Unusual metallic-like transport near the percolation threshold

In multiwall carbon nanotube (MWNT)-polystyrene (PS) composites, a weak temperature dependence of conductivity has been observed at a percolation threshold of 0.4 wt %. The power law [sigma(T)proportional to T-0.3] behavior indicates metallic-like behavior, unlike the usual activated transport for systems near the percolation threshold. The low field positive magnetoconductance follows H-2 dependence, due to the weak localization in disordered metallic systems. The marginal metallic nature of MWNT-PS at percolation threshold is further verified from the negligible frequency dependence of conductivity, in the temperature range of 300 to 5 K. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3455895]

Sol-Gel Synthesis, characterization and optical properties of Tio2 thin films deposited on ITO/Glass substrates

TiO2 thin films have been deposited on glass and indium tin oxide (ITO) coated glass substrates by sol-gel technique. the influence of annealing temperature on the structural , morphological and optical properties has been examined. X-ray diffraction (XRD) results reveal the amorphous nature of the as-deposited film whereas the annealed films are found to be in the crystalline anatase phase. The surface morphology of the films at different annealing temperatures has been examined by atomic force microscopy (AFM). The in situ surface morphology of the as-deposited and annealed TiO2 films has also been examined by optical polaromicrograph (OPM). TiO2 films infatuated different structural and surface features with variation of annealing temperature. The optical studies on these films suggest their possible usage in sun-shielding applications.

Structural and optical properties of (100-x)Li2B4O7 center dot x(Ba5Li2Ti2Nb8O30) glasses and glass nanocrystal composites

Optically clear glasses of various compositions in the system (100-x)Li2B4O7 center dot x(Ba5Li2Ti2Nb8O30) (5 <= x <= 20, in molar ratio) were fabricated by splat quenching technique. Controlled heat-treatment of the as-quenched glasses at 500 degrees C for 8 h yielded nanocrystallites embedded in the glass matrix. High Resolution Transmission Electron Microscopy (HRTEM) of these samples established the composition of the nano-crystallites to be that of Ba5Li2Ti2Nb8O30. B-11 NMR studies revealed the transformation of BO4 structural units into BO3 units owing to the increase in TiO6 and NbO6 structural units as the composition of Ba5Li2Ti2Nb8O30 increased in the glass. This, in turn, resulted in an increase in the density of the glasses. The influence of the nominal composition of the glasses and glass nanocrystal composites on optical band gap (E-opt), Urbach energy (Delta E), refractive index (n), molar refraction (R-m), optical polarizability (alpha(m)) and third order non-linear optical susceptibility (chi(3)) were studied.

The fracture toughness of bulk metallic glasses

Stiffness, strength, and toughness are the three primary attributes of a material, in terms of its mechanical properties. Bulk metallic glasses (BMGs) are known to exhibit elastic moduli at a fraction lower than crystalline alloys and have extraordinary strength. However, the reported values of fracture toughness of BMGs are highly variable; some BMGs such as the Zr-based ones have toughness values that are comparable to some high strength steels and titanium alloys, whereas there are also BMGs that are almost as brittle as silicate glasses. Invariably, monolithic BMGs exhibit no or low crack growth resistance and tend to become brittle upon structural relaxation. Despite its critical importance for the use of BMGs as structural materials, the fracture toughness of BMGs is relatively poorly understood. In this paper, we review the available literature to summarize the current understanding of the mechanics and micromechanisms of BMG toughness and highlight the needs for future research in this important area.

Analysis of Dynamic Heterogeneity in a Glass Former from the Spatial Correlations of Mobility

The growth of characteristic length scales associated with dynamic heterogeneity in glass-forming liquids is investigated in an extensive computational study of a four-point, time-dependent structure factor defined from spatial correlations of mobility, for a model liquid for system sizes extending up to 351 232 particles, in constant-energy and constant-temperature ensembles. Our estimates for dynamic correlation lengths and susceptibilities are consistent with previous results from finite size scaling. We find scaling exponents that are inconsistent with predictions from inhomogeneous mode coupling theory and a recent simulation confirmation of these predictions.

Hexagonally ordered arrays of metallic nanodots from thin films of functional block copolymers

We demonstrate a new and simple route to fabricate highly dense arrays of hexagonally close packed inorganic nanodots using functional diblock copolymer (PS-b-P4VP) thin films. The deposition of pre-synthesized inorganic nanoparticles selectively into the P4VP domains of PS-b-P4VP thin films, followed by removal of the polymer, led to highly ordered metallic patterns identical to the order of the starting thin film. Examples of Au, Pt and Pd nanodot arrays are presented. The affinity of the different metal nanoparticles towards P4VP chains is also understood by extending this approach to PS-b-P4VP micellar thin films. The procedure used here is simple, eco-friendly, and compatible with the existing silicon-based technology. Also the method could be applied to various other block copolymer morphologies for generating 1-dimensional (1D) and 2-dimensional (2D) structures. (c) 2010 Elsevier Ltd. All rights reserved.

Temperature and pressure dependence of direct current electrical resistivity in the Na2O-ZnO-B2O3 glass system

Two series of glasses were prepared, xNa2O, yZnO, 100 - x - yB2O3 and 30 - xNa2O, xZnO, 70B2O3 (mol%). The temperature dependence of the direct current resistivity was measured from room temperature to about 700 K and in both series of glasses we observed a simple Arrhenius type of temperature dependence. However, the resistivity of the binary alkali glass increased steeply by as much as two orders of magnitude with the addition of even a small quantity of ZnO and remained virtually unaffected by further addition of ZnO. The resistivity decreases gradually with increasing pressure in Na2O-B2O3 but increases with increasing pressure with the addition of ZnO.

Photoacoustic Investigation of Glass Transition in AsxTe1-x Glasses

Photoacoustic (PA) technique is used to study glass transition and temperature dependence of thermal diffusivity in AsxTe1-x glasses with 0.25 0.60. PA amplitude goes through a minimum and the phase shows a maximum at glass transition temperature Tg. The variation of thermal diffusivity with temperature shows sharp decrease near Tg. The variation of thermal diffusivity with composition shows maximum at = 0.40 for all temperatures T Tg.

Evolution of magnetic order in the system LaNi1-xMnxO3 (x < 0.10)

Magnetic susceptibility measurements on dilute solid-solutions LaNi1-xMnxO3 (x < 0.1) have been carried out. With increasing x the magnetic susceptibility behaviour changes from Pauli paramagnetic to Curie-Weiss type. The temperature coefficient of resistance (TCR) changes sign around x = 0.03 but the system seems to be metallic in terms of showing a finite extrapolated conductivity at 0 K even when x = 0.10. The x = 0.10 system shows indications of spin-glass like behaviour.

A series of metallic oxides of the formula La3LnBaCu5O13+δ (Ln = rare earth or Y)

Oxides of the formula La3LnBaCu5O13+δ (Ln = Nd, Sm, Gd, Dy, or Y) exhibiting metallic resistivity have been prepared and characterized. In the case of yttrium, a composition close to La2Y2BaCu5O13+δ, which is also metallic, could be prepared.

Effect of prestrain on fatigue crack propagation in mild steel containing non-metallic inclusions

The effect of tensile prestrain on fatigue crack propagation behaviour of commercial mild steel with significant amount of stringer inclusions has been studied. In prestrained materials the usual stable stage II crack growth region is preceded by a phase wherein a retardation in crack growth rate occurs. No such behaviour is observed in annealed material. The amount of retardation is found to increase with increase in prestrain. A mechanism for the observed retardation in crack growth rate is also presented.

Evidence for double valence fluctuation in metallic oxides of lead

The possible role of double valence fluctuation in both lead and oxide ions with reference to metallization in oxides of lead is examined by x-ray-photoemission spectroscopy, ultraviolet-photoemission spectroscopy (UPS), and 207Pb NMR studies. The double valence fluctuations may be viewed as Pb4++2O2-⇄Pb2+O22-. While the insulating oxides PbO, Pb3O4, and Sr2PbO4 show a single oxide ion, O2- characterized by O(1s) at 529.7 eV, the insulating peroxide BaO2 is characterized by the ion O22- with a single O(1s) at 533 eV. The metallic PbO2, BaPbO3, BaBiPbO3, and SrPbO3 showed the occurrence of both O2- and O22- ions. The valence band in these compounds has also been studied by UPS, and clear evidence for the coexistence of O2- and O22- is seen in PbO2. A simultaneous study of 207Pb NMR suggests that the Pb ion could also exist in mixed-valence states. Qualitative arguments are presented to rationalize the existence of such mixed valences of the anion in metal oxides in general and their role in superconductivity.

A computer study of the miscibility limits of Pd-Cu-Si

Metallic glasses are of interest because of their mechanical properties. They are ductile as well as brittle. This is true of Pd77.5Cu6Si16.5, a ternary glassy alloy. Actually, the most stable metallic glasses are those which are alloys of noble or transition metals A general formula is postulated as T70–80G30-20where T stands for one or several 3d transition elements, and includes the metalloid glass formers. Another general formula is A3B to A5B where B is a metalloid. A computer method utilising the MIGAP computer program of Kaufman is used to calculate the miscibility gap over a range of temperatures. The precipitation of a secondary crystalline phase is postulated around 1500K. This could produce a dispersed phase composite with interesting high temperature-strength properties.

Lack of Cu3+, Pb4+, and Bi5+ ions in metallic and superconducting oxides

With a view to understand the oxidation states of metal ions involved in oxide superconductors, oxidation behavior of Cu, Pb, and Bi metals have been studiedi employing x-ray-photoemission and ultraviolet-photoemission spectroscopy. Pb and Bi have distinct 6p (0 to 4 eV) and 6s (7.5 to 10 eV) bands and upon oxidation, only the 6p electrons are ionized forming PbO and Bi2O3 with the simultaneous development of the O2-(2p) band (3 to 7 eV). We show that the 6s band of metals lies below the O2-(2p) band, and hence 6s electrons cannot be ionized to form Pb4+ and Bi4+ as expected in PbO2, BaPbO3, BaBiO3, and BaPb0.75Bi0.25O3. Instead these oxides are stabilized with lower valent O22- along with O2- ions with metals remaining in +2 and +3 states. Similarly, it is shown that the Cu2+(3d) band overlaps with the O2-(2p) band in the YBa2Cu3O6.95 completely and the excess oxygen can be stabilized through lower valent oxide ions instead of Cu3+.