Steroids and related natural products. VII. Boron trifluoride etherate-lithium aluminum hydride reduction of smilagenin acetate

Reduction of smilagenin acetate (Va) using a boron trifluoride etherate-lithium aluminum hydride reagent, followed by hydrogen peroxide oxidation and acetylation, was found to yield: 3β-ethoxysmilagenin (Vb), 3β-ethoxydihydrosmilagenin acetate (VIa), dihydrosmilagenin diacetate (VIb), and a complex mixture of partially acetylated products. Similar reaction conditions were employed to convert dihydrodiosgenin (II) to dihydrochlorogenin (III). Boron trifluoride etherate-lithium aluminum hydride reduction of 3β-acetoxy-5α-cholestane and 3β-acetoxy-5α-lanostane (VIIIa) was shown to yield the corresponding 3β-ethoxy (e.g., VIIIb) derivatives.

Novel Nanocomposites Made of Boron Nitride Nanotubes and a Physical Gel

This article describes successful incorporation of multiwalled boron nitride nanotubes (BNNTs) and various functionalized BNNTs by Lewis bases such as trioctylamine (TOA), tributylamine (TBA), and triphenylphosphine (TPP), etc., in organogels formed by triphenylenevinylene (TPV)-based low molecular weight gelator (LMWG) in toluene and consequent characterization of the resulting gel nanocomposites. Functionalized BNNTs were synthesized first,and the presence of tubular structures with high aspect ratio and increased diameter compared to the starting BNNTs was confirmed by SEM. TEM, and Raman spectroscopy. The micrographs of composites of I and BNNTs showed evidence of wrapping of the gelator molecules on to the BNNT surface presumably brought about by pi-pi stacking and van der Waals interactions, This leads to the formation of densely packed and directionally aligned fibrous networks. Such ``reinforced'' aggregation of the gelator molecules in presence of doped BNNTs led to an increase in the sot-to-gel transition temperature and the solidification temperature of the gel nanocomposites as revealed from differential scanning calorimetry. Rheological investigations of the gel nanocomposites indicate that the flow properties of the resulting materials become resistant to applied stress upon incorporation of even a very low wt % of BNNTs. Finally, the increase in thermal conductivity of the nanocomposite compared to the gelator alone was observed for the temperature range of 0-60 degrees C which may make these composites potentially useful in various applications depending on the choice and the amount of BNNT loading in the composite.

Improved procedures for the Beckmann rearrangement: the reaction of ketoxime carbonates with boron trifluoride etherate

A variety of ketoxime ethyl carbonates-easily prepared from the oximes and ethyl chloroformate-undergo the Beckmann rearrangement upon treatment with 1 equivalent of boron trifluoride etherate, in dichloromethane solution at room temperature in excellent yields (generally 75-99%). (C) 2000 Elsevier Science Ltd. All rights reserved.

Improved procedures for the Beckmann rearrangement: the reaction of ketoxime carbonates with boron trifluoride etherate

A variety of ketoxime ethyl carbonates-easily prepared from the oximes and ethyl chloroformate-undergo the Beckmann rearrangement upon treatment with 1 equivalent of boron trifluoride etherate, in dichloromethane solution at room temperature in excellent yields (generally 75-99%). (C) 2000 Elsevier Science Ltd. All rights reserved.

Electrochemical reduction of oxygen on gold and boron-doped diamond electrodes in ambient temperature, molten acetamide-urea-ammonium nitrate eutectic melt

The electrochemical reduction of oxygen has been studied on gold, boron-doped diamond (BDD) and glassy carbon (GC) electrodes in a ternary eutectic mixture of acetamide (CH3CONH2), urea (NH2CONH2) and ammonium nitrate (NH4NO3). Cyclic voltammetry (CV), differential pulse voltammetry (DPV), chronoamperometry and rotating disk electrode (RDE) voltammetry techniques have been employed to follow oxygen reduction reaction (ORR). The mechanism for the electrochemical reduction of oxygen on polycrystalline gold involves 2-step. 2-electron pathways of O-2 to H2O2 and further reduction of H2O2 to H2O. The first 2-electron reduction of O-2 to H2O2 passes through superoxide intermediate by 1-electron reduction of oxygen. Kinetic results suggest that the initial 1-electron reduction of oxygen to HO2 is the rate-determining step of ORR on gold surfaces. The chronoamperometric and ROE studies show a potential dependent change in the number of electrons on gold electrode. The oxygen reduction reaction on boron-doped diamond (BOO) seems to proceed via a direct 4-electron process. The reduction of oxygen on the glassy carbon (GC) electrode is a single step, irreversible, diffusion limited 2-electron reduction process to peroxide. (C) 2010 Elsevier Ltd. All rights reserved.

High temperature deformation processing maps for boron modified Ti-6Al-4V alloys

The alloy, Ti-6Al-4V is an alpha + beta Ti alloy that has large prior beta grain size (similar to 2 mm) in the as cast state. Minor addition of B (about 0.1 wt.%) to it refines the grain size significantly as well as produces in-situ TiB needles. The role played by these microstructural modifications on high temperature deformation processing maps of B-modified Ti64 alloys is examined in this paper.Power dissipation efficiency and instability maps have been generated within the temperature range of 750-1000 degrees C and strain rate range of 10(-3)-10(+1) s(-1). Various deformation mechanisms, which operate in different temperature-strain rate regimes, were identified with the aid of the maps and complementary microstructural analysis of the deformed specimens. Results indicate four distinct deformation domains within the range of experimental conditions examined, with the combination of 900-1000 degrees C and 10(-3)-10(-2) s(-1) being the optimum for hot working. In that zone, dynamic globularization of alpha laths is the principle deformation mechanism. The marked reduction in the prior beta grain size, achieved with the addition of B, does not appear to alter this domain markedly. The other domains, with negative values of instability parameter, show undesirable microstructural features such as extensive kinking/bending of alpha laths and breaking of beta laths for Ti64-0.0B as well as generation of voids and cracks in the matrix and TiB needles in the B-modified alloys. (C) 2010 Elsevier B.V. All rights reserved.

29Si NMR chemical shifts in silicates

29Si chemical shifts in a wide variety of silicates in crystalline, glassy and gel states have been related to a parameter, P, which takes into account the electronegativity and the structural description of the silicate units as well as the ionic potential of the modifier cation. The relation, δ(ppm)=28.4 [1−exp(−P)]−110.5, besides having predictive value, satisfactorily accounts for all the available chemical-shifts data on silicates and shows the right kind of limiting behaviour, with δ approaching the Q0 value at large P.

On characterization of deformation microstructure in Boron modified Ti-6Al-4V alloy

Addition of boron in small quantities to various titanium alloys have shown significant improvement in mechanical behavior of materials. In the present study, electron back-scatter diffraction (EBSD) techniques have been applied to investigate the deformation microstructure evolution in boron modified two-phase titanium alloy Ti-6Al-4V. The alloy was hot compressed at 750 degrees C up to 50% height reduction at two different strain rates (10(-3) s(-1) and 1 s(-1)). The EBSD analyses indicated significant differences in deformed microstructure of the base alloy and the alloy containing boron. A strong subgrain formation tendency was observed along with inhomogeneous distribution of dislocations inside large a colonies of Ti64. In contrast, a colonies were relatively strain free for Ti64 + B, with more uniform dislocation density distribution. The observed difference is attributed to microstructural modifications viz, grain size refinement and presence of TiB particles at grain boundary produced due to boron addition. (C) 2010 Elsevier B.V. All rights reserved.

X-ray diffraction line profile analysis of deformation microstructure in boron modified Ti-6Al-4V alloy

X-ray diffraction line profile analysis (XRDLPA) techniques have been applied to investigate the deformed microstructure of a recently developed boron modified two-phase titanium alloy Ti-6Al-4V. The alloy was hot compressed at 750 degrees C up to 50% height reduction at two different strain rates (10(-3) S-1 and 1 S-1). Microstructural parameters like average domain size, average microstrain within the domain and dislocation density of the two phases were determined using X-ray diffraction line profile analysis. The results indicate an increase in the microstrain and dislocation density for the alpha-phase and decrease for the beta-phase in the case of boron modified alloys as compared to the normal material. Microstructural modifications viz, the grain refinement and the presence of hard, brittle TiB particles in the case of boron modified alloy are held responsible for the observed difference in the dislocation density. (C) 2010 Elsevier Inc. All rights reserved.

Kinetics of oxidation of boron powder

The kinetics of the oxidation of electrodeposited boron powder and the boron powder produced by the reduction process were studied using thermogravimetry (TG). The oxidation was carried out by heating boron powder in a stream of oxygen. Both isothermal and non-isothermal methods were used to study the kinetics. Model-free isoconversional method was used to derive the kinetics parameters. A two step oxidation reaction (exothermic) was observed. The oxidation reaction could not be completed due to the formation of glassy layer of boric oxide on the surface of boron powder which acts as a barrier for further diffusion of oxygen into the particle. The activation energy obtained using model-free method for electrodeposited boron is 122 +/- 7 kJ mol(-1) whereas a value of 205 +/- 9 kJ mol(-1) was obtained for boron produced by the reduction process (commercially procured boron). Mechanistic interpretation of the oxidation reaction was done using model based method. The activation energy was found to depend on the size distribution of the particles and specific surface area of the powder. (C) 2010 Elsevier B.V. All rights reserved.

Edge Stabilities of Hexagonal Boron Nitride Nanoribbons: A First-Principles Study

We investigate the comparative stability of sp(2) bonded planar hexagonal boron nitride (h-BN) nanoribbon (BNNR) edges, using first principles calculations. We find that the pristine armchair edges have the highest degree of stability. Pristine zigzag edges are metastable, favoring planar reconstructions in the form of 5-7 rings] that minimizes the energy. Our investigation further reveals that the pristine zigzag edges can be stabilized against 5-7 reconstructions by passivating the dangling bonds at the edges by other elements, such as hydrogen (H) atoms. Electronic and magnetic properties of nanoribbons depend on the edge shapes and are strongly affected by edge reconstructions.

Texture Evolution in Boron Modified Ti-6Al-4V Alloy

Owing to their high strength-to-weight ratio, excellent mechanical properties and corrosion resistance, titanium (Ti) and its alloys, especially (alpha+beta) alloys like Ti-6Al-4V is the backbone materials for aerospace, energy, and chemical industries. Trace boron addition (similar to 0.1 wt. %) to the alloy Ti-6Al-4V produces a reduction in as-cast grain size by roughly an order of magnitude resulting in enhanced ductility, higher stiffness, strength and good fracture resistance. Boron addition could also affect the evolution of texture and microstructure in the material. The solidification microstructures of Boron free as well as Boron containing Ti-6Al-4V are found to be almost homogeneous from periphery towards the center of as-cast ingot in terms of both alpha-colony size and distribution. Boron addition substantially reduces alpha-colony size (similar to 50-80 mu m). A gradual change in alpha texture from periphery towards the center has been observed with orientations close to specific texture components suggesting the formation of texture zones. The mechanism of texture evolution can be visualized as a result of variant selection during solidification through (alpha+beta) phase field.

Quantum Dots and Nanoroads of Graphene Embedded in Hexagonal Boron Nitride

The quest for novel two-dimensional materials has led to the discovery of hybrids where graphene and hexagonal boron nitride (h-BN) occur as phase-separated domains. Using first-principles calculations, we study the energetics and electronic and magnetic properties of such hybrids in detail. The formation energy of quantum dot inclusions (consisting of n carbon atoms) varies as 1/root n, owing to the interface. The electronic gap between the occupied and unoccupied energy levels of quantum dots is also inversely proportional to the length scale, 1/root n-a feature of confined Dirac fermions. For zigzag nanoroads, a combination of the intrinsic electric field caused by the polarity of the h-BN matrix and spin polarization at the edges results in half-metallicity; a band gap opens up under the externally applied ``compensating'' electric field. For armchair nanoroads, the electron confinement opens the gap, different among three subfamilies due to different bond length relaxations at the interfaces, and decreasing with the width.

Reactivation kinetics of boron acceptors in hydrogenated silicon during zero bias anneal

The reactivation kinetics of passivated boron accepters in hydrogenated silicon during zero bias annealing in the temperature range of 65-130 degrees C are reported, For large annealing times and high annealing temperatures, the reactivation process follows second-order kinetics and is rate limited by a thermally activated <(H)over tilde (2)> complex formation process, For short annealing times and low annealing temperatures, the reactivation rate is found to be larger than that due to <(H)over tilde (2)> complex formation alone. We conclude that the faster reactivation is caused by the diffusion of the liberated hydrogen atoms into the bulk as well as <(H)over tilde (2)> complex formation. The effective diffusion coefficient of hydrogen is measured and found to obey the Arrhenius relation with an activation energy (1.41 +/- 0.1) eV. (C) 1997 American Institute of Physics.