Iranian Journal of Materials Science and Engineering

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Abstract: Multiwalled carbon nanotubes (MWCNTs) were coated with MgO nano particles using simple precipitationmethod. The growth of Mg(OH)2particles was controlled by adjusting the alkaline concentration, salt concentrationand feed rate in simple precipitation method. The nanometer-sized Mg(OH)2particles were precipitated on the surfaceof functionalized MWCNTs by reaction between MgSO4 solution and NH4OH. The samples have been characterizedby scanning electron microscopy, energy dispersive X-ray spectrometry, X-ray diffraction and thermal gravimetricanalysis. The results showed a nominally complete MgO coating over the entire outer surface of MWCNTs resulting inimprovement of their oxidation durability.

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Colloidal silica bonded refractory castables have been developed recently. It was found that colloidal silica is one of the best binders can substitute other binders such as cement in No Cement Castable (NCC) and Ultra Low Cement Castable (ULCC) refractories. Also composition of colloidal silica with appropriate additives resulted in a gel form which makes the initial strength. Moreover, the nano size silica particles are extremely reactive in high alumina castables and may encourage the mullite formation in the microstructure. In the current study, four castables were prepared. The sample containing 6wt % microsilica was a reference, then microsilica was replaced by different amount of colloidal silica (2.5, 5, 7.5 wt %). Silica and water content was kept constant. It’s concluded that the castables containing the optimum amount of silica sol shows remarkable increase in both castable fluidity and mechanical strength (CCS and MOR) in dried and sintered state. It was also found that nanosilica particles increase the rate of needle-shaped mullite formation during sintering at 1400°C. According to FTIR results, the addition of Calcium Aluminate Cement (CAC) to the silica sol may be responsible for the increment of siloxane bridges (Si-O-Si).

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Metakaolinite-based geopolymer has been synthesized at about 25 °C from metakaolin which has been calcined in different temperatures (600-900 °C) and different Na2O/SiO2 ratio activator (0.3-1.1). Compressive strength and microstructure of cement pastes after 7-28 days curing at ambient temperature were measured. Compressive strength tests on the samples showed that the sample made with calcined kaolin at 700 °C and molar ratio of 0.6 has highest average compressive strength of 32 MPa after 28 days of curing. Evaluation of infrared spectroscopy (FTIR) and microstructure showed that geopolymer cement developed and new molecular structure established

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An accurate prediction of reaction kinetics of silicon nitridation is of great importance in designing procedure of material production and controlling of reaction. The main purpose of the present study is to investigate the effect of temperature on the kinetics of reaction bonded silicon nitride (RBSN) formation. To achieve this, nitrogen diffusion in the silicon nitride layer is considered as a reaction controlling factor and sharp interface method based on this theory is used to develop the analytical model. In the developed model, the variations in the size of silicon particles are calculated for the whole reaction. In the experimental phase, the extent of nitridation is measured for different reaction temperatures and 4 different reaction times and then, the occurrence of full nitridation is shown by EDS analysis. Furthermore, an analytical approach was established for describing the kinetics of compound formation and the performance of the developed model is evaluated through statistical analysis. There was good agreement between experimental data and predictions of the developed model which demonstrates the accuracy of considered presumptions and reaction mechanisms. An accurate prediction of reaction kinetics of silicon nitridation is of great importance in designing procedure of material production and controlling of reaction. The main purpose of the present study is to investigate the effect of temperature on the kinetics of reaction bonded silicon nitride (RBSN) formation. To achieve this, nitrogen diffusion in the silicon nitride layer is considered as a reaction controlling factor and sharp interface method based on this theory is used to develop the analytical model. In the developed model, the variations in the size of silicon particles are calculated for the whole reaction. In the experimental phase, the extent of nitridation is measured for different reaction temperatures and 4 different reaction times and then, the occurrence of full nitridation is shown by EDS analysis. Furthermore, an analytical approach was established for describing the kinetics of compound formation and the performance of the developed model is evaluated through statistical analysis. There was good agreement between experimental data and predictions of the developed model which demonstrates the accuracy of considered presumptions and reaction mechanisms.

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In the present study, a soft computing method namely the group method of data handling (GMDH) is applied to develop a new and efficient predictive model for prediction of conversion percentage of silicon. A comprehensive database is obtained from experimental studies in literature. Several effective parameters like time, temperature, nitrogen percentage, pellet size and silicon particle size are considered. The performance of the model is evaluated through statistical analysis. Moreover, the silicon nitridation was performed in 1573 k and results were evaluated against model results for validation of the model. Furthermore, the performance and efficiency of the GMDH model is confirmed against the two most common analytical models. The most effective parameters in estimating the conversion percentage are determined through sensitivity analysis based on the Gamma Test. Finally, the robustness of the developed model is verified through parametric analysis.