In-situ formation of bulk and porous h-AlN/SiC-based ceramics from geopolymer technique

In this paper, bulk and porous AlN/SiC based ceramics were in-situ prepared from graphite/geopolymer composites based on the carbon thermal reduction reaction under high temperatures. Thermal evolution of graphite/geopolymer composites were systematically investigated by XRD, TG/DTA, DIL, SEM, and TEM. The results show that graphite co-exists with amorphous geopolymer below 900 degrees C, whereas leucite crystallizes from geopolymer matrix and thus results in graphite/leucite composites in the temperature range of 1000 degrees C-1400 degrees C. After being treated at 1500 degrees C, h-AlN emerges due to the carbon thermal reduction reaction. By further increasing the heating temperature to 1700 degrees C, h-AlN or SiC with flocculent structures dominate the resulting high-temperature products, depending on the sintering atmosphere. It has been proved that h-AlN is the predominant phase of the resulting products after being treated at 1700 degrees C in NZ atmosphere. Nevertheless, SiC becomes the dominant one in Ar atmosphere, ceteris paribus. Moreover, both h-AlN and SiC ceramics derived by this technique show porous features, with the porosity of 61% and 68% and considerable compressive strength of 0.76 MPa and 0.63 MPa, respectively.