Carbothermal Reduction Nitridation Process Research Articles

NMR study on the formation mechanism of β-SiAlON from zeolite by nitridation using ammonia gas

β-SiAlON was synthesized from a zeolite by NH 3 gas nitridation and its formation mechanism was investigated using X-ray diffraction and 29 Si and 27 Al NMR spectroscopy. It was revealed that most of the Si and Al atoms react to form β-SiAlON via amorphous forms of Si-Al-O-N and Ο-SiAlON. Nitridation using NH 3 gas is an effective means of preventing mullite formation and promoting the introduction of nitrogen into aluminosilicate materials at lower temperatures than temperatures required by the carbothermal reduction nitridation process. Further, the NMR spectra showed that the siliceous part of the system changed into low z-value of Si 6-z Al z O z N 8-z (β-SiAlON) and the incorporation of Al components into the β-SiAlON was promoted in the later stages of the reaction.

Transformation Potential of Kalemaden Clay – 220 to Technological Ceramics

In this study, the Clay – 220 mineral of Turkish origin from Kalemaden A.S., having a composition of the quarts, kaolinite, feldspar and the others with the weight percent of 36.20, 12.66, 48.03 and 3.11, respectively, was used as a raw material to investigate its transferability potential to a technological ceramic. The mineral was in the form of conglomerate rocks, which were prepared in the consecutive mineral processing procedures before taken it to the stage where carbothermal reduction - nitridation (CRN) was performed in an atmosphere controlled environment. Various process parameters, i.e., temperature, time and N2 – flow rate were tested in order to optimise the CRN process. SiAlON was formed at temperatures as low as 1300oC and its content increased with reaction temperature up to 1475oC. Above 1425oC, SiC becomes more predominant phase in the powdered product after CRN. Fine powders formed at temperature of 1350oC, which is the lowest value reported among the current literature, contain single crystalline phase of β-SiAlON. The results indicated that the best transformations and yield to the SiAlON ceramic were occurred on the previously prepared Clay – 220 samples in powder form that were processed at 1350oC for 6 hours with nitrogen flow rate of 1 lt/min.

Influence of Mg2+ on the formation of β-SiAlON by the carbothermal reduction-nitridation of homogeneous gel

Homogeneous gels with Al–O–Si bonds were prepared by the sol–gel method using aluminum iso-propoxide (AIP) and silicon ethoxide (TEOS) containing Mg2+ added as Mg(NO3)2. Via carbothermal reduction-nitridation (CRN) of the as-prepared gels, β-SiAlON was prepared, the yield of which was determined by XRD measurements. At 1500 and 1550°C in the CRN process, single-phase β-SiAlON was prepared from the precursor gel with a molar ratio (Mg(NO3)2/(AIP+TEOS)) of 0.5 and 1.0, respectively. Mullite and χ-SiAlON were obtained using other preparation conditions. The concentration of Mg2+ in the gel affected the yield and z value of β-SiAlON, that is, the yield and the z value increased with increasing concentration of Mg2+ in the gel. From energy dispersive X-ray analysis (EDX), it was found that no Mg was present in β-SiAlON prepared above 1500°C. The process of the formation of β-SiAlON above 1400°C is postulated on the basis of X-ray diffraction and EDX analyses.

Sepiolite-PAN intercalation used as Si3N4 forming precursor

Silicon nitride (Si3N4) formation was investigated using sepiolite and polyacrylonitrile as silicon and carbon source, respectively. It was found that purified sepiolite could readily adsorb a sufficient amount of acrylonitrile (AN) reagent without pre-treatment. Polymerisation of sepiolite-AN mixtures and subsequent cyclization of the polymerised complex yielded a precursor, which was found ideal as a starting material in carbothermal reduction-nitridation (CRN) for the formation of silicon nitride powders. The quantity of silicon nitride, grain size and morphology were found to be dependent on the reaction conditions and starting reagent. Fine grain size, high surface area (up to approx. 12.4 m2/g) powders of mainly β-Si3N4 were obtained via pyrolysis of sepiolite-PAN complex after 4 h heating at 1400°C in 1000 ml/min nitrogen flow with a heating rate of 300°C/h. Mg retained in the molecular structure of the mineral must have promoted the formation of β-grains in CRN process.