Carbothermal Reduction Of SiO2 Research Articles

The microstructural analysis of SiC nanorods synthesized through carbothermal reduction

SiC nanorods were synthesized through carbothermal reduction of SiO by C nanocapsules (CNCs) or amorphous activated C (AAC). The synthesized nanorods were characterized by high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectrometry (EDS). The nanorods formed during a reaction between SiO and CNCs were straight. They displayed a preferential orientation of the growth axis; Which was either parallel or normal to the [111] direction. The nanorods formed during reaction between SiO and AAC consisted of straight and curled parts or chains of SiC nanoparticles. The straight parts were aligned in the [111] direction. A surface of the rod tip was covered with a thin amorphous layer, 1∼3 nm thick; Containing Si; C and traces of O. Formation mechanism of the SiC nanorods was proposed.

Synthesis and characterization of nanowires and nanocables

The synthesis and characterization of nanowires and nanocables of carbides and nitrides were reported. The carbothermal reduction of SiO 2 xerogel containing carbon nanoparticles in argon was used to prepare SiC nanowires and SiC nanowires sheathed with amorphous SiO 2 coatings (SiC-cored nanocables). Si 3N 4 nanowires were also made by the carbothermal reduction, but the reaction should be carried out in N 2 atmosphere. The gas reaction of Ga 2O vapor with flowing NH 3 was used to form GaN nanowires on a porous alumina template.

Growth and characterization of nanostructured β-SiC via carbothermal reduction of SiO 2 xerogels containing carbon nanoparticles

Sol–gel route was used to prepare SiO 2 xerogels containing carbon nanoparticles. Three kinds of β-SiC nanostructures were obtained by carbothermal reduction of the xerogels under different conditions. XRD and IR studies show that the powders, obtained from carbothermal reduction at 1650° for 2.5 h in static argon atmosphere, are β-SiC. TEM reveals that the powders are nanoparticles with diameters ranging from 30 to 100 nm. The specific surface area of the powders are 15.7 m 2/g. TEM, SAED and EDX show that the products, synthesized by carbothermal reduction at 1650°C for 1.5 h, and followed by heating to 1800°C, holding for 30 min in flowing argon atmosphere, are nanowires that consist of 10- to 25-nm-diameter crystalline β-SiC cores surrounded by amorphous SiO 2 sheathes with outer diameters of between 20 and 70 nm. While TEM and SAED demonstrate that the products, formed by carbothermal reduction at 1650° for 2.5 h in flowing argon atmosphere, are bare SiC nanowires with diameters in the range of 15–30 nm.

Stacking Faults in β‐SiC Formed during Carbothermal Reduction of SiO2

Factors affecting stacking fault formation during the synthetic fabrication of β‐SiC were investigated in the present study. Two main reaction routes in the carbothermal reduction of SiO2—solid–solid and solid‐gas reactions—led to the formation of β‐SiC, depending on the vapor pressure of the SiO gas. The β‐SiC formed by the solid‐gas (carbon‐SiO gas) reaction showed a whisker morphology with a high stacking fault density (SFD), whereas that formed by the solid‐solid (Si‐carbon) reaction exhibited spherical particles with a low SFD. The average size of the synthesized particles decreased with decreasing reaction temperature and time, and their stacking fault content, measured by X‐ray diffraction, was higher than the true value, possibly because of a size effect. The stacking fault density increased with increase in the heating rate because of an increased reaction rate.

Mechanism of Si3N4 nucleation during carbothermal reduction of silica

The process of carbothermal reduction of SiO2 in a nitrogen flow at temperatures of 1673–1723 K was investigated. It was established that mixtures obtained by the sol-gel technique are not microhomogeneous and consist of silica and carbon aggregates, inside which the processes of structure and phase ordering occur during heat treatment. The contact region of these aggregates is the place where SiO2 reduction occurs. In this region the following transformation takes place: partial SiO2 reduction, appearance of a film of melt of the composition SiOx, where x<2, enveloping of the carbon particles by this melt, destruction-activation of the carbon particles, implantation of nitrogen in the highly oxygen-defective melt, formation of silicon oxynitride and, subsequently, silicon nitride. α-Si3N4 forms in the presence of a large carbon nucleus, β-Si3N4 forms in its absence (or on two-dimensional particles).

Effect of gas phase on SiC and Si3N4 formations from SiO2

During the synthesis of SiC, Si3N 4 and sialon whiskers by carbothermal reduction of SiO 2, a localized formation of amorphous phases or Si2N20 powders was observed beneath these whiskers. Because these whiskers were formed by the vapour/solid mechanism, the controlling gas phase was of primary importance to obtain whiskers of tailored morphology and chemistry. To elucidate the effect of the gas phase composition on the reaction mechanisms of SiC and Si3N 4, the oxygen partial pressure was measured during the synthesis with a Zr02 solid electrolyte. The carbothermal reduction of Si02, as well as evolution of gases, were accelerated by a formation of a molten fluorosilicate with an auxiliary halide bath. The oxygen partial pressure steadily increased with increasing temperature and reached a maximum level of 10-11-10 -12 atm in the early stage of reaction at 1623 K, then decreased again towards the end of reaction in both cases. Effects of the gas phase on the SiC and Si3N 4 formations were not the same: Pco and Pc02 and their ratio were important factors in the SiC formation, while the higher Po02 formed an oxynitride phase in the Si3N 4 formation.

Carbon Removal and Oxidation of SiC Powder Synthesized by Carbothermic Reduction of Silica

Three kinds of SiC powders were synthesized by the carbothermic reduction of SiO2 at 1973K in an Ar atmosphere from reagent graphite, electrode graphite and carbon black. The oxidation rates of free carbon and SiC component in these powders were measured in an O2 gas stream at temperatures from 923 to 1573K by the thermogravimetric analysis and the determination of CO2 evolved. As the powders synthesized from the reagent graphite and carbon black were surrounded continuously by SiC layer, the free carbon could not be removed by oxidation. The free carbon in the powder synthesized from the electrode graphite was partly removed. The activation energy for the oxidation of free carbon was 173kJ/mol. This oxidation was controlled by a chemical process. The oxidation of SiC component in the powder initiated at 1073K. The oxidation rate was significantly dependent on surface state of the powder. Jander's rate equation was applicable to the oxidation rate of SiC. The activation energy varied from 217 to 283kJ/mol. The oxidation of SiC is probably controlled by the diffusion of O2- ion in the SiO2 film formed around the SiC particles.

Toxicity of silicon carbide whiskers

There is growing interest in the use of ceramic materials, such as A1203, Si3N 4 and SiC in the form of whiskers for the reinforcement of ceramics and metals [1 3]. Epidemiological studies have established that exposure to certain fibrous minerals of natural occurrence is associated with increased incidence of mesothelioma a malignant tumour of the pleural or peritoneal cavity. Studies in experimental animals have shown that carcinogenicity (or at least the ability to induce mesothelioma) is restricted to long (> 10 #m), thin ( < 1 ~tm diameter) fibres and is relatively independent of chemical nature, providing the fibres are durable in the body (e.g. [4, 5]). Many whisker products consist of, or contain, fibres in the sub-micrometre diameter range and some have been shown to produce tumours in experimental animals [6]. We have prepared sub-micrometre diameter SiC whiskers by the carbo-thermic reduction of SiO2 and subjected this material to an in vitro cell culture test in which the response correlates well with the results of injection or implantation studies in animals [7]. A comparison has been made with crocidolite (blue) asbestos, which is well established as a causative agent for mesothetioma in human populations. The cellular response to the SiC whiskers is of the same order as crocidolite suggesting that in the absence of any additional information, such whiskers should be considered as potentially carcinogenic with appropriate handling precautions applied. A whisker-containing product of the carbo-thermic reaction (Fig. 1) contained a proportion of nonfibrous materials. The diameter range of the fibrous component is shown in Fig. 2 and no attempt to separate fibrous from non-fibrous material was made. The presence of free silica can lead to confusing results in the in vitro cell tests used and to avoid this the material was extracted with hot aqueous NaOH and washed in distilled water prior to use in the test. X-ray examination showed the SiC preparation to consist of highly crystalline cubic/?-SIC together with a minor amount of hexagonal c~-SiC and traces of mullite and corundum (A1203) from contamination by the crucible. The cell culture test has been previously described [8] and its specificity in terms of fibre size has been established [9]. Survival of V79-4 Chinese hamster lung cells was determined by cloning efficiency from a single-cell suspension. Cell suspension (20ml) was added to the appropriate quantity of test fibre in I ml of physiological saline in a sterile McCartney bottle. Aliquots of this suspension were seeded into 60-mm diameter plastic Petri dishes and the surviving cells allowed to grow and form colonies for 5 to 6 days. Colonies were then washed with physiological saline

Carbothermic reduction of silicon dioxide— a thermodynamic investigation

The carbothermic reduction of silicon dioxide at high temperature was studied using a free energy minimization (FEM) program. The stability boundaries of Si(l), SiC(s), and SiO2(l), and the equilibrium compositions of the gaseous species at various pressures, temperatures, and total composition of the system have been calculated based on available thermodynamic data. At 101.325 kPa total pressure, formation of Si(l) is possible if the initial C/SiO2 ratio in the system is kept between 1 and 2.34. Above this ratio, only SiC(s) is observed. The temperature range over which only Si(l) is present in the condensed phase depends on the composition and pressure of the system. For example, at an initial C/SiO2 ratio of 1.75 and 101.325 kPa pressure, Si(l) is stable from 3080 K to 2813 K, below which SiC(s) begins to form. The possibility of producing elemental silicon using carbothermic reduction of SiO2 in a plasma reactor is discussed.

Formation of silicon nitride from silica

The formation of Si3N4, during the carbothermic reduction of SiO2 in the presence of small Fe additions combined with nitridation is a complex process involving several solid-solid, gas-solid, gas-liquid-solid, and gas-gas reaction stages. The catalytic action of iron manifests itself in the formation of an intermediate compound — iron carbosilicide. The product of the nitridation of FeSixCy isα-Si3N4, and that of the nitridation of silicon, β-Si3N4.