Critical phenomena in electrical resistance of binary liquids: A study of the frequency dependence

The electrical resistance is measured in two binary liquid systems CS2 + CH3NO2 and n-C7H16 + CH3OH in the critical region as a function of frequency from 10 Hz to 100 kHz. The critical exponent b ≈ 0.35 in the singularity of dR/dT α (T - Tc)−b near Tc has no appreciable dependence upon the frequency. Thus any contribution from dielectric dispersion to the critical resistivity is not appreciable. The universal behaviour of the dR/dT anomaly does not seem to be followed in binary liquid systems.

Orientation dependence of the indentation impression morphology in a Mg alloy

The formation of anomalous indentations, with two opposite faces describing a pin-cushion effect and the other two faces normal, in long elongated grains of an extruded Mg-2Al-1Zn alloy is reported. Subsurface microstructural observations combined with Schmid factor calculations suggest that extension twinning accompanied by basal slip are the reasons for these. Johnson's expanding cavity model is invoked for further substantiation. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Texture and grain-size dependence of tensile properties in warm rolled cadmium

The development of crystallographic texture and the change in the grain size during warm rolling (300 deg K) and their effect on the tensile yield strength at 77 and 300 deg K are studied in 99.9% pure Cd. Both longitudinal and transverse specimens are tested. The yield strength obeys the Hall--Petch relation. The Hall--Petch slope, k, is lower and the intercept sigma o is higher in the warm worked material in comparison with the corresponding values for annealed Cd. The differences are attributed to the change in 1013 < and 0001 textures that are developed during warm rolling.26 refs.--AA

Compositional dependence of crystallization of As-Te glasses

Thermal crystallization studies have been carried out on bulk, semiconducting AsxTe100−x glasses of different compositions using Differential Scanning Caloritmery. AsxTe100−x glasses with x < 40, are found to exhibit one glass transition and one crystallization. On the other hand, glasses with composition 40 less-than-or-equals, slantxless-than-or-equals, slant 50 show one glass transition and two crystallization reactions. It has been found that in glasses with x greater-or-equal, slanted 40, the two crystallization reactions progressively merge with an increase in arsenic concentration. Consequently AsxTe100−x glasses with x greater-or-equal, slanted 50 show only one crystallization. The composition dependence of crystallization temperatures and activation energies for crystallization estimated by Kissinger's method, show marked deviations at a composition x = 40. These observations can be explained in terms of the changes in the local structure of the material with composition.

Anomalous behaviour in the composition dependence of the photoacoustic properties of Si---As---Te glasses

The optical bandgap and thermal diffusivity of Si10AsxTe90−x (10 ≤ x ≤ 50) and Si15AsxTe85−x (5 ≤ x ≤ 40) glasses have been measured using the photoacoustic technique. The anomalous behaviour observed in these properties at the mean coordination number left angle bracketrright-pointing angle bracket = 2.60 is interpreted by reference to the formation and development of a layered structure in these glasses.

On The Temperature-Dependence of Regioselectivity in the Amination of Bromobenzotropones

The ipso/cine ratio in the amination of 5-bromo-2,3-benzo- or 2-bromo-4,5-benzotropone shows a dependence upon the temperature at which the reaction is conducted, changing in favour of the ipso-product when the temperature is maintained high, ruling out an aryne-type mechanism. A comparison of independent mechanisms envisaged for the formation of the two isomeric products suggests a two-part reason: (i) at a higher reaction temperature, C-protonation, a step necessary for the formation of the cine-product, could be retarded when a direct internal mode is interfered with by a less efficient external one, and (ii) reketonisation by elimination of bromide, needed to form the ipso-product, is likely to have a high temperature coefficient enabling the rate of its formation to overtake that of the cine-product.

Temperature dependence of hydrogen evolution reaction on nickel oxide electrode in sealed nickel/cadmium cells

The impedance of sealed nickel/cadmium cells around a cell e.m.f. of 0.0 V was measured at five different temperatures between � 10 and +30 °C. The results show that the behaviour is similar at all temperatures. Based on the experimental results, the relation between charge-transfer resistance (Rct) and temperature (T) has been established for the Volmer reaction. Further, the value of cathodic transfer coefficient (?) has been estimated.

Size dependence of the bulk modulus of semiconductor nanocrystals from first-principles calculations

The variation in the bulk modulus of semiconductor nanoparticles has been studied within first-principles electronic-structure calculations using the local density approximation (LDA) for the exchange correlation. Quantum Monte Carlo calculations carried out for a silicon nanocrystal Si87H76 provided reasonable agreement with the LDA results. An enhancement was observed in the bulk modulus as the size of the nanoparticle was decreased, with modest enhancements being predicted for the largest nanoparticles studied here, a size just accessible in experiments. To access larger sizes, we fit our calculated bulk moduli to the same empirical law for all materials, the asymptote of which is the bulk value of the modulus. This was found to be within 2-10% of the independently calculated value. The origin of the enhancement has been discussed in terms of Cohen's empirical law M.L. Cohen, Phys. Rev. B 32, 7988 (1985)] as well as other possible scenarios.

Dependence of magnetism on GdFeO3 distortion in the t(2g) system ARuO(3) (A = Sr, Ca)

We have examined the stability of the ferromagnetic (FM) state in CaRuO3 and SrRuO3 as a function of the GdFeO3 distortion. Model calculations predict the dependence of the FM transition temperature (T-c) on the rotation angle theta to vary as cos(2)(2 theta) for e(g)-electron systems. However, here, we find an initial increase and then the expected decrease. Furthermore, a much faster decrease is found than predicted for e(g)-electron systems. Considering the specific case of CaRuO3, a larger deviation of the Ru-O-Ru angle from 180 degrees in CaRuO3 as compared to SrRuO3 should result in a more reduced bandwidth, thereby making the former more correlated. The absence of long-range magnetic order in the more correlated CaRuO3 is traced to the strong collapse of various exchange interaction strengths that arises primarily from the volume reduction and increased distortion of the RuO6 octahedra network that accompanies the presence of a smaller ion at the A site.

Improved mechanical properties of polymer nanocomposites incorporating graphene-like BN: Dependence on the number of BN layers

The mechanical properties of composites of polymethylmethacrylate (PMMA) with two-dimensional graphene-like boron nitride (BN) have been investigated to explore the dependence of the properties on the number of BN layers. This study demonstrates that significantly improved mechanical properties are exhibited by the composite with the fewest number of BN layers. Thus, with incorporation of three BN layers, the hardness and elastic modulus of the composite showed an increase of 125% and 130%, respectively, relative to pure PMMA. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Importance of orientational rearrangement during vitrification of hydrocarbons: dependence on molecular shape

On the basis of Monte Carlo calculations of 2,2-dimethylpropane (neopentane), n-pentane, and 2,2-dimethylbutane (neohexane) at several temperatures, thermodynamic properties and radial distribution functions as well as dimerization and bonding energy distribution functions are reported for both liquid and glassy states. Changes in the radial distribution functions on cooling depend on whether the groups are accessible (peripheral) or inaccessible. Peaks in the radial distribution functions corresponding to peripheral groups do not shift to lower distances on cooling and at times display a large increase in the intensity of the first peak. The peaks due to inaccessible groups, on the other hand, shift to lower distances on cooling. The magnitude of the reorientational contribution in determining the resulting structure of the glass is estimated for the different hydrocarbon molecules investigated. The reorientational contribution is highest for neopentane (26%) followed by isopentane (24%), neohexane (22%), and n-pentane (0%). It appears that molecular geometry has an important role in determining the magnitude of the reorientational contribution to the structure of the glass.

Dependence of the strength of interaction of carbon monoxide with transition metal clusters on the cluster size

The interaction of CO with Cu, Pd, and Ni at different coverages of the metals on solid substrates has been investigated by He II and core-level spectroscopies, after the nature of variation of the metal core-level binding energies with the coverage or the cluster size is established. The separation between the (1 pi + 5 sigma) and 4 sigma levels of CO increases with a decrease in the size of the metal clusters, accompanied by an increase in the desorption temperature. In the case of Cu, the intramolecular shakeup satellite of CO disappears on small clusters. More importantly, CO dissociates on small Ni clusters, clearly confirming that metal-CO interaction strength increases with a decrease in the cluster size.

Diffusion of sorbates in zeolites Y and A novel dependence on sorbate size and strength of sor ba t e-Zeoli t e interaction

A systematic investigation of monatomic spherical sorbates in the supercages of zeolites Y and A by molecular dynamics technique is presented. Rates of intercage diffusion, rates of cage visits, and the diffusion coefficients have been calculated as a function of the sorbate-zeolite interaction strength. These properties exhibit markedly different dependences on interaction strength for the two zeolites. The observed behavior is shown to be a consequence of the two principal mechanisms of intercage diffusion and the energetic barrier associated with them. The diffusion coefficient and other properties associated with intercage diffusion are found to be directly proportional to the reciprocal of the square of the sorbate diameter when the sorbate diameter is significantly smaller than the window diameter. As the sorbate diameter increases, a peak is observed in all the transport properties investigated including the diffusion coefficient. We call this surprising effect as the ring or levitation effect and it explains several anomalous results reported in the literature and suggests a breakdown of the geometrical criterion for diffusion of sorbates. It shows that under certain conditions nongeometrical factors play a major role and geometrical factors become secondary in the determination of the molecular sieve property. A generalized parameter has been proposed which suggests conditions under which one can expect the ring or levitation effect in any porous medium. Inverse size selectivity becomes operative under these conditions.