Research On Silicon Carbide Platelets Prepared By Double-hating Technique

Silicon carbide platelets are regarded as a good alternative to silicon carbide whiskers due to its good integrality, few defects, little environmental pollution and little health hazard to humans. At the present time, the production of silicon carbide platelets mainly adopts conventional heating equipment which has disadvantages such as high energy consumption, high cost and low production yield. After analyzing the characteristic of conventional heating and the electric feature of the raw materials, double-heating technique for preparing silicon carbide platelets is put forward. In order to put the new technique into practice, novel crucibles and double-heating furnace are designed and prepared. In order to testify this new technique and new equipment, the technologies of synthesis and post-processing are first studied. After that, silicon carbide platelets obtained are examined by the methods of XRF, XRD, SEM, etc. Finally, the growth mechanism of silicon carbide platelets is also studied.In this paper, powder heating releaser with adjustable resistance is prepared using phenolic resin and high temperature insulation dielectric as materials. Compounding planks with high resistance are obtained using powder heating releaser and high temperature adhesive as materials. High resistance carbon compounding crucibles are prepared using compounding planks, graphic planks and high temperature adhesives. Then double-heating furnace is prepared after special structure design and careful material choice. In double-heating furnace, the sample is affected by the inner and outer heating system at the same time. The outer one uses powder heating releaser and crucibles as heating units, while the heating unit of the inner one is the sample itself. After optimizing the powder heating releaser, high resistance carbon compounding crucibles and the structure of double-heating furnace, difficulties such as large energy consumption, small hearth volume, uneven temperature field, high synthesis temperature and long synthesis time are overcome.The synthesis technology, purification technology and classification technology of silicon carbide platelets are systematically studied. It is proved that double-heating furnace can fully meet the need of preparing silicon carbide platelets on a large scale with low cost.The results of synthetic experiments show that the optimum parameters are as follows.Carbon black and white carbon black are the desirable raw materials and the reasonable mass ratio is 1.9:1. The ideal quantity of catalyst is 5% of white carbon black. Also, silicon carbide whiskers are used and the quantity is 4% of white carbon black. Heated by alternating current at a heating velocity of 5℃·min-1, silicon carbide platelets are synthesized at a temperature of 1900℃for a holding time of 3h.The results of purification experiments reveal that platelets with 85.0% grade can be obtained after flotation. The remaining carbon can be removed by oxidation at 700℃for 1.5h in self-prepared de-carbon furnace. Metal impurity and SiO2 can be eliminated effectively after acid developing at 60℃for 5h with mixture of hydrofluoric acid and hydrochloric acid. The results of classification experiments show that platelets can be classified by sift method, free settlement and hydrocyclone separation with tertiary series connection successively. After classification, silicon carbide platelets are separated from particles and whiskers to a certain extent and platelets with diameter of less than 50μm and grade of 95.6% are enriched.The examination results indicate that silicon carbide platelets synthesized by double-heating technique under the optimum condition have higher performance index. Silicon carbide platelets are well-dispersed with the average diameter of 50～100μm and the average thickness of 2～10μm. The productivity of platelets is as high as 85% and the content of SiC in it is up to 99.83%. The XRD pattern of the platelets demonstrates that the crystals belong toα-SiC.Thermodynamics and kinetics analyses of the synthesis process are also studied. Combined with the appearance examination of the platelets, two kinds of growth mechanisms are determined. The nucleation and growth stages of the platelets can be well explained by the mechanism of planar two-dimension nucleation. It is also found that some platelets are prepared through transformation from silicon carbide whiskers at high temperature.The research results indicate that double-heating technique is an effective measure to produce silicon carbide platelets on a large scale with the advantages of low energy consumption, short synthesis time and high efficiency.