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Öğe 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.Öğe Fully reacted high strength geopolymer made with diatomite as a fumed silica alternative(Elsevier Ltd, 2017) Bağcı, Cengiz; Kutyla, Gregory P.; Kriven, Waltraud M.Geopolymers are formed by mixing of aluminosilicate sources with alkaline meta-silicate solution at room temperature. In the current study, diatomite of Turkish origin was fully utilized as a fumed silica alternative for the preparation of geopolymer, having a typical formula of K2O•Al2O3•4SiO2•11H2O. From XRD of this sample, a broad peak centered at 28° 2? indicated the well-known formation of amorphous geopolymer, as well as a fully reacted microstructure of geopolymer as seen by scanning electron microscopy. Additionally, geopolymer having the same formula was made by using fumed silica, in order to compare with geopolymers prepared from diatomite. The Weibull modulus was calculated from four-point bending and compressive strength testing of both geopolymer composites. The use of diatomite as a fumed silica substitute in geopolymer production resulted in a very close flexure strength 9.2 (± 4.2 MPa) when compared to geopolymer made from fumed silica 10.2 (± 3.3 MPa). There was a significantly higher compressive strength 71 (± 13.9 MPa) and Weibull modulus (5.4), than comparable properties of geopolymer made from fumed silica, which had a compressive strength 54 (± 25.8 MPa) and Weibull modulus of 2.0. The discrepancy was attributed to some self-reinforcement of the geopolymer matrix due to unreacted diatomite. © 2017 Elsevier Ltd and Techna Group S.r.l.Öğe In situ carbothermal reduction/nitridation carbon-nano powder added geopolymer composites(American Ceramic Society, 2014) Bağcı, Cengiz; Kutyla, Gregory P.; Kriven, Waltraud M.Geopolymer was prepared by mixing metakaolin (Al2O3 center dot SiO2) with sodium water glass solution and then the activated carbon nano powder was added the geopolymer. To determine the best composition for carbothermal reduction, one series geoploymer was made with and without carbon by hand. The viscous and homogenous slurry was poured into a mold to obtain bar samples at ambient temperature and cured in a humidity-controlled, constant temperature oven at 50 degrees C for 24 h. The composition of Na2O center dot Al2O3 center dot 4.5SiO(2)center dot 12H(2)O+9C was determined to be best for carbothermal reduction. The carbothermal reduction process of the samples were carried out in an atmosphere controlled tube furnace both under argon (denoted as CTR) and nitrogen (denoted as CTRN) flow of 5 cm(3).min(-1) at temperatures varying between 1450-1550 degrees C for 2h. XRD and SEM analyses were used to determine in situ transformation and morphology of all the products after CTR and CTRN. The results were briefly discussed with respect to possibility of in situ conversion of geopolymers to its carbide and nitride analogues.Öğe Synthesis and characterization of silicon carbide powders converted from metakaolin-based geopolymer(Blackwell Publishing Inc., 2016) Bağcı, Cengiz; Kutyla, Gregory P.; Seymour, Kevin C.; Kriven, Waltraud M.Silicon carbide (SiC) ceramic powders were synthesized by carbothermal reduction in specific geopolymers containing carbon nanopowders. Geopolymers containing carbon and having a composition M2O·Al2O3·4.5SiO2·12H2O+18C, where M is an alkali metal cation (Na+, K+, and Cs+) were carbothermally reacted at 1400°C, 1500°C, and 1600°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°C and underwent significant weight loss. SiC was seen as the major phase resulting from Na-based geopolymer heated to ?1400°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°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 ~5 ?m to nanosize, as seen by X-ray diffraction as well as SEM and TEM, respectively. © 2016 The American Ceramic Society
