Papers by Keyword: Carbothermal Nitridation

Abstract: The effect of nitrogen gas flow rate on the morphology of silicon nitride fibers obtained via carbothermal nitridation heat treatment method was investigated. A precursor containing silicon, oxygen and carbon was obtained by a sol-gel method from a mixture of tetraethyl orthosilicate, polyvinyl alcohol, H₂O and ethanol. A white wool-like product was obtained by heat treating the precursor placed in an alumina crucible under a 0.5 MPa nitrogen gas pressure at 1500oC with different nitrogen gas flow rates. The mass-based production rates of the samples obtained from the precursor powder were 20-30% for the different nitrogen gas flow rates. X-ray diffraction analysis revealed that the samples contained α-Si₃N₄ as the major phase along with β-Si₃N₄, Si₂N₂O and a small amount of amorphous product as minor phases. Unique twisted fibers with diameters of several hundreds of nanometers were found among the straight fibers by SEM observation. Elemental analysis using energy dispersive X-ray spectroscopy indicated that silicon and nitrogen were contained in the twisted fibers along with approximately 68 at.% of oxygen and several at.% of aluminum, which might have come from the crucible material. The SiAlON-like structures might have been formed by the partial dissolution of Al and O in the Si₃N₄ fibers. It was considered that the twisted morphology of some fibers might be formed by co-existing of β-SiAlON and/or amorphous phase regions in the Si₃N₄ fiber and resultant distortion of the fibers.

Abstract: In this paper, the synthesis of MgAl2O4-SiAlON composite powders by forsterite, alumina and carbon black was studied. The samples were calcined under nitrogen atmosphere at 1500 °C, 1600 °C, 1650 °C for 3 hrs respectively. The phase compositions and microstructure of the synthesized powders were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The result shows that an optimal alumina/forsterite content was a molar ratio of 2.0:1.0, the products consisted mainly of MgAl2O4 and Si3Al3O3N5 at 1500 °C, MgAl2O4 and Mg1.25Si1.25Al2.5O3N3 at 1600 °C and 1650 °C. The temperature can improve the formation of the octahedral spinel and flakiness Mg1.25Si1.25Al2.5O3N3.

Abstract: In this study, our objective is to deposit an alumina or an aluminum nitride layer on a turbostratic carbon substrate. The coatings are synthesized via a sol-gel route followed by a heat treatment in order to obtain -alumina or hexagonal aluminum nitride by carbothermal nitridation of alumina. The synthesis of such layers on carbon substrates is not reported in literature. Several slurries were elaborated using various solvents and catalysts, and aluminum-tri-sec-butoxide as an aluminum precursor. The coating is obtained by dropping the substrate in the sol. After drying and pyrolysis, the amorphous alumina layer obtained has a thickness ranging from 500 nm to 1 µm. The material is finally heat treated. Several treatment conditions were evaluated. A thermodynamic study of the Al-C-O-N system will be drawn, and the composition of the synthesized sol-gel and heat treatment parameters will be detailed. Then, the layer’s morphology and structure will be characterized thanks to Scanning Electron Microscopy and X-Ray Diffraction analyses. The impact of heat treatment parameters will be discussed and experimental results will be compared to the theoretical thermodynamic results.

Abstract: In this paper, a carbothermal reduction and nitridation (CRN) synthesis for cubic aluminum oxynitride (γ-AlON) powders is reported. The CRN reaction was conducted via a two-step synthesis procedure at a heating rate of 100 °C/min. In the first step, the mixture of fine γ-Al2O3 and soluble starch was annealed at 1550~1580 °C for different duration. It is found that the mixture of α-Al2O3 and AlN with an appropriate ratio can be obtained by CRN reaction. The content of AlN in mixture is closely related with the reaction condition. In the second step, the as-received mixture of α-Al2O3 and AlN was heat-treated at 1700 °C for 10 min. The single phase γ-AlON powders with particle size less than 3 µm were obtained.

Abstract: Rod-like α-sialon was synthesized successfully using pure SiO2 and AlN as the starting materials, carbon black as reductant, CaF2 and Y2O3 as addition agent by carbothermal reduction-nitridation. The effects of reaction temperature (1450°C, 1500°C, 1600°C and 1700°C) and additive (Li2CO3, CaF2, Y2O3 and Y2O3+CaF2) on phases and microstructure of the final products were studied by XRD and SEM. The results showed that α-sialon was synthesized by carbothermal reduction-nitridation at 1700°C for 3 hours. The morphology of the synthesized α-sialon was rod-like.

Abstract: The kaolinite-polyacrylamide intercalation compound was prepared by the displacement reaction of the kaolinite-formamide intercalation precursor with acrylamide and the polymerization under 140°C for 15h with the catalysis of dibenzoyl peroxide. Sialon was synthesized from kaolinite-polyacrylamide intercalation compound by carbothermal reduction and nitridation (CRN) processing at 1400°C. The kaolinite-carbon mixture and its CRN product were also prepared for the comparison with the CRN result of intercalation compound. XRD, FT-IR, and TEM were used to characterize the phases, structure and morphology of products. In the CRN product from the intercalation compound, β′-sialon is the main phase. And the reduction and nitridation rate from the intercalation compound was greater than that from the mixture. There was higher sialon phase’s content and lower mullite phase’s content in the CRN product from the intercalation compound than that from the mixture. The CRN processing of kaolinite intercalation compound is a novel and effective method of sialon synthesis.

Abstract: Bulk ceramic materials on the base of β-sialon were prepared from hydrosilicates. Except of some mechanical properties (HV, KIC) also the corrosion resistance of β-sialon against Cl-, iron and steel was investigated. The results were compared with β-sialon of similar composition z = 3.8, which was prepared from commercial powders Si3N4, AlN and Al2O3. The differences in mechanical properties of β-sialons prepared from natural hydrosilicates and synthetic powders are discussed.