Processing of mesocarbon microbeads to high-performance materials: Part II. Reaction bonding by in situ silicon carbide and nitride formation

The processing of carbon-ceramic composites by utilizing the unique sintering ability of mesocarbon microbeads (MCMB) is reported. The ceramic constituents (silicon nitride and silicon carbide) are formed in situ by reactions between MCMB and silicon in different atmospheres. In comparison with direct addition of ceramic (SiC, Si₃N₄) phases, in situ formation shows several appealing features. By inducing the reaction of silicon with MCMB, the sintering ability of the composite is enhanced via reaction bonding mechanisms. Similarly, it is demonstrated that composite porosity is limited owing to silicon reaction with nitrogen. The reactive formation of nanoscale ceramic reinforcements via decomposition of the silicon-containing polymer (e.g. poly-carbomethyl-silane) is also reported. This approach results in formation of uniform nanosized (>100 nm) SiC layers strongly bonded to the surface of the carbon particles. The presented results contribute to the development of carbon-ceramic materials with high-operational properties.     

Organosilicium-Chemie: Vom Molekül zum Werkstoff

Organosilicon Chemistry: From Molecules to Materials The transition from basic to applied organosilicon chemistry is described for the research fields that are studied by the author's group: silaethene chemistry, organosilicon polymer chemistry with silacycles and the chemistry of higher coordinated silicon species. Early investigations started with the study of the pyrolysis of silacyclobutanes that yielded silaethenes in the gas phase. However, these investigations are tedious and are of little use for a widespread study of silaethene chemistry. Therefore the generation of silaethenes in solution (from vinylchlorosilanes and tert-butyllithium) is the preferred method. Numerous differently substituted silaethenes have been generated in situ and their reactivity, especially in cycloaddition reactions, has been studied. Silaethenes that have two chlorine substituents at the silicon atom (especially Cl₂SiCHCH₂Bu^t) show an unusual cycloaddition reactivity and yield unexpected products with dienes and trienes (e. g. from [2+2] and [6+2] reactions); some of the silenes with donorfunctional groups at the α-C-atom undergo interesting intramolecular rearrangements. The cycloadducts of Si-dichlorofunctional silenes also show rearrangements when thermolyzed (e. g. [2+2] → [4+2] adducts). The reactivity of these silaethenes and their cyclo-adducts is due to the two chlorine atoms at the silicon atom and their influence can be explained by theoretical models. Silacycles synthesized from silaethenes have been incorporated or have been transformed into silicon containing polymers. Model compounds for nucleophilic substitution reactions at silicon centers and for the hydrolysis of chlorosilanes are higher coordinated silicon species up to coordination number „seven”︁. Some structures that describe such reactions have been characterized by X-ray crystallography.     

CW-Laser-Induced Solid-State Reactions in Mixed Micron-Sized Particles of Silicon Monoxide and Titanium Monoxide: Nano-Structured Composite with Visible Light Absorption

Silica–titania mixed oxides and composites have been broadly examined for their physical and catalytic properties, but titanium monoxide–silicon monoxide counterparts have yet drawn very little attention. Here we report that laser-induced processing of mixed silicon and titanium monoxides results in reactions yielding a nanostructured Ti/Si/O composite absorbing visible light. Changes in the composition of intimately mixed μm-sized particles of silicon monoxide (SiO) and titanium monoxide (TiO) induced by scanning of a cw CO₂ laser beam along the TiO–SiO pellet surface were examined by FTIR and Raman spectroscopy, X-ray diffraction and electron microscopy. They are shown to consist in the evolution of TiO₂ (rutile and anatase), titanium suboxides (Ti_4.5O₅, Ti₂O₃), silica and amorphous binary SiO_x, TiO_x and ternary Si_xTi_yO_z nano-phases which contain less or more O than SiO and TiO monoxides. These products are ascribed to concurrent silicothermal reduction of TiO and O-transfer between SiO and TiO due to interdiffusion of Si- and Ti-based species. These reactions taking place under transient localized heating are not inhibited by passivation shells around SiO and TiO particles. The laser-produced Ti/Si/O composite shows absorption band at 425 nm tailing up to 1100 nm. Its solar-light photocatalytic activity in decolorization of Methylene Blue is compared to that of the unheated SiO and TiO powders absorbing only in UV region.     

Electrochemical characteristics of porous TiO2 encapsulated silicon anode

TiO₂ coated silicon, which was prepared by the modified sol–gel method, was employed as the anode material for lithium secondary batteries and the relationship between the diffusivity and electrochemical characteristics was investigated. The results showed that the physical properties of the samples, such as their diffusivity and pore size distribution, enhanced the cycling efficiency of the TiO₂ coated silicon, probably due to the reduction of the side reactions, which may be closely related to the pore size distribution of the TiO₂ coating layer. The pore size of the coating layer plays an important role in retarding the lithium ion diffusion. In the experimental range studied herein, higher capacity retention was exhibited for the TiO₂ coated silicon prepared at pH 10.7.     

Kinetic regularities of the interaction of aluminum oxide and silicon dioxide single crystals with phosphoric acid

The kinetics of the interaction of aluminum oxide and silicon dioxide single crystals with phosphoric acid is investigated under isothermal treatment at temperatures in the range 250–350°C. The probable mechanism of the reactions of Al₂O₃ and SiO₂ with phosphoric acid is considered. The influence of the structure of the Al₂O₃ and SiO₂ oxides on their interaction with phosphoric acid is analyzed.     

Current improvement of porous silicon photovoltaic devices by using double layer porous silicon structure: applicable in porous silicon solar cells

The photoluminescence (PL) phenomena of porous silicon (PS) samples with different etching times were examined to find out a relationship between PL emission energy (experimental value of PS band gap energy) and the etching time for fabrication of double (two) layer porous silicon sample on one silicon substrate. The dependence of PL Peak energy with etching time was discussed. A double layer PS structure was formed by using two electrochemical reactions with different etching times of 20 and 10 min, respectively. The photovoltaic (PV) properties of mono layer and double layer porous silicon PV devices were examined and compared. The main result is the enhanced short-circuit current (I_sc) of double layer PS structure compared to monolayer ones.     

Sintering of mixtures of the MgO-MgCr2O4-MeC (Me=Si,Ti) system

A thermodynamic calculation of the probable reactions of interaction in mixtures of the MgO-MgCr₂O₄-MeC (Me=Si, Ti) system is presented. Sintering of MgO+MgCr₂O₄ mixture with additions of titanium and silicon carbides is investigated. It is established that the maximum density (96% of the theoretical value) of sintered specimens of MgO+MgCr₂O₄ mixture is attained at a 0.5% mass fraction of titanium or silicon carbide and a firing temperature of 1850°C.Translated from Ogneupory, No. 11, pp. 11–13, November, 1995.     

Combustion of carbonized coal residue in a mixture of ammonium nitrate and supercritical water

The possibility of low-temperature oxidation of a solid carbonized coal residue in a mixture of NH₄NO₃ and supercritical water (723 K and 30 MPa) is shown for the first time and its mechanism is described. Conjugate processes of oxidation of the carbonized residue and formation of combustible gases H₂ and CH₄ caused by the participation of H₂O in redox reactions was found. It was established that the ash residue has a high porosity and consists of agglomerated nanoparticles of silicon and metal oxides.     

Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.     

Effects of residual radicals on compositional and structural stability of silicon nitride fibers

In this study, amorphous silicon nitride fibers were prepared through the nitridation of cured polycarbosilane fibers. It was observed that their composition and properties can be controlled by adjusting the flow of NH₃ during the nitridation process. Based on their compositional and structural stability, the samples could be divided into two classes: stable fibers and unstable fibers. As indicated by the electron spin resonance spectra, the amount of the residual free radicals in the unstable fibers was significantly higher than that in the stable fibers. Because of the reactions of the radicals with the moisture in the air, the oxygen content of the unstable fibers increased day by day until the residual radicals were exhausted. Thus, to develop silicon nitride fibers with both low oxygen content and low carbon content, the amount of NH₃ used should be optimized to eliminate the free radicals. These results suggest that it is possible to tailor the nitridation conditions for preparing high-purity silicon nitride materials so that they exhibit desirable properties, such as compositional and structural stability, good mechanical properties, and high electrical resistivity.     

Coal demineralization using sodium hydroxide and acid solutions

A coal product which exceeds the purity requirements for ash, iron and silicon in Hall Cell anode carbon was prepared by a unique leaching sequence involving caustic treatment, followed by treatments with sulphuric and nitric acids. The two acid leaching steps for the three-step process can also be combined with no adverse effect on the coal quality for anode purposes. In the process, the majority of the pyrite reacts with NaOH, forming Fe₂O₃ and Na₂S. Na₂S is dissolved in the leaching liquor and Fe₂O₃ is trapped in the coal matrix. The incorporation of sodium with organic sulphur increases the ash content and decreases the heating value of caustic leached coal on a dry basis. The silicon and aluminium contents are lowered by reactions with NaOH. While most of the reaction products are soluble sodium silicates and aluminates, the remaining silica and alumina form noselite. Dilute sulphuric acid dissolves nearly all of the sodium salts. The remaining iron compounds are dissolved by nitric acid.     

The fast pyrotechnic reaction of silicon and red lead: heats of reaction and rates of burning

Heats of reaction were measured with a bomb calorimeter and linear rates of burning by an electronic timer. The former, reached a maximum at 10% Si for all sizes tested; the latter at 30% Si for coarse silicon and at 15% Si for fine silicon. An attempt was made to correlate both properties with rate of heat evolution. Reactions of mixtures containing <20% Si were violent owing to rapid rise of temperature to a high level, causing vaporization of products and rapid expansion of air above the delay element. Mixtures containing 20–50% Si were adaptable for practical use and reactions were mainly of the solid/solid type. The products are SiO₂, Pb, and unreacted Si.     

Thermal stability and crosslinking of hydropolysilanes for use as silicon carbide precursors

Thermal stability of [(CH₃SiH)₃₀(C₆H₅SiCH₃)₇₀]_n a hydropolysilane copolymer, in vacuum and its crosslinking reactions with vinylic silanes as crosslinking agents was evaluated in order to obtain high yields of oxygen-free silicon carbide ceramics. It was found that the polymer was thermally stable in vacuum up to 140 °C for 20 hrs based on Fourier transform infrared spectroscopy analysis. The crosslinking reactions of the polymer occurred to various extents depending on the type of vinylic silanes used as evidenced by Fourier transform infrared spectroscopy, ultraviolet spectroscopy, gel permeation chromatography, thermogravimetry and solubility data. The additions of vinylic silanes to Si-H in the hydropolysilane were found to obey anti-Farmer's rule, despite Farmer's addition of unsaturated hydrocarbons to Si-H.     

Kinetics and mechanism of the hydrolysis and alcoholysis of alkoxysilanes

As part of our work with the use of silane coupling agents, we have been investigating the transformations of alkoxysilancs into siloxanes. The influence of a para-substituted phenyl group attached to the silicon was investigated and the rates of acid catalyzed hydrolysis and alcoholysis of these para-substituted phenylalkoxysilanes have been determined. The kinetics and mechanism of the reactions are presented. These reactions are of interest because of their role in the use of silane coupling agents as adhesion promoters, in the preparation of zinc-rich silicate coatings, in the sol-gel process and in the preparation of silicones in general. The hydrolysis reaction was found to be first order in acid, zero order in water and to have a Hammett p of -1.42 when catalyzed by sulfuric acid. These results are consistent with current opinion that the reaction mechanism is SN₁, and involves a positive intermediate, possibly a siliconium intermediate. The alcoholysis reaction was found to be first order in both the silane and the catalysts, and of intermediate order in the alcohol, when catalyzed by carboxylic acids. When catalyzed by dichloroacetic acid, the reaction has a Hammet p value of +0.43. This is consistent with a concerted displacement reaction between the alkoxysilane and the complex involving the alcohol and the carboxylate anion. The intermediate is a negatively charged intermediate, probably a penta-substituted silicon species.     

Oligomer and Polymer Formation in Hexamethylcyclotrisiloxane (D<Subscript>3</Subscript>) – Hydrosilane Systems Under Catalysis by tris(pentafluorophenyl)borane

Summary Reactions of hexamethylcyclotrisiloxane, D₃, with 1,1,3,3-tetramethyldisiloxane, ^HMM^H, 1,1,1,3,3-pentamethyldisiloxane, ^HMM, phenyldimethylsilane and phenylmethylsilane catalyzed by tris(pentafluorophenyl)borane were studied. These reactions lead to ring opening of D₃ by the SiH reactant producing open chain oligomers with hydrosilane functionality at one or both chain ends. The reactivity of the hydrosilanes toward D₃ decreases in the series: PhMeSiH₂ > ^HMM^H > PhMe₂SiH > ^HMM. Competitive self-oligomerization of ^HMM^H and ^HMM also occurs. Primary products of these processes are able to enter into reactions with the SiH and D₃ reactants; some also undergo cyclization. Thus, consecutive and competitive processes lead to a series of various oligohomologues. Gas chromatography in conjunction with chemical ionization mass spectroscopy permitted identification of structure and determination of the basic directions of these oligomerization processes. Polysiloxanes of higher molecular weight may be also formed in some of these systems. The reactions, which occur in the systems studied, are rationalized on the basis of the mechanism involving the hydride transfer from silicon to trivalent boron. This includes the transient formation of tertiary trisilyloxonium borate which decomposes by the hydride transfer to one of the silicon atoms of the trisilyloxonium center. Footnote: This paper is dedicated to Professor Ian Manners in recognition of his significant contributions to the field of organometallic polymers.     

Study on rapid nitridation process of molten silicon by thermogravimetry and in situ Raman spectroscopy

The melt of silicon, hindering nitridation for its agglomeration, should be avoided in the direct nitridation of silicon to synthesize silicon nitride powders, although liquid phase facilitates nitridation. Therefore, we proposed a method to nitride molten silicon without agglomeration. Thermogravimetric and in situ Raman studies on the nitridation process of molten silicon were performed. The as-prepared silicon nitride samples were found to be micron clusters composed of submicron grains with high α-Si₃N₄ content. The nitridation of molten silicon at 1500°C was completed after 500 s and 109 times faster than the nitridation of solid silicon at 1350°C. β-Si₃N₄ is produced dominantly by α–β-phase transition. Less nitridation time and low temperature can decrease the β-Si₃N₄ content. The rapid nitridation was owning to core–shell structure Si@Si₃N₄, which was formed after the initial nitridation of silicon particles and hindered the agglomeration of molten silicon.     

SiC modified self-healing ytterbium disilicate materials for potential environmental barrier coating application

Environmental barrier coatings (EBCs) are crucial to the reliability and durability of SiC_f/SiC composite components seeking applications in hot sections of next-generation advanced aero-engines. The cracks initiated and developed in EBCs owing to various reasons during service greatly undermine their lifespans. To address this problem, in this work, silicon carbide (SiC) in the forms of particles and whiskers with various amounts have been introduced to ytterbium disilicate (Yb₂Si₂O₇), the mainstream EBC topcoat materials, so as to gain some self-healing potential. The results reveal that, the SiC inclusions in Yb₂Si₂O₇ in the presence of ytterbium monosilicate (Yb₂SiO₅) can trigger the following reactions. Specifically, SiC self-healing agents are oxidized to form viscous SiO₂, which actively reacts with Yb₂SiO₅ upon encountering it, forming Yb₂Si₂O₇. This has brought twofold beneficial effects including ① silicon supplementation of disilicate topcoat, whose silicon element tends to be “dragged out” by water vapor, leading to the deterioration of thermal mismatch; as well as ② crack self-healing resulting from the volume expansion induced by the above reactions. Then the two aspects of self-healing agents, namely the “promptness” and “sustainability,” have been discussed in detail. The former is unveiled to be more pertinent to the repairing of large cracks, whilst the latter is more relevant to the self-healing of tiny cracks at initiation or early stage of propagation. The current work sheds some lights on the design and development of more durable and robust EBCs with self-healing capability.     

Self-hardening coatings for protection of graphite products from oxidation

Results of testing coatings for silicided graphite that are based on a self-hardening mixture (Al₂O₃ powder modified with silicon alkoxide and ethyl silicate binder) are presented. The dense intermediate layer created, reinforced with mullite and SiC inclusions that resulted from mechanochemical reactions and firing, is shown to provide high resistance to oxidation for graphite up to 2003 K. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 5, pp. 19–23, May, 1997.     

Interaction of oxide phases of graphite-containing crucibles with secondary silumins

A thermodynamic investigation of the interaction between the oxide phases Al₂Si₂O₇ and SiO₂ of graphite-containing crucibles and metallic melts of the Al — Si system is presented. Al₂O₃ and metallic silicon are shown to be the equilibrium products of the reactions.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 8, pp. 15 – 16, August, 1996.     

Silica to Silicon: Key Carbothermic Reactions and Kinetics

The primary carbothermic reactions for the reduction of silica to produce silicon were defined and the reaction kinetics were determined. Most possible reactions between silicon oxide and carbon or carbon compounds were studied by a series of thermogravimetric analyses at temperatures up to 2000°C. Four key sequential reactions occur with SiC and SiO as intermediate reactants; two reactions involve SiO₂ and two involve SiO. Reaction rate versus temperature, activation energy, and preexponential factors were determined for each of six reactions involving SiO₂ or SiO. These kinetic studies show that SiO, when combined with either carbon or Sic, reacts in the gaseous state, and the sublimation of SiO is not the rate-limiting reaction for forming silicon.