Unreacted Si Research Articles

Effect of Mg/B2O3 molar ratio and furnace temperature on the phase evaluation and morphology of SiC–B4C nanocomposite prepared by MASHS method

In this study, SiC–B4C nanocomposite has been synthesized in situ successfully by Mechanical activated combustion synthesis method (MASHS). Initially Si, C, B2O3 and Mg powders as raw materials were weighed according to different molar ratio of Mg to B2O3. In next step theses materials were milled in a planetary mill under Ar atmosphere. The synthesis step was performed in a tube furnace equipped with controlled atmosphere system. The different furnace temperature was investigated on the phase synthesis and morphology of the products. The specimens in the various steps were studied by XRD analysis for evaluation of the phase compositions and calculation of the average crystallite size of them. The morphology of synthesized products was investigated by scanning and transmission electron microscopes (SEM&TEM). The final product contains main phases MgO, B4C and SiC. Also, In this sample byproducts were characterized such as Mg3B2O6, Mg2B2O5 and remaining carbon. XRD pattern of synthesized sample showed the considerable effect of increasing Mg to B2O3 molar ratio on reducing the amount of Mg3B2O6, Mg2B2O5 and remaining carbon. SiC–B4C composite was synthesized with more homogenous morphology by reducing the furnace temperature from 1000 to 900 °C, but reduction of temperature up to 800 °C give rise to uncompleted reaction whereas some unreacted Si remains. Average crystallite sizes of optimal sample were calculated 10.5 and 8 nm for SiC and B4C respectively. These values are consisted of the TEM results somehow while grain size was less than 70 nm. Also SEM observation showed fine grains with sizes falling in the nanometer range.

Influence of metallic palladium on thermal properties of polysiloxane networks

Polysiloxane networks differing in cross-link densities were prepared by hydrosilylation of linear polysiloxanes containing various amounts of vinyl groups uniformly distributed in their chains. Thus, D2V polymer with a vinyl group at every third Si atom in the macromolecule and 4D3V3 copolymer with vinyl groups located in a block constituting one fifth of its chain were reacted with hydrogensiloxanes of different molecular structures and functionalities in the presence of Karstedt catalyst. The process was conducted at excessive amounts of Si–H groups with respect to vinyl ones which ensured that unreacted Si–H groups remained in the polysiloxane networks formed. Such systems were treated with palladium(II) acetate solution in THF. As established by X-ray diffraction and FTIR spectroscopic studies, the redox reaction between Pd2+ ions from the solution and Si–H groups of the networks occurred which resulted in the appearance of metallic Pd particles in the systems. Systematic investigations conducted using thermogravimetry coupled with mass spectrometry allowed to conclude that the presence of Pd modifies thermal properties of polysiloxane networks, influencing mainly redistribution of Si bonds taking place during thermolysis of the systems. It was found that higher cross-link densities of the D2V polymer-derived networks than those of the 4D3V3 copolymer-based ones are beneficial for thermal stability of the systems with incorporated Pd. The type of the cross-linking agent, in turn, decides on residual mass remaining after pyrolysis of the Pd-containing samples conducted at 1000 °C in Ar flow.

Fabrication and properties of Si−Hf alloy melt-infiltrated Tyranno ZMI fiber/SiC-based matrix composites

The fabrication of Tyranno ZMI fiber/SiC-based matrix composites by a melt infiltration (MI) process using Si−8.5at%Hf alloy was investigated. The Si−Hf alloy was utilized instead of pure Si for MI processing in order to lower the MI temperature to below 1400°C as well as to inhibit the thermal degradation of amorphous SiC fibers. Microstructural characterization revealed that the composite fabricated by Si−8.5at%Hf alloy MI at 1375°C had a highly dense matrix comprising reaction-formed SiC and unreacted Si−HfSi2 phases. The Si−Hf alloy melt-infiltrated composite exhibited approximately 35% higher bending strength than a conventional Si melt-infiltrated composite fabricated at 1450°C. Further, it was confirmed that Si−Hf alloy melt-infiltrated composite maintained its strength up to 1200°C in an Ar atmosphere. These results clearly demonstrated that the Si−8.5at%Hf alloy offers sufficient infiltration ability and reactivity to form a dense SiC-based matrix at 1375°C.

Silylation of Layered Silicate RUB-51 with SiCl4 and Conversion of the Silylated Derivative to a Crystalline Microporous Material

A novel crystalline microporous material is synthesized by silylation of layered silicate RUB-51 with tetrachlorosilane (SiCl4), hydrolysis, and heat treatment for interlayer condensation. One SiCl4 molecule first undergoes ordered silylation with two confronting groups, Si–O– and Si–OH, on the interlayer surfaces of RUB-51 (bidentate silylation). Hydrolysis of the unreacted Si–Cl groups of the immobilized silyl groups by a mixture of H2O and dimethyl sulfoxide (DMSO) affords geminal Si–OH groups on the surfaces without simultaneous interlayer condensation because of intercalation with DMSO. Subsequent heat treatment leads to condensation between neighboring layers, and the condensed material is composed of layers of RUB-51 displaced by a half-unit cell along the a axis. Hydroxyl groups are present after interlayer cross-linking, which is one of the unique features of the method using SiCl4. The obtained sample adsorbs CO2 molecules, while RUB-51 itself without silylation cannot. The amount of adsorbed CO2 on the microporous material is larger than that of CH4, suggesting the potential of this material as a separation medium between CO2 and CH4. This study indicates that the preparation of microporous materials, using layered silicates as a building block and various silylating agents, is useful for precise design of both porosity and functional groups on pore surfaces, which drastically affect the properties of crystalline microporous materials, including catalytic selectivity and separation capability.

Scalable Synthesis of Interconnected Porous Silicon/Carbon Composites by the Rochow Reaction as High‐Performance Anodes of Lithium Ion Batteries

AbstractDespite the promising application of porous Si‐based anodes in future Li ion batteries, the large‐scale synthesis of these materials is still a great challenge. A scalable synthesis of porous Si materials is presented by the Rochow reaction, which is commonly used to produce organosilane monomers for synthesizing organosilane products in chemical industry. Commercial Si microparticles reacted with gas CH3Cl over various Cu‐based catalyst particles to substantially create macropores within the unreacted Si accompanying with carbon deposition to generate porous Si/C composites. Taking advantage of the interconnected porous structure and conductive carbon‐coated layer after simple post treatment, these composites as anodes exhibit high reversible capacity and long cycle life. It is expected that by integrating the organosilane synthesis process and controlling reaction conditions, the manufacture of porous Si‐based anodes on an industrial scale is highly possible.

Convenient Melt-Growth Method for Thermoelectric Mg2Si

We have succeeded in growing single-crystalline-like n-type Mg2Si bulk crystals by a convenient melt-growth method that requires no vacuum or inert gas. The Sb-doped, n-type Mg2Si crystals had a density equivalent to the theoretical ideal of 1.99 g cm3 to 2.00 g cm−3 and well-developed crystalline grains. Powder x-ray diffraction measurements and scanning electron microscopy observations confirmed the single-phase Mg2Si nature of the grown crystals, with no MgO or unreacted Si and Mg observed. The crystals had high Hall mobility and power factor compared with Sb-doped sintered Mg2Si crystals. The achieved ZT values were 0.10 at 300 K and 0.36 at 600 K for 0.317 at.%Sb-doped Mg2Si.

Salt-assisted combustion synthesis of β-SiAlON fine powders

A novel method of producing submicron-size β-SiAlON powders by combustion synthesis using NaCl as a diluent was proposed. The combustion synthesis was carried out with the raw materials Si, SiO2, and Al powders, and varying amounts of NaCl under a nitrogen pressure of 1 MPa. Phase compositions and particle morphologies of the synthesized powders were analyzed. The results revealed that as the z-values increased, the amount of NaCl needed to complete the reaction increased, which in turn decreased the particle size of the product. With small z-values, single-phase products were obtained; however, with large z-values, NaCl and unreacted Si remained in the products due to the large amount of liquid phase. NaCl acted not only as a diluent by absorbing the heat generated by the reaction but also as a diffusion barrier between β-SiAlON particles, which greatly limited the growth of β-SiAlON crystals.

Fabrication of diamond/SiC composites by Si-vapor vacuum reactive infiltration

Diamond/SiC composites were fabricated by a rapid gaseous Si vacuum reactive infiltration of porous carbon-containing diamond preforms at 1600°C for 1h. The obtained composites consisted of diamond, β-SiC and a small fraction of residual unreacted Si; nearly fully dense composites were achieved. Graphitization of diamond did not occur during the infiltration at 1600°C under vacuum. The transgranular fracture mode of diamond crystals and the perfect interface between diamond and SiC suggested a strong interfacial bonding strength. Relatively-low coefficient of thermal expansion (3.6×10−6/K, 50–400°C) and high thermal conductivity (562W/mK) of diamond/SiC composites showed great potential for thermal management applications.

Microstructural Characterization of Diamond/SiC Composites Fabricated by RVI

Diamond/Si/C porous preform was prepared with phenolic resin, Si, Diamond and graphite. Subsequent reactive vapor infiltration (RVI) of gaseous silicon at 1600 °C for 2 h in vacuum atmosphere resulted in the formation of a compact Diamond/SiC composite. The influence factors of perform porosity were investigated, including process pressure and raw materials. Density and microstructure of Diamond/SiC composites were also discussed. The results showed that the open porosity of preform was mainly influenced by the pressing pressure. It can also be affected by the content, morphology and the particle size of diamond. The preform with open porosity higher than 20% after RVI treatment can obtain Diamond/SiC composites with high density. However, over-high porosity of the perform will cause a mass of unreacted Si exists in the composite which is unfavorable for its application. In order to reduce the content of residual Si, open porosity of preform should be no higher than 40%.

Exhaustive and partial alkoxylation of polymethylhydrosiloxane by copper-catalyzed dehydrocoupling with alcohols: Synthesis, crosslinking behavior, and thermal properties of the products

Polymethylhydrosiloxane (PMHS) reacts with aliphatic and aromatic alcohols at room temperature in the presence of [CuH(PPh3)]6 complex catalyst to give poly[(methyl) (alkoxy)siloxane]s in high yields. Reactivity of alcohols decreases in the order of p-methoxyphenol > p-cresol > phenol > benzyl alcohol > allyl alcohol > ethanol > isopropanol > tert-butyl alcohol. Partially p-cresylated polymers, which still retain unreacted SiH bonds, react further with ethylene glycol or water to form cross-linked polymers, which, depending on the extent of cross linking, gelate during the cross-linking process. Propargyl alcohol reacts with PMHS very rapidly to give exhaustively and partially propargyloxylated PMHS. Resulting polymers, upon heating, undergo crosslinking. Partially propargyloxylated polymers display high thermal stability [Td5 (temperature of 5% weight loss) > 500 °C] as compared with starting PMHS (243 °C) and exhaustively propargyloxylated one (414 °C). © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Effect of Si content on H 2 production using Al–Si alloy powders

The effect of Si content in Al–Si alloy powder with NaOH on H 2 production was investigated. The total amount of H 2 produced decreased as Si content increased, which is inconsistent with the results predicted by the chemical reaction. Si caused a delay in the rate of H 2 production. Energy dispersive spectrometry showed that a large amount of unreacted Si remained in the matrix, and the unreacted fraction increased as the Si content increased. As the evolution reaction of Al and Al–Si alloys is exothermic, the temperature of all the specimens increased. Si addition reduced the hydroxide removal rate, which decreased the average H 2 production rate. The initiation time for H 2 evolution depends on the elimination rate of the oxide film formed during production of the powder. On increasing the Si content, SiO 2 was formed, which is harder to eliminate than Al 2O 3; this delayed the initiation.

Molten salt synthesis of silicon carbide nanorods using carbon nanotubes as templates

Silicon carbide (SiC) nanorods were synthesised by reacting multi-walled carbon nanotubes (CNTs) with Si particles in a NaCl–NaF binary salt for 4 h at 1100–1200 °C in Ar. Reaction products were analysed by a combination of X-ray diffraction (XRD) and transmission electron microscopy (TEM), including aberration corrected lattice imaging and electron energy loss spectroscopy (EELS). At 1100 °C, 3C–SiC became detectable by XRD but unreacted Si and CNTs were still dominant, while, at 1150 °C, SiC increased at the expense of Si and CNTs. Finally, at 1200 °C, only SiC peaks were seen, indicating the complete conversion from CNTs to SiC nanorods. This synthesis temperature is 200–250 °C lower than that required by the conventional vapour–solid reaction process. The resultant SiC nanorods mostly followed the morphologies of the as-received CNTs, indicating that CNTs had not only served as carbon source but also acted as the templates for the nanorod growth. Most of the resultant SiC nanorods had complex stacking sequences containing various forms of stacking faults, and only some exhibited regular fault-free stacking sequences.

Effects of α-Si 3N 4 and AlN addition on formation of α-SiAlON by combustion synthesis

Preparation of Yb α-SiAlON was investigated by self-propagating high-temperature synthesis (SHS) from α-Si 3N 4- and α-Si 3N 4/AlN-diluted powder compacts under nitrogen of 2.17 MPa. For the AlN-free samples, the molar ratio of Si 3N 4/Si varies between 0.22 and 0.5. The starting stoichiometry of the AlN-added samples comprises a constant proportion of Si 3N 4/Si equal to 0.22, but a broad range of AlN/Al from 0.33 to 1.0. The self-sustaining combustion wave propagated in the spinning mode on account of highly diluted samples adopted in this study. The overall reaction exothermicity increases with Si 3N 4/Si ratio for the AlN-free samples, while decreases with AlN/Al ratio for the AlN-added powder compacts. As a result, the amount of unreacted Si left in the final product was significantly reduced and the formation of nearly single-phase Yb α-SiAlON was achieved in the sample with Si 3N 4/Si = 0.5. Moreover, the growth of elongated α-SiAlON grains was enhanced in the samples with high contents of Si 3N 4. In contrast, the nitridation of Si was only improved to a certain extent with the addition of AlN and no further improvement was attained by increasing the AlN content. Due to the lack of sufficient liquid phases during combustion and the weak reaction exothermicity, the samples with high contents of AlN were inclined to produce α-SiAlON grains in a fine equiaxed form.

Effects of Carbon Fiber Architecture on the Microstructure and Mechanical Property of C/C-SiC Composites

C/C-SiC composites were rapidly fabricated by a two-steps processing. Firstly a short-cut carbon fiber felt (SC) and a 2D carbon fiber felt (2D) were densified to C/C composites by a thermal gradient chemical vapor infiltration (CVI) method with vaporized kerosene as a precursor in 2h, 3h, 4h and 5h, respectively. Then the C/C composites were infiltrated and reacted with melting silicon to obtain C/C-SiC composites. The results show that, with increase of the CVI time, the densities of the two types of C/C-SiC composites decrease in the range of 2.28g/cm3 to 2.00g/cm3; their porosities increase ranging from 1.3% to 7.5%; the contents of the β-SiC and the unreacted Si phases in the composites decline. The flexural strength of the 2D_C/C-SiC composite is much higher than that of the SC_C/C-SiC composite when prepared in the same condition.

Dispersibility in organic solvents of nanosized silica particles used in semiconductor package substrates

This study examined the influence of different types of silane coupling agents on the dispersion of nanosilica particles with diameters of ca. 10 nm and 25 nm in two organic solvents, methylisobutylketone (MIBK) and cyclohexanone (CXN). The nanosilicas were prepared by the sol–gel process, and the following coupling agents were used: 3-glycidylpropyltrimethoxysilane (EpoSi), N-phenyl-3-aminopropyltrimethoxysilane (AmiSi), 3-mercaptopropyltrimethoxysilane (MerSi), and alkyltrimethoxysilane (AlkSi). While the 25-nm silica particles displayed dispersibility similar to that of submicron silica particles (ca. 150 nm and 250 nm), the dispersibility of the 10-nm silica particles proved to be dependent on the combinations of the agents and solvents; for example, EpoSi and MerSi were found to be suitable agents for MIBK and CXN, respectively. The slurries were also subjected to heating at 40 °C for 24 h, which resulted in a considerably higher stability of the dispersion state when CXN was employed as the solvent, regardless of which silica coupling agents were used. FTIR measurement revealed the presence of unreacted free Si–OH at 3740 cm −1 for some nanosilicas. Combustion of the organic moieties introduced onto the silica surfaces also proved that the amount of organic groups present in a unit surface area of the nanosilicas were only 1/3 or 1/2 of that found on submicron silicas. From these observations, the low dispersibility of the nanosilicas was inferred to be due to the imperfect coverage of the silanol groups by the silane coupling agents.

Phase stabilization by rapid thermal annealing in amorphous hydrogenated silicon nitridefilm

We have studied the effect of rapid thermal annealing (RTA) in the context ofphase evolution and stabilization in hydrogenated amorphous silicon nitride(a-SiNx:H) thin films having different stoichiometries, deposited by an Hg-sensitized photo-CVD(chemical vapor deposition) technique. RTA-treated films showed substantialdensification and increase in refractive index. Our studies indicate that a mereincrease in flow of silicon (Si)-containing gas would not result in silicon-richa-SiNx:H films. We found that out-diffusion of hydrogen, upon RTA treatment, plays a vital role in theoverall structural evolution of the host matrix. It is speculated that less incorporation of hydrogenin as-deposited films with moderate Si content helps in the stabilization of the silicon nitride(Si3N4) phase and may also enable unreacted Si atoms to cluster after RTA. These studies are ofgreat interest in silicon photonics where the post-treatment of silicon-rich devices isessential.

Thermal Conductivities of β-SiAlONs by Mechanically Activated Combustion Synthesis

–sialons (Si6-zAlzOzN8-z, z=3) synthesized by mechanically activated combustion synthesis (MA-CS) at a low N2 pressure of 1 MPa, were sintered by Spark Plasma Sintering (SPS) and thermal conductivity was measured at room temperatures. Specimens were fully densified at 1600oC for 10 mins. and showed only –sialon phases confirmed by x-ray diffraction patterns though un-reacted Si was present as impurities after MA-CS. Thermal conductivities increased with sintering temperature and had a maximum value 5.49 W m-1 K-1 for specimens sintered at 1700oC.

Fabrication, chemical etching, and compressive strength of porous biomimetic SiC for medical implants

BioSiC is a biomimetic SiC-based ceramic material fabricated by Si melt infiltration of carbon preforms obtained from wood. The microstructure of bioSiC mimics that of the wood precursor, which can be chosen for tailored properties. When the remaining, unreacted Si is removed, a SiC material with interconnected porosity is obtained. This porous bioSiC is under study for its use as a medical implant material. We have successfully fabricated bioSiC from Sipo wood and studied the kinetics of Si removal by wet etching. The results suggest that the reaction is diffusion-limited, and the etch rate follows a t−0.5 law. The etching rate is found to be anisotropic, which can be explained attending to the anisotropy of the pore distribution. The compressive strength was studied as a function of etching time, and the results show a quadratic dependence with density. In the attainable range of densities, the strength is similar or better than that of human bone.

Porous Si/SiC Composite Based on Nonwoven Cellulosic Fabrics: Synthesis and Characterization

The microstructure and properties of porous Si/silicon carbide (SiC) composites manufactured from nonwoven cellulosic fabrics have been investigated. A fibrillar reaction product with a microstructure reproducing the initial preform morphology consisting of β‐SiC and unreacted Si was obtained. Improvement of strength is expected to be achieved by increase of SiC content and reduction of porosity.

Investigation of the Chemical Purity of Silicon Surfaces Reacted with Liquid Methanol

The reaction of hydrogen-terminated Si(111) and oxide-terminated silicon surfaces with neat anhydrous liquid methanol (CH3OH) has been studied with Fourier transform infrared spectroscopy (FTIR) as a function of solution temperature and immersion time. At 65 °C, reaction of atomically smooth H−Si(111) surfaces with CH3OH (l) results in partially methoxylated silicon surfaces that are free of any detectable subsurface oxidation (Si−O−Si bonds); this is in contrast to observable oxidation found after similar reactions on H−Si(100) surfaces. At long reaction times (t > 3 h), the Si(111) surface saturates with Si-OCH3 sites at a coverage of approximately 30% of a monolayer, with the residual ∼70% comprised of unreacted Si−H sites. The lack of any detectable silicon oxide makes it possible to conclude the following: (i) Reaction mechanisms involving insertion of oxygen atoms from the CH3OH molecule into the subsurface Si−Si back bonds cannot be dominant for (111)-oriented silicon under these conditions. (ii) The vibrational modes of the oxide-free surface are very sharp and can be clearly distinguished from blue-shifted modes observed for methoxyl groups chemisorbed on oxidized surfaces. For surfaces that display subsurface oxidation, no evidence for oxygen atoms directly below atop Si−H sites has been observed. Instead, FTIR analysis demonstrates that subsurface oxidation selectively exists underneath atop Si-OCH3 sites. Finally, H-terminated oxide surfaces, prepared by reacting trichlorosilanes on OH-terminated SiO2 surfaces, react with methanol to form a methoxy-terminated oxide surface.