Fly-ash Beads Preparing Sic Department Of Material Research

Fly ash from coal-fired power stations is the largest amount of industrial residue. Comprehensive utilization of fly ash is of great significance to economy and environment.In recent years,many experts and scholars have taken fly ash as the main raw materials to prepare high performance ceramic-based materials,such as Sialon powders,SiC-Al₂O₃ composite powders etc.Although fly ash as a low-cost raw materials can reduce the costs of ceramic,large quantity of spherical particles among fly ash——fly ash microspheres were ignored.Microspheres can be used not only as traditional building materials of low value,but also directly for the production of thermal insulation,fire resistance materials,or be utilized for high-strength, abrasion resistance,shielding,filtering and other functional materials through proper surface activation or modification.Although many efforts have done about value-added utilization of microspheres,but preparation of SiC,SiC/Mullite hollow spheres with fly ash microspheres has not reported.In this paper,according to fly ash chemical composition and particle morphology, fly ash microspheres and carbon black were taken as raw materials to prepare SiC/Mullite composite spheres,SiC hollow spheres and SiC/AlN-polytype composite spheres by microwave carbothermal reduction method.The effects of process conditions such as synthesis temperature,holding time,mole ratio of carbon to silicon and different carbon on the synthetic products were investigated.The reaction process and synthesis mechanism of various spheres also were analyzed.This paper aimed to explore a new way to synthesize high-performance SiC-based composite materials with relatively low cost,and to increase the level of value-added comprehensive utilization of fly ash.1 SiC/Mullite composite spheres prepared by microwave carbothermal reduction reaction with fly ash microspheres and carbon blackThe results show that:（1） The optimum conditions of SiC/Mullite composite spheres are n（C）:n（SiO₂）=4.2,1h at 1200℃,argon atmosphere.（2） The reaction mechanism mainly associates with solid-solid reaction between C（s） and SiO₂（s） on the surface of fly ash microspheres with its spherical shape unchanging when the temperature is between 1100℃～1300℃.（3） When the temperature is higher than 1300℃,SiC is easier orientation growth to whisker through VLS mechanics.Mullite is involved in the reaction;therefore,fly ash microspheres cann’t maintain its spherical shape.（4） The spherical morphology of prepared product is related to the survivorship of mullite.Excess content of carbon black barrier the adhesion among microspheres,which is necessary to maintain the shape of SiC/Mullite spheres.2 SiC hollow spheres prepared by microwave carbothermal reduction reaction with fly ash and carbon sphere templateThe results show that:（1） SiC hollow spheres could be prepared at n（C）:n（SiO₂） =4.2,0.5h at 1300℃,argon atmosphere.The SiC hollow spheres have retained the similar size with the original carbon sphere template.（2） The shell of SiC hollow sphere is composed of a lot of curving SiC wires.The formation of curving SiC wires should be attributing to the VLS and VS mechanism.（3） The shell thickness can be modulated by the n（C）:n（SiO₂） radio.Properly increasing the ratio is beneficial to synthesize SiC hollow spheres.Conversely,it is easy to synthesize SiC solid spheres.3 SiC/AlN-polytype composite spheres prepared by microwave carbothermal reduction nitridation reaction with fly ash spheres and carbon sphere templateThe results show that:（1） SiC/AlN-polytype composite spheres with gradient structure could be prepared at n（C）:n（SiO₂）= 4.2,0.5h at 1300℃,nitrogen atmosphere. （2） SiC/AlN-polytype composite spheres are composed of three layers:the shell （AlN-polytype）,the transition layer（SiC nanowires）,and the core（SiC whiskers and flower-like SiC crystals）.（3） The formation process of SiC/AlN-polytype composite spheres as follows:First,SiC hollow spheres which is composed of SiC nanowires formed.Then the Al,Al₂O steam decomposed from the Al₂O₃ in nitrogen atmosphere deposit on the surface of SiC hollow spheres to form the outer layer of AlN- polytype. After that,the surface densification results in internal and external gas exchange slowing down,which changing the gas saturation of growth of internal SiC,so that SiC grows along the different directions of the side line which result in the formation of flower-liked SiC crystals.