Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

Vitamin A and Bone Health: A Review on Current Evidence

Vitamin A is a fat-soluble micronutrient essential for growth, immunity, and good vision. The preformed retinol is commonly found in food of animal origin whereas provitamin A is derived from food of plant origin. This review summarises the current evidence from animal, human and cell-culture studies on the effects of vitamin A towards bone health. Animal studies showed that the negative effects of retinol on the skeleton were observed at higher concentrations, especially on the cortical bone. In humans, the direct relationship between vitamin A and poor bone health was more pronounced in individuals with obesity or vitamin D deficiency. Mechanistically, vitamin A differentially influenced the stages of osteogenesis by enhancing early osteoblastic differentiation and inhibiting bone mineralisation via retinoic acid receptor (RAR) signalling and modulation of osteocyte/osteoblast-related bone peptides. However, adequate vitamin A intake through food or supplements was shown to maintain healthy bones. Meanwhile, provitamin A (carotene and β-cryptoxanthin) may also protect bone. In vitro evidence showed that carotene and β-cryptoxanthin may serve as precursors for retinoids, specifically all-trans-retinoic acid, which serve as ligand for RARs to promote osteogenesis and suppressed nuclear factor-kappa B activation to inhibit the differentiation and maturation of osteoclasts. In conclusion, we suggest that both vitamin A and provitamin A may be potential bone-protecting agents, and more studies are warranted to support this hypothesis.     

Quantification of curium isotopes in environmental samples: drawbacks,speciation and specific tracer

Journal of Radioanalytical and Nuclear Chemistry - In this study, drawbacks of the use of 243Am as a tracer in order to quantify curium isotopes during a complex chemical procedure were...     

The reversible addition‐fragmentation chain transfer process and the strength and limitations of modeling: Comment on “the magnitude of the fragmentation rate coefficient”

There is appreciable uncertainty concerning the magnitude of the fragmentation rate coefficient of the intermediate radical in reversible addition‐fragmentation chain transfer (RAFT) polymerizations. A large proportion of the experimental and theoretical evidence suggests that it is a stable species with a lifetime longer than 0.0001 s. This is particularly the case when the intermediate macro‐RAFT radical is stabilized by a phenyl group attached to the radical center or has a poor leaving group. Although the occurrence to some extent of irreversible termination reactions cannot be excluded, we argue that such reactions are more likely to be a result of slow fragmentation of the intermediate macro‐RAFT radical. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2828–2832, 2003     

Shellable nonpure complexes and posets. II

This is a direct continuation of Shellable Nonpure Complexes and Posets. I, which appeared in Transactions of the American Mathematical Society 348 (1996), 1299-1327.

     

Étude thermochimique des sels de nickel et de cuivre de quelques amino-acides

Standard enthalpies of formation of:nickel and copper salts of several amino acids (α-alanine, valine, norvaline, isoleucine, norleucine);mixed complexes of copper:. Cu(α-alanine) (valine), Cu(α-alanine) (isoleucine), and Cu(valine) (isoleucine) are determined by calorimetry.The variations of enthalpy linked to the transformation of amminated and hydrated salts of the valine and of the isoleucine into the corresponding anhydrous compounds were determined by two different procedures: indirect calorimetry and differential calorimetric analysis.     

Enzymatically generated polyphenols as array-based metal-ion sensors

Phenolic polymers containing fluorescent reporter molecules were prepared via soybean hull peroxidase (SBP) catalysis, and used as metal-based sensor components. Specifically, we generated a variety of phenolic homopolymers containing p-cresol, p-phenylphenol, or p-methoxyphenol containing one of the two fluorophores: fluorescein and calcein. The fluorescence of the resulting polyphenols was dependent on the presence of metal ions, including Cu²⁺, Fe²⁺, Zn²⁺, Mn²⁺, and Fe³⁺. The fluorescence response was dependent on both, the metal ion and the phenolic monomer used in polyphenol synthesis, indicating that significant control over the selectivity and sensitivity of metal sensing can be achieved by varying the components that comprise the phenolic polymer. In some cases, metal-ion concentrations as low as 10 μM can be analyzed. The broad reactivity of SBP toward phenolic monomers enables the use of a wide array of structures and chemistries that can assist in metal-ion detection.     

Sur la synthèse d'alkoxy-2 nitro-3 2H-chroménes

2-Alkoxy-3-nitro-2H-chromenes have been obtained by the reaction of β-nitroacetaldehyde dialkyl acetals with some salicylic aldehydes in the presence of triethylamine p-toluene sulfonate.     

Glycosidic linkage rotations determine amylose stretching mechanism

The primary mechanism for relieving tensile strain in alpha-linked polysaccharides (e.g., [alpha-d-Glc-(1-->4)alpha-d-Glc]n) is achieved by complex rotations of the glycosidic linkages. This is shown through computer simulations of atomic force microscopy stretching experiments in combination with free energy calculations of the unfolding pattern for amylose. The experimental force-extension curve is reproduced and analyzed to reveal that the chair-to-boat conversions play a smaller role than previously proposed.     

Pyrolysis of mesoporous silica-immobilized 1,3-diphenylpropane. Impact of pore confinement and size

Mesoporous silicas such as SBA-15 and MCM-41 are being actively investigated for potential applications in catalysis, separations, and synthesis of nanostructured materials. A new method for functionalizing these mesoporous silicas with aromatic phenols is described. The resulting novel hybrid materials possess silyl aryl ether linkages to the silica surface that are thermally stable to ca. 550 degrees C, but can be easily cleaved at room temperature with aqueous base for quantitative recovery of the organic moieties. The materials have been characterized by nitrogen physisorption, FTIR, NMR, and quantitative analysis of surface coverages. The maximum densities of 1,3-diphenylpropane (DPP) molecules that could be grafted to the surface were less than those measured on a nonporous, fumed silica (Cabosil) and were also found to decrease as a function of decreasing pore size (5.6-1.7 nm). This is a consequence of steric congestion in the pores that is magnified at the smaller pore sizes, consistent with parallel studies conducted using a conventional silylating reagent, 1,1,3,3-tetramethyldisilazane. Pyrolysis of the silica-immobilized DPP revealed that pore confinement leads to enhanced rates and altered product selectivity for this free-radical reaction compared with the nonporous silica, and the rates and selectivities also depended on pore size. The influence of confinement is discussed in terms of enhanced encounter frequencies for bimolecular reaction steps and pore surface curvature that alters the accessibility and resultant selectivity for hydrogen transfer steps.