Sol - Gel, Microwave Carbothermal Reduction Synthesis Of Sialon, Sic Ultrafine Powder And Its Dielectric Properties

β-Sialon, O'-Sialon and SiC ultrafine powders are prepared by sol-gel and microwave carbothermal reduction method with tetraethoxysilane and aluminum nitrate as raw material. The influence of process conditions such as mole ratio of carbon to silicon, temperature, soaking time, the amount of crystal seeds and the amount of additive are studied. The absorbability and dispersion properties ofβ-Sialon and SiC ultrafine powders are characterized. The high temperature oxidation resistance process ofβ-Sialon ultrafine powders is also studied by non-isothermal kinetics. Besides, the dielectric properties ofβ-Sialon and SiC ultrafine powders are studied.The results of synthesizingβ-Sialon ultrafine powders preparation show that: (1) Excessive carbon is positive for synthesizingβ-Sialon ultrafine powders. The SiC powders bed for auxiliary heating is negative. To prolong soaking time and more N₂ flux is also positive for synthesizingβ-Sialon ultrafine powders. The relative content ofβ-Sialon ultrafine powders is increased with the increase of temperature, but when the temperature is above 1400℃,β-Sialon ultrafine powders converted to 15R phase and SiC. The optimum preparation temperature is 1300℃. The addition of crystal seeds shows very little effect on the preparation ofβ-Sialon. The relative content ofβ-Sialon ultrafine powders noticeably increases with Fe₂O₃ addition, but the effect of adding TiO₂ is negative. (2) The result of FE-SEM shows thatβ-Sialon ultrafine powders is granular and the primary particle size is less than 100nm; Analysis of particle-size analyzer and specific surface shows that agglomeration occurred and the secondary particle size is about 500nm. (3) The absorbability and dispersion properties ofβ-Sialon ultrafine powders are more excellent than that ofβ-Sialon powders prepared by conventional method. (4) BP artificial neural network was used to investigate the relationship between the mole ratio of carbon to silicon, the preparation temperature and the relative content ofβ-Sialon ultrafine powders. A non-linear model with well prediction results is established.The results of O'-Sialon synthesis indicate: Under the given condition, O'-Sialon can be synthesized by sol-gel and microwave carbothermal reduction nitridation method, but it is difficult form pure O'-Sialon in the case of mole ratio of carbon to silicon is 1:3.5, temperature is 1400℃and soaking time is 2h. Excessive carbon is positive for synthesizing O'-Sialon ultrafine powders. Prolong the soaking time also promotes the process. There is little effect on O'-Sialon preparation with crystal seeds addition.The results of SiC ultrafine powders preparation show that: (1) Excessive carbon is positive for synthesizing SiC ultrafine powders. With the increase of temperature, the crystal shapes grow to perfect. The optimum temperature is 1300℃. To prolong the soaking time promotes the synthesis process. There is little effect on SiC preparation with crystal seeds addition. (2) The result of FE-SEM shows that SiC ultrafine powders is granular and its primary particle size is 60nm more or less; The analysis of particle-size analyzer and specific surface shows that there occurs agglomeration and the secondary particle size is about 200nm.(3) The absorbability of SiC ultrafine powders is excellent.The result of oxidation resistance process ofβ-Sialon ultrafine powders shows that: The activity energy is changing with oxidation the process continuing. The oxidation mechanism ofβ-Sialon is different in different period. In initial stage, the mechanism oxidation ofβ-Sialon ultrafine powders is three-dimensional diffusion, whose function is reverse Janderequation . . The activity energy and preexponential factor is 240.5kJ/moland 5.7×10³/s, respectively. In final stage, the mechanism of oxidation is formation and growth of nuclei, whose function is Avrami-Erofeev equation [-ln(1-α)]³, the reactionorder is 3. The activity energy and preexponential factor is 410.7kJ/mol and 3.5×10¹¹/s, respectively.The results of dielectric properties ofβ-Sialon and SiC ultrafine powders indicate that: The permittivity ofβ-Sialon and SiC ultrafine powders change little with the frequency increasing. The values of permittivity ofβ-Sialon ultrafine powders prepared by microwave carbothermal reduction nitridation method are higher than that ofβ-Sialon powders prepared by conventional method. The imaginary part ofβ-Sialon ultrafine powders is almost zero, which is to say it is transparent for microwave. The real part and imaginary part of permittivity of SiC ultrafine powders is about 14 and 1.2, respectively. Lichtenecker formula is most suitable for calculation of the intrinsic dielectric properties ofβ-Sialon and SiC ultrafine powders.