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    Formation of alpha/beta-Si(3)N(4 )nanoparticles by carbothermal reduction and nitridation of geopolymers
    (Wiley, 2019) Bağcı, Cengiz; Yang, Qun; Kriven, Waltraud M.
    Silicon nitride (Si3N4) particles with various alpha/beta-Si(3)N(4 )ratios were fabricated from geopolymer (GP)-carbon compositions (M2O center dot Al2O3 center dot 4.5SiO(2)center dot 12H(2)O+18C), where M is an alkali ion (Na+, K+ and Cs+). They were made by carbothermal reduction and nitridation at 1400 degrees, 1500 degrees, and 1600 degrees C for 2 hours under flowing nitrogen. Characterization of carbothermally reacted GP-carbon compositions was based on XRD, SEM-EDS, HRTEM, and selected area electron diffraction analyses. Depending on the alkaline composition of GP, the carbon content and the reaction temperature, a compositionally variable alpha/beta-Si3N4 or SiAlON was achieved. Crystallization of the GPs gradually increased by heat treatment over 1400 degrees C with corresponding weight loss. It was found that NaGP, KGP, and CsGP crystallized into a major phase of alpha-Si3N4, beta-Si3N4, and SiAlON, respectively. Prolonged heating at 1600 degrees C led to an increase in the alpha/beta-Si3N4 ratio in NaGP due to the formation of aluminum nitride, while it led to a decrease in alpha/beta-Si3N4 ratio in KGP. In the case of CsGP, SiAlON replaced the pollucite which mainly formed at lower temperatures. Transmission electron microscopy revealed that the needle-like particles were of ~0.5 mu m in size and consisting of alpha/beta-Si3N4 mixtures.