Reactive diffusion in W–Mo–Si thin films

This study reports the phase formation in the ternary thin films system Mo–W–Si. The metallic films were deposited onto Si (100) substrate by sputtering. Two kinds of samples were prepared, either by sequential deposition or by co-deposition. The phase formation was investigated by In situ X-ray diffraction measurements from 300 to 900 °C. The influence of the sample preparation, namely sequential deposition and co-deposition, on the mechanism of phase formation has been evidenced.     

Monomer-Dimer Equilibrium of Rhodamine B Encapsulated in Si–Al and Si–Ti Binary-Oxide Thin Films

Si—Al and Si—Ti binary-oxide thin films including Rhodamine B (RB) have been prepared. They were dip-coated as a function of time after mixing of each sol-gel reaction system. The absorption and fluorescence spectra of the individual films have been observed. These spectra were analyzed in order to clarify the behavior of RB along with the change in the environment around the RB molecules, caused by the progress of the sol-gel reaction, in the fluid sol and the prepared thin films. Some amount of the RB dimers (H- and J-types) were formed in the Si—Al and Si—Ti binary-oxide films (Si : M = 75 : 25) prepared at the initial stage of the sol-gel reaction and aged under relative humidity of 60%. In the case of Si—Al binary-oxide films, the amount of the J-dimer decreased along with the reaction time at which the films were prepared, indicating that growing polymer networks of metal alkoxides around the RB molecules prevent the formation of the J-dimer. On the other hand, larger amounts of the H- and J-dimers were formed in the Si—Ti binary-oxide films prepared at longer reaction time of the solution. RB interacts more strongly with —TiOH compared with —AlOH. In the case of the Si—Ti binary-oxide films, with the progress of the sol-gel reaction, RB molecules in the prepared films easily cohere around the —TiOH and form the dimers because of increase in the amount of the —TiOH and contraction in the volume of the spaces where RB molecules exist.     

Synthesis of complex Si–O–N–H derivatives by molecular bombardment of silicon and silicon dioxide

Bombardment of a silicon target in a high vacuum with a molecular beam (mixture of high energy H₂+N₂, obtained by charge exchange) and a thermal beam of O₂ produces on the target a variety of compounds. The target is then bombarded by the same molecular beams which produce, extracted by an electrostatic field at an energy of about 10 keV, molecular ions due to the compounds thus formed. These ions are analysed (electromagnet) to give a primary spectrum of ions according to their mass, which are individually selected and dissociated in a collision cell (same H₂+N₂ mixture). Mass analysis of the dissociation fragments leads to the identification of silicon clusters (Si)_n and of Si–O–N–H derivatives, the fragmentations of which permit a definitive determination of their molecular complexity. Dissociation spectra have thus been obtained for some of the most intense peaks of the primary spectrum, on the one hand, and on the other hand for some peaks of lower intensity but of special interest to us (see below). The composition of the fragments is confirmed by the study of the satellite ions derived from the natural 28, 29 and 30 isotopes of silicon, and by the use of deuterium instead of hydrogen. None of the Si–O–N–H derivatives obtained was apparently known earlier. It is shown that some of these molecules (those ‘of special interest to us’) may be identical with sila-analogues of standard amino acids and of nucleic bases: the fact that their fragmentations are identical with those of the corresponding carbon analogues speaks in favour of a structural identity. However, one cannot yet distinguish between the various possible isomeric arrangements, as none of them has been independently prepared, which excludes a direct comparison with reference samples, and as these isomers might give identical fragments; we hope to be able to resolve this ambiguity later. Anyhow, the substances formed are the most complex molecular silicon derivatives so far produced: e.g. Si₂O₂NH₅, Si₃O₂NH₇, Si₄O₃NH₉, Si₄O₂N₂H₄, Si₄ON₃H₅, Si₅O₃N₂H₁₀, Si₅O₂NH₁₁, corresponding to sila-glycine, sila-alanine, sila-threonine, sila-uracile, sila-cytosine, sila-valine, sila-glutamine, – or isomers. Similar results have been obtained using a silicon dioxide target and high energy molecular beams of hydrogen and of nitrogen, without thermal oxygen, or with a carborundum target. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASmolecular impact / silicon derivatives / amino-acid silicon analogues     

Thermodynamic analysis of precipitation processes in Nb–Ti-microalloyed Si–Al TRIP steel

The work deals with the thermodynamic calculations of precipitation processes in austenite of the Nb–Ti-microalloyed steel with increased Si and Al content dedicated for the automotive industry. The analysis is based on the equilibrium precipitation of individual MX-type interstitial phases, as well as the effect of various Mn and Si additions is included. The solubility products and corresponding limits of the mutual solubility of microalloy and metalloid additions in austenite were calculated. The temperature sequence of the precipitation under equilibrium conditions was determined. The Dutta–Sellars model has been applied for determination of recrystallization stop temperature of austenite and time needed for Nb(C,N) precipitation. The calculations were verified by microstructure investigations including revealing prior austenite grain size as a function of austenitizing temperature and the identification of complex carbonitrides using transmission electron microscopy. The model calculations are in good agreement with experimental results.     

Thermal stability and conductivity of hot-pressed Si3N4–graphene composites

This study concerns new Si₃N₄–graphene composites manufactured using the hot-pressing method. Because of future applications of silicon nitride for cutting tools or specific parts of various devices having contact with high temperatures there is a need to find a ceramic composite material with good mechanical and especially thermal properties. Excellent thermal properties in the major directions are characteristic of graphene. In this study, the graphene phase is added to the silicon nitride phase in a quantity of up to 10 mass%, and the materials are sintered under uniaxial pressure. The mixture of AlN and Y₂O₃ is added as sintering activator to the composite matrix. The studies focus on thermal stability of produced composites in argon and air conditions up to the temperature of 1,000 °C. The research also concerns the influence of applied uniaxial pressure during the sintering process on the orientation of graphene nanoparticles in the Si₃N₄ matrix. The study also presents research on anisotropy of thermal diffusivity and following thermal conductivity of ceramic matrix composites versus the increasing graphene quantity. Most of the presented results have not been published in the literature yet.     

Thermodynamics,structure and kinetics in the system Ga–O–N

Within the ternary system Ga–O–N we performed experimental and theoretical investigations on the thermodynamics, structure and kinetics of new stable and metastable compounds.We studied the ammonolysis of β-Ga₂O₃ at elevated temperatures by means of ex situ X-ray diffraction, ex situ neutron diffraction, and in situ X-ray absorption spectroscopy (XAS). From total diffraction pattern refinement with the Rietveld method we analyzed the anionic occupancy factors and the lattice parameters of β-Ga₂O₃ during the reaction. Within the detection limits of these methods, we can rule out the existence of a crystalline oxynitride phase that is not derived from wurtzite-type GaN. The nitrogen solubility in β-Ga₂O₃ was found to be below the detection limit of about 2–3 at.% in the anionic sublattice. The kinetics of the ammonolysis of β-Ga₂O₃ to α-GaN and of the oxidation of α-GaN to β-Ga₂O₃ was studied by means of in situ X-ray absorption spectroscopy. In both cases the reaction kinetics could be described well by fitting linear combinations of β-Ga₂O₃ and α-GaN spectra only, excluding that other crystalline or amorphous phases appear during these reactions. The kinetics of the ammonolysis can be described well by an extended Johnson–Mehl–Avrami–Kolmogorow model with nucleation and growth of GaN nuclei, while the oxidation kinetics can be modeled by a shrinking core model where Ga₂O₃ grows as a layer. Investigations by means of TEM and SEM support the assumptions in both models.To investigate the structure and energetics of spinel-type gallium oxynitrides (γ-galons) we performed first-principles calculations using density-functional theory. In addition to the ideal cubic γ-Ga₃O₃N we studied gallium deficient γ-galons within the Constant-Anion-Model.In highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO_1.2 we found at a temperature around 670 K an insulator–metal transition, with a conductivity jump of seven orders of magnitude. We demonstrate through experimental studies and density-functional theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric β-Ga₂O₃ within the metastable oxide matrix. By doping with nitrogen the critical temperature and the conductivity in the highly conducting state can be tuned.     

Assessment of intermolecular N–H…F and N–H…Cl hydrogen bonding in stabilising hetero- and homodimers in solution

This paper describes the first assessment of intermolecular weak N–H…F and N–H…Cl hydrogen bonding in stabilising hetero- and homodimers in solution. Aromatic amide and urea monomers have been designed and synthesised. The association constants of the heterodimers formed by two complementary monomers and the homodimers formed by self-complementary monomers have been determined by using ¹H titration and dilution experiments. The results show that both N–H…F and N–H…Cl hydrogen bonds are able to stabilise the corresponding dimers to a measurable extent, even though the stability of the dimers is generally low.     

Reaction pathway and phase transitions in Al–Ti–Si system during differential thermal analysis

The present paper mainly studied the phase formation and reaction pathway of the Al–Ti–Si system in detail by thermal analysis combined with XRD and SEM observations. The phase formation sequence in Al–Ti–Si system from starting mixtures to final products with increasing temperature can be described as following: Al_(l) + Ti_(s) + Si_(s) → (Al–Si)_(l) + Ti_(s) + Si_(s) → Ti(Al,Si)_3(s) + Si_(s)Ti₅(Si,Al)₃ + Al_(l). More importantly, the solubility of Si in Ti(Al,Si)₃ decreased gradually while that of Al in Ti₅(Si,Al)₃ increased with temperature increasing, suggesting the transportation of Si atoms from intermediate aluminides Ti(Al,Si)₃ to final stable silicides Ti₅(Si,Al)₃ and hence further confirming the formation of Ti₅(Si,Al)₃ at the expense of Ti(Al,Si)₃.     

Thermodynamic simulation and experimental investigation of reactive chemical vapor deposition in the Ta–C–Si–O–F system

Reactive deposition processes of tantalum carbide are studied experimentally and theoretically in the Ta–C–Si–O–F system. It is shown that Sio₂ substantially affects the carbide formation process. This is expressed in a decreased efficiency of tantalum transfer from the source zone to the crystallization zone, the possibility of bilateral transport of tantalum to carbon and carbon to tantalum, the complicated composition of condensed phases in the equilibrium with the gas phase. Aspects of preparing single crystal tantalum oxyfluoride are considered.     

A novel N–O ligand for palladium-catalyzed Mizoroki–Heck reaction in neat water

1-(Morpholin-1-yl-pyridin-2-yl-methyl)-2-naphthol as a novel efficient N–O ligand has been developed for palladium-catalyzed Mizoroki–Heck reaction in neat water without the protection of an inert atmosphere. The reactions proceed smoothly and give the desired products in moderate to excellent yields. The catalyst system is reusable.     

C–H...O and C–H...N interactions in RNA structures

Small molecule studies indicate that C–H...X interactions (X: O,N) constitute weak H-bonds. We have performed a comprehensive analysis of their occurrence and geometry in RNA structures. Here, we report on statistical properties of the total set of interactions identified and discuss selected motifs. The distance/angle distribution of all interactions exhibits an excluded region where the allowed C–H...X angle range increases with an increasing H...X distance. The preferred short C–H...X interactions in RNA are backbone-backbone contacts between neighbour nucleotides. Distance/angle distributions generated for various interaction types can be used for error recognition and modelling. The axial C2′(H)...O4′ and C5′(H)...O2′ interactions connect two backbone segments and form a seven-membered ring that is specific for RNA. An AA base pair with one standard H-bond and one C–H...N interaction has been identified in various structures. Despite the occurrence of short C–H...X contacts their free energy contribution to RNA stability remains to be assessed. Received: 17 May 1998 / Accepted: 4 August 1998 / Published online: 2 November 1998     

Direct-space analysis of the Si–Si bonding pattern in the π-bonded chain reconstructed Si(111)(2 × 1) surface

Atomic and bond properties of silicon atoms in the buckled π-bonded chain reconstructed Si(111)(2 × 1) system were investigated by applying the quantum theory of atoms in molecules to a number of wavefunctions from periodic ab initio calculations using a slab model for the surface and geometries from experiment. Reconstruction involves much larger surface-cell charge distortions than in the unrelaxed surface and drastic changes in the atomic polarizations of the surface layer atoms. The effect of buckling is to largely differentiate the properties (charge, energy, volume, atomic polarizations) of the two unique atoms of each surface layer. The direction of electronic charge transfer in the topmost chain (from the “up” to the “down” atom) was found to be opposite to what was claimed previously. The π conjugation is not strictly localized along the topmost layer chains (where it is also largely incomplete), but rather it extends over a 2D array of bonds between the topmost and the lower surface layers. Received: 19 July 2000 / Accepted: 2 October 2000 / Published online: 23 January 2001     

Experimental investigation of the Nd–Al–Si system

The isothermal section of the Nd–Al–Si ternary system at 500 °C has been investigated using differential thermal analysis, X-ray diffraction analysis, scanning electron microscopy and electron micro-probe analysis. Four ternary intermetallic compounds were confirmed: NdAl₂Si₂ (τ₁), hP5-CaLa₂O₂ structure type, Nd₂Al₃Si (τ₂), hP3-AlB₂ structure type, NdAl_1−x Si_1+x , 0.25 ≤ x ≤ 0.3 (τ₃), tI12-αThSi₂ structure type and Nd₂Al_1−x Si_1+x , 0 ≤ x ≤ 0.2, (τ₅), oS8-CrB structure type. A new ternary intermetallic phase (τ₄) was found: Nd₄Al₃Si₃, orthorhombic oS20, isotypic with Pr₄Al₃Ge₃.     

N‐Alkylation of Indole Compounds in Modified Wittig–Horner Reaction

N‐Ethyl‐3‐styrylindoles are prepared under modified Wittig–Horner reaction conditions.     

Effects of bulky substituent groups on the Si–Si triple bonding in RSiSiR and the short Ga–Ga distance in Na2[RGaGaR]: A theoretical study

The steric and electronic effects of bulky aryl and silyl groups on the Si–Si triple bonding in RSiSiR and the short Ga–Ga distance in Na₂[RGaGaR] are investigated by density functional calculations. As typical bulky groups, Tbt = C₆H₂-2,4,6-{CH(SiMe₃)₂}₃, Ar′ = C₆H₃-2,6-(C₆H₃-2,6-iPr₂)₂, Ar¹ = C₆H₃-2,6-(C₆H₂-2,4,6-iPr₃)₂, SiMe(SitBu₃)₂, and SiiPrDis₂ (Dis = CH(SiMe₃)₂) are investigated and characterized. The importance of large basis sets is emphasized for density functional calculations.     

Convenient approach to Si–H-containing polymers

New synthetic routes to organosilicon polymers containing SiH₂ groups and organic -electron units in the polymer main chain are described. These polymers are expected to be useful precursors for ceramics. The polymer backbone is formed by condensation of ,-bis(trifluoromethylsulfonyloxy)-substituted organo-silicon compounds containing SiH₂ groups with the organometallic dinucleophiles Li₂C₂, Li₂C₄, and 1,4-BrMg–C₆H₄–MgBr. We could confirm the formation of the silicone polymers at low temperatures, in short reaction times, and with high yields. The structural characterization is based on ²⁹Si, ¹³C, and ¹H NMR spectroscopy. The narrow ²⁹Si NMR signals of the products indicate the regular alternating arrangement of the building blocks in the polymer backbone resulting from the fact that the condensation reactions are not accompanied by exchange processes analogous to metal halogen exchange. Weight-average molecular weights in the range of M_w = 10000–20000, relative to polystyrene standards, were found by GPC. The pyrolytic behaviour of [H₂Si–H₂Si–C C–]_n 2a is compared with the behaviour of the methylated derivative [Me₂Si–Me₂Si–C C–]_n.     

Si3N4陶瓷的连接研究进展

在系统介绍陶瓷与陶瓷、金属的连接方法基础上 ,重点评述了Ｓｉ3Ｎ4 陶瓷的直接钎焊法和Ｓｉ3Ｎ4 陶瓷的间接钎焊法的连接工艺进展及其存在的问题和解决的措施 ,同时对目前研究较少的Ｓｉ3Ｎ4 陶瓷的玻璃焊法也进行了评述 ,并简介了Ｓｉ3Ｎ3陶瓷固相压力扩散焊法。Ｉｎｔｈｉｓｐａｐｅｒ,ａｓｙｓｔｅｍａｔｉｃｄｅｓｃｒｉｐｔｉｏｎｏｆｊｏｉｎｉｎｇｔｅｃｈｎｉｑｕｅｓｏｆｃｅｒａｍｉｃｔｏｉｔｓｅｌｆａｎｄｗｉｔｈｍｅｔａｌ,ｅｓｐｅｃｉａｌｌｙｔｈｅｄｅｖｅｌｏｐｍｅｎｔｏｆＳｉ3Ｎ3ｃｅｒａｍｉｃｄｉｒｅｃｔｂｒａ…     

Influence of variation in the silicon content on the silicon precipitation in the Al–Si binary system

Ti-based amorphous alloys produced by ultra-rapid melt cooling represent an excellent option as biomaterials because of their mechanical properties and corrosion resistance. However, complete elimination of toxic elements is affecting the glass-forming ability and amorphous structure could be obtained only for thin ribbons or powders that are subsequently processed by powder metallurgy. Amorphous ribbons of special Ti₄₂Zr₄₀Ta₃Si₁₅ alloy, which is completely free of any toxic element, were produced by melt spinning, and the thermostability of resulting material was investigated in order to estimate its ability for further heat processing. Isochronal differential scanning calorimetry (DSC) was used to determine transformation points such as glass transition temperature T _g or crystallization temperature T _x. The activation energy for crystallization of amorphous phase was calculated based on Kissinger method, using heating rates ranging between 5 and 20 °C min^?1. Amorphous structure of resulting ribbon was evidenced by means of X-rays diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). It was determined that amorphous Ti₄₂Zr₄₀Ta₃Si₁₅ alloy has a high activation energy for crystallization, similar to other Ti-based amorphous alloys, which provides good thermal stability for subsequent processing, especially by means of powder metallurgy techniques.     

Kinetics of aquation of trans-dichlorobis(N,N′-dimethylethylenediamine)cobalt(III) ion in water–methanol in water–propan-2-ol media

The complex trans-[Co(dmen)2Cl2]Cl (dmen=N,N-dimethylethylenediamine) has been prepared and characterized by elemental analysis, u.v.-vis. and i.r. spectra. The kinetics of the primary aquation of trans-[Co(dmen)2Cl2] in H2O, H2O–MeOH and H2O–i-PrOH have been examined over a wide range of solvent compositions and temperatures (40–55°C). Plots of rate constants (log k) versus the reciprocal of the dielectric constant of the medium (Ds–1) and Grunwald–Winstein values of the solvent (Y) were found to be non-linear. The variation of enthalpies (H) and entropies (S) of activation with solvent composition has been determined. Plots of H or S versus the mole fraction of each solvent exhibit extrema at x2=ca. 0.16 and 0.27 for MeOH and at x2=ca. 0.03 and 0.14 for i-PrOH. Furthermore, the cycle relating the free energy of activation in H2O to that in H2O–co-solvent shows that the stabilizing influence of the changes in the solvent structure is greater on the emergent five-coordinate cation in the transition state than that on the complex ion in the initial state, with the difference becoming greater as the mole fraction of the co-solvent increases.     

Ligand-free C–N bond formation in aqueous medium using a reusable Cu–Mn bimetallic catalyst

A general ligand-free protocol has been described for the recyclable and reusable Cu–Mn catalyzed C–N bond forming cross coupling reaction of arylboronic acids with various amines to form N-arylated amine products in aqueous medium affording excellent yields under ambient conditions, in 3–4 h.