New ways to polysilanes

New cyclosilanes with substituents, which are functional groups, open the possibility of forming new polycyclic silanes in an aimed synthesis. This is possible on five- and six-membered rings. Also, mixed systems with different ring sizes were prepared. The UV spectra are discussed. An annelid dicycle Si₁₀Me₁₈ was also prepared. With this compound a radical anion can be formed. New aspects of the electrochemical synthesis of polysilanes are presented and a chlorine stable anode is proposed for the first time. The polymerization reaction of hydrogen-containing methyldisilanes is discussed. This reaction yields polysilanes with a general composition (SiMeH) _n and a ceramic yield over 88%. The reaction is very different from the known reaction of monosilanes. Disilanes react faster and to high polymers. Two reaction mechanisms seem to be working simultaneously, i.e., a mechanism as postulated in monosilanes and a disproportionation reaction. These mechanisms are discussed.     

Polysilanes Bearing a Phenoxypropyl Side Chain

A series of polysilanes bearing a 3‐phenoxypropyl side chain was prepared applying the Wurtz coupling reaction of dichlorosilanes. All prepared polymers ([R‐Si(CH₂)₃OPh]_n, R = Me, Pent, Hex, Hept, (CH₂)₃OPh, Ph) were investigated by GPC, UV, fluorescence, and ¹H, ¹³C, ²⁹Si NMR spectroscopy. Among these polysilanes only [HeptSi(CH₂)₃OPh]_n ( 3d ) exhibits a thermochromic behaviour showing at least two transitions.     

用作陶瓷先驱体的可溶性聚硅烷的合成与表征

以甲基氢二氯硅烷、二甲基二氯硅烷为原料，利用均聚和共聚的方法，合成了几种不同组成的可溶性聚硅烷，研究了反应条件对可溶性聚硅烷产率的影响，并用IR、XRD对产物进行了表征。结果表明：甲基氢二氯硅烷在反应体系中的比例越高，可溶聚硅烷的产率越高，增加反应时间和降低反应温度可提高可溶聚硅烷的产率。     

Conductivities of polysilanes

Measurements of the electrical conductivities of three types of polysilanes were carried out over a range of dc fields and temperatures. These polymers are varied in substituents and structures with formulas of [C₆H₅SiCH₃]_n, [(C₆H₅SiCH₃)₇₀(CH₃SiH)₃₀]_n, and [(C₆H₅SiCH₃)₅₀(C₆H₅SiH)₃₀(CH₃Si)₂₀]_n. Undoped polysilanes behaved as insulators since their conductivities were observed in the range of 10⁻¹⁰ to 10⁻¹³ S cm⁻¹, while SbF₅-doped polysilanes of all kinds behaved as semiconductors with conductivities in the range of 10⁻² to 10⁻⁴ S cm⁻¹. No significant difference in conductivities was observed among three SbF₅-doped polysilanes although these polymers are very different in chemical properties. These experimental results suggest that electrical conductivities of polysilanes are associated with the Si Si main-chain backbone rather than with the side groups. It is evident that the dopant is able to diffuse throughout the bulk of the polymer and the conductivity of the doped polymer is a function of the dopant concentration from the result of in situ monitoring of the resistance of the silane homopolymer during SbF₅ doping at room temperature. The conductivities of polysilanes appeared to be temperature-dependent. The activation energy for the conduction of SbF₅-doped silane copolymer was found to change at its glass transition temperature. © 1996 John Wiley & Sons, Inc.     

On the formation of permethylcyclopolysilanes in the würtz-type reaction of Me2SiCl2: The influence of solvent and complexing agent

Permethylcyclopolysilanes are most often prepared by Würtz-type coupling of Me₂SiCl₂ with Na K alloy in THI: When nonpolar solvents like benzene were used, only linear polysilanes (Me₂Si)_n were produced, but the addition of a catalytic amount of crown ether 18-crown-6 was found to increase significantly the amount of cyclic oligomers. The ratio of (Me₂Si)₆ to (Me₂Si)₅ decreased from 18:1 to 6:1 in the presence of crown ether in a typical synthesis in THF. An explanation based on solvated electron theory has been proposed.     

Effect of initial compositions on boron carbide synthesis and corresponding growth mechanism

ABSTRACT

The yield of boron carbide (B₄C) synthesised by carbothermal reduction (CTR) is low (40–50%) owing to the inappropriate initial composition. In this paper, thermodynamic equilibrium simulations for effects of initial compositions on B₄C synthesis were performed, while initial compositions on the quality and yield of B₄C synthesised by CTR from B₂O₃-C mixture at atmospheric pressure were systemically investigated. The reaction mechanisms for B₄C synthesis process were thermodynamically analysed. Moreover, growth mechanisms for B₄C particles were speculated based on the thermodynamic and experimental results. The results show that the simulations agree well with experimental results, and optimal initial composition is 1.906 (B₂O₃/C weight ratio). Under optimal condition, B₄C powder prepared at 1973?K is high-quality with well crystallinity, purity of 93.58% and yield of 90.05%. Additionally, the prepared B₄C particles show two types of morphologies, aggregate of small equiaxed particles and needle-like particle, which are formed through different reaction mechanisms, respectively.     

Thermal characterization of Se78 − x Te20Sn2Pb x (0 ≤ x ≤ 6) glassies for phase change optical recording technique

New multi-component glasses of Se_{78 ? x} Te₂₀Sn₂Pb _x (0 ≤ x ≤ 6) system have been synthesized using melt-quench technique. Differential Scanning Calorimetric measurements are performed at different heating rates under non-isothermal conditions to study the glass transition kinetics of prepared chalcogenide glass. Different kinetic parameters such as the activation energy of glass transition, thermal stability, glass forming ability etc have been determined. The composition dependence of kinetic parameters of glass transition is also discussed.     

Preparation of polycarbosilanes and polysilanes from (trimethylsilylmethyl)chlorosilanes

The one-step conversion of (Me₃SiCH₂)SiCl₃ to polycarbosilanes by reaction with three molar equivalents of the alkali metals lithium, sodium, or sodium potassium alloy is reported. The expected network polysilanes were not isolated. Under similar conditions (Me₃SiCH₂)SiHCl₂ gives polysilanes. The low molecular weight polymers have been characterized by elemental analyses, gpc, and FT-IR, NMR, UV-vis, and mass spectroscopies.Presented in part at the XXVIth Silicon Symposium, Indiana University-Purdue University at Indianapolis, March 26–27, 1993.     

Skeletal isomerization kinetics of 1-pentene over an HZSM-22 catalyst

Synthesis of the oxygenate fuel additive TAME (tertiary amyl methyl ether) is based on the reaction between reactive isopentene compounds (2-methyl-1-butene and 2-methyl-2-butene) and methanol. Skeletal isomerization of n-pentenes is an important reaction route in production of the raw material for TAME synthesis. In this work the kinetics of skeletal isomerization of 1-pentene was studied using HZSM-22 as a catalyst, and the effect of temperature (523–598 K) and WHSV (weight hourly space velocity) on product composition were investigated. The observed main reaction products were cis- and trans-2-pentenes, 2-methyl-2-butene, 2-methyl-1-butene and 3-methyl-1-butene. Minor products found in the output stream were C₂, C₃, C₄, C₆, C₇, C₈ and C₁₀ – alkenes and – alkanes. Only traces of methane and C₉ compounds were detected. Two different reaction mechanisms have been proposed in literature for skeletal isomerization of n-pentenes and their side reactions (dimerization and fragmentation); the monomolecular and the bimolecular skeletal isomerization mechanisms. In the current work a pseudohomogeneous reaction model for both mechanisms is fitted to experimental data. The fitting results show that both reaction mechanisms explain the skeletal isomerization of 1-pentene to isopentenes well. However, the monomolecular reaction mechanism explained the formation of dimerization and fragmentation products slightly better. The reliability of the estimated Arrhenius parameters was analyzed using Bayesian inference and Markov chain Monte Carlo methods (MCMC) which are rarely used in chemical engineering.     

Characterisation of BaZr0.9Y0.1O3‐δ Prepared by Three Different Synthesis Methods: Study of the Sinterability and the Conductivity

BaZr_0.9Y_0.1O_3‐δ has been synthesised by three different methods: the solid‐state reaction, the spray pyrolysis and the spray drying. Significantly different apparent lattice parameters (between 0.4192 nm for the sample prepared by the solid‐state reaction method and sintered at 1,500 °C and 0.4206 nm for the sample prepared by the solid‐state reaction method and sintered at 1,720 °C) are observed after calcination and sintering, depending on the synthesis method and the sintering temperature. The bulk conductivity values also vary over several orders of magnitude (–7.2< log σ_b <–3.6 at 300 °C) depending on the synthesis method and the sintering temperature. These variations of the bulk conductivity and also the activation energy are correlated with variations of the apparent lattice parameter. The influence of the preparation method on the electrical properties is discussed. The grain boundaries are more resistive than the bulk. The variation of the grain boundary conductivity could be correlated to the microstructure in terms of the grain size.     

Chemical kinetic modelling of the effect of NO on the oxidation of CH4 under fluidized bed combustor conditions

The ignition and burnout of the volatiles in fluidized bed combustor are essential for its performance and emissions. NO_x are known to sensitize the oxidation of hydrocarbons, CO, and H₂. This effect is relevant especially for fluidized bed combustors, which are operated at relatively low temperatures (i.e. about 850 °C). Different reaction mechanisms and modifications to existing mechanisms have been proposed in the literature to account for these low temperature interactions of NO_x and hydrocarbons. In this work, an existing widely used reaction mechanism is adapted and tested for its capability to describe the NO sensitized oxidation of CH₄ under conditions relevant to fluidized bed combustion. NO lowers the ignition temperature to about 300 °C under the conditions investigated. Three different oxidation paths for the oxidation of CH₄ have been identified and discussed. Their relative importance strongly depends on combustion temperature, indicating that the presence of NO_x significantly affects the oxidation of the volatiles in fluidized bed combustion.     

Evaluation and characterization of Mn-Cu mixed oxide catalysts for ethanol total oxidation: Influence of copper content

Mixed oxides of manganese and copper with different wt% of copper have been prepared and evaluated in ethanol combustion. The co-precipitation method used for the synthesis of MnxCuy mixed oxides is adequate to obtain catalysts with excellent catalytic performance in combustion reactions. Catalysts were characterized by means of XRD, FT-IR, TPR and O₂-TPD. A small amount of copper prevents manganese oxide reaching a crystalline structure. This poor crystalline structure of manganese oxide may improve the existence of oxygen vacancies giving a best performance in ethanol combustion to CO₂. When the copper content increases, an extent of solid state reaction between Cu and Mn is favored and the partial oxidation of ethanol becomes more important. The incorporation of manganese into incomplete spinel structure diminishes CO₂ yield.     

The effects of PVP surfactant in the direct and indirect hydrothermal synthesis processes of ceria nanostructures

CeO₂ nanostructures with completely different morphologies were successfully prepared using the same cerium source and mineralizer through the direct and indirect hydrothermal methods with different introducing strategies of PVP surfactant. The CeO₂ nanostructures tend to form the morphologies of nano-flowers and nano-cubes through the indirect and direct hydrothermal methods, respectively. X-ray diffraction (XRD) analyses revealed that both as-prepared nanostructures are composed of CeO₂ with a standard fluorite structure. The different synthesis mechanisms and corresponding chemical evolutions of the as-prepared CeO₂ nanostructures are discussed based on the different introducing strategies of PVP surfactant in the direct and indirect hydrothermal processes. Investigation of the UV-shielding ability of both CeO₂ nanostructures suggested that the UV absorbance of the nano-flowers is much higher than that of the nano-cubes.     

Direct Oxyfluorination of Hydrocarbons on Metal Fluorides

A critical overview of the recent literature data and of our results on the oxyfluorination of C–H to C–F bond with metal fluorides is presented. Some important aspects that require further investigation, in order to obtain a better understanding of the reaction mechanism and to develop a general approach to catalysts design are discussed. In particular, the oxyfluorination of ethene, utilizing different metal fluorides, in pulse and continuous reactors, was studied. Obtained results indicated that the presence of mixed fluorides with a fluoroperovskite structure, such as AgCuF₃, led to improved reactant conversion and higher selectivity to vinyl fluoride, in comparison to the single metal fluorides. CO₂ is formed from secondary parallel reaction upon ethene oxidation, because of the presence of metal oxides in the catalysts. The difficulties met in the regeneration of the Ag° formed during oxyfluorination, addressed the search for new metal fluorides as starting materials for the synthesis of perovskite-like materials. Still, among the prepared mixtures only in the case of AgCuF₃ the formation of perovskite-like materials was evidenced by means of XRD; such behaviour is explained by means of the Goldschmidt factor t.     

A novel route for synthesis of alumina–chromium carbide composites from PTFE-activated Cr2O3/Al/C combustion

Fabrication of alumina-chromium carbide composites was investigated by PTFE-activated Cr₂O₃/Al/C combustion synthesis. PTFE was employed as not only a reaction promoter, but a carburizing agent. Three reaction systems were prepared with different contents of carbon for the synthesis of Cr₂₃C₆, Cr₇C₃, and Cr₃C₂. The amounts of PTFE were selected to ensure combustion synthesis in the SHS (self-propagating high-temperature synthesis) mode and to provide carbon in quantities of 15 and 25?mol% of the total carbon. Experimental results showed that the combustion wave velocity and temperature decreased with increasing carbon, but increased with PTFE. A correlation between combustion wave velocity and temperature contributed to determination of the activation energy E_a =?89.15?kJ/mol for the combustion reaction. The increase of PTFE also improved formation of chromium carbides. As a result, the Cr₂₃C₆– and Cr₇C₃–Al₂O₃ composites were produced with almost no impurities. Due to a loss of carbon in carbothermic reduction, the Cr₃C₂–Al₂O₃ composite was obtained with Cr₇C₃ as the secondary carbide. SEM micrographs and DES analyses indicated that spherical carbide grains with a size of 0.5–3.0 μm were synthesized.     

The role of Al in the reactive synthesis of porous Mo2Ti2AlC3 ceramics

The mixed powders of TiH₂, molybdenum, aluminum and graphite with molar ratios of 2/2/n/2.85 (n ranges from 1.0 to 1.4 mol with an interval of 0.1 mol) were used as raw material powders for this work, and a novel porous Mo₂Ti₂AlC₃ was synthesized via reactive synthesis. Through systematic research on the pore structure parameters of porous Mo₂Ti₂AlC₃ prepared with different aluminum content, the results show that there is a clear correlation between the aluminum content and the pore structure parameters. With the aluminum content rising from 1.0 to 1.2 mol, the viscous permeability coefficient and pore size decreased, while the porosity increased; When the aluminum content increased from 1.2 to 1.4 mol, the pore structure parameters of porous Mo₂Ti₂AlC₃ displayed an opposite trend. The reasons for the evolution laws of these pore structure parameters were also discussed in depth. In addition, the pore structure forming mechanism of porous Mo₂Ti₂AlC₃ ceramics during the activation reaction sintering process has been explored. This work can provide an important reference for the subsequent preparation of quaternary porous MAX phase ceramics.     

Anatase–rutile transformation in TiO2–V2O5 catalyst coatings for ceramic foams

The aim of this investigation was to study the thermal stability of TiO₂–V₂O₅ catalysts prepared from aqueous powder suspensions based on pigment grade anatase and sub-micron vanadia particles. Such catalysts are proposed for dip coating of ceramic foams for application in a new reactor design for the synthesis of phthalic anhydride. Catalyst mixtures with vanadia contents up to 10% were annealed in air in the temperature range from 400 to 1200 °C. The influence of temperature and vanadia content on the anatase to rutile transformation and the specific surface were monitored, employing XRD, SEM and BET measurements. Results show that the addition of V₂O₅ strongly affects the temperature of phase transformation. This process is accompanied by a large decrease of surface area. The structural changes can produce a pronounced effect on catalytic performance for the oxidation reaction of o-xylene. Possible mechanisms and the consequences for catalytic performance are discussed.     

Chemically Cross-linked Polysilanes as Stable Polymer Precursors for Conversion to Silicon Carbide

Cross-linked polysilanes were prepared by the co-polymerization of Me₂SiCl₂ or PhMeSiCl₂ with varying amounts of divinylbenzene (2–15% by weight) using molten sodium as the dehalogenating agent. All the cross-linked polysilanes were stable to air and could be processed thermally for conversion to silicon carbide. Polymers containing from 5–15% of the cross-linking agent underwent a uniform shrinkage during thermal treatment (1500 °C) to afford β-SiC in good yields. The ceramic was characterized by a variety of techniques including Raman and infrared spectroscopy, powder XRD, as well as Scanning Electron Microscopy (SEM). Dedicated to Prof. C. W. Allen in recognition of his outstanding contributions to inorganic polymers. Deceased in a tragic car accident in July 2004.     

Fabrication of Ti3SiC2-based ceramic matrix composites by a powder-free SHS technique

A powder-free technique for fabrication of Ti₃SiC₂-based composite ceramics via self-propagating high-temperature synthesis (SHS) is developed. The essence of the method is that the SHS-compaction of a multilayer stack comprised alternating layers of titanium foils and polymer films filled with either microsized silicon carbide particles or microsized silicon carbide particles mixed with charcoal particles. It is shown that SHS-compaction of non-powder materials can be used to synthesize dense particulate-reinforced ceramic matrix composites of Ti₃SiC₂TiSi₂SiC_p. Effects of the initial reactant composition and the synthesis conditions on the microstructure of the prepared materials are discussed.