An ab-initio study of the role of lone pairs in the structure and insulator-metal transition in SnO and PbO

We have performed density functional calculations on tetragonal SnO and PbO (litharge) in the space group P4/nmm with the specific intention of examining the role played by Sn 5s and Pb 6s lone pairs in stabilizing the structure, and in giving rise to semi-metallic behavior (of SnO at ambient pressure and of PbO in the gamma phase). Use of the electron localization function has permitted real-space visualization of the lone pair in these structures. We also discuss the electronic structure of the orthorhombic PbO (massicot, space group Pbma) which again has localized lone pairs, contrary to some earlier expectation. (C) 2002 Editions scientifiques et medicales Elsevier SAS. All rights reserved.

Reaction of La2CuO4 with binary metal oxides in the solid state: Metathesis, addition, and redox metathesis pathways

Investigation of the reaction of La2CuO4 with several binary metal oxides in the solid state at elevated temperatures has revealed three different reaction pathways. Reaction of La2CuO4 with strongly acidic oxides such as Re2O7, MoO3, and V2O5 follows a metathesis route, yielding a mixture of products: La3ReO8/La2MoO6/LaVO4 and CuO. Oxides such as TiO2, MnO2, and RuO2 which are not so acidic yield addition products: La2CuMO6 (M = Ti, Mn, Ru). SnO2 is a special case which appears to follow a metathesis route, giving La2Sn2O7 pyrochlore and CuO, which on prolonged reaction transform to the layered perovskite La2CuSnO6. The reaction of La2CuO4 with lower valence oxides VO2 and MoO2, on the other hand, follows a novel redox metathesis route, yielding a mixture of LaVO4/LaCuO2 and La2MoO6/Cu, respectively. This result indicates that it is the redox reactivity involving V-IV + Cu-II --> V-V + Cu-I and Mo-IV + Cu-II --> Mo-VI + Cu-0, and not the acidity of the binary oxide, that controls the nature of the products formed in these cases. The general significance of these results toward the synthesis of complex metal oxides containing several metal atoms is discussed.

Lubricity of an oil in water emulsion in metal cutting: The effect of hydrophilic/lypophilic balance of emulsifiers

A steel disc is cut using a single point tool. The coefficient of friction of the nascent cut surface is measured by a spherical steel pin situated in close proximity of the point of cutting. The tool, disc and the friction pin are immersed in an oil in water emulsion bath during the experiment. The purpose of the experiments conducted here is to record the effect of hydrophilic/lypophilic balance (HLB) of the emulsifier on the lubricity experienced in the cutting operation. The more lypophilic emulsifiers were found to give greater lubricity than what is recorded when the emulsifier is more hydrophilic. XPS and FTIR spectroscopy are used to explore the tribofilm generated on the nascent cut surface to indicate a possible rationale for the effect. (C) 2011 Elsevier B.V. All rights reserved.

On the mechanism of metal nanoparticle synthesis in the brust-schiffrin method

Brust-Schiffrin synthesis (BSS) of metal nanoparticles has emerged as a major breakthrough in the field for its ability to produce highly stable thiol functionalized nanoparticles. In this work, we use a detailed population balance model to conclude that particle formation in BSS is controlled by a new synthesis route: continuous nucleation, growth, and capping of particles throughout the synthesis process. The new mechanism, quite different from the others known in the literature (classical LaMer mechanism, sequential nucleation-growth-capping, and thermodynamic mechanism), successfully explains key features of BSS, including size tuning by varying the amount of capping agent instead of the widely used approach of varying the amount of reducing agent. The new mechanism captures a large body of experimental observations quantitatively, including size tuning and only a marginal effect of the parameters otherwise known to affect particle synthesis sensitively. The new mechanism predicts that, in a constant synthesis environment, continuous nucleation-growth-capping mechanism leads to complete capping of particles (no more growth) at the same size, while the new ones are born continuously, in principle leading to synthesis of more monodisperse particles. This prediction is validated through new experimental measurements.

A composition-dependent ``re-entrant'' crystallographic phase transition in the substitutional metal acetylacetonate complex (Cr1-xGax)(acac)(3)

The formation of a complete solid solution between acetylacetonate (acac) complexes of chromium and gallium, (Cr1-x,Ga-x)(acac)(3) for 0.1 investigated through the co-synthesis method. Wellcrystallised, subliming solids are found to form for each nominal value of x, with thermal analysis confirming each composition to have a congruent melting point, making it a substitutional complex. Whereas the pure complexes (i.e. the end members of the solid solution, x = 0 and x = 1) are both centro-symmetric, a composition-dependent crystallographic phase transition to a non-centrosymmetric structure is found to occur for compositions with 0.4 < x < 0.9. Such a ``re-entrant'' crystallographic transition is interpreted to be due to the drive to overcome the disorder present in certain centrosymmetric chromium-rich compositions, by going over to a non-centrosymmetric structure with a doubling of the unit cell. The substitutional complex is shown to lead to a substitutional oxide with the beta-gallate structure. (C) 2014 Elsevier Ltd. All rights reserved.

Film thickness mediated transition in the kinetics of electric current induced flow of thin liquid metal films

Application of high electric-field between two points in a thin metallic film results in liquefaction and subsequent flow of the liquid-film from one electrode to another in a radially symmetric fashion. Here, we report the transition of the flow kinetics driven by the liquid film thickness varying from 3 to 100 nm. The mechanism of the flow behavior is observed to be independent of the film thickness; however, the kinetics of the flow depends on the film thickness and the applied voltage. An analytical model, incorporating viscosity and varying electrical resistivity with film thickness, is developed to explain the experimental observations. (C) 2014 AIP Publishing LLC.

Functionality preservation with enhanced mechanical integrity in the nanocomposites of the metal-organic framework, ZIF-8, with BN nanosheets

Metal-organic frameworks (MOFs) and boron nitride both possess novel properties, the former associated with microporosity and the latter with good mechanical properties. We have synthesized composites of the imidazolate based MOF, ZIF-8, and few-layer BN in order to see whether we can incorporate the properties of both these materials in the composites. The composites so prepared between BN nanosheets and ZIF-8 have compositions ZIF-1BN, ZIF-2BN, ZIF-3BN and similar to ZIF-4BN. The composites have been characterized by PXRD, TGA, XPS, electron microscopy, IR, Raman and solid state NMR spectroscopy. The composites possess good surface areas, the actual value decreasing only slightly with the increase in the BN content. The CO2 uptake remains nearly the same in the composites as in the parent ZIF-8. More importantly, the addition of BN markedly improves the mechanical properties of ZIF-8, a feature that is much desired in MOFs. Observation of microporous features along with improved mechanical properties in a MOF is indeed noteworthy. Such manipulation of properties can be profitably exploited in practical applications.

Differential response of cholesterol based pyrimidine systems with oxyethylene type spacers to gelation and mesogen formation in the presence of alkali metal ions

A new series of lipophilic cholesteryl derivatives of 2,4,6-trichloro-pyrimidine-5-carbaldehyde has been synthesized. Oxyethylene spacers of variable lengths were inserted between the hydrogen bonding promoting pyrimidine core and the cholesteryl tail in order to understand their effect on the selfassembly of these compounds. Only compound 1a with the shortest spacer formed a gel in organic solvents such as n-butanol and n-dodecane. While other members (1b and c) having longer spacers led to sol formation and precipitation in n-butanol and n-dodecane respectively. The self-assembly phenomena associated with the gelation process were investigated using temperature-dependent UVVis and CD-spectroscopy. The morphological features of the freeze-dried gels obtained from different organic solvents were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The solid phase behaviours of these molecules and their associated alkali metal ion complexes were explored using polarized optical microscopy (POM) and differential scanning calorimetry (DSC). The molecular arrangements in the xerogel and in the solid state were further probed using a wide-angle Xray diffraction (WAXD) technique. Analysis of the wide-angle X-ray diffraction data reveals that this class of molecules adopts a hexagonal columnar organization in the gel and in the solid state. Each slice of these hexagonal columnar structures is composed of a dimeric molecular-assembly as a building block. Significant changes in the conformation of the oxyethylene chains could be triggered via the coordination of selected alkali metal ions. This led to the production of interesting metal ion promoted mesogenic behaviour.

Design Optimization of Thin-Film Transistors Based on a Metal-Substrate-Semiconductor Architecture for High DC Voltage Sensing

We discuss the potential application of high dc voltage sensing using thin-film transistors (TFTs) on flexible substrates. High voltage sensing has potential applications for power transmission instrumentation. For this, we consider a gate metal-substrate-semiconductor architecture for TFTs. In this architecture, the flexible substrate not only provides mechanical support but also plays the role of the gate dielectric of the TFT. Hence, the thickness of the substrate needs to be optimized for maximizing transconductance, minimizing mechanical stress, and minimizing gate leakage currents. We discuss this optimization, and develop n-type and p-type organic TFTs using polyvinyldene fluoride as the substrate-gate insulator. Circuits are also realized to achieve level shifting, amplification, and high drain voltage operation.

New insight into the architecture of oxy-anion pocket in unliganded conformation of GAT domains: A MD-simulation study

Human Guanine Monophosphate Synthetase (hGMPS) converts XMP to GMP, and acts as a bifunctional enzyme with N-terminal ``glutaminase'' (GAT) and C-terminal ``synthetase'' domain. The enzyme is identified as a potential target for anticancer and immunosuppressive therapies. GAT domain of enzyme plays central role in metabolism, and contains conserved catalytic residues Cys104, His190, and Glu192. MD simulation studies on GAT domain suggest that position of oxyanion in unliganded conformation is occupied by one conserved water molecule (W1), which also stabilizes that pocket. This position is occupied by a negatively charged atom of the substrate or ligand in ligand bound crystal structures. In fact, MD simulation study of Ser75 to Val indicates that W1 conserved water molecule is stabilized by Ser75, while Thr152, and His190 also act as anchor residues to maintain appropriate architecture of oxyanion pocket through water mediated H-bond interactions. Possibly, four conserved water molecules stabilize oxyanion hole in unliganded state, but they vacate these positions when the enzyme (hGMPS)-substrate complex is formed. Thus this study not only reveals functionally important role of conserved water molecules in GAT domain, but also highlights essential role of other non-catalytic residues such as Ser75 and Thr152 in this enzymatic domain. The results from this computational study could be of interest to experimental community and provide a testable hypothesis for experimental validation. Conserved sites of water molecules near and at oxyanion hole highlight structural importance of water molecules and suggest a rethink of the conventional definition of chemical geometry of inhibitor binding site.

Particulate Size Effects in the Particle-Reinforced Metal-Matrix Composites

The influences of I,article size on the mechanical properties of the particulate metal matrix composite;are obviously displayed in the experimental observations. However, the phenomenon can not be predicted directly using the conventional elastic-plastic theory. It is because that no length scale parameters are involved in the conventional theory. In the present research, using the strain gradient plasticity theory, a systematic research of the particle size effect in the particulate metal matrix composite is carried out. The roles of many composite factors, such as: the particle size, the Young's modulus of the particle, the particle aspect ratio and volume fraction, as well as the plastic strain hardening exponent of the matrix material, are studied in detail. In order to obtain a general understanding for the composite behavior, two kinds of particle shapes, ellipsoid and cylinder, are considered to check the strength dependence of the smooth or non-smooth particle surface. Finally, the prediction results will be applied to the several experiments about the ceramic particle-reinforced metal-matrix composites. The material length scale parameter is predicted.