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Wang et al. also performed a computer analysis of the expected reactions and found several differences between the experimental and calculated data. Although the differences that were found are not important, the cause of these differences was worth noting. The computer program used to predict chemical reactions at different combinations of temperature and pressure was dependent upon the database used, and thus could not predict products not contained in the database. The program used by Wang et al. could also analyze only a closed system at equilibrium. Any reactions (e.g., formation of interfacial layer) that may retard further reaction would prevent equilibration. Insufficient time for reactions to proceed to completion would also contribute to the differences, because the computer program based upon minimization of total free energy of formation cannot predict the kinetics of the reactions. Thus, one should remember that calculated reactions based upon thermodynamics is only a portion of any study, and only an indication of what should be expected during actual experimentation.

Alumino-Silicatcs

Arnulf Muan has provided a considerable amount of experimental data concerning the atmospheric effects upon the phase equilibria of refractory materials. One such article was reprinted in the Journal of the American Ceramic Society [5.93] as a commemorative reprint. This article stressed the importance of the oxygen partial pressure in determining the phases present in the reaction of iron oxides with alumino-silicate refractories. Under oxidizing conditions, large amounts of ferric iron can substitute for aluminum in the various aluminum-containing phases; however, under reducing conditions, this substitution is negligible. Large volume changes accompany some of the phase changes that occur with damaging results to the refractory. In addition, the temperature at which liquid phase develops decreases as the oxygen partial pressure decreases.

Reactions that have occurred between alumino-silicate refractories and the gaseous exhaust in glass furnace regenerators

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at about 1100–1200°C forming nepheline and noselite are shown below:

(5.36)

The nepheline formed then reacts with SO3 and more Na2O vapor of the exhaust forming noselite:

(5.37)

Although the precise mechanisms that take place have not been determined, most likely, the alumina and any silica available form more nepheline. Large volume expansions (10–15%) accompany these reactions resulting in spalling or shelling. Historically, these reactions have been a serious problem to the glass manufacturer, as they cause plugging of the regenerator and a less efficient combustion process. Various regenerator design and material changes have essentially eliminated this problem; however, it is a reaction that may still occur when the conditions are appropriate.

Magnesia-Containing Materials

McCauley and coworkers [5.94–5.97] have studied the effects of vanadium upon the phase equilibria in magnesia-containing materials. This work was initiated in an effort to understand the effects of vanadium impurities in fuel oils on basic refractories. It has been found that only small amounts of V2O5 are needed to alter the phase assemblages in high magnesia materials. The reactions that occur generally form low melting vanadates (i.e., tricalcium and trimagnesium vanadates with melting points of 1380 and 1145°C, respectively, and magnesium-calcium-vanadium garnet with its melting point of 1167°C) and, depending upon the exact compositions, can develop appreciable amounts of liquid at service temperatures.

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Although the initial reaction is a gaseous phase reaction, it quickly converts to a liquid attack.

Ready [5.98] has shown that magnesia containing a small percentage of carbon black or graphite can form CO/CO2 bubbles in oxidizing atmospheres at temperatures ranging from 1200 and 1500°C. These bubbles, which formed primarily at grain boundaries, were the source of intergranular fracture. For the unoxidized materials, the fracture was 100% transgranular, because no grain boundary separation (caused by the presence of bubbles) was present.

Zirconia

Lepistö et al. [5.99] studied the effects of humid conditions at 150°C for up to 1000 hrs upon the stability of metastable tetragonal phase in tetragonal zirconia polycrystals (TZP). Several different materials containing a small amount of yttria were tested, and all were found to contain increasing amounts of monoclinic zirconia as the exposure time increased. It was believed that surface finish along with the grain size and yttria content all had an effect upon the transformation of the tetragonal-to-monoclinic zirconia. In a later study, Lepistö and Mäntylä [5.100] concluded that the stability of yttria-containing TZP in humid atmospheres was through the dissolution of ZrO2 at grain boundaries with the subsequent relief of localized stresses, followed by the transformation of the tetragonal-to-monoclininc phase. The proposed mechanism of dissolution was through the dissociation of adsorbed water molecules on the zirconia surface. The oxygen ions formed were then proposed to anneal the oxygen vacancies present within the yttria-containing zirconia. As a result, water is formed, which then allowed continued dissolution. The water adsorption was greatest at defect sites, and thus was dependent upon the yttria concentration. Lepistö and Mäntylä found that both yttria and alumina increased the dissolution rate, whereas ceria did not, as it did not change the vacancy concentration. Thus the tetragonal phase stability toward humid atmospheres can be increased by using ceria as the dopant.

Copyright © 2004 by Marcel Dekker, Inc.