Lignin–silica hybrids as precursors for silicon carbide

Lignin, prepared by digesting cedar with acetic acid, and tetraethoxysilane have been allowed for a sol–gel reaction in tetrahydrofuran using H₂SO₄ as catalyst to yield lignin–SiO₂ hybrids in the bulk gel form. The solid-state ¹³C nuclear magnetic resonance spectra of the hybrids and products formed solely from the lignin under the acidic conditions revealed that the lignin underwent crosslinking during the sol–gel reaction. The degree of crosslinking increased with an increasing amount of H₂SO₄. The powdered hybrids have been heated at 1500°C in Ar for carbothermal reduction, resulting in the formation of SiC powders. The lignin-to-tetraethoxysilane mixing ratio of the starting solutions varied free carbon content in the SiC powders. In addition, the amount of carbonaceous residue formed from the lignin upon heating depended on the degree of crosslinking of the lignin. Thus, to adjust the amounts of both the lignin and H₂SO₄ was necessary for producing the hybrids suitable for precursors for SiC powders with high purity. Critical adjustment of the amounts led to the formation of SiC powders with a free carbon content of 0.57 wt %, implying that the lignin is a beneficial carbon source for the production of SiC powders by the hybrid route. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1321–1328, 1999     

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.     

Effect of ultrafine carbon precursors on the morphology of silicon carbide nanoparticles

A comparative analysis of carbon precursors used for the synthesis of silicon carbide nanoparticles has been carried out. Phenol-formaldehyde resin, colloidal graphite, poly(vinyl alcohol), carbon black obtained as a byproduct of fullerene synthesis, and multiwall carbon nanotubes have been used as carbon sources in the synthesis of the starting ultrafine composition nC + mSiO₂. After carbothermic synthesis, the morphology of silicon carbide nanoparticles was studied by scanning and transmission electron microscopy. The morphologic features and particle-size distribution of the materials were comparatively studied.     

Mechanical properties and microstructure of biomorphic silicon carbide ceramics fabricated from wood precursors

Silicon carbide-based, environment friendly, biomorphic ceramics have been fabricated by the pyrolysis and infiltration of natural wood (maple and mahogany) precursors. This technology provides an eco-friendly route to advanced ceramic materials. These biomorphic silicon carbide ceramics have tailorable properties and behave like silicon carbide based materials manufactured by conventional approaches. The elastic moduli and fracture toughness of biomorphic ceramics strongly depend on the properties of starting wood preforms and the degree of molten silicon infiltration. Mechanical properties of silicon carbide ceramics fabricated from maple wood precursors indicate flexural strengths of 344±58 MPa at room temperature and 230±36 MPa at 1350 °C. Room temperature fracture toughness of the maple based material is 2.6±0.2 MPa√m while the mahogany precursor derived ceramics show a fracture toughness of 2.0±0.2 MPa√m. The fracture toughness and the strength increase as the density of final material increases. Fractographic characterization indicates the failure origins to be pores and chipped pockets of silicon.     

In-situ growth of silicon carbide nanowire (SCNW) matrices from solid precursors

In an effort to develop highly porous silicon carbide for high temperature air filtration, an alternative approach to forming silicon carbide nanowires (SCNW) was developed by blending carbon containing materials with silicon powder and heating these precursors to 1400?°C. The mixing ratio of precursor materials and processing temperature were investigated with respect to the formation of SCNWs. Results indicate that anthracite and starchy materials can yield high purity SiC ceramics, yet these combinations did not produce SiC nanowires. SCNWs were successfully grown from a combination of guar gum and silicon powder precursors at 1400?°C, when held for 4?h with an argon flow rate of 1?L/min. The produced SiC is a high purity product with nanowire diameters of approximately 40?nm and ranging in length from about 100?nm to several micrometers in length. Iron was used to catalyze the nanowire growth through vapor-liquid-solid (VLS) mechanisms by adsorbing the silicon and carbon vapor at the iron rich tip, which then led the nanowire growth. TEM analysis revealed the growth of SCNWs followed the [1,1,2] direction. A wafer comprised of the synthesized SiC nanowire matrix has much higher hardness compared with a wafer of the porous commercially available cordierite.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

Perhydropolysilazane precursors to silicon nitride ceramics

Preceramic perhydropolysilazane has been synthesized by the ammonolysis of dichlorosilane pyridine adduct. Perhydropolysilazane was polymerized and cross-linked by heat treatment with or without ammonia by dehydrogen condensation between SiH and NH and disproportionation at the SiH site in the polymers in the presence of pyridine. Ceramic yields of 82–93% were observed for perhydropolysilazane. The ceramic yield depended on the branching-group, i.e., –SiH < and –N<, content of the polymer. Controlled molecular weight distribution and elemental composition of the polymers obtained by heat treatment in the presence of pyridine meet various requirements for the production of shaped ceramics.     

Nonabrasive silicon carbide as rammed lining for boiler furnaces

Conclusions Nonabrasive silicon carbide (sludge), a waste product in the manufacture of abrasives, can be used as additive to the silicon carbide ramming compounds used for lining the stay-supported panels of boiler furnaces in place of standard fine-dispersion silicon carbide powders (Nos. 4–6).The fine-dispersion silicon carbide sludge can be used in place of less abundant materials. It increases lining life by a considerable margin and improves the properties of the lining.Translated from Ogneupory, No. 7, pp. 32–34, July, 1975.     

以水玻璃为硅源喷雾干燥制备SiC前驱体

以廉价的水玻璃和炭黑为原料,通过制备均匀混合的前驱体,利用碳热还原反应合成出超细SiC粉体。比较研究了前驱体喷雾干燥与搅拌干燥两种制备方法对体系碳热还原反应的影响,重点考察了喷雾干燥制备过程中主要工艺参数对前驱粉体密度及收率的影响。结果表明:喷雾制备的前驱体具有更高的反应活性,在1550℃下反应2h就可使SiO2转化率达到89.4%,搅拌干燥制备的前驱体转化率只有65.2%。喷雾干燥过程中,适当提高进口温度、喷雾头转速并降低料液的固体含量,可获得高收率、高产率的前驱粉体,利于整个制备工艺生产效率及产率的提高。     

Evaporation-condensation derived silicon carbide membrane from silicon carbide particles with different sizes

Silicon carbide ceramic is a promising membrane material because of the high corrosive and high temperature resistance, and the excellent hydrophility. Here, a silicon carbide ceramic membrane with both substrate layer and separate layer composed of pure silicon carbide phase was successfully prepared. The effect of particle size on the microstructure and properties was investigated. The substrates were prepared from three silicon carbide particles at 2200 ℃. With the content increase of fine particle, the average pore size increased from 5.6 μm to 14.1 μm; meanwhile, the flexural strength of the substrate increased from 14.1 MPa to 24.6 MPa. The separation layers were made from particles of 3.0 μm and 0.5 μm. When sintered at 1900 ℃, the separation layer formed pore network with homogeneous structure. Such silicon ceramic membrane can be used in harsh conditions, including high temperature wastewater and strongly corrosive wastewater.     

Sialon-containing silicon carbide refractories

Conclusions We worked out the technology of sialon-containing silicon carbide refractories for lining the shafts of blast furnaces. It was shown that addition of MgO in combination with the waste products of the oxides of the rare-earth elements improves the strength and the other properties of the products fired at 1550°C in nitrogen atmosphere.The sialon-containing silicon carbide refractories exhibit a high resistance to the action of slags and are recommended for lining the lower portion of the shaft and the bosh of blast furnaces.Translated from Ogneupory, No. 5, pp. 6–8, May, 1992.     

Synthesis and characterization of poly(dimethylsilylene-diphenylsilylene) as a precursor to silicon carbide

Poly(dimethylsilylene-diphenylsilylene) copolymer, [(SiMe_{2 x}(SiPh₂)]_n, where x=1.0 to 8.0, was synthesized by condensation polymerization of dimethyldichlorosilane and diphenyldichlorosilane in xylene. In the thermogravimetric analysis of the copolymer, weight loss started at 170‡C and the weight residue at 1,000‡C was 32%. The copolymer was stabilized by thermal oxidation at 80 to 110‡C for 5 hr in air, and its thermal stability increased as the temperature increased. The copolymer heat treated above 700‡C showed no organic bonds of C-H and Si-CH₃, while Si-C and Si-0 bonds were observed. Amorphous SiC was obtained by pyrolysis at 1,000‡C and β-SiC at 1,200 to 1,400‡C, in which better crystallinity of β-SiC was observed at a higher temperature.     

Foamed nitride-bonded silicon carbide refractories

Conclusions We obtained foamed silicon carbide insulating material with a nitride bond. The nitride bond is formed from a mixture of silicon nitride of the- and-modifications upon reaction with nitrogen. The formation of the nitride commences at about 1200°, and is practically complete at 1450°; the rate of nitriding of the foam ceramic is much greater than in dense specimens. The material has a homogeneous phase composition which is an advantage compared with foamed silicon carbide containing a complex bond obtained in carbon fillings.The foamed insulating brick based on nitride bond possesses a high thermal-shock resistance and structural strength at elevated temperatures. The strength is increased with an increase in the apparent density. The material is characterized by moderate mechanical strength, typical for silicon carbide refractories of this type [1, 3, 4].The foamed silicon carbide brick with a nitride bond can be recommended for service at high temperatures in nonoxidizing atmospheres when substantial thermal-shock resistance is required.Translated from Ogneupory, No. 7, pp. 48–51, July, 1970.     

Oxidation of polycrystalline silicon carbide

The oxidation of densified silicon carbide has been studied by micrography and infrared reflection spectrometry, in addition to gravimetric techniques and X-ray diffraction. Of particular interest was the relative oxidation resistance of varieties of the material treated in various ways. The hot-pressed type oxidized less readily than the sintered, and annealing was found to impart substantial resistance to oxidation; these are thought to be impurity effects.