClO4-SN electrolyte. Systematic observations reveal that the presence of water in varying amounts promote ion-pair dissociation in the electrolyte. While trace amounts (approximate to 1-15 ppm) do not affect the trans-gauche isomerism of SN, the presence of water in large amounts (approximate to 5500 ppm) submerges the plasticity of SN. Subjugating the electrolyte to different thermal protocol resulted in enhancement of trans concentration only. This is an interesting observation as it demonstrates a simple and effective procedure involving utilization of an optimized set of external parameters to decouple solvation from trans-gauche isomerism. Observations from the ionic conductivity of various samples were accounted by changes in signature isomer and ion-association bands in the mid-IR regime and also from plastic to normal crystal transition temperature peak obtained from thermal studies. (C) 2010 Elsevier B.V. All rights reserved.

Large negative magnetoresistance and metal-insulator transition observed in MnO/C composite coatings grown on ceramic alumina by metalorganic chemical vapor deposition

MnO/C composite coatings were grown by the metalorganic chemical vapor deposition process on ceramic alumina in argon ambient. Characterization by various techniques confirms that these coatings are homogeneous composites comprising nanometer-sized MnO particles embedded in a matrix of nanometer-sized graphite. Components of the MnO/C composite coating crystalline disordered, but are electrically quite conductive. Resistance vs. temperature measurements show that coating resistance increases exponentially from a few hundred ohms at room temperature to a few megaohms at 30 K. Logarithmic plots of reduced activation energy vs. temperature show that the coating material undergoes a metal-insulator transition. The reduced activation energy exponent for the film under zero magnetic field was 2.1, which is unusually high, implying that conduction is suppressed at much faster rate than the Mott or the Efros-Shklovskii hopping mechanism. Magnetoconductance us. magnetic field plots obtained at various temperatures show a high magnetoconductance (similar to 28.8%) at 100 K, which is unusually large for a disordered system, wherein magnetoresistance is attributed typically to weak localization. A plausible explanation for the unusual behavior observed in the carbonaceous disordered composite material is proposed. (C) 2010 Elsevier Ltd. All rights reserved.

Photoluminescence enhancement and quenching in metal-semiconductor quantum dot hybrid arrays

Hybrid monolayer arrays of metal and semiconductor quantum dots have been prepared to study the exciton-plasmon interaction. We observed crossover from strong quenching to enhancement in photoluminescence of the quantum dots as a function of the emission wavelength for fixed interparticle spacings. Remarkably, the enhancement is observed even for extremely short separation at which strong quenching has been observed and predicted earlier. A significant redshift in emission maxima is also observed for quantum dots with quenched emission. The possible role of collective phenomena as well as strong interactions in such ordered hybrid arrays in controlling the emission is discussed. (C) 2011 American Institute of Physics. doi:10.1063/1.3553766]

Lattice embedding and equilibrium geometry of metal-halogen chains in the two-band extended Hubbard model

Two-band extended Hubbard model studies show that the shift in optical gap of the metal-halogen (MX) chain upon embedding in a crystalline environment depends upon alternation in the site-diagonal electron-lattice interaction parameter (epsilon(M)) and the strength of electron-electron interactions at the metal site (U(M)). The equilibrium geometry studies on isolated chains show that the MX chains tend to distort for alternating epsilon(M) and small U(M) values.