Quarzsprung

While the destruction of refractories by carbon deposition in CO atmospheres is still a relevant issue for manufacturers and users of refractory products, the influences on the CO resistance of refractories are largely unexplored due to a lack of quantitative measurement data on the carbon deposition in the refractory structure. Thermogravimetrical measurements might be capable of generating this missing data and making a contribution to the targeted development of CO resistent refractory materials.

The present standardized test methods to determine the CO resistance of refractory materials are based on a classification of tested materials into groups "A" to "D" according to ISO 12676 and ASTM C 288 and thus do not allow any quantitative, reproducible statements regarding the CO resistance of refractories. This hinders the targeted development of CO resistant refractory materials in the refractory industry. Based on the results of previous research carried out at Forschungsgemeinschaft Feuerfest e. V., a new, quantitative and reliable testing method was developed in cooperation with Koblenz University of Applied Science. In this newly developed testing methodology, the rate of carbon deposition in refractory materials (under service conditions) can be determined using in situ thermogravimetry and thus can be correlated with varying test and material parameters such as the flow rate of CO and the concentration of iron compounds within the refractory materials. This new testing method enables a reliable and reproducible evaluation of the CO resistance of refractory materials.

In this study the influence of parameters like heating rate and dwell time on the kinetics of the nitridation process and microstructural development of nitride bonded silicon carbides was investigated. A combination of in situ and ex situ methods was used with special focus on the spatial formation of nitride phases.The nitridation of SiC–Si green bodies was carried out at heating rates of 1, 3 and 5 K/min and different multi-stage firing profiles were used. All samples were analysed by Raman spectroscopy and SEM. It was observed that with higher heating rates the number of α-Si3N4 whiskers in the matrix was reduced. An effect that was achieved with the direct firing to a temperature above the silicon melting point at 1409 °C as well. However, high heating rates have the disadvantage of a higher amount of residual silicon in the matrix after cooling.

The disintegration of refractory materials in CO containing atmospheres, due to the deposition of carbon in the microstructure in the presence of reducible iron phases, is a relevant issue for both refractory manufacturers and users. With a new testing method, developed previously by Forschungsgemeinschaft Feuerfest e. V., reproducible and quantifiable in situ information on the reaction kinetics of this destruction process in unshaped refractory materials can be determined. With this method, the reaction kinetics of the carbon deposition was investigated in dependency of the pore structure of the refractory material by measuring the open porosity together with thermogravimetrical analysis to collect data on reaction kinetics of the carbon deposition in CO containing atmospheres. The results of these investigations are helpful to develop new CO resistant refractory materials and to optimize already existing material systems for the application in reducing conditions.