Heterogeneously catalysed crosslinking of polycarbosilane with divinylbenzene

Crosslinking of polycarbosilane (PCS) with divinylbenzene (DVB) is readily accomplished using heterogeneous catalysis with platinum chloride in heptane to provide a silicon carbide precursor that produces a ceramic with significantly reduced oxygen content. The ceramic yield after crosslinking increased from 47% to between 72% and 78%; however, crosslinking may be influenced by dehydrogenative silylation of hydroxyl groups. Solid-state ¹³C NMR spectroscopy of the crosslinked PCS showed peaks assignable to the aromatic group at 144 and 126.7 ppm. Monitoring of the crosslinking reaction by ¹H NMR spectroscopy indicated 40% consumption of the vinyl bonds of DVB within 10 min and complete consumption within 16 h. Infrared spectroscopy showed no increase in the peak at 3,650 cm⁻¹ due to O–H stretching in Si–OH, demonstrating that hydrosilylation crosslinking is a highly effective non-oxidative crosslinking technique. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Quantitative hydrogen measurements in PECVD and HWCVD a-Si:H using FTIR spectroscopy

A series of hydrogenated amorphous silicon films have been deposited using plasma enhanced chemical vapour deposition (PECVD) and hot-wire chemical vapour deposition (HWCVD) techniques. The total concentration of bonded hydrogen in the films was varied between 3% and 18% as determined by hydrogen effusion measurements. Fourier transform infra-red (FTIR) spectra of the PECVD and HWCVD samples exhibit strong absorption peaks that correspond to Si–H bend and stretch modes, and Si–H₂ stretch modes. A quantitative fit of the FTIR peak areas to the hydrogen effusion concentrations reveals that there is reasonable agreement between the required proportionality constants in PECVD and HWCVD material for the Si–H bend and stretch modes. The uncertainty error for the FTIR proportionality constants is consistently greater for the HWCVD data set, however, which may indicate that the effective dynamical charge of the Si–H dipoles is perturbed in the HWCVD material by bonded impurities that are sourced from the tungsten wire.     

Wettability of NiAl by a liquid Ni–Si–B alloy

An investigation of the wettability of the intermetallic compound NiAl by a liquid Ni–4.5 wt% Si–3.2 wt% B filler metal is presented in this paper. Dynamic observations of spreading of Ni–Si–B droplets, conducted using hot-stage light microscopy, are correlated with post-cooling microscopy and analysis. The paper examines the influence of the oxide layer on the NiAl substrates, on the progression of spreading of the Ni–Si–B liquid. Termination of spreading of the Ni–Si–B droplets by the onset of isothermal solidification at the spreading front is considered. Spreading of the Ni–Si–B droplets was found to be rapid until the onset of isothermal solidification at the spreading front. However, once isothermal solidification commenced, negligible further spreading was observed. The Ni–Si–B filler metal was observed to spread by undermining of the substrate oxide. However, a marked reaction occurred between the substrate oxide and the Ni–Si–B filler metal. This reaction served to remove the substrate oxide layer. The paper contrasts the mechanisms of substrate oxide undermining and isothermal solidification of liquid Ni–Si–B droplets on NiAl with those occurring during the spreading of the same liquid on pure nickel and Ni–Cr alloys. This revised version was published online in November 2006 with corrections to the Cover Date.     

Phase separation and liquid-crystal state of cellulose triacetate in nitromethane

The existence of the liquid-crystal state of cellulose acetate in nitromethane has been shown on the basis of the calculation data, rheological and x-ray structural analysis, and polarization- and electron-microexamination. The conditions for the formation of the lyotropic phase have been revealed and the states of its concentration-temperature boundary have been determined and marked on the diagram. It is shown that the ratio between acetate and hydroxyl groups in the polymer strongly influences the temperature and concentration ranges of realization of a highly oriented state. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 6, pp. 1178–1187, November–December, 2008.     

Liquid crystalline elastomers based on cyclic cyclosiloxane and mesogenic crosslinking units

Cholesteric liquid crystalline elastomers (LCEs) IP–VIP were graft copolymerized by a one-step hydrosilylation reaction with cyclo(pentamethylhydrogeno)siloxane, a cholesteric liquid crystalline (LC) monomer cholesteryl 3-(4-(undec-10-enoyloxy)phenyl)acrylate and crosslinking LC monomer biphenyl-4,4′-diyl bis(4-(allyloxy)benzoate). The effect of crosslinking mesogens on mesomorphic properties of the chiral LCEs was studied by swelling experiments. All the samples IP–VIP showed cholesteric mesophase when they were heated and cooled. The glass transition temperature (T _g) of elastomers increased slightly with increase of crosslinking mesogens in the polymer systems, but mesophase–isotropic phase transition temperature (T _i) decreased slightly, suggesting that the temperature range of mesophase became narrow with increase of crosslinking mesogens for all the elastomers. In XRD curves, all the samples IP–VIP exhibited diffuse reflections at 2θ ≈ 17° suggesting lack of the smectic-layered packing arrangement, and the intensity of these diffuse diffraction peaks decreases with increase of mesogenic crosslinking units, suggesting that the order between two neighbor LC molecules disturbed by the mesogenic crosslinking agents. The maximum reflection bands shift slightly to long wavelength and become broad at the same temperature, indicating that the helical structure is partially disrupted because of both the constraint of chemical crosslinking agents and the different mesogenic units.     

Polymer–ceramic conversion of a highly branched liquid polycarbosilane for SiC-based ceramics

Liquid polycarbosilane (LPCS) with a highly branched structure was characterized by fourier-transform infrared spectrometry (FT-IR) and ¹H, ¹³C, ²⁹Si nuclear magnetic resonance spectrometry (NMR). The LPCS was then cured and pyrolysized up to 1,600 °C under flowing argon. The structural evolution process was studied by thermogravimetric analysis and differential scanning calorimetry (TG-DSC), FT-IR, and X-ray diffraction (XRD). Hydrosilylation, dehydrocoupling, and polymerization cross-linking reactions between Si–H and C=C groups occurred at low temperatures, which mainly accounted for the high ceramic yield (70%) up to 1,400 °C. The organic groups gradually decomposed and the structure rearranged at high temperatures. The FT-IR analysis revealed that Si–CH₂–Si chains, the backbone of original polymer, can be retained up to 1,200 °C. At temperatures higher than 1,200 °C, the Si–CH₂–Si chains broke down and crystalline SiC began to form. The final crystalline products were β-SiC and a small amount of carbon.     

Decarbonization mechanisms of polycarbosilane during pyrolysis in hydrogen for preparation of silicon carbide fibers

Polycarbosilane (PCS) fibers are cured by electron beam irradiation in helium. Then, the cured fibers are pyrolyzed under hydrogen. The mechanisms of carbon removal during pyrolysis are investigated using chemical elemental analysis, FTIR, Raman, and AES analysis. The development of microstructure and phase is examined by SEM, TEM, and XRD. The results show that the thermal cleavage of relatively weak Si–H and Si–CH₃ bonds takes place first during pyrolysis in hydrogen, generating free radicals. The free radicals then react with C–H bonds or with each other to form Si–CH₂–Si groups, releasing hydrogen and methane. As temperature increases, the Si–CH₂–CH₂–Si groups in PCS begin to dissociate and react with hydrogen to form methane, resulting in the further removal of carbon and giving silicon-rich silicon carbide fibers (i.e. C/Si <1).     

Low frequency dielectric spectroscopy characterization of microcrystalline cellulose, tablets and paper

Cellulose fibres and particles in the form of powder, tablets and paper sheets have been investigated by very low frequency dielectric spectroscopy using a novel form of dielectric cell, in which two planar electrodes have been mounted in fixed positions at right angles. The broad pattern of response obtained from the samples is independent of the structural form of the cellulose sample, a loss peak in the 0.1–100 Hz range, and at lower frequencies a dispersion process which is dominated by an imperfect charge transport. Moisture has a significant influence on the rate of charge transport. In dry samples the dipolar loss peak was not evident but as the moisture content increased it appeared. Using a humidity normalizing technique the dielectric response for microcrystalline cellulose has been characterized over the equivalent of 14 decades in frequency. It has also been shown that there is a linear response between the capacitance and the density of microcrystalline cellulose samples. The consolidation of powder into tablets is discussed with respect of the observations of changes in capacitance, loss peak frequency and imperfect charge transport efficiency. Furthermore it was found possible to investigate differences between the dipolar relaxation rate “in” and “out” of the plane of paper in the stack. The relaxation time for dipoles “out” of the paper plane is 7 to 8 times longer than for dipoles “in” the paper plane. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical properties of rapidly solidified ribbons of some AI–Si based alloys

Continuous uniform ribbons of Al–16 Si, Al–12.5 Si–1 Ni and Al–12.5 Si–1 Mg were prepared by melt spinning. Microhardness was measured. The as-melt spun values were 1280, 1370 and 1500 MN m-2 which relax on thermal ageing to 700, 700 and 800 MN m-2 for Al–16 Si, Al–Si–Ni and Al–Si–Mg, respectively. The hardness values of the melt spun ribbons are higher than the as-cast rods from which the ribbons were produced by a factor ranging from 1.8–2.2 times. Tensile testing at room temperature shows that the load–elongation curves are linear with a change of slope occurring in some of the specimens. These curves also show serrations in the case of as-melt spun and the intermediately annealed Al–Si specimens, while no serration was observed in the fully annealed samples. No serration was observed in the Al–Si–Ni and Al–Si–Mg alloys. UTS values were 420, 270 and 100 MN m-2 for Al–16 Si, Al–Si–Ni and Al–Si–Mg, respectively. These values show that the rapid solidification process improved the tensile properties significantly in Al–16 Si and Al–Si–Ni alloys while no significant improvement can be detected for Al–Si–Mg alloy. A discussion is given on hardness relaxation and tensile testing results in terms of silicon precipitation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of plasma treatment on the microstructure and electrical properties of MIM capacitors with PECVD silicon oxide and silicon nitride

Metal–insulator–metal (MIM) capacitors with plasma enhanced chemical vapor deposited (PECVD) nitride exhibit trap-induced dispersive behavior and electrical hysteresis, which lead to degradation in capacitor linearity at low frequencies. The dominant defect was suggested to be silicon dangling bonds originated from nitrogen deficiency. Previous methods to eliminate the dispersive behavior and electrical hysteresis include use of oxide–nitride–oxide (ONO) stacks and/or plasma pre-treatment of silicon substrate before nitride deposition [Van Huylenbroeck S, Decoutere S, Venegas R, Jenei S, Winderickx G (2002) IEEE Electron Device Lett 23:191; Lau WS (1990) Jpn J Appl Phys 29:L690]. In this study, the plasma post-treatment method was employed; MIM capacitors with PECVD oxide and nitride were treated with N₂O and SiH₄/NH₃ plasma, respectively, after deposition of the dielectric layer. No apparent change in film microstructure is observed after plasma treatment. Plasma post-treatment is effective in eliminating the electrical hysteresis shift of the nitride capacitors. Fourier transform infrared (FTIR) absorption spectra suggest an increase of the Si–H bond after SiH₄/NH₃ plasma bombardment of the nitride films. Auger depth profiling indicates a slight increase of nitrogen to silicon ratio after plasma treatment. The increase of the Si–H bonds as well as the raise of nitrogen to silicon ratio are two possible causes for the elimination of the hysteresis shift of the plasma-treated nitride capacitors. The time dependent dielectric breakdown testing indicates a decrease in both the leakage current and the lifetime of the MIM capacitors treated with plasma. Possible dielectric degradation mechanism is explored.     

纤维素接枝丙烯酰胺高吸水树脂的制备与表征

通过溶液聚合的方法,以丙烯酰胺和浆板制备了纤维素系高吸水树脂;研究了单体量、合成温度、引发剂量、交联剂量、水解浓度对树脂吸水能力的影响,并确定树脂的最佳制备条件为:浆板3g,单体量18g,交联剂量0.08g,引发剂量0.6g,合成温度70℃,水解浓度0.7mol/L,此时吸水倍率最高,达到633.3g/g;采用红外光谱(FT-IR)、X-射线衍射(XRD)、热重分析(TGA)和扫描电镜(SEM)分别对树脂进行表征并分析了高吸水树脂的结构。     

Wet and dry highly porous cellulose beads from cellulose–NaOH–water solutions: influence of the preparation conditions on beads shape and encapsulation of inorganic particles

Wet (swollen-in-water) and dry highly porous cellulose beads were prepared from cellulose–8%NaOH–water solutions by making droplets and coagulating cellulose in water bath. The shape of the beads varied from very flat plates to spheres. The influence of the preparation conditions (cellulose concentration, delay time, bath temperature, and the distance between the pipette and the bath surface) on the shape of the beads is discussed. Higher is the solution viscosity and weaker is the shock of the falling droplet on the bath surface, more spherical are the beads. Gelation of cellulose–NaOH solutions is shown to be a very important factor controlling bead shape. A simple way of encapsulation of various powders in cellulose beads is demonstrated and wet cellulose–inorganic composites were prepared. Highly porous (Aerocellulose) beads carrying encapsulated particles were made by drying coagulated cellulose under CO₂ in supercritical conditions.     

Electrical and structural properties of LiNbO<Subscript>3</Subscript> films,grown by RF magnetron sputtering

Nanocrystalline films of LiNbO₃ on substrates (001)Si and (001)Si–SiO₂ were synthesized by the method of RF magnetron sputtering. The elemental composition, structure of the LiNbO₃ films, and also—electrical properties of heterostructures (001)Si–LiNbO₃ and (001)Si–SiO₂–LiNbO₃ were studied. The dielectric constant of the LiNbO₃ films calculated from the capacitance at the accumulation region was about 28. The resistivity was 1·10⁹ ohm cm for films on (100)Si and 1.6·10¹¹ ohm cm for films on (001)Si–SiO₂. It has been determined that transmission of the current in the studied structures during direct biases is defined by hopping conduction, and, during reverse biases—by the Poole–Frenkel effect.     

Potential of short Si–Ti–C–O fiber-reinforced epoxy matrix composite as electromagnetic wave absorbing material

The effects of fiber electrical properties on electromagnetic wave absorbing potential in short Si–Ti–C–O fiber-dispersed epoxy matrix composites were studied. Six kinds of short Si–Ti–C–O fibers with different respective electrical resistivity were incorporated into an epoxy matrix and the dielectric properties of the composites in a frequency range from 1 MHz to 1 GHz were measured. The penetration depth of electromagnetic wave, which is defined as the distance to reduce 1/e of the incident electromagnetic wave power, is obtained from the measured dielectric properties. It is found that the dielectric properties of the composites are strongly dependent on the electrical resistivity of the fiber: the use of lower electrical resistivity fiber leads to a shorter penetration depth. Independent of the electrical resistivity of fiber, the penetration depth decreases with increase in the frequency. This result demonstrates the potential of the composite as a thin electromagnetic wave absorbing material.

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Dielectric properties of solution-grown-undoped and acrylic-acid-doped ethyl cellulose

Dielectric capacities and losses were measured, in the temperature (50–170°C) and frequency (01–100 kHz range), for undoped and acrylic acid (AA) doped ethyl cellulose (EC) films (thickness about 20 μm) with progressive increase in the concentration of dopant in the polymer matrix. The variation of capacity with temperature is attributed to thermal expansion in the lower temperature region to the orientation of dipolar molecules in the neighbourhood of glass transition temperature (T _g) and random thermal motion of molecules aboveT _g. The dielectric losses exhibit a broad peak. Doping with AA is found to affect the magnitude and position of the peak. AA is found to have a two-fold action. Firstly, it enhances the chain mobility and secondly, it increases the dielectric loss by forming charge transfer complexes.     

The effect of Copper alloying additions on the crystallization of an amorphous Fe–Si–B alloy

The effect of Cu alloying additions on the crystallization of Fe–Si–B alloy was studied. The selected alloys compositions are Fe_77.5Si_13.5B₉ and Fe_76.5Si_13.5B₉Cu₁. By comparing their crystallization temperatures, activation energy of crystallization, phase formation and microstructural evolution after heat treatment, the effect of Cu alloying additions was determined. It was found that Cu alloying additions reduced the crystallization temperature as well as the activation energy of the crystallization. Although the phases formed in both alloys were Fe₃Si and Fe₃B phases the microstructures were dramatically different: a dendritic microstructure was observed in the case of the Fe–Si–B alloy, while spheriodal crystals around 100 nm in size were observed in the case of the Fe–Si–B–Cu alloy. Cu alloying addition increased the saturation magnetization during primary crystallization whereas it decreased the saturation magnetization after secondary crystallization began. Interestingly, for both alloys the same trends of the magnetization and coercivity measurements were observed except that the extent of the changes were higher in the case of the Fe–Si–B–Cu alloy than that of the Fe–Si–B alloy.     

Effect of doping on TSD relaxation in cellulose acetate films

Thermally stimulated depolarization current (TSDC) studies have been performed on solution grown cellulose acetate films doped with different concentrations of acrylic acid (AA) prepared at the poling temperatures (40–75°C) with poling fields (10–50 kV/cm). The TSDC spectra of pure and AA doped CA films reveal two relaxation peaks at 80°C and 180 ±2°C, having activation energies centred around 0.25 and 0.55 eV. The phenomena of the existence of these current maxima have been analyzed and discussed in terms of the molecular motion of the polar side groups and release of the remaining part of the frozen dipoles by their cooperative motion with adjoining segments of the main polymer chain. The peak currents, released charge and activation energies associated with the peaks are affected by AA doping. The effect of doping with acrylic acid on the discharge current indicates the formation of molecular aggregates     

Mechanism of cyclohexene vapor curing polycarbosilane fibers

Cyclohexene vapor, instead of air, is applied to cure polycarbosilane (PCS) fibers. The cured fibers are characterized by infrared (IR), electron spin resonance (ESR), elements analysis (EA) and simulated through the HyperChem^TM program for comparison. The curing process is investigated by thermoanalysis. The results indicate that the Si–H and Si–CH₃ bonds in PCS are induced by cyclohexene to cleavage and form Si-central radicals. A fully developed cross-linking fibers come into being through the combination of these radicals, and the byproducts, some cyclohexyls bonded onto PCS derived from cyclohexene, introduce the variations in IR spectra, weight gain and carbon contents increase of PCS. On the basis of investigation and simulation, a likely mechanism of curing reaction is presented.     

Development of Fe<Subscript>3</Subscript>C,SiC and Al<Subscript>4</Subscript>C<Subscript>3</Subscript> compounds during mechanical alloying

Mechanical alloying process, as a solid-state technique, is a very useful method for fabrication of high melting point compounds like metal carbides and nitrides, which additionally have nanocrystalline structure with improved properties. In this work the development of several carbides including iron, aluminium and silicon carbides by the mechanical alloying process and the effect of subsequent heat treatment were investigated. Mixtures of elemental powders of Fe–C, Si–C and Al–C were mechanically alloyed, nominally at room temperature using a laboratory planetary ball mill. Structural changes of samples were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the aluminium carbide (Al₄C₃) could not be synthesized by mechanical alloying process alone, even after long milling times. A suitable subsequent heat treatment was required to allow Al–C reaction to take place kinetically. In contrast mechanical alloying of Fe–C as well as Si–C systems directly led to the formation of Fe₃C and SiC carbides after sufficient milling time. In all cases the end product had a nanosized structure.     

Wear characteristics of spray formed Al-alloys and their composites

In the present investigation, different Al based alloys such as Al–Si–Pb, Al–Si, Al–Si–Fe and 2014Al + SiC composites have been produced by spray forming process. The microstructural features of monolithic alloys and composite materials have been examined and their wear characteristics have been evaluated at different loads and sliding velocities. The microstructural features invariably showed a significant refinement of the primary phases and also modification of secondary phases in Al-alloys. The Pb particles in Al–Si–Pb alloy were observed to be uniformly distributed in the matrix phase besides decorating the grain boundaries. The spray formed composites showed uniform distribution of SiC particles in the matrix. It was observed that wear resistance of Al–Si alloy increases with increase in Pb content; however, there is not much improvement after addition of Pb more than 20%. The coefficient of friction reduced to 0.2 for the alloy containing 20%Pb. A sliding velocity of 1 ms⁻¹ was observed to be optimum for high wear resistance of these materials. Alloying elements such as Fe and Cu in Al–Si alloy lead to improved wear resistance compared to that of the base alloy. The addition of SiC in 2014Al alloy gave rise to considerable improvement in wear resistance but primarily in the low pressure regime. The wear rate seemed to decrease with increase in sliding velocity. The wear response of the materials has been discussed in light of their microstructural features and topographical observation of worn surfaces.