Synthesis and Characterization of Silicon Carbide Powders Converted from Metakaolin-Based Geopolymer

Silicon carbide (SiC) ceramic powders were synthesized by carbothermal reduction in specific geopolymers containing carbon nanopowders. Geopolymers containing carbon and having a composition M2O center dot Al2O3 center dot 4.5SiO(2)center dot 12H(2)O+18C, where M is an alkali metal cation (Na+, K+, and Cs+) were carbothermally reacted at 1400 degrees C, 1500 degrees C, and 1600 degrees C, respectively, for 2 h under flowing argon. X-ray diffraction and microstructural investigations by SEM/EDS and TEM were made. The geopolymers were gradually crystallized into SiC on heating above 1400 degrees C and underwent significant weight loss. SiC was seen as the major phase resulting from Na-based geopolymer heated to >= 1400 degrees C, even though a minor amount of Al2O3 was also formed. However, phase pure SiC resulted with increasing temperature. While a slight increment of the Al2O3 amount was seen in potassium geopolymer, Al2O3 essentially replaced cesium geopolymer on heating to 1600 degrees C. SEM revealed that SiC formation and a compositionally variable Al2O3 content depended on the alkaline composition. Sodium geopolymer produced high SiC conversion into fibrous and globular shapes ranging from similar to 5 mu m to nanosize, as seen by X-ray diffraction as well as SEM and TEM, respectively.