Anomalous temperature dependence of resistance observed for high TC Y-Ba-Cu-O thin films

Anomalies in the resistance versus temperature plots of some Y-Ba-Cu-O sputtered thin films have been observed. Compositional variations over some portion of the film are found responsible for the appearance of the anomalies. The need for exercising caution in the interpretation of such anomalies is emphasized.     

Transition Metal Chemistry of Main Group Hydrazides. 17. Triphosphine derived from phosphanyl hydrazide as a building block for hetero trimetallic compounds. Synthesis and coordination chemistry of ((Me2P)2N?N(Me)(PMe2))

The new triphosphine (Me₂P)₂N? N(Me)(PMe₂) ( 1 ), has been synthesized in pure form by the reaction of methylhydrazine with dimethylchlorophosphine in the presence of triethylamine. This triphosphine represents a bridge between phosphinoamine (>P? N(R)? P<) and phosphanyl hydrazide (>P? N(R)? N(R)? P<) backbones. Reaction of 1 with cis-[W(CO)₄(NHC₅H₁₀)₂] proceeds via two coordination modes to give four-membered M? P? N? P and five-membered M? P? N? N? P metallacyclic frameworks. The tungsten complex cis-[W(CO)₄{(Me₂P)₂NN(Me)(PMe₂)}] ( 2 ) possessing an uncoordinated phosphine moiety to prepare multimetallic organometallic compounds. For example, reactions of 2 with PdCl₂(PhCN)₂ and PtCl₂(COD) produced the trimetallic complexes consisting of W(0)? Pt(II) and W(0)? Pd(II) centers respectively in good yields. The different isomers of these trinuclear complexes have been clearly identified by ³¹P NMR spectroscopy.     

Evolution of mechanical response of sodium montmorillonite interlayer with increasing hydration by molecular dynamics

The mechanical response of the interlayer of hydrated montmorillonite was evaluated using steered molecular dynamics. An atomic model of the sodium montmorillonite was previously constructed. In the current study, the interlayer of the model was hydrated with multiple layers of water. Using steered molecular dynamics, external forces were applied to individual atoms of the clay surface, and the response of the model was studied. The displacement versus applied stress and stress versus strain relationships of various parts of the interlayer were studied. The paper describes the construction of the model, the simulation procedure, and results of the simulations. Some results of the previous work are further interpreted in the light of the current research. The simulations provide quantitative stress deformation relationships as well as an insight into the molecular interactions taking place between the clay surface and interlayer water and cations.     

Development of novel water-soluble, organometallic compounds for potential use in nuclear medicine: synthesis, characterization, and (1)H and (31)P NMR investigations of the complexes fac-[ReBr(CO)3L] (L=bis(bis(hydroxymethyl)phosphino)ethane, bis(bis(hydroxymethyl)phosphino)benzene)

The bidentate, water-soluble phosphine ligands, bis(bis(hydroxymethyl)phosphino)benzene (HMPB, 1) and bis(bis(hydroxymethyl)phosphino)ethane (HMPE, 2) were reacted with the organometallic precursor fac-[ReBr(3)(CO)(3)](2-), 3, to produce the complexes fac-[Re(OH(2))(CO)(3)L](+) and fac-[ReBr(CO)(3)L] (L = HMPE, HMPB), respectively, in good yields. The rhenium complexes fac-[ReBr(CO)(3)HMPB], 5, and fac-[ ReBr(CO)(3)HMPE], 8, were characterized using (1)H and (31)P NMR spectroscopy. The structure of fac-[ReBr(CO)(3)HMPB] was confirmed by single-crystal X-ray spectroscopy. The substitution reactions of HMPE/HMPB with the rhenium precursor 3 in aqueous solution were monitored using time-dependent (31)P NMR techniques. A significant discrepancy in the reaction kinetics and the substitution mechanism between the two bidentate ligands could be observed presumably due to the different chemical backbones.     

Finite difference methods and their convergence for a class of singular two point boundary value problems

Summary We discuss the construction of three-point finite difference approximations and their convergence for the class of singular two-point boundary value problems: (x y)=f(x,y), y(0)=A, y(1)=B, 0<<1. We first establish a certain identity, based on general (non-uniform) mesh, from which various methods can be derived. To obtain a method having order two for all (0,1), we investigate three possibilities. By employing an appropriate non-uniform mesh over [0,1], we obtain a methodM ₁ based on just one evaluation off. For uniform mesh we obtain two methodsM ₂ andM ₃ each based on three evaluations off. For =0,M ₁ andM ₂ both reduce to the classical second-order method based on one evaluation off. These three methods are investigated, theirO(h ²)-convergence established and illustrated by numerical examples.     

Phosphinimine complexes of technetium(VII): X-ray crystal structure of [Ph3P=NH+2][TcO−4]1

An X-ray diffraction study and the solution chemistry is reported for the very stable ion pair [Ph₃P=NH⁺ ₂][TcO^– ₄]. This compound crystallizes in the orthorhombic system, in space group P2₁2₁2₁, with cell dimensions of a = 11.1454(6), b = 11.449(2), and c = 14.570(2)Å, and refining to R = 0.023 and R _W = 0.031. Strong hydrogen bonds are formed between the iminium protons and the pertechnetate oxygens (1.89(6) and 2.23(6) Å) in the solid state, and similar hydrogen bonding apparently occurs in aqueous solution to stabilize the phosphiniminium cation to hydrolysis.     

Speckle free redundant holography

A new technique with experimental results for producing speckle free redundant holograms without a spurious back ground pattern is reported.     

Preparative liquid chromatography

Summary Historically, liquid chromatography has been a preparative technique. Its applications have been limited, however, until a decade ago. The needs of modern chemical, pharmaceutical and biochemical industries have motivated this period of phenomenal growth which is being witnessed now. Novel packing materials, new packing technologies, and advancements in instrumentation and process technologies have appeared in rapid succession. Instruments using columns with diameters ranging from a few inches to a few feet, can be packed with efficiencies comparable or better than analytical columns having the same packing material. This permits the development of new applications covering a wide variety of problems. The empirical approach, followed until recently for the development of new applications, is being improved by insights derived from a better understanding of the theory of large concentration chromatography. With increased computer power and a greater comprehension of the theoretical aspects, a fundamental approach to design and optimization of the operating parameters is being developed. Investigation of the components of the cost of industrial production is also in its early stages. Historical trends, theoretical treatments, column technologies, operating modes and guidelines for optimization will be discussed and reviewed.     

Local heat transfer distribution between smooth flat surface and impinging air jet from a circular nozzle at low Reynolds numbers

An experimental investigation is performed to study the effect of jet to plate spacing and low Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length-to-diameter ratio (l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 500 and 8,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are obtained using thermal images from infrared thermal imaging technique. It was observed that at lower Reynolds numbers, the effect of jet to plate distances covered during the study on the stagnation point Nusselt numbers is minimal. At all jet to plate distances, the stagnation point Nusselt numbers decrease monotonically with the maximum occurring at a z/d of 0.5 as opposed to the stagnation point Nusselt numbers at high Reynolds numbers which occur around a z/d of 6.